What Is a Concrete Core Test?
The Concrete Core Test Is required when concrete is hardened and laboratory cube test result shows negative result then to ensure the quality or strength of concrete work the Core Test on Concrete is performed.
In the Concrete Core test, the sample or core collect from the hardened concrete and extracted using the core cutter machine and then these cores are used to determine the compressive strength of the concrete.,
A Core Test is generally performed for the concrete to determine the suspected hardened concrete is fitted with acceptance criteria or not.
Codal Provision for Concrete
IS Code – 456: 2000, Reference clause 17.3
In that case where doubt regarding the low grade of concrete is used either poor workmanship, is identified by the core test, strength test and load test.
If the average equivalent strength is equal to at least 85 per cent of the cube strength of the grade of concrete for the individual core strength is not more than 75 per cent.
the IS Code for Concrete Core test is IS – 516 PART- 4 : 2018, Reference clauses 3
Why Concrete Core Test of Concrete Required?
- The core test is required when the result of concrete cubes are not giving satisfactory result.
- In case of doubt regarding the grade of concrete used either due to poor workman ship or based on result of cube strength test, compressive strength test of concrete core may be carried out.
- In addition core cutter test is used for determining the existing structural strength of concrete also evaluated structural capacity or safety assessment of the existing structure.
- Also the case of rehabilitation in the existing structure, the concrete element is the most important factor so this concrete is tested with the help of core cutter test.
- Core cutter test is finding the strength of existing structure.
Sample Preparation For Concrete Core Test
Number of Cores
How many numbers are core is required for the test is depend on the discretion of the engineer in charge and the engineer must be representative of the concrete test and quality.
Diameter of Cores
- The diameter of the cores shall be specified before testing.
- The ratio of diameter to the nominal maximum size of aggregate shall be greater than 3, the core diameter shall be 100 mm to 150 mm.
- Preferred diameter being 100 mm for a nominal maximum size up to 20 mm.
- Other smaller diameters which may make drilling easier and reduce the damage to the element may be used if the effect of diameter on the accuracy of the result.
Length of Core
- The length of core sample to be used, which is specified by an engineer in charge and the total length of cores to be extracted.
- The length of the core sample shall be decided based on,
- The diameter of the core Specimen.
- Whether comparison will be made with cylindrical strength of cube strength.
Length / Diameter ratio (L / D)
The preferred length / Diameter ratio shall be 2,l/d value from 1 to 2 may also be permitted.
In the case of using a specimen with a length/ diameter ratio smaller than 2 the test value of the compressive strength should be corrected corresponding to a value of l/d of 2.
Marking and Identification
Immediately after drilling mark each core ideally and clearly. Record its location and orientation within the element from which it was drilled if a core is subsequently cut to produce a number of specimens mark each specimen to indicate its position and orientation.
Drilling through reinforcement shall be avoided. The core shall not be containing any reinforcement bars along or near its longitudinal axis cores containing cross reinforcement.
It shall be trimmed off to obtain core free from reinforcement the reinforcement detector such as cover meter can be used for selecting the drilling location free of reinforcement.
Preparation of Concrte Core Test Sample
- The core specimen shall be sawn perpendicular to its longitudinal axis And the size of the specimen shall be adjustable.
- The load–bearing surface can be prepared by either capping or grinding to improve contact with the loading machine.
- Various types of capping material required different maximum compressive strength,that is given in the below table,
|Sr. No.||Anticipated Strength||Preparation Method|
|1||For any value of strength||Grinding|
|2||Up to 50 MPa Mortar||Capping With Calcium Aluminates Cement|
|3||Up to 100 MPa||Capping With High strength sulphur|
The Procedure Taking Core Smaple
- The specimen’s lower end is held vertically into a pool of molten sulphur mixture on a horizontal plate or mould. Allow the specimen to harden before repeating the procedure of other ends.
- Uses a capping frame that will ensure that both capped surfaces are parallel.
- Mineral oil shall be used for realizing the plate or moulds.
