Early age properties of self-compacting concrete - Effects of fine aggregate and limestone filler
Doctoral thesis, 2007
Self-compacting concrete (SCC) is a sensitive mix, strongly dependent on the composition and the characteristics of its constituents. It has to possess the incompatible properties of high flowability together with high segregation resistance, a balance made possible by the dispersing effect of water-reducing admixture combined with cohesiveness produced by a high concentration of fine particles. These fines and their effects on the early age properties of the SCC have been in focus in this present dissertation.
The effect of the specific surface area of gravel and limestone filler on the rheology of SCC was evaluated. Performed experiments clearly demonstrated that traditional methods for geometric characterization of the fines (size distribution, water absorption, fineness modulus, etc.) are not sufficient to ensure consistent quality of SCC. By measuring the specific surface area with a simplified gas adsorption method, BET(H2O), it was found that the specific surface area of a normal gravel, accepted by traditional methods for production of SCC, can vary up to 7000 m2/kg. A model is proposed, based on an assumption that 30 full molecular layers of water covering the particle surface are required to provide lubrication sufficient to create flowability, where a change in specific area is translated to a change in water demand for the concrete mix. It is suggested that an increase in BET(H2O)-area of 1000 m2/kg corresponds to an increase in water demand by approximately 0.85% by mass of the filler or gravel content for constant flowability.
Furthermore, the influence of mix design and fines BET(H2O)-area on the SCCs early-age deformation was demonstrated. The autogenous (sealed) deformation was measured with a specially developed concrete dilatometer, together with capillary pore pressure and temperature. It is suggested that that autogenous shrinkage and rate of evaporation are the main factors promoting the risk of plastic shrinkage cracking. An increased particle surface decreased the rate and magnitude of evaporation, and consequently reduced the plastic cracking tendency, despite an increase in autogenous shrinkage.
specific surface area