Keywords

1 Introduction

The crack resistance of concrete has always been a technical challenge of concern. Cracks can accelerate concrete carbonization and steel corrosion, and create a vicious cycle, seriously damaging the safety and durability of concrete structures [1,2,3,4]. There are many factors that can cause concrete cracks, but most of them are related to shrinkage. Controlling shrinkage will be an important measure to prevent concrete cracks [5,6,7].

The cracking of concrete has always been a key factor in its structural life and durability, and is also a technical challenge. Cracks in concrete can usually be divided into load induced cracks and non load induced cracks. As for non load cracks, they mainly include various shrinkage cracks, including plastic shrinkage cracks, temperature shrinkage cracks, dry shrinkage cracks, self shrinkage cracks, and carbonization shrinkage cracks. To avoid the occurrence of non load cracks, in addition to strengthening early maintenance and controlling hydration heat, measures should also be taken to limit and compensate for shrinkage. The main purpose of adding expansion agents to concrete is to reduce and compensate for shrinkage, especially for concrete with high crack resistance requirements, which generally requires the addition of expansion agents. In steel fiber reinforced concrete, in order to fully encapsulate steel fibers and maximize their performance, it is necessary to increase the amount of cementitious materials, which will inevitably lead to significant shrinkage. Therefore, it is necessary to study the effect of expansion agents on the performance of steel fiber reinforced concrete. The purpose of this part of the experiment is to analyze the effect of expansion agents on the performance of concrete and determine whether it is still necessary to add expansion agents in steel fiber reinforced concrete.

The shrinkage of concrete can be divided into plastic shrinkage, temperature shrinkage, self shrinkage, drying shrinkage, and carbonization shrinkage. For ordinary concrete structures, due to the use of lower amounts of cementitious materials and the limited size of components, the influence of temperature shrinkage is relatively small. The common destructive factors are mainly dry shrinkage and plastic shrinkage. The influence of fibers (mainly hybrid fibers) and expansion agents on the dry shrinkage of concrete has also been studied by many scholars [8,9,10]. This article studies the effects of fibers (polypropylene fibers, steel fibers, and hybrid fibers) and expansion agents on the dry shrinkage of concrete from the perspectives of limiting shrinkage and compensating shrinkage.

2 Experimental Design and Mix Proportion

The effect of expansion agents on the dry shrinkage of concrete: Adding expansion agents to plain concrete, polypropylene fiber concrete, and hybrid fiber concrete, testing the effect of expansion agents on the dry shrinkage of these three types of concrete;

  1. 1.

    The effect of steel fiber content on concrete shrinkage: Add 32 and 64 kg m−3 steel fibers (with a length of 15 mm) to the concrete, and test their impact on concrete shrinkage;

  2. 2.

    The effect of steel fiber type on concrete shrinkage: The influence of different steel fibers on concrete shrinkage was tested by adding 64 kg m−3 of 15 mm long steel fibers and 30 mm long steel fibers to the concrete.

Test using 100 mm × 100 mm × 515 mm. After the specimen is formed and removed, its initial length is measured, and then the specimen is sent to the standard curing room for immersion curing for 7 days, and then moved to the constant temperature and humidity chamber to measure its 7d Free expansion rate, and finally its corresponding strain is measured at the specified age. The mix proportions tested in this article are shown in Table 1.

Table 1 Test mix proportions for dry shrinkage of concrete
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3 Dry Shrinkage Test Results and Analysis

3.1 Effects of Expansion Agents on the Dry Shrinkage of Concrete

In the experiment, the effects of expansion agents on the dry shrinkage of plain concrete (S1 group), polypropylene fiber concrete (S3 group), and hybrid fiber concrete (S7 group) were tested. The results are shown in Figs. 1, 2, and 3, respectively. The horizontal coordinate “7w” in the figure represents day 7 of immersion curing, while the rest represents the curing time in a constant temperature and humidity chamber.

Fig. 1
A dual-line graph compares the strain versus test age of S 3 and S 4. Both lines start at (0, 0). S 3 passes through (3, 55), (14, negative 25), (45, negative 175), to (90, negative 175). S 4 passes through (3, 155), (14, negative 24), (45, negative 100), to (90, negative 170). Values are approximate.

