Relationships between concrete composition and boundary layer composition to optimise concrete pumpability

摘要

Concrete pumpability is determined by the friction at the interface between the concrete and the wall of the pumping pipes (which are generally made of steel); called the concrete-steel interface. This friction is related directly to the thickness and composition of the boundary layer (BL) that occurs during the movement of fresh concrete in a pipe. These BL characteristics are rigorously linked with concrete composition parameters. To highlight this complicated relationship, an apparatus called a 'tribometer' was used in two experimental programs. This apparatus allows measurement of the steel-concrete interface friction and deduction of interface parameters (also called the pumping parameters), namely viscous constant and interface yield stress. The first program focuses on the effect of the concrete composition on pumping parameters, and the second program focuses on the relationships between the concrete composition and the BL composition. The results of the first program show that increases in cement paste volume, water/cement ratio and superplasticiser dosage enhance concrete pumpability. Increased content of fine sand in concrete induces negative effects on the interface frictions and on the pumpability. The results of the second program show that the BL is formed by water, cement and fine sand particles with a diameter lower than 0.25 mm. It also shows that the proportions of water and cement volume in BL and micro-concrete (concrete with the largest grain diameter lower than 0.25 mm) are almost the same. The relative enrichment produced by fine sand in the BL decreases with increasing proportion of fine sand volume in the concrete.