CN107402152B - Method for prefabricating cement-based test piece crack, prefabricated test piece and prefabricating system - Google Patents
Method for prefabricating cement-based test piece crack, prefabricated test piece and prefabricating system Download PDFInfo
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Abstract
The invention discloses a method for prefabricating a crack of a cement-based test piece, a prefabricated test piece and a prefabricating system.
Description
Technical Field
The invention relates to the technical field of cement-based material self-repairing, in particular to a method for prefabricating a cement-based test piece crack, a prefabricated test piece and a prefabricating system.
Background
Cement-based materials are the most widely used materials in modern building materials, particularly concrete materials. The method for searching for improving the durability of the cement-based material is of great significance for improving the service performance of the building, and is a research hotspot of civil engineering materials at present. The development trend of the concrete material and the structure field is widely accepted as a mode of endowing the cement-based material with self-repairing performance to improve the integral performance of the building structure, and the development trend also accords with the strategic development direction of China.
In the self-repairing research of cement-based materials, particularly when small cement-based test pieces are researched, cracks of the small cement-based test pieces generally need to be prefabricated manually. The traditional crack making mode is mainly to make cracks by directly carrying out a bending test or a pressure test through a pressure tester, but the positions, the widths and the number of the generated cracks are difficult to control by directly using the two methods, and particularly, the cracks are easy to directly damage by pressure for small-sized test pieces. The size and location of the concrete sample cracks are problematic.
In order to solve the problems, the method of preparing the notch can be adopted to induce the crack generation, but the method can only control the initial position of the crack of the test piece and the approximate area of the crack propagation. The cement-based material is a brittle material, once the material is cracked, the crack can be rapidly developed to form a through crack, so that the size and the position of the crack generated by the method cannot well meet the requirement. In addition, the prior art also has a method for manufacturing artificial cracks by pulling out the embedded steel sheets, but the shape of the cracks formed by the method is far from the shape of actual stress cracks.
Thus, there is a need for improvements and enhancements in the art.
Disclosure of Invention
In view of the defects of the prior art, the invention aims to provide the method, the prefabricated test piece and the prefabricating system for prefabricating the cement-based test piece crack, which not only can realize the control of the size and the position of the crack formed by the prefabricated test piece, but also have the advantage that the formed crack is close to the crack generated by the real stress cracking of the concrete.
The technical scheme adopted by the invention for solving the technical problem is as follows:
a method for prefabricating a cement-based test piece crack comprises the following steps:
manufacturing a structural net: the structural net is a latticed structural net with a rectangular grid shape, the grids of a preset area of the latticed structural net are removed, and the network wires of the preset area form a fracture which is used as a weak area of the structural net;
manufacturing a cement-based test piece: after a first layer of cement base layer is poured in a mould, a structural net is placed above the mould, the two side directions of a fracture are recorded on the mould, a second layer of cement base layer is poured to cover the structural net, the mould is removed after a cylindrical cement-based test piece is formed through maintenance, and marking is carried out according to the record of the mould at the corresponding position of the side surface of the cement-based test piece;
and (3) prefabricating and installing cracks: after the cement-based test piece is maintained to a specified age, placing the cement-based test piece between an upper loading plate and a lower loading plate of a pressure testing machine, and selecting a loading point according to the mark;
and (3) crack preparation: and starting a pressure testing machine, applying a vertical load to the cement-based test piece, acquiring width data of a crack generated by the test piece in real time through a laser displacement sensor, and stopping loading and load-holding of the pressure testing machine when the width of the crack reaches a target width.
In the method for prefabricating the cement-based test piece crack, the cement-based test piece is manufactured by the following steps: after a first layer of cement-based layer is poured in the mould, a structural net is placed above the mould, the directions of two sides of a fracture are recorded on the mould, a second layer of cement-based layer is poured to cover the structural net, the mould is removed after a cylindrical cement-based test piece is formed through maintenance, and the steps of marking the corresponding position of the side surface of the cement-based test piece according to the record further comprise:
and continuously repeating the pouring steps for a plurality of times, namely continuously placing a new configuration net on the uppermost cement base layer of the formed cement-based test piece, wherein the size and the orientation of the fracture of the new configuration net are consistent with those of the fracture of the placed configuration net, and pouring a new cement base layer on the new configuration net.
