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Improvement of fiber corrosion resistance of ultra-high-performance concrete by means of crack width control and repair
Abstract
This study aims to investigate the influences of the pullout state and pre-crack width on steel fiber corrosion in ultra-high-performance concrete (UHPC) and its implication on the interfacial bond and tensile performances. For this, two pullout states, that is, partial and full debonding, and five pre-crack widths, ranging from 0.02 to 0.5 mm, were considered. An epoxy-based crack repair process was also proposed, and its benefits on limiting steel fiber corrosion were evaluated. The average bond strength of steel fiber from UHPC could be improved by 54%–59% after exposure to a corrosive environment for 4 weeks, mainly due to partial surface corrosion. The debonding region was the main passage of the NaCl solution and led to the growth of ferric oxide. The crack width of ultra-high-performance fiber-reinforced concrete (UHPFRC) clearly affected the degree of steel fiber corrosion and the tensile performance. The tensile behavior of the micro-cracked UHPFRC with a small crack width below 0.15 mm was insignificantly influenced by the 4 week corrosion; whereas, the 0.3-mm cracked UHPFRC provided 10%–14% higher tensile strength and maintained higher stress levels in the softening region because of the moderately corroded fiber surface. Given the wider pre-crack condition (0.5 mm), no increase in the tensile strength was detected by partial ruptures of steel fibers. The steel fiber corrosion in cracked UHPFRC could be effectively prevented by the crack repair process, and no change in tensile behavior was thus obtained after exposure to a corrosive environment for 4 weeks.
... The production of UHPC requires a very large quantity of cement, which exceeds 1,000 kg/m 3 [27][28][29] or a combination of cement and silica fume with the silica fume quantity generally higher than 175 kg/m 3 [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. The large amount of cement consumed in UHPC production makes this concrete not an environmentally friendly construction material since cement has been reported to be one of the major contributors to the production of greenhouse gas emissions, especially CO 2 [44]. ...
... Some of the additives usually added include fly ash [48,49], nano-silica [32,33], silica powder or silica flour [38,39], limestone powder [50][51][52][53][54], ground granulated blast furnace slag [43,[54][55][56][57], quartz powder [51,57,58], rice husk ash [59], and glass powder [60,61]. Moreover, the production process requires using a special type of sand such as quartz [28,56] or silica sands [35][36][37][38][39][40]43]. It is also important to note that the curing process is also special as indicated by steam [2,34,38,58], moist [49,56], or heat treatment [5,13,18,28,29,[35][36][37][38][39]55,[35][36][37][38][39][40]62] curing. ...
... Moreover, the production process requires using a special type of sand such as quartz [28,56] or silica sands [35][36][37][38][39][40]43]. It is also important to note that the curing process is also special as indicated by steam [2,34,38,58], moist [49,56], or heat treatment [5,13,18,28,29,[35][36][37][38][39]55,[35][36][37][38][39][40]62] curing. This means that it has a very high production cost and its application is limited [63,64]. ...
Calcined diatomaceous earth (CDE) with a maximum grain size of 143 μm was used to partially replace 5 and 10% of cement in ultra-high-performance concrete (UHPC) mixtures. The other materials used in producing the concrete include Ordinary Portland Cement, iron ore powder, and river sand with maximum grain sizes 112.5, 231, and 766.2 μm, respectively. Moreover, the UHPC specimens designed with a water-cement ratio of 0.2 and a superplasticizer of 1.5% from the cement weight were tested for flow, compressive strength, flexural strength, splitting tensile strength, durability against NaCl and Na 2 SO 4 attack, and resistance to 400, 500, and 600°C temperatures. The results showed that the use of 5 and 10% CDE to replace cement was able to increase the com-pressive strength, flexural strength, splitting tensile strength, the durability of UHPC against NaCl, and Na 2 SO 4 , as well as its resistance to high temperatures but reduced the mixture flow.
... The production of UHPC requires a very large quantity of cement, which exceeds 1,000 kg/m 3 [27][28][29] or a combination of cement and silica fume with the silica fume quantity generally higher than 175 kg/m 3 [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. The large amount of cement consumed in UHPC production makes this concrete not an environmentally friendly construction material since cement has been reported to be one of the major contributors to the production of greenhouse gas emissions, especially CO 2 [44]. ...
... Some of the additives usually added include fly ash [48,49], nano-silica [32,33], silica powder or silica flour [38,39], limestone powder [50][51][52][53][54], ground granulated blast furnace slag [43,[54][55][56][57], quartz powder [51,57,58], rice husk ash [59], and glass powder [60,61]. Moreover, the production process requires using a special type of sand such as quartz [28,56] or silica sands [35][36][37][38][39][40]43]. It is also important to note that the curing process is also special as indicated by steam [2,34,38,58], moist [49,56], or heat treatment [5,13,18,28,29,[35][36][37][38][39]55,[35][36][37][38][39][40]62] curing. ...
... Moreover, the production process requires using a special type of sand such as quartz [28,56] or silica sands [35][36][37][38][39][40]43]. It is also important to note that the curing process is also special as indicated by steam [2,34,38,58], moist [49,56], or heat treatment [5,13,18,28,29,[35][36][37][38][39]55,[35][36][37][38][39][40]62] curing. This means that it has a very high production cost and its application is limited [63,64]. ...
Calcined diatomaceous earth (CDE) with a maximum grain size of 143 μm was used to partially replace 5 and 10% of cement in ultra-high-performance concrete (UHPC) mixtures. The other materials used in producing the concrete include Ordinary Portland Cement, iron ore powder, and river sand with maximum grain sizes 112.5, 231, and 766.2 μm, respectively. Moreover, the UHPC specimens designed with a water–cement ratio of 0.2 and a superplasticizer of 1.5% from the cement weight were tested for flow, compressive strength, flexural strength, splitting tensile strength, durability against NaCl and Na2SO4 attack, and resistance to 400, 500, and 600°C temperatures. The results showed that the use of 5 and 10% CDE to replace cement was able to increase the compressive strength, flexural strength, splitting tensile strength, the durability of UHPC against NaCl, and Na2SO4, as well as its resistance to high temperatures but reduced the mixture flow.
... Previous studies have reported that the uncracked dense matrix with high alkalinity can protect the steel fibers from erosive environmental exposure [4,5], showing a high corrosion resistance in UHPFRC [6,7]. However, cracks generated by damage of thermal and/or mechanical loading exist in almost all reinforced concrete during the service life [8][9][10][11][12], as well as in UHPFRC [13][14][15][16][17][18]. Such micro-cracks of UHPFRC are inevitable to produce as a consequence of for instance the cases under the pressure of thermal stress caused by cement hydration, especially in large volumes, and loading during service life that impacts the durability over time. ...
... Once cracked, fiber corrosion would certainly occur and trigger the surrounding matrix deterioration through the existing open pore, causing deterioration in porosity and strength. Pre-cracked UHPFRC has often been performed to investigate the self-healing process [9,19], other than focusing on the corrosion risk and the deterioration of the damaged UHPFRC. A well-known fact is that cracks and reinforcement-matrix interface are the significant factors affecting reinforcement corrosion in concrete. ...
... Numerous studies have been conducted to quantify the corrosion resistance of steel fibre reinforced concretes (SFRC) [e.g., [8][9][10]]. To assist in the design of the experimental work contained in this paper, and to provide guidance for future studies, a statistical overview of 342 individual test results obtained from 20 studies [8,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] is contained in the associated Data-in-Brief article. This overview shows that tests to quantify the impact of corrosion have considered a relatively narrow range of compressive strengths (typically either 60-70 MPa or greater than 150 MPa) and with up to 2 % fibre content. ...
... A challenge with this work is that the method of corrosion (pre-corroding) yields and upper bound estimate of the impact of material properties because it does not allow for any protection that is provided by the concrete of the fibres and therefore any influence of the spatial variation of fibre corrosion. A further challenge with the work of both Tran et al. [10], Yoo et al. [16,17], and Yoo et al. [19] is that the results have been obtained from specimens that have thicknesses Fig. 1. Test specimen geometry and location for crack-width measurement with microscope. ...
When UHPFRC is exposed to a corrosive environment, steel fibres may undergo corrosion which may affect the interfacial bond and therefore the overall tensile behaviour of UHPFRC. Previous studies have predominately investigated the influence of fibre corrosion on the tensile behaviour of UHPFRC using the flexural test method. In this approach, the significant variation in crack width may lead to variation in the level of corrosion, making it more difficult to develop material models. In this study, the influence of corrosion on the tensile resistance of UHPFRC has been studied using direct tensile test of thirty-nine grooved prisms with a range of different pre-cracking conditions and corrosion levels. The results show that even when crack widths are large, fibre corrosion has a relatively minor impact on the tensile performance of UHPFRC, and the expected level of deterioration is reduced due to self-healing of cracks and continued hydration of the cement when immersed in the chloride solution. When uncracked specimens are subjected to corrosive environments it was shown that the tensile strength was significantly increased due to enhanced hydration of the binder and associated pozzolanic reaction with silica fume. Despite the degree of experimental scatter and the competing mechanisms of corrosion and self-healing, the residual response of the corroded specimens has been found similar to that of the control specimens.
... On the one hand, the fibers inside the sound UHPFRC are challenging to be corroded except for near the exposed surface (0-2 mm), as shown in Fig. 13, but a high steel fiber amount (more than 2%) may lead to an increased risk of electrochemical corrosion [93]. On the other hand, for cracked UHPFRC, the fibers around the cracks are corroded, and the corrosion phenomenon gradually spreads to the fibers in the embedded region [94]. A wider crack will lead to corrosion over a wider area, and worse damages inevitably occur during service life, eventually resulting in the continuous weakening of corrosion resistance and reduction of performance. ...
... This phenomenon is consistent with chloride ion penetration in literature [88]. Yoo and Shin [94] also pinpointed that crack repair had a strong inhibitory effect on the steel fiber corrosion in the pre-cracked UHPFRC. ...
Ultra-high performance fiber reinforced concrete (UHPFRC) is well known for its superior workability, strength, ductility as well as durability, but its intrinsic self-healing ability is rarely valued and developed. This review focuses on the inherent potential or superiority, characterization, and mechanism of autogenous healing UHPFRC, aiming to obtain fundamental data for its mixture innovation, design, and application. High potentialities of autogenous self-healing UHPFRC depend on its excellent component requirements (fiber; abundant binding particles), mix design (high cementitious materials content, low water-binder ratio, moderate fiber content), rehydration capacity, and shrinkage or loading-initiated cracking features. Meantime, the generation of cracks makes the internal substances include active ingredients exposed to the external environment such as air, water, and temperature, which induces physical, chemical, and mechanical interaction between them at cracks. Intrinsic partial or entire sealing of the multiple cracks in UHPFRC has been proven to improve the safety and durability of UHPFRC infrastructures. A higher healing rate exists in cracks with a width of 75–175 μm, which is connected with crack healing kinetics, and the width of total healing cracks can reach up to 162 μm, which is mainly filled with calcium carbonate. Continuous accumulation of healing products at cracks can effectively improve the mechanical properties and suppress the decay of transport performance and steel fiber corrosion. Furthermore, mild fiber corrosion contributes to the partial restoration of flexural strength during the self-healing process.
