Keywords

1 Introduction

The application of high-performance concrete in modern large and medium-sized engineering is very extensive. Professor Wu Zhongwei's definition of High performance concrete (HPC) is a new type of high-tech concrete, which is made using modern concrete technology on the basis of significantly improving the performance of ordinary concrete. It is designed with durability as the main indicator and focuses on durability, construction, applicability, strength Key guarantees for volume stability and economy [1]. One of the important indicator in the high performance concrete’s durability is dry shrinkage (expansion) and crack resistance, which is particularly important in the underground engineering like civil air defense engineering and subway engineering. And domestic scholars have also conducted extensive research on related content in these years [2,3,4,5,6]. This article attempts to draw meaningful conclusions through experiments.

2 Dry Shrinkage Test

The concrete’s dry shrinkage is related to its component, which mainly includes cement, fly ash, slag and water-cement ratio. Twelve different mix proportions’ HPC specimens are designed and the raw materials for each cubic meter of concrete are shown in the Table 1. The A100 is the reference concrete and the A110 is a reference concrete which adds HLC anti crack and anti-seepage agent. The test is mainly about ten mix proportion HPC from A111 to A136.From the first table, we can get twelve types mix proportion HPC which is the Table 2.

Table 1 C30 HPC mix ratio
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Table 2 The test result of C30 high-performance pumped concrete
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When we determine C30 pumping concrete water cement ratio and the influence of raw materials such as cement, sand and gravel, and additives on the work ability and mechanical properties of concrete, we use ordinary silicon 32.5 cement, first grade ash, slag powder, river sand and crushed stone HLC anti crack and anti-seepage agent to test C30 pumping concrete’s comprehensive performance such as work ability, physical and mechanical properties, volume stability and durability. We use that to determine the optimal mix ratio of concrete and ensure the service life of civil air defence engineering concrete exceed a hundred years.

The comprehensive performance test of C30 high-performance pumped concrete shows the concrete mix ratio in Table 1.

In the table, the A100 is the reference concrete and it’s water-cement ratio is 0.43; the A110 is reference concrete which add HLC anti crack and anti-seepage agent, and it’s ware-cement ratio is 0.43; A111 is a concrete with a single addition of 28% fly ash and the ware-cement ratio of it is 0.38; A112 is a concrete with a single addition of 37% fly ash and the ware-cement ratio of it is 0.38; A121 is a concrete with a single addition of 40% slag powder and the ware-cement ratio of it is 0.38; A122 is a concrete with a single addition of 60% slag powder and the ware-cement ratio of it is 0.38; A131 is a double mixed concrete with 15% fly ash and 25% slag micro powder and the ware-cement ratio of it is 0.38; A132 is a double mixed concrete with 18% fly ash and 28% slag micro powder and the ware-cement ratio of it is 0.38; A133 is a double mixed concrete with 28% fly ash and 28% slag micro powder and the ware-cement ratio of it is 0.38; A134 is a double mixed concrete with 18% fly ash and 37% slag micro powder and the ware-cement ratio of it is 0.38; A135 is a double mixed concrete with 28% fly ash and 37% slag micro powder and the ware-cement ratio of it is 0.38; A136 is a double mixed concrete with 18% fly ash and 46% slag micro powder and the ware-cement ratio of it is 0.38.

3 Concrete Shrinkage Test

Dry shrinkage (expansion) test of C30 high-performance pumped concrete refer to the “Highway Civil Testing Regulations” (JTGE40-2007) for testing. After three days’ age, we dismantle the mold and measure the base length. And then, make test-pieces stay in water at 20 ± 3 ℃ to test the first, seventh and fourteenth day’s length. After that, we move the test-pieces in the drying chamber to measure the twenty-eighth and ninetieth day’s length.

The detection results are shown in Table 2, and the analysis of the result is shown in Figs. 1,2 and 3.

Fig. 1
A multi-line graph plots the expansion and shrinkage rate versus age for A 300, A 310, A 311, and A 312. A 312 has the highest expansion and shrinkage rate with around 3.3 times 10 to the power negative 4 at 7 and 14 days. A 300 and A 310 have the lowest with negative 2 times 10 to the power negative 4 at 90 days.

The influence of fly ash content on the expansion and dry shrinkage rate of concrete

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Fig. 2
A multi-line graph plots the expansion and shrinkage rate versus age for A 300, A 310, A 321, and A 322. A 322 has the highest expansion and shrinkage rate with around 3.6 times 10 to the power negative 4 at 14 days. A 300 and A 310 have the lowest with negative 2 times 10 to the power negative 4 at 90 days.

