According to national standards, the strength of cement is measured by mortar method. The strength of cement is to mix cement and standard sand according to the ratio of 1: 3, add a specified amount of water, make a specimen according to the specified method, and maintain it according to the specified requirements, and measure its compressive strength and flexural strength at 3d and 28d respectively. Determine the strength grade of cement according to the test results.
1 range
This standard specifies the mortar composition, instruments and equipment, test steps and result calculation of cement mortar dry shrinkage test.
This standard is applicable to Portland cement, ordinary Portland cement, slag Portland cement, pozzolanic Portland cement,
Fly ash portland cement, composite portland cement, road portland cement and other cement pastes designated to adopt this standard.
2 reference standard
The clauses contained in the following standards constitute the clauses of this standard by reference in this standard. When this standard was published,
All versions displayed are valid. All standards will be revised, and all parties using this standard should explore the following standards.
Possibility of version.
GB 177-85 Test method for strength of cement mortar
GB 178-77 Standard Sand for Cement Strength Test
GB/T 24 19-94 determination method of fluidity of cement mortar
GB 3350. 1-82 cement physical inspection instrument-mortar mixer
Gb/t17671-1999 cement mortar strength test method (ISO method)
3 method principle
The method adopts a 25mm×25mm×280mm 1:2 mortar sample with spherical nail heads at both ends,
After curing in air with a certain humidity, the drying shrinkage of cement mortar is determined by measuring the length change of specimens at different ages with a hydrometer.
Performance.
4 Instruments and equipment
4. 1 mortar mixer
Comply with the provisions of GB3350.l
4.2 The platform, frustum die, die sleeve, round rod and vernier caliper used in the fluidity test meet the requirements of GB24 19.
4.3 mold testing
The trial die is a triple die, which consists of vertical partition, end plate, base and positioning screw. The structure is shown in figure 1.
Show. Each component can be disassembled, and the inner wall size of each component is 25mmX25ntmX280mm. There are three measuring nails on the end plate.
The position of the head hole should ensure that the measuring nail head of the specimen is on the axis of the specimen after molding.
4.3. 1 The measuring nail head is made of stainless steel or copper, and the specifications are shown in Figure 2. When the sample is molded, the nail head extending into the end plate of the test mold is measured.
The depth is 10 1 mm.
4.3.2 The partition and end plate are made of 45 # steel, and the surface roughness is not more than 6.3 microns. ..
4.3.3 The base is made of HT20-40 gray cast iron, and the roughness of the upper surface of the base is not more than 6.3 microns. The base is not machined.
After painting, there are no flow marks.
4.4 tamping rod
Tamping includes square tamping and notch tamping, and the specifications are shown in Figure 3, both of which are made of metal. Compressive area of square tamping rod
It's 23 mm x 23 mm. Notch rammer is used for tamping and measuring rubber sand on both sides of nail head.
4.5 Sand scraping plate
It is made of metal materials that are not easy to rust and are not corroded by cement slurry. See Figure 4 for specifications.
4.6 Cement mortar shrinkage curing humidity control box
4.6. 1 curing box structure
Made of plastic that is not easily corroded by drugs, its smallest unit can maintain 6 specimens and form its own sealing system, and the smallest one is single.
The structure of this element is shown in the figure. The effective volume is 340mmX220mmX200mm, and there are five bars for placing specimens, which are divided into upper and lower bars.
In the next two parts, the width of the bar is 10mm and the height is 15mm, the interval between them is 45 mm, and the space above the bar is 65mm high.
The lower part of the bar is used to place a medicine tray and control the humidity of the unit. The medicine tray is made of plastic and its size should not be limited by the lower part of the device.
In and out, the volume is about 2.5L.
Schematic diagram of drying shrinkage curing humidity control box unit
4.7 Instruments with Specific Length
It consists of a dial indicator, a bracket and a correcting rod, with a scale value of 0.0lmm and a maximum base length of not less than 300mm.
The distance is 10mm, and the middle contact part of the straightening rod should be covered with heat preservation.
Note: Other forms of length measuring instruments are allowed, but the accuracy must meet the above requirements, and the ratio should be used in arbitration inspection.
The document shall prevail.
5 kinds of test materials
5. 1 The cement sample shall pass through a 0.9mm square hole sieve in advance, record the sieve residue and mix it evenly.
5.2 Standard sand shall comply with GB 178.
5.3 The test water shall be clean fresh water.
6 Laboratory temperature and humidity
6. 1 temperature of sample forming chamber 17 ~ 25℃, and the relative humidity is more than 50%.
6.2 The temperatures of cement samples, mixing water, standard sand and instruments shall be consistent with those in the laboratory.
6.3 The drying shrinkage curing temperature of the specimen is 20 3℃ and the relative humidity is (50 4%).
7 composition of mortar
7. 1 lime sand ratio
The ratio of cement to standard sand in mortar is 1∶3. [ 1]
In order to measure the drying shrinkage of cement mortar, three samples should be molded, and 450±2g cement sample and1350 5g standard sand should be taken for molding. [ 1]
7.2 mortar water consumption
The water-cement ratio of mortar is usually 1∶2. [1] The water consumption of mortar is determined according to the fluidity of the mortar 130 ~ 140mm. The fluidity of mortar shall be determined according to GB24l9, but the proportion and weighing of mortar shall meet the requirements of 7. 1 of this standard.
