Keywords

1 Foreword

With the completion of the “eight vertical and eight horizontal” main network of China's railway, the railway network between major cities in central and eastern regions is becoming more and more perfect. At present, in order to complement the short board of railway network construction in China, the state is actively promoting the pace of railway network construction in the western and southwest regions. The difficulties of railway construction in western and southwest China include complicated geological conditions, lack of high-quality raw materials, and great difficulty in construction. In Yunguichuan area, more than half of the railway lines are tunnel structures, and the proportion of tunnels on individual lines can reach more than 70%, which is significantly different from the central and eastern regions where the railways are mainly bridge structures. The most important structure in railway tunnel is lining concrete structure, which has the characteristics of dense reinforcement, narrow construction space, concrete is not easy to vibrate and compact, vault part is easy to empty, lining concrete is not easy to cure, so the construction of railway tunnel lining concrete has always been a difficult point in railway engineering. Due to the “high complexity and strong concealment” of tunnel structure, once the quality disease occurs in railway operation, the maintenance and replacement difficulty is far greater than that of bridge structure and subgrade structure. Therefore, how to ensure the construction quality of lining concrete has become the primary concern of engineering and technical personnel.

This paper analyzes the causes and influencing factors of railway tunnel lining concrete cracking, and focuses on the influence of mix ratio parameters on the cracking sensitivity of lining concrete.

2 Causes and Influencing Factors of Railway Tunnel Lining Concrete Cracking

2.1 Causes of Lining Concrete Cracking

Among the quality defects of railway tunnel lining concrete, cracking is the most common quality defect that affects the safety and durability of the concrete structure, second only to cavitation and uncompaction. In the statistical classification of railway tunnel operation risks[1], the quality diseases of lining concrete cracks accounted for 20.0% of the 5 types and 15 risks of the whole railway tunnel operation safety risks, second only to the quality diseases of lining concrete caverns 28.8%.

Concrete is a kind of brittle material. It is easy to crack under load and non-load, which seriously affects the durability of concrete structure. Data [2,3,4] show that cracking of tunnel lining concrete mainly occurs during construction and within 1–2 years of tunnel through. Many railway tunnel projects show that the tunnel lining concrete cracking is mainly divided into three forms: circumferential cracking, longitudinal cracking and oblique cracking, among which the circumferential cracking is the main. According to the statistical results of Wang Jiahe [5], among the cracks of 696 tunnels at home and abroad, the average proportion of circumferential cracks is 64.5%, and the highest proportion is 71%. Research [6] shows that the circumferential cracking of tunnel lining concrete is mainly caused by non-load action, that is, the cracking is caused by shrinkage and deformation of lining concrete when it is not under load. The longitudinal cracks and oblique cracks are mainly caused by loading action. Therefore, the cracking of tunnel lining concrete is mainly caused by non-load factors, that is, it is closely related to the shrinkage and deformation of concrete. For lining concrete, there are four main reasons for its greater shrinkage. First, temperature stress shrinkage is greater. The general thickness of lining concrete is about 0.5m, and the thickness of special sections can reach 1m, which is similar to mass concrete. The temperature inside the tunnel is relatively stable before the tunnel passes through, and its internal hydration temperature rises higher. In addition, it is difficult to carry out warming and moisturizing maintenance after the mold removal of the lining concrete, so it is easy to crack due to temperature stress shrinkage. Second, the lining concrete itself shrinks and deforms greatly. Because it is required to smoothly pour in a close space with dense reinforcement, the amount of cementing material in the lining concrete is high, and the slump is large, especially in the vault part, to ensure that the pouring is dense and not empty, the concrete slump is larger, usually more than 220mm, so the plastic shrinkage, autogenous shrinkage and dry shrinkage of the concrete are relatively large. Once the concrete is not cured well, it is easy to crack due to greater shrinkage. Third, the waterproof board on the back of the lining and the initial spray concrete will constrain the shrinkage deformation of the lining concrete, resulting in tensile stress on the concrete surface, thus causing cracking. Fourth, the concrete construction and maintenance is poor. Holes or non-compaction caused by the inadequacy of vibration, and segregation or bleeding caused by overvibration, will increase the risk of concrete cracking. The lack of timely maintenance after mold removal of lining concrete also aggravates the water loss of concrete surface, resulting in shrinkage cracking.

