It’s a fresh new week and since it’s Monday, Let’s talk about serious technical stuff.

In this article, I will give you the secret on how to estimate the number of bags of cement that would be consumed in the strip foundation for a simple project of a room, using the plan shown above.

In building projects, most owners don’t have an idea on how their materials are being utilized on site. This eventually gives the building Engineer an edge to make money off materials that will be used during the project.
One of the common materials the building Engineer easily makes money from is cements.

Strip foundations consist of a continuous strip, usually of concrete formed centrally under load bearing walls. This continuous strip serves as a level base on which the wall is built and is of such a width which is necessary to spread the load on the foundations, to an area of subsoil capable of supporting the load without undue compaction.

Fig. 1.2. A Diagram showing the section of a strip foundation

After digesting this, it will give you an edge, to checkmate your building Engineer on the request for bags of cements for your building project.

Firstly, there are some important things you need to know:

mm = millimeters
m= meters
‘’= inches
‘ = feet
ft = feet
 = approximately
m3= cubic meter
In this project we are using 6inches blocks.

• The length of a block (6inches (6’’) or 9inches(9’’) ) is 450mm = 0.450m = 18’’ (18inches)
• The width of 6’’ blocks is150mm=0.150m
• The volume of a bag of cement is 0.035m3 but to cater for shortages we are going to use 0.03m3.
• The width of the foundation trench will be 150mm x 3 = 450mm = 0.450m
• From the building plan above The total length of the foundation trench is :
16’0’’+13’0’’+16’0’’+13’0’’ which is 16ft+13ft+16ft+13ft = 58ft =17.6784m17.68m i.e the length of the foundation trench is 17.68m
• We are going to make the thickness of the foundation blinding/strip 6inches(for a good soil) i.e 150mm=0.15m.

NOTE:The thickness of a foundation trench depends on the nature of the soil. But for the purpose of this work I chose 6’’= 0.15m.
• THEREFORE THE VOLUME OF CONCRETE for the total length of foundation trench can be calculated as :
0.450m (width of foundation trench) x 17.68m (length of foundation trench) x0.15m (thickness of foundation trench) = 1.1934m3.

Moreover, There are two major mix ratios for concrete 1:2:4 or 1:3:6, there are others, but we are going to base our calculations on 1:2:4. It means one portion of cement is mixed with an equivalent volume or ratio of two of same portion of volume of cement in sharp sand and four of same portion of volume of cement in granite. That is, if I have a cement of 1m3, then I will need to mix it with 2m3 of sharp sand and 4m3 of granite stones. However one bag of cement is calculated to be approximately two head pans of sand or granite. Therefore for one bag of cement in ratio 1:2:4, you will add four (4)head pans of sharp sand, and eight ( 8)head pans of granite, this is the standard.

NOTE experience has shown that materials properties vary; hence there are always little variations in quantities used on site.

Now, for the plan above, the numbers of bags of cement needed in the strip foundation is simply calculated by dividing the total volume of cement in concrete by a volume of one bag which we took as 0.03.

• To get the total volume of cement in the mix ratio 1:2:4, we will add the numbers in the mix ratio together i.e 1 + 2 + 4 = 7.

• To Get The Volume of Cement in Concrete : To get the volume of cement in concrete we are going to divide the volume of concrete (1.1934m3) by 7

i.e 1.1934m3 = 0.1705m3.
7

Remember that the volume of one bag of cement is 0.03m3.

Therefore to get the numbers of bags of cement we will divide 0.1705m3 by 0.03m3.

i.e. 0.1705m3 = 5.683 = 6 bags
0.03
We are going to make provision for 5% waste; 5% of 6bags is 0.3bags
So 0.3bags + 6bags will give 6.3bags  7 bags
So the number of bags of cement that will be required in the strip foundation is 7 bags.