A method for determining the saturation of cement-based materials based on linear strain under low temperature environment

Abstract

The present invention discloses a method for determining the saturation of cement-based materials based on the linear strain of cement-based materials under low temperature conditions. The method for determining the saturation is as follows: (1) sampling and testing the linear strain ε under low temperature conditions; (2) using the Multi-R-R model to calculate and / or experimental methods such as mercury intrusion method and nitrogen adsorption method to obtain the pore size distribution of cement-based material samples; (3) assuming the saturation of cement-based material samples, obtaining the distribution of liquid and gas before freezing inside the material and the distribution of liquid, ice crystals and gas after freezing; (4) dividing the freezing state of cement-based materials under low temperature conditions, thereby calculating the linear strain generated at different freezing temperatures; (5) comparing the calculated linear strain with the linear strain obtained by the test in step (1), and finally obtaining the correct saturation. The present invention determines the saturation by monitoring the linear strain of cement-based materials under low temperature conditions, thereby predicting the freeze-thaw damage of concrete, thereby mastering the actual service conditions of concrete structures.

Description

Technical Field

[0001] The invention belongs to the field of civil engineering materials and relates to a method for determining the saturation of cement-based material linear strain under a low temperature environment. Background Art

[0002] Cement-based materials are one of the most important building materials in the field of civil engineering and are widely used in industrial and civil buildings, bridge engineering, water conservancy projects, etc. However, in low temperature environments, freeze-thaw damage seriously reduces the durability of concrete structures. The freeze-thaw damage of concrete structures is closely related to the saturation of the material. In the actual engineering field, most structures are in an unsaturated state. Not only that, the saturation of the underwater and above-water parts of structures such as bridge piers and dams is different, and during the freeze-thaw cycle, they absorb moisture from the outside due to the low-temperature cold absorption effect. The structure has a phenomenon of moisture transfer from the outside to the inside, resulting in differences in saturation in the same part. The traditional saturation test method requires a series of treatments such as sampling, drying, and saturation of the structure, which is a complicated and cumbersome process. Summary of the invention

[0003] In view of the above problems, the present invention proposes a method for determining the saturation by monitoring the linear strain of cement-based materials in a low temperature environment, thereby understanding the actual service status of the concrete structure.

[0004] The present invention provides a method for determining the saturation of cement-based material linear strain under low temperature environment, comprising the following steps:

[0005] (1) Sampling and testing of cement-based materials: obtain cement-based material samples and test their linear strain ε under low temperature conditions;

[0006] (2) Pore size distribution test of cement-based materials: The pore size distribution of cement-based material samples is obtained by using the Multi-Rayleigh-Ritz (Multi-RR) model calculation and / or experimental methods such as mercury intrusion porosimetry and nitrogen adsorption;

[0007] (3) Distribution of liquid, gas, and ice crystals in the pores of cement-based materials: Assuming the saturation of the cement-based material sample, the distribution of liquid and gas in the material before freezing and the distribution of liquid, ice crystals, and gas after freezing at a certain freezing temperature are obtained from the pore size distribution of the cement-based material sample in step (2), and the freezing temperature is consistent with the temperature during the linear strain test in step (1);

[0008] (4) Classification of the freezing state of cement-based materials and their linear strain: Classify the freezing state of cement-based materials in a low-temperature environment, and obtain the distribution of liquid, ice crystals and gas after freezing based on the saturation assumed in the above step (3), the distribution of liquid and gas before freezing and the freezing temperature, so as to calculate the linear strain generated at different freezing temperatures;

[0009] (5) Determination of saturation of cement-based materials: Compare the linear strain calculated in step (4) with the linear strain obtained by testing in step (1). If the two sets of data are similar, the saturation assumed in step (3) is correct. Otherwise, a new saturation is established and the calculation is repeated in step (3) until the correct saturation is obtained.

[0010] The present invention forms a new technology for determining the saturation of linear strain of cement-based materials under low temperature environment.

