Analysis on the method of testing cement strength

1. Specimen preparation

1. Preparation of mortar: mix 450±2g of cement, 1350±5g of standard sand, and 225±1g of water in a mixer for mechanical mixing of mortar. Put the water into the pot first, then add the cement, put the pot on the fixed rack, and raise it to a fixed position. Then immediately start the machine, stir at low speed for 30s, and add sand evenly at the same time as the two 30s start. When each level of sand is sub-packed, start from the coarse-grained level, and add up the required amount of each level of sand in turn. Turn the machine to high speed and mix for another 30 seconds. Stop mixing for 90 seconds, and use a rubber scraper to scrape the sand on the blade and the pot wall into the middle of the pot within .1 15s. Continue to stir at high speed for 60 seconds.

2. Forming immediately after the mortar is prepared. Fix the empty test mold and the mold sleeve on the vibrating table, and use a suitable spoon to load the mortar into the test mold in two layers directly from the mixing pot. When installing one layer, put about 300g of mortar in each tank, use a large spreader to vertically spread the material on the top of the mold sleeve and back and forth along each mold slot once, and then vibrate 60 times. Load the second layer of mortar again, spread it flat with a small spreader, and vibrate it again 60 times. Remove the mold sleeve, remove the test mold from the vibrating table, use a metal ruler to put a metal ruler on one end of the top of the test mold at an angle of approximately 90? Move one end to scrape off the mortar that exceeds the part of the test mold at a time, and use the same straight ruler to smooth the surface of the test body almost horizontally. Mark or add a note on the test mold to indicate the number of the test piece and the position of the test piece relative to the tapping table.

3. Clamp the test mold and the blanking funnel to the center of the vibrating table while mixing the mortar. Put all the mixed mortar evenly into the blanking funnel, start the vibrating table, and the mortar will flow into the test mold through the funnel. Vibrate 120s±5s to stop. After the vibration is completed, remove the test mold, use a flat ruler to scrape and smooth the rubber sand that is higher than the test mold. Then mark the test mold or use a note to indicate the test piece number.

2. Specimen maintenance

Remove the mortar left around the mold, and put the test mold on a horizontal shelf in a fog room or wet box for curing. The moist air should be able to contact all sides of the test mold. During maintenance, the test mold should not be placed on other test molds. Remove the demold until the specified demolding time, and mark the sample before demolding. Then put the test body in 20℃±1℃ water for curing. When placed horizontally, the scraping plane should face upwards. The test piece is placed on a non-perishable shelf and kept a certain distance between each other to allow water to contact the six surfaces of the test piece. During the curing period, the interval between the specimens or the water depth on the upper surface of the specimen shall not be less than 5mm. Except for the specimens that are 24h old or delayed to 48h demolding, any specimens that have reached the age should be taken out of the water 15min before the test (broken type). Wipe off the deposits on the surface of the test body and cover it with a damp cloth until the test.

3. Test procedure

1. Determination of flexural strength

Place one side of the test body on the supporting cylinder of the testing machine, the long axis of the test body is perpendicular to the supporting cylinder, and the load is uniformly applied to the opposite side of the prism at a rate of 50N/s±10N/s through the loading cylinder. Until it breaks. The flexural strength is calculated as follows:

Rf = 1.5FtL/b3

Where: Ft-the load applied to the middle of the prism when it is broken;

L-the distance between the supporting columns;

B-the side length of the square cross-section of the prism.

2. Determination of compressive strength

Load evenly on the side of the half prism at a rate of 2400N/s±200N/s until it breaks. The compressive strength is calculated as follows:

Rc = Fc/A

In the formula: Fc-large load at the time of failure;

A-The area of ​​the compressed part.

4. Confirmation of test results

1. Take the average value of the flexural strength results of a group of three prisms as the test result. When the three strength values ​​exceed the average value by ±10%, the average value should be eliminated and then taken as the flexural strength test result;

2. Take the arithmetic average of the six compressive strength measured values ​​obtained on a set of three prisms as the test result. If one of the six measured values ​​exceeds ±10% of the average value of the six, this result should be eliminated , And the average of the remaining five as the compressive strength test results. If any of the five measured values ​​exceeds ±10% of their average, this set of results is invalid.

The cement strength can only be used in construction projects after passing the above tests and meeting the standards. This is responsible for production safety.