Experimental investigation of the web-shear strength of deep hollow-core units

Estimation of the shear strength of precast, prestressed concrete hollow-core units in the United States follows the shear provisions of ACI 318-08. These provisions are based on experimental research conducted mainly on members other than hollow-core units. These provisions have been extended to hollow-core units based on a limited number of hollow-core unit shear tests. Recent shear tests performed on thick hollow-core units (depths greater than 12 in. [300 mm]) by several U.S. manufacturers have shown that these units fail in the web-shear tension mode at forces smaller than those predicted by the Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318 R-05). Some fail at shear forces equal to 60% of the ACI 318-05 predicted strength. As a consequence, ACI 318-08 now requires minimum shear reinforcement to be supplied in hollow-core units with depths greater than 12.5 in. (318 mm) if the factored shear force exceeds 50% of the design shear strength of the concrete or the web-shear capacity must be reduced by 50%. A research program was conducted to investigate the prediction of the web-shear capacities of thick hollow-core units and to quantify the reasons why these thicker units are failing at lower shear forces than those predicted using the ACI 318-05 provisions. The reduced shear capacity appears to correlate with increased end slip of the strand and with poorer consolidation of the concrete.