Civil Engineering Reference
In-Depth Information
Advice to specifiers
The old adage that no one is more difficult to teach than those who are
convinced they already know it all is nowhere more apparent than in the
specification and control of concrete quality. It explains why two of the
world's most respected sources of knowledge about concrete—American
Concrete Institute (ACI) and United Kingdom/Europe—are lagging almost
30 years behind developments in Australia.
1
Structural designers in concrete are certainly expected and entitled to
specify the properties they have assumed in their design, including such
items as strength, shrinkage, and resistance to anticipated sources of dete-
rioration. Unfortunately few structural designers are also expert concrete
technologists and may be reluctant to admit this. There is a tendency to
assume that if you do not have much detailed knowledge of a subject, then
there is not much to know about it.
1.1 MIX SELECTION
Existing codes accept that concrete strength tends to be a normally dis-
tributed variable and therefore needs to be considered in terms of mean
strength and standard deviation rather than an absolute limit. Instances of
lower than specified strength should be dealt with by analysing a number
of recent results to determine whether the low result constitutes a genuine
downturn or an isolated statistical aberration. An investigation by coring
or otherwise may be undertaken to confirm the diagnosis.
Although early age strengths may be determined, correction normally
tends to be based on a number of 28day results. In the case of the United
Kingdom a technique known as a V-mask may be applied to a cusum graph
of strength. A CUSUM graph (see Chapter 10) is a more sensitive detector
of change and a V-mask automatically applies a precise statistical test to
detect a significant downturn. There are actually two basic requirements