... As the environment in which the project is located becomes increasingly complex and harsh, the corrosive effect of various erosive media on high-performance concrete is becoming more and more serious, and under the action of various destructive factors, the stability and durability of concrete components gradually attenuate and deteriorate, and the working service life decreases dramatically [1][2][3][4], which not only brings huge cost pressure to the later maintenance, but also the potential damage is not handled in time and is very likely to have bridge collapse and other major safety accidents, which bring extremely negative impacts to the society [5][6][7]. In order to resist the erosion of environmental factors such as freezing and thawing, carbonation and chemical media and reduce the cost of post-care, the durability of high-performance concrete has put forward higher requirements [8][9][10]. High-performance concrete with excellent durability has been increasingly emphasized and has become an inevitable trend in the development of high-performance concrete. ...
The extreme high temperature and erosive environment service environment in bridge construction puts forward higher requirements for high performance concrete and other aspects of performance. In this paper, compound mineral admixture is selected as a research breakthrough, and X-ray diffraction analysis (XRD) and Raman spectroscopy are used to explore the micromechanical behavior of compound mineral admixture in high-performance concrete. In the Raman spectral analysis, the stress distribution of the fitted curve of the compound mineral admixture is more flat and uniform, and the offset of the G’ peak position is higher than that of the reference concrete and the single-mineral-admixture concrete, and the stress can reach 2.5 MPa under 1% strain, showing good interfacial bond, stress transfer efficiency, etc. The physical phase data of the XRD also shows the frost resistance of compound mineral admixture, with the ability to mitigate carbon dioxide, and the ability to reduce the carbon footprint of the concrete, with the ability to reduce the carbon dioxide. The XRD data also show the frost resistance of the compound mineral admixture, which has the performance of slowing down carbonization. The NSGA-II algorithm is introduced and improved to propose a concrete proportion optimization model. The final evaluation function converges from 35 generations and the final value is 0.4558, which achieves the adaptive optimization of compound mineral admixture.
... However, the incorporation of steel fibers in UHPC mix designs may result in surface corrosion, particularly in harsh environments. As an alternative, synthetic fibers have been investigated, encouraging researchers to study their impact on the mechanical properties of UHPC [21][22][23][24][25][26][27][28]. ...
Ultra-high-performance concrete (UHPC) is considered a highly applicable composite material due to its exceptional mechanical properties, such as high compressive strength and ductility. UHPC deep beams are structural elements suitable for short spans, transfer girders, pile caps, offshore platforms, and bridge applications where they are designed to carry heavy loads. Several key factors significantly influence the shear behavior of UHPC deep beams, including the compressive strength of UHPC, the vertical web reinforcement (ρsv), horizontal web reinforcement (ρsh), and longitudinal reinforcement (ρs), as well as the shear span-to-depth ratio (λ), fiber type, fiber content (FC), and geometrical dimensions. In this paper, a comprehensive literature review was conducted to evaluate factors influencing the shear behavior of UHPC deep beams, with the aim of identifying research gaps and enhancing understanding of these influences. The findings from the literature were systematically classified and analyzed to clarify the impact and trends associated with each factor. The analyzed data highlight the effect of each factor on the shear behavior of UHPC deep beams, along with the overall trends. The findings indicate that an increase in compressive strength, FC, ρsv, ρs, and ρsh can enhance the shear capacity of UHPC-DBs by up to 63.36%, 63.24%, 38.14%, 19.02%, and 38.14%, respectively. Additionally, a reduction of 61.29% in λ resulted in a maximum increase of 49.29% in the shear capacity of UHPC-DBs.
... Figure 1c-e depicts a large amount of reddish-brown corrosion products that appeared on the surface of the SFs, even affecting the concrete around the SFs, indicating severe corrosion of the SFs, where Fig. 1e shows the magnified detail of the portion of the concrete surface with the presence of SFs after 36 wet and dry cycles using a digital microscope, which allows for a clearer view of the adverse effects caused by the corrosion of the SFs. Thus, the corrosion of SFs represents a significant factor influencing the durability of concrete [29,30]. Due to the aforementioned defects, the application of SFs in practical engineering has been greatly limited, indicating that SFs are not the optimal choice for use in concrete. ...
Steel fibers (SFs) are commonly used to enhance the performance of concrete. However, they possess limitations such as susceptibility to corrosion, agglomeration during construction and high cost. To overcome these shortcomings, alternative fiber options have been explored. This study investigates and compares the mechanical properties of ultra-high-performance cementitious composites (UHPCC) reinforced with SFs and imitation steel fibers (ISFs). Various tests, including uniaxial compression, quasi-static splitting and dynamic splitting, are conducted to examine the effects of fiber aspect ratio (Lf /df = 20, 30, 40), fiber content (Vf = 0.5%, 1%, 2%) and fiber type (steel fiber and imitation steel fiber) on the force and deformation properties of UHPCC. The test results are analyzed to discern the patterns of tensile strength and energy absorption variations in static and dynamic tensile tests concerning fiber type, aspect ratio and content. Furthermore, the damage process of the specimens is captured using a high-speed camera, enabling the investigation of damage patterns in UHPCC reinforced with different fibers under strain rates of 34–56 s⁻¹ and 109–125 s⁻¹ in dynamic splitting tests. Additionally, fragments generated during impact loading are meticulously collected, sieved and weighed to assess the extent of specimen fragmentation under different splitting load conditions.
... The potential impact of this surface corrosion on the performance of UHPC is a concern addressed by many researchers. [14][15][16][17][18][19]. Lv, Wang, Xiao, Fang and Tan [14] conducted electrochemical accelerated corrosion tests on UHPC and proposed that corroded steel fibers tend to form interlaced rust expansion cracks, leading to delamination of UHPC. ...
... Numerous researchers have investigated the corrosion of steel in cracked concrete [6][7][8][9][10]. Yoo et al. [11] reported that small cracks below 0.15 mm had a minor influence on the corrosion of steel after 4 weeks of corrosion. Kim et al. [7] stated that the half-cell potential increased with increasing crack width (0.5 mm-1.5 mm) after a 35 d cyclic salt spraying test. ...
... The experimental results of these findings match previous studies exploring the effects of fiber reinforcement and high temperatures on concrete properties. Doo et al. [72] and Doo and Shin [73] found that fibers enhance splitting tensile strength and crack resistance, consistent with current findings. Similarly, Chen et al. [45] showed that high temperatures decrease splitting tensile strength more in concrete without fiber reinforcement, in line with the experimental results. ...
This paper investigates the effect of high temperatures on the compressive strength, flexural strength, and splitting tensile strength of ultra-high-performance concrete (UHPC), and ultra-high-performance, fiber-reinforced concrete (UHPFRC). The experimental variables in this study were fiber type, fiber content, and high-temperature exposure levels. Three different types of fibers were evaluated, including steel fibers, polypropylene (PP), and polyvinyl alcohol (PVA) fibers. Six concrete mixes were prepared with and without different combinations of fibers. One mix was made with no fibers. Others were made with either steel fibers alone; a hybrid of steel fibers and PVA; and a hybrid system of steel, PP, and PVA fibers. These mixes were tested under a range of temperatures and compared for strength. The UHPC and UHPFRC were exposed to high temperatures at 100°C, 300°C, 400°C, and 500°C for 3 hours. The results showed that UHPFRC did not exhibit any significant degradation when exposed to 100°C. However, reductions of approximately 18% to 25%, 12% to 22%, and 14% to 25% in the compressive strength, splitting tensile strength, and flexural strength were observed when the UHPFRC was exposed to 400°C. UHPFRC made of steel fibers showed higher mechanical properties after exposure to 400°C compared to UHPFRC made of PP and PVA fibers. The results also demonstrate the use of PVA and/or PP fibers, along with steel fiber, to withstand the effects of highly elevated temperature and prevent spalling of UHPC after exposure to elevated temperature. The observed spalling was a direct result of the melting and evaporation of PVA and/or PP fibers when exposed to high temperature, an effect that was confirmed using scanning electron microscopy.
... So, mechanical degradation in UHPFRC is not fully characterized from microscopic perspective. In reality, stress area of steel fiber is damaged when concrete structure is subjected to corrosive environment [27,28]. Besides, with aggravation of corrosion, hardness of matrix decreases sharply, resulting in the decrease of bonding property between fiber and local matrix [29,30]. ...
Chloride-induced corrosion of ultra-high-performance fiber-reinforced concrete (UHPFRC) inevitably affects structural durability. However, the process of multi-fiber corrosion and mechanical deterioration still lacks sufficient understanding. This work aims to reveal the fiber corrosion degradation mechanism from a microscopic to macroscopic view, applying multiple analytical analyses of atomic absorption spectrometry, SEM-EDS, nano-indentation, polarization, and macroscopic mechanical testing. Results show that the flexural strength of specimens decreases significantly with the increase of corrosion degree, and a clear reduction of up to 47% is found at a high corrosion degree. Elastic modulus and nano-hardness of corroded samples vary in a wide range of 30-189 GPa and 0.16-6.41 GPa. With the increase in fiber content, two distinctive corrosion mechanisms are proposed. The corrosion path deteriorates from fiber edge to inner by the invasion of erosive solution through the matrix at low contents (<2 vol%). Considering impurities, greater interfacial defects and macro-cell potential differences at high contents (≥2 vol%), another corrosion path originates from the fiber inner outward to the matrix. Fiber corrosion damages the fiber's structural integrity and induces matrix deterioration, the micro-mechanics of the matrix along the fiber edge 20 μm decreases at least 10% more than the concrete matrix. This work firstly sheds light on the mechanical deterioration of UHPFRC from the perspective of fiber corrosion paths considering different initiation scenarios.
... 9,10 Incorporating appropriate polyethylene fibers 11 and epoxy-type materials can effectively reduce the crack width and improve the tensile strain capacity. 12 The self-healing effect of cracks 13,14 in UHPC is considered as a novel technique for controlling crack width. In addition to experimental investigations, some analytical models have been developed to predict crack propagation. ...
Studying the crack propagation of ultrahigh-performance concrete (UHPC) helps us understand its mechanical mechanism and assess its structural performance. A novel method for crack separation and its characteristic evaluation was developed in this work. The proposed method introduces robust principal component analysis (RPCA) to decompose a data matrix from video streams stacked into a low-rank matrix and a sparse matrix, in which the sparse matrix represents the crack information. Compared with the cracks in a binary image, the obtained sparse matrix preserves rich crack information that can be used to quantify crack characteristics. The statistical characteristics of the crack area, the major and minor axes of the equivalent ellipse for crack regions, and the power spectral density are investigated and compared continuously. The proposed method is demonstrated by the crack development of UHPC under tensile loading. The analysis results indicate that RPCA can accurately separate cracks from the background. In the frequency domain by performing the Fourier transform of the sparse matrix, cracks are concentrated at small wavenumbers and the magnitude of small wavenumbers increases with an increase in the crack width. The relationship between the crack propagation and the stress–strain is also discussed. This work provides insight into the crack propagation of UHPC and an accumulated crack database for predicting the damage evolution of UHPC.
... Accordingly, steel fibers delay corrosion initiation and decrease the corrosion rate of bars [28][29][30]. There is also evidence that steel fibers are effective in controlling and reducing crack opening in the corrosion propagation period [31][32][33]. With regard to the bond performance, Berrocal et al. [32] demonstrated the effective role of steel fibers in maintaining the bond strength between the corroded bars and concrete. ...
