Royce W. Floyd

Measured Transfer Lengths of 0.7 in. (17.8 mm) Strands for Pretensioned Beams

The use of 0.7 in. (17.8 mm), Grade 270 (1860) prestressing strands has advantages over 0.5 and 0.6 in. (12.7 and 15.22 mm) strands. This study provides design guidelines for estimating transfer length of 0.7 in. (17.8 mm) strands. Sixteen pretensioned concrete beams using a single prestressing strand were fabricated with conventional concrete and self-consolidating concrete. The concrete strengths at 1 day ranged from 5.9 to 9.2 ksi (40.7 to 63.4 MPa). Transfer lengths were determined using concrete surface strains along with the Average Maximum Strain Method. Initial strand end slips were also measured for predicting transfer length at release using an empirical formula. Experimental results indicated ACI 318 and American Association of State Highway and Transportation Officials (AASHTO) specifications are applicable for estimating transfer length of 0.7 in. (17.8 mm) strands at release and at 28 days. The results also showed that a coefficient of 2.32 was the most appropriate value for estimating transfer lengths at release from initial strand end slips.

Measured Development Lengths of 0.7 in. (17.8 mm) Strands for Pretensioned Beams

The use of 0.7 in. (17.8 mm), Grade 270 (1860) prestressing strands in construction is slow regardless of the engineering advantages of these types of strands. The limited research data and unavailable design guidelines partially account for the slow use. This study measured development length for 16 pretensioned concrete beams, in which the prestressing strand was tensioned to 75% of its ultimate strength. The beams were fabricated with conventional concrete or self-consolidating concrete (SCC). The concrete compressive strengths at 28 days of age varied from 9.2 to 13.4 ksi (63.5 to 92.5 MPa). The development length was determined by conducting bending tests at different embedment lengths. The experimental results indicated that the measured development lengths did not show a good correlation with concrete compressive strength. The ACI 318 equation significantly overpredicts the measured development lengths. A simple equation was proposed to predict development length of 0.7 in. (17.8 mm) prestressing strands.