Testing of Concrete in Structures: Fourth Edition

Cores 135from flexurally cracked tensile zones must be regarded as unreliable, whilstYip (149) has demonstrated that compressive load history of the concretebefore coring may lead to reductions in measured strength of up to 30% dueto internal microcracking. This latter effect may be quite significant evenat relatively low stress levels and adds to interpretation uncertainties. Theestimated cube strengths obtained from core compression tests may tendto underestimate the true in-situ capacity in all these situations. Strengthchanges with age may also be considered when interpreting core results, butany allowances must be carefully considered as discussed in Section 1.5.2.The principal limitations of core testing are those of cost, inconvenienceand damage, and the localized nature of the results. It is strongly recommendedthat core testing is used in conjunction with some other form oftesting which is less tedious and less destructive. The aim is to providedata on relative strengths within the body of the concrete under test. Thesize of core needed for reliable strength testing can pose a serious practicalproblem; ‘small’ cores may be worthy of consideration with slendermembers. It may also be appropriate to consider using a larger numberof ‘small’-diameter cores to obtain an improved spread of test locationswhere large volumes of concrete are involved. The cutting effort for three50 mm cores may be as low as one-third of that for one 150 mm specimen.A comparable overall strength accuracy may be expected (see Section 5.3.2)provided that maximum aggregate size is less than 17 mm. Where cores areused for other purposes, it will often be possible to use a ‘small’ diameterwith considerable savings of cost, inconvenience and damage.Apart from physical testing, cores often provide the simplest method ofobtaining a sample of the in-situ concrete for a variety of purposes, butcare must be taken that the effects of drilling, including heat generated byfriction, or the presence of water, do not distort the subsequent results. Asample taken from the centre of a core may conveniently overcome thisproblem. Chemical analysis can often be performed on the remains of acrushed core, or specimens may be taken specifically for that purpose.Visual inspection of the interior of the concrete may be extremely valuableboth for the assessment of compaction and workmanship, and for obtainingbasic data about concrete for which no records are available. In cases wherestructural assessments of old structures are required, cores may also provevaluable in confirming covermeter results concerning the location and sizeof reinforcement.5.3 Small coresAlthough standards normally require cores to have a minimum diameterof 100 mm for compressive strength testing, cores of smaller diameteroffer considerable advantages in terms of reduced cutting effort, time anddamage. For applications such as visual inspection, density or voidage