Concrete is tested for strength by breaking cylinders.

Using the ACI 211.1 method, here is an abbreviated run-through on how to design a mix:

  1. Choose the target slump
  2. Choose the maximum aggregate size—remember that the larger the better for reducing shrinkage and curling.
  3. Estimate the water and air content using ACI 211.1 Table 6.3.3.
  4. Select the water-cement ratio.
  5. Calculate the cement content by dividing the water content by the water-cement ratio.
  6. Estimate the coarse aggregate content.
  7. Estimate the fine aggregate content.
  8. Adjust for aggregate moisture—wet aggregate can significantly reduce the amount of water to be added.
  9. Make trial batches to see what you've got.

So you see that after all these fancy calculations, the bottom line is that concrete is still designed by experience with previous mixes or by making trial batches in the lab and testing the concrete. Nothing beats experience with a mix. Many decorative contractors will have 4 or 5 mixes that they use for different applications or weather conditions. If you have these mixes already in your ready mix provider's office, he can quickly pull out the design and you can tell him to bring your mix #3 or #5.