Freeze-thaw-sulfate erosion test method for sprayed concrete under different service lives

Abstract

The invention discloses a freeze-thaw-sulfate erosion test method for sprayed concrete under different service lives, relates to the technical field of sprayed concrete, and generally directly gives sulfate dry-wet cycle times and freeze-thaw cycle times in a traditional cycle test. Therefore, the influence of freeze thawing-sulfate erosion on the durability of the concrete structure under different service lives in the actual situation cannot be accurately verified. The invention provides a sprayed concrete freeze thawing-sulfate erosion test method under different service lives, which comprises the following steps: step 1, selecting raw materials, and manufacturing a concrete test piece; 2, determining the number of times of freeze-thaw cycle tests and the number of days of sulfate dry-wet cycle tests; step 3, soaking the concrete test piece in a sulfate solution for pretreatment; and step 4, performing freeze-thaw cycle test and sulfate dry-wet cycle test treatment on the concrete test piece. The durability of the sprayed concrete can be evaluated by simulating freeze thawing-sulfate erosion environments of the sprayed concrete under different service lives.

Description

Technical Field

[0001] The present invention relates to the technical field of shotcrete, and particularly to a method for freeze-thaw-sulfate erosion test of shotcrete under different service life. Background Art

[0002] Concrete is widely used in many fields such as underground engineering (such as tunnel lining) and slope protection. In these application scenarios, it is long-term in complex and harsh environmental conditions. For example, in tunnel projects in cold regions, concrete has to bear the erosion of chemical substances such as sulfates carried by groundwater and the action of freeze-thaw cycles. Such a complex environment will cause the deterioration of concrete performance, and thus affect the safety and durability of the structure.

[0003] Concrete structures will be affected by various factors such as freeze-thaw cycles and sulfate erosion in practical applications, resulting in the decline of the durability of the structure. Concrete structures in cold regions with sulfate environment are affected by the combined action of freeze-thaw and sulfate erosion in the cold season, and are only affected by sulfate erosion in the non-cold season.

[0004] Traditional cyclic tests generally directly specify the number of sulfate wet-dry cycles and the number of freeze-thaw cycles, resulting in their inability to accurately verify the influence of freeze-thaw-sulfate erosion on the durability of concrete structures under different service life in actual situations. Therefore, a new method is needed to determine the number of freeze-thaw-sulfate cycles in actual situations, and then conduct an accelerated aging test on shotcrete materials to evaluate their durability. Summary of the Invention

[0005] Aiming at the above existing problems, the present invention aims to provide a method for freeze-thaw-sulfate erosion test of shotcrete under different service life, which can simulate the actual environmental conditions, conduct an accelerated aging test on shotcrete materials to evaluate their durability, and is applicable to the durability evaluation of tunnels, dams and other shotcrete structures in cold regions with freeze-thaw-sulfate environment.

[0006] The main idea of the technical solution adopted by the present invention: Starting from the actual situations in different regions, based on the climate conditions in that region and the concentration of erosion ions in the actual service environment of shotcrete, calculate to determine the number of freeze-thaw cycle tests and the number of days of sulfate wet-dry cycle tests, and then formulate a freeze-thaw-sulfate wet-dry cycle test method to simulate the freeze-thaw-sulfate erosion environment of shotcrete under different service life, so as to evaluate its durability.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for freeze-thaw-sulfate erosion test of shotcrete under different service life, comprising the following steps:

[0009] Step 1: Select raw materials and prepare concrete specimens;

[0010] Step 2: Determine the number of freeze-thaw cycles and the number of days of sulfate wet-dry cycles in the test;

[0011] Step 3: Pretreat the concrete specimens;

[0012] Step 4: According to the number of freeze-thaw cycles and the number of days of sulfate wet-dry cycles obtained in Step 2, conduct freeze-thaw cycle tests and sulfate wet-dry cycle tests on the concrete specimens.

[0013] Through the above technical solutions, further: Step 2 includes the following steps:

[0014] Step 2.1: Design an expression for the average temperature of the concrete specimens under immersion and drying in the sulfate wet-dry cycle test;

[0015] Step 2.2: Design expressions for the temperature acceleration coefficient and the erosion solution concentration acceleration coefficient in the sulfate wet-dry cycle test, and an expression for the freeze-thaw deterioration ratio coefficient in the freeze-thaw cycle test;

[0016] Step 2.3: Design expressions for the number of freeze-thaw cycles and the number of days of sulfate wet-dry cycles.