2.2 Influencing Factors on Shrinkage Cracking of Lining Concrete

Amount of Cementing Material.

The amount of cementing material is the main factor affecting the shrinkage and deformation of concrete. The greater the amount of cementing material, especially the higher the amount of cement in the cementing material, the greater the self-shrinkage of the hydration product and the easier the concrete cracking. In addition, the amount of cementing material determines the amount of aggregate, which is an important factor affecting the shrinkage and deformation of concrete [7, 8]. The ratio of shrinkage Sc of concrete to shrinkage Sp of cement paste depends on the aggregate content α, that is, Sc = Sp(1-α)n, where n is the empirical coefficient. The higher the aggregate content, the smaller the shrinkage deformation of concrete [8]. When the water-binder ratio is fixed, the amount of cementing material reflects the proportion of cement slurry and aggregate, that is, the slurry aggregate ratio[8]. The higher the amount of cementing material, the larger the slurry aggregate ratio, and the larger the shrinkage deformation of concrete. In order to ensure the strength and working performance of tunnel lining concrete, a higher amount of cementite material is usually used.

Water Consumption Per Cubic Meter of Concrete.

The drying shrinkage of concrete is mainly caused by the continuous evaporation of excess water in concrete. Generally, the amount of water required for cement hydration is only about 25% of the cement mass, that is, the water-cement ratio is 0.25, but the water-cement ratio of concrete applied in actual engineering is much larger than this. It is beneficial to reduce the drying shrinkage of concrete by using concrete with less water consumption.

Aggregate Varieties.

Aggregate varieties also have a great influence on the dry shrinkage of concrete[9]. General low shrinkage aggregates are quartzite, limestone, granite and basalt. In addition, the drying shrinkage of concrete is reduced when the aggregate size is large, and the drying shrinkage of concrete is increased when the aggregate moisture content is high.

Mineral Admixtures.

Railway tunnel lining concrete is mainly mixed with fly ash. An Mingzhe [10] showed that the mixture of fly ash reduced the self-shrinkage of concrete, especially in the early stage. However, the reduction effect of fly ash on concrete shrinkage is mainly reflected in the later stage of concrete hardening. In the early stage of hydration, due to the slow hydration rate of fly ash, more free water can be evaporated in concrete, so it is easier lose water because of dry, which aggravates the early shrinkage of concrete. Therefore, it is very necessary for the early moisture curing of the lining concrete mixed with fly ash. The addition of slag powder will increase the shrinkage deformation of concrete, which is unfavorable to the crack prevention of the lining concrete.

Moisture Curing.

Early moisture curing is the most effective measure to reduce the shrinkage of concrete, but because of the structural characteristics of lining concrete, it is difficult to achieve effective moisture curing in actual construction, which is unfavorable to the crack prevention of lining concrete.

3 Experiment

3.1 Raw Material and Experiment Mix Ratio

In this paper, Beijing Jinyu P·O42.5 grade ordinary Portland cement was used, with specific surface area of 347 m2/kg and 3d and 28d compressive strength of 23.0 MPa and 47.0 MPa, respectively. Fly ash was produced by Shenyang Guodian Kangping New Materials Co., LTD.,which is Class F grade I fly ash, with fineness of 11%, firing loss of 1.42%, water requirement ratio of 94%. Limestone powder was used with MB value of 0.9 and mobility ratio of 105%. The water reducing agent was produced by Jiangsu Subote New Materials Co., LTD., which is PCA-I retarding water reducing agent. The fine aggregate is made of machine-made sand with fineness modulus of 2.8, stone powder content of 6% and apparent density of 2650 kg/m3. The coarse aggregate is 5 mm–31.5 mm continuously graded three-graded gravel, with an apparent density of 2700 kg/m3 and a mud content of 0.5%.

