In every BOQ there are two major things. First one is quantities and second is per unit rate of quantity. For accurate extracting of unit rate; this working is necessary and now a days computer software’s are developing in construction companies for accurate per unit cost on monthly basis and linked with market rates. Until you have not basic item quantities you can not calculate accurate rate. In the same way if any one have 500 or 1000 items they got accurate unit rate with current market basic rates.
For example generally these calculations are taken for 3000 PSI
For 100 Cft of Concrete or 2.83 M³ Quantities taken as under:
if you need 100 cft volume for a structural member with 1:2:4 mix can be made with 1:2:4 mix ratio FOR 3000 PSI
Remember that for 100 cft wet concrete you need 154 cft dry ingredients of cement sand and crush.
quantity of green concrete required = 154 cft (100*1.54)
Quantity of cement = 154*1/7 = 22 cft (17.6 Bags of 50Kg)
where (7=1+2+4 according to mix ratio required for 1:2:4 concrete)
Quantity of sand = 154*2/7= 44cft
Quantity of crush = 154*4/7= 88 cft
Accordingly you can calculate also for 4000, 5000 PSI etc with the help of design mix
As per ACI code and today’s concept, the ratios like 1:2:4 is in limbo. The actual quantities of materials shall be determined based on Trial Mix (ACI 301) and measured in weight rather than volume. The standard specification of concrete generally prepared by Design Consultants according to the section 03 30 00 (CSI-MF), the quantities of concrete materials shall be ascertained by trial mix according to the Cylinder strength (ACI 318) and the contractor shall submit the result of this trial mix for Engineer’s approval. Quality of material etc. shall follow ACI and ASTM codes.
If your problem is the budget, get quotes (at least two) from ready-mixed concrete companies in order to be on the safe side.
The unit rate depends on the total quantity, type of construction and location of the project.
Unit rates based on analysis by the book will be theoretical and rather far away from reality, because factors such as the type of quarry and the distance from the project, the percentage of waste, additives etc are possibly more important for setting the price.
Industry has solve these problems so let’s use their experience.
ready mix already have the exact proportion for a design mix… in our company we have already ready mix given by our batching plant, they have design mixes such as:
cement- 450 kg
sand- 240 kg
crushed stone 1/3- 470 kg
gravel 3/8- 220 kg
gravel 8/15- 500 kg
gravel 15/25 – 600 kg
water – 190 liters
w/c – 0.42
density – 2.67
and other design mix for 30 , 25 MPA.