Concrete specimens and characterization methods of the effects of freeze-thaw and size on concrete strength

Abstract

This invention discloses a concrete specimen and a method for characterizing the effects of freeze-thaw and size on concrete strength. The concrete specimen is composed of the following materials in a proportional ratio: 195 kg / m³ of water, 418 kg / m³ of cement, 46 kg / m³ of fly ash, 1184 kg / m³ of coarse aggregate, and 557 kg / m³ of a material to be predicted as quasi-brittle. The coarse aggregate is graded crushed stone. This concrete specimen, made with the material to be predicted as quasi-brittle, provides a standardized material benchmark for studying freeze-thaw-size coupling effects and is particularly suitable for constructing life prediction models for projects sensitive to freeze-thaw damage.

Description

Technical Field

[0001] The invention belongs to the technical field of material performance prediction and structural engineering, and particularly relates to a concrete specimen and a method for characterizing the effects of freeze-thaw and size on concrete strength. Background Art

[0002] Concrete is widely used in cold-region engineering projects, but its long-term mechanical properties are significantly affected by freeze-thaw cycles, which can cause the expansion of microcracks and pores within the material, leading to a decrease in strength. In addition, concrete strength also has a significant size effect. For engineering construction and disaster prevention, the design of concrete structures of different sizes under the action of freeze-thaw cycles and disaster prevention require a mechanistic model.

[0003] Therefore, it is particularly urgent to develop a model to describe the coupled effects of freeze-thaw cycles and specimen size on concrete strength.

[0004] Under non-freeze-thaw cycling conditions, concrete strength typically decreases with increasing specimen size, a phenomenon known as the decreasing size effect. However, Guan et al. conducted tests on the compressive strength of concrete under the influence of specimen size and freeze-thaw cycling and discovered a novel size effect phenomenon. For uniaxial compressive strength, a decreasing size effect was observed when the number of freeze-thaw cycles was less than 30; however, the size effect disappeared after 30 freeze-thaw cycles. When the number of freeze-thaw cycles exceeded 30, the compressive strength increased with increasing specimen size, demonstrating an increasing size effect. Guan et al. introduced a quadratic function to fit the relationship between concrete strength loss and specimen size and the number of freeze-thaw cycles, and used this quadratic function to modify the Type I Băzant size effect model. The modified Type I Băzant size effect model effectively predicts the strength of wastewater concrete under the coupled effects of size and freeze-thaw cycling. However, existing simulation and experimental results have shown that as the size of unnotched concrete specimens increases, their uniaxial tensile and compressive strengths gradually decrease and eventually approach a constant. The Type I Băzant size effect model is unable to predict this size effect characteristic of unnotched concrete specimens. This is because the Type I Băzant size effect model was proposed to describe the size effect of notched specimens. Furthermore, the modified Type I Băzant size effect model by Guan et al. assumes uniform damage distribution within the specimen, which is inconsistent with the gradient attenuation of freeze-thaw damage from the surface to the interior of the specimen. Summary of the Invention

[0005] In view of this, the main purpose of the present invention is to provide a concrete specimen and a method for characterizing the effects of freeze-thaw and size on concrete strength.

[0006] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0007] A concrete specimen is composed of the following materials in proportion: 195 kg / m³ of water, 418 kg / m³ of cement, 46 kg / m³ of fly ash, 1184 kg / m³ of coarse aggregate, and 557 kg / m³ of brittle material to be predicted;

[0008] The coarse aggregate is graded crushed stone.

[0009] Preferably, the brittle material to be predicted is sand;

[0010] The sand comprises a mixture of river sand and aeolian sand, wherein the aeolian sand replacement rate (ASRR) of the mixed sand is specifically selected from one of the following mass percentages:

[0011] When ASRR=25%, 418kg / m³ of river sand and 139kg / m³ of aeolian sand are used to form the sand;

[0012] When ASRR=50%, 278.5kg / m³ of river sand and 278.5kg / m³ of aeolian sand are used to form the sand;

[0013] When ASRR=75%, river sand 139kg / m³ and aeolian sand 418kg / m³ are used to form the sand;

[0014] When ASRR=100%: river sand 0kg / m³ and aeolian sand 557kg / m³ are used to form the sand.

[0015] A method for characterizing the effects of freeze-thaw and size on concrete strength comprises the following steps:

[0016] After the materials are proportioned and evenly mixed, they are poured into the test mold to produce concrete specimens of different sizes; each concrete specimen is cured;

[0017] Uniaxial compression tests were performed on concrete specimens of different sizes without freeze-thaw cycle treatment using a pressure testing machine to obtain the initial uniaxial compressive strength of concrete specimens of different sizes without freeze-thaw cycle treatment.

