Crack Width Calculation Method for Concrete in Hogging Moment Region of Steel–UHPC–NC Composite Girder with Integrated Piers

The application of ultra-high performance concrete (UHPC) in the hogging moment region significantly enhances the crack resistance of concrete slabs of composite girders with integrated piers, while also providing economic benefits. To investigate the crack resistance performance and develop a calculation method for crack width in hogging moment region of steel–UHPC–normal concrete (NC) composite girders, a full-scale bending test was conducted. Based on the test results, the post-cracking residual tensile strength of UHPC was determined according to the energy equivalence principle. A calculation method for reinforcement stress incorporating the tensile contribution of UHPC at a cracked section was proposed and then the applicability for current design codes for crack width calculation was evaluated. For the UHPC–NC interface, a corresponding crack width calculation method was developed. The results indicate that cracks initiated on the surface of the NC layer beneath the UHPC overlay at the cantilever root. Then cracks developed in sequence at the top surface of the UHPC layer cantilever root, the UHPC–NC interface, and the mid-plane of the girder-to-pier joint. Ultimately, UHPC cracks exhibited a “numerous and closely spaced” distribution, whereas NC cracks were “few and widely spaced.” When the residual tensile strength of UHPC at cracked section was considered, the mean value and average coefficient of variation in the ratios of calculated to measured reinforcement stresses for different sections were 1.07 and 0.10, respectively, which can be further used for crack width calculation. The mean ratios of code-predicted to measured UHPC crack widths for different sections using the Chinese code, French code, and European code were 1.10, 0.98, and 1.13, respectively, with corresponding average coefficients of variation of 0.25, 0.33, and 0.28; the Chinese code is recommended for UHPC crack width prediction. For the UHPC–NC interface, an expression for crack width calculation was derived using the comprehensive theory, and the mean ratio of calculated to measured values and the coefficient of variation were 1.08 and 0.18, respectively, demonstrating good predictive accuracy.