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Alkali-silica reaction is responsible for concrete cracking, but when initial microcracking is present, does it influence the reaction and, if so, how? This was the problem the authors tried to solve through the following experiments. Four sets of 7 x 7 x 28-cm test concrete bars were prepared with a potentially reactive aggregate. One set was kept as a control, while two others were mechanically microcracked by applying stresses corresponding to 75 and 100 percent of the breaking stress. The fourth set was used to determine the minimum stress that could be applied to the bars. The resulting microcracking was estimated by measuring the ultrasonic wave velocity and by scanning electron microscopy. The evolution of the disorders was tracked by measurement of dimensional variations. The bars were cured at 38 C (100 F) with a moisture content of 100 percent in accordance with standard testing procedure. After 2 years of observation, the authors noted the following developments. The original microcracking had significantly increased the speed of the material's response to the alkali reaction; at the same time, the number of disorders that were consequences of the reaction seemed noticeably higher. Also, cyclic behavior was evident, which induced a dormant stage corresponding to the filling of the microcracking by the reaction gel, and also induced an active stage leading to additional microcracking. Such a sequence of dormant and active stages should affect all the bars tested, but was actually totally evident only on the bars that were initially subjected to significant cracking. This study clearly shows the important role played by initial microcracking on the future of concrete and, consequently, the choice and implementation of solutions that could reduce concrete disorders.

Although aggregates in the concrete matrix are regarded primarily as inert, certain aggregates have been identified as deleterious due to their chemical reactivity in an alkaline environment. Despite extensive research on the various aspects of this problem, a rational model that comprehensively explains the rate of the chemical reaction and resulting expansion has not yet been presented. Paper deals primarily with modeling of the chemical reactions and ensuing expansion in the case of alkali-silica reaction. The chemical reaction phase has been assumed to be governed by the rate of diffusion of hydroxide and alkali ions into the aggregate. The model also assumes the existence of a porous zone around the aggregate and that expansion is initiated only after the amount of reaction products exceeds the volume of this porous zone. An attempt has also been made to discuss some experimental results in the light of the proposed model and provide some of the analytical results arrived at using the model. It was found that by carrying out a slightly modified version of the quick chemical test, the apparent diffusion coefficients of the hydroxide ions can be estimated and the results can be used to accurately estimate the expansion ensuing during the mortar bar tests. Analytical results also indicate that certain characteristic features of alkali-aggregate reaction-related expansion, such as the existence of an incubation period before the onset of expansion, varying rates of expansion, and the shapes of the expansion-time curves, can be explained using the model proposed by the authors.

Inspection and service reports from concrete platforms up to 20 years old in the hostile environment of the North Sea are positive. The need for remedial work has been minimum and these high-quality concretes demonstrate excellent performance under marine conditions. To meet the demands for durability and development in structural design and construction methods, a continuous effort has been made to advance concrete materials. Improved concrete properties are predominantly a consequence of improved cement qualities, more efficient admixtures, and better controlled processing of aggregates. The soundness of the aggregates has been verified from the start. Examinations of concrete specimens drilled out from different elevations of some of the platforms have revealed rather high concentrations of chlorides close to the concrete surface. For most of the specimens, the chloride content is, however, negligible at the reinforcement. Epoxy coating applied to the concrete surface in the splash zone of some platforms has shown to be efficient in preventing ingress of chlorides.

Despite the excellent resistance of high-performance (HP) concretes in the presence of aggressive agents, instances of application have shown that the microstructure of the concrete surface can be greatly disturbed by the curing method, thereby compromising durability on the part covering the reinforcement. Paper presents results of a study on three concrete design mixes (one reference concrete and two HP concretes with and without silica fume), each subjected to three curing methods and three durability tests. Results on carbonation, variation in free lime, and microcracking indicate that HP concrete with silica fume is more sensitive to the curing method than the reference concrete or concrete without silica fume, as evidenced by increased carbonation and a larger reduction in alkalinity. The study of microcracking in the various concretes showed that desiccation causes more microcracking in the HP concrete with silica fume than in the HP concrete without silica fume. Results of microstructural inspection and physical and chemical tests explain these variations in mechanical properties and carbonation behavior of various concretes, depending on the curing method.

Investigates the effect of high temperatures (70 to 600 C) on the residual strength of ordinary and high-performance concretes made with the same cement and aggregates. Measurements of weight losses and residual strengths were carried out. Between 25 and 600 C, the mass loss was about 8 percent of the wet concrete weight. The tests showed that after being exposed to a temperature of 600 C and then cooled, the concrete retains 38 to 46 percent of its initial compressive strength. Experimental results indicated further that mercury porosimetry measurements were suitable for obtaining information about microstructural changes resulting from thermal exposure. The distribution function of the pore system indicated that no remarkable changes had taken place in its shape and location up to 120 C. The residual porosity increased with temperature, particularly after 300 C, and the pore size distribution was significantly modified. Approximately one-third of the high-strength concrete samples failed through explosion at about 300 C. With the results obtained, the authors were able to analyze the phenomenological aspects susceptible to explain the observed behavior. This behavior might be caused by a tension in the solid microstructure produced by thermal stresses and by the pore vapor pressure.