Q. We are involved in a quality claim in which the testing lab reports the hardened concrete did not achieve the specified design strength. Our testing lab broke cylinders from that same job and all achieved proper strength. We are confident that we will achieve proper strength if the coring is properly conducted.
Are there any guidelines that we can provide the testing lab to ensure that the sampling is properly conducted?
A Strength testing indicating low results requires that steps be taken to assure that the load-carrying capacity of a structure is not reduced. If the likelihood of low-strength concrete is confirmed and calculations indicate that load-carrying capacity is significantly reduced, core tests may be required. Core tests can be problematic and incorrectly indicate low core strength if they are not performed, stored, and tested accurately.
The American Society of Concrete Contractors recently published a document that spells out the current best practices for this important sampling. Written from the contractor's perspective, the “Technical Checklist for Concrete Core Testing” can help ensure that drilled cores are obtained and tested properly.
The four-page brochure provides a series of checklists that will help concrete contractors and producers confirm that core strengths represent the true condition of the in-place concrete. Items in the checklist are based on ASTM C 42-04, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” The list covers sampling, specimens, measurement, testing, and reporting.