[0018] Uniaxial compression tests were performed on concrete specimens of different sizes after different freeze-thaw cycles using a pressure testing machine to obtain the uniaxial compressive strength under freeze-thaw cycles.

[0019] The uniaxial compressive strength and initial uniaxial compressive strength under at least seven freeze-thaw cycles are fitted to obtain the compressive elastic limit strength of the inner area of ​​the concrete specimen. f ce 、 Ratio of the ultimate compressive elastic strength of the concrete interior to the initial uniaxial compressive strength K、 Decay constant of unfrozen concrete 、Concrete material constant t 0 、 Concrete material constant 、 Nominal damage modulus Size effect Nonlinear index m、 The ratio of the ultimate compressive strength of the concrete interior to its true compressive strength b and the size of the local damage zone of the concrete specimen under uniaxial compression D 0 ;

[0020] According to the f ce 、K、 、t 0 、 ,m,b and D 0 Determine the size-effect model of energy balance considering freeze-thaw cycles;

[0021] The energy balance size effect model considering freeze-thaw cycles is used to predict the uniaxial compressive strength of concrete specimens made of the quasi-brittle material to be predicted under any freeze-thaw cycle number and any size.

[0022] Preferably, the uniaxial compression test is performed on concrete specimens of different sizes after being subjected to different freeze-thaw cycles by a pressure testing machine to obtain the uniaxial compressive strength under freeze-thaw cycles, specifically comprising:

[0023] Concrete specimens of different sizes are subjected to different numbers of freeze-thaw cycles using a rapid freeze-thaw tester. After a preset number of freeze-thaw cycles, uniaxial compressive strength tests are performed on the concrete specimens of different sizes using a pressure testing machine until the concrete specimens fail, thereby obtaining the failure loads of concrete specimens of different sizes and with different numbers of freeze-thaw cycles.

[0024] The uniaxial compressive strength under freeze-thaw cycles is determined based on the failure load of concrete specimens with different freeze-thaw cycles and different sizes.

[0025] Preferably, determining the uniaxial compressive strength under freeze-thaw cycles according to the failure loads of concrete specimens of different freeze-thaw cycles and different sizes specifically includes: determining the uniaxial compressive strength according to uniaxial compressive strength=failure load / compressed area of ​​concrete specimen.

[0026] Preferably, the constructed energy balance size effect model considering freeze-thaw cycles specifically includes:

[0027] Construct a strength model of freeze-thaw damage structures;

[0028] Integrate the strength-energy-size effect model of the undamaged internal region;

[0029] An energy balance size effect model considering freeze-thaw cycles is obtained based on the freeze-thaw damaged structural strength model combined with the strength energy size effect model of the undamaged internal area.

[0030] Preferably, the freeze-thaw damage structural strength model is , where the parameter t 0 and is the concrete material constant; parameter and are the initial uniaxial compressive strength and decay constant of unfrozen-thawed concrete, respectively; N is the number of freeze-thaw cycles; It is the nominal strength of the undamaged internal area of ​​the concrete specimen.

[0031] Preferably, the strength-energy-size effect model of the undamaged internal area is: , where m >0, which reflects the exponent of the nonlinearity of the size effect of the nominal damage modulus; b = f c / f ce ,and f ce ≤ f c ; 、 f ce and f c are the nominal compressive strength, compressive elastic limit strength and true compressive strength of the inner region of the concrete specimen, respectively; D 0 is the size of the local damage zone (LDZ) in the uniaxial compression specimen, and its dimension is length; D is the size of the concrete specimen during the test.

[0032] Preferably, the energy balance size effect model considering freeze-thaw cycles is:

[0033] ;in, K is the compressive elastic limit strength of the concrete inner area f ce Initial uniaxial compressive strength The ratio of is the growth power function ; An exponential decay model ; m is the nonlinear index of size effect of nominal damage modulus; is the decay constant of unfrozen concrete、t 0 is the concrete material constant 、 is the concrete material constant 、b is the ratio of the compressive elastic limit strength of the concrete interior area to the true compressive strength and D 0 is the size of the local damage zone of the concrete specimen under uniaxial compression.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention adopts concrete specimens made of the quasi-brittle material to be predicted to provide a standardized material benchmark for the study of freeze-thaw-size coupling effects, and is particularly suitable for the construction of life prediction models for projects sensitive to freeze-thaw damage.

