Effects of Substrate Texture and Moisture Conditions on Overlay Bond Strength

Laboratory direct tension (pull-off) tests were conducted to supplement field implementation of a non-proprietary ultra-high performance concrete (UHPC) overlay on an existing normal strength concrete (NSC) bridge deck in Socorro, New Mexico, USA. A mock placement of the UHPC overlay prior to construction revealed that improper surface preparation of the substrate concrete resulted in lack of bond. To demonstrate the importance of surface preparation to the contractor, a series of pull-off tests were performed on overlaid laboratory slabs. The NSC laboratory slabs had surface moisture conditions that included pre-wetted and dried prior to overlay application. Pre-wetted slab specimens had a tined, tined-light sand blasted, or tined-medium sand blasted substrate surface texture. The dried slab specimens had either a tined or an exposed aggregate substrate surface texture. Pre-wetted specimens with tined, tined-light sand blasted, and tined-medium sand blasted surface textures achieved average direct tension bond strengths of 134 psi (0.924 MPa), 211 psi (1.45 MPa), and 283 (1.95 MPa), respectively. Specimens with dried substrate surface conditions prior to overlay placement had zero bond strength. Results from this study showed that pre-wetting was essential for achieving adequate overlay bond. Tined-light sand blasted specimens with proper pre-wetting achieved adequate bond strengths, and properly pre-wetted, tined-medium sand blasted specimens produced excellent bond strengths. This type of information allows specific guidance to be provided to contractors and communicates the consequences of not following best practices.