CN109856374B - Concrete self-shrinkage test method - Google Patents
Concrete self-shrinkage test method Download PDFInfo
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Abstract
The invention relates to a concrete self-shrinkage testing method, and belongs to the concrete shrinkage deformation testing technology. Immediately covering a moisture-preserving maintenance film after concrete pouring, removing a mold after final setting, sticking a test copper sheet, putting into a hygrometer, wrapping and sealing by using the moisture-preserving maintenance film, vertically fixing on a shrinkage test frame, testing shrinkage by using a contact method vertical length measuring mode, and recording humidity and shrinkage values. The invention adopts the moisturizing curing film to replace the traditional plastic film to seal the test piece, can eliminate the drying shrinkage influence caused by the permeability of the plastic film, maintains the humidity in the film and accurately tests the self-shrinkage values of different concretes.
Description
Technical Field
The invention relates to a concrete self-shrinkage testing method and a material, belonging to the concrete shrinkage deformation testing technology.
Background
The shrinkage of concrete after setting and hardening mainly includes chemical shrinkage, self-shrinkage and drying shrinkage. Chemical shrinkage refers to the reduction in absolute volume of solids and liquids in the slurry due to cement hydration. Self-shrinkage refers to the shrinkage caused by self-drying as a result of cement hydration. Drying shrinkage refers to the shrinkage of concrete in unsaturated air due to the loss of moisture to the outside. Among them, since chemical shrinkage can be compensated by the formation of microscopic pores, it is not entirely reflected on macroscopic volumetric deformation of concrete, whose macroscopic behavior is called self-shrinkage.
The self-shrinkage phenomenon is very obvious in high-strength concrete, self-compacting concrete and mass concrete, and even can cause concrete cracking under the constant-temperature water-curing condition, so that the accurate test of the self-shrinkage of the concrete has important significance for guiding the application of the concrete. For accurate testing of self-contraction, it is critical to exclude the effects of: (1) the humidity of the outside atmosphere is invaded and evaporated; (2) raising and lowering the temperature; (3) stress caused by external loads or restraints. At present, the traditional domestic self-shrinkage test method is to seal the hardened concrete by a plastic film and place the concrete in a constant temperature environment for testing. While research on self-contraction test methods has focused mainly on eliminating stress disturbances and starting the test from casting. Chinese patent (publication No. CN 202182880U) discloses a self-contraction tester, which is to pour concrete into a test mould, seal the mould with a plastic film, and finally test the self-contraction of the concrete from the beginning of pouring to any age period in an environment of 20 +/-2 ℃. Chinese patent (publication No. CN 102590485B) discloses a frame type concrete shrinkage testing device, which eliminates stress interference of a template to concrete. The test methods adopt plastic film sealing to eliminate the interference of humidity change, but the plastic film sealing performance is limited, the internal humidity is difficult to guarantee, and drying shrinkage is inevitably generated when the concrete is in an environment with the relative humidity lower than 100%. Therefore, the measured self-contraction data is large, and the self-contraction of the concrete cannot be accurately reflected.
Disclosure of Invention
In view of the defects of the existing testing method and material performance, the invention adopts a new material to improve the testing method, and designs a method which can eliminate the influence of drying shrinkage and accurately test the self-shrinkage of the concrete after final setting. The method is simple and easy to operate, does not need complex and expensive devices, and can accurately test the self-contraction values of different concretes.
The technical scheme adopted by the invention is as follows: a self-contraction test method for concrete adopts a moisture-preserving curing film as a sealing material to test the self-contraction of the concrete.
The maintenance membrane consists of an airtight plastic film surface layer, a microporous permeable and breathable non-woven fabric bottom layer and a water storage high polymer material middle layer.
The curing film can maintain the air humidity of the concrete surface to be more than 95% in a close-contact state.
Comprises the following steps: 1) forming concrete samples, wherein every three concrete samples form a group; 2) immediately covering a curing film sprayed with water on a molding surface after molding; 3) after final setting, removing the mold, and adhering a humidity sensor at the middle part of the test piece and a copper sheet at the middle position of one end of the test piece; 4) wrapping the sealed test piece by using a pre-sprayed maintenance film to enable the maintenance film to be closely attached to the concrete test piece; 5) vertically fixing the test piece on a shrinkage test frame provided with a dial indicator, wherein the end of the test piece stuck with the copper sheet faces upwards; 6) placing the test piece in an environment with the temperature of 20 +/-2 ℃ and the relative humidity of 60 +/-5 percent for maintenance; 7) humidity sensor readings and dial gauge readings are recorded periodically.
