Concrete forms a major part in many construction projects. With a high compressive strength, concrete is used for many load bearing and transferring components of any structure. Whether it is a slab on the ground, or being used to strengthen retaining, all estimators and quantity surveyors will need to understand how to perform accurate takeoffs.
Providing accurate quantities for concrete is paramount when bidding on projects. When tendering, over-estimating quantities can lead to an inflated price and the bid not being won. Under-estimating quantities on the other hand may win the bid but put financial strain on the building company. Likewise, when generating quantities for ordering inaccuracies can lead to over-ordering which causes waste, or under-ordering bringing delays to the project. In order to provide the most accurate quantities, we must first understand what makes up a concrete takeoff.
What makes up a concrete takeoff?
Concrete takeoffs can be broken down into three main components – materials, equipment and labour. There is no simple formula that covers all aspects of a construction project, as quantities of these three components will change for slabs, footings and any other concrete structures. This information can usually be found within the Engineer’s certified structural drawings. Structural drawings provide us with the details we need to perform an accurate takeoff, but these are not always available in a tendering situation.
When tendering on projects without structural details allowances will still need to be made. These can be based on previous projects, reference from the National Construction Code or a standard detail suggested by an engineer. It is important to note what allowances are made when submitting the tender to minimise any risk to the builder. Regardless of whether structural documents are supplied, or standard details are used, the takeoff process will be the same.
From the ground up
Starting with our ground floor slab, we need to determine the strength of the concrete being used. This is represented by its MPa rating and will determine the material cost of the concrete. The thickness of the concrete slab will also be detailed in the structural drawings. With the thickness known, we can now measure the area of the ground floor slab to determine the volume of concrete needed and the respective labour component.
Poured concrete and labour are not the only elements that make up a slab takeoff. Having already worked out the area of our slab, we can use this to figure out the rest of our materials. Steel mesh forms a crucial part of the slab, and the type of mesh is detailed within the structural drawings. Ordered per sheet, quantifying steel reinforcement mesh requires calculating how many sheets are required to suit the area of the slab. Our slab area is also used to quantify the permeable membrane that sits under the slab, the chairs for the mesh as well as any pest control items under or around the slab.
This method of generating quantities for a slab can also be applied to suspended slabs. Again, the strength of the concrete and the thickness of the slab will be noted on the details, as well as the type of reinforcement needed. It is important to note that for a suspended slab formwork will need to be allowed to shape the slab. Of equal importance is to identify the thicknesses of any balconies. A balcony will generally be thinner than the main floor slab to accommodate a step-down. While this may only be a 50mm difference, it can work out to a huge discrepancy on larger projects.
Put your best footing forward
With an understanding of how to take off a concrete slab, we can apply these principles to all other components of our project. Concrete footings in a building are the final point where load is transferred to the ground. Unlike our slabs where we determine the volume of concrete from the area and its thickness, calculating the concrete needed for footings is slightly different.
Structural details for footings show us the depth and the width of our footings, giving us the area. In order to determine the volume, we then need to measure the length of each footing. It’s important to note that on structural drawings the footings will be detailed out by type, as some footings may be wider or deeper than others. This process can then be applied to any thickenings under the slab, as well as pad footings that may be needed.
Based on the detail of our footings, we may also have a need for steel reinforcement. Quantifying reinforcement in strip footings is different to the mesh found in a slab, but the concept is the same. Rather than a sheet of steel mesh, footing or trench mesh is ordered in lengths that must be tied in to the slab. Having measured the length of each footing already, we can easily calculate how many lengths of mesh will be needed for the project.
Know your site, know your risks
Knowing what quantities of materials are needed will help us to identify the labour required for the project, but what about the equipment we need? Here is where we need to get an understanding of our site and the project as a whole. Concrete pumps come in different boom lengths, so it is vital to order the right length if our site is inaccessible from the street. Cranes may also be needed to lift any steel beams that form the suspended slab system.
Key documents that help to determine the details for a concrete slab and footing system are the Site Classification report or a geotechnical report. In Australia, the ground in a building site is given a classification based on the expected amount of surface movement. The scale, ranging from A through to P, is determined based on the presence of reactive components in the ground such as clay. If a site is quite reactive, strip footings and slabs will be deeper and thicker than those of a non-reactive site and there may be a requirement for additional reinforcement.
Whether we have full documentation or are relying on standard details from past projects, by following these guidelines we should now be able to calculate our concrete components.