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*By
KAUSHAL KISHORE
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, m^{3} |
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 m^{3} 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. |

**ACCEPTANCE CRITERIA**

(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.

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**(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/mm^{2}

b) The strength determined from any test result is not less than specified characteristic strength less 0.3 N/mm^{2}

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/mm^{2} Minimum |
Individual Test Results in N/mm^{2}Minimum |

(1) | (2) | (3) |

M15 and above | f_{ck} + 0.825 X established Standard deviation (round off to nearest 0.5 N/mm^{2}
or f |
f_{ck} – 3 N/mm^{2} |

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 m |

(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/mm^{2} at 28 days age

In all the case average of three 150 mm cubes shall be taken.

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**Table 3: Site concrete acceptance. **

**In one shifts 4 m ^{3} foundation concrete was done.**

Shift | Cube test results N/mm^{2} |
Average fav N/mm^{2} |
0.85 fav N/mm^{2} |
1.15 fav N/mm^{2} |
Acceptance 25+4 = 29 N/mm^{2}(Min) |

1. | 19, 26, 16 | 20.3 | 17.3 | 23.3 | Rejected due to:
a) Minimum strength of 29 N/mm 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 m ^{3} foundation concrete was done.**

Shift | Cube test results N/mm^{2} |
Average fav N/mm^{2} |
0.85 fav N/mm^{2} |
1.15 fav N/mm^{2} |
Acceptance f_{ck} + 4 N/mm^{2} 25+4 = 29 MinIndividual f _{ck} – 2 N/mm^{2}25 – 2 = 23 N/mm ^{2}(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/mm^{2} |
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(1) All the cubes strength are within +15% of average (2) From shifts 1, 2 and 3 all cubes strength are > 23 N/mm (3) Average cubes strength of shift 1, 2 and 3 is 29.3 N/mm Foundation concrete is accepted as M25 grade. |

**Table 5: Site concrete acceptance. In 6 shifts 75 m ^{3} roof slab concrete was done.**

Shift | Cube test results N/mm^{2} |
Average fav N/mm^{2} |
0.85 fav N/mm^{2} |
1.15 fav N/mm^{2} |
Acceptance 25+0.825×4 = 28.3 N/mm^{2}
Rounded of 28.0 N/mm |

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/mm (3) Average of shift 2, 3, 4, 5 cubes is 30.8 N/mm (4) Average of shift 3,4, 5, 6 cubes is 30.5 N/mm (5) All the cubes strength are > 22 N/mm 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.

**REFERENCES**

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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Difference Between Crushing Strength & Compressive Strength

how to calculated +-15% in tha concrete

Avg of 3 individual sample X .85 is -15%

Avg of 3 individual sample X 1.15 is +16%

Grade M:30,

7 day result was – 21.38, 26.99, 27.19

What will i consider the average result?

Please give me full calculation.

Is grade of concrete or design strength that is provided in the ACI 211.1-91 table 6.3.4(a)?

Is there any IS std for how many cube results are necessary for passing cube test out of three casted and checked.

all of three should be pass if any one of them fail it means your result of cube are fail.

As per IS -456 cube sampling is correct ? i would like to know suppose 5m3 concrete done at site as per IS 456 we have to check 7 days and 28 days it means we need 6 nos of cube required, but as per 456 3 cubes only how to determine the 28 days strength please guide me

Only 28 days cube strengths are valid

What is the mean of is 456:2000,page no.30,table no.11

what criteria are to be use for 28 days ,m-30 grade of concrete

I HAVE GOT COMPRESSIVE STRENGTH AS 31.5 N PER MILLIMETER SQUARE IN MY LABORATORY FOR M30 GRADE CONCRETE.IS THIS CORRECT FOR A MIX RATIO OF 1:1.66:3.03 OF M30 GRADE CONCRETE.

TABLE 5, THEARE REMARKS 3 WHY THE MEAN OVERLAP? HERE CODE SAY THAT 4 NON OVERLAPPING CONSECUTIVE…. PLEASE EXPLAIN

What is the max percentage of cube strength are acceptable for any grade after 28 days?

Is there M-25 grade = 35 accepted after 28 days?