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*By
KAUSHAL KISHORE
Materials Engineer, Roorkee*

For concrete roads, flexural strength of concrete is the design criteria. For all major projects, flexural strength of the mix shall be determine by third point loading of flexural beams size 150 mm x 150 mm x 700 mm as per IS: 516. Determination of flexural strength by correlating with cube strength (compressive strength) shall not be allowed for major projects, as the correlation is not well established.

However in small projects where facilities for testing beams with three point loading are not available, in a such case, the mix design may be carried out by using compressive strength values and there after flexural strength will be determined as per correlation between flexural strength with compressive strength given below

f_{cr} = 0.7 x √f_{ck}

where f_{cr} is the flexural strength in N/mm^{2} and f_{ck} is the characteristic compressive strength in N/mm^{2} as per IS: 456-2000.

**QUALITY CONTROL**

Samples from fresh concrete shall be taken as per IS: 1199 and cube and beam specimens shall be made, cured and tested at 28 days as per IS: 516.

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The minimum frequency of sampling of concrete shall be one sample per 200 cu.m of concrete. Each sample shall comprise of 3 test specimens of beams and cubes. These shall be tested for 28 days strength. However, in case of major projects test pertaining to beams shall govern. For each day’s work, number of specimens shall not be less than six beams taken out of 2 different batches of concrete. Additional six cubes may also be got cast for reference, record and co-relation, if desired by the Engineer.

The test results of the sample shall be the average of the strength of three specimens, comprising the sample. The individual variations of any specimen (beam/cube/core) shall not be more than +15 percent of the average (of the three specimen comprising the sample). In case, if it is more, than the sample will be rejected.

Flexural strength shall be used for quality control and for acceptance purposes. The flexural strength be determined by modulus of rupture under third point loading as per IS: 516. The preferred size of beam shall be 150 mm x 150 mm x 700 mm and cubes of 150 mm x 150 mm x 150 mm for the maximum size of aggregate of 31.5 mm. Determination of flexural strength by correlating with cube strength shall not be allowed for major projects, as the correlation is not well established.

A quality control chart indicating the strength values of individual specimens shall be maintained for continuous quality assurance. Where the requirements are not met with or where the quality of the concrete or its compaction is suspect, the actual strength of the concrete in the slab shall be ascertained by carrying out tests on cores cut at the rate of 2 cores for every 150 cu.m. of concrete. The diameter of cores shall not be less than 150 mm.

If, however, the tests on cores also confirm that the concrete is not satisfying the strength requirements, then the concrete corresponding to the area from which the cores were cut should be replaced, i.e., at least over an area extending between two transverse joints where the defects could be isolated or over larger area, if necessary, as assessed by additional cores and their test results.

With regard to age factor, it is recommended that if the cores are cut within 90 days of casting the slab, no allowance for age factor is required, the strength of the core may be considered as at 28 days. However, if the cores are cut after 90 days, an age factor of 115 percent as compared to 28 days characteristic compressive strength of core may be applied.

**Acceptance Criteria of Strength**

*Flexural strength*

The concrete will be said to comply with the specified flexural strength, when the following conditions are met with,

i) The mean strength determined from any group- of 4 consecutive samples (each sample containing 3 beam specimen i.e. 4×3 = 12 beam specimens) at 28 days should exceed the specified characteristic flexural strength by atleast 0.3 MPa (N/mm^{2}).

ii) The strength of any specimen is not less than the specified characteristic flexural strength minus 0.3 MPa (N/mm^{2})

*Compressive strength**

When both the following conditions are met, the concrete complies with the specified compressive strength;

i) The mean strength determined from any group of 4 consecutive samples (4×3 = 12 cubes specimens) at 28 days should exceed the specified characteristic compressive strength by 3 MPa (N/mm^{2}).

ii) The strength of any sample is not less than the specified characteristic compressive strength minus 3 MPa (N/mm^{2}).

*Acceptable for smaller projects, where design is based on compressive strength.

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At compressive strength ii) in place of: The – strength of any sample should be The – strength of any specimen.(IS:456-2000 mentioned individual Test Results)

At flexural strength at 11) is also mentioned any specimen

The acceptance criteria is best illustrated by the following example:

Characteristic flexural strength (Required in the field at 28 days age)

= 4.5 N/mm^{2}

This correspond to characteristic compressive strength using relationship given else where = 41.4 N/mm^{2}

Laboratory design mean flexural strength of good quality control. The concrete is to be used on urban streets.

= 4.5 + 1.65 x 0.4 = 5.2 N/mm^{2}

at 28 days age

and laboratory design mean target compressive strength:

41.4 + 1.65 x 5 = 49.7 N/mm^{2}

at 28 days age

with the given correlation this gives flexural strength of

0.7 x √49.7= 4.93 N/mm^{2}

whereas the required target flexural strength is 5.2 N/mm^{2}. Therefore, the design target compressive strength shall be

5.2 = 0.7 x √x

√x = 5.2/0.7

√x=(7.429)^{2}

= 55 N/mm^{2} in place of 49.7 N/mm^{2}

**SITE ACCEPTANCE**

(a) Characteristic flexural strength = 4.5 N/mm^{2}

(b) Characteristic compressive strength = 41.4 N/mm^{2}

Beams size is 150 mm x 150 mm x 700 mm and cubes size 150 mm x 150 mm x 150 mm. All tested at 28 days age.

