An Experimental Investigation of Interface Bond Strength of Concrete Using Large Powder-Driven Nails

Delaminations of freshly placed overlays from existing concrete substrates have been reported in bonded concrete overlays (BCOs). An experimental study was undertaken to determine the performance of special large powder-driven nails used to provide positive bonding for BCOs. The investigation consisted of four parts. (1) Nail pull-out strength was investigated (366 tests). Test variables were strength of concrete and nail location with regard to cracks and test slab edges. (2) Interface shear strengths and mechanism of shear transfer with and without nails were investigated by push-off tests (116 tests). Test variables included concrete strength, interface roughness (shotblasted, sandblasted, and troweled), bonded versus unbonded interface, interface reinforcement, and crack effects. Interface strengths were also tested at early ages. (3) Beam fatigue tests were conducted to determine the interface shear strength deterioration. (4) A full-scale experimental BCO was constructed near El Paso, Texas, and nails were used in two of eight test sections. The condition of overlays in sections with and without nails was compared in terms of overlay drying shrinkage crack and interface strength development and extension of delaminated interfaces. In the laboratory studies, nail pull-out strengths of 36 kN and 44 kN were determined in normal strength concrete and high strength concrete, respectively. The push-off test results indicated that nails were effective in improving the interface shear strengths of bonded-rough interfaces. An unbonded-rough interface with nails effectively resisted the shear force and limited the interface slip. Overlay curing methods at early ages significantly influenced the strength development. Interface strength did not deteriorate after 3,000,000 fatigue cycles in tests with nails and bonded-rough interfaces, but deterioration of composite action of unbonded interfaces with nails was observed after half a million cycles. In the experimental BCO, good bond (1,360 kPa or higher in tension) was developed in the slab interior region but interface delaminations and low bond strengths were determined near cracks and slab corners and edges. Test sections with nails performed significantly better than those without nails in terms of overlay drying shrinkage crack and interface bond strength development. The study results were used to make recommendations regarding the use of nails in a BCO to be constructed on IH-10 in downtown El Paso.