i am trying to cement a garden tier in order to patio on top however i am struggling with how to know how many bags i will need to complete the job. i am working to the ratio of 1 part cement to 4 parts ballast measurements are 570 cm x 305 cm x 3 cm deep how many 25kg bags of ballast do i need and how many 25kg bags of cement do i need?? thank you all in advance for your help
That looks like quite a large area (5.7m x 3m), but very shallow, I'm not sure the concrete will bind properly being only 3cm thick. Its about 1.6 times the volume of the shed base I laid which was 7 mixer loads. You'll need a bit more than one 1 ton bags of ballast and around 6 bags of cement I reckon. A 1 ton bag is actually around 900Kg ish, so probably 40-45 bags at 25kg each is my guess. 1:4 ratio is quite strong, 1:6 would be fine - 12 shovels of ballast to 1/2 bag of cement. There are various useful calculators here http://www.pavingexpert.com/calcall.htm
This calculator is more useful http://www.diydata.com/general_building/concrete_calculators/concrete_calculator_2_m.php 5.7 * 3 * 0.03 = .513 m3 which equates to 1.139 tonne (46 x 25kg bags of all in ballast) and 8 x 25kg of cement.
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