The quality control is a corporate, dynamic program to assure that all the aspects of materials, equipment, and workmanship are well looked after. The quality control should have conformity to the specification, no more, no less.
For the manufacturer of the concrete, the quality control process will involve material, personnel, equipment and workmanship in all stages of concreting.
The concrete is generally produced in batches at the site with the locally available materials at variable characteristics. It is therefore likely to vary from one batch to the other. The magnitude of this variation is depending upon several factors like:
- 1. The variation of quality of constituent materials
- 2. Variation in the mix proportion
- 3. Variation in the quality of batching and mixing
- 4. The quality of overall workmanship and supervision at the site.
Moreover, concrete undergoes a number of operation like transportation, placing, compacting and curing. During these operations, considerable variations occur partly due to the quality of the plant available and partly due to the differences in the efficiency of the techniques used.
Hence, there is no attributes to define the quality of concrete. Under such situations, the concrete is treated as good, poor or fair. Hence it is necessary to judge the concrete based on the performance characteristics, economics, safety, aesthetics, durability and other factors.
The main aim of quality control of concrete is to reduce the above variations and produce uniform material providing the characteristics desirable for the goals.
The following are the three main factors that affect the quality of concrete.
- 1. Personal Factors
- 2. Material, Equipment, and Workmanship
- 3. Field Control Factors
The success of the quality control plan is the availability of experienced, knowledgeable and trained workers at all the levels. The designer and the specification writer should have the knowledge of the construction operation as well.
For the uniform quality of concrete, the ingredient (particularly the cement) should preferably be used from a single source. When ingredients from different sources are used, the strength and other characteristics of the materials are likely to change and therefore these materials should be u sed only after the proper evaluation and testing.
- Cement should be tested initially once from each source of supply and subsequently once for every month. Adequate storage under cover is necessary for protection from moisture set cement with hard lumps is to be rejected.
- Aggregates should be well graded, free from deleterious substances. Grading, maximum size, shape and moisture content of aggregate are the major sources of variability.
- Equipment used for batching, mixing and vibration should be the right capacity. Weight batchers should be frequently checked for their accuracy weigh – batching of materials is always preferred to volume batching.
- The vibrators should have the required frequency and amplitude of the vibration.
- The fresh concrete should be handled, transported and placed in such a manner that it does not get segregated. The time interval between the mixing and placing the concrete should be reduced to the minimum as possible. The expected targets of strength, impermeability and durability of concrete can be achieved only by thorough and adequate compaction.
- 1% of air content left in concrete due to incomplete compaction can lower the compressive strength by nearly 5%. Adequate curing is essential for handling and development of strength of the concrete.
- Long-term compressive strength of concrete moist cured for 3 days or 7 days will be about 60% or 80% respectively, given the concrete is cured for 28 days or more.
The field control i.e. inspection and testing, is an important factor to be considered for quality control. Concrete should be tested at its fresh and hardened stages to assess their strength. Accelerated strength tests by which a reliable idea about the potential 28 day strength can be obtained within few hours are effective control tools.
- Quality Control means a rational use of the available resources after testing their characteristics and reduction in the material cost.
- · In the absence of quality control at the site, the designer is tempted to overdesign, so as to minimise the risks. This adds to the overall cost.
- · Quality Control reduces the maintenance cost
- · In the absence of quality control, there is no guarantee that overspending in one area will compensate for the weakness in another
- · Checks at every stage of the production of concrete and the rectification of the faults at the right time expedites completion and reduces delay.