CN108956325B - Method for measuring bending moment strength of concrete - Google Patents
Method for measuring bending moment strength of concrete Download PDFInfo
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- CN108956325B CN108956325B CN201810731697.8A CN201810731697A CN108956325B CN 108956325 B CN108956325 B CN 108956325B CN 201810731697 A CN201810731697 A CN 201810731697A CN 108956325 B CN108956325 B CN 108956325B
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/20—Investigating strength properties of solid materials by application of mechanical stress by applying steady bending forces
Abstract
The invention discloses a concrete bending-resistant moment strength measuring method which comprises a bending-resistant moment strength testing device, a bending-resistant limit moment calculation formula, a bending-resistant moment strength calculation formula, a bending-resistant moment damage standard and a testing result analysis method. The measured bending moment strength index eliminates the size effect caused by the test method, is closer to the real situation of the bending strength of the concrete, and has good consistency with the tensile strength of the concrete. The invention provides a means for accurately measuring the bending resistance and the bearing capacity of concrete, adopts the bending resistance moment strength as a design parameter, and has the beneficial effect of obviously improving the design reliability and the safety of beams and plate structures.
Description
Technical Field
The invention belongs to the technical field of concrete structure performance testing, and particularly relates to a method for measuring the bending moment strength of concrete.
Background
Bending is the main stress mode of concrete beams and plate structures, and the limit state of the bearing capacity of the bending is used for determining the safety of the structure, so that the accurate measurement of the bending resistance parameters of concrete is very important. In the current structural design, the bending problem is evaluated by using a bending tensile stress intensity index, and the bending tensile stress intensity index obtained by using a tensile strength parameter in the design process cannot reflect the real situation of the bearing capacity of the structure.
The Korean dam and Huangshuanghua put forward a non-zero moment of stress elasticity theory, completely separate bending from stretching, and correct the existing elasticity theory; the Xiaojianhua establishes a strain geometric field theory and a stress-strain relation geometric field theory, so that the independent and accurate analysis and measurement of the bending resistance and the bearing capacity of the concrete structure become possible. The invention starts from a non-zero moment-of-stress theory and a strain geometric field theory, provides a concrete moment-of-stress strength measuring method for controlling damage of the moment-of-stress through different damage mode analysis of concrete, solves the problem of measuring the bending resistance bearing capacity design parameters of concrete beams and plate structures, and provides theoretical and technical support for the design of the concrete beams and the plate structures and the acquisition of the parameters.
Disclosure of Invention
The invention aims to provide a method for measuring the bending resistance moment strength of concrete, which provides a reliable means for obtaining bending resistance parameters for concrete structure design, thereby improving the safety of concrete beams and plate structures.
The invention is realized by the following technical scheme:
a method for measuring the bending moment strength of concrete comprises the following steps:
1) carrying out test design by using a non-zero moment theory, wherein a test piece for controlling damage by bending moment resistance adopts a square-section beam type test piece;
2) loading by adopting a simple supporting beam three-point and two-point mode, wherein the supporting mode is a simple support, one end of the supporting mode is a fixed support, and the other end of the supporting mode is a movable support provided with 2 spherical hinges;
3) adjusting a movable support, measuring the span, placing a test piece on the support, placing a loading device with a pouring surface of the test piece facing right ahead and a side surface facing upwards, aligning the geometric center of the loading device with the geometric center of the test piece, and enabling the support and the loading point to be stably and uniformly contacted with the test piece;
4) selecting a universal testing machine which accords with highway engineering cement and cement concrete test regulations for loading;
5) and after fracture failure, calculating the bending moment strength according to a bending moment strength formula.
The standard size of the square-section beam test piece in the determination method is 150mm multiplied by 550 mm.
The measuring method adopts the three-point and two-point loading of the simply supported beam, the high span ratio is 3.0, and the span is 450 mm.
In the measuring method, one end of a loading device is provided with 1 loading point of a spherical hinge, and the other end of the loading device is provided with 2 loading points of the spherical hinge.
The measuring method of the invention has the loading speed of 0.05 MPa/s-0.08 MPa/s, records the load-deflection curve during loading, and determines the ultimate deflection value according to the load-deflection curve.
