The Counterbalance of the Adverse Effect of Abrasion on the Properties of Concrete Incorporating Nano-SiO 2 and Polypropylene Fiber Based on Pore Structure Fractal Characteristics

: Abrasion damage is a typical hydraulic structure failure and considerably impacts the durability of buildings. In severe circumstances, it can even prevent hydraulic structures from being used and operated normally. Thus, it is essential to research abrasion-resistant hydraulic systems that are more durable, inexpensive, safe, and ecologically friendly, given its unavoidable characteristics. In this context, ﬁve dosages of nano-SiO 2 and three dosages of ﬁbers are selected to evaluate and analyze the modiﬁcation effect of nano-SiO 2 and polypropylene ﬁbers on the abrasion resistance of concrete. The evolution of the concrete properties was characterized based on the abrasion resistance strength. Moreover, the mineralogical composition and microstructure characterization were investigated through X-ray diffraction and scanning probe microscope. Mercury intrusion porosimetry was applied to determine the pore-structure parameters of concrete, such as pore-size distribution and the fractal characteristics. The results indicate that nano-SiO 2 improves the abrasion resistance of concrete by densifying the pore structure and promoting the formation of hydration products. Results reveal that the excessive dosage of ﬁbers agglomerates in the concrete to form an unsubstantial pore structure due to poor dispersibility. The fractal dimension of the pore structure exhibits a close relationship with the abrasion resistance strength of concrete. The implications of these ﬁndings inform the design of abrasion and erosion resistance for hydraulic engineering structures.


Introduction
The abrasion damage of hydraulic concrete is caused by the continuous friction and impact of the sediment, gravel, sand, ice, and other debris mixed in seawater or river water on the concrete surface [1]. In actual engineering, spillway skirts, scuppers, stilling pools, culverts, and bridge abutments are easily damaged by erosion and erosion, resulting in the wear, cracking, and shedding of hydraulic concrete surfaces and even corrosion of steel bars.
Incorporating different materials to improve the abrasion resistance of concrete is the most effective and widely used method in engineering [2][3][4][5][6]. Combined with the form and process of concrete being damaged by abrasion, the materials currently used are mainly through the following three ways: (1) Improve the strength of concrete cement paddle by adding mineral admixtures to form a harder cement shell to resist the damage of highspeed sand-containing water flow, such as silica fume concrete, high-strength concrete, HF concrete, and a series of concrete materials [7]. These concrete materials often have a considerable increase in compressive strength while the abrasion resistance is improved as well. (2) By controlling the cracks generated by the concrete under the action of high-speed sand-containing water flow, the development of cracks is inhibited, the integrity of the  First, nano-SiO 2 and polycarboxylate superplasticizer were added into an aqueous solution and stirred for two minutes to make a uniformly dispersed nano-SiO 2 solution. Next, coarse aggregate, cement, fly ash, and fine aggregate were added into the mixer for two minutes. Then, the remaining water and nano-SiO 2 solution were added to the mixer and mixed for two minutes. Finally, polypropylene fibers were added and stirred for another two minutes to prepare a paddle with good fluidity. After vibrating the concrete mixture loaded into the concrete mold on the vibrating table, it was left to stand for 24 h and then demolded.
All specimens were prepared; refer to Specifications for the mix proportion design of ordinary concrete (JGJ55-2011) and cured at 20 ± 2 • C and 95% RH until an age of 28 days [18], as shown in Figure 1.