- The level of mixture in the melting pot shall be never to allowed fall too low as there will be an increased risk of the production of sulphur vapor.
- The core specimen shall be checked to ensure that the capping material has adhered to both ends of the specimen.
- If a capping layer sounds hollow it shall be removed and the capping operation repeated.
- The cap shall not fracture or fail before the concrete fails when the specimen is tested.
- The compression test on the test core specimen shall not be carried out until at least 30 min have elapsed.
Concrete Core Test Procedure
The concrete core test procedure as per IS described below,
- Before placing the core specimen in the testing machine weigh it and also determine its length.
- Testing machines shall be wiped, clean and freeform any loose material shall be checked.
- The core cutter specimen shall contact the compression plates.
- The core specimen shall be placed in the machine in such a manner that the load shall be applied to the top and bottom prepared surface.
- The axis of the specimen shall be carefully aligned with the center of thrust.
- The spherical–shaped block is bear to the core specimen and the movable portion shall be rotated gently by the hand.
- So that uniform block seating is obtained.
- The load shall be applied on the core specimen without shock and increase continuously at the rate of 14N/MM2/min.
- Until the core specimen breakdown and no greater load can be sustained.
- The maximum load applied to the specimen then recorded the reading and the check appearance of the concrete.
- Also check any unusual feature in the type of failure shall be noted.
Assessment of Type of Failure
- For core specimen if the failure is satisfactory. This fact shall be recorded if the failure patterns us unsatisfactory.
- The record type of failure using the pattern latter and considered the closet value of the result.
Calculation of Concrete Core Result
The compressive strength of the core specimen shall be determined by dividing the maximum load applied to the specimen.
That is expressed to the nearest N/mm2 value.
The production of the corrected factor for core diameter as given in the table, and measured compressive strength shall be known as the corrected compressive strength.
|Sr.No||Diameter of Core||Correction Factor|
The Concrete Core Test Result Calculation,
This Correction factor is depending upon the L/D ratio of the core specimen.
F = 0.11N + 0.78
F = Factor of Correction
N = Length / Diameter Ratio
What Is Capping and Its Role In The Result?
- The capping is a preparation of the ends of cylindrical concrete specimen this perpendicular surface to the applied axial load during compressive strength.
- Inside the structure collect the core sample which is not having a smooth surface that reason capping is used. This capping also distributed load on the entire surface of the core specimen.
- The core specimen axis is not perpendicular to the end of face; it will be causes stress concentration within the test specimen and reduce the measured strength.
- Using capping is to ensure that the compressive load is applied evenly for each and every strength test.
- The strength of the cap should be such that the ends cap should not fail or fracture when the specimen is being tested.
What Is Requirements of Good Capping?
- The end of cylindrical specimen that is not plane within 0.05 mm shall be capped.
- Before capping you should check the surface of core cutter, which is free from loose materials and clean.
- The material of capping is having greater compressive strength as compare to concrete core in cutter.
- Cap shall not be thick more than 5 mm and shall not fracture or flow before the concrete fail when the specimen is tested.
Report of Core rest
The following information shall be included in the concrete core test report,
- Date of test
- Identification mark
- Age of Specimen
- Maximum Nominal Size of Aggregate
- Condition of curing
- Date of manufacture the specimen in the field
- Density of Specimen
- Which types of method is used for preparation of specimen
- Cross Sectional Area of the Specimen
- Diameter of the Specimen
- Maximum Load
- Measured Compressive test
- Corrected Compressive Test
- Corrected Cylindrical Strength
- Equivalent Cube Strength
- Appearance of fracture on the surface of specimen
- Types of Fracture
Factor Affecting The Strength of Core
The following types of factors are affecting the strength of the concrete core test,
- Micro – Cracking
- Moisture Content
- Place of drilling the Core
What are the requirements of good capping?
What is the Factor affecting the strength of the core?
The following types of factors are affecting the strength of the core,
Micro – Cracking
Place of drilling the Core
What is a Core test?
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