**Explanation:**

To understand why the maximum permissible size of aggregates for casting the ribs of a slab is 10 mm, we need to consider the factors that influence the choice of aggregate size in concrete construction.

**1. Workability of Concrete:**
The workability of concrete refers to its ability to be easily mixed, placed, and compacted without segregation or bleeding. The size and shape of aggregates have a significant impact on the workability of concrete. Smaller aggregates provide better workability as they can easily flow through the gaps between reinforcement bars and fill the formwork properly. On the other hand, larger aggregates may result in a more rigid and less workable concrete mix.

**2. Reinforcement Placement:**
In the case of ribbed slabs, steel reinforcement bars are placed within the ribs to provide additional strength and load-bearing capacity. The maximum size of aggregates used should be small enough to allow easy placement of the reinforcement bars without obstruction. If the aggregates are too large, it may be challenging to properly place the reinforcement bars within the ribs.

**3. Concrete Cover:**
Concrete cover refers to the thickness of concrete between the reinforcement bars and the outer surface of the slab. It is necessary to maintain an adequate concrete cover to protect the reinforcement from corrosion and provide sufficient fire resistance. The maximum size of aggregates used should be small enough to ensure proper cover is achieved. If the aggregates are too large, it may be difficult to achieve the desired concrete cover.

**4. Bond Strength:**
The bond strength between the concrete and reinforcement bars is crucial for the structural integrity of the ribbed slab. Smaller aggregates provide a larger surface area for bonding with the reinforcement bars, resulting in higher bond strength. Larger aggregates may reduce the bond strength between the concrete and reinforcement bars.

Based on these factors, the maximum permissible size of aggregates for casting the ribs of a slab is determined to be 10 mm. This size allows for good workability, easy placement of reinforcement bars, adequate concrete cover, and satisfactory bond strength. Using larger aggregates may lead to issues such as poor workability, difficulty in reinforcement placement, inadequate concrete cover, and reduced bond strength, compromising the structural performance of the ribbed slab.