Method for determining the flexural strength of concrete after freeze-thaw cycles

Abstract

The application discloses a method for determining the flexural strength of concrete after freeze-thaw cycles, and comprises the following steps: measuring the dynamic elastic modulus of the concrete; taking the dynamic elastic modulus, the initial flexural strength of the concrete, the flexural strength loss coefficient and the air pressure influence coefficient as inputs, and calculating the flexural strength of the concrete based on a flexural strength prediction formula; the application analyzes the dynamic elastic modulus and the flexural strength of the concrete, establishes the relationship between the dynamic elastic modulus and the flexural strength of the concrete with the increase of the freeze-thaw cycle times, proposes a prediction formula for the flexural strength of the concrete after freeze-thaw cycles, so that the dynamic elastic modulus of the concrete only needs to be detected, and the flexural strength can be obtained through corresponding calculation, damage to the existing concrete during measurement is avoided, and nondestructive testing of the concrete is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of concrete flexural strength measurement, and particularly relates to a method for determining the flexural strength of concrete after freeze-thaw cycles. BACKGROUND

[0002] The flexural strength of concrete is an important quality control index of concrete, which is related to the strength performance of concrete, especially the frost resistance of concrete.

[0003] At present, the measurement of the flexural strength of concrete is mainly carried out when the concrete is prepared, so as to obtain the flexural strength performance of the prepared concrete.

[0004] The common test method for the flexural strength of in-use concrete is destructive test, which needs to take cores in the concrete structure and make standard test pieces for flexural test. However, in actual engineering, the existing concrete structure will be damaged, thereby affecting the service life of the existing concrete structure. SUMMARY

[0005] The purpose of the application is to provide a method for determining the flexural strength of concrete after freeze-thaw cycles, so as to avoid damage to the existing concrete during measurement.

[0006] The application adopts the following technical scheme: a method for determining the flexural strength of concrete after freeze-thaw cycles, comprising the following steps:

[0007] measuring the dynamic elastic modulus of the concrete;

[0008] inputting the dynamic elastic modulus, the initial flexural strength of the concrete, the flexural strength loss coefficient and the air pressure influence coefficient, and calculating the flexural strength of the concrete based on a flexural strength prediction formula;

[0009] wherein the flexural strength prediction formula is F(N) is the flexural strength of the concrete after N freeze-thaw cycles, N is the number of freeze-thaw cycles, F0 is the initial flexural strength, E r (N) is the relative dynamic elastic modulus after N freeze-thaw cycles, a is the air pressure influence coefficient, and c is the flexural strength loss coefficient.

[0010] Further, the construction method of the flexural strength prediction formula is as follows:

[0011] constructing a freeze-thaw damage model and a flexural strength attenuation model of the concrete;

[0012] integrating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles to obtain the flexural strength prediction formula.

[0013] Further, the freeze-thaw damage model is as follows:

[0014] E(N+ΔN)-E(N)=-αβE(N)ΔN,

[0015] Where E(N+ΔN) is the dynamic elastic modulus of concrete after (N+ΔN) freeze-thaw cycles, E(N) is the dynamic elastic modulus of concrete after N freeze-thaw cycles, α is the air pressure influence coefficient, β is the dynamic elastic modulus loss rate, and Δ(N) is the increase in the number of freeze-thaw cycles of concrete.

[0016] Furthermore, incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles includes:

[0017] The freeze-thaw damage model was transformed and deformed to obtain

[0018] Furthermore, the flexural strength attenuation model is as follows:

[0019]

[0020] Where F(N+ΔN) is the flexural strength of concrete during freeze-thaw cycles (N+ΔN), and η is the flexural strength loss rate of concrete.

[0021] Furthermore, incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles also includes:

[0022] The flexural strength attenuation model was transformed and deformed to obtain Where F0 is the initial flexural strength of the concrete.

[0023] Furthermore, incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles also includes:

[0024] Will Integrate From

[0025] right A transformation is performed to obtain the flexural strength prediction model; where,

[0026] Furthermore, the flexural strength loss coefficient was obtained by conducting freeze-thaw cycle tests using concrete with the same mix proportions.