**Table 1 – In a small road project in 2 days 90 cu.m of concrete was layed**

Batch | Cube test results N/mm^{2} |
Average fav N/mm^{2} |
0.85 fav N/mm^{2} |
1.15 fav N/mm^{2} |
Acceptance exceed average 41.4+3 = 44.4 N/mm^{2}Individual 41.4-3 = 38.4 N/mm ^{2} Minimum |

Day one Batch I |
36.5, 39.0, 40.7 | 38.7 | 32.9 | 44.5 | Rejected 36.5 is less than 38.4 |

Batch II | 41.5, 37.3, 38.4 | 39.1 | 33.2 | 45.0 | Rejected 37.3 is less than 38.4 |

Day two Batch III |
45.5, 40.2, 39.5 | 41.7 | 35.5 | 48.0 | |

Batch IV | 46.7, 40.9, 48.7 | 45.4 | 38.6 | 52.2 | |

Average | – | 41.2 | – | – | Concrete rejected as average could also not exceed 44.4 N/mm^{2} |

**Remarks**: As concrete is rejected the option is cores should be taken out and tested for strength and compliance.

**Table 2 – In a major road project in 6 days 2300 cu.m **

**of concrete was layed**

Batch |
Beam test results N/mm^{2} |
Average fav N/mm^{2} |
0.85 fav /mm^{2} |
1.15 fav N/mm^{2} |
Acceptance exceed average 4.5+0.3 = 4.8 N/mm^{2}Individual 4.5- 0.3 = 4.2 N/mm ^{2} Minimum |

Day 1 Batch I |
3.32, 5.79, 3.41 | 4.17 | 3.55 | 4.80 | Rejected due to variation in strength |

Batch II | 5.53, 4.95, 5.41 | 5.30 | 4.50 | 6.10 | |

Day 2 Batch III |
5.21, 4.32, 4.20 | 4.58 | 3.89 | 5.27 | |

Batch IV | 5.55, 5.00, 4.97 | 5.17 | 4.40 | 5.95 | |

Day 3
Batch V |
5.20, 4.72, 4.59 | 4.84 | 4.11 | 5.57 | |

Batch VI | 4.62, 5.31, 4.87 | 4.93 | 4.19 | 5.67 | |

Day 4 Batch VII | 4.95, 5.71, 5.66 | 5.44 | 4.62 | 6.26 | |

Batch VIII | 4.51, 4.82, 4.77 | 4.70 | 4.00 | 5.41 | |

Day 5 Batch IX | 5.78, 5.91, 5.33 | 5.67 | 4.82 | 6.52 | |

Batch X | 4.21, 4.71, 4.96 | 4.63 | 3.94 | 5.32 | |

Day 6 Batch XI | 5.01, 4.92, 4.57 | 4.83 | 4.11 | 5.55 | |

Batch XII | 5.43, 5.98, 6.21 | 5.87 | 4.99 | 6.75 |

**Remarks**:

1) The concrete of batch I is rejected by site engineer due to variation in strength. The strength values are out of range +15 percent of average value. By taking into consideration the strength of batch II and also other batches, it is concluded that this may be due to care was not taken in taking sample and in casting of beams. By visual inspection and rebound hammer readings, batch I portion of concrete found to be identical with batch II portion of concrete. Accordingly the concrete was accepted without doing any remedial measures.

2) All the beams strength of batch II, III, IV, V, VI, VII, VIII, IX, X, XI and XII are within +15 percent of average strength.

3) All the beams strength of batch II, III, IV, V, VI, VII, VIII, IX, X, XI and XII are equal to or more than 4.2 N/mm^{2}.

4) The average beams strength of batch II, III, IV and V is 4.97 N/mm^{2} which exceed 4.8 N/mm^{2}.

5) The average of beams strength of batch III, IV, V, VI is 4.88 N/mm^{2} which exceed 4.8 N/mm^{2}.

6) The average of beams strength of batch IV, V, VI and VII, is 5.10 N/mm^{2} which exceed 4.8 N/mmm^{2}.

7) The average of beams strength of batch V, VI, VII and VIII is 4.98 N/mm^{2} which exceed 4.8 N/mm^{2}.

8) The average of beams strength of batch VI, VII, VIII and IX is 5.19 N/mm^{2}, which exceed 4.8 N/mm^{2}.

9) The average of beams strength of batch VII, VIII, IX and X is 5.11 N/mm^{2} which exceed 4.8 N/mm^{2}.

10) The average beams strength of batch VIII, IX, X and XI is 4.96 N/mm^{2} which exceed 4.8 N/mm^{2}.

11) The average of beams strength of batch IX, X, XI and XII is 5.25 N/mm^{2} which exceed 4.8 N/mm^{2}.

The above test results shows that the concrete has compliance of characteristic flexural strength of 4.5 N/mm^{2}.

**REFERENCES**

REFERENCES

1. IS: 456-2000, Plain and reinforced concrete – code of practice (Fourth Revision) BIS, New Delhi.

2. IS: 516-1959, Method of test for strength of concrete, BIS, New Delhi.

3. IS: 1199-1959, Methods of sampling and analysis of concrete, BIS, New Delhi.

4. IRC: 15-2011, Standard specifications and code of practice for construction of concrete roads (Fourth Revision), Indian Roads Congress, New Delhi.

5. Kishore Kaushal, Mix design for concrete Roads as per IRC: 15-2011 , Civil Engineering Materials and Testing, pp.115-122 Standard Publishers Distributors 1705, B, Nai Sarak, Delhi – 110006.

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*We at engineeringcivil.com are thankful to Sir. Kaushal Kishore, for submitting this very useful paper to us.*