In the method for prefabricating the cement-based test piece crack, after the cement-based test piece is maintained to a specified age period, the step of placing the cement-based test piece between an upper loading plate and a lower loading plate of a compression testing machine and selecting a loading point according to the mark comprises the following steps:
after the test piece is maintained to the specified age, the cement-based test piece is vertically arranged between an upper loading plate and a lower loading plate of a pressure testing machine;
and selecting a loading point of the compression testing machine according to the marking position of the side surface of the cement-based test piece, and arranging a gasket at the loading point, wherein the contact surface of the gasket and the cement-based test piece is an inwards concave arc surface matched with the side surface of the cement-based test piece.
In the method for prefabricating the cement-based test piece crack, the method for starting the pressure testing machine to apply a vertical load to the cement-based test piece, acquiring width data of the crack generated by the test piece in real time through a laser displacement sensor, and stopping loading of the pressure testing machine when the width of the crack reaches a target width further comprises the following steps:
arranging a laser displacement sensor at the side of the pressure testing machine, and connecting the laser displacement sensor and the pressure testing machine with a computer;
and the camera of the laser displacement sensor is aligned to the middle part of the test piece to finish calibration.
In the method for prefabricating the cement-based test piece crack, the step of starting the pressure testing machine, applying a vertical load to the cement-based test piece, acquiring the width data of the crack generated by the test piece in real time through a laser displacement sensor, and stopping loading and load-holding of the pressure testing machine when the width of the crack reaches a target width comprises the steps of;
starting a pressure testing machine, vertically loading a test piece by adopting a displacement control mode, and controlling the loading speed to be lower than a preset value so as to avoid the generation of cracks and the damage of the test piece caused by the over-high expansion speed;
the target width of the crack is set on the computer, the width data of the crack generated by the test piece is collected in real time through the laser displacement sensor and sent to the computer, and when the width of the crack analyzed by the computer reaches the target width, the loading is stopped and the load is preserved.
In the method for prefabricating the cement-based test piece crack, the structural net is a steel wire net or a fiber net.
A prefabricated test piece for prefabricating a cement-based test piece crack is a cylindrical cement-based test piece and comprises a configuration net and cement base layers positioned on two sides of the configuration net;
the structural net is a grid structural net with a rectangular grid shape, and comprises an encryption area and a reserved weak area positioned in the encryption area, and the network cables in the reserved weak area are removed to form a fracture;
and marks are arranged on the lateral surfaces of the prefabricated test pieces at the two sides of the fracture towards the fracture.
The prefabrication system for prefabricating the cement-based test piece crack comprises a pressure testing machine, a laser displacement sensor and the prefabrication test piece, wherein the prefabrication test piece is arranged between an upper loading plate and a lower loading plate of the pressure testing machine, and the loading point of the pressure testing machine corresponds to the mark of the prefabrication test piece.
In the prefabrication system for prefabricating the cement-based test piece crack, a gasket is arranged between a mark of the prefabricated test piece and a loading point of a pressure testing machine;
one surface of the gasket is a plane matched with a loading plate of the compression testing machine;
the other side of the gasket is an inward concave arc surface matched with the side surface of the prefabricated test piece.
The prefabricating system for prefabricating the cement-based test piece cracks is characterized in that the laser sensor and the pressure testing machine are connected with a computer, the laser sensor is located on one side of the pressure testing machine, and a camera of the laser sensor is aligned to the middle of the prefabricated test piece.
Compared with the prior art, the method for prefabricating the cracks of the cement-based test piece, the prefabricated test piece and the prefabricating system provided by the invention have the advantages that the construction type net of the weak area is generated by removing the net lines to form the fracture, and the construction type net is placed in the prefabricated test piece in the process of pouring to form the prefabricated test piece, so that when the prefabricated test piece is loaded by a pressure testing machine, the size and the position of the cracks formed on the prefabricated test piece can be controlled through the construction type net, and the formed cracks are close to the cracks generated by actual stress cracking of concrete.
Drawings
FIG. 1 is a side view of a pre-fabricated test piece for pre-fabricating a fracture in a cement-based test piece according to the present invention.
