Testing - Concrete Mix Design, Quality Control and Specification - page 139

mainly of use if rejection of trucks is contemplated, but this should be
abnormal.
8. The test results should be analysed to detect, at the earliest possible
time, any departure from the previously acceptable concrete proper-
ties. This can best be done by drawing cusum graphs of early-age
and 28-day results, slump, temperature, cylinder density, 28-day pair
difference, and early-age to 28-day strength gain.
Such graphs are of substantial value not only in showing a strength
downturn quickly and obviously but also in making it much easier to
see whether the downturn is due to basic concrete quality, weather
conditions, site abuse (excessive waiting time, water addition, etc.), or
only the testing process.
9. It is very desirable to separate the functions of mix amendment and
contractual acceptance. Mix amendment should take place based on
early-age results and can be reversed without excessive cost having
being incurred if found unnecessary a few days later. It can there-
fore be done on relatively slender evidence. Contractual acceptance is
best regulated by a cash penalty or cash bonus based on a statistical
analysis of at least thirty 28-day results.
Physical rejection of hardened concrete, or even its further investi-
gation by coring and so on, should be virtually unnecessary if these
recommendations are followed. One very desirable result of a cash
penalty/bonus specification is that it avoids any need to argue about
a possible mix amendment based on slender evidence at an early
age. The decision can happily be left to the supplier, as it is his pen-
alty/bonus that is at risk rather than the structural integrity of the
concrete.
The implementation of the aforementioned principles enables excellent
control of concrete quality at very low sampling frequencies. The reduced
volume of testing easily pays for the analysis, but much larger savings are
made by the elimination of disputes, investigations, delays to program,
rejections, and so forth. Day (1989) certainly did not advocate a greater
expenditure on control by adding the cost of elaborate analysis to the cost
of the present level of testing. The proposal was rather to minimise the total
cost of a given degree of assurance of concrete of a given minimum quality.
This cost includes the necessary minimum cost of the concrete, any extra
costs imposed by restrictive specification requirements; the cost of testing;
the cost of test result analysis; and any costs imposed by failures, including
further investigation, partial demolition, legal costs, program delays, and
wasted time in meetings.
A rapid check on water content of the fresh concrete using AASHTO
T318 microwave procedure is another useful tool available to the concrete
technologist.