The fracture failure mode in the determination method is the small-angle fracture failure of the pure bending section, the fracture is positioned in the pure bending section, and the included angle between the upward expansion of the fracture and the vertical direction is less than 30 degrees.
The bending-resistant moment strength formula of the invention is as follows:
wherein, tauwBending moment strength (MPa); f is a limit load (N); b is the specimen width (mm); h is the specimen height (mm).
The invention has the beneficial effects that:
1. the invention provides a concrete bending moment strength test method, which can be used for measuring the bending moment strength index, eliminating the size effect caused by the test method, having good consistency on different structures and different stress working conditions and conforming to the actual stress condition of a concrete structure.
2. The invention provides a concrete bending moment strength test method, which adopts bending moment strength indexes in concrete beam and plate structure design, can improve the reliability of structure bending design, and is beneficial to the design of new structures and the generation of new findings.
Drawings
FIG. 1 is a graph of loading patterns, internal force distribution, and moment stress distribution for one embodiment of the present invention; wherein a is a loading mode, b is internal force distribution, and c is moment stress distribution.
FIG. 2 is a graph of bending moment versus bending tensile strength for one embodiment of the present invention;
FIG. 3 is a graph of bending moment versus split tensile strength for one embodiment of the present invention;
FIG. 4 is a graph showing the relationship between bending moment strength, bending tensile strength and split tensile strength according to an embodiment of the present invention;
FIG. 5 is a graph showing the relationship between the bending moment strength of the standard test piece and the bending moment strength of the non-standard test piece according to an embodiment of the present invention.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to specific embodiments of the present invention, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
To achieve the above objects and other advantages in accordance with the present invention, there is provided a method for measuring bending moment strength of concrete, comprising the steps of:
step one, designing a bending-resistant moment strength test device by utilizing a non-zero moment theory, and determining the size range and the loading mode of a test piece damaged by the bending-resistant moment.
And 2, preferably selecting a loading mode, adopting a simple supported beam three-division-point double-point loading mode, recording the limit load and the deflection, wherein the high span ratio is 3.0, the span is 450 mm.
And step two, establishing a mechanical analysis method for controlling damage of the bending moment and a bending moment strength test method, and giving a bending moment strength calculation formula.
in the formula, mwThe ultimate bending moment (N/mm) of a pure bending beam; f is the ultimate load (N)) (ii) a L is the distance between the supports, namely the span (mm); b is the specimen width (mm); h is the specimen height (mm).
wherein, tauwBending moment strength (MPa); f is a limit load (N); b is the specimen width (mm); h is the specimen height (mm).
And step three, determining the bending moment strength test step, giving a test result processing method and a credibility analysis method, and evaluating and utilizing the test result.
And 2, preferably, recording a load-deflection curve, and determining a limit deflection value according to a linear section of the load-deflection curve.
And 3, preferably, the fracture damage mode is small-angle fracture damage of the pure bending section, the fracture is positioned in the pure bending section, and the condition that the included angle between the upward expansion of the crack and the vertical direction is less than 30 degrees is qualified.
And 4, preferably, after the bending-resistant moment strength test, measuring the splitting tensile strength by adopting 6 test piece broken blocks, wherein the bending-resistant moment strength close to the splitting tensile strength is qualified.
The invention provides a method for testing the bending resistance, moment stress and strength of concrete. The method can quickly and accurately obtain the strength parameters of the bending-resistant moment of stress of the concrete, and improve the reliability and safety of the design of the concrete beam and the plate structure. The overall flow of one implementation is as follows:
test design, determining the size and loading mode of a test piece damaged by the bending-resistant moment; testing the bending moment strength, determining the testing steps, a calculation method of the bending moment strength and the testing record content; and analyzing the test result, and determining the conformity of the failure mode, the data processing method and the error of the test result.
First, experimental design
Tension, shear, bending and torsion are four typical stress modes of a deformable body structure, the failure of which is controlled by tensile (compression) strength, shear strength, bending and torsion stresses, respectively. The concrete bending resistance test usually adopts a simple supported beam mid-span concentrated loading or simple supported beam three-point loading mode, and adopts the bending tensile stress of the beam bottom to represent the bending resistance of the concrete structure. Because the concentrated loading mode of the span of the simply supported beam is easily influenced by the vertical shear stress, the bending resistance bearing capacity of the concrete is measured by adopting a three-point loading mode of the simply supported beam in a standard bending strength test, and the bending tensile stress of the bottom of the pure bending section beam is adopted to represent the bending tensile strength of the concrete.
