A User’s Guide to Concrete Test Reports — Part 2

As we mentioned last month, Portland cement concrete is probably the most common material found at a construction site. The concrete test report would probably be the second most common, with the possibility of hundreds of individual reports representing thousands of test specimens.

Test Requirements

Evaluation of these project concrete results requires “valid” test data (test specimens must be 28 days old) and the test reports considered must provide the minimum information required by ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. Furthermore, according to American Concrete Institute (ACI) 318, “… a strength test is valid provided at least two individual 6 by 12 in. cylinder strengths or at least three 4 by 8 in. cylinders are averaged”. Single cylinder compressive strength test results should not be included or considered in involution of concrete used in a project.

The acceptance criteria stated in ACI 318 (section 5.6.3.3) mandates:

• The average of three consecutive strength tests equal or exceed design compressive strength f’c .
• No individual strength tests falls below f’c by more than 500 psi when f’c is 5,000 psi or less or by more than 10% of f’c when f’c is greater than 5,000 psi.

(Note: f’c = compressive strength of the specified concrete)

Benefits of the Concrete Test Report

The concrete test report can provide important information in the investigation of non-conforming concrete. A complete report should include:

• Batch, arrival and completion times;
• Slump and air content;
• Concrete temperature;
• Quantity of water additions to the concrete; and
• Weather conditions.

Part of the reason that all of this information is required (and important) is because data collected at the time of a test will sometimes explain why a test sample may be in non-compliance. For example:

• Delays in arrival or placement may alter the characteristics of the concrete and affect strength.
• High slump or air content can reduce compressive strength.
• High ambient and/or concrete temperatures reduce the concrete strength potential.
• Freezing temperature can significantly retard the strength gain of concrete.

When problems are found, it is also prudent to request that the testing lab conduct an internal review to verify that proper testing procedures were followed and if there were any deviations from normal practice in handling and curing the specimens at the project. Concurrently, the concrete supplier should verify that the correct concrete mix was delivered and determine if there were any batching issues with the load in question. If no significant issues are found in the batching and testing of the concrete, field investigation of the questionable concrete may be warranted. Any investigation should be co-ordinated with the project structural engineer regarding test methods, sampling and critical locations.

Part 3 of this series shall review evaluation methods for questionable concrete.