FIG. 2 is a top view of a formation type network for a prefabricated test piece for prefabricating a cement-based test piece fracture provided by the present invention.
FIG. 3 is a schematic structural diagram of a prefabrication system for prefabricating cracks of a cement-based test piece.
FIG. 4 is an elevation view of a preparation system for preparing a fracture of a cement-based test piece provided by the present invention.
FIG. 5 is a side view of a preparation system for preparing a fracture of a cement-based test piece according to the present invention.
FIG. 6 is a flow chart of a method for preparing a fracture of a cement-based test piece according to the present invention.
FIG. 7 is a flowchart of step S300 of the method for preparing a crack of a cement-based test piece according to the present invention.
FIG. 8 is a flowchart of step S400 of the method for preparing a crack of a cement-based test piece according to the present invention.
Wherein: 10. a cement base layer; 20. forming a network; 21. an encryption area; 22. a region of weakness; 30. marking a line; 31. an upper gasket; 32. a lower gasket; 41. an upper loading plate; 42. a lower loading plate; 50. a laser displacement sensor; 100. and (6) prefabricating a test piece.
Detailed Description
In order to make the objects, technical solutions and effects of the present invention clearer and clearer, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Referring to fig. 1, which is a side view of a prefabricated test piece for prefabricating a crack of a cement-based test piece according to the present invention, the prefabricated test piece 100 is a cylindrical cement-based test piece and includes a structural net 20 and cement-based layers 10 located on both sides of the structural net 20. Referring to fig. 2, the structural net 20 is a grid structural net with a rectangular grid shape, and includes a dense area 21 and a reserved weak area 22 in the dense area 21, the grid lines in the reserved weak area 22 are removed to form a fracture, and the side surface of the prefabricated test piece is provided with a mark at the two sides of the fracture, wherein the mark is a mark line 30 perpendicular to the bottom surface of the cement-based test piece.
Please refer to fig. 3, which is a schematic structural diagram of a prefabricating system for prefabricating a cement-based test piece crack according to the present invention, the prefabricating system includes a pressure tester (not shown in the figure), a laser displacement sensor 50, and the prefabricated test piece 100, the prefabricated test piece 100 is disposed between an upper loading plate 41 and a lower loading plate 42 of the pressure tester, and a loading point of the pressure tester corresponds to a mark of the prefabricated test piece 100.
Wherein, the structural net 20 is a steel wire net or a fiber net. When the pre-cast test piece 100 is loaded by the compression testing machine, the configuration net 20 will provide the pre-cast test piece 100 with a load-bearing force against the loading force. However, as the loading process progresses, the pre-weakened areas 22 in the configuration network 20 may lose their load-bearing capacity first and cause the pre-fabricated test piece 100 to form microcracks. And as the external loading force increases, the crack will continue to propagate here to form a macro crack, i.e. the position where the crack is induced upon loading can be achieved according to the position of the reserved weak area 22. In addition, the use of the encrypted zone 21 of the configuration net 20 increases the toughness of the sample, so that the crack propagation can be delayed, and the control of the crack width in the loading process is facilitated. Therefore, the size and the position of the crack formed by the prefabricated test piece 100 can be controlled through the structural net 20, and the formed crack and the crack generated by the actual stress cracking of the concrete can have the same crack tip micro-crack area, crack tortuosity and crack wall surface roughness.
Further, referring to fig. 4, a gasket is arranged between the mark point of the prefabricated test piece 100 and the loading point of the pressure tester, and is an upper gasket 31 and a lower gasket 32; one surface of the gasket is a plane matched with a loading plate of the compression testing machine; the other side of the gasket is an inward concave arc surface matched with the side surface of the prefabricated test piece 100. By using the spacer, the movement of the test specimen during the installation and loading of the test specimen can be limited, and the stress concentration at the contact surface between the test specimen 100 and the upper and lower load plates 41 and 42 during the loading can be reduced, thereby preventing the occurrence of local damage at the contact surface and the influence on the formation of cracks.
