During the construction works at site, we need to know and perform different cement test to check the quality of cements.
At site, cube tests should be carried out for each 30 cum of concrete / each batch of cement. Samples from fresh concrete should be taken. At least a set of 6 cubes of 150mm shall be prepared and cured.
Similarly for compressive strength test 3 Cubes each at 7 days and 28 days shall be tested . After that the test results should be submitted to engineer for approval.
Here, in this post the folowing important test to be perform for quality check are described in detail.
Fineness test:
This is the most important property of cement that determines, to a great extent, the quality of the cement. Generally speaking, finer the cement in grain size, the better is its quality. Finer cement reacts quicker with water and develops early strength through the ultimate strength is not affected. However, finer cement increases the shrinkage and cracking of concrete.
The fineness is tested by the following two methods:
a. Sieve analysis: 90-micron sieve is used.
b. Specific surface: Blaine Air permeability apparatus is used.
Consistency Test
This test is conducted to determine the percentage of water required for preparing cement pastes of standard consistency for other tests (e.g. setting time, soundness, compressive and tensile tests, etc.)
This test is performed with the help of Vicat’s apparatus.
The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10mm diameter and 50mm length to penetrate to a depth of 33-35mm from the top of the mould. The test is required to be conducted in a constant temperature 27±2o
Initial setting time test: C and constant humidity 90%.
This test is carried out by preparing the standard paste of cement. This paste is placed in the Vicat’s mould. The needle of the 1mm2 cross-section is fitted with the Vicat apparatus and brought in contact with the surface of the paste.
Then the needle is quickly released to allow penetration. This process is carried out until the needle penetrates the paste 33-35mm from the top (i.e. 5-7mm from the bottom). The time elapsed from placing water to the cement and allowing the standard penetration is the initial setting time of cement.
Final setting time test
The final setting time test is carried out first by preparing the standard paste. Then the paste is filled in the Vicat’s mould and placed over the Vicat’s apparatus. The annular collar is fitted with the apparatus. This needle is brought to touch the surface of the paste and released gently to allow penetration. The final setting is said to occur when the central needle makes an impression over the surface of the paste but circular cutting edge fails to do so.
Soundness test:
It is very important that the cement after setting shall not undergo any appreciable change in volume. Certain cement has been found to undergo a large expansion after setting causing disruption of the set and hardened mass. This will cause serious difficulties for the durability of structures when such cement is used. This expansion is sometimes responsible for the crack of cement after setting. This is an unsound quality.
By soundness of cement is therefore understood its capacity to form a non-disintegration, hard, and uniformly strong mass on the setting. This depends on its original composition, proper burning or manufacturing, and grinding process. Presence of lime in a free state, an excess of sulphate or magnesia, and a very fine size may be the cause of unsoundness in cement. Le- Chatelier apparatus is used for finding soundness of cement.
Compressive strength test:
The compressive strength of hardened cement is the most important of all the properties. Therefore, it is not surprising that the cement is always tested for its strength at the laboratory before the cement is used in important works. Strength test are not made on neat cement paste because of difficulties of excessive shrinkage and subsequent cracking of neat cement.
The strength of cement is indirectly found on cement sand mortar in specific proportions. The standard sand is used for finding the strength of cement. The ratio of cement to sand should be 1:3. Take cement and sand in a non-porous enamel tray and mix them with a trowel for one minute, then add water of quantity P/4+3.0 % of the combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform color. The time of mixing should not be less than 3 minutes not more than 4 minutes. Immediately after mixing, the mortar is filled into a cube mould of size 7.06cm.
The area of the face of the cube will be equal to 50sq.cm. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating equipment. The compressive strength for 3 days and 7 days of OPC should not be less than 10.5N/mm2 and 17.5N/mm2.
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