Abrasion Resistance Strength
The concrete abrasion test was carried out by using the HKCS-2 testing machine designed in DL/T5332-2005 [19]. The high-speed centrifugal force was used to rotate the impeller to make the sand-containing water flow impact the inner ring of the concrete to determine the ability of the concrete surface to resist the impact and wear damage of the high-speed, sand-containing water flow after 28 and 60 days. The test equipment and specimen are shown in Figure 2. Each group consisted of three replicate specimens. The sand content of water flow is 20% by mass. The single flushing time was 30 min, then the abrasive was replaced, and the operation was repeated 4 times. Additionally, when the saturated surface of the specimen was dry, the mass (g) of the specimen was weighed, and the total abrasion resistance of the specimen was calculated by Equation (1): where fa (h/ (kg/m 2 )) is the abrasion resistance strength, t (h) is the cumulative abrasion time of specimens, A (m 2 ) is the scour area the specimen, and ΔM is the accumulated mass change of the specimen.

Abrasion Resistance Strength
The concrete abrasion test was carried out by using the HKCS-2 testing machine designed in DL/T5332-2005 [19]. The high-speed centrifugal force was used to rotate the impeller to make the sand-containing water flow impact the inner ring of the concrete to determine the ability of the concrete surface to resist the impact and wear damage of the high-speed, sand-containing water flow after 28 and 60 days. The test equipment and specimen are shown in Figure 2. Each group consisted of three replicate specimens.

Abrasion Resistance Strength
The concrete abrasion test was carried out by using the HKCS-2 testing machi signed in DL/T5332-2005 [19]. The high-speed centrifugal force was used to rotate t peller to make the sand-containing water flow impact the inner ring of the conc determine the ability of the concrete surface to resist the impact and wear damage high-speed, sand-containing water flow after 28 and 60 days. The test equipmen specimen are shown in Figure 2. Each group consisted of three replicate specimens The sand content of water flow is 20% by mass. The single flushing time was 3 then the abrasive was replaced, and the operation was repeated 4 times. Additio when the saturated surface of the specimen was dry, the mass (g) of the specime weighed, and the total abrasion resistance of the specimen was calculated by Equati = where fa (h/ (kg/m 2 )) is the abrasion resistance strength, t (h) is the cumulative ab time of specimens, A (m 2 ) is the scour area the specimen, and ΔM is the accumulated change of the specimen. The sand content of water flow is 20% by mass. The single flushing time was 30 min, then the abrasive was replaced, and the operation was repeated 4 times. Additionally, when the saturated surface of the specimen was dry, the mass (g) of the specimen was weighed, and the total abrasion resistance of the specimen was calculated by Equation (1): where f a (h/(kg/m 2 )) is the abrasion resistance strength, t (h) is the cumulative abrasion time of specimens, A (m 2 ) is the scour area the specimen, and ∆M is the accumulated mass change of the specimen.

Microstructural Monitoring
The mineral phases, morphology, and pore structure of the concrete samples were characterized to assess the alterations caused by the abrasion damage process. Utilizing a Bruker diffractometer with a copper target operating at 45 kV and 40 mA, we qualitatively analyzed the amorphous substances in the altered sample. The XRD patterns were recorded in a 2θ angle range of 5 • -50 • with a counting time of 50 s/step and a step size of 0.016 • . MIP at a contact angle of 130 • and surface tension of 485 dynes/cm (Hg) evaluated the PSD of the mortars; additionally, the fractal dimension of the specimen was calculated according to the results of PSD. SEM was performed on a Zeiss Sigma 500 microscope to identify the morphology and microstructure of the samples on a microscale. Electron probe microanalysis (8050G, EPMA) was used to characterize and image the demineralization process and its consequence on matrices in the mortar samples after abrasion damage.

Fractal Dimension
Fractal geometry can be used to quantitatively characterize the irregularity, disorder, and complexity of shapes [20,21]. The pore network of eroded concrete exhibits significant fractal geometrical characteristics. The fractal modeling of porous materials is based on the principle of the Menger sponge (Karl and Menger, 1935). A cube with a side length R is divided into m 3 equal cubes, some small cubes are removed according to a rule, and the remaining small cubes have a length N 1 (m). This process is carried out iteratively. Hence, the number of remaining cubes continues to increase, while the sizes of the cubes continue to decrease. The remaining infinite number of small cubes constitute the matrix of the material, and the removed small cubic spaces of different orders make up the pore grid of the material. After performing the removal process by k times, the remaining cube size is r k = R/m k , and its number can be calculated using Equation (2): where D is the volume fractal dimension of the geometry. D is obtained using Equation (3).
