Cement and Concrete Composites

Published by Elsevier
Online ISSN: 0958-9465
Publications
Article
A brief overview of research on AAR presented at the 10th AARC is given and new areas of research and future needs are highlighted. Some recent experimental data are presented as examples of details that we would need to understand in order to be able to interpret the effects of parameters such as temperature and chemical environment on the nature of the reactions occurring in the concrete, and on the results of tests carried out to determine the expansion behaviour of aggregates or concrete mixtures. Similar examples are needed for other areas and it appears that understanding AAR and its practical management need further research efforts.
 
Article
Sixty eight case studies of applications of self-compacting concrete (SCC) have been analysed. These were published from 1993 to 2003, the period of increasingly widespread use of SCC in many countries.They were selected for analysis on the basis of including details of concrete formulations and properties. The ranges of properties, component materials and mix proportions show the diverse nature of SCC, and confirm that it should be considered as a family of mixes suitable for a wide range of applications with widely varying requirements.The outcome of the analysis of the above factors is given in statistical terms—ranges, frequencies, cumulative distributions, medians and deciles. This will be of value to those new to SCC, current users and researchers.
 
Article
The search for durable and sustainable construction materials inspires the developments in the world of cement concrete, as well as in the world of concrete–polymer composites. Both worlds recognize, strive for and accept each other’s contribution to the synergetic effects that are realized by the combination of classical building materials and polymers. A better knowledge of materials behaviour, especially in the field of admixtures, and a better understanding of curing processes allowed the development of highly performing mineral or modified mineral concretes, mortars and grouts. CPC-science becomes an invaluable element in the development of sustainable construction materials. ICPIC brings together practitioners and scientists, dealing with concrete–polymer composites in all industrial fields, but with emphasis on construction industry. The 11th International ICPIC Congress took place in Berlin, 2–4th June 2004. New trends and evolutions have been presented and discussed. The highlights of the Congress, and the synergies for the construction world that emerge from this congress on polymers in concrete in combination with cement concrete, are presented.
 
Article
During the 1995 Hanshin-Awaji earthquake, underground subway structures suffered significant damage, including middle column shear failure which has never been experienced in the past. The present paper describes analytical studies for damage verification and for failure mechanism investigation. Focusing on box culvert structure with middle columns at Kamisawa station, Kobe city municipal subway, the following three series of analytical studies have been conducted. In the first study, soil deposit dependent ground response are investigated using equivalent linear response analysis computer program based on multi-reflection theory. The increase of the deposit layer thickness provides less acceleration, but more displacement responses, which causes more in the way of damage in the present underground structure. In the second study, structure and ground responses are investigated with using two-dimensional soil-structure interaction computer program. As a result, the horizontal motion dependent flexural shear section force has more of an effect, but the vertical motion dependent axial force has less of an effect on the middle column damage. That section force is more affected by displacement amplification of ground dependent on deposit layer. In the third study, three-dimensional FE static nonlinear analysis is conducted to investigate failure mechanism of the damaged structure subjected to earth pressure load, which is obtained in the second study. Analytical results predict shear failure of the top story middle column prior to the flexural yielding of the slabs and walls.
 
Article
Polymers in concrete have received considerable attention over the past 25 years. Polymer-impregnated concrete (PIC) was the first concrete polymer composite to receive widespread publicity. PIC has excellent strength and durability properties, but it has few commercial applications. Polymer concrete (PC) became well known in the 1970s and is used for repair, thin overlays for floors and bridges, and for precast components. Polymer-modified concrete (PMC) has been used primarily for repair and overlays. Several limitations have slowed the use of concrete polymer materials. However, there are many current and future uses for these materials that will effectively use their unique properties. Improved, automated repair methods, improvements in materials, replacements for metals, structural applications, and architectural components will prove to be popular uses of concrete-polymer materials.
 
Article
Alkali activated slag concrete (AASC) is made by activating ground granulated blast furnace slag with alkalis without the use of any Portland cement. This study investigates the level of microcracking which occurs in AASC when subjected to various types of curing regimes. The corresponding compressive strength developments of AASC were monitored. The level of microcracking were measured using three different types of tests: (1) frequency and size of surface cracks using crack-detection microscope (2) water sorptivity tests measuring absorption of water by capillary attraction and (3) mercury intrusion porosimetry (MIP) tests which measured the pore size distribution of AASC and AAS pastes (AASPs). The results show that the lack of moist curing of AASC increased the level of microcracking measured using all three different tests mentioned above. The strength development of AASC is also significantly reduced by lack of moist curing.
 
Article
Lime mortars have been used for centuries in civil engineering construction. Considering ancient monuments and historical buildings it seems that these mortars have proved to be durable and reliable materials although they are of low strength in comparison with cement mortars. Nowadays, they are used for the repair of monuments and for the manufacture of renderings and plasters.In the present paper the role of aggregates on the structure and behaviour of lime mortars is examined by studying the influence of the aggregate content and the grain size on strength, porosity and volume stability of the mortars. Capillary porosity by suction was also measured as an indicator of resistance to weathering. It is shown that coarse aggregates contribute to the volume stability of lime mortars independent of strength enhancement when adequate compaction reduces the capillary pores. The highest strength values, and consequently, the low porosity, were attained by lime mortars of low binder/aggregate ratio which contained aggregates of maximum size 0–4 mm.
 