[0017] Through the above technical solutions, further, in Step 2.1, the expression for the average temperature of the concrete specimens under immersion and drying in the sulfate wet-dry cycle test is:

[0018] T 均 =[T 烘箱 ×t1 + T1×t2 + t3×(T 烘箱 + T1)] / 24 (1)

[0019] Where: T 均 is the average temperature under immersion and drying, T 烘箱 is the oven air temperature, t1 is the duration of oven drying, T1 is the annual average temperature of the region under the Celsius temperature scale, t2 is the sulfate immersion time, and t3 is the air-drying time.

[0020] Through the above technical solutions, further, in Step 2.2, the expression for the temperature acceleration coefficient is:

[0021]

[0022] Where: K a is the temperature acceleration coefficient; E / R is the activation energy, with a value of 14242; T2 is the annual average temperature of the region under the Kelvin temperature scale;

[0023] The expression for the erosion solution concentration acceleration coefficient is:

[0024]

[0025] Where: K b is the erosion solution concentration acceleration coefficient; is the sulfate concentration in the erosion solution; is the average sulfate concentration in the actual environment; C 试验 is the erosion ion concentration collected in the indoor accelerated test; C 实际 is the erosion ion concentration in the actual service environment of the concrete;

[0026] The freeze-thaw deterioration ratio coefficient: that is, the ratio of the effect of a single freeze-thaw cycle under natural conditions to the effect of a single freeze-thaw cycle test in the laboratory, and the value is 12.

[0027] Through the above technical solutions, further, in step 2.3, the expression for the number of freeze-thaw cycles is: the number of freeze-thaw cycles = the annual average number of freeze-thaw cycles ÷ the freeze-thaw deterioration ratio coefficient, where the annual average number of freeze-thaw cycles refers to the annual average number of freeze-thaw cycles under natural conditions;

[0028] The expression for the number of days of sulfate wet-dry cycle test is: the number of days of sulfate wet-dry cycle test = the number of days of sulfate wet-dry cycle ÷ K a ÷K b , where the number of days of sulfate wet-dry cycle refers to the annual average number of sulfate wet-dry cycles under natural conditions.

[0029] Through the above technical solutions, further, in step 3, the pretreatment method is: the concrete specimens are immersed in a sodium sulfate solution with a mass fraction of 5% for 4 d.

[0030] Through the above technical solutions, further: in step 4, the freeze-thaw cycle test and the sulfate wet-dry cycle test are carried out alternately, and before the start of the nth (n≥2) freeze-thaw cycle test, the concrete specimens are immersed in a sodium sulfate solution with a mass fraction of 5% for 1 d.

[0031] Through the above technical solutions, further: in step 4, the freeze-thaw cycle test includes a spraying section, a cooling section, a low-temperature constant-temperature section, a heating stage, and a high-temperature constant-temperature section that are carried out in sequence. The durations of the spraying section, the cooling section, the low-temperature constant-temperature section, the heating stage, and the high-temperature constant-temperature section are: 5 min, 2 h, 2 h, 0.5 h, and 1.5 h, respectively.

[0032] Through the above technical solutions, further: in step 4, the temperature of the low-temperature constant-temperature section is set at -20°C, the temperature of the high-temperature constant-temperature section is set at +25°C, and after the end of the high-temperature constant-temperature section, the freeze-thaw box automatically sprays three times, each lasting 1 min.

[0033] Through the above technical solutions, further, the operation of the sulfate wet-dry cycle test is as follows: the concrete specimen is immersed in a 5% sulfate solution for 16 h, and then the concrete specimen is subjected to air drying, drying and cooling treatments in sequence. The drying temperature is 65 °C and the drying time is 6 h.

