Keywords

1 Introduction

Bag concrete is an integral structure formed by pouring fluidity concrete into the bag with high pressure pump and solidifying after the excess water oozes from the fabric gap. The bag is made of geosynthetics woven into two layers with a certain thickness. This technology has the characteristics of fast construction speed, save labor and time, easy operation and strong terrain adaptability. It can be widely used in road, reservoir, river channel, seawall and other projects. In the aspect of lining technology of concrete channel for mold bag at home and abroad, researches have been carried out mainly on the concrete structure, material and application effect of mold bag. Li yatong [1] studied the bag concrete channel in hetao irrigation area of Inner Mongolia from the perspective of mechanical properties. Wang jun, huo yizhen et al. [2] proposed the optimized mix ratio of mold bag concrete. Zhang hailing, huo yizhen et al. [3] discussed the concrete roughness of mold bag and its influencing factors. Gong jibin [4] introduced the selection of mold bag in the concrete construction of mold bag, the technical index of concrete, the construction method of pouring concrete and the matters needing attention, based on the concrete laying and protecting project of the seven-star canal mingsha section. Wang jun, guo yanfen et al. [5] analyzed and studied the freeze resistance and macroscopic mechanical properties of mold bag concrete by measuring the mass loss rate and relative dynamic elastic modulus of test blocks with different mixing ratios. At present, the research on thermal insulation and antifreeze technology of polystyrene board used in precast concrete and cast-in-place concrete channel is relatively common [6,7,8,9,10,11,12,13,14,15], while the thermal insulation and antifreeze technology of polystyrene board laid in concrete channel of mold bag has not been studied yet. Therefore, combined with the characteristics of soil and climate in Hetao irrigation area of Inner Mongolia, this paper carried out an experimental study on the laying of polystyrene board with different thickness under the conditions of mold bag concrete.

2 Materials and Methods

In hetao irrigation area, Inner Mongolia, a platform for concrete treatment of different mold bags was set up. Each test platform has an area of 4 m × 4 m, and a total of 6 die bag concrete treatment platforms are set. Treatment 1~3 laid 10 cm, 12 cm and 15 cm mold bag concrete without heat preservation. Polystyrene insulation boards with thickness of 4 cm, 6 cm and 8 cm were laid under the concrete of 10 cm mold bags from 4 to 6. The 6 cm thick polystyrene board was laid vertically around each treatment block at a depth of 1 m to prevent interference from horizontal frost heaving.

The schematic diagram of the frost heave test platform is shown in Fig. 1, and the test processing is shown in Table 1. The test was conducted from November 2015 to April 2018, with a total of three freezing cycles.

Fig. 1.
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Sketch map of frost heave test platform of different molded bag concrete treatment

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In hetao area, the concrete construction thickness of the main channel mold bag is generally 10~15 cm, so considering the economic efficiency, the insulation board is laid under the 10 cm mold bag concrete in the test design, and the effect of insulation and anti-frost heave of the lining structure of mold bag concrete + insulation board is discussed.

Table 1. Design test of different molded bag concrete treatment
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3 Results and Analysis

3.1 Different Bag Concrete Treatment Insulation Effect

The variation law of daily mean temperature of concrete treated with different mold bags.Statistical analysis was conducted on the daily mean temperature of concrete treatment with different mold bags (buried 30 cm deep) in the test site from 2015 to 2016, and the daily mean temperature variation process of concrete treatment with six different mold bags was drawn, as shown in Fig. 2.

Fig. 2.
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Curve of daily average temperature variation of concrete treated with different mould bags in 2015~2016

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As can be seen from the Fig.above, the daily mean temperature of the six treatments decreased first and then increased with time. The changes of the three treatments of 10 cm, 12 cm and 15 cm mold bag concrete are basically the same, indicating that the increase of the thickness of mold bag concrete does not significantly increase the insulation effect. Under the concrete condition of 10 cm mold bag, the changes of the three treatments of laying 4 cm, 6 cm and 8 cm polystyrene boards are basically the same, indicating that the difference of daily average temperature between laying 4 cm, 6 cm and 8 cm polystyrene boards is also small.

