A User’s Guide to Concrete Test Reports — Part 3

In this final article dealing with the concrete evaluation process, we are going to discuss core testing in-place concrete, the last step in the evaluation process.

Beginning the Process

First and foremost, the project design engineer should be involved in all steps associated with the development of the in-place concrete evaluation process. This will help to ensure that the final results are accurate and provide that kind of information that will be most helpful to the project.

When the low strength is confirmed and it has been determined by calculation that the load-capacity of the element or area in question could be compromised, the in-place concrete should be sampled and tested per ASTM C42. The American Concrete Institute (ACI) document 318 provides the guidance needed to devise the evaluation process.

The use of nondestructive testing methods (i.e.: Impact Hammer, Windsor Probe) to detect zones of relative weakness – and cover meter or GPR to detect reinforcing steel in the concrete – can be helpful in minimizing the area of investigation. In addition, it is sometimes wise to secure additional samples for potential petrographic investigation. These specimens would be useful in determining the cause of the low strength.

Sample Evaluation

ASTM C42 mandates that a minimum of three core specimens be secured for each area in question. Whenever possible, the specimens should be at least three or more inches in diameter and have a length of two times the specimen diameter (capped). When conditions do not allow for this requirement, the standard provides for correction factors for shorter cores.

In order for a core to be deemed in compliance with applicable standards, the tested cores must have the following conditions:

• The average compressive strength of 3 cores equals at least 85% of the design compressive strength of the concrete.
• No single core in the set shall be less than 75% of the design strength.

Should the compressive strength of the cores not achieve the required minimum values, the design engineer shall determine if the concrete in question should be removed, or if other corrective action should be taken.

Conclusion

A good quality assurance/quality control plan cannot guarantee that there will be no problems with the concrete in a project. However, an effective QA/QC program should provide continuous information monitoring for changes in the concrete performance that could lead to a failure.

Feel free to contact your local F&R office for additional information about the concrete testing process and to discuss how our professionals can be of service to you.