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Code Changes
Durability Chapter of ACI 318 Building Code Streamlined
Since 1989, the ACI 318 Building Code has emphasized the importance
of durability at the design stage by having all durability requirements
in one stand-alone location, Chapter 4. Since that time, and through
the most recent version in 2005, the code and accompanying commentary
explicitly alert the concrete designer that durability requirements
must be considered before designing a concrete mix conforming with
the provisions of Chapter 5. However, the requirements for durability
in ACI 318-05 are cumbersome to follow and sometimes misinterpreted.
For the 2008 edition of Building Code Requirements for Structural
Concrete, ACI Committee 318 has proposed a significant restructuring
of the durability chapter. Without significantly changing the current
prescriptive requirements, ACI 318-05 has been modified to include
exposure classes based on the existing requirements. Not only does
this improve clarity and flow of the durability provisions, it simplifies
how a specification can be written. Table 1 shows the four main
durability-related exposure categories which will now be covered
in ACI 318-08: freezing and thawing, sulfates, corrosion, and permeability.
Table 1: ACI 318-08 Exposure Categories
| F |
| Freezing and Thawing |
 |
|
F0 (Not applicable) – for
concrete not exposed to cycles of freezing and thawing |
| F1 (Moderate) – Concrete exposed to
freezing and thawing cycles and occasional exposure to moisture
(and no deicing chemicals) |
| F2 (Severe) – Concrete exposed to freezing
and thawing cycles and in continuous contact with moisture |
| F3 (Very Severe) – Concrete exposed
to freezing and thawing cycles that will be in continuous contact
with moisture and exposure to deicing chemicals |
| S |
| Sulfates |
 |
|
S0 (Not applicable)
– Soil: SO4 <0.10%
– Water: SO4 <150 ppm |
S1 (Moderate)
– Soil: 0.10% ≤
SO4 < 0.20%
– Water: 150 ppm ≤ SO4 <1500 ppm (and Seawater) |
S2 (Severe)
– Soil: 0.20% ≤ SO4 < 2.0%
– Water: 1500 ppm ≤ SO4 <10,000 ppm |
S3 (Very severe)
– Soil: SO4 > 2.0%
– Water: SO4 >10,000 ppm |
| C |
| Corrosion |
 |
|
C0 (Not applicable) - Concrete
that will be dry or protected from moisture in service |
| C1 (Moderate) - Concrete exposed
to moisture but not to an external source of chlorides in service |
| C2 (Severe) - Concrete exposed to moisture
and an external source of chlorides in service |
| P |
| Permeability |
 |
|
P0 (Not applicable) - Concrete
where low permeability to water is not required |
| P1 - Concrete required to have low permeability
to water |
Refer to Lobo
(2007) for more information on the four categories (F, S, C,
and P), subcategories, and their respective concrete requirements.
In essence, the designer has to select the relevant exposures for
each component of the concrete structure, and pick the one that
requires the greatest resistance in terms of:
- the lowest water-to-cement ratio
- the highest minimum concrete strength
- other requirements as given in ACI 318
Figure 1 shows an example of different exposure classes for individual
elements of a model structure.
Additional durability concerns specific to a project (for example
abrasion or alkali-aggregate reactivity) also need to be addressed
by the design professional. There is adequate guidance on this in
ACI 201.2R-01 Guide to Durable Concrete, PCA’s Specifier’s
Guide to Durable Concrete and other documents.
Fig. 1 Example of Durability Exposure Categories for Elements of
a Concrete Structure
International Standards Related to Concrete
Durability
The exposure class approach is essentially the method used by most
other international codes to address concrete durability. In specifications
used in Australia, New Zealand, South Africa, the European Union,
and Canada, exposure classes are defined for specific regional durability
concerns. Only the Australian, New Zealand, and Canadian standards,
however, have definitive provisions for specifying on a performance
basis. Bickley
et al. (2006) contains an extensive review of current international
specifications related to durability performance requirements.
References:
Bickley, J. A.; Hooton, R. D.; and Hover, K. C., “Preparation
of a Performance-Based Specification for Cast-in-Place Concrete,”
RMC Research Foundation, January 2006, 155 pages
Lobo, C., “New
Perspective on Concrete Durability,” Concrete infocus,
National Ready Mixed Concrete Association, Silver Spring, Maryland,
Spring 2007, pages 24 to 30.
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