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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.

SP-145 In 1994, The Canada Centre for Mineral and Energy Technology (CANMET) in association with the America Concrete Institute, sponsored a third international conference on the Durability of Concrete This Special ACI publication presents the 65 conference papers accepter for publication. For Your Convenience, Durability of Concrete has been divided into two parts. Part 1, which contains 34 papers, covers the areas of: 1. Deicer Salt Scaling of Concrete 2. Freezing and Thawing Phenomenon 3. Performance of Concrete in Marine Environments 4. Corrosion of Steel to Fluoride-Ion Attach 5. And other Topics Part 2, containing 31 papers, covers the areas of: 1. Alkali-Aggregate Reactivity 2. Coatings for Concrete 3. Carbonation 4. High-Volume Fly Ash Concrete 5. Durability of Concrete

The transport of ions related to the penetration of chlorides into concrete has been studied in the field by drilling 100-mm concrete cores from a marine bridge column. A 4-year-old concrete column in Sweden was selected. The concrete was of high quality (i.e., frost- and sulfate-resistant, with a low-heat, low-alkali portland cement with a maximum water-cement ratio of 0.40) according to new Swedish recommendations. Concrete cores were drilled from the submerged, splash, and atmospheric zones. Selective rinding from the concrete surface (profile grinding) revealed concentration profiles of acid-soluble chlorides, carbonates, sulfates, and water-soluble alkalies. Selected parts of the concrete surface were examined by SEM and thin-section microscopy for microstructural studies. Laboratory estimates of chloride diffusivities were carried out on 6-month-old laboratory concrete of similar mix proportions, and also on unexposed parts of drilled concrete cores. Chloride diffusivities obtained from laboratory exposure were then compared with the values obtained from the field concentration profiles, from both the bridge column and a field station, using Fick's second law of diffusion. Maximum chloride diffusivities calculated from the field profiles after 4 years of exposure were more than ten times lower than those obtained from the same concrete in the laboratory. Clearly, there are important mechanistic problems associated with laboratory procedures, resulting in serious misjudgments, if such laboratory tests are used for linear extrapolation of the service life for marine concretes.

The repair and rehabilitation of bridge decks in the western countries and reinforced concrete structures in the countries along the Arabian Gulf is a major challenge to civil engineers. The need for repair of these structures results from concrete deterioration caused mainly by reinforcement corrosion. The use of deicer salts in North America and Europe accelerates reinforcement corrosion in bridge decks. Aggressive environmental conditions in the Arabian Gulf are responsible for deterioration of concrete structures in this area. This investigation was carried out to evaluate the durability performance of various repair materials. The repair materials were exposed to thermal variations to evaluate their performance in arid environments, such as in the Arabian Gulf. Durability performance was evaluated by measuring water and chloride permeability, and resistance to reinforcement corrosion. Results indicate that the water permeability in all the repair materials was less than that in plain concrete. Water permeability was significantly increased in all the specimens that were subjected to thermal variations, compared to those cured in the laboratory temperature. Ordinary cement mortar specimens indicated higher chloride permeability and lower corrosion resistance than other repair materials and plain concrete, which could be attributed to its lower electrical resistivity in saturated condition.