Continuing with the Demo Day project discussed below, we did 7 day breaks on three 4x8 cylinders this week. All three ranged between 2600 and 2800 psi. Using the commonly accepted "7 day strength is 75% of 28 day strength" rule of thumb, this suggests 28 day strength of 3600 psi. This more than adequate for many uses.
Two items of note: (1) The "75%" rule of thumb is only an approximation and will vary with mix design. (2) This mix design used fly ash for 30% of the cementitious material. Fly ash mix designs typically show lower 7 day strengths than Portland-only mixes, but higher 28 day strengths.
Here's the mix design: Portland 724 lbs, fly ash 310 lbs, sand 2200 lbs, water 430 lbs with mid range water reducer. When we publish a mix design, it is with the understanding that it is only guideline and carries no assurances that you will get the same results. Local materials, batching techniques, and mixing can and will affect final results. As an example, fly ash varies from one generation plant to another. This used no coarse aggregate, though most of our recent mix designs in this category have included coarse aggregate. It was also somewhat "cement rich" even after the addition of foam, but money was still saved.
BOTTOM LINE: Cellular Concrete Saves Money
and Solves Problems
