Autogenous healing of engineered cementitious composites at early age

Autogenous healing of early ages (3 days) ECC damaged by tensile preloading was investigated after exposure to different conditioning regimes: water/air cycles, water/high temperature air cycles, 90%RH/air cycles, and submersion in water. Resonant frequency measurements and uniaxial tensile tests were used to assess the rate and extent of self-healing. The test results show that ECC, tailored for high tensile ductility up to several percent and with self-controlled crack width below 60 μm, experiences autogenous healing under environmental exposures in the presence of water. However, the recovery for these early age specimens is not as efficient as the recovery for more mature specimen, for the same amount of pre-damage and exposure to the same environment. Even so, the self-healing for these early age specimens demonstrates high robustness when the preloading strain is limited to 0.3%. This conclusion is supported by the evidence of resonant frequency and stiffness recovery of the healed ECC materials.     

Macrocell and microcell corrosion of steel in ordinary Portland cement and high performance concretes

Microcell corrosion is the term given to the situation where active corrosion and the corresponding cathodic half-cell reaction take place at adjacent parts of the same metal. Macrocell corrosion can occur when the actively corroding bar is coupled to another bar which is passive, either because of its different composition or because of different environment. The present study was undertaken to determine the influence of concrete type and properties on the relative microcell and macrocell corrosion rates. The samples were monitored for more than 3 years and the results confirm that microcell corrosion is the major mechanism in corrosion of steel reinforcing bars in concrete. Furthermore, results show that, for high performance concrete, the difference between microcell and macrocell corrosion is far more significant than for ordinary Portland cement concrete because of its high resistance to ionic flow.     

Durable fiber reinforced self-compacting concrete

In order to produce thin precast elements, a self-compacting concrete was prepared. When manufacturing these elements, homogenously dispersed steel fibers instead of ordinary steel-reinforcing mesh were added to the concrete mixture at a dosage of 10% by mass of cement. An adequate concrete strength class was achieved with a water to cement ratio of 0.40. Compression and flexure tests were carried out to assess the safety of these thin concrete elements. Moreover, serviceability aspects were taken into consideration. Firstly, drying shrinkage tests were carried out in order to evaluate the contribution of steel fibers in counteracting the high concrete strains due to a low aggregate-cement ratio. Secondly, the resistance to freezing and thawing cycles was investigated on concrete specimens in some cases superficially treated with a hydrophobic agent. Lastly, both carbonation and chloride penetration tests were carried out to assess durability behavior of this concrete mixture.     

Strength, permeability, and carbonation of high-performance concrete

This investigation is aimed at developing high-performance concrete and form part of an investigation into the optimization of a blended cementitious system for the development of high-performance concrete. Binary and ternary blended cementitious systems based on ordinary Portland cement (OPC), pulverised fuel ash (PFA) and silica fume (SF) were investigated. PFA up to 40% was used, and to these blends, 0%, 5%, 10% and 15% SF were incorporated as partial cement replacements. Results of compressive strength, tensile strength, oxygen permeability and carbonation of concrete are reported. A water-binder (w/b) ratio of 0.27 was used for the main group of mixes and w/b ratios of 0.40 and 0.50 were used for some selected mixes. Based on the experimentally obtained results, prediction models were developed which enabled the establishment of isoresponse contours showing the interaction between the various parameters investigated. It was found that the incorporation of 8-12% SF as cement replacement yielded the optimum strength and permeability values.     

Microstructural and chemical effects of wet/dry cycling on pulp fiber-cement composites

The microstructural and chemical mechanisms responsible for pulp fiber-cement composite degradation during wet/dry cycling are being investigated through environmental scanning electron microscopy (ESEM), energy dispersive spectroscopy (EDS), and mechanical testing. Based on these results, a three-part progressive degradation mechanism for cast-in-place kraft pulp fiber-cement composites is proposed, which involves: (1) initial fiber-cement or fiber interlayer debonding, (2) reprecipitation of needle-like or sheath-like ettringite within the void space at the former fiber-cement interface or between the S1 and S2 fiber layers, and (3) fiber mineralization due to reprecipitation of calcium hydroxide filling the spaces within the fiber cell wall structure. This investigation also revealed that kraft pulp fibers exhibit poor resistance to degradation due to their inferior dimensional stability, as compared to thermomechanical pulp (TMP) fibers. TMP fibers contain significant amounts of lignin, which is alkali sensitive. Despite this, TMP fiber-cement composite exhibit improved resistance to degradation during wet/dry cycling. It is proposed that this improvement in durability may be attributed to the presence of lignin in the cell wall restricting fiber dimensional changes during wetting and drying, and hence, minimizing fiber-cement debonding. Additionally, it is proposed that lignin acts as physical barrier to calcium hydroxide formation within the fiber cell wall, minimizing fiber mineralization of TMP fibers.     

