Ultra-high performance concrete （UHPC）， with its exceptional tensile strength and strain-hardening characteristics， offers the possibility of designing non-reinforced lightweight structures. However， current design codes often underestimate its tensile contribution， and traditional analytical methods struggle to accurately quantify the complex influence of different fibers on the flexural capacity of members. During the casting of thin-walled elements or narrow cross-sections， the mold boundaries impose significant constraints on fiber orientation—a phenomenon known as the “wall effect”—leading to anisotropic fiber distribution and consequently a substantial impact on the flexural performance of the element. In this study， flexural performance tests were conducted on three non-reinforced UHPC slabs and compared with prior experimental results. The findings revealed that as the cross-sectional size decreased， the influence of the wall effect on fiber orientation intensified， significantly enhancing the nominal flexural strength of the element. Particularly for narrow and deep cross-sections， the side-wall effect increased the effective tensile strength by more than 30% compared to that of the matrix. Building on this， the ultimate flexural mechanism of non-reinforced UHPC simply-supported slabs was systematically investigated. By introducing an equivalent rectangular stress block model， a calculation method for the flexural capacity of UHPC simply-supported slabs with three fiber types was proposed. The theoretical model presented in this paper can effectively predict the ultimate bearing capacity of UHPC slabs across various geometric dimensions.
XIAO et al. (Sun,) studied this question.