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.