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1. Introduction
The concrete is considered one of the most important materials in
modern construction, besides also is the material more produced
in world in terms of volume. Such relevance achieved by this com-
pound is, basically, due to excellent water resistance, ease of use
with diversity of shapes and sizes, low cost, worldwide availability
(MEHTA e MONTEIRO [1]), and their potential of use, which has
been expanded over the years, with technological development.
Another factor that contributed to the increasing use of concrete
in the world market, beyond technological development, was the
rationalization of the production process, which created an indus-
try with the concrete batching plants. From this industrialization
may result reduced costs and increment of production capacity,
while maintaining the performance and quality required of con-
crete. Today, in major cities worldwide, most of the concrete used
is produced at concrete batching plants1 and the trend is that this
process becomes increasingly prevalent, especially in developing
countries like Brazil, seen their advantages compared to the pro-
duction at construction site.
The ready mix concrete can be mixed in the plant itself and trans-
ported with truck agitator or mixed during transport by truck mixer,
being the latter predominant in the country (Brazil). Such choice is
probably due to cost savings in the deployment of concrete batch-
ing plant, without stationary concrete mixer (fixed site), increase
in term and distance of transport, and especially the type of taxa-
tion2. However, as Mehta and Monteiro [1], the quality control of
concrete mixed in truck mixer is not as good as the one produced
in stationary mixer.
The growing use of concrete, added with the pressure of civil con-
struction market by reducing costs while maintaining the minimum
requirements for project safety, increased the importance and
necessity of technological control of the material. The parameter
usually adopted as the standard for verifying compliance with the
minimum requirements of the project is the compressive strength
of concrete, since the property is easily to measure, it is able to
expose the changes in the quality of the concrete and is related to
several other properties of the material (NEVILLE [7]; HELENE e
TERZIAN [8]).
The control of mechanical properties is performed in a standard
specimen, molded and cured according to standard prescriptions,
and the strength testing realized, usually, at 28 days old of con-
crete. For purposes of acceptance, from as the result is verified
compliance with the requirements of project resistance and safety
of the structure built.
The specimens should be molded with concrete samples collected
during discharge from truck mixer, according to the prescriptions of
NBR 7212 [9] and NBR NM 33 [10], which recommend not using,
for testing purposes, the 15 % initial and final from total volume of
the concrete mixer. However, the current practice in most construc-
tion sites, the sample is collected immediately from the first portion
of concrete discharged. Such procedure is due to the difficulty in
collect the concrete during the discharge, often in areas of difficult
access, and the ease and speed that you have to collect a single
sample for slump test and molding of specimens for compressive
strength test in an accessible place. The NBR 7212 [9] define a
concrete batch of the same mixture as homogeneous for purposes
of testing and control without, however, specifying parameters and
tests to ensure the efficiency of the mixer as their primary function:
produce a uniform mixture of concrete. Furthermore, it is known
that concrete mixers have wear of its components, according to
the use and therefore, may lose efficiency so as not satisfacto-
rily perform its function. Countries like the U.S. and England have
norms with guidelines for assessing the performance of mixers,
such as ASTM C94/C94M [11] and BS 3963 [12], among others,
which limits the variability of the mixture and ensures uniformity of
concretes produced.
The NBR 12655 [13] recommends doing a uniformity test of mix-
tures in concrete mixers, when show up signs of heterogeneity of
compositional or consistency during discharge, but does not pro-
vide more detailed information. Already ABECE [14] makes a rec-
ommendation a little more complete, indicating regular quality con-
trol of concrete mixers by comparing resistances between three
different collection points: the first shortly after discharge 15% of
the total volume, intermediate in the middle of the discharge, and
the last one shortly after discharge 85% of the total volume. As
control we have that the difference between the highest and low-
est resistance of the first and last point should not differ more than
15% of the resistance of the intermediate point.
In the country there is not specific normalization for assess and
control the quality of mixers, so there is not guarantee of repre-
sentativeness of samples of concrete, even if collected between
discharge 15% and 85% of total volume of batch, as recommend-
ed. This lack of guarantees for uniformity of ready mix concrete,
and the common practice of collecting samples outside the rec-
ommended standards, can affect the credibility of quality control,
creating uncertainties about tests of lots and up to the safety of
structures.
Since the common practice of collecting samples differ from the
norm, the lack of recommendations for measuring the quality of
the mixer and the importance of quality control testing of concrete
to certify the safety of structures, this study aims to analyze the
uniformity of concrete mixtures in concrete mixer truck, during un-
loading the material, to determine if there is significant variation in
195 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 1
R. MASCOLO | A. B. MASUERO | D. C. C. DAL MOLIN
1 At developed countries like USA and Japan, 75 and 71%, respectively, of the consumption of cement are used by ready mixed concrete, and in
European countries like Germany, England and France this percentage is above 50% (NRMCA [2]; ERMCO [3 ]). At Brazil, the current consumption of
cement is still sprayed with 55% of sales to small consumers and only 18% for the production of ready mixed concrete. However it is clear that cement
consumption by concrete batching plants is growing in the country since 2006, when only 13% of cement consumption were intended for ready mixed
concrete (SNIC, [4]).
2 Mixing the constituents of concrete (cement, aggregates, water and eventually additives and additions) in truck mixer along the path between the plant
and the local of service delivery (construction site), concreting companies are classified as service providers and thus subject only to the ISS (Service
Tax), while the mixture in stationary mixer equipment at concrete batching plant, ie, off-site service delivery, the ready mix concrete shall be classified as a
commodity and then there incidence ICMS (Tax on Circulation of Goods and Services). For companies concreting the ISS rate in the city of Porto Alegre
is 4% of the value of the invoice, while the ICMS is 17% (state tax-rate basis for the Rio Grande do Sul), which explains the preference of concrete
companies (BRASIL - Lei complementas n 116 [5]; SEFAZ-RS [6].