Decoding the Shear Strength of Sand–Concrete Interfaces: The Role of Surface Texture and Bentonite

Bentonite slurry is frequently used as a support fluid in the construction of drilled shafts. During the piling process, the slurry acts as a sealant and slightly penetrates the nearby soil. However, the degree to which bentonite slurry penetrates the soil affects the resulting frictional capacity of the bored piles. This experimental study examines the extent of this phenomenon, arising from the formation of what is typically known as the bentonite filter cake or mud cake. The frictional properties of the filter cake are examined through three groups of direct shear tests, employing three pre-cast concrete blocks positioned on a sand layer that has been subject to bentonite slurry for varying durations. To ensure comparison, a similar pre-cast concrete block was utilized in each test series, resulting in uniform surface roughness in the concrete. A handheld surface roughness device was utilized to measure the roughness profile of each concrete block, assessing the surface roughness of all concrete surfaces. The outcomes of the direct shear test performed were subsequently normalized based on the assessed roughness of the concrete surface. Experimental results showed that the friction capacity of the soil–concrete interface for granular materials (“sand–concrete interface”) decreases with longer exposure to bentonite slurry. Specifically, the shear strength is inversely proportional to the square root of the bentonite slurry exposure time. Tests on the internal friction angle of Québec Valcartier granitic sand and the friction angles at sand–concrete interfaces with and without bentonite slurry exposure revealed that the non-exposed sand–concrete interface achieves a peak friction angle equal to 77% of the peak internal friction angle of Québec Valcartier granitic sand. This value represents 69% and 60% of the peak friction angle of the sand tested for bentonite exposure durations of 2 and 4 h, respectively.