Page 173 - Riem-Vol6_nº2

344 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?
3. Results and discussion
3.1 Compressive strength
Polesello (2012) recorded all individual results for compressive
strength as well as standard deviation values and their corre-
sponding coefficients of variance. Figures 3 and 4 illustrate the
mean values of compressive strength for concrete sampled in the
laboratory and in the mixing plant, respectively.
Results show that even when a concrete sample is mixed for a
period of up to 6 hours, using a superplasticizer as described
in this study, its mean compressive strength value at 28 days
is maintained. To check for the influence of w/c ratios, produc-
tion site and mixing periods, as well as for interactions of these
variables on the results, individual values were checked using
analysis of variance (ANOVA). The results of this analysis are
shown in table 3.
ANOVA results show a statistically significant influence for the
three control variables (w/c ratio, production site and mixing time)
on compressive strength at 28 days. The isolated behavior of each
of these variables is shown in figure 5.
Figure 5a presents the effect of the w/c ratio in isolation. It shows
that compressive strength values decrease as the w/c ratios in-
crease, a confirmation of conventional wisdom in the technical
field. Mixing times display a significant effect as illustrated in fig-
ure 5c. The mean results in the 5-hour interval show a slight drop
in compressive strength followed by an increase. After 6 hours,
compressive strength was identical to the one in freshly mixed con-
crete. With reference to the statistical significance of the produc-
tion site factor, this could be explained by the fact that the volume
of concrete mixed in real-life conditions is much larger than in a
laboratory, which results in differences in evaporation and mixing
processes.
The fact that the mean compressive strength at 28 days was main-
tained in concrete mixed for a period of 6 hours could be explained
by the loss of water to the environment, which would cause w/c
ratios in the mixture to drop. Kirca et al (2002) recorded this loss of
water in their study, and found an increase in compressive strength
in two concrete compositions of different classes, C25 and C35,
Figure 4 – Average results of compressive strength at 28 days of concrete produced in ready-mixed concrete plant
Table 3 – ANOVA results for compressive strength at 28 days
Font SQ MQ Test F Prob. Significance GDL
A: w/c ratio
B: production site
C: mixing time
AB
AC
BC
ABC
Error
Total
7745,70
15,08
36,01
1,34
29,29
21,60
8,76
174,31
8032,08
3872,85
15,08
9,00
0,67
3,66
5,40
1,10
2,91
–
1333,10
5,19
3,10
0,23
1,26
1,86
0,38
–
–
0,00%
2,51%
1,95%
79,41%
27,45%
12,48%
93,03%
–
–
S
S
S
NS
NS
NS
NS
–
–
2
1
4
2
8
4
8
60
89
SQ - sum square; GDL - degree of freedom; MQ - averae square; Test F - calculated value of F.; Prob. - significance level associated with the
calculated value of F.; S - significant; NS - not significant.