[0036] The present invention simplifies the freeze-thaw damage of the specimen into a damaged surface layer enveloping an undamaged internal area, breaking through the uniform damage assumption. First, a freeze-thaw damage structural strength model is constructed, and then, combined with the strength-energy-size effect model of the undamaged internal area, a theoretical model is constructed to describe this novel size effect phenomenon, providing a theoretical basis for engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings described herein are used to further understand the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0038] Figure 1 A flowchart of a method for characterizing the effects of freeze-thaw and size on concrete strength is provided for an embodiment of the present invention;

[0039] Figure 2 A surface plot obtained by fitting using MATLAB software in a method for characterizing the effects of freeze-thaw and size on concrete strength is provided in an embodiment of the present invention;

[0040] Figure 3 The present invention provides a method for characterizing the effects of freeze-thaw and size on concrete strength, wherein an energy balance size effect model considering freeze-thaw cycles is used to fit parameters using MATLAB software.

[0041] Figure 4 This is an analysis diagram of a sample with a damaged surface layer surrounding an undamaged internal area. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0043] The same or similar numbers in the drawings of this embodiment correspond to the same or similar parts; in the description of the present invention, it should be understood that the terms "upper", "lower", "left", "right", "inner", "outer", etc. indicating the orientation or position relationship are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0044] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, article, or device comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, article, or device comprising the element.

[0045] An embodiment of the present invention provides a concrete specimen, which is composed of the following materials in a proportion: 195 kg / m³ of water, 418 kg / m³ of cement, 46 kg / m³ of fly ash, 1184 kg / m³ of coarse aggregate, and 557 kg / m³ of a brittle material to be predicted.

[0046] Taking sand as an example, the brittle material to be predicted may include a mixture of river sand and aeolian sand, wherein the aeolian sand replacement rate (ASRR) of the mixed sand is specifically selected from one of the following mass percentages:

[0047] When ASRR=25%, 418kg / m³ river sand and 139kg / m³ aeolian sand are used to form the sand;

[0048] When ASRR=50%, 278.5kg / m³ of river sand and 278.5kg / m³ of aeolian sand are used to form the sand;

[0049] When ASRR=75%, river sand 139kg / m³ and aeolian sand 418kg / m³ are used to form the sand;

[0050] When ASRR=100%: river sand 0kg / m³ and aeolian sand 557kg / m³ are used to form the sand.

[0051] The cement is ordinary Portland cement (P·O42.5), with a density of 3151 kg / m³, an initial setting time of 145 min, and a final setting time of 210 min;

[0052] The fly ash is Grade II fly ash, with a specific surface area of ​​352 m² / kg and a density of 2150 kg / m³;

[0053] The coarse aggregate is graded crushed stone with a particle size of 5-20 mm, a bulk density of 1652 kg / m³, and an apparent density of 2640 kg / m³;

[0054] The fineness modulus of the river sand is 2.64, and the fineness modulus of the aeolian sand is 0.8.

[0055] For example, taking the aeolian sand as the surface aeolian sand of the Maowusu Desert as an example, the bulk density is 1564 kg / m³, the apparent density is 2610 kg / m³, the moisture content is 0.5%, the mud content is 2%, and its particle grading curve shows a uniform distribution feature.

[0056] The embodiment of the present invention also provides a method for characterizing the effects of freeze-thaw and size on concrete strength, such as Figure 1-4 As shown, the following steps are included:

[0057] Step 101: After the materials are proportioned and evenly mixed, they are poured into a test mold to produce concrete specimens of different sizes; each of the produced concrete specimens is cured;

[0058] For example, take a concrete specimen made from aeolian sand as an example:

[0059] Determine the mix ratio: Determine the mix ratio of concrete based on the actual project conditions or test requirements. The mix ratio is composed of the following materials in proportion: water 195kg / m³, cement 418kg / m³, fly ash 46kg / m³, coarse aggregate 1184kg / m³, and sand 557kg / m³, so as to ensure that the quality and performance of the concrete meet the test requirements.

[0060] Specimen preparation: According to the determined mix ratio, mix the materials evenly and pour them into the pre-prepared test mold to make concrete specimens of specified sizes.

[0061] For example, a cubic specimen of 100mm×100mm×100mm or a cylindrical specimen of Φ50mm×100mm.

[0062] During the production process, the density and dimensional accuracy of the specimen must be ensured. At the same time, marks should be made on the surface of the specimen to record the production date, number and other information of the specimen.