[0034] The beneficial effects of the present invention are as follows:

[0035] Starting from the actual conditions in different regions, based on the climatic conditions in the region and the concentration of erosion ions in the actual service environment of shotcrete, the annual freeze-thaw-sulfate cycle times of the concrete structure are determined by calculation, and then a freeze-thaw-sulfate wet-dry cycle test method is formulated, and further, the freeze-thaw-sulfate erosion environment of shotcrete under different service years is simulated, so as to evaluate its durability. It is applicable to the durability evaluation of tunnels, dams and other shotcrete structures in cold regions under freeze-thaw-sulfate environments. Description of the Drawings

[0036] Figure 1 It is a flowchart of a simulation test method for a concrete component serving for N years in a cold region under a freeze-thaw-sulfate environment provided by the present invention;

[0037] Figure 2 It is a flowchart of a freeze-thaw-sulfate wet-dry cycle alternating test regime provided by the present invention;

[0038] Figure 3 It is a schematic diagram of the temperature rising and falling mechanism of a freeze-thaw test provided by the present invention;

[0039] Figure 4 It is an actual temperature change diagram of a freeze-thaw box provided by the present invention. Detailed Embodiments

[0040] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Usually, the components of the embodiments of the present application described and illustrated herein can be arranged and designed in various different configurations.

[0041] The inventors have found through research that in actual applications, concrete structures are affected by various factors such as freeze-thaw cycles and sulfate erosion, resulting in a decline in the durability of the structure. In cold regions, concrete structures in sulfate environments are subjected to the combined action of freeze-thaw and sulfate erosion in the cold season and only sulfate erosion in the non-cold season. Traditional cycle tests generally directly specify the number of sulfate wet-dry cycles and freeze-thaw cycles, resulting in their inability to accurately consider the impact of freeze-thaw-sulfate erosion on the durability of concrete structures under different service years in actual situations. Therefore, a new method is needed to determine the number of freeze-thaw-sulfate cycles in actual situations.

[0042] Based on the above findings, the present application proposes a freeze-thaw-sulfate erosion test method for shotcrete under different service years. Starting from the actual situations in different regions, according to the climate conditions in the region and the concentration of erosion ions in the actual service environment of shotcrete, the annual freeze-thaw-sulfate cycle times of the concrete structure are determined through calculation, and then a freeze-thaw-sulfate dry-wet cycle test method is formulated, and further the freeze-thaw-sulfate erosion environment of shotcrete under different service years is simulated, so as to evaluate its durability performance.

[0043] Example 1

[0044] Refer to Figures 1-4 , the present application discloses a freeze-thaw-sulfate erosion test method for shotcrete under different service years, including the following steps:

[0045] Step 1, select raw materials and make concrete specimens;

[0046] Step 2, determine the number of freeze-thaw cycle tests and the number of days of sulfate dry-wet cycle tests;

[0047] Step 3, pre-treat the concrete specimens;

[0048] Step 4, according to the number of freeze-thaw cycle tests and the number of days of sulfate dry-wet cycle tests obtained in Step 2, conduct freeze-thaw cycle tests and sulfate dry-wet cycle test treatments on the concrete specimens.

[0049] Example 2

[0050] The following further describes the application principle of the present invention in combination with the test.

[0051] In Step 1, the raw materials include:

[0052] ① Cement: The cement used in this test is Conch brand P.O42.5R ordinary Portland cement;

[0053] ② Fine aggregate: Natural river sand;

[0054] ③ Coarse aggregate: Crushed stone with a particle size distribution of 4 - 9 mm;

[0055] ④ Water: Ordinary tap water;

[0056] ⑤ Salt solution: Sodium sulfate solution with a mass fraction of 5%;

[0057] ⑥ Water reducing agent.

[0058] The concrete specimens are made by the shot large plate cutting method. During construction, the concrete is sprayed into the mold. After the concrete reaches a certain strength, it is processed into cube specimens and cured under standard conditions for a period of time before the test.

[0059] Select concrete specimens according to actual requirements, and calculate the number of freeze-thaw-sulfate cycles of the concrete specimens under actual conditions.

[0060] In a sulfate environment in cold regions, concrete structures are subjected to the combined action of freeze-thaw and sulfate erosion in the cold season and only sulfate erosion in the non-cold season. By immersing concrete specimens in a sulfate solution to carry out freeze-thaw cycle tests, the sulfate erosion and freeze-thaw cycle effects on concrete structures in the cold season can be simulated; by sulfate dry-wet cycle tests, the sulfate erosion on concrete structures in the cold and non-cold seasons can be simulated.

