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Materials Engineer, Roorkee
Strength of concrete is commonly considered its most valuable property, although in many practical cases, other characteristics, such as durability and permeability, may in fact be more important. However, the strength of concrete is almost invariably a vital element of structural design and is specified for compliance purposes.
Table 1 : Frequency (IS: 456-2000 clause 15.2.2)
The minimum frequency of sampling of concrete of each grade shall be in accordance with the following:
|Quantity of concrete in the work, m3||Number of samples|
|1 – 5||1|
|6 – 15||2|
|16 – 30||3|
|31 – 50||4|
|51 and above||4 plus one additional sample for each additional 50 m3 or part thereof|
|NOTE: At least one sample shall be taken from each shift where concrete is produced at continuous production unit, such as ready-mixed concrete plant, frequency of sampling may be agreed upon mutually by suppliers and purchasers.|
(A) Compressive strength
The concrete shall be deemed to comply with the strength requirement when both the following conditions are met:
a) The mean strength determined from any group of four non-overlapping consecutive test results, complies with the appropriate limits col. 2 of Table 2.
b) Any individual test result complies with the appropriate limits in col. 3 Table 2.
(B) Flexural strength
When both the following conditions are met, the concrete complies with the specified flexural strength.
a) The mean strength determine from any group of four consecutive test results exceed the specified characteristic strength by at least 0.3 N/mm2
b) The strength determined from any test result is not less than specified characteristic strength less 0.3 N/mm2
The quantity of concrete represented by a group of four consecutive test results shall include the batches from which the first and last samples were taken together with all intervening batches.
Three test specimens shall be made for each sample for testing at 28 days. Additional specimens may be required for 7 days strength. In all the cases 28 days strength shall alone be the criterion for acceptance or rejection of the concrete.
The test results of the sample shall be the average of the strength of three specimens. The individual variation should not be more than +15 percent of the average. If more, the test results of the sample are invalid.
Table 2 : Characteristic compressive strength compliance requirement
IS: 456-2000 with amendments Table 11 (Clause 16.1 and 16.3)
|Specified Grade||Mean of Group of 4 Non-Overlapping Consecutive test results in N/mm2 Minimum||Individual Test Results in N/mm2
|M15 and above||fck + 0.825 X established Standard deviation (round off to nearest 0.5 N/mm2
fck + 3 N/mm2 whichever is greater
|fck – 3 N/mm2|
|NOTE 1: In the absence of established value of standard deviation, the values given in Table 8 (IS: 456-2000) may be assumed, and attempt should be made to obtained results of 30 samples as early as possible to establish the value of standard deviation.
NOTE 2: For concrete quantity up to 30 m3 (where the number of samples to be taken is less than four) as per frequency of sampling given in 15.2.2, the mean of test results of all such samples shall be fck + 4 N/mm2, minimum and the requirement of minimum individual test results shall be fck – 2 N/mm2, minimum. However, when the number of samples is only one as per 15.2.2, the requirement shall be fck + 4 N/mm2, minimum.
(Values of column 2 and 3 are equal to or more than)
The acceptance criteria is best illustrated by the following examples:
Grade of concrete : M25
Laboratory design mean target strength for
good quality control: 25 + (1.65 x 4)= 31.6 N/mm2 at 28 days age
In all the case average of three 150 mm cubes shall be taken.
Table 3: Site concrete acceptance.
In one shifts 4 m3 foundation concrete was done.
|Shift||Cube test results N/mm2||Average fav N/mm2||0.85 fav N/mm2||1.15 fav N/mm2||Acceptance 25+4 = 29 N/mm2(Min)|
|1.||19, 26, 16||20.3||17.3||23.3||Rejected due to:
a) Minimum strength of 29 N/mm2 not achieved
b) Variation in strength cubes 26 and 16 are out of range +/-15% of average
NOTE: Due to doubtful concrete the work was stop. Concrete was tested by rebound hammer and drilled cores. From test results the foundation concrete found to be of M25 grade.
The work was started only when new cube moulds, properly calibrated compression testing machine was purchased and laboratory staff was trained in all the site testing work.
Table 4: Site concrete acceptance. In 3 shifts 27 m3 foundation concrete was done.
|Shift||Cube test results N/mm2||Average fav N/mm2||0.85 fav N/mm2||1.15 fav N/mm2||Acceptance fck + 4 N/mm2 25+4 = 29 Min
fck – 2 N/mm2
25 – 2 = 23 N/mm2(Min)
|1.||33, 29, 32||31.3||26.6||36.0||Av = 31.3|
|2.||24, 32, 28||28.0||23.8||32.2||Av = 28.0|
|3.||25, 29, 32||28.7||24.4||33.0||Av = 28.7|
|Average = 29.3 N/mm2|
(1) All the cubes strength are within +15% of average
(2) From shifts 1, 2 and 3 all cubes strength are > 23 N/mm2.
(3) Average cubes strength of shift 1, 2 and 3 is 29.3 N/mm2 which is > 29 N/mm2.
Foundation concrete is accepted as M25 grade.
Table 5: Site concrete acceptance. In 6 shifts 75 m3 roof slab concrete was done.
|Shift||Cube test results N/mm2||Average fav N/mm2||0.85 fav N/mm2||1.15 fav N/mm2||Acceptance 25+0.825×4 = 28.3 N/mm2
Rounded of 28.0 N/mm2 Individual
|1.||22, 28, 26||25.3||21.5||29.1||Av = 25.3|
|2.||26, 24, 28||26.0||22.1||29.9||Av = 26.0|
|3.||31, 35, 33||33.0||28.1||38.0||Av = 33.0|
|4.||32, 31, 33||32.0||27.2||36.8||Av = 32.0|
|5.||31, 32, 33||32.0||27.2||36.8||Av = 32.0|
|6.||26, 25, 24||25.0||21.3||28.0||Av = 25.0|
(2) Average of shift 1, 2, 3, 4 cubes is 29.1 N/mm2 whichis > 28.0 N/mm2.
(3) Average of shift 2, 3, 4, 5 cubes is 30.8 N/mm2 which is > 28.0 N/mm2.
(4) Average of shift 3,4, 5, 6 cubes is 30.5 N/mm2 which is > 28.0 N/mm2.
(5) All the cubes strength are > 22 N/mm2
It is concluded that the roof slab concrete comply with compressive strength requirements of M25 grade
A sample consist of three cubes/specimens . It is better for the same concrete sample cast more than three cubes, so that any defective cube should not be tested or any doubt full test result due to testing may be rejected and not included in the average of three cubes.
1. IS: 456-2000 (Fourth Revision) with amendments plain and reinforced concrete – Code of Practice, BIS, New Delhi.
2. Kishore Kaushal, “Quality Control of Construction – Testing of Concrete Cubes”, Indian Construction, Apr. 2010 pp. 21-24.
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