Title: Influence of Microcracking on the Onset and Development of Alkali Silica Reaction
Author(s): J. S. Guedon and A. Leroux
Publication: Symposium Paper
Appears on pages(s): 713-724
Keywords: alkali-aggregate reactions; concretes; microcracking; stresses; scanning electron microscope; Materials Research
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.