Effects of expansion agent on dry shrinkage of ordinary concrete

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Fig. 2
A dual-line graph compares the strain versus test age of S 1 and S 2. Both lines start at (0, 0). S 1 passes through (3, negative 60), (14, negative 175), (45, negative 200), to (90, negative 275). S 2 passes through (3, 125), (14, negative 75), (45, negative 125), to (90, negative 170). Values are approximate.

Effects of expansion agent on the dry shrinkage of polypropylene fibers concrete

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Fig. 3
A dual-line graph compares the strain versus test age of S 2, S 9, and S 10. All lines start at (0, 0). S 2 passes through (7, 25), (45, negative 175), to (90, negative 185). S 9 passes through (7, 150), (45, negative 225), (90, negative 250). S 10 passes through (7, 140), (45, negative 200), to (90, negative 210). Values are approximate.

Effects of expansion agent on dry shrinkage of hybrid fibers reinforced concrete

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From Figs. 1, 2, and 3, it can be seen that the addition of expansion agents reduces the drying shrinkage of concrete at various stages, especially in the early stages. Among them, the expansion agent has a significant effect on reducing shrinkage of plain concrete and hybrid fiber concrete, and the dry shrinkage can be reduced by about 30% at day 60. The principle is that after adding an expansion agent to the concrete, a chemical substance with expansion properties is generated, which improves the compactness of the matrix and fills, blocks, and seals the pores of the concrete, preventing the migration of water, thereby compensating for and reducing the drying shrinkage of the concrete itself.

3.2 The Effects of Steel Fibers Content on the Dry Shrinkage of Concrete

In the experiment, the effect of steel fibers (with a length of 15 mm) mixed with 32 and 64 kg m−3 on the dry shrinkage of concrete was tested, as shown in Fig. 4. From Fig. 4, it can be seen that the addition of steel fibers does not reduce the dry shrinkage of concrete, but instead increases it. However, the dry shrinkage of concrete mixed with 64 kg m−3 steel fibers is smaller than that mixed with 32 kg m−3. Under certain water cement ratio conditions, the dry shrinkage of concrete is closely related to its pore structure. Due to the high density and equivalent diameter of steel fibers, the number of steel fibers per unit mass is relatively small. A single length of steel fibers is difficult to improve the pore structure of concrete, and even has certain adverse effects (related to the quality of vibration forming), thereby increasing the dry shrinkage of concrete. This fully indicates that steel fibers cannot effectively reduce the dry shrinkage of concrete.

Fig. 4
A dual-line graph compares the strain versus test age of S 7 and S 8. Both lines start at (0, 0). S 7 passes through (3, 25), (14, negative 100), (45, negative 200), to (90, negative 250). S 8 passes through (3, negative 180), (14, 0), (45, negative 125), to (90, negative 150). Values are approximate.

Effects of steel fibers content on concrete dry shrinkage

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3.3 The Influence of Steel Fibers Types on the Dry Shrinkage of Concrete

Under a certain amount of steel fiber, the influence of different steel fibers on the dry shrinkage of concrete was tested, as shown in Fig. 5. From Fig. 5, it can be seen that after adding 64 kg m−3 of 15 mm long steel fibers and 30 mm long steel fibers, namely S10 and S11 groups, the dry shrinkage of concrete is also greater than the reference group S2. Among them, the ability of 15 mm long steel fibers to limit dry shrinkage is better than that of 30 mm long steel fibers. The main reason is that the equivalent diameter and length of steel fibers with a length of 15 mm are relatively small, and there are more fibers per unit volume, which is beneficial for improving the pore structure of concrete.

Fig. 5
A dual-line graph compares the strain versus test age of A 2, A 10, A 11, and A 12. All lines start at (0, 0). A 11 has the lowest strain with negative 250 at 90 days, and A 12 has the highest with 200 at 7 weeks. All values are approximate.

Effects of steel fiber types on concrete dry shrinkage

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When two types of steel fibers are mixed with 32 kg m−3 each, the dry shrinkage ratio of concrete in reference group A2 is slightly smaller, with a decrease of nearly 20% in dry shrinkage after 60 days. The reason is that hybrid steel fiber fibers can improve the pore structure of concrete at different scales, especially under the synergistic effect of expansion agents, which can refine the pore size, reduce water leakage, and thus reduce dry shrinkage.

