Page 175 - Riem-Vol6_nº2

346 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Is it possible to exceed the time limit specified by Brazilian Standard NBR 7212 for mixing and
transporting concrete?
recorded. A possible reason for this behavior (i.e. maintenance or
increase in compressive strength) may be the continuous agitation
of concrete during the hydration phase, which may cause the initial
hydration products (larger and more fragile) to break down.
3.2 Slump loss and superplasticizer consumption
Slump loss in concrete occurs at a specific rate that is affected
mainly by time, temperature, concrete composition and the type
of additions used (MEHTA; MONTEIRO, 2008). Table 4 lists ini-
tial slump values and their respective decrease over a period of 2
hours, when no superplasticizer was added, alongside concrete
production conditions. The loss recorded in the first two hours can
be better seen in figure 6.
As expected, for lower w/c ratios and therefore greater cement
consumption, a greater decrease in slump is observed. In situa-
tions when this decrease was the same or smaller, for different w/c
ratios, the influence of relative humidity on slump loss is notice-
able, as shown in figure 6.
After 2 hours of mixing, the superplasticizer was added to the mix
at the intervals specified above to restore slump to its initial value.
The slump measured at each time as well as increases resulting
from the addition of the superplasticizer for the laboratory samples
and the mixing plant can be seen in figures 7 and 8, respectively.
Slump behavior along the mixing interval was similar at both
production sites. However, in laboratory conditions, final slump
values show smaller differences, because it is easier to visualize
the concrete in the mixer at the moment of adding the superplas-
ticizer. It is also noticeable that slump losses tend to increase as
longer mixing periods are used. This happens because at the end
of the test, slump is more affected by the addition and less by the
water content.
The concentrations of superplasticizer used (by weight of ce-
ment) to correct slump values along the test period of 6 hours are
shown in figures 9 and 10. It should be remembered that the use
of additions such as plasticizers or superplasticizers can affect
setting times.
More superplasticizer was needed to restore slump to its initial val-
Figure 6 – Slump loss in 2 hours with the registry of relative humidity
Figure 7 – Slump and increase in slump by the addition of the superplasticizer
for concrete produced in the laboratory, during the 6 hours of mixing