Core test perform under the following situation

When standard 28 days cube strength test givens lower strength and acceptable and primary aim of core test is to ascertain whether the structural element is of adequate strength. To detect the segregation honey combing and to check the bond at construction joint or to verify the thickness of pavement.

Core test serve as a good support test for any field investigation special electric or gasoline driven core drill machine have been develop to cut core from the structural members. Core of 25mm to 300mm diameter can be cut including reinforcements. The core can be tested for compressive strength, chemical analysis, petrographic examination and evaluation physical parameter. The strength testing of core provide almost a direct evidence on the quality of concrete as it exist on the structure.

The core samples can also be used for the following:

Strength and density determination

Depth of carbonation of concrete

Chemical analysis

Water/gas permeability

Petrographic analysis

ASHTO Chloride permeability test

Factors Affecting Strength of Concrete Cores

Following are the factors which affect the compressive strength of extracted concrete cores:

Size of stone aggregate

If the ratio of diameter of core to maximum size of stone aggregate is less than 3, a reduction in strength is reported. For concrete with 20mm size aggregate, 50mm dia core has been tested to give 10% lower results than with 10mm dia cores.

Presence of transverse reinforcement steel

It is reported that the presence of transverse steel causes a 5 to 15% reduction in compressive strength of core. The effect of embedded steel is higher on stronger concrete and as its location moves away from ends, i.e. towards the middle. However presence of steel parallel to the axis of the core is not desirable.

H/D ratio

This has been already discussed above. However its value should be minimum 0.95 and maximum 2. Higher ratio would cause a reduction in strength

Age of concrete

No age allowance is recommended by the Concrete Society as some evidence is reported to suggest that in-situ concrete gains little strength after 28 days. Whereas others suggest that under average conditions, the increase over 28 days¡¯ strength is 10% after 3 months, 15% after 6 months. Hence it is not easy to deal the effect of age on core strength.

Strength of concrete

The effect in reducing the core strength appears to be higher in stronger concretes and reduction has been reported as 15% for 40 MPa concrete. However a reduction of 5 50 7% is considered reasonable.

Drilling operations

The strength of cores is generally less than that of standard cylinders, partly as a consequence of disturbance due to vibrations during drilling operations. Whatever best precautions are taken during drilling, there is always a risk of slight damage.

Site conditions vis-a-vis standard specimens

Because site curing is invariably inferior to curing prescribed for standard specimens, the in-situ core strength is invariably lower than the standard specimens taken and tested during concreting operations.

Testing Of Core

The core shall be placed in water at a temperature $24^o$ to $30^oC$ for 48 hours before testing. Center the core carefully on the lower platen of the machine without shock apply and increase the load continuously at constant rate with in range of $0.2 N/(mm^2/s)$ to $0.4 N/(mm^2/s)$ until no greater load can be sustained. Note any unusual failures and the appearance of the concrete. Calculate the compressive strength of each core by dividing the maximum load by the cross-sectional area, calculated from the average diameter. Express the results to the nearest $0.5 N/mm^2$.

Throughout the world core testing is acceptable method for the determination of strength and quality of concrete in the structure.