[0011] In step (1) of the present invention, when testing in the laboratory, it is recommended that the sample size be set to a rectangular parallelepiped of 40 mm × 40 mm × 160 mm; when applied to actual engineering, core sampling is performed at the location where saturation needs to be measured, and the sampling size is recommended to be The cylinder can also be sized according to actual conditions. For the accuracy of linear strain testing, the specimen length should not be less than 100 mm. When it is inconvenient to sample the structure and it is in a low temperature environment, the strain gauge can be attached to the structure to directly test the linear strain of the concrete structure with temperature changes.

[0012] The testing device for the linear strain of the cement-based material sample in step (1) comprises a high-low temperature and humidity test chamber, a test line and a strain tester. A strain gauge is pasted on the cement-based material sample, and the sample is placed in the high-low temperature and humidity test chamber. The strain gauge passes through a test hole on the high-low temperature and humidity test chamber through the test line and is connected to the strain tester. The high-low temperature and humidity test chamber is provided with a control interface for adjusting the temperature inside the test chamber.

[0013] The linear strain test steps in step (1) are specifically as follows:

[0014] ① Obtain cement-based material samples and paste strain gauges on them. The strain gauges are KFG series general foil strain gauges with an operating temperature range of -196°C to 150°C. The length of the strain gauge is based on the length of the sample. It is recommended to use a 100mm length, leaving 30mm on both sides for easy pasting, with a spacing of 10mm. Figure 2 When the sample is a cylinder in actual engineering, the patch method is similar to it;

[0015] ②Then seal the cement-based material sample with a plastic film to prevent moisture exchange with the outside world and maintain an unsaturated state; place the sample in a high and low temperature humidity test chamber and connect it to the strain tester through a test line;

[0016] ③ Set the lowest freezing temperature through the control interface of the test chamber, such as -20°C, and measure the linear strain of the test specimen at -5°C, -10°C, -15°C, and -20°C. If the measured linear strain value is small at this temperature, set the lowest freezing temperature to -30°C or -40°C, and measure the linear strain at 4 temperatures according to the equal temperature gradient. The strain values of the specimen are denoted as ε1, ε2, ε3, and ε4.

[0017] Since the working environment of the strain gauge is a low-temperature environment, the present invention uses a Wheatstone circuit to reduce the change in the resistance value of the strain gauge caused by temperature changes. When measuring the linear strain, connect the circuit, and a temperature compensation sheet can also be placed beside the cement-based specimen in the high and low temperature and humidity test chamber. The temperature compensation sheet is placed in the same environment as the strain gauge to eliminate the measurement error caused by low temperature. Because quartz glass has almost no linear strain in a low-temperature environment, quartz glass is used as the temperature compensation sheet.

[0018] In step (2) of the present invention, there are two methods to obtain the pore size distribution of cement-based materials. One is to use the Multi-Rayleigh-Ritz (Multi-R-R) model [1] (Xi Y, Z P, Jennings H M. Moisture diffusion in cementitious materials-Adsorption isotherms[J]. Advanced Cement-Based Materials, 1994, 1(6):248-257.) to calculate, and the other is to obtain it by experimental methods such as mercury intrusion method and nitrogen adsorption method.

[0019] The specific calculation process of the Multi-Rayleigh-Ritz (Multi-R-R) model is as follows:

[0020] Applying the Multi-Rayleigh-Ritz (Multi-R-R) model to calculate the pore size distribution of cement-based materials covers gel pores smaller than 2.5 nm, small capillary pores of 2.5 - 50 nm, large capillary pores and microcracks of 50 - 10 4 nm.

[0021] The cumulative pore size distribution function and probability density function of the cement-based material obtained by the model are:

[0022]

[0023]

[0024] In the formula, φ(r p <r) and f d(r) respectively represent the aperture distribution function and probability density function of the accumulated aperture from small to large; φ t Indicates the total porosity of cement-based materials; φ i represents the ratio of the porosity of quasi-pores to the total porosity (i=1, 2, 3, 4, representing gel pores, small capillary pores, large capillary pores, and microcracks, respectively); B i It indicates the pore radius corresponding to the peak value on the pore size logarithmic density distribution diagram, nm, which reflects the distribution of the pores.

