Specification - Concrete Mix Design, Quality Control and Specification - page 110

all possible materials. However this would require the specifier to assume
responsibility for the resulting concrete and would contribute to “dumbing
down” the industry and keeping in business less technically competent
producers. It would also be likely to substantially increase the cost of the
concrete, since a high variability must be assumed, necessitating a large
margin over the minimum requirement, having financially disadvantaged
the more competent producers. There are certain situations where such a
prescriptive specification may be the best option, such as where the local
producer has no experience of achieving the required performance, and
the precautions that he would add to the concrete mix would be excessive.
However, even in this situation, including a performance-based specification
as an alternative to the prescriptive one would help encourage producers to
develop appropriate mixes and knowledge.
In the past, the concrete industry generally focused on compressive
strength. However there are legitimate reasons for specifying more than just
a minimum strength. It may be useful to specify a number of requirements
in particular cases:
1. A peak temperature and maximum temperature differential to limit
thermal cracking or potential delayed ettringite formation
2. A test for reactive aggregates where aggregates without a proven
record are considered
3. An air content and spacing factor for freeze-thaw resistance
4. An early strength required for stripping, prestressing, depropping,
and so on
5. A drying shrinkage limit
6. A requirement for self-consolidation or extreme pumpability
7. A penetrability test limit for either water penetration or durability in
aggressive conditions
8. Maximum crack width for water-retaining structures
9. A bleeding limit, especially where a good off-form finish is required
10. Segregation resistance; it could be specified that the concrete shall not
display any tendency to segregation at the proposed workability
11. Abrasion resistance
The problem is that some of these performance goals may be difficult to
establish by test at an early age. Some of the test procedures are complex
and costly. In addition, the statistical variability of many parameters may
not be known and therefore it can be hard to apply the concept of a char-
acteristic performance. Some performance requirements oppose others.
The following discussion considers the practical issues associated with
performance specification to achieve particular engineering goals. However,
having established an acceptable mix, variation from it can be detected by
an early-age strength test, even though strength may not be what matters.