CN109849152B - method and device for prefabricating microcracks of concrete member - Google Patents
method and device for prefabricating microcracks of concrete member Download PDFInfo
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- CN109849152B CN109849152B CN201810380162.0A CN201810380162A CN109849152B CN 109849152 B CN109849152 B CN 109849152B CN 201810380162 A CN201810380162 A CN 201810380162A CN 109849152 B CN109849152 B CN 109849152B
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Abstract
The invention relates to a method and a device for prefabricating microcracks of a concrete member, wherein the thickness of a required heat shrinkable film is determined according to the size of the microcracks to be prefabricated, and a corresponding shape is cut out; preparing concrete and pouring the concrete into a steel mould; when pouring to the position where the micro cracks are to be prefabricated, embedding a heat shrinkage film, then continuously pouring until the concrete is completely filled in the mold, and curing the concrete for 28 days under standard conditions after vibrating and compacting; and (3) placing the concrete sample which is cured on a heating plate for heating, changing the contact surface of the sample and the heating plate in the heating process, and generating micro cracks in the sample after the thermal shrinkage film shrinks. The method and the device have the advantages that the opening width of the micro-crack prefabricated in the concrete member at the part close to the surface of the sample is equal to the thickness of the heat shrinkable film embedded in the micro-crack in advance, the precision is high, and the micro-crack can be produced in the concrete test block in a short time.
Description
Technical Field
The invention belongs to the technical field of civil engineering and concrete construction, and particularly relates to a high-precision prefabricating method and device for micro cracks of a concrete member.
background
Cracks are a significant potential hazard affecting the safe operation of concrete buildings during service. The large-size visible cracks originate from the microcracks, and the microcracks can be found as early as possible and corresponding protective measures can be developed to effectively avoid the large-size cracks. In recent years, attention has been paid to the idea of finding microcracks in concrete structures by using an ultrasonic-excitation infrared thermography method, but the method is still in a test exploration stage. One of the important reasons limiting the popularization of the method is that the opening width magnitude of the detected crack cannot be determined, and the detection mechanism is still unclear. The root of both problems is that microcracks with precise opening width cannot be effectively produced. At present, cracks are mainly made in a test block in a mode of loading a concrete test block, and the test block containing the cracks is used for testing the excitation effect of an ultrasonic thermography. The opening width of the real crack generated by the load action can not be controlled. This is mainly manifested in two ways: firstly, the opening width cannot be actively set; second, the variation in the opening width is random, and the opening width of the crack at the surface of the test block may be greatly different from that at the near surface in the depth direction thereof. It is the uncontrollable opening width that makes it difficult to effectively define the opening width of a crack detected by ultrasonic thermography. Other credible microcrack prefabricating methods are that a thin steel sheet is pre-embedded when concrete is poured, then the steel sheet is pulled out at a certain proper time during the concrete curing period, and a cavity body left after the steel sheet is pulled out is the microcrack. According to the method, the steel sheet is taken out when the concrete is not formed, so that rubbing on the inner surface of a micro-crack to be formed is inevitable, the volume of a micro-crack cavity after the steel sheet is taken out is possibly changed in the curing process of the concrete, and the accuracy of the opening width of the prefabricated micro-crack in the formed concrete is difficult to ensure.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method and a device for quickly and accurately prefabricating micro cracks in a concrete member.
in order to achieve the technical purpose, the invention adopts the following technical scheme:
a high-precision prefabricating method for microcracks of concrete members comprises the following steps:
Determining the thickness of the standard body according to the size of the to-be-prefabricated microcrack, and cutting out a corresponding shape;
Preparing concrete, pouring the concrete into a steel mould, burying a standard body when the concrete is poured to a position where micro cracks are to be prefabricated, then continuously pouring the concrete until the concrete is completely filled in the mould, and curing the concrete for 28 days under standard conditions after the concrete is vibrated and compacted;
And (3) heating the concrete sample after the curing in a heating device, changing the contact surface of the sample and the heating plate in the heating process, and generating micro cracks in the sample after the standard body shrinks.
the method comprises the following steps:
1) And determining the thickness of the standard body according to the size of the to-be-prefabricated microcrack, and cutting out a corresponding shape.
2) The concrete is prepared and poured into a steel mould. When pouring to the position where the micro cracks are to be prefabricated, the standard body is buried. And then, continuously pouring until the concrete completely fills the mould. In the process of embedding the standard body, the surface of the concrete contacted with the upper surface and the lower surface of the standard body is ensured to be flat. And curing the concrete for 28 days under standard conditions after the concrete is vibrated and compacted.
