Effects of polymer and bentonite support fluids on concrete-sand interface shear strength

Support fluids are widely used for the construction of deep bored piles and diaphragm walls. Specifications for the use of these fluids vary, and a thorough understanding of their effect on pile shaft resistance has not yet been developed. This paper presents the results of a set of concrete-sand interface shear tests carried out using both polymer and bentonite support fluids, with water as a reference fluid. It was found that polymer fluids had little effect on the interface shear strength when compared with water. Furthermore, in contrast to the results of earlier studies, concrete curing time was found to have little effect on the results. However, when bentonite slurry was used, the interface shear strength was found to decrease approximately linearly with the square root of the filtration time, until the strength of the pure filter cake was reached. This was due to the development of a bentonite filter cake at the interface so that only aggregate protruding through the filter cake made contact with the sand. It was found that the full concrete-sand shearing resistance could be mobilised when the concrete-sand contact area was greater than about 50% of the total area.