Page 29 - Riem-Vol6_nº2

200 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Ready mixed concrete: variability analysis of the compressive strength and physical properties along
the unloading of the truck mixer
of ± 2 cm, with some exceptions, and in addition there is a greater
number of occurrence of points P1 and P2 with greater consis-
tency than P3 and the converse for the points P4 and P5, which
typically have a consistency smaller than the midpoint. This is due,
probably, to the time of testing of each point collected, seeing that,
as previously mentioned, the samples were used in the same order
of collection (point 1 to 5), so the starting point was tested first, fol-
lowed by second, third, fourth, and last, which remained a longer
time stored and had greater mixing time.
Table 4 shows a summary of the variations in coarse aggregate
content of each point relative to the midpoint. Analyzing the per-
centage of occurrences for each zone, is verified that for the points
P1 and P5 there is a higher concentration of coarse aggregate
that the midpoint, centered in the range of up to +5%. The higher
coarse aggregate content at the starting point corroborates the
statements of HELENE [19] and RECENA [20] that there is a con-
centration of coarse aggregate in the beginning of the discharge.
From the results of the compressive strength of the specimens
and the coarse aggregate content were realized statical analyses
to determine the statistical significance of variations between lots
of the same dosage and the influence of the different sampling
points on the compressive strength of the concrete. In table 5 and
6 shows the analysis of variance for the compressive strength,
considering three results each point, and the coarse aggregate
content, respectively.
Based on analysis of variance of the compressive strength, was
found a significant difference between batches and not significant
between points, to a confidence level of 95%, i.e., there is variation
in compressive strength according to the batch analyzed, while to
the collection point there isn’t interference if the point is taken at
the beginning, middle or end of the discharge. For the content of
coarse aggregate was found that there is significant difference be-
tween the lots and the points.
4. Conclusions
The dispersions of compressive strength within the batches, usu-
ally were concentrated in zone ± 5%, in relation to the reference
point (central, P3). Seeing the occurrence of many profiles varia-
Table 3 – Summary of percentage of occurrence
of the consistency of each point relative
to point P3
Zone P1 P2 P3 P4 P5
<-4cm 0% 0% 0% 0% 0%
-2-4cm 8% 0% 0% 0% 23%
até -2cm 8% 8% 0% 46% 54%
0 15% 15% 0% 31% 8%
até 2cm 62% 69% 0% 23% 15%
2 a 4cm 8% 8% 0% 0% 0%
>4cm 0% 0% 0% 0% 0%
Table 4 – Summary of percentage of occurrence
of coarse aggregate content of each point relative
to point P3
Zone P1 P2 P3 P4 P5
<-10% 0% 0% - 0% 0%
-5-10% 0% 0% - 0% 0%
até -5% 15% 62% - 62% 38%
até 5% 85% 38% - 38% 54%
5 a 10% 0% 0% - 0% 8%
>10% 0% 0% - 0% 0%
Table 5 – Variance analysis for compressive strength
Source of variation SQ GL MQ Fcalc. Ftab. PROB. SIGN.
Lot 3848,873 12 320,739 155,210 1,827 0,00% S
Collection Point 16,929 4 4,232 2,048 2,441 9,14% NS
Interactions 217,931 48 4,540 2,197 1,455 0,02% S
Error 268,644 130 2,066
Total 4352,376 194
Table 6 – Variance analysis for coarse aggregate
Source of variation SQ GL MQ Fcalc. Ftab. PROB. SIGN.
Lot 0,079 12 0,007 34,599 1,960 0,00% S
Collection Point 0,003 4 0,001 4,093 2,565 0,62% S
Error 0,009 48 0,000
Total 0,091 64