According to the non-zero moment elasticity theory, although the beam bottom has bending elongation in the bending test process, the normal stress on the section does not exist, the bending damage of the beam is controlled by the bending moment, and the maximum bending moment of the beam bottom is the real physical quantity causing the bending damage of the concrete trabecula. However, in practical engineering, it is found that although most brittle bending failures conform to the non-zero moment theory, and the failure is controlled by the bending moment, the non-zero moment theory does not conform to the failure mode of large-deflection thin plates and shell structures and deep beam structures with narrow sections, small spans and large heights.
According to the geometric field theory of strain and the geometric field theory of stress-strain constitutive relation, the bending process of the concrete beam really has elongation strain, and the deformation process of the structure is strictly restricted by the volume of the micro-element. When the elongation strain produced by bending exceeds the ultimate elongation strain of the concrete, the concrete can produce tensile failure, and the failure is controlled by the tensile strain. Therefore, the large-deflection thin plate shell structure is reasonable in stress theory, but the bending resistance bearing capacity is not controlled by the tensile stress of the beam bottom, but by the equivalent shear stress generated by the stress moment acting on the beam bottom. There is no physical basis for improving the tensile strength of concrete by bending.
According to the non-zero moment theory and the strain geometric field theory, when the resultant moment generated by the bending moment is not destroyed and the vertical shear stress generated by the concentrated load at the edge of the pure bending section exceeds the shear strength of the concrete, the shear destruction of the concrete occurs. Or when the resultant moment generated by the bending moment is not damaged yet and the transverse shear stress and the equivalent torque generated by the pure bending section for balancing the bending moment exceed the shearing resistance and the bearing capacity of the concrete, the concrete is longitudinally sheared and cracked.
Therefore, the bending-resistant moment strength test method for concrete provided by the invention needs to determine the loading mode, the size of the test piece and the application range.
1. And designing a loading mode. The designed loading mode has the following characteristics: generating a large enough bending moment to control the damage of the concrete test piece by the moment; the influence of the shear stress is small enough to avoid the occurrence of shear failure; the bending elongation generated in the test process is small enough to avoid tensile failure.
The beam test piece three-point loading mode is preferred. The beam type test piece has larger height and can bear larger bending load; a three-point loading mode is adopted, a pure bending section is generated at the midspan section, only transverse shear stress is generated in the bending process, as long as the transverse shear stress is small enough, the bending moment can be balanced, and the influence of the shear stress on the pure bending section is small; the beam type test piece has smaller deflection in the test process, and can avoid tensile damage caused by large-deflection bending.
2. The size of the test piece is preferably selected to be a square section test piece, so that the existing standard section test piece is convenient to form and use. In order to avoid tensile damage caused by large-deflection bending, reasonable span and high span ratio need to be determined; to avoid deep beam shear failure, a reasonable beam height and high span ratio needs to be determined.
According to the bending stress theory, the bending tensile strength, namely the maximum bending tensile stress of the beam bottom, is as follows:
wherein f isfTensile strength against bending (MPa); f is a limit load (N); l is the distance between the supports, namely the span (mm); b is the specimen width (mm); h is the specimen height (mm).
According to the bending moment theory, the bending moment, i.e. the maximum bending moment of the beam bottom, is as follows:
in the formula, mwThe ultimate bending moment (N/mm) of a pure bending beam; the remaining symbols have the same meanings as above.
The ultimate bearing capacity for avoiding large-deflection bending damage of the bending beam is that the damage load controlled by bending stress is less than or equal to the damage load controlled by bending tensile stress, so that the beam is firstly subjected to bending damage under the stress control, and the bending damage stress does not reach the ultimate state, then:
finishing to obtain:
wherein the bending moment mwThe concrete tensile strength is a physical quantity related to the beam height, the unit of the physical quantity is N/mm, the numerical value is equal to 70-100 times of the tensile strength of the concrete, and the numerical value is 90 times; tensile strength against bending ffIs a derived physical quantity in units of MPa or N/mm2According to the existing test results, the relationship between the numerical value and the splitting tensile strength is as follows:
wherein f isfTensile strength against bending (MPa); f. ofspThe tensile strength at split (MPa).