3.4 The Effect of Hybrid Fibers on the Dry Shrinkage of Concrete

To better limit and reduce the dry shrinkage of concrete, polypropylene fibers and composite steel fibers were mixed and added, and the dry shrinkage results are shown in Fig. 6. From Fig. 6, it can be seen that mixed fiber concrete (S8 group) has the smallest dry shrinkage. The 60 day shrinkage of the S8 group decreased by 23.5% compared to the reference group S2, which fully demonstrates the composite effect between different fibers. Mixing polypropylene fiber and composite steel fiber can improve the pore structure of concrete at different scales and levels in three-dimensional space, refine the size of pores, improve the uniformity of concrete, and effectively prevent the escape of water. Meanwhile, polypropylene fibers and steel fibers with different elastic moduli can better disperse capillary shrinkage stress, prevent local stress concentration, and reduce dry shrinkage.

Fig. 6
A dual-line graph compares the strain versus test age of 2, A 8, and A 12. All lines start at (0, 0). A 2 passes through (7, 150), (45, negative 150), to (90, negative 200). A 8 passes through (7, 75), (45, negative 140), (90, negative 150). A 12 passes through (7, 75), (45, negative 140), to (90, negative 200). Values are approximate.

The effects of mixed fibers on the dry shrinkage of concrete

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In this part of the experiment, the mechanical properties, fracture mechanical properties, and dry shrinkage properties of hybrid fibers were mainly compared and studied. Experiments have shown that hybrid fibers have better performance than pure steel fibers or polypropylene fibers. However, it is not that polypropylene fibers and any volume fraction of steel fibers have good reinforcement effects, but rather that there is an optimal volume addition. When 0.9 kg/m3 polypropylene fiber is mixed with 0.8% volume fraction composite steel fiber, it has better comprehensive performance. The effect of hybrid fibers on improving the compressive strength of concrete is not significant, but the improvement of flexural strength, splitting tensile strength, and initial crack strength is more prominent, which can be increased by about 26%, 13%, and 25%, respectively. In terms of dry shrinkage, expansion agent JM—III can effectively reduce the dry shrinkage of concrete, with a 60 day dry shrinkage reduction of about 30%. The effect of a single variety of steel fibers on reducing concrete shrinkage is not significant, which is consistent with the research results in the first part. However, steel fiber composites with different lengths effectively reduce the dry shrinkage of concrete, with a reduction of nearly 20% after 60 days. Hybrid fibers, especially polypropylene fibers and composite steel fibers, have the best effect in reducing dry shrinkage, with a 60 day dry shrinkage reduction of about 23.5%.

Expansion agents can effectively reduce the dry shrinkage of concrete, but compared to concrete with expansion agents, the addition of steel fibers (with a volume content of less than 1.5%) cannot effectively reduce the dry shrinkage of concrete, and even if there is some inhibition, the effect is relatively limited. The effect of Shanghai Becarte steel fiber on limiting dry shrinkage is better than Jiangxi Guomao steel fiber.

In order to meet the requirements of good process performance, mechanical performance, and anti shrinkage performance, the technology of adding expansion agents and steel fibers should be used. The volume content of steel fiber should be controlled at around 1.0%, the length should be controlled at around 30 mm, and the maximum particle size of coarse aggregate should be less than 20 mm. Medium sand should be used, and high-efficiency additives should be used.

4 Conclusion

  1. (1)

    Expansion agents can effectively reduce the dry shrinkage of plain concrete or fiber reinforced concrete, with a 60 day dry shrinkage reduction of about 30%.

  2. (2)

    The effect of reducing concrete shrinkage with a single variety of steel fibers is not significant. However, steel fiber composites with different lengths effectively reduce the dry shrinkage of concrete, with a reduction of nearly 20% after 60 days.

  3. (3)

    Hybrid fibers, especially polypropylene fibers and composite steel fibers, have the best effect in reducing dry shrinkage, with a 60 day dry shrinkage reduction of about 23.5%.