[0063] Specimen curing: The prepared concrete specimens are cured under specified conditions, usually in a standard curing room with a temperature of 20℃±2℃ and a relative humidity greater than 95% for a specified age, such as 28 days, to reach the design strength grade; during the curing period, the specimens should be regularly sprinkled with water to prevent the surface from drying out.

[0064] Step 102: performing uniaxial compression tests on concrete specimens of different sizes that have not been subjected to freeze-thaw cycles using a pressure testing machine to obtain initial uniaxial compressive strengths of the concrete specimens of different sizes that have not been subjected to freeze-thaw cycles;

[0065] Specifically, initial uniaxial compressive strength = failure load / compressive area of ​​concrete specimen.

[0066] The initial uniaxial compressive strength is the strength in the original state without being affected by freeze-thaw cycles, and provides a reference benchmark for the strength attenuation after subsequent freeze-thaw cycles.

[0067] Step 103: Performing uniaxial compression tests on concrete specimens of different sizes after being subjected to different freeze-thaw cycles using a pressure testing machine to obtain uniaxial compressive strength under freeze-thaw cycles;

[0068] Specifically, concrete specimens of different sizes are subjected to different numbers of freeze-thaw cycles using a rapid freeze-thaw tester. After a preset number of freeze-thaw cycles, uniaxial compressive strength tests are performed on the concrete specimens of different sizes using a pressure testing machine until the concrete specimens fail, thereby obtaining the failure loads of concrete specimens of different sizes and with different numbers of freeze-thaw cycles.

[0069] During the freeze-thaw cycle, some specimens are taken out for uniaxial compressive strength test after a certain number of freeze-thaw cycles, such as every 5 or 10 times.

[0070] During the test, the specimen is placed in the center of the pressure plate of the pressure testing machine and loaded at a certain loading rate, such as about 0.5mm / min, until the specimen is destroyed, and the failure load of the specimen is recorded.

[0071] Afterwards, the uniaxial compressive strength under freeze-thaw cycles was determined based on the failure loads of concrete specimens with different freeze-thaw cycle numbers and sizes;

[0072] The uniaxial compressive strength is determined according to uniaxial compressive strength = failure load / compressive area of ​​concrete specimen.

[0073] Step 104: Fit the uniaxial compressive strength and initial uniaxial compressive strength of at least seven groups of freeze-thaw cycles to obtain the compressive elastic limit strength of the inner area of ​​the concrete specimen. f ce 、Ratio of the ultimate compressive elastic strength of the concrete interior to the initial uniaxial compressive strength K、 Decay constant of unfrozen concrete 、 Concrete material constant t 0 、 Concrete material constant 、 Nominal damage modulus Size effect Nonlinear index m、 The ratio of the ultimate compressive strength of the concrete interior to its true compressive strength b and the size of the local damage zone of concrete under uniaxial compression D 0 ;

[0074] Specifically, the least squares method in the fitting toolbox in MATLAB software is used for fitting.

[0075] The uniaxial compressive strength and initial uniaxial compressive strength of at least seven groups of freeze-thaw cycles were fitted and input into the MATLAB software, based on the energy balance size effect model considering freeze-thaw cycles:

[0076] Perform fitting to obtain f ce 、K、 、t 0 、 ,m,b and D 0 ;

[0077] like Figure 2 As shown in the figure, the surface obtained by fitting with Matlab software is Figure 2 The black dots in the middle are the “uniaxial compressive test results of concrete specimens with different freeze-thaw cycles and sizes”, and the curved surface is the prediction result of the “energy balance size effect model considering freeze-thaw cycles”; the parameter fitting results of the “energy balance size effect model considering freeze-thaw cycles” are shown on the left side of the figure, namely f ce 、K、 、t 0 、 ,m,b and D 0 .

[0078] Step 105: According to the f ce 、K、 、t 0 、 ,m,b and D 0 Determine the size-effect model of energy balance considering freeze-thaw cycles;

[0079] Specifically, an energy balance size effect model considering freeze-thaw cycles was constructed;

[0080] Firstly, a freeze-thaw damage structural strength model was constructed based on the simplified structural characteristics of the specimen freeze-thaw damage as a damaged surface layer enclosing an undamaged internal area.

[0081] The freeze-thaw damage structural strength model is , where the parameter t 0 and is the concrete material constant; parameter and are the initial uniaxial compressive strength and decay constant of unfrozen-thawed concrete, respectively; N is the number of freeze-thaw cycles; is the nominal strength of the undamaged internal area.