Transport properties of water and glycol in an ultra high performance fiber reinforced concrete (UHPFRC) under high tensile deformation

Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability. Those characteristics make them very attractive for the rehabilitation of existing structures and the conception of new structures. To define the range of admissible tensile deformation in those materials, the influence of imposed tensile deformation and subsequent cracking on permeability and absorption was studied. The transport properties of water and glycol were assessed in order to estimate the effect of the interaction of water with a specific UHPFRC. The experimental results demonstrate that permeability and absorption increase steadily until a residual tensile deformation of 0.13% is reached in the material, then water seeping rises distinctly. During experiments, the interaction of water with the UHPFRC decreases by 1 to 3 orders of magnitude the permeability and reduces absorption by approximately 50 to 85%. Test results reveal the high capability of the material to seal cracks and improve its water-tightness with time.     

Induced anisotropic permeability due to drying of concrete

Structural strength, porous space, and permeability of concrete are strongly affected by mechanical, hydrous, and thermal loading. These various loadings may lead to drying shrinkage, one of the main characteristics of this type of material, which has to be involved in the behaviour modelling and experimental investigations being the subjects of this paper. Experimental devices and principal parameters studied are first presented. Drying shrinkage and loss of mass in time were measured on prismatic samples while uniaxial compression tests were performed on cylindrical samples. Gas permeability tests, carried out on a concrete cylinder 30 mm in diameter, form the second part of this study. The samples used for these measurements were cored from each prismatic sample at the end of 10 months or 2 years of drying, either from the transverse direction of sample (privileged direction of drying) or from the longitudinal direction. Gas permeability procedure, using micropulse test technique, is described as well as the experimental process. Experimental results are finally commented on and discussed with a view on induced anisotropy due to desiccation. Such an anisotropy is clearly observable in permeability, which is also increasing with drying time.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

Strength properties of nylon- and polypropylene-fiber-reinforced concretes

The strength potential of nylon-fiber-reinforced concrete was investigated versus that of the polypropylene-fiber-reinforced concrete, at a fiber content of 0.6 kg/m³. The compressive and splitting tensile strengths and modulus of rupture (MOR) of the nylon fiber concrete improved by 6.3%, 6.7%, and 4.3%, respectively, over those of the polypropylene fiber concrete. On the impact resistance, the first-crack and failure strengths and the percentage increase in the postfirst-crack blows improved more for the nylon fiber concrete than for its polypropylene counterpart. In addition, the shrinkage crack reduction potential also improved more for the nylon-fiber-reinforced mortar. The above-listed improvements stemmed from the nylon fibers registering a higher tensile strength and possibly due to its better distribution in concrete.     

Compared imbibitions of ordinary and high performance concrete with null or positive water pressure head

A method to simultaneously measure the moisture diffusion coefficient, D_θ, of unsaturated concrete, and the saturated concrete hydraulic conductivity, K_l, was developed for cylindrical specimens placed on a container filled with water that could be maintained at a given hydraulic pressure. Ordinary Portland cement Concrete (OPC) with a moderate and High Performance Concrete (HPC) with a low water to cement ratio were tested. The time dependent distribution of water content in the specimens was measured using a non-intrusive method based on gamma-ray attenuation. The measurements were conducted with varying hydraulic head (positive or null). Boltzmann's transformation was used to analyze the experimental results obtained at different hydraulic pressures and the difference between the null (or atmospheric) and positive pressure results is used to accurately determine K_l and also D_θ . This paper will present the results obtained using this original method, possible interpretations and future research.     

Properties of concretes produced with waste concrete aggregate

An environmentally friendly approach to the disposal of waste materials, a difficult issue to cope with in today's world, would only be possible through a useful recycling process. For this reason, we suggest that clearing the debris from destroyed buildings in such a way as to obtain waste concrete aggregates (WCA) to be reused in concrete production could well be a partial solution to environmental pollution. For this study, the physical and mechanical properties along with their freeze-thaw durability of concrete produced with WCAs were investigated and test results presented. While experimenting with fresh and hardened concrete, mixtures containing recycled concrete aggregates in amounts of 30%, 50%, 70%, and 100% were prepared. Afterward, these mixtures underwent freeze-thaw cycles. As a result, we found out that C16-quality concrete could be produced using less then 30% C14-quality WCA. Moreover, it was observed that the unit weight, workability, and durability of the concretes produced through WCA decreased in inverse proportion to their endurance for freeze-thaw cycle.     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

