Core Test

IS 516 – Part 4 – 2018 / ASTM – C42 – 20 / EN – 12504 – 2019

Principle & ProcedureThis is one of the very reliable tests adopted for checking the compressive strength of the ‘In situ concrete”. Other physical properties such as density, water absorption can also be measured from the core concrete. In addition chemical properties of concrete specimen for its cement content, carbonation depth, chloride and sulphate content may be measuredThough this test may become partially destructive for beams / columns , but it can be used for slabs, walls, where partial destruction of concrete due to core cutting do not disturb the stability of the member. In this method concrete cores of sizes raging from 20 mm to 150 mm in diameter and 50 mm to 500 mm long are drilled out by a diamond cutters. The recommended diameters are 100 to 150 mm, but if the drill depth is insufficient as in of case slabs , then smaller diameters may be used but not less than three times nominal aggregate size. The core diameter to length ratio shall be normally between 1.0 to 2.0 ( preferably 2.0) The core diameter shall be at least three times the nominal maximum size of aggregate. Reinforcement shall be avoided in the core. At least three cores shall be tested for acceptable accuracy. These cylindrical concrete cores are then made smooth at both ends ( if required ) and then tested for compressive strength. If required capping of the faces shall be done. The strength of capping material shall be higher than that of concrete in the core. Cap shall be as thin as practicable. The specimen shall be cured in water for 48 hours before testing.

The cylindrical strength is then co-related to cube strength. IS – 516 suggest a multiplying factor of 1.25 for converting cylindrical strength to equivalent cube strength. In addition a correction factor for height to diameter ratio and diameter shall be applied as given in IS – 516. IS – 456 states that the concrete in the member represented by a core test shall be considered acceptable, if the average equivalent cube strength of core is equal to at least 85 % of the cube strength of the grade of concrete specified, but no individual core has strength less than 75 %.

Diamond Core Cutting Machine

Diamond Core Bit

ApplicationThe core cutting is mainly conducted for :-

Determining “In situ” compressive strength of structure
Small cores for chemical tests
Inserting water supply, plumbing pipes
Inserting conducts for electrical cables
Making pockets for machine foundation for inserting bolts
Making weep holes in walls

Core Sample Extraction from a Dam

Core Sample Extraction from a Beam

Core Extraction from a RCC Chimney

Core Extraction from Box Girder Bridge

Factors that affect core strength results –

Systematic variation of in-place strength along member or throughout structure
Random variation of in-place strength, both within one batch & among batches
Low test results attributable to flawed test specimen or improper test procedure
Effect of size, aspect ratio and moisture condition of the test specimen
Additional uncertainty attributable to the testing that is present even for standardize tests
Size, H/d ratio
Direction of Core – upto 7 % less strength is noticed for horizontally drilled cores
Reinforcement in the core

Reliability & Limitations

As this test gives actual in situ strength of concrete is more acceptable, but due to partial destructiveness needs to be used very carefully. The reliability of small cores i.e. 40 – 50 mm is less as compared to normal cores. The detection of reinforcement shall be perfect. If the quality of concrete is not good, one may not even get a complete core for testing. The cost of core cutting is more compared with other ND tests, as it consumes diamond bits, which are costly.

Some Observations & Recommendations –

100 to 150 mm diameter cores are recommended. But due to reinforcement spacing many a times 60 to 75 mm dia. cores are extracted
Analysis and Interpretation of core test data is complex.
Strength interpretation shall be made by an Expert Concrete Technologist and Structural Engineer

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