No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of Elevated Temperatures on Mechanical Performance of Normal and Lightweight Concretes Reinforced with Carbon Nanotubes
Abstract
An experimental program was developed to evaluate the effect of multi-walled carbon nanotubes (MWCNTs) inclusion on elevated temperature properties of normal weight concrete (NWC) and lightweight concrete (LWC). The mechanical performance was assessed by conducting material property tests namely compressive strength (f’c ,T ), tensile strength (f’t,T ), mass loss (MT ), elastic modulus (ET ), compressive toughness (Tc ) and stress–strain response under unstressed and residual conditions in the range of 23°C to 800°C. The mechanical properties were measured by heating 100 × 200 mm cylindrical specimens to 200°C, 400°C, 600°C and 800°C at a heating rate of 5°C/min. Results show that the inclusion MWCNTs in cementitious matrices enhanced the fire endurance. The relative retention of mechanical strength and mass of concretes modified with MWCNTs was higher. The stress–strain response of specimens modified with MWCNTs was more ductile. Microstructural study of cyrofractured samples evidenced the homogenous dispersion of nano-reinforcements in host matrix. Furthermore, the data obtained from high temperature material property tests was utilized to develop mathematical relationships for expressing mechanical properties of modified mixes as a function of temperature.
... Then, nanoinclusions can refine the composite's pore structure, amplify the hydration process, and prevent crack development [4,48,49]. For instance, carbon-based nanomaterials have presented promising improvements in the mechanical performance of cementitious composites at high temperatures due to their high strength, fractural toughness, and thermal stability [48,[50][51][52][53][54][55][56][57][58]. ...
... MWCNT were found to increase the resistance to thermal cracking of concretes due to the improvements in the bridging effects [48,[63][64][65]. They also improved the residual mechanical properties of concretes exposed to fire because they provided a more uniform distribution of thermal stresses [50]. Lower CBN contents also improved the thermal resistance of mortars due to improvements in the densification of their pore system, bondage between the nanofillers and the cement matrix, and the chemical stability of C-S-H [66]. ...
... However, no previous research focused on the evaluation of the effects of rehydration on the mechanical properties of cement-based materials containing carbon nanomaterials after their exposure to high temperatures. The scientific motivation for investigating the role of MWCNT in the process of portfire curing was the quantification of possible improvements in this concrete repair method due to nucleation effects, crack bridging effects, and improvements in adhesion to hydration products provided by MWCNT [48,50,67,68]. The scientific motivation for studying the role of CBN in the rehydration process was the possibility of quantifying the mechanical properties recovery of fire-damaged composites containing these nanoparticles due to the high water adsorption, nano-core, filling, and bonding effects of CBN [47,66,69,70]. ...
Concrete elements exposed to fire can partially recover strength and stiffness if recured in water or in a moist environment. This paper reports a pioneering investigation of the influence of post-fire curing on residual mechanical properties and microstructure of cement mortars containing different concentrations of carbon-black nanoparticles (CBN) or multi-walled carbon nanotubes (MWCNT). A total of 84 specimens were subjected to various maximum exposure temperatures (200, 400, or 600 °C) and post-fire curing (water-recuring for 1 day followed by air-recuring for 27 days). X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS) were used to investigate the microstructure of different types of specimens. Experimental tests were carried out to determine the compressive strength, static modulus of elasticity, and dynamic modulus of elasticity of rehydrated specimens. Nanofillers enhanced the strength recovery of composites exposed to temperatures up to 400 °C, since they improved rehydration reactions. In this case, the rehydration process provided values of relative residual strength and elastic modulus factors higher than 85%. Composites containing 0.4% MWCNT and 3% CBN presented the best strength and stiffness recovery. Therefore, small contents of carbon nanomaterials provided additional nucleation sites for dehydration products to sediment and being rehydrated. Moreover, water adsorbed by the nanomaterials during the water-recuring process was used later to provide greater rehydration. Composites containing 1.2% MWCNT and 9% CBN presented lower mechanical properties recovery, as high contents of nanomaterials made it harder for water to reach decomposed C-S-H gel, hindering the development of rehydration reactions. After 600 °C, lower relative residual factors were obtained, but always higher than 40%. It happened because nanofillers were not able to provide those rehydration improvements, since their thermal decomposition started at about 500 °C.
... Some carbon nano-scale reinforcements such as SWCNT and MWCNT provided cementitious materials with higher resistance to thermal cracking due to the improvements in the bridging effects [25]. These carbon-based nanomaterials also improve the residual mechanical performance of fire-damage concretes because they provide a more uniform distribution of thermal stresses [36]. Previous studies also reported that graphite nanoplatelets (GNP) increased the thermal conductivity of cementitious composites, which was beneficial for releasing heat of hydration, maintaining temperature uniformity, decreasing thermal stresses, and providing lower strength degradation [37,38]. ...
... Therefore, previous studies [25,36,[39][40][41][42][43][44][45] reported increases in residual mechanical properties and ductility of fire-damaged cement-based materials with MWCNT, graphene oxide and graphene sulfonate nanosheets. These carbon-based nanofillers can develop a crack bridging action, reduce the number of capillary pores, improve the distribution of thermal stresses, work as channels for the release of autoclaving steam and prevent the damage caused by the high-pressure steam. ...
... However, the thermocouple measurements obtained in the present work indicated that a hold time of (60 ± 5) min was not enough to ensure a constant temperature from the surface to the core of all of the tested specimens. In fact, previous experimental studies have selected very distinct hold times to stabilize the temperature over the entire cross-section, such as 60 min [10,31,42], 90 min [41], 120 min [13,15,30], 150 min [25,36,38], 180 min or even more [34,72,73]. In the face of such variation, the methodology proposed by Kumar et al. [32] and Oliveira et al. [3] was used in the present work: the desired maximum temperature inside the furnace chamber (TF thermocouple) was maintained for the time interval enough to ensure a constant temperature from the surface to the core of the cylindrical specimens (TN and TF thermocouples). ...
Carbon-based nanomaterials can provide improvements of the mechanical behavior of cementitious materials at high temperatures. This research investigated mechanical properties and microstructure of 92 composites containing different concentrations of multi-walled carbon nanotubes (MWCNT) and carbon black nanoparticles (CBN), and exposed to temperatures up to 600 °C. A comprehensive comparison between the post-fire performances of these different carbon-based nanomaterials was made. Values of moisture loss and microstructure changes due to temperature increases elucidated variations of different mechanical properties. The addition of 3% of CBN and 0.4% of MWCNT increased the post-fire residual compressive strength, static and dynamic elastic modulus of plain mortars.
... Apart from significant deteriorated columns, RC element with viable structural health conditions were assessed to compare the results. The compressive strength of concrete is an important mechanical property to evaluate the structural strength of concrete [10,11]. Impact rebound hammer (IRH) and ultrasonic pulse velocity method (UPV) both predict the compressive strength of concrete. ...
... CSH gel tends to decompose and loses its structural integrity and binding properties with the rise in temperature which consequently affects the strength and durability of concrete matrix [17]. Scanning electron microscopy has been used by several researchers to analyze the decomposition of hydrates, development of cracks and the interfacial zone deterioration patterns of fire exposed concrete [11,17,18]. Concrete is a two component system i.e. paste and aggregate, the boundary separating the two is known as interfacial transition zone (ITZ). ...
... This loss is mainly associated to the evaporation of freely attached water and the development of minor-cracking. At 400°C, the retained strengthwas 84% (3417 psi), the loss in strength is due to the increased pore area fraction and subsequent increase in cracks[11]. At 600°C, the cracks in the matrix were doubled as evident from the micrographs shown inFigure 7(d), with severe deterioration in CH and CSH gel structure and a result the strength retention was about 67% (2726 psi). ...
After fire exposure the reinforced concrete structures lose strength and durability. Mapping the damage caused by fire exposure becomes extremely important to assess the serviceability of a fire exposed building. This case study covers in brief the structural health assessment of a fire damaged structure. To unravel the heating history various techniques were adopted. Non-destructive testing (NDT) methods were employed to assess the residual durability properties. Similarly, core extraction tests were performed to evaluate the residual mechanical properties. Further, a framework was developed with the help of microstructural and thermal analyses to reasonably estimate the exposure temperatures of various structural units. This methodology has efficiently predicted the degraded quality of Reinforced concrete based on Microstructural Structural Analysis and Non-Destructive Testing. Keywords: Non-destructive testing, Structural heath assessment, Ultrasonic pulse velocity, Microstructural analysis, Compressive strength
... Akçaözoğlu and Akçaözoğlu (2017) studied the residual physical and mechanical properties of fire exposed lightweight concrete produced with expanded clay aggregate or calcium aluminate cement, tested using different cooling regimes [19]. Baloch et al. (2018) studied the residual mechanical properties of fire exposed lightweight concrete incorporating carbon nanotubes [20]. ...
... Akçaözoğlu and Akçaözoğlu (2017) studied the residual physical and mechanical properties of fire exposed lightweight concrete produced with expanded clay aggregate or calcium aluminate cement, tested using different cooling regimes [19]. Baloch et al. (2018) studied the residual mechanical properties of fire exposed lightweight concrete incorporating carbon nanotubes [20]. ...
... A total of 289 data points compiled from eight papers [13][14][15][16][17][18][19][20] were used to develop an empirical formulation for relative compressive strength (fc/fc0) of lightweight concrete after exposure to high temperatures. Residual test type where the specimen is placed in the furnace after the water curing is completed was used by all eight papers. ...
This study proposes a new model for the residual compressive strength of structural lightweight concrete after exposure to elevated temperatures up to 1000 °C. For this purpose, a database of residual compressive strengths of fire exposed lightweight concrete was compiled from the literature. Database consisted a total number of 289 data points, used for generating training and testing datasets. Symbolic regression was carried out to generate formulations by accounting for various input parameters such as heating rate, cooling regime, target temperature, water content, aggregate type, and aggregate content. Afterwards, predictions of proposed formulation is compared to experimental results. Statistical evaluations verify that the prediction performance of proposed model is quite high.
... O emprego de nanoinclusões à base de carbono também tem sido proposto para melhoria do comportamento mecânico durante e após exposição a tratamentos térmicos severos. Por exemplo, nanotubos de carbono (CNTs) são excelentes alternativas para o aumento da resistência à fissuração térmica de concretos por promoverem efeitos de ponte (em inglês, bridging effects) [11], além de garantirem uma distribuição uniforme de tensões decorrentes de gradientes de temperatura [12]. Estudos anteriores também indicaram que nanoplateletes de grafite (GNP) reduzem tensões térmicas, mantêm uniformidade de temperatura e diminuem a degradação de propriedades mecânicas [13,14]. ...
... Melhoras na resistência à compressão residual de compósitos cimentícios contendo baixos teores de CNTs também foram reportadas por Baloch et al. [11,12] (dosagem de CNT de 0.08% da massa de aglomerante), Zhang et al. [33] (0.1% e 0.2% de CNT) e Sikora et al. [34] (0.125-0.5% de CNT). Os benefícios promovidos pelos baixos teores de CNT utilizados nesses estudos e na presente pesquisa podem ser atribuídos aos conhecidos efeitos de ponte promovidos por esses nanomateriais e pela liberação de pressão de vapor através da nanoestrutura vazada dos mesmos, o que previne a propagação de fissuração devido ao aumento de temperatura. ...
Este trabalho apresenta os efeitos da incorporação de diferentes tipos de nanomateriais de carbono no desempenho mecânico de matrizes cimentícias após exposição a elevadas temperaturas. Foram avaliados a resistência à compressão, os módulo de elasticidade estático e dinâmico, e a microestrutura de argamassas contendo teores de 0,0%, 0,4%, 0,8% e 1,2% (por massa de cimento) de nanotubos de carbono (CNT), ou 0,0%, 3,0%, 6,0% e 9,0% (por massa de cimento) de nanopartículas de negro de fumo (CBN), após exposição a diferentes temperaturas máximas (200, 400 e 600 ºC). A incorporação de pequenos teores de nanomateriais (3% de CBN, 0,4% ou 0,8% de CNT) proporcionou a densificação da matriz, melhoria da estabilidade química dos produtos de hidratação, e/ou contenção da evolução de fissuras, proporcionando ganhos de resistência e rigidez a argamassas antes e após exposição a temperaturas entre 200-600 ºC. Elevados teores de nanomateriais (9% de CBN e 1.2% de CNT), no entanto, reduziram as propriedades mecânicas de argamassas expostas a temperaturas de até 400 ºC, devido a dificuldades de dispersão dos nanomateriais e consequente formação de pontos de fraqueza no interior da matriz. Todos os compósitos nanomodificados apresentaram melhor desempenho mecânico após exposição a 600 ºC, em comparação aos compósitos de referência. Isso ocorreu porque a decomposição de nanomateriais de carbono após exposição a 500 ºC proporcionou reduções na degradação térmica devidas à mitigação da propagação de fissuras associadas à liberação de vapor de alta pressão pela decomposição térmica de hidratos da matriz cimentícia.