[0061] Figure 1 is a schematic diagram of the simulation test method for concrete specimens in service for N years in a freeze-thaw-sulfate environment in cold regions. Among them, N represents the service life of the concrete specimens, N d represents the number of freeze-thaw cycles suffered by the concrete specimens within 1 year of service, and N f represents the number of dry-wet cycles suffered by the concrete specimens within 1 year of service. Therefore, the concrete specimens in service for N years in a cold region with complex saline soil environment have suffered N×N d times of freeze-thaw cycle effects in the cold season, and a total of N×N f times of dry-wet cycle effects in the cold and non-cold seasons.

[0062] In step 2, the following specific steps are included:

[0063] Step 2.1, design an expression for the average temperature of the concrete specimens during immersion and drying in the sulfate dry-wet cycle test;

[0064] Step 2.2, design expressions for the temperature acceleration coefficient and the erosion solution concentration acceleration coefficient in the sulfate dry-wet cycle test, and an expression for the freeze-thaw deterioration ratio coefficient in the freeze-thaw cycle test;

[0065] Step 2.3, design expressions for the number of freeze-thaw cycle tests and the number of days of the sulfate dry-wet cycle test.

[0066] In step 2.1, the expression for the average temperature of the concrete specimens during immersion and drying in the sulfate dry-wet cycle test is:

[0067] T 均 =[T 烘箱 ×t1 + T1×t2 + t3×(T 烘箱 + T1)] / 24 (1)

[0068] Where: T 均 is the average temperature during immersion and drying, and T 烘箱is the oven temperature, t1 is the duration of oven drying, T1 is the annual average temperature of the region under the Celsius temperature scale, t2 is the sulfate immersion time, and t3 is the air-drying time.

[0069] In step 2.2, the expression for the temperature acceleration coefficient is:

[0070]

[0071] Where: K a is the temperature acceleration coefficient; E / R is the activation energy, which is taken as 14242 here; T2 is the annual average temperature of the region under the Kelvin temperature scale.

[0072] Erosion solution concentration acceleration coefficient:

[0073]

[0074] Where: K b is the erosion solution concentration acceleration coefficient; is the sulfate concentration in the erosion solution; is the average concentration of sulfate in the actual environment; C 试验 is the concentration of erosion ions collected in the indoor accelerated test; C 实际 is the concentration of erosion ions in the actual service environment of the concrete.

[0075] Through research, it is obtained that the comparison relationship between the number of freeze-thaw cycles in the laboratory and the number of freeze-thaw cycles under natural conditions is 1:10 - 1:15, and the value is taken as 1:12, that is, one freeze-thaw cycle in the laboratory is equivalent to 12 freeze-thaw cycles under natural conditions.

[0076] Therefore, the freeze-thaw deterioration ratio coefficient is the ratio of the effect of a single freeze-thaw cycle under natural conditions to that of a single freeze-thaw cycle test in the laboratory, and the value is taken as 12.

[0077] In step 2.3, first, the annual average number of freeze-thaw cycles in this region needs to be determined.

[0078] Adopt the test regime of freeze-thaw - sulfate wet-dry cycle to simulate 1 year of the actual environment. The freeze-thaw cycle considers the acceleration of freeze-thaw deterioration, and the sulfate wet-dry cycle considers the temperature acceleration and the erosion solution concentration acceleration.

[0079] Therefore, the expression for the number of freeze-thaw cycles is obtained as: the number of freeze-thaw cycles = the annual average number of freeze-thaw cycles ÷ 12; where the annual average number of freeze-thaw cycles refers to the annual average number of freeze-thaw cycles under natural conditions.

[0080] Since there is one freeze-thaw cycle per day and night, the number of days of freeze-thaw in a year: the annual average number of freeze-thaw days = the annual average number of freeze-thaw cycles.

[0081] Number of sulfate wet-dry cycle days = Number of days in a year - Annual average number of freeze-thaw cycle days;

[0082] The expression for obtaining the number of sulfate wet-dry cycle test days is: Number of sulfate wet-dry cycle test days = Number of sulfate wet-dry cycle days ÷ K a ÷K b , where the number of sulfate wet-dry cycle days refers to the annual average number of sulfate wet-dry cycle days under natural conditions.

