Final answer:
The assertion that smaller coarse aggregates are always more economical in concrete mix design is false. Larger aggregates can be more economical because they require less cement paste and can reduce shrinkage, while providing adequate strength and durability when properly incorporated into the concrete.
Step-by-step explanation:
The statement that for economy, the maximum size of coarse aggregate should be as small as possible is false. In concrete mix design, the size of the coarse aggregate depends on various factors, including the particular requirements of the project, the spacing between reinforcement bars, the dimensions of the structural member, and the method of compaction. Coarse aggregates are usually chosen to achieve a balance of strength, workability, and durability of the concrete. Larger aggregates are typically more economical as they reduce the amount of cement paste required and reduce shrinkage, thereby minimizing costs. However, they must be adequately covered by the cement paste to ensure proper bonding and avoid weak spots.