The concrete test piece size is 100mm 400mm, the copper sheet is a square block with the thickness of 1 mm-2 mm and the side length of 20 mm-40 mm, the measuring range of the hygrometer is 0-100% RH, the resolution is 0.1% RH, and the precision of the dial indicator is +/-0.001 mm.
The pre-watering amount of the maintenance film before wrapping the test piece is 80-180 g/m2。
And regularly observing humidity data and the attaching condition, and replenishing water in time for the maintenance film and re-attaching tightly when the humidity is lower than 95%.
Reading the value of the dial indicator periodically, and taking the average value of the three test values as a test result when the deviation between the test value and the average value of a group of three test pieces is less than 15%; when the deviation of the test value of one test piece from the average value is more than 15 percent, and the difference between the other two test values is less than 15 percent, taking the average value of the other two values as a test result; otherwise, retesting.
The invention has the advantages that: when the polymer water storage material in the curing film absorbs a proper amount of water, the water can not flow, the water is released to the test piece, the permeability of the plastic film can be compensated, and the humidity in the film is maintained to be more than or equal to 95%. The method is simple and easy to operate, and complex and expensive devices are not needed; compared with the traditional plastic film sealing, the method can compensate the water vapor permeability of the plastic film and the reduction of the system humidity caused by cement hydration, maintain the internal humidity of the system and eliminate the influence of drying shrinkage; the self-contraction value from final setting to each age of different concrete can be accurately tested.
Detailed Description
In order to explain the features of the present invention and the effects achieved, the following description is made in detail with reference to the embodiments.
Example 1
C30 concrete mixing ratio:
TABLE 1C 30 concrete mix proportion (unit g/m)3）
| Cement | Fly ash | Sand | Stone (5 mm to 10 mm) | Stone (10 mm to 20 mm) | Water (W) | Water reducing agent | Air entraining agent |
| 288 | 72 | 716 | 467.2 | 700.8 | 155 | 1.8 | 0.4 |
According to the mixing ratio of table 1, the concrete is mixed and loaded into a test mold of 100 × 400mm, and immediately covered with 100g/m of pre-sprinkled water2After final setting, the curing film is removed, and 30 to 30mm concrete is adhered to one end of the curing filmA copper sheet. Then dividing the test piece into three groups, and spraying water in advance to 100g/m2The water-saving moisture-preserving curing film is wrapped and put into a hygrometer, the copper sheet is exposed, and the lap joint is sealed by an adhesive tape. And finally, vertically fixing the copper sheet of the test piece on a shrinkage test frame provided with a dial indicator, and placing the test piece in a curing room with the temperature of 20 +/-2 ℃ and the relative humidity of 60 +/-5%.
Humidity was recorded periodically, dial gauge readings were taken for 28 consecutive days, and shrinkage values were calculated. The results are shown in tables 2 and 3;
TABLE 2 humidity recording table in the test piece
TABLE 3 test piece shrinkage record sheet
| Age/d | 1 | 3 | 7 | 14 | 28 |
| Shrinkage microstrain | 0.00 | -4.17 | -13.67 | -20.83 | -25.00 |
As can be seen from the data in table 2: the water-saving moisturizing maintenance film has good moisturizing stability;
as can be seen from the data in Table 3, the C30 concrete 28d age self-contracting microstrain was-25.00.
Example 2
C50 concrete mixing ratio:
TABLE 4C 50 concrete mix proportion (unit g/m)3）
| Cement | Fly ash | Mineral powder | Sand | Stone (5 mm to 10 mm) | Stone (10 mm to 20 mm) | Water (W) | Water reducing agent |
| 363 | 48.5 | 72 | 704 | 422 | 633 | 155 | 2.5 |
According to the mixing ratio of Table 4, the concrete was mixed and loaded into a test mold of 100X 400mm, and immediately covered with 100g/m of pre-sprinkled water2The blank control group is covered with a plastic film; and after final setting, removing the mould, and immediately sticking a 30 x 30mm copper sheet on one end of the concrete. Then dividing the test piece into three groups, and spraying water in advance to 100g/m in the experimental group2The water-saving moisture-preserving curing film is wrapped and put into a hygrometer to expose the copper sheet, and the lap joint is sealed by an adhesive tape; the blank control group was not treated. And finally, vertically fixing the two groups of test piece copper sheets on a shrinkage test frame provided with a dial indicator, and placing the test piece copper sheets in a curing room with the temperature of 20 +/-2 ℃ and the relative humidity of 60 +/-5%.