Article
The sustainability of cement-based materials is a core technical issue of national importance. The serviceability of the concrete infrastructure, especially bridges, has recently received considerable media attention. Concurrently, the nanoscience of cements has emerged as a new discipline with the potential to positively manipulate the nanostructure of calcium silicate hydrate (C–S–H) in order to achieve sustainability objectives. Organic/inorganic C–S–H nanocomposites are being developed to enhance durability and long-term engineering behavior. Results of a 29Si MAS NMR study of phase pure C–S–H (prepared with C/S ratios 0.6–1.6) modified by interaction with hexadecyltrimethyl ammonium (HDTMA), methylene blue (MB), polyethylene glycol (PEG), polyvinylalcohol (PVA) and polyacrylic acid (PAA) are reported. It is apparent that the degree of silica polymerization is affected by the presence of the various organic molecules and the C/S ratio of the C–S–H. Possible mechanisms responsible for this effect are described. The implications of these nanostructural changes for sustainability are discussed. It is suggested that the interaction of polymers with the nanostructure of C–S–H that increases silicate polymerization may improve volume stability and promote mechanical properties of such systems. The resistance of polymer-modified C–S–H systems in chemically aggressive environments is likely to be increased as the defect sites in the silicate structure are eliminated by the presence of polymer molecules.
 
Article
A survey is given of stereology-based two-dimensional (2D) and three-dimensional (3D) approaches to shape assessment of embedded or non-embedded particles. Firstly, the development is outlined of global parameters characterizing geometric structure of cementitious materials. Secondly, these parameters are combined to yield effective shape estimators to be applied to section images of embedded particles or to projection images of non-embedded particles. This application can be just 2D, denoted as quantitative image analysis, giving information on what is displayed of the particle(s) in the section or projection image plane only. However, the researcher should strive for geometrical–statistical (stereological) extrapolation of the 2D observations to the real world’s third dimension. This is demonstrated superior over the 2D approach, however, requires a careful sampling strategy for providing representative information on structure.
 
Article
The influence of location relative to the casting position, on porosity and pore size distribution of cement pastes, was investigated. Three different pastes were prepared at a constant water/binder ratio of 0.45. The pastes were the control paste (CP) in which Portland cement was used and no cement replacement materials were added, pastes with 22% and 9% replacement (by mass) of cement with fly ash (FA) and silica fume (SF), respectively. Paste specimens were cast in cube moulds and were either cured in air at a temperature of 45 °C and relative humidity of 25% for 28 days or moist cured for 14 days after casting at 45 °C, followed by air curing at 45 °C and 25% relative humidity for further 14 days. Samples were taken from various locations of the cube specimens. Porosity and pore size distribution were conducted on the paste samples using the mercury intrusion porosimetry technique.The results show that large differences in porosity and pore size distribution exist between samples taken from different locations relative to casting positions. These differences are larger in pastes subjected to dry curing as compared to pastes subjected to some initial moist curing. The influence of sample location relative to casting position on porosity and pore size distribution of paste is compared with absorption of concrete performed in a previous investigation. The correlation between pore volume of paste and water absorption of concrete is also conducted.
 
Article
Owing to the presence of finely divided calcite, mortars and concretes made with Portland-limestone cements are particularly susceptible to damaging thaumasite formation during sulfate attack at lower temperatures. This work reports the results of investigations on mortars made according to DIN/EN 196 and pastes (w/c ratio of 0.5) with CEM I 42,5 R, as well as with mixtures of cement with limestone filler. Some of the samples were heat-treated at 95 °C. The length changes and resonant frequencies of the samples were measured during long-term water-storage at 20 and 5 °C. There was no evidence from X-ray diffraction data of thaumasite formation in the samples. Only for pastes containing 30 wt.% limestone filler were small areas found by SEM and X-ray microanalysis whose chemical analysis matched thaumasite or a thaumasite–ettringite solid solution.
 
Article
The overall thrust of this paper is to show that an integrated material and structural design strategy needs to be adopted to develop techniques that are meaningful and effective for the identification, evaluation and rehabilitation of concrete elements and concrete structures affected and damaged by AAR. It is shown that exposure to environmental and climatic changes is the major factor influencing the rate of expansion and total expansion of concrete in real structures. AAR is also closely and intimately involved with testing and test methodologies so that material and structural rectification requires a global approach involving diagnostic methods, tests to establish the potential of future expansion, selective sealing of cracks and protection from environmental attack, structural evaluation using non-destructive test techniques and structural strengthening.
 
Article
Three repair materials were prepared using cement-based, geopolymeric, or geopolymeric containing steel slag binders. Their mechanical performances such as compressive strength, bond strength and abrasion resistance were examined experimentally. The test results showed that the geopolymeric materials had better repair characteristics than cement-based repair materials, and the addition of steel slag could improve significantly the abrasion resistance of geopolymeric repair. By means of scanning electron microscopy (SEM) it can also be concluded that the steel slag was almost fully absorbed to take part in the alkali-activated reaction and be immobilized into the amorphous aluminosilicate geopolymer matrix.
 
Article
With more and more severity of electromagnetic environment pollution, the study on building materials that can prevent electromagnetic interference (EMI) has caused great attention. This paper mainly reviews the progress and prospective future of cement-based EMI shielding and wave absorbing building materials.
 
Article
Double-layer cementitious composites filled with Mn–Zn ferrite as microwave absorbers were designed based on the impedance matching theory and electromagnetic wave propagation laws. The results showed that the addition of silica fume can improve the impedance matching between the cementitious composites and free space. Comparing with the single-layer structure, the reflectivity of the double-layer cementitious plates can decrease by 6–8 dB and decrease by 15 dB maximum with 30 wt.% ferrite; in addition, the reflectivity of electromagnetic wave is lower than 10 dB in the frequency range of 11.4–18 GHz. These composites can be potentially used as electromagnetic interference (EMI) materials for buildings.
 