The bond deterioration behavior of reinforcements in steel fiber-reinforced concrete (SFRC) subjected to chloride-induced corrosion has not yet been fully elucidated. This study investigates the corrosion characterization, resistivity, corrosion-induced cracking, and associated effects on the bond performance of reinforcing bars in SFRC with milled-cut steel fibers (M fibers), referred to as MCSFRC. Pull-out tests were performed following galvanostatic corrosion, with the M fiber, concrete cover, and corrosion level being the main variables. A closer look at the bar corrosion was conducted using 3D laser scanning technology, which allowed for estimating the spatial-domain corrosion pattern and bond index degradation that characterize the rib-concrete interaction. The results indicated that, with the increase in corrosion levels, the bond index decreased exponentially as a result of pits propagating from the rib root to the rib peak. The bond strength of the MCSFRC specimens deteriorated less than that of the control counterparts and hook-end fiber-reinforced specimens in the literature. It is further revealed that the crack opening and bond index reduction are the bond deterioration mechanisms under corrosion, reducing the contact area between the bar and concrete through the ribs. On that basis, a mechanics-based model for bond strength deterioration was developed. The proposed model considered the two aforementioned effects, as well as the fiber reinforcement effect on crack suppression, showing good agreement with the experimental results.
... To reduce cement consumption, many studies, which proposed the replacement of some amount of cement with pozzolanic materials as a binder, have been conducted. Some of these materials include industrial byproducts such as fly ash, ground granulated blast-furnace slag, and silica fume [30][31][32][33][34][35][36]. Moreover, some waste or recycled materials such as palm oil fuel ash, rice husk ash, waste ceramics, and glass powder have been used [37][38][39][40][41]. ...
In this study, the effects of calcined diatomaceous earth (CDE), polypropylene fiber (PF), and glass fiber (GF) on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) were observed, and a total of 33 UHPFRC mixtures, consisting of 3 mixtures without fiber, 15 mixtures with PF, and 15 mixtures with GF were prepared. Subsequently, the fresh concrete mixtures were tested for flow, while the hardened concrete specimen’s mechanical properties were analyzed. These tests include compression, splitting tensile, and flexural tests. The test results showed that the use of 5 and 10% CDE as a binder for cement replacement improved the compressive strength, splitting tensile strength, and flexural strength of the UHPFRC. Furthermore, the addition of PF and GF contents of up to 1% of the concrete volume increased the compressive strength of the UHPFRC, while their contents of up to 1.5% improved their splitting tensile strength and flexural strength. It is also important to note that the workability of the UHPFRC reduced as the fiber and CDE contents increased. Finally, based on the experimental data tested in this study, the relationship between splitting tensile strength, flexural strength, and compressive strength of the UHPFRC containing PF and GF were proposed. Moreover, the reduction in flow value, which is a function of the volumetric content of both PF and GF, with the CDE contents was also proposed.
... The self-healing phenomenon itself is inherent in cementitious materials, but it is particularly important and obvious in UHPC. Furthermore, due to its lower water-cement ratio and more unhydrated cementitious materials, UHPC has a higher potential self-healing capacity than regular concrete [33,34], yet there is some debate over whether re-curing is beneficial or detrimental to the recovery of damaged concrete properties [35,36]. Li [37] found that the effect of post-fire curing was only manifested when UHPC was exposed to 600 • C, with no significant difference before 600 • C. Akca et al. [38,39] found that the strength of air re-cured concrete after elevated temperature continued to decrease, while that of water re-cured concrete increased. ...
Ultra-high performance concrete containing coarse aggregates (UHPC-CA) is more cost-effective, with its low cost and easy access to materials, but its high-temperature performance and self-healing capability after post-fire curing remain to be further explored. In this paper, after high temperature, the compressive strength increases by approximately 20 MPa within 400 • C, and the micro-characteristics reveal that this is mainly because the temperature promotes the cement hydration reaction and pozzolanic reaction, which plays the role of secondary accelerated curing. Due to the continuous decomposition of substances, the flexural strength and ultrasonic pulse velocity gradually decrease; the mass loss, total porosity, and the amount of absorbed water increase, especially the sorptivity coefficient, which increases by more than 100 times after 800 • C. The residual values of all properties of UHPC-CA are best maintained by adding all of the finer PP fibers. Relying on many unhydrated gelling materials, UHPC has a more significant self-healing potential, improving its mechanical properties and permeability in all three post-fire curing environments. The most significant effect is seen in the water environment , where its recovery rate of strength exceeds 50 % in all cases. The self-healing enhancement of UHPC does not come from a single source, but from multiple combinations of physical and chemical reaction processes.
... At present, there has been the problem of insufficient durability of reinforced concrete structures in the application of marine engineering. Ultra-high performance concrete (UHPC) has aroused rising attention in civil engineering for its high strength, good durability, and good fatigue resistance [1][2][3][4][5]. Glass fiber reinforced plastic reinforcement (GFRP) has been progressively employed in marine engineering for its light * Authors to whom any correspondence should be addressed. ...
Glass fiber reinforced plastic reinforcement (GFRP) and ultra-high performance concrete (UHPC) were combined into a new composite beam, which was applied in ocean engineering to improve the durability of structures. To enhance the stiffness and durability of composite beam and lower the cost of structure, prefabricated construction technology was adopted to reserve holes for pouring UHPC. Through the quasi-static test of prefabricated GFRP-UHPC composite beams, the interface between concrete and GFRP was monitored using piezoelectric smart aggregate. The damage index was obtained in accordance with wavelet packet energy analysis theory to examine the interface damage of prefabricated composite beams. Experimental results show that active monitoring of assembled GFRP-UHPC composite beams with piezoelectric smart aggregate can effectively reflect the degree of interface peeling damage of composite beams. The monitoring results reveal that interface damage of specimens with reserved continuous holes is less than that of specimens with reserved discontinuous holes. Moreover, peeling damage will occur not only between GFRP and UHPC, but also at the interface between concrete and UHPC.
... Nevertheless the part of the fibers that had been directly exposed to the wet and dry cycles presented corrosion as can be seen at Fig. 10 sample B26, for example. However this phenomenon did not interfere on the self-healing evaluation since there was no corrosion on the fibers inside of bulk concrete and the crack openings are relatively small resulting on insignificant on the tensile behavior as reported by others researchers [37,38]. Products of corrosion were not observed in the crack filling material and only fiber pullout was observed in the specimens submitted to tensile test after 3 months of wet and dry cycles. ...
This paper presents the results of a research on the influence of sisal fibers on the self-healing capacity of slag-cement UHPFRC reinforced with steel fibers. In order to evaluate the composite healing capacity, specimens were submitted to pre-cracking by tensile test and then to a 3 months of wetting and drying cycles treatment. Treated specimens were resubmitted to tensile test and evaluated by Optical and Electronic microscopy and CT Scan. Results indicate that the sisal fiber works as a healing conveyor improving by about 20% and 15% the tensile stress and post cracking energy of treated specimens, respectively. Sisal fibers played a major role densifying the interface, working as a vehicle for the healing agents into small interface cracks. All specimens with cracks under 80 μm could completely self-seal mainly by calcium carbonate. Wider cracks could not be completely sealed, though the specimens exhibited a recovery of the mechanical behavior essentially due to late slag-cement hydration.
... As a result, steel fiber reinforced UHPC has excellent energy absorbing ability and toughness [2]. However, steel fibers still have some shortcomings, such as high density, high price and low resistance for corrosion [26][27][28][29]. Although most of the steel fibers currently used are coated with copper or brass, the corrosion of copper or brass coated steel fibers on UHPC surface has been observed [30]. ...
This paper presents a comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC). Firstly, based on the modified Andreasen & Andersen packing model, a UHPC with ultra-low cement content is produced. Then, the steel fibers and polyoxymethylene (POM) fibers are added into this UHPC matrix separately and together. The obtained experimental results show that the addition of POM fiber has limited contribution to the compressive strength of UHPC, while it has a positive influence on its flexural strength and high temperature resistance. The results of nanoindentation indicate that POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Based on the nanoindentation results, it can be found that the interfacial transition zone (ITZ) is about 50 μm in the case of UHPC with only steel fibers, while this value gradually increase to about 60 μm when POM fiber is added individually. This results further demonstrate that the POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Therefore, to efficiently apply the POM fiber in UHPC, it is suggested to combine it with steel fibers and its optimal content should be around 1% (vol.). It can be concluded that it is difficult for POM fiber to fully replace steel fiber in referencing UHPC, while the hybrid fibers should be a good choice to produce excellent UHPC composite.
... Steel fibers are reported to help increase the crack resistance and thus the corrosion resistance of UHPC by bridging and restricting cracks in the matrix [10,14]. Literature shows that the corroded steel fibers in uncracked UHPC specimens will not adversely affect the tensile performance, which is also the case for cracked specimens with crack width smaller than 45 µm [15][16]. Corroded steel fibers barely influence flexural strength and toughness of uncracked specimens [17], and the moderately corroded fibers can increase the flexural strength of specimens with pre-crack width of 0.3 mm [18]. ...
Intact ultra-high-performance concrete (UHPC) can properly protect embedded steel rebar from corrosion. However, cracks initiate and propagate in UHPC beams under loading, which will facilitate the corrosion of steel rebar. In this study, UHPC beams made with different steel fiber contents (0.5 vol% and 2.0 vol%) were pre-cracked to generate different primary crack widths (0.1, 0.2 and 0.4 mm) on the tensile surfaces. The electrochemical behaviors and corrosion states of steel rebar embedded in these cracked beams were investigated in 3.5 wt% NaCl solution for 174 days. The results indicated that the corrosion rate of steel rebar in UHPC with cracks was reduced with time, and corrosion was not amplified and was restrained within the initially generated corrosion spots. When the primary crack width in UHPC beams was limited to 0.1 mm, steel rebar likely remained in the uncorroded state. Higher content of steel fibers (2%) could provide better bridging effects to reduce the corrosion rate without introducing galvanic corrosion on steel rebar. Besides, the Stern-Geary coefficients B of steel rebar embedded in UHPC beams with cracked matrix are determined to be 13.5 and 14.1 mV for active and passive corrosion, respectively. Also, this paper found the UHPC has low efficiency in autogenous self-healing.
... After 180 days, the crack opening width for a nonloaded specimen was 3.43 mm, where the tensile stress started to drop sharply from a peak of 9 MPa. A similar phenomenon for UHPFRC was also reported by another research work [39,40,42,53], where the sharp drop of tensile stress occurred from an average peak of 8 MPa to 11 MPa. It was observed that the crack opening width reduced with the increase of tensile strength. ...
The research presented in this paper is associated with the time-dependent tensile behavior of UHPFRC, especially on tensile creep for the long-term. This study outlines the experimental program for the implication of the newly developed testing rig, installation and preparation of test specimens, and studies of the research findings. The time-dependent tensile behavior was evaluated in terms of tensile creep under different sustained tensile loads. The sustained tensile loads were harnessed following the different percentages (50% and 75%) of cracking loads. The cracking loads were determined from the instantaneous tensile tests of unreinforced UHPFRC prism specimens at 28-day. A newly designed test rig was introduced to perform the tensile creep test for the long-term, and the test rig was designed in a way to overcome limitations found throughout the critical literature review. This study demonstrates that the tensile creep of UHPFRC is significantly influenced by the shrinkage strain. The experimental results demonstrate that under higher sustained tensile stress directs to a higher tensile creep rate for the first 13 days. Afterward, the total tensile creep strain was dominated by the shrinkage strain.
