Concrete freezing resistance evaluation method suitable for high-altitude low-pressure environment

Abstract

The invention discloses a concrete freezing resistance evaluation method suitable for a high-altitude low-pressure environment. The method comprises the following steps: stirring and forming a concrete test piece in a low-pressure environment; soaking a group of test pieces for 4 days until the test pieces are saturated with water, putting the test pieces into a freezing stress test barrel, setting freezing and thawing cycle test air pressure and frozen concrete test piece center temperature according to the actual operation environment of the high-altitude cold region project, and drawing a freezing stress development curve in the freezing and thawing cycle process; finally, a freezing stress inflection point value is obtained through the freezing stress development curve in the freezing and thawing cycle process, the lower the freezing stress inflection point value is, the more excellent the anti-freezing performance is, and it is judged that concrete is subjected to freezing and thawing damage according to inflection points changing from large to small in the freezing stress development curve. According to the method, the anti-freezing performance of the concrete in the low-pressure environment is evaluated through the freezing stress, the defect of controlling the gas content of a concrete mixture in the high-altitude low-pressure environment is effectively avoided, and the anti-freezing durability in the actual engineering concrete service process can be evaluated more accurately.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and in particular to a method for evaluating the frost resistance of concrete applicable to high-altitude and low-pressure environments. Background Art

[0002] Using an air-entraining agent to introduce a certain number of fine air bubbles into concrete is a common method to improve the frost resistance of concrete in plain areas. During specific operations, the air content of the concrete mixture is determined according to the test method for the air content of concrete mixtures in the "Test Code for Hydraulic Concrete" DL / t5150-2017; when the concrete mixture contains a certain amount of air content (usually 3% - 5%), the concrete can have certain frost resistance. However, in high-altitude and low-pressure environments, there are situations where the fine air bubbles introduced into the concrete are unstable and prone to rupture, and even the air-entraining agent has difficulty in entraining air, resulting in a decrease in the quality of the introduced air bubbles. The frost resistance durability under the same air content is significantly inferior to that in the normal pressure environment, leading to a poor applicability of the measure of ensuring the frost resistance of concrete by controlling the air content of the concrete mixture. In the "Test Method for Frost Resistance of Hydraulic Concrete" DL / t5150-2017, a mass loss rate exceeding 5% or a relative dynamic elastic modulus retention value lower than 60% is used as the judgment basis for the occurrence of freeze-thaw damage of concrete specimens. However, in high-altitude and low-pressure environments, the internal microscopic water migration, ice crystal growth, and crystallization pressure in the concrete will all be affected, and the inaccuracy of using the mass loss rate and relative dynamic elastic modulus as macroscopic characterization methods to evaluate the frost resistance performance is further increased. Therefore, in high-altitude and low-pressure environments, a method that can directly evaluate the frost resistance performance of concrete through the freeze-thaw damage mechanism without being affected by air pressure is required. Summary of the Invention

[0003] In view of the problems existing in the above background art, the present invention provides a method for evaluating the frost resistance of concrete applicable to high-altitude and low-pressure environments, which evaluates the frost resistance performance of concrete by measuring the freezing stress of the concrete during the freeze-thaw cycle in a climate simulation chamber, thereby eliminating the inaccuracy of using the mass loss rate and relative dynamic elastic modulus characterization methods to evaluate the frost resistance performance caused by air pressure.

[0004] To achieve the above object of the invention, the present invention provides the following technical solutions:

[0005] A method for evaluating the frost resistance of concrete applicable to high-altitude and low-pressure environments, comprising the following steps:

[0006] (1) Preparation and curing of test specimens: Set a low-pressure environment in a climate simulation chamber to stir and form concrete specimens, and cure them in a low-temperature and low-pressure environment of 5°C after forming. Stirring is carried out under low pressure during the test process, and the curing temperature is 20°C; simulate the low-pressure environment during the preparation and curing stages of the concrete to eliminate the influence of the high-altitude and low-pressure environment;

[0007] The low-pressure environment is an environment with an altitude exceeding 3000 m and a pressure lower than 70 kPa;

[0008] (2) Concrete freezing stress test: Take a group of concrete specimens prepared in step (1). After soaking them in water for 4 days to saturation, place them in a freezing stress test barrel. Set the freeze-thaw cycle test air pressure and the central temperature of the frozen concrete specimens according to the actual operating environment of the project in high-altitude cold regions. Set the central melting temperature of the specimens to 8 °C, and complete the melting process in 1.5 h. Each freeze-thaw cycle is completed within 3 hours; Read the concrete freezing stress data every 25 freeze-thaw cycles, take the average value of all specimens, and draw the development curve of the freezing stress during the freeze-thaw cycle;

