A method for obtaining concrete restraint stress

Abstract

The present invention discloses a method for obtaining concrete constraint stress, that is, calculating concrete constraint stress based on obtaining real and effective concrete elastic modulus; the concrete elastic modulus is obtained by simulating the real temperature history and stress history of large-volume concrete. The present invention couples factors such as temperature, constraint, and microcracks generated by long-term load holding into the measured elastic modulus at one time, so that the concrete constraint stress obtained is truly coupled with the real temperature and stress history of concrete; the present invention is based on truly and accurately carrying out effective elastic modulus tests of concrete under actual environmental changes and development conditions, and obtains the constraint stress of concrete under real conditions, providing a basis with extremely high reference value for computer simulation analysis and calculation of actual projects and formulation of various construction plans.

Description

Technical Field

[0001] The invention relates to a method for obtaining concrete constraint stress under real temperature history and stress history of concrete, and belongs to the technical field of water conservancy and hydropower engineering. Background Art

[0002] Due to the thick cross-section of mass concrete, the heat generated by the hydration reaction of the cementitious material cannot be dissipated into the air in time, resulting in an increase in its internal temperature, which then slowly drops to the ambient temperature. The temperature inside the mass concrete rises from low to high and then drops; in the early stage, when the temperature inside the concrete rises, it expands and generates compressive stress due to the constraint of the surface concrete. In the later stage, when the temperature inside the concrete drops, it generates tensile stress due to the constraint of the surface concrete. Once the tensile stress exceeds the tensile strength of the concrete body, the concrete will crack. Anti-cracking is an important task that needs to be considered during the construction of concrete dams. Effective temperature control and anti-cracking is the most important means to be taken during the construction of concrete dams. In order to formulate scientific and effective temperature control and anti-cracking measures, it is necessary to accurately grasp the concrete constraint stress, and to accurately grasp the concrete constraint stress, it is necessary to accurately grasp the concrete elastic modulus. The concrete elastic modulus is one of the key parameters for calculating the concrete constraint stress.

[0003] Due to the large size of mass concrete, it is difficult to carry out indoor prototype tests, so numerical analysis and calculation have become a commonly used evaluation method in the construction process of mass concrete.

[0004] For decades, technical personnel in this field have been conducting indoor tests in accordance with the "GB / T 50081-2019 Standard for Test Methods for Physical and Mechanical Properties of Concrete" using standard curing specimens to obtain the elastic modulus of concrete. That is, at a constant temperature and without load, the concrete specimens are cured to the test age and tested to calculate and obtain the elastic modulus of concrete, and then the concrete constraint stress needed to formulate temperature control and crack prevention measures is calculated.

[0005] Although the traditional method of obtaining the elastic modulus of concrete and then calculating the constraint force of concrete has been used for decades, it strictly complies with the "GB / T 50081-2019 Standard for Test Methods for Physical and Mechanical Properties of Concrete", and the test results have certain significance. However, in fact, the test conditions for obtaining the elastic modulus of concrete are completely different from the time temperature change (i.e., temperature history) and the constraint conditions (i.e., stress history) of mass concrete after pouring. Therefore, directly applying its test results (i.e., concrete elastic modulus) to the simulation analysis and calculation of mass concrete has natural limitations. The results of simulation numerical analysis and calculation cannot reflect the actual situation of mass concrete. This is also one of the reasons why cracking still exists after taking temperature control and crack prevention measures.

[0006] Therefore, a new method for obtaining the elastic modulus and constraint stress of concrete is proposed in the study to directly measure the real elastic modulus of concrete under comprehensive influencing factors, and then calculate the real concrete constraint stress. It is very necessary to formulate temperature control and crack prevention measures for large-volume concrete based on this method. Summary of the invention

[0007] In view of the above reasons, the purpose of the present invention is to provide a method for obtaining the real effective restraint stress of concrete. The method calculates the concrete restraint stress based on the real and effective concrete elastic modulus, changes the limitations and shortcomings of the existing concrete elastic modulus test method that cannot reflect the actual temperature history and stress history of concrete, and thus leads to the problem of inaccurate calculated concrete restraint stress, thereby improving the effectiveness and accuracy of temperature control and crack prevention measures for large-volume concrete.

[0008] To achieve the above object, the present invention adopts the following technical solutions: a method for obtaining concrete restraint stress, that is, calculating the concrete restraint stress based on obtaining the real and effective concrete elastic modulus;

[0009] The concrete elastic modulus is obtained by simulating the actual temperature history and stress history of large-volume concrete.

