STRENGTH TEST RESULTS- HOW TO INTERPRET THEM?
I am sampling all those concrete specimens, as per requirements. Curing them, as per requirements. Crushing them, as per requirements. How do I analyze those results? AS PER REQUIREMENTS.
Specified Strength versus Strength Results
The best answer as to how to accept concrete cylinder strength results is by comparing them to the specified strength. The keyword here is “specified” meaning as written in the tender documents or project specifications. The acceptance criteria shall be provided in the contract documents by listing them in the project specifications, general notes or otherwise showing them on the drawings.
What to specify?
The acceptance criteria are normally referred to some standard documents or specifications such as ACI 301 “Specifications for Structural Concrete”, or ACI 318 “Building Code”, or ASTM C94 “Standard Specification for Ready-Mixed Concrete” or EN 206.
But because such standards and codes are updated frequently, it is critical that the contract documents indicate exactly which version of these documents shall be applicable.
Rules of the game
Once the acceptance criteria are clear, further interpretation to referenced documents will tell the details of precisely how the concrete is to be sampled and tested.
These rules like in ACI 301 and ASTM C94 also include a lot of other important information. For example, both have clear requirements that test results are to be reported to the owner, architect/engineer, contractor, and concrete supplier.
ACI 301 also makes it clear that “when it appears that material furnished or work performed by contractor fails to conform to contract documents, the testing agency will immediately report such deficiency to architect/engineer, owner, contractor, and concrete supplier.”
ACI 301 goes on to state that “the owner’s testing agency and its representatives are not authorized to revoke, alter, relax, enlarge, or release requirements of contract documents, nor to accept or reject any portion of work.” When encountering apparently unacceptable work or materials, therefore, the testing agency is to inform all the principal parties so that those parties can make decisions about how to proceed.
What does it mean? The testing agency shall only report tests and cannot replace the quality control Engineer or act as Quality Assurance.
we set? … It is not only fail or pass what matters?
ACI 318 and ACI 301 provide control requirement of a mixture that is over-performing through required strength f’cr. The same codes require immediate modifications to a mixture that is under-performing. When we say under-performing, it does not mean to be a failure in strength only. Thus, timely distribution of test results is needed to rectify the quality of the produced concrete.
Tracking test results
Not only that, tracking results and quality charts relating strength versus age, concrete slump or air temperature over the duration of your project can be used as preventive action. Actually the strength versus the sequential order of samples is required by ACI 318, ACI 301, and ASTM C94 in the fundamental criteria for acceptance of concrete on the basis of compressive strength as follow:
The strength of concrete is considered satisfactory provided that:
• Average of three consecutive strength tests equals or exceeds the specified compressive strength f´c
• No strength test result falls below f´c by more than 3.5MPa when f´c is 35MPa or less, or by more than 0.10 f´c when f´c is more than 35MPa.
Noting that a single strength-test result is not the result of breaking a single cylinder, rather it is defined by averaging the strengths of preferable three but not less than two 150x300 mm cylinders or at least three 100x200mm cylinders.
Let’s consider a concrete mixture for which there are test results for forty 150x300 mm cylinders, placed at different intervals of time and tested at an age of 28 days. The specified 28-day strength is 30MPa. The field testing technician carefully fabricated a pair of cylinders at each desired age, so that for each batch (truck-load) of concrete that was sampled, two cylinders were subsequently tested in the lab at 28 days.
How to evaluate?
The average of those two cylinder strengths is reported as the single 28-day compressive strength of that batch after checking the outlier or discrepancy. The strength of that batch and of subsequent batches would be considered acceptable if each two-cylinder average from the batch is not lower than the specified strength by more than the specified strength minus 3.5MPa (in this example 30 – 3.5 = 26.5MPa).
On the other hand, the average of the strength values for the first, second, and third batches should equal or exceeds 30MPa. For the concrete mixture to remain in compliance, the same requirement applies to the average of the 2nd, 3rd and 4th; 3rd, 4th and 5th etc.
Taking the above graph, even though we have two points that are below f’c =30.0 however they are meeting the requirements above whereby:
• The individual results are above 26.5 MPa
• The running average is above 30.0 MPa
Of course tracking your results requires that you “do the math”. By tracking your records, you are able to know exactly how you are doing in terms of compliance and if you are trending higher than needed or alarmingly too close to specified strength line.
If you don’t manage to properly sample your data and track it properly you will end up with concrete without compliance criteria. Not only that, you will not know what went wrong to correct the situation and it will be used against you.