In order to gain the stress–strain relationship of ultra-high performance concrete (UHPC) reinforced with basalt fiber reinforced polymer (BFRP) minibars under uniaxial compression, nine kinds of UHPC mixtures with BFRP minibars were designed. In addition, other three kinds of UHPC mixtures were designed to study the effect of fiber modulus on uniaxial compression behavior, the UHPC which were reinforced with no fiber, polypropylene (PP) fibers and steel fibers, respectively. The flowability, porosity, elastic modulus, compressive strength and toughness of UHPC were systematically tested and analyzed. Based on the test results, BFRP minibar significantly enhance the uniaxial compression behavior of UHPC, the stress–strain relationship was gained and it was expressed by a classical model with a good agreement, resulting in a promotion of the structural design for UHPC structures. The compressive strength increased with fibers modulus, content, aspect ratio, and bonding strength between UHPC and fibers. A semi-empirical model with a relative error rate less than 15% was proposed for compressive strength prediction, the model which was based on fibers modulus, content, aspect ratio, and bonding strength. The findings based on this research could enrich the technical basis for the performance design of UHPC in practical applications.
Ultra-high performance concrete (UHPC), characterized by its high strength and toughness as well as durability, provides a promising solution for the construction of offshore wind towers (OWTs). This paper comprehensively reviews the durability and the dynamic mechanical properties of UHPC for OWTs under the impacts of the marine environment. Furthermore, the modifying effects of additives, including supplementary cementitious materials (SCMs) and reinforcing fibers, as well as nanofillers on UHPC are explored. Overall, UHPC possesses a dense microstructure that impedes the intrusion of harmful substances, and owing to the incorporation of additives, UHPC exhibits outstanding dynamic mechanical properties, making it an ideal material for applications in OWTs subjected to vibration fatigue and dynamic impact loads. Incorporating SCMs into UHPC can improve the durability and environmental benefits while maintaining similar dynamic mechanical properties concurrently. Nanofillers can serve as a beneficial supplement to steel fibers providing improved durability and dynamic mechanical properties by endowing UHPC dense microstructure and high system energy. Various models of marine environmental and loading actions on UHPC, examining ion transport, matrix degradation, and constitutive models, are concluded to gain insight into the underlying destructive mechanisms. These underlying mechanisms and the theoretical models further deepen the understanding of the service performance of UHPC in marine environments, thus providing the design guidance for the potential applications of UHPC in OWTs.
Enhancing the interfacial deformability of UHPC positively impacts its toughness and durability. In this work, a novel interfacial toughening strategy was proposed and employed for UHPC, in which the aggregates were treated with polyacrylate emulsion (PL) or PL modified by silica fume or carbon nanotubes to form an interfacial flexible layer (FL). The flexural characteristics of the prepared UHPC were comprehensively investigated, with attention to the damage evolution based on acoustic emission. Meanwhile, the corresponding toughening mechanism was discussed. The results showed that the FL modified by carbon nanotubes effectively enhanced the flexural deformation capacity, energy absorption capacity, and toughness of UHPC, while maintaining flexural strength. Introducing FL reduced ringing count and acoustic emission energy and mitigated damage rate of UHPC. The FL altered the flexural damage mode of UHPC by alleviating stress concentration to prevent sudden matrix cracking and fiber debonding. During the elastic stage, FL and the UHPC matrix jointly sustained tensile cracks, enhancing the matrix's energy absorption capacity, which correlated positively with the percentage of tensile cracks. In the softening stage, this capacity correlated positively with the percentage of shear cracks. Moreover, FL reduced the probability of microcracks at the interface. Although the FL reduced the average microhardness at the interface, it stabilized the performance of hydration products and increased their maximum microhardness. The FL promoted interfacial energy dissipation and synergistically bridged microcracks with steel fibers, ultimately enhancing the flexural toughness of UHPC.
This work numerically explores the anisotropy, impact phase wave propagation, buckling resistance, and natural vibration of ultra-high performance concrete (UHPC) and UHPC-steel interpenetrating phase composite (IPC) with triply periodic minimal surfaces (TPMSs), including sheet and solid Gyroid, Primitive, Diamond, and I-WP. The experiment is conducted verifying the accuracy of the numerical model in terms of Young's modulus of polylactic acid (PLA)-based TPMS lattices and PLA-cement IPCs with TPMS cores, with the highest percent difference of 15% found for IPCs and 17% found for lattice. The results indicate that UHPC material with sheet Gyroid exhibits the least extreme anisotropy in response to the varying orientation among other lattices regardless of the change of solid density, making it the ideal candidate for construction materials. Interestingly, compared to UHPC-based TPMS lattice, IPCs possess a much smaller anisotropy and exhibit almost isotropy regardless the variation of solid density and TPMS topology, offering a free selection of TPMS type to fabricate IPCs without much care of anisotropy. The phase wave and buckling resistance of UHPC- and IPC-based beams with TPMSs nonlinearly decrease with a drop of TPMS solid density, but it is the almost linear pattern for the case of natural vibration frequency. UHPC material and IPC with sheet Gyroid lattice are found to possess the lowest phase wave velocity and exhibit the least anisotropy of wave propagation, showing it as an ideal candidate for UHPC material to suppress the destructive energy induced by the external impact.
For marine reinforced concrete structures exposed to saline water, the rate of penetration of chloride ions into concrete is crucial to their performance. To resist the chloride penetration within marine structures, the incorporation of basalt fibers into M40-grade concrete was pursued. The current research develops into two primary investigations. The first pertains to the determination of chloride penetration depths and the associated diffusion coefficients of concrete cubes, while the second focuses on the structural behaviour of basalt fiber-reinforced concrete beams under chloride diffusion. The chloride diffusion coefficients were determined using experimental methods for eight concrete cubes with varying proportions of basalt fiber (0%, 0.5%, 0.75%, and 1% v/v). These coefficients were subsequently validated through numerical methods, applying Fick’s law. Additionally, numerical techniques were employed to calculate chloride concentration and the corresponding flux at various diffusivity time intervals, leading to the development of corresponding chloride concentration equations. A corelation was developed in between chloride penetration depth, time period and dose of basalt fiber. The present model can be used as a tool for analysis to represent the real condition of concrete deterioration brought on by chloride action. In terms of examining structural responses, a comprehensive evaluation was conducted encompassing load deflection behaviors, crack propagation tendencies, and stress-strain analyses of concrete beams reinforced with basalt fibers. The stress block diagram had been modified by determining various stress block parameters (alpha and beta) of chloride-diffused basalt fiber reinforced concrete beam under cyclic load.
This study summarizes the durability of ultra-high performance concrete (UHPC) under chloride ion erosion in marine and saline-alkali environments. The influence of chloride ion migration on the performance of UHPC is the focus of this study, with various factors affecting the microstructure of chloride ion migration in UHPC being summarized, including mineral admixtures, water binder ratio, fibres, curing temperature, and chloride exposure environment. The impact of these factors on the chloride ion diffusion coefficient and mechanical properties of UHPC is also discussed, and methods to improve the chloride ion permeability are proposed. Furthermore, an overview of research progress on chloride ion migration in UHPC components is provided, and the corrosion behaviour of reinforcement in UHPC members is discussed. Through the review, it was revealed that corrosion of reinforcement or fibres in UHPC is primarily caused by chloride ions, thereby reducing the stability of the structure. Raw materials and external environments played a significant role in the chloride ion migration process of UHPC. Steel fibres could enhance UHPC's resistance to chloride ion penetration and help reinforcement resist corrosion. Finally, addressing the limitations and deficiencies in current research, several directions and approaches for further studying the chloride ion permeability of UHPC were proposed, including strengthening research on the coupling effects of multiple factors, deepening the understanding of the influence of steel fibres and mineral admixtures on chloride ion migration, establishing a unified chloride ion migration testing standard, and focusing on chloride ion migration in components and structures. These findings and suggestions hold important innovative significance for promoting the application of UHPC in special environments such as marine and saline-alkali areas.
Ultra-high-performance concrete (UHPC) has gained significant attention as a construction material owing to its exceptional mechanical properties and durability. Steel fibers are widely utilized as a reinforcement material for UHPC. Achieving excellent bond and tensile performances is considered to be a predominant issue for the utilization of steel fiber reinforcement. This comprehensive review presents recent research progress on the bond and tensile properties of steel-fiber-reinforced UHPC. First, an overview of the experimental methods for evaluating pullout and tensile performance is provided. Subsequently, the factors influencing these properties are discussed in detail. The review then comprehensively examines several analytical models for steel-fiber-reinforced UHPC, ranging from traditional approaches to innovative methods such as artificial neural network models, genetic algorithms, deep learning methods, inverse analysis, and micromechanical damage models. Furthermore, the correlations between pullout behavior, tensile performance, and flexural strength are explored in detail. Finally, the review addresses essential considerations and summarizes various modification techniques for improving the pullout and tensile performances, including physical and chemical methods of modifying the steel fiber surface and UHPC matrix. This review serves as a valuable reference for researchers and engineers in relevant fields, promoting further research and application of steel fiber-reinforced UHPC.
Ultra High-Performance Concrete (UHPC) has superior mechanical properties, including high compressive strength, tensile strain hardening behavior, and self-healing capacity. However, there has been limited focus on developing predictive models for UHPC's self-healing properties, despite extensive research in the aforesaid respect. While multi-physics modeling has made progress in predicting the coupled chemical, physical, and mechanical phenomena in cement-based materials, data-driven models, including Artificial Intelligence (AI) and Machine Learning (ML), are gaining popularity in predicting some concrete properties. In this study, a machine learning model was developed to predict UHPC's self-healing performance using three meta-heuristic algorithms, i.e., whales optimization algorithm (WOA), grey wolf optimization (GWO), and flower pollination algorithm (FPA), combined with extreme gradient boosting tree (Xgboost). The dataset used for the model was obtained from original experimental tests on UHPC's crack sealing performance under sustained through crack tensile stress and exposure to various aggressive environments for up to six months. The model's predictive performance was assessed using four mathematical indicators. The regression error characteristic (REC) and Taylor diagrams also showed the optimal models’ performance were found to be consistent and reliable across different optimization algorithms. SHapley Additive exPlanation (SHAP) results revealed that exposure time and crack width were most critical features for predicting self-healing performance. The study demonstrated the potential of using machine learning for predicting UHPC's self-healing performance and provided insights into the most critical factors affecting the process.
