Basic HTML Version
699
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5
J. J. L. Tenório | P. C. C. Gomes | C. C. Rodrigues | T. F. F. de Alencar
3.3 Water absorption, air content
and air-permeability time
The results of water absorption and air content are shown in figure
6, and the results of the air-permeability time are shown in figure 7.
In figure 6, the RAC generally has a larger absorption and a larger
air content than the reference concrete, demonstrating that the
recycled aggregate negatively influences the porosity of the com-
posite. It is also possible to notice a normal tendency of decrease
in the water absorption and in the air content when moving from
the recycled fine aggregate to the sand and of the coarse aggre-
gate G1 to G2, that is, when less porous aggregate is used in the
concrete. On the other hand, so much for the RAC as for the refer-
ence concrete, the two properties seemed not to be sensitive to
the alteration of the w/c ratio. Noticed that any concrete with similar
composition (for instance xxMNG1) had an absorption value or an
air content very similar to the three studied w/c ratios. Therefore
demonstrating that in any concrete, the absence of a clear relation
inversely proportional between the porosity of the material and its
compressive strength. It is still observed the fact that the tests they
give indications (figure 6) that they are not sensitive, or they are
little sensitive, to the alterations of the w/c ratio.
In figure 7, the property air-permeability time of the RAC was sen-
sitive to the changes in the types of aggregates, it also happened
with the water absorption and the air content. The test, unlike the
absorption and of the air content, measuring the air-permeability
time was sensitive to the alteration of the w/c ratio, decreasing as
this increased, and to the aggregate type, increasing when the fine
aggregate is normal and the recycled coarse aggregate is G2. The
relation between the permeability and compressive strength of the
concretes can be seen in the figure 8, in which, for each concrete,
each point corresponds to a w/c ratio.
In figure 8, the air-permeability time demonstrated a relation with
the compressive strength of the concrete, the larger the air-perme-
ability time the larger the resistance to the concrete. The concretes
tended to be more resistant the smaller the permeability, confirm-
ing that for this composite, independent of the type of aggregate
used. The relation between porosity and resistance is valid proven
by the Science of the Materials. Part of the porosity of the RAC
might have been caused by the migration of part of the water pre-
soaked of the aggregates recycled for the paste.
4. Conclusions
This study developed the following final considerations:
n
The composition and properties of the recycled aggregates in-
fluence on the production and in the fresh and hardened prop-
erties. In the mixture, the absorbed water from the aggregates
needs to be controlled; and the density of the recycled coarse
aggregate may be a parameter for selection of recycled aggre-
gates with structural purpose.
n
The values of the strength obtained were inside of the struc-
tural concrete norms and specified in the Brazilian norm for the
calculation of structures of armed concrete.
n
The compressive strength of the RAC was approximately 20
MPa for the w/c = 0.67, the strength was approximately 30
MPa for the w/c = 0.50 and was approximately 40 MPa for the
smallest w/c = 0.40. This behavior happened independently
from the type of aggregate used, and therefore shows that all
concretes obeyed the Abrams Law, which is with the increase
of the relation, the compressive strength is decreased.
n
The properties of compressive strength and those related to
the durability of the concretes generally showed better per-
formance in the concretes with recycled coarse aggregate of
larger density (G2), compared to the concretes with recycled
aggregate of smaller density (G1) independent of the type of
fine aggregate.
n
The concretes with recycled fine aggregate normally had their
properties inferior to the one of the concretes with natural fine
aggregate.
n
The RAC were more porous and permeated than the conven-
tional concretes, with exception to the 40MNG2 concrete which
refers to the permeability. The more porous concretes and per-
meated, tend to be less durable for providing, in the case of the
applications structural, smaller protection to the steel against
external aggressive agents that can penetrate the composite.
Figure � � �ir-permeability time in function of concrete compressive strength
(Ref) R2 = 0,9152
(MRG1) R2 = 0,9666
(MRG2) R2 = 0,9817
(MNG1) R2 = 0,9747
(MNG2) R2 = 0,8003
3
8
13
18
23
18
22
26
30
34
38
42
46
Compressive strength (MPa)
Figg air-permeability time (sec)
Ref
MRG1
MRG2
MNG1
MNG2
Ref
MRG2
MNG1