Mechanisms and influential variables on the abrasion resistance hydraulic concrete

2.3.1 Repairing materials

It is an effective method to use repairing materials in damaged hydraulic structures to prolong the period of anti-abrasion, and particular restoration techniques appropriately used will guarantee the hydraulic structures’ safe functioning. However, repairing hydraulic concrete structures is a challenging task requiring much workforce, materials, and financial resources. Mai et al. [24], by conducting abrasion tests under sandy water flowing at a velocity of 40 m/s, discovered that the abrasion resistance of epoxy resin with a “sea-island structure” was significantly improved compared to C70 concrete. Sun et al. [25] investigated the properties and application scope of using SK abrasion and scratch-resistant polyurea coatings for protecting the overflow surface of Fuchunjiang Hydropower Station against abrasion, and demonstrated that their durability, abrasion resistance, and bonding with concrete were excellent. In another work, Wang et al. [26] using the block test on-site comparison method, compared the repairing conditions of seven various abrasion-resistant epoxy mortars in the Xiaolangdi sand drainage tunnel. Comparing results revealed that the construction of epoxy mortars should match the requirements for abrasion resistance performance, while preventing a soft surface so as to lead to the best improvement effect. In all kinds of large and even ultra-large water conservancy projects being carried out vigorously now, hydraulic structures will face the challenge of abrasion damage caused by the higher flow rate and velocity of water flows. Thus, the difficulty of repairing hydraulic structures affected by abrasion damage from high-speed sandy water flows also rises linearly.

2.3.2 Different materials for modifying concrete abrasion resistance

There is a general agreement that using adequate enhancement material in concrete will be the most effective improvement to increase the abrasion resistance of concrete, therefore, most scholars have started research focused on the use of different materials to enhance the concrete’s abrasion resistance. At present, research shows that the abrasion resistance of concrete is mainly dominated by the strength of the surface cement mortar, the ability to control the degree of crack development after damage, and the hardening degree of coarse aggregate. Therefore, the research on incorporating different materials into concrete for improving its abrasion resistance can be categorized into the following aspects.

2.3.3 Mineral admixtures

Silica fume is the most extensively utilized substance to enhance the concrete abrasion resistance currently. The silica fume was primarily used for filling the micro voids of the concrete matrix and the interfacial zones between coarse aggregate and mortar. In addition, silica fume is pozzolanic, i.e., it reacts with calcium hydroxide to produce more calcium–silicate–hydrate, thereby reducing porosity and increasing the strength. Generally, the abrasion resistance of concrete can be improved by adding silica fume [27]. Besides, Ashish and Verma [28] Stated that the loss in permeability can be compensated up to some extent with the addition of silica fume. As a result, incorporating silica fume into concrete optimized the cement paste, and markedly enhanced the abrasion resistance of concrete.

Some of the researchers studied the concrete abrasion resistance that only incorporates silica fume. Santanu and Bratish [29] based on the 28 days strength of concrete, discovered that the ideal silica fume content of concrete was not a constant but depended on the water-to-cement ratio of the concrete. But, replacing cement with 15–25% of silica fume will considerably improve the mechanical characteristics of the concrete in the general case. Yogendra et al. [30] discovered that for the properties of concrete with a higher water-to-cement ratio, the greater enhancement of concrete properties would appear in the larger admixtures replacement of silica fume situation. As demonstrated in Figure 6, the abrasion resistance of silica fume concrete improved with increase in its silica fume content due to the higher content of hydrated calcium silicate regardless of the test method: the underwater abrasion or the blown sand abrasion [31]. Regarding the silica fume content of concrete, Wu et al. [32] studied the interfacial bond strength of concrete containing 0–7% of silica fume at the same mass substitution of cement. They found that the early strength of the bonding interface of concrete was higher at 5% than 4% of silica fume, but the 28 days strength of the bonding interface of the concrete was higher at a lower silica fume content. However, the amount of silica fume in the concrete mixture should not generally exceed 30%, because the excessive silica fume causes severe early cracking of the concrete, which is not conducive to the abrasion resistance of concrete under a high-speed sand-containing water flow. Simultaneously, including a large amount of silica fume will significantly impact the workability and fluidity of concrete, which is detrimental to the actual construction process. Most importantly, silica fume concrete has various drawbacks, such as sizeable early shrinkage and high hydration heat concentration, imposing some limitations on the hydraulic structure applications demanding abrasion resistance.

