What Are Concrete Classes?
Concrete classes (also called strength classes) describe the compressive strength of concrete after 28 days of curing. The designation follows the pattern CXX/YY, where XX is the cylinder compressive strength and YY is the cube compressive strength in N/mm2 (MPa).
The classes are standardized in DIN EN 206 and DIN 1045-2. Four classes are relevant for home use: C12/15, C20/25, C25/30 and C30/37. Higher classes (C35/45 to C100/115) are used in professional building and bridge construction.
C12/15: Lean Concrete for Simple Tasks
C12/15 is the lowest strength class for load-bearing concrete. It is also called lean concrete because it contains relatively little cement (approx. 250 kg/m3). The mixing ratio is 1:4:6 (cement:sand:gravel) with a w/c ratio of 0.60.
Typical applications: Blinding layers under foundations, curb bedding, setting posts in concrete, fill concrete between stones. C12/15 is not approved for structural members.
Cost: C12/15 is the cheapest concrete due to less cement. Per cubic meter self-mixed: approx. 50-65 EUR. A 40 kg bag of premix yields approx. 0.015 m3.
C20/25: The All-Rounder for Home and Garden
C20/25 is the standard class for most private construction projects. With 300 kg cement per cubic meter and a mixing ratio of 1:3:5, it offers a good balance of strength and economy.
Typical applications: Strip foundations, floor slabs for garages and terraces, retaining walls, stair steps, concrete paving. C20/25 is the minimum class for reinforced concrete in residential construction.
Processing: C20/25 handles well by hand or mixer. After placing, keep moist for at least 24 hours (cover with plastic or water) to prevent drying cracks.
C25/30: For More Heavily Loaded Foundations
C25/30 is used when higher strength requirements apply. Cement content rises to 350 kg/m3, the mixing ratio is 1:2.5:4 and the w/c ratio drops to 0.50.
Typical applications: Foundations for single-family homes, basement walls, retaining walls with earth pressure, heavy-duty floor slabs. C25/30 is the standard class for engineering structures.
Processing: Due to higher cement content, C25/30 is somewhat stiffer than C20/25. Aftercare (keeping moist) is even more important as the higher cement content generates more hydration heat and increases the risk of early shrinkage cracks.
C30/37: Heavy-Duty Concrete
C30/37 is the highest class still practical for self-building. With 400 kg cement per cubic meter and a mixing ratio of 1:2:3.5, it achieves very high compressive strength.
Typical applications: Underground parking decks, industrial floors, heavy-duty foundations, machine anchoring. Rarely needed in home building except for special loads.
Important: C30/37 requires precise work. The low w/c ratio of 0.45 makes the concrete stiff and hard to compact. Without a vibrator (poker vibrator), voids and honeycombing form that drastically reduce strength.
Premix vs. Self-Mixing: What Pays Off?
Premix concrete from the hardware store (bagged, usually 40 kg) is convenient but expensive. Per cubic meter you need approximately 67 bags at 40 kg, costing 130-180 EUR. Self-mixing with cement, sand and gravel costs 50-80 EUR per cubic meter but requires a concrete mixer and more time.
From 0.5 m3 upward, self-mixing almost always pays off. For small quantities (under 50 liters), premix is more practical since you do not need to store three different aggregates.
Tip: For larger quantities (from 3-5 m3), order a ready-mix concrete truck. The concrete arrives pre-mixed in the correct consistency and strength class. Costs are 80-120 EUR/m3 plus delivery fee.
Curing: What Happens in 28 Days?
Concrete reaches its nominal strength after 28 days of curing (hydration). This process is not linear: after 3 days approximately 50% of final strength is reached, after 7 days approximately 70%, after 14 days approximately 85% and after 28 days full nominal strength.
Curing only proceeds with sufficient moisture and above 5 degrees Celsius. In frost, hydration stops completely. Freshly placed concrete must therefore be protected from frost (with insulation mats or radiant heaters).
Walkable after 24 hours. Load-bearing (e.g., car traffic) after 7 days. Fully cured and load-bearing after 28 days. These times apply at 20 degrees Celsius -- at cooler temperatures, proportionally longer.