# Experimental Study on Mechanical Properties of Graded EPS-Steel Fiber-Reinforced Concrete
## 3 Analysis of Experimental Phenomena and Results
### Evaluation of Compressive Strength and Flowability
Steel fibers, distributed parallel to the aggregate boundaries, create a wall effect that provides a slight lateral restraint on crack propagation. As stress continues to increase, the internal cracks in the concrete lead to failure, extending into the hardened cement paste. Steel fibers then act as a reinforcement, slowing the rate of crack propagation, resulting in a small increase in compressive strength. It is worth noting that in Figure 5c, at the 28-day standard curing age, all grades of fine (gravel) concrete specimens with a steel fiber volume fraction of 2.25% and EPS volume fraction of 15% achieve the standard compressive strength of concrete. This indicates that, under these proportions, graded EPS-steel fiber concrete can be directly selected for practical engineering without the need to increase the grade. For other combinations of steel fiber and EPS volume fractions, based on actual engineering requirements for compressive strength, the concrete grade may need to be increased by one to two grades to meet the design specifications. The Standard for Testing Methods of Performance of Concrete Mixes, GB/T 50080-2002, stipulates that the slump and slump flow values of concrete mixtures are measured in millimeters, with precision to 1 mm and the result rounded to the nearest 5 mm [29]. Since the influence of steel fiber volume fractions of 1.5% and 2.25% on the slump of concrete, after rounding, is consistent with the value for a volume fraction of 1.75%, a steel fiber volume fraction of 1.75% was selected as the representative. The study investigates the effect of different gradation EPS volume fractions on the flowability of concrete. For a steel fiber volume fraction of 1.75%, the slump values of concrete corresponding to different gradation EPS volume fractions in the gravel and crushed stone groups are shown in Figure 6. Overall, the increase in gradation EPS volume fraction is negatively correlated with the flowability of concrete, and the slump decreases at an accelerating rate. In Figure 6a for fine aggregate concrete, when the EPS volume fraction increases from 15% to 25%, the slump of all three concrete grades decreases by 10 mm. When the EPS volume fraction continues to increase to 35%, the slump decreases by 20 mm for all three grades. In Figure 6b, for gravel aggregate concrete, when the EPS volume fraction increases from 15% to 25%, the slump of C30 and C40 concrete decreases by 10 mm, while that of C50 decreases by 5 mm. However, when the EPS volume fraction increases from 25% to 35%, the slump of C50 decreases by 20 mm, while the slump reduction for the other grades is 15 mm. The influence of steel fibers and graded EPS on concrete flowability can be attributed to four main reasons:  
1. **Aggregate replacement:** The 5 mm particles in graded EPS replace some coarse aggregates, and the 1 mm particles replace part of the fine aggregates. This improves the embedding effect of the concrete matrix structure, increases frictional resistance, and, at the same time, the lower density of EPS particles compared with aggregates reduces the vertical and diagonal component forces from the weight of the concrete matrix during the slump test, leading to a decrease in the slump (positive effect). 2. **Reduced water content:** the incorporation of graded EPS reduces the amount of water required for concrete [30], increasing the consistency and reducing the slump (positive effect). 3. **Hydrophobic nature of EPS particles:** EPS particles are hydrophobic, preventing the retention of free water on their surfaces. As a result, there is no liquid bridge connection between EPS particles and fine aggregate particles. According to the Young-Laplace theory, the absence of capillary suction between the two types of particles leads to an increase in the concrete slump (negative effect). 4. **Steel fiber length:** The length of steel fibers is much greater than the diameter of the aggregates. When steel fibers are incorporated, they consume more cement paste to surround them.