Effect of Shrinkage Reducing Admixture on Time-dependent Pre-stress Losses in Pre-tensioned Beams

Sixteen large scale pre-stressed concrete beams were tested under monotonic center point loading to determine the effect of shrinkage reducing admixture (SRA) on the time-dependent (TD) pre-stress losses in pre-tensioned beams. The nominal dimensions of the beam were 4.0 m x 0.20 m x 0.25 m (13 ft. 1.5 in. x 7.9 in. x 10 in.). The beams were reinforced with two straight, seven-wire, low relaxation strands of Grade 270. The SRA was added at dosage rates of 0%, 1% and 2%. The beams were instrumented and tested at variable ages ranging from 3 to 945 days. The flexural cracking load was measured and used to calculate the pre-stress force at the time of testing using principles of mechanics, and thus the pre-stress loss was evaluated at different ages. The results showed that the addition of SRA significantly reduced the pre-stress loss in the beams, particularly when added at 2%. The SRA reduced the TD pre-stress losses by up to 41.9 % and 37.6% at the ages of 735 and 945 days, respectively.