Concrete Slab Cost Calculation

Construction consumes a huge fortune due to the high costs of labor and materials. That’s why a precise calculation of the cost is essential to avoid any losses. This accurate calculation will be based on an accurate calculation of the quantity of materials, including cement, sand, aggregate, and steel reinforcement, as well as an updated price for these materials. In addition, the cost of construction, including the equipment and labor, has to be accurately calculated.

In this article, we will walk you through the involved steps for calculating the price of construction for a 25 m2 reinforced concrete slab.

Step 1: Amount of Concrete materials

Concrete consists of several materials, including cement, which acts as a binding material; aggregate, which is divided into fine and coarse aggregate; water; supplementary cementitious materials such as fly ash and silica fume; and, in some cases, plasticizers, which may be needed to achieve the target workability.

But how do we calculate the amount of these materials?

The answer is based on the mixture design. So, the engineer does the mixture design process depending on the target strength and other durability considerations according to the governing code of practice to obtain the amount of constituent materials in concrete that satisfy those requirements.

The amount of these materials is typically given as kilograms per cubic meter. For example, let us assume the amount of material is as follows:

• Mass of Cement (C) = 400 kg/m3
• Mass of Sand (S):  400 kg/m3
• Mass of Aggregate (A): 800 kg/m3

Step 2: Amount of Steel

Next, we need to know the amount of steel that will be used in the construction of the slab. This will depend on the structural design. So, based on the structural drawings that are provided by the structural engineer, we can calculate the amount of steel that would be implemented in construction.

Depending on the thickness of the slab, one or two layers of steel reinforcement will be used.

In general, for each layer, we have main bars and secondary bars. If the slab is a solid slab, the main bars will be in a short direction, and if the slab is a flat slab, the main bars will be in a longer direction.

Let us assume that our slab is 150mm thick and has only one layer of steel (5 T10/m’) in each direction.

Step 3: Amount of concrete materials and steel

In this step, we will calculate the total amount of concrete materials and steel for the entire slab.

We know from the beginning of the article that our slab has an area of 25 m2, with a thickness of 150 mm.

Total volume of slab = area x thickness; this is known as wet volume.

Wet Volume = 25 x 0.15 = 3.75 m3

Dry volume

Wet volume is known to have pores and spaces that are filled with water and bubbles; these can be removed using a vibrator or compressor machine, necessitating the use of extra material.

The term “dry volume” refers to the number of voids and pores in a mixture of cement, sand, and aggregates that evaporate when the concrete is wet. The volume of wet concrete decreases by 54%, while the volume of dry concrete increases by 54%. Therefore, the dry volume can be calculated by multiplying the wet volume by 1.54.

Dry volume = wet volume x 1.54 = 3.75 * 1.54 = 5.8 m3

Total Mass of Cement (C) = 400 kg/m3 * 1.54 = 616 kg / 50 kg (mass of a bag) = 13 bags

Mass of Sand (S): 400 kg/m3 * 1.54 = 616 kg [can be converted to m3 knowing the density of sand].

Mass of Aggregate (A): 800 kg/m3 * 1.54 = 1232 kg [can be converted to m3 knowing the density of aggregate].

Volume of steel = area of a bar * total length of bars

Area of T10 bar = 78.5 mm2

Length of bars = 5T10/m’ *  5 m [length of one bar] * 2 [two directions] = 5 * 5 * 2 = 50 m

Volume = 50 * 78.5/10^4 = 0.3925 m3

Weight = density of steel * volume = 7.85 tons/m3 * 0.3925 = 3.1 tons

Step 4: Cost of concrete materials and steel

Depending on the current price of materials, you can multiply the rate of each material with the amount to obtain the total price