Keywords

1 Introduction

Professor Mehta [1] has pointed out that the freeze–thaw cycle is one of the main reasons for the reduction of concrete durability. In northeast and northwest of China, a large number of concrete structures are damaged due to long-term freeze–thaw cycles of varying degrees, seriously affecting the service life and safety performance of concrete structures, and causing certain economic losses [2, 3]. Therefore, improving the frost resistance of concrete material is helpful to improve the durability of concrete structures.

Under the action of freeze–thaw cycle, the micro-cracks and pores in concrete gradually expand under the influence of hydrostatic pressure and osmotic pressure, and finally cause the freeze–thaw damage to concrete. To study the influence of fiber on the frost resistance of concrete, scholars at home and abroad has done a lot of research and found that the addition of fiber can effectively slow down the cracking of concrete and improve its frost resistance [4,5,6,7]. With the deepening of research, the fibers mixed in concrete have gradually developed from single fibers to incorporate fibers of different types and sizes. When two or more fibers were incorporated, they were able to achieve different effects on the concrete [8, 9].

In this paper, the freeze–thaw cycle experiment of steel -PVA hybrid fiber concrete was carried out by orthogonal experiment to study the effects of different steel fiber content, PVA fiber content and steel fiber type on the mass-loss rate and relative dynamic elastic modulus of the specimen, and the mechanism of high frost resistance of steel -PVA fiber concrete was expounded.

2 Experiment

2.1 Raw Materials of Experiment

The strength of concrete used in the experiment is C35. The cement is P.O42.5 ordinary Portland cement; the coarse aggregate is crushed stone with 5 ~ 10 mm continuous gradation, and the fine aggregate is natural medium sand with fineness modulus of 2.54. The water reducing agent is polycarboxylate superplasticizer with a water-reducing rate of ≥ 10%; Three types of steel fibers are selected: end hook type with a length of 30 mm, a diameter of 0.6 mm, an aspect ratio of 50, indentation type with a length of 32 mm, a diameter of 0.7–1.2 mm, an aspect ratio of 45.7–26.2, and high-strength ultra-fine steel fiber with a length of 13 mm, a diameter of 0.18–0.25 mm and an aspect ratio of 72.2–52. The PVA fiber is 12 mm in length, 14 μm in diameter and 845 in length-diameter ratio.

2.2 Design of Mix Proportion

To study the influence of steel fiber content, PVA fiber content and steel fiber types on the frost resistance of concrete, the experiment was designed by orthogonal test method, each influencing factor was selected at three levels, and the L9(33) orthogonal table was used to design the orthogonal test with three factors and three levels, as shown in Table 1.

Table 1 Orthogonal factor table
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According to the orthogonal factor table, 9 groups of mix proportion are designed, as shown in Table 2.

Table 2 Mix proportions of hybrid fiber concrete
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3 Experimental Results and Analysis

3.1 Experimental Results

The mass-loss rate and relative dynamic elastic modulus of steel -PVA fiber concrete during freeze–thaw cycles are tested, and the specific test results are shown in Table 3.

Table 3 Orthogonal test results
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3.2 Range Analysis

To investigate the influence of three factors on the mass-loss rate and relative dynamic elastic modulus of steel -PVA fiber concrete, the range analysis of each test result is carried out, and the analysis results are shown in Fig. 1.

Fig. 1
Two increasing line graphs with the first one plotting a range of mass loss rate versus the number of freeze thaw cycles and the second one plotting a range of relative dynamic modulus versus the number of freeze thaw cycles for steel fiber content, P V A fiber content, and steel fiber type. Lines in both the graphs plots an increasing trend.

Range analysis result

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It can be seen from Fig. 1a that with the increase of freeze–thaw cycles, the influences of three factors on the mass-loss rate of concrete gradually increase. When the number of freeze–thaw cycles is the same, the steel fiber content has the greatest influence on the mass-loss rate of concrete, followed by the type of steel fiber, and the PVA fiber content has the smallest influence on it.

It can be seen from Fig. 1b that with the increase of freeze–thaw cycles, the influence of three factors on the relative dynamic elastic modulus of concrete gradually increases. When the number of freeze–thaw cycles is the same, the steel fiber content has the greatest influence on the relative dynamic elastic modulus of concrete, and the PVA fiber content and the type of steel fiber have similar influences on it.

3.3 Factor-Index Analysis

According to the orthogonal experimental method, the mass-loss rate and relative dynamic elastic modulus are called indexes. Combined with Tables 2 and 3, the average values of the three factors at each level can be obtained, as shown in Figs. 2 and 3.

