12.3 Non-Destructive Testing, Core Sampling & Structural Concrete Repairs (ACI 318 & ACI 562)

12.3 Non-Destructive Testing, Core Sampling & Structural Concrete Repairs

Low test results and visible defects call for different evidence. Locate the affected placement and verify its records. The licensed design professional and, where applicable, the building official determine the structural investigation and disposition. The inspector documents the test or condition and verifies the approved corrective work.

1. Apply the right strength criteria

ACI 318-19 Section 26.12.3.1(a) sets two criteria for the strength level of a mixture based on standard-cured specimens. Every average of three consecutive strength tests must reach the specified compressive strength $f'_c$. An individual strength test can be below $f'_c$ by no more than 500 psi if $f'_c$ is 5,000 psi or less, or by no more than 10% of $f'_c$ when it exceeds 5,000 psi. Compare strength tests at the specified test age. A strength test combines the specimens for one sample. It differs from a moving average of three separate tests.

If either requirement fails, Section 26.12.3.1(b) requires steps to increase subsequent strength test results. The individual low-result trigger also invokes Section 26.12.6 under Section 26.12.3.1(c). Section 26.12.6.1(a) addresses that case and cases where field-cured cylinders indicate deficiencies in curing or protection. When the likelihood of low strength is confirmed and calculations indicate significantly reduced structural adequacy, Section 26.12.6.1(b) permits ASTM C42 cores from the affected area, three for each standard strength test below its applicable lower limit.

2. Verify core geometry and conditioning

Use the authorized sampling plan and locate reinforcement or tendons before drilling. Verify the sampled member, core ID and condition. ASTM C42/C42M-20 Section 7.1 uses a minimum diameter of 3.70 inches or twice the nominal maximum coarse aggregate size, whichever is greater, with specified exceptions for access or geometry and reporting of a smaller diameter. The preferred prepared length is 1.9 to 2.1 diameters for comparison to cylinder strength. Section 7.2.1 excludes prepared specimens shorter than their diameter and addresses the minimum length before capping. Section 7.9.1 gives the correction factors at length-to-diameter ratios of 1.75 or less, including 0.98 at 1.75 and 0.96 at 1.50. A measured 4,000 psi at a ratio of 1.50 gives $4000\times0.96=3840$ psi after correction.

Under ACI 318-19 Section 26.12.6.1(d), condition cores in watertight bags or containers in accordance with the applicable ASTM C42 procedure. The code default is testing at least five days after last wetting and no later than seven days after coring, unless otherwise approved by the licensed design professional or building official. Record drilling, last wetting and testing dates.

Worked example: Check the two core limits separately

For $f'_c=4{,}000$ psi, Section 26.12.6.1(e) requires a three-core mean of at least 3,400 psi and each core to reach at least 3,000 psi. Suppose the corrected results are 3,700, 3,600 and 2,900 psi. Their mean is 3,400 psi, but one core is 100 psi below the individual limit. The set does not establish structural adequacy under this criterion. Report both comparisons and obtain the disposition. Additional cores may be permitted for erratic results. Where the criteria are unmet and adequacy remains in doubt, the responsible authority can order an evaluation under Chapter 27.

3. Use investigative methods for their intended measurements

4. Verify the approved repair

For new construction, use the applicable design disposition and construction specification. ACI 301-20 Section 5.3.7.3, for example, gives a default method for surface defects other than tie holes when applicable. It includes a 1/2-inch-deep perpendicular perimeter saw cut, removal to sound concrete, preparation and curing. Tie-hole patching is addressed separately. A structural repair may have different requirements. ACI 562 addresses assessment and repair of existing concrete structures where applicable under the governing code. ACI 546 provides repair guidance.

Record the approved limits and confirm removal depth, reinforcement condition, substrate preparation and repair material against the repair detail. Check that the product’s surface moisture condition, application and curing requirements are followed. If removal exposes a larger void or damaged steel beyond the approved limits, document it and obtain the revised disposition before concealing it. A repair’s geometry and method follow its design and specification.