The volume of the solid structure is then calculated using Equation (4).
When K → ∞ or r k → 0, V k → V(r), and we can obtain Equation (5).
According to the pore volume of the concrete V p (r) = R 3 − V(r), we obtain Equation (6): where V p is the cumulative invasion volume of mercury under pressure P, r is the pore radius of the concrete specimen, and D is the fractal dimension. The pore volume of concrete was measured using the MIP method, and the fundamental principle was expressed using the Washburn equation and calculated using Equation (7): where T is the surface tension of mercury, θ is the contact angle between mercury and the solid, and P is the pressure exerted when mercury invades. Equations (6) and (7) were used to establish the relational expression of the fractal dimension, as expressed in Equation (8). Figure 3 shows the change in the abrasion resistance strength of modified concrete. Table 4 lists the abrasion resistance strength of the selected sample. Compared with the PFC and NC specimens, it is found that the incorporation of polypropylene fibers can effectively enhance the abrasion resistance of concrete, and the abrasion rates of concrete after curing for 28 days and 60 days are reduced by 3.04% and 2.17%, respectively, compared with ordinary concrete. This is mainly due to the fact that the surface cement slurry is washed and worn away in the process of concrete being eroded and damaged by high-speed water flow, and the polypropylene fibers will form an overlapping network structure inside the concrete, which will have a restraining effect on the aggregates in the concrete. Additionally, for the broken cement block, the bond between the polypropylene fiber and it can also keep it on the concrete, continuing to resist the impact of high-speed sand-containing water flow abrasion damage, and the corresponding concrete abrasion resistance is also improved. When the content of polypropylene fiber is 1.2 Kg/m 3 , it cannot be well-dispersed in the concrete, and the agglomeration of the fiber causes it to become a weak area inside the concrete, which has a negative impact on the formation of the dense structure inside the concrete. Nevertheless, the restraining effect of fibers on the block damaged by impact wear and the reinforcing effect of nanoparticles on the cement matrix still make the abrasion resistance strength of the PF12-NS3 samples enhanced. Comparing the abrasion resistance strength of 28 days and 60 days, it can be found that the addition of excess fiber has a great influence on the formation of the compact structure inside the concrete. Overall, the addition of nano-SiO 2 could enhance the abrasion resistance of concrete. When the nano-SiO 2 content was 5%, the concrete had the highest abrasion resistance strength. However, when the content of nano-SiO 2 was increased to 7%, with too much nano-materials agglomerate in the concrete, the wrapped water molecules hindered the hydration of the cement so that the fibers were not firmly bonded to the matrix in the concrete, and the internal structure of the concrete was weakened. Consequently, the abrasion resistance strength of concrete was reduced. When the nano-SiO2 content was 5%, the concrete had the highest abrasion resistance strength. However, when the content of nano-SiO2 was increased to 7%, with too much nano-materials agglomerate in the concrete, the wrapped water molecules hindered the hydration of the cement so that the fibers were not firmly bonded to the matrix in the concrete, and the internal structure of the concrete was weakened. Consequently, the abrasion resistance strength of concrete was reduced.    Figure 4 illustrates XRD spectrum of the nano-SiO2 and polypropylene fiber modified concrete with different dosages when the curing age was 28 days. With the increase of curing age, the characteristic peak value of C3S in concrete decreased more obviously, indicating that most of the cement had been hydrated into C-S-H gel, and the addition of nano-SiO2 will make the content of C3S decrease heavily, indicating the greater degree of cement hydration. At the same time, the change of fiber content also affected the change of C3S content in concrete. Comparing several groups of ratios of fiber content change, when the content was 0.9 kg/m 3 , it had the best effect of promoting hydration.  Figure 4 illustrates XRD spectrum of the nano-SiO 2 and polypropylene fiber modified concrete with different dosages when the curing age was 28 days. With the increase of curing age, the characteristic peak value of C 3 S in concrete decreased more obviously, indicating that most of the cement had been hydrated into C-S-H gel, and the addition of nano-SiO 2 will make the content of C 3 S decrease heavily, indicating the greater degree of cement hydration. At the same time, the change of fiber content also affected the change of C 3 S content in concrete. Comparing several groups of ratios of fiber content change, when the content was 0.9 kg/m 3 , it had the best effect of promoting hydration.

