Concrete Quality Control Issues

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Concrete Quality Control Issues

Even with the best mixture designs, failures occur in concrete structures, some of which are attributable to design and the others to the material itself. However, more often than not, the failure is a result of negligence and lack of attention to quality on the jobsite during construction.

Quality control (QC) is a check of the quality of the material and construction performed by the builder, while Quality Assurance (QA) is performed by an independent authorized agent hired by the owner.

Statistical QC

An effective and economical system of QC must be based on statistical methods. The most important item as far as concrete is concerned is the sampling of test specimens. Sampling should be random, and should be as representative of the entire material as possible.

Generally, a Gaussian normal distribution is assumed for the property under investigation (most QC is done for concrete compressive strength, since the conventional design is also based on strength). The distribution may be presented either using the strength variable, or a transformed variable called the standard normal variable, which is defined as:
Z = (X – µ) / σ, where X is the strength variable that follows a normal distribution, µ is the mean strength of the population, and σ is the standard deviation of the population.

Figure 1 shows the probability density function associated with the standard normal variable. The definitions of characteristic strength of concrete are based upon this function. According to the definition, 95% if the specimens should possess a strength greater than the characteristic compressive strength (fck) of concrete. From the probability density function, this corresponds to a value of –1.65 for the standard normal variable. According to IS, the target strength of the concrete mixture is defined as:
Target strength = fck + 1.65 σ, where s is the standard deviation.

The standard deviation σ can be initially based on prior experience, and later determined from trial results.

Quality control charts

Control charts are typically prepared for concrete strength (see Figure 2). According to the probability density function for a normal random variable, 99.9 % of the area is enclosed between Mean ± 3σ. Thus, warning and action limits are typically set at 2σ and 3σ, respectively.

Three types of presentations of the compressive strength (or any other QC parameter) can be used: (1) individual strength values, (2) moving average based upon the average of five previous sets of tests (each set is an average of 3 specimens), and (3) moving average for the range of strengths, where each point represents the average range of the 10 previous sets of tests (each set – 3 specimens). The occasional outliers in the individual strength values need not be significant. The moving average of strength can smooth out the data, while the moving average of the range of strengths can indicate the reproducibility of the test results.

Acceptance criteria according to Indian standards

As per the IS code (Clause 16 of IS 456:2000), for a given set of tests, the compressive strength is taken as the average of three tests, no one test differing from the average by more than 15%. The strength requirements are deemed to meet standards in the following conditions are satisfied.
Compressive strength:

Mean of 4 test results > fck + 0.825 σ, or fck + 4 MPa (whichever is greater)
Individual strength result > fck – 4 MPa
Flexural strength (ft is the characteristic flexural strength):
Mean of 4 test results > ft + 0.3 MPa
Individual strength result > ft – 0.3 MPa

Quality factors

For a good quality concrete construction, one must ensure the four Cs:

• Ensure design Cover is maintained
• Ensure sufficient Cement and proper w/c
• Ensure adequate Compaction so there is no honeycombing
• Ensure good Curing so that design strength is obtained

Cores removed from concrete sections typically show a lower strength compared to specimens cats and cured in the standard lab conditions. According to ACI, if at least 3 cores are removed from a representative part of concrete and none of them shows strength less than 75% of the characteristic strength (also, average not less than 85% of characteristic strength), then the concrete is in a sound condition.

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