Contents:

**Testing of Concrete Cores for Strength**

The diameter of core specimens for the determination of concrete compressive strength should preferably be at least three times the nominal maximum size of the coarse aggregate used in the concrete, and must be at least twice the maximum size of the coarse aggregate in the core sample.
The length of the specimen, when capped, should be nearly as practicable twice its diameter. A core having a maximum height of less than 95% of its diameter before capping or a height less than its diameter after capping must be rejected. It is preferable to test the cores in moist condition.
**Table-1: Correction Factor for Ratio of Length of Cone to its Diameter**

Ratio of length of cylinder to diameter (L/D) |
Strength correction factor |

1.75 | 0.98 |

1.50 | 0.96 |

1.25 | 0.93 |

1.00 | 0.87 |

**Fig: Core sampling and testing of concrete**

**The following guidelines are of particular importance in core sampling:**

- The number, size, and location of core samples should be carefully selected to permit all necessary laboratory tests. If possible, use virgin samples for all tests so that there will be no influence from prior tests.
- For determination of strength the core must have a minimum diameter of greater of three times the maximum nominal size of the coarse aggregate, or 50 mm.
- For strength tests, the cores must have a length of at least twice their diameter.
- Reinforcing steel should not be included in a core to be tested for strength.
- During drilling of cores, the electrical conduits or prestressing steel must not be disturbed.
- It is better to drill core through full depth of member to avoid the need of its breaking for extraction. An extra 50 mm is usually drilled to allow for possible damage at the base of the core.
- At least three cores must be removed at each location in the structure for strength determination.

*R*) on the strength is given as under:

**For horizontally drilled cores:**

**Corrected cylinder strength = Core strength x[(2.5x0.8)/(1+(1/R))]**

**For vertically drilled cores:**

**Corrected cylinder strength = Core strength x[(2.3x0.8)/(1+(1/R))]**