Figure 6

The effect of silica fume on the abrasion resistance of concrete [31].

To overcome the drawbacks of abrasion resistance of silica fume concrete, the most effective way to enhance the inadequacy of silica fume concrete is to combine different materials with silica fume, using the advantages of other materials to compensate for the shortcomings of silica fume concrete.

The primary purpose of mixed other mineral admixtures was to reduce concrete cracking due to the excessive heat of hydration [33,34]. Liu and Fang [35] discovered that fly ash (FA) had a low hydration activity, which could delay the hydration rate of cement, thereby effectively reducing the high hydration heat of silica fume concrete in the early stages and preventing the formation of early cracks; Gil and Golewski [36] stated that the basic physical mechanism of interaction of FA and silica fume with concrete is primarily to seal the composite microstructure; de Gutiérrez et al. [37] found that FA particles were unique, spherical, glassy, and bead-shaped, which could effectively disperse the cement particles and fill the pores in the cement paste, thereby densifying the cement paste; Naik et al. [38] experimented with investigating the impact of FA on the abrasion resistance of concrete by using three different types of materials. Their results demonstrated that the type of FA significantly impacted the abrasion resistance of concrete, FA reduced the abrasion resistance of concrete regardless of its type, and the weakness of concrete was more pronounced in the early curing periods than in the later stages. Finally, there is a best content for FA to influence concrete abrasion resistance, which in this study will be 40%, abrasion resistance will be greatly reduced if this content is exceeded, as depicted in Figure 7. However, it is still an effective way to add FA into silica fume concrete to enhance its mechanical property. Through experiments, Ma [34] found that the strength of concrete was enhanced by increasing the silica fume content in the range of 3–8%, but the viscosity of concrete will also be increased at the same time, considerably raising the possibility of early cracking of concrete. Therefore, incorporating FA into concrete can dramatically improve its abrasion resistance compared to concrete mixed solely with silica fume.

Figure 7

The effect of FA content on the abrasion resistance of concrete at a curing period of (a) 28 days and (b) 91 days [38].

Although the addition of FA declines the concrete abrasion resistance strength when the dosage of silica fume is significant [31,39], it could reduce the amount of cement, ease the burden of temperature control, alleviate the risk of early concrete cracking, and enhance the workability and fluidity of concrete under the premise of satisfying various technical requirements. Hence, considering the resistance of concrete to cracking, abrasion resistance, and the construction performance, it is still recommended that 20% FA be mixed with cement to ensure that the crack resistance of the concrete is not too poor to affect its regular use. Meanwhile Ashish and Verma [40] have pointed out that when using supplementary cementitious materials like silica fume, FA, etc., in concrete, a water–cement ratio in addition to pozzolanic material by weight must be considered. There is a close relationship among water–cement and water–cementitious material ratio, and water–cementitious material ratio could provide the particular amount of water needed for the hydration process. Therefore, starting the research focus on the content of supplementary cementitious materials in concrete have become a prominent necessity.

Besides, there existed the study on different additives mixed with silica fume concrete to examine their impact on the abrasion resistance of concrete. Additives primarily considered the regularly used additives such as water-reducing agents, shrinkage reducers, and expansion agents. Scholars such as Hu [3] and Ding [41] have reached a more consistent conclusion, that among these additives, shrinkage reducing agents and expansion agents negatively influence the abrasion resistance of concrete and should be avoided in engineering practice to ensure project quality. However, Li et al. [42] reported that the addition of expansion agents could prevent the cracking of the high-strength concrete (strength C60–C80) mixed with silica fume and equipped with stressed steel bars. Nevertheless, studies have reported both positive and negative impacts of water-reducing agents on the abrasion resistance of concrete depend on the type and rate of water reduction, and the actual influence of water-reducing agents is not noteworthy. Thus, they can be employed in engineering applications.

