Concrete coring is done to verify the strength of concrete in-place and to measure the thickness of completed concrete pavement.
Sampling
- Concrete coring shall be undertaken to obtain the required number of specimens. It should be done after the concrete has become hard enough to permit sample removal without disturbing the bond between the mortar and coarse aggregate. In general, the concrete shall be 14 days old before specimens are taken.
- Samples from concrete coring that show abnormal defects or samples that have been damaged in the process of removal shall not be used.
- Samples for determining compressive strength that contains embedded reinforcement should be avoided or trimmed to eliminate the reinforcement, provided that a length-ti-diameter ratio (L/D) of at least 1.0 can be obtained.
Core Specimens
The diameter of core specimens for thickness determination should be at least 3 times the nominal maximum size of the coarse aggregate used in the concrete and must be at least twice the nominal maximum size of the coarse aggregate in the core sample. The length of the specimens, when capped, shall be as nearly as practicable twice its diameter.
Concrete Coring for Thickness Determination
A completed pavement is accepted on a lot basis. A lot shall be considered as 1000 linear meters of single-lane pavement. Or 500 linear meters of pavement if the two-lane pavement is poured concurrently. While other areas such as intersections, entrances, cross-overs, ramps, etc. will be grouped to form a lot. Each lot will be divided into five (5) equal segments and one concrete core will be obtained from each segment. When the measurement of any core is less than the specified thickness by more than 25mm, additional cores will be taken at no less than 5-meter intervals parallel to the center line in each direction, from the affected location.
Concrete coring will only be stopped when a core that is not deficient in thickness by more than 25mm is found in each direction.
Evaluation of Concrete Samples for thickness determination
- Concrete core samples should be measured at least nine (9) measurements of length, one at the center, and one each at eight additional locations spaced at equal intervals along the circumference of the circle.
- The average length of the core is then obtained from the nine measurements. Each measurement will be subject to the following;
- Individual measurements in excess of the specified thickness by more than 5mm will be considered as specified thickness plus 5mm.
- Individual measurements less than the specified thickness by more than 25mm shall not be included in the average.
- When the average thickness for the lot is deficient, the contract unit price is adjusted as follows:
Deficiency in the Average Thickness per Lot, (mm) | Percent of Contract Price per Lot, % Payment |
0-5 | 100 |
6-10 | 95 |
11-15 | 85 |
16-20 | 70 |
21-25 | 50 |
More than 25 | Remove and Replace |
No Payment |
Example:
Concrete coring was made on a section of a newly constructed pavement. A core specimen taken from the concrete pavement was measured for thickness determination. The required pavement thickness is 230mm.
Position | Sample (mm) | Corrected(mm) |
1 | 250 | 235 |
2 | 220 | 220 |
3 | 235 | 235 |
4 | 245 | 235 |
5 | 215 | 215 |
6 | 243 | 235 |
7 | 253 | 235 |
8 | 241 | 235 |
9 | 211 | 211 |
Average | 228 |
Analysis:
The deficiency in thickness taken from the computed average of the corrected thickness is only 2mm. Therefore, the section where the concrete cores represent will be paid in full.
Concrete Coring Strength Determination
Concrete coring on pavements will only be done when the strength test results of concrete beam samples are not acceptable. At least three representative cores shall be taken from each area of concrete in-place considered deficient. Obtained concrete cores will be subjected to compressive strength tests wherein the average strength test results will be computed. Concrete in an area represented by cores can only be considered adequate when the average strength of the cores is equal to 85% of, and if no single core is less than 75% of, the specified strength.
Deficiency in Strength of Concrete Specimens, Percent (%) | Percent (%) of Contract Price Allowed |
Less than 5 | 100% |
5 to less than 10 | 80% |
10 to less than 15 | 70% |
15 to less than 20 | 60% |
20 to less than 25 | 50% |
25 or more | 0 |
Example:
In a section, the required concrete strength at 28 days is 24.1 Mpa (3500 psi). Concrete coring was made and three cores were taken from the section and were tested for strength verification. The results are the following;
CORE ID | Strength, Mpa (psi) |
1 | 23.31 (3378) |
2 | 15.94 (2310) |
3 | 24.79 (3593) |
Analysis:
- The required strength is 24.1 Mpa (3500 psi)
- The strength test result for Core ID no. 2 is less than 75 percent of the required strength.
- Concrete in the section represented by the cores is considered inadequate.
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