The factors that are significant which influence the core compressive strength of concrete are: 1. Moisture and Voids 2. Length/Diameter Ratio of Core 3. Diameter of Core 4. Position of Cut out Concrete in Structure 5. Direction of Drilling 6. Effect of Age.
Factor # 1. Moisture and Voids:
The moisture condition of the core influences the measured strength. It has been observed that a saturated specimen has a value of 10 to 15% lower than comparable dry specimen. Thus while estimating the actual in-situ concrete strength the relative moisture conditions of the core and the in-situ concrete should be taken into consideration.
Voids in the core concrete will reduce the measured strength. Peterson found that the ratio of core strength to standard cylinder strength at the same age is always less than 1.0, and decreases with the increase in the strength of cylinder. Upto cylinder strength of 20 MPa it is just less than 1 and 0.7 for 60 MPa strength.
Factor # 2. Length/Diameter Ratio of Core:
It has been observed that as the l/d ratio increases, the measured strength decreases due to the effect of specimen shape and stress distribution during the test. For establishing a relation between core strength and standard cube strength, ratio of l/d = 2.0 is taken as the basis of computation.
Factor # 3. Diameter of Core:
The diameter of the core may influence the measured strength and variability. Measured concrete strength decreases with the increase in the size of specimen. This effect is significant. However this effect will be small for sizes above 100 mm, but for smaller sizes this effect is significant.
Factor # 4. Position of Cut out Concrete in Structure:
Cores taken from near the top surface have usually lowest strength may it be a column, beam, or wall or slab. With the increase in depth below the top surface the strength increases, but at depths more than 300 mm, there is no further increase in strength. The difference may be 10 to 20%. In case of slabs, poor curing increases this difference.
Factor # 5. Direction of Drilling:
Due to the layering effect, the measured strength of specimen drilled vertically relative to the direction of casting is likely to be greater than that for a horizontally drilled specimen from the same concrete. The average value of 8% of this difference has been reported in literature.
Factor # 6. Effect of Age:
It has been observed that in-situ, concrete gains little strength after 28 days. Tests on high strength concrete have shown that, though the core strength increases with age upto 1 year, but remains lower than the 28 days standard cylinder strength as shown in Fig. 19.6 and Table 19.3. On the other hand Petersons has suggested the increase in core strength over that of 28 days cylinder strength as 10% after 3 months and 15% after 6 months. Thus the effect of age is not easy to discuss, but in the absence of definite moist curing, no increase in strength should be expected.
Curve A of Fig. 19.6 shows the strength development of a standard cylinder. Curve B shows the strength of well cured slab core tested dry. Curve C shows strength development of well cured slab core tested wet. Curve D shows strength development of poorly cured slab, core tested dry and curve E shows strength development of poorly cured slab core tested wet.