2.3.4 Fibers

Fibers are also a widely used material to enhance the concrete abrasion resistance at present. Unlike silica fume, fibers are mainly used in concrete to bridge cracks formed by stress shrinkage, temperature, or dry shrinkage during hydration, thereby effectively relieved the stress concentration at the tip of cracks, delayed the development of cracks, and controlled the internal cracks in concrete. In this case, the internal cracks in concrete were effectively controlled, strengthened the continuity of the concrete, and prevented the cracks from further expanding after the concrete cracks, thus, the resistance of concrete to high-speed sandy water flows is enhanced. From the works conducted in this context [43,44,45,46,47], it can be concluded that, unlike ordinary concrete, the cement matrix peeled off and left pits on the surface of the concrete after abrasion damage. Fiber-reinforced concrete made the damaged pieces stay on the surface of the concrete due to the restraining effect of fibers, allowing it to withstand the abrasion damage. Therefore, the resistance of concrete abrasion has significantly improved.

Concrete is known to be intrinsically porous and brittle making surface cracking inevitable, and the abrasion of concrete that results from impact of water borne particles and matrix removal by fracture is a mechanical degradation process, and fiber used can effectively control it. Therefore, the different lengths, thicknesses, and shapes of fiber modified in concrete had been conducted by many researchers to examine the impact on abrasion resistance of hydraulic concrete. For example, Nihat [48] studied the effects of the length of basalt fibers (12 and 14 mm), the fiber content, and the water-to-binder ratio on the abrasion resistance of concrete. As shown in Figure 8, the abrasion resistance of the concrete improved as its water-to-binder ratio declined, for the characteristics of the basalt fibers. At higher fiber contents of the concrete, the abrasion resistance weakened as the length of the basalt fibers increased. However, when the fiber content of the concrete is at a low situation, longer basalt fibers will improve the abrasion resistance of concrete more prominently. Moreover, many researchers have proposed the study of combining fibers of the same type but with different morphological characteristics, using different types of fibers, or employing fibers together with other materials. Then they compared the results to the single-fiber composite concrete’s situation, to complete the research on the mechanism of the abrasion resistance of fiber-reinforced concrete [49,50,51]. For example, the use of the polypropylene (PP) fiber effects in limited solidifying contraction and the use of steel fiber increases the compression and tensile strength. In conclusion, fiber reinforced concrete has only high strength but also high durability in specific environmental conditions.

Figure 8

The effect of the basalt fiber content on the abrasion resistance of concrete [48].

PP fibers have been widely used on concrete to improve its abrasion resistance. The concretes with PP fiber are beneficial in restricting the drying shrinkage, increasing the split tensile strength, and restraining the initiation and development of cracks in cement pastes and concrete. Horszczaruk [52] investigated the effect of three different PP fibers on the concrete abrasion resistance using a fiber content of 0.9%, and discovered that regardless of the length and Young’s modulus of PP fibers, the PP-fiber-reinforced concrete samples’ surface will be uniformly abrasive, indicating the significant enhancement of the abrasion resistance of hydraulic concrete. Meanwhile, Liu and Wang [44] reported that PP fibers could directly improve the resistance of concrete to micro-cutting wear damage. de Gutiérrez et al. [37] also found that when PP fibers were evenly distributed inside the concrete, the compressive strength of concrete change slightly, and in a few cases, there might even be a modest reduction in compressive strength. Nonetheless, PP fibers still dispersed the energy generated by concrete shrinkage, enhanced the internal continuity of the concrete material, reduced the scale of micro-cracks, and weakened the concrete stress concentration during the stressing process, resulting in an improvement in the brittleness and abrasion resistance of concrete. Similar results were also reported by Lu et al. [45] and Andrzej [53]. Additionally, Zoran et al. [18] conducted a corresponding experiment on the influence of different PP fibers on the abrasion resistance of concrete. They concluded that in the case of a low water-to-binder ratio, the PP fiber compacted the pore and decreased the pore size to better enhance the abrasion resistance of concrete, as shown in Figure 9. It can be concluded that the group of composed fibrillated fibers have a better performance in concrete mechanical properties, than the monofilament fibers with a circular cross-section and smooth surface. On the whole, the abrasion resistance of fiber-reinforced concrete is far preferable to that of ordinary concrete.

Figure 9

The effect of PP fiber type on the abrasion resistance of concrete [18].

