Mix Design Aids – what do C.R.R.I. charts relate to? C.R.R.I. (Central Road Research Institute) charts are commonly used to obtain a relationship between concrete strength and which primary mix parameter?

Introduction / Context:
Empirical charts are used in preliminary mix design to select proportions that achieve target strengths without excessive trial mixes. The most influential parameter for strength in conventional concretes is the water–cement (w/c) ratio.

Given Data / Assumptions:

• Normal-density aggregates and ordinary Portland cement.
• Standard curing and test age (e.g., 28 days).
• Charts compiled from calibrated laboratory data.

Concept / Approach:
Concrete strength decreases as w/c increases, due to higher capillary porosity. C.R.R.I. charts conveniently plot characteristic/compressive strength against w/c for given materials and conditions, enabling the designer to pick an initial w/c for the target grade.

Step-by-Step Solution:
Identify the most direct predictor of strength → water–cement ratio.Recognize that workability and grading are adjusted separately to meet placement needs without sacrificing the selected w/c (use admixtures as needed).

Verification / Alternative check:
Classic relationships (e.g., Abram’s law) correlate strength inversely with w/c, which the charts operationalize for practice.

Why Other Options Are Wrong:

• Workability: primarily controlled by water content and admixtures but not a direct strength index.
• Grading and fineness modulus affect paste demand and finishability, not strength as directly as w/c.
• “None” is incorrect because charts explicitly use w/c.

Common Pitfalls:
Changing water to improve slump after selecting w/c; instead, adjust with admixtures or re-balance aggregate grading to keep strength on target.

Final Answer:
Water–cement ratio