What Do Concrete Grades Mean?
Concrete is classified into different grades based on its compressive strength after 28 days of proper curing. In standard metric notation, the letter "M" stands for "Mix," and the following number represents the characteristic compressive strength in megapascals (MPa) or Newtons per square millimeter (N/mm²). In the US, these grades map directly to PSI (Pounds per Square Inch) ratings.
Standard Concrete Grades and Mix Ratios
| Grade | Volumetric Ratio (Cement : Sand : Stone) | Approximate Strength (28-day) | Common Applications |
|---|---|---|---|
| M15 | 1 : 2 : 4 | 2,200 PSI | Walkways, pathways, fence posts, non-structural slabs. |
| M20 | 1 : 1.5 : 3 | 3,000 PSI | Residential driveways, garage slabs, foundation footings. |
| M25 | 1 : 1 : 2 | 3,600 PSI | Columns, heavy beams, retaining walls, suspended slabs. |
The Dry Volume Shrinkage Factor (1.54)
When estimating raw ingredients (cement, sand, aggregate) to mix concrete manually, you cannot simply add the volume shares of the dry ingredients. When water is added, the cement particles dissolve and fill the micro-voids between the sand grains, which in turn fill the voids between the larger stones. This compaction reduces the overall volume of the mixture.
To produce 1 cubic meter of wet concrete, you need approximately **1.54 cubic meters of dry ingredients**. This is known as the **Dry Volume Factor**.