Page 26 - Riem-Vol6_nº2

197 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
R. MASCOLO | A. B. MASUERO | D. C. C. DAL MOLIN
construction site, protected from direct exposure to sunlight, and
then were transported to the laboratory, where they were demold-
ed and identified. After these steps the specimens were sent to
submerge curing in water saturated with lime, in a wet room with a
temperature of 23 ± 2 ° C.
One day prior of rupture, the specimens were removed from the
cure for base preparation previous the compression test. The
specimens from the same batch were always demolded, placed
and removed from curing, prepared for testing of compressive
strength, and tested in short periods of time to reduce any possible
influence of other factors.
3. Results and discussions
Table 1 shows the slump test results, compressive strength and
coarse aggregate content for each of the five points analyzed in
each of the thirteen batches of ready mixed concreto. Figure 1
shows the variation of the compressive strength, where each row
of dots represents the results of the three specimens that compose
the five samples from each batch, being the upper and lower ends,
higher and lower resistance, respectively, and at the midpoint the
intermediate value of three brothers specimens. The batch lowest
average value got 36.7 MPa, while the higher 55.3 MPa, which
represents an amplitude of 18.6 MPa, a value of the same order
of size as found in other studies of the kind such as Borges [18],
wherein the resistance variation for a 30 MPa concrete, mixed in a
mixer truck, was 19.2 MPa.
To facilitate understanding of data, is showed the figures 2 to 4,
with the results of compressive strength, slump test and coarse
aggregate content, for each of five sampling points of the batch-
es tested.
In Figures 2-4, the results of slump test are presented in unit of
“cm”, and their possible variation, as the limits of the slump test,
from 0 to 30cm. About the results of the compressive strength, was
used the highest value among the three specimens that composing
each sample, i.e. each point is represented by resistance potential.
For data compression strength and coarse aggregate content is
adopted, for each batch, the third sampling point collected from the
To each of 5 sampling points per batch were performed: (1) slump
test according to the specifications of NBR NM 67 [15], (2) com-
pression test-axial of specimens, following the requirements of
NBR 5738 [16] and NBR 5739 [17] and (3) determine the coarse
aggregate content as recommended by the ASTM C94/C94M [11].
Whereas the main goal of the research is to check the variabil-
ity of the compressive strength and physical properties of ready-
mix-concrete within the same batch, has been in the collection of
samples from several batches one of the fundamental stage for
the development of research. In research, each batch is defined
as the volume of concrete of the same mixture, so each lots has
between 7 and 8 m³, which is the maximum capacity of the truck
mixers used in the region.
For the tests were taken five samples of fresh concrete in each
of the thirteen lots: one at the beginning of unloading (point 1),
another during the initial third (point 2), the third in the middle third
(point 3), the penultimate in the final third (point 4) and the last one
near the end of the discharge (point 5). In percentage terms, the
sampling points described above were collected, approximately,
after unload of 5%, 25%, 50%, 75% and 95% of the total volume
of the truck mixer.
During discharge of the concrete had not precisely the real volume
unloaded, then the points were determined from the experience of
the person responsible for the operation of sampling, so the actual
collection points vary around the percentages cited before.
The sample collection of concrete was made at construction site,
directly at the point of discharge into 20l pails, without interrupting
the flow of material discharge. A shovel was used to assist in the
collection due to the impossibility of moving the chute of discharge.
As soon as the end of the sampling collection, samples, properly
stored, covered and identified, were sent to the test site, mold-
ing and preparation of the samples, inside the construction site.
Initially, in the first portion collected from the point in question,
was realized slump test, casting the specimens and preparation of
samples for subsequent wash off the construction site. The same
procedures occurred for the portions of the following points, obey-
ing the order of collection. Thus, the starting point was tested first,
followed by the second, third, fourth, and last one, which conse-
quently remained a longer stored.
Each sample of a representative point consisted of three specimens,
i.e. one more than recommended by the NBR 12655 [13], and usu-
ally performed in quality control. This choice of the number of speci-
mens was adopted to obtain greater security for the results and to
evaluate the uniformity and efficiency of operations and test control.
To determine coarse aggregate content, was separated a sample
by point, with volume around 2l, from the main sample of 20l, which
was added concrete retarder in order that the mixture keep fresh
state for a long period until transportation to the laboratory.
The wash of fresh concrete samples, to determine the coarse ag-
gregate content was performed at laboratory because of the need
of precision balance, usually not available in a site construction.
For determination of coarse aggregate content, the sample was
weighed and then forwarded to wash with water under 4.8 mm
mesh sieve. After the material retained in the sieve remained until
losing all excess water and subsequently was weighted, as recom-
mended by ASTM C94/C94M [11].
The casting of the cylindrical specimens was performed using
manual density with a metal rod, as recommended by the NBR
5738 [16]. The specimens remained for a period of 24 hours on
Figure 1 – Compressive strength of specimens