Mineral Phases by XRD
The addition of nano-SiO 2 increased the content of Ca(OH) 2 in the concrete, but then its characteristic peak had a significant decrease. It shows that in the early stage of curing, the addition of nano-SiO 2 into the concrete will promote the hydration of the cement, and the amount of generated Ca(OH) 2 will be enhanced. Subsequently, due to the small specific surface area and high chemical activity of nano-SiO 2 , the hydration of Ca(OH) 2 is further promoted to form C-S-H gel. When curing for 28 days, the amount of Ca(OH) 2 crystals in the concrete was the least when the content of nano-SiO 2 was 3%, which is consistent with the strength change exhibited by the macroscopic mechanical properties. When the nano-SiO 2 content reached 7%, it can be seen that the effect of nano-SiO 2 on the further hydration of the generated Ca(OH) 2 crystals was small, and the quantity of C-S-H gels generated in the concrete as also hard to notice.

Pore Structure Fractal Characteristic by SEM and EPMA
In order to further understand the effect of nano-SiO2 and polypropylene fiber-modified concrete on its properties and abrasion resistance, the concrete samples under proportions were observed by SEM technology. Overall, the incorporation of nano-SiO2 made the internal microstructure of concrete more compact. As shown in Figure 5c-f, the content of calcium hydroxide crystals inside the nano-SiO2 modified concrete decreased greatly, and only a small amount of exposed ettringite was found in the C-S-H gel stacking structure. At this time, the C-S-H gel in the concrete had formed a relatively dense structure under the early hydration of nano-SiO2.
When the curing age continued to increase to 28 d, the larger content of C-S-H gel completely wrapped the network fiber structure, and the structures were stacked in an

Pore Structure Fractal Characteristic by SEM and EPMA
In order to further understand the effect of nano-SiO 2 and polypropylene fibermodified concrete on its properties and abrasion resistance, the concrete samples under proportions were observed by SEM technology. Overall, the incorporation of nano-SiO 2 made the internal microstructure of concrete more compact. As shown in Figure 5c-f, the content of calcium hydroxide crystals inside the nano-SiO 2 modified concrete decreased greatly, and only a small amount of exposed ettringite was found in the C-S-H gel stacking structure. At this time, the C-S-H gel in the concrete had formed a relatively dense structure under the early hydration of nano-SiO 2 .   Figure 6 illustrates BSE image associated with O, Al, Si, S, and Ca mappings of the PF-NS3 and PF-NS5 sample after abrasion damage. The lap joint of the fibers inside the damaged concrete is discontinuous, and the hydration products inside the concrete show the characteristics of regional distribution, resulting in poor continuity of the concrete microstructure After the abrasion damage, a large number of micro-cracks appeared inside the concrete. These micro-cracks expanded each other and became weak areas of concrete When the curing age continued to increase to 28 d, the larger content of C-S-H gel completely wrapped the network fiber structure, and the structures were stacked in an orderly manner, forming a continuous structure with high uniformity. Additionally, its internal Ca(OH) 2 and AFt crystals were mostly hydrated into C-S-H gels under the promotion of nano-SiO 2 . Moreover, part of the fly ash's surface-active substances was also consumed by hydration, exposing iron oxides within it that cannot be hydrated. When the content of nano-SiO 2 was 3%, the microstructure of the sample was the densest. Combined with the effect of polypropylene fibers, the microstructure of the C-S-H gel was optimized. The denser polypropylene fiber network structure makes the packing morphology of C-S-H gel more compact. At the same time, needle-like AFt crystals can also fill the pores in the middle of the network structure and hydrate to form C-S-H gel under the promotion of nano-SiO 2 , which ensures its stability and further compacts the microstructure of concrete. When the dosage was 5%, although there exists a small part of nano-SiO 2 particles that was not well dispersed, it did not become a weak area inside the concrete; consequently, the strength of the concrete was promoted as well.
