How do f'cr and strength requirements work in Mix Designer?

Mix Designer works from a required average strength, f'cr, and checks predicted strength against up to three strength-and-age requirements. Understanding f'cr and the controlling age explains most of what you'll see in the strength predictions.

f'cr is the required average strength: the over design target the mix is designed to reach. You can edit it in Mix Designer. As general context, f'cr is typically set above the strength a specification requires, to allow for the normal variability in concrete production and testing.

Mix Designer supports f'cr at the design age, plus up to two additional age-and-strength pairs. For example, a project might need one strength at the design age and a lower strength at an earlier age.

• Each requirement is checked independently against the predicted strength.
• The controlling age is the one with the smallest margin between predicted and required strength.
• Check which age is controlling, and how close the curve is to each requirement.
• Review which limit is holding the mix back, and widen it where that's appropriate for the project.
• Try the Project strength goal in Enhance, which reduces excess predicted strength toward f'cr while keeping all requirements.
• [Link: How accurate is the Predictive Strength Model, and how do I read the Estimated Strength Curve?]
• [Link: What is Enhance, and which goal should I choose?]
• [Link: What is Mix Designer, and what information does it use?]

For example, suppose a mix has a comfortable margin at its design age but a much tighter margin at an earlier age. The earlier age has the smallest margin, so it's the controlling age. (Illustrative only.)

If another project limit is reached first, the predicted strength may remain above f'cr. For example, Enhance adjusts a mix toward f'cr within your limits. If w/cm, SCM replacement or paste content reaches its limit first, strength can't come down any further. This is expected, not an error.

The shaded band on the Estimated Strength Curve is the model's own uncertainty. It isn't added on top of f'cr. Enhance designs the mix so that the expected strength, the center of the band, meets f'cr.