[0009] In step (2), the group of concrete specimens is at least 3 pieces and are Φ100 mm × 400 mm cylindrical concrete specimens;

[0010] (3) Evaluation of concrete frost resistance: Obtain the inflection point value of the freezing stress through the development curve of the freezing stress during the freeze-thaw cycle described in step (2). The freezing stress reflects the situation of the internal microscopic water migration, crystallization, and development in the concrete. The lower the stress value corresponding to the inflection point of the freezing stress development curve during the freeze-thaw process of the concrete, the better its frost resistance. And judge the freeze-thaw damage of the concrete by the inflection point where the freezing stress changes from large to small in the freezing stress development curve.

[0011] Furthermore, the present invention can also optimize and screen the concrete mix ratio scheme with good frost resistance through the inflection point value of the freezing stress on the freezing stress development curve obtained in step (3), that is, optimize and screen the concrete mix ratio scheme with good frost resistance according to the size of the inflection point value of the freezing stress.

[0012] The present invention measures the freeze-thaw situation by testing the magnitude of the freezing stress during the freeze-thaw process of the concrete. The freeze-thaw damage of the concrete is mainly caused by the internal pressure condition exceeding the mechanical strength of the material, resulting in cracks and frost damage of the material. Measure and draw the development curve of the concrete freezing stress during the freeze-thaw cycle in the climate simulation chamber, and measure the frost resistance of the concrete according to the size of the inflection point value of the freezing stress. And the "Test Code for Hydraulic Concrete"

[0013] In the test method for concrete frost resistance in DL / t5150 - 2017 "Test Method for Concrete Frost Resistance", a mass loss rate exceeding 5% or a relative dynamic elastic modulus retention value lower than 60% is used as the judgment basis for the concrete specimens to have suffered freeze-thaw damage.

[0014] The present invention takes another group of 100 mm × 100 mm × 400 mm concrete specimens prepared in step (1), tests the mass loss rate and relative dynamic elastic modulus of the concrete under the same freeze-thaw environment as in step (2), and establishes the freezing stress p of the concrete before freeze-thaw damage according to the test in steps (2) and (4)f The relationship between the relative dynamic elastic modulus P and the mass loss rate W is as follows:

[0015] W = (0.60p f 2 - 0.84p f + a) × 100%

[0016] P = (100 - 3.5p f 2 - 0.8p f + b) × 100%

[0017] where a is the temperature coefficient of the mass loss rate and b is the temperature coefficient of the relative dynamic elastic modulus.

[0018] Through the above relational expressions, the threshold range of the freezing stress corresponding to 5% mass loss and 60% retained dynamic elastic modulus of the concrete specimens is obtained, so as to determine whether it is feasible to evaluate the frost resistance performance by using the freezing stress in the present invention. When the central temperature of the freeze-thaw cycle specimen is -30°C, a = 1.0, b = -3.0, and the inflection point value of the freezing stress of high frost-resistant concrete should not be greater than 2.4 MPa.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] Evaluating the frost resistance of concrete in a low-pressure environment through freezing stress effectively avoids the disadvantages of controlling the air content of concrete mixtures in a high-altitude and low-pressure environment, and can more accurately evaluate the frost resistance durability of concrete during the service process of actual projects; moreover, the freezing stress measurement can be continuously and automatically tested at the beginning of the freeze-thaw cycle test, avoiding errors generated during the test operation process and having good operability. Specific embodiments

[0021] In order to make the content described in the present invention easier to understand, the following further illustrates the technical solutions described in the present invention with specific embodiments, but the present invention is not limited thereto.

[0022] Embodiment

[0023] Set a low-pressure environment in a climate simulation chamber to mix and form concrete specimens. After forming, cure them in a low-temperature and low-pressure environment at 5°C. During the test process, the mixing is carried out under low pressure, and the curing temperature is 20°C; the low-pressure environment is an environment with an altitude exceeding 3000 m and a pressure lower than 70 kPa; prepare F400 frost-resistant design concrete through the above tests, with a water-binder ratio of 0.39 and an air content of 5.6%. The mix proportion is shown in Table 1 below.