... Carbon-based nanoparticles, ceramic nanoparticles, and metallic nanoparticles have displayed the capability to strengthen the physical, structural, and longevity characteristics of concrete mixtures under atmospheric temperatures [22,23]. Baloch et al. [24] described the use of carbon-based carbon nanotubes to create fire-resistant masonry. The researchers proposed that the adoption of such nano-fillers not only address the thermal degradation but also lowered temperature resilience. ...
... SDBS had been employed to be the surfactant in the current investigation. According to Baloch et al. [24], the surfactant decreases Van der Waals attractions, therefore aids in the dispersion of nano-sized particles. Using a surfactant-based ultrasonication method, nano-MgO particles have been disseminated. ...
Owing to the exceptional characteristic features of substances at their nano-scale, the inclusion of nanoparticles in cementitious materials is being evolved as a potential phenomenon under investigation. The main purpose of this present research work is to examine the sustainability aspects of concrete that has been mixed with a nano-sized filler. Obviously, this study investigates the effect of magnesium oxide nanoparticles (nano-MgO) on the durability of structural concrete at different concentrations. Three distinct nano-MgO dosages such that 2 %, 4 %, and 6 % had been used. The intensity of moisture absorption, the number of cracks, the gross ruptured region per unit area, and the sorptivity of the nano-concrete samples have been determined to assess the reliability of concrete. The results indicate that whenever the nano-MgO percent of the nano-concrete is maintained within a certain limit of 4.0 %, the addition of nano-MgO would considerably boost its durability by improving inhibition to porosity and fracture. The durability of nano-concrete enhanced first and then decreased when the nano-MgO content increased over 4.0 %. The degree of moisture absorption of the typical cementitious material had been increased by 44.73 % with a 4 percent inclusion of nano-MgO. Similarly, with a 4.0 % nano-MgO inclusion, the total fracture region per unit area of nano-concrete was reduced by 82.70 percent. An overabundance level of nano-MgO may reduce overall durability of the nano-concrete. If the nano-MgO proportion was maintained at 4.0 %, then the nano-concrete has become more reliable.
... RAC is a multi-phase and multi-scale material, and its properties greatly depend on the quality and composition of its constituent materials. On exposure to fire, the disintegration of its materials occurs due to a phenomenon called thermal inconsistency of ingredients [53][54][55]. Furthermore, the exposure of RAC to elevated temperature results in several physical and chemical alterations, as shown in Table 1, which consequently result in major strength degradation. ...
... The thermal endurance of fiber-reinforced recycled concrete during fire is vital for its application in modern-day buildings and other infrastructures. It has been widely reported that the mechanical properties of conventional recycled aggregate concrete highly degrade at elevated temperatures [54,131]. To overcome this issue, the technique of fiber reinforcing in recycled aggregate concrete is used, which at present is one of the most promising and persuasive strengthening techniques that results in better thermal endurance of FRAC. ...
In recent times, the applications of fiber-reinforced recycled aggregate concrete (FRAC) in practical engineering have gained greater popularity due to its superior mechanical strength and fracture properties. To apply FRAC in buildings and other infrastructures, a thorough understanding of its residual mechanical properties and durability after exposure to fire is highly important. According to the established research, the properties and volume fractions of reinforcing fiber materials, replacement levels of recycled concrete aggregate (RCA), and heating condition would affect the thermal–mechanical properties of FRAC. This review paper aims to present a thorough and updated review of the mechanical performance at an elevated temperature and post-fire durability of FRAC reinforced with various types of fiber material, specifically steel fiber (SF), polypropylene (PP) fiber, and basalt fiber (BF). More explicitly, in this review article the residual mechanical properties of FRAC, such as compressive strength, splitting tensile capacity, modulus of elasticity, mass loss, spalling, and durability after exposure to elevated temperatures, are discussed. Furthermore, this study also encompasses the relationship among the dosages of fibers, replacement levels of recycled aggregate, and the relative residual mechanical properties of FRAC that would help in the optimum selection of the fiber content. Conclusively, this study elaborately reviews and summarizes the relevant and recent literature on recycled aggregate concrete containing SF, PP fiber, and BF. The study further provides a realistic comparison of these fibers in terms of the residual mechanical performance and durability of FRAC that would help in their future enhancements and applications in practical engineering.
... The elemental composition of NGPs obtained from EDX spectroscopy and the mineral oxide composition of NGPs obtained from XRF analysis is discussed in Section 3.1 along with the SEM analysis and XRD pattern of NGPs. Acacia gum, utilized in past studies also for dispersion of NGPs [37,38], was utilized as a natural surfactant in this study. The elemental composition of acacia gum [37,38] is shown in Table 3. ...
... Acacia gum, utilized in past studies also for dispersion of NGPs [37,38], was utilized as a natural surfactant in this study. The elemental composition of acacia gum [37,38] is shown in Table 3. ...
Past works have used electronic waste (E-waste) plastic as a raw material for the production of plastic aggregate to be used as a substitute for natural aggregates. However, the results have shown that the plastic aggregate reduces the strength properties of resulting concrete limiting its application to non-structural components. Nano graphite platelets (NGPs) are nanofillers that significantly improve the density and hardness of the cementitious composite due to reduction in porosity and reinforcement in microstructure. In the current study, NGPs have been utilized to increase the strength of plastic concrete. The natural coarse aggregates are partially substituted by 25% of E-waste plastic coarse aggregate. Locally available NGPs are utilized, and their structural characteristics were assessed by scanning electron microscopy (SEM), X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) spectroscopy. A dispersion test is carried out and a ratio of 0.6:1 (surfactant:NGPs) is found to yield maximum dispersion. Different dosages of NGPs (1%, 3%, and 5%) have been introduced into plastic concrete. This research work aims to explore the fresh, mechanical, durability, and microstructure characteristics. The mechanical properties of the cementitious composites declined with the incorporation of E-waste plastic aggregates which were significantly enhanced by incorporating NGPs. The specimens containing 25% of plastic aggregate and 5% of NGPs showed the hardened density, compressive strength, tensile strength, and flexural strength to increase by 2.16%, 13.56%, 15.53%, and 31.42%, respectively compared to the samples without NGPs. Sorptivity and ultrasonic pulse velocity values are found to decrease by 56.5% and increase by 2.53%, respectively for plastic concrete with 5% NGPs compared to the control mix. SEM-EDX spectroscopy was carried out to examine microstructure investigation and it was found that by the inclusion of NGPs in the concrete composites, the hydrated crystals stacked one over the other and became massive and thicker implying that the addition of NGPs is a promising reinforcing agent in plastic concrete.
... Thus far, studies have depicted analyses concerning the effect of high temperatures on the physical properties of lightweight concrete [22,23], including lightweight concrete with the addition of perlite [24][25][26][27]. However, these publications do not address issues pertaining to chimney blocks in that respect. ...
... It is also worth mentioning that the average compressive strength (4.03 MPa) after fire trial was higher than that of leca concrete blocks readily available on the market, whose strength is approx. 3 MPa at a bulk density of 1200 kg/m 3 [22,23]. ...
Chimneys are structures designed to convey exhaust gases from heating devices to the outside of buildings. The materials from which they are made have a great impact on their fire safety, as well as on the safety of the whole building. As current trends in the construction industry are moving towards improving the environmental impact and fire safety, changes to building materials are constantly being introduced. This also applies to the development of chimney technology, as there is still a recognised need for new solutions when it comes to materials used in the production of chimney systems. This article presents the findings of tests carried out on a chimney made from innovative perlite concrete blocks. Four different perlite concrete blocks that differed in bulk densities were analysed. The obtained results were then compared with widely used leca (lightweight expanded clay aggregate) concrete blocks. The test results confirmed high insulation properties of the perlite concrete block, from which the innovative chimney casing was made. The fire safety level was maintained even in high temperatures that occur during soot fire (1000 °C). These properties were retained despite there being no additional insulation of the flue duct. Even though the thermal load decreased the compressive strength of the chimney blocks, they still displayed sufficient average strength of 4.03 MPa. Additionally, the test results confirmed the possibility of recovering heat from the chimney with the efficiency of 23–30%, which constitutes a considerable increase compared to chimneys made from leca concrete blocks.
... Because in many conditions, reinforced concrete members may be exposed to elevated temperatures causing serious damages, the importance of investigating the mechanical properties is vitally important. Some previous studies examined the mechanical properties of concrete in the cooled state [6,[11][12][13][14][15][16], and others tested in the hot state [17,18]. The former determines the residual mechanical properties, and the latter determines the resistance capacity during the simultaneous effect of high temperature and load-bearing. ...
... A rise in temperature increases the rate of weight loss of geopolymer concrete [19]. It was observed that the weight loss of normal concrete at 600 and 800 C was 8% and 14% [17]. Demirel and Keleştemur [15] assessed the effect of high temperature on the specimens containing pumice and silica fume. ...
The ever-increasing construction demand for the human and high energy consumption of the production of the raw material has a significant impact on environmental pollution. One of the challenges facing civil engineers is to understand the behavior of concrete during exposure to elevated temperatures, and their service life performance after exposure. In this study, the possibility of partial replacement of cement with metakaolin on the improvement of concrete properties is investigated. The dosage of metakaolin is 10, 15, and 20% of cement weight. After determining the optimum percentage of metakaolin at ambient temperature, the specifications of the specimens are tested at temperatures of 28–800 °C. Although high temperatures damage the mechanical and durability characteristics of normal concrete, the substitution of 15% of cement weight with metakaolin can be recommended as a practical solution to improve the mechanical and durability properties of concrete exposed to elevated temperatures.
... The incorporation of high concentrations of nano fillers not only enables sensing capabilities but also enhances the mechanical durability of cement composites, showing over 5 % improvement in resistance to elevated temperatures compared to conventional concrete [106]. Environmental factors, particularly temperature, induce gradual or continuous changes in the electrical resistivity signals of CNT-composites, while dynamic loads create transient and sudden variations [107]. ...
... In this regard, the merits and proper use of clays as a cement alternative in concrete have been studied for decades (Akbar et al., 2013). In the past, various substances, such as polypropylene, were employed to enhance the general behavior of concrete subjected to high temperatures (Shihada, 2011), fibers, carbon nanotubes (Baloch et al., 2018), and metakaolin (Andrejkovičová et al., 2015). However, these resources are either too expensive or not easily accessible. ...
... In addition, the mineralogy of the aggregate also affects the thermal expansivity (a) of the interface between the aggregate and the cement paste. Lower a in LECA leads to better compatibility with the cement paste in comparison with normal weight counterpart, resulting in a more stable bond between the aggregate and the surrounding matrix [26][27][28]. A strong bond between the aggregate and the cement paste is essential for maintaining the mechanical properties of the concrete under high-temperature conditions. ...
While normal weight aggregate concrete (NWAC) can experience significant strength loss and spalling at high temperatures, lightweight aggregate concrete (LWAC) can maintain its structural integrity. Stress–strain relationship of concrete is an important test to perform during designing phase of concrete infrastructures. Therefore, this study focuses on exploring the stress–strain behavior of NWAC and LWAC under uniaxial compression at temperatures ranging from 20 to 750°C. In addition, pruning long short-term memory (P-LSTM) networks to create a predictive model for the stress–strain relationship of NWAC and LWAC is also utilized. Concrete mixture designs containing ordinary Portland cement, silica fume, and lightweight expanded clay aggregate, were first optimized to reduce the number of experiments using the response surface method. Subsequently, 30 mixture designs were fabricated and subjected to compression tests, following exposure to varying temperatures that ranged from 20 to 750°C, to evaluate their stress–strain relationship and determine associated mechanical properties. Experimental results were then utilized to develop a P-LSTM model used to forecast the stress–strain relationship of concrete at varying temperatures. The P-LSTM model developed in this study improved the prediction accuracy and stability beyond conventional LSTM model, which would be useful in the design and optimization of NWAC and LWAC structures. Additionally, the P-LSTM model has a lower computational cost and less likelihood of over-fitting as compared to typical LSTM networks.
... Up to 350°C, concrete with 40 and 50% GGBFS replacement ratios exhibited no significant change in residual strength, but higher replacement ratios resulted in lower strength [31]. Furthermore, alternative materials such as polypropylene fibres [9], steel fibres [32], carbon nanotubes [33], nano silica, nano titanium [34][35][36], and nano alumina [37] have been employed in the production of concrete. Although these materials are effective in improving the performance of concrete in terms of mechanical and durability properties, the cost of these materials is a concern. ...
