Characteristic Compressive Strength Acceptance Criteria as per IS456: 2000
Characteristic
Compressive Strength Acceptance Criteria as per IS456: 2000
1. Introduction
Strength of Concrete is commonly considered its most
valuable property, although in many practical cases, other characteristics, such
as durability and permeability may infact be more important.However,the
strength of concrete is almost invariably a vital element of strength of
structural design and is specified for compliance purpose.
2. Characteristic Strength
As per Clause 6.1.1 of IS456:2000, the characteristic
strength is defined as the strength of material below which not more than 5
percent of the test results arc expected to fall.
3. Frequency of sampling
A random sampling procedure shall be adopted to ensure
that each concrete batch shall have a reasonable chance of being tested that
is, the sampling should be spread over the entire period of concreting and
cover all mixing units.
The minimum frequency of sampling of concrete of each
grade as per Clause 15.2.2 of IS456:2000 shall be in accordance withthe
following:
|
Quantity of
Concrete in Work(Cum) |
Number of
Samples |
|
1 to 5 |
1 |
|
6 to 15 |
2 |
|
16 to 30 |
3 |
|
31 to 50 |
4 |
|
51 and above |
4 plus one additional sample for each additional 50
Cum 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. |
|
4. Sample and test Specimen
A sample shall
have three test specimens. Three test specimens shall be made for each sample
for testing at 28 days. Additional samples may be required for various purposes
such as to determine the strength of concrete at 7 days or at the time of
striking the formwork, or to determinethe duration of curing or to checkthe
testing error. Additional samples may also be required for testing samples
cured by accelerated methods as described in IS: 9103. The specimen shall be tested
as described in IS: 516
5. Test Results of Sample
·
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.
6. Acceptance Criteria for Compressive Strength
The concrete shall be deemed to comply with the
strength requirements when both the following condition is met:
a) The
mean strength determined from any group of four consecutive test results complies
with the appropriate limits (as per Table 11,Col2 of IS 456:2000) and
b) Any
individual test result complies with the appropriate limits in Col 3 of Table
11) as under:
Table 11
Characteristic Compressive Strength Compliance Requirement (Clauses 16.1 and
16.3) (Revised as per amendment No.4 in 2013)
Table 8 : Assumed
Standard Deviation
|
Grade of
Concrete |
Assumed
Standard Deviation N/mm2 |
|
M10 |
1.5 |
|
M15 |
|
|
M20 |
4.0 |
|
M25 |
|
|
M30 |
5.0 |
|
M35 |
|
|
M40 |
|
|
M45 |
|
|
M50 |
Note:
The above values correspond to the site control having proper storage of
cement; weigh batching of all material; controlled addition of water; regular
checking of all materials,aggregate grading and moisture content; and
periodical checking of workability and strength. Where there is deviation from
the above, the values given in the above table shall be increased by 1 N/mm2.
7. Acceptance Criteria for Flexural Strength
When both the following conditions are met the
concrete complies with the specified flexural strength:
a) The mean strength determined from any group of four
consecutive test results exceeds the specified characteristic strength by at
least 0.3 N/mm2.
b) The
strength determined from any test result is not less than the specified
characteristic strength less 0.3 N/mm2.
8. Acceptance Criteria with Example
Case-I: When number of
sample is only 1(Concrete quantity 1 to 5 cum)
(Note: One sample strength
consists average of three test specimens)
Minimum requirement of strength of sample=fck+4
Example-(a)
If strength of three specimen of a sample of
Concrete grade M-30 are as under:
Specimen-1 Specimen-2 Specimen-3
35N/mm2, 33
N/mm2 35 N/mm2
Mean of three specimen i.e. strength of one sample i.e.(35+33+35)/3=34.33
N/mm2 which is greater than fck+4
i.e.30+4=34 N/mm2.
Therefore, concrete
is acceptable.
Example-(b)
If the
strength of three specimen of a sample of Concrete grade M-30 are as under:
Specimen-1 Specimen-2 Specimen-3
33N/mm2, 33 N/mm2 35 N/mm2
Mean of three specimen i.e. strength of one sample
i.e.(33+33+35)/3=33.67 N/mm2 which is less than
fck+4 i.e.30+4=34 N/mm2.
