M60 and M80 grades of high-strength concrete are available. M10 and M15 concrete grades are commonly used for leveling courses and bedding for footings. Standard concrete and concrete of grade M20 are appropriate for Reinforced Cement Concrete (RCC) work such as slabs, beams, and columns.

The M value indicates the minimum required strength when specified as a single layer. The maximum design depth for M60 concrete is 20 feet; for M80, 40 feet.

The type of reinforcement used in concrete plays **an important role** in determining its strength. The three main types are plain concrete, steel rebar, and precast concrete sections. Each has its own advantages and disadvantages. The choice of reinforcement should be based on the expected load requirements and the cost of construction materials. For example, if you expect the concrete floor to carry heavy loads, then it makes sense to use rebar instead of plain concrete. On the other hand, if the floor will only carry light weight or no weight at all, then plain concrete is enough.

Concrete strength is also affected by **its quality**. Quality control during construction ensures that adequate strength is achieved before any reinforcing is added. For example, if the concrete is mixed too vigorously, then it will not have time to develop its full strength before it sets. This kind of concrete needs more reinforcing to support **its initial weight** after placement. Otherwise, the structure will likely fail under **its own weight**.

For routine labor, the ratios are 1.5:3 and 1:2:4. For foundations and bulk concrete constructions, use 1: 3: 6 and 1: 4: 8. Concrete grades lower than M20 are not permitted in RCC work, according to IS 456: 2000.

The required proportions of cement to coarse and fine aggregates (for ordinary strength) are given in table 1. The ratio of water to cement (the water/cement ratio or w/c) should be maintained at a level of about 0.35 for normal setting times. If the mixture sets slowly, add **more cement**; if it sets too quickly, add more sand.

Table 1: Proportions of components in concrete

Component% of mix by volume Coarse aggregate 40-60% Fine aggregate 20-40% Water 25-35% Cement

Concrete kinds and their applications

Concrete Grade | Mix Ratio (cement : sand : aggregates) | Compressive Strength |
---|---|---|

M5 | 1 : 5 : 10 | 725 psi |

M7.5 | 1 : 4 : 8 | 1087 psi |

M10 | 1 : 3 : 6 | 1450 psi |

M15 | 1 : 2 : 4 | 2175 psi |

Design of Cement Mix (M) = Cement: Sand: Aggregate. Concrete grades are also designated as C16/20, C20/25, C25/30, and so on, which stands for **Concrete Strength Class** (C). The figure behind C denotes the compressive strength of concrete measured in N/mm 2 with a cylinder or cube. For example, a C20 concrete means that it has **20 times the strength** of regular concrete.

The maximum aggregate size for most construction mixes is 0.5 mm. Some construction mixes may include 1.0 or 1.5 mm aggregates to increase their durability. Large aggregates such as 2.0 mm stones or 4.0 mm gravel can be used to create a decorative concrete floor or patio.

Concrete mix designs vary depending on the type of mix needed for **a particular project**.

OPC 53 Grade cement is suggested for **all RCC constructions** such as footings, columns, beams, and slabs where initial and ultimate strength are critical. It is also used for **general industrial applications** such as building bridges, dams, and highways.

OCP 67 grade cement is suitable for use in masonry piers and walls up to 3 feet thick. This cement has an equivalent calcium carbonate content of 32 percent with the remaining 68 percent made up of silicon dioxide and aluminum oxide. The higher percentage helps prevent cracking caused by **moisture absorption**. OPC 53 grade cement is a premium product that is more expensive than typical portland cement but offers **improved performance** in terms of strength and durability.

OPC 70 grade cement is used in applications where high heat resistance and low water permeability are required such as roof tiles, flooring, and boiler plates. This type of cement has an equivalent calcium carbonate content of 31 percent with the remaining 69 percent composed of silica, alumina, iron oxides, and magnesium oxide.

OPC 73 grade cement has a higher percentage of calcium carbonate (45 percent) than OPC 70 grade cement (41 percent). This means that OPC 73 grade cement sets faster and provides better early-age strength than OPC 70 grade cement.