3) The sample after completion of the above steps was placed on a stage with a heating system. A heating plate made of a specific material and with a specific thickness is arranged between the sample and the workbench; a heat insulating layer is arranged between the heating plate and the workbench. A temperature sensor was placed on the upper surface of the sample (the surface corresponding to the surface contacting the hot plate) and the stage power was adjusted to meet the shrinkage conditions of the heat shrinkable film in the sample.
4) And switching on a circuit of the workbench to heat the sample, and changing the contact surface of the sample and the heating plate in the heating process. After the standard body shrinks, microcracks appear in the sample.
As a further improvement of the invention, the standard body is a heat shrinkable film having a maximum thickness of less than 0.1 mm.
As a further improvement of the invention, when the standard body is embedded, a thin cement powder layer is scattered on the surface of the concrete in contact with the heat shrinkage film and is leveled in order to make the surface of the standard body in contact with the concrete smooth and reduce pores on the contact surface as much as possible.
As a further improvement of the invention, the power for heating the concrete sample is not less than 2000W.
As a further improvement of the invention, when the concrete sample is heated, each surface of the concrete sample penetrated by the standard body is required to be in direct contact with the heating plate for more than 3 min. Further, after the surface temperature of each of the concrete samples penetrated by the standard body reached 130 ℃.
another object of the present invention is to provide a micro-crack prefabricating apparatus for a concrete member.
The technical purpose of the invention is realized by the following technical scheme:
a micro-crack prefabricating device for a concrete member comprises a workbench, a heating plate and a temperature sensor;
the heating plate is fixed on the workbench, a heat insulation layer is arranged between the heating plate and the workbench, and a concrete sample is placed on the heating plate;
The temperature sensor is arranged on the upper surface of the concrete sample and used for measuring temperature;
The temperature sensor is limited in the rail and can transversely slide along the rail.
As a further improvement of the invention, the worktable is internally provided with a copper coil, and the heating power of the worktable is not less than 2000W. Furthermore, the insulating layer material selects silicon dioxide aerogel, and its thickness is no more than 6 mm. The heating plate is a metal plate with good magnetic conductivity, the melting point of the heating plate is more than 1000 ℃, and the thickness of the heating plate is between 1 and 2 mm. The selected temperature sensor should resist electromagnetic interference, and the upper limit of measurement is more than 130 ℃.
The invention relates to a method for quickly prefabricating microcracks in a concrete member according to the heat shrinkage characteristic of a heat shrinkage film after concrete is formed. In the whole prefabricating process, the shrinkage temperature of the heat shrinkable film is about 130 ℃, so that the heat shrinkable film cannot be changed in shape due to heat release of cement hydration in the concrete curing process; when the concrete is heated, the temperature of a sample is basically controlled to be above 130 ℃ and below 300 ℃, and when the temperature is lower than 350 ℃, the mechanical property of the concrete material is basically unchanged compared with the normal temperature; the thickness of the thermal contraction film is accurate, the concrete sample is molded when being heated, and the mechanical property of the concrete sample is not influenced by heating conditions, so the opening width of the micro-crack prefabricated in the concrete member at the part close to the surface of the sample by the method and the device of the invention is equal to the thickness of the thermal contraction film embedded in the micro-crack in advance, and the precision is high. In addition, the electromagnetic heating system has high energy utilization rate, and can produce micro cracks in the concrete test block in a short time.
drawings
FIG. 1 is a schematic structural diagram of a micro-crack prefabricating device for a concrete member according to the invention;
FIG. 2 is a portion of a sample surface with microcracks made in accordance with the present invention; wherein a is a micro-crack prefabricated by a PVC film with the thickness of 0.02 mm; b is a micro-crack prefabricated by a PVC film with the thickness of 0.03 mm; c is a micro-crack prefabricated by a PVC film with the thickness of 0.05 mm; d is a micro-crack prefabricated by a PVC film with the thickness of 0.08 mm;
FIG. 3 is the residual condition of the heat shrinkable film on the side wall of the microcrack cavity in the sample after heat shrinkage; wherein a is the residual condition of the heat shrinkable film on the left side wall of the crack cavity after heat shrinkage; b is the residual condition of the heat shrinkable film on the right side wall of the crack cavity after heat shrinkage;
The notation in the figure is:
1-1 workbench, 1-2 heat insulating layers, 1-3 heating plates, 1-4 temperature sensors, 1-5 supports, 1-6 concrete samples, 1-7 heat shrinkage films and 1-8 sliding rails.
