The delivery note of a concrete truck carries a line like this: C28/35, XC2, S4, Dmax 22, CEM II/A-LL 42.5 R. In a few codes the producer declares the strength, the exposure class, the workability, the maximum aggregate size and the type of cement. The first code is the most quoted and also the most misunderstood, because it holds two numbers and because designer, laboratory and standard do not always use the same one.
What the two numbers mean
In C28/35 the first number is the characteristic compressive strength measured on cylindrical specimens (150 mm diameter, 300 mm height) at 28 days, in N/mm2. The second is the same quantity measured on cubes with a 150 mm side. It is called characteristic because it is the 5 per cent fractile: only one result in twenty is expected to fall below that value. The cylinder value is written fck, the cube value Rck. The Italian building code (NTC 2018) uses Rck for site acceptance control and fck for design checks.
The cube gives a higher number than the cylinder because the platens of the testing machine hold the specimen laterally and confine it. In design checks the NTC use fck = 0.83 Rck. The standard classes, however, are tabulated with rounded values and the actual ratio between the two numbers ranges from 0.78 to 0.83, so the two numbers of a class should never be derived from one another by division: they are read from the EN 206 table.
From characteristic strength to design values
To size a member you need the design strength, obtained by dividing the characteristic one by a partial safety factor: fcd = αcc fck / γc, with αcc = 0.85 for long-term effects and γc = 1.5. For a C25/30 this gives fcd = 0.85 x 25 / 1.5 = 14.2 N/mm2. This is the value that enters the ultimate limit state checks, which is why the strength written on the delivery note never coincides with the one used for design.
The other parameters derive from fck. The mean compressive strength is fcm = fck + 8 N/mm2. For classes up to C50/60 the mean tensile strength is fctm = 0.30 fck2/3 and the secant modulus of elasticity is Ecm = 22,000 (fcm/10)0.3, with fcm in N/mm2. The ultimate compressive strain is 3.5 per mille.
| Class | fck (N/mm2) | Rck (N/mm2) | fcm (N/mm2) | fctm (N/mm2) | Ecm (GPa) | fcd (N/mm2) |
|---|---|---|---|---|---|---|
| C20/25 | 20 | 25 | 28 | 2.2 | 30 | 11.3 |
| C25/30 | 25 | 30 | 33 | 2.6 | 31 | 14.2 |
| C28/35 | 28 | 35 | 36 | 2.8 | 32 | 15.9 |
| C30/37 | 30 | 37 | 38 | 2.9 | 33 | 17.0 |
| C32/40 | 32 | 40 | 40 | 3.0 | 33 | 18.1 |
| C35/45 | 35 | 45 | 43 | 3.2 | 34 | 19.8 |
| C40/50 | 40 | 50 | 48 | 3.5 | 35 | 22.7 |
| C45/55 | 45 | 55 | 53 | 3.8 | 36 | 25.5 |
| C50/60 | 50 | 60 | 58 | 4.1 | 37 | 28.3 |
For reinforced concrete structures the NTC 2018 set a minimum of C16/20, and C28/35 for prestressed concrete; in practice durability almost always leads to at least C25/30. Above C45/55 we speak of high-strength concrete, which requires preliminary studies and stricter controls.
Which class to use, and where
The class is not chosen from structural calculation alone. EN 206 and, in Italy, UNI 11104 link the exposure environment to a maximum water/cement ratio, a minimum cement content and a minimum strength class. A column in a dry building (XC1) can stay at C25/30. The same column in a car park where de-icing salts get in (XD3) requires at least a C35/45. Very often it is durability, not statics, that sets the class.
| Class | Typical environment | Max w/c | Minimum class | Min cement (kg/m3) |
|---|---|---|---|---|
| XC1 | dry or permanently wet interiors | 0.60 | C25/30 | 280 |
| XC2 | foundations, wet and rarely dry | 0.60 | C25/30 | 280 |
| XC3 | moderate humidity, exteriors sheltered from rain | 0.55 | C30/37 | 300 |
| XC4 | exteriors with wet and dry cycles | 0.50 | C32/40 | 320 |
| XD1 | airborne non-marine chlorides, moderate humidity | 0.55 | C30/37 | 300 |
| XD3 | chlorides with wet and dry cycles (bridges, car parks) | 0.45 | C35/45 | 350 |
| XS1 | salt-laden air, near the coast | 0.50 | C32/40 | 320 |
| XS3 | tidal and splash zone | 0.45 | C35/45 | 350 |
How it is checked on site
Acceptance control is carried out on cube specimens taken at the time of casting. In type A control under the NTC 2018, which refers to no more than 300 m3 of homogeneous mix, three samples of two specimens each are taken. The lowest value among the three samples must not be less than Rck minus 3.5 N/mm2, and the mean of the three must exceed Rck by at least 3.5 N/mm2. Type B control, required for pours of homogeneous mix above 1,500 m3, applies instead a statistical criterion based on the standard deviation.
The laboratory result describes the specimen, not the structure. In-place concrete can turn out weaker because of insufficient compaction, segregation or missing curing. On an existing building, cores are taken and the results interpreted with EN 13791, often alongside indirect tests such as the rebound hammer and ultrasonic testing, which are not enough on their own.