The splitting tensile strength and the axle center tensile strength of the concrete are equal, and the minimum height of a bending moment test piece is as follows:
wherein h is the minimum specimen height (mm); f. ofspThe tensile strength at split (MPa).
For the non-reinforced concrete of C30-C200, when the splitting tensile strength is 3.0-10.0 MPa, the minimum test piece height only needs 18-25 mm, and the damage is completely controlled by the bending moment. For the reinforced concrete, the bending tensile stress of the beam bottom is borne by the steel bar, the ductility and the tensile strength are obviously improved, and the required minimum beam height is increased.
The maximum beam height is controlled by avoiding shear failure of the deep beam. According to the non-zero moment theory, the shear stress generated by the maximum shearing force at the two edges of the pure bending section is distributed in a triangular shape, the shear stress at the top surface and the bottom surface of the beam is maximum, the shear stress at the neutral axis of the beam is zero, the conclusion obtained by the material mechanics formula is completely opposite, and the method accords with the engineering practice. The shear capacity of the beam is controlled by the average shear stress across the section, allowing the shear stress at the bottom and top of the beam to be up to 2 times the average shear stress. This has been confirmed by a number of experimental results. Thus, the shear strength determined according to the non-zero moment theory is:
wherein τ is shear strength (MPa); f is a limit load (N); b is the specimen width (mm); h is the specimen height (mm).
When shear failure is avoided, the following conditions should be satisfied:
and is
The required span and high span ratio are:
numerically, the shear strength tau of the concrete is equal to the tensile strength, and the tensile strength can adopt the splitting tensile strength fsp(MPa); bending moment mw(N/mm) is numerically equal to 90 times the tensile strength of the concrete. Therefore, the temperature of the molten metal is controlled,
L≥90mm
preferably, the width b multiplied by the height h of the square-section test piece is 150mm multiplied by 150mm, the length is 550mm, and the span L is 450mm, so that all requirements of minimum height, minimum span and maximum height-span ratio of the section of the bending moment damage control are met. Meanwhile, the test piece of 150mm multiplied by 550mm is a standard test piece for testing the bending tensile strength of concrete in highway engineering cement and cement concrete test regulations, is easy to form, and has abundant and mature experience for testing the bending tensile strength of concrete. According to the maximum aggregate particle size and research requirements, non-standard test pieces with other sizes can be selected for testing, and the test result can eliminate the size effect caused by the difference of the test methods. And the test piece forming and maintaining method is determined according to the research content requirement and the test design.
3. The invention is suitable for testing the bending resistance moment strength of various common concrete, and is used for analyzing and designing the bearing capacity of concrete beams and plate structures. The bending moment strength of other materials can be tested by adopting the method when the bending moment strength is proved to be in accordance with the failure mode of the invention. Wherein:
(1) and (4) testing the bending moment strength of the cement mortar and the cement mortar. The performance of the cement mortar is based on the anti-fracture and broken-end compression tests of the standard trabecula with the size of 40mm multiplied by 160mm, and the loading mode, the minimum height of the section, the span and the high-span ratio all meet the requirements. The performance test of the cement mortar shows that the section size of the standard test piece is 70mm multiplied by 70mm, and the requirements are also met.
(2) The concrete can adapt to larger bending deformation and can work with cracks, but the shear resistance and the bearing capacity are improved a little, and the size control requirement of the test piece of the invention is easy to meet.
Second, bending moment strength test
The bending moment strength test comprises three steps of test preparation, test loading and bending moment strength calculation. The following are specific test methods:
1. preparation of the test
And (3) molding a test piece: each set of test forms 3 test pieces, preferably a standard test piece for bending tensile strength test, and the test piece size is 150mm multiplied by 550 mm. Depending on the type of material and the requirements of the study, other non-standard size test pieces can be used, but must be converted into the bending moment strength values of the standard test pieces.
A loading device: preferably, the three-point double-point loading bending-resistant tensile test device has the span of 450mm and the supporting mode of simple support, wherein one end of the device is a fixed support, and the other end of the device is a movable support provided with 2 spherical hinges; one end of the loading device is provided with 1 loading point of the spherical hinge, and the other end of the loading device is provided with 2 loading points of the spherical hinge.
