CN109297865B - Method for measuring early-age structural stress of cement concrete pavement slab - Google Patents
Method for measuring early-age structural stress of cement concrete pavement slab Download PDFInfo
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Abstract
The invention belongs to the technical field of cement concrete pavements of highways and urban roads, and relates to a method for measuring early-age structural stress of a cement concrete pavement slab, which comprises the following steps of 1) predicting the strain acquisition initial time of the cement concrete pavement slab; 2) calculating the elastic strain of the cement concrete pavement slab by taking the initial strain acquisition time of the cement concrete pavement slab determined in the step 1) as a starting point; 3) and calculating the early-age structural stress of the cement concrete pavement slab in the measured period according to the elastic strain of the cement concrete pavement slab calculated in the step 2). The invention improves the accuracy of actual measurement of strain in two aspects of determining the initial strain acquisition time and extracting the elastic strain, and further provides a method for measuring the early-age structural stress of the cement concrete pavement slab.
Description
Technical Field
The invention belongs to the technical field of cement concrete pavements of highways and urban roads, and relates to a method for measuring early-age structural stress of a cement concrete pavement slab, in particular to a method for measuring the early-age structural stress of the cement concrete pavement slab for the highways and the urban roads.
Background
In the highway engineering monitoring and early warning technology, the calculation of the structural stress sigma of the cement concrete pavement slab mainly depends on the accurate measurement of the strain xi and the modulus E, and along with the gradual improvement of the early modulus measurement technology, the reliability of the actually measured strain becomes a decisive factor for the correctness of the stress calculation.
The cement concrete is a multi-phase and multi-component composite material, and the material gradually develops from a flowing state to a plastic state and a hardened state along with the changes of cement hydration, water loss, temperature and load conditions. In this process, the strain of the concrete is changing all the time, but the structural stresses do not develop until the moment when the concrete begins to harden. Namely: the moment of "zero" stress of the concrete structure exists and this moment is the moment when the strain is zero. Therefore, when the elastic stress of the cement concrete pavement slab structure is calculated by adopting the actually measured strain and the elastic modulus, the initial moment of the measured strain must be matched with the zero-stress moment. Otherwise, the initial time is not properly selected and the stress state may be severely biased. Further, from the viewpoint of stress composition, it can be classified into two types: elastic and inelastic strains, early-age strains such as creep, shrinkage, and the like, are inelastic strains. Since the cement concrete pavement slab has not yet developed strength at the early stage of hydration, the inelastic strain at the early stage does not generate structural stress. Therefore, in the stress measurement of the early-age structure of the cement concrete pavement slab, the elastic strain before the strength of the pavement slab is formed is separated from the measured total strain, otherwise, the deviation of the stress calculation result is caused.
However, at present, the strain of cement concrete road slabs is mainly measured by strain gauges, and is characterized by the increment of strain between the initial moment and the measured moment in the measuring period. In both the resistance strain gauge and the chordal or fiber grating strain gauge, the initial time is difficult to cooperate with the zero-stress time, and a user often uses the strain at the zero clearing time of the measuring instrument as the initial time of strain acquisition and calculates the strain increment at a certain subsequent time by using the time as a starting point, wherein the increment includes the inelastic strain caused by creep, shrinkage factors and the like. The stress is calculated by multiplying the piecewise accumulated strain by the corresponding modulus based on the increment, whereby the calculated stress is essentially the stress increment over the measurement period without distinguishing between elastic and inelastic strain.
Disclosure of Invention
In order to solve the problem of unreliable actual measurement strain in the existing cement concrete pavement slab early-age stress measurement technology in the background technology, the accuracy of the actual measurement strain is improved from two aspects of determining the initial time of strain acquisition and extracting elastic strain, and further, the method for measuring the early-age structural stress of the cement concrete pavement slab is provided.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for measuring early-age structural stress of a cement concrete pavement slab is characterized by comprising the following steps of: the method for measuring the stress of the cement concrete pavement slab early-age structure comprises the following steps:
1) predicting the strain acquisition initial time of the cement concrete pavement slab;
2) calculating the elastic strain of the cement concrete pavement slab by taking the initial strain acquisition time of the cement concrete pavement slab determined in the step 1) as a starting point;
3) and calculating the early-age structural stress of the cement concrete pavement slab in the measured period according to the elastic strain of the cement concrete pavement slab calculated in the step 2).
