International Concrete Abstracts Portal

For the calculation of civil engineering structures, designers employ the mechanical aspect underestimating the physicochemical phenomena in connection with the hydration of cement paste. Although the mechanical approach is widely sufficient for classical structures, this is not the case for large structures like dams, where thermophysical phenomena play a leading part. After a short analysis of the degradation observed on a roller compacted concrete dam, showing the importance of the control of hydration effects on mass concrete, the authors present a thermomechanical model able to describe the main evolutions of concrete properties with aging. Application to the Riou dam shows the ability of the approach to simulate temperature, strains, and stresses and, as a consequence, the risk of damage for the structure. Cracks in the middle of the dam are properly represented. This approach permits determination of the position and number of construction joints and setting the schedule of construction as thickness of concrete layers or maximum delay between two layers.

Reinforcement corrosion due to chloride ingress is the most common cause of concrete deterioration in Norway. A wharf with dimensions of 270 x 25 m was built in 1965 to 1966 and required partial repairs in 1980, 1986, and 1989 to 1990. The repair work included some research and development. The conclusion of the 1989 inspection was that no corrosion activity was evident in the earlier repaired areas. Repair mortar with silica fume had somewhat lower chloride ingress and significantly higher electrical resistivity than mortar without silica fume. Latex addition to the repair mortar showed the same effect, as well as a reduced water content. The main conclusion is that materials and working procedures used for the 1980 repair have resulted in a satisfactory service life of at least 10 years.

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.

Under the Strategic Highway Research Program (SHRP), Contract C-206, "Optimization of Highway Concrete Technology," constructibility and performance of concrete for early opening of highway repairs were evaluated. A variety of concretes mixed using different types of rapid strength cements and admixtures were used for full-depth repair (slab replacement) of concrete pavements and for bridge deck overlays in the states of Ohio, Kentucky, and Georgia. For pavement applications, eight mixtures with different strength-gain capacities allowing for a variety of traffic opening times ranging from 2 to 24 hr were evaluated. Latex-modified concrete with Type III cement and silica fume mixes were used for bridge deck overlays. Durability evaluation of these mixtures included freeze-thaw resistance, characterization of the air-void system and deicer scaling tests, and measurement of chloride permeability. Specimens for these tests were prepared in the field and were subject to standard field curing. Tests were also conducted on cores taken from pavements and overlays at opening time. Freeze-thaw tests on beams were conducted following a modified procedure of ASTM Method C 666B, using specimens wrapped in towels during the air freeze to reduce drying from the surface during the freeze cycle. Follow-up surveys were conducted to examine the performance of these concretes under the effects of environmental exposure and traffic loading. Test results showed that overlay mixes have excellent freeze-thaw resistance. Latex-modified concrete mixes showed moderate scaling using the deicer scaling test. Chloride permeability of cores taken from silica fume overlays were lower than those of latex-modified concrete overlays. Poor freeze-thaw performance of many of the pavement repair mixes indicates that many questions still remain regarding durability of concretes designed for early opening applications. Proper air content and adequate air-void systems are necessary, but not sufficient, conditions for obtaining the desired freeze-thaw durability. Microcracking in the concretes may account for some of the poor performance in freeze-thaw testing. The use of calcium chloride should be avoided, as it contributes to reduced freeze-thaw resistance.