Keywords

1 Introduction

In hydraulic and hydroelectric projects, high speed mud, sand and stone flows wash hydraulic structures for a long time. This leads to the aggravation of abrasion and cavitation on the overflow surface of concrete buildings, resulting in large-area denudation of surface concrete, which greatly shortens the service life of buildings. The research and application of using high performance concrete (HPC) instead of traditional concrete to improve the impact and wear resistance of hydraulic structures has gradually become a hot spot. The studies show that [1,2,3], when adding coarse aggregate into HPC, it can reduce the amount of cementitious materials and cut the cost. This can not only reduce the plastic shrinkage of HPC, but also improve the overall stability and strength of concrete because of the bite action between aggregates. The study [4] pointed out that the low water binder ratio of HPC makes the internal structure of concrete more compact. In particular, the interfacial bonding ability between coarse aggregate and matrix is much higher than that of ordinary concrete [5]. This conclusion provides a theoretical basis for adding coarse aggregate to HPC. At present, the research on HPC containing coarse aggregate mostly focuses on its mechanical properties and durability, but the research on abrasion resistance is rarely reported.

Based on the above analysis, high performance grouting material is used as the base material, and different proportions of coarse aggregates are added. Some of the coarse aggregates are then replaced with sand to prepare hydraulic high performance abrasion resistant concrete, and performance tests are conducted. Analyze the influence of aggregate content on the abrasion resistance strength of concrete, and obtain the optimal aggregate content. This can provide a basis for the application of hydraulic abrasion resistant concrete engineering, thereby improving the economic efficiency of abrasion resistant concrete.

2 Test Overview

2.1 Raw Materials

2.1.1 Grouting Material

A high performance grouting material is used as the base material, including cement, sand, admixture and other materials. The design strength of high performance grouting material is C80. Table 1 shows the test results of physical and mechanical properties of grouting materials.

Table 1. Physical and mechanical property index of grouting material
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2.1.2 Aggregate

The test aggregates are all artificial aggregates, in which the fineness modulus of sand is 2.6, the particle size of small stone is 5 mm–20 mm, and the particle size of medium stone is 20 mm–40 mm. The physical performance test results of aggregates are shown in Table 2.

Table 2. Physical performance indicators of aggregates
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2.2 Experimental Design

High abrasion resistance concrete is prepared by adding different proportions of fine aggregate and coarse aggregate into high performance grouting material. The ratio of grouting material to coarse aggregate is 5:5 and 4:6. The ratio of small stone to medium stone is 5:5. The replacement rate of fine aggregate (sand) is 0%, 5%, 10% and 15%. The water consumption is 10% of the quality of grouting material. The workability of concrete can be improved by adding a certain amount of fine aggregate. Each concrete is calculated as 100 kg and its mix proportion is shown in Table 3.

Table 3. Test mix proportion
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3 Test Method and Analysis of Test Results

3.1 Slump Test and Result Analysis

The slump test of concrete mixture shall be carried out according to Chinese specification DL/T 5150-2017. And the workability of the mixture is analyzed. The slump test process of high abrasion resistance concrete mixture at a ratio of 5:5 is shown in Fig. 1. The mixture state and slump test process of high abrasion resistance concrete with a ratio of 4:6 are shown in Fig. 2 and Fig. 3, respectively.

Fig. 1.
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Slump test process of concrete (5:5)

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Fig. 2.
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State of concrete mixture (4:6)

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Fig. 3.
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Slump test process of concrete (4:6)

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It can be seen from Fig. 1 that when the ratio of slurry to stone is 5:5, the cohesion of the mixture is good. The aggregate shall be evenly wrapped and there shall be no water precipitation on the surface. From Figs. 2 and 3, it can be seen that when the ratio of slurry to stone is 4:6, the concrete mixture basically has no slump. Due to the lack of sufficient water for mixing, the proportion of cement slurry and inclusion in aggregate can not be completely reduced.

It can be seen from Fig. 1 that the slump of concrete mixture is 80 mm–154 mm, and the slump decreases with the increase of sand substitution rate. When the sand substitution rate is 5% and 10%, the concrete slump is 32% and 48% lower than that of 5:5–0% group respectively. This is because the sand has a large specific surface area and needs more cement paste to wrap, which reduces the free water in the concrete mixture and reduces the fluidity of the concrete. With the increase of sand replacement rate, the wrapping of cement mortar to aggregate and the workability of mixture are improved. However, when the sand replacement rate exceeds 15%, the specific surface area of aggregate increases, resulting in the relative decrease of water binder ratio of concrete mixture. The workability is reduced to a certain extent.