Preferably, the specific implementation manner of step 1) adopted by the invention is as follows:
1.1) calculating the change of a temperature representative value (capable of representing the temperature of the whole concrete test piece) of the reduced-size concrete test piece after self-molding under the constant temperature and humidity condition along with time;
1.2) drawing a temperature representative value and a time-varying curve of a strain self-test piece after forming on the basis of the temperature representative value of the reduced-size concrete test piece after self-forming under the constant temperature and humidity condition obtained by calculation in the step 1.1);
1.3) determining the zero stress moment of the reduced-size concrete test piece under the constant temperature and humidity condition and the temperature representative value of the test piece corresponding to the moment according to the characteristics of the two curves obtained by drawing in the step 1.2), wherein the temperature representative value of the test piece corresponding to the moment is the curing temperature (the temperature representative value of the concrete test piece when the zero stress moment is reached under the constant temperature and humidity condition);
1.4) constructing a relation model between the curing temperature and the zero stress moment of the reduced concrete test piece under the constant temperature and humidity condition;
1.5) calculating the equivalent curing temperature (the representative value of the temperature of the concrete test piece when the zero stress moment is reached under the general environmental condition) of the cement concrete pavement slab of the highway and the urban road under the general environmental condition, which is equivalent to the curing temperature under the constant temperature and humidity condition, according to the maturity equivalence principle;
1.6) substituting the equivalent curing temperatures of the cement concrete pavement slabs of the roads and the urban roads obtained in the step 1.5) into the relation model established in the step 1.4) under the general environmental condition to obtain the zero-stress moment of the cement concrete pavement slabs under the general environmental condition, namely the initial moment of strain acquisition.
Preferably, the specific implementation manner of step 1.1) adopted by the invention is as follows:
1.1.1) carrying out a reduced scale test by adopting the raw materials and the mixing ratio which are the same as those of a cement concrete slab to be tested, and measuring the temperature change of different layers of a reduced scale concrete test piece along with time under the conditions of constant temperature and constant humidity;
1.1.2) determining the temperature representative value of the reduced-size concrete sample under the constant temperature and humidity condition by adopting a layered summation method on the basis of the temperatures of different layers of the reduced-size concrete sample under the constant temperature and humidity condition, which are measured in the step 1.1.1):
wherein:
Tfis a scale mixingTemperature representative value of the concrete specimen, deg.C;
Tiis the measured temperature at point i, deg.C;
hiis the height, cm, from the test point i to the upper surface of the test piece;
h is the thickness of the reduced-scale concrete sample;
n is the total number of layers of the reduced-scale concrete test piece, i is more than or equal to 1 and less than or equal to N, and i is a natural number;
hi+1is the height, cm, from the test piece upper surface at the test point i + 1;
Ti+1is the measured temperature at point i +1, deg.C.
Preferably, the specific implementation manner of step 1.2) adopted by the invention is as follows:
1.2.1) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the strain change of the corresponding layer;
1.2.2) drawing a change curve of the strain and the temperature representative value along with time after the reduced scale test piece is molded on the basis of the temperature representative value obtained in the step 1.1.2) and the strain obtained in the step 1.2.1).
Preferably, the specific implementation of step 1.3) adopted by the invention is as follows:
1.3.1) determining transition time of temperature representative values and strain changes from homodromous to heterodromous on the two curves obtained in the step 1.2) based on Glisic hypothesis, wherein the time is zero-stress time of the reduced-size concrete test piece under the constant temperature and humidity condition;
1.3.2) the temperature representative value corresponding to the transition moment is the curing temperature.
Preferably, the specific implementation of step 1.4) adopted by the invention is as follows: and changing the temperature and humidity conditions, carrying out parallel tests, and establishing a relation model between the curing temperature of the reduced-scale concrete test piece and the zero stress moment under the constant temperature and humidity conditions.