The growth space of calcium hydroxide crystals in concrete will be compressed with the increase of nano-SiO 2 content, which will make the hardened cement structure loose and reduce the impact and wear resistance of concrete. As is shown in Figure 5f, due to the high water absorption of nanoparticles, at the same age, the distribution of calcium silicate hydrate inside the concrete was less, and the contents of AFt and Ca(OH) 2 increased. The microstructure of PF-NS7 concrete is mainly composed of needle-like and flake-like crystals and agglomerated nanoparticles, and the consistency and continuity of the structure are poor, which corresponds to the poor macroscopic mechanical properties of concrete mixed with 7% nano-SiO 2 .
In the ratio of changing the content of fibers by fixing the content of nano-SiO 2 , the microstructure of concrete formed by the change of fibers was also different. When the fiber content was 0.6 kg/m 3 , the lap joint of the fibers in the concrete was relatively discontinuous, and the hydration products in the concrete had obvious regional distribution characteristics, resulting in poor continuity of the concrete microstructure and the accumulation of C-S-H gel, which makes its internal microscopic appearance uneven. It is worth noting that the uneven surface accelerates the cavitation damage of high-speed water flow during the abrasion process. Figure 6 illustrates BSE image associated with O, Al, Si, S, and Ca mappings of the PF-NS3 and PF-NS5 sample after abrasion damage. The lap joint of the fibers inside the damaged concrete is discontinuous, and the hydration products inside the concrete show the characteristics of regional distribution, resulting in poor continuity of the concrete microstructure After the abrasion damage, a large number of micro-cracks appeared inside the concrete. These micro-cracks expanded each other and became weak areas of concrete deterioration. The microstructure of concrete with a nano-SiO 2 content of 3% was the densest. Under the synergistic toughening of polypropylene fibers, the abrasion resistance of C-S-H gel increased to a certain extent. This is because that the network structure formed by polypropylene fibers makes the packing of C-S-H gel denser, and at the same time, nano-SiO 2 can be filled between the network structures formed by AFt crystals, thereby promoting the formation of C-S-H gel by hydration. The synergistic effect of the two substances increased the degree of the compaction of the concrete matrix while maintaining the structural stability. Figure 7 depicts the PSD of selected sample on the basis of five predefined pore ranges. Concrete pore-size distribution is separated into five categories: "harmless pores" (d ≤ 20 nm), "less harmful pores" (20 nm ≤ d ≤ 50 nm), "harmful pores" (50 nm ≤ d ≤ 200 nm), and "more harmful pores" (d > 200 nm) [11]. As shown in the figure, with the increase of nano-silica, the proportion of harmless pores and less harmful pores in concrete increases, and the proportion of harmful pores and porous pores decreases. Apparently, this is due to the self-filling effect of nano-SiO 2 and its promotion in the cement hydration process. The observations derived herein are in agreement with those presented in ref. [22]. Moreover, the increase in fiber dosage caused the opposite effect. Poor dispersibility leads to the agglomeration of a large amount of fiber inside the concrete pores, increasing