The prescriptive approach to durability in EN 206-1 maintains the durability grade principle of Deacon and Dewar (1982) that underpins the advice in concrete design standards such as those published in the UK since the 1980s. This approach is based on the principle that a clear relationship exists between durability expectation and minimum concrete grade, minimum binder content and maximum water/binder ratio. Exposure classifications based on a range of deterioration mechanisms are used and these are further sub-divided to take account of varying degrees of severity in each major class. The subclasses relevant to freeze/thaw attack are presented in Table 8.3. The recommended limiting values of concrete composition and properties are presented in Table F.1 of the Standard, which is generally adapted in locally determined national annexes.
The recommended limiting values are based on the following assumptions:
· an intended working life of 50 years
· concrete made with cement type CEM I
· maximum nominal upper size of aggregate in the range of 20 to 32 mm
· cover to reinforcement in compliance with the minimum requirements of European design standard EN 1992-1-1:2004 (Eurocode 2).
Research on deterioration rates is being conducted in many institutes to better inform requirements for service lives in excess of 50 years. However, in the case of freeze-thaw resistance, Hobbs et al. (1998) argue that the limiting values for a 100-year life should be the same as for 50 years. This is due to the event- dependence of the process and the fact that concrete capable of resisting freeze- thaw events on an on-going basis should be durable, irrespective of age.
Section 4 of Eurocode 2 is concerned with durability and cover to reinforcement. It is harmonised with EN206-1 in respect of exposure classes. Although a clear trade-off table between cover and strength, a feature of earlier codes, does not appear the concept survives in the form of different values for different `structural classes’. The recommended structural class (design working life of 50 years) is `Structural Class S4′ but this aspect is not very significant in the case of freeze-thaw, since it is the quality of the pore structure of the concrete that determines durability in cold climates.
The set of four exposure subclasses differentiated in respect of freeze-thaw attack, designated XF1, XF2, XF3, and XF4, is based on a matrix of two degrees of saturation (`moderate’ and `high’) and the absence or presence of deicing agent or seawater. Harrison (2000) explained that CEN intended the terms `moderate saturation’ and `high saturation’ to imply a moderate and high risk of damage, respectively. However, Hobbs et al. (1998) noted that the differentiation is not entirely clear and could be on the basis of a lower number of freeze-thaw cycles per annum or a lower likelihood of freeze-thaw events whilst saturated, when distinguishing the exposure classes XF1 and XF2 from XF3 and XF4.
Control of the pore distribution and permeability of the concrete is approached in one of two ways. One approach involves keeping the free water content so low that the amount of expansion will not be deleterious. This is achieved by minimising the permeability and porosity of the concrete. The second approach involves increasing the porosity of the concrete in a controlled manner through air entrainment so that the pores can act as safety valves, providing space for the freezing water to expand without stressing the concrete. In either case the amount of free water in the concrete is minimised by specifi- cation of moderately low water/cement ratio concrete. This prevents inclusion of excessive amounts of mix water and limits ingress from external sources during service.
The exposure classes in EN 206-1 take account of the degree of saturation and the presence, if any, of external sources of salt from deicing agents and seawater. As described in Section 8.4 the presence of salt increases the risk of damage due to the detrimental influence of solute concentration gradient.
The informative annex in EN 206-1 presents indicative limits for maximum water/cement ratio, minimum cement content, minimum strength class, minimum air content (except for class XF1) and a requirement for freeze-thaw resisting aggregates for each exposure condition, as presented in Table 8.4. It cannot be over-emphasised that these are informative values only, based on the mean of values representative of European practice; specifiers must consult the appropriate advice on limiting values in national annexes or complementary standards valid in the place of use of the concrete. For example, the informative annex to EN 206-1 shows a requirement for freeze-thaw resisting aggregates in accordance with the recommendations of the harmonised aggregate standard. Unanimity on this requirement is not apparent in national annexes and complementary standards in respect of exposure class XF1. Exposure class XF1 covers the case of concrete exposed to significant attack by freeze-thaw cycles whilst wet and having `moderate water saturation’. Examples include the vertical surfaces of members exposed to rain and freezing.
Exposure class XF2 covers concrete exposed to significant attack by freeze- thaw cycles whilst wet, having `moderate water saturation’ in the presence of deicing agent. Examples therefore include the vertical surfaces of road structures exposed to freezing and airborne deicing agents. The values for class XF2 in the informative table in Annex F of EN 206-1 indicate typical requirements that are the same as in class XF1 but with the inclusion of a minimum air content of four per cent and consequently the minimum strength class is lower. A higher minimum strength class is also implied as an alternative to an air-entrained concrete, through performance testing. Experience in some countries indicates that the use of non-air-entrained mixes of minimum strength class C40/50 can provide adequate resistance.
Exposure class XF3 covers concrete exposed to significant attack by freeze- thaw cycles whilst wet and described as having `high water saturation’. Examples of concrete in this context include horizontal surfaces of members exposed to rain and freezing.
Exposure class XF4 covers the case of concrete exposed to significant attack by freeze-thaw cycles whilst wet and having `high water saturation’ in the presence of deicing agent or seawater. Examples of concrete covered by class XF4 include bridge decks, surfaces exposed to spray containing deicing agents, and surfaces in the splash zone of marine structures. Some initial difficulty may arise in categorising the exposure of an element in a highway structures as either class XF2 or XF4. If the consequences of failure are significant, in terms of access for repair or threat to passing traffic, it may be prudent to opt for XF4, the severest class.