FIG. 1 is a graph showing the loading pattern, bending moment and transverse shear stress distribution in one embodiment of the present invention. Wherein, FIG. 1(a) is a three-point loading bending moment test device; FIG. 1(b) is a distribution diagram of bending moment of a pure bending section, which is a symmetrical triangular distribution; the bending moment of the beam bottom is used as a limit bending moment; FIG. 1(c) is a distribution diagram of shear stress of a pure bending section, which is a symmetrical triangular distribution. The pure bending section is damaged under the action of bending moment. The transverse shear stress is simply the equivalent shear stress that balances the bending moments to produce torsion.
2. Test loading
And (3) mounting a test piece: adjusting the movable support, and measuring the span to reach 450mm and be accurate to 1 mm; the test piece is placed on the support, the pouring surface of the test piece faces right ahead, the side face of the test piece faces upwards, the loading device is placed, the geometric center of the loading device is aligned with the geometric center of the test piece, and the support and the loading point are stably and uniformly contacted with the test piece.
Test loading: the preferred universal testing machine that accords with highway engineering cement and cement concrete test regulation (JTGE30), the precision of testing machine is 1%, and the test piece destroys the load and is greater than 20% and less than the universal testing machine of the whole journey 80%, and the testing machine has computer control loading speed and vertical displacement sensor, can automatic record and draw the load-deflection curve.
Recording the content: the loading speed is 0.05 MPa/s-0.08 MPa/s, and the load-deflection curve and the limit load F (N) of the bending fracture damage are recorded. The fracture position and crack propagation direction are described in detail. And (5) researching a load-deflection curve, and recording the deflection of the nearly straight-line segment during limit load.
3. Calculation of bending moment
According to the size, loading mode and breaking load of the test piece, the bending resistance moment calculation formula is as follows:
in the formula, mwThe ultimate bending moment (N/mm) of a pure bending beam; f is a limit load (N); b is the specimen width (mm); h is the specimen height (mm).
4. Calculation of bending moment strength
The vertical neutral axis of the beam is taken as an x axis, the height direction of the beam is taken as a y axis, the plane direction perpendicular to the beam is taken as a z axis, and the origin of coordinates is established on a fulcrum. According to the non-zero moment theory, the stress-moment equilibrium equation in the y direction is:
wherein m isyz＝0，mzz＝0，
Wherein M (x) is a bending moment; i SzI is absolute static moment, and for a rectangular section beam, the calculation formula is as follows:
the pure bending section Q (x) is 0, and the shear force balanced with the bending moment is realized by the shear force on the cross section of the loading point at the two ends of the pure bending section. The equilibrium shear force q (x) resulting from the bending moment is therefore calculated by the formula:
shear stress tauxyIn the value of and
are equal. Therefore, the shear stress of the balanced bending moment is equal to the bending moment strength of the concrete, and the calculation formula is as follows:
wherein, tauh/2The horizontal shear stress (MPa). The remaining symbols have the same meanings as above.
According to the geometric field theory of stress-strain relationship, the measured bending moment strength is based on the size after elongation, and the size effect of the bending moment strength is obtained by the relationship between the elongation strain and the curvature radius as follows:
wherein β is the size effect coefficient; epsilonyFor the elongation strain caused by bending, the calculation formula is:
wherein | y | is the absolute value of the y coordinate of the beam bottom; rho is curvature radius, and is calculated according to the following formula according to the deflection:
the expression for determining the size effect coefficient is:
wherein β is a measure of the size effect coefficient; f is a deflection value (mm); h is the height of the test year (mm); l is span (mm).
Third, analysis of test results
The analysis of the test results includes: determining the conformance of the failure mode, a data processing method and test result error analysis.
1. Compliance with failure modes
The bending moment failure mode is that the bottom of a pure bending section beam cracks upwards, and under the action of transverse torsion moment, the crack of the bottom of the beam is obliquely crossed with the plane of the beam, and the oblique crossing angle depends on the bending moment and the torque led out by the transverse shear stress generated by the bending moment. The crack expands upwards and has a certain inclination, but the included angle between the crack and the vertical direction is not large and is generally 10-20 degrees.
