International Concrete Abstracts Portal

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.

Polycarboxylate type Superplasticizers have become the most widely used in SCC since they show outstanding performance regarding water reduction and flow retention. In some cases, depending on polymer structure and binder used, incompatibility problems like rapid slump loss may be observed. In order to overcome such problems, the mode of action of different polycarboxylates was studied by measuring the adsorption of the polymer on cement, the flow in cement paste, the flow and strength development in mortar as well as in standard concrete and self compacting concrete. The structure of the polycarboxylate (molecular weight, length of side chains, grafting degree) strongly influences its adsorption on cement and thus the performance as Superplasticizer. The increase of flow of the cement paste and polymer adsorption follows a quasi linear relationship. However the polycarboxylate with the highest adsorption is not necessarily the best performing in SCC applications. The outstanding performance of polycarboxylates in SCC and the importance of using a knowledge based approach for reaching the optimal properties of this materials is demonstrated in case studies.

A new type of admixtures for cement-based materials presented in this work is based on silane-modified acrylic polymers prepared by emulsion polymerization of acrylic monomers and organofunctional silane precursor. The acrylic polymer chain was functionalized with organofunctional silane molecules to build an organic-inorganic polymer structure with terminal siloxane groups. The advantage of this type of polymers over already known polymer modifiers for concrete and mortars is the ability to act as a coupling agent between different phases in the cementitious system. The presence of siloxane groups enables strong chemical interaction among mineral phases of inorganic matrix (cement, aggregate) and direct chemical bonding of organic polymer chain to inorganic phases of the cement based material. The influence of silane-modified acrylic latex admixture on the properties of cement paste and mortar was evaluated and compared to unmodified acrylic latex. Mortars modified with this type of hybrid inorganic-organic polymers showed improved physical-mechanical properties like higher mechanical strength, better adhesion to the substrate, lower shrinkage, better thermal conduction and better resistance to different sources of corrosion.

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.