Article
Long-term movements in masonry can be predicted by composite modelling, however, the accuracy of prediction is reduced if the unbonded brick and mortar phases do not reflect the behaviour of the bonded components. It is apparent that the water absorbed by the unit soon after laying produces changes in the behaviour of the bonded components of the masonry especially of the mortar joints, which have reduced creep and shrinkage. This paper presents a study of the transfer of moisture between the brick and mortar components from 30 min after laying of bricks to 120 days. Subsequently, unit water absorption modification factors are developed which improved the accuracy of predicting creep and moisture movement strain of masonry by composite models to within 10% of the measured values.
 
Article
X-ray absorption measurements have been applied to studying film formation at the exposed surface during the drying of cementitious tile adhesive mortars as a function of ingredients and mixture proportions. Preliminary observations suggested that in addition to a drying front, concurrently, a densification front is observed proceeding from the exterior of the specimens inward. Due to the extremely high viscosity of the pore “solution” in these mortars, an analysis based on Stokes equation actually suggests that some of the smaller cement particles will be “carried” along with the drying pore solution to the top surface of the specimen where they are sequentially deposited. To verify this hypothesis, further experiments were conducted using either a coarse cement or a fine limestone as a total replacement for the commonly used cement. The X-ray absorption results are further supported by concurrent mass loss measurements on equivalent specimens for each experiment and by particle size distribution analysis of the final dried limestone/silica sand system as a function of depth.
 
Article
The main objective of this study was to investigate the potential utilisation of rubber waste in cementitious matrix, as fine aggregates, to develop lightweight construction materials. Composites containing different amounts of rubber particles, as partial replacement to cement by volume, were characterised by destructive and non-destructive testing. Five designated rubber contents varying from 10% to 50% by volume were used. The 28-days physical, mechanical and hydraulic transport properties of the cement composite were determined. Analyses included dry unit weight, elastic dynamic modulus, compressive and flexural strengths, strain capacity, and water absorption. Test results of the physico-mechanical behaviour indicated that the increase in rubber content decreases the sample unit weight with a large reduction in the strengths and elastic modulus values of the composites. Results have only shown that the introduction of rubber particles significantly increases the strain capacity of the materials. However, rubbers into cement paste enhances the toughness of the composite. Although the mechanical strengths were reduced, the composite containing 50% of rubber particles satisfies the basic requirement of lightweight construction materials and corresponds to “class II”, according to the RILEM classification. Test-results of the hydraulic transport properties revealed that the addition of rubber particles tends to restrict water propagation in the cement matrix and reduces water absorption of the composite. The decrease of the sorptivity-value is favourable to the durability of the specimen structures.
 
Article
The W/C ratio is generally considered to be the governing parameter, which affects the strength and durability of concrete. In this paper, the effect of paste volume and of water content on capillary absorption and strength is investigated on concrete mixes having the same Wef/C ratio. Four Wef/C ratios (0.3, 0.4, 0.5, 0.6) were used and for each Wef/C ratio four mixes were prepared with effective water contents 140, 180, 220 and 260 l/m3. It is found that although the Wef/C ratio is kept constant, strength increases and capillary absorption decreases when the volume of the water or the volume of the paste decreases. The effect of the paste or water content by volume on strength is stronger for lower W/C ratios and for water contents at the lower or upper values (140 l/m3, 260 l/m3) used in the investigation. Similarly capillary absorption is higher the higher is the value of Wef/C ratio and, with constant Wef/C ratio, increases approximately linearly as the paste content increases. The rate of increase is lower for lower values of Wef/C ratios.
 
Article
AC-impedance spectroscopy (AC-IS) was combined with time-domain reflectometry (TDR) to investigate the impedance response of fiber-reinforced cement (FRC) composites with multi-walled carbon nanotubes (MWCNTs). In Nyquist plots (−imaginary impedance vs. +real impedance) three impedance arcs/features were observed, similar to Nyquist plots for macrofiber and microfiber FRCs. The intersection of the electrode arc and the intermediate frequency feature (RDC(FRC)) corresponds to the DC resistance of the composite. The intersection of the two bulk features (Rcusp) corresponds to the AC resistance of the composite. Reductions in (RDC(FRC)) from the matrix resistance are indicative of a nanotube percolating network. Reductions in Rcusp from the matrix resistance are indicative of a discontinuous fiber–fiber path. Both shifts increased with fiber loading. AC-IS measurements are therefore able to discriminate percolation vs. discontinuous fiber effects in CNT-FRCs, with the potential for characterizing dispersion issues (e.g., clumping/aggregation) in nanocomposites.
 
Article
The freezing of water in portland cement paste was investigated using ac impedance spectroscopy (ACIS) techniques. Length change and impedance measurements on cooling and warming were obtained simultaneously using a specially designed coupling technique. A new descriptor of frost resistance is described in terms of the high-frequency arc (HFA) depression angle in the impedance plane. Curves of the depression angle parameter φ versus temperature are shown to be indicative of the pore structure and its effect on mass transfer of water and nucleation and growth of ice crystals. The φ–temperature and resistance–temperature curves are used in combination to assess structural damage. Results are compared with those obtained for model pore systems, i.e., Vycor glass and clay brick.
 
