Compressive Strength Test of Concrete Core

The examination and compression testing of cores cut from hardened concrete is a well – established method, enabling visual inspection of the interior regions of a member to be coupled with strength estimation.

IS: 456-2000 specified that the points from which cores are to be taken and the number of cores required shall be at the discretin of the engineer-in-charge and shall be representative of the whole of concrete concerned in no case, however, shall fewer than three cores be tested. Core shall be prepared and tested as described in IS: 516.

Diameter of Concrete Core

The diameter of the core specimen for the determination of compressive strength in load bearing structural members shall be at least 3.70 inch [94 mm].

For concrete with nominal maximum size of aggregate greater than or equal to 1.5 inch [37.5 mm], the preferred minimum core diameter shall be, three times the nominal maximum size of coarse aggregate but it should be at   least two times the nominal maximum size of the coarse aggregates.

Length of Concrete Core

The preferred length of capped specimen is between 1.9 and 2.1 times the diameter. High lengths can be trimmed and for specimens having low length, correction factor has to be applied in compressive test.

Moisture Conditioning of Concrete Core

After cores have been drilled, wipe off the surface drilled water and allow the surface moisture to evaporate. When surface appears dry, but not greater than 1 hour after drilling, place cores in separate bags or non absorbent containers and seal to prevent moisture loss.

Maintain cores at ambient temperatures and protect from exposure to direct sunlight. Transport the cores as soon as possible to laboratory. Cores can be taken out of the bags for a maximum time of 2 hrs to permit capping before testing.

If water is used for grinding or sawing the core ends, complete these operations as soon as possible, but not later than 2 days after drilling. Minimize the duration of exposure to water during end preparation.

Allow the cores to remain in the sealed plastic bags or non-absorbent containers for at least 5 days after last being wetted and before testing.

Sawing of the ends of concrete core

The ends of core specimen shall be flat, and perpendicular to the longitudinal axis. Sawing should be such that prior to capping the following requirements are met:

a)   Projections, if any shall not extend more than 0.2 inch[5mm] above the end surfacesb)   The end surfaces shall not depart from perpendicularity to the longitudinal axis by a slope of more than 1.8 d or 1:0.3d where d is the average core diameter.

Determining the core density

Determine the density by weighing the core before capping but after grinding and dividing the mass by the volume of the core calculated from the average diameter and length.

Capping of concrete core

v  If the ends of cores do not confirm to the perpendicularity and plainness requirement, they shall be sawed or ground or capped.

v  If cores are capped, the capping device shall accommodate actual core diameters and produce caps that are concentric with the core ends.

v  The material used for capping shall be such that it’s compressive strength is greater than that of the concrete in the core.

v  Caps shall be made as thin as practicable and shall not flow or fracture before the concrete fails when specimen is tested.

v  Capped surface shall be right angles to the axis of the specimen and shall not detach depart from a plane by more than 0.05 mm.

v  Measure core lengths to the nearest 0.1 inch [2 mm] before capping.

Measurement of Concrete Core

v  Before testing, measure the length of the capped or ground specimen to the nearest 0.1 inch  [2 mm] and compute this to calculate  the length diameter [L/D] ratio.

v  Determine the average  diameter  by  averaging the  two measurements taken at right angles to each other at the mid height of the specimen.

v  Measure core diameters to the nearest 0.01 inch[.2 mm] when the difference in core diameters does not exceed 2% of their average, otherwise measure to the nearest 0.1 inch[2mm].

v  Do not test cores if the difference between smallest and largest diameter of the core exceeds 5% of their average.

Testing of concrete core

Centre the core carefully on the lower platen of the machine without shock apply and increase the load continuously at constant rate within range of 0.2 N/(mm²/s) to 0.4N/(mm²/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/mm².

Test the specimen within 7 days after coring.

Calculation of Compressive Strength of Concrete Core

Calculate the compressive test of the specimen using the computed cross sectional area based on average diameter of the specimen. If the L/D ratio is 1.75 or less, correct the result obtained by multiplying with correction factors as given below:

L/D Ratio	Correction Factor
1.75	0.98
1.5	0.96
1.25	0.93
1.0	0.87

The value obtained after multiplying with correction factor is called corrected compressive strength, this being equivalent strength of a cylinder having L/D ratio of 2. The equivalent cube strength can be calculated by multiplying the corrected cylinder strength by 5/4.

Report of Compressive Strength Test

Report the results with addition of the following information:

a)  Length of core drilled to the nearest 5 mm.

b)  Length of the specimen before and after capping to nearest 2 mm and average diameter of core to the nearest 0.2 mm or 2 mm.

c)  Compressive strength to the nearest 0.1 MPa when diameter is measured to nearest 0.2 mm and to the nearest 0.5 MPa when diameter is measured to the nearest 2mm after correction for L/D ratio.

d)  Direction of application of load with respect the horizontal plane of the concrete as placed.

e)  Moisture conditioning history.

f)  If water was used during end preparation, the date and time end preparation was completed and core was placed in concealed bags.

g)  The date and time when tested.

h)  Nominal maximum size of the aggregates.