Preferably, the specific implementation of step 1.5) employed in the present invention is:
1.5.1) calculating the maturity of a reduced-scale concrete test piece from the beginning of molding to the zero-stress moment under the constant temperature and humidity condition by adopting a strip division method; the maturity is the sum of the products of the concrete curing time and the temperature representative value;
1.5.2) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the change of the environmental temperature of the cement concrete pavement slab of the highway and the urban road along with the time under the common environmental condition; drawing a change curve of the ambient temperature along with time;
1.5.3) calculating the equivalent curing temperature of the cement concrete pavement slab of the highway and the urban road under the general environment according to the following formula based on the principle of equivalent maturity:
wherein:
Tf,nthe equivalent curing temperature is DEG C of cement concrete pavement slabs of highways and urban roads in general environment;
Tf,ais the ambient temperature, deg.C;
eta is a correction coefficient, said
Tf,20The temperature representative value of the reduced-scale concrete test piece is DEG C under the condition of constant humidity at 20 ℃;
t is the zero stress moment of the reduced-scale concrete specimen under the constant humidity condition of 20 ℃.
Preferably, the specific implementation manner of step 2) adopted by the invention is as follows:
2.1) installing a strain gauge: preparing a first embedded concrete strain gauge, a second embedded concrete strain gauge, a PVC pipe 1 section and a plastic film in the same production batch; the length of the PVC pipe is 1.3-1.6 times of the axial length of the strain gauge; the diameter of the PVC pipe is 1.3-1.6 times of that of the flange plate of the strain gauge; the second concrete strain meter is arranged in the center of the PVC pipe, the periphery of the PVC pipe is densely filled with concrete to be measured, a data line of the strain meter is led out from one end of the PVC pipe, then the two ends of the PVC pipe are wrapped and sealed by a plastic film, and the moisture and heat transfer between the concrete in the pipe and the outside are blocked; pouring a cement concrete pavement slab, and respectively installing a first concrete strain gauge and a second concrete strain gauge arranged in a PVC pipe at preset positions, wherein the first concrete strain gauge and the second concrete strain gauge are positioned on the same section and at the same depth, and the installation angles, the environments and the boundary conditions of the first concrete strain gauge and the second concrete strain gauge are the same;
2.2) acquisition and extraction of strain: after the cement concrete pavement slab is poured and the strainometer is installed, recording the reading xi of the second concrete strainometer2,oAnd monitoring for changes therein; recording the reading xi of the second concrete strain gauge when the initial moment of strain acquisition of the cement concrete pavement slab estimated in the step 1) is reached2,pAnd recording the reading xi of the first concrete strain gauge1,oMonitoring the change of the first concrete strain gauge; when the moment of the required test is reached, the reading xi of the first concrete strain gauge is recorded1,p(ii) a Calculating the elastic strain xi of the cement concrete pavement slab in the measured period according to the following formulae：
ξe＝ξ1,p-ξ1,o-(ξ2,p-ξ2,o)。
Preferably, the specific implementation manner of step 3) adopted by the invention is as follows:
calculating the early-age structural stress of the cement concrete pavement slab according to the following formula:
σe＝ξe·E
wherein:
σethe early-age structural stress of the cement concrete pavement slab in the measured period;
ξeis the elastic strain of the cement concrete pavement slab in the measured period;
and E is the modulus of elasticity of the cement concrete at the moment of measurement.
The invention has the following advantages:
the method for measuring the early-age elastic strain of the cement concrete pavement slab by using the strain gauge provides a basis for measuring the early-age elastic strain of the cement concrete pavement slab and other concrete structures, and solves the problems that the elastic strain and the total strain are mixed together and the elastic strain and the inelastic strain cannot be separated in the prior art; the method expands the method for determining the zero stress moment of the concrete under the constant temperature and humidity condition to the common environmental condition by a maturity equivalent method, and provides a basis for selecting the initial moment of early-age strain measurement of the cement concrete pavement slab. The method eliminates inelastic strain, calculates structural stress on the basis of elastic strain, and can be used for measuring the structural stress of cement concrete pavement slabs and other concrete structures in the early stage. The method for measuring the early-age structural stress of the cement concrete pavement slab by using the strain gauge provides a basis for measuring the early-age structural stress of the cement concrete pavement slab and other concrete structures, clarifies the error region of bringing inelastic strain into the structural stress range in the past and solves the problem of larger measurement result of the early-age structural stress of the cement concrete pavement slab in the past.