Article
A number of tests are available for assessing the potential of aggregates for alkali-aggregate reaction. These are listed, for example, in ASTM C33-93. The majority of these test procedures have been widely used in researching aggregates, but there are serious reservations concerning the application of some of these tests to the selection of materials for structures. In recent years, many papers have been published detailing investigations of the tests. An examination of the literature shows that most of the tests are of little value for making engineering decisions. This paper reviews the literature relating to three commonly used tests, the ASTM C-289 Rapid Chemical Test, the ASTM C-227 Mortar Bar Test and what has become ASTM C-1260-94 Accelerated Mortar Bar Expansion Test. Attention has been given to a detailed practical study of the accelerated mortar bar tests since these are becoming very widely used. The experimental work, in essence, represents a determination of the errors to be expected from application of the test to a single aggregate and the factors that influence these errors. The repeatability of the tests suggests an upper bound standard deviation of 0.0365% expansion. This figure is used to establish discriminant criteria for the recognition of potentially damaging materials. It is evident that diagnosis of potentially highly reactive materials can be made with confidence, but that a large number of tests are required to assess marginal types. Petrographic evaluation of the mortar following the test is essential, particularly where the results are borderline, though the presence of traces of gel has to be interpreted with caution. Much of the uncertainty in the test results derives from inherent aggregate variability. A test schedule can be designed that has small contributions to the error from the experimental steps, but the error due to aggregate variability remains. This can lead to the requirement for the manufacture of multiple batches of mortar for establishing acceptance criteria for even a single aggregate batch, and further consideration is needed as to the procedures necessary for the assessment of aggregate sources.
 
Article
The mechanisms of ageing and environmental degradation involving exposure to cyclic hot/cold temperatures, wetting/drying movements as well as exposure to a carbon dioxide (CO2)-rich environment, have different effects on the microstructure of interfaces within cementitious composites. This paper presents results of an investigation into changes occurring in fibre pull-out and composite tensile behaviour in Textile Concrete (TC) after exposure to accelerated ageing conditions. The microstructure of the matrix at the fibre/matrix interface, and fibre properties, were found directly to affect the mechanical behaviour at the macro-level. The study illustrated that exposure of TC to a CO2-rich environment improves the fibre/matrix bond significantly; no major changes were observed in the mechanical behaviour of the composites after exposure to hot/cold and wetting/drying environments.
 
Article
Agricultural residues offer great promise and new challenges as replacement for wood in engineered wood products. Wheat straw, for example, offers desirable geometric and mechanical attributes for replacement of wood in cement-bonded particleboard. The inhibitory effects of wheat straw on hydration of cement, however, represent the major obstacle against development of cement-bonded strawboard. Accelerated processing techniques involving carbonation reactions were successfully employed to produce cement-based board products incorporating wheat straw. Comprehensive mechanical, durability and physical tests confirmed the high potential of cement-bonded strawboard as a versatile building product capable of meeting the requirements in demanding applications.
 
Article
For assessing the applicability of a newly proposed Chinese accelerated mortar bar test (CAMBT) to overseas aggregates and determining the appropriate aggregate size fraction for the test, the influence of aggregate particle size on ASR expansion was studied at 0.15–0.80 mm, 1.25–2.50 mm and 2.5–5.0 mm size fractions on nine aggregates from a range of sources. Correlation between expansions in the CAMBT and in the accelerated mortar bar test (AMBT), and correlations between the two accelerated tests and the Concrete Prism Test (CPT) were examined. The results indicate that, for most aggregates tested, 0.15–0.80 mm is not the most sensitive aggregate size to expansion in the CAMBT, especially at early period before 10 days. The 1.25–2.50 mm size fraction of all the nine aggregates, gives the highest early expansion (first 10 days). Correlation between expansions in the CAMBT and expansions in the AMBT is satisfactory. However, the correlations in expansions of both AMBT and CAMBT with the CPT are very poor. A better correlation between expansions in the modified CAMBT and in the CPT is obtained when 2.5–5.0 mm aggregate particles was used, but further tests are necessary to establish the full reliability of the test.
 
Article
A large number of ultra-accelerated test procedures, for determining the potential alkali reactivity of aggregates, have been developed, particularly in the past 15 years. An ultra-accelerated test method is defined as one which yields results within a few days or, at most, a few weeks. A number of ultra-accelerated test methods have been adopted as ‘standard tests’, but few have been adequately evaluated. The rapid globalization of the construction industry will require the harmonization of National Standard Test Methods. The major requirement of ultra-accelerated test methods is that they should correctly predict the potential reactivity of aggregates in greater than 95% of the cases. Due to the complexity and variability in the composition and grain size of aggregates, it is improbable that a single test method will be developed which would be appropriate for evaluating all types of aggregates. Another major requirement for ultra-accelerated test methods is that the interlaboratory coefficient of variation should be low, preferably less than 12%. At present, only the NBRI accelerated mortar bar method has been subject to adequate inter-laboratory evaluation. However, a more limited inter-laboratory investigation showed that the autoclave mortar bar test also shows considerable potential, as a satisfactory ultra-accelerated test method. Further refinement of the NBRI and autoclave methods is required to improve their performance with a wide variety of aggregates.
 
