ANS:C - 10 mm

Using aggregates in concrete mixtures for ribbed slabs involves several considerations, including workability, compaction, and the ability to achieve the desired surface finish. Here's a more detailed explanation of why a maximum permissible size of 10 mm aggregates is commonly used:

1. Workability: Smaller aggregates generally improve the workability of concrete. For ribbed slabs, where concrete is placed in relatively narrow spaces between ribs, it's essential to ensure that the concrete can flow easily and fill all the voids without excessive effort. Using 10 mm aggregates provides a good balance between workability and mechanical properties.
2. Compaction: Proper compaction of concrete is crucial to achieve the desired strength and durability. Smaller aggregates allow for better compaction, reducing the likelihood of voids or honeycombing in the concrete. This is particularly important in ribbed slabs, where ensuring full compaction between ribs can be challenging.
3. Surface Finish: The use of smaller aggregates helps in achieving a smoother surface finish, especially in thin sections like the ribs of a slab. Larger aggregates may protrude from the surface, leading to an uneven finish or difficulty in achieving the desired architectural appearance.
4. Structural Considerations: While larger aggregates can be used in concrete mixtures for structural elements, such as beams or columns, ribbed slabs are primarily thin sections where smaller aggregates are preferred to ensure proper consolidation and homogeneity of the concrete mixture.

Overall, considering the requirements for workability, compaction, surface finish, and structural performance, a maximum permissible size of 10 mm aggregates is commonly chosen for casting the ribs of a slab. This size strikes a balance between these factors and is well-suited for achieving the desired quality and performance of the ribbed slab.