For R. C. C. work the plaster ratio is :

Question

Accepted Answer

The correct option is 2:
The plaster ratio recommended for Reinforced Cement Concrete (R. C. C.) work, such as ceilings, beams, and columns, is generally **1:3** (1 part cement to 3 parts sand by volume). This ratio is selected for its high strength and, most importantly, its superior **adhesion** to the smooth, non-porous surface of cured concrete. Unlike brick masonry, which has high porosity and a rough surface that provides a strong mechanical key for the mortar, R. C. C. surfaces are dense and smooth. A richer mix, like 1:3, is necessary to ensure the cement paste creates a strong, chemical and physical bond that resists cracking and peeling over time. Additionally, plaster applied to R. C. C. surfaces is typically thin, often ranging from 6 mm to 10 mm. This thin application also mandates a richer mix like 1:3 or sometimes 1:4 to achieve the required strength and prevent detachment, as a thin layer of lean mortar would be structurally weak and susceptible to bond failure.

The incorrect option is 1:
The ratio **1:2** is considered excessively rich for standard plastering work, including on R.C.C. surfaces. This high proportion of cement makes the mortar extremely prone to **shrinkage cracks** as it cures and dries, which would defeat the purpose of applying a smooth, protective layer. Furthermore, using a 1:2 mix is uneconomical due to the high consumption of cement. While a 1:2 ratio is occasionally used for highly specialized applications such as waterproofing treatments or as the top layer for a Damp Proof Course (DPC), it is not the standard or recommended ratio for general finishing plaster on R.C.C. elements. Therefore, selecting this option would lead to material waste and significant surface defects in the finished structure.

The incorrect option is 3:
The ratio **1:5** (1 part cement to 5 parts sand) is a **leaner** mix, meaning it has a lower proportion of cement. Such mixes are typically suitable for interior plastering on **brick masonry** walls, especially where the wall surface is already somewhat rough and porous, allowing for a good mechanical bond. However, when applied to a smooth R. C. C. surface, a lean mix like 1:5 will not generate the sufficient tensile strength or bond-to-substrate required to hold the thin plaster layer firmly in place. This lack of adequate bonding strength on a smooth R.C.C. element increases the high risk of the plaster cracking, sagging, or completely detaching (delaminating) from the concrete substrate in the long term.

The incorrect option is 4:
The ratio **1:6** is the **leanest** mix among the options and is generally reserved for non-structural masonry work or for the internal plastering of less exposed or non-critical brick walls. For any work involving R. C. C., which requires a strong, durable, and highly adhesive finish, the 1:6 ratio is completely unsuitable. Applying a 1:6 cement mortar to an R. C. C. surface will result in a weak bond that is almost guaranteed to fail, particularly on ceiling surfaces where gravity constantly pulls the plaster down. Moreover, a mix this lean would not provide the necessary resistance to the forces of curing and drying, making it highly susceptible to major bond failure and the formation of extensive cracks across the plastered area.