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Compressive Strength Test on Concrete Core ‚Äď Calculation and Results

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Compressive Strength Test on Drilled Concrete Cores is required to determine the strength of hardened concrete in structure. Following are the specification for drilled concrete cores to be suitable for compressive strength test:

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 surfaces b)  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.

Capping of concrete core

Measurement of Concrete Core

Testing of concrete core

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. Also Read: Why do We Test Concrete Compressive Strength after 28 Days? Compressive Strength of Concrete -Cube Test, Procedure, Results Compressive Strength Test on Concrete Cylinders Tips for Core Extraction & Testing of Concrete
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