Concrete stress monitoring method based on wet-force coupling effect and capillary water theory

Abstract

The invention discloses a concrete stress monitoring method based on a wet-force coupling effect and a capillary water theory, and the method comprises the steps: measuring the capillary pressure change of pores in concrete through a tensiometer, and calculating the concrete stress according to the wet-force coupling effect and the capillary water theory. The invention relates to the technical field of concrete materials. According to the concrete stress monitoring method based on the wet-force coupling effect and the capillary water theory, real-time monitoring of a structure can be achieved, the cost is low, compared with common strain measurement, the novel stress monitoring method is provided, and the method is based on an early-age concrete wet-force coupling model and the capillary water theory and has the advantages of being high in practicability and easy to popularize. The stress of the concrete structure is monitored by measuring the capillary pressure change of pores in the concrete, and the real-time safety of the structure is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of concrete materials, in particular to a concrete stress monitoring method based on moisture-mechanical coupling effect and capillary water theory. Background Art

[0002] The problem of early concrete cracking is widely present in various buildings and infrastructures. These cracks will greatly accelerate the transmission of moisture and harmful molecules, thereby accelerating the carbonization of the concrete structure and the corrosion of the internal steel bars, resulting in a significant reduction in the durability of the concrete structure. According to research, more than 80% of concrete cracks are non-structural stress cracks, which are mostly caused by temperature deformation and shrinkage deformation. When the volume deformation is constrained, there will be constrained stress. When this stress is greater than the tensile strength of the early-age concrete, there will be a risk of cracking. Therefore, it is very important to monitor the stress of the concrete structure.

[0003] Most existing stress monitoring technologies use pre-buried strain gauges to measure the strain of concrete and then convert it into stress. This monitoring method generally has poor accuracy because its essence is to measure strain, and the conversion calculation from strain to stress needs to consider the influence of many factors, such as the elastic modulus, stress relaxation, shrinkage creep, etc. of early-age concrete, which will affect the accuracy of the results. A new stress monitoring method is proposed here. From the perspective of moisture-mechanical coupling effect, combined with capillary water theory, the capillary pressure of the internal pores of concrete is measured to monitor the structural stress.

[0004] The moisture-mechanical coupling effect of early-age concrete usually refers to the uneven volume shrinkage produced by the drying process of concrete, which in turn generates constraint stress. In fact, the humidity field and stress field of concrete are not unidirectionally coupled, and stress also has a significant impact on the humidity field. Due to the porous material properties of concrete, when subjected to stress, it will produce sponge-like characteristics, that is, the redistribution of the internal humidity field, which is specifically manifested as: compressive stress leads to an increase in relative humidity, and tensile stress leads to a decrease in relative humidity. This phenomenon can be explained by the capillary water theory: when the internal pores of concrete are under pressure, the adsorbed water between adjacent cementitious material particles will be squeezed, resulting in the breakdown of the water balance, and the adsorbed water will transfer to the capillary water, which will increase the radius of the meniscus of the capillary water, thereby reducing the capillary pressure and increasing the relative humidity. Similarly, when concrete is subjected to tension, the capillary water will transfer to the adsorbed water, resulting in a decrease in the radius of the meniscus of the capillary water, an increase in the capillary pressure, and a decrease in the relative humidity of the pores. For details, see the attached. Figure 1 Therefore, there is a certain relationship between the stress of concrete and relative humidity and capillary pressure. As long as this relationship is determined, the stress of concrete can be deduced by measuring the change in relative humidity.

[0005] In reality, due to this phenomenon, the change in relative humidity is very small, usually within 2%, and the measurement effect of traditional humidity sensors is not ideal. Considering that the change in capillary pressure is more sensitive than relative humidity, usually reaching tens to hundreds of Kpa, a tensiometer is used to measure the capillary pressure of concrete, and then the relative humidity is linked to the capillary pressure through the Kelvin and Laplace equations, so that the stress on the concrete can be deduced from the change in capillary pressure. Summary of the invention

[0006] 1. Technical issues to be solved

[0007] In view of the shortcomings of the prior art, the present invention provides a concrete stress monitoring method based on the moisture-force coupling effect and the capillary water theory. The capillary pressure change of the pores inside the concrete is measured by a tensiometer, and the concrete stress is calculated according to the moisture-force coupling effect and the capillary water theory. This method can realize real-time monitoring of the structure at a low cost.

