International Concrete Abstracts Portal

The mechanisms underlying physical disintegration of concrete by crystallization of mirabilite (Na 2SO 4 10H 2O) and thenardite (Na 2SO 4) were studied by a series of laboratory experiments. In contrast to chemical sulfate attack, which manifests itself in the formation of gypsum or ettringite, the deterioration investigated in this study did not involve chemical attack on the cement paste in concrete. Rather, the damage was strictly of a physical nature, caused by phase changes within the sodium sulfate-water system. Several possible mechanisms of distress were investigated, including pressure caused by hydration, evaporation, and temperature effects. Rapid temperature changes were found to be the dominant mechanism of deterioration. In particular, rapid decreases in temperature resulted in supersaturation, rapid crystallization, and a net increase in the sodium sulfate-water system. Consequently, significant hydraulic pressure, similar to that observed in the classical freeze-thaw phenomenon, would develop if drainage conditions within the concrete were not adequate to allow for the volume increase of the sodium sulfate-water system.

In this study, structural-grade concretes with characteristic strength of 20 to 45 MPa were made with general purpose portland cement (ASTM Type I) and fly ash blends. High volumes of fly ash (ASTM Class F) in the range of 40 to 50 percent by weight of total binder were used. It was found that for an equivalent 28-day strength and slump, structural concretes with high-volume fly ash can provide a number of advantages over plain cement concretes, including lower drying shrinkage and better creep characteristics. Similar flexural strengths and elastic modulus were observed between equivalent plain cement and high-volume fly ash concretes. Experience obtained in field trials of high-volume fly ash concretes showed that they can be mixed, transported, placed, and finished using conventional concreting equipment and techniques. Laboratory studies of blended cements with high percentages of fly ash as cement replacement material indicated that steel passivation characteristics improved with age of hydration and that there was no negative effect caused by pozzolanic reaction. Electrochemical data using polarization resistance techniques on paste samples immersed in NaCl solution are given. The results indicated that, even with limited initial curing of 7 days, the corrosion rates of steel in 40 percent fly ash blend by weight were very similar to that of plain cement at high water-to-binder ratio (>0.6) and were lower than that of plain cement at low water-to-binder ratio ( 0.6). Data obtained from mortar samples subjected to sulfate environments suggested that the use of blended cements with high fly ash replacement could be beneficial in the case where the pH of the environment is low, such as that experienced by concrete structures in sewerage works.

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.

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.