The construction industry commonly employs concrete as a construction material, which sometimes may be subjected to fire exposure. It is important to adopt fire safety measures while planning and constructing such structures to ensure the safety of the occupants and the structural integrity of the concrete. So, determining its performance at elevated temperatures is of utmost importance. The main objective of this study was to investigate the impact of mineral incorporations, namely, nano bentonite clay (NBC) and nano fly ash (NFA), on the retained properties of concrete at normal (27°C) and at elevated temperatures. The feasibility of partly substituting ordinary Portland cement utilizing a mixture of NBC (0–5%) and NFA (0–50%) in concrete was assessed under the exposure to an elevated temperature ranging from 200 to 600°C. Several parameters were examined, including compressive strength, flexural strength, split tensile capacity, water penetration, loss of mass, ultrasound pulse velocity, and microstructure properties. After the experimental analysis, it was observed that the fire endurance was shown to be improved with the inclusion of nanoparticles (BC and FA). A reduction in the loss of mass by samples subjected to elevated heat was observed with the addition of nano bentonite and NFA. The mechanical strength results were obtained as maximum for the concrete specimens with 2% NBC and 20% NFA and further, the specimens performed better when exposed to elevated temperature as compared with normal concrete specimens. The microstructure of the concrete also upgraded with better impermeability owing to the use of NBC and NFA.
... Langaroudi et al. [31,32] studied the mechanical performance of cementitious composites with nano-clay addition. Some researchers also found that the fire resistance of cement-based materials can be significantly improved by adding nanomaterials such as multiwalled carbon nanotubes, nano-iron oxide, nano-silica and nano-calcium carbonate [33][34][35][36][37]. ...
Due to improved mechanical properties, nano-alumina (NA) addition has been considered as an interesting method to promote the performance of cement composites. To investigate the enhancing effect of NA on the fire resistance of cement-based composites, the physical and mechanical properties of NA-modified cement composites (NAMCCs) were experimentally measured after exposure to high temperatures (up to 800 °C). The variation mechanism of the physical properties of NAMCCs with increasing temperature was explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Increasing temperature leads to a gradual decrease in density and ultrasonic wave velocity but an overall increase in mass loss. The addition of NA particles effectively improved the mechanical properties of hardened cement pastes after heat treatment at all the temperatures studied. The compressive strength, elastic modulus and flexural strength all gradually decrease with increasing temperature. Combined with the changes in XRD and SEM, three regions are identified for the variation in mechanical performance in the temperature range of 20~400 °C, 400~600 °C and 600~800 °C. Finally, the relation between the physical and mechanical parameters of these regions was evaluated.
... Unlike the SCC samples, which showed a reduction in excess of 90% at 800°C, all the ECC mixtures presented very similar flexural strength losses of around 76-87%. The carbonation of limestone aggregates (Baloch et al., 2018a) at 800°C may have also advanced the degradation of the matrix/aggregates, ITZs and influenced negatively the flexural strength loss of SCC compared with the ECCs. ...
The influence of various supplementary cementitious materials (SCMs) and fibres on the fire resistance of composite systems that combine engineered cementitious composites (ECCs) in tension with self-compacting concrete (SCC) in compression was examined. The study was designed to determine the ideal ECC formulation for optimising mechanical properties and bonding performance at ambient and elevated temperatures. The SCC and ECC were hot-joined without vibration or surface preparation, using a fresh-to-fresh casting method. Modifications to the chemical composition of the ECCs included the addition of class F fly ash (FAF), class C fly ash (FAC) or slag, as well as polyvinyl alcohol (PVA) fibres or steel fibres. The samples were exposed to temperatures of 200°C, 400°C, 600°C and 800°C, followed by comprehensive testing to evaluate their flexural strength, tensile strength and interfacial properties. The results indicate that the incorporation of an ECC layer within the SCC system significantly improved mechanical strength and thermal stability, both at ambient temperatures and under high-temperature conditions. Notably, the addition of FAF (rather than FAC or slag) in the ECC layer offered superior thermal stability and ensured the retention of desirable residual mechanical properties. Moreover, steel fibre reinforcement greatly improved the SCC/ECC bonding, outperforming PVA reinforcement at elevated temperatures.
... The presence of nanomaterials is probably responsible for the development in compressive strength since the former accelerates C 3 A hydration. Furthermore, the nanomaterials provide a uniform distribution of thermal stress on the surrounding mortar [62]. However, the percentage reduction due to the CNTs addition was only 2.4% at 800 • C; this is attributed to the loss of the chemical bonding and adhesion between CNTs and cement paste at elevated temperatures [26]. ...
In this study, the influence of high temperature on corrosion resistance of steel rebars and the mechanical strength of nano-modified mortars is investigated. Two (2) different mixtures were prepared and tested in the lab; mortars containing 0.2 wt% multi-walled carbon nanotubes (MWCNTs) and mortars without nanomaterials for comparison reasons. The specimens were heated in a furnace up to 800οC for 1 h. Destructive and non-destructive tests were used to evaluate the fire resistance and durability performance of cementitious materials. In particular, the corrosion protection provided by nanomaterials was investigated using electrochemical and mass loss measurements on reinforcing steel, whilst strength tests of mortars were also carried out to evaluate their mechanical behavior after exposure in elevated temperatures. SEM and XRD techniques were used to analyze the surface and microstructure of the steel bars and mortars. The results showed, that the CNT addition slightly improves the strength and durability of mortars exposed to high temperatures as compared with the plain OPC mortar. At the same time, the corrosion resistance of steel rebars was enhanced using carbon nanotubes in mortars after exposure to high temperatures.
... In this regard, the characteristics and suitable use of clays as an alternative to cement in concrete have been studied for decades [5]. Previously, different materials were used to improve the overall behaviour of concrete exposed to elevated temperature, such as polypropylene fibres [6], steel fibres [7], carbon nanotubes [8], fly ash [9] and metakaolin [10]. However, these materials are either expensive or not freely available. ...
This study investigated the effect of elevated temperature on the mechanical, physical and durability properties of normal strength concrete, modified with bentonite. The bentonite concrete was cast by substituting cement with bentonite content in proportions of 5, 10 and 15% by weight. Mechanical tests were conducted (compressive and splitting tensile strength). Furthermore, durability performance (mass loss and sorptivity) and specimen properties (elastic modulus, stress–strain behaviour, ductility and energy absorption) were evaluated and discussed. The results demonstrated that samples incorporating bentonite showed better fire endurance than the control mix. The inclusion of bentonite in concrete decreased the mass loss of the specimens exposed to high temperatures. The performance of bentonite concrete was better in terms of mechanical behaviour (compressive and tensile strength) than that of conventional concrete, and it had high resistance to water absorption. Higher ductility and energy absorption capacity were observed for the concrete specimen containing bentonite than its counterpart control specimens.
... Based on the experimental results of the current study recorded in the temperature range of 23-800°C mathematical relationships are established. The proposed equations will help assess the post-fire residual performance of structural concrete [6,62]. The relationships established are derived on the basis of linear regression analysis by using a commercial data analysis software, Minitab. ...
This research explores the effect of basalt fibers (BFs) added in high
strength concrete (HSC) to tailor its mechanical and microstructural response on being exposed to fire. Thermo-mechanical behavior of reference and modified formulations was monitored in ambiance alongside the exposed elevated temperatures till 100C, 200C, 400C, 600C, and 800C. The dose of BFs was maintained as 1% and 2% by weight of cement in modified HSC formulations. The specimens were subjected to the controlled heat ramp of 5C/min, to attain the desired temperature elevation with a hold time of 150 min. The heated specimens were then cooled by air till ambiance to test for residual properties. The test results revealed significant improvement in thermo-mechanical properties of BFs reinforced HSC formulations after being exposed to fire. Micro-forensics evidenced the homogenized distribution of BFs alongside their crack arresting actions throughout the matrix, contributing to the added residual performance. The matrix morphology of BFs reinforced HSC was also monitored to endorse any physical or chemical change in the hydration product. Moreover, numerical equations based on statistical analysis were formulated that can predict the fire behavior of modified HSC samples with BFs.
... CNT can improve the high-temperature resistance of cementbased materials. One study explored the influence of CNT reinforced cement-based materials at high temperatures and found that the addition of CNT improved strength more than in ordinary concrete (Baloch et al., 2018). The influence on the resistivity signal of CNT composites is continuous or gradual, while the effect of dynamic load on the resistivity signal of CNT composites is instantaneous and abrupt. ...
Carbon nanotube (CNT) is a promising nanomaterial with excellent mechanical, electrical, thermal, and chemical stability. It has received extensive attention due to its unique multifunctional properties in engineering materials. Researchers have explored the preparation and characterization of CNT reinforced cement-based materials. Studies have shown that adding CNT will significantly improve the performance of cement-based materials. This article introduces the techniques for the dispersion characterization of CNT and summarizes the advantages and disadvantages of these techniques. The functionalized applications of CNT in cement-based materials are reviewed, including sensing performance, structural health monitoring of concrete, electromagnetic shielding, and other applications. In addition, the application and development prospects of CNT in 3D printing concrete have been prospected. Finally, we discussed the existing problems and challenges in developing and applying CNT in cement-based materials and suggested future research.
... irdly, the salary structure is unreasonable, the salary level is low, the salary system is not competitive, especially for the salary of management, and technical personnel are not consistent with their contributions and does not play a due incentive role; finally, the lack of scientific selection means and the neglect of staff career development make the frequent loss of staff and difficult to retain talents. erefore, the efficiency of human resources has become the focus of tourism, and how to improve the efficiency of human resource management has become a widespread concern [4][5][6]. e performance evaluation of tourism human resource management is not only directly related to the rational allocation of personnel, but it also plays an important role in realizing the optimization of enterprise resources [7]. e problem of low performance of the tourism industry is increasingly exposed, and one of the important reasons is the lack of scientific enterprise performance evaluation ideas and methods. ...
Tourism human resources performance management is an important means of enterprise core competitiveness; the assessment results is good or bad will directly affect the staff's work enthusiasm and attitude; the use of tourism human resources, configuration, and performance carries on the scientific, reasonable, and effective comprehensive evaluation and can fully stimulate the potential of employees. To continuously improve the work efficiency, quality, and level of staff, which has certain application value and practical significance for the long-term and healthy development of tourism industry, this article analyzes the current situation of tourism human resource performance management, introduces the data mining technology and fuzzy data mining technology, and then gives the fuzzy data mining algorithm and detailed fuzzy data mining steps, through the fuzzy data mining technology to evaluate the performance of tourism human resource management. It also analyzes the key factors and existing problems of human resource management in tourism industry and puts forward innovative thinking countermeasures for the performance management of human resources in the tourism industry.
... For effective dispersion of NGPs, a natural surfactant, called acacia gum, was used. Acacia gum (AG) was found to be very effective for this purpose [30,31]. Table 3 shows the elemental composition of acacia gum obtained through EDX spectra. ...
Nano graphite platelets (NGPs) belong to the carbon family and have a huge impact on the construction industry. NGPs are used as multi-functional fillers and have the potential to develop reinforcing within cementitious composites. In this paper, NGPs were incorporated in cementitious composites to investigate the effects of NGPs on the fresh, mechanical, durability, and microstructural properties of concrete. Five mixes were prepared with intrusion of NGPs (0%, 0.5%, 1.5%, 3%, and 5% by weight of cement). The properties studied involved workability, air content, hardened density, compressive strength, tensile strength, flexural strength, sorptivity, ultrasonic pulse velocity (UPV), water absorption, and external sulfate attack. The workability and percent air content decrease by 22.5% and 33.8%, respectively, for concrete with 5% NGPs compared to the control mix. The specimens containing 5% of NGPs revealed the hardened density, compressive, tensile, and flexural strength to increase by 11.4%, 38.5%, 31.6%, and 44.34%, respectively, compared to the control mix. The results revealed that the incorporation of 5%NGPs in cementitious composites reduces the sorptivity and water absorption by 32.2% and 73.9%, respectively, whereas, it increases the UPV value by 7.5% compared to the control mix. Furthermore, the incorporation of NGPs provided better resistance against external sulfate attacks. SEM–EDX spectroscopy was carried out to investigate its microstructural analysis.
... Khandaker M. Anwar Hossain [19] studied high-strength Scoria aggregate concrete at 800 • C for strength and durability, and compared with a high-strength concrete control, this material showed a better performance in terms of the residual strength, resistance to chloride ion attack, and resistance to high-temperature deterioration. Waqas Latif Baloch [20] studied the effect of incorporation of multi-walled carbon nanotubes on concrete. The results showed that the incorporation of multi-walled carbon nanotubes could improve the strength of concrete both prior to and after exposure to fire. ...