The total accumulated temperature and the effect of increasing the temperature of the concrete with different thickness.The daily average temperature of concrete treatment of 6 different mold bags in 3 freezing-thawing periods in the test site was accumulated to obtain the total accumulated temperature value of each treatment, and the average total accumulated temperature and average warming effect value of each treatment were calculated, and the average total accumulated temperature and average warming effect of concrete treatment of different mold bags were drawn, as shown in Fig. 3.

Fig. 3.
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Curve of Total accumulated temperature and increasing temperature effect of different Mould bags

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As can be seen from Fig. 3, the average total accumulated temperature of concrete in mold bags with no insulation treatment in 10 cm, 12 cm and 15 cm is 278.35 ℃, 289.30 ℃ and 304.01 ℃, and there is no significant difference between them. The average total accumulated temperature of concrete in the three types of treatment with insulation board was 856.29 ℃, 955.10 ℃ and 1036.17 ℃, respectively, which were significantly higher than those without insulation. On the basis of 10 cm mold bag concrete, add 2~5 cm mold bag concrete, the heating effect is 3.93%~9.22%, and laid 4~8 cm polystyrene board, the heating effect is 207.63%~272.25%, indicating that mold bag concrete does not have the heat preservation effect, and the heat preservation board has a good insulation effect.

Effect of thermal insulation plate on soil temperature at different depths. The average ground temperature distribution diagram of 0~50 cm soil layer without heat preservation (comparison section) is shown in Fig. 4.

Fig. 4.
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Soil temperature distribution map of different treatments

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According to the analysis of the ground temperature at different depths, with the increase of the thickness of the insulation board, the ground temperature of the 0–10 cm soil layer on the surface increased less, indicating that the laying of the insulation board had less influence on the ground temperature of the 0–10 cm soil layer on the surface. With the increase of the thickness of the insulation board, the ground temperature of the 10–40 cm soil layer changed greatly, especially without the insulation treatment and the laying of 4 cm polystyrene board, the ground temperature of the 10–40 cm soil layer changed most significantly. The results showed that the insulation board laid on the concrete foundation of mold bag had the greatest influence on the ground temperature of 10–40 cm soil layer.

3.2 Concrete Treatment of Different Mold Bag Anti - frost Heaving Effect

The maximum frost heaving capacity and the reduction of frost heaving capacity are handled by different mold bags of concrete. Table 2, 3 and 4 shows the maximum frost heave and reduction rate of concrete treated with different mold bags during 3 complete freezing-thawing periods from 2015 to 2018.

Table 2. Maximum Frost heaving of different mold bags concrete treatment in 2015~2016
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Table 3. Maximum Frost heaving of different mold bags concrete treatment in 2016~2017
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Table 4. Maximum Frost heaving of different mold bags concrete treatment in 2017~2018
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As can be seen from the above Table, the maximum frost heave of the three treatments of mold bag concrete + insulation board are all below 3 cm, while the frost heave of the three treatments of mold bag concrete without insulation board is 5.4 cm~10.3 cm. On the basis of 10 cm mold bag concrete, add 2~5 cm mold bag concrete, reduce the frost heave rate of 18.28%~55.44%; And laying 4~8 cm polystyrene board, the frost heave reduction reached 71.43%~96.6%. It can be seen that the laying of insulation board under the concrete mold bag can significantly reduce the frost heave of the foundation soil, and the thicker the laid insulation board is, the more frost heave will be reduced, and the frost heave effect of polystyrene board is significantly greater than that of concrete mold bag.

3.3 Variation Law of Freezing Depth of Concrete Treatment with Different Mold Bags

Variation law of freezing depth in concrete treatment of different mold bags. Figure 5 shows the freezing depth variation process lines of the subsoil treated with 10 cm mold bag concrete, 12 cm mold bag concrete, 15 cm mold bag concrete and 10 cm mold bag concrete +4 cm polystyrene board, 10 cm mold bag concrete +4 cm polystyrene board, 10 cm mold bag concrete +4 cm polystyrene board in the south of the test site during a complete freezing-thawing period from 2015 to 2016.