[0083] In step 3, after determining the number of freeze-thaw cycle tests and the number of sulfate wet-dry cycle test days, before the start of the first freeze-thaw cycle test, first soak all concrete specimens in a sodium sulfate solution with a mass fraction of 5% for 4 days to make them in a completely saturated water state.

[0084] In step 4, the freeze-thaw cycle test and the sulfate wet-dry cycle test are carried out alternately, and before the start of the nth (n≥2) freeze-thaw cycle test, the concrete specimens are soaked in a sodium sulfate solution with a mass fraction of 5% for 1 day.

[0085] After the soaking is completed, take out the concrete specimens and place them in the freeze-thaw box in turn, and place wooden strips under each concrete specimen to prevent the bottom of the concrete specimen from being affected by the freezing of the remaining water at the bottom of the freeze-thaw box. Keep a certain distance between adjacent concrete specimens to avoid sticking together, so that each surface is evenly affected by the temperature change of the freeze-thaw box.

[0086] A freeze-thaw cycle period includes a spraying section, a cooling section, a low-temperature constant-temperature section, a heating stage, and a high-temperature constant-temperature section. The duration of each stage is: 5 minutes, 2 hours, 2 hours, 0.5 hours, 1.5 hours, totaling 6 hours and 5 minutes. In the spraying section, place the concrete specimens under the nozzles of the freeze-thaw box for 5 minutes of continuous water replenishment. The temperature of the low-temperature constant-temperature section of the freeze-thaw box is set at -20°C to ensure that the concrete specimens can complete the freezing process at this temperature. The high-temperature constant-temperature section is set at +25°C. After the end of the high-temperature constant-temperature section, the freeze-thaw box automatically sprays three times, each time lasting 1 minute, to keep the concrete specimens in a water-retaining state.

[0087] The operation method of the sulfate wet-dry cycle test is: soak the concrete specimens in a 5% sulfate solution. The time from when the concrete specimens are first put into the solution to the end of the soaking process is 16 hours. After the soaking process is completed, take out the concrete specimens and place them at a designated position to air-dry for 1 hour. After the air-drying process is completed, put the concrete specimens into a blast drying oven, turn on the oven switch, and after the temperature rises to 65°C, maintain the temperature at about 65°C and dry for 6 hours. After drying is completed, cool the concrete specimens for 1 hour until the surface temperature of the concrete specimens cools to room temperature, and then continue the above freeze-thaw cycle test and sulfate wet-dry cycle test.

[0088] Since the ettringite, the sulfate erosion product in the concrete specimen, may decompose at 70°C, which may change the erosion mechanism. Therefore, it is necessary to ensure that the drying temperature is 65°C ± 5°C, and the drying temperature in this sulfate wet-dry cycle test is 65°C.

[0089] Example 3

[0090] Taking the Jiuquan area in Gansu Province as an example, the annual average temperature in the Jiuquan area is 3.9°C to 9.3°C, and the average value is 6.6°C. According to the soil quality detection at the Jiuquan test site, the ion content of SO4 2- is 1536 mg / kg, and the salt solution concentration is 3% Na2SO4. Therefore, the actual environmental erosion ion concentration is taken as 3%.

[0091] Average temperature under immersion and drying:

[0092] T 均 = [65×6 + 6.6×16 + 1×(65 + 6.6)] / 24 = 23.6°C;

[0093] Temperature acceleration coefficient:

[0094]

[0095] Erosion solution concentration acceleration coefficient:

[0096]

[0097] The annual freeze-thaw cycle times in the Jiuquan area of Gansu are 100 - 130 times. In this example, the average value of 115 times is taken, and it is considered that there is one freeze-thaw cycle per day and night. Therefore, the number of days with freeze-thaw in a year is 115 days.

[0098] The number of days of sulfate wet-dry cycle = the number of days in a year - the number of days with freeze-thaw cycle = 365 - 115 = 250 days. The freeze-thaw - sulfate wet-dry cycle test system is adopted to simulate the actual environment for 1 year.

[0099] The freeze-thaw cycle considers the acceleration of freeze-thaw deterioration, and the sulfate wet-dry cycle considers temperature acceleration and erosion solution concentration acceleration.

[0100] The number of freeze-thaw cycle tests: 115÷12 = 9.6 times; The number of days of sulfate wet-dry cycle test: 250÷18.5÷1.67 = 8.1 d.