Scoria aggregate concrete (SAC) as new green material has been gradually used in some construction projects for its lightweight and high strength, which can reduce the environmental impact of construction materials. In this paper, the residual mechanical properties and intact compressive stress-strain relationships of polypropylene (PP) fiber-reinforced Scoria aggregate concrete after high-temperature exposure at 20, 200, 400, 600, and 800 °C were investigated. The failure modes of PP fiber-reinforced Scoria aggregate concrete specimens and the effect of high temperatures on the peak stress, secant modulus, and peak strain were obtained. The results showed that the residual compressive strength of heated concrete is significantly reduced when the temperature exceeds 400 °C. The residual strength and residual secant modulus of PP fiber-reinforced Scoria aggregate concrete are significantly higher than those of ordinary concrete. The Scoria aggregate concrete specimens with PP fibers exhibited fewer surface cracks and fewer edge bursts under high temperatures. The residual stress-strain equation of the Scoria aggregate concrete was established by regression analysis, which agreed well with the experimental results.
... Generally, CNT can improve the resistance of cement composites against high temperatures. Baloch et al. [98] studied the behavior of CNT-composites against elevated temperature effects. The compressive strength of 100 × 200 mm cylindrical specimens were measured by heating to 200 • C, 400 • C, 600 • C and 800 • C at a heating rate of 5 • C/ min. ...
Carbon nanotubes (CNT) as a functional filler can increase the electrical conductivity property of concrete and thus provide intrinsic self-sensing properties with no need for external sensors to monitor the behavior of concrete infrastructure and structures containing CNT. CNT-cement composites also improve mechanical strength and have higher energy absorption capacity. The sensitivity of their electrical conductivity to external physical parameters, such as strain, stress, load, temperature, displacement, and pressure, makes them suitable for structural health monitoring (SHM) applications. This paper presents a comprehensive review of the CNT properties, fabrication process, composition, and sensing characteristics as well as challenges for applying CNT concrete as part of a self-sensing structure. Furthermore, the self-healing property of CNT, as an integral feature of future smart concrete infrastructure is discussed.
... Similarly, the naturally found aggregates are usually stable till 350 • C, but the thermal instability beyond this temperature aids the accrescent cracking. When the exposure increases beyond 400 • C, the decomposition of portlandite and CSH gel causes a rapid decrease in the concrete mechanical strength [126]. At 800 • C, the de-carbonation of aggregates and the complete dissociation of CSH gel renders the concrete extremely weak [127]. ...
There has been an increasing interest in the field of rehabilitation, mainly for repair and strengthening of reinforced concrete (RC) structures. One of the most effective techniques comprises externally bonded concrete composites, in which the efficacy of rehabilitation depends considerably on the integrity and durability of the bond established between the overlaying and the substrate concrete. This paper presents a detailed review of the critical durability issues associated with the structural bond for rehabilitation purposes, with a special focus on the performance of modern overlay materials such as ultra-high performance concrete (UHPC) and Engineered Cementitious Composites (ECC). A number of factors determining the durability of concrete-concrete bonded joints have been identified and grouped into two categories related to materials – joint characteristics and in-service conditions. For each of these factors, the main findings obtained in previous studies are summarized and discussed and the aspects that need further investigation are outlined.
... Be to, bandiniuose su MWCNTs pastebėtas porų vidutinio skersmens sumažėjimas. Baloch et al. (2018) darbe išanalizuotas MWCNTs poveikis putbetonio atsparumui ugniai. Nustatyta, kad MWCNTs pagerina atsparumą ugniai ir bandinių tamprumo modulį. ...
... Mathematical equations have been developed to determine the response of fire-damaged reinforced concrete structural members for computation in the temperature range of 23°C till 800°C. These sets of mathematical equations, based on literature [63,70] can help quantify the residual performance of concrete members exposed to fire. A commercial statistical software, Minitab [71] was utilized to perform regression analyses to contemplate these empirical relationships. ...
Carbon nano fibers (CNFs) were added into high strength concrete (HSC) to evaluate the fire endurance of incumbent matrix. The residual mechanical performance was evaluated by conducting material property tests namely compressive strength, tensile strength, stress-strain response, elastic modulus, compressive toughness and mass loss made with 0.1% CNF and 0.2% CNF as cement replacement in the temperature range of 23 oC to 800 oC. The internal structure damage on exposure to fire was analyzed by ultrasonic
pulse velocity (UPV) in macro-phase while scanning electron microscopy (SEM) is used for micro and nano forensics. Being exceptionally conductive and resilient, these fibers reduce spalling potential via low thermal inertia and crack bridging action as established in forensic analysis. The test results revealed better retention in mechanical and physical properties of high strength concrete containing CNFs. Mathematical relationships based on statistical analysis have been proposed for predicting mechanical, durability and energy related properties of HSC modified with varying percentages of CNFs at elevated
temperatures.
... To achieve a well dispersed aqueous solution of CNTs require low concentrations of carbon nanotubes, typically 0.1 wt % or even less. Therefore, maximum of 0.08 wt% of nanoinclusions were adopted based on previous studies [28,34,36,37]. Superplasticizer was added with respect to cement in a proportion of 0.75 wt% to achieve a workable cement mortar [38,39]. ...
Nano-engineered cement composites using a hybrid of multiwalled carbon nanotubes (MWCNTs) and graphite nano-micro particles (GNMPs) integrates the superior mechanical performance of MWCNTs and the better dispersibility and shielding capability of GNMPs. Combinations of MWCNTs and GNMPs at total of 0.08% over cement weight were ultrasonically dispersed in aqueous solution to make the cement mortar. The results show that incorporation of graphite platelets can synergistically improve the dispersion and compatibility of MWCNTs with surrounding cementitious matrix. Furthermore, the incorporation of 2-dimenstional GNMPs provided better resistance against water absorption and external sulphate attack. We assert that by limiting the hybridization of MWCNTs and GNMPs to a ratio of 1:0.125 (C1G0.125), a cementitious composite with enhanced mechanical performance can be developed. Young's modulus, flexural strength and flexural toughness was improved by 87%, 61%, and 51% respectively. Furthermore, the findings of this study provide the new insights on the design of hybrid nano reinforced cement composites with improved mechanical properties for various applications.
... Many researchers [3][4][5]9,10,17,[22][23][24][25][26][27][28][29][30][31][32] studied the residual mechanical properties of LWC at room temperature after exposure to high temperatures because, to some extent, they represent conditions present after a fire event. Compared with ordinary aggregates, LWA has a larger porosity, so its water absorption is relatively higher. ...
In this study, the effects of individual and mixed fiber on the mechanical properties of lightweight aggregate concrete (LWC) after exposure to elevated temperatures were examined. Concrete specimens were divided into a control group (ordinary LWC) and an experimental group (fiber-reinforced LWC), and their compressive strength, elastic modulus, and flexural strength after heating to high temperatures of 400–800 °C were investigated. The four test parameters included concrete type, concrete strength, fiber type, and targeted temperature. The test results show that after exposure to 400–800 °C, the variation in mechanical properties of each group of LWC showed a trend of increasing first and then decreasing. After exposure to 400 °C, the residual mechanical properties of all specimens did not attenuate due to the drying effect of the high temperature and the more sufficient cement hydration reaction. However, after exposure to 800 °C, the residual mechanical properties significantly reduced. Overall, the mixed fiber-reinforced LWC showed a better ability to resist the loss of mechanical properties caused by high temperature. Compared with the loss of compressive strength, the flexural strength was relatively lost.
... The aforementioned properties of carbon nano-inerts can be useful to improve the post fire strength and durability of concretes. Similar insights can be found on recent studies by Waqas et al. [71,72] who studied the fire enduring capacity of normal and lightweight concretes modified with CNTs and reported modest gains. ...
Fire poses serious threat to the life of structure and occupants, as it induces detrimental effects at structural level both short term and long term. Keeping in view the importance of fire performance of concrete , various researchers have evaluated the behavioral characteristics of concrete under such conditions. The safety and serviceable life of concrete structures after exposure to fire depend on two important factors i.e. residual strength and durability. In this paper, a comprehensive and updated review on the durability of concrete containing various mineral admixtures after being exposed to elevated temperatures is presented. Durability of concrete after fire exposure is measured through indirect tests such as chloride ion permeability, water permeability, absorption and sorptivity. Thus the test methods and available literature on the aforementioned techniques has been elaborately reviewed and summarized. The recent trends followed to improve the durability performance of concrete in the post-fire conditions have also been discussed and future research needs have been identified. However, to properly understand the durability performance of fire exposed concrete structures constructed with sustainable con-cretes, a great deal of work is required in this particular domain.
Bentonite is a type of clay with a very high proportion of clay mineral montmorillonite, resulting from
the decomposition of volcanic ash. Bentonite is highly water-absorbent and has high shrinkage and
swelling characteristics. In this study, the partial replacement of Cement with Bentonite (source:
Nizampur) at 5%, 10%, 15% and 20% mixes were prepared, and their compressive strength was
compared with the control mix. The workability of the bentonite mixes is comparatively lower than that
of the control mix because Bentonite is much more water-absorbent. The comparative compressive
strength analysis indicated that at the age of 7 days of testing, the mixes containing Bentonite showed
lower strength than the control mix, while at the age of 28 days of testing, the mixes having different
percentages of Bentonite showed good strength when compared to the control mix. The main conclusion
drawn from this research work is that Bentonite can be used where later-stage strength is required. The
durability of the Bentonite concrete was measured in terms of resistance to the penetration of sulphate
ions. It was seen from the results that the bentonite result was poor in the early stage and later stages.
Compressive strength was reasonable when compared with Control samples. Although the strength of
bentonite mixes was not higher than the control mix, as the percent of bentonite increases, the strength
of concrete increases. The later age strength of 5% and 10% bentonite concrete was higher than the
15% and 20% bentonite mix, respectively. The strength of 5% and 10% bentonite was approximately
equal to the control mix. For chemical analysis, the X-ray fluorescence test is performed on cylinder
specimens. From the results, we obtain that three dominant compounds, i.e. SiO2, Al2O3, Fe2O3 and
their occurrence in abundance are responsible for the strength factor of bentonite mixes. At the same
time, other minerals are present in small amounts. The XRF test was performed to know the chemical
compositions of the minerals in the clay. The XRD test was performed on different bentonite mixes to
determine mineral composition.
This paper delves into the intricate realm of cementing operations in extreme environments, where challenging conditions pose formidable obstacles to well integrity. The exploration begins by dissecting the diverse and multifaceted challenges associated with cementing in harsh conditions, including but not limited to high temperatures, elevated pressures, exposure to corrosive fluids, and the unique complexities of unconventional formations. Each of these challenges demands innovative and tailored solutions to ensure the reliability and effectiveness of cement barriers, addressing issues such as fluid migration and maintaining zonal isolation in the face of extreme conditions. Subsequently, the paper systematically explores a myriad of field applications of cementing technologies that have been specifically designed and optimized for deployment in extreme environments. The scrutiny of case studies, methodologies, and advancements in cement formulations, additives, and placement techniques unveils a nuanced understanding of the strategies employed to tackle the unique challenges posed by extreme conditions. Insights garnered from successful field applications underscore the adaptability, resilience, and efficacy of cementing practices in mitigating the adverse effects associated with extreme temperatures, pressures, and geological complexities. This synthesis of challenges and field applications provides readers with a comprehensive and up-to-date overview of the state-of-the-art approaches in cementing for extreme environments. By shedding light on the latest advancements and sharing valuable insights derived from practical experiences, this paper significantly contributes to the evolving understanding of cementing technologies, which are crucial for the success and longevity of wells operating in the harshest and most demanding conditions.
The paper investigates the impact of carbon nanotube-based modifiers on the performance characteristics of lightweight foamed concrete (LFC). The method involves saturating quartz sand with a solution containing a catalyst for carbon nanotube (CNT) growth, followed by the subsequent chemical vapor deposition (CVD) synthesis of CNTs. Evaluation of nanomodified sand samples was conducted using SEM and TEM, thermogravimetry, Raman spectroscopy, and XRD. Compression and flexural strength tests of (LFC) specimens indicated that the optimal proportion of nanomodified sand introduced is 1% by weight with a particle size of 0.16 mm. This resulted in a notable 35% increase in compressive strength and an approximately 32% improved in flexural strength. Furthermore, the modified sample with CNT-based sand exhibited a 27% reduction in water absorption. The paper also presents a potential mechanism to explain the impact of carbon nanotube-based modified sand on the evolving structure of (LFC).
