Page 171 - Riem-Vol6_nº2

342 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?
mixing times might result in more dramatic changes in concrete
properties.
Concrete slump was measured according to Brazilian standard
NBR NM 67 (1998) to determine fresh concrete workability. The
test program attempted to maintain the workability of concrete
by adding a polycarboxylate superplasticizer at specific intervals
along a period of 6 hours after concrete mixing started, which is
when cement particles first come into contact with water.
In laboratory conditions, the materials were mixed for five min-
utes to ensure materials were thoroughly mixed before mea-
suring concrete workability. Slump was then measured at the
intervals of 120 min (2h), 180 min (3h), 240 min (4h), 300 min
(5h) and 360 min (6h) along rest (15 minutes) and agitation
cycles (5 minutes). After each measurement, the superplasti-
cizer was added to restore slump to its original value (120±20
mm) and test specimens (TS) were cast for each time interval.
It should be noted that the time intervals for the slump test
checks in the mixer trucks are slightly different from the inter-
vals used in the laboratory mixer. This was necessary because
the drums in mixer trucks must be kept revolving. In order to
simulate real life construction site conditions, the drum was
kept at 2 rpm during the rest intervals. Just before measuring
slump, drum speed was increased to 16 rpm. The process of
restoring slump can be seen in figure 2.
and in real life conditions in a ready-mixed concrete plant for all
control variables selected, resulting in a total of 30 compositions,
as shown in figure 1.
The cement type used is readily available from mixing plants in
the city of Porto Alegre and surrounding areas, which includes
the plant in which the laboratory tests were reproduced in real
life production. The three w/c ratios used were selected be-
cause they correspond to different cement consumption thresh-
olds and therefore provide a better assessment of compressive
strength behavior in three different levels. As it was necessary
reproduce the test results in real life conditions, the concentra-
tions used followed the patterns used at the mixing plant (ta-
ble 2), characterized by the amounts of cement, natural sand,
crushed aggregate #0, crushed aggregate #1, water and super-
plasticizer admixture. The coarse aggregate consists of 85%
crushed aggregate 19mm and 15% crushed aggregate 9.5mm.
The concentration of superplasticizer was 0.6% by weight of ce-
ment and slump was set at 120±20mm.
Tests were carried out in freshly mixed concrete, after 6 hours of
mixing and at intervals in between these limits to provide a bet-
ter picture of concrete strength behavior. Two-hour intervals were
used. The test program allowed a fifth sample to be tested so an
interval of 5 hours of mixing was used (instead of 3 hours, which
was another possible sampling interval) as it was felt that longer
Figure 1 – Combinations between the controllable variables of research
Table 2 – Dosages of the concretes used in this study
Mix proportion
1 : m
Unit mix proportion,
in mass
Mortar
level (%)
w/c
ratio
3 Amount of materials per m (kg)
Cement Water Fine
aggregate
Coarse
aggregate
1 : 3,2
1 : 4,5
1 : 6,2
514
395
300
206
205
204
660
776
869
1000
997
996
1 : 1,28 : 1,95
1 : 1,97 : 2,53
1 : 2,90 : 3,32
54
54
54
0,40
0,52
0,68