Concrete Class A vs B vs C: Choosing the Right Mix

Philippine concrete estimating typically groups mixes into three classes — A, B, and C — each defined by a cement-to-sand-to-gravel ratio that determines both the strength of the finished concrete and the material quantities in your take-off. Getting the class right isn't just a specification detail; it directly changes how many cement bags and how much sand and gravel a given volume of concrete actually requires.

The Three Classes, By the Numbers

Per cubic meter of concrete, standard mix design tables call for approximately:

Class A — 9.00 bags of cement, 0.50 m³ sand, 1.00 m³ gravel. The richest of the three, used for structural elements carrying significant load: columns, beams, and elevated slabs.

Class B — 7.50 bags of cement, 0.50 m³ sand, 1.00 m³ gravel. A step down in cement content, common for slabs on grade and lighter structural work.

Class C — 6.00 bags of cement, 0.50 m³ sand, 1.00 m³ gravel. The leanest mix, typically reserved for non-structural fill, lean concrete beds, or blinding layers under footings.

Why This Matters for Your Estimate

Specifying Class A where Class C would do inflates your material cost for no structural benefit — cement is consistently the most expensive component per cubic meter. Specifying Class C where Class A is actually required is a structural problem, not just a cost one. The class should follow the structural engineer's design, not the estimator's assumption, but knowing the ratios lets you sanity-check a BOQ at a glance: if a column line item is priced at Class C rates, that's worth a second look before the estimate goes out.

Worked Example: 10 Cubic Meters of Each Class

For 10 m³ of concrete, the three classes require noticeably different cement quantities while sand and gravel stay fixed. Class A needs 90 bags of cement, 5.0 m³ of sand, and 10.0 m³ of gravel. Class B needs 75 bags of cement for the same 5.0 m³ of sand and 10.0 m³ of gravel. Class C needs just 60 bags of cement, again against the same 5.0 m³ of sand and 10.0 m³ of gravel. At a typical cement price, the gap between specifying Class A and Class C on that same 10 m³ pour is 30 bags of cement — cost that either belongs on the estimate because the structural design genuinely requires it, or doesn't belong there at all.

Common Mistakes

The most common estimating error isn't picking the wrong class outright — it's letting one class default across an entire BOQ because that's what a previous project used, without checking whether the current structural drawings actually call for something different element by element. A slab-on-grade and an elevated beam on the same project can legitimately call for different classes, and a take-off that applies one blanket class to every concrete line item will be wrong for at least some of them. The second common mistake is reading "Class A" as simply "the strongest, safest option" and defaulting to it out of caution — which quietly inflates cement cost on every non-structural pour (blinding layers, lean concrete beds, non-structural fill) where a leaner mix would perform identically for a fraction of the cement. Class selection should always trace back to the structural engineer's specification, not the estimator's risk tolerance.

Quick Reference

Per m³: Class A — 9.00 bags cement / 0.50 m³ sand / 1.00 m³ gravel. Class B — 7.50 / 0.50 / 1.00. Class C — 6.00 / 0.50 / 1.00. Sand and gravel are identical across all three; cement content is the only variable, and it's the one figure worth double-checking against the structural notes before a BOQ goes out.