Construction Materials Testing Laboratory: Obtaining And Testing Drilled Cores of Concrete

Obtaining And Testing Drilled Cores of Concrete

ASTM Designation : C42 / C 42M

AASHTO Designation : T 24

Scope:

This test method covers obtaining, preparing and testing core drilled from concrete for length or compressive or splitting strength.

Apparatus:

Core Drill, for obtaining cylindrical core specimens with diamond impregnated bits attached to core barrel.

Sampling:

Samples of hardened concrete for use in the preparation of strength test specimens shall not be taken until the concrete strong enough to permit sample removal without disturbing the bond between the mortar and the coarse aggregate. when preparing strength test specimens from samples of hardened concrete, samples that show defects or samples that have been damaged in the process of removal shall not be used.

specimens containing embedded reinforcement shall not be used for determining splitting tensile strength and specimens for determining flexural strength shall not be used if reinforcement is embedded in the tensile portion of the specimen.

Core Drilling:

A core specimen taken perpendicular to horizontal surface shall be located, when possible, so that its axis is perpendicular to the bed of the concrete as originally placed and not near formed joints or obvious edges of a unit of deposit. A specimen taken perpendicular to vertical surface with a batter, shall be taken from near the middle of a unit of deposit when possible and not near formed joints or obvious edges of a unit of deposit.

Length of Drilled Cores:

Core specimens drilled through a structure for the purpose of measuring structural dimensions shall have a

diameter of at least 3.75 in. [95 mm]. For cores which are not intended for measuring structural

dimensions, measure the longest and shortest lengths on the cut surface along lines parallel to the core axis. Record the average length to the nearest 1⁄4 in. [5 mm].

Cores for Compressive Strength:

Test Specimen—The nominal diameter of core specimens for the determination of compressive strength shall be at least 3.75 in. [95 mm]. Core diameters less than 3.75 in.[95mm] are permitted when it is impossible to obtain cores with length to diameter (L/D) ratio > 1 for compressive strength evaluations in cases other than load bearing situations. For concrete with nominal maximum aggregate size greater

than 11⁄2 in. [37.5 mm], the nominal diameter should preferably be at least three times the nominal maximum size of the coarse aggregate and must be at least twice the nominal maximum size of the coarse aggregate. The preferred length of the capped specimen is between 1.9 and 2.1 times the diameter. If the ratio

of the length to the diameter of the core specimen exceeds 2.1, reduce the length of the specimen so that the ratio is between 2.1 and 1.9. Core specimens with length-to-diameter ratios less than 1.8 require corrections to the measured compressive strength. A core having a maximum length of less than 95 % of its diameter before capping or a length less than its diameter after capping shall not be tested.

End Preparation:

The ends of core specimens to be tested in compression shall be essentially smooth, perpendicular

to the longitudinal axis, and of the same diameter as the body of the specimen. If necessary, saw the ends of the specimens until the following requirements are met:

Projections:

if any, shall not extend more than 0.2 in.[5 mm] above the end surfaces,

The end surfaces shall not depart from perpendicularity to the longitudinal axis by more than 0.5°, and

The diameters of the ends shall not depart more than 0.1 in. [2.5 mm] from the mean diameter of the specimen.

Moisture Conditioning:

Test specimens shall be tested in a moisture condition representative of the in-place concrete or as directed by the specifying authority.

Compressive strength test results are usually used for the evaluation of the in-place concrete strength; therefore, the cores shall be conditioned in a moisture condition most representative of the in-place strength. If the concrete service condition is dry, the cores can be tested in either an “as received condition” after allowing the drilling moisture to evaporate or tested in a “dry condition” where the cores are air dried in a temperature range of 60 to 80°F [16 to 27°C] at a relative humidity less than 60 % for seven days, as directed by the specifying authority.

The following procedure is used to bring the cores to the “as received condition.” After drilling, transport the cores to the testing laboratory within 24 h. Dry the cores for 12 to 24 h in air at a temperature between 60 and 80° [16 to 27°C] and at less than 50 % relative humidity. Cap or grind the cores, and

test them within 48 h of receipt.

Capping:

The ends of the cores shall conform to the tolerance requirements of Test Method C 39. The ends shall be

sawed or ground to tolerance or capped in accordance with capping procedures for hardened concrete specimens of Practice C 617.

Calculation:

Calculate the compressive strength of each specimen using the computed cross-sectional area based

on the average diameter of the specimen.

If the ratio of length to diameter (L/D) of the specimen is 1.75 or less, correct the result obtained in 7.7 by multiplying by the appropriate correction factors shown in the following

Ratio of Length to Diameter (L/D) Strength Correction Factor

1.75 0.98

1.50 0.96

1.25 0.93

1.00 0.87

Use interpolation to determine correction factors for L/D values not given in the table.

Drilling Core at R.C. Box Culvert Bottom Slab