[0025] φ4 and B4 are obtained by fitting the mercury intrusion test. i and B i (i = 1, 2, 3, representing gel pores, small capillary pores, and large capillary pores, respectively) The water saturation S during the isothermal adsorption process r The relationship curve with the change of relative humidity h is obtained [2] (Huang Q, Jiang Z, Gu X, et al. Numerical simulation of moisture transport in concrete based on a pore size distribution model [J]. Cement & Concrete Research, 2015, 73 (17): 67-69.). Its expression is:

[0026]

[0027] The expressions for parameters C and k are:

[0028]

[0029] Where T represents the absolute temperature of isothermal adsorption, K; n1 represents the number of adsorption layers under saturation.

[0030] Consider the water-cement ratio w / c, cement type c t and curing time t c The impact on n1, the expression of n1 is:

[0031] n1=N tc (t c )N ct (c t )N wc (w / c) (5)

[0032] N tc (t c )=2.5+15 / t c (6)

[0033]

[0034] N wc (w / c)=0.33+2.2w / c (8)

[0035] In the formula, Type I: ordinary cement, such as ordinary Portland cement; Type II: medium sulfate-resistant cement, such as slag Portland cement; Type III: high early strength cement, such as Portland cement; Type IV: low water-heat cement, such as fly ash Portland cement.

[0036] At this point, the water saturation S is established. r and water-cement ratio w / c, cement type c t , maintenance time t c Relationship with relative humidity h.

[0037] The saturation of water S r It can also be expressed as:

[0038]

[0039] in:

[0040] r c =r k +t a (10)

[0041]

[0042] γ=7.5796×10 -2 -1.45×10 -4 (T-273)-2.4×10 -7 (T-273) 2 (12)

[0043]

[0044] In the formula, r c represents the critical pore radius for capillary condensation, nm; r k is the radius of the capillary condensate, nm; t a is the thickness of the adsorbed water layer on the pore wall, nm; γ represents the surface tension of water, N / m; ν L represents the molar volume of water, taking 1.8×10 － 5 m 3 / mol; represents the ideal gas constant, which is 8.314J / mol / K.

[0045] The water saturation S can be obtained from formulas (9) to (13): r and the aperture probability density function f d (r) and f dWhen (r) changes, the curve represented by formula (9) changes accordingly. When this curve is the best fitting curve of the isothermal adsorption curve expressed by formula (3), f can be determined. d (r),φ i and B i , thereby obtaining the pore size distribution of cement-based materials.

[0046] In step (3) of the present invention, assuming the saturation of the cement-based material sample, the distribution of liquid and gas before freezing inside the sample and the distribution of liquid, ice crystals and gas after freezing are obtained from the pore size distribution of the cement-based material sample. The pore distribution diagram is as follows: Figure 3 shown.

[0047] in:

[0048]

[0049]

[0050] In the formula, r c1 and r c2 are the critical pore radius of gas and ice crystal, the critical pore radius of ice crystal and liquid water, respectively, in nm; r max and r min Indicates the maximum and minimum pore radius in the pore structure, nm; S J Indicates the saturation of the J phase (J = L, C, G, representing liquid phase, solid phase, and gas phase, respectively).

[0051] The cement-based material sample is given an initial saturation S0. The setting of S0 is determined according to the linear strain measured in step (1). If ε is greater than 0, S0>0.917 is set; if ε is negative, S0≤0.917 is set.

[0052] Before freezing, there is only liquid and gas in the pores. c1 =r c2 According to the initial saturation S0 and formulas (14) and (15), the critical pore radius of the liquid and gas can be determined, and the distribution of the liquid and gas can be obtained.

[0053] After the phase change of water in the pores, liquid, ice crystals and gas exist in the pores, among which:

[0054]

[0055] In the formula, γ CL is the interfacial energy between ice and water, taken as 0.0409 J / m 2 ; ΔS m is the melting entropy of ice crystal per unit volume, which is 1.2MPa / K; T mis the freezing temperature of bulk water, K; T is the current temperature, K; δ is the thickness of the water film between the crystal and the pore wall, which is 1.2 nm.

[0056] From formula (16), we can get r c2 According to the initial saturation S0 and formulas (14) and (15), the distribution of liquid, ice crystals and gas in the pores can be determined.