Drawings
FIG. 1 is a flow chart of the early-age structural stress of a cement concrete pavement slab according to the present invention;
FIG. 2 is a schematic view of the mounting of a strain gauge (in cm) to which an embodiment of the invention is applied;
FIG. 3 is a graph showing the measurement of strain and temperature (unit: cm) by a reduced-scale test using an example of the present invention;
FIG. 4 is a graph of the characteristic temperature versus the zero stress time under constant temperature and humidity conditions using an embodiment of the present invention.
Detailed Description
The invention provides a method for measuring early-age structural stress of a cement concrete pavement slab, which comprises the following steps of:
1) the method for predicting the strain acquisition initial moment of the cement concrete pavement slab specifically comprises the following steps:
1.1) calculating the change of a temperature representative value (capable of representing the temperature of the whole concrete sample) of the reduced-size concrete sample after self-forming along with time under the conditions of constant temperature and constant humidity:
1.1.1) carrying out a reduced scale test by adopting the raw materials and the mixing ratio which are the same as those of a cement concrete slab to be tested, and measuring the temperature change of different layers of a reduced scale concrete test piece along with time under the conditions of constant temperature and constant humidity;
1.1.2) determining the temperature representative value of the reduced-size concrete sample under the constant temperature and humidity condition by adopting a layered summation method on the basis of the temperatures of different layers of the reduced-size concrete sample under the constant temperature and humidity condition, which are measured in the step 1.1.1):
wherein:
Tfthe temperature representative value of the reduced-scale concrete specimen is DEG C;
Tiis the measured temperature at point i, deg.C;
hiis the height, cm, from the test point i to the upper surface of the test piece;
h is the thickness of the reduced-scale concrete sample;
n is the total number of layers of the reduced-scale concrete test piece, i is more than or equal to 1 and less than or equal to N, and i is a natural number;
hi+1is the height, cm, from the test piece upper surface at the test point i + 1;
Ti+1is the measured temperature at point i +1, deg.C.
1.2) drawing a temperature representative value and a time-varying curve of a strain self-test piece after the self-molding of the temperature representative value on the basis of the temperature representative value of the scaled concrete test piece after the self-molding under the constant temperature and humidity condition obtained by the calculation in the step 1.1):
1.2.1) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the strain change of the corresponding layer;
1.2.2) drawing a change curve of the strain and the temperature representative value along with time after the reduced scale test piece is molded on the basis of the temperature representative value obtained in the step 1.1.2) and the strain obtained in the step 1.2.1).
1.3) determining the zero stress time of the reduced-size concrete test piece under the constant temperature and humidity condition and the temperature representative value of the test piece corresponding to the time according to the characteristics of the two curves obtained by drawing in the step 1.2), wherein the temperature representative value of the test piece corresponding to the time is the curing temperature (the temperature representative value of the concrete test piece when the zero stress time is reached under the constant temperature and humidity condition):
1.3.1) determining transition time of temperature representative values and strain changes from homodromous to heterodromous on the two curves obtained in the step 1.2) based on Glisic hypothesis, wherein the time is zero-stress time of the reduced-size concrete test piece under the constant temperature and humidity condition;
1.3.2) the temperature representative value corresponding to the transition moment is the curing temperature.
1.4) constructing a relation model between the curing temperature and the zero stress moment of the reduced concrete test piece under the constant temperature and humidity condition; and changing the temperature and humidity conditions, carrying out parallel tests, and establishing a relation model between the curing temperature of the reduced-scale concrete test piece and the zero stress moment under the constant temperature and humidity conditions.