Article
A variety of additives and admixtures are added to shotcrete to improve strength, adhesiveness, cohesiveness, freezing/thawing and abrasion resistance characteristics, and reduce rebound. Accelerators are being used increasingly in both dry- and wetprocess applications. Accelerators are common in the dry process to increase early strength and reduce dust and rebound and in the wet process are used to achieve rapid set and early strength. The choice of a particular accelerator and its dosage is largely governed by the setting time required for the shotcrete application. Various watersoluble salts of the alkali metals can be used to accelerate the setting of cement. Most of the set accelerators used today are based on alkali aluminates in combination with carbonates and hydroxides and produced in both liquid and powder form. The performance of these accelerators depends on the cement chemical composition and fineness, and the presence of mineral additions such as flyash, and blastfurnace slag. This performance is generally evaluated using setting tests on cement/ accelerator pastes despite the belief of some researchers that this procedure can produce misleading results. This paper describes the main accelerators used in shotcrete and presents some results of field tests performed on shocrete panels evaluating the behavior of different accelerators. Particular attention is given to a new liquid alkalifree shotcrete accelerator that showed a very interesting behavior at very early ages and no strength loss at later ages. The early strengths were evaluated using non-destructive tests (Constant Depth Penetrometer and Constant Energy Penetrometer).
 
Article
Polymers in concrete have received considerable attention over the past 25 years. Water-soluble sulfonated acetophenone–formaldehyde (SAF) resin was produced in the laboratory from the reaction between acetophenone–formaldehyde, and sodium bisulfite. Its performance as a concrete admixture was evaluated through its effect on the (w/c) ratio, air content, setting time, compressive strength at different ages, water absorption and permeable pores. Also, the performance of the concrete when subjected to acidic environment using sulfuric acid (pH = 4), and sulphate attack using magnesium sulphate (pH = 6.5) were investigated. SAF resin could be classified as a high-range water reducer with retarding effect (Type F and G according to ASTM C494). It was found that concrete mixtures incorporating SAF resin-based admixture yielded higher compressive strength results compared with the control concrete mixtures, as well as they are more resistant to aggressive environments investigated due to the higher resistance to water movement.
 
Article
This paper describes the influence of morphologically altered cellulose fibers on the acoustic and mechanical properties of cellulose–cement composites. Three fiber morphologies were considered (macro-nodules, discrete fibers, and petite nodules). The main parameters studied include the normal incident acoustic absorption coefficient (α), specific damping capacity (ψ), loss tangent (tanδ), storage modulus (E′), and loss modulus (E′′=E′tanδ). The acoustic absorption coefficient was found to increase with an increase in fiber volume for all three fiber types investigated, though “macro-nodule” fibers were found to be the most effective. Stiffness–loss relationships are reported for these composites and the behavior of cellulose–cement composites with soft cellulose fiber inclusions was found to be similar to a Voigt (series) composite model. Low volumes of fibers had a minimal effect on the loss tangent; however the stiffness was considerably reduced. Predictive equations for loss modulus as a function of fiber volume at different moisture conditions were developed. These relations compare well with the experimental values as well as the idealized Voigt composite behavior. This suggests that there is an optimum fiber volume, which maximizes the loss modulus for saturated composites while the loss modulus is practically independent of fiber volume for dry composites.
 
Article
The possibility of employment of acrylic fibres as reinforcing material in cement paste, mortars and concretes has been studied by the authors of the present paper according to two features characterizing this kind of composite: 1.(a) Reinforcing capacity of poliacylonitrile fibres2.(b) Resistance to chemical attack of the aforementioned fibresThe present paper is the first part of a study projected on the long run; accordingly, in the future more data could be furnished in order to study more closely some specific aspects that can complement these first results. Thus for instance, in pastes of Portland cement, it has been proved that the presence of this type of fibre improves their impact resistance upto around 30–40 times whereas flexural resistances for 1 1·5% wt of addition of fibres have an increase, with respect to those obtained for pure paste, of around 50% in some cases. Simultaneously, it has been proved that the chemical resistance or attackability of acrylic fibres in high alkaline media is quite good.
 
Article
Microsilica and acrylic polymer dispersions were used in different types of fibre treatments and matrix modifications. The effects of the various treatments/modifications on the flexural properties, the failure modes and the interfacial changes after different periods of ageing were investigated. The fibre treatment was found to be more effective in controlling the interfacial changes and properties of the aged composites, compared to the matrix modification. The relative effectiveness of the different fibre treatments, however, depended greatly on the bundle size of the fibre reinforcement used.A new technique based on an unique microindentation apparatus was developed and used to carry out micro-strength testing in the fibre-matrix interfacial zone and particularly within the fibre bundle. Results suggested that a soft/flexible fibre bundle core combined with a strong bonding at the fibre-matrix interface was desirable for the optimal improvement of the long term performance of the composites.
 
Article
Pozzolans play an important role when added to Portland cement because they usually increase the mechanical strength and durability of concrete structures. The most important effects in the cementitious paste microstructure are changes in pore structure produced by the reduction in the grain size caused by the pozzolanic reactions pozzolanic effect (PE) and the obstruction of pores and voids by the action of the finer grains (physical or filler effect). Few published investigations quantify these two effects. Twelve concrete mixtures were tested in this study: one with Portland cement (control), nine mixtures with 12.5%, 25% and 50% of replacement of cement by fly ash, rice husk ash and limestone filler; two with (12.5+12.5)% and (25+25)% of fly ash and rice husk ash. All the mixtures were prepared with water/binder ratios of 0.35, 0.50, and 0.65. The compressive strength for the samples was calculated in MPa per kg of cement. The remaining contents of calcium hydroxide and combined water were also tested. The results show that the pozzolanic and physical effects have increased as the mineral addition increased in the mixture, being higher after 91 days than after 28 days. When the results for the same strength values are compared (35 and 65 MPa), it was observed that the filler effect (FE) increased more than the pozzolanic effect. The PE was stronger in the binary and ternary mixtures prepared with rice husk ash in proportions of 25% or higher.
 