the proportion of macropores in the concrete (see Figures 5 and 6). of C-S-H gel increased to a certain extent. This is because that the network structure formed by polypropylene fibers makes the packing of C-S-H gel denser, and at the same time, nano-SiO2 can be filled between the network structures formed by AFt crystals, thereby promoting the formation of C-S-H gel by hydration. The synergistic effect of the two substances increased the degree of the compaction of the concrete matrix while maintaining the structural stability.  Figure 7 depicts the PSD of selected sample on the basis of five predefined pore ranges. Concrete pore-size distribution is separated into five categories: "harmless pores" (d ≤ 20 nm), "less harmful pores" (20 nm ≤ d ≤ 50 nm), "harmful pores" (50 nm ≤ d ≤ 200 nm), and "more harmful pores" (d > 200 nm) [11]. As shown in the figure, with the increase of nano-silica, the proportion of harmless pores and less harmful pores in concrete in- of nano-silica, the proportion of harmless pores and less harmful pores in concrete increases, and the proportion of harmful pores and porous pores decreases. Apparently, this is due to the self-filling effect of nano-SiO2 and its promotion in the cement hydration process. The observations derived herein are in agreement with those presented in ref. [22]. Moreover, the increase in fiber dosage caused the opposite effect. Poor dispersibility leads to the agglomeration of a large amount of fiber inside the concrete pores, increasing the proportion of macropores in the concrete (see Figures 5 and 6).  Table 5 shows the fractal dimension of the selected sample. As shown in the table, the addition of nano-SiO 2 increases the fractal dimension of concrete. This indicates that nano-SiO 2 optimizes the pore size distribution in concrete, changes the macroporous structure into small pores, and increases the complexity of the internal pore structure of concrete [23]. This is due to the micro-aggregate filling effect of nano-SiO 2 . When the content of nano-SiO 2 increases to 7%, the fractal dimension of concrete begins to decrease, and the proportion of harmless pores and harmful pores also decreases. Firstly, when the dosage of nano-SiO 2 is too high (7%), too much nano-materials agglomerate in the concrete, and the wrapped water molecules hinder the hydration of the cement so that the fibers are not firmly bonded to the matrix in the concrete, and the internal structure of the concrete is weakened. Secondly, under the synergistic effect of nanomaterials and fibers, the proportion of harmful pores and harmful pores in concrete with 3% nano-SiO 2 dosage is relatively high, and the complexity of pores is relatively low, so its fractal dimension is relatively low compared with specimen incorporating 3% nano-SiO 2 . The abrasion resistance of concrete is closely related to the internal pore structure. Figure 8 shows the relationship between the fractal dimension of concrete and the abrasion resistance strength. It can be seen from the fitting results in Figure 8 that the fractal dimension is linearly correlated with the abrasion resistance strength, and the correlation coefficient is high. The greater the fractal dimension of concrete, the greater the abrasion resistance under high-speed flow scouring and the better the toughness. PF-NS5 concrete has the highest fractal dimension [24], so its anti-abrasion performance is the highest.

