Concrete estimation is a core part of construction cost estimating. Concrete reaches a project in one of two ways: ready-mixed, or mixed on the job site.
Most commercial and residential projects use ready-mix concrete. The quality control requirements, quantity, strength, cement type, and other requirements go to the ready-mix supplier, who quotes the cost.
On large projects, and on projects far from cities, concrete is often mixed on site. Estimating it then means working out every material in detail — sand, aggregate, cement, admixtures, and the water ratio.
On-site mixing also requires planning for a batch plant. The size of the project and the quantity of concrete determine the size and number of batch plants.
Finally, assess where the materials will come from. Distance from the site is the main consideration, but sometimes a specified material quality isn't available from nearby suppliers, which can seriously affect the cost of the concrete.
It's also wise to consult a mix design specialist to determine the right proportions of concrete ingredients.
Below are specific tips to help you estimate concrete accurately. First, a few general guidelines.
General guidelines for concrete estimates
- Concrete is estimated in cubic yards. Calculations from the drawings are made in cubic feet and then converted to cubic yards, because ready-mix suppliers usually price by the cubic yard.
- Roof slabs, floor slabs, pavements, sidewalks, and slabs on grade are measured by length, width, and thickness.
- Footings, columns, and beams are computed by multiplying their running length by their cross-sectional area.
- Irregular slab shapes are broken into smaller, regular shapes so they can be measured accurately.
- Footing lengths can be tricky. Count corners only once as you work around the building, and remember that footings usually run longer than the walls they support because they extend past the wall ends.
- Openings smaller than 2 square feet are generally ignored, and the space taken up by accessories, embeds, and reinforcing bars isn't deducted from the volume.
- Typical waste allowances for wall footings, columns, beams, and slabs range from 5 to 8 percent.
These guidelines apply to any concrete estimate, but every project has its own characteristics and requirements. The following tips will improve the accuracy and effectiveness of your concrete estimating.
Tip 1: Review the specifications thoroughly
Review the project specifications for the floor slabs, footings, walkways, and other elements. For each element, list:
- Type of concrete
- Strength of concrete
- Color of concrete
- Any special curing or testing requirements
Tip 2: Match the drawings to the specifications
Review the construction drawings thoroughly and match every concrete item shown on them to the specification requirements.
If you find discrepancies, contact the owner, engineer, or architect so an addendum can be issued.
Tip 3: Take quantities from the working drawings
- Check footing sizes on every wall section and on the foundation plans — different walls may have different footings.
- Information for estimating concrete slabs is most often found on wall sections, structural details, and floor plans.
- Review the site plan and section details to estimate exterior walks and driveways.
Tip 4: Give formwork special attention
Forming is often the biggest cost in concrete construction.
- Selecting the forms best suited to the job is a key part of the estimate.
- Forms are estimated in square feet of contact area.
- Enough forms must be available to keep the placing equipment and crew working efficiently.
- Form-in-place costs include all necessary bracing and shoring.
- Pay close attention to labor rates and crew productivity.
Tip 5: Price each major component separately
For an accurate concrete estimate, consider each of these components individually rather than burying them in a single unit rate:
- Reinforcing steel (see our rebar takeoff services)
- Reinforcing accessories — beam and slab bolsters, high chairs, bar ties, and tie wire
- Formwork
- Ready-mix concrete
- Placing the concrete
- Finishing the top surface
Conclusion
Many variables affect a concrete estimate, and every project has its own requirements. Concrete technology keeps improving as well, with ongoing research into reinforcing materials, admixtures, and placement techniques.
The American Concrete Institute (ACI) publishes guides and reports on the latest developments and is a good place for further reading. For estimating and bidding, it pays to stay current.