We can conclude that the strengthening mechanism of PP fibers will be controlling, easing, and restricting [18,47,54]. The PP fibers distributed in a large number in the unit volume of concrete, hindered the development of micro-cracks, and consumed a large amount of energy, making it more difficult for cracks to spread further. When cracks form, PP fibers restricted the expansion of the crack tips. Therefore, a large amount of energy must be consumed to break or bypass the fibers so that cracks can continue developing. In other words, PP fibers can control the crack expansion and ease the stress concentration. Furthermore, after high-speed sand-containing water damages the surface of concrete, the shattered concrete blocks were strongly constrained and stayed on the concrete surface, so the impact of cavitation damage correspondingly weakens, which indirectly improved the abrasion resistance of the concrete.

However, the work of Sheng and Fang [55] on the bonding of PP fibers with concrete demonstrated that the bonding between the cement matrix and untreated PP fibers is very weak, the fibers drawn from the matrix have a smooth surface and hardly stick to cement. Thus, the effect of untreated PP fibers on improving the abrasion resistance of concrete is minimal, so they must be modified. Changing the hydrophobicity or increasing the surface roughness of PP fibers to enhance their bonding with the cement matrix, is illustrated in Figure 10. Hence, using PP fibers to achieve stronger abrasion resistance of concrete can be a research direction. For this purpose, Liao et al. [56] modified PP fibers by blending and spinning them with additives and reported that the “wall-attachment” effect of low-molecular-weight additives boosted the hydrophilicity of the fibers while guaranteeing that the fibers were well dispersed in the cement. Shi [57] also modified PP fibers with additives and carried out convex and concave thread treatments on the surface of the PP fibers to further strengthen their bonding with concrete. The modified PP fibers enhanced the toughness and abrasion resistance of concrete prominently.

Figure 10

The bonding between (a) unmodified and (b) modified PP fibers and cement [55].

Regarding the improvement in the abrasion resistance of concrete reinforced with steel fibers, Ji et al. [58], through experimentation, found that steel fibers acting as the micro-reinforcement of cracks in concrete during the hardening stage of cement due to their high elastic modulus, which better transfers the force applied to the concrete matrix, effectively limited the formation of internal cracks in the concrete, and prevented the further expansion of the cracks. Shi and Xie [59] also stated that when steel fibers were evenly distributed inside the concrete, they considerably improved the ability of controlling the development of internal cracks in the concrete and thus enhanced the abrasion resistance of concrete due to their strong toughness and flexural tensile strength. In addition, different fibers with a low elastic modulus, such as polyethylene fibers, steel fibers etc., generally increase the compressive strength of concrete considerably. Ju et al. [60] compared the effect of different types of fibers on the abrasion resistance of concrete and showed that the unit water consumption of concrete mixed with steel fibers was relatively higher than that of the concrete mixed with other fibers under a similar slump and air content. Nevertheless, steel fibers still caused the most significant increase in the compressive strength of concrete and enhanced its abrasion resistance markedly. Besides, Liu [61] stated that a minimum dosage of steel fibers in concrete was required to form a network structure. In fact, below this optimum content of steel fibers, the strength of the concrete cannot increase, and a weak interlayer is formed inside the concrete, which creates a peeling layer at the fiber–cement matrix interface, making it easier for the concrete to separate under the action of abrasion, thus, the performance of all aspects of the concrete declines. Abid et al. [62] also investigated the abrasion resistance of C30 concrete containing three different amounts of steel fibers and compared the results with the data on the abrasion resistance of C40 and C50 concrete (Figure 11). Their results showed that steel fibers effectively promoted the abrasion resistance of the concrete, and the abrasion resistance improved as the steel fiber content of the concrete increased, the performance of the C30 concrete reached that of the C40 concrete but was still lower than that of the C50 concrete. It was also observed that the reinforcing effect of steel fibers had a similar increasing tendency at different curing periods.

Figure 11

Different concretes in the depth decrement at: (a) 7 days, (b) 28 days, and (c) 90 days [62].

