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.

The rubble from construction is generally used in landfill or thrown to natural areas at the end of their service life. Because continuously increasing production of concrete consumption, recycling of concrete waste materials will provide environmental protection and economical benefits. In this study, effects of fine and coarse recycled concrete aggregates on Hot-Mix Asphalt (HMA) performance were investigated. In performed experiments, Marshall Mixtures were prepared by using recycled concrete aggregates in the proportion of 10%, 20%, and 30% by mixture weight for the replacement of limestone in suitable gradation. Six different bitumen ratios were added to each mixture, respectively. Void %, flow and stability values were examined on 54 specimens. Furthermore, indirect tensile strength experiments were examined on the specimens having optimum 4.5% bitumen content and 30% recycled concrete aggregates. The results indicate that waste concrete can be used in HMA as aggregate to obtain the required Marshall stability and indirect tensile strength of the mixtures. However, the void percent of the mixture are not desirable due to the dense gradation of aggregate. Hence, gradation change is needed to Marshall Design criteria.

Among the harmful substances carried into reinforced concrete by water, none is potentially more damaging than chloride ions, which destroy the passivity layer on steel in alkaline environment leading to corrosion processes of reinforcement and loss of durability. In this work we present direct potetiometric measurements of chloride ions in the pore water of mortar samples by using Ag/AgCl indicator electrodes. The long-term stability of electrodes in highly alkaline environment of cementitious materials and selectivity to chloride ions were improved with the deposition of polymer membrane consisting of polyvinyl chloride matrix and chloride ionophore. The chloride ingress experiments were performed on cylindrical mortar samples with over 50 sensors embedded into the solid body of the sample. The ingress of dissolved chloride ions into the mortar sample was monitored for more than six months. The concentration profiles of chlorides at different places in the sample were determined and used for the evaluation of diffusion processes of chlorides through the pores of the microporous matrix.