09-09-2014, 10:51 AM
ASSESSMENT OF RECYCLED AGGREGATE CONCRETE
ASSESSMENT OF RECYCLED.pdf (Size: 165.74 KB / Downloads: 86)
ABSTRACT
Use of recycled aggregate in concrete can be useful for environmental protection and economical terms.
Recycled aggregates are the materials for the future. The application of recycled aggregate has been started in
many construction projects in many European, American and Asian countries. Many countries are giving many
infrastructural laws relaxation for increase the use of recycled aggregate. Paper reports the basic properties of
recycled fine aggregate and recycled coarse aggregate. It also compares these properties with natural aggregates.
Basic changes in all aggregate properties were determined and their effects on concreting work were discussed
at length. Similarly the properties of recycled aggregate concrete were also determined and explained here.
Basic concrete properties like compressive strength, flexural strength, workability etc are explained here for
different combinations of recycled aggregate with natural aggregate. Codal guidelines of recycled aggregates
concrete in various countries were stated here with their effects, on concreting work. In general, present status of
recycled aggregate in India with their future need and its successful utilization were discussed here in detail.
INTRODUCTION
Recent research by the Fredonia group has
established that the global demand for
construction aggregates may exceed 26 billion
tones by 2011[1]. Leading this demand, are the
single user: China (25%), EU (12%) and the
USA (10%). However, because of
industrialization and significant infrastructure
and construction development, there are
expected to be significant increase in use of
aggregates in India (which is already one of
the major national markets at 3%) beyond
2011. In India, about 14.5 MT of solid wastes
are generated annually from construction
industries, which include wasted sand, gravel,
bitumen, bricks, and masonry, concrete.
However, some quantity of such waste is being
recycled and utilized in building materials and
share of recycled materials varies from 25% in
old buildings to as high as 75% in new
building
CONCRETE AND THE ROLE OF THE AGGREGATE
Regarding concrete, which is the construction
material of our era, the protection of the
environment concerns three basic axes:[4]
• Use of high amounts of raw materials
(aggregates for the production of cement
and concrete) which result in the decrease
of available natural resources which is
continuously sub-graded.
• Consumption of high amounts of energy
for the production, transport, use of raw
materials and final ones, as cement and
concrete.
• Creation of big volumes of old concrete
from old construction works (demolition
wastes).
The cementing medium has two main
functions.
NECESSITY FOR THE RE-USE OF RCA
Due to issues relating to sustainability and
limited natural resources, it is clear that the use
of recycled and secondary aggregates (RSA),
for example crushed concrete and asphalt and
industrial byproducts such as fly ash and blast
furnace slag, will grow. However, currently, it
is only in the USA, Japan, parts of Western
CURRENT KNOWLEDGE BASE ON THE USE OF RECYCLED AGGREGATES IN CONCRETE
Regarding that the quality data of old concrete
is often unknown (w/c ratio, kind and amount
of admixtures, aggregates origin and
gradation, etc.), as well as the differentiation
of its properties during its performance time,
the knowledge and tests of RCA should refer
to four categories:
(a) Historical data of RCA referring to the
composition of old concrete, masonry etc.
petrography characteristics, data of old
structures.
(b) Physical characteristics, especially in water
absorption, specific gravity, amount of
chlorides and sulphates, amount of contained
foreign ingredients, possibility of creation of
alkali–silica reaction.
MIX PROPORTIONS
The compressive strength increased with a
decrease in a/c ratio and is directly
proportional to strength of the blended
aggregate. However, when used at a higher level of
replacement, the high water absorption ability
of recycled aggregate results in a higher total
water demand. This renders the control of the
free water-to-cement ratio (w/c) and the
workability of fresh concrete difficult and,
results in a higher shrinkage and creep of the
hardened concrete when compared with the
concrete prepared with natural aggregates. The
extent to which the properties of concrete are
affected by the use of recycled aggregate
depends on the water absorption, crushing
value and soundness of the recycled
aggregate
STRENGTH AND DEFORMATION CHARACTERISTICS
It was concluded that concrete strength
decreases when recycled concrete was used[21]
and the strength reduction could be as low as
40%[22, 23]. However, no decrease in strength
was reported for concrete containing up 20%
fine or 30% coarse recycled concrete
aggregates, but beyond these levels, there was
a systematic decrease in strength as the content
of recycled aggregates increased[24]. The
strength characteristics of concrete was not
affected by the quality of recycled aggregate at
high water/cement ratio, it was only affected
when the water/cement ratio is low[25, 26]. The
higher the water/cement ratio, the less
reduction in compressive strength[23–25]
.
However a conservative value for replacement
of aggregate is 20% by mass was later adopted
in BS 8500-2[27
DURABILITY
In contrast, many tests for carbonation have
shown that the carbonation depth (for
concretes of equal compressive strength) after
a given length of exposure decreases as the
recycled aggregate content of concrete
increases, i.e. recycled aggregate concrete has
better resistance to carbonation than natural
aggregate concrete[30]
It has been purported that this is due to the fact
that recycled aggregate concretes have higher
INTERNATIONAL EXPERIENCE ON RCA
The research produced general guidance
supporting the wider use of recycled
aggregates in concrete as well as grouping
aggregate particle composition into three
classes of recycled aggregate suitable for
different applications, as given below.
• Class A: recycled aggregates for use in a
wide range of concrete including marine
environments,
• Class B: covering most combinations of
natural and recycled aggregate and
suitable for most ‘moderate’ exposure
conditions, and
• Class C: those aggregates suitable for only
the ‘mildest’ exposure conditions.
ECONOMICS AND BEST PRACTICABLE OPTIONS FOR RCA
The main alternative to using recycled
aggregates is, of course, natural aggregate and
these are still relatively low cost materials.
However, in a purely economic balance, the
cost of processing to recycled aggregates in
the UK is becoming less than that of disposing
of the demolition waste and purchasing new
aggregates, due to increases in landfill tax and
the newly introduced aggregates levy. If
recycled aggregates have to be transported a
significant distance from the place of
production to the place of use, then both the
cost and environmental benefits may become
more questionable.[28]
In India, the cost of construction materials is
increasing incrementally. As a result, in India,
the informal sector and secondary industries
recycle 15–20% of solid wastes in various
building components
CONCLUDING REMARKS
This paper has shown that:
• There is significant potential for growth of
recycled and secondary aggregates as an
appropriate and “green” solution to the
anticipated increased world – wide
construction activity and with it the
demand for RSA.
• Major research world – wide has shown
that RA can be used in concrete – and that
there are few (if any) applications issues
related to its use. However, further work is
needed to gradually develop the much
wanted knowledge base and the necessary
tools for the industry to be able to use
these resources routinely and with
confidence.
• Significant steps are being taken to
improve the quality of RA and new
standards are easing its use in higher value
applications. Nonetheless, this is very
much limited to few countries and the
message has to travel worldwide to make a
meaningful difference to the sustainable
use of RSA in concrete