[0027] Furthermore, freeze-thaw cycle tests were conducted using concrete with the same mix proportions, including:

[0028] Obtain the original concrete mix proportions and prepare test specimens;

[0029] The specimens were subjected to freeze-thaw cycles of varying numbers, and the flexural strength and dynamic modulus of elasticity of the specimens were measured after the tests.

[0030] The measured flexural strength and dynamic elastic modulus are substituted into the flexural strength prediction model and calculated to obtain the flexural strength loss coefficient.

[0031] The beneficial effects of the present application are: the present application analyzes the dynamic elastic modulus and flexural strength of concrete, establishes the relationship between the dynamic elastic modulus and flexural strength of concrete with the increase of freeze-thaw cycle times, proposes a prediction formula for the flexural strength of concrete after freeze-thaw cycles, so that only the dynamic elastic modulus of concrete needs to be detected, and the flexural strength can be obtained by corresponding calculation, avoiding damage to the existing concrete during measurement, and realizing nondestructive testing of concrete. DETAILED DESCRIPTION

[0032] The present application will be described in detail below in conjunction with specific embodiments.

[0033] In view of the problems pointed out in the background art, it is of great significance to carry out nondestructive testing. The present application discloses a method for determining the flexural strength of concrete after freeze-thaw cycles, comprising the following steps: measuring the dynamic elastic modulus of concrete; taking the dynamic elastic modulus, the initial flexural strength of concrete, the flexural strength loss coefficient and the air pressure influence coefficient as inputs, and calculating the flexural strength of concrete based on a flexural strength prediction formula; wherein the flexural strength prediction formula is F(N) is the flexural strength of concrete after N freeze-thaw cycles, N is the number of freeze-thaw cycles, F0 is the initial flexural strength, E r (N) is the relative dynamic elastic modulus after N freeze-thaw cycles, a is the air pressure influence coefficient, and c is the flexural strength loss coefficient.

[0034] Specifically, the initial flexural strength of concrete and the air pressure influence coefficient can be obtained by querying the environmental parameters of the location of the concrete structure, and the flexural strength loss coefficient can be obtained by freeze-thaw cycle test on concrete with the same proportion.

[0035] The present application analyzes the dynamic elastic modulus and flexural strength of concrete, establishes the relationship between the dynamic elastic modulus and flexural strength of concrete with the increase of freeze-thaw cycle times, proposes a prediction formula for the flexural strength of concrete after freeze-thaw cycles, avoids damage to the existing concrete during measurement, and realizes nondestructive testing of concrete.

[0036] In one embodiment, the method for constructing the flexural strength prediction formula is: constructing a freeze-thaw damage model and a flexural strength decay model of concrete; incorporating the freeze-thaw damage model into the flexural strength decay model based on the number of freeze-thaw cycles to obtain the flexural strength prediction formula.

[0037] Specifically, the freeze-thaw damage model is:

[0038]

[0039] The formula (1) is transformed as follows:

[0040] E(N+ΔN)-E(N)=-αβE(N)ΔN (2)

[0041] Thus, we have:

[0042]

[0043] Integrating the formula (3), we have:

[0044]

[0045] That is,

[0046] E r (N)=e -αβN (5)

[0047] wherein E(N+ΔN) is the dynamic elastic modulus of the concrete after the freeze-thaw cycle (N+ΔN) times, the unit is MPa, E(N) is the dynamic elastic modulus of the concrete after the freeze-thaw cycle N times, α is the air pressure influence coefficient, β is the dynamic elastic modulus loss rate, Δ(N) is the number of freeze-thaw cycles, E r (N) is the relative dynamic elastic modulus of the concrete after the freeze-thaw cycle N times, E r (N)=E(N) / E0, E0 is the initial dynamic elastic modulus of the concrete, the initial dynamic elastic modulus is the dynamic elastic modulus of the concrete just prepared, α=P / P0, P0 is the standard atmospheric pressure, P is the atmospheric pressure of the place where the concrete is located, the value of α is shown in Table 1, β is the dynamic elastic modulus loss rate of the concrete in the freeze-thaw cycle in the relative atmospheric pressure and is a constant, which is obtained by experiment.

