## 3 Results and Discussion  
### Difference between Underwater Method and Abrasion Test on Waterborne Sand Impact  
The underwater method entails using circulating water (1200 rpm) that in turn moves the steel balls on the concrete surface, producing the desired abrasion effects. Figure 3 depicts a photo of the concrete specimen at the end of 72 h of testing. This photo indicates that the surface mortar layer has been worn away, the aggregate has been exposed, and the low strength mortar region has formed an indentation. The surface of concrete prepared with a high w/cm ratio of 0.50 exhibited a strongly nonregular structure, whereas concrete prepared with a low w/cm ratio of 0.28 exhibited a slightly nonregular surface. However, these nonregular surfaces were smooth. As illustrated in Figure 4, steel fibers added to the concrete, when exposed, were flattened by the rotating steel balls and covered by the mortar during the test period. The hard and the highly erosion-resistant steel fibers acted as shields, preventing the concrete material behind the fibers from being abrad  
Figure 3: Concrete surface at the end of 72 h of testing (w/cm = 0.50, ASTM C1138). According to the aforementioned observations, the friction wear involved mainly abrasive action resulting from the rolling water and steel balls. The principal resistances were shear strength and surface hardness. Therefore, concrete made with high compressive strength or containing high hardness materials, such as chert, quartzite and steel fiber, could exhibit improved resistance to friction abrasion. Similar findings have been reported in references [5; 16]. Observations of the specimen after being subjected to a waterborne sand jet test revealed that transient hydraulic rim-pulls impinged on the specimen and caused local tensile stresses in the top layer of the exposed concrete. In accordance with the theory of energy conservation, the intensity of the tensile stresses varied with respect to the impact momentum of the hydraulic jet forces. These tensile stresses are the prime culprits for causing cracks in the hardened mortar and fractures around the aggregate particles that eventually led to impact abrasion. Figure 5 shows various impact abrasions of the concrete after testing. The cement matrix exhibited significant indenting by the exposed erodent, and the aggregate grain appeared to peel away; specifically, the mortar layer was worn away and interfacial cracks became visible on the concrete prepared with a high w/cm ratio and impacted at 90\({}^{\circ}\) (Figure 5a), whereas it appeared to be rather smooth on the concrete prepared with a low w/cm ratio and impacted at 45\({}^{\circ}\) (Figure 5b). SEM revealed the formation of cracks in the cement matrix and the interface between the aggregate grain and the cement matrix (Figure 6a) for the concrete impacted at 90\({}^{\circ}\). Additionally, the concrete impacted at 90\({}^{\circ}\) displayed a rougher and more rugged surface than did the concrete impacted at 45\({}^{\circ}\) (Figure 6b). Figure 4: Typical abrasion site in steel fiber concrete abraded by friction abrasive action (w/cm = 0.36, ASTM C1138). Figure 5: \(Cont\). The presence of steel fibers in the concrete caused some notable phenomena. As Figure 7a illustrates, the partial fiber length separated from the matrix was exposed and the front region tapered to a fine point. Figure 7b shows that the entire circumference of the fiber was exposed to a certain depth, and the amount of exposed fiber was comparatively high for the fiber concrete impacted at 90\({}^{\circ}\). The exposed steel fiber could shield the concrete material from being removed. Another notable observation was the loss of bonding between both the fiber and the matrix for the concrete impacted at 45\({}^{\circ}\) and 90\({}^{\circ}\) (Figure 7c). This observation demonstrates the weak interface between the fiber and the concrete matrix. Figure 5: Images of worn concrete surfaces after impact test.