[0082] Then, the strength-energy size effect model of the undamaged inner region is combined;

[0083] The strength-energy-size effect model of the undamaged internal area is: , where m >0, which reflects the exponent of the nonlinearity of the size effect of the nominal damage modulus; b=f c / f ce ,and f ce ≤ f c ; 、 f ce and f c are the nominal compressive strength, compressive elastic limit strength and true compressive strength of the inner region of the concrete specimen, respectively; D 0 is the size of the local damage zone (LDZ) in the uniaxial compression specimen, and its dimension is length; D is the size of the concrete specimen during the test.

[0084] Finally, an energy balance size effect model considering freeze-thaw cycles is obtained based on the freeze-thaw damaged structural strength model combined with the strength energy size effect model of the undamaged internal area.

[0085] The energy balance size effect model considering freeze-thaw cycles is:

[0086] ;

[0087] in, K is the compressive elastic limit strength of the concrete inner area f ce Initial uniaxial compressive strength The ratio of K = f ce / s 0; is the growth power function , describes the increase of surface layer thickness with the number of freeze-thaw cycles N; An exponential decay model , describing the intensity attenuation of the surface layer; m is the nonlinear index of size effect of nominal damage modulus; is the decay constant of unfrozen concrete 、t 0 is the concrete material constant 、 is the concrete material constant 、b is the ratio of the compressive elastic limit strength of the concrete interior area to the true compressive strength and D 0 is the size of the local damage zone of the concrete specimen under uniaxial compression;

[0088] when When the uniaxial compressive strength s nc = = s s The reason is that when the number of freeze-thaw cycles is constant, for concrete specimens of different sizes, the thickness of the damaged surface layer is considered to be t and strength s s constant.

[0089] when When , the concrete specimen is completely damaged surface layer.

[0090] The parameters obtained by fitting f ce 、K、 、t 0 、 ,m,b and D 0After substituting into the energy balance size effect model considering freeze-thaw cycles, the unknown variable of the energy balance size effect model considering freeze-thaw cycles is only the number of freeze-thaw cycles. N , the size of the concrete specimen D , and the corresponding uniaxial compressive strength .

[0091] The parameters obtained by fitting f ce 、K、 、t 0 、 ,m,b and D 0 After substituting into the energy balance size effect model considering freeze-thaw cycles, we get Figure 3 The surface shown.

[0092] like Figure 3 As shown in the figure, the "uniaxial compressive test results of concrete specimens with different freeze-thaw cycles and sizes" for which parameters are determined by fitting are represented by blue, while the other part of the test results that are not used to determine parameters are represented by red dots.

[0093] Step 106: predicting the uniaxial compressive strength of a concrete specimen made of the quasi-brittle material to be predicted under any number of freeze-thaw cycles and any size using the energy balance size effect model considering freeze-thaw cycles.

[0094] Specifically, it is only necessary to obtain the size of the concrete specimen that needs to be predicted D , and the number of freeze-thaw cycles that need to be predicted N The uniaxial compressive strength can be predicted by considering the energy balance size effect model of freeze-thaw cycles. .

[0095] Experimental data: 1. The strength data of wastewater concrete specimens after different freeze-thaw cycles collected from the literature are as follows:

[0096] Table 1 Freeze-thaw and strength data of different materials

[0097]

[0098] 2. Use the data in Table 1 to fit and determine the parameters of the energy balance size effect model considering freeze-thaw cycles f ce 、K、 、t 0 、 ,m,b andD 0 ,The fitting method is: using the least square method in the fitting toolbox in matlab software;

[0099] Table 2 Fitting parameter values ​​of energy balance size effect model considering freeze-thaw cycles

[0100]

[0101] 3. Substituting the fitting parameter values ​​in Table 2 into the energy balance size effect model considering freeze-thaw cycles, we can obtain Figure 3 The wastewater concrete data shown are compared with the predictions of the energy balance size effect model considering freeze-thaw cycles, as shown by Figure 3 It can be seen that the energy balance size effect model considering freeze-thaw cycles can accurately describe the nonlinear characteristics between the uniaxial compressive strength of wastewater concrete specimens and the freeze-thaw cycles and sizes, which verifies the correctness of the energy balance size effect model considering freeze-thaw cycles in this application.

[0102] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention.