[0010] The method for obtaining the elastic modulus of concrete is:

[0011] 1) Pour the mixed concrete into the concrete temperature stress testing machine, set the temperature history and stress history of the concrete temperature stress testing machine, remove the positioning concrete specimen after the concrete is finally set, and paste the strain gauge for measuring the deformation of the concrete on the concrete specimen to start the test;

[0012] The temperature history is directly derived from the internal temperature of the concrete measured after the mass concrete is poured;

[0013] According to the restraint state of different areas of the mass concrete, the restraint degree γ of the concrete temperature stress testing machine is set. R ;

[0014] 2) Observe the stress condition of the concrete specimen, and start measuring the elastic strain of the concrete specimen when the concrete specimen changes from a compressive state to a tensile state;

[0015] 3) From the beginning of the concrete tension, 6 hours later at t1, the restraint system is turned off, and the tensile stress σ1 of the concrete specimen is unloaded to 0.1MPa. After stabilization for 60 seconds, the strain ε is recorded. 0.1 , reload to σ1, stabilize for 60 seconds, record the strain ε1, and then repeat the above steps twice to calculate the elastic modulus E(t1), ε 0.1 and ε1 are the data recorded in the last loading process;

[0016]

[0017] 4) After clearing the strain data, restart the constraint system and continue the temperature stress test;

[0018] 5) After 6 hours, repeat steps 3)-4) and use formula (2) to calculate the effective elastic modulus E(t i ), i = 2, 3, 4, ..., until the concrete cracks;

[0019]

[0020] 6) Obtain the effective elastic modulus E(t ei , γ R );

[0021] Get the effective age of concrete after standardizing the temperature factor t e :

[0022]

[0023] In the formula, E h is the activation energy, which is taken as 33500 J / mol, R is the ideal gas constant, which is taken as 8.31 J / mol / K, T0 is the reference temperature, which is taken as 20°C; T is the actual temperature of the concrete;

[0024] The effective age of concrete after standardizing the temperature factor obtained by formula (3) is t ei Replace t in formula (2) i , and obtain the real effective elastic modulus E(t ei , γ R ).

[0025]

[0026] The method for calculating the concrete confinement stress based on the obtained concrete effective elastic modulus is:

[0027]

[0028]

[0029] Where: σ(t i+1 / 2 ) is the concrete at age t i+1 / 2 Stress at time, (Δσ) j is the stress increment of concrete in the jth time period, γ R is the degree of constraint, E(t ei , γR ) is the concrete confinement degree γ R and effective age t ei The elastic modulus under the condition, ε(t i+1 / 2 ) is the concrete at age t i+1 / 2 The actual strain at the moment, ε 0 (t i+1 / 2 , t0) is the concrete from loading age t0 to age t i+1 / 2 The free strain within a time period is the sum of the autogenous volumetric strain and the temperature strain of concrete. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flow chart of the method for obtaining concrete restraint stress of the present invention;

[0031] Figure 2 It is the concrete temperature and loading process curve when testing the effective elastic modulus of concrete in the present invention (the constraint degree is 100%);

[0032] Figure 3 It is a comparison chart of the effective elastic modulus of concrete obtained by the present invention and the effective elastic modulus of concrete obtained by the traditional standard test. DETAILED DESCRIPTION

[0033] The structure and features of the present invention are described in detail below in conjunction with the accompanying drawings and embodiments. It should be noted that various modifications can be made to the embodiments disclosed herein, and therefore, the embodiments disclosed in the specification should not be regarded as limiting the present invention, but are merely examples of embodiments, the purpose of which is to make the features of the present invention obvious.

[0034] The method for obtaining the real effective restraint stress of concrete of the present invention comprises the following steps: calculating the concrete restraint stress based on the real effective concrete elastic modulus; the concrete elastic modulus is obtained by simulating the real temperature history and stress history of large volume concrete.

[0035] In order to accurately obtain the concrete restraint stress, it is first necessary to obtain the real and effective concrete elastic modulus. The method of obtaining the concrete elastic modulus of the present invention is:

[0036] 1) Pour the mixed concrete into the concrete temperature stress testing machine, set the temperature history and stress history of the concrete temperature stress testing machine, remove and position the concrete specimen after the concrete is finally set, and paste the strain gauge for measuring the deformation of the concrete on the concrete specimen to start the test.

[0037] The temperature history is directly derived from the internal temperature of the concrete measured after the mass concrete is poured; the constraint degree γ of the concrete temperature stress testing machine is set R , simulating the stress history of concrete.

[0038] 2) Observe the stress conditions of the concrete specimen, and start measuring the elastic strain of the concrete specimen when the concrete specimen changes from a compressive state to a tensile state.

