Interface shear strength of concrete at early ages

The development of interface shear strength at early ages following placeme nt of new concrete overlaying existing concrete was studied experimentally. Thirty-two pushoff tests were conducted to determine interface strength. T est variables were age of overlay and overlay curing method. Special powder -driven shear connectors were used across the interface in most specimens t o provide some reinforcement between old and new concrete layers. Tests wer e performed when the age of the overlays was between 12 hr and 35 days. Tes t results showed that the interface shear strength was 1370 kPa or higher 2 4 hr after casting, and strength improved rapidly during the first 3 days. The interface strength was influenced significantly by the overlay curing m ethod. Test results suggested that good curing of the overlays at early age s, especially during the first 3 days when the interface strength is increa sing rapidly, resulted in higher interface shear strengths.