1. What is a Core?
In civil engineering, a core refers both to the process of extracting a cylindrical sample from hardened concrete and to the sample itself. Core samples are used to evaluate the current condition of a structure, determine the concrete’s strength, and verify its compliance with the concrete grade specified in the project.

Specifically:

• New buildings: for quality control

• Existing structures: to determine actual strength before strengthening or repair

2. Core Extraction Process
Core extraction is performed using special diamond-tipped core drilling machines. The process includes the following steps:

• Site selection: Critical points on structural elements (columns, beams, slabs) are identified according to the structural design. Reinforcement and utilities are scanned.

• Drilling: Cylindrical samples, usually 50–150 mm in diameter, are extracted using water-cooled drills.

• Sample preservation: Extracted cores are labeled and transported to the laboratory carefully to prevent damage.

• Repair of the hole: The drilled area is filled with epoxy or repair mortar to maintain structural continuity.

3. Core Sample Dimensions

• Diameter: 50–150 mm (typically 100 mm)

• Length-to-diameter ratio: Approximately 2:1; otherwise, a correction factor is applied in compressive strength tests.

• Number of samples: Determined based on the project and structure size; a single sample is not sufficient for decision-making.

4. How is the Core Test Performed?

a) Sample Preparation

• End faces are ground or a special capping is applied.

• Samples are stored under appropriate moisture conditions in the laboratory.

b) Compressive Strength Test

• The sample is placed in a compression testing machine.

• The breaking strength is measured at standard loading rates.

• Results are compared to the project’s specified concrete class.

c) Additional Tests (if necessary)

• Density and water absorption

• Carbonation depth

• Microstructure analysis

These tests provide information about the long-term performance and durability of the concrete.

5. Standards
Core procedures in civil engineering are performed according to the following standards:

• EN 12504-1: Testing concrete in structures — taking, examining, and testing cores

• ASTM C42 / ACI: Taking and testing concrete cores

• TS EN compliant standards: Used in Turkey

6. Interpretation of Results

• Core test results are not directly comparable to cube/cylinder tests of fresh concrete; correction factors must be considered.

• Concrete strength should be evaluated together with reinforcement density and construction quality.

• If low results are obtained, additional cores should be taken to verify the findings.

7. Impact on the Structure

• When performed correctly, core extraction does not damage the structure.

• Critical points must be selected to avoid weakening the structural system, the number of cores should be limited, and drilled holes must be properly repaired.

Core testing is the most reliable method to determine the strength of existing concrete in the construction industry. Samples should be taken according to relevant standards, laboratory tests should be carefully conducted, and results should be evaluated together with structural calculations. For structural safety, core test results should not be used alone but as part of a comprehensive assessment.

References

• BS EN 12504-1:2019 — Testing concrete in structures — Cored specimens

• ASCC – Concrete Core Testing Checklist (ASTM C42 / ACI Guidelines)

• Concrete Society — Core testing guidance

• KTÜ – Core extraction and cutting (technical document)

• TS EN standards and civil engineering practices in Turkey