Concrete Grades and Mix Ratios: What C15 to C30 Actually Mean
What the number means
A concrete grade is a performance specification, not a recipe. C25, or Class 25, means the concrete reaches a characteristic compressive strength of 25 megapascals at twenty-eight days, tested on standard cubes.
That distinction matters. The grade tells you what the concrete must achieve. It deliberately does not tell you how to achieve it, because the mix that gets there depends on your aggregate, your sand and your cement.
The grades and where they belong
Class 15. Blinding under foundations, mass fill, pathways. Non-structural, and it should never be carrying load.
Class 20. Ground-bearing residential slabs, light foundations, hardstanding.
Class 25. The usual structural residential grade in Kenya. Suspended slabs, columns, beams.
Class 30. Heavy foundations, transfer beams, water-retaining structures, anything with an aggressive exposure.
Nominal mix ratios, and their limits
Kenyan sites still work extensively to nominal ratios: 1:3:6 for lean work, 1:2:4 as the general structural mix, 1:1.5:3 for higher strength. Cement, sand, coarse aggregate, by volume.
They are useful shorthand and they have a real weakness. A ratio specifies the ingredients and says nothing about the outcome. The same 1:2:4 gives materially different concrete depending on how the aggregate is graded, how wet the sand was when it was gauged, and above all how much water went in.
Measuring by headpan adds error on top of that, because a headpan of damp sand and a headpan of dry sand are not the same quantity of sand.
Water is the variable that decides everything
If you take one thing from this page: strength is governed principally by the water-to-cement ratio, and adding water reduces strength roughly in proportion.
Concrete that is stiff is harder to place, so water gets added at the chute to make the day easier. It works, immediately and visibly. It also converts the Class 25 you paid for into something appreciably less, and nobody finds out until a cube test or a failure.
If a mix is too stiff to place, the correct answer is a plasticiser or a different slump specified at the plant, not a hose on site.
Why batching beats site mixing
A batching plant weighs the constituents, corrects for the moisture already in the aggregate, and records what went into the load. Site mixing gauges by volume, cannot correct for moisture, and records nothing.
For non-structural work the difference rarely matters. For anything holding a building up, the batch ticket is the evidence that what you specified is what you got, and it is the only such evidence you will ever have.