1.5) calculating the equivalent curing temperature of the cement concrete pavement slab of the highway and the urban road under the common environmental condition equivalent to the curing temperature under the constant temperature and humidity condition according to the maturity equivalent principle (the representative value of the temperature of the concrete test piece when the zero stress moment is reached under the common environmental condition):
1.5.1) calculating the maturity of a reduced-scale concrete test piece from the beginning of molding to the zero-stress moment under the constant temperature and humidity condition by adopting a strip division method; the maturity is the sum of the products of the concrete curing time and the temperature representative value;
1.5.2) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the change of the environmental temperature of the cement concrete pavement slab of the highway and the urban road along with the time under the common environmental condition; drawing a change curve of the ambient temperature along with time;
1.5.3) calculating the equivalent curing temperature of the cement concrete pavement slab of the highway and the urban road under the general environment according to the following formula based on the principle of equivalent maturity:
wherein:
Tf,nthe equivalent curing temperature is DEG C of cement concrete pavement slabs of highways and urban roads in general environment;
Tf,ais the ambient temperature, deg.C;
eta is a correction coefficient for the correction coefficient,
Tf,20the temperature representative value of the reduced-scale concrete test piece is DEG C under the condition of constant humidity at 20 ℃;
t is the zero stress moment of the reduced-scale concrete specimen under the constant humidity condition of 20 ℃.
1.6) substituting the equivalent curing temperatures of the cement concrete pavement slabs of the roads and the urban roads obtained in the step 1.5) into the relation model established in the step 1.4) under the general environmental condition to obtain the zero-stress moment of the cement concrete pavement slabs under the general environmental condition, namely the initial moment of strain acquisition.
2) Calculating the elastic strain of the cement concrete pavement slab by taking the initial strain acquisition time of the cement concrete pavement slab determined in the step 1) as a starting point;
2.1) installing a strain gauge: preparing a first embedded concrete strain gauge, a second embedded concrete strain gauge, a PVC pipe 1 section and a plastic film in the same production batch; the length of the PVC pipe is 1.3-1.6 times of the axial length of the strain gauge; the diameter of the PVC pipe is 1.3-1.6 times of the diameter of the flange plate of the strain gauge; the second concrete strain meter is arranged in the center of the PVC pipe, the periphery of the PVC pipe is densely filled with concrete to be measured, a data line of the strain meter is led out from one end of the PVC pipe, then the two ends of the PVC pipe are wrapped and sealed by a plastic film, and the moisture and heat transfer between the concrete in the pipe and the outside are blocked; pouring a cement concrete pavement slab, and respectively installing a first concrete strain gauge and a second concrete strain gauge arranged in a PVC pipe at preset positions, wherein the first concrete strain gauge and the second concrete strain gauge are positioned on the same section and at the same depth, and the installation angles, the environments and the boundary conditions of the first concrete strain gauge and the second concrete strain gauge are the same;
2.2) acquisition and extraction of strain: recording the second concrete strain gauge after the cement concrete pavement slab is poured and the strain gauges are installedReading xi2,oAnd monitoring for changes therein; recording the reading xi of the second concrete strain gauge when the initial moment of strain acquisition of the cement concrete pavement slab estimated in the step 1) is reached2,pAnd recording the reading xi of the first concrete strain gauge1,oMonitoring the change of the first concrete strain gauge; when the moment of the required test is reached, the reading xi of the first concrete strain gauge is recorded1,p(ii) a Calculating the elastic strain xi of the cement concrete pavement slab in the measured period according to the following formulae：
ξe＝ξ1,p-ξ1,o-(ξ2,p-ξ2,o)。
3) Calculating the early-age structural stress of the cement concrete pavement slab in the measured period according to the elastic strain of the cement concrete pavement slab calculated in the step 2):
calculating the early-age structural stress of the cement concrete pavement slab according to the following formula:
σe＝ξe·E
wherein:
σethe early-age structural stress of the cement concrete pavement slab in the measured period;
ξeis the elastic strain of the cement concrete pavement slab in the measured period;
and E is the modulus of elasticity of the cement concrete at the moment of measurement.