Effect of slag content on concrete expansion and shrinkage rate

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Fig. 3
A multi-line graph plots the expansion and shrinkage rate versus age for D 300, D 310, A 331, A 332, A 333, A 334, A 335, and A 336. A 336 has the highest expansion and shrinkage rate with around 6.8 times 10 to the power negative 4 at 14 days.

Effect of dual dosage of admixtures on the expansion and shrinkage rate of concrete

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From the experimental results, it can be seen that after adding HLC anti crack and anti-seepage agent, strengthening wet curing can slow down the occurrence of early shrinkage cracking in concrete. But the shrinkage rate of concrete is still relatively high, which is comparable to the concrete specimens. Without HLC anti crack and anti-seepage agent.

From Fig. 1, it can be seen that after adding high-quality fly ash to replace some cement in the concrete, the early expansion rate of the concrete increases, and the corresponding dry shrinkage rate of the concrete decreases, but the decrease rate of dry shrinkage rate is consistent with the benchmark concrete added with HLC anti crack and anti-seepage agent. The addition of high-quality fly ash is beneficial for slowing down the occurrence of early shrinkage cracking in concrete. The continued increase in fly ash content (from 28 to 37%) is not conducive to improving the early cracking resistance of concrete.

From Fig. 2, it can be seen that after adding slag powder to replace some cement in concrete, the early expansion rate of concrete increases, and increases with the increase of slag powder content, but the rate of decrease in dry shrinkage is consistent with the reference concrete. The experimental results show that adding an appropriate amount of slag powder can slow down the occurrence of early shrinkage cracking in concrete.

From Fig. 3, it can be seen that after using double fly ash and slag micro powder to replace some cement in concrete, the early expansion rate of concrete increases, and increases with the increase of double admixture content. However, the rate of decrease in dry shrinkage is also consistent with the reference concrete. The experimental results show that an appropriate amount of dual admixtures can slow down the occurrence of early shrinkage cracking in concrete.

4 Analysis of Concrete Crack Resistance

4.1 Analysis of Concrete Expansion Mechanism

The early expansion mechanism of concrete mixed with HLC crack resistance and anti-seepage agent is the formation of ettringite, namely, trisulfide calcium sulphoaluminate (C6A\(\overline{{\text{S}} }\)3H32), at the initial stage of cement hydration. When this mineral is formed, its solid phase volume increases by 1.27 times. Ettringite begins to form in the first hour of cement hydration, and then its quantity increases in the first day. When the original sulfate is consumed, in the case of insufficientsulfate, ettringite will dissolve and react with Al(OH)3 to convert into single sulfur type calcium sulphoaluminate (C4A \(\overline{{\text{S}} }\)H12), and the solid volume will not increase when the single sulfur type calcium sulphoaluminate is formed. Which phase is stable depends on the sulfate concentration in the pore solution, and monosulfide type calcium aluminate is stable at low sulfate ion concentrations [7].

The amount of ettringite in concrete gel is determined by the content of effective aluminate, sulfate and calcium ions in concrete binding materials. Research has shown that large expansion occurs only when Ca(OH)2 is fully supplied. When Ca(OH)2 is insufficient, the expansion of hydrated calcium sulfoaluminate is very small and does not develop into a solid matrix [8].

The chemical composition of the cementitious material used for C30 pumping concrete in underground civil air defense engineering is as follows.

The chemical composition testing results of 32.5 ordinary silicon cement used are shown in Table 3.

Table 3 Chemical composition test results of Tianbao 32.5 ordinary silicon cement
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The results of the chemical composition detection of Grade I ash used are shown in Table 4.

Table 4 Chemical composition test results of Nanre grade I ash
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The chemical composition testing results of the slag powder used are shown in Table 5.

Table 5 Chemical composition test results of slag powder
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After replacing 28% cement with Grade I ash, the content of effective Al2O3 in the cementitious material decreases, the proportion of ettringite converted into single sulfur calcium sulphoaluminate decreases, and the early expansion of concrete increases. With the replacement amount of fly ash increases to 37%, although the effective Al2O3 content in the cementitious material decreases more, the content of Ca(OH)2 in the gel pore decreases significantly, which may be the main reason for the reduction of the early expansion amount of concrete, which is lower than the expansion amount of 28% cement concrete specimens replaced by fly ash.