[0008] (II) Technical solution

[0009] To achieve the above objectives, the present invention is implemented through the following technical solutions: A concrete stress monitoring method based on the moisture-mechanical coupling effect and capillary water theory specifically comprises the following steps:

[0010] S1. Select several monitoring points on the concrete structure and drill holes. Bury a tensiometer at a certain depth from the surface to measure the capillary pressure of the pores inside the concrete in real time.

[0011] S2. Calibrate the original capillary pressure of concrete on site, i.e. bury a tensiometer at the same depth as the monitoring position to obtain the original capillary pressure of concrete at that depth;

[0012] S3, comparing the real-time measured capillary pressure with the original capillary pressure curve to obtain the stress influence difference;

[0013] S4. The stress that causes this difference is calculated using the wet-mechanical coupling model and capillary water theory.

[0014] Preferably, a reasonable monitoring position is selected in step S1. To ensure measurement accuracy and protect the sensor from damage, moisture-proof and safety protection measures should be considered when burying the tension meter.

[0015] Preferably, in step S2, it is considered that the development trends of concrete capillary pressure in different convection environments and different depths from the surface are different. Therefore, it is necessary to bury a tensiometer at the same depth from the surface according to the concrete drying environment on site to measure the original capillary pressure at the depth from the surface.

[0016] Preferably, in step S3, the measured capillary pressure is compared with the original capillary pressure. If the measured value is larger, it means that the stress at the measuring point is tensile stress. If the measured value is smaller, it means that the stress at the measuring point is compressive stress.

[0017] Preferably, in step S4, a stress calculation formula needs to be derived based on the coupling model and capillary theory, and the specific derivation process is as follows:

[0018] A1. Measure the capillary pressure P of the concrete internal pores in real time at the specified measuring point in step S1. c2 , combined with the original capillary pressure P in step 2 c2 , and obtain the capillary pressure difference (P c2 -P c1 );

[0019] A2. According to the Lapace equation, the relationship between capillary pressure and meniscus radius can be determined:

[0020]

[0021] Where: P c - capillary pressure in the pores of concrete;

[0022] γ-surface tension of the liquid in the capillary;

[0023] θ - solid-liquid contact angle of the pores inside concrete, usually 0 for cement-based materials;

[0024] r - radius of the meniscus of the liquid in the capillary;

[0025] The capillary pressure P is obtained from c Relationship with relative humidity RH:

[0026]

[0027] Then the capillary pressure difference (P c1 -P c2 ) is rewritten as:

[0028]

[0029] A3. Volumetric strain Δε caused by relative humidity change VH for:

[0030] Δε VH =(AP c S+P c2 ΔS)(1 / K-1 / K s );

[0031] Where: Δε VH- Volumetric strain due to changes in relative humidity;

[0032] ΔP c - Capillary pressure change caused by relative humidity change, i.e. (P c2 -P c1 );

[0033] S-concrete saturation;

[0034] ΔS - change in concrete saturation, obtained from the adsorption isotherm as a function of capillary pressure;

[0035] K-bulk modulus;

[0036] K s - Bulk modulus of solid materials;

[0037] get:

[0038]

[0039] Therefore, as long as the change in capillary pressure is known, the difference in volume strain can be calculated, and then the magnitude of the stress can be deduced, that is:

[0040]

[0041] Preferably, according to the pore water redistribution theory, in step A3, when the stress increment manifests as a dynamic response, the water in the pores does not have time to redistribute at the moment of application, so the relative humidity does not change basically. When the stress increment manifests as a static response, the pore water has sufficient time to redistribute, so the relative humidity change caused by the stress increment can be explained by the difference in volume strain calculated by the dynamic and static elastic moduli.

