International Concrete Abstracts Portal

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

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.

Abrasion resistance of 10 concrete mixes with compressive strength ranging from 0.24 to 0.42 MPa was evaluated. Mixes studied contained silica fume, fly ash or natural pozzolan, and addition of superplasticizer in some cases to reduce mixing water. Concrete workability remained constant. Tests were carried out following a Portuguese standard similar to a Brazilian standard and German Standard DIN 52108, using the Dorry apparatus. Porosity and compressive strength of concrete were also determined. The main conclusions are as follows: 1) cement replacement by mineral admixtures always reduced the abrasion resistance at rates between 10 and 25 percent, while less satisfactory results were obtained with condensed silica fume concretes; 2) addition of superplasticizer increased the abrasion resistance about 25 percent; 3) abrasion resistance varied inversely with water-cement ratio, cement paste volume, and concrete porosity; 4) general correlation was poor between abrasion resistance and compressive strength, indicating a strong influence from cementing material type, mainly in the case of silica fume; 5) there was evidence that poor performance of condensed silica fume concrete can be ascribed to self-desiccation; 6) even the worst results obtained in this test series were equivalent to abrasion resistance at least six times higher than that of ordinary concrete with 20 MPa compressive strength.

In 1980, dead-burned dolomite particles removed from a cement kiln were inadvertently distributed in aggregates that were later used in concrete. These particles were of coarse aggregate size (38 mm) and contained approximately 55 percent calcium oxide (CaO) and 35 percent magnesium oxide (MgO). When this contaminated aggregate was used to make concrete in 1980, it caused some relatively large popouts (up to possibly 230-mm-diameter). Subsequent periodic visual evaluations of this contaminated concrete were performed to verify the acceptability of the concrete and the durability of popout repairs. To the authors' knowledge, only one structure was removed and repaired. In 1989, another such incidence occurred, but this time the portland cement was contaminated with smaller (<9.5-mm) dead-burned dolomite particles with approximately the same proportions of CaO and MgO. Paper reports on how data developed from the 1980 incident was extended for use in evaluating the concrete contaminated in 1989, and how instrumentation was used to effectively determine the actual volume of dead-burned dolomite in the contaminated concrete and degree of hydration of the particles. Such information is being used to predict the long-term effects of the contamination.