Article
The freeze–thaw resistance of all cement-based materials is improved by incorporating a fine air bubble system in them. For acceptable life expectancy, incorporated air bubble volume should be about 25% of the cement paste. The specific surface of the air bubble system need to be above 25 mm²/mm³ and a spacing factor below about 0.16 mm. Powers explained these on the basis of his saturated flow hydraulic pressure mechanism. According to Powers’ mechanism, the chemical nature of the air-entraining agent has no part in this improvement in performance.
 
Article
The alkali–silica reaction in waterglass-alkali-activated slag (waterglass-AAS) and ordinary Portland cement (OPC) mortars was evaluated using three types of (siliceous and calcareous) aggregates. The tests were conducted to the ASTM C1260-94 standard test method. The mortars were studied by volume stability, mechanical strength and Hg intrusion porosity. The ASR products were studied with XRD, FTIR and SEM/EDX techniques.According to the results obtained, under the test conditions applied in this study, waterglass-AAS mortars are stronger and more resistant to alkali-aggregate reactions than OPC mortars. When the mortars were made with a reactive siliceous aggregate, expansion was four times greater in the OPC than in the AAS material. When a reactive calcareous (dolomite) aggregate was used, no expansion was detected in any of the mortars after 14 days, although the characterization results showed that the dolomite had reacted and calcareous-alkali products (brucite) had in fact formed in both mortars. These reactive processes were more intense in OPC than in AAS mortars, probably due to the absence of portlandite in the latter. When the calcareous aggregate was non-reactive, no expansions were observed in any of the mortars, although a substantial rise was recorded in the mechanical strength of AAS mortars exposed to the most aggressive conditions (1 M NaOH and 80 °C).
 
Article
Developments in the application of electrical methods in the study of the early hydration reaction kinetics of alkali-activated slag are presented. Work focuses on monitoring the change in both conductance and capacitance over the initial 24–48 h hydration with electrical measurements taken over the frequency range 1–100 kHz. In the current study, sodium hydroxide and sodium silicate are used as activators. The work highlights the frequency dependence of capacitance and, to a lesser extent conductance; in addition, over the duration of the test, both these electrical parameters undergo significant changes which can be interpreted in terms of mechanisms of hydration. It is shown that electrical methods could be exploited as a convenient technique in characterising the reactivity of these materials with alkaline activators.
 
Article
A lightweight composite material with alkali-activated aluminosilicate binder is investigated. The intended use of this material is the high-temperature applications, such as the fire-protecting layers for Portland-cement based structures. Therefore, a heat-resistant mixture of expanded vermiculite and electrical porcelain is used as aggregates. Basic physical characteristics, mechanical properties and water- and water vapor transport properties are studied as functions of previous heat treatment up to 1200 °C. Experimental results show that the studied material has very good high-temperature properties which are quite superior to Portland-cement concrete. The open porosity increases only up to 7% between room temperature and 1000 °C. The material keeps 35% of its original compressive strength and 66% of its flexural strength even in the worst case of 800 °C pre-heating. After pre-heating to 1200 °C the compressive strength is found 30% higher and flexural strength three-and-half times higher than in the reference state. Liquid moisture diffusivity is after the heat treatment up to three orders of magnitude higher than in reference room-temperature conditions. The water vapor transport parameters allow fast removal of water vapor and other gaseous compounds over the whole studied heat-treatment range.
 
Article
An experiment was performed to investigate the properties of the hardened paste of fly ash by alkali activation and to determine the possible use of the paste in the production of lightweight aggregates. The highest compressive strength was 33.9 MPa, for paste with 10% NaOH, 15% sodium silicate, and 5% MnO2, cured at room temperature after 24 h of moisture curing at 50 °C. The hardened paste of fly ash was granulated to produce AFLA (alkali-activated fly ash lightweight aggregate). AFLA exhibited specific gravity (SSD, OD), water absorption, unit weight, and solid volume percentages of 1.85 (SSD), 1.66 (OD), 11.8%, 972 kg/m3, and 58.6%, respectively. The results of the heavy metals leaching test met US EPA regulations. The concrete using AFLA exhibited a compressive strength of 26.47 MPa and good freeze–thaw resistance at 6.0% entrained air content.
 
Article
Structural evolution in pastes produced from alkali silicate-activated granulated blast furnace slag (GBFS)/metakaolin (MK) blends is assessed. In the initial period of the reaction, the addition of MK leads to an increase in the total setting time, reduces the heat release, and affects the reaction mechanism by introduction of a large quantity of additional Al. This effect is more significant when an activating solution with a higher silicate modulus is used, and leads to a slight reduction in the final mechanical strength of mortars but a significant increase in setting time, which is valuable in the development of alkali-activated slag binders as these are known to sometimes harden more rapidly than is desirable. High-energy synchrotron X-ray diffractometry reveals that the main reaction products in alkali-activated GBFS/MK blends are segregated and partially crystalline calcium silicate hydrate and aluminosilicate phases, including a small component with a zeolitic (gismondine) structure. No hydrotalcite-type phases are observed in these samples, which are synthesized from a low-Mg slag. A secondary reaction product (Na-substituted C–S–H) is also identified in pastes activated with a modulus of solution of 2.0. Infrared spectroscopy carried out over a period of 180 days shows the development of the gel structure, with aluminum incorporation leading to an increase in the extent of crosslinking, and higher alkalinity giving a more depolymerized gel structure.
 