Ultra-High-Performance Concrete (UHPC) has been the subject of tremendous research over the years and is being used in a higher and higher number of engineering applications, especially in highly demanding structural service scenarios, thanks to both its superior mechanical performance and likewise significant durability in the un-cracked state. However, as a cement-based material, also UHPC in service condition almost unavoidably works in a cracked state, though its signature tensile behavior allows damage to be spread into multiple thinly opened and tightly spaced cracks instead of localizing it into a single wider crack. Cracking can lead to deterioration of performance, particularly when UHPC structures are exposed, as above, to harsh environments. The development of self-healing concrete technologies in recent years, has opened new possibilities to enhance the durability of UHPC in cracked state, empowering with additional capabilities its inborn autogenous self-healing capacity, which is due to the synergy between its peculiar mix-design composition (high binder content and low w/b ratios) and the signature crack pattern as above. It is therefore important to explore the development of self-healing properties of UHPC under different scenarios, not only as a mean to regain the impermeability lost upon cracking but also to retain the (tensile) mechanical performance over time, which also results into the capacity of slowing down the degradation of the overall structural performance. This paper reviews the existing self-healing technologies used in UHPC and summarizes the self-healing properties and their effect on mechanical properties of UHPC when exposed to different environments, also including salt water, geothermal water, wet/dry and freeze/thaw cycles and low temperatures.
This work aims at investigating the effect of multiple microcracks in Ultra-High Performance Reinforced Concrete (UHPFRC) occurring in service conditions on the apparent chloride diffusion which is critical to guarantee the design lifetime. First, series of preliminary static bending tests were carried out on three UHPFRC beams to characterize their flexural response as well as their microcrack distribution measured by Digital Image Correlation (DIC). Then, a special test set-up was designed to apply and maintain a sustained bending moment on the UHPFRC beams. Finally, some accelerated migration tests were carried out on the micro-cracked UHPFRC beams under sustained bending load. The results showed different chloride penetration rates and profiles in tested UHPFRC beams under different loading levels, and the influence of the microcrack’s width on the apparent chloride diffusion coefficient of the considered UHPFRC was clearly identified, which is a precious information for better predicting the service lifetime of a UHPFRC structure.
There are many laboratory researches for microbial self-healing concrete (MSC) but few engineering applications. Demonstration application is an essential link from laboratory to practical engineering. This paper describes a demonstration application case of MSC on the sidewall of an underground engineering, aiming to provide practical experience for the large-scale application of MSC. Firstly, microbial healing agent (MHA) with core-shell structure produced using automatic equipment, and laboratory tests were carried out to evaluate the effects of MHA on concrete performance. Then, the feeding method of MHA in concrete mixing station was studied. Finally, the MSC’s application effect were studied through embedded sensor and field monitoring. Results show that MHA has no negative effect on concrete properties and does not affect engineering use. Using an external cylinder to deliver MHA can ensure the feeding effect and safe production. Compared with ordinary sidewall, MHA can effectively heal cracks and reduce the number of cracks and the average length, width, and depth. This paper can provide a good demonstration for the popularization and application of MSC and a new technical scheme for the prevention and treatment of sidewall cracks in underground engineering.
This study investigates the effect of surface corrosion on the pullout behavior of straight steel fibers embedded in ultra-high-performance concrete (UHPC). To this aim, straight steel fibers, either with or without surface corrosion, were utilized, and various corrosion degrees from 2% to 15% by weight were considered. To evaluate the implication of rust layer on the pullout behavior of corroded steel fibers from UHPC, both washed and unwashed conditions were considered. The surface roughness of plain and corroded steel fibers was analyzed by means of scanning electron microscope and atomic force microscope (AFM) images. The test results indicated that surface corrosion is effective in enhancing the pullout resistance of straight steel fibers in UHPC when the fibers are completely pulled out from the matrix without breakage. The maximum average bond strength and pullout energy of moderately corroded fibers in UHPC were found to be 18.5 MPa and 715.7 N·mm, approximately 2.7 and 1.8 times higher than those of plain fibers in the same matrix at the aligned condition. The benefits of moderate surface corrosion on improving the pullout resistance were mitigated by inclining the fibers. A higher corrosion degree led to a better pullout resistance up to a certain value (2 or 5%); however, beyond such value, the resistance decreased significantly due to the rupture of fibers. A threshold value of 2% for the corrosion degree was thus suggested to achieve an excellent fiber bridging capability. The washed corroded fibers exhibited higher bond strength and pullout energy than the unwashed ones with the same degree of corrosion at aligned condition; however, the benefits of washing vanished when the fibers were inclined and ruptured prematurely. An obvious correlation between the bond strength and the surface roughness was observed from the AFM images.
While fiber-reinforced concrete has been proposed for corrosion damage control of reinforced concrete, long-term behavior of these composites remains largely uninvestigated. In this study, reinforced concrete and reinforced hybrid fiber-reinforced concrete (HyFRC) were subjected to a chloride environment for 2.2 years. Samples were mechanically loaded prior to chloride exposure to induce varied matrix cracking characteristics. When precracked, the time to corrosion initiation is correlated to flexural stiffness degradation. After corrosion initiation, fiber reinforcement restricts corrosion-induced cracking, causes more extensive diffusion of corrosion products into the cementitious matrix, and lowers overall mass loss of steel reinforcing bars. The mechanical responses of corroded samples are also reported.
An overall review of the structural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) elements subjected to various loading conditions needs to be conducted to prevent duplicate research and to promote its practical applications. Thus, in this study, the behavior of various UHPFRC structures under different loading conditions, such as flexure, shear, torsion, and high-rate loads (impacts and blasts), were synthetically reviewed. In addition, the bond performance between UHPFRC and reinforcements, which is fundamental information for the structural performance of reinforced concrete structures, was investigated. The most widely used international recommendations for structural design with UHPFRC throughout the world (AFGC-SETRA and JSCE) were specifically introduced in terms of material models and flexural and shear design. Lastly, examples of practical applications of UHPFRC for both architectural and civil structures were examined.
The capability of processing robust Engineered Cementitious Composites (ECC) materials with consistent mechanical properties is crucial for gaining acceptance of this new construction material in various structural applications. ECC’s tensile strain-hardening behavior and magnitude of tensile strain capacity are closely associated with fiber dispersion uniformity, which determines the fiber bridging strength, complementary energy, critical flaw size and degree of multiple-crack saturation. This study investigates the correlation between the rheological parameters of ECC mortar before adding PVA fibers, dispersion of PVA fibers, and ECC composite tensile properties. The correlation between Marsh cone flow rate and plastic viscosity was established for ECC mortar, justifying the use of the Marsh cone as a simple rheology measurement and control method before fibers are added. An optimal range of Marsh cone flow rate was found that led to improved fiber dispersion uniformity and more consistent tensile strain capacity in the composite. When coupled with the micromechanics based ingredient-tailoring methodology, this rheological control approach serves as an effective ECC fresh property design guide for achieving robust ECC composite hardening properties.
A study on the influence of crack widths and type of bars (plain and deformed) on corrosion of steel bars in cracked concrete is presented here. Microcell and macrocell corrosions of plain and deformed steel bars were investigated on 10 X 10 X 40 cm single crack specimens with crack widths of 0.1, 0.3, and 0.7 mm. Water-to-cement ratios were 0.3, 0.5, and 0.7. Electrochemical investigations were also conducted on 15 X 15 x 125 cm multicrack specimens with plain and deformed bars. For these specimens, water-to-cement ratios were 0.5 and 0.7 and crack widths were varied from 0.1 to 0.4 mm. After electrochemical investigations, chloride ions in concrete, corroded areas, weight losses, and pit diameters of the steel bars were investigated. The entire study was carried out in an artificially created chloride ion-induced corrosion environment. The study concludes that the relationship between crack widths and corrosion rate is observed at the very early age of exposure. Water-to-cement ratio and corrosion rate relationship is clearer than crack widths and corrosion rate relationship. Deformed bars are more prone to corrosion than plain bars.
For the purpose of global sustainability the long life of structures is essential and the durability performance of reinforced concrete structures is one of the key issues to be resolved. This paper reports the results of a series of long term corrosion tests onfiber reinforced cementitious composites containing polyethylene (PE) alone and hybrid steel cord (SC) and PE fibers. The results are also compared with ordinary mortar. The specimens are subjected to accelerated corrosion for one year by applying external potential to the steel bar anode and a cathode made out of a steel wire mesh placed outside the concrete. Durability performances of the specimens are examined through regular monitoring of the time to initiate corrosion, the corrosion area ratio, corrosion depth, and the amount of steel loss. Results show that the hybrid fiber reinforced cementitious composites (HFRCC) containing hybrid SC and PE fibers exhibited excellent performance compared to mortar and fiber reinforced cementitious composites (FRCC) containing PE fiber. The order of the durability performance is HFRCC, FRCC, and Mortar. It is observed that the sacrificial corrosion of some of the SC fibers in the HFRCC specimen played an important role in the significant reduction of steel bar corrosion in the specimen.
Cracks in concrete structures have always posed a big threat on the durability of concrete. Cracking of concrete is a random process, highly variable and influenced by many factors. Among the crack repair methods is the use of epoxy either by injection or by gravity filling in order to bond the crack and restore its structural integrity.Prior to the use of epoxy compounds and due to their versatility and the wide range of available physical and chemical properties of epoxy resin systems, one has to be completely aware and informed before entering the world of epoxies; other than crack repair, epoxy compounds have found a wide variety of uses in the concrete industry.In this study, 15 concrete cubes, six including cracks without repair, six including cracks bonded with gravity filled epoxy and three with no cracks were crushed and their compressive strengths were obtained. It was found that the cracks caused a reduction in compressive strength up to 40.93% whereas the epoxy system, when properly applied, restored the compressive strength by decreasing the reduction down to 8.23%.
This paper presents a brief overview of the tensile test methods for concrete and cementitious composites. Comparisons of uniaxial tension test results for a round robin test conducted as part of a project of the Japan Concrete Institute Tech-nical Committee (JCI-TC) for Ductile Fiber-Reinforced Cementitious Composites (DFRCC) are introduced. Four types of tensile test methods for four types of DFRCC were used in this round robin test. The results differ according to the testing method and compacting direction of DFRCC. The relationships between the tensile test results and tensile char-acteristics calculated from bending test results are discussed. The possibility of establishing a standard test method for the evaluation of the tensile characteristics of DFRCC has been discussed by the Japan Concrete Institute Standard Committee. This discussion was based on the report of the JCI-TC and the results of the round robin test. Items that were discussed in further detail were (a) difficulties of uniaxial tension test as a standard test method, (b) treatment of DFRCC that does not have a strain hardening branch in tension, (c) adaptability of strain-based evaluation for cracked materials, and (d) relationship between uniaxial tensile characteristics and bending characteristics. The Standard Committee proposed the standard test method using the 4-point bending test to obtain bending moment–curvature curves. An evaluation method for the tensile strength and ultimate strain of DFRCC was added as an appendix of non-mandatory information. This method is considered to be one of the evaluation methods for the tensile characteristics of DFRCC.
Although concrete is the most utilised building material nowdays, this material has a large shortcoming: it has a good resistance against compressive stresses, but a very low resistance against tensile stresses. Usual way to solve this problem is the application of steel reinforcement in concrete structures. Other possibility is the application of different types of fibres in concrete, for example steel or synthetic fibres: this material is then called "fibre concrete". In the past, many types of fibre concrete were developed. For many of them, the added value of fibres was rather low: almost no improvement of tensile strength could be achieved. In this research project, an innovative type of fibre concrete is developed, with improved both the tensile strength and the ductility: the Hybrid-Fibre Concrete (HFC). The expression "Hybrid" is here applied to point out to the hybridisation of fibres: short and long steel fibres were combined together in one concrete mixture. This is opposite to the conventional steel fibre concretes, which contain only one type of fibre. Within this research project, all important aspects needed for successful development and applications of Hybrid-Fibre Concrete have been considered. In total 15 mixtures, with different types and amounts of steel fibres were developed and tested in the fresh state (workability, with the self-compactability as a main goal) as well as in the hardened state (uniaxial tensile tests, flexural tests, pullout tests on single fibres and compressive tests). A new analytical model for bridging of cracks by fibres was developed and successfully implemented for tensile softening response of HFC. At the end, the utilisation of HFC in the engineering practice was discussed, including a case-study on light precast prestressed beams made of HFC, without conventional reinforcement. There many other possibilities for the applications of this material in building engineering, civil infrastructure, geotechnical and tunnelling engineering. It is also expected that the durability of the Hybrid-Fibre Concrete is better compared to conventional concrete, which could be its another big advantage.