[0057] In step (4) of the present invention, the method for dividing the freezing state of the cement-based material under low temperature environment is: dividing the unsaturated state into: initial saturation less than critical saturation, initial saturation equal to critical saturation, initial saturation greater than critical saturation according to the critical saturation:

[0058] Initial saturation is less than critical saturation: In low temperature environment, the water content in the pores is small. After the temperature drops, the water and ice crystals in the pores do not fill all the pores. At this time, there is generally no ice crystallization pressure. This is the first type of freezing state;

[0059] Initial saturation is equal to critical saturation: Under low temperature conditions, the water content in the pores is higher than that in the first type. After the temperature drops, the water and ice crystals in the pores can just fill all the pores. At this time, the unfrozen water is not squeezed by the ice crystals. This is the second type of freezing state and has experienced the first type of freezing state.

[0060] The initial saturation is greater than the critical saturation: in a low temperature environment, the water content in the pores is higher than that of the second type but still belongs to an unsaturated state. After the temperature drops, the water and ice crystals in the pores further freeze on the basis of filling all the pores. At this time, the water around the freezing area is squeezed and there is hydrostatic pressure. This is the third type of freezing state and has experienced the first and second types of freezing states.

[0061] In the present invention, the critical saturation theory holds that all materials have their critical saturation S for freezing damage. cr During the freeze-thaw process, when the saturation of the cement-based material exceeds the critical saturation, the cement-based material is in an expansion state, and the material is prone to freeze-thaw damage; when the saturation of the cement-based material is lower than the critical saturation, the cement-based material is in a contraction state, and the freeze-thaw damage is very small. This saturation is the critical saturation of the material. The present invention sets the critical saturation to 0.917.

[0062] In the present invention, the three freezing states are as shown in the attached Figure 4 As shown in the figure, the first type of frozen state refers to the small content of water in the pores of cement-based materials. After the temperature drops, the water and ice crystals in the pores do not fill all the pores, that is, S L +S C +S G =1. It can be considered that p C =p G=p atm At this time, the liquid pressure p L for:

[0063]

[0064] The linear strain of cement-based materials during freezing is:

[0065]

[0066]

[0067] b J =bS J (20)

[0068] In the formula, p J Indicates the pressure of phase J, MPa; p atm is the standard atmospheric pressure, which is 101.325 kPa; ν J represents the molar volume of the phase, ν C Take 1.998×10 -5 m 3 / mol, ν L Take 1.8×10 -5 m 3 / mol; a is the thermal volume expansion coefficient of the solid skeleton, which is 18×10 -6 K -1 ; K represents the bulk modulus of the cement-based material skeleton, and the cement paste is 13.3×10 3 MPa, the mortar sample is 17.7×10 3 MPa, other cement-based materials can be obtained through experiments; k s is the bulk modulus of the solid matrix of cement-based materials, and the cement paste is 31.8×10 3 MPa, the mortar sample is 42.4×10 3 MPa, other cement-based materials can be obtained through experiments; b J is the Biot coefficient.

[0069] The second type of freezing state refers to the fact that the water content in the pores is higher than that in the first type. After the temperature drops, the water and ice crystals in the pores can just fill all the pores. At this time, the unfrozen water is not squeezed by the ice crystals. L +S C =1, the pressure of each phase and the volume strain of the material still satisfy equations (17) and (18).

[0070] The third freezing state refers to when the freezing process of water in the pores has gone through the first and second freezing states, and when the temperature drops further, there are no pores to provide space for the freezing of water in the pores. After that, the freezing process of water is restricted. At this time, SL +S C =1.

[0071] The liquid pressure and linear strain are:

[0072]

[0073]

[0074] in:

[0075]

[0076] Where, T 0m It represents the new freezing point corresponding to the lowering of the freezing point due to the increase of pressure, K; p0 is the standard atmospheric pressure.

[0077] In the present invention, the process of calculating the linear strain of the cement-based material sample and determining the correct saturation S0 is as shown in the attached figure. Figure 5 shown.