It can be concluded that when steel fibers are uniformly distributed in concrete, they can alleviate and weaken the intense stress concentration generated by defects such as micro-cracks and pores in concrete formed under the action of high-speed sand-containing water flows. Moreover, similar to PP fibers, they can reduce the expansion of cracks, improve the toughness of concrete, and enhance its abrasion resistance, and it is improves the abrasion resistance of concrete. However, there are certain distinctions between these two types of fibers in enhancing the abrasion resistance of concrete. By observing the surface of the concrete structure damaged by a high-speed sandy water flow, Wu [63] reported that the cementitious material on the concrete surface peeled off when the ultimate tensile strength of concrete was lower than the concentrated tensile stress. Since the tensile strength of steel fibers is much higher than that of concrete and the elastic modulus of steel-fiber-reinforced concrete increases with the fiber content, steel fibers can effectively control the mass loss of concrete to mainly bear the load after cracking, which enhances the abrasion resistance of the concrete. Nevertheless, due to their low modulus of elasticity, PP fibers cannot play a role in strengthening the concrete, similar findings can also be found in the studies of Liu [61] and Hu et al. [64]; however, according to Liu [61], during abrasion damage to concrete, steel fibers are generally removed from the surface of the specimen, as shown in Figure 12. Therefore, the impact of steel fibers on the abrasion resistance of concrete is often weaker than PP and PP fibers.

Figure 12

The exposed steel fibers due to the mass loss from the surface of steel-fiber-reinforced concrete [61].

In other words, steel fiber due to its high elastic modulus can significantly improve the strength of concrete; however, to the ductility and toughness of concrete, the enhancement was relatively insignificant when compared to the use of lower elastic modulus fibers like PP fiber. And combined with the higher cost, therefore, for the modified abrasion resistance of hydraulic concrete, steel fibers may not be the best choice to apply in practical engineering.

2.3.5 Special aggregate

Because the internal aggregate can continue to endure the abrasive damage after the cement surface of concrete was abraded by high-speed sand-containing water flow, the high-strength aggregate used can eminently enhance the abrasion resistance of concrete.

According to Liu et al. [65], the anti-abrasion properties of each constituent of concrete, especially the coarse aggregate, can significantly impact the overall abrasion resistance of concrete. In general, the more rigid the aggregate was, the stronger its bonding with the cement matrix became, and thus the more substantial the abrasion resistance of concrete was. The data on the concrete exposed to high-speed sandy water abrasion for a long time indicated that the particle size of coarse aggregate influences the formation of pores in the concrete damaged by abrasion, and the mechanism for the direct bonding between coarse aggregate and mortar plays a vital role in the abrasion resistance of concrete. Li and Yang [66] also demonstrated that the selection of coarse aggregate markedly impacted the abrasion resistance of concrete and recommended that gravel be chosen instead of pebbles as the coarse aggregate as much as possible for abrasion-resistant concrete. The fundamental reason for this recommendation is that the mechanical characteristics of pebbles and crushed stones vastly differ, under high-speed sand-containing water scouring, the weak particles of pebbles made the concrete surface uneven and formed other deficiencies. Moreover, the strength of the bonding of pebbles with cement is inferior to that of crushed stones with cement. As a result, the resistance of pebble aggregate to the abrasion damage from high-speed sand-containing water flows is weaker than that of the crushed stone, so the improvement in the abrasion resistance of the concrete containing pebble aggregate is relatively limited.

At present, the Los Angeles abrasion test is standard and reliable for determining the wear loss of aggregate. Considering the limitations of the existing tests, Mohajerani et al. [67] studied using modified compaction to replace the Los Angeles wear test. Esra and Kursat [68] also established a prediction of the wear resistance of coarse aggregate and proposed a convenient way to measure the wear resistance of coarse aggregate, making it easier to decide on using coarse aggregate to improve the abrasion resistance of concrete. However, geographical variables limit employing special aggregate in practical engineering, so while they have an excellent anti-abrasion function, they still have several limitations in large-scale applications.