The method for measuring early-age structural stress of a cement concrete pavement slab provided by the invention is described in detail by combining specific embodiments as follows:
referring to fig. 1, the present invention provides a method for measuring early-age structural stress of a cement concrete pavement slab, which measures the structural stress of concrete 10 hours after the cement concrete pavement slab is poured, and the method comprises the following steps:
(1) determining the initial moment of strain acquisition of the concrete to be detected to be 256min, and specifically calculating the following steps:
calculating the initial moment of strain acquisition:
1.1) calculation of concrete curing temperature under constant temperature and humidity conditions
1.1.1) adopting the mixing ratio shown in the table 1, pouring 6 groups of concrete according to the size and the position shown in the figure 3, and respectively embedding the strain gauge and the temperature sensor. After pouring, the mixture is respectively placed under the conditions of constant temperature (10 ℃, 20 ℃, 30 ℃, 40 ℃ and 50 ℃) and constant humidity (95%) and is kept outdoors for curing. And recording the change of strain and temperature along with time by taking 5min as a counting interval.
TABLE 1 compounding ratio
1.1.2) the representative temperature values of the scaled concrete samples under the constant temperature and humidity conditions were calculated according to equation 1, where N is 5, and the representative temperature values at a certain time are listed in table 2:
in the formula:
Tfthe temperature representative value of the reduced-scale concrete specimen is DEG C;
Timeasured temperature at point i is measured in degrees centigrade;
hithe height from the measuring point i to the upper surface of the test piece is cm;
h is the thickness of the test piece.
TABLE 2 temperature representative value of concrete at a certain time under constant temperature and humidity conditions
1.2) estimation of concrete zero stress moment under constant temperature and humidity conditions
1.2.1) on the basis of the step 1.1), drawing a change curve of strain and temperature representative value of the reduced-size concrete specimen along with time under the conditions of constant temperature and constant humidity.
1.2.2) based on Glisic hypothesis, determining a temperature representative value on the curve and a transition time of the strain from the homodromous change to the heterodromous change, wherein the time is a zero-stress time, and the temperature representative value corresponding to the time is a curing temperature.
The calculated curing time at 10 ℃, 20 ℃, 30 ℃, 40 ℃ and 50 ℃ when the concrete reaches the zero stress state, namely the zero stress time is shown in table 3:
TABLE 3 zero stress moment of concrete under constant temperature and humidity conditions
1.2.3) obtaining a relation model of the zero stress moment and the curing temperature of the concrete under the conditions of constant temperature and constant humidity by adopting Matlab through fitting, wherein the prediction formula of the zero stress moment is as follows as shown in FIG. 4:
T＝133.7Tf，t -0.9(R2＝0.95)
in the formula: t is the zero stress moment; t isf，tIs the curing temperature.
1.3) calculating the equivalent curing temperature of the cement concrete pavement slab in the general environment:
1.3.1) according to 1.2.2), the zero stress time of the concrete under the conditions of 20 ℃ and 95 percent is 6.9 h;
1.3.2) plotting the ambient temperature T under typical environmental conditionsf,aCalculating the maturity of the cement concrete slab in the interval of 0 h-6.9 h along with the change curve of time;
1.3.3) calculating the equivalent curing temperature of the concrete under the general environment according to the formula 2) based on the principle of maturity equivalence.
Correction factor
Wherein:
Then
1.4) estimating the zero stress moment of the concrete under the general environment: will Tf，nSubstituting the relation model established in the step 2) at 45.9 ℃, and obtaining the zero-stress time of 256min under the general environment.
2) According to the position shown in figure 2, a concrete panel with the thickness of 26cm is formed, 1 vibration wire type strain gauge 1 and 1 vibration wire type strain gauge 2 are respectively installed at the position with the thickness of 13cm, and a PVC pipe is 40mm in diameter and 120mm in length; pouring concrete, connecting with a strain acquisition instrument, setting the acquisition frequency to be 2min, and recording the reading xi of the strain gauge 22,o。
When the concrete pouring is finished for 256min, the reading xi of the strain gauge 2 is recorded2,pRecording the reading xi of the strain gauge 11,o；
When the cement concrete pavement slab is poured for 10 hours, the reading xi of the strain gauge 1 is recorded1,p. According to the formula (1), calculating the elastic strain xi in the measured time periode：
ξe＝ξ1,p-ξ1,o-(ξ2,p-ξ2,o)＝218-159-(98-86)＝47
3) Through measurement, when the concrete is poured for 10 hours, the elastic modulus of the concrete is 5480MPa, and the structural stress at the moment is as follows: 5480 × 47/1000000 ═ 0.258 MPa.