[0042] Preferably, the Kelvin equation in step A2 gives the relationship between the meniscus radius and the saturated and unsaturated vapor pressures:

[0043]

[0044] Where: P-actual vapor pressure of concrete pores;

[0045] P0-saturated vapor pressure of concrete pores, P / P0 is relative humidity RH;

[0046] M w - molar mass of water;

[0047] R-gas ideal constant;

[0048] T-absolute temperature;

[0049] ρ – density of water.

[0050] Preferably, the volume strain difference Δε caused by the dynamic and static load responses in step A3 is VH for:

[0051]

[0052] Where: Δε VS - Volumetric strain differences caused by dynamic and static loads;

[0053] σ-stress;

[0054] v - Poisson's ratio;

[0055] E s - static elastic modulus of concrete;

[0056] E d -Dynamic elastic modulus of concrete.

[0057] (III) Beneficial effects

[0058] The present invention provides a concrete stress monitoring method based on the moisture-force coupling effect and the capillary water theory. Compared with the prior art, the method has the following beneficial effects: the concrete stress monitoring method based on the moisture-force coupling effect and the capillary water theory measures the capillary pressure change of the internal pores of the concrete through a tensiometer, and calculates the concrete stress according to the moisture-force coupling effect and the capillary water theory. This method can realize real-time monitoring of the structure at a low cost. Compared with the common strain measurement, it provides a new stress monitoring method. This method is based on the early-age concrete moisture-force coupling model and the capillary water theory, and realizes the monitoring of the stress of the concrete structure by measuring the capillary pressure change of the internal pores of the concrete, thereby ensuring the real-time safety of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a schematic diagram of pore water redistribution in the present invention;

[0060] Figure 2 This is a schematic diagram of burying the tensiometer of the monitoring object of the present invention;

[0061] Figure 3 This is a schematic diagram of the buried tensiometer that is the calibration object of the present invention. DETAILED DESCRIPTION

[0062] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0063] See also Figure 1-3 The embodiment of the present invention provides two technical solutions: a concrete stress monitoring method based on the moisture-mechanical coupling effect and the capillary water theory, which specifically includes the following embodiments:

[0064] Embodiment 1: A concrete stress monitoring method based on the moisture-mechanical coupling effect and capillary water theory specifically comprises the following steps:

[0065] S1. Select several monitoring points on the concrete structure and drill holes. Bury a tensiometer at a certain depth from the surface to measure the capillary pressure of the pores inside the concrete in real time. See the attached Figure 2 ;

[0066] S2. Calibrate the original capillary pressure of concrete on site, that is, bury the tensiometer at the same depth as the monitoring position to obtain the original capillary pressure of concrete at that depth. Figure 3 ;

[0067] S3, comparing the real-time measured capillary pressure with the original capillary pressure curve to obtain the stress influence difference;

[0068] S4. The stress that causes this difference is calculated using the wet-mechanical coupling model and capillary water theory.

[0069] In the embodiment of the present invention, a reasonable monitoring position is selected in step S1. To ensure the measurement accuracy and protect the sensor from damage, moisture-proof and safety protection measures should be considered when burying the tension meter.

[0070] In the embodiment of the present invention, in step S2, it is taken into account that the development trends of concrete capillary pressure in different convection environments and at different depths from the surface are different. Therefore, it is necessary to bury a tensiometer at the same depth from the surface according to the concrete drying environment on site to measure the original capillary pressure at the depth from the surface.

[0071] In the embodiment of the present invention, in step S3, the measured capillary pressure is compared with the original capillary pressure. If the measured value is larger, it means that the stress at the measuring point is tensile stress. If the measured value is smaller, it means that the stress at the measuring point is compressive stress.