The test adopts the mix ratio of lining concrete with strength grade C40 commonly used in railway tunnel engineering, the gas content is between 4.0% and 5.0%, and the slump is 200 mm. The base mix ratio is shown in Table 1.

Table 1. Mix proportion（kg/m3）
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3.2 Experiment Method

Drying shrinkage can directly reflect the shrinkage of concrete, flexural strength and splitting tensile strength can also effectively characterize the cracking sensitivity of concrete. Dry shrinkage, flexural strength and splitting tensile strength were used to evaluate the influence of key parameters on the cracking sensitivity of lining concrete. The flexural strength and splitting tensile strength of concrete were tested according to the “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019), and the drying shrinkage of concrete was tested according to the “Standard for test methods of long-term performance and durability of ordinary concrete” (GB/T 50082-2009).

4 Influence of Key Parameters on Cracking Sensitivity of Lining Concrete

4.1 Content of Cementitious Materials

Based on the base mix proportion in Table 1, the influence of different cementitious materials (total content of cement and fly ash) on the dry-shrinkage of lining concrete were studied when they were 360 kg/m3, 380 kg/m3, 400 kg/m3 and420 kg/m3, respectively, while the water-binder ratio and other parameters remained unchanged. The results were shown in Fig. 1.

Fig. 1.
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Influence of content of cementitious materials on dry-shrinkage of concrete

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The results of Fig. 1 show that the content of cementitious materials has a significant effect on the dry-shrinkage of lining concrete, and the shrinkage value of concrete increases by about 8% with the increase of 20 kg/m3 cementitious materials. The content of cementitious materials increases, and the content of aggregate decreases accordingly when the water-binder ratio is fixed, therefore, the change in the content of cementitious materials actually reflects the change of bone pulp ratio. The greater the content of cementitious materials, the smaller the bone pulp ratio, and the greater the dry-shrinkage value of concrete.

4.2 Unilateral Water Consumption

Based on the base mix ratio in Table 1, the influence of water consumption of 155 kg/m3, 150 kg/m3, 145 kg/m3, 140 kg/m3 and 135 kg/m3 on flexural strength and splitting tensile strength of concrete were studied while keeping other parameters constant. The results were shown in Figs. 2 and 3.

Fig. 2.
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Influence of water consumption on flexural strength of concrete

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Fig. 3.
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Influence of water consumption on splitting tensile strength of concrete

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From Figs. 2 and 3, it can be seen that with the decrease of unilateral water consumption, both the flexural strength and splitting tensile strength of concrete tend to increase significantly, especially before 28d. The splitting tensile strength is more sensitive to the change of unilateral water consumption, when the unilateral water consumption is lower than 145 kg, the increase rate of the splitting tensile strength decreases significantly and shows a decreasing trend at 56d. When the unilateral water consumption is lower than 140kg, the increase rate of the flexural strength also decreases significantly. The flexural strength test is a test method to measure the tensile stress of the reaction material under the action of transverse load, which is usually tested by three-point bending method. The splitting tensile strength is another commonly used test method to characterize the tensile strength of concrete materials, which is usually tested by the two-point concentrated load method. Both methods are used to replace the direct tensile strength test which is difficult to operate to test the tensile strength of the material. The tensile strength is a key index to determine their cracking properties for concrete material [11]. Therefore, flexural strength and splitting tensile strength are often used to characterize the cracking sensitivity of concrete material. The results show that the cracking sensitivity of lining concrete firstly increase and the decrease with the decrease of unilateral water consumption, and there is an optimal unilateral water consumption. At the same time, the unilateral water consumption should not be too low in order to ensure the working performance and hydration characteristics of concrete, it should be controlled at about 140 kg/m3–145 kg/m3.

