Concrete and its grades

Concrete is a construction material that consists of cement (commonly Portland cement), aggregate (generally gravel and sand), water and admixtures.

Engineers usually specify the required compressive strength of concrete, which is normally given as the 28 day compressive strength in mega pascals (MPa) or pounds per square inch (psi). Twenty eight days is a long wait to determine if desired strengths are going to be obtained, so three-day and seven-day strengths can be useful to predict the ultimate 28-day compressive strength of the concrete.

Potable water can be used for manufacturing concrete. The w/c ratio (mass ratio of water to cement) is the key factor that determines the strength of concrete. A lower w/c ratio will yield a concrete which is stronger and more durable, while a higher w/c ratio yields a concrete with a larger slump, so it may be placed more easily.

The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution, both fine and coarse aggregates are utilised to make up the bulk of the concrete mixture.

Regular concrete is the lay term describing concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying strength from about 10 MPa to about 40 MPa, depending on the purpose, ranging from blinding to structural concrete respectively. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.

High-strength concrete has a compressive strength generally greater than 6,000 pounds/square inch (40 MPa). High-strength concrete is made by lowering the water-cement (w/c) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.

High-performance concrete (HPC) is a relatively new term used to describe concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength.

M-15 = 1:2:4 (cement:aggregate:sand)

M-20= 1:1.5:3 (cement:aggregate:sand)

M-25= 1:1:2 (cement:aggregate:sand)

M-30 - 1:1:2 (cement:aggregate:sand)

They refer to compressive strength of cement. A cubical block made with that cement can take a load of 15 MPa, 20 MPa, 25 MPa resepctively.M stands for mix & 15,20 & 25 value stands for required compressive strength in Mpa.

  GRADES OF CONCRETE

Classification Grade Applications Ordinary M10 PCC (Plain Cement Concrete) e.g. Levelling course, bedding for footing, concrete roads etc. M15 PCC e.g. Levelling course, bedding for footing, concrete roads etc. M20 RCC (Reinforced Cement Concrete) e.g. Slabs, beams, columns, footing etc. (for mild exposure) Standard M25 RCC (Reinforced Cement Concrete) e.g. Slabs, beams, columns, footing etc. M30 RCC e.g. Slabs, beams, columns, footing etc. M35 RCC e.g. Slabs, beams, columns, footing etc. M40 RCC e.g. Pre-stressed concrete, slabs, beams, columns, footing etc. M45 RCC e.g. Runways, Concrete Roads (PQC), Prestressed Concrete Girders,beams, RCC Columns M50 RCC e.g. Runways, Concrete Roads (PQC), Prestressed Concrete Girders,beams, RCC Columns M55 RCC e.g. Prestressed Concrete Girders & Piers High Strength M60-M80 RCC work where compressive strength is required such as high rise building, long span bridges, ultra-thin white topping etc and constructions in aggressive e.g. Spilways of dams coastal construction