Concrete Core Test

The Concrete Core Test Is required when concrete is hardened and the laboratory cube test result shows a 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 is collected 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 whether the suspected hardened concrete is fitted acceptance criteria or not.

Concrete Core Test

If the average equivalent strength is equal to at least 85 percent of the cube strength of the grade of concrete the individual core strength is not more than 75 percent.

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, cleaned, 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 was then recorded as 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 the 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 later and considered the closest 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
1	75	1.03
2	< 70	1.06

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

Where,

F = Factor of Correction

N = Length / Diameter Ratio

Sample Preparation For Concrete Core Test

Number of Cores

Concrete Core Test Sample — Number of Cores

How many numbers are core 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.
The preferred diameter is 100 mm for a nominal maximum size of 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 the 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,
The diameter of the core Specimen.
Whether a comparison will be made with cylindrical strength or 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.

Reinforcement

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 a core free from reinforcement the reinforcement detector such as a cover meter can be used for selecting the drilling location free of reinforcement.

Preparation of Concrete 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 Sample

The specimen’s lower end is held vertically into a pool of molten sulfur mixture on a horizontal plate or mold. 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 molds.
The level of mixture in the melting pot shall 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 has elapsed.

Code Provision for Concrete

IS Code – 456: 2000, Reference clause 17.3

Testing

In that case where doubt regarding the low grade of concrete is used poor workmanship, is identified by the core test, strength test, and load test.

the IS Code for the Concrete Core test is IS – 516 PART- 4: 2018, Reference clauses 3

Why is the Concrete Core Test Concrete Required?

The core test is required when the results of concrete cubes are not giving satisfactory result.
In case of doubt regarding the grade of concrete used either due to poor workmanship or based on the result of the cube strength test, a compressive strength test of the concrete core may be carried out.
In addition, the core cutter test is used for determining the existing structural strength of the concrete and also evaluating the structural capacity or safety assessment of the existing structure.
Also the case of rehabilitation of the existing structure, the concrete element is the most important factor so this concrete is tested with the help of the core cutter test.
The core cutter test is finding the strength of the existing structure.

What Is Capping and Its Role In The Result?

The capping is the preparation of the ends of a cylindrical concrete specimen on a perpendicular surface to the applied axial load during compressive strength.
Inside the structure collects the core sample which is not having a smooth surface that the 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 the face; it will cause 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 Are the Requirements of Good Capping?

The end of the cylindrical specimen that is not plane within 0.05 mm shall be capped.
Before capping you should check the surface of the core cutter, which is free from loose materials and clean.
The material of capping is having greater compressive strength as compared to the concrete core in the 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 of the specimen in the field
Density of Specimen
Which type of method is used for the preparation of a 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
The appearance of fracture on the surface of the specimen
Types of Fracture

Factors Affecting The Strength of Core

The following types of factors are affecting the strength of the concrete core test,

Curing
Micro – Cracking
Moisture Content
Compaction
Place of drilling the Core

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Concrete Core Test