[0101] Through these steps, the freeze-thaw - sulfate erosion environment of shotcrete under different service years can be simulated, so as to evaluate its durability performance. This method is not only applicable to the Jiuquan area, but also can be adjusted according to the climate conditions and erosion ion concentrations in other regions to meet the test requirements of different regions.

[0102] Example 4

[0103] Taking Alar City as an example, the annual average temperature in Alar City in 2023 was 11°C. The SO4 content in the surface layer of saline soil in this area was determined by titration, and the actual environmental erosion ion concentration was taken as 1.99%. 2-

[0104] Average temperature under dipping and drying:

[0105] T 均 = [65×6 + 11×16 + 1×(65 + 11)] / 24 = 26.75°C;

[0106] Temperature acceleration coefficient:

[0107]

[0108] Erosion solution concentration acceleration coefficient:

[0109]

[0110] The number of annual freeze-thaw cycles in Alar City is 70 - 100 times. In this embodiment, the average value of 85 times is taken, and it is considered that there is one freeze-thaw cycle per day and night. Therefore, the number of days with freeze-thaw in a year is 85 days.

[0111] The number of days of sulfate wet-dry cycle = the number of days in a year - the number of days with freeze-thaw cycle = 365 - 85 = 280 days.

[0112] Adopt the test regime of freeze-thaw - sulfate wet-dry cycle to simulate the actual environment for 1 year.

[0113] The freeze-thaw cycle considers the acceleration of freeze-thaw deterioration, and the sulfate wet-dry cycle considers temperature acceleration and erosion solution concentration acceleration.

[0114] The number of freeze-thaw cycle tests: 85÷12 = 7.1 times; The number of days of sulfate wet-dry cycle tests: 280÷13.9÷2.5 = 8.1 d.

[0115] Example Five

[0116] Taking Haidong City in Qinghai Province as an example, the annual average temperature in Haidong City is 5.4°C - 8.6°C, and the average value of 7°C is taken. Referring to the SO4 content in the saline soil of the test site in Haidong City, the actual environmental erosion ion concentration is taken as 2%. 2-

[0117] Average temperature under dipping and drying:

[0118] T 均 = [65×6 + 7×16 + 1×(65 + 7)] / 24 = 23.9°C;

[0119] Temperature acceleration coefficient:

[0120] ​​

[0121] Erosion solution concentration acceleration coefficient:

[0122]

[0123] The number of freeze-thaw cycles in Haidong City per year is 100 - 130 times. In this embodiment, the average value of 115 times is taken, and it is considered that there is one freeze-thaw cycle per day and night. Therefore, the number of days with freeze-thaw in a year is 115 days.

[0124] Number of days of sulfate wet-dry cycle = Number of days in a year - Number of days with freeze-thaw cycle = 365 - 115 = 250 days.

[0125] Adopt the test regime of freeze-thaw - sulfate wet-dry cycle to simulate the actual environment for 1 year.

[0126] For the freeze-thaw cycle, consider the acceleration of freeze-thaw deterioration. For the sulfate wet-dry cycle, consider the acceleration of temperature and the acceleration of erosion solution concentration.

[0127] Number of freeze-thaw cycle tests: 115 ÷ 12 = 9.6 times; Number of days of sulfate wet-dry cycle tests: 250 ÷ 18.1 ÷ 2.5 = 5.5 d.

[0128] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A freeze-thaw-sulfate attack test method for shotcrete under different service years, characterized in that: The following steps are involved: Step 1, selecting raw materials and making concrete specimens; Step 2, determining the number of freeze-thaw cycle tests and the number of days for sulfate dry-wet cycle tests; Step 3, pre-treating the concrete specimen; Step 4: According to the number of freeze-thaw cycle tests and the number of days of sulfate dry-wet cycle tests obtained in step 2, the concrete specimens are subjected to freeze-thaw cycle tests and sulfate dry-wet cycle tests.

2. The freeze-thaw-sulfate attack test method for shotcrete under different service life according to claim 1, characterized in that: Step 2 includes the following steps: Step 2.1, design an expression for the average temperature of the concrete specimens during the sulfate dry-wet cycle test; Step 2.2, design the expressions of the temperature acceleration coefficient and the corrosion solution concentration acceleration coefficient in the sulfate dry-wet cycle test, and the freeze-thaw degradation proportional coefficient in the freeze-thaw cycle test; Step 2.3, design expressions for the number of freeze-thaw cycle tests and the number of days for the sulfate dry-wet cycle test.

