CN109668782B - Method for measuring fracture initiation fracture toughness of concrete - Google Patents
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- 230000000977 initiatory effect Effects 0.000 title claims abstract description 40
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- 238000012360 testing method Methods 0.000 claims abstract description 85
- 239000000945 filler Substances 0.000 claims abstract description 10
- 238000004364 calculation method Methods 0.000 claims abstract description 9
- 239000010687 lubricating oil Substances 0.000 claims abstract description 6
- 229910000831 Steel Inorganic materials 0.000 claims description 9
- 239000010959 steel Substances 0.000 claims description 9
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- 238000013001 point bending Methods 0.000 description 4
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- 238000006073 displacement reaction Methods 0.000 description 3
- 238000004088 simulation Methods 0.000 description 3
- 238000004458 analytical method Methods 0.000 description 2
- 239000002390 adhesive tape Substances 0.000 description 1
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Abstract
The invention belongs to the technical field of concrete fracture performance research in civil and hydraulic engineering, and provides a method for measuring fracture toughness of concrete. The test piece is a concrete cubic test piece. The testing device is divided into two parts, and the first part is an antifriction device and comprises a bottom plate and two layers of antifriction pieces. The friction reducing piece is arranged between the test piece and the bottom plate, and lubricating oil is coated on the friction reducing piece to realize smooth contact between the test piece and the bottom plate. The second part is a loading device which comprises a wooden filler strip, an arc loading head and a load sensor. And monitoring the strain value of the tip of the crack of the test piece by using a resistance strain gauge, and determining the crack initiation load according to the hysteresis of the strain value in the loading process. And (4) bringing the crack initiation load into a cubic crack initiation toughness calculation formula to obtain the crack initiation toughness of the concrete. The cubic test piece provided by the method has the advantages of convenience in manufacturing, small self weight, core drilling and sampling and the like in form.
Description
Technical Field
The invention belongs to the technical field of concrete fracture performance research in civil and hydraulic engineering, and relates to a method for determining concrete fracture initiation fracture toughness by a cubic test piece with a slit at the bottom.
Background
The formation of cracks in concrete structures is divided into three stages, namely crack initiation, stable crack propagation and unstable crack propagation. The concept of fracture initiation toughness and fracture instability toughness, which are critical points for distinguishing the crack propagation state, is put forward by a double-K fracture model taking a stress intensity factor as a parameter. In recent years, fracture propagation criteria based on fracture initiation fracture toughness have been widely used, which considers that a fracture starts to propagate with a fracture tip stress intensity factor greater than the fracture initiation fracture toughness of the material and treats each propagation of the fracture as a new initiation. The fracture initiation fracture toughness of the material can be regarded as a basis for analyzing the whole process of fracture propagation, so that how to determine the fracture initiation fracture toughness of the material is widely concerned by domestic and foreign scholars.
At present, common concrete fracture mechanical test methods comprise a three-point bending beam method and a wedging and splitting method, and research results of a large number of scholars at home and abroad prove that the two methods can be used for measuring the fracture initiation fracture toughness of concrete. In 2005, the three-point bending beam method and the wedging and splitting method are listed as hydraulic concrete fracture mechanics standard test methods in China. The three-point bending beam method can easily determine a stable test curve and thus fracture parameters. However, due to the limitation of the span-height ratio, the volume of the large aggregate test piece is often large, which not only brings inconvenience to the pouring and carrying work of the test piece, but also has great influence on the determination of the fracture parameters due to the self weight. Compared with a three-point bending beam method, the wedge splitting method has the advantages that the influence of the self weight of the test piece on the measurement result of the fracture parameter is eliminated or weakened. But the test device is comparatively complicated, and has higher requirement on the operation technique of the tester. In addition, when wedge-in split-pulling test pieces with different height sizes are manufactured, if the loading groove is not increased in proportion with the increase of the size of the test piece, an additional bending moment is generated at the tip of a crack due to the fact that the vertical component force transmitted to the test piece by the clamp, the self weight of the test piece and the counter force of the support are not collinear, and the authenticity of the measured fracture toughness is influenced to a certain extent.
Disclosure of Invention
Based on the analysis and combining the long-term research results in the field of concrete fracture of the applicant, the method for accurately and conveniently determining the fracture initiation fracture toughness of the concrete material by using the cubic test piece with the kerfs on the bottom edge is provided. The cubic test piece provided by the method has the advantages of convenience in manufacturing, small self weight, core drilling and sampling and the like in form. The test method has the advantages of simple and convenient operation, convenient popularization and the like. In addition, the applicant obtains a calculation formula for determining the fracture toughness at break of concrete by using a cubic test piece based on experimental research and theoretical analysis.