Claims (8)
1. A method for measuring early-age structural stress of a cement concrete pavement slab is characterized by comprising the following steps of: the method for measuring the stress of the cement concrete pavement slab early-age structure comprises the following steps:
1) predicting the strain acquisition initial time of the cement concrete pavement slab;
2) calculating the elastic strain of the cement concrete pavement slab by taking the initial strain acquisition time of the cement concrete pavement slab determined in the step 1) as a starting point;
3) calculating the early-age structural stress of the cement concrete pavement slab in the measured period according to the elastic strain of the cement concrete pavement slab calculated in the step 2);
the specific implementation manner of the step 1) is as follows:
1.1) calculating the change of a temperature representative value of a reduced-scale concrete sample after self-forming under the constant temperature and humidity condition along with time;
1.2) drawing a temperature representative value and a time-varying curve of a strain self-test piece after forming on the basis of the temperature representative value of the reduced-size concrete test piece after self-forming under the constant temperature and humidity condition obtained by calculation in the step 1.1);
1.3) determining the zero stress moment of the reduced-size concrete test piece and the temperature representative value of the test piece corresponding to the moment under the constant temperature and humidity condition according to the characteristics of the two curves obtained by drawing in the step 1.2), wherein the temperature representative value of the test piece corresponding to the moment is the curing temperature;
1.4) constructing a relation model between the curing temperature and the zero stress moment of the reduced concrete test piece under the constant temperature and humidity condition;
1.5) calculating the equivalent curing temperature of the cement concrete pavement slab of the highway and the urban road under the common environmental condition equivalent to the curing temperature under the constant temperature and humidity condition according to the maturity equivalent principle;
1.6) substituting the equivalent curing temperatures of the cement concrete pavement slabs of the roads and the urban roads obtained in the step 1.5) into the relation model established in the step 1.4) under the general environmental condition to obtain the zero-stress moment of the cement concrete pavement slabs under the general environmental condition, namely the initial moment of strain acquisition.
2. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 1, wherein the method comprises the following steps: the specific implementation manner of the step 1.1) is as follows:
1.1.1) carrying out a reduced scale test by adopting the raw materials and the mixing ratio which are the same as those of a cement concrete slab to be tested, and measuring the temperature change of different layers of a reduced scale concrete test piece along with time under the conditions of constant temperature and constant humidity;
1.1.2) determining the temperature representative value of the reduced-size concrete sample under the constant temperature and humidity condition by adopting a layered summation method on the basis of the temperatures of different layers of the reduced-size concrete sample under the constant temperature and humidity condition, which are measured in the step 1.1.1):
wherein:
Tfthe temperature representative value of the reduced-scale concrete specimen is DEG C;
Tiis the measured temperature at point i, deg.C;
hiis the height, cm, from the test point i to the upper surface of the test piece;
h is the thickness of the reduced-scale concrete sample;
n is the total number of layers of the reduced-scale concrete test piece, i is more than or equal to 1 and less than or equal to N, and i is a natural number;
hi+1is the height, cm, from the test piece upper surface at the test point i + 1;
Ti+1is the measured temperature at point i +1, deg.C.
3. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 2, wherein the method comprises the following steps: the specific implementation manner of the step 1.2) is as follows:
1.2.1) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the strain change of the corresponding layer;
1.2.2) drawing a change curve of the strain and the temperature representative value along with time after the reduced scale test piece is molded on the basis of the temperature representative value obtained in the step 1.1.2) and the strain obtained in the step 1.2.1).
4. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 3, wherein the method comprises the following steps: the specific implementation manner of the step 1.3) is as follows:
1.3.1) determining transition time of temperature representative values and strain changes from homodromous to heterodromous on the two curves obtained in the step 1.2) based on Glisic hypothesis, wherein the time is zero-stress time of the reduced-size concrete test piece under the constant temperature and humidity condition;
1.3.2) the temperature representative value corresponding to the transition moment is the curing temperature.
5. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 4, wherein the method comprises the following steps: the specific implementation manner of the step 1.4) is as follows: and changing the temperature and humidity conditions, carrying out parallel tests, and establishing a relation model between the curing temperature of the reduced-scale concrete test piece and the zero stress moment under the constant temperature and humidity conditions.
6. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 5, wherein the method comprises the following steps: the specific implementation manner of the step 1.5) is as follows:
1.5.1) calculating the maturity of a reduced-scale concrete test piece from the beginning of molding to the zero-stress moment under the constant temperature and humidity condition by adopting a strip division method; the maturity is the sum of the products of the concrete curing time and the temperature representative value;
1.5.2) measuring the temperature of different layers of the reduced-size concrete test piece under the constant temperature and humidity condition in the step 1.1.1), and simultaneously measuring the change of the ambient temperature of the cement concrete pavement slab of the highway and the urban road along with time under the general environmental condition; drawing a change curve of the ambient temperature along with time;
1.5.3) calculating the equivalent curing temperature of the cement concrete pavement slab of the highway and the urban road under the general environment according to the following formula based on the principle of equivalent maturity:
wherein:
Tf,nthe equivalent curing temperature is DEG C of cement concrete pavement slabs of highways and urban roads in general environment;
Tf,ais the ambient temperature, deg.C;
eta is a correction coefficient, said
Tf,20The temperature representative value of the reduced-scale concrete test piece is DEG C under the condition of constant humidity at 20 ℃;
t is the zero stress moment of the reduced-scale concrete specimen under the constant humidity condition of 20 ℃.
7. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 6, wherein the method comprises the following steps: the specific implementation manner of the step 2) is as follows:
2.1) installing a strain gauge: preparing a first embedded concrete strain gauge, a second embedded concrete strain gauge, a PVC pipe 1 section and a plastic film in the same production batch; the length of the PVC pipe is 1.3-1.6 times of the axial length of the strain gauge; the diameter of the PVC pipe is 1.3-1.6 times of that of the flange plate of the strain gauge; the second concrete strain meter is arranged in the center of the PVC pipe, the periphery of the PVC pipe is densely filled with concrete to be measured, a data line of the strain meter is led out from one end of the PVC pipe, then the two ends of the PVC pipe are wrapped and sealed by a plastic film, and the moisture and heat transfer between the concrete in the pipe and the outside are blocked; pouring a cement concrete pavement slab, and respectively installing a first concrete strain gauge and a second concrete strain gauge arranged in a PVC pipe at preset positions, wherein the first concrete strain gauge and the second concrete strain gauge are positioned on the same section and at the same depth, and the installation angles, the environments and the boundary conditions of the first concrete strain gauge and the second concrete strain gauge are the same;
2.2) acquisition and extraction of strain: after the cement concrete pavement slab is poured and the strainometer is installed, recording the reading xi of the second concrete strainometer2,oAnd monitoring for changes therein; to be treatedWhen the initial moment of strain acquisition of the cement concrete pavement slab estimated in the step 1) is reached, recording the reading xi of the second concrete strain gauge2,pAnd recording the reading xi of the first concrete strain gauge1,oMonitoring the change of the first concrete strain gauge; when the moment of the required test is reached, the reading xi of the first concrete strain gauge is recorded1,p(ii) a Calculating the elastic strain xi of the cement concrete pavement slab in the measured period according to the following formulae：
ξe＝ξ1,p-ξ1,o-(ξ2,p-ξ2,o)。
8. The method for determining early-age structural stress of a cement concrete pavement slab according to claim 7, wherein the method comprises the following steps: the specific implementation manner of the step 3) is as follows:
calculating the early-age structural stress of the cement concrete pavement slab according to the following formula:
σe＝ξe·E
wherein:
σethe early-age structural stress of the cement concrete pavement slab in the measured period;
ξeis the elastic strain of the cement concrete pavement slab in the measured period;
and E is the modulus of elasticity of the cement concrete at the moment of measurement.
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