This study aims to investigate the effects of pre-formed microcrack properties and immersion duration in a 3.5% NaCl solution on the corrosion degree of steel fibers in ultra-high-performance concrete (UHPC). Two types of steel fiber, i.e., straight and twisted, two pre-strain levels causing different microcrack properties, i.e., 0.45% and 0.6%, and four immersion durations of 0, 4, 10, and 20 weeks were investigated. Energy-dispersive X-ray spectroscopy (EDX) analysis was conducted using scanning electron microscope images for a quantitative evaluation of the degree of surface corrosion. Test results indicate that steel fibers embedded in the multi-cracked UHPC are oxidized due to permeation of the NaCl solution, and longer immersion durations lead to a higher corrosion degree in general. Better tensile performance is achieved if both the straight and twisted steel fibers are moderately corroded. The twisted-fiber-reinforced UHPC is more susceptible to corrosion than the straight-fiber-reinforced UHPC, which is indicated by the earlier deterioration in tensile performance and lower tensile parameter ratios. Given identical immersion durations, the composites with fewer and smaller microcracks formed provide better tensile performance due to the moderately corroded steel fibers. Steel fiber corrosion influences the energy absorption capacity more significantly than the tensile strength, irrespective of the fiber type and microcrack property.
This study evaluated steel fiber corrosion and tensile behaviors of plain and self-healed ultra-high-performance fiber-reinforced concrete (UHPFRC) exposed to 3.5% sodium chloride (NaCl) solution. The degree of steel fiber corrosion was quantitatively evaluated via energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM) image analyses. Test results indicate that, even after a 20-week immersion in the NaCl solution, only few steel fibers located near the surface of the non-cracked UHPFRC samples were slightly corroded, and they insignificantly affected the tensile behavior. A slightly better tensile performance was achieved by self-healing process, and it was further improved after exposure to the NaCl solution for a longer duration due to the moderately corroded steel fibers through the partially self-healed cracks. The surface roughness of the pulled-out steel fibers from the composites increased due to the self-healing and corrosion processes, relevant to the enhanced tensile performance, and by increasing the immersion duration.
In this study, the effect of steel fiber corrosion on the tensile behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) was examined. Macro straight steel fibers with five different corrosion degrees ranging from 0 to 8% were used, and unwashed and washed corrosion fibers were simultaneously used to evaluate the effect of the rust layer formed at the fiber surface on the tensile performance. The post-cracking tensile response of the strain-hardening UHPFRC is strongly affected by the fiber bridging capacity, which is related to the interfacial bond resistance. Thus, the surface roughness of plain and corroded steel fibers was also quantitatively evaluated based on scanning electron microscope (SEM) images. The test results indicated that the surface roughness of steel fiber increases with an increase in the corrosion degree, and a severe and irregular roughened surface is found at a high-corrosion degree of greater than 8% by weight. The tensile behavior of UHPFRC was generally improved by the steel fiber corrosion up to a certain value of 4 or 6%, leading to the complete pullout failure mode without any breakage, whereas beyond the threshold value, it was rather deteriorated due to the fiber ruptures. Even though there was deterioration in the tensile performance, UHPFRC with the maximum corrosion degree of 8% even satisfied the international recommendations on the strength and energy absorption capacity limits. Lastly, the unwashed, corroded fibers exhibited a better tensile performance than the washed ones because of the appropriately moderated bond resistance.
This study investigates the effects of matrix shrinkage, fiber geometry, and loading rate on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For varying the matrix shrinkage, different quantities of calcium sulfoaluminate-based (CSA) expansive agent (EA) were adopted, ranging from 0% to 8%. In addition, moderately and highly deformed, i.e., half-hooked (HH) and twisted (T), steel fibers were used along with three different loading rates ranging from 0.018 (static) to 793 mm/s (impact). The test results indicated that the addition of the CSA EA is effective in reducing the shrinkage strains of UHPC only when its quantity is greater than 6%. The addition of the CSA EA improved the static average bond strengths of all the HH- and T-fibers; however, its effectiveness on the pullout energy was only valid when these fibers were aligned. The addition of the CSA EA enhanced the dynamic bond strengths of HH- and T-fibers, and the improvement was more obvious in the T-fiber specimens than in the HH-fiber specimens and in the aligned fibers than in the inclined ones. However, its implication on the dynamic pullout energies of the HH- and T-fibers was ambiguous. Both the addition of the CSA EA and a faster loading rate increased the probability of rupture failures of the deformed steel fibers in UHPC. The loading rate sensitivity was the highest in the straight steel fiber in UHPC, followed by the HH- and T-fibers, respectively. Consequently, straight or moderately deformed steel fibers are recommended for use in reinforcing UHPC under extreme loads, such as impact and blast.
This study investigates the pullout behaviors of various steel fibers embedded in ultra-high-performance concrete (UHPC) under static and impact loading conditions. For this purpose, four types of steel fibers, i.e., straight, hooked, twisted, and half-hooked, and three different loading rates applied by static and impact pullout test machines were adopted. To examine the effects of the inclination angle on the pullout behavior, four different inclination angles of 0°, 30°, 45°, and 60° were considered. Test results indicate that the highest average bond strengths were found for the hooked and twisted fibers for static and impact loads, respectively, whereas the straight fibers exhibited the lowest bond strength at all inclination angles. The effectiveness of using half-hooked fibers increased when they were inclined, and maximized at an inclination angle of 45° compared to straight and highly deformed fibers. The use of the twisted and half-hooked fibers was also more effective in static pullout energies than the hooked and straight fibers. The pullout resistance of all the steel fibers in UHPC was improved under impact loading conditions, and the order of the loading rate effectiveness regarding both the average bond strength and pullout energy was as follows: straight fibers > half-hooked fibers > twisted fibers > hooked fibers. The change in the failure mode from pullout to rupture, due to the increase in the loading rate, adversely affected the rate sensitivity of the bond strength and energy absorption capacity. Since the deformed steel fibers were easily ruptured under impact loads, their failure by rupture needs to be prevented to achieve an excellent bond strength and energy absorption capacity.
Despite tremendous advances made in fracture mechanics of concrete in recent years, very little information has been available on the nature of fracture processes and on reliable test methods for determining parameters for the different models. Moreover, most texts on this topic discuss numerical modeling but fail to consider experimentation. This book fills these gaps and synthesizes progress in the field in a simple, straightforward manner geared to practical applications.
Experimental results on the effects of chloride content of ultra high performance concrete (UHPC) are presented. The experimental variables are the amount of sodium chloride, ranging from zero to 3.0% per cement weight, and matrix strength, including normal mortar and high strength mortar. Sodium chloride is directly mixed with cement matrix in order to simulate harsh environments by promoting the corrosions of fibers and matrix. The effects of chloride content are evaluated in terms of visual observation, electrical resistivity change, compressive strength, bending strength, and dynamic Young's modulus. The experimental results show that UHPC has the superior capability to resist chloride ions due to its dense microstructures, which prevent the growth of rust crystals. Furthermore, the experimental findings suggest that the potentials for corrosion of steel fibers and for corrosion-induced matrix cracking are inconsequential in UHPC even if chloride ions penetrate into UHPC.
This study comprehensively investigates impact and blast resistances of ultra-high-performance fiber-reinforced concrete (UHPFRC) by considering various influential factors. At a material level, rate-dependent fiber pullout behavior, dynamic compressive behavior, and impact tensile and flexural behaviors were examined in detail, and the benefits of using UHPFRC to improve the impact resistance of ordinary concrete were discussed. It was obvious that (1) UHPFRC is able to dissipate much higher energy by impact than ordinary concrete with and without fibers, (2) the use of long straight steel fiber is effective in improving the impact resistance of UHPFRC compared to that of deformed steel fibers at high volume fractions, (3) fiber orientation significantly influences the impact resistance of UHPFRC: when more fibers are aligned in the tensile load direction, better impact resistance is achieved, and (4) size effect on the dynamic increase factor versus strain-rate relationship is insignificant. Impact and blast resistances of UHPFRC beams, slabs, columns, and composite structures were also examined at structural level, and several useful conclusions were drawn. (1) UHPFRC is favored for impact- or blast-resistant structures as compared with ordinary concrete due to its much better impact and blast resistance at identical dimensions, reinforcement configuration, and load magnitude, (2) the use of high-strength steel rebar provides the better blast resistance of UHPFRC beams or slabs as compared with that of normal-strength steel rebar, and (3) seismic detailing applied in UHPFRC columns leads to better blast resistance than is seen for columns without seismic detailing. Further research is suggested to address the remaining complicated problems or conflicts and to inspire proper design of structural UHPFRC members in an attempt to increase the use of UHPFRC.
Single fiber pullout tests enable a deeper understanding of the behavior of fiber reinforced cementitious materials. The vast majority of fiber pullout tests in the literature are quasi-static and conducted with fibers aligned in the loading direction. Studies that focus on dynamic or inclined pull out behavior are not common and those that combine both effects are rare. In this paper, the experimental study investigates the effects of embedment inclination and pullout rate on the behavior of high strength steel fibers embedded in an ultra-high performance concrete (UHPC) matrix. The experimental variables are fiber type (straight smooth, hooked and twisted), embedment inclination, which varies from zero (aligned with load) to 45°, and loading rate, which ranges from 0.018 mm/s (representing quasi-static loading) to 1800 mm/s (representing impact loading). Test results show that the load and energy dissipation capacities for straight smooth fibers generally increase with loading rate and inclination angle up to 45°. The hooked and twisted fibers exhibit less consistent trends and their peak load and energy dissipation capacities occur at inclination angles that range from 0° (aligned with load) to 30°. The straight smooth fibers exhibit the most sensitive response to loading rate and achieve a load capacity dynamic increase factor (DIF) as high as 2.32. The DIFs are generally less for hooked fibers and drop below 1.00 for twisted fibers, especially at higher inclination angles.
This study investigated the effects of fiber type and matrix strength on the fiber pullout behavior of high-performance fiber-reinforced cementitious composites (HPFRCC). The correlation between single fiber pullout behavior and flexural behavior of HPFRCC was also evaluated. Two different steel fibers, i.e., straight and hooked fibers, and three different matrix strengths were adopted. Test results indicate that the fiber pullout performance was improved with increasing matrix strength. The hooked fibers exhibited higher bond strengths and pullout work than the straight fibers, but at large slips, they showed smaller shear stress at the interface than their counterpart. In addition, the straight fibers were more effective in improving the pullout performance with the matrix strength than were the hooked fibers. For the straight fibers, the shorter fibers provided higher bond strengths and maximum shear stress at the interface than the longer fibers. The flexural performance of HPFRCC beams was improved with increasing matrix strength. The beams with medium-length straight fibers (lf/df = 19.5/0.2 mm/mm) gave the best flexural performance, whereas those with hooked fibers exhibited the worst flexural performance. Therefore, due to several influential factors, the correlation between the single fiber pullout behavior and flexural behavior of HPFRCC beams is quite low.
