The Compressive Strength Test on Drilled Concrete Cores is a crucial method for assessing the strength of hardened concrete in structures. To ensure the accuracy and reliability of this test, specific specifications must be met when obtaining concrete cores for testing. These specifications dictate the necessary conditions that the drilled concrete cores must meet to be deemed suitable for compressive strength testing.
Diameter of Concrete Core
The diameter of the core specimen used to determine the compressive strength in load bearing structural members must be at least 3.70 inches (94 mm). In the case of concrete with a nominal maximum size of aggregate greater than or equal to 1.5 inches (37.5 mm), it is preferred to have a minimum core diameter that is three times the nominal maximum size of coarse aggregate. However, it should be at least two times the nominal maximum size of the coarse aggregates.
Length of Concrete Core
The ideal length of a capped specimen falls within the range of 1.9 to 2.1 times the diameter. If the length exceeds this range, it can be trimmed down to fit within the preferred length. On the other hand, if the specimen’s length is too short, a correction factor must be applied during compressive testing to account for the deviation from the preferred length.
Moisture Conditioning of Concrete Core
After drilling cores, it’s important to wipe off any surface water and allow the moisture on the surface to evaporate. Once the surface appears dry, within a maximum of 1 hour after drilling, the cores should be placed in separate bags or non-absorbent containers and sealed to prevent further moisture loss. It’s crucial to maintain the cores at ambient temperatures and protect them from direct sunlight during transportation. The cores should be transported to the laboratory as soon as possible.
If capping is required before testing, the cores can be taken out of the bags for a maximum of 2 hours. If water is used for grinding or sawing the core ends, these operations should be completed as soon as possible, but not later than 2 days after drilling. It’s important to minimize the duration of exposure to water during end preparation. After the end preparation is completed, the cores should be allowed to remain in the sealed plastic bags or non-absorbent containers for at least 5 days after the last time they were wetted before testing.
Sawing of the ends of concrete core
The ends of the core specimen must be flat and perpendicular to the longitudinal axis. Prior to capping, sawing should be done in a way that ensures the following requirements are met: a) Any projections on the ends shall not extend more than 0.2 inch [5mm] above the end surfaces, and b) The end surfaces shall not deviate from perpendicularity to the longitudinal axis by a slope of more than 1.8 times the average core diameter (1.8d) or 1:0.3 times the average core diameter (1:0.3d).
Capping of concrete core
If the ends of cores do not meet the requirements of perpendicularity and plainness, they must be either sawed, ground, or capped. When capping is done, the capping device must be able to accommodate the actual core diameters and produce caps that are concentric with the core ends. The material used for capping must have a compressive strength greater than that of the concrete in the core. The caps should be made as thin as possible and should not flow or fracture before the concrete fails during testing of the specimen.
The capped surface of the core must be at right angles to the axis of the specimen and should not deviate from a plane by more than 0.05 mm. Prior to capping, the core lengths must be measured to the nearest 0.1 inch [2 mm].
Measurement of Concrete Core
Before conducting the test, measure the length of the capped or ground specimen with an accuracy of up to 0.1 inch (2 mm). Use this measurement to calculate the length-to-diameter (L/D) ratio of the specimen. To determine the average diameter, take two measurements at right angles to each other at the midpoint of the specimen’s height and calculate their average.
When measuring core diameters, use an accuracy of up to 0.01 inch (0.2 mm) if the difference between the diameters does not exceed 2% of their average. Otherwise, use an accuracy of up to 0.1 inch (2 mm).
Do not test cores if the difference between the smallest and largest diameter of the core exceeds 5% of their average.
Testing of concrete core
The specimen must be tested within a 7-day timeframe from the coring process.
Calculation of Compressive Strength of Concrete Core
The compressive test of a specimen can be calculated using the computed cross-sectional area, which is based on the average diameter of the specimen. However, if the length-to-diameter (L/D) ratio of the specimen is 1.75 or less, the result obtained needs to be corrected using correction factors as provided below:
| L/D Ratio | Correction Factor |
| 1.75 | 0.98 |
| 1.5 | 0.96 |
| 1.25 | 0.93 |
| 1.0 | 0.87 |
The corrected compressive strength is the value obtained after multiplying the raw strength with a correction factor. This corrected strength is equivalent to the strength of a cylinder with a length-to-diameter (L/D) ratio of 2. To find the equivalent cube strength, the corrected cylinder strength is multiplied by 5/4. This calculation allows for a comparison of the strength of different specimens, taking into account the differences in geometry and size, and provides a standardized measure of strength for analysis and reporting purposes.
Report of Compressive Strength Test
The results of the core drilling test include the length of the core drilled, rounded to the nearest 5 mm. The length of the specimen before and after capping is measured to the nearest 2 mm, and the average diameter of the core is measured to the nearest 0.2 mm or 2 mm. The compressive strength is reported to the nearest 0.1 MPa when the diameter is measured to the nearest 0.2 mm, and to the nearest 0.5 MPa when the diameter is measured to the nearest 2 mm after correction for the L/D ratio.
The direction of application of load with respect to the horizontal plane of the concrete as placed is noted. The moisture conditioning history is recorded, including whether water was used during end preparation, and the date and time when end preparation was completed and the core was placed in concealed bags. The date and time of testing are also reported.
Additionally, the nominal maximum size of the aggregates used in the concrete is recorded. All measurements are rounded to the nearest appropriate unit as specified (e.g. 5 mm, 2 mm, 0.2 mm, 0.1 MPa, 0.5 MPa), and all relevant information, such as length, diameter, compressive strength, direction of load application, moisture conditioning history, and dates and times, are included in the report for accurate documentation of the core drilling test results.