ANS:D - all the above.

A ribbed slab is typically provided for all of the purposes mentioned: a plain ceiling, thermal insulation, and acoustic insulation. Let's explore each purpose in more detail:

1. Plain Ceiling: Ribbed slabs are commonly used in buildings to create a plain, flat ceiling surface. The ribs in the slab provide structural support and help distribute loads, while the spaces between the ribs allow for the installation of utilities such as lighting fixtures, HVAC ducts, and electrical wiring. This configuration results in a clean and unobstructed ceiling, making it suitable for various architectural styles and interior designs.
2. Thermal Insulation: Ribbed slabs can also serve as a means of thermal insulation in buildings. The voids between the ribs can be filled with insulating materials such as expanded polystyrene (EPS), extruded polystyrene (XPS), or mineral wool. These materials help reduce heat transfer between the interior and exterior of the building, improving energy efficiency and maintaining comfortable indoor temperatures. By incorporating thermal insulation into the ribbed slab, the building's overall thermal performance can be enhanced.
3. Acoustic Insulation: The voids in ribbed slabs can also be utilized to improve acoustic insulation in buildings. Sound-absorbing materials such as acoustic batts or panels can be installed within the voids to absorb sound energy and reduce noise transmission between floors or rooms. This helps create a more comfortable and acoustically pleasing indoor environment by minimizing noise disturbances and echoes. Ribbed slabs with integrated acoustic insulation are commonly used in spaces such as offices, classrooms, auditoriums, and residential buildings to enhance sound quality and speech intelligibility.

Therefore, ribbed slabs are versatile structural elements that can fulfill multiple functions, including providing a plain ceiling surface, offering thermal insulation, and enhancing acoustic comfort. Depending on the specific requirements of the building and the desired performance criteria, ribbed slabs can be designed and constructed to accommodate these various purposes simultaneously.