What to do When Cube Strength is Low
In the incident of receiving low strength results for concrete cylinders, a procedure should be followed to minimize delays in construction without endangering either the safety of the structure or the interests of the involved parties. In the The National Ready Mixed Concrete Association guidelines, a 6-step procedure to investigate the structural adequacy of the concrete in question is presented.
But before taking the decision to investigate, keep in mind that as per ACI 318-77 cylinders are accepted if two of the requirements are met:
No individual strength test (average of two cylinders) can be more than 500 psi (3.45 MPa) below f’c and
The average of all sets of three consecutive tests must equal or exceed f’c.
Also, it should not be overlooked that low measurements will occur on an average of once or twice in every 100 tests simply due to normal statistical variability. If the sample does not meet the two requirements, the significance of the low strength measurement should be investigated in the following sequence.
VERIFY ACCURACY & CHECK REQUIREMENTS
Step 1 – Verify Testing Accuracy: in many low strength result cases, deviations from the sampling and testing standards are to blame. If the investigation of the testing accuracy found that the standards followed were satisfactory, then it may be necessary to continue to Step 2.
Step 2 – Compare Structural Requirements with Strength: the structural engineer should determine if the low-strength concrete affects the load-carrying capacity of the structure. If not, then the structural engineer can accept the low-strength concrete and no further investigation is needed. If it does, then further investigations are needed.
IN-PLACE & LABORATORY STRENGTH EVALUATION
Step 3 – Non-destructive Tests: the first stage of evaluation is using non-destructive testing technologies such as a rebound hammer, ultrasound pulse velocity, impact echo, and penetration probe. Generally these methods do not provide values that can directly translated into cylinder or core strengths. However, in the hands of a skilled operator, they can yield useful information on the concrete in place by comparing readings on portions of the structure represented by the low strength tests with those on other portions considered acceptable. Such readings can indicate quite convincingly whether or not the questioned concrete differs appreciably from concrete judged acceptable. If one or more of the tests show inadequate quality of concrete, then taking cores may be necessary.
Step 4 – Core Tests: taking cores from the structure in question provide a measure of in-place concrete strength. The coring, testing, and interpretation of results all require great care and sound experience. If the results of properly made core tests are low (if the average strength of three cores is less than 85% of that specified, with cores less than 75%, as per ACI 318) further action may be required as described in Step 5.
Step 5 – Load Tests: in the last testing stage, load testing is performed to check the load bearing capacity of structural members whose strength is in doubt. The load test is usually performed on flexural members such as slabs and beams, but can be used for other types. In some other cases, in-situ stress measurement may be required for structural elements in high compression zones (e.g. core wall of a high rise building).
Step 6 – Corrective Measures: if the structural member fails the load test, or when structural analysis of untestable members indicates an inadequacy, appropriate corrective measure must be taken.
Reduce the load rating to a level consistent with the concrete strength actually obtained.
Augment the construction to bring its load-carrying capacity up to original expectations. This might involve adding new structural members, or increasing the size of existing members using concrete/steel jacketing, or wrapping elements with Fiber Reinforced Polymers (FRP).
Replace the unacceptable elements.
ACI 318-11 Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute
National Ready Mixed Concrete Association’s Publication Number 133-79, “In-Place Concrete Strength Evaluation—A Recommended Practice,” developed by the NRMCA Committee on Research, Engineering and Standards.