Testing - Concrete Mix Design, Quality Control and Specification - page 187

From one viewpoint, the strength of the concrete in the structure is what
really matters. However, even if this is accepted, we still have to consider
whether what matters is the current strength of the concrete in the struc-
ture or its eventual strength. If the requirement is to assess readiness for
early stripping or prestressing, or termination of curing protection, then the
current strength is the more important. If it is the load carrying capacity of
the structure or its durability, then the eventual strength will probably be
more significant.
If the intention is to regulate the proportions of the concrete mix cur-
rently being produced, it is equally not obvious whether the potential stan-
dard specimen strength or the current actual strength in the structure is
what matters. If considerations of eventual strength and durability in a
particular structure require a 30 MPa (4350 psi) strength but construction
efficiency requires 22 MPa (3190 psi) at 22 hours for prestressing, then
the latter requirement will clearly rule. If day-to-day temperatures vary
very widely (as they do in parts of Australia) then it could be necessary
to supply concrete of 40 MPa (5800 psi) 28-day strength one day and 60
MPa (8700 psi) 28-day strength the next. Of course it is always possible
that it is economically preferable to supply 60 MPa throughout, rather than
complicate the situation, but this option can be ignored for the purposes of
this example.
In the more usual case, a particular concrete mix will have already been
assessed as suitable for its intended purposes and testing will be undertaken
only to determine when any change takes place in that mix. In this case any
extraneous factor that affects the test result, such as variable compaction of
the test specimen, or variable temperature, either of the supplied concrete
or of the specimen during curing, will add to apparent variability and so
reduce the efficiency of the control process.
Assessing the above range of possibilities, it appears that the only case
in which NDT testing could be considered as a total replacement for
typical compression testing of standard specimens is where an early age
requirement ensures such a large excess of 28-day strength that control
of that strength is unnecessary. Even in this circumstance, standard test-
ing may still be desirable if any problems are encountered, as otherwise
it may be difficult to establish whether the problems are mix problems or
usage problems. To some extent the decision would depend on the quanti-
ties of concrete involved since the cost of control measures may be to a large
extent “per pour”, whereas the cost of providing excess strength to avoid
or reduce control is definitely per unit volume of concrete. Thus if a few
small units totaling, say, 1 cubic metre of concrete per day were involved, it
would be economical to use an excessively high strength and do little test-
ing of any kind. However, if 200 cubic metres per day were used in floor
slabs to be prestressed at an early age, both specimen testing and some form
of in situ testing would be obviously justified.