Use of tribometer to estimate interface friction and concrete boundary layer composition during the fluid concrete pumping

The friction at the steel–concrete interface (on the boundary layer) which occurs when concrete flows in a drain plays an important role in its pumpability. This friction is directly related to the composition of the formed boundary layer, which depends on the concrete composition parameters. In order to highlight the relationship between concrete composition and interface friction, the authors initially developed and validated an apparatus (called “tribometer”) allowing steel–concrete interface friction measurements and deduce interface parameters. It is simple to use at building sites. The paper highlights the rigorous relationship between the boundary layer (BL) composition and the concrete composition. The results show that the BL is formed by water, cement and fine sand particles of concrete with a diameter lower than 0.25 mm. It was also shown that the water–cement ratio of the BL and that of the concrete are almost the same. The relative enrichment of fine sand in the BL decreases with the increase of the fine sand voluminal proportion in the concrete.