Detailed Description
The invention is further described below with reference to examples and figures.
Example 1
This embodiment will be further described with reference to the accompanying drawings.
The micro-crack prefabricating device for the concrete member shown in FIG. 1 comprises a workbench 1-1, heating plates 1-3 and temperature sensors 1-4; the heating plate 1-3 is fixed on the workbench 1-1, a heat insulation layer 1-2 is arranged between the heating plate 1-3 and the workbench 1-1, and a concrete sample 1-6 is placed on the heating plate 1-3;
The temperature sensors 1-4 are arranged on the upper surfaces of the concrete samples 1-6 and used for measuring temperature;
The temperature sensor is characterized in that supports 1-5 are arranged on two sides of the workbench 1-1, a hollow structure is arranged in the middle section of a beam on the upper portions of the supports 1-5, rails 1-8 are arranged in the hollow section, and the temperature sensor 1-4 is limited in the rails 1-8 and can slide transversely along the rails 1-8.
In this embodiment, the worktable 1-1 is provided with a copper electromagnetic coil and a temperature control system, electromagnetic heating is adopted, the heating power is not less than 2000W, and the temperature control system can automatically cut off the circuit of the worktable when the surface temperature of the worktable is too high.
The heating plates 1-3 are metal plates with good magnetic conductivity, and the melting point is more than 1000 ℃; the thickness of the heating plate 1-3 is 1-2 mm.
The heat insulation layer 1-2 is made of silicon dioxide aerogel; the thickness of the heat insulating layer 1-2 is not more than 6 mm.
The selected temperature sensor is an anti-electromagnetic interference temperature sensor, and the upper measurement limit of the temperature sensor is more than 130 ℃.
Example 2
This example illustrates a method for performing microcrack prefabrication in a concrete structure using the apparatus of example 1.
The heating power in the embodiment is 3000W; the thickness of the heat insulation layer 1-2 is 6 mm; stainless steel is selected as the heating plate 1-3, and the thickness is 1 mm. The upper measurement limit of the temperature sensor is 300 ℃. 1-7 the heat shrinkable film is a PVC film.
the concrete member micro-crack prefabricating method comprises the following specific steps:
(1) PVC films with the thickness of 0.02mm, 0.03mm, 0.05mm and 0.08mm are selected to be prefabricated into micro cracks in concrete, and the length and the width of the films are 40 mm.
(2) C30 concrete was prepared and poured into steel molds of 40mm by 160 mm. When poured into the position where the microcracks are to be preformed, the heat shrinkable film is embedded. Wherein, only one PVC film with one thickness is embedded in one steel mould. And then, continuously pouring until the concrete completely fills the mould. In the process of embedding the heat shrinkable film, a thin layer of cement powder should be sprinkled on the concrete surface in contact with the upper and lower surfaces of the film. And curing the concrete for 28 days under standard conditions after the concrete is vibrated and compacted.
(3) and (3) placing the samples 1-6 after the steps are finished on a stainless steel heating plate 1-3, wherein PVC heat shrinkage films 1-7 are pre-embedded in the samples 1-6. Setting temperature sensor 1-4 on the upper surface of sample 1-6, and the measuring range of temperature sensor 1-4 is-10-300 deg.C. The heating power of the stage 1-1 was adjusted to 3000W.
(4) The circuit of the working table 1-1 is switched on, and the sample 1-6 is heated. And changing the contact surface of the samples 1-6 and the heating plate 1-3 in the heating process, so that the front and back surfaces of the samples 1-6 penetrated by the PVC heat shrinkable film 1-7 are in direct contact with the heating plate 1-3 for more than 3min, and stopping heating when the temperatures of the front and back surfaces reach 130 ℃. At this time, microcracks occurred in the samples at the positions of the pre-buried PVC heat shrinkable films 1 to 7 due to the shrinkage of the heat shrinkable films 1 to 7.
