Title: Influence of surface roughness on interface shear strength in composite concrete members
Date Published: November - December 2026
Volume: 71
Issue: 6
Page Numbers: 47 - 71
Authors: Shih-Ho Chao, Guillermo Palacios, and Regina N. Waweru
https://doi.org/10.15554/pcij71.6-01
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Abstract
This study investigates the interface shear strength between precast concrete girders and cast-in-place concrete slabs, with particular emphasis on the influence of surface roughness on the cohesion factor c and friction factor μ _used in the shear-friction provisions of the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications. Push-off tests were conducted on specimens with cold joints and without interface shear reinforcement, using surface finishes including wood-float and rake finishes, as well as International Concrete Repair Institute concrete surface profiles (CSPs) ranging from CSP 6 to CSP 9. The results indicate that both cohesion and friction factors generally increase with increasing surface roughness, though the cohesion and friction values are ultimately governed by the strength of the cement paste and coarse aggregate. Wood-float finishes exhibited the lowest cohesion and friction values, whereas excessively rough surfaces beyond CSP 7 provided limited additional benefits. Increasing concrete compressive strength leads to an increase in interface shear capacity, particularly for sections with larger interface areas. Overall, increasing surface roughness is more effective in regions with closely spaced shear reinforcement, such as near girder ends, while its influence becomes less significant in regions with larger reinforcement spacing, such as near midspan. This study provides values of c and μ _for different levels of surface roughness. This information is important because there is significant variability in achieved surface roughness among precast concrete plants, which suggests that interface shear strength may vary considerably in composite members depending on fabrication practices. The results of this study help clarify the influence of surface preparation on these parameters and may support AASHTO and state departments of transportation in developing more specific recommendations to achieve consistent surface roughness and reliable interface shear strength.