# Experimental Investigation of TR-UHPC Composites and Flexural Behavior of TR-UHPC Composite Slab
## 3 Stacking and Bending Tests on BFT-UHPC Pedestrian Walkway Thin Slabs
### Bending Test
#### 3.4.2 Failure Mode and Stiffness Analysis  
Figure 11 depicts the load-displacement curve of the BFT-UHPC slab, from which the damage process of the test slab can be divided into five stages. Stage 1: Linear elasticity stage. Prior to UHPC cracking, the mid-span deflection varies linearly with increasing load. Stage 2: Crack development stage (14.8 kN). Initially, numerous microcracks appeared at the bottom of the BFT-UHPC slab, causing no significant decrease in structural stiffness due to the strain-hardening properties of UHPC. The steel fibers in the matrix bridged the cracks and continued to provide tensile resistance. As loading continued, cracks developed  
Figure 11: Load–displacement curve of the test slab. Figure 10: Flexural loading of the specimens and arrangement of the measuring points. at the top of the slab near the support (Zone-E in Figure 10), resulting in a decrease in specimen stiffness. After the top UHPC cracked, the BFT started to perform, limiting the further development of UHPC cracks at the top while slowing the development of the bottom UHPC cracks. Stage 3: Rebar yielding stage (84.2 kN). After the reinforcement yielded, UHPC entered the softening stage, gradually diminishing its contribution, which was accompanied by a decline in the specimen's stiffness. In this stage, the primary stressed section was the negatively bending section. The UHPC at the top of the slab entered the multi-cracking stage, and the textile and short steel fibers bore more external forces. With an increasing load, basalt fiber bundles and the matrix bridge anchorage between the fibers in the cracks played a dual role in transferring bridging forces, leading to the phenomenon of multi-crack development. When the UHPC crack spacing was insufficient, the basalt fiber bundles entered the tensile stage, gradually reaching the ultimate bearing capacity. Stage 4: Peak load stage (94.7 kN). Due to the brittleness of the fibers, the test slab failed with the fiber rupture at the top of the support section. Prior to the rupture of fibers at the support section, the steel bars at the mid-span bottom yielded, causing a faster development in the crack width of the mid-span bottom than that of the support section. The negative bending zone could not bear more load after the fiber ruptured, leading to a decrease in the bearing capacity, and ultimately causing the test slab to fail. As the loading continued, the basalt fiber bundles sequentially reached their ultimate strength and ruptured, causing a reduction in the bearing capacity of the test slab. The UHPC crack at the top of the slab rapidly propagated to form a main crack, and the bearing section failed gradually. After the bottom of the slab took over as the main stress section, maintaining a certain level of bearing capacity, the UHPC crack development at the slab bottom accelerates. UHPC cracks gradually penetrated along the transverse direction at the mid-span (Zone-C), while on the other mid-span (Zone-D), UHPC cracks appeared to be partially closed. Stage 5: Load declining stage. With consecutive loading, the top of the mid-span exhibited clear UHPC crushing, accompanied by the sound of steel fibers pulling out and UHPC matrix powder falling. The deflection at the crack location increased rapidly, leading to a significant decrease in the bearing capacity. The final failure mode of the BFT-UHPC slab is presented in Figure 12.  
The test results indicate that upon reaching the failure state, the UHPC at the slab top was crushed, resulting in a substantial deflection without actual fracture. It was observed that the test slab not only exhibited a main crack in the negatively bending section, but also developed a main crack in the mid-span of the positively bending section on one side (Zone-C) during the load declining stage. The location of the lateral cracks generally corresponded to the main cracks on the slab soffits, and a multitude of small, dense UHPC cracks appeared in the softit near the main crack.