International Concrete Abstracts Portal

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.

The long-term durability of a number of special types of concretes has been tested by exposing various large-scale concrete specimens to different outdoor conditions. The concretes tested include concretes made with different types of cement and incorporating various quantities of fly ash and silica fume (microsilica) as well as mixtures thereof. In one test series, full-scale precast concrete units were installed as functional members of a fish-ladder. The units, comprising concrete with up to 50 percent of silica fume (related to the cement content), are subject to the action of lake water and freezing and thawing. They have been in service for nearly 15 years, and the results demonstrate the excellent durability of concrete with silica fume. In another test series, concrete panels and slabs are exposed to Danish outdoor climates (freezing and thawing during winters) in connection with frequent use of deicing salts. The test series also comprises large-size panels, installed in a harbor at the west coast of Denmark. For this test series, 10-year results are now available.

Corrosion of reinforcing steel is a major cause of concrete deterioration and, consequently, of loss of serviceability of concrete structures. Presents the results of a laboratory investigation to assess the effects of fly ash on the resistivity and rate of corrosion of reinforcing steel in concrete. Environmental exposure conditions were simulated in the laboratory, and corrosion tests were carried out on specimens corroded naturally or under accelerated conditions. Results show that fly ash is a very effective addition to improve the resistivity of concrete and to reduce the rate of corrosion of reinforcing steel. The resistivity of fly ash concrete is approximately double that of the resistivity of an equivalent normal portland cement concrete. Results are used to propose a model relating resistivity, porosity, and permeability of concrete with the rate of corrosion of reinforcing steel.

In Australia, the Cement and Concrete Association has sponsored a number of research projects addressing aspects relating to deterioration of concrete structures caused by corrosion of the reinforcement. The overall objectives of these projects were to identify the factors influencing steel corrosion and to quantify their effect on initiating corrosion and on the rate of corrosion. The ultimate objective was to provide practicing engineers with the relevant parameters that can be used in the design and specification of concrete structures to minimize the risk of corrosion of reinforcing steel. Reviews the major corrosion research carried out in Australia. Article attempts to correlate research findings to the conditions in practice and to quantitatively predict design life of reinforced concrete structures in an environment simulating severe exposure conditions in Australia. The design life predictions presented should be considered within the context of the assumptions and approximations made. Data presented in this paper showed that the influence of binder type is more in the medium-strength concretes in terms of time to potential jump (initiation) and corrosion rate (propagation). Therefore, it is recommended to optimize concrete mixture proportioning with respect to binder type in this range of concrete strengths to utilize the benefits possible from different binders.