2.3.6 Other materials

Many other materials have been employed to enhance the abrasion resistance of concrete in addition to the widely used silica fume, fibers, and coarse aggregate. For example, Gurpreet and Rafat [69] incorporated waste foundry sand with a particle size of 150–600 µm in concrete as fine sand and found that the formation of a dense matrix helped improve the abrasion resistance of concrete, and the abrasion resistance of concrete improved with the increase in the waste foundry sand content, regardless of the curing period; Thomas et al. [70] analyzed the influence of substituting waste tire rubber particles for part of fine aggregate on the abrasion resistance of ordinary and high-strength concrete. They found that the weak bonding between the cement paste and the rubber particles did not affect the abrasion resistance of the resultant concrete, so the rubber particles could enhance the abrasion resistance of concrete. Xie and Lou [71] investigated the abrasion resistance of the concrete reinforced with rubber particles, and found that the content of the rubber particles was the most crucial factor affecting the abrasion resistance of concrete. They reported that the best improvement in the abrasion resistance of concrete was achieved at 15% of rubber particles, while a rubber particle content of 30% reduced the abrasion resistance of the concrete. Feng et al. [72] also proposed using controlled permeability formwork liner construction technology to pour concrete. It markedly enhanced the abrasion resistance of concrete and was applied to the overflow parts such as drain grooves and gate piers that require high abrasion resistance.

Even enhancing the abrasion resistance of concrete by adding different materials is considerable, but there are some drawbacks to limiting its application, such as the selection of different materials, the comparison to traditional materials, and lack of more systematic research. As a result, further research is required to conduct for the application of these additives.

3.2.1 Nano-silica

Focused on the research of added nanomaterials in concrete to improve its abrasion resistance, nano-SiO₂ particles are the widely used materials at present. As an inorganic chemical material, nano-SiO₂ is non-toxic, pollution-free, and tasteless, with a spherical, flocculent, and reticulated quasiparticle microstructure and insoluble in water [94]. Referring to the practicalities of using silica fume to enhance the concrete abrasion resistance, Nano-SiO₂ benefits from the higher specific surface area, and smaller particle size has better reinforcement in abrasion resistance of concrete. Golewski [95] modified siliceous FA, silica fume, and nano-silica in ordinary Portland cement, and it was found that the modified concrete was characterized by a well-developed structure and had high values of mechanical parameters, besides, it also showed a significant reduction in CO₂ emissions.

The modification effects of nano-SiO₂ in concrete can be concluded as:

1. Accelerate pozzolanic reaction. In the early stage of the concrete curing period, nano-SiO₂ will react with calcium hydroxide in hydration products to produce more calcium silicate hydrate gel, which with higher specific surface area and stronger gluing capacity, increases the degree of polymerization of C–S–H gel and orderliness of the structure, resulting in improved density of concrete and enhanced bonding between the cementitious paste and aggregates [96,97,98].

2. Crystal nucleus effect. Due to the high specific surface area, the nano-SiO₂ will be as the hydrate seed to facilitaing hydration, and the hydration will be carried out by encompassed with nano-SiO₂, the C–S–H gel grows on the surface of nano-silica, therefore, more outstanding optimization of the internal microstructure and microscopic defects of concrete can be seen [99,100].

3. Filling effect. Enhance the density of matrix, and reduce the porosity [101].

The research on using nano-SiO₂ in concrete to enhance its abrasion resistance is inadequate currently. The influence of SiO₂ nanoparticles in concrete is similar to the silica fume, but with a more outstanding performance. Therefore, in order to meet the demand for higher abrasion resistance of concrete in hydraulic structures, adding nano-SiO₂ in concrete to replace silica fume, to analyze the mechanism of nano-SiO₂ materials to improve the concrete abrasion resistance, is a new research direction for higher abrasion resistance new concrete materials. In the research conducted by Jo et al. [102], they monitored the amount of calcium hydroxide to determine the extent of the pozzolanic hydration, and found that the addition of nano-SiO₂ accelerated the early hydration process of concrete, and was also far more effective than the application of silica fume. Besides, Hou et al. [103], through the SEM micrograph of cement mortar mixed with 1.5% nano-SiO₂ at 7 days, found that the microstructure of cementitious materials at this period was neatly distributed and dense. Meanwhile, due to the reaction between nano-SiO₂ and Ca(OH)₂, a large amount of hydration heat will be released, which will promote the hydration reaction process, resulting in a restricted content of Ca(OH)₂ crystals, and has a great enhancement for the early concrete strength [103,104]. As can be seen in Figure 15, concrete added with 5% nano-SiO₂ particles has a smoother microstructure surface, denser matrix, and higher integrality when compared to the normal concrete [105], which represents a higher abrasion resistance. In the relatively later stage of concrete curing, because of the larger specific surface area of nano-SiO₂, the surface adsorption of water is larger, making the cement’s continued hydration less effective in the later stage. Under the situation of huge content of nano-SiO₂, it will even decrease the concrete strength. In general, the incorporation of nano-SiO₂ has a significant enhancement on the concrete’s early strength [106]. Although the growth rate of early self-shrinkage strain in concrete is higher with increase in the nano-SiO₂ content [107,108], and increases in the amount of water used in concrete will prominently improve the strength of concrete even with a small dosage of nano-SiO₂ [109].