Cement-based sensors, widely used in monitoring the health of various structural elements, are constantly exposed to different environmental conditions, especially a wide range of temperatures and different moisture levels. Concrete is a dynamic material that exchanges ions with its environment and undergoes various chemical reactions over time. Moisture, various chemical agents, and temperature changes can significantly affect the behavior of cement sensors. Sensors based on cement-based composites have been developed with precise composition, dimensions, and setup to measure strain and stress, locate and assess damage severity, and detect temperature and moisture. To accurately evaluate the behavior and performance of cement sensors and improve the validity of their outputs, it is necessary to examine the variables affecting their behavior and propose strategies to reduce the unwanted effects of these factors. The aim of this research is to examine how temperature and moisture changes influence the conductive and multifunctional properties of filler compounds, like carbon fibers and carbon nanotubes. This study aims to investigate the impact of environmental factors on the durability and performance of cement sensors, and to suggest strategies and recommendations for managing and mitigating the adverse effects of these factors, thus ensuring the accurate operation of the sensors. The results show significant improvements in smart cement-based sensors, which have been used in most structural fields, especially in determining the health status of structures, traffic detection, moisture detection, and fire alarm, indicating promising prospects for more practical use in various industries.
Exposure of concrete to elevated temperatures causes irreversible damage to the concrete structure and poses a serious threat to its service life. Due to the importance of concrete fire performance, extensive research has been conducted to investigate its behavior under different conditions of elevated temperatures. The properties of concrete are significantly affected by various factors, including heating temperatures, heating durations, and cooling methods. Among these factors, the residual compressive strength of concrete is considered the most crucial characteristic after exposure to elevated temperatures. This paper aims to develop mathematical models for analyzing and predicting the relative residual compressive strength of concrete at high temperatures. Three independent factors were identified in this study: heating temperatures, heating duration, and cooling method. Two groups of datasets on the relative residual compressive strength of concrete under elevated temperatures were reviewed and collected from previous studies, serving as the benchmark dataset and validation dataset, respectively. Response Surface Methodology (RSM) was employed to analyze the datasets. The results of various statistical parameters, such as the coefficient of determination, sum of squares, F-value, and P-value, indicate the significance of the predicted model for estimating concrete's relative residual compressive strength under elevated temperatures. The RSM analysis reveals that heating temperatures have the most significant effect on the relative residual compressive strength of concrete. In summary, the RSM model shows a strong correlation with the validation datasets, with a coefficient of determination (R2) of 0.7869.
Phase change materials (PCMs) are latent heat storage materials that can store a large amount of thermal energy while changing their phase and are usually incorporated into concrete for improving thermal properties. However, the fire performance of concrete incorporated with PCMs is adversely affected at elevated temperatures as PCMs have weaker fire endurance and burn out instantly in case of fire exposure. This research is focused on improving the fire endurance of PCMs introduced into the lightweight aggregate (LWA) using the vacuum infusion technique and incorporated in concrete encapsulated with fire resistant coating materials to achieve energy efficiency. In two encapsulation layers, the first encapsulation of epoxy sealed paraffin inside the pores of LWA at ambient conditions, while the second melamine–formaldehyde layer prevented the leakage of PCM into the concrete matrix at elevated temperatures and consequently shielded the flammable reaction of PCM. By shielding PCM, the fire performance of concrete was improved, as PCM inside concrete effectively remained serviceable up to 250°C temperature.
Lightweight, high energy reservation and excellent functional performance are the main benefits of cellular concrete. Its performance mainly depends on the pore characteristics, governed by different factors. This review paper presents the state-of-the-art techniques in foam stabilisation for maximising the performance of foam concrete. The kinetics of foam degradation, current progress, trending applications in building and construction sectors, as well as potential challenges encountered in developing foam concrete were also elaborated. Overall, the current state of strategies for high foam stability involves the optimal selection of binder combination, aggregate substitution, internal reinforcement, curing regime, treatment methods, gas-liquid interface modified by nanomaterials, polymer chains and surfactants monomers. The breakthrough in current 3D printable foam concrete technology and the main challenges with mixing protocols, long-term durability, technological development, energy efficiency and economic feasibility were discussed. These contribute to a holistic approach for future research and development in manufacturing a novel foam concrete.
The need for finding sustainable alternative sources of coarse aggregates as an ingredient for concrete has been increasing globally. Recycled and brick aggregates are two viable options in this regard. Many properties of recycled and brick aggregate concrete remain to be investigated to predict their behaviour accurately and to set proper code guidelines. In this study, a few properties of recycled and brick aggregate concrete and their behaviour in flexural members have been investigated. In particular, the stress–strain relationship, modulus of elasticity and splitting tensile strength of concrete made of brick, recycled and natural aggregate have been analysed. Using these properties, some of the flexural properties of reinforced concrete members made with these three types of aggregates have been determined and compared. To the best of our knowledge, for the first time, a comparative analysis of these properties of brick, recycled and natural aggregate concrete has been conducted in this study. Results from the study show that while the code equations for predicting some of the important mechanical properties of concrete are suitable for natural aggregate concrete, they are not appropriate for recycled or brick aggregate concrete. The moment–curvature response and deflection properties in flexural members made of recycled and brick aggregate concrete are affected by the difference in the properties of the aggregate. Finally, it is suggested that the utilization of recycled and brick aggregate in concrete is likely a feasible option without any significant degradation in the mechanical and flexural properties of the structural members.
This paper investigates the performance of composite systems combining self-consolidating concrete (SCC) and normal strength concrete (NSC) under fire exposure. SCC was used in the compression zone while NSC was overlaid in the tensile zone. The resulting systems were exposed to high temperatures ranging from 23 to 800 °C. Two types of cooling regimes have been considered i.e. water and air cooling. The primary objective of the study was to analyze the interfacial bond deterioration and the variation in mechanical strength. Test results indicated that the mechanical strength and mass decreases with rise in exposure temperature. The interface remained intact without excessive damage. Further, the thermal shock experienced during the water cooling negatively impacts the composite system.
یکی از مهمترین عوامل آلودگی محیط زیست تولید سیمان و یکی از عوامل آسیب به سازههای بتن آرمه دماهای
باال میباشد. در این مطالعه تاثیر جایگزینی 10 و 20 درصد وزنی سیمان با زئولیت بر مشخصات مکانیکی و دوام سازههای
بتنی بررسی شده است. مشخصات مکانیکی شامل مقاومت فشاری و مقاومت کششی در حالت داغ و مشخصات دوام شامل
جذب آب سطحی، عمق نفوذ آب، مقاومت الکتریکی و افت وزنی بتن حرارت دیده است. دماهای بررسی شده بازه دمایی
28 تا 800 درجه سانتیگراد است. نتایج نشان داد که جایگزینی سیمان با زئولیت موجب کاهش جزئی مقاومت فشاری
و کششی 28 و 42 روزه بتن معمولی شده است. این بررسی در دماهای باال نشان داد که اگرچه جایگزینی درصدی از
سیمان با زئولیت مقاومت فشاری بتن معمولی را به میزان کمی کاهش داده است، در رابطه با مقاومت فشاری نرمال شده
در بیشتر دماهای آزمایش موجب بهبود این پارامتر شده است. عالوه بر این، مالحظه گردید برای جایگزینی 10 و 20
درصد وزنی سیمان با زئولیت مقاومت کششی بتن در دماهای باال را به ترتیب 21 و 13 افزایش داده است. این بررسی در
رابطه با مقاومت کششی نرمال شده بتن در دماهای باال برای درصدهای یاد شده، به ترتیب افزایش 22 و 14 درصدی را
نشان داد. در رابطه با مشخصات دوام بتن مالحظه گردید که اگرچه افزایش دمای آزمایش مشخصات دوام بتن معمولی را
دچار آسیب کرده است، جایگزینی زئولیت با سیمان موجب بهبود آن مشخصات شده است. نتایج به نحوی بود که بهترین
عملکرد دوام بتن، در نمونههای حاوی درصدهای بالاتر زئولیت حاصل گردید.
In the present study, the mechanical properties and the residual stress–strain behavior of lightweight concrete (LWC) containing pumice coarse aggregate and rock wool waste (consisting of mineral fibers) were explored prior to and following thermal loading. Key variables included the volume percentage of rock wool waste (0%, 2.5%, 5%, 7.5%, and 10%) and exposure temperature (20°C, 200°C, 400°C, and 600°C). Here, parameters playing a role in the compressive performance of LWC containing rock wool waste were examined. These parameters included the elastic modulus, compressive strength, strain at peak stress, ultimate strain, toughness index, stress–strain relationship, and failure mode. Then, several empirical relationships were proposed to predict different mechanical characteristics in terms of temperature and volume percentage of rock wool. Furthermore, the compressive strength, elastic modulus, and strain at peak stress values were compared to the prediction results of the ACI 216, EN 1994-1-2, ASCE, and CEB-FIP 1990 codes. The results demonstrated that the mechanical properties of the LWC specimens were degraded with temperature. The highest degradation in the temperature range under study occurred at 600°C, with around 50% and 80% drop in the compressive strength and elastic modulus, respectively. Furthermore, after exposure to 600°C, an increase of 2 to 2.8 folds occurred in the strain at peak stress and an increase of 2.6 to 3.4 folds occurred in the ultimate strain of the specimens relative to those at the ambient temperature. In the end, two stress–strain models were presented to capture the compressive performance of LWC including rock wool waste after elevated temperature exposure based on the empirical equations obtained for the mechanical characteristics. These models showed a relatively good correlation with the experimental data.
In this study, uniaxial compression tests and digital speckle collection technology are used to study the applicability of carbon nanotube-reinforced foam concrete, which is prepared with micro-doped carbon nanotubes, as a pressure-relieving energy-absorbing material. Moreover, the effects of different carbon nanotube contents on the load-deformation characteristics, including the stress-strain response, displacement field, maximum shear strain field, Poisson's ratio and energy drop release law, of carbon nanotube-reinforced foam concrete materials are investigated. The results show that the ultimate strain of the carbon nanotube-reinforced foam concrete materials is much larger than that of brittle materials, such as ordinary concrete and rock. As the carbon nanotube content increases, the carbon nanotube-reinforced foam concrete specimen provides higher post-peak compressive resistance under certain deformation conditions. This property is a performance requirement of surrounding rock support pressure relief materials. It is proposed that the deformation localization starting conditions of the carbon nanotube-reinforced foam concrete specimens are divided into strain decision conditions and stress decision conditions. The strain decision condition is 86.8% of the peak strain, and the stress decision condition is 80.2% of the peak stress. It is suggested that the strain condition is more reasonable than the stress condition as the starting condition for deformation localization in the carbon nanotube-reinforced foam concrete specimens. So on after deformation localization starts, the corresponding Poisson's ratio-strain curve exhibits a steep increase. The energy release process of carbon nanotube-reinforced foam concrete is measured by the energy drop release rate. The energy release process of the carbon nanotube-reinforced foam concrete specimens with 0.05% carbon nanotubes best satisfies the surrounding rock support pressure relief requirements.
An experimental program was designed to investigate the residual properties of high strength concrete (HSC) modified with different percentages of graphite nano/micro platelets (GNMPs) after exposure to elevated temperature. Mechanical properties such as compressive and tensile strength, stress–strain response, elastic modulus and mass loss have been studied at elevated temperature in 23–800 °C range. Non-destructive testing technique such as ultrasonic pulse velocity (UPV) was used to investigate the quality of concrete in the post fire exposure. Further, the energy dissipation characteristics have also been observed in detail. Scanning electron microscopy (SEM) was used for the study of morphological changes such as crack formation, microstructural damage and the dispersions of GNMPs. Results showed better retention of mechanical and physical properties of high strength concretes containing GNMPs. Data obtained was utilized to formulate mathematical relationships for expressing mechanical, durability and energy related properties of HSC modified with different percentages of GNMPs as a function of temperature.
One of mankind's biggest achievements has been to harness fire which has quickened the pace of technological advancements. While we have benefitted greatly, we have suffered none the less. Structural fires have been a huge problem for the past few decades and the loss of property and life has been immense. Concrete structures are generally considered as potent against fire, given the intrinsic lower thermal conductivity of concrete and its enduring nature. However, irreversible physical and chemical changes occur in the cement matrix over long periods of fire exposure. Concrete is a multi-phase, multi-scale material, with each phase behaving differently under fire conditions. This difference of behavior often leads to deterioration of mechanical strength of concrete which can cause structural failures. Human population is on the rise and so is the need for its habitation, thus the building materials are being used today at a higher pace than ever. The advanced building materials are light, strong, durable and long lasting; however, the problem could be with their resistance to fire. This is where nano-technology offers innovative and effective solutions, by contributing to develop building materials that are not only durable and sustainable in nature, but at the same time are fire enduring.
Some have downloaded this article for free at sci-hub.
