How to determine volumes of sand and stone when relaying a 4" concrete floor over an existing damaged concrete floor outside a church building

by Kwasi
(Kumasi, Ghana)

Q: Sir, I have been tasked to spearhead the laying of a 4" concrete slab over an existing damaged outside floor of my local church. our first task is to put together a budget - to get a good estimate of the cost of the project.

We propose to lay the new floor in 3 meter square blocks. If we could calculate the quantity of materials for each block we will be able to get an good estimate of the cost. The portland cement is sold in 50 kg bags. Our problem is the quantity of sand and agregate. We plan to use the following ratio 1:2:3 We do not have concrete sellers here and will have to do the mixing ourselves.

We are finding it difficult as to how to translate the weight of sand and aggregate given by the calculator into volumes as we purchase the sand and aggregate in volumes and we cannot weigh the required quantity as we do not have a scale to do that. Please help us. We are in Ghana.

A: To make figuring easy, I have calculated each quantity, cement, sand, and stone per 1 square meter for a 1:2:3 mixing ratio like you mentioned.

To make 1 square meter of concrete at 4" thick you will need: 31kg of cement, 63.5kg of sand, and 93kg of stone (aggregate).

Without a scale to measure weight, the easiest way to calculate volumes of sand and stone for your purpose is to make a wooden box that will hold exactly 31kg of cement (or a little over half the bag).

Then you can use this box to determine the volumes of sand and stone per cubic meter of concrete. 2 boxes filled for sand, 3 boxes filled for stone per cubic meter of concrete. It might not be perfect but it will get the job done.

Once you determine the volumes you need, you can double or triple the size of the boxes to do it on a larger scale.