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 tie 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.

Concrete-Core-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:

  • If the ends of cores do not confirm to the  perpendicularity and  plainness requirement, they shall be sawed or ground or capped.
  • If cores are capped, the capping device shall  accommodate  actual  core diameters and produce caps that are concentric with the core ends. 
  • The material used for capping shall be such that it’s compressive strength is greater than that of the concrete in the core.
  • Caps shall be made as thin as practicable and shall not flow or fracture before the concrete fails when specimen is tested.
  • 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.
  • Measure core lengths to the nearest 0.1 inch [2 mm] before capping.

Measurement of concrete core:

  • 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.
  • Determine the average  diameter  by  averaging the  two measurements taken at right angles to each other at the mid height of the specimen.
  • 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].
  • Do not test cores if the difference between smallest and largest diameter of the core exceeds 5% of their average.

Testing of concrete core:

Test the specimen within 7 days after coring.

Calculation of compressive strength:

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: Tips for Core Extraction & Testing of Concrete