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What compressive strength of concrete? How determine the cube compressive strength?
The compressive strength of the concrete is the important parameter as per the design requirements in structures. It is the ability of the structural elements that will carry the load without any cracks or deflections. The compressive strength of concrete is generally determined by using universal testing machine (UTM) in the laboratory for the cube specimens.
The strength is generally determined at the age of 3 days, 7 days, 14days, 28days, 56days, 90day and 180 days curing periods.
The compressive strength of concrete cube is determined by using below formulae
Compressive strength = P/A
Where P = Maximum applied load on cube specimen.
A= Cross sectional area of cube specimen.
Process of determination of compressive strength of concrete
The complete compressive strength of concrete specimens is determined by using below specified three stages
- Preparation of test specimens
- Testing the compressive strength using UTM
- Calculation of compressive strength
Preparation of test specimens
The preparation of test specimens is the process of making concrete cube from mix design calculation to curing period process.
The below specified steps involves the making the cube specimen in the compressive strength testing.
Step 1: Mix design calculation
Mix design is the process of determining the proportions of materials used to make concrete. Example cement, fine aggregates, coarse aggregates and water. In order to calculate the mix design of concrete specimens we required material properties like specific gravity, fineness modulus, fineness, initial and final setting times, grading of aggregates, water absorption etc. The mix design of concrete is determined by using IS 10262 and IS 456 codes as per the Indian conditions. And other methods like British codes, ACI codes and trial and adjustment methods are also available to make proportions. The complete process of steps used in mix design of concrete and mix calculation of M20 grade concrete is explained in my previous posts read them with below links.
Calculation of mix design of M20 grade concrete by using IS10262 and IS 456 code
What is mix design of concrete and steps followed in mix design calculation
Step 2: Batching of material
After determining the mix proportions as per the mix proportions we need to follow the batching of materials. The batching is the process of determining the quantity of materials required to make the concrete cube specimens. The batching is of two types one is volume batching and second one is weight batching. For the perfect concrete mixes we will use weight batching other than volume batching. Volume batching of concrete is used if the density of the materials is less.
Step 3: Mixing of concrete
The next step in the preparation of samples is mixing of concrete. Initially the coarse aggregates and fine aggregates are mixed properly then cement is added to the mixture of aggregates and again mix the concrete until it gets uniformity. The water is added to the mix as per the calculations and finally mix again until it gets to uniformity. The prepared concrete is called as fresh concrete and workability tests are used for studying the concrete behavior.
Step 4: Casting of the specimens and demoulding
The cubes of 150mmX150mmX150mm or 100mmX100mmX100mm moulds are used in the casting of cube. After mixing the concrete fill the mould with 1/3rd level and compact with tampering rod with 25 blows, again fill with one more 1/3rd level and give 25 blows with tampering rod and finally fill the concrete with total concrete (another 1/3rd) and give 25 blows tampering and finish the top surface. After casting process is done leave the specimen for 24 hours to get hardening and demould then from the cube moulds.
Step 5: Curing of the test specimen
After demoulding is done from the cube moulds the cubes are released in to water tank. The curing of text specimens are generally done for 3days, 7days, 14days, 28days, 56days and 90days time period. Clear portable water is generally used in the curing process of concrete.
Percentage of strength gain by cube
The below table shows the percentage of strength gain for the cube specimens for the period of 1day, 3days, 7days, 14days and 28days.
|
S. No |
Curing period |
Percentage of strength gain |
|
1 |
1 day |
16% |
|
2 |
3 days |
30% |
|
3 |
7 days |
65% |
|
4 |
14 days |
90% |
|
5 |
28 days |
99% |
For the 1day the percentage strength gained by 16%, for 3days the percentage strength gained by 30%, for 7days the percentage strength gained by 65%, for 14 days the percentage strength gained by 90%, and finally for 28days the percentage strength gained by 99%.
Test machine used in compressive strength testing
The test machine of universal testing machine (UTM) is used to determine the compressive strength of concrete.
Test procedure to determine the compressive strength of concrete
The following test procedure is used to determine the compressive strength of concrete
- Take the test specimen from the curing days required time period.
- Remove the moisture content with wet cloth around the specimen.
- Keep it in the sun light for the 2 to 3 hours in order to removing the moisture content from the cube specimen.
- Now place the test specimen in between the two plates of the UTM machine.
- Now apply the load at the rate of 140kg/m2/min for the specimen.
- Apply load until the failure in the specimen is obtained and note the load at which specimen fails.
Calculation of compressive strength of concrete example
The below specified calculation shows the sample compressive strength calculation 300kN load.
Compressive strength = Failure load/Area
= 300X103/ (150X150)
= 13.333 N/mm2
Follow our previous posts with below links
What are the different types of foundations used in building construction
What are the basic parameters considered in seismic analysis of building
Loads and different types of loads used in building design as per structural design
Conclusions on how to determine the compressive strength of concrete
The compressive strength of the concrete is the amount of maximum load applied on the cube specimen at which failure occurs. The above explained concepts are related to the complete concept of what is compressive strength of concrete and casting of the cube specimen and test procedure to determine the compressive strength of concrete for 1day, 3days, 7days, 14days, 28days, 56days, 90days and 180days curing periods.
For more information related to the civil engineering concepts follow my YouTube channel Civil engineering by shravan.
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