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 object of this publication is to determine the relationship between the cracking in loaded reinforced concrete and the corrosion of embedded steel. Paper deals with the synthesis carried out for 7 years on reinforced concrete elements kept in a loading state, in a confined salt fog. The test specimens are 3-m-long beams, which is a sufficient size to be representative of the actual operating conditions of the reinforced concrete structures. Steel electrode potential measurements are performed on all beams kept in a salt fog environment to evaluate the state of the steel corrosion by a nondestructive way. The development of damage in concrete specimens is monitored by means of a scanning electronic microscope. The bending of the beams leads to the development of cracks that are neither preceded nor accompanied by microcracks, but the cement paste-aggregate interfaces are damaged in the tensile areas. On the other hand, the corrosion of steel leads to a secondary cracking that is branched out. Chloride penetration in concrete is obtained by measuring out free chlorides and bound chlorides in different samples extracted from different locations of the beams. Then, several parameters are studied, such as the effect of the location of the beam in the storage enclosure, the loading state of the beam area, and the intensity of the mechanical stress applied. Moreover, a relation is established between the rate of free chlorides measured and the embedded steel corrosion. The rate of corrosion is influenced by the concrete cover thickness even in the presence of cracks.

Current manufacturing practice used in the fabrication of concrete pavers produces a final product that has high density and strength, low permeability, and inadequate pore structure. Although high strength and low permeability should keep the pore structure from becoming critically saturated, lack of sufficient amount of air (entrapped and/or entrained) makes paving blocks potentially vulnerable to freezing and thawing damage. Presents experimental results relevant to the freezing and thawing performance of various concrete paving blocks using a standard accelerated laboratory test, ASTM C 666, Procedure A. The parameters investigated included bulk properties and weight loss. Test results indicate that ASTM C 666 can be successfully utilized to evaluate the relative performance of various block pavers. Weight loss correlates well with the cement content (aggregate-cement ratio) of the matrix. Minimum cement content, instead of minimum strength or maximum absorption, is the most suitable requirement for insuring a reasonable freezing and thawing resistance for concrete block pavers.

In 1989, a field inspection of concrete structures in selected geological areas showed that nearly all known structures with alkali-aggregate reaction (AAR) were situated within or close to areas with known potentially reactive bedrock of metarhyolite, metasandstone, metagreywacke, and phyllite. Such areas may, therefore, be interpreted as areas with a high risk of AAR, when local aggregate deposits and Norwegian high-alkali cements are used. To get an overview of the extent of AAR in the whole of Southern Norway, a survey inspection of randomly distributed road bridges and dams older than 10 years was carried out during the summer of 1990. The decision was made to use map cracking as an indicator for AAR and to measure maximum crack width and estimate area percent of map cracking. The data from the inspections of 468 structures have been put into a database (as well as other available information, e.g., laboratory analyses results). A distribution map plot of inspected structures has been constructed and used to locate structures into two types of geological areas, namely, structures located in potentially reactive bedrock areas, and structures located in supposed innocuous bedrock areas. Data processing shows that map cracking is common in structures in Southern Norway and is more frequent in potentially reactive bedrock areas. In potentially reactive bedrock areas, average maximum crack widths and area percent of map cracking is larger than in supposed innocuous bedrock areas. A surprising peak height of increased maximum crack width has been revealed to occur in structural elements built around 1950 to 1960. This peak height is most significant in structures situated in supposed innocuous bedrock areas. The cause of this increased cracking in structures built around the 1960s is unknown but could be caused by a very high alkali content in Norwegian cement at that time.