TEST CORES QUALITY OF HARDENED CONCRETE

Core tests are useful for assessing the quality of hardened concrete in site. Not only can the comprehensive strength of the concrete be assessed, but a description of the aggregate, including maximum size, shape, surface texture and type, can be obtained and the degree of compaction noted.
The results of the tests can be used to give the 'estimated in-site cube strength' or to give an estimate of the potential strength of the standard test cube. While the relationship of core strength with in-site cube strength is fairly straightforward, the relationship between core strength and standard cube strength is complex and will vary with particular conditions.

Before deciding to drill cores for compressive testing, full consideration must be given to the aims and value of the results that will be obtained. Reference should be made to specialist literature for the number of cores and the assessment of results - BS 6089, BS EN 13791, Concrete Society Digest No. 9 and Concrete Society Technical Report No. 11.

The usual diameter of a core (Figure 42) is 150 mm or 100 mm and the length/diameter ratio must be between 1 and 2, preferably between 1 and 1.2. In thin members, or where reinforcement is congested, smaller diameter cores may be necessary but these give less reliable strength test results. Core cutting is a skilled operation, usually performed by specialist subcontractors.

Testing cores

Core tests may be used when the result of a cube test has proved unsatisfactory. A cube may have failed to give a desired result because of a defect in the testing procedure, in which case it is usual to examine the concrete in the structure in an attempt to assess its properties. Core tests need careful interpretation because the strength of a core is dependent on:

n Quality of concrete

n Degree of compaction

n Location in the structure

n Curing

n Presence of reinforcement

n Method and direction of cutting

n Preparation of specimen

n Testing procedure

n Age at test

In contrast, the cube test, if properly performed, uses standardized preparation and testing procedures in an attempt to eliminate all variables except that of concrete quality. Concrete in a structure cannot be expected to have had the same treatment as a standard cube. Accordingly, cores give variable results and the equivalent cube strengths are usually lower than the standard cube strength of the concrete.

Because of the variability of the core test a number of cores are needed to form a reasonable estimate as to the acceptability of the concrete. If possible a set of cores should also be tested from comparable concrete that is known to be acceptable.

Cores must be prepared to give end surfaces that are plane, parallel and at right angles to the axis. The length and diameter of the core are accurately measured because they are necessary for the calculation of strength. Core tests should be carried out only in a UKAS accredited laboratory; details of the preparation and test procedure are given in BS 1881 : Part 120 and BS EN 12504-1.

The compressive strength of each core is calculated by dividing the maximum load by the cross-sectional area calculated from the average diameter. Correction factors dependent upon the length/diameter ratio of the specimen after preparation of the ends, the direction of drilling and the presence of reinforcement are applied to give the estimated in-situ cube strength. Other factors may then have to be applied if an estimate of the standard cube strength is required.

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