[0048] Table 1

[0049] Altitude / m Local atmospheric pressure / KPa Standard atmospheric pressure / KPa Atmospheric pressure influence coefficient 0 100 101.325 0.98 500 95.45 101.325 0.94 1000 91.30 101.325 0.90 2000 79.75 101.325 0.78 3000 70.46 101.325 0.69 4000 62.06 101.325 0.61 5000 54.49 101.325 0.53

[0050] Then, the freeze-thaw damage model is transformed as follows:

[0051]

[0052] In one embodiment, the freeze-thaw cycle number of the flexural strength loss rate is constant. It is assumed that the flexural strength F(N) of the concrete after freeze-thaw cycle N times is differentiable, the flexural strength before freeze-thaw is F0, and η is the strength loss rate after freeze-thaw. The flexural strength attenuation model of the concrete freeze-thaw cycle N times to N+ΔN times is as follows:

[0053]

[0054] Wherein, F(N+ΔN) is the flexural strength of the concrete after freeze-thaw cycle (N+ΔN), and η is the flexural strength loss rate of the concrete.

[0055] As a specific implementation, the freeze-thaw damage model is integrated into the flexural strength attenuation model based on the number of freeze-thaw cycles, and the integration includes:

[0056] The flexural strength attenuation model is transformed by moving terms to obtain:

[0057] F(N+ΔN)-F(N)=-ηF(N)ΔN (8)

[0058] That is:

[0059]

[0060] Integrating (9) obtains:

[0061]

[0062] The formula (6) is fused into the formula (10) to obtain:

[0063]

[0064] Wherein, F0 is the initial flexural strength of the concrete.

[0065] According to the above derivation, the flexural strength prediction model can be obtained by transforming the formula (11). Specifically, the freeze-thaw cycle concrete flexural strength damage model is established as shown in formula (12):

[0066]

[0067] Wherein,

[0068] Another transformation of formula (12) can obtain the flexural strength prediction model of the concrete after freeze-thaw cycle, that is, formula (13):

[0069]

[0070] In actual engineering, the dynamic elastic modulus of the concrete after freeze-thaw cycle is measured by the flat measurement method, and the flexural strength of the concrete can be obtained by formula (13).

[0071] More specifically, the flexural strength loss coefficient is obtained by freeze-thaw cycle test of concrete with the same proportion. That is, the original proportion of the concrete is obtained and the test piece is prepared; the test piece is subjected to different number of freeze-thaw cycle tests, and the flexural strength and dynamic elastic modulus of the test piece after the test are measured; the measured flexural strength and dynamic elastic modulus are substituted into the flexural strength prediction formula and calculated to obtain the flexural strength loss coefficient.

[0072] In addition, after obtaining the above anti-bending strength prediction formula, the accuracy of the formula can be verified by experiments. The specific steps are as follows:

[0073] Step one, prepare the test piece, the cement, water, fine aggregate, coarse aggregate and admixture used shall meet the "Standard for Mixing Water for Concrete", "Standard for Quality and Test Method for Sand and Stone for Ordinary Concrete", "Technical Specification for Application of Concrete Admixture", and the concrete mix proportion shall meet the "Design Specification for Concrete Mix Proportion"; more preferably, the same concrete with the same mix proportion as the concrete to be tested is used for testing.

[0074] Step two, prepare the test piece, use a prism test block with a concrete size of 100mmx100mmx400mm. The test piece is taken out 24 days before the curing age, and then the freeze-thaw test piece is placed in water at a temperature of 20℃±2℃ for 4 days. The water surface should be higher than the top surface of the test piece. When the age reaches 28 days, the freeze-thaw test is carried out.

[0075] Step three, place the 100mmx100mmx400mm test piece in a test box with a size of 115mmx115mmx500mm, add water to submerge the test piece by more than 1cm. After placing the test piece in the middle of the freeze-thaw box, add frozen liquid to the height of the other test pieces, and insert a thermometer into the center of the test piece to measure the internal temperature of the concrete during the freeze-thaw cycle. At the same time, place three thermometers on both sides and in the middle of the freeze-thaw box to monitor the temperature of the frozen liquid, ensuring that the temperature difference during the freeze-thaw cycle is within ±2℃.