[0039] like Figure 2 As shown, after the concrete is poured, the internal temperature rises, the concrete expands and deforms, and compressive stress is generated after being constrained. As the temperature drops, the concrete begins to shrink and deform, and the compressive stress gradually weakens after being constrained and turns into tensile stress. Since the cracking of large-volume concrete is caused by tensile behavior, the present invention focuses on the effective elastic modulus of concrete tested in the tensile stage, that is, starting from time t0.

[0040] 3) If Figure 2 As shown, from the beginning of the concrete tension, 6 hours later at t1, the restraint system is turned off, and the tensile stress σ1 of the concrete specimen is unloaded to 0.1MPa. After stabilization for 60 seconds, the strain ε is recorded. 0.1 , reload, that is, the load system of the concrete temperature stress tester adjusts the stress from 0.1MPa to σ1, stabilizes for 60 seconds, records the strain ε1, and then repeats the above steps twice to calculate the elastic modulus E(t1) of the concrete at that age, ε 0.1 and ε1 are the data recorded during the last loading process.

[0041]

[0042] 4) After clearing the strain data, restart the constraint system and continue the temperature stress test;

[0043] 5) After 6 hours, repeat steps 3) to 4) and use formula (2) to calculate the effective elastic modulus E(t i ), i = 2, 3, 4, ..., until the concrete cracks.

[0044]

[0045] 6) Obtain the effective elastic modulus E(t ei , γ R ).

[0046] Since the effective elastic modulus of concrete obtained by the present invention is based on the actual temperature history and stress history of concrete, and the obtained elastic modulus has already included the influence of the degree of constraint, the present invention adjusts the influence of the temperature history by the effective age through formula (3) to obtain the effective age of concrete after the temperature factor is standardized: e :

[0047]

[0048] In the formula, Eh is the activation energy, which is taken as 33500 J / mol, R is the ideal gas constant, which is taken as 8.31 J / mol / K, T0 is the reference temperature, which is taken as 20°C; T is the actual temperature of the concrete;

[0049] The effective age of concrete after standardizing the temperature factor obtained by formula (3) is t ei Replace t in formula (2) i , and obtain the effective elastic modulus E(t ei , γ R ).

[0050]

[0051] Figure 3 The constraint degree obtained by the present invention is 100%, and the real effective elastic modulus of concrete under the real temperature history and stress history. It can be seen that the effective elastic modulus of concrete obtained by the present invention has a certain degree of decrease compared with the traditional experimental results. The reason is that microcracks are generated inside the concrete under long-term load holding (under stress), resulting in a decrease in the elastic modulus of concrete. Since the concrete elastic modulus obtained by the present invention truly reflects the working state of mass concrete, the constraint stress of mass concrete calculated based on this more truly and accurately reflects the working state of mass concrete.

[0052] Since the volume of mass concrete (such as concrete dam) is large, the degree of constraint in different areas of the dam body is different. Therefore, the present invention can set the degree of constraint of the concrete temperature stress tester according to the specific conditions of different areas of the mass concrete to simulate the stress history of mass concrete at different positions. Repeat the above steps and set different degrees of constraint (such as 80%, 20%, etc.) to obtain the effective elastic modulus of different areas of the mass concrete under corresponding conditions.

[0053] Since the degree of constraint in different parts of large-volume concrete is different, the strong constraint area (constraint degree is 100%) is taken as an example to illustrate how to calculate the concrete constraint stress based on the real effective elastic modulus of concrete. The calculation methods for other constraint areas are similar.

[0054] Strong constraint condition (constraint degree γ R When the total deformation of concrete is 100%,

[0055]

[0056] Where σ(τ) is the stress of concrete at the time of loading, E(τ) is the elastic modulus of concrete at the time of loading, C(t, τ) and C(t, τ i ) is the concrete loading age τ and τ iThe creep degree, Δσ(τ i ) is the concrete age τ i The stress increment at time, E(τ i ) is the concrete age τ i The elastic modulus at the moment, ε 0 (t,τ i ) is the concrete loading age τ i The free strain within the time period up to age t is the sum of the autogenous volumetric strain and temperature strain of concrete.