Claims

1. A method for characterizing the effects of freeze-thaw and size on concrete strength, characterized in that: The following steps are involved: 195 kg / m³ of water, 418 kg / m³ of cement, 46 kg / m³ of fly ash, 1184 kg / m³ of coarse aggregate, and 557 kg / m³ of the material to be predicted as quasi-brittle are mixed and poured into a test mold to produce concrete specimens of different sizes; each of the produced concrete specimens is cured; the coarse aggregate is graded crushed stone; the material to be predicted as quasi-brittle is sand; the sand comprises a mixture of river sand and aeolian sand; Uniaxial compression tests were performed on concrete specimens of different sizes without freeze-thaw cycle treatment using a pressure testing machine to obtain the initial uniaxial compressive strength of concrete specimens of different sizes without freeze-thaw cycle treatment. Uniaxial compression tests were performed on concrete specimens of different sizes after different freeze-thaw cycles using a pressure testing machine to obtain the uniaxial compressive strength under freeze-thaw cycles. The uniaxial compressive strength and initial uniaxial compressive strength under at least seven freeze-thaw cycles are fitted to obtain the compressive elastic limit strength f of the inner area of ​​the concrete specimen. ce , the ratio of the ultimate compressive elastic strength of the concrete interior area to the initial uniaxial compressive strength K, and the decay constant of the unfrozen-thawed concrete , concrete material constant t0, concrete material constant , the nonlinear index of size effect of nominal damage modulus m, the ratio of the compressive elastic limit strength of the internal area of ​​concrete to the true compressive strength β and the size of the local damage area D0 of the concrete specimen under uniaxial compression; According to the f ce , K, ,t0, , m, β and D0 determine the energy balance size effect model considering freeze-thaw cycles; Predicting the uniaxial compressive strength of a concrete specimen made of the quasi-brittle material to be predicted under any number of freeze-thaw cycles and any size using the energy balance size effect model considering freeze-thaw cycles; The energy balance size effect model considering freeze-thaw cycles specifically includes: Construct a strength model of freeze-thaw damage structures; Integrate the strength-energy-size effect model of the undamaged internal region; An energy balance size effect model considering freeze-thaw cycles is obtained based on the freeze-thaw damaged structural strength model combined with the strength energy size effect model of the undamaged internal area; The freeze-thaw damage structural strength model is , where the parameter is the initial uniaxial compressive strength of unfrozen concrete; N is the number of freeze-thaw cycles; is the nominal strength of the undamaged internal area of ​​the concrete specimen; The strength-energy-size effect model of the undamaged internal area is: , where m>0; β=f c / f ce , and f ce ≤f c ;f c is the true compressive strength of the inner area of ​​the concrete specimen; the dimension of D0 is length; D is the size of the concrete specimen during the test; The energy balance size effect model considering freeze-thaw cycles is: 。 2. The method for characterizing the effects of freeze-thaw and size on concrete strength according to claim 1, characterized in that: The uniaxial compression test is performed on concrete specimens of different sizes after being subjected to different freeze-thaw cycles by a pressure testing machine to obtain the uniaxial compressive strength under freeze-thaw cycles, specifically including: Concrete specimens of different sizes are subjected to different numbers of freeze-thaw cycles using a rapid freeze-thaw tester. After a preset number of freeze-thaw cycles, uniaxial compressive strength tests are performed on the concrete specimens of different sizes using a pressure testing machine until the concrete specimens fail, thereby obtaining the failure loads of concrete specimens of different sizes and with different numbers of freeze-thaw cycles. The uniaxial compressive strength under freeze-thaw cycles is determined based on the failure load of concrete specimens with different freeze-thaw cycles and different sizes.

3. The method for characterizing the effects of freeze-thaw and size on concrete strength according to claim 2, wherein: Determining the uniaxial compressive strength under freeze-thaw cycles according to the failure loads of concrete specimens of different freeze-thaw cycles and different sizes specifically includes: determining the uniaxial compressive strength according to uniaxial compressive strength=failure load / compressed area of ​​concrete specimen.

4. The method for characterizing the effects of freeze-thaw and size on concrete strength according to claim 3, wherein: The aeolian sand replacement rate ASRR of the mixed sand is specifically selected from one of the following mass percentages: When ASRR=25%, 418kg / m³ of river sand and 139kg / m³ of aeolian sand are used to form the sand; When ASRR=50%, 278.5kg / m³ of river sand and 278.5kg / m³ of aeolian sand are used to form the sand; When ASRR=75%, river sand 139kg / m³ and aeolian sand 418kg / m³ are used to form the sand; When ASRR=100%: river sand 0kg / m³ and aeolian sand 557kg / m³ are used to form the sand.

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