[0078] In the above description, the distribution of liquid, ice crystals, and gas in the pores is obtained from the initial pore structure, initial saturation S0, and freezing temperature T. If S0≤S cr , substitute into equations (17) and (18) to calculate the linear strain ε kk ;

[0079] If S0>S cr , S L +S C <1, still substitute into equations (17) and (18) to calculate the linear strain ε kk ;

[0080] If S0>S cr , S L +S C = 1, substitute into equations (21), (22), (23) to calculate the linear strain ε kk .

[0081] In step (5) of the present invention, the calculated value is compared with the measured value. When the measured value ε is equal to the calculated value ε kk When the two sets of values ​​are approximately equal, the saturation of the sample is S0; when the two sets of values ​​differ greatly, S0 is reset for calculation. kk When ε<ε, reduce S0. kk When the value of S0 is 0, increase S0 until the correct initial saturation S0 is obtained.

[0082] Compared with the prior art, the present invention has the following advantages:

[0083] The present invention is a non-destructive and precise detection technology. Without damaging the internal structure of concrete, it determines the saturation of cement-based materials by detecting the linear change of cement-based materials, thereby predicting the freeze-thaw damage of concrete, providing a solid basic theory and practical core technology for improving the durability of concrete structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Figure 1 This is a schematic diagram of the deformation testing device for cement-based material samples under low temperature environment according to the present invention.

[0085] Figure 2 This is a schematic diagram of the structure of the cement-based material sample in a low-temperature environment according to the present invention.

[0086] Figure 3 This is a schematic diagram of the pore distribution of the cement-based material of the present invention.

[0087] Figure 4 These are three different types of freezing states of the cement-based material in an unsaturated state according to the present invention.

[0088] Figure 5 The present invention is a flow chart for determining the saturation of cement-based materials.

[0089] Figure 6 This is the pore size distribution diagram of cement paste with a water-cement ratio of 0.5 in the embodiment of the present invention.

[0090] Figure 1-2 The markings in the figure are: 1 high and low temperature humidity test chamber, 2 cement-based material specimen, 3 control interface, 4 test hole, 5 test line, 6 strain tester, 7 strain gauge. DETAILED DESCRIPTION

[0091] The technical solution of the present invention is further described below in conjunction with specific examples, but is not limited thereto. Any method and process that modifies or replaces the technical solution of the present invention without departing from the technical solution of the present invention should be included in the protection scope of the present invention.

[0092] Example 1

[0093] A method for determining the saturation of cement-based material linear strain under low temperature environment comprises the following steps:

[0094] (1) Obtain cement-based material samples and measure their linear strain under low temperature conditions;

[0095] (1.1) This example uses a cement paste sample prepared in the laboratory. The sample size is a rectangular parallelepiped of 40 mm × 40 mm × 160 mm, with a water-cement ratio of 0.5. Sample A is prepared and placed in saturated lime water for curing to 56 days, after which sample A is frozen.

[0096] (1.2) Place the prepared cement paste sample in a high-low temperature and humidity test chamber for freezing treatment, and record the strain values ​​of the sample at -5°C, -10°C, -15°C, and -20°C. The four strain values ​​of sample A are 54με, 190με, 411με, and 579με, respectively.

[0097] (2) Obtaining the pore size distribution of cement paste. In this example, the pore size distribution model mentioned above is used for calculation;

[0098] (2.1) The cumulative porosity of the sample is calculated to be 0.294 using formulas (1) to (13). The pore size distribution diagram is shown in the attached figure. Figure 6 As shown;

[0099] (3) Assuming the saturation of the sample, the distribution of liquid and gas before freezing and the distribution of liquid, ice crystals and gas after freezing are obtained from the pore size distribution of the cement paste;

[0100] The four strain values ​​ε of sample A are all greater than 0, and S0=0.960>S cr , then S can be obtained from the pore size distribution L =0.960,S G =0.040, critical pore size is 57.4nm;

[0101] When the temperature is -5°C, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample A is 29.6 nm, S L =0.787, S C =0.189, S G =0.024;

[0102] When the temperature is -10℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample A is 16.0nm, S L =0.543, S C =0.455, S G =0.002;

[0103] When the temperature is -15℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample A is 11.4nm, S L =0.408, S C =0.592, S G =0;