8 sample molding
8. 1 Mold test preparation
Wipe the test mould clean before molding, and the contact surface between the template and the surrounding base should be coated with Huang Gan oil, which should be assembled tightly to prevent leakage.
Slurry, evenly brush a thin layer of engine oil on the inner wall. Then clean the nail head, dip a little Huang Gan oil in the round end of the nail head, and embed the nail head.
Insert it into the hole of the trial mold and turn it left and right in the hole to make the nail head match the hole accurately.
8.2 Preparation of Mortar
The preparation of mortar shall be carried out according to GB 177, but the ratio of lime to sand and the weighing shall conform to this standard 7. 1, and the water consumption of mortar shall conform to this standard.
Requirements of standard 7.2.
8.3 Forming of Specimen
The prepared mortar is divided into two layers and put into a test mold with nail heads at both ends. After the first layer of mortar is put into the trial mold,
Draw back and forth with a knife first, especially on both sides of the nail head. Draw several times if necessary, and then scrape the main height above the test height with a sand scraper.
3/4 of the mortar, and then with a square tamping rod of 23mmX23mm from the inside of the nail head, from one end to the other end in turn 10 times,
Return to ramming 10 times, ramming for 20 times, ramming twice on both sides of the nail head with notch rammer, and then loading the remaining mortar into the mold.
In the same way, draw evenly with a knife, the depth of the knife should pass the surface of the first layer of mortar, and then start from one end with a 23mmX23mm tamping rod.
Tamp 12 times in sequence, and tamp 24 times back and forth (tamp the mortar surface first and then tamp it hard every time). stuffing
Should be uniform and stable, and shall not be tamped), after tamping, put the rubber mortar on the edge of the test mold back into the test mold with a knife, and use a triangular scraper.
Number them after scraping, and put them into a curing box with a temperature of 20 3℃ and a relative humidity of more than 90% for curing.
9 Maintenance, preservation and measurement of specimens
9. 1 The sample shall be demoulded after curing in water for 24 hours. Then put the sample into water at a temperature of 20 2℃ for curing.
If demoulding is difficult, the demoulding time can be extended. The extended time shall be indicated in the test report and counted from the water conservation time.
Deduct.
9.2 After the specimen is cured in water for 2 days, take it out of the water and wipe off the moisture and dirt on the surface of the nail head with a wet cloth for long-term use.
The meter measures the initial reading. Before use, the length meter should be calibrated with a calibration rod, and its transfer point can only be tried after it is confirmed to be correct.
Volume measurement (zero is a reference number, not necessarily zero). After measuring the initial reading, the zero-point addition should be rechecked with a correction rod.
If the zero change exceeds 1, the whole batch of samples shall be re-determined. Then move the sample into the grille of the humidity control box for drying and curing.
On the curing strip, there should be gaps between samples, and the same batch of water samples can only be placed in one curing unit at most.
At the same time, two groups of effluent samples were put, and 0.5kg of relative humidity control drugs were put on the branches of the medicine tray in each group. Drugs can usually make
Potassium thiocyanate solid can be used, and other salts that can control the specified relative humidity can also be used, but salts that are harmful to human body and environment cannot be used.
Things. Tighten the bolt of the device to make it airtight and isolated from the outside world. The ambient temperature around the box is controlled at 20 3℃. Drugs at this time
The relative humidity in the device shall be (50 4%).
Dry-shrinkage specimens can also be cured under the condition of meeting the specified relative humidity and temperature, but it should be specially noted in the test report.
Needless to say, if there is any contradiction, the result of drying, shrinkage and curing of the humidity control box shall prevail.
9.3 When the sample is placed in the box for 4d,11d. 18d and 25d, (i.e. 7d from molding, 14d,
2 1d, 28d), and take out the measured length respectively.
Note: According to the curve of drying shrinkage of different types of cement with age, the measured age can be increased or decreased as needed.
9.4 The length of the sample shall be measured in the laboratory of 17 ~ 25℃, and the length meter shall be kept at constant temperature in the laboratory before it can be made.
Use.
9.5 When measuring, the upper and lower positions of the specimen in the length meter should always be the same. When reading, rotate the sample left and right.
Make the nail head of the test body contact the length gauge correctly, and the pointer swing shall not be greater than 2 squares. The reading should be recorded to 0.005 mm.
After measurement, the zero point shall be calibrated with a calibration rod. When the zero change exceeds 1 grid, the whole batch of samples shall be re-measured.
10 result calculation and processing
10. 1 The dry shrinkage St(%) of cement mortar samples at all ages is calculated according to the formula (1), and it is 0.00 1%.
L0-Lt
ST =────× 100 ……( 1)
250
Where: l0-initial measurement reading, mm;
Lt-the measured reading of a certain age, mm;
250-Effective length of sample, mm
10.2 result processing
If the drying shrinkage of one sample exceeds the middle value of 15%, the average shrinkage of three samples is taken as the drying shrinkage result of the sample.
Taking the middle value as the drying shrinkage result of the sample; When two samples exceed the median value of 15%, the test shall be repeated.