Batching is the process of weighing or volumetrically measuring and introducing into the mixer the ingredients for a batch of concrete. One batch of concrete means the quantity of concrete produced at a time. Weight-batching is recommended as it provides greater accuracy and simplicity and avoids the problem created by bulking of damp sand. However, volume-batching, is used for concrete mixed in a continuous mixer and for certain places where weighing facilities are not available.
While estimating concrete quantities for given specifications, estimator should more concern about the concrete batching as it has big cost impact. Almost all the time BOQ specifications for concrete indicate the batch by volume so as to most practical mixing concrete based on volume. The reason for workers to prefer volume batching in general construction is that volume batching which involves measuring the volume of the ingredients (cement, coarse and fine aggregates, etc.) is easier, simpler and faster, whereas weigh batching involves tedious work- transporting the mixtures to weighing machines, weighing; if it exceeds the desired amount, it has to be unloaded etc. And moreover laborers at work sites are not technical and hence they tend to ignore complicated processes.
But in estimator point of view it is better to consider the concrete batch by weight as it guides to build up accurate and competitive rate for concrete. This can be proved through understanding the difference between volume and weight.
To answer the question – why batch by weight, let us find out how much 25 m3 of sand will weigh? Let us weigh 25 m3 of sand in moist and loose condition. It will probably weigh around X kgs. Again, weigh 25 m3 of the same sand in dry and compacted conditions it will very likely weigh 2X kgs. This is twice as much as the first result. Thus 25 m3has an indefinite quantity. It may weigh X kg (if loose and wet) or 2X kg if dry and compact.
Therefore this needs a small correction for the amount of moisture present. That is the reason for ICTAD specified “while measuring aggregates from 400x350x250mm box, shaking, ramming or heaping shall not be done and proportioning of sand on dry volume, if damp sand should allow for bulkage as given in Appendix 4C”.
By considering the above example we can clearly see that volume batching is trying to misguide the estimator during build up the concrete rate according to
-quality of water
-size of aggregates and its condition (wet/dry)
-water cement ratio,
Volume of the batch is varying and based on that estimated cost may high or low. During estimating estimator has to identify all those facts individually otherwise estimate will unrealistic. This is risk for estimator as he cannot predict material conditions during estimating.
Therefore weight batching helps estimator to build up more realistic concrete rate as it is standard proportioning. Weight of materials are directly related to the space (volume) occupied by them. Hence weigh batching is both accurate and logical to use.
In weigh batching the mix proportions are usually based on 50 kg bag of cement. A mix may be specified as: 50 kg cement: 128 kg sand: 189 kg broken stone: 22.5 liters of water (0.45 water cement ratio) The execution of this in the field is quite accurate because the cement is bagged by weight in the factory. Also it is easy to build up cost for the concrete as no need to concern above variables.
Thus if estimator has idea about concrete grades, volumes and required material weights for those volumes, even though the specifications given from volume estimator can simply convert it to weight and can build up rate accurately.
Quantity surveyors always concern about time, cost and quality and the ultimate target of the estimating/pricing is to build up competitive bid. Weight batching of concrete fulfills all 3 factors as quality assurance during pricing and construction, combination with making economical concrete mix in rational time period.