Investigation of bond in unreinforced concrete interfaces for partial depth repairs and new construction

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Becker, Maggie Elizabeth

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This thesis presents an experimental study on the horizontal shear strength of reinforced concrete slabs and beams with unreinforced concrete-to-concrete interfaces. Current design provisions for new construction and repair applications in ACI 318-19 and ACI 562-19 codes attribute a low capacity for concrete-to-concrete interfaces that are unreinforced. Specifically, the nominal strength of an intentionally roughened, unreinforced interface is 80 psi, and an unreinforced interface without intentional roughening is assumed to have no shear strength. The goal of this research is to provide recommendations for redefining horizontal shear provisions for unreinforced interfaces in partial depth repairs and precast construction. To quantify interface strength, experimental tests were performed in two discrete phases: direct shear and direct tensile pull-off tests on slabs, then flexural tests on beams, each of which having differing surface roughnesses and topping slab workability. The roughnesses used for this experimental program was representative of both partial depth repair and new construction. Direct shear strengths from guillotine testing ranged from 400 psi to 1000 psi, with float (smooth) conditions providing the lowest strength and hydrodemolition roughening providing the highest strength. Direct tensile pull-off strengths ranged from 190 psi to 420 psi for toppings made of concrete with moderate workability and different roughnesses, while bond of toppings made of concrete with low workability was so weak that it failed during pull-off test preparations. The average horizontal shear strength along the unreinforced interface of beams subjected to flexure ranged from 530 psi to 550 psi for broom, tine and hydrodemolition roughening conditions. The maximum horizontal shear stress towards the topping ends of these beams were estimated in between 800 psi and 900 psi. However, beam specimens with smoother interfaces (floated and sandblasted) presented signs of debonding prior to flexural testing. The research results indicate that a sound (i.e., not bruised or microcracked by concrete removal), laitance and defect free interface with uniform and sufficiently rough surface texture in combination with well consolidated the repair or topping material are keys to high shear and tensile bond strengths. The results show that unreinforced interfaces with these characteristics can achieve interface shear strengths significantly higher than the ACI nominal shear strength of 80 psi.


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