# Experimental Study on Mechanical Properties of Graded EPS-Steel Fiber-Reinforced Concrete
## 3 Analysis of Experimental Phenomena and Results
### Flexural Strength and Failure Mechanism  
The specimen will experience four distinct stages of deformation as the applied stress changes, As shown in Figure 7: the overall working stage (elastic deformation), crack development stage, steel fiber-dominated working stage (significant crack development), and specimen rupture (ductile failure). In the initial stage of loading, the specimen remains in the linear elastic phase. As the three-point loading gradually increases, the stress rises rapidly, and the specimen remains in the elastic phase, where the deformation can recover upon unloading (OA segment). However, as the stress increases quickly to 50-60% of the peak value at point A, small vertical cracks will appear inside the specimen, with no  
Figure 6: The effect of graded EPS volume fraction on the fluidity of concrete. external visible damage. At this point, the skeleton structure made up of coarse aggregates, fine aggregates, cement, and other materials is under stress. As the tensile stress continues to increase, the EPS and aggregates inside the skeleton are compressed and experience some damage. However, since EPS has good elasticity, the specimen only experiences minor plastic deformation, and fine cracks develop at the bottom. As the stress continues to increase to a peak at point A, the specimen formally enters the crack development stage. The tensile stress generated by the external load exceeds the load-bearing capacity of the skeleton structure. The neutral axis quickly moves upward, and the bottom of the specimen is pulled apart. Developmental cracks emerge and rapidly extend toward the top of the specimen. At this stage, the skeleton structure still dominates the load-bearing capacity, and steel fibers begin to participate in load-bearing (AB segment). When the strain reaches point B, the cracks at the bottom of the specimen continue to develop. Several smaller micro-cracks appear adjacent to the main crack. As the deformation continues to increase, the main crack extends toward the top of the specimen. From point B onward, the steel fibers play a dominant role in bearing the tensile stress, thereby slowing the rate of development of the micro-cracks (BC segment). As the strain increases to point C, the tensile stress in the specimen suddenly changes. At this point, the main crack width rapidly increases and extends to the top of the specimen. The specimen is about to rupture completely. However, due to the effect of the steel fibers, the failure is not linear (CD segment), indicating that the failure has a certain degree of ductility. Based on the load-deflection curves of concrete with different gradations of EPS and steel fiber volume fractions, as shown in Figure 8, it can be observed that for the same grade, fine aggregate concrete reaches the peak stress in the elastic-plastic deformation phase earlier than gravel concrete, and the peak stress is also slightly higher. Additionally, high-grade concrete reaches the peak stress at a larger strain compared with low-grade concrete. Figure 7: Graded EPS–steel fiber-reinforced concrete bending failure stage. Overall, for all three EPS volume fractions, the flexural strength of each concrete grade increases as the steel fiber volume fraction increases from 1.5% to 2.25%. The change in steel fiber volume fraction has a minimal impact on the deflection value at the inflection point A during both the overall working stage (OA segment) and the crack development stage (AB segment). The deflection values at point A for all load-deflection curves fall within the range of (0.48, 1.12) mm, with the exception of the C50 gravel group, where the deflection at point A exceeds 1 mm. The A points for the other grades are more concentrated. The steel fiber volume fraction is negatively correlated with the deflection value at point B, meaning that as the fiber content increases, the deflection value at point B decreases. However, the steel fiber volume fraction is positively correlated with the length of the BC segment.