Measurement of Transfer and Development Lengths of 0.7 in. Strands on Pretensioned Concrete Elements

The implementation of 0.7 in. (17.8 mm), Grade 270 (1860), low-relaxation prestressing strands in construction is slow regardless of its advantages over the use of 0.6 in. (15.2 mm) and 0.5 in. (12.7 mm) strands. The limited research data and unavailable design guidelines partially account for the slow utilization. This study measured transfer and development length, and evaluated applicable strand spacing of 0.7 in. (17.8 mm) prestressing strands for 24 pretensioned concrete beams. Each beam contained one prestressing strand or two prestressing strands placed at spacing of 2.0 in. (51 mm). The beams were fabricated with high strength, conventional concrete or high strength, self-consolidating concrete. The concrete compressive strengths varied from 5.9 ksi to 9.8 ksi (40.7 MPa to 67.6 MPa) at 1 day, and from 9.2 ksi to 13.4 ksi (63.4 MPa to 92.4 MPa) at 28 days. Transfer lengths were determined using concrete surface strains along with the 95% Average Maximum Strain method. Initial strand end-slips were also measured for predicting transfer length at release using an empirical formula. The development lengths were determined by conducting bending tests with different embedment lengths. Experimental results indicated ACI 318 and AASHTO specifications are applicable to predict transfer length of 0.7 in. (17.8 mm) strands at release and at 28 days. A coefficient of 2.38 was the most appropriate value to estimate transfer length at release from initial strand end-slip. Concrete compressive strength had little effect on the measured development lengths. The ACI 318 and AASHTO equations significantly over-predicted the measured development lengths. The use of strand spacing of 2.0 in. (51 mm) has no significant effect on the measured transfer end development lengths. Two simple equations were proposed to predict transfer length and development lengths of 0.7 in. (17.8 mm) prestressing strands