[0057] Rewrite formula (5) into the form of sum:

[0058]

[0059] In the formula, It is concrete j Load at time t i+1 / 2 The creep coefficient at time,

[0060] Extract the last stress expression, we have

[0061]

[0062] Then the stress increment in the i-th time period can be expressed as

[0063]

[0064] It can be seen from formula (8) that the creep coefficient of concrete is required to calculate the concrete stress increment. Thanks to the fact that the concrete effective elastic modulus obtained by the present invention has coupled the real parameters obtained by the temperature history and long-term damage (i.e., constraint state) of concrete, after substituting formula (4), formula (8) can be rewritten as

[0065]

[0066] The confinement stress of concrete is:

[0067]

[0068] Where: σ(t i+1 / 2 ) is the concrete at age t i+1 / 2 Stress at time, (Δσ) j is the stress increment of concrete in the jth time period, γ R is the degree of constraint, E(t ei , γ R ) is the concrete confinement degree γ R and effective age t ei The elastic modulus under the condition, ε(t i+1 / 2) is the concrete at age t i+1 / 2 The actual strain at the moment, ε 0 (t i+1 / 2 , t0) is the concrete from loading age t0 to age t i+1 / 2 The free strain within a time period is the sum of the autogenous volumetric strain and the temperature strain of concrete.

[0069] Using formulas (9) and (10), we can obtain the constraint degree γ R The constraint stress value of the concrete is obtained based on the real and effective concrete elastic modulus that has been coupled with the concrete temperature history and stress history. It truly reflects the constraint stress of large-volume concrete; at the same time, it eliminates the need to measure the concrete creep coefficient in advance and simplifies the engineering simulation analysis and calculation process.

[0070] The beneficial effect of the present invention is that the present invention couples factors such as temperature, constraint and micro cracks generated by long-term load into the measured elastic modulus at one time, so that the concrete constraint stress obtained thereby truly couples the actual temperature and stress history of the concrete. The present invention is based on truly and accurately carrying out effective elastic modulus tests of concrete under actual environmental changes and developments, obtains the constraint stress of concrete under real conditions, and provides a basis with extremely high reference value for computer simulation analysis and calculation of actual projects and formulation of various construction plans.

[0071] Finally, it should be noted that the above-described embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some or all of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for obtaining concrete restraint stress, characterized in that: Based on obtaining the real and effective concrete elastic modulus, the concrete restraint stress is calculated; The concrete elastic modulus is obtained by simulating the real temperature history and stress history of large-volume concrete; The method for obtaining the elastic modulus of concrete is: 1) Pour the mixed concrete into the concrete temperature stress testing machine, set the temperature history and stress history of the concrete temperature stress testing machine, remove the positioning concrete specimen after the concrete is finally set, and paste the strain gauge for measuring the deformation of the concrete on the concrete specimen to start the test; The temperature history is directly derived from the internal temperature of the concrete measured after the mass concrete is poured; According to the restraint state of different areas of the mass concrete, the restraint degree γ of the concrete temperature stress testing machine is set. R ; 2) Observe the stress condition of the concrete specimen, and start measuring the elastic strain of the concrete specimen when the concrete specimen changes from a compressive state to a tensile state; 3.1) From the time the concrete begins to be tensile, 6 hours later, t1, the restraint system is turned off; 3.2) Unload the tensile stress σ1 of the concrete specimen to 0.1MPa, stabilize it for 60 seconds, and record the strain ε 0.1 , reload to σ1, stabilize for 60 seconds, and record the strain ε1; 3.3) Repeat the above step 3.2) twice; 3.4) Calculate the elastic modulus of concrete at the current age E(t1), ε 0.1 and ε1 are the data recorded in the last loading process; 4) After clearing the strain data, restart the constraint system and continue the temperature stress test; 5) Repeat steps 3.1)-4) and use formula (2) to calculate the effective elastic modulus E(t i ), i = 2, 3, 4, ..., until the concrete cracks; 6) Obtain the effective elastic modulus E(t ei ,γ R ); Get the effective age of concrete after standardizing the temperature factor t e : In the formula, E h is the activation energy, which is taken as 33500 J / mol, R is the ideal gas constant, which is taken as 8.31 J / mol / K, T0 is the reference temperature, which is taken as 20°C; T is the actual temperature of the concrete; The effective age of concrete after standardizing the temperature factor obtained by formula (3) is t ei Replace t in formula (2) i , and obtain the real effective elastic modulus E(t ei ,γ R ): The calculation method of the concrete restraint stress is: Where: σ(t i+1 / 2 ) is the concrete at age t i+1 / 2 Stress at time, (Δσ) j is the stress increment of concrete in the jth time period, γ R is the degree of constraint, E(t ei ,γ R ) is the concrete confinement degree γ R and effective age t ei The elastic modulus under the condition, ε(t i+1 / 2 ) is the concrete at age t i+1 / 2 The actual strain at the moment, ε 0 (t i+1 / 2 ,t0) is the concrete load from age t0 to age t i+1 / 2 The free strain within a time period is the sum of the autogenous volumetric strain and the temperature strain of concrete.

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