4.3 Sand Ratio

Generally speaking, the best sand ratio of the concrete mix in good condition ranges from 36% to 45%. Based on the base mix ratio in Table 1, the influence of different sand ratio (36%, 39% and 42%) on the flexural strength, splitting tensile strength and dry-shrinkage of concrete under the same amount of cementitious materials were studied. The results were shown in Figs. 4, 5 and 6 respectively.

Fig. 4.
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Influence of sand ratio on flexural strength of concrete

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Fig. 5.
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Influence of sand ratio on splitting tensile strength of concrete

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From Figs. 4 and 5, it can be seen that the flexural strength and splitting tensile strength of the lining concrete before 28d showed a decreasing trend in general and basically the same after 56d with the increase of sand ratio. The results show that the increase of sand ratio is unfavorable to the early cracking sensitivity of lining concrete, because the limiting ability of microcrack propagation in concrete is reduced with the reduction of coarse aggregate content.

Fig. 6.
figure 6

Influence of sand ratio on dry-shrinkage of concrete

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From Fig. 6, it can be seen that the dry-shrinkage of lining concrete presents a linear growth trend with the increase of sand ratio, and the 56d dry-shrinkage value increases by 10% with the increase from 36% to 39% sand ratio. The 56d dry-shrinkage value increase by 6% compared with the sand rate of 39% When sand ratio continues to increase to 42%. The increase of sand ratio means the decrease of coarse aggregate content and the increase of pulp to bone ratio, which is equivalent to the increase of the amount of cementitious materials. Although river sand dose not participate in hydration reaction, it will also increase the dry-shrinkage of concrete.

4.4 Limestone Powder Content

Machine-made sand is used to produce concrete on nearly half of railway lines with the increasing shortage of natural river sand and high-quality fly ash resources in the country, and as a by-product of machine-made sand production, limestone powder is used to prepare concrete has become an inevitable trend as a new mineral admixture. Based on the base mix ratio in Table 1, when limestone powder is used to replace fly ash, the influence of limestone powder content of 10%, 20% and 30% respectively (including 6% limestone powder in machine-made sand) on flexural strength and splitting tensile strength of concrete were studied under the total amount of cementing material remains unchanged at 420kg/m3. The results were shown in Fig. 7 and Fig. 8.

Fig. 7.
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Influence of limestone powder content on flexural strength of concrete

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Fig. 8.
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Influence of limestone powder content on splitting tensile strength of concrete

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From Figs. 7 and 8, it can be seen that the flexural strength and splitting tensile strength of different aged lining concrete decreases significantly with the increase of limestone powder content, and the splitting tensile strength being more evident. In addition, the flexural strength and splitting tensile strength of concrete has little difference when the stone powder content is 10% and 20%, but the reduction is significant when the stone powder content reaches 30%.As an inert material, limestone powder does not participate in cement hydration, but mainly accelerates cement hydration by forming crystalline nucleation points and plays a filling role in concrete. Due to the extremely weak hydration, limestone powder will reduce the hydration rate of cement, reduce the generation of hydration gel products, and reduce the flexural strength and splitting tensile strength of concrete after exceeding a certain dosage. At the same time, due to the delay of cement hydration rate, the amount of free water that can be evaporated in concrete will be more, and the dry shrinkage of concrete will be increased to a certain extent.

5 Conclusion

  1. (1)

    Among the mix ratio parameters that affect the cracking sensitivity of lining concrete, the amount of cementitious materials and unilateral water consumption have obvious effects on the shrinkage cracking of concrete. The content of cementitious materials should be as low as possible, and the water content should be controlled at about 140 kg/m3 –145 kg/m3.The increase of sand ratio is unfavorable to the shrinkage of lining concrete, so it is advisable to choose as low sand ratio as possible to prepare lining concrete.

  2. (2)

    As a new mineral admixture, the excessive addition of limestone powder will increase the crack sensitivity of the lining concrete, and the limestone powder content should be controlled within 20%.