Civil's Guide
Durability and Exposure Classes
Durability
Aggressive Chemical Attack
Concrete placed in the ground (i.e, foundations) are susceptible to chemical attacks due to ground contamination, gas, sulphate attack and chemicals in groundwater. Due to the unpredictability of ground conditions, precaustion should be taken when designing foundations and sulphate-restsiing concrete can specified. Sulphate-resisting concrete is where the amount of tricalcium aluminate is restricted in the cement mix, which reduces the formation of sulphate ions. This reduces the chance of sulpahate attack on concrete. In addition, Special BRE digest 1 (concrete in aggressive grounds) should be read as it discuess modes of chemical attack and how to deal with these issues
Chloride Attack
Water is added to the cement which creates cement paste that coasts the aggregate and chemically reacts with cement (hydration). The volumne of water added affects the strength and workability and the lower the water-cement ratio, the stronger the mix but less workability. A balance is needed between the right mix of concrete but typical water-cement ratios are between 0.35 – 0.5.
Aggregates
Course aggregates are crushed gravel or stone, (particles greater than 5mm) and fine aggregates are sand, (particles less than 5mm). They make up the bulk of the concrete mixture (between 60% – 80%).
Reinforcement
Reinforced concrete is a composite material with steel bars added due to the weak tensile capacity of concrete. Reinforced concrete is generally designed to resist tensile stresses,cracking and failure of concrete members.
What are the Exposure classes?
Exposure classes are defined in accordance to the condition of the concrete when it’s been built. (Is the concrete located indoors or outside where it is exposed to rain and freezing).
The Eurocodes have developed a table which can be found in BS 8500 which defines each of these conditions.
| Class Designation | Class Description | Examples |
|---|---|---|
| No risk or corrosion or attack (X0 Class) | ||
| X0 | Very Dry | Unreinforced concrete completely buried in soil classed as AC‑1. Unreinforced concrete permanently submerged in non‑aggressive water. Unreinforced concrete surfaces in cyclic wet and dry conditions not subject to abrasion, freezing or chemical attack. Reinforced concrete surfaces exposed to very dry. |
| Corrosion induced by carbonation (XC Classes) (where concrete containing reinforcement or other embedded metal is exposed to air and moisture) | ||
| XC1 | Dry or permanently wet | Reinforced and prestressed concrete surfaces inside enclosed structures except voided superstructures and areas of structures with high humidity. Reinforced and prestressed concrete surface permanently submerged in non‑aggressive water |
| XC2 | Wet, rarely dry | Reinforced and prestressed concrete surfaces permanently in contact with soil not containing chlorides. |
| XC3 and XC4 | Moderate humidity or cyclic wet and dry | External reinforced and prestressed concrete surfaces sheltered from, or exposed to, direct rain. Reinforced and prestressed concrete surfaces subject to high humidity (e.g. poorly ventilated bathrooms,kitchens) Reinforced and prestressed concrete surfaces exposed to alternate wetting and drying Interior concrete surfaces of pedestrian subways not subject to de‑icing salts, voided superstructures or cellular abutments |
| Corrosion induced by chlorides other than from sea water (XD Classes) (where concrete containing reinforcement or other embedded metal is subject to contact with water including chlorides including de-icing salts, from sources other than from seawater) | ||
| XD1 | Moderate humidity | Reinforced and prestressed concrete wall and structure supports more than 10 m horizontally from a carriageway Parts of structures exposed to occasional or slight chloride conditions Bridge deck soffits more than 5 m vertically above the carriageway |
| XD2 | Wet, rarely dry | Reinforced and prestressed concrete surfaces totally immersed in water containing chlorides Buried highway structures more than 1 m below adjacent carriageway |
| XD3 | Cyclic wet and dry | Reinforced and prestressed concrete walls and structure supports within 10 m of a carriageway Bridge parapet edge beams Buried highway structures less than 1 m below carriageway level Reinforced pavements and car park slabs |
| Corrosion induced by chlorides from sea water (XS Classes) (where concrete containing reinforcement or other embedded metal is subject to contact with seawater or airborne salt origination from sea water) | ||
| XS1 | Exposed to airborne salt but not in direct contact with sea water | External reinforced and prestressed concrete surfaces in coastal areas |
| XS2 | Permanently submerged | Reinforced and prestressed concrete surfaces and completely submerged or remaining saturated, e.g. concrete below mid-tide level |
| XS3 | Tidal, splash and spray zones | Reinforced and prestressed concrete surfaces in the upper tidal zones and the splash and spray zones, including exposed soffits above sea water |
| Freeze‑thaw attack (XF classes) (where concrete is exposed to significant attack from freeze‑thaw cycles whilst wet) | ||
| XF1 | Moderate water saturation without de‑icing agent | Verticl concrete surfaces such as facades and columns exposed to rain and freezing Non‑vertical concrete surfaces not highly saturated, but exposed to freezing and to rain or water |
| XF2 | Moderate water saturation with de‑icing agent | Concrete surfaces such as parts of bridges, which would otherwise be classified as XF1, but which are exposed to de‑icing salts either directly or as spray or run‑off |
| XF3 | High water saturation without de‑icing agent | Horizontal or near horizontal concrete surfaces, which are exposed to freezing whilst wet Concrete surfaces subjected to frequent splashing with water and exposed to freezing |
| XF4 | High water saturation with de‑icing agent or sea water | Horizontal concrete surfaces, such as roads and pavements, exposed to freezing and to de-icing salts either directly or run off Concrete surfaces subjected to frequent splashing with water containing de-icing agnets and exposed to freezing |