fig. 2 shows the shape of the microcracks on the sample surface, produced using PVC heat shrink films of four thicknesses. Wherein, the microcrack of FIG. 2(a) is formed by heat shrinkage of a 40mm × 40mm × 0.02mm heat shrinkable film; the microcracks of FIG. 2(b) are formed by heat shrinking a 40mm by 0.03mm heat shrinkable film; the microcracks of FIG. 2(c) are formed by heat shrinking a 40mm by 0.05mm heat shrinkable film; the microcracks of FIG. 2(d) are formed by heat shrinking a 40mm by 0.08mm heat shrinkable film. Figure 3 shows a typical residual condition (as indicated by the arrows) after heat shrinking of the heat shrinkable film. The heat shrinkable film is completely filled in the crack cavity before heating, and as can be seen from the figure, according to the microcrack manufacturing method and by means of a special device, microcracks are manufactured in the concrete member, the middle part of the cavity is reserved with a part of the heat shrinkable film, and the remained heat shrinkable film accounts for about 15 percent of the total amount of the heat shrinkable film; but no heat shrinkable film remains in the portions close to the sample surface (portions 1, 2, 3 and 4 in the figure), and can be used to calibrate the detection accuracy of the detection technique. Therefore, as can be seen from fig. 2, the width of the microcrack cavity (1, 2, 3, and 4 in the figure) near the surface of the sample is standard, and no thermal shrinkage film remains in the microcrack cavity, the microcrack near the surface can be used for calibrating the detection accuracy of the micro-size detection technology such as ultrasonic thermography.
Claims (9)
1. A method for prefabricating micro-cracks of a concrete member, comprising:
determining the thickness of the standard body according to the size of the to-be-prefabricated microcrack, and cutting out a corresponding shape;
preparing concrete, pouring the concrete into a steel mould, burying a standard body when the concrete is poured to a position where micro cracks are to be prefabricated, then continuously pouring the concrete until the concrete is completely filled in the mould, and curing the concrete for 28 days under standard conditions after the concrete is vibrated and compacted;
Placing the concrete sample which is finished maintenance in a heating device for heating, changing the contact surface of the sample and a heating plate in the heating process, and stopping heating when the surface temperature of each concrete sample penetrated by the standard body reaches over 130 ℃; after the standard body shrinks, microcracks appear in the sample.
2. the method of claim 1 wherein the standard body is a heat shrinkable film having a maximum thickness of less than 0.1 mm.
3. A method according to claim 1, wherein the standard body is embedded by applying a thin layer of cement powder to the surface of the concrete which is in contact with the standard body and leveling the concrete.
4. the method of claim 1, wherein the power to heat the concrete sample is not less than 2000W.
5. The method of claim 1, wherein each of the surfaces of the concrete samples penetrated by the standard body is in direct contact with the heating plate for more than 3 minutes when the concrete samples are heated.
6. the micro-crack prefabricating device for the concrete member used in the method of any one of claims 1 to 5, characterized by comprising a worktable, a heating plate and a temperature sensor;
The heating plate is fixed on the workbench, a heat insulation layer is arranged between the heating plate and the workbench, and a concrete sample is placed on the heating plate;
The temperature sensor is arranged on the upper surface of the concrete sample and used for measuring temperature;
the temperature sensor is limited in the rail and can transversely slide along the rail.
7. The device of claim 6, wherein the worktable is internally provided with a copper electromagnetic coil and a control system; the copper coil induces current in the heating plate through electromagnetic effect, the current drives the heating plate to generate heat, and the heating power is not less than 2000W; the control system cuts off the working circuit when the surface temperature of the workbench is overhigh, and the heating device is protected.
8. the device of claim 6, wherein the insulating layer is made of silica aerogel; the thickness of the heat insulating layer is not more than 6 mm.
9. The device of claim 6, wherein the heating plate is a metal plate with good magnetic permeability, and the melting point is more than 1000 ℃; the thickness of the heating plate is 1-2 mm.
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| CN113358756B (en) * | 2021-07-23 | 2022-05-17 | 水利部交通运输部国家能源局南京水利科学研究院 | Standard unit and method for testing contact state of inner wall of concrete microcrack in vibration excitation |
| CN113933122B (en) * | 2021-10-11 | 2022-05-13 | 水利部交通运输部国家能源局南京水利科学研究院 | Method for manufacturing three-dimensional axisymmetric smooth curved surface internal crack |
| CN114474326B (en) * | 2022-03-09 | 2023-10-10 | 内蒙古科技大学 | Prefabricated well bore for manufacturing prefabricated cracks and manufacturing method of prefabricated concrete test block |
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| CN103822807A (en) * | 2014-02-11 | 2014-05-28 | 中国水利水电科学研究院 | Method, device and test piece for judging low-temperature cracking resistance of material as well as preparation method thereof |
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