The proper and accurate measurement of all the material used in concrete making is necessary to ensure uniformity of proportions and aggregate grading in succeeding batches.
For volumetric batching, it is convenient to make steel or wooden boxes (farmas) of various sizes/ volumes. It is generally preferred to have boxes of various sizes readily available at site so that any adjustment which is required to be made by way of change in mix design or due to bulkage of sand can be done easily.
Also Read: What do you mean by Batching in Concrete?
- Batching of cement:
Generally one bag cement is used for each batch mix. Actual volume of a given weight of cement depends upon how it is filled into the farmas. The density of cement may vary from about 1.12 g/cm3 if it is lightly poured into the farma and 1.60 g/cm3 if tamped down sufficiently hard.
Hence as an ideal case, the volume of the farma is made equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. The farmas are made comparatively deeper with narrow surface rather than shallow with wider surface to facilitate easy estimation of top level.
It is generally recommended to have the following boxes (farmas) at site:
|Width (cm)||Height (cm)||Depth (cm)||Volume (litres.)||Qty. (Nos.)|
Measurement boxes are prepared based on volume of cement bag. Hence if you need concrete of proportion 1: 2: 4, you need to
1 Bag of cement
2 Measurement box of sand
4 measurement box of coarse aggregate & 25 litre water (if water cement ratio is 0.5)
- Batching of aggregate:
Aggregates can be measured by volume. Volume of fine aggregate (sand) in a loose condition weigh much less than the same volume of dry compacted fine aggregate (sand). For bulkage testing of fine aggregate (sand) click here.
The reduction in the ratio of sand causes a reduction in the quantity of concrete produced with each bag of cement.
- Batching of water:
Proper control of mixing water is also important for concrete making. The strength and other desirable properties of a concrete mix depend entirely upon ratio of the quantity of water used to the bag of cement.
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. Water shall be measured by volume in a calibrated tank.
The quantity of water required is a product of water/cement ratio and the weight of cement; for 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.
Volume batching is not a preferable method for proportioning the material because of the difficulty it offers to measure finer material in terms of volume. Volume batching may be allowed only where weigh batching is not practical and for unimportant concrete or for any small job, concrete may be batched by volume.
As the strength and durability of concrete is largely dependent on proportion of ingredients and water cement ratio, weigh batching is must and recommended.