Production of cement, aluminum, magnesium, titanium, and steel structural materials generate more than 2 gigatonnes of CO2 globally per year. Replacement of structural materials with lightweight, stronger carbon nanotube (CNT) composites reduces the structural material's production aggregate requirements by achieving the same strength with less material. This averts a massive CO2 emission in the production of structural materials. CNTs have the highest measured tensile strength of all materials and form strong composites, but until recently, they were produced only by high carbon footprint processes. CNTs are synthesized in this study from CO2 (are carbon negative) by low-energy C2CNT (CO2 to CNT) molten electrolysis. Four tonnes of CO2 electrolyzed forms one tonne of CNTs. This avoids several hundred tonnes of CO2 by replacing structural materials with CNT composites. For example, a 2-tonne cement block with 0.001 tonne of CNTs has the same strength as a 3-tonne block without CNTs. The 1-tonne cement avoided eliminates its CO2 production emission. Specifically, a 0.048 wt% CNT-cement composite eliminates 840 tonne of CO2/tonne CNT. CO2 is eliminated from the anthropogenic carbon cycle at less than $1 per tonne. High carbon footprint materials such as aluminum trigger larger CO2 composite elimination effects, and 1 tonne of CNT extraordinarily eliminates more than 4000 tonne CO2/tonne CNTs.
The addition of high performance nano materials like carbon fibers, carbon nanotubes, graphene etc. in the cement and concrete is gaining attention for achieving multifunctional composite materials with enhanced mechanical, physical and electrical properties. The nano-metric size range and the exceptionally high mechanical properties of carbon nanotubes possess very great potential for their utilization in cementitious composites for obtaining remarkable properties. Billions of ton of concrete is used every year in construction industry and its quantity may be reduced to a large extent only by improving the mechanical and durability properties. One way of achieving the enhanced mechanical properties of cement composite is the utilization of thoroughly dispersed carbon nanotubes in the composite matrix. In the present research, small fractions of multiwall carbon nanotube (MWCNTs) i.e. 0.05 and 0.10 wt.% of cement have been incorporated into the cement concrete and their influence on the mechanical properties of the resulting composites have been studied. It is a well-known fact that the uniform dispersion of the MWCNTs in the composite matrix holds the key for the performance improvement. Therefore, special attention was paid to this aspect and uniform dispersion of MWCNTs was achieved through the use of high energy sonication in the presence of modified acrylic based polymer (acting as a surfactant). The concrete specimens were tested in splitting tensile, flexure and compressive strength after 3, 7, 28 and 56 days of immersed water curing. It was observed that the addition of 0.05wt.% MWCNTs increased the splitting tensile strength by 20.58%, flexural strength by 26.29% and compressive strength by 15.60% as compared to the control mix at 28 days of curing. The strength enhancements for the concrete mixes containing MWCNTs may be regarded to the phenomenon of bridging, pinning and branching of the cracks at nano/micro level due to the presence of MWCNTs. Beside strength enhancements, it was also observed that the MWCNTs had tremendously enhanced the fracture energy and breaking strains of the concrete mixes as observed in three-point bending tests. The research concludes that very low amounts of MWCNTs incorporated in the cement concrete mixes improve their mechanical strengths and fracture behavior remarkably but the thorough dispersion of MWCNTs in the matrix have to be insured.
In the present work, carbon nano/microparticles obtained by controlled pyrolysis of peanut (PS) and hazelnut (HS) shells are presented. These materials were characterized by Raman spectroscopy and field emissionscanning electron microscopy (FE-SEM). When added to cement paste, up to 1 wt%, these materials led to an increase of the cement matrix flexural strength and of toughness. Moreover, with respect to plain cement, the total increase in electromagnetic radiation shielding effect when adding 0.5 wt% of PS or HS in cement composites is much higher in comparison to the ones reported in the literature for CNTs used in the same content.
In this study, material properties of calcium aluminate cement concrete (CACC) were investigated at various temperatures of 23, 200, 400, 600 and 800 °C. Material properties namely compressive strength, splitting tensile strength, elastic modulus, stress-strain response, mass loss and compressive toughness were measured using unstressed and residual test methods. High temperature performance of CACC was compared with conventional normal strength concrete (NSC). Data from high temperature tests of CACC revealed that the presence of alumina as a binding agent showed considerable enhancement in the mechanical performance compared to NSC. At elevated temperatures, reduction in the stress-strain response was observed in both CACC and NSC; however, increase in axial strain was more in case of CACC. Compressive toughness was higher in case of CACC as compared to NSC which increases up to 200 °C, but decreases beyond this temperature. Scanning electron microscope (SEM) was used to differentiate the microstructural changes in both types of concrete at temperatures up to 600 °C. Visual investigations after high temperature exposure revealed that CACC exhibits low cracking with less color changes as compared to NSC. Data generated from material property tests was utilized to develop simplified relations for expressing material properties of CACC as a function of temperature.
In recent decades, the use of structural high performance concrete (HPC) sandwich panels made with thin plates has increased as a response to modern environmental challenges. Fire endurance is a requirement in structural HPC elements, as for most structural elements. This paper presents experimental investigations on the fire behaviour of HPC thin plates (20 or 30 mm thick) being used in lightweight structural sandwich elements. Tests were undertaken using a standard testing furnace and a novel heat-transfer rate inducing system (H-TRIS), recently developed at the University of Edinburgh. The parametric assessment of the specimen performance included: thickness of the specimen, testing apparatus, and concrete mix (both with and without polypropylene fibres). The results verified the ability of H-TRIS to impose an equivalent thermal boundary condition to that imposed during a standard furnace test, with good repeatability, and at comparatively low economic and temporal costs. The results demonstrated that heat induced concrete spalling occurred 1 to 5 min earlier, and in a more destructive manner, for thinner specimens. An analysis is presented combining the thermal material degradation, vapour pore pressure, stress concentrations, and thermo-mechanical energy accumulation in the tested specimens. Unexpectedly, spalling at the unexposed surface was observed during two of the tests, suggesting a potentially unusual, unwanted failure mode of very thin-plates during fire. On this basis it is recommended to favour 30 mm thick plates in these applications, since they appear to resist spalling better than those with 20 mm thickness.
Ultra-high-performance concrete (UHPC) incorporates a relatively large volume fraction of very dense cementitious binder with microscale fibers. The dense binder in UHPC can effectively interact with nano- and microscale reinforcement, which offers the promise to overcome the brittleness of UHPC. Nanoscale reinforcement can act synergistically with microscale fibers by providing reinforcing action of a finer scale, and also by improving the bond and pullout behavior of microscale fibers. Carbon nanofiber (CNF) and polyvinyl alcohol (PVA) fiber were used as nano- and microscale reinforcement, respectively, in UHPC. An optimization experimental program was conducted in order to identify the optimum dosages of CNF and PVA fiber for realizing balanced gains in flexural strength, energy absorption capacity, ductility, impact resistance, abrasion resistance, and compressive strength of UHPC without compromising the fresh mix workability. Experimental results indicated that significant and balanced gains in the UHPC performance characteristics could be realized when a relatively low volume fraction of CNF (0.047 vol.% of concrete) is used in combination with a moderate volume fraction of PVA fibers (0.37 vol.% of concrete).
The knowledge of high temperature thermal properties is critical for evaluating the fire response of concrete structures. This paper presents the effect of temperature on the thermal properties of different types of high-strength concrete (HSC). Specific heat, thermal conductivity, and thermal expansion are measured for three concrete types, namely, HSC, self-consolidating concrete (SCC), and fly ash concrete (FAC), in the temperature range from 20–800°C. The effect of steel, polypropylene, and hybrid fibers on thermal properties of HSC and SCC is also investigated. Results from experiments show that SCC possesses higher thermal conductivity, specific heat, and thermal expansion than HSC and FAC in the 20–800°C temperature range. Data generated from tests is utilized to develop simplified relationships for expressing different thermal properties as a function of temperature. The proposed thermal property relationships can be used as input data for evaluating the response of concrete structures under fire conditions.
Compression toughness tests were carried out on concrete cylinders reinforced with three different aspect ratios of hooked-end steel fibers 60, 75, and 83 and six different percentages of steel fibers 0.5, 1.0, 1.25, 1.5, 1.75, and 2.0% by volume of concrete. The w/c ratio used for the normal strength steel fiber reinforced concrete mixes (NSSFRC) was 0.55, and the water-cementitious ratio (w/c+s) for the high strength fiber reinforced concrete mixes (HSSFRC) was 0.31. For each mix, three test cylinders were tested for compression specific toughness. The effect of fiber reinforcement index: volume of fibers × length/diameter ratio on compression specific toughness and also on the relationship between these two properties is presented in this paper. As a result, (a) equations are proposed to quantify the effect of fibers on compression toughness ratio of concrete in terms of FRI, (b) equations obtained in terms of FRI and compression specific toughness of plain concrete to estimate both compression specific toughness of NSSFRC and HSSFRC (N.m), (c) equations obtained which represent the relationship between compression toughness index and FRI for NSSFRC and HSSFRC, respectively, and (d) equations obtained to quantify the relationship between compression specific toughness index and fiber reinforcement index for NSSFRC and HSSFRC, respectively. The proposed equations give good correlation with the experimental values.
This paper reports the main findings from a comprehensive study undertaken to evaluate the thermal and mechanical properties of lightweight foamed concrete (LFC) exposed to high temperatures. Experimental and analytical studies were carried out to develop quantification models to obtain thermal and mechanical properties of LFC at ambient and elevated temperatures. LFC of densities of 650 and 1000 kg/m(3) was comprehensively tested; supplementary tests on LFC densities of 800, 1200 and 1400 kg/m(3) were carried out for additional data. To quantify thermal conductivity, LFC is treated as a porous material and the effects of radiant heat transfer within the pores are included. This thermal conductivity model produced results in very good agreement with the experimental results obtained from guarded hot plate tests. The elevated mechanical property models consist of two parts: the prediction of compressive modulus of elasticity and peak stress as functions of porosity and the prediction of strength and stiffness retention factors as functions of temperature. The reduction in strength and stiffness of LFC at high temperatures can be predicted using the mechanical property models for normal-strength concrete, provided that the LFC is based on Portland cement CEM1. This paper describes the models in full and presents comparisons with test results.
This research work was carried out to evaluate the performance of High Performance Concrete (HPC) made with Fly Ash (FA) and Metakaolin (MK) at elevated temperature through Stiffness Damage Test (SDT). Variables of the test program include partial replacement of cement with MK (10% and 20%) and FA (20% and 40%) and temperature from 27 to 400 degrees C. To quantify the damage, chord loading modulus, unloading stiffness, plastic strain, damage index and non-linearity index were evaluated from SDT. Correlation among SDT parameters and in between SDT parameters and elevated temperature were also studied. According to the test results, the SDT parameters showed that the stiffness and elasticity decreased and damage increased with the increase in temperature. The stiffness changes were evident from chord modulus and unloading stiffness, elasticity changes from plastic strain and damage to concrete from damage index. For all mixes, significant change in the SDT parameters occurred at 300 degrees C. Therefore, 300 degrees C can be considered as critical temperature. At elevated temperatures (300 degrees C and 400 degrees C), FA mixes showed lower values of plastic strain and damage index than MK mixes. Thus, it may feasible to increase the allowable working temperature for FA mixes. The SDT parameters were found to be sensitive to elevated temperatures caused by changes in the microstructure of concrete. Test results also revealed that plastic strain and damage index correlate well with loading elastic modulus with values of coefficient of correlation equal to 0.9657 and 0.9835 respectively. Therefore, plastic strain and damage index measured from SOT can be used to estimate the percentage residual chord loading modulus of concrete affected by fire. Strong correlation also exists between PS and DI with coefficient of correlation equal to 0.985.
Excellent mechanical, thermal, and electrical properties of carbon nanotubes (CNTs) and nanofibers (CNFs) have motivated the development of advanced nanocomposites with outstanding and multifunctional properties. After achieving a considerable success in utilizing these unique materials in various polymeric matrices, recently tremendous interest is also being noticed on developing CNT and CNF reinforced cement-based composites. However, the problems related to nanomaterial dispersion also exist in case of cementitious composites, impairing successful transfer of nanomaterials' properties into the composites. Performance of cementitious composites also depends on their microstructure which is again strongly influenced by the presence of nanomaterials. In this context, the present paper reports a critical review of recent literature on the various strategies for dispersing CNTs and CNFs within cementitious matrices and the microstructure and mechanical properties of resulting nanocomposites.
The extensive use of eco-friendly materials in concrete has led to the demand to fully understand the effect of fire on concrete. This research was carried out to evaluate high performance concrete (HPC) made with fly ash and metakaolin with replacement level of 20 percent by weight of cement after elevated temperatures exposure (200 °C, 400 °C, 600 °C and 800 °C). The mechanical performance was assessed from compressive strength while the durability was assessed from chloride permeability and water sorptivity test. Qualitative analysis of the microstructure of heated and unheated concrete was performed by SEM while quantitative analysis was performed on SEM images using Image Pro-plus software. Based on the qualitative and quantitative analysis of SEM images, the distribution of the number, type and surface area fraction of flaws were identified and changes in the structure of Interfacial Transition Zone (ITZ) were classified into different temperature ranges.
This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.
