Avoid the False Alarm for Low-Strength Concrete

by Kim Basham, PhD PE FACI
Created: January 16, 2013

Standard concrete cylinders are test cylinders made and cured in accordance with ASTM C31 and tested in accordance with ASTM C39.

Kim Basham

Make cylinders in strict accordance with ASTM C31 and store cylinders in a concrete cure box for the initial curing period.
Make cylinders in strict accordance with ASTM C31 and store cylinders in a concrete cure box for the initial curing period.
Kim Basham

Concrete cure box
Concrete cure box
Kim Basham

After initial curing, start final curing by storing cylinders in a water storage tank or moist room.
After initial curing, start final curing by storing cylinders in a water storage tank or moist room.
Kim Basham

Concrete test cylinder moist room.
Concrete test cylinder moist room.
Kim Basham

Compressive strengths are determined by loading cylinders in a testing machine until concrete failure occurs. Strengths are computed by dividing the maximum compressive load obtained during the test...
Compressive strengths are determined by loading cylinders in a testing machine until concrete failure occurs. Strengths are computed by dividing the maximum compressive load obtained during the test by the average cross-sectional area of the cylinder. Example for a 6 by 12 inch cylinder: 116,190 lbs ÷ 28.27 sqin = 4,110 psi.
Table 1
Table 1
Table 2
Table 2

Low-strength example for 4,000 psi concrete

In Table 2 below, strength 3,250 psi for test 5 (column #3) falls below 4,000 psi by more than 500 psi; therefore, it fails to satisfy Requirement #1. Also, the average of three consecutive strengths for test #3 (column #4) falls below 4,000 psi. Therefore, test 3 fails to satisfy Requirement #2. Subsequently, strengths for tests 3 and 5 are low and not acceptable.

Unacceptable strengths

When strength tests fail to satisfy Requirement #1, the building code requires steps to be taken to assure the load-carrying capacity of the structure has not been jeopardized. In addition to assuring safety of the structure, the building code also suggests the engineer and/or building official should use judgment determining the significance of low-strength tests and to decide whether the low-strength tests indicate a need for concern.

Because the building code does not specify the steps for investigating low-strength test results, consider these:

Review the concrete testing procedures to confirm or deny the possibility of low-strength tests. This review should consider the procedures for sampling fresh concrete, making cylinders, initial and final curing, and transporting and testing cylinders. Essentially, determine if low-strength tests were most likely caused by improper testing or inadequate concrete.
If the likelihood of low-strength concrete is confirmed, the engineer should determine if the low-strength concrete affects the load-carrying capacity of the structure. If the engineer determines the load-carrying capacity of the structure has not been jeopardized, then the engineer can accept the low-strength concrete and no further investigation is needed.
If the likelihood of low-strength concrete is confirmed and calculations indicate that load-carrying capacity is significantly reduced, tests of cores drilled from the area in question should be considered to determine the in-place concrete strength. Then, the engineer can determine the appropriate actions to be taken to ensure the load-carrying capacity of the structure is adequate.

When strength tests fail to satisfy Requirement #1 and/or Requirement #2, the building code requires steps to be taken to increase the average of subsequent strength tests. Steps could include one or more of the following:

Check mix design and batching procedures and correct errors
Adjust mix proportions (e.g., increase cementitious materials)
Closer control of water content (e.g., check and monitor moisture content of aggregates)
Closer control of air content (as air content increases, strength decreases)
Improve quality of concrete testing including strict compliance with ASTM standard testing procedures

For future projects, evaluate concrete cylinder strengths properly and avoid the low-strength false alarm. If strength tests do fail to meet the strength acceptance requirements when evaluated properly, perform a low-strength investigation to determine the appropriate actions to ensure the safety of the structure. In many cases, expensive options and construction delays associated with drilling cores, tearing out and replacing concrete may not be necessary.