Saturday, 25 February 2017

Quality control of Ready Mix Concrete plant.


Introduction

As per Indian Standard code of practice (IS 4926) Ready Mixed Concrete (RMC) is defined as the concrete delivered in plastic condition and requiring no further treatment before being placed in position in which it is to set and harden. Instead of being batched and mixed on site, concrete is delivered for placing from central batching plant. First RMC was patented in Germany in 1903, but the transportation system it had not done properly introduced to achieve proper workability of concrete. The first delivery of RMC was made in Baltimore in 1913, and the transit mixer was born in 1926. In 1931 erected a plant at Bedfont, west of London and launched a company named as Ready Mixed Concrete Ltd.

What is need of RMC plant?

The consumer wanted his concrete delivered to the job in a ready-to-place condition. Volume batching is completely replaced by the weight batching and presently computerized weigh batchers are used in most of the batching plants. Aggregates are stored in properly installed bins and cement and flyash are stored in silos. Conveyors are used to transport the aggregates. Cement and flyash is pumped into the central mixer with pneumatic pumps. Electronic moisture meters, digital admixture dispensers are used in fully automatic batching plants. So due to such practices in RMC plant the workable concrete can produce which may be more durable, because of that there is a need of RMC plant.


What do you mean by quality control?

Quality control is a process by which entities review the quality of all factors involved in production.

What is the need of quality control?

Concrete is the most universal of all the construction material and is frequently considered as the most economical one and is strong and durable material. The quality of concrete has also a very direct effect on the strength & durability of the structure as a whole. It consists of those processes, resources, materials and activities that influence your product and service quality also produce the better quality of concrete, eliminate of storage space for basic materials at site, elimination of procurement or hiring of plant and machinery and avoid the wastage of basic materials.

Process of quality control of concrete at RMC plant as follow:-

1.  As per the requirement the ready mix concrete the design create for mix (design mix) by using the standard procedure of the mix design but the all RMC plant now days having the excel spreadsheet which made with all calculations and all assumptions of mix design.
Ex. Design mix of M35, M30 grade of concrete.

2. After design mix of concrete of particular grade then it forward for the approval from the concerned authority. The establishment of standard QA/QC lab where the modification of mix design done, occurrence of change notice like out of control situation.

3. When concern authority allow then it transfer for the casting of trial cubes for verification of target strength.

4.The inspection of incoming of raw material also follow with some process like,
     1. Visual checks raw material,
     2. Acceptance of sampling,
     3. Test as per the IS test procedure and weighing of material.

The raw materials found unsatisfactory in the visual checks should be rejected and as per requirement confirmatory tests should be conducted before rejection. The supplier should be immediately informed about the rejection of his material and should be warned so that if he repeats the same, his contract would be terminated. Standard tests for raw materials, fresh concrete and hardened concrete have to be performed as per Indian Standard (IS) testing procedures.

5. Material stored in the storage bins/silos then by using the conveyors or loaders it transfer to the central mixer. Cement and flyash should be stored in cement silo and flyash silo respectively. It is desirable to transport the cement in bulkers and directly pump it into the silo through pneumatic pumps. The cement if required to be stored in godowns should be well protected from rains. The godown should be covered on all sides and should be provided with adequate size of lockable entry or exit gates. Cement should always be stored on a plain cement concrete base which is about 200 mm thick or brickwork base which is about 300 mm above the ground. The oldest lot should be used first. It is always better to cover the cement bags with polythene sheets particularly in pre-monsoon and monsoon seasons. The workmen should compulsorily wear safety goggles and helmet while working with the pneumatic pumps.

6. During the mixing of the material all material weighed  properly for the unit cum production with the designed amount of water and admixture as per the requirement mix in the central mixer.

7. Properly weighing control takes place during mixing and excess amount of water and admixture control by the all machine for proper mix.

8. RMC plant undergo the test of fresh concrete with corresponding temperature of the concrete means in summer when the outside temperature is about 40 to 45 degree Celsius the drum of the transit mixer should be covered with wet jute cloth and desirably some ice cubes should be added to the fresh concrete. The slump of the mixes to be placed by concrete pumps should be at least 180mm to 200 mm at the time of pumping. Thus use of flyash in the mix is always desirable for pumpable mixes as flyash due to ball bearing action increases the pump ability of a mix. As per the standard test of the cubes the sample place for curing for 3 days,7 days and 28 days and all test conducted to check the strength of sample.

9. Ready mix concrete again check by the EWMA control chart then check the in control situation and then continue with the same mix design.

10. With established standard QA/QC lab test casting of the cubes, slump test and temperature test made on the concrete.

11. Mix concrete load to the transit mixer and by using the transit mixer it transported to the site, and place at the site.

12. At site the cube casting for site testing also done for checking of the workability of concrete.

13. But if the maximum time form batching to placing of concrete not more than 2 hours i,e the check time, slump, temperature, uniformity & cohesiveness matter during the batching to placing process at respective site. If all these test and time not proper or delay in placing so it tends to reject to concrete.


14. The QA /QC (Quality Assurance (QA)/ Quality Control (QC)) team for the proposed model should be headed by a QA/QC Manager. He is the key person involved in the decision making process. He should be assisted by QA/QC In charge whose job is to co-ordinate and implements the QA /QC guidelines and test procedures for the entire batching plant. Minimum two QA/QC Engineers should assist the in charge. The engineers should be involved in physically conducting the tests of the incoming raw materials and the final product i,e. RMC. They should be physically present to check that all the equipment are calibrated properly. Also they should carry out the daily or weekly quality monitoring as per the proposed model. The QA/QC In charge will take decisions about the assignable causes noticed during the monitoring phase. At least four lab technicians should be employed per plant to carry out the sampling and testing.

By :- Parag kamlakar pal. ( Civil engineer, NICMAR)

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