The measurement of materials for making concrete is known as batching. It is the process of measurement of specified quantities of cement, aggregates, water and admixture, i.e., ingredients of concrete in correct proportion. There are two methods of batching.
1) Volume Batching
Volume batching is not a good method for proportioning the material because of the difficulty it offers to measure granular material in terms of volume. Volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. The amount of solid granular material in a cubic metre is an indefinite quantity. Because of this, for quality concrete material have to be measured by weight only. However, for unimportant concrete or for any small job, concrete may be batched by volume.
Cement is always measured by weight. It is never measured in volume. Generally, for each batch mix, one bag of cement is used. The volume of one bag of cement is taken as thirty-five (35) litres. Gauge boxes are used for measuring the fine and coarse aggregates. The typical sketch of a gauge box is shown in Fig.1. The volume of the box is made equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. The gauge boxes are made comparatively deeper with narrow surface rather than shallow with wider surface to facilitate easy estimation of top level. Sometimes bottomless gauge-boxes are used. This should be avoided. Correction to the effect of bulking should be made to cater for bulking of fine aggregate, when the fine aggregate is moist and volume batching is adopted.
Gauge boxes are generally called ‘farmas’. They can be made of timber or steel plates. In a major site it is recommended to have the following gauge boxes at site to cater for change in mix design or bulking of sand. The volume of each gauge box is clearly marked with paint on the external surface.
Water is measured either in kg or litres as may be convenient. In this case, the two units are same, as the density of water is one kg per litre. The quantity of water required is a product of water/cement ratio and the weight of cement; consider an example, if the water/cement ratio of 0.5 is specified, the quantity of mixing water required per bag of cement is 0.5 x 50.00 = 25 kg or 25 litres. The quantity is inclusive of any surface moisture present in the aggregate.
2) Weigh Batching
Weigh batching is the correct method of measuring the materials. For important concrete, weigh batching system should be adopted. Use of weight system in batching, facilitates accuracy, flexibility and simplicity. Different types of weigh batchers are available and the particular type to be used depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment. The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers. In this, further complication will come to adjust water content to cater for the moisture content in the aggregate. In smaller works, the weighing arrangement consists of two weighing buckets, each connected through a system of levers to spring-loaded dials which indicate the load. The weighing buckets are mounted on a central spindle about which they rotate. Thus one can be loaded while the other is being discharged into the mixer skip. A simple spring balance or the common platform weighing machines also can be used for small jobs.
On large work sites, the weigh bucket type of weighing equipment is used. This fed from a large overhead storage hopper and it discharges by gravity, straight into the mixer. The weighing is done through a lever-arm system and two interlinked beams and jockey weights. The required quantity of coarse aggregate is weighed, having only the lower beam in operation. After balancing, by turning the smaller lever, to the left of the beam, the two beams are interlinked and the fine aggregate is added until they both balance. The final balance is indicated by the pointer on the scale to the right of the beams. Discharge is through the swivel gate at the bottom.
Automatic batching plants are available in small or large capacity. In this, the operator has only to press one or two buttons to put into motion the weighing of all the different materials, the flow of each being cut off when the correct weight is reached. In their most advanced forms, automatic plants are electrically operated on a punched card system. This type of plant is particularly only suitable for the production of ready-mixed concrete in which very frequent changes in mix proportion have to be made to meet the varying requirements of different customers. In some of the recent automatic weigh batching equipment, recorders are fitted which record graphically the weight of each material, delivered to each batch. They are meant to record and check the actual and designed proportions.
Aggregate weighing machines require regular attention if they are to maintain their accuracy. Check calibrations should always be made by adding weights in the hopper equal to the full weight of the aggregate in the batch. The error found is adjusted from time to time. In small jobs, cement is often not weighed; it is added in bags assuming the weight of the bag as 50 kg. In reality, though the cement bag is made of 50 kg at the factory, due to transportation, handling at a number of places, it loses some cement, particularly, when jute bags are used. In fact, the weight of a cement bag at the site is considerably less. Sometimes, the loss of weight becomes more than 5 kg. This is one of the sources of error in volume batching and also in weigh batching, when the cement is not actually weighed. But in important major concreting jobs, cement is also actually weighed and the exact proportion as designed is maintained.
Components of a Batching Plant
- Aggregate bins for various types of aggregates
- Feeding mechanisms such as scrappers, conveyors or hoists etc. to transfer aggregate to scales (balances)
- Balance and measuring system
- Cement silos and a conveyor screw or bucket conveyor
- The storage tank for water and water measuring system
- Dispenser for chemical (liquid) admixture
Measurement of Water
When weigh batching is adopted, the measurement of water must be done accurately. Addition of water by graduated bucket in terms of litres will not be accurate enough for the reason of spillage of water etc. It is usual to have the water measured in a horizontal tank or vertical tank fitted to the mixer. These tanks are filled up after every batch. The filling is so designed to have a control to admit any desired quantity of water. Sometimes, water meters are fitted in the main water supply to the mixer from which the exact quantity of water can be let into the mixer.
In modern batching plants sophisticated automatic microprocessor controlled weigh batching arrangements which not only accurately measures the constituent materials, but also the moisture content of aggregates. Moisture content is automatically measured by sensor probes and corrective action is taken to deduct that much quantity of water contained in sand from the total quantity of water.
(Ref : Concrete Technology – M S Shetty)
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