classes of concrete

by philemon
(Nairobi Kenya)

When going through building plans I encounter construction jargon where I am instructed to cast class 25, 20 or 30 of concrete. What are these classes and how do I determine them where not given?

The grade 15, 25, 30 means the concrete compression resistance(or strength) at 28 days. It is done in kg/cm2. Usually for most of the qualities of Portland Cement, sand, and gravel, the proportion is done in terms of volume: 1:2:3 for 1 cement sack volume: 2 sand volume: 3 gravel volume.

This proportion variable depends of the size of the aggregates, physical characteristics, and quality of cement and water used. However, in most cases for small projects it is a practice to use 1:2:3 for grade 30; 1:2:4 for grade 25; and 1:2:5 for grade 15.

If the class of concrete is not determined for a specific project it may require you to hire an engineer to properly determine the correct class (strength) of concrete needed.