[0104] When the temperature is -20℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample A is 9.2nm, S L =0.330, S C =0.670, S G =0;

[0105] (4) According to the freezing state of cement-based materials in low temperature environment, the samples of sample A at -5℃ and -10℃ are in the first type of freezing state, and the samples at -15℃ and -20℃ are in the third type of freezing state. Substitute them into formulas (17), (18) and formulas (21), (22), and (23) respectively to calculate and obtain ε kk1 =-36με,ε kk2 =-53με,ε kk3 =178με,ε kk3 =316με;

[0106] (5) It can be determined that the A sample ε kk <ε, then increase the initial saturation and recalculate; when the saturation is 0.982, the strain values ​​at -5℃, -10℃, -15℃, and -20℃ are calculated to be 47με, 241με, 473με, and 606με, respectively. kk ≈ε, the initial saturation of the unsaturated cement-based material can be considered to be 0.982.

[0107] Example 2

[0108] A method for determining the saturation of cement-based material linear strain under low temperature environment comprises the following steps:

[0109] (1) Obtain cement-based material samples and measure their linear strain under low temperature conditions.

[0110] (1.1) This example uses a cement paste sample prepared in the laboratory. The sample size is a rectangular parallelepiped of 40 mm × 40 mm × 160 mm, and the water-cement ratio is 0.5. Sample B is prepared and placed in saturated lime water for curing to 56 days. After that, sample B is placed in a curing box with a temperature of (50±2)℃ and a humidity of (80±3)%RH for 3 days to reduce its saturation, and then frozen.

[0111] (1.2) Place the prepared cement paste sample in a high and low temperature humidity test chamber for freezing treatment, and record the strain values ​​of the sample at -5°C, -10°C, -15°C, and -20°C. The four strain values ​​of sample B are -207με, -359με, -534με, and -687με, respectively.

[0112] (2) Obtaining the pore size distribution of cement paste. In this example, the pore size distribution model mentioned above is used for calculation;

[0113] (2.1) The cumulative porosity of the sample is calculated to be 0.294 using formulas (1) to (13). The pore size distribution diagram is shown in the attached figure. Figure 6 As shown;

[0114] (3) Assuming the saturation of the sample, the distribution of liquid and gas before freezing and the distribution of liquid, ice crystals and gas after freezing are obtained from the pore size distribution of the cement paste;

[0115] The four strain values ​​ε of specimen B are all less than 0, and S0 is set to 0.880. cr , then S can be obtained from the pore size distribution L =0.880, S G =0.100, critical pore size is 43.1nm;

[0116] When the temperature is -5°C, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample B is 29.6 nm, S L =0.787, S C =0.101, S G =0.112;

[0117] When the temperature is -10℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample B is 16.0nm, S L =0.543, S C =0.367, S G =0.090;

[0118] When the temperature is -15℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample B is 11.4nm, S L =0.408, S C =0.514, S G =0.078;

[0119] When the temperature is -20℃, substituting into formula (16), the critical pore size of the liquid and ice crystal of sample B is 9.2nm, S L =0.330, S C =0.600, S G =0.070;

[0120] (4) According to the freezing state of cement-based materials in low temperature environment, the B specimens are all in the first type of freezing state. Substituting into formulas (17) and (18) for calculation, ε kk1 =-167με,ε kk2 =-301με,ε kk3 =-435με,ε kk3 = -509με;

[0121] (5) It can be determined that the ε of sample B kk ​>ε, then reduce the initial saturation and recalculate; when the saturation is 0.850, the strain values ​​at -5℃, -10℃, -15℃, and -20℃ are calculated to be -201με, -363με, -522με, and -610με, respectively. kk ≈ε, the initial saturation of the unsaturated cement-based material can be considered to be 0.850.

[0122] Finally, it is noted that the above disclosures are only two specific embodiments of the present invention, but the embodiments of the present invention are not limited thereto, and all changes related thereto should fall within the protection scope of the present invention.