The bending elongation stress failure mode has the deflection larger than the predicted value by the theory of elasticity, the plane where the pure bending section beam bottom crack and the beam are located is nearly vertical, the included angle between the upward expansion direction of the crack and the vertical direction is very small and nearly vertical upward expansion is realized, and the bending tensile strength is obviously lower than the predicted value by the theory of moment of reaction.
And in a shear failure mode, the deflection is obviously lower than the predicted value by an elasticity theory, the crack initiation point is positioned on the beam bottom surface near the load action point of the pure bending section, the beam bottom crack is obliquely crossed with the plane of the beam, the included angle between the upward expansion direction of the crack and the vertical direction is large and reaches 30-45 degrees, and the bending tensile strength is obviously higher than the predicted value by a stress theory.
2. Data processing method
Geometric parameters are as follows: calculating the span, wherein the span is determined by measuring the horizontal distance between the central lines of the two supports, the span is 450mm, and the accuracy is 1 mm; the width and height of the test piece are determined by measuring the width and height of the cross section, the standard value is 150mm multiplied by 150mm, and the precision is 1 mm. The cross center is centered with the loading center of the testing machine, the loading position is centered with the cross center line of the test piece, and the error is not more than 1 mm.
Mechanical parameters are as follows: the limit load is recorded and read by a computer of the testing machine and is accurate to 1N; the deflection is determined by a load-vertical displacement curve through data processing. And during data processing, taking a near straight line segment, making a straight line to extend to zero load, wherein the intersection point of the load-displacement and the displacement axis is a zero point, and subtracting the zero point reading from the displacement corresponding to the ultimate load to obtain a deflection value which is accurate to 0.01 mm.
Calculating the bending moment: according to the calculation formula provided by the invention, the ultimate bending moment of the beam bottom is taken as the bending moment. The bending moment test result takes the arithmetic average value of the bending moment measurement values of 3 test pieces as the measurement value, and the accuracy is 1N/mm.
And (3) calculating the bending moment strength: according to the calculation formula provided by the invention, the ultimate bending moment strength of the beam bottom is taken as the bending moment strength. The bending moment strength test result takes the arithmetic mean value of the bending moment strength measured values of 3 test pieces as the measured value, and the accuracy is up to 0.01 MPa.
3. Error analysis of test results
If the difference between the maximum value or the minimum value and the median value exceeds 15 percent of the median value in the 3 test pieces, the maximum value and the minimum value are discarded, and the median value is taken as a measured value. If the maximum value and the difference between the minimum value and the median value exceed 15 percent of the median value, the test result is invalid.
If one fracture surface is positioned outside the loading point in the 3 test pieces, the difference between the larger value and the smaller value of the other two test pieces does not exceed 15 percent of the smaller value, and the average value of the two measured values is taken as the measured value; if there are two fracture surfaces outside the load point, or if the error between the two measurements exceeds 15% of the smaller value, the test result is invalid.
FIG. 2 is a graph of bending moment versus bending tensile strength for one embodiment of the present invention. The bending moment is h/3 times of the bending tensile strength, and is 50 times in the embodiment.
FIG. 3 is a graph of bending moment versus split tensile strength for one embodiment of the present invention. In the embodiment, the bending moment is equal to 71.6-86.9 times of the splitting tensile strength of the concrete in numerical value, and the higher the strength of the concrete is, the lower the strength of the concrete is. Considering that the split tensile strength test is affected by the loading filler strip and is significantly higher than the axial tensile strength, which is difficult to load, it is attempted to demonstrate by this example that the bending moment strength is numerically equivalent to the concrete tensile strength.
FIG. 4 shows the relationship between the bending moment strength, bending tensile strength and split tensile strength of a three-point loading system according to an embodiment of the present invention. The bending moment strength is an objective physical quantity and has good consistency with the tensile strength of concrete, and the measured value is lower than the tensile strength, which proves that the concrete is not broken in tension; the flexural tensile strength and tensile strength were also in good agreement, but the measured values were much higher than the tensile strength. The beam slab and the structural design adopt the tensile strength of concrete, the brittleness coefficient needs to be considered for the design strength value, and the design tensile strength value is basically the same as the bending-resistant moment strength of the concrete after the brittleness coefficient is considered.
