A Brief Analysis Of The Method Of Testing Cement Strength

Specimen preparation

1. Preparation of mortar: Mechanically stir mortar with 450±2g of cement, 1350±5g of standard sand and 225±1g of water in a mixer. First add water to the pot, then add cement, place the pot on the fixed rack and raise it to a fixed position. Then start the machine immediately, stir at low speed for 30 seconds, and add the sand evenly at the beginning of the second 30 seconds. When each grade of sand is to be packed separately, start from the coarse grade and add the required amount of sand for each grade in sequence. Turn the machine to high speed and mix for 30 seconds. Stop mixing for 90 seconds, and within 15 seconds, use a rubber scraper to scrape the glue sand on the blades and pot walls into the middle of the pot. Continue stirring at high speed for 60 seconds.

2. Molding is carried out immediately after the mortar is prepared. Fix the empty test mold and mold sleeve on the shaking table, and use an appropriate spoon to directly load the glue sand into the test mold in two layers from the mixing pot. When installing one layer, put about 300g of mortar in each groove, use a large spreader to stand vertically on the top of the mold sleeve and spread the material back and forth once along each mold groove, and then tap it 60 times. Then put in the second layer of mortar, spread it evenly with a small spreader, and then tap it 60 times. Remove the mold cover, remove the test mold from the vibration table, place a metal ruler on one end of the test mold top at an angle of approximately 90 degrees, and then slowly move it toward the other end with a transverse sawing motion along the length of the test mold. Scrape off the glue sand that exceeds the test mold at a time, and use the same ruler to smooth the surface of the test body almost horizontally. Mark or add a note on the test mold to indicate the test piece number and the position of the test piece relative to the vibration table.

3. While stirring the mortar, clamp the test mold and the unloading funnel in the center of the vibrating table. Put all the mixed mortar evenly into the unloading funnel, start the vibrating table, and the mortar flows into the test mold through the funnel. Stop after vibrating for 120s±5s. After the vibration is completed, remove the test mold, use a scraper to scrape off the glue sand that is higher than the test mold and smooth it. Then mark the test mold or use a note to indicate the test piece number.

Specimen maintenance

Remove the glue left around the mold, and place the test mold on a horizontal shelf in a mist chamber or wet box for curing. The moist air should be able to contact all sides of the test mold. Test molds should not be placed on other test molds during maintenance. Take out the demoulding until the specified demoulding time, and mark the specimen before demoulding. Then put the specimen in 20℃±1℃ water for curing. When placed horizontally, the scraping surface should face upward. The specimens are placed on non-perishable racks and spaced apart from each other to allow water to come into contact with all six sides of the specimens. During the curing period, the distance between specimens or the water depth on the upper surface of the specimen shall not be less than 5mm. Except for specimens aged 24 hours or whose demolding is delayed to 48 hours, any aged specimens should be taken out of the water 15 minutes before testing (breaking). Wipe off the sediment on the surface of the test object and cover it with a damp cloth until the test.

Test procedure

1. Determination of flexural strength

Place one side of the specimen on the support cylinder of the testing machine, with the long axis of the specimen perpendicular to the support cylinder, and apply the load vertically and uniformly on the opposite sides of the prism at a rate of 50N/s±10N/s through the loading cylinder until it breaks. The flexural strength is calculated according to the following formula:

Rf = 1.5FtL/b3

In the formula: Ft-the load applied to the middle part of the prism when it breaks;

L - distance between supporting cylinders;

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

2. Determination of compressive strength

Load the side of the half prism uniformly at a rate of 2400N/s±200N/s until failure. The compressive strength is calculated according to the following formula:

Rc = Fc/A

In the formula: Fc - maximum load at failure;

A-The area of the compressed part.

Test results confirmed

1. Use the average of the flexural strength results of a set of three prisms as the test result. When any of the three strength values exceeds the average ±10%, it should be eliminated and then the average value should be taken as the flexural strength test result;

2. The arithmetic mean of the six compressive strength measurements obtained on a set of three prisms is used as the test result. If one of the six measured values exceeds the average value of the six by ±10%, this result should be discarded and the average of the remaining five should be used as the compressive strength test result. If any of the five measured values ​​exceeds ±10% of their average, the results of this set will be invalid.

Cement strength must pass the above test and meet the standards before it can be used in construction projects. This is responsible for production safety.