Further, referring to fig. 5, the laser displacement sensor 50 and the pressure testing machine are both connected to a computer, the laser displacement sensor 50 is located at one side of the pressure testing machine, and a camera of the laser displacement sensor 50 is aligned with the middle of the prefabricated test piece 100. Before loading, the calibration is completed by aligning the camera of the laser displacement sensor 50 to the middle of the prefabricated test piece 100, so that the measurement of the width of the crack is prevented from being inaccurate. Secondly, in the loading process, the computer controls the loading speed of the prefabricated test piece 100 according to the displacement control mode of the pressure testing machine, and the crack initiation and expansion speed of the prefabricated test piece is ensured to be very slow so as to avoid the brittle failure of the prefabricated test piece and influence the formation of the crack.
In addition, in a traditional crack prefabrication system, a crack width is usually controlled by measuring the transverse displacement of concrete by using an LVDT displacement sensor to represent the crack width. However, the process of removing the sensor can generate secondary disturbance to the sample with cracks, thereby affecting the manufacturing accuracy. The laser displacement sensor 50 adopted by the system is arranged on one side of the prefabricated test piece 100, and is not contacted with the prefabricated test piece 100 in the measuring process, so that secondary disturbance is effectively avoided.
With continued reference to FIG. 6, the present invention further provides a method for preparing a fracture of a cement-based test piece, comprising the steps of:
s100, manufacturing a structural net: the structural net is a latticed structural net with a rectangular grid shape, the grids of a preset area of the latticed structural net are removed, and the network wires of the preset area form a fracture which is used as a weak area of the structural net;
s200, manufacturing a cement-based test piece: after a first layer of cement base layer is poured in a mould, a structural net is placed above the mould, the two side directions of a fracture are recorded on the mould, a second layer of cement base layer is poured to cover the structural net, the mould is removed after a cylindrical cement-based test piece is formed through maintenance, and marking is carried out according to the record of the mould at the corresponding position of the side surface of the cement-based test piece;
s300, prefabricating and installing a crack: after the cement-based test piece is maintained to a specified age, placing the cement-based test piece between an upper loading plate and a lower loading plate of a pressure testing machine, and selecting a loading point according to the mark;
s400, crack preparation: and starting a pressure testing machine, applying a vertical load to the cement-based test piece, acquiring width data of a crack generated by the test piece in real time through a laser displacement sensor, and stopping loading of the pressure testing machine when the width of the crack reaches a target width.
Further, after the step S200, the method further includes:
and continuously repeating the pouring steps for a plurality of times, namely continuously placing a new configuration net on the uppermost cement base layer of the formed cement-based test piece, wherein the size and the orientation of the fracture of the new configuration net are consistent with those of the fracture of the placed configuration net, and pouring a new cement base layer on the new configuration net. In the embodiment, the pouring step is preferably repeated once, and compared with a cement-based test piece with a single-layer structure net structure, the cement-based test piece with a double-layer structure net structure can control the crack generation position more strictly, and the formed crack is closer to the crack generated by the actual stress cracking of the concrete.
Further, referring to fig. 7, the step S300 specifically includes:
s301, after maintaining the test piece to a specified age, erecting the cement-based test piece between an upper loading plate and a lower loading plate of a pressure testing machine;
s302, selecting a loading point of the compression testing machine according to the marked position of the side face of the cement-based test piece, and arranging a gasket at the loading point, wherein the contact surface of the gasket and the cement-based test piece is an inwards concave arc surface matched with the side face of the cement-based test piece.
In addition, before the step S400, the method further includes:
arranging a laser displacement sensor at the side of the pressure testing machine, and connecting the laser displacement sensor and the pressure testing machine with a computer;
and the camera of the laser displacement sensor is aligned to the middle part of the test piece to finish calibration.
With reference to fig. 8, the step S400 specifically includes;
s401, starting a pressure testing machine, carrying out vertical load on a test piece by adopting a displacement control mode, and controlling the loading speed to be lower than a preset value so as to avoid the test piece from being damaged due to the generation of cracks and the over-high expansion speed;
s402, setting a target width of the crack on a computer, acquiring width data of the crack generated by the test piece in real time through a laser displacement sensor, sending the data to the computer, and stopping loading when the computer analyzes that the width of the crack reaches the target width.