Claims

1. A method for determining the saturation of cement-based material linear strain under low temperature environment, characterized in that: The steps include: (1) Sampling and testing of cement-based materials: obtain cement-based material samples and test their linear strain ε under low temperature conditions; (2) Pore size distribution test of cement-based materials: The pore size distribution of cement-based material samples was obtained by using the Multi-Rayleigh-Ritz (Multi-RR) model calculation and the experimental methods of mercury intrusion and nitrogen adsorption. (3) Distribution of liquid, gas, and ice crystals in the pores of cement-based materials: Assuming the saturation of the cement-based material sample, the distribution of liquid and gas in the material before freezing and the distribution of liquid, ice crystals, and gas after freezing at a certain freezing temperature are obtained from the pore size distribution of the cement-based material sample in step (2), and the freezing temperature is consistent with the temperature during the linear strain test in step (1); (4) Classification of the freezing state of cement-based materials and their linear strain: Classify the freezing state of cement-based materials in a low-temperature environment, and obtain the distribution of liquid, ice crystals and gas after freezing based on the saturation assumed in the above step (3), the distribution of liquid and gas before freezing and the freezing temperature, so as to calculate the linear strain generated at different freezing temperatures; (5) Determination of saturation of cement-based materials: Compare the linear strain calculated in step (4) with the linear strain obtained by testing in step (1). If the two sets of data are similar, the saturation assumed in step (3) is correct. Otherwise, a new saturation is established and the calculation is repeated in step (3) until the correct saturation is obtained.

2. The saturation determination method according to claim 1, characterized in that: In step (1), when the linear strain test is carried out in the laboratory, the sample size is set to a rectangular parallelepiped of 40 mm × 40 mm × 160 mm; when applied to actual engineering, core sampling is carried out at the location where saturation needs to be measured, and the sampling size is a cylinder of φ40 mm × 160 mm or the size is set according to actual conditions, and the sample length is not less than 100 mm.

3. The saturation determination method according to claim 1 or 2, characterized in that: The testing device for the linear strain of the cement-based material sample described in step (1) comprises a high-low temperature and humidity test chamber (1), a test line (5) and a strain tester (6); a strain gauge (7) is pasted on the cement-based material sample (2), the sample is placed in the high-low temperature and humidity test chamber (1), and the strain gauge (7) passes through a test hole (4) on the high-low temperature and humidity test chamber (1) through the test line (5) and is connected to the strain tester (6); the high-low temperature and humidity test chamber (1) is provided with a control interface (3) for adjusting the temperature inside the test chamber.

4. The saturation determination method according to claim 3, characterized in that: The linear strain test steps in step (1) are specifically as follows: ① Obtain cement-based material samples and paste strain gauges on the samples. The strain gauges are KFG series general foil strain gauges with an operating temperature range of -196℃ to 150℃. The length of the strain gauge is based on the length of the sample. When it is inconvenient to sample the structure and it is in a low temperature environment, paste the strain gauge on the structure to directly test the linear strain of the concrete structure with temperature changes. ②Then seal the cement-based material sample with a plastic film to prevent moisture exchange with the outside world and maintain an unsaturated state; place the sample in a high and low temperature humidity test chamber and connect it to the strain tester through a test line; ③ Set the minimum freezing temperature to -20℃ through the control interface of the test chamber, and test the linear strain of the sample at -5℃, -10℃, -15℃, and -20℃. If the linear strain value measured at this temperature is small, set the minimum freezing temperature to -30℃ or -40℃, and test the linear strain at four temperatures according to the equal temperature gradient. The strain values ​​of the sample are recorded as ε1, ε2, ε3, and ε4.

5. The saturation determination method according to claim 4, characterized in that: The strain gauge described in step ① is 100 mm long. When it is pasted on the cement-based material sample, 30 mm is left on both sides for easy pasting, and the interval is 10 mm.

6. The saturation determination method according to claim 4, characterized in that: Since the working environment of the strain gauge is a low temperature environment, a Wheatstone circuit is used to reduce the change in the resistance value of the strain gauge caused by temperature changes; when testing the line strain, a temperature compensation sheet is placed next to the cement-based specimen in the high and low temperature and humidity test chamber. The temperature compensation sheet is placed in the same environment as the strain gauge, and quartz glass is used as the temperature compensation sheet.

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