Compressive Strength of Concrete at 7 days, 28 days Calculation & Formula is required to get the compressive strength of the related concrete structure. Here we described the procedure of compressive strength test of cubes at 7 days and 28 days with calculation and formula as per IS Code.
What is Compressive Strength of Concrete
Overall Strength of a structure like flexural resistance and abrasion directly depends on the compressive strength of concrete. According to Wikipedia, the Compressive Strength of cement is defined as size concrete cubes tested in 28 days’ strength. Why do we test concrete cubes at 7, 14 & 28 days?
Concrete is a macro content with Sand, Cement, & Coarse aggregate as its micro ingredient and gains its 100% strength with time at the hardened state. Have a look at the below table. Concrete Strength Overtime – As you can see the concrete gains its strength rapidly until 7th & 14th Days. Then progressively increases from there.
Compressive Strength of Concrete IS Code
These IS Code used for:
Sampling procedure of concrete for cube casting – IS 1199
Cube casting, curing, and testing procedure – IS 516
Specific Limit for compressive strength – MORTH 5th revision, section – 1700
Apparatus and Tools Required for Compressive Strength
1. Compressive testing machine
a) The compressive testing machine or CTM shall have sufficient capacity to test any grade of concrete ( usually used in any type of project ). I recommend a minimum 2000 KN capacity.
b) The rate of loading shall be set at 140 kg/sq cm/minute ( 14 N/mm2/minute ).
c) The permissible error of compressive testing machine shall be not greater than ± 2 percent of the maximum load.
d) The bearing surface of the platens used in the compressive testing machine shall be maintained with a permissible variation limit of 0.02 mm.
e) The bearing face of the bottom plate can be rotated freely and tilted through small angles in any direction.
f) Compressive testing machine must be under calibration.
2. Weighing balance with 1 gram accuracy
Compressive Strength of Concrete Test Procedure
We described before –
- Concrete cube specimens kept in water shall be tested immediately on removal from the water and if they are still in the wet condition.
- Surface water and dirt shall be wiped off the specimens.
- Concrete cube specimens, if received dry, shall be kept in water for 24 hours before testing.
- The dimensions of the specimens to the nearest 0.2 mm (mostly not used) and their weight shall be noted before testing.
- The bearing surfaces of the compressive strength testing machine shall be wiped clean and any loose sand or other material removed from the surfaces of the cube specimen which are to be in contact with the compression machine platens.
- The axis of the cube specimen shall be carefully aligned in CTM with the center of the spherically seated platen.
- No packing shall be used between the faces of the test cube specimen and the steel plate of the compression testing machine.
- The load shall be applied in compression testing machine without shock and increased continuously at a rate of approximately 140 kg/sq cm/min until the resistance of the cube specimen to the increasing load reading breaks down and no greater load reading can be sustained.
- The maximum load applied to the concrete cube specimen shall be recorded.
Compressive Strength Testing at Ages
The ages of concrete cube specimens shall be calculated from the time of the addition of water to the dry ingredients.
Compressive strength tests shall be made at recognized ages of the test specimens, the most usual being 7 and 28 days.
Ages of 13 weeks and one year are recommended if compressive tests at greater ages are required. If the early strengths of concrete cube specimens required, tests may be made at the ages of 24 hours ± ½ hour and 72 hours ± 2 hours.
Compressive Strength of Concrete Calculation
The compressive strength of the cube specimen shall be calculated by dividing the maximum load applied to
the cube specimen during the test by the cross-sectional area and shall be expressed to the nearest kg per sq cm or N/mm2.
The average of three values shall be taken as the representative of the batch provided.
The individual variation shall not be more than ± 15 percent of the average. Otherwise, repeat tests shall be made.
Compressive Strength of Concrete Formula
Compressive strength = maximum load / cross-sectional area
suppose the load is 1000 KN and cross-sectional area (fixed for concrete cube) is 150 mm * 150 mm = 22500 mm2
compressive strength = 1000 KN / 22500 mm2
= 1000 * 1000 / 22500 N/mm2 ( 1 KN = 1000 N, so 1000 KN = 1000 * 1000 N )
= 1000000 / 22500 = 44.44 N/mm2
Shortcut formula :
Compressive strength = Load (KN) / 22.5
= 1000 / 22.5 = 44.44 N/mm2
Compressive Strength of Concrete Report
The following points shall be included in the report for each test specimen:
- Identification mark,
- date of the cast,
- date of the test,
- age of the specimen,
- weight of the specimen,
- dimensions of the specimen ( mostly not used for cube )
- maximum load,
- compressive strength, and
- average compressive strength
Required Compressive Strength at 7 days, 28 days
|Sr. no.||Grade of Concrete||7 days (minimum)||28 days (minimum)|
Specification Limit to Pass for Each Grade
- The average compressive strength from any group of four consecutive non-overlapping cube samples shall not be less than specified characteristic strength plus 3.
- The compressive strength of any cube sample shall not be less than specified characteristic strength minus 3.
- The individual variation in cube specimens’ compressive strength shall not be more than +/- 15 percent. If it is, then the sample is invalid.
Here we will discuss the relation between compressive strength of concrete and tensile resistance of concrete. The theoretical strength of concrete is eight times bigger than its tensile strength. This means a fixed connection between the compressive and tensile resistance of concrete.
The proportion of tensile to compressive power is lower for higher compressive strengths. Experimental results have also shown that concrete in compression and anxiety are closely associated, but the connection is not of direct proportionality type. The relationship between tensile strength to compressive strength depends on the strength of concrete.
Therefore higher the compressive strength, higher the electrical power, but the rate of growth of tensile strength is of diminishing order.
The tensile resistance of concrete is more very sensitive to improper curing compared to compressive strength. This is sometimes a result of the subsequent many reasons: Formation of lower quality gel due to improper curing. Development of more shrinkage cracks because of improper curing. The uses of pozzolanic materials have proven the escalation in tensile strength.
The Central Road Research Institute Delhi has completed an extensive study for demonstrating the connection between the tensile and compressive power of concrete to the construction of concrete streets. On the basis of the evaluation data of the study, CRRI has indicated the following relationship between flexural power and compressive power of concrete: alpha = 11 x 3.4 – in which y is the compressive power of concrete from MPa and x its flexural strength.
This relation depends upon the size of the coarse aggregate. The power is found to vary with this nature and size of aggregate. The connection is reproduced below: For 20 mm Crushed Stone alpha = 15.3 x 9.0 – For 20 mm Natural tombstone y = 14.3 x 10.4 – For 40 mm Crushed Stone alpha = 9.9 x -5.5 – For 40 mm Natural tombstone y = 9.8 x 2.5 – The flexural power of concrete was found to be 8 to 11% of the compressive power of concrete of higher power concrete of this order of 25 MPa and 9 to 12.8% for concrete of power less than 25 MPa see Table 13.1: The ratio between bending power to compressive power was found greater for 40 mm maximum size aggregate compared to that of 20 mm max sized aggregate.
The modulus of rupture is significantly given by this connection. Fb = 2p x a\/bd2 – where. Fb = modulus of rupture – p = load applied upon specimen in kg. A = distance between this crack and this closest support – b = width of beam from cm – d = depth of specimen at failure point in cms – The value of modulus of rupture varies from 11% to 23% of the compressive power of the same concrete.