For better understanding of the present invention, the method for preparing the cracks of the cement-based test piece provided by the present invention is described in detail below with reference to fig. 1 to 8:
firstly, preparing a steel wire mesh, namely cutting two steel wire meshes (a commercial 304 stainless steel galvanized steel wire mesh with the diameter of 1.5mm and the grid size of a square grid of 15mm multiplied by 15 mm) according to the size of a cement-based test piece (a mortar sample with the diameter of phi 100mm multiplied by 50 mm), and cutting a middle vertical steel wire of the two steel wire meshes by 50mm.
And then, pouring in layers to form a cement-based test piece, namely after the first cement base layer is poured in the mold (the height of the first cement base layer is 25 mm), placing a cut steel wire mesh above the first cement base layer 10, and making marks on the mold according to the directions of two sides of a fracture of the steel wire mesh.
And continuously pouring the cement base layer 10, and placing another steel wire mesh to enable fracture directions of the two steel wire meshes to be consistent. And after the last cement base layer is poured on the second steel wire mesh, placing the formed cement base test piece in a standard curing room. And when the mold is removed after the curing is carried out for 24 hours, drawing a corresponding mark line on the side surface of the cement-based test piece according to the mark on the mold, and then continuing the curing until the test age.
After the maintenance is finished, the cement-based test piece is placed on the lower gasket, and the joint position between the cement-based test piece and the lower gasket is adjusted, so that the marking line is superposed with the central line of the lower gasket. And then placing the two on a lower loading plate of a compression testing machine together, arranging an upper gasket, starting the compression testing machine to enable the upper loading plate and the lower loading plate to be in contact with the gasket, and controlling the loading load to be not more than 100N.
And a laser displacement meter is arranged on the side of the pressure testing machine, so that a camera of the laser displacement sensor is aligned to the middle part of the sample, and the laser displacement meter is connected with a computer to finish calibration.
And continuously applying load to the cement-based test piece through a pressure testing machine, and controlling the loading speed of 0.1mm/min according to a displacement control mode. And stopping loading when the width of the crack reaches the target width of 0.5mm detected by the laser displacement sensor, unloading after the load is kept for 30s, and taking down the cement-based test piece from the pressure testing machine to finish the manufacture of the crack of the cement-based test piece.
Therefore, according to the method, the prefabricated test piece and the prefabricating system for prefabricating the crack of the cement-based test piece, provided by the invention, in the process of pouring and forming the cement-based prefabricated test piece, the construction type net which generates the weak area by removing the net line to form the fracture is used, so that when the prefabricated test piece is subjected to the loading action of a compression testing machine, the size and the position of the crack formed by the prefabricated test piece can be controlled through the construction type net, and the formed crack is close to the crack generated by the actual stress cracking of concrete.
It should be understood that equivalents and modifications of the technical solution and inventive concept thereof may occur to those skilled in the art, and all such modifications and alterations should fall within the scope of the appended claims.
Claims (7)
1. A method for prefabricating a cement-based test piece crack is characterized by comprising the following steps:
manufacturing a structural net: the structural net is a latticed structural net with a rectangular grid shape, grids in a preset area of the latticed structural net are removed, and a fracture is formed by network wires in the preset area to serve as a weak area of the structural net;
manufacturing a cement-based test piece: after a first layer of cement base layer is poured in a mould, a structural net is placed above the mould, the two side directions of a fracture are recorded on the mould, a second layer of cement base layer is poured to cover the structural net, the mould is removed after a cylindrical cement-based test piece is formed through maintenance, and marking is carried out according to the record of the mould at the corresponding position of the side surface of the cement-based test piece;
and (3) prefabricating and installing cracks: after the cement-based test piece is maintained to a specified age, placing the cement-based test piece between an upper loading plate and a lower loading plate of a pressure testing machine, and selecting a loading point according to the mark;
and (3) crack preparation: and starting a pressure testing machine, applying a vertical load to the cement-based test piece, acquiring width data of a crack generated by the test piece in real time through a laser displacement sensor, and stopping loading and load-holding of the pressure testing machine when the width of the crack reaches a target width.