[0072] Embodiment 2: The difference between the embodiment of the present invention and the embodiment 1 is that: in step S4, a stress calculation formula needs to be derived based on the coupling model and capillary theory. The specific derivation process is as follows:

[0073] A1. Measure the capillary pressure P of the concrete internal pores in real time at the specified measuring point in step S1. c2 , combined with the original capillary pressure P in step 2 c2 , and obtain the capillary pressure difference (P c2 -P c1 );

[0074] A2. According to the Laplace equation, the relationship between capillary pressure and meniscus radius can be determined:

[0075]

[0076] Where: P c - capillary pressure in the pores of concrete;

[0077] γ-surface tension of the liquid in the capillary;

[0078] θ - solid-liquid contact angle of the pores inside concrete, usually 0 for cement-based materials;

[0079] r - radius of the meniscus of the liquid in the capillary;

[0080] The capillary pressure P is obtained from c Relationship with relative humidity RH:

[0081]

[0082] Then the capillary pressure difference (P c1 -P c2 ) is rewritten as:

[0083]

[0084] A3. Volumetric strain Δε caused by relative humidity change VH for:

[0085] Δε VH =(ΔP c S+P c2 ΔS)(1 / K-1 / K s );

[0086] Where: Δε VH - Volumetric strain due to changes in relative humidity;

[0087] ΔP c - Capillary pressure change caused by relative humidity change, i.e. (P c2 -P c1 );

[0088] S-concrete saturation;

[0089] ΔS - change in concrete saturation, obtained from the adsorption isotherm as a function of capillary pressure;

[0090] K-bulk modulus;

[0091] K s - Bulk modulus of solid materials;

[0092] get:

[0093]

[0094] Therefore, as long as the change in capillary pressure is known, the difference in volume strain can be calculated, and then the magnitude of the stress can be deduced, that is:

[0095]

[0096] In the embodiment of the present invention, according to the pore water redistribution theory in step A3, when the stress increment manifests as a dynamic response, the water in the pores does not have time to redistribute at the moment of application, so the relative humidity does not change basically. When the stress increment manifests as a static response, the pore water has sufficient time to redistribute. Therefore, the relative humidity change caused by the stress increment can be explained by the difference in volume strain calculated by the dynamic and static elastic moduli.

[0097] In the embodiment of the present invention, the Kelvin equation in step A2 gives the relationship between the meniscus radius and the saturated and unsaturated vapor pressures:

[0098]

[0099] Where: P-actual vapor pressure of concrete pores;

[0100] P0-saturated vapor pressure of concrete pores, P / P0 is relative humidity RH;

[0101] M w - molar mass of water;

[0102] R-gas ideal constant;

[0103] T-absolute temperature;

[0104] ρ – density of water.

[0105] In the embodiment of the present invention, the volume strain difference Δε caused by the dynamic and static load responses in step A3 is VH for:

[0106]

[0107] Where: Δε VS - Volumetric strain differences caused by dynamic and static loads;

[0108] σ-stress;

[0109] v - Poisson's ratio;

[0110] E s - static elastic modulus of concrete;

[0111] Ed -Dynamic elastic modulus of concrete.

[0112] In summary, the present invention measures the capillary pressure change of the internal pores of concrete by a tensiometer, and calculates the concrete stress according to the wet-force coupling effect and the capillary water theory. This method can realize real-time monitoring of the structure, with low cost. Compared with the common strain measurement, it provides a new stress monitoring method. This method is based on the early-age concrete wet-force coupling model and capillary water theory, and realizes the monitoring of concrete structure stress by measuring the capillary pressure change of the internal pores of concrete, thereby ensuring the real-time safety of the structure.

[0113] Meanwhile, the contents not described in detail in this specification belong to the prior art known to those skilled in the art.

[0114] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0115] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A concrete stress monitoring method based on the moisture-mechanical coupling effect and capillary water theory, characterized in that: The specific steps include: S1. Select several monitoring points on the concrete structure and drill holes. Bury a tensiometer at a certain depth from the surface to measure the capillary pressure of the pores inside the concrete in real time. S2. Calibrate the original capillary pressure of concrete on site, i.e. bury a tensiometer at the same depth as the monitoring position to obtain the original capillary pressure of concrete at that depth; S3, comparing the real-time measured capillary pressure with the original capillary pressure curve to obtain the stress influence difference; S4. The stress that causes this difference is calculated using the wet-mechanical coupling model and capillary water theory.