Fig. 2
Three line graphs of mass loss rate versus the number of freeze thaw cycles have fluctuating curves for various measurements of steel fiber content, P V A fiber content, and steel fiber type. Lines in three graphs plots an decreasing trend.

Relationship between three factors and mass-loss rate

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Fig. 3
Three line graphs of relative dynamic elastic modules versus the number of freeze thaw cycles have fluctuating curves for various measurements of steel fiber content, P V A fiber content, and steel fiber type. Lines in three graphs plots an decreasing trend.

Relationship between three factors and relative dynamic elastic modulus

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It can be seen from Fig. 2 that the greater the steel fiber content, the greater the mass-loss rate of concrete; With the increase of freeze–thaw cycles, the mass-loss rate of concrete gradually decreases, but the PVA fiber content has no obvious relationship with the mass-loss rate. With the increase of freeze–thaw cycles, different types of steel fibers have different effects on the mass-loss rate of concrete; generally speaking, the end hook steel fiber has the largest mass-loss rate, followed by indentation steel fiber and superfine steel fiber.

It can be seen from Fig. 3 that the greater the steel fiber content, the greater the relative dynamic elastic modulus of concrete; The greater the PVA fiber content, the greater the relative dynamic elastic modulus of concrete. The relative dynamic elastic modulus of hook-shaped steel fiber is the largest, and the relative dynamic elastic modulus of indentation steel fiber is close to that of superfine steel fiber.

3.4 Mechanism of High Frost Resistance of Steel—PVA Fiber Concrete

It can be seen from the above analysis that adding PVA fiber based on steel fiber can refine the pore structure, reduce the appearance of initial cracks and inhibit the development of freeze–thaw cracks. This is because the mixture of steel fiber and PVA fiber can play a role in different stages and scales of crack development. The effect model of high frost resistance of steel -PVA fiber concrete is shown in Fig. 4. PVA fibers are evenly dispersed in the concrete to bridge the micro-cracks in the concrete. Hook-shaped steel fiber can bridge the macro-cracks, transfer the stress on both sides of the cracks, reduce the stress at the crack tip, and effectively prevent the macro-cracks from further expanding. Moreover, the interface transition zone between the smooth fiber surface and cement paste has good hydration and compact structure. For indented steel fiber, the hydration effect of cement around it is worse than that around the end hook steel fiber, because of uneven surface curvature and uneven water distribution. The shape of superfine steel fiber is similar to PVA, so it is difficult for them to give full play to the complementary advantages of fiber length in concrete.

Fig. 4
An illustration of the fiber distribution includes pore, short fiber, long fiber, and macro crack from the top to the bottom.

Hybrid fiber distribution in concrete

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To sum up, the frost resistance of end-hooked steel—PVA hybrid fiber concrete is better, mainly because dense hydration products can be formed around the smooth surface of end hook fiber, which reduces the permeability of concrete. PVA fiber has small volume and many elements per unit volume, which can refine pores, reduce connected pores, make the air in the concrete evenly distributed and bridge micro-cracks. Therefore, the end hook steel fiber -PVA fiber hybrid can realize the complementary functions of the two fibers, resist the freeze–thaw damage more effectively, and show the "positive" hybrid effect of fibers.

4 Conclusion

Through the freeze–thaw cycle experiment of steel -PVA hybrid fiber concrete, the mass-loss rate and relative dynamic elastic modulus of the specimen are analyzed, and the frost resistance mechanism is explored, so as to obtain the optimal mix ratio of high frost resistance concrete, which provides a certain reference value for practical projects. The main conclusions are as follows:

  1. (1)

    Different factors have different effects on the mass-loss rate and relative dynamic elastic modulus of concrete. The steel fiber content has the greatest influence on them, the PVA fiber content has a great influence on the relative dynamic elastic modulus, and the content of steel fiber has a great influence on the mass-loss rate. With the increase of freeze–thaw cycles, the influence of three factors on the frost resistance of concrete gradually increases.

  2. (2)

    The more steel fiber content, the better frost resistance of concrete. The more PVA fiber content, the better frost resistance of concrete. Steel fiber concrete with end hook has the best frost resistance. When the fiber content is 1.0% end hook steel fiber + 0.3%PVA fiber, the frost resistance of concrete is the best.

  3. (3)

    The end hook steel fiber and PVA fiber are overlapped and complementary to each other in the concrete to improve the initial strength and permeability resistance of concrete, effectively prevent the development of micro-cracks in concrete, and thus improve the frost resistance of concrete.

At present, there are many studies on the frost resistance of concrete, but the problem of freeze–thaw damage of concrete has not been solved from the root. It is necessary to further study the mechanism and measures of frost resistance of fiber reinforced concrete and do more in-depth research on the frost resistance of concrete.