After using slag powder to replace part of cement, as slag powder is mixed with 3%CaSO4·2H2O, the content of calcium sulfate in the cementitious material is sufficient, and the content of calcium oxide is rich, so the content of ettringite in the gel is more, and the early expansion of concrete specimens is larger, and with the increase of slag powder replacing cement, the early expansion of concrete increases. Based on the same reasons mentioned above, the early expansion rate of concrete increases with the addition of fly ash and slag micropowder to replace some cement, and increases with the increase of the amount of double admixtures.

4.2 Analysis of Concrete Crack Resistance

Under constrained conditions, cracks occur in concrete when the sum of dry shrinkage strain, self generated volume shrinkage strain, chemical reduction strain, carbonization shrinkage strain, and temperature difference shrinkage strain exceeds the ultimate tensile rate of the concrete. It is very difficult to improve the ultimate tensile rate of concrete, so the measure to improve the crack resistance of concrete is to reduce temperature difference and concrete shrinkage [9, 10].

Due to the fact that autogenous shrinkage of concrete mainly occurs 14 d before hydration, ensuring 14 d of wet curing after concrete formation can eliminate most of the shrinkage deformation of concrete. At the same time, strengthening wet curing is conducive to the full formation and stability of ettringite, the source of concrete expansion. And through 14 d of sufficient moisture curing, the temperature difference and shrinkage deformation of concrete can be reduced.

The measurement results of dry shrinkage strain of concrete with different mix ratios are shown in Figs. 2 and 3. When measuring the shrinkage strain of concrete, it already includes the autogenous shrinkage strain, chemical shrinkage strain, and carbonization shrinkage strain of concrete.

From the comparison between the shrinkage strain of concrete and the ultimate tensile rate of concrete, it can be seen that the absolute value of the 28 d shrinkage strain of the benchmark concrete without the addition of HLC crack resistance and anti-seepage agent and active admixture is greater than the ultimate tensile rate of the concrete, indicating that even if early water saturation curing is strengthened, the concrete is still prone to shrinkage cracks.

After adding HLC anti crack and anti-seepage agent, the absolute value of the 28 d dry shrinkage strain of concrete is less than the ultimate tensile rate of concrete at 28 d, which can meet the anti crack requirements without the influence of temperature difference shrinkage strain. However, the absolute value of dry shrinkage strain in the later stage of concrete is relatively large, and shrinkage cracks may still occur under constrained conditions.

After adding an appropriate amount of high-quality fly ash and slag micro powder, the absolute values of dry shrinkage strain of concrete at 28 and 90 d are less than the ultimate tensile rate of concrete at 28 d. Without considering the influence of temperature difference shrinkage strain, the concrete can meet the crack resistance requirements. If the influence of temperature difference shrinkage strain of concrete is considered, the cracking resistance performance of A111 single mixed 28% fly ash concrete, A132 double mixed 18% fly ash + 28% slag powder concrete, A133 double mixed 28% fly ash + 28% slag powder concrete, and A134 double mixed 18% fly ash + 37% slag powder concrete is better. Replacing cement with a certain amount of fly ash and slag powder in concrete can also reduce the hydration heat of concrete, reduce the temperature difference and shrinkage strain of concrete, and improve the crack resistance of concrete.

A122 single addition of 60% slag micro powder concrete, A135 double addition of 28% fly ash + 37% slag micro powder concrete, and A136 double addition of 18% fly ash + 46% slag micro powder concrete. Due to the high early expansion rate of the concrete, under constrained conditions, the compressive stress of the concrete is relatively high. At this time (4–5d), the compressive strength of the concrete is still low, and the concrete may experience expansion and cracking damage, but it cannot play a crack resistance role. Taking A135 double mixed 28% fly ash + 37% slag micro powder concrete as an example, the expansion rate of the concrete after 5 d of water addition is about 0.046%. The compressive modulus of the concrete is calculated at 30.0 GPa, and the compressive stress of the concrete is about 12–15 MPa, which is close to the stress zone of concrete compressive crack propagation.

5 Conclusion

  1. (1)

    Adding high-quality fly ash and slag powder in moderation is beneficial for improving the workability of concrete. However, when the amount of slag micro powder and double fly ash plus slag micro powder is too large (greater than 60%), the delayed setting time of concrete is too long, which is not conducive to normal construction of concrete.

  2. (2)

    From the perspective of improving the crack resistance of concrete, it is advisable to choose A111 single mixed 28% fly ash concrete, A132 double mixed 18% fly ash + 28% slag micro powder concrete, A133 double mixed 28% fly ash + 28% slag micro powder concrete, and A134 double mixed 18% fly ash + 37% slag micro powder concrete.

  3. (3)

    Ensuring 14 d of wet curing after concrete formation is a necessary measure to prevent and reduce concrete shrinkage cracks.