Figure 15

The transition zone: (a) normal concrete and (b) concrete modified with 5% nano-SiO₂ [105].

The research on incorporating nano-SiO₂ into concrete to enhance its abrasion resistance was mainly focused on the content of nano-SiO₂ in concrete at present. Similar to the use of silica fume, it s not increase the content of SiO₂ nanoparticles in concrete will have a better improvement effect in concrete. It’s mainly because the slump of concrete will significantly decrease as the added with nano-SiO₂, and also the SiO₂ nanoparticles evenly dispersed inside concrete were difficult. So, combined with different experiment conditions and materials, a degree of disagreement exists in the suggestion for the optimum dosage of nano-SiO₂ in concrete. Li [110], by controlling the content of nanomaterials, found that the addition of nano-SiO₂ will significantly increase the concrete’s early strength and durability, but the final strength’s change is not obvious, and recommended that the optimal dosage of nano-SiO₂ should not be over 3% in concrete, otherwise it will have a negative impact on the concrete fluidity and workability. Huang et al. [111] studied high-content FA concrete compounded with nano-SiO₂ and PVA fibers (polyvinyl alcohol fibers), and the results suggested that the dosage of nano-SiO₂ should be between 0.5 and 1.5%, the larger content leading to an excessive reduction in the fluidity of the concrete, which is detrimental to the dispersion of fibers and affects the strength of concrete. Li et al. [112] stated that the enhancement of concrete strength in the situation of 4% content of nano-silica is already weaker than the 3% situation, just because the limitations of dispersion method. But Xu [13] studied the C30 concrete and found that the content of nano-SiO₂ reached 7%, and the strength of the concrete would still increase with the extent of nanomaterials. However, for C60 concrete, the strength of the concrete already decreased with the nano-SiO₂ at 3%. Figure 16 demonstrated that concrete abrasion resistance will be remarkably enhanced when nano-SiO₂ and PP fibers were reinforced, and the abrasion resistance of concrete decreased with the increase in the content of nanoparticles [113]. Rahmanzadeh et al. [114] kept the content of nano-silica at 5%, changed the water–cement ratio, and studied its abrasion resistance, the results showed that the abrasion resistance of concrete decreased with the increase in the water–cement ratio, but due to the influence of nano-SiO₂, the abrasion resistance will still be enhanced, as shown in Figure 17. Besides, Alireza [115] found that the difference in the average size of nano-SiO₂ will result in different content suggestions, when the size of nano-SiO₂ is 15 nm, the cement replacement is suggested at 1.0%, but when the size is 80 nm, the best content in concrete will be 1.5%.

Figure 16

The effect of different materials on the concrete abrasion resistance [113].

Figure 17

Different mechanical properties change compared to concrete W/C ratio at 0.46 [114].

At the same time, cooperating nanomaterials with other mineral admixtures were also a popular research direction for improving the concrete property. For example, Ji et al. [116] studied the concrete mechanical properties by adding FA and nano-SiO₂, and found that the 7 days and 28 days compressive and flexural strengths could be significantly improved with 0.5% nano-SiO_2. Zhang et al. [117] added nano-SiO₂ to concrete mixed with 50% FA or slag, and results showed that when the nano-SiO₂ was mixed at 1%, the concrete setting time was significantly reduced. Sun et al. [118] stated that high content FA concrete, like incorporating 2% nano-SiO₂ in 40% FA concrete, would result in the optimum early mechanical properties of concrete; Musa et al. [119] used the Box–Behnken design method to test the concrete mixed with nano-silica and crumb rubber, the results showed that the abrasion resistance of concrete decreased with an increase in crumb rubber, and enhanced with an increase in the addition of nano-silica; Nazari and Riahi [120] found that Portland cement partially replaced with SiO₂ and Al₂O₃ nanoparticles will enhance the concrete abrasion resistance, and increase in the nanoparticles content have found to increase the abrasion resistance of concrete which was cured in water and saturated limewater. Similar study has been applied by Shadi and Ali [121] where Portland cement was partially replaced up to 2.0 wt% with SiO₂ and CuO nanoparticles, and it was found that concrete mixed with SiO₂ nanoparticles has a better enhancement than CuO nanoparticles, as can be seen in Figure 18. However, completely different results can be seen in Figure 19, Gupta and Kumar [122] through experiments found that added coir fiber and nano-SiO₂ will decrease the concrete abrasion resistance, but the increase in the content of nano-SiO₂ from 2 to 3% will compensate for the loss in weight, thereby indicating that nano-silica really can fill the voids and make the concrete more homogeneous.