Fig. 5.
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Freeze depth variation of different mold bag concrete treatment in 2015~2016

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From the above analysis, it can be seen that during the complete freezing-thawing period from 2015 to 2016, the freezing depth of concrete treatment for each mold bag has the same change rule. All treated subsoil began to freeze in mid-November, reached its maximum depth in mid-late February, began to melt in early march, and melted in mid-April. The freezing depth of polystyrene board was significantly lower than that of concrete bag without heat preservation, and the freezing depth of concrete bag with 8 cm polystyrene board had the most gentle change, indicating that the freezing depth had the least change.If the concrete in mold bags of 10 cm is increased by 2~5 cm, the reduction rate of freezing depth is 17.74%~29.15%. Adding 4~8 cm polystyrene plate, the reduction rate of freezing depth reached 71.24%~85.08%. The results show that increasing the thickness of concrete in mold bag does not significantly reduce the freezing depth, while laying polystyrene board under the concrete in mold bag can significantly reduce the freezing depth of foundation soil.

Change law of frost heaving rate in different mold bag concrete treatment. Based on the analysis of the freezing depth and ground temperature data of different bag concrete treatment from 2015 to 2016, the frost heave rate of each treatment in the process of frost heave and melting was calculated. The fitting curve of frost heave rate changing with temperature was fitted, as shown in Fig. 6.

Fig. 6.
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Relationship between frost heave ratio and soil temperature in different mold bag concrete treatment

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As can be seen from Fig. 6, there is a linear relationship between frost heave rate of foundation soil and soil temperature under different mold bag concrete treatment, and the frost heave rate decreases with the increase of soil temperature. Under the condition of no insulation, the maximum frost heave rate of the 10 cm mold bag concrete is 22.2%. While the increase of 2–5 cm mold bag concrete, the maximum frost heave rate is 16% above, which decreases by 28%. The maximum frost heave rate of laying 4~8 cm polystyrene board treated was 14%~9%, which decreased by 38%~60%. It can be seen that the polystyrene board can significantly reduce the frost heaving rate of subsoil.

The slope of the curve fitting frost heave rate and soil temperature reflects the slowness of soil freezing process. The higher the absolute slope, the faster the freezing rate. When increasing 2–5 cm mold bags concrete on the foundation of 10 cm mold bags concrete, the absolute slope of the curve fitting the frost heave rate and soil temperature does not change. The results show that increasing the thickness of concrete does not delay the freezing process of foundation soil. When 4~8 cm polystyrene board was laid on the foundation of 10 cm mold bag concrete, the absolute slope of the curve fitting the frost heave rate and the soil temperature decreased by 44.6%~58.7%. The results showed that the change range of frost heave rate was small and the freezing duration was long after laying insulation board. That is, the laying of polystyrene board significantly slows down the freezing speed of the subsoil and prolongs the freezing duration, thus playing a thermal insulation effect.

4 Conclusion and Discussion

Laid bag concrete almost does not have the thermal insulation effect, the thermal insulation effect is far lower than the polystyrene board thermal insulation effect. Polystyrene insulation board is laid on the mold bag concrete, which has the greatest influence on the ground temperature in 10–40 cm soil layer.

The frost heaving reduction rate is only 18.28%~55.44% when 2~5 cm mould bag concrete is laid on the basis of 10 cm mould bag concrete, while the frost heaving reduction rate is 71.43%~96.6% when 4~8 cm polystyrene board is laid. The frost heaving reduction effect of polystyrene board is significantly greater than that of mould bag concrete. Increasing the thickness of the mold bag concrete does not significantly reduce the freezing depth, while laying polystyrene board under the mold bag concrete can significantly reduce the freezing depth of the foundation soil. Laying polystyrene board can significantly delay the freezing speed of foundation soil and prolong the freezing time, thus achieving the effect of heat preservation.