Acceptance of Concrete Test Results
ACI-5.6 (2008) recommends that concrete tests be conducted by highly skilled field-testing technicians on concrete in fresh, hardened, or old states. The technicians are responsible for collecting and preparing test specimens according to the provided specifications. The acceptance of concrete cube test, cylinder test, and core test results is determined based on several factors.
1. Acceptance Based on Frequency of Testing the specimens
The frequency of testing specimens plays a crucial role in determining the validity of test results. In the case of concrete, the strength test involves using a minimum of two cylinders measuring 150 x 300mm or three measuring 100 x 200mm, which are made from the same concrete. The test is conducted on these samples after 28 days or on the date specified by standard codes to determine the compressive strength of the concrete, denoted as fc’.
To ensure accurate and representative results, the frequency of selecting samples for testing must meet specific criteria. This includes conducting the test at least once a day, once for every 110 cubic meters of concrete used, or once for every 460 square meters of surface area for slabs or walls. If the average thickness of the wall or slab is less than 240mm, then this criterion necessitates more frequent sampling than once for every 110 cubic meters of concrete used.
Another criterion for selecting samples for testing is to randomly select at least five samples for a given class of concrete. This helps to ensure that the strength of the concrete is consistent throughout the project.
However, according to the American Concrete Institute (ACI), strength tests may not be required for a given class of concrete if the total quantity used is less than 38 cubic meters. In such cases, evidence of satisfactory strength must be submitted to and approved by the building official.
2. Acceptance Based on Standard Cured Specimens
When determining the strength of newly prepared concrete using a standard-cured specimen, two factors are taken into consideration to determine the acceptance of the test results. The first factor is the arithmetic average of any three consecutive strength tests. If the average of these tests is equal to or greater than fc’, the test result is accepted.
The second factor considers the deviation of the strength tests from fc’. When fc’ is 35 MPa or less, no strength test can fall below fc’ by more than 3.5 MPa. If fc’ is greater than 35 MPa, no strength test can fall below fc’ by more than 0.10 fc’. This means that the test results will only be accepted if the deviation from fc’ is within the specified limits.
These two factors are critical in determining the strength of newly prepared concrete using a standard-cured specimen. By ensuring that the average of the strength tests and the deviation from fc’ fall within the specified limits, accurate and reliable test results can be obtained. This information is crucial in determining the quality of the concrete and its suitability for various applications.
3. Acceptance Based on Field Cured Specimens
The field testing samples, also known as test cylinders, must be prepared using the same material and time frame as the laboratory testing samples. Although the test results of both samples will not be used as an acceptance criterion, they will provide us with valuable insights into the curing procedures performed in the field.
If the strength designated for determining the fc’ (compressive strength) for the field cured cylinder is less than 85% of the laboratory cured cylinder’s strength, then it is essential to focus on protecting and curing the concrete to enhance its strength in the field. On the other hand, if the field cured concrete strength exceeds the laboratory tested sample by 3.5MPa, we need not consider the 85% limitation.
In conclusion, it is crucial to ensure that the test cylinders used for field testing are prepared with the same sample material and time frame as those used for laboratory testing. By doing so, we can gain insights into the effectiveness of the curing procedures conducted in the field. Additionally, if the strength of the field cured concrete is lower than 85% of the laboratory tested sample, steps must be taken to protect and cure the concrete to enhance its strength. Conversely, if the field cured concrete strength exceeds the laboratory tested sample by 3.5MPa, the 85% limitation does not need to be considered.
Low Strength Test Results
In situations where the test results for concrete cylinders do not meet the acceptance conditions, certain criteria must be followed to avoid under construction. If the tested cylinders are found to be deficient in strength or curing, appropriate steps must be taken. In cases where the strength of a particular concrete is found to be low, three core samples must be taken from the structure for an under strength test. It is important to note that a licensed design professional must provide proper guidance throughout this process.
The core samples must be tested within 48 hours, but no later than 7 days, and the strength of concrete is deemed acceptable if the average of the three core strengths is equal to a minimum of 85% of fc’. Additionally, the strength of a single core must not be less than 75% of fc’. Deviating from these test criteria will require the extraction of new cores and the testing of additional ones. It is important to note that the cores taken for testing will be aged beyond the testing age recommended for fc’.
If the core test fails to provide a satisfactory result, it is recommended to conduct a structural strength evaluation or a load test. This is necessary to ensure that the construction meets the required safety standards and is able to support the intended load. Therefore, it is important to follow the appropriate testing procedures and seek professional guidance when necessary to ensure that the construction meets the necessary safety requirements.