Are you paying for cement the spec does not need?
One cubic meter of concrete, priced on the powders that go into it, against a specification you type in. The mix a supplier quotes when nobody asks a question costs $50.66. The cheapest compliant one costs $44.45.
Words on this sheet
- Contribution: What is left of the income after the costs that come with it, before the fixed costs are paid.
Construction Starter Optimization free
After you install, this is the model to open.
Which Concrete Mix Meets Spec for the Least Money?
- In your spreadsheet, click the Sortia icon in the strip of icons down the right-hand edge. No strip? Click the arrow at the bottom-right to open it. You can also use Extensions, then Sortia, then Open Sortia.
- Click Start from a template and put that name in the search box.
- Pick the card with that name and click Load this template. It arrives on a new tab with real numbers already in it.
The answer
One cubic meter, and then the whole raft:
- Optimized cost
- $44.45 a cubic meter
- Opening mix
- $50.66 a cubic meter
- Saving
- 12.3% $6.21 a cubic meter
- Across a 450 m³ raft
- $20,001 against $22,797
What is strange about the answer is that it uses more material: 380 kg of powder against 320, because cheap fly ash lets the mix reach the same strength index on 194.5 kg of cement instead of 320, and the extra volume costs less than the cement it replaces. Four limits come back binding, which is the report telling you the answer sits exactly on the corner where those rules meet: raise the fly ash ceiling to 35% and the mix comes back at $44.02, another $191 across the raft. One warning, because this is a structural material: the sheet prices a blend against limits you type, it is not a mix design, water and aggregate are not modeled, and no optimizer should be the last word on a pour.
The model
Cement, slag, fly ash and limestone filler, each with a delivered price and a strength contribution. Three limits are the specification and two headroom rows are the percentage rules written so the optimizer can read them.
| Powders | cement, slag, fly ash, filler |
| Cementitious minimum | 320 kg |
| Total powder minimum | 380 kg |
| Strength index minimum | 300 cement-equivalent kg |
| Replacement ceilings | fly ash 30%, total 45% |
Once it is in your sheet
- The model arrives with real numbers in it and runs as it stands, so you can press the button first and understand it second.
- Change the numbers to yours. The sheet marks which cells are inputs and which hold formulas, and most labels carry a note explaining the row.
- Press the run button at the bottom of the panel. It is labeled for the tool you are in, and the result lands on its own tab, with a written reading of it beside the figures.
Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.
Next question
- Crew B is 16% ahead. Is that the crew, or the fortnight?Do the Three Crews Work at the Same Rate?
- Should you chase this tender at all?How Should Two Bidders Price One Tender?
- Was your best day on site really your best?Which Days on Site Went Wrong?
- Is your markup quietly costing you margin?What Markup Gets This Bid to Target Margin?
- Is the extra crew cheaper than the damages?How Many Crews, When the Weather Is a Guess
- A construction site risk register pricing five risks where every mitigation pays for itself, plus the exposure left for the contingency line. Free template.What Could Go Wrong on Site?
Every model like this one, and the method behind them: Optimization in Google Sheets.