The technical scheme of the invention is as follows:
a method for measuring the fracture toughness of concrete crack initiation is characterized in that the crack initiation load of a concrete test piece is obtained through a test piece which is simple to manufacture and a loading device which is convenient to operate, and the fracture toughness of the concrete is determined by combining a formula obtained by fitting a numerical simulation result, and the method comprises the following specific steps:
the novel test method for determining the crack initiation fracture toughness of concrete by adopting a cubic test piece with a joint cut at the bottom comprises a test piece and a test device;
the test piece is a concrete cubic test piece with a through crack at the middle position of the bottom;
the testing device is divided into two parts, wherein the first part is an antifriction device and comprises a bottom plate and two layers of antifriction pieces; two layers of friction reducing pieces are arranged between the test piece and the bottom plate, and lubricating oil is smeared between the two layers of friction reducing pieces to realize smooth contact between the test piece and the bottom plate; the second part is a loading device which comprises a wooden filler strip, an arc loading head and a load sensor; the wooden filler strip is made of three layers of plywood, and the length of the wooden filler strip is not less than the length of the test piece; the arc loading head consists of an arc cushion block and a steel block, the widths of the upper surfaces of the steel block and the arc cushion block are the same, and the length of the arc loading head is the same as that of the test piece;
the measured crack initiation load is brought into a cubic test piece crack initiation toughness calculation formula to obtain the crack initiation toughness of the concrete; the calculation formula is as follows:
wherein, P is an external load; b is the side length of the cubic test piece; a is the crack length; k is the stress intensity factor.
The invention has the beneficial effects that: the cubic concrete test piece is simple to manufacture and convenient to test and operate, and the crack initiation fracture toughness of the concrete can be directly obtained according to a given calculation formula.
Drawings
FIG. 1 is a schematic view of a combination of a test piece and an apparatus.
In the figure: 1, a bottom plate; 2 a first friction reducing sheet; 3, lubricating oil; 4 a second friction reducing sheet; 5, a cubic test piece; 6, wood cushion strips; 7, an arc loading head; 8 load sensors.
Detailed Description
The following further describes a specific embodiment of the present invention with reference to the drawings and technical solutions.
A method for measuring the fracture toughness of concrete crack initiation is provided, a loading device which is simple in manufacturing test piece form and convenient to operate is provided, so that the crack initiation load of a concrete test piece is obtained, the fracture toughness of concrete is determined by combining a formula obtained by fitting a numerical simulation result, and the concrete steps are as follows:
a novel test method for determining the crack initiation fracture toughness of concrete by using a cubic test piece with a joint cut at the bottom relates to a test piece and a test device; the test piece is a concrete cubic test piece, the side length is 150mm, and a through crack with the length of 45mm and the width of 2mm is arranged at the middle position of the bottom of the test piece; the testing device is divided into two parts, the first part is an antifriction device and comprises a bottom plate and two layers of PVC antifriction pieces, and the size of the bottom plate of the support is 400mm long, 400mm wide and 40mm thick. The PVC friction reducing sheet is arranged between the test piece and the base plate, and lubricating oil is coated on the PVC friction reducing sheet to realize smooth contact between the test piece and the base plate. The size of the lower PVC friction reducing piece (friction reducing piece 1) is 300mm long, 200mm wide and 0.4mm thick, and the size of the upper PVC friction reducing piece (friction reducing piece 2) is 200mm long, 150mm wide and 0.4mm thick. The second part is a loading device which comprises a wooden filler strip, an arc loading head and a 30t load sensor. The wooden filler strip is made for the three-layer plywood, and the width is 20mm, and thickness is 3 ~ 4mm, and length is not less than test piece length. The arc loading head is composed of a steel arc cushion block with the radius of 75mm and a steel block, the steel arc cushion block is 20mm high, the widths of the upper surfaces of the steel block and the cushion block are the same, the cross section of the steel block is as shown in figure 1, and the length of the arc loading head is the same as that of a test piece.
(1) Test piece manufacture
A standard cube test mold is adopted to manufacture a cube concrete test piece, a mold release agent is coated on the inner side of the mold, concrete is poured, the concrete is compacted through vibration, and the cube concrete test piece is conveyed into a standard curing room within 10 minutes after pouring, wherein the temperature is 20 ℃, and the relative humidity is 95%. And removing the mold after 24 hours, and then sending the mold into a standard curing room for curing for 28 days and taking out the mold.
And (3) taking out the test piece, standing for 24h in a laboratory environment, and cutting a through crack with the width of 2mm and the height of 45mm at the middle position of the bottom of the test piece by using a cutting machine, wherein the schematic diagram is shown in fig. 1.
