How to Determine Quantities
Measuring a Pavement Job.To determine the amount of asphalt you will need for a paving job you first need to know the area to be paved. This may be simple if the area to be paved is square, rectangular or constant width of road. However, this is seldom the case since there may be bumpouts or islands in the area to be paved, the width of the road may vary, or there are culsdesac. This may seem to be impossible in some cases, but by dividing the pavement area into simple geometric shapes and adding and subtracting these areas, the total area to be paved can be quickly determined. The following are basic formulas for calculating the area of different geometric shapes.
Lets look at the parking lot in Figure 1, as it has many of the sorts of features that can complicate the calculation of area.
When you first look at this it may seem complicated because it is not a square, triangle, or circle. However, it is actually a combination of squares, triangles, and circles, as can be seen in Figure 2.
By dividing a project into these simple shapes you can now easily calculate the area to be paved.
Area A is a rectangle 100' × 100' = 10,000 sq.ft.
Area B is a triangle 30' × 100' ÷ 2 = 1,500 sq.ft.
Area C is a rectangle 20' × 120' = 2,400 sq.ft.
Area D is a circle 70^{2} ÷ 4 × 3.14 = (70 × 70) ÷ 4 × 3.14 = 3,846.5 sq.ft.
To determine the total area of paving all we have left to do is add the areas together with one exception, the islands in the middle of the parking area. For these, we have to calculate the area of the islands and subtract that area from the sum of the other areas. I will leave the calculation of the islands' area up to you. Just remember, the islands are just rectangles with semicircles at each end.
Therefore, the total paved area is:
10,000 + 1,500 + 2,400 + 3,846.5 − (Islands' Area) = 17,257 sq.ft.
OK, I did calculate the planter area, I just wanted you to have a try at this. I also rounded off to the nearest whole number. This is all the accuracy you are likely to need on a paving job.
You may notice that there are small areas around the curb returns at the culdesac that were not calculated. In this case, the areas are small and will not make a significant difference in the total areas. If you have many areas like this on a project it would probably be best to draw the area to scale on graph paper and estimate the area by counting the number of squares and multiplying the number of squares by the area of each square. For example, if you drew the area at a scale of 1"=20' and the graph paper was divided into 0.10" increments, each square would have an area of 4 sq.ft. (2' × 2'). If you counted 20 squares, the total area would be 80 sq.ft. (20' × 4').
Figure 3 illustrates an example of this. In this case there are approximately 20 squares. One of the tricks is to combine partial squares. For example if 3/4 of one square is covered by the area and 1/4 of another square is also covered by the area, they can be combined and counted as a full square.
Now that you have the area calculated, you next need to calculate the quantities. Quantities are usually specified by weight (tons) or volume (cubic yards). No matter how it is specified you will first need to calculate the volume. Volume is easy to calculate since it is simply Area × Thickness. One of the keys in calculating volume is to always use the same dimension. Therefore, if we measure the area in square feet, we have to convert thickness measurement to feet too. For example, if you are placing 6" of aggregate base, the thickness would be converted to:
6" ÷ 12"/foot = 0.5 feet.
To compute the volume we simply multiply this thickness by the area previously computed.
Volume = Area × Thickness = (17,257 sq.ft.) × (0.5 feet) = 8,628.5 cu.ft.
Aggregate base and other material are often specified or purchased by the cubic yard; therefore, you need to convert cubic feet to cubic yards. Since there are 27 cubic feet per cubic yard (3'/yd. × 3'/yd. × 3'/yd.) you would need to divide the cubicfoot volume by 27.
Volume = 8,628.5 cu.ft. ÷ 27 cu.ft/cu.yd. = 320 cu.yds.
To calculate the weight of material needed for a project you will need to know the compacted density of the material. Density is the weight of material per volume. For example pounds per cubic foot. The density of asphalt pavement mixtures does vary depending on mix proportions and the type of aggregate. If you do not know the inplace density of mixes, you must ask your supplier.
To determine the weight of material multiply the volume by the density of material:
Weight = Volume × Density
Using the previous example and assuming an asphalt pavement mixture density of 145 pounds per cubic foot (pcf) and a thickness of 4" we would perform the following calculations.
Now it's your turn to try your new skills. Using the paving area in Figure 2, calculate the cubic yards of aggregate base and tons of asphalt pavement mixture needed, assuming the following:
Answers:
A note about measuring areas of roadways with curvesWhen determining the area of a roadway with a curve it is important that the length be measured along the centerline of the road. By doing this the area is simply the length of the arc along the centerline times the width.