2. The concrete stress monitoring method based on the moisture-mechanical coupling effect and capillary water theory according to claim 1 is characterized in that: In the step S1, a reasonable monitoring position is selected. To ensure the measurement accuracy and protect the sensor from damage, moisture-proof and safety protection measures should be considered when burying the tension meter.

3. The concrete stress monitoring method based on the moisture-mechanical coupling effect and capillary water theory according to claim 1 is characterized in that: In step S2, it is considered that the development trends of concrete capillary pressure in different convection environments and different depths from the surface are different. Therefore, it is necessary to bury a tensiometer at the same depth from the surface according to the concrete drying environment on site to measure the original capillary pressure at the depth from the surface.

4. The method for monitoring concrete stress based on moisture-mechanical coupling effect and capillary water theory according to claim 1, characterized in that: In step S3, the measured capillary pressure is compared with the original capillary pressure. If the measured value is larger, it means that the stress at the measuring point is tensile stress. If the measured value is smaller, it means that the stress at the measuring point is compressive stress.

5. The method for monitoring concrete stress based on moisture-mechanical coupling effect and capillary water theory according to claim 1, characterized in that: In step S4, a stress calculation formula needs to be derived based on the coupling model and capillary theory. The specific derivation process is as follows: A1. Measure the capillary pressure P of the concrete internal pores in real time at the specified measuring point in step S1. c2 , combined with the original capillary pressure P in step 2 c2 , and obtain the capillary pressure difference (P c2 -P c1 ); A2. According to the Laplace equation, the relationship between capillary pressure and meniscus radius can be determined: Where: P c - capillary pressure in the pores of concrete; γ-surface tension of the liquid in the capillary; θ - solid-liquid contact angle of the pores inside concrete, usually 0 for cement-based materials; r - radius of the meniscus of the liquid in the capillary; The capillary pressure P is obtained from c Relationship with relative humidity RH: Then the capillary pressure difference (P c1 -P c2 ) is rewritten as: A3. Volumetric strain Δε caused by relative humidity change VH for: Δε VH =(ΔP c S+P c2 ΔS)(1 / K-1 / K s ); Where: Δε VH - Volumetric strain due to changes in relative humidity; ΔP c - Capillary pressure change caused by relative humidity change, i.e. (P c2 -P c1 ); S-concrete saturation; ΔS - change in concrete saturation, obtained from the adsorption isotherm as a function of capillary pressure; K-bulk modulus; K s - Bulk modulus of solid materials; get: Therefore, as long as the change in capillary pressure is known, the difference in volume strain can be calculated, and then the magnitude of the stress can be deduced, that is:

6. The method for monitoring concrete stress based on moisture-mechanical coupling effect and capillary water theory according to claim 5, characterized in that: According to the pore water redistribution theory in step A3, when the stress increment manifests as a dynamic response, the water in the pores does not have time to redistribute at the moment of application, so the relative humidity basically does not change. When the stress increment manifests as a static response, the pore water has sufficient time to redistribute. Therefore, the relative humidity change caused by the stress increment can be explained by the difference in volume strain calculated by the dynamic and static elastic moduli.

7. The method for monitoring concrete stress based on moisture-mechanical coupling effect and capillary water theory according to claim 5, characterized in that: The Kelvin equation in step A2 gives the relationship between the meniscus radius and the saturated and unsaturated vapor pressures: Where: P-actual vapor pressure of concrete pores; P0-saturated vapor pressure of concrete pores, P / P0 is relative humidity RH; M w - molar mass of water; R-gas ideal constant; T-absolute temperature; ρ – density of water.

8. The method for monitoring concrete stress based on moisture-mechanical coupling effect and capillary water theory according to claim 5, characterized in that: The volume strain difference Δε caused by the dynamic and static load responses in step A3 VH for: Where: Δε VS - Volumetric strain differences caused by dynamic and static loads; σ-stress; v - Poisson's ratio; E s - static elastic modulus of concrete; E d -Dynamic elastic modulus of concrete.

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