3.2 Compressive Strength

The compressive strength test of concrete shall be carried out according to the test method in standard DL/T 5150-2017. The compressive test piece is a standard cube with a side length of 150 mm. The 28 d compressive strength comparison of high-performance impact resistant and wear-resistant concrete is shown in Fig. 4 and Fig. 5.

Fig. 4.
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Compressive strength of concrete (5:5)

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Fig. 5.
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Compressive strength of concrete (4:6)

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As shown in the Fig., when the ratio of slurry to stone is 5:5, the 28 d compressive strength of high abrasion resistance concrete is between (84–92) MPa. And the compressive strength slightly increases with the increase of sand replacement rate. This is because a certain proportion of sand can fill the gaps in the coarse aggregate, making the aggregate grading more uniform. While improving the workability of concrete mixtures, it also makes the interior of the concrete denser, thereby improving the compressive strength of the concrete.

When the ratio of slurry to stone is 4:6, the 28 d compressive strength of high abrasion resistance concrete is between (75–80) MPa. The sand replacement rate has little effect on the compressive strength of concrete. On the one hand, due to the inherent high proportion of coarse aggregate, the workability of concrete decreases sharply after sand replaces a portion of coarse aggregate, leading to an increase in internal defects in the concrete. On the other hand, when the amount of aggregate added increases too much, the cement slurry cannot fully wrap around the aggregate interface or the wrapping layer is relatively thin, which will cause insufficient adhesion between the aggregate and the matrix, resulting in a decrease in the compressive strength of the concrete.

3.3 Abrasion Resistance

The high abrasion resistance concrete containing coarse aggregate shall be tested for abrasion resistance according to the underwater steel ball method in chinese specification DL/T 5150-2017. This method can be used to measure the relative resistance of concrete to underwater flow medium wear and evaluate the relative wear resistance of concrete surface.The abrasion index of concrete is expressed by the abrasion strength. The abrasion strength according to Eq. (1). Figure 6 shows the abrasion strength of high abrasion resistant concrete at 28 days under two kinds of coarse aggregate content.

$$ fa = \frac{tA}{{\Delta m}} $$
(1)

where fa represents the impact wear strength, which is the time required per unit area to be worn per unit mass, in h/(kg/m2)；t represents the cumulative test time, in h; A represents the area of the specimen subjected to impact and wear, in m2; Δ represents the cumulative mass loss of the specimen after grinding during the t period, in kg.

Fig. 6.
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Abrasion strength of concrete

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It can be seen from Fig. 4 that after adding 50% and 60% coarse aggregate into the grouting material, the abrasion strength of high abrasion resistant concrete at 28 days reaches 23.66 h/(kg/m2), 20.76 h/(kg/m2) respectively. With the increase of aggregate proportion, the slump of concrete mixture decreases obviously, and the workability becomes worse, which increases the internal defects of concrete and leads to the reduction of impact and wear strength. After sand replaces part of coarse aggregate, the impact and abrasion strength of concrete in 28 days is reduced to (13–18) h/(kg/m2). The abrasion strength of concrete decreases slightly with the increase of sand replacement rate. This is because the impact of fine aggregate on the impact and abrasion resistance of concrete is more significant than that of coarse aggregate. The artificial aggregate used in the study has high crushing index and relatively low aggregate strength, which will change the first failure trend of the interface transition zone. Therefore, the abrasion strength of concrete will be greatly reduced after adding fine aggregate.

Figure 7 shows the impact abrasion test failure diagram of high abrasion resistance concrete when the coarse aggregate content is 50%. It can be seen from the Fig. 7 that the form of abrasion resistance is surface aggregate exposure, indicating that the interfacial bonding strength between aggregate and rubber material is high. And the exposure degree of aggregate increases with the increase of sand substitution rate.

Fig. 7.
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Damage diagram of abrasion sample

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Based on the analysis of the above test results, when the content of coarse aggregate is 50% and the sand replacement rate is 5%, the workability and abrasion resistance of concrete are better. Under this mix proportion, the slump of concrete mixture is 105 mm. The compressive strength is 90.7 MPa. And the abrasion strength is 17.39 h/(kg/m2).

4 Conclusion

  1. (1)

    The ratio of slurry to aggregate has a certain impact on the workability, compressive strength, and impact and wear resistance of high impact and wear resistance concrete.

  2. (2)

    When the ratio of slurry to aggregate is 5:5, the slump and impact wear strength of high abrasion resistance concrete decrease with the increase of sand replacement rate, while the compressive strength increases with the increase of sand replacement rate.

  3. (3)

    When the ratio of slurry to aggregate is 5:5 and the sand replacement rate is 5% (5:5–5), the workability of concrete is good, with a compressive strength of 90.7 MPa and an abrasion resistance of 17.39 (h/(kg/m2). This ratio not only ensures certain mechanical properties and impact wear resistance, but also reduces engineering costs.