# Construction - Estimating Concrete

# Construction - Estimating Concrete

Most building projects require the use of concrete at some point. Concrete is the most common material for foundations and basements as it can be formed to just about any shape and becomes very hard once it has cured. Concrete is created by mixing cement, gravel, water, and sometimes other additives. When freshly mixed it is a thick fluid mixture that is poured into forms. In most situations, the forms are removed once the concrete has cured at which point the concrete will maintain its shape for many years if designed and cared for properly.

Concrete is typically mixed at a concrete plant and then delivered in specialized trucks. Because fluid concrete has a fairly short lifespan, approximately 2 hours, it is important that the correct amount is ordered. If there is too little concrete then more will need to be ordered and the new batch will often not adhere well to the original pour resulting in a weakened structure as well as look unsightly. If there is too much concrete then it wastes money.

Estimating the amount of concrete to order is simply a matter of finding the volume of the forms to be filled. This can very straight forward for something simple like a sidewalk but can be quite complicated when creating complex shapes that need to be completed in multiple pours. For instance, in the case of a basement, forms are created for the footings (a base that the wall sits on), the footings are poured, and then the footings are stripped once the concrete has cured adequately. Next, the forms for the walls are built, the walls are poured, and then the forms are stripped off. The concrete order must be calculated separately as the pours are often days apart.

In the United States, and some other countries, the standard unit to order is a cubic yard, commonly referred to as a yard. In countries that use metric measurements the order will be placed in cubic meters. Although you will use the abbreviated term "yard" when ordering, remember that this is actually a measure of volume so it is technically a cubic yard. Since a cubic yard is 3 feet tall, 3 feet wide, and 3 feet deep there are 27 cubic feet in a cubic yard. [math]3*3*3=27[/math] This means that you can convert cubic feet to cubic yards by dividing by 27.

Because running short can be very costly (having another truck come with a small amount or ending up with a job that doesn't meet requirements) it is very important to calculate extra. This is usually calculated as a percentage of the total pour with 3%-5% being quite common. To add 5% to your order multiple by 1.05 which calculates the 5% and adds it to the original amount.

EXAMPLE:

Hannah and John want to build a sidewalk to their back door. The sidewalk will be 32' long, 3' wide, and 4" deep. To calculate this we can begin by [math]32 xx 3 = 96[/math] which means that the sidewalk will cover [math]96[/math] square feet of the ground. To find the volume we must multiple the height but this is in inches. [math]96 xx 4/12=32[/math] This means that we will need [math]32[/math] cubic feet of concrete.

To convert to cubic yards we divide by [math]27[/math] which gives [math]1.18[/math] cubic yards. Multiply by [math]1.05[/math] to add our safety margin and we come to [math]1.24[/math] cubic yards. Most companies will mix to a quarter of a yard so we can order [math] 1 1/4[/math] yards. Some companies will only mix to a half a yard in which case we would need to order [math] 1 1/2[/math] yards.

These steps apply to any application but for most projects, the numbers will not be this simple and there may be multiple pours but the steps are the same.

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