2. The method for prefabricating the cement-based test piece crack as recited in claim 1, wherein the manufacturing of the cement-based test piece: after a first layer of cement-based layer is poured in the mould, a structural net is placed above the mould, the directions of two sides of a fracture are recorded on the mould, a second layer of cement-based layer is poured to cover the structural net, the mould is removed after a cylindrical cement-based test piece is formed through maintenance, and the steps of marking the corresponding position of the side surface of the cement-based test piece according to the record further comprise:
and continuously repeating the pouring steps for a plurality of times, namely continuously placing a new configuration net on the uppermost cement base layer of the formed cement-based test piece, wherein the size and the orientation of the fracture of the new configuration net are consistent with those of the fracture of the placed configuration net, and pouring a new cement base layer on the new configuration net.
3. The method for prefabricating cement-based test piece cracks as claimed in claim 1, wherein said step of placing the cement-based test piece between upper and lower load plates of a compression testing machine after curing the cement-based test piece to a specified age and selecting a load point according to the mark comprises:
after the test piece is maintained to a specified age, erecting the cement-based test piece between an upper loading plate and a lower loading plate of a pressure testing machine;
and selecting a loading point of the compression testing machine according to the marked position of the side surface of the cement-based test piece, and arranging a gasket at the loading point, wherein the contact surface of the gasket and the cement-based test piece is an inwards concave arc surface matched with the side surface of the cement-based test piece.
4. The method for prefabricating the cement-based test piece crack according to claim 1, wherein the step of starting the compression testing machine, applying a vertical load to the cement-based test piece, acquiring width data of the crack generated by the test piece in real time through a laser displacement sensor, and stopping loading and maintaining the loading of the compression testing machine when the width of the crack reaches a target width further comprises the following steps:
arranging a laser displacement sensor at the side of the pressure testing machine, and connecting the laser displacement sensor and the pressure testing machine with a computer;
and the camera of the laser displacement sensor is aligned to the middle part of the test piece to finish calibration.
5. The method for prefabricating the cement-based test piece crack according to claim 4, wherein the step of starting the compression testing machine, applying a vertical load to the cement-based test piece, acquiring width data of the crack generated by the test piece in real time through a laser displacement sensor, and stopping loading of the compression testing machine when the width of the crack reaches a target width comprises;
starting a pressure testing machine, applying a vertical load to a test piece by adopting a displacement control mode, and controlling the loading speed to be lower than a preset value so as to avoid the generation of cracks and the damage of the test piece caused by the over-high expansion speed;
the target width of the crack is set on the computer, the width data of the crack generated by the test piece is collected in real time through the laser displacement sensor and sent to the computer, and when the computer analyzes that the width of the crack reaches the target width, the loading is stopped and the load is preserved.
6. Method for prefabricating the cracks of cement-based test pieces according to claims 1-5, characterized in that the structural mesh is a steel or fiber mesh.
7. A prefabrication system for prefabricating cement-based test piece cracks comprises a pressure testing machine and a laser displacement sensor and is characterized by further comprising a prefabricated test piece;
the prefabricated test piece is a cylindrical cement-based test piece and comprises a configuration net and cement base layers positioned on two sides of the configuration net;
the structural net is a grid structural net with a rectangular grid shape, and comprises an encryption area and a reserved weak area positioned in the encryption area, and the network cables in the reserved weak area are removed to form a fracture;
marks are arranged at the facing positions of the two sides of the fracture on the side surface of the prefabricated test piece;
the prefabricated test piece is arranged between an upper loading plate and a lower loading plate of a pressure testing machine, and the loading point of the pressure testing machine corresponds to the mark of the prefabricated test piece;
a gasket is arranged between the mark of the prefabricated test piece and the loading point of the pressure testing machine;
one surface of the gasket is a plane matched with a loading plate of the compression testing machine;
the other side of the gasket is an inwards concave arc surface matched with the side surface of the prefabricated test piece;
the laser sensor and the pressure testing machine are both connected with a computer, the laser sensor is located on one side of the pressure testing machine, and a camera of the laser sensor is aligned to the middle of the prefabricated test piece.
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| CN115290409A (en) * | 2022-08-03 | 2022-11-04 | 燕山大学 | Method for inducing and quantitatively characterizing cracks of reinforced concrete test piece |
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