How many yards of concrete do I need? This is one of the most asked questions in everyone’s mind before any general construction. Especially with the do-it-your self-projects, calculating your concrete yardage is pretty much the first important step.
Yes, figuring out the volume is pretty straightforward as we did it in school. Even for complex structures, we can separate them into pieces and calculate the concrete volume one by one. Let’s try one!
Calculate the concrete volume
Assume a column and a footing with the following dimensions
Length = 6 ft.
Width = 6 ft.
Thickness = 1 ft.
Height above the footing = 6 ft.
Column cross section data = 9 in × 9 in
There are many online calculators to compute the concrete yardage. But I always prefer hand calculation.
First, calculate the footing volume.
Here the length and the width are somewhat exact values. You can measure along with the formwork. However, the thickness can vary due to uneven ground surface. So it is important to check the thickness at different locations and calculate the average thickness of the footing.
Footing thickness = (T1 + T2 + T3 + …..Tn) / number of points
Footing volume = length x width x thickness
= 6 × 6 × 1
= 36 ft3
Now, let’s calculate the column volume
Column volume = cross section x height
= 9/12 × 9/12 × 6
= 3.375 ft3
The exact concrete volume you need is (36 + 3.375 = 39.375 ft3). But, it’s better to consider spillages and waste during handling the concrete, so adding 5% is thoughtful.
Additionally, consider another 5% for concrete placements on uneven surfaces like soil. Footings, pavements, ground slabs, and parking areas often need this correction to the volume.
Finally, the required concrete volume will be
= (36 × 105% + 3.375) × 105%
= 43.23 ft3
Finally, the concrete yardage will be = 43.23/27 = 1.6 yd3
Ready mixed or site mixed concrete
After you calculate the quantity and double-checking it, you can ask a ready mix supplier to deliver the concrete to your door. Things are easy!
However, imagine if you need to mix the concrete at the site. Then estimating material quantity is a bit of work. In some countries, the most preferred way is mixing at the site. You can use the mix design proportions to calculate the material quantity.
Two types of mix proportions are available in the construction field. Volumetric mix proportions and weighted mix proportions. Among them, weighted mix proportions are most popular. However, due to the easiness of measuring, volumetric proportions are still used. Following are the main volumetric mix proportions.
|Volumetric ratio Cem : Sand : Agg||Strength Grade (MPa)||Water content in liters|
|1 : 1 : 2||30||20|
|1 : 1 ½ : 3||25||22.5|
|1 : 2 : 4||20||25|
|1 : 3 : 6||10||35|
|1 : 4 : 8||–||40|
Ordering premix bags is one option for you. Volume conversion for 30lb, 40lb, and 60lb bags as follows. Click for the online calculator.
|Premix bag||Volume in cubic feet||No of bags for 1 cubic yard|
Be aware of the maximum water content you can use with pre-mix concrete bags. Adding too much water will drastically reduce the strength. See the specification for manufacture recommendations.
Estimate the material amount
In case if you need to estimate the raw material quantity to order them to the site, the following step-by-step calculation will guide you.
Assume the required C 25 concrete volume is 43.23 ft3
Cement : Sand : Aggregate = 1 : 1 1/2 : 3
Assume no mixing and compaction first
So, the total un mixed volume will be = 1 + 1.5 + 3 = 5.5
Let’s calculate the corresponding volume for materials
Water mix with the cement, so no volume will increase due to water
Cement volume = 1/5.5 × 43.23 ft3= 7.86 ft3
Sand volume = 1.5/5.5 × 43.23 ft3= 11.79 ft3
Coarse aggregate volume = 3/5.5 × 43.23 ft3= 23.58 ft3
After you place the concrete, giving good compaction is essential for the strength to gain. So we have to back-calculate the fresh concrete volumes in order to mix them.
When compacting concrete, the air inside the concrete releases, and concrete volume reduce by 36%. The use of a proper vibrator is essential for the compaction of concrete and its strength. Here is my guide for concrete vibrators!
Now let’s apply the factor to calculate the compacted concrete volume
Uncompact concrete volume = 43.23 ft3× 136% = 58.8 ft3
Now, to gain 43.23 ft3of compacted concrete volume, you have to mix concrete of 58.8 ft3 fresh volume. To convert into cubic yards, divide the number by 27.
Required material volumes will be,
Cement = 7.86 × 136% = 10.7 ft3 = 0.396 yd3
Sand = 11.79 × 136% = 16.0 ft3 = 0.593 yd3 Coarse aggregate = 23.58 × 136% = 32.1 ft3 = 1.189 yd3
Figuring out the concrete yardage is crucial before any concrete construction. Use the above guide to calculate the volume of cement, sand, and aggregate amount according to your volumetric mix ratio. Always keep in mind to consider the volume drop because of the compaction giving to the concrete. Hope you nailed the estimation and good luck with the project!