Abstract
The bond strength between ultra-high-performance concrete (UHPC) and normal-strength concrete (NC) plays an important role in governing the composite specimens’ overall behaviors. Unfortunately, there are still no widely accepted formulas targeting UHPC–NC interfacial strength, either in their specifications or in research papers. To this end, this study constructs an experimental database, consisting of 563 and 338 specimens for splitting and slant shear tests, respectively. Moreover, an additional 35 specimens for “improved” slant shear tests were performed, which could circumvent concrete crushing and trigger interfacial debonding. Additionally, for the first time in our tests, the effect of casting sequence on UHPC–NC bond strength was identified. Based on the database, an artificial neural network (ANN) model is proposed with the following inputs: namely, the normal stress perpendicular to the interface, the interface roughness, and the compressive strengths of the UHPC and NC materials. Based on the ANN analyses, the explicit expression of UHPC–NC bond strength is proposed, which significantly lowers the prediction error. To be fully compatible with the specifications, the conventional shear-friction formula is modified. By splitting the total force into adhesion and friction forces, the modified formula additionally takes the casting sequence into account. Although sacrificing accuracy to some extent compared to the ANN model, the modified formula relies on a solid physical basis and its accuracy is enhanced significantly compared to the existing formulas in specifications or research papers.
Highlights
Recent decades have witnessed the emergence of ultra-high-performance concrete (UHPC) as a promising cementitious material in buildings [1], bridges [2], and tunnels [3], etc., due to its excellent tensile and compressive strength [4], agreed post-cracking behaviors [5], and enhanced durability [6]
Since this study focuses on UHPC–normal-strength concrete (NC) bond strength for specimens subject to normal environmental conditions, we merely take the surface roughness and material strengths into account
The surface roughness R of the UHPC–NC interface is identified as the main factor influencing the extent of adhesion in the database
Summary
Published: 30 September 2021Recent decades have witnessed the emergence of ultra-high-performance concrete (UHPC) as a promising cementitious material in buildings [1], bridges [2], and tunnels [3], etc., due to its excellent tensile and compressive strength [4], agreed post-cracking behaviors [5], and enhanced durability [6]. Extensive application of UHPC materials to fully replace normal-strength concrete (NC) would render the cost prohibitively high [7]. Composite UHPC–NC has aroused the attention of engineers, in terms of, but not limited to, retrofitting existing structures [8], enhancing durability [9], and accelerating construction speed [10]. These UHPC–NC composite members, especially those without any rebars crossing their interface, would possibly face the risk of debonding when subject to larger external loading [11]. Accurate prediction of interfacial strength between UHPC and NC is critical to estimate the composite specimen’s strength
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