3. The freeze-thaw-sulfate attack test method for shotcrete under different service years according to claim 2, characterized in that: In step 2.1, the expression for the average temperature of the concrete specimen under immersion drying in the sulfate dry-wet cycle test is: T 均 =[T 烘箱 ×t1+T1×t2+t3×(T 烘箱 +T1)] / 24 (1) Where: T 均 is the average temperature during immersion and drying, T 烘箱 is the oven temperature, t1 is the duration of oven drying, T1 is the average annual temperature of the region in Celsius, t2 is the sulfate soaking time, and t3 is the air drying time.

4. The freeze-thaw-sulfate attack test method for shotcrete under different service life according to claim 3, characterized in that: In step 2.2, the expression of the temperature acceleration coefficient is: Where: K a is the temperature acceleration coefficient; E / R is the activation energy, which is 14242; T2 is the annual average temperature of the region in Kelvin scale; The expression of the acceleration factor of the corrosion solution concentration is: Where: K b is the acceleration factor of the corrosion solution concentration; is the sulfate concentration in the etching solution; is the average concentration of sulfate in the actual environment; C 试验 is the concentration of corrosive ions used in indoor accelerated tests; C 实际 is the concentration of corrosive ions in the actual service environment of concrete; Freeze-thaw degradation proportional coefficient: the ratio of the effect of a single freeze-thaw cycle under natural conditions to the effect of a single freeze-thaw cycle test in the laboratory, and the value is 12.

5. The freeze-thaw-sulfate attack test method for shotcrete under different service life according to claim 4, characterized in that: In step 2.3, the expression of the number of freeze-thaw cycle tests is: number of freeze-thaw cycle tests = annual average number of freeze-thaw cycles ÷ freeze-thaw degradation ratio coefficient, where the annual average number of freeze-thaw cycles refers to the annual average number of freeze-thaw cycles under natural conditions; The expression of sulfate dry-wet cycle test days is: sulfate dry-wet cycle test days = sulfate dry-wet cycle days ÷ K a ÷K b Among them, the sulfate dry-wet cycle days refers to the annual average sulfate dry-wet cycle days under natural conditions.

6. A freeze-thaw-sulfate attack test method for shotcrete under different service years according to claim 5, characterized in that: The pretreatment method in step 3 is: the concrete specimen is immersed in a sodium sulfate solution with a mass fraction of 5% for 4 days.

7. A freeze-thaw-sulfate attack test method for shotcrete at different service lives according to claim 6, characterized in that: In step 4, the freeze-thaw cycle test and the sulfate dry-wet cycle test are carried out alternately, and before the start of the nth (n≥2) freeze-thaw cycle test, the concrete specimens are immersed in a sodium sulfate solution with a mass fraction of 5% for 1 day.

8. The freeze-thaw-sulfate attack test method for shotcrete at different service life according to claim 7, characterized in that: The freeze-thaw cycle test in step 4 includes a spraying section, a cooling section, a low-temperature constant temperature section, a heating stage and a high-temperature constant temperature section in sequence. The durations of the spraying section, the cooling section, the low-temperature constant temperature section, the heating stage and the high-temperature constant temperature section are 5 min, 2 h, 2 h, 0.5 h and 1.5 h respectively.

9. A freeze-thaw-sulfate attack test method for shotcrete at different service lives according to claim 8, characterized in that: In step 4, the temperature of the low-temperature constant temperature section is set to -20°C, and the temperature of the high-temperature constant temperature section is set to +25°C. After the high-temperature constant temperature section ends, the freeze-thaw box automatically sprays three times, each time lasting 1 minute.

10. A freeze-thaw-sulfate attack test method for shotcrete at different service lives according to claim 9, characterized in that: The operation of the sulfate dry-wet cycle test is as follows: after the concrete specimens are immersed in a 5% sulfate solution for 16 hours, the concrete specimens are sequentially air-dried, dried and cooled, with the drying temperature being 65°C and the drying time being 6 hours.

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