Transfer Length in Prestessed Self-Consolidating Concrete

Abstract

Over the years much research into the transfer lengths of prestressed beams cast with conventional-slump concrete has been performed, but very little has been done in regard to beams cast with self-consolidating concrete (SCC). Currently, expressions for predicting transfer length of prestressed specimens in the ACI and AASHTO building codes are based on conventional-slump concrete. An experimental study was performed to determine the effects that SCC would have on transfer length. Currently, expressions for predicting transfer length of prestressed specimens in the ACI and AASHTO building codes are based on conventional-slump concrete. Thirty-six concentrically prestressed concrete specimens were cast and used to measure transfer lengths. One conventional-slump mixture and four different SCC mixtures were tested. The SCC mixtures were made up of a high- and low-strength mixture for each type of mineral admixture, Class C fly ash or ground-granulated blast furnace slag. The high-strength mixtures had compressive strengths at prestress transfer ranging from 8,700 to 9,700 psi. The low-strength SCC mixtures and the conventional-slump concrete varied from 5,000 to 6,250 psi at prestress transfer. It was found that prestressed specimens cast with SCC had transfer lengths 14% longer on average than prestressed beams cast with conventional-slump concrete. The transfer lengths on the cut ends were significantly longer than those on the dead ends of the specimens. Transfer lengths were found to be indirectly proportional to the square-root of the concrete compressive strength. It was also observed that the transfer length growth stabilized within the first week after prestress transfer. An analysis of all transfer lengths from the current study was done to evaluate the adequacy of the ACI 318, AASHTO Standard, and AASHTO LRFD equations and other proposed equations in predicting transfer length. It was determined that the ACI 318, AASHTO Standard, and AASHTO LRFD equations for transfer length do not adequately predict transfer length for prestressed specimens cast with SCC or conventional concrete. These expressions predict shorter transfer lengths then were actually measured.