Introduction.
A good quality concrete is essentially a homogeneous mixture of cement, coarse and fine aggregates and water which consolidates into a hard mass due to chemical action between the cement and water.
Each of the four constituents has a specific function. The coarser aggregate acts as a filler. The fine aggregate fills up the voids between the paste and the coarse aggregate. The cement in conjunction with water acts as a binder. The mobility of the mixture is aided by the cement paste, fines and nowadays, increasingly by the use of admixtures. Most of the properties of the hardened concrete depend on the care exercised at every stage of the manufacture of concrete.
A rational proportioning of the ingredients of concrete is the essence of the mix design. However, it may not guarantee of having achieved the objective of the quality concrete work. The aim of quality control is to ensure the production of concrete of uniform strength from batch to batch. This requires some rules to be followed in the various stages of concrete production and are discussed as follows.
The stages of concrete production are:
1. Batching or measurement of materials
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Curing
7. Finishing
Batching of materials.
For good quality concrete a proper and accurate quantity of all the ingredients should be used. The aggregates, cement and water should be measured with an accuracy of ± 3 per cent of batch quantity and the admixtures by 5 per cent of the batch quantity.
There are two prevalent methods of batching materials, the volume batching and the weigh batching. The factors affecting the choice of batching method are the size of job, required production rate, and required standards of batching performance.
For most important works weigh batching is recommended.
1. Volume batching.
Volume batching is generally recommended for small jobs only. The amount of each solid ingredient is measured by loose volume using standard box known as gauge box. The amount of solid granular material in a cubic meter will be an indefinite quantity.
As an example, volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. Cement is always measured by weight, irrespective of the method of batching.
The volume of a bag of cement (50 kg) is 0.035 m3. The volume of one gauge box is made equal to 0.035m3.
The gauge boxes are made comparatively deeper with narrow top surface to facilitate easy estimation of top level.
Correction to the effect of bulking of fine aggregate should be made if volume batching is adopted since the density of water is in kg/L, water is measured either in kg or litres
2. Weigh batching.
For all important works weigh batching is used. Different types of weigh batchers are used. For smaller works manual batching is done. All the operations of weighing and batching of the ingredients are done manually. The weighing may also be done by ordinary platform weighing machines. For large size works weigh bucket equipments are used. The weigh buckets are fed from hoppers and these discharge the ingredients by gravity, straight into the mixer.
Automatic batching plants ranging from small to large capacity and manually or electrically operated are available. However, they operate on same principle. By pressing a button the weighing of all the different materials is put into motion and the flow of each is cut-off when the correct weight is reached.
Some of the recent batchers record graphically the weight of each material delivered to each batch. In weigh batching water is not added by graduated buckets as the water may spill over during its addition. A horizontal or vertical tank is fitted to the mixer.
The filling is so designed to have a control to admit any desired quantity of wate
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