The design of ultra-high-performance concrete (UHPC) depends basically on the packing density and particle-size distribution (PSD) of its ingredients. The PSD of cement exhibits a gap at the micro scale that needs to be filled with finer materials such as silica fume (SF). Filling this gap solely with SF requires a high amount of SF (25% to 30% by wt. of cement) due to its extreme fineness. The use of high SF volume negatively effects concrete rheology. In addition, the limited resources and high cost of SF impede the wide use of UHPC in concrete market. This paper reports on a study to determine the possibility of producing and using fine glass powder (FGP) as a SF partial replacement in UHPC. The results show that FGP with a mean particle size (d50) of 3.8 µm could be recommended as an optimal PSD to fill the gap between cement and SF particles. The results demonstrate that compressive strength values of 235 and 220 MPa under 2 days of steam curing can be achieved, respectively, when replacing 30% and 50% of SF with FGP, compared to 204 MPa for the reference UHPC containing 100% SF. The rheology of fresh UHPC is improved by replacing SF particles with nonabsorptive glass particles.
Extensive studies have shown that young Engineering Cementitious Composites (ECCs) have the potential to achieve effective self-healing. The present study investigates the medium-term self-healing performance of cracks in ECCs that are relevant in the medium and long-term stages of the material service life. For this purpose, the prepared ECC specimens are pre-cracked at an age of 180 days. The major experimental variables are the weight fraction of fly ash in ECCs (a fly ash to cement ratio of 1.2, 1.6, or 2.0) and the healing duration (7, 28, or 90 days). The medium-term self-healing performance is quantified using a resonant frequency test followed by a uniaxial tensile test. In addition, scanning electron microscopy and energy dispersive X-ray analyses are employed to observe the micro-structure of the healed crack and identify the medium-term healing product, respectively. The results suggest that as long as water is present in the environment, ECCs have moderate medium-term self-healing ability, and can partially recover their tensile mechanical properties. In particular, effective medium-term self-healing performance can be achieved within 90 days of conditioning for ECCs with a pre-strain of less than 1%.
Research on the pullout behavior of single steel fibers embedded in ultra-high-performance concretes (UHPCs) was conducted to investigate the bond properties of straight and deformed steel fibers. The main research objective was to compare the physicochemical interfacial bond properties between brass-coated straight steel fibers and the ultra-high-performance cementitious matrix with the mechanical bond properties of hooked-end and twisted steel fibers embedded in the same matrix. The results show that the enhanced bond properties provided by the ultra-high-performance cementitious matrix led to the failure of fibers having a high mechanical bond component. Tailoring of the fiber strength and mechanical bond to the matrix strength is needed for optimal pullout behavior. It is observed that the equivalent bond strength of deformed fibers embedded in UHPC reaches up to 47 MPa (6.8 ksi) - that is, almost five times the equivalent bond strength of straight fibers (10 MPa [1.4 ksi]) embedded in the same matrix. Furthermore, the equivalent bond strength of straight steel fibers, which are commonly used in ultra-high-performance fiber-reinforced concrete (UHP-FRC), can be doubled to a value exceeding 20 MPa (2.9 ksi) by optimizing the UHPC matrix through composition and particle size distribution, leading to an atypical pullout load-slip-hardening behavior. Such behavior is desirable for high tensile strength, high-energy-absorbing, strain-hardening UHP-FRC.
Water regimes are known to vary markedly in their aggressivity towards cement. It has long been appreciated that the damaging regimes are associated with the presence of dissolved CO2, Historial efforts to correlate cement performance with groundwater compositions are reviewed, especially in respect of CO2 behaviour, as functions of its ‘aggressive’ and ‘non-aggressive’ speciations. The basic physical chemistry of the relevant aqueous solutions is explored, leading to new definitions of aggressivity and a computer-based iterative method of solving the necessary equations. Interpretation of the results is complicated by the formation of passivating layers of CaCO3 and SiO2 (gel) at the cement-water interface. These factors are explored and ‘coverage mapping’ is developed as a guide to their likely protective value. The model is related to dynamic situations; for example, the process of crack reheating which can occur spontaneously in concrete.
To clarify the corrosion mechanism and the dominant influencing variables, especially the influence of crack width, laboratory tests were performed on cracked reinforced concrete beams. Test results and a mathematical model1 were then used to calculate the effect of crack distance and the effect of a crack width limitation by reducing the rod diameters on the steel removal rates due to chloride-induced corrosion. The results show that after local depassivation of the steel surface by chlorides penetrating through cracks in concrete the steel in the cracked zone acts as an anode (iron removal) and the steel between the cracks acts as a cathode (oxygen reduction). Therefore the corrosion rate in the crack zone is influenced considerably by the conditions between the crack. It has been found that thickness and quality of concrete cover influence the corrosion rate much more than the crack width. By simplified calculations it was shown that a crack width limitation by reducing rod diameters from about 0.4 mm (0.016 in) to lower crack widths results in increasing losses of steel diameter. As a consequence, corrosion protection must be assured primarily through adequate concrete quality and cover.
The effectiveness of epoxy resins and cementitious repair materials, including silica fume cement, in improving the functional performance of beams and slabs with corroded reinforcement was evaluated. Reinforcement corrosion in the beams and slabs was accelerated by application of a direct current for various periods of time. The deteriorated specimens were repaired and tested for flexural strength. The results indicate that not all the repair materials are able to restore the original strength of the components. The improvement in the load-carrying capacity was related to the increase in the bond between the parent concrete and the repair material, inter alia, the steel reinforcement for an effective load transfer. One of the epoxy resin mortars investigated, as well as silica fume cement concrete, to some extent, were able to restore the original strength of the component. Furthermore, the improvement in the functional performance of the repair materials, vis-à-vis, epoxy resin mortar, was observed to be dependent on the degree of reinforcement corrosion. The repair using this material was only effective when the degree of reinforcement corrosion was less than 10%.
Although it is generally recognized that cracks promote the ingress of chloride in concrete, the lack of sufficient knowledge on this subject does not yet allow reliable quantification of their effects. In the current study, the influence of artificially created, parallel-wall cracks with widths ranging from 0.08 to 0.68 mm on chloride ingress was examined. The effect of crack wall surface roughness was evaluated as well. Cracked and uncracked samples were exposed to a 40-day chloride bulk diffusion test. Lateral movement of chlorides from the crack wall into the bulk of the sample was analyzed using scanning electron microscopy with energy dispersive x-ray (SEM/EDX). Based on the results, it was concluded that chloride diffusion in concrete was independent of either crack width or the crack wall roughness for the ranges studied. The transecting, parallel-wall cracks were found to behave like a free concrete surface, resulting in a case of two-dimensional diffusion and greatly promoting chloride ingress.
The objective of this research was to investigate the pullout behavior of straight high-strength steel fibers embedded in different ultrahigh-performance concretes (UHPCs) with a compressive strength ranging from 190 to 240 MPa (28 to 35 ksi). Particular attention was placed on obtaining matrixes with high packing density to enhance the physicochemical bond with the embedded fiber. The parameters investigated included the use of different sand ratios, silica fume (SF) and glass powder with different mean particle sizes, different superplasticizers, and the addition of hydrophilic or hydrophobic nanosilica particles. Thus, by tailoring the matrix composition, significantly different bond stress versus slip-hardening behaviors were achieved. This is atypical for straight smooth steel fibers, which are normally characterized by a bond-slip softening behavior. Microscopical studies revealed that scratching and delaminating of the brass-coated fiber surface by fine sand and by abrading matrix particles is one reason for this phenomenon, and help explain the maximum equivalent bond strength observed of up to 20 MPa (2.9 ksi).
The well-known practical phenomenon of autogenous healing in cracks plays a significant role in relation to the functional reliability of structures subjected to water-pressure loads. Due to the autogenous healing, the water flow through the cracks gradually reduces with time, and in extreme cases, the cracks seal completely. In the past, there has been no deliberate technical exploitation of self-healing, because too little is known about the phenomenon itself and about the chemical/physical processes involved. Based on theoretical and experimental research, the effect of crack healing was investigated on a larger scale for the first time. The experimental studies showed the formation of calcite in the crack to be almost the sole cause for the autogenous healing. A comprehensive theoretical discussion of the laws, which govern the calcite nucleation and the subsequent crystal growth of water-bearing cracks in concrete, revealed that the crystal growth responds to two different crystal growth processes that are determined by the changes in the chemical and physical conditions in the crack. Further, the crystal growth rate is dependent on the crack width and water pressure, whereas concrete composition and water hardness have no influence on autogenous healing. On the basis of the experimental studies, an algorithm that can be used to estimate the reduction in water over time as a result of autogenous healing was developed.
The results of an extensive experimental investigation of the deterioration of steel fiber reinforced concrete due to fiber corrosion is presented. About 1200 specimens were tested. Two parallel test programs were conducted: one dealt with the effect of corrosion on steel fiber reinforced mortar specimens, and the other dealt with the effects of using precorroded fibers in mortar specimens. Different exposure periods with typical 3 days, cycling in 3.5 percent standard sodium-chloride solution and in laboratory air, respectively, were used with different solution temperatures. All experimental results seem to indicate that after a certain degree of corrosion has occurred, strength and toughness decrease with an increase in the degree of corrosion and that these mechanical properties are primarily affected by the reduction in minimum fiber diameter.
Service loads well below the yield strength of steel reinforcing bars lead to cracking of reinforced concrete. This paper investigates whether the crack resistance of Hybrid Fiber Reinforced Concrete (HyFRC) reduces the corrosion rate of steel reinforcing bars in concrete after cyclic flexural loading. The reinforcing bars were extracted to examine their surface for corrosion and compare microcell and macrocell corrosion mass loss estimates against direct gravimetric measurements. A delay in corrosion initiation and lower active corrosion rates were observed in the HyFRC beam specimens when compared to reinforced specimens containing plain concrete matrices cycled at the same flexural load.
The effectiveness of epoxy resins and cementitious repair materials, including silica fume cement, in improving the functional performance of beams and slabs with corroded reinforcement was evaluated. Reinforcement corrosion in the beams and slabs was accelerated by application of a direct current for various periods of time. The deteriorated specimens were repaired and tested for flexural strength. The results indicate that not all the repair materials are able to restore the original strength of the components. The improvement in the load-carrying capacity was related to the increase in the bond between the parent concrete and the repair material, inter alia, the steel reinforcement for an effective load transfer. One of the epoxy resin mortars investigated, as well as silica fume cement concrete, to some extent, were able to restore the original strength of the component. Furthermore, the improvement in the functional performance of the repair materials, vis-a-vis, epoxy resin mortar, was observed to be dependent on the degree of reinforcement corrosion. The repair using this material was only effective when the degree of reinforcement corrosion was less than 10%. (A)
In this study, the combined effect of shrinkage-reducing admixture (SRA) and expansive admixture (EA) on the shrinkage and cracking behaviors of restrained ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs was investigated. For this investigation, six full-scale UHPFRC slabs with three different thicknesses (h = 40, 60, and 80 mm) were fabricated using two different mixtures. Test results indicated that the combined use of 1% SRA and 7.5% EA is beneficial to improve the mechanical strengths and to reduce the free shrinkage strain of approximately 36–42% at 7 days. Regardless of SRA and EA contents, the slabs with the lowest thickness of 40 mm showed shrinkage cracking at a very early age, while the slabs with higher thicknesses of 60 and 80 mm showed no cracking during testing. However, the UHPFRC slab including 1% SRA and 7.5% EA exhibited a shallow crack with a very small maximum crack width of below 0.04 mm, while the slab without SRA and EA showed through cracks with a large maximum crack width of 0.2 mm.
