What is the benefit of smaller dia steel instead of larger dia steel in slab and beam?

There are many benefits of using a large number of small dia bars than a small number of large dia bars. The reasons are as follows.

1. to provide ductility to the structure, i.e. (beam and slab ) which is not available in the concrete.
2. to provide stability to the structure as the bars have rib design on it which helps to prevent the bars from slippage as we have to provide enough development length.
3. also because of the complicated shape of the structure we have to provide small dia bars.

and because of these reasons, it is not possible to provide all these factors using a single bar, and therefore we have to provide a small number of large diameter bars.

Having smaller diameter rebar distributed throughout the structure also distributes the load from the concrete to the steel more effectively as there is a greater area of contact for adhesion between the two.

The structure is made with Reinforce cement concrete (RCC) material.

It is a combination of bars and cement with strong bonding between them. Bars are provided to resist tensile as well as compressive stresses. Thus it's very important to make a strong bond between concrete and Reinforced bars.

If small diameter bars(also called distribution bars) are used more in quantity, increase the surface area of steel, and thus bonding area of reinforcement bars and cement increase. Thus bond becomes stronger and avoids cracks in the future.

For in-depth insights, refer below link:

https://theconstructor.org/structural-engg/reinforcement-detailing-of-rcc-slab-openings-cutouts/8478/

In the case of Reinforced cement concrete (Rcc), the concrete is reinforced by steel bars for carrying more stress, such as tensile cum compressive. For this reason, easy transfer of load from concrete to steel is a crucial criterion, and it can be achieved if steel and concrete have proper bonding with each other.
So making RCC stronger and stable and also making concrete more Reinforced, we should provide more number of the smaller diameter of the bar than the larger one, then it will help in good bonding between steel and concrete.
So that’s the main reason.

Benifit of smaller dia over larger dia steel in rcc member

• Required less development length
• Better crack prevention
• Better confinement

Basically in beam we use 16-20 mm dia of bar,

In column we use 16-25mm dia,

In slab we use 10-16mm dia in ordinary residential building