Conclusions
By experimentally determining the abrasion resistance strength, mineral phases alterations, morphology, and pore structure and calculating the fractal dimension of different dosages of nano-SiO2 and polypropylene fiber, our results elucidate the mechanism for synergistic effect of nano-SiO2 and polypropylene fiber on the properties of concrete. The observations derived here might may help steer the definition of more appropriate recommendations in actual engineering. From the macroscopic and microscopic results discussed above, the following conclusions can be drawn: (1) When the dosage is between 1% and 5%, nano-SiO2 can compact the microstructure of concrete and promote the hydration of cement to generate more Ca(OH)2 and AFt crystals in the early stage of curing. Due to its small specific surface area and high chemical activity, nano-SiO2 can continue to play the role of control and refinement during the hydration process, reducing the proportion of macro-pores inside concrete and increasing the formation of C-S-H gel. When the dosage of nano-SiO2 is 3%, the internal structure of concrete is the optimum. However, excessive dosage will increase the content of harmful pores and inhibit the hydration process of cement. (2) When the content of polypropylene fibers is 0.9kg/m 3 , the fibers form a stable structure, overlapping each other inside the concrete, which promotes a smoother and flatter microstructure of the matrix. At the same time, under this dosage, the fibers can restrain the separation of the broken cement blocks during the process of abrasion damage, thereby improving the abrasion resistance of concrete. However, a toohigh fiber dosage, such as 1.2 kg/m 3 , will produce a vulnerable pore structure due to the unfavorable dispersibility of fibers, resulting in an increase in pore volume and a decrease in concrete performance. (3) The internal pore structure of concrete with nano-SiO2 and polypropylene fiber shows obvious fractal characteristics. At the same time, the fractal dimension has a close positive correlation with the concrete abrasion resistance strength. The larger the fractal dimension, the higher the abrasion resistance strength and toughness of concrete. (4) In general, the incorporation of nano-SiO2 and polypropylene fibers in concrete improves the abrasion resistance of concrete. Nano-SiO2 can promote cement hydration, compact the microstructure of concrete, and enhance the bond between the cement matrix and the fibers so that the fibers can fully exert the restraint effect on the broken

Conclusions
By experimentally determining the abrasion resistance strength, mineral phases alterations, morphology, and pore structure and calculating the fractal dimension of different dosages of nano-SiO 2 and polypropylene fiber, our results elucidate the mechanism for synergistic effect of nano-SiO 2 and polypropylene fiber on the properties of concrete. The observations derived here might may help steer the definition of more appropriate recommendations in actual engineering. From the macroscopic and microscopic results discussed above, the following conclusions can be drawn: (1) When the dosage is between 1% and 5%, nano-SiO 2 can compact the microstructure of concrete and promote the hydration of cement to generate more Ca(OH) 2 and AFt crystals in the early stage of curing. Due to its small specific surface area and high chemical activity, nano-SiO 2 can continue to play the role of control and refinement during the hydration process, reducing the proportion of macro-pores inside concrete and increasing the formation of C-S-H gel. When the dosage of nano-SiO 2 is 3%, the internal structure of concrete is the optimum. However, excessive dosage will increase the content of harmful pores and inhibit the hydration process of cement. (2) When the content of polypropylene fibers is 0.9kg/m 3 , the fibers form a stable structure, overlapping each other inside the concrete, which promotes a smoother and flatter microstructure of the matrix. At the same time, under this dosage, the fibers can restrain the separation of the broken cement blocks during the process of abrasion damage, thereby improving the abrasion resistance of concrete. However, a too-high fiber dosage, such as 1.2 kg/m 3 , will produce a vulnerable pore structure due to the unfavorable dispersibility of fibers, resulting in an increase in pore volume and a decrease in concrete performance. (3) The internal pore structure of concrete with nano-SiO 2 and polypropylene fiber shows obvious fractal characteristics. At the same time, the fractal dimension has a close positive correlation with the concrete abrasion resistance strength. The larger the fractal dimension, the higher the abrasion resistance strength and toughness of concrete. (4) In general, the incorporation of nano-SiO 2 and polypropylene fibers in concrete improves the abrasion resistance of concrete. Nano-SiO 2 can promote cement hydration, compact the microstructure of concrete, and enhance the bond between the cement matrix and the fibers so that the fibers can fully exert the restraint effect on the broken cement blocks. Moreover, the polypropylene fiber in the concrete will also play a role in controlling the cracks that are easily generated in the early stage after the incorporation of nano-SiO 2. The results derived show that when the contents of nano-SiO 2 and polypropylene fiber are 3% and 0.9 kg/m 3 , the effect of improving the abrasion resistance of concrete is the best. In practical engineering, it is recommended to use the mix proportion of the PF-NS3 specimen to improve guidance on the mixture design of concrete when exposure to abrasion is expected in the field.