International Concrete Abstracts Portal

Heat of hydration of cement and resulting thermal gradient has a great influence on the quality of concrete and concrete structures particularly in the mass concrete for dams. In roller compacted concrete (RCC) method for dam construction, surface of concrete layers are very large in comparison with thickness of the layers. Therefore the thermal condition in the center of layers is almost adiabatic in horizontal direction. t means that, the generated heat of hydration mostly flows in the vertical direction and a great proportion of heat dissipates through the upper face of the layer before the next layer is placed. Low thermal conductivity of concrete layers has a great influence on the dissipation of generated heat. Thermal gradient induced by generated and remaining heat in the layers can cause thermal cracking in RCC dams which have no post-cooling system. In this investigation a laboratory model is set up to optimize the layer thickness and required time for dissipating of generated heat and controlling thermal cracking. The laboratory model consists of a 90x90 cm cylinder filled with 3 layers of concrete with 30cm thickness each layer. In this simulation the variables were the thickness of layers and the exposure time for each before placing the subsequent layer. Temperature variations were recorded at the center of each layer and at different distances from the center of the laboratory model. From the results of this research, the thickness of the layers and their related exposure time were determined for various concrete mixtures in order to minimize the heat problem and thermal crack prevention. The proposed guide for placing concrete in RCC dams seems to be beneficial for the construction of such dams under various conditions.

A beneficial use of carbonation technology to sequester exhaust CO2 in concrete through accelerated curing was studied. The carbonation took place in a chamber under 500 kPa pressure, at ambient temperature, for two hours and with a 100% concentration CO2 to simulate the recovered CO2 from a flue gas. Both Type 10 and Type 30 Portland cements were used in concrete containing 0%, 25%, 50% and 75% of either quartz aggregates or lightweight aggregates. The performance of carbonated concrete was evaluated by strength development and mass gain. For a 15-mm thick sample, a 9-16% CO2 uptake in two hours was achieved. Analysis by X-ray diffraction indicated calcite formation. Samples collected from the surface or the core, immediately after 2-hour carbonation or 7 days later, contained consistent carbon content. The strength after 2-hours carbonation was close to that of reference samples cured 7 days in a moist environment. Carbonated concrete demonstrated a much finer and denser microstructure and a much higher resistance to atmospheric carbonation shrinkage.

SP235 The Canadian Centre for Mineral and Energy Technology (CANMET) of Natural Resources Canada, Ottawa, Canada, has played a significant role in Canada for over 40 years in the broad area of concrete technology. In recent years, CANMET has become increasingly involved in research and development dealing with supplementary cementing materials, high-performance normalweight and lightweight concretes, and alkali-aggregate reactions. In May 2006, CANMET, in association with the American Concrete Institute and several organizations in Canada and the U.S.A., sponsored the Eighth CANMET/ACI International Conference on Recent Advances in Concrete Technology in Montreal, Canada. The proceedings of the conference consisting of 17 refereed papers, were published as ACI SP-235. In addition to the refereed papers, more than 20 other papers were presented and distributed at the conference. During the conference, a special symposium was held in honor of retired Professor Marc-André Berubé of Laval University, Quebec City, Canada, for his outstanding and sustained contribution in the broad areas of alkali-aggregate reactivity. The proceedings of this symposium, consisting of more than 20 papers, have been published as a separate publication by CANMET. Thanks are extended to the members of the CANMET/ACI paper review panel who met in Budapest, Hungary, in May 2005 to review the papers. Without their prompt review and constructive comments, it would not have been possible to bring out the ACI special publication for distribution at the conference in Montreal. The cooperation of the authors in accepting reviewers’ suggestions and in revising their manuscripts accordingly is greatly appreciated. Particular thanks are extended to Dr. Pawan Gupta, G.D. Brearley, and Colleen Mansfield-Joiner for their help in the processing of draft manuscripts.

The growing use of fluid concrete increases the need for understanding the conditions under which these materials can undergo bleeding and segregation. However, the interfacial and colloidal phenomena, which control water and solids migration in cementitious systems, are inherently complicated by the hydration of the cement components. Hence, to unravel the specific role of chemical admixtures on the stability of cement-based systems, the mode of action of these admixtures should also be investigated in dense colloidal slurries of ‘un-reactive’ minerals. Several highly insoluble minerals, having specific surface areas comparable to that of a Portland cement, were thus evaluated for this purpose. The state of flocculation of these materials in dilute and concentrated slurries was examined through sedimentation and rheological measurements under various conditions, and the results compared to observations on similar slurries containing cements. The comparison showed that calcium carbonate (CaCO3) exhibits surface and colloidal properties very similar to “un-hydrating” cement particles. In fact, CaCO3 pastes can be made to accurately reproduce most of the kinetic properties of a cement paste, including bleeding, sedimentation and all dynamic viscosity parameters. It is therefore proposed that CaCO3 pastes can be used to adequately model ‘physical-type’ effects occurring in cementitious systems at very early stage of hydration, i.e., in the first hour.

The interior of a large-size concrete member is heated by internal storage of the heat of cement hydration in early ages. The temperature rise that occurs in the interior of a large-size concrete member affects its strength development. Moreover, the use of high-strength concrete causes higher temperature rises due to the large volume of cement incorporated in the concrete. This paper describes a study on the temperature rise and the long-term strength development of high-strength concrete in large-size concrete members with different shapes and cross sections, especially columns and walls. In addition, we attempt to elucidate the effect of high-temperature curing in early ages on the hydration of cement and the microstructure formation of hydrated cement paste. The greater the cross section of a concrete member, the smaller the compressive strength of a concrete member at 28 and 365 days. This tendency is caused by the fact that the curing temperature of a concrete member rises with increasing cross section. There was a good correlation between the maximum temperature and compressive strength of concrete. The compressive strength of concrete at 28 and 365 days decreased with increasing maximum temperature. High-temperature curing in early ages resulted in the formation of a porous microstructure in later ages. This is why high-temperature curing that occurs in the interior of a thick concrete member inhibits later strength development.