Quality control of ready-mixed concrete may be divided into three components,
1. Forward control,
2. Immediate control and
3. Retrospective control.
Forward control and consequent corrective action are essential aspects of quality control. Forward control includes the following.
• Control of purchased material Quality
• Control of Materials storage
• Mix design and mix design modification
• Transfer and Weighing Equipment: The producer shall be able to demonstrate that a documented calibration procedure is in place. The use of elector-mechanical weighing and metering systems, that is, load cells, flow meters etc. is preferable over purely mechanical system, that is, knife edge and lever systems.
• Plant mixers where present and truck mixers used shall be in an operational condition.
Immediate control is concerned with instant action to control the quality of the concrete being produced or that of deliveries closely following. It includes the production control and product control.
a) Production Control:
The production of concrete at each plant shall be systematically controlled.
This is to ensure that all the concrete supplied shall be in accordance with these requirements and with the specifications that has formed the basis of the agreement between the producer and purchaser.
Each load of mixed concrete shall be inspected before dispatch and prior to discharge.
The workability of the concrete shall be controlled on a continuous basis during production and any corrective action necessary taken.
For each load, written, printed or graphical records shall be made of the mass of the materials batched, the estimated slump, the total amount of water added to the load, the delivery ticket number for that load, and the time the concrete was loaded into the truck.
Regular routine inspections shall be carried out on the condition of plant and equipment including delivery vehicles.
b) Product Control:
Concrete mixes shall be randomly sampled and tested for workability and where appropriate, plastic density, temperature and air content. Where significant variations from target values are detected, corrective action shall be taken.
It is important to maintain the water cement ratio constant at its correct value. The amount of added water shall be adjusted to compensate for any observed variations in the moisture contents in the aggregates.
Suitable adjustments should also be made in masses of the aggregates due to this variation (see IS 456). Any change in water content due to change in aggregate grading shall be taken care of by forward control by suitable modifications to mix design.
Retrospective control is concerned with those factors that influence the control of production.
Retrospective control may cover any property of materials or concrete, such as aggregate grading, slump, or air content, but is particularly associated with 28-day cube strength because by its very nature it is not property which can be measured ahead of, or at the time of, manufacture.