[0076] Step four, the duration of a single freeze-thaw cycle is set to 2 to 4 hours, and in this test, the freeze-thaw cycle is set to 3.2 to 4 hours, and the center temperature of the test piece is 5℃ at the highest and -17℃ at the lowest. The temperature rising and falling process time is within 10±1min. After every 25 freeze-thaw cycles, take out 3 parallel test pieces of concrete from the freeze-thaw box, carefully wash off the debris and mortar on the surface of the concrete, and dry them.

[0077] Step five, dynamic elastic modulus detection; the test uses a non-metal ultrasonic wave detection analyzer to collect the ultrasonic wave speed of the concrete under different freeze-thaw cycle numbers. When performing ultrasonic detection, a suitable amount of medical vaseline should be applied to the part of the concrete test piece in contact with the transducer; the ultrasonic wave emission frequency is 50kHz; the flat measurement method is used; a single-channel ultrasonic wave detector is used for ultrasonic wave testing; in order to obtain more accurate results, each test point is tested repeatedly, and the average value is taken as the test result of the test point.

[0078] Step six, conduct the bending test before and after the freeze-thaw cycle of the concrete; the bending strength detection is carried out according to the standard GB / T 50081-2002 for mechanical property test method of ordinary concrete.

[0079] Step 7: Data processing and analysis results. The flexural strength and dynamic modulus of elasticity of concrete before and after the freeze-thaw cycle are processed and analyzed for comparison, thereby verifying the above-mentioned formula for predicting flexural strength.

[0080] In addition, to more clearly demonstrate the method of the present invention and verify the accuracy of the formula, indoor test data from a road project in Nagqu City, Tibet Autonomous Region, were introduced for verification.

[0081] The local altitude is about 4500m and the atmospheric pressure is about 57.75kPa. To meet the local engineering requirements, P.O52.5 grade silicate cement is used. The fine aggregate is natural river sand with a fineness modulus of 3.02. The coarse aggregate is 4.75mm to 31.5mm of mine crushed stone, of which 4.75mm to 10mm is small crushed stone, 10mm to 20mm is medium crushed stone, and 20mm to 31.5mm is large crushed stone.

[0082] Made of polyester fiber, produced by Jiangsu Subote Company -Ⅳ High-efficiency air-entraining agent for concrete, -9 series water-reducing agents, -Ⅳ High-efficiency concrete expansion agent. Concrete mix design examples are shown in Table 2.

[0083] Table 2

[0084]

[0085] P0 is the standard atmospheric pressure, and P is the atmospheric pressure at the local location. The influence coefficient of air pressure, α, can be calculated to be 0.57. The test values ​​of dynamic elastic modulus and flexural strength of concrete after freeze-thaw cycles are shown in Table 3.

[0086] Table 3

[0087]

[0088] Substituting the dynamic modulus and flexural strength test values ​​from Table 3 into Equation (12), and performing nonlinear fitting on the flexural strength loss coefficient c, we obtain that the flexural strength loss coefficient c for ordinary fiber-reinforced concrete is 0.3120, and the flexural strength loss coefficient c for expanded fiber-reinforced concrete is 0.3112.

[0089] Finally, the dynamic elastic modulus test value is substituted into equation (13) to calculate the predicted value of the flexural strength of concrete after freeze-thaw cycles, and compared with the measured value to obtain the relative error. The results are statistically shown in Table 4.

[0090] Table 4

[0091]

[0092] As shown in Table 4, the established formula for predicting the flexural strength of concrete has high accuracy and can effectively predict the flexural strength of concrete under freeze-thaw cycle damage, and the relative error of the flexural strength does not exceed 7%.

[0093] This invention utilizes rapid freeze-thaw tests on concrete to measure its dynamic modulus of elasticity and flexural strength. It establishes the relationship between the dynamic modulus of elasticity and flexural strength as the number of freeze-thaw cycles increases, and proposes a predictive formula for the flexural strength of concrete after freeze-thaw cycles. This formula effectively reflects the damage to the flexural strength of concrete after freeze-thaw cycles. This invention uses the dynamic modulus of elasticity as the evaluation criterion for flexural strength; specifically, it rapidly derives the flexural strength of concrete damaged by freeze-thaw cycles using a curve equation relating the dynamic modulus of elasticity and flexural strength.

