Evaluation of Bond Properties between Fiber Reinforced Concrete Overlay and Substrate Concrete
Alshammari, Emad Okrush
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Bonded concrete overlay is the most economical and quick option in concrete rehabilitation methods to provide strength for the existing structure. Bonded concrete overlay strength rely on properties of both layers (substrate and new layers). Poor bonding in interface zone leads to two main failures of concrete overlays; debonding and cracks. There are several traditional factors affect bond strength such as water to cement (w/c) ratio, moisture condition, and surface roughness for substrate layer. However, the effect of using fiber consider a new variable affecting bond strength. Fiber reinforced concrete (FRC) is one type of overlay system which has been used for applications such as pavements, slope stabilization, arches, and domes. Adding discrete fibers in the mortar or concrete mixture significantly enhance the bond strength between substrate layer and overlay by increasing the cohesion at the interface, decreasing curling strain, restraining the development of cracks and spread them into several finer cracks. The objective of this study is to investigate how the bond strength between the existing and overlay layer under two loading conditions (indirect tension and direct shear) is influenced by adding Polyvinyl alcohol fiber (PVA) fiber and Polypropylene fiber. This study was performed by using two bond tests: splitting prism tensile test and direct double shear test. The splitting prism test form indirect tensile stress on the interface zone while the direct double shear test form direct shear stress on interface zone. The substrate surfaces were roughened by wire brush and the specimens were cured for 28 days of curing. Both layers use a 0.46 (w/c) ratio. Two types of fiber were added to overlay layer with different volume fraction dosages. Analysis of study results found that the bond strength (tension and shear) significantly increases when using fiber reinforced concrete (FRC) as an overlay layer. A relation was found between increasing fiber content and increasing bond strength. However, there is no relation found between bond strength and fiber type. In most FRC cases, the bond strength was greater than the control case, and the improvement was more than 600% in some cases. Adding higher fiber volume fraction dosage leads to higher bond strength. However, volume fraction dosage more than 1.5% causes less workability for the concrete mixture which leads to a decrease in bond strength. Low volume fraction dosage (≤ 0.5%) did not show a significant improvement in tensile and shear bond strength. Strong correlation was found between shear bond strength and tensile bond strength.