Article
Fly ashes (FA) are byproducts of electricity production from mineral coal in thermoelectric power plants. The pozzolanic properties of FA have been utilized in various applications, including structural concrete, yet the large part of FA is still discarded into the environment. To promote greater FA usage, this study aims to produce a dense matrix, with mechanical properties satisfactory for civil engineering projects, from alkali-activated fly ash-based geopolymers. Three variables were studied: the Na2O/SiO2 molar ratio (N/S 0.20, N/S 0.30 and N/S 0.40); curing temperature in the first 24 h (50, 65 and 80 °C); and age (1, 7, 28, 91 and 180 days). For this study, alkali-activated fly ash pastes and mortars were prepared. In pastes, morphology was studied using scanning electron microscopy (SEM/EDS) and microstructural properties with X-ray Diffraction (XRD) analysis. Mortars were evaluated according to their mechanical performance measured using compression strength tests. Compression strength results were analysed using ANOVA. The results show that the N/S molar ratio plays an important role in the mechanical and morphological characteristics of geopolymers. The mortars prepared with a N/S 0.40 molar ratio had the greatest compression strength. The analysis of paste morphology revealed that N/S 0.40 pastes had a denser appearance, which is in agreement with results of compression strength tests.
 
Article
The present paper studies the corrosion behaviour of alkali-activated fly ash (AAFA) mortars manufactured using two alkaline solutions, with and without chloride additions (0.4% and 2%) in an environment of constant 95% relative humidity. Measurements were performed at early age curing up to 180 days of experimentation. A new, lower-cost type of austenitic stainless steel (SS) with a low nickel content was studied. Conventional austenitic AISI 304 SS and carbon steel rebar were also tested for comparative purposes. The evolution with time of corrosion potential, polarization resistance and electrochemical impedance spectroscopy were studied.
 
Article
Low calcium ground fly ash and metakaolin were activated with a sodium-silicate solution and cured under ambient and heat conditions. The resulting mature aluminosilicate composites were indented by several series of grids consisting of approximately 100 indents in each. The effective material volume affected by an indent was ≈1 μm3. Statistical histogram plots of elastic properties of the main reaction product (N-A-S-H gel) as well as of other material phases were constructed. The deconvolution of histograms via four Gaussian distributions testified that the mature N-A-S-H gel has almost the constant intrinsic Young’s modulus ≈17–18 GPa, irrespective of the curing procedure and activated material, i.e. fly ash or metakaolin.
 
Article
Various activation techniques, such as physical, thermal and chemical were adopted. By adopting these methods of activation, hydration of fly ash blended cement was accelerated and thereby improved the corrosion-resistance and strength of concrete. Concrete specimens prepared with 10%, 20%, 30% and 40% of activated fly ash replacement levels were evaluated for their compressive strengths at 7, 14, 28 and 90 days and the results were compared with ordinary Portland cement concrete (without fly ash). Corrosion-resistance of fly ash cement concrete was studied by using anodic polarization technique. Electrical resistivity and ultrasonic pulse velocity measurements were also carried out to understand the quality of concrete. The final evaluation was done by qualitative and quantitative estimation of corrosion for different systems. All the studies confirmed that upto a critical level of 20–30% replacement; activated fly ash cement improved both the corrosion-resistance and strength of concrete. Chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.
 
Article
Mechanically activated granulated blast furnace slag (GBFS) was used in the range of 50–95% to replace clinker in portland slag cement (PSC). The slag and clinker were activated separately using an attrition mill and mixed to prepare cement formulations. Use of activated slag resulted in a remarkable increase in strength vis-à-vis commercial slag cement. Both 1-day and 28-day strength were found to increase with an increase in slag content up to 70%. The strength of the sample containing 80–85% slag was comparable to the commercial cement used as a reference. It was observed that mechanical activation of slag was more critical from the point of view of strength development. The hydrated cement samples were characterised using powder X-ray diffraction (XRD), scanning electron microscopy with X-ray microanalysis (SEM-EDS) and simultaneous thermogravimetry and differential thermal analysis (TG/DTA). It is established that microstructural changes resulting from enhanced reactivity of slag and densification are related with the improvement in cement strength.
 
Article
Bleed tests were performed in accordance with ASTM C232-92 on concretes in which up to 85% of the cement was replaced with ground granulated blastfurnace slag (GGBS) obtained from a number of different sources. The time at which the bleed test was started was varied from 30 to 120 min to simulate site conditions. The addition of up to 55% slag increased the bleed capacity by 30% (compared to the plain Portland cement (OPC) mix) but had little effect on bleed rate. Increasing the slag content to 85% had no further significant effect on bleeding. The source of slag was also found to have little effect on the bleeding but comparisons made with results from 10 years ago suggest that now the present day slags have a much less marked effect on bleeding probably because they are ground finer. Delaying the start of the bleed test from 30 to 120 min reduced the bleed capacity of the OPC mix by more than 55% compared with 32% for the slag mixes. The reduction in bleed rate was similar for all mixes at about 45%.
 
Article
The influence of the matrix formulation and different amounts of synthetic fiber on physical and mechanical performance of asbestos free fiber cement was evaluated. Polyvinyl alcohol (PVA) fiber was tested as reinforcement in combination with mechanical and kraft cellulose pulps. Silica-fume, metakaolin and fly ash were used as pozzolanic additions in proportions up to ∼14% by mass in combination with ordinary Portland cement and carbonatic filler. Bulk densities of composites varied from 1.5 to 1.7 g/cm3. Synthetic fiber contents higher than 2% by mass (from 4% to 5% by volume of the composite) were unable to promote any further improvement in the mechanical performance of the composites at the age of 28 days. Formulations with silica fume showed better strength performance for the composites after accelerated aging test. The toughness measurements of composites after exposition to soak and dry cycles also showed that silica fume seems to prevent cellulose fiber degradation.
 
