According to the ASTM Standard C 42, the testing of concrete cores is conducted using a specific procedure for strength determination. This involves taking core samples from the concrete structure and subjecting them to various tests in order to assess their strength.
The core sampling process involves drilling cylindrical cores from the concrete structure using a diamond-tipped drill. These cores are typically 4 inches in diameter and can range in length from 2 to 4 times their diameter, depending on the intended use of the cores.
Once the cores are extracted, they are typically tested within 48 hours to prevent any moisture loss or other factors that may affect the results. The testing process involves measuring the length and diameter of the cores, as well as their weight, before subjecting them to compressive strength testing.
During compressive strength testing, the cores are placed in a specialized machine that applies a load to the top and bottom surfaces of the core until it fractures. The load required to fracture the core is then used to calculate its compressive strength, which is typically reported in units of pounds per square inch (psi).
Overall, the ASTM Standard C 42 provides detailed guidance for the testing of concrete cores, including procedures for core sampling and strength determination. By following these guidelines, engineers and technicians can obtain accurate and reliable information about the strength of concrete structures, which is critical for ensuring their safety and durability.
Testing of Concrete Cores for Strength
When determining the compressive strength of concrete using core specimens, it is recommended that the diameter of the cores be at least three times the nominal maximum size of the coarse aggregate used in the concrete. Additionally, the diameter of the core samples must be at least twice the maximum size of the coarse aggregate. The length of the specimen, when capped, should be as close to twice its diameter as possible. Any core sample with a maximum height of less than 95% of its diameter before capping, or a height less than its diameter after capping, should be rejected. It is preferable to test the cores in a moist condition.
The ASTM standard specifies a procedure for conducting compression tests on test specimens. According to the standard, the specimens should be submerged in lime-saturated water at a temperature of 23.0 +/- 1.7° C for a minimum of 40 hours immediately prior to the compression test. It is important to test the specimens promptly after they are removed from water storage. During the period between removal from water storage and testing, the specimens should be kept moist by covering them with a wet blanket made of burlap or other absorbent fabric.
If the ratio of the length to diameter of the specimen is less than 1.94, correction factors should be applied as per the guidelines provided in Table 1.
Table-1: Correction Factor for Ratio of Length of Cone to its Diameter
| Ratio of length of cylinder to diameter (L/D) | Strength correction factor |
| 1.75 | 0.98 |
| 1.50 | 0.96 |
| 1.25 | 0.93 |
| 1.00 | 0.87 |
ASTM C 42 provides guidelines for the correct removal of concrete samples through core drilling. When the cores are intended for strength testing, diamond-studded core bits must be used. Alternatively, for vertically drilled cores, a shot drill may be considered acceptable for other purposes. However, it is recommended to use diamond-studded core bits for drilling in other orientations.
Fig: Core sampling and testing of concrete
In core sampling, it is crucial to carefully select the number, size, and location of core samples to allow for all necessary laboratory tests. Whenever possible, virgin samples should be used to avoid any influence from prior tests.
For strength determination, the core must have a minimum diameter of three times the maximum nominal size of the coarse aggregate or 50 mm, whichever is greater. The length of the core should be at least twice its diameter for strength testing.
Reinforcing steel should not be included in a core that is being tested for strength, and care should be taken to avoid disturbing electrical conduits or prestressing steel during core drilling.
To avoid the need for breaking the core for extraction, it is recommended to drill the core through the full depth of the member. An additional 50 mm is usually drilled to account for possible damage at the base of the core.
At least three cores should be removed at each location in the structure for strength determination. The hole left after core extraction should be filled with packaged repair material. For slabs, a thixotropic material that does not fall under gravity is required. Alternatively, a precast cylinder of concrete may be fitted in the core hole using cement grout or epoxy resin. The minimum core diameter is typically 100 mm, but 75 mm and 50 mm diameters may be used in special cases. The number of 50 mm diameter cores should be three times the number of 100 mm diameter cores for equivalent accuracy.
The top 20 percent portion of the member, with a minimum of 50 mm and a maximum of 300 mm, and a side cover of 50 mm within the member, should preferably not be included in the portion of the core to be tested.
Before testing the cores, various information such as dimensions, density, shape, presence of well-graded or gap-graded aggregates, position of cracks, damage due to drilling, and presence of steel should be recorded.
The ends of the cores should be prepared using a high-speed wet grinding machine with diamond-faced grinding wheels, or capping with strong materials may be carried out. It is recommended to soak the cores in water for 40 hours prior to testing, as per British standard CSTR No. 11, to ensure accurate strength results.
For horizontally drilled cores:
The given context describes a formula to calculate the corrected cylinder strength. According to the formula, the corrected cylinder strength is equal to the core strength multiplied by the expression [(2.5×0.8)/(1+(1/R))].
To calculate the corrected cylinder strength, one needs to know the core strength of the cylinder. The core strength refers to the inherent strength of the cylinder material without any defects or damages. Once the core strength is known, it needs to be multiplied by the expression [(2.5×0.8)/(1+(1/R))] to obtain the corrected cylinder strength.
The expression in the formula involves two constants – 2.5 and 0.8 – and a variable, R. The variable R represents the ratio of the diameter of the cylinder to its length. The value of R can vary depending on the dimensions of the cylinder. The expression [(2.5×0.8)/(1+(1/R))] is used to adjust the core strength of the cylinder based on its dimensions.
Overall, the given context provides a formula to calculate the corrected cylinder strength, which takes into account the core strength of the cylinder and its dimensions. By using this formula, one can obtain an accurate estimate of the strength of a cylinder, which can be useful in various engineering applications.
For vertically drilled cores:
The given context is a formula for calculating the corrected strength of a cylinder. According to the formula, the corrected strength of a cylinder can be determined by multiplying its core strength by a factor calculated using the following equation:
(2.3 x 0.8) / (1 + (1/R))
where R represents the radius of the cylinder.
The numerator of the equation is the product of two constants, 2.3 and 0.8. The denominator includes the radius of the cylinder, which is used to calculate the correction factor.
The corrected strength of the cylinder is then obtained by multiplying the core strength of the cylinder by the correction factor calculated using the above equation.
It is important to note that this formula is specific to cylinders and is used to determine their corrected strength based on their core strength and radius.