Humidity was recorded periodically, dial gauge readings were taken for 28 consecutive days, and shrinkage values were calculated. The results are shown in tables 5 and 6;
TABLE 5 humidity recording table inside test piece
TABLE 6 shrinkage chart of test piece
As can be seen from the data in table 5: the water-saving moisture-preserving curing film can ensure the saturation humidity in the experimental group.
As can be seen from the data in Table 6, the total shrinkage microstrain for the concrete 28 d-age was-358.33 and the self-shrinkage microstrain was-79.17, representing 22.09% of the total shrinkage.
Comparative example
C50 concrete mixing ratio:
TABLE 7C 50 concrete mix proportion (unit g/m)3）
| Cement | Fly ash | Mineral powder | Sand | Stone (5 mm to 10 mm) | Stone (10 mm to 20 mm) | Water (W) | Water reducing agent |
| 363 | 48.5 | 72 | 704 | 422 | 633 | 155 | 2.5 |
According to the mixing proportion of table 7, the concrete is mixed and filled into a test mould with 100 × 400mm, and the test group and the blank control group are immediately covered with plastic films; and after final setting, removing the mould, and immediately sticking a 30 x 30mm copper sheet on one end of the concrete. Then dividing the three test pieces into a group, wrapping the experimental group by using a plastic film, placing the experimental group into a hygrometer, exposing the copper sheets, and sealing the lap joints by using an adhesive tape; the blank control group was not treated. And finally, vertically fixing the two groups of test piece copper sheets on a shrinkage test frame provided with a dial indicator, and placing the test piece copper sheets in a curing room with the temperature of 20 +/-2 ℃ and the relative humidity of 60 +/-5%.
Humidity was recorded periodically, dial gauge readings were taken for 28 consecutive days, and shrinkage values were calculated. The results are shown in tables 8 and 9;
table 8 humidity recording table in test piece
TABLE 9 shrinkage recording chart of test piece
As can be seen from the data in table 8: the humidity inside the experimental group wrapped by the plastic film is always reduced in the 28d curing period, and the humidity of the environment where the concrete is located does not reach the saturated humidity;
as can be seen from the data in Table 9, the self-shrinking microstrain measured with plastic film sealing was-258.33, representing 72.09% of the total shrinkage value, with the self-shrinkage value being significantly greater.
Claims (4)
1. A concrete self-contraction test method is characterized by comprising the following steps: 1) forming concrete samples, wherein every three concrete samples form a group; 2) immediately covering a curing film sprayed with water on a molding surface after molding; 3) after final setting, removing the mold, and adhering a humidity sensor at the middle part of the test piece and a copper sheet at the middle position of one end of the test piece; 4) wrapping the sealed test piece by using a pre-sprayed maintenance film to enable the maintenance film to be closely attached to the concrete test piece; 5) vertically fixing the test piece on a shrinkage test frame provided with a dial indicator, wherein the end of the test piece stuck with the copper sheet faces upwards; 6) placing the test piece in an environment with the temperature of 20 +/-2 ℃ and the relative humidity of 60 +/-5 percent for maintenance; 7) regularly recording the reading of the humidity sensor and the reading of a dial indicator;
the pre-watering amount of the maintenance film before wrapping the test piece is 80-180 g/m2；
Regularly observing humidity data and a pasting condition, and replenishing water in time for the maintenance film and re-pasting tightly when the humidity is lower than 95%;
the maintenance membrane consists of an airtight plastic film surface layer, a microporous permeable and breathable non-woven fabric bottom layer and a water storage high polymer material middle layer.
2. The concrete self-contraction test method according to claim 1, wherein: the curing film can maintain the air humidity of the concrete surface to be more than 95% in a close-contact state.
3. The concrete self-contraction test method according to claim 1, wherein: the concrete test piece size is 100mm 400mm, the copper sheet is a square block with the thickness of 1 mm-2 mm and the side length of 20 mm-40 mm, the measuring range of the humidity sensor is 0-100% RH, the resolution is 0.1% RH, and the accuracy of the dial indicator is +/-0.001 mm.
4. The concrete self-contraction test method according to claim 1, wherein: reading the value of the dial indicator periodically, and taking the average value of the three test values as a test result when the deviation between the test value and the average value of a group of three test pieces is less than 15%; when the deviation of the test value of one test piece from the average value is more than 15 percent, and the difference between the other two test values is less than 15 percent, taking the average value of the other two values as a test result; otherwise, retesting.
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