The effect of wet–dry cycles in tap water and in 3.5% NaCl solution on the passivating products of galvanized steel after concrete carbonation was investigated. The results show that carbonation destroys the passivating layer of calcium hydroxyzincate with the formation of the amorphous products ZnCO3 and Zn5(CO3)2(OH)6, which are also protective. The exposure to wet–dry cycles in tap water after concrete carbonation does not significantly increase the corrosion rate of the galvanized steel with respect to the values measured in non-carbonated concrete. On the contrary, the exposure to chloride solution after concrete carbonation leads to a marked decrease in the chloride threshold for galvanized steel corrosion with respect to that found in non-carbonated concrete.
This study investigates the effect of fiber length and placement method on the flexural behavior, tension-softening curve, and fiber distribution characteristics of ultra-high-performance fiber-reinforced concrete (UHPFRC). Four different fiber lengths (Lf = 13, 16.3, 19.5, and 30 mm) were considered for two different placement methods. The ultimate flexural strength increased with increasing fiber length up to 19.5 mm, despite no noticeable difference in the first crack strength. Conversely, fiber length of 30 mm showed deterioration of flexural performance due to the decrease of fiber number existed across the crack surface. Both of first crack and ultimate flexural strengths were affected by the placement method; the specimen with concrete placed in the center (at maximum moment region) exhibited higher strength than that with concrete placed in the corner. The reasons were confirmed by image analysis that poorer fiber dispersion and fewer fibers across the crack surface were obtained for the specimen with concrete placed in the center than its counterpart. Finally, a tri-linear softening curve for UHPFRC was suggested based on inverse analysis and verified through comparison between the finite element analyses and the test data.
This paper reports results of a study conducted to assess the performance of commonly utilized repair systems when exposed to some selected exposure conditions, such as marine, belowground, fire, acid, and sulfur fumes. The performance of the selected repair systems was assessed by exposing large-sized repaired concrete specimens to the selected exposure conditions in addition to thermal variations. After the completion of the exposure, the repaired specimens were visually examined for damage to the surface coating and presence of rust stains, salt scaling, etc. The bond of the coating with the substrate was evaluated and then the specimens were crushed to retrieve reinforcing steel bars that were examined for the extent of corrosion, if any. The data developed in this study were utilized to recommend repair systems suitable for the selected exposure conditions.
Under continuous hydrothermal treatment the strength of portland cement paste decreases with curing time and the pore structure coarsens. It was found in this study that the compressive strength of slag cement paste containing 67.5 wt.% ggbfs also decreases with time after 24 hour hydrothermal processing, but with a small addition of silica fume to the slag cement, the cement strength increases and the pore structure densifies when processed under comparable conditions. Based on observations XRD and SEM, these changes are attributed to: (a), changes in the hydration reactions and products by highly reactive silica fume, such that amorphous products dominate and the strength reducing phase α-C2SH does not form; (b), slower hydration of slag, partially caused by the decreased pH of the pore solution, favors the formation of a dense pore structure; and (c), the space filling properties of the micro particles of silica fume.
With the attractive possibility of offshore concrete casting in mind, the effects of marine curing on the pull-out behavior of steel fibers were investigated. Single fiber pull-out specimens were chosen to clearly examine the behavior of individual fibers. Three curing temperatures of 2°, 22°, and 38° C were chosen. Deformed fibers with hooks on both ends were chosen. The effects of silica fume addition were also investigated. Pull-out resistances were continuously monitored by conducting tests starting at the age of 1 day up to about 3 months. In some cases, the tests were extended to an age of 1 year. Some fibers were retrieved and examined in a scanning electron microscope. Curing at a low temperature of 2° C was not found to adversely affect the pull-out resistance even after one year of continuous marine exposure. High temperatures, however, were found to promote an early corrosion leading to substantial reductions in pull-out resistances. The presence of silica fume was not found to promote strength retrogression in any particular way. The deformed locations of the fibers, perhaps because of the residual stresses, were found to be particularly susceptible to anodic pitting.
This paper presents a study of the tensile fracture properties of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) considering the effects of the fiber content. To investigate the impact of fiber content, notched 3-point bending tests were executed, where the fiber volume ratio was varied from 0% to 5%. From the bending tests, it was found that the flexural tensile strength of UHPFRC linearly increases with increasing fiber volume ratio and the rule of mixture can be applied to UHPFRC. Furthermore, an inverse analysis was performed to determine the tensile fracture model of UHPFRC and a tri-linear tensile softening model is suggested. The suggested model successfully represents the increase of the stress-constant bridging zone and the decrease of the stress-resisting zone with increasing fiber content. The proposed model for various fiber content levels is simple and versatile and can be readily applied to structural design or numerical analysis of UHPFRC.
In this study, as a part of research to characterize the tensile properties of steel fiber reinforced ultra-high strength cementitious composites, pullout tests of steel fiber were performed to evaluate the effect of fiber inclination angle on the load direction and an analytical pullout model was derived considering this effect. The fiber inclination angles considered in the pullout tests were 0°, 15°, 30°, 45°, and 60°. From the pullout tests, it was observed that the largest peak load was obtained at an angle of 30° or 45°, and the peak slip increased as the fibers were oriented at a more inclined angle. Based on the experimental results, an analytical pullout behavior model considering fiber inclination was proposed. In order to take into account the effect of fiber inclination in the pullout model, apparent shear strengths (τ(app)) and slip coefficient (β) were introduced to express the variation of pullout peak load and the augmentation of peak slip as the inclined angle increases. These variables are expressed as functions of the inclined angle (ϕ).
Crimped steel fibers with large diameters are often used in concrete as reinforcement. Such large diameter fibers are inexpensive, disperse easily and do not unduly reduce the workability of concrete. However, due to their large diameters, such fibers also tend to be inefficient and the toughness of the resulting fiber reinforced concrete (FRC) tends to be low. An experimental program was carried out to investigate if the toughness of FRC with large diameter crimped fibers can be enhanced by hybridization with smaller diameter crimped fibers while maintaining workability, fiber dispersability and low cost. The results show that such hybridization indeed is a promising concept and replacing a portion of the large diameters crimped fibers with smaller diameter crimped fibers can significantly enhance toughness. The results also suggest, however, that such hybrid FRCs fail to reach the toughness levels demonstrated by the smaller diameter fibers alone.
In this study, the effect of the fiber orientation distribution on the tensile behavior of Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC) was investigated. The tensile behavior was explored separately in two stages; pre-cracking and post-cracking tensile behaviors. Pre-cracking tensile behavior is expressed using the mechanism of elastic shear transfer between the matrix and the fiber in the composites. Post-cracking tensile behavior was expressed as the combined behavior of the resistance by the fibers and the matrix, considering a probability density distribution for the fiber orientation distribution across crack surface and a pullout model of steel fiber. The effect of the fiber orientation distribution was found to be very small on pre-cracking behavior, but to be significant on post-cracking behavior of UHPFRCC. The predicted results were compared with the experimental results, and the comparison presented satisfactory agreement.
The corrosion of steel fibres in the cracked section has been under investigation by many researchers since the last 15 years. It is reported widely that in case of steel fibres reinforced concrete (SFRC), corrosion is less active as compared with steel bars. In the cracked section, the durability of the material depends on the performance of the bridging capacity of the fibres embedded in the concrete. The corrosion of the fibres not only could produce the spalling of concrete but it could also reduce the sectional area of the fibres, turning the durability of structures in danger. This study focuses on those two aspects of fibre corrosion. The tests were performed on cracked SFRC samples with 0.5-mm crack mouth openings (CMOs) exposed to marine-like environment for 1 year. The results confirm the small sensitivity of SFRC to corrosion. Surprisingly, they made appear an increase of the flexural strength after corrosion. The factors affecting the corrosion of the fibres and the reasons for the increase in flexural strength after corrosion are discussed.
Development of an ultra-high strength ductile concrete designated RPC (Reactive Powder Concrete), was made possible by the application of a certain number of basic principles relating to the composition, mixing and post-set heat curing of the concrete.RPC 200, which can be used under job site conditions similar to those for conventional high performance concretes, can be used in the construction of prestressed structures incorporating no passive reinforcement. RPC800 is suitable for precasting, and can achieve compressive strength values exceeding 600MPa. A value of 810MPa has been obtained with a mixture incorporating steel aggregate.
The self-healing behavior of a series of pre-cracked fiber reinforced strain hardening cementitious composites incorporating blast furnace slag (BFS) and limestone powder (LP) with relatively high water/binder ratio is investigated in this paper, focusing on the recovery of its deflection capacity. Four-point bending tests are used to precrack the beam at 28 days. For specimens submerged in water the deflection capacity can recover about 65–105% from virgin specimens, which is significantly higher compared with specimens cured in air. Similar conclusion applies to the stiffness recovery in water cured specimens. The observations under ESEM and XEDS confirmed that the microcracks in the specimens submerged in water were healed with significant amount of calcium carbonate, very likely due to the continuous hydration of cementitious materials. The self-healing cementitious composites developed in this research can potentially reduce or even eliminate the maintenance needs of civil infrastructure, especially when repeatable high deformation capacity is desirable, e.g. bridge deck link slabs and jointless pavements.
Design and Field Testing of Tapered H-Shaped Ultra High Performance Concrete Piles
- T L V Voort
T.L.V. Voort, Design and Field Testing of Tapered H-Shaped Ultra High
Performance Concrete Piles, MS Thesis, Iowa State University, Iowa, USA, 2008,
p. 229.
Tension-softening behavior and chloride ion diffusivity of cracked ultra-high strength fiber reinforced concrete
- K Hashimoto
- T Toyoda
- H Yokota
- T Kono
- T Kawaguchi
K. Hashimoto, T. Toyoda, H. Yokota, T. Kono, T. Kawaguchi, Tension-softening
behavior and chloride ion diffusivity of cracked ultra-high strength fiber reinforced
concrete, in: RILEM-fib-AFGC International Symposium on Ultra High Performance
Fibre-Reinforced Concrete, France, Marseille, 2014, pp. 257-264.
Repair of Cracked Reinforced Concrete: Assessment of Corrosion Protection
- A J J Calder
- D M Thompson
A.J.J. Calder, D.M. Thompson, Repair of Cracked Reinforced Concrete: Assessment
of Corrosion Protection, vol. 150, Transport and Road Research Laboratory,
Department of Transport, Research Report, 1998.
Ultra-high Performance Concrete, ACI Fall Convention
ACI Committee 239, Ultra-high Performance Concrete, ACI Fall Convention,
Toronto, Ontario, Canada, 2012.