Article
This paper shows how a new powder polymer additive (PPA), containing a waterproofing agent, a rheology control agent and air-entrainers, affects the workability, mechanical properties and setting times of polymer-lightweight mortar composites (PLMC). The waterproofing agent was a mixture of redispersible polyethylene vinyl acetate and redispersible silane based polymer powder. The rheology control agent was a redispersible hydroxypropyl carboxymethyl ether of patato starch based polymer powder. Air-entraining agent was a redispersible and an unmodified sodium laurly sulphate based polymer powder. Pumice fine aggregate at 0–3 mm size fraction was used as lightweight aggregate throughout the research work. In order to examine the effects of powder polymer additive on flowability and the performance when the additive is mixed in a mortar, the mixture proportions were set in four trial batches. The volume proportions of cement and pumice lightweight fine aggregate were fixed at 1:9, 1:8, 1:7 and 1:6, respectively, defining the mixture of mortar for measuring the compressive strength and workability of lightweight mortar. In this research study, PLMC mortars with 28 different mixture proportions (M1–M28) by weight of cement contents of 0.2%, 0.4%, 0.6%, 0.8%, 1.0% and 1.2% were adopted for the mortar mixture batches, respectively. Flow value of mortar was measured using a flow table method in accordance with the regulation in ASTM C230, “flow table for use in tests of hydraulic cement”. The target flow was fixed at 130 mm for each mixture proportion, which is regarded as the most suitable fluidity to secure workability at a site. For each mixture, 12 fresh plastic mortar samples were prepared according to the method specified in ASTM C305 and cured in a humidified atmosphere for 24 h, removed from the mould after 24 h, cured in water for 7 days, and then cured in air. The compressive strength test results were evaluated in accordance with ASTM C270.
 
Article
Mortar serves as the basis for the workability properties of self-compacting concrete (SCC) and these properties could be assessed by self-compacting mortars (SCM). In fact, assessing the properties of SCM is an integral part of SCC design. The objective of this study was to evaluate the effectiveness of various mineral additives and chemical admixtures in producing SCMs. For this purpose, four mineral additives (fly ash, brick powder, limestone powder, and kaolinite), three superplasticizers (SP), and two viscosity modifying admixtures (VMA) were used. Within the scope of the experimental program, 43 mixtures of SCM were prepared keeping the amount of mixing water and total powder content (portland cement and mineral additives) constant. Workability of the fresh mortar was determined using mini V-funnel and mini slump flow tests. The setting time of the mortars, were also determined. The hardened properties that were determined included ultrasonic pulse velocity and strength determined at 28 and 56 days. It was concluded that among the mineral additives used, fly ash and limestone powder significantly increased the workability of SCMs. On the other hand, especially fly ash significantly increased the setting time of the mortars, which can, however, be eliminated through the use of ternary mixtures, such as mixing fly ash with limestone powder. The two polycarboxyl based SPs yield approximately the same workability and the melamine formaldehyde based SP was not as effective as the other two.
 
Article
This paper evaluates and compares the water retention in the fresh state and adhesion or bond strength in the hardened state of powdered and aqueous polymer-modified mortars. The polymer-modified mortars using various powdered and aqueous cement modifiers were prepared with different polymer-cement ratios, and tested for water retention in the fresh state and adhesion in tension in the hardened state. In conclusion, the powdered as well as aqueous polymer-modified mortars show markedly improved water retention and adhesion in tension, which increase with a rise in the polymer-cement ratio regardless of the type of cement modifiers used. The magnitude of improvement in the water retention and adhesion in tension of the powdered and aqueous polymer-modified mortars, however, depends upon the type of cement modifiers used, polymer-cement ratios or both. Moreover, the failure mode distribution of the powdered and aqueous polymer-modified mortars depends on the type of cement modifiers used, polymer-cement ratio, or both.
 
Article
In this paper, the ultrasonic methods were used to assess an adhesion between polymer composite and concrete substrate. The usability of indirect (surface) ultrasonic methods was evaluated on the example of commercial polymer coating. The relationships between pull-off strength and propagation of ultrasonic wave were established and analyzed. The effect of chemical composition and thickness of PC system was discussed. The results confirmed usefulness of indirect ultrasonic method for non-destructive mapping of adhesion between polymer composite and concrete substrate.
 
Article
The early age strength development of concrete is determined by a proposed maturity method, which considers not only the curing temperature history at the core of the concrete specimens, but also the relative humidity of the environment. The humidity factor is incorporated with the original rate constant model to form a new maturity function for the prediction of concrete strength development. In order to calibrate this humidity-adjusted rate constant, compression tests were conducted on concrete cylinders cured at different conditions of temperature and humidity. By comparing the ratio of rate constants of concrete cylinders cured at the same temperature but different humidities, the humidity factor could be quantified. Verification programs (including concrete cylinders cured at the programmed temperature and relative humidity, submerged in water baths and cured outdoors, and air-dried outdoors) were used to determine the applicability of the prediction model. Experimental results show that this new maturity function is able to predict the in-place strength development of ordinary concrete cylinders at early age with a maximum difference of 10%.
 
Top-cited authors
Chi Sun Poon
  • The Hong Kong Polytechnic University
Surendra P. Shah
  • University of Texas at Arlington
Nemkumar Banthia
  • University of British Columbia - Vancouver
D. P. Bentz
  • National Institute of Standards and Technology
S.C. Kou
  • The Hong Kong Polytechnic University