A comparison of bending strength between adhesive and steel reinforced concrete with steel only reinforced concrete

Cracking of brittle cementitious composites subjected to excessive loading causes a potential reduction in material performance. Steel bars or metal fibers typically act as tensile reinforcing in concrete composites to increase the material's structural capacity in bending and to delay or prevent matrix cracking.The goal of this research is to determine whether the performance in bending strength and material integrity of a typically reinforced cementitious composite may be improved through the release of “healing” chemicals, such as adhesives, from hollow fibers into cracks induced by loading in addition to the metal reinforcing. Adhesive-filled repair fibers are intended to break immediately upon cracking in the concrete thereby activating the healing process with the release of a sealing or adhering substance. This self-repair occurs whenever and wherever cracks are generated.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

Modelling the pressure dependence and the influence of added polymeric fibers on the permeability of refractory concretes

In order to facilitate the water evacuation during drying, polymeric fibers are added to refractory concretes. A permeability model using Bruggeman's approach is developed to predict the permeability increase due to the fibers addition. This model (without adjustable parameters) knowing the fibers geometry (given by the manufacturer) and added amount (mass fraction varying between 0 and 0.20%) is validated by a thorough comparison with experimental results. The refractory permeability is measured at ambient temperature varying the quantity of added organic fibers after the samples were fired at different temperatures (from 80 °C to 500 °C). The analysis of results is especially careful to take into account the influence on the permeability, on both pressure (Klinkenberg effect) and firing temperature. The agreement between theoretical and experimental results is shown to be very satisfactory.     

Correlation of hollow and solid cylinder dynamic pressurization tests for measuring permeability

An experimental apparatus and analytical derivation are developed for quantifying the permeability of cementitious materials using dynamic pressurization of a hollow cylinder. Experimental results from the newly developed hollow dynamic pressurization technique for measuring permeability are then compared to results obtained using the solid dynamic pressurization test. The measured permeabilities obtained from testing Vycor^® glass and hardened cement paste indicate close agreement between the two test methods, which supports the validation of the hollow dynamic pressurization test as an accurate and repeatable method for measuring the permeability of cementitious materials. Three different pore fluids with widely varying viscosities were tested, each yielding equivalent intrinsic permeabilities. Additionally, the permeability values from this study agree reasonably well with relevant values presented in the recent literature.     

Studies on expansion properties in mortar containing waste glass and fibers

The utilization of waste glass in concrete can cause cracking and weakening due to expansion by alkali-silica reaction (ASR). In this study, ASR expansion and properties of strength were analyzed in terms of waste glass content, glass color (brown, green), fibers (steel fiber, polypropylene fiber) and fiber content, in anticipation of reducing ASR expansion.Results showed that green waste glass was more usable than brown because its expansion was less than that of brown glass. Using the accelerated ASTM C 1260 test of waste glass, no pessimum content was found. Furthermore, when fibers and waste glass were combined, there was an effect on the reduction of expansion and strength loss due to ASR between the alkali in the cement paste and the silica in the waste glass. In particular, adding 1.5 vol.% of steel fiber to concrete containing 20% waste glass reduced the expansion ratio by 40% and increased flexural strength by up to 110%, a vast improvement when compared with using only waste glass (80 °C H₂O curing) by itself.     

Interaction between loading, freeze-thaw cycles, and chloride salt attack of concrete with and without steel fiber reinforcement

In this paper, the deterioration of concrete subjected to the combined action of four-point bending—loading, freeze-thaw cycles, and chloride salt attack—is discussed. Test results show that concrete tested in chloride salt solution scaled much more severely than in fresh water, and its weight loss in chloride salt solution was twice that in water. However, dynamic modulus of elasticity (DME) of concrete in chloride salt solution dropped more slowly than that in water due to supercooling resulting from chloride salt. It is also shown that the degradation process of concrete simultaneously exposed to loading, freeze-thaw cycles, and chloride salt attack was significantly accelerated. The higher the stress ratio exerted, the lesser the freeze-thaw cycles that concrete could resist and, consequently, the shorter the service life. When a relatively high steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the three damaging processes is considerably reduced.     

Study of the Joule effect on rapid chloride permeability values and evaluation of related electrical properties of concretes

Concrete specimens were tested at different temperatures using the ASTM C1202 rapid chloride permeability test (RCPT). The influence of temperature on the RCPT results was analyzed and activation energies were calculated. An Arrhenius equation was presented to account for the Joule effect due to heating. Electrical properties of concrete derived from the test were evaluated and correlations were explained based on the effect of temperature on the kinetics of the pore fluid and on the microstructure of concrete. The results support the modification of ASTM C1202 by changing it to a 1-min conductivity test.