(2) Testing device
And loading by adopting a 300t electro-hydraulic servo testing machine. The friction reducing sheet 1 is flatly laid in the center of the base plate of the testing machine and fixed along the periphery by the transparent adhesive tape, and the lubricating oil is uniformly coated on the upper surface of the friction reducing sheet 1. And the friction reducing pieces 2 are symmetrically paved on the upper surfaces of the friction reducing pieces 1 along the center line of the bottom plate of the testing machine and are respectively 1mm away from the center line, and air bubbles between the upper friction reducing pieces and the lower friction reducing pieces are discharged. And placing the concrete test piece on the antifriction sheet 2 to enable the left and right crack surfaces of the test piece to be respectively flush with the edge of the antifriction sheet 2. A wooden filler strip, an arc loading head and a 30t load sensor are sequentially arranged above the concrete sample from bottom to top.
(3) Loading and monitoring scheme
(3.1) Loading scheme
And a loading device (a load sensor and an arc loading head) is connected, and the testing machine is adjusted to enable the arc loading head to be in uniform contact with the pad strip, so that eccentric loading is prevented. And (3) continuously and uniformly loading the test piece to be damaged at the speed of 0.024mm/min by adopting a displacement loading mode. To prevent the tester from being damaged, the loading is stopped if the vertical shear displacement reaches 5 mm.
(3.2) monitoring protocol
And monitoring the strain value of the tip of the crack of the test piece by using a resistance strain gauge, and determining the crack initiation load according to the hysteresis of the strain value in the loading process. Before loading, 1 strain gauge is pasted at the position of 5mm on each of the left side and the right side of the crack tip on the front surface and the rear surface of the test piece, the size of a grid is 10mm multiplied by 2mm, the strain gauges are connected to a data acquisition system, and strain in the loading process is monitored in real time. And meanwhile, the sensor and the data acquisition system are connected, and load and strain data of the whole loading process are recorded in real time.
(4) Data processing
(4.1) determination of the initiation load
And after the test is finished, drawing a load-strain curve according to test data, wherein the load corresponding to the strain hysteresis point is the crack initiation load, and the average value of the crack initiation loads obtained from 4 groups of data is used as the crack initiation load of the test piece.
(4.2) calculation of fracture initiation toughness
Based on the results of numerical simulation, usingAnd determining a cubic fracture toughness calculation formula by a data fitting method. Calculating stress intensity factor by using displacement extrapolation method provided in ANSYS software, in order to simulate stress of each point at the tip of crack
Singularity, the node in each edge of the isoparametric element around the fracture tip was moved to the point 1/4 near the fracture tip. The stress intensity factor expression suitable for the cubic test piece is obtained by fitting through adjusting different seam height ratios and test piece sizes of the test piece (the seam height ratio is 0.05-0.95, the interval is 0.01, the side length of the test piece is 50-2000 mm, and the interval is 50 mm):
p external load (kN); b cube side length (mm); a crack length (mm); k stress intensity factor (MPa mm)1/2)；
When P is the crack initiation load, substituting into a formula to obtain the crack initiation toughness Kini。
(5) Results
Obtaining a load-strain curve of the whole loading process; determining the crack initiation load of the concrete; obtaining accurate concrete crack initiation toughness.
Claims (1)
1. The method for measuring the fracture initiation fracture toughness of the concrete is characterized by comprising the following steps of:
the method comprises a test piece and a test device;
the test piece is a concrete cubic test piece with a through crack at the middle position of the bottom;
the testing device is divided into two parts, wherein the first part is an antifriction device and comprises a bottom plate and two layers of antifriction pieces; two layers of friction reducing pieces are arranged between the test piece and the bottom plate, and lubricating oil is smeared between the two layers of friction reducing pieces to realize smooth contact between the test piece and the bottom plate; the second part is a loading device which comprises a wooden filler strip, an arc loading head and a load sensor; the wooden filler strip is made of three layers of plywood, and the length of the wooden filler strip is not less than the length of the test piece; the arc loading head consists of an arc cushion block and a steel block, the widths of the upper surfaces of the steel block and the arc cushion block are the same, and the length of the arc loading head is the same as that of the test piece;
substituting the measured crack initiation load into a cubic test piece crack initiation fracture toughness calculation formula to obtain the crack initiation fracture toughness of the concrete; the calculation formula is as follows:
wherein, P is an external load; b is the side length of the cubic test piece; a is the crack length; k is the stress intensity factor.
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| CN110501126B (en) * | 2019-08-28 | 2021-01-22 | 湘潭大学 | Beam structure damage identification method based on support counterforce and deflection curvature |
| CN110487578B (en) * | 2019-08-28 | 2021-01-22 | 湘潭大学 | Beam structure damage identification method based on support reaction force and strain |
| CN111610099B (en) * | 2020-07-08 | 2022-08-02 | 郑州大学 | Rubber concrete fracture performance analysis method based on temperature and humidity changes |
| CN113325164B (en) * | 2021-06-04 | 2022-08-09 | 大连理工大学 | Method for analyzing three-dimensional expansion process of concrete I-shaped crack |
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