Therefore, concrete
is not acceptable.
Case-II: When number of sample is less than 4(Concrete quantity 6 to 30 cum)
(Note: One sample
strength consists average of three test specimens)
·
Requirement of Minimum individual test of sample=fck-2
N/mm2
·
Mean of samples=fck+4 N/mm2(minimum)
Example-(a)
If the strength or test result of three samples (9
specimens) of Concrete grade M-25 are as under:
Sample-1 Sample-2 Sample-3
28 N/mm2, 26 N/mm2 23
N/mm2
Requirement of Minimum individual test of sample=fck-2
N/mm2 i.e.(25-2)=23 N/mm2.
All the three samples are fulfilling the criteria.Hence;
strength is conforming individual strength criteria.
Mean of three samples= (28+26+23)/3=25.67 N/mm2 which
is less than fck+4 i.e.25+4=29 N/mm2.Hence, strength is not conforming mean
strength criteria.
Therefore, concrete
is not acceptable.
Example-(b)
If the strength or test result of three samples (9
specimens) of Concrete grade M-25 are as under:
Sample-1 Sample-2 Sample-3
29 N/mm2, 30 N/mm2 29 N/mm2
Requirement of Minimum individual test of sample=fck-2
N/mm2 i.e.(25-2)=23 N/mm2.
All the three samples are fulfiling this
criteria.Hence, strength is conforming individual strength criteria.
Mean of three samples=(29+30+29)/3=29.33 N/mm2 which
is more than fck+4 i.e.25+4=29 N/mm2.Hence,
concrete is conforming mean strength criteria.
Therefore, concrete
is acceptable.
Case-III: When number of sample is 4 or more (Concrete qty. 31 to 50 cum
or more)
(Note: One sample
strength consists average of three test specimens)
·
Requirement of Minimum individual test of sample ≥ fck-3 N/mm2
·
Mean of the group of 4 Non overlapping consecutive
test results in N/mm2(Min) ≥
fck+0.825x established standard deviation(rounded off to nearest 0.5 N/mm2) Or
≥ fck
+3 N/mm2
Whichever
is greater.
Consecutive 4 non-overlapping test results (12
specimens) are depicted below:
5 4 2
Ist Set of four test
results IInd set of four test results IIIrd
set of four test results and so on
Example-(a)
If Strength of four consecutive non overlapping samples
of Concrete grade M-30 are as under:
Sample-1 Sample-2 Sample-3 Sample-4
36 N/mm2
38 N/mm2 27 N/mm2 37 N/mm2
Requirement of Minimum individual test of sample=fck-3
N/mm2 i.e.(30-3)=27 N/mm2.
The strength of all the four samples are fulfilling
the criteria.Hence, strength is conforming individual test criteria.
Mean of the group of 4 non overlapping consecutive
test results, fck+0.825x established standard deviation (rounded off to nearest
0.5 N/mm2)
·
Mean of group of four non overlapping consecutive test
results=(36+38+27+37)/4=34.50 N/mm2 ,which is more than the requirement of fck+0.825x
established standard deviation( or assumed standard deviation as per table 8
above of IS 456:2000) i.e.30+0.825x5=34.125 N/mm2 =34 N/mm2(after rounding off)
·
fck+3= 33 N/mm2
So greater of
the above two values i.e. 34 N/mm2 shall be considered for acceptance.
Mean of group of four non overlapping consecutive test
results is 34.50 N/mm2 which is greater than 34 N/mm2.
Therefore, concrete
is acceptable.
9. Conclusion
It is very important to maintain the desired quality
of concrete as concrete production does not involve homogeneity. A good quality
control from material management to placement of concrete to sampling and
testing is required. Concrete production process is full of heterogeneity and
therefore careful testing, assessment and analysis of all its constituents is
required as all its constituents are of heterogeneous nature. The testing
process of concrete material and the concrete can only be done based on
sampling and the sample output is considered valid for the population.
Due care need to be taken during acceptance of
concrete.The concrete sampling shall be done as per provisions laid down in
IS456:2000 and concrete shall be accepted only after the concrete fullfills the
acceptance criteria mentioned in IS456:2000(with up-to-date amendments).
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