The binding phase of cementitious materials, calcium–silicate–hydrates, can be described as nanogranular and as an inorganic hydrogel. Similar to other hydrated “soft matter,” the water confined within the nano- to microscale pores of such cementitious materials plays a crucial role in the structure and properties of cement pastes. When compared to organic hydrogels, non-stoichiometric calcium–silicate–hydrates (C–S–H) are relatively robust against changes in humidity and temperature. However, under extreme physical environments, changes in the amount, and location, and physical state of water can limit damage tolerance and sustainability of otherwise stiff and strong cementitious macrostructures. Here, we employed Grand Canonical Monte-Carlo and Molecular Dynamics simulations to investigate the effect of temperature on the water content within and between C–S–H grains constituting the cement microstructure, and on the associated physical and mechanical properties of this material. We found water content within grains decreased with increasing relative temperature up to T/T* = 2 (where T* is the transition temperature at which the bulk liquid and gas are in equilibrium for a given pressure), and that C–S–H grains densified with attendant increases in heat capacity, stiffness, and hardness. Although intragranular cohesion increased monotonically with increasing relative temperature over this range, intergranular cohesion increased up to a relative temperature of T/T* = 1.1 and then decreased at higher relative temperatures. This finding suggests a rationale for the decreased mechanical performance of cement paste and concrete at high relative temperatures, and supports previous claims of peak hardness in C–S–H at an intermediate relative temperature between 1 and 2.61. Further, these atomistic simulations underscore the important role of confined water in modulating the structure and properties of calcium–silicate–hydrates upon exposure to extreme environments.
An appropriate dispersion of carbon nanotubes (CNTs) is a prerequisite for their use in improving the mechanical properties of cement-based composites. In this study two types of carbon nanotubes (CNTs) having different morphologies were investigated. To obtain a uniform distribution of CNTs in the cement matrix, the effect of sonication on the deagglomeration of CNTs in combination with anionic and nonionic surfactants in varying concentrations was quantitatively investigated when preparing aqueous dispersions of CNTs for the subsequent use in cement paste. The relationships between the quality of CNT-dispersion on the one hand and the sonication time and surfactant concentration on the other were determined using UV–vis spectroscopy. After dispersion, nitrogen-doped CNTs were found mostly as individual, broken CNTs. In contrast, after the treatment of the mixture of single-, double-, and multi-walled CNTs, a net-like distribution was observed where destruction of the CNTs due to sonication could not be distinguished. Modification of the cement pastes with dispersions of CNTs led to a pronounced increase, up to 40%, in compressive strength and, in some cases, to a moderate increase in tensile strength under high strain-rate loading. However, no significant improvement in strength was observed for quasi-static loading. Microscopic examination revealed that the bridging of the C–S–H phases differed depending on the type of CNT. This explained, at least partly, the observed effects of CNT-addition on the mechanical properties of hardened cement pastes.
Carbon nanofibers/tubes (CNF/Ts) are very strong and stiff and as a result, are expected to be capable of enhancing the mechanical properties of cementitious materials significantly. Yet there are practical issues concerning the utilization of CNF/Ts in cementitious materials. This study summarizes some of the past efforts made by different investigators for utilizing carbon nanofilaments in cementitious materials and also reports recent experimental research performed by the authors on the mechanical properties of CNF-reinforced hardened cement paste. The major difficulties concerning the utilization of CNF/Ts in cementitious materials are introduced and discussed. Most of these difficulties are related to the poor dispersibility of CNF/Ts. However, the findings from the research presented in this work indicate that, despite these difficulties, carbon nanofilaments can significantly improve the mechanical properties of cementitious materials. The results show that CNFs, even when poorly dispersed within the cementitious matrix, can remarkably increase the flexural strength and cracking resistance of concrete subjected to drying conditions.
This paper deals with the description of tests on concrete subjected to a high temperature and then cooled in two ways. Two series of cylinder specimens (103 mm in diameter, 200 mm in height) made of concrete C25/30 and C40/50 were tested. Specimens were heated in the electric furnace to 270, 370 and 500 °C. After heating, some of specimens were left in the open air and some of them were put in water for 10 sec and then left in the open air. Next day the compressive strength of concrete was tested. Test results are presented. In addition, the paper includes some test results taken from the literature.
h i g h l i g h t s Enhancing mechanical properties of oil well cement (OWC) pastes using different nanomaterials. Increasing curing temperature causes a notable increase in the compressive strength. 0.1% CNTs causes a good enhancement in the mechanical properties of the hardened OWC pastes. Optimum addition of NS to hardened OWC pastes is 1% at 25 and 90 °C. a b s t r a c t Nanotechnology has shown great potential in different applications and presents solutions to some of the upstream and downstream challenges in the oil and gas industry. This research work aimed to improve the mechanical properties of oil well cement (OWC) pastes using multiwall carbon nanotubes (CNTs), nano-silica (NS) and nano-metakaolin (NMK). The effect of different curing temperatures, namely; room temperature and elevated temperature (90 °C) on the hydration reaction of OWC pastes admixed with different nano materials was investigated. XRD, TG/DTG, and SEM techniques were used to investigate the phase composition and the microstructure of some selected hardened specimens. The obtained results indicated that all of the nano materials used in this study cause a notable improvement in the mechanical properties of hardened OWC pastes at both curing temperatures. In addition, NMK – OWC hardened pastes present the highest improvement in the mechanical properties among all of other tested mixes.
La profesion –formacion- docente es un tema crucial en los actuales debates educativos. La existencia de dos decretos y el desplazamiento del verdadero sentido del ser maestro reclaman de los analisis un ejercicio de comprension del orden discursivo oficial. La calidad es el sustrato de la sociedad de control. En este marco se agencia nuevas practicas de subjetivacion del maestro los cuales podriamos situar en la calidad, flexibilidad, adaptabilidad, eficiencia, eficacia. En cualquier caso, el esfuerzo por hacer del maestro un intelectual de la educacion fue borrado. La gran cuestion consiste en saber que discursos regula el saber del docente a la luz de la sociedad de control.
The lightweight aggregate–cement paste interface is characterized by a mechanical interlocking in combination with a chemical interaction in the form of a pozzolanic reaction. The lightweight aggregates have a porous surface due to which some part of the binder will penetrate into the aggregate, which will subsequently decrease the internal bleeding water zone. There are two types of pores in the lightweight aggregates—open and closed pores. Open pores are the pores that are interconnected and take part in the permeation, whereas the closed pores are sealed and not interconnected thus not taking part in the permeation. The total porosity of a material is the sum of the closed and open pores, whereas the permeability will depend only upon the interconnected pores. The simple way to assess the interconnectivity of the pores is by measuring the water absorption property. This chapter concludes with a discussion on inter-relation of microstructure and the strength of lightweight aggregate cement.
In this study, both fire tests and low-frequency cyclic loading tests after fire were conducted on three conventional high strength concrete (HSC) shear walls and a superimposed HSC shear wall with precast recycled aggregate concrete (RAC) panels. The RAC in this paper was made with recycled concrete aggregate. When specimens suffered the fire exposure on one side for 45 min, 90 min, and 135 min separately, spalling of concrete, temperature distribution and deformation of specimens were investigated as indicators of fire response. When specimens were subjected to cyclic load after fire, hysteresis curves were obtained, based on which the secant stiffness degradation and energy dissipation capacity of walls were analyzed. The results indicated that HSC would suffer severe spalling during the fire and that fire response of the superimposed wall including spalling was smaller than that of conventional walls. Using RAC panel as a thermal barrier was found to be effective to alleviate spalling, as it reduced more than 60% of spalling of HSC compared with bare walls. Based on the seismic tests results, the fire exposure deteriorated the load bearing capacity, lateral stiffness and energy dissipation capacity of walls, whereas the application of RAC panels improved the load bearing capacity by about 10% even when the superimposed wall was exposed to the fire for a long time.
Nanotechnology in construction and building materials has attracted great attention
in recent years. Results demonstrated that nanomodification of cementitious materials
can lead to significant improvement of the mechanical property, compactness, and
durability. In this paper, based on the major characteristics of nanomaterials when
used in cement-based materials, the recent progresses of nanomodification of cemen-
titious materials with the mostly used nanomaterials such as nano-SiO 2 , nanoclay,
nano-Al 2 O 3 ,and carbon nano materials (carbon nanotubes and nanofibers) are
reviewed. Modification effects and their influencing mechanisms of nanomaterials
introduced by their specific features, such as the hydration seeding effect, the filling
effect, the thixotropy-modifying effect, and the chemical reactivity feature on the
properties of cementitious materials are reviewed. It is suggested that a stronger,
greener, and more durable cementitious material can be obtained through the help of
nanomodification.
In this study, the possibility of using granulated blast-furnace slag (GBFS) as partial or full natural silica sand replacement in alkali-activated slag (AAS) mortar was investigated. The ratio of binder to fine aggregate was 1:2. Natural sand was replaced with GBFS at levels of 0%, 25%, 50%, 75% and 100%, by weight. Compressive strength at ages of 7 and 28d was measured. The behaviour of the investigated mortar mixtures after exposure to 200, 400, 600 and 800°C for 2h was evaluated quantitatively by measuring the residual compressive strength and qualitatively by visual inspection. The various decomposition phases formed and the morphology of formed hydrates were identified by using X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results indicated that the compressive strength of the mortar specimens before and after firing increased with increasing GBFS sand content.
The effect of transient high temperature on strength and load-deformation behavior of high-strength concrete was investigated. The concrete strength varied between 4500 to 12,900 psi (31.1 to 89 MPa) and the temperature exposure was in the range of 23 to 800 C (73 to 1472 F). The presence of loads in real structures was simulated by preloading the test specimens before exposure to elevated temperature. Exposure to temperatures in the range of 100 to 300 C (212 to 572 F) was observed to decrease the compressive strength of high-strength concrete by 15 to 20 percent. At temperatures in the range of 400 to 800 C (752 to 1472 F), the compressive strength of concrete decreased to about 30 percent of its strength at room temperature. The exposure to high temperature also decreased the modulus of elasticity of concrete.
Exposing concrete to high temperature causes progressive breakdown of the cement gel structure and consequently severe deterioration and loss in the structure's load bearing capacity. This paper presents an experimental study on the mechanical and physicochemical properties of lightweight self-consolidating concrete (LWSCC) subjected to high temperatures. Four LWSCC mixes and one normal-weight self-consolidating concrete (NWSCC) were tested. The specimens underwent two different tests: a fire test and a thermal characterization test. The first is the ISO fire test, which consists of heating the prismatic specimens according to the standard fire curve up to 600°C. The second test is the thermal characterization test, which consists of heating the specimens at a rate of 1°C/min up to 400°C. Ultimate and residual compressive strength, loss of mass, density, water porosity, spalling characteristics, and other physicochemical properties before and after the fire tests were recorded. The LWSCC performed differently from the NWSCC with respect to mechanical properties and spalling resistance. Scanning electron microscopy and backscatter electron images analysis were performed to study the microstructure of both heated and unheated specimens.
The influence of multi-walled carbon nanotubes (MWCNTs) additive on characteristics of non-autoclaved aerated concrete (NAC) and autoclaved aerated concrete (AAC) was investigated. It was established that MWCNTs used as additive in NAC and in AAC production process acts as nucleators of crystallization influencing the hydration process and structure formation leading to increase in crystallinity of hardened binding material, as well as to increase in flexural and compressive strength of concretes and decrease in shrinkage during heating. The influence of MWCNTs on characteristics of AAC is more significant than on the characteristics of NAC. Investigation results allow to premise that NAC and as well AAC containing MWCNTs additive should be more stable during exploitation than those concretes without this additive.
Experimental studies were carried out to determine the thermal properties of fibre-reinforced concrete at elevated temperatures. The effect of steel-fibres on thermal conductivity, thermal expansion, specific heat and mass losss of fibre-reinforced concrete at elevated temperatures was investigated. Test data indicate that the steel-fibre-reinforced concrete, under elevated temperatures, exhibits thermal properties that are slightly different from those of plain concrete. These studies were carried out as part of a research program aimed at developing methods capable of predicting the fire resistance of fibre-reinforced concrete columns. Des études expérimentales ont été menées dans le but de déterminer les propriétés thermiques du béton renforcé de fibres aux températures élevées. On a examiné plus particulièrement l'incidence des fibres d'acier sur la conductibilité thermique, la dilatation thermique, la chaleur massique et la perte massique du béton renforcé de fibres aux températures élevées. Les données des essais indiquent que le béton renforcé de fibres d'acier affichait aux températures élevées des propriétés thermiques légèrement différentes de celles du béton ordinaire. Ces études ont été réalisées dans le cadre d'un programme de recherche visant le développement de méthodes permettant de prédire la résistance au feu des poteaux en béton renforcé de fibres. RES
The use of effectively dispersed multiwalled carbon nanotube (MWCNT)/aqueous/surfactant suspensions in cement based materials have been shown to substantially improve their mechanical properties. The produced MWCNT suspensions have a high aqueous content, which corresponds to the mixing water. In the present work, a method for preparing highly concentrated MWCNT suspensions is presented, thus reducing the volume of the resulting admixture that is required in cement based materials. A centrifugal process, that uses two different ultracentrifuge rotors, was employed to reduce the quantity of water in the suspensions. Optical absorbance spectroscopy shows that the ultracentrifugation process increases the concentration of the MWCNT suspensions by a factor of 5. Using the highly concentrated MWCNT suspensions following dilution results in nanocomposites with mechanical properties that are comparable to the performance of samples prepared using the non-concentrated suspensions. These results verify that the ultracentrifugation concentration method successfully preserves the solubility of the MWCNT suspensions without affecting the reinforcing properties of the admixture. In this manner, the ultracentrifugation concentration method may constitute an effective preparation step for large-scale implementation of MWCNT admixtures.
