AS 1012.14:2018 Methods of testing concrete
A3 CALCULATION OF ADJUSTED COMPRESSIVE STRENGTH
When calculating the adjusted compressive strength of cores, the following needs to be taken into account:
(a) Effect of core diameter Early work on the effect of core diameter on strength seemed to imply higher strengths for small diameters, but recent work indicates that there is no significant difference for cores having diameters within the range of 100 mm to 150 mm, provided the cores are taken from the centre of the mass of concrete. There can be a strength gradient within the concrete, increasing with depth below the surface resulting from curing and consolidation effects. This should be considered, particularly when securing or assessing small diameter cores. Recent work shows that smaller cores exhibit lower strengths. The adjustment factor required to compensate strength reduction is considered to be around 1.06 for smaller core diameter of 75 mm compared to 100 mm diameter core where the adjustment factor is taken as 1.0.
(b) Effect of damage due to drilling It is recognized that the coring process causes damage to the concrete core samples when the core bit cuts through the aggregates and the concrete matrix. The cutting process weakens the bond between the aggregate and the enveloping cementitious matrix, and may induce microcracks due to the shearing forces between the coring bit and the concrete surface. The adjustment factor required to compensate strength reduction due to damage during the drilling process is considered to be around 1.06.
(c) Effect of coring direction Concrete cores drilled vertically in the direction of concrete placement and compaction exhibit higher strength than concrete cores drilled horizontally. This difference may be attributed to the development of a weaker bond and minor flaws below the coarse aggregate at the cementitious paste interface due to the effects of bleed water. These may be exacerbated further by induced microcracks during the coring process. Whereas in the case of the vertically cut concrete cores these microcracks and flaws close up during the test load application, in the case of the horizontally cut cores these flaws open up during testing thereby having a strength decreasing effect. The adjustment factor required to compensate strength reduction due to the effect of horizontal coring is considered to be around 1.07.