[0024] Table 1 Mix proportion of F400 frost-resistant design concrete

[0025]

[0026] Prepare 3 cylindrical concrete specimens with a size of Φ100mm×400mm and 3 specimens with a size of 100mm×100mm×400mm to conduct the freezing stress freeze-thaw cycle test and the rapid freeze-thaw method frost resistance test in accordance with the "Test Code for Hydraulic Concrete" (DL / T 5150-2017). The freezing temperature for the freezing stress test is set at -30°C and the air pressure is 65 kPa, corresponding to an altitude of 3600 m. The test results are as follows.

[0027] Table 2 Freezing Stress Freeze-Thaw Test Results of F400 Frost-Resistant Design Concrete

[0028]

[0029] It can be known from the freezing stress test results that when the freeze-thaw cycle reaches 225 times under the environment of -30°C and 65 kPa, the freezing stress of the concrete reaches the maximum value of 2.45 MPa, which is higher than the proposed control threshold of 2.4 MPa for the inflection point of the freezing stress of high frost-resistant concrete. According to the freezing stress p f The estimated mass loss rate W calculated is 2.5%, and the relative dynamic elastic modulus P is 74%. According to the evaluation method of freezing stress frost resistance performance, it is judged that the specimen has suffered freeze-thaw damage.

[0030] According to the frost resistance performance evaluation test method specified in the current specification DL / T 5150-2017, when the freeze-thaw cycle reaches 350 times, the mass loss rate exceeds 5% and the relative dynamic elastic modulus reaches 60%. According to the evaluation of the mass loss rate and relative dynamic elastic modulus specified in DL / T 5150-2017, it is judged that the specimen has suffered freeze-thaw damage.

[0031] Table 3 Rapid Freeze-Thaw Test Results of F400 Frost-Resistant Design Concrete

[0032]

[0033] Therefore, under the high-altitude and low-air-pressure environment, according to the frost resistance performance method based on freezing stress provided by the invention, it is judged that the concrete specimen has suffered freeze-thaw damage after 225 freeze-thaw cycles. The test results are more accurate and more suitable for the evaluation of the frost resistance performance of concrete in high-altitude, low-air-pressure and cold environments.

Claims

1. A method for evaluating the frost resistance of concrete applicable to high-altitude and low-atmospheric-pressure environments, characterized in that It includes the following steps: (1) Preparation and curing of specimens to be tested: Set a low-pressure environment in a climate simulation chamber to stir and mold concrete specimens. After molding, cure them in a low-temperature and low-pressure environment at 5°C. During the test process, stirring is carried out under low pressure, and the curing temperature is 20°C; (2) Concrete freezing stress test: Take a group of concrete specimens prepared in step (1). After soaking in water for 4 days to saturation, place them in a freezing stress test barrel. Set the freeze-thaw cycle test pressure and the central temperature of the frozen concrete specimens according to the actual operating environment of the project in high-altitude cold regions. The central melting temperature of the specimens is set at 8°C, and the melting process is completed in 1.5 hours. Each freeze-thaw cycle is completed within 3 hours; Read the concrete freezing stress data every 25 freeze-thaw cycles, take the average value of all specimens, and draw the development curve of freezing stress during the freeze-thaw cycle; (3) Evaluation of concrete frost resistance: Obtain the inflection point value of freezing stress through the development curve of freezing stress during the freeze-thaw cycle described in step (2). The freezing stress reflects the situation of internal microscopic water migration, crystallization, and development in concrete. The lower the inflection point value of freezing stress that appears during the freeze-thaw process of concrete, the better its frost resistance. And judge the freeze-thaw damage of concrete through the inflection point where the freezing stress changes from large to small in the development curve of freezing stress.

2. The method for evaluating the frost resistance of concrete applicable to high altitude and low air pressure environment according to claim 1, wherein The low-pressure environment described in step (1) is an environment with an altitude exceeding 3000 m and a pressure lower than 70 kPa.

3. The concrete frost resistance evaluation method applicable to high altitude and low air pressure environment according to claim 1, wherein, The group of concrete specimens described in step (2) is at least 3 pieces and is cylindrical concrete specimens with a diameter of Φ100 mm and a height of 400 mm.

4. A method for evaluating the frost resistance of concrete applicable to high altitude and low air pressure environments according to claim 1, characterized in that, The inflection point value of freezing stress obtained through step (3) can be used to optimize and screen the concrete mix proportion plan with good frost resistance, that is, to optimize and screen the concrete mix proportion plan with good frost resistance according to the size of the inflection point value of freezing stress.

Images