Testing - Concrete Mix Design, Quality Control and Specification - page 164

at a temperature of 20°C. It can be imagined that if compression speci-
mens were stored at anywhere from 10°C to 30°C and tested at anywhere
between 10 and 40 days, poor correlation would be obtained with w/c. This
is what we are currently doing with slump tests (i.e., ignoring time and tem-
perature effects). The development in admixtures and especially workability
retainers that maintain workability for long periods without retardation
does overcome the temperature effect to some extent.
It would be quite easy to arrange for a slump value to be converted into
its equivalent value as it is entered into a computer, although less easy
to arrange for this to be available during a field acceptance test. What
becomes quite clear when these matters are considered is the absurdity of
some rejection decisions currently taken in the field. A slump of say 150
mm taken 15 minutes after batching on a cold morning may indicate a
lower water content, and therefore a stronger concrete, than a slump of 50
mm taken an hour after batching on a hot afternoon. Rules of thumb could
be developed to provide some allowance approximately for this effect with
at least more equity and realism than assuming that a slump is a slump and
that's it.
With the above points considered, adequate attention given to correct
sampling and remixing of the sample; correct bedding, cleaning and moist-
ening of a rigid metal baseplate; and use of a square mouth scoop (because
a round mouth scoop leaves mortar behind in the sampling tray) the slump
test can give more reliable guidance than is often the case. Nevertheless one
does encounter the occasional cheeky operator who asks what you would
like the slump to be before carrying out the test. Suitably instructed, such
persons are at least usually competent, since they obviously know what
causes incorrect results
7.5.2 Assessing the workability of
self-compacting concrete
Several special tests have been devised to measure the workability of SCC.
These include the U box, L box, fill box, Orimet, and J ring in addition
to rheometers, and are adequately described on the website http://www.
efnarc.org (EFNARC being a European federation dedicated to specialist
construction chemicals and concrete systems). These are essentially labora-
tory tools to be used in devising SCC mixes and are too cumbersome to be
likely to find site use except in major products.
The test likely to become the standard for site use is the slump flow test.
This test uses the current standard slump cone but, instead of measuring the
height of the cone, the diameter of spread is measured. The time for the out-
ward flow to reach a diameter of 500 mm, (20"), known as the T500 time, is
desirably also recorded. A further variant is to surround the slump cone by a
steel ring of 300 mm diameter with evenly spaced “feet” of vertical 100 mm