Contractions volumétriques des bétons aux ajouts cimentaires influence des caractéristiques du réseau de pores

Abstract

At the present time, high-performance concrete (HPC) has become widely used throughout the world in various construction fields. Producing such type of concrete requires a decrease in water/cement (W/C), an increase in cement content and the addition of cementitious materials in replacement of portland cement. Even though this concrete has shown good mechanical properties and appropriate durability, this material is also known for its high sensitivity to early-age cracking related to the high thermal and self-desiccation shrinkages produced during the progress of binder hydration. This study highlights fundamental aspects (pore system and shrinkage) resulting from the use of mineral admixtures in HPC. Usually, self-desiccation shrinkage measurement is very complex, particularly when dealing with massive concrete for which hydration heat (thermal strains) and self-desiccation shrinkage evolve simultaneously. The results obtained are mainly dependent on the measurement set-up used and the choice of the starting point of autogenous shrinkage strains measurement. Several approaches have been proposed to determine this important point when dealing with low W/B concrete. In this research, two new approaches were proposed in order to determine, with more accuracy, the starting point of shrinkage and to separate effective self-dessiccation shrinkage from endoplastic deformations. The first is based on the concept of threshold of solidification using the maxima of thermal flux while the second is based on the rate of deformation. Concrete mixtures with three W/B of 0.35, 0.30, and 0.26, and seven types of binders were used in this research. Shrinkage was measured on sealed massive blocks consisting of 0.6 m cubes that perfectly simulate reality. Pore system characterization was performed at different ages using the mercury intrusion porosimetry technique (MIP). Self-desiccation shrinkage measured on massive concrete is mainly affected by W/B of concrete and the binder type as well as curing time. The use of a binary system with glass frit as alternative cementitious material and metakaolin were harmful for shrinkage development, particularly at early age. However, a ternary system with silica fume and fly ash has shown lower magnitude of shrinkage and finer pore system among all cementitious systems investigated. This system is the best choice combining low shrinkage and excellent mechanical properties. The pore system of concretes tested is characterized by very fine capillary pores smaller than 60 nm. Generally, pores ranging between 50 and 10 nm dominate the pore population. Refinement of these pores mainly depends on the type, proportion, and fineness, as well as particle size distribution of the mineral admixtures incorporated. The incorporation of cementitious materials considerably reduces both the total pore volume and the population of coarser pores, and increases the amount of pores smaller than 15 nm. It was also established that the self-desiccation shrinkage is intimately related to change in the capillary pore's structure, especially the evolution of the threshold pore radius.