Concrete is a mixture of various materials including cement, water and aggregates; other ingredients may be added to improve performance. Depending on the percentage of each material used during the preparation of the product, different properties will be achieved.
Concrete is divided into different classes, according to the characteristic compressive strength of standardised specimens, cured for 28 days under specific temperature and humidity conditions.
The classification is defined in the Eurocode, taken from EN 1992-1-1:2010 (Eurocode 2: “Design of Concrete Structures. Part 1-1: General Rules and Rules for Buildings); it consists of a capital letter C followed by the ratio between two quantities:
- The first is the compressive strength in N/mm2 in a cylindrical specimen (150 mm in diameter and 200 mm in height).
- The second is the compressive strength in N/mm2 in a cubic specimen (150 mm long sides).
The Spanish Structural Code classifies concrete types according to the characteristic compressive strength of cylindrical specimens (150 m in diameter, 200 mm in height) cured for 28 days, distinguishing mass, reinforced and prestressed concrete.
The different heavy and normal hardness concrete classes, according to the aforementioned European standard, are detailed in the following table:
| Compressive strength class | Minimum characteristic strength in cylindrical specimen fck,cyl | Minimum characteristic strength in cubic specimen fck,cub | Classification of mass concrete in the Spanish Structural Code | Classification of reinforced concrete in the Spanish Structural Code |
|---|---|---|---|---|
| C8/10 | 8 | 10 | — | — |
| C12/15 | 12 | 15 | — | — |
| C16/20 | 16 | 20 | — | — |
| C20/25 | 20 | 25 | HM20 | HA20 |
| C25/30 | 25 | 30 | HM25 | HA25 |
| C30/37 | 30 | 37 | HM30 | HA30 |
| C35/45 | 35 | 45 | HM35 | HA35 |
| C40/50 | 40 | 50 | HM40 | HA40 |
| C45/55 | 45 | 55 | HM45 | HA45 |
| C50/60 | 50 | 60 | HM50 | HA50 |
| C55/67 | 55 | 67 | HM55 | HA55 |
| C60/75 | 60 | 75 | HM60 | HA60 |
| C70/85 | 70 | 85 | HM70 | HA70 |
| C80/95 | 80 | 95 | HM80 | HA80 |
| C90/105 | 90 | 105 | HM90 | HA90 |
| C100/115 | 100 | 115 | HM100 | HA100 |
It should be noted that there are many variables that affect the hardening stage of concrete, making it difficult to know its exact strength, so this value will depend on the number of days elapsed from the date of manufacture to the time of testing (the less time, the less strength and vice versa).
For all these reasons, the Spanish Structural Code includes an empirical equation to calculate the strength of concrete, and offer estimated values regarding an increase or decrease in strength compared to the strength after 28 days:
| Age of concrete [days] | 3 | 7 | 28 | 90 | 365 |
|---|---|---|---|---|---|
| Normal-hardening concrete | 0,46 | 0,68 | 1,00 | 1,18 | 1,32 |
| Fast-hardening concrete | 0,60 | 0,78 | 1,00 | 1,12 | 1,20 |
This means, for example, that fast-hardening concrete at 7 days will have a 22% lower strength than it would have at 28 days.
Latest revision: FAQ12 rev0