Multi-walled carbon nanotubes (MWCNTs) modified by anionic gum arabic have been incorporated into Portland cement pastes to investigate the effect on flexural toughness. The flexural toughness of the cement composites were investigated, and the results showed that the addition of treated nanotubes significantly improved both the fracture energy and flexural toughness index of Portland cement pastes. The porosity and pore size distribution of the composites were measured using mercury intrusion porosimetry, and the results indicate that cement paste containing MWCNTs had lower porosity and a more uniform pore size distribution. The microstructure of samples was investigated using field emission scanning electron microscopy. This showed that MWCNTs act as bridges across cracks and voids and form a network that transfers the load in tension.
An experimental investigation was carried out to evaluate the performance of Fly Ash (FA) and Metakaolin (MK) mortar at elevated temperatures. Variables of the test program include partial replacement of cement with MK from 5% to 20%, FA from 20% to 60% and temperatures from 27 °C to 800 °C. The mechanical performance was assessed from compressive strength while the durability was assessed from chloride permeability test. Qualitative analysis of the microstructure of heated and unheated mortar was performed by Scanning Electron Microscope (SEM) while quantitative analysis was performed on SEM images using Image Pro-plus software.Test results show that for all mixes compressive strength decreased while charge passed increased with the increase in temperature from 27 °C to 800 °C. For all mixes, major strength and durability loss occurred after 400 °C. Therefore, 400 °C can be considered as critical temperature from the standpoint of strength and durability loss. Qualitative and quantitative analysis of SEM were found to be consistent with the results of strength and durability loss. The observation of SEM images and image analysis of area fractions of hardened cement paste (hcp) of different mortar mixes indicated that with the increase in temperature the pore area fraction increased while hydrated paste area fraction decreased. These factors resulted in the degradation of microstructure and affected the strength and durability of mortar. Major drop in hydrated paste area and rise in pore area fraction occurred at 400 °C. Therefore, 400 °C could be regarded as the critical temperature for change in the properties of mortar. In general, fly ash mix (FA20) showed better performance in all aspects.
The influence of volume fraction and aggregate properties on the mechanical performance of lightweight aggregate concrete (LWAC) were studied. A new Shape Index that described the shape characteristics of lightweight aggregate (LWA) has been proposed and its effects on resulting LWAC were evaluated. Test results substantiate that the proposed Shape Index had a great influence on the mechanical properties of LWAC.Crack development, failure characteristics and stress strain curves of LWAC were studied and the concept of the proportional strain ratio was introduced to evaluate the brittleness of LWAC. Test results showed that higher volume content of LWA resulted in a more brittle failure. It was also found out that for similar concrete strengths, LWAC was comparatively brittle than normal weight concrete (NWC).
This research focuses on reinforced lightweight aggregate concrete walls. After performing a standard temperature rising fire-resistance test, the fire resistance performance and mechanic behavior of the wall sample are studied under a lateral horizontal load. Reinforced lightweight aggregate concrete walls and reinforced normalweight concrete walls are given the fire-resistance test under the same conditions, then their fire resistance performance and mechanical behaviors are compared. Taking into consideration the steel spacing, aggregate type, wall size, and high temperature as the wall sample variables.The research results showed that the reinforced lightweight aggregate concrete wall is superior to the reinforced normalweight concrete wall on ultimate load, yield load, cracked load, stiffness, ductility, and inter-story drift after a high-temperature fire-resistance test. In terms of failure patterns, for a reinforced lightweight aggregate concrete wall after a high-temperature fire-resistance test, the smaller the steel spacing the higher the yield load and ultimate load, but the worse the ductility. This result matched the results of the reinforced lightweight aggregate concrete wall without the high-temperature fire-resistance test and revealed that the reinforced lightweight aggregate concrete wall retained its mechanics after the fire-resistance test.
Self consolidating lightweight concrete (SCLWC) mixtures were prepared by using two different lightweight coarse aggregates and by replacing normal weight crushed coarse limestone aggregate at a constant water/powder ratio. One of the SCLWC mixtures was also prepared at different water/powder ratios. All the mixtures were exposed to 300, 600 and 900 °C, respectively. Lightweight aggregate type and water/powder ratio affect water transport characteristics and resistance of the mixtures to elevated temperatures. The microstructural investigation findings are consistent with thermal strain and residual mechanical properties of the mixtures after exposure to high temperatures.
This paper reports the results of an experimental and analytical study to quantify the thermal properties of lightweight foamed concrete (LFC) at high temperatures. The density of LFC ranges from 600 to 1800 kg/m3. The primary objective of this study is to obtain the thermal conductivity of LFC at high temperatures so as to obtain material property data for prediction of fire resistance of LFC based systems. In the analytical approach, LFC is considered to be a two phase material with solid cement and air pores. Therefore, it is assumed that the thermal conductivity of LFC is a function of its porosity and pore size. The porosity of LFC can be easily obtained from the volume of foam inside the material. The effective pore size is based on the dominant internal pore size of the foamed concrete. The Hot Guarded Plate (HGP) test was carried out at different elevated temperatures for foamed concrete of different densities. The HGP test and analytical prediction results are in close agreement. To validate the thermal property results, transient heating tests were conducted in an electric kiln on LFC slabs and the recorded temperatures were compared with a validated one-dimensional heat transfer program in which the aforementioned thermal properties were treated as input data. Close agreement between the measured and predicted temperature results confirms the thermal property results.
The present study investigated the effects of the method of surface decoration on the wettability of multi-walled carbon nanotubes (MWCNTs) and the mechanical properties of the cement paste incorporating these dispersions. The results showed that stable and homogeneous dispersions of MWCNTs in water can be obtained by using surface functionalization combined with decoration using polyacrylic acid polymers. The cement paste specimens incorporating these dispersions revealed good workability and an increase in compressive strength of nearly 50% even with only a small addition of the MWCNTs, namely 0.045 – 0.15% of the cement weight. These results indicate the existence of chemical bonds between the OH groups of the functionalized MWCNTs and probably the C-S-H phase of the cement matrix, which enhanced the transfer of stresses. A second method that was studied included decoration of MWCNTs with polyacrylic acid polymers and gum Arabic. These dispersions appeared to be homogeneous only for approximately 2 h after which a progressive sedimentation occurred. Good workability was found for the cement pastes produced with all of the dispersions; the only significant difference being the slower hydration of the cement incorporating gum Arabic. The mechanical properties of the cement pastes incorporating MWCNTs treated with polyacrylic polymers were unchanged.
In this study, the effect of high temperature on compressive and splitting tensile strength of lightweight concrete containing fly ash was investigated experimentally and statistically. The mixes incorporating 0%, 10%, 20% and 30% fly ash were prepared. After being heated to temperatures of 200, 400 and 800°C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. This article adopts Taguchi approach with an L16 (45) to reduce the numbers of experiment. Two control factors (percentage of fly ash and heating degree) for this study were used. The level of importance of these parameters on compressive and splitting tensile strength was determined by using analysis of variance (ANOVA) method.
Due to their exceptional mechanical properties, carbon nanotubes (CNTs) are considered to be one of the most promising reinforcing materials for the next generation of high-performance nanocomposites. In this study, the reinforcing effect of highly dispersed multiwall carbon nanotubes (MWCNTs) in cement paste matrix has been investigated. The MWCNTs were effectively dispersed in the mixing water by using a simple, one step method utilizing ultrasonic energy and a commercially available surfactant. A detailed study on the effects of MWCNTs concentration and aspect ratio was conducted. The excellent reinforcing capabilities of the MWCNTs are demonstrated by the enhanced fracture resistance properties of the cementitious matrix. Additionally, nanoindentation results suggest that the use of MWCNTs can increase the amount of high stiffness C–S–H and decrease the porosity. Besides the benefits of the reinforcing effect, autogenous shrinkage test results indicate that MWCNTs can also have a beneficial effect on the early strain capacity of the cementitious matrix, improving this way the early age and long term durability of the cementitious nanocomposites.
The investigation performed was aimed at showing the influence of high temperatures on the mechanical properties and properties that affect the measurement by non-destructive methods (rebound hammer and pulse velocity) of concrete containing various levels (10% and 30%) of pozzolanic materials. Three types of Pozzolans, one natural pozzolan and two lignite fly ashes (one of low and the other of high lime content) were used for cement replacement. Two series of mixtures were prepared using limestone and siliceous aggregates. The W/b and the cementitius material content were maintained constant for all the mixtures. Concrete specimens were tested at 100, 300, 600 and 750 °C for 2 h without any imposed load, and under the same heating regime. At the age of 3 years, tests of compressive strength, modulus of elasticity, rebound hummer and pulse velocity were come out. Results indicate that the residual properties of concrete strongly depend on the aggregates' and the binder type. Relationships between strength of concrete as well as rebound and pulse velocity versus heating temperatures are established. The above results are evaluated to establish a direct relationship between non-destructive measurements and compressive strength of concrete exposed to fire.
The remarkable mechanical properties of carbon nanotubes (CNT) suggest that they are ideal candidates for high performance cementitious composites. The major challenge however, associated with the incorporation of CNTs in cement based materials is poor dispersion. In this study, effective dispersion of different length multiwall carbon nanotubes (MWCNTs) in water was achieved by applying ultrasonic energy and in combination with the use of a surfactant. The effects of ultrasonic energy and surfactant concentration on the dispersion of MWCNTs at an amount of 0.08wt.% of cement were investigated. It is shown that for proper dispersion the application of ultrasonic energy is absolutely required and for complete dispersion there exists an optimum weight ratio of surfactant to CNTs. For a constant ratio of surfactant to MWCNTs, the effects of MWCNT type (short and long) and concentration on the fracture properties, nanoscale properties and microstructure of nanocomposite materials were also studied. Results suggest that MWCNTs improve the nano- and macromechanical properties of cement paste.
Synopsis
The paper describes tests on 100 mm concrete cubes heated to temperatures in the range of 200–800°C to determine the effect of varying time of exposure and rates of heating and cooling on the residual compressive strength of concrete. These variables were found to have a significant effect on concrete heated to the lower range of temperatures but their effect became less pronounced at high temperatures. Almost all the loss of compressive strength occurred within two hours of exposure to the maximum temperature. Pulse velocity measurements did not predict the residual strengths accurately and appeared to be more indicative of the levels of temperature to which the concrete had been exposed.
Stable homogeneous suspensions of multi-walled carbon nanotubes (MWCNTs) were prepared using gum arabic (GA) as a dispersant. By measuring adsorption isotherm, the surface chemical properties of the MWCNTs were investigated. The dispersion of MWCNTs in aqueous solution was examined with transmission electron microscopy and ultraviolet visible spectroscopy. The stabilization mechanism of the MWCNTs in aqueous solution using the GA surfactant was discussed. The isotherm of GA adsorption on MWCNTs indicates a typical two-step mechanism of adsorption (namely "SL"), and the adsorption isotherm reaches the saturation plateau at a GA concentration of 0.45 g x L(-1). The concentration measurement of MWCNTs shows that the optimum concentration of GA is 0.45 g x L(-1).
The compressive strength of normal strength concrete at elevated temperatures up to 700∘C and the effect of cooling regimes were investigated and compared in this study. Thus, two different mixture groups with initial strengths of 20 and 35 MPa were produced by using river sand, normal aggregate and portland cement. Thirteen different temperature values were chosen from 50 to 700∘C. The specimens were heated for 3 h at each temperature. After heating, concretes were cooled to room temperature either in water rapidly or in laboratory conditions gradually. The residual strengths were determined by an axial compressive strength test. Strength and unit weight losses were compared with the initial values. Throughout this study, ASTM and Turkish Standards were used. It was observed that concrete properties deteriorated with the heat; however, a small increase in strength was observed from 50 to 100∘C. Strength loss was more significant on the specimens rapidly cooled in water. Both concrete mixtures lost a significant part of their initial strength when the temperature reached 700∘C. Copyright © 2008 John Wiley & Sons, Ltd.