Figure 18

Abrasion resistance of concrete cured in (a) water and (b) saturated limewater [121].

Figure 19

Abrasion resistance of concrete for different W/C ratio after 28 days [122].

Although modifying the abrasion resistance of hydraulic concrete with nanoparticle materials was effective and outstanding, the influence of many factors such as curing medium, age of curing, water to cement ratio, aggregate type, nanoparticles content, etc., on the abrasion resistance of concrete incorporating nanoparticles remain unknown. Therefore, the study about nano-SiO₂ used in concrete still needs to pay more attention on it, to obtain higher anti-abrasion concrete new material in the future.

3.2.2 Other nanoparticles

Because the main cement hydration product C–S–H gel belongs to the nanoscale, so aside from the wide application of nano-SiO₂ mixed in concrete, many other nanoparticles have also been used in concrete to improve its abrasion resistance.

For example, Tian et al. [123] studied the different contents of nano-CaCO₃ influence on concrete abrasion resistance, the results showed that nano-CaCO₃ mixed with concrete will promote the cement hydration, reinforce the concrete microstructure, and improve the concrete density. Therefore, the abrasion resistance can be enhanced effectively. There is approximately 6 times increase in concrete abrasion resistance compared to normal concrete when the Nano-CaCO₃ content is at 1.5%. Besides, due to adding a little fraction of graphene oxide (GO) can improve the transport properties, enhance Young’s modulus, and increase the mechanical properties. So, researchers have been interested in the application of GO to enhance concrete abrasion resistance. Du et al. [124] found that the addition of GO (up to 0.1% by weight of cement) can enhance the concrete abrasion resistance by accelerating the hydration of cement and providing nucleation sites for hydration, facilitating the cement clinker consumption, and producing more hydration products. Du et al. [125] also points out that the surface abrasion resistance of high-volume fly ash (HVFA) mixtures will be modified by GO, and the micro-hardness, scratch surface roughness, and scratch hardness of the HVFA mixture first improved and then deteriorated with the increase in GO content. The suggestions on GO content in HVFA mixtures is 0.05 wt%. Li and Gao [126] also studied the impact of nano-MgO and nano-Al₂O₃ on concrete abrasion resistance, and through experiments found that nano-MgO was a benefit to the abrasion resistance of ultra-high performance concrete, but nano-Al₂O₃ will inhibit the cement hydration, thus there was no obvious influence on concrete abrasion resistance.

Meanwhile, many scholars have applied nanoparticles to protective covering to enhance its abrasion resistance. The addition of nanomaterials is one of the most effective ways to reduce the abrasive damage, aside from improving the abrasion resistance, it will also make materials acquire some new properties like corrosion protection, electrical conductivity, self-cleaning, and photocatalytic properties [127]. For example, Xu and Cai [128] used the combined ultrasonic cavitation and high-speed dispersion, and with the help of dispersant uniformly dispersed nano-Al₂O₃/ZrO₂ in epoxy resin. They found that the new coating has excellent abrasion resistance, salt spray resistance, and anti-aging properties, at the same time, the bonding strength between the coating and concrete will be remarkably improved due to the high surface activation energy of nanoparticles.

In summary, thanks to the outstanding performance of nanoparticles, concrete will be effectively enhanced when mixed with nanoparticles, therefore, more and more research has been applied in this field to study the mechanism of nanoparticle reinforcement. However, the research on concrete abrasion resistance modified with nanoparticles is still in the preliminary stage, still more efforts are needed to study the influence of nanoparticles on concrete abrasion resistance.