Effects of Substrate Texture and Moisture Conditions on Ultra-High Performance Concrete and Silica Fume Concrete Overlay Bond Strengths

Abstract

Direct tension tests were conducted to investigate the effects of substrate moisture conditions and texture on ultra-high performance concrete (UHPC) overlay bond strengths. Improper substrate surface preparation can result in inadequate bond strengths and, in severe cases, lack of bond. To demonstrate the importance of surface preparation, pull-off tests were performed on overlaid slabs that had two extreme substrate surface moisture conditions (saturated and dried) prior to overlay application. Saturated slabs had a tined, tined-light sand blasted, or tined-medium sand blasted substrate surface texture. Dried slabs had either a tined or an exposed aggregate surface texture. Saturated specimens with tined, tined-light sand blasted, and tined-medium sand blasted surface textures achieved average bond strengths of 0.924, 1.45, and 1.95 MPa, respectively. Dried substrate surfaced specimens had zero bond strength. Surface moisture conditions that ranged from saturated to dry were also investigated by allowing the substrate surfaces to dry for 15, 30, 45, and 60 minutes prior to application of an UHPC overlay. Tined-light sand blasted specimens with surfaces that dried for 15, 30, 45, and 60 minutes achieved average bond strengths of 2.86, 2.01, 1.59, and 0.165 MPa, respectively. Results showed tined-light sand blasted specimens with proper saturating achieved adequate bond strengths, and properly saturated, tined-medium sand blasted specimens produced excellent bond strengths. Results also exposed the drastic consequences of not maintaining a saturated substrate surface prior to overlay application and delaying overlay application up to 60 minutes can drastically reduce bond strength.