[0094] This invention employs a flat measurement method, using a non-metallic ultrasonic testing and analysis instrument to test the dynamic elastic modulus. The change in dynamic elastic modulus is used as the basis for evaluating the strength of the specimen, establishing the relationship between the flexural strength and dynamic elastic modulus of concrete under different freeze-thaw cycles.

[0095] This invention utilizes rapid freeze-thaw tests on concrete to measure its dynamic modulus of elasticity and flexural strength. It establishes the relationship between the dynamic modulus of elasticity and flexural strength of concrete with increasing freeze-thaw cycles, and proposes a predictive formula for the flexural strength of concrete after freeze-thaw cycles. This formula effectively reflects the damage to the flexural strength of concrete after freeze-thaw cycles. This invention uses the dynamic modulus of elasticity as the evaluation criterion for flexural strength; specifically, it rapidly derives the flexural strength of concrete damaged by freeze-thaw cycles using a curve equation relating the dynamic modulus of elasticity and flexural strength.

[0096] By conducting rapid freeze-thaw tests on concrete, the dynamic elastic modulus and flexural strength of concrete were tested. The relationship between the dynamic elastic modulus and flexural strength of concrete with the increase of freeze-thaw cycles was established. A formula for predicting the flexural strength of concrete after freeze-thaw cycles was proposed, providing a theoretical basis for determining the flexural strength of concrete after freeze-thaw cycles.

Claims

1. A method for determining the flexural strength of concrete after freeze-thaw cycles, characterized in that, Includes the following steps: Measure the dynamic elastic modulus of concrete; Using the dynamic elastic modulus, the initial flexural strength of concrete, the flexural strength loss coefficient, and the air pressure influence coefficient as inputs, the flexural strength of concrete is calculated based on the flexural strength prediction formula. The formula for predicting flexural strength is as follows: , Let N be the flexural strength of the concrete after N freeze-thaw cycles. The initial flexural strength, Let N be the relative dynamic elastic modulus after N freeze-thaw cycles. , For freeze-thaw cycles The dynamic elastic modulus of concrete at this time. This represents the initial dynamic elastic modulus of concrete. is the air pressure influence coefficient, and c is the flexural strength loss coefficient.

2. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 1, characterized in that, The method for constructing the flexural strength prediction formula is as follows: Construct freeze-thaw damage models and flexural strength attenuation models for concrete; The freeze-thaw damage model is incorporated into the flexural strength attenuation model based on the number of freeze-thaw cycles to obtain the flexural strength prediction formula.

3. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 2, characterized in that, The freeze-thaw damage model is as follows: ， in, For freeze-thaw cycles The dynamic elastic modulus of concrete at this time. For freeze-thaw cycles The dynamic elastic modulus of concrete at this time. This is the air pressure influence coefficient. The rate of loss of dynamic elastic modulus. The number of freeze-thaw cycles for concrete is increased.

4. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 3, characterized in that, Incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles includes: The freeze-thaw damage model is transformed and deformed to obtain .

5. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 4, characterized in that, The flexural strength attenuation model is as follows: ， in, For freeze-thaw cycles The flexural strength of concrete at that time This represents the flexural strength loss rate of concrete.

6. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 5, characterized in that, Incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles also includes: The flexural strength attenuation model was transformed and deformed to obtain ;in, This represents the initial flexural strength of the concrete.

7. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 6, characterized in that, Incorporating the freeze-thaw damage model into the flexural strength attenuation model based on the number of freeze-thaw cycles also includes: Will Integrate From ; right The flexural strength prediction formula is obtained by performing a transformation; where, .

8. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 7, characterized in that, The flexural strength loss coefficient was obtained by conducting freeze-thaw cycle tests using concrete with the same mix proportions.

9. The method for determining the flexural strength of concrete after freeze-thaw cycles as described in claim 8, characterized in that, Freeze-thaw cycle tests using concrete with the same mix proportions include: Obtain the original concrete mix proportions and prepare test specimens; The specimens were subjected to freeze-thaw cycles of varying numbers, and the flexural strength and dynamic modulus of elasticity of the specimens were measured after the tests. The measured flexural strength and dynamic elastic modulus are substituted into the flexural strength prediction formula and calculated to obtain the flexural strength loss coefficient.

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