FIG. 5 is a graph illustrating the bending moment strength dimensional effect in one embodiment of the present invention. The bending moment strength has a certain size effect, but is obviously smaller than the size effect of the bending tensile strength; the size effect of the bending-resistant moment strength of the concrete is more effective than the size effect of the bending-resistant moment strength of cement paste and mortar, and the size effect is mainly related to the grain size of coarse aggregates.
One typical application is that when the bending moment strength is used for analyzing and designing the structural limit state of the cement concrete pavement, the design formula is as follows:
γr(σpr+σtr)≤τw
γr(σp,max+σt,max)≤τw
wherein, γrIs a reliability coefficient; tau iswBending moment strength (MPa); sigmaprLoad fatigue stress (MPa); sigmatrTemperature fatigue stress (MPa); sigmap,maxThe maximum load stress (MPa); sigmat,maxThe maximum temperature stress (MPa).
By adopting the bending-resistant moment-stress strength design, the early fatigue fracture of the cement concrete pavement and the cracking of the panel caused by the board bottom void and the cracking of the panel caused by the temperature stress can be more reasonably explained. The bending moment strength is the real breaking strength, the size effect is eliminated, and the numerical value is greatly lower than the bending tensile strength of the concrete and is also obviously lower than the tensile strength of the concrete. Therefore, the reliability and the safety of the cement concrete pavement structure can be obviously improved by adopting the bending-resistant moment-resistant strength design.
As described above, according to the invention, the bending failure essence of the concrete and the root cause of the size effect in the bending moment strength test are found, the provided moment strength test method measures the objective physical quantity of the bending bearing capacity of the concrete, eliminates the size effect generated by the test method, and improves the reliability of the test parameters. The bending resistance moment strength is adopted to carry out design and analysis of the beam and the plate structure, so that the reliability and the safety of the structure can be greatly improved.
The invention is suitable for the bending moment strength test of the C30-C80 concrete. The bending-resistant moment strength is adopted to replace the axial tensile strength, and the method is used for analyzing and designing the bearing capacity of a bent beam and a plate structure; or replace the bending tensile strength of the cement concrete of the pavement to be used for the structural design of the pavement. The cement concrete pavement structure is converted from bending tensile strength design into bending moment strength design, so that the reliability and the safety of the cement concrete pavement structure can be greatly improved.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (5)
1. The method for measuring the bending moment strength of the concrete is characterized by comprising the following steps of:
1) carrying out test design by using a non-zero moment theory, wherein a test piece for controlling damage by bending moment resistance adopts a square-section beam type test piece;
2) loading by adopting a simple supporting beam three-point and two-point mode, wherein the supporting mode is a simple support, one end of the supporting mode is a fixed support, and the other end of the supporting mode is a movable support provided with 2 spherical hinges;
3) adjusting a movable support, measuring the span, placing a test piece on the support, placing a loading device with a pouring surface of the test piece facing right ahead and a side surface facing upwards, aligning the geometric center of the loading device with the geometric center of the test piece, and enabling the support and the loading point to be stably and uniformly contacted with the test piece;
4) selecting a universal testing machine which accords with highway engineering cement and cement concrete test regulations for loading;
5) the fracture failure mode is small-angle fracture failure of the pure bending section, the fracture is positioned in the pure bending section, the included angle between the upward expansion of the crack and the vertical direction is less than 30 degrees, and the bending moment strength is calculated according to a bending moment strength formula after the fracture failure; the bending moment strength formula is as follows:
wherein, tauwThe bending moment strength is obtained; f is a limit load; b is the width of the test piece; h is the specimen height.
2. The method for measuring the bending moment strength of concrete according to claim 1, wherein the standard size of the square-section beam specimen is 150mm x 550 mm.
3. The method for measuring the bending moment strength of concrete according to claim 1, wherein the simple beam is loaded with three points, two points, the high span ratio is 3.0, and the span is 450 mm.
4. The method for measuring the bending moment strength of concrete according to claim 1, wherein the loading device is provided with loading points with 1 spherical hinge at one end and 2 spherical hinges at the other end.
5. The method for measuring the bending moment strength of concrete according to claim 1, wherein the loading speed is 0.05 MPa/s-0.08 MPa/s, the load-deflection curve is recorded while loading, and the ultimate deflection value is determined according to the load-deflection curve.
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