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[P.K.Mallick]

VARIABILITY OF STRENGTH OF CONCRETE CORES.
In the article “Core Tests: Easy to Perform, not Easy to Interpret”, the Author Adam Neville writes that there is a scatter in the strength of cores even when cores are taken in close vicinity to one another. This scatter of strength of cores is large because coring itself introduces variability. It may be worth repeating that, however carefully performed; the coring operation causes some damage to the core: there may be a weakening of the bond of large particles of aggregates near the surface of the core, and there may be the dynamic effect of drilling which includes some fine cracks.
In order to reduce the variability of strength of concrete cores IS: 516 (Part-4), Hardened Concrete – Method of Test, Part-4, Sampling, Preparing and Testing of Concrete Cores has suggested various methodologies.
1)     Location of Core: The Core shall be taken away from the edges of the Concrete Joint and places where reinforcement is present. So use of Cover meter to locate the reinforcement is a must. The Clause 5.9 of IS: 516 (Part-4) states that drilling through reinforcement shall be avoided. The Core shall not contain any reinforcement bars along or near its longitudinal axis. Cores containing cross reinforcement shall be trimmed off to obtain core free from reinforcement. EN 12504-1, Testing of Concrete in Structures – Part 1- Cored Specimens- Taking, Examining and Testing in Compression states that it should be ensured that Cores for determination of Compressive Strength do not contain any reinforcing bars in, or close to the direction of longitudinal axis of the core. In the article, Core Tests: Easy to Perform, not Easy to Interpret, the Author Adam Neville does not suggest  for coring for heavily reinforced member for the sake of Structural Integrity of the member. However, he writes, if, for whatever reason, core cuts through the reinforcement, then the cores can be tested for compressive strength, only if there is no reinforcement parallel to axis of the core. The British Standard BS EN 13791:2019, Assessment of in-situ Compressive Strength in Structures and precast Concrete Components states that any core that contains reinforcement in the direction of coring or close to the direction of coring shall be rejected immediately and a further core shall be taken from same Test location. Where a core contains reinforcement that is arranged perpendicular to the direction of loading, this shall be recorded and evaluated separately.
              If reinforcement is present, it should be predominantly horizontal to the direction of testing and not more than 2 percent of volume of specimen. In addition, with 2:1 specimens, the reinforcement should be contained within 30mm of each side.

  

Fig-1 Marking of Reinforcement using reinforcement detector –Cover Meter. (Ref: b)

Fig-2 Placing of Core Bit within the location marked using reinforcement detector. (Ref: b)

2)     Preparation of Cores: Cores not prepared properly before testing shall induce variability in Core Test results. The intended load-bearing surfaces of cores shall be prepared either by grinding or by capping to improve the contact with the loading machine. Cutting and grinding shall be carried out in such a way that structural changes of test specimen are avoided. IS: 516, Part-4 provides a table giving a relation between “Anticipated Strength of Concrete” and “Preparation of load bearing surface of Specimens”.


No     Anticipated Strength of Concrete        Preparation Method
1     For any value of Strength                      Grinding.
2     Up to 50 Mpa                              1)Capping with Calcium
                                                            
                                                                    Aluminate Cement Mortar.
                                               2)Capping with Sulphur
                                                                       Mixture.

3     Up to 100 Mpa                           Capping with high Strength Sulphur
                                                             mixture.

Other Capping materials may also be used provided that, at the time of testing, it has strength at least equal to the anticipated strength of Concrete.

3)     Some of the important points pertaining to Grinding and Capping as per IS:516 (Part-4):

a)     The ends of specimen shall be ground to the tolerance as given below:

FLATNESS.

PERPENDICULARITY AND PARALLELISM.

Fig-3 Permissible Tolerances for prepared Core Specimens. (Ref-a)

These tolerances are for 100mm diameter Core. If the Core is with diameter less than 100mm, these tolerances shall be reduced in proportion to tolerances specified for 100mm diameter.
b)     The cap shall be as thin as possible and shall not, on average, be greater than 5mm thick, small local deviations are permissible.
c)      If a capping layer sounds hollow, it shall be removed and the capping operation shall be repeated.
d)     The Cap shall not fail or fracture before the Concrete fails when the core specimen is tested.

4) Assessment of type of failure.

IS: 516 (Part4) provides two set of images i.e. Images of Satisfactory failure of specimens and Images of Unsatisfactory failure of specimens. The clause No B-1(f) of IS: 516 (Part-4) states that if cracking is observed during testing from failure pattern, the core test result can be discarded. So in order to reduce the viability of core strength, the tested cores should be observed for satisfactory failure or unsatisfactory failure.

Fig-4 Satisfactory failure of specimen. (Ref-a)

Fig-5 Unsatisfactory failure of specimen. (Ref-a)

5)     Other factors which induces variability of Core Strength:

a)     Place of drilling the core.
b)     Micro-cracking.
c)     Curing.
d)     Compaction.
e)     Moisture Content.

6)     Conclusion: It would be very apt, if we conclude by quoting Adam Neville, “Core Tests: Easy to perform, Not easy to Interpret”.

7)     References:
a)     IS: 516 (Part-4), Hardened Concrete – Method of Test, Part-4, Sampling, Preparing and Testing of Concrete Cores.
b)     Core Test for Hardened Concrete by Delhi Metro Rail Corporation Ltd.
c)     BS EN 13791:2019 Assessment of in-situ Compressive in Structures and precast Concrete Components.
d)     BS EN 12504-1 Testing Concrete in Structures- Part-1: Cored Specimens- Taking, examining and testing in Compression.
e)     ECP 203-2007 Egyptian Code for Design and Construction of Concrete Structure.
f)     Core Tests: Easy to perform, Not Easy to Interpret by Adam Neville.