The Effect of Slurry Type on Drilled Shaft Cover Quality-Article

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The study examined how slurry type affects the quality of concrete cover in tremie-placed drilled shafts. The authors tested 24 laboratory shafts, each 42 inches in diameter, cast using water, polymer, or bentonite slurry. They assessed surface voids and texture, corrosion potential, and concrete strength.<br /><br />The results indicate that bentonite slurry promotes laitance-filled creases, or “mattressing,” where concrete flows through the reinforcing cage and into the cover. Bentonite-cast shafts had substantially greater surface void volumes than water- or polymer-cast shafts, and their cover concrete showed poorer corrosion protection and lower, less uniform strength. Average cover strengths were 2,886 psi for bentonite, 4,346 psi for water, and 4,943 psi for polymer. Seventy per cent of bentonite specimens had median corrosion-potential readings below the study’s 350 mV threshold, while water and polymer specimens generally showed no elevated corrosion risk. These effects appeared across the tested bentonite viscosities.<br /><br />Water and polymer shafts generally had better surface condition and more consistent strength profiles. However, laitance creases were also detected in a water-cast shaft, indicating that the phenomenon is not exclusive to bentonite.<br /><br />The authors conclude that bentonite slurry can compromise drilled-shaft cover quality and durability, potentially creating pathways for corrosion and reducing structural capacity. The findings come from laboratory specimens, and the authors note that additional coring and chemical analyses would help confirm the observed trends.

Keywords

drilled shafts

tremie concrete

bentonite slurry

polymer slurry

water slurry

concrete cover

surface voids

corrosion potential

laitance

concrete strength