Core Testing in Hardened Concrete
Concrete, as a structural material, is subject to rigorous
testing to ensure its integrity and strength over time. One well-established
method for this is the examination and compression testing of cores cut from
hardened concrete. This approach allows for visual inspection of the interior
regions of a concrete member alongside strength estimation. In this article, we
delve into the intricacies of core testing, covering its procedures, equipment,
and key considerations.
Core Testing Procedures
Selection of Cores: According to IS: 456-2000, the
selection of core points and the number of cores required should be at the
discretion of the engineer-in-charge. However, a minimum of three cores should
be tested to ensure representativeness. Cores should be prepared and tested as
per the guidelines provided in IS: 516.
Acceptance Criteria: Concrete represented by core tests
is deemed acceptable if the average equivalent cube strength of the cores is at
least 85 percent of the specified concrete grade strength for the corresponding
age. Additionally, no individual core should have a strength less than 75
percent.
Load Testing: In cases where core test results do not
meet acceptance criteria or when such tests are not feasible, load testing can
be an alternative approach.
Core Cutting Machines and Drilling
Equipment: Cores are typically cut using rotary cutting
tools with diamond bits. Portable core drilling machines are employed for this
purpose, though they require firm support against the concrete to prevent
distortion or breakage. Water supply is essential for lubrication during
drilling.
Core Preparation: Cores should not be removed from the
structure until the concrete has sufficiently hardened to maintain bond
integrity. Ideally, cores should be drilled at 28 days old concrete, but a
minimum of 14 days is required before removal.
Capping: To ensure perpendicularity and planeness, cores
may require sawing, grinding, or capping with high alumina cement mortar or
sulfur-sand mixture. The thickness of caps should be minimal, preferably 1.5 to
3 mm, and must not be weaker than the concrete.
Key Considerations
Diameter and Length: Cores should have a
diameter-to-maximum aggregate size ratio not less than 3. While 100 mm diameter
cores are preferred, 50 mm cores may be used with caution, considering their
lower and more variable compressive strength.
Length-to-Diameter Ratio: The length of the capped or
ground specimen should ideally be between 1.9 and 2.1 times the diameter.
Corrections to measured compressive strength are required for L/D ratios equal
to or less than 1.75.
Measurement and Testing
Measurement: Before testing, the average length and
diameter of the specimen are measured. The density is determined by weighing
the core and calculating its volume.
Testing: Cores are submerged in water for 48 hours
before testing. Compressive strength is determined by applying a continuous
load until failure, with results recorded and reported accurately.
Core testing is a globally accepted method for assessing the
strength and quality of concrete in structures. By following standardized
procedures and employing suitable equipment, engineers can obtain valuable
insights into the integrity of concrete elements, ensuring safety and
longevity.
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