To
effectively face rising ecological and environmental threat from waste concrete
and from some of the
industrial wastes, recycling has become need of the modern civilization. In
this article recycling
of the following waste materials from constriction is discussed.
1.
Concrete Rubble
 |
| recycle concept |
2. Crusher Dust
3. Fly
Ash
4.
Silica Fume
5. Spent
Foundry Sand
6. Blast
Furnace Slag
7. Red
mud
8.
Polythene
Recycling Waste Concrete
Every
year millions of tonnes of concrete rubble is generated in this world due to
various reasons such as:
(a)
Distructions of cities during the wars and earthquakes.
(b)
Rejected precast concrete elements.
(c)
Testing of concrete cubes.
(d)
Concrete pavements reaching the end of their useful life
It is
estimated that in USA itself approximately 150 million tonnes of concrete
rubble is generated
annually. Earlier disposal of concrete rubble was for filling land fills like
borrow pits of soils
or depleted quarries. But environmental consciousness, depletion of quarry
aggregate and economical
consideration have made it necessary to reuse concrete rubble.The
research in this field started immediately after second world war in Europe.
Nowadays from
many concrete laboratories in India also encouraging results have been
reported.Concrete
rubble is mainly recycled as aggregate for new concrete. The recycle aggregate
has slightly
less specific gravity and has more absorption value. Impact value and crushing
values are also
reduced.The
strength of recycled aggregate concrete is about 10 to 15 per cent less as
compared to concrete
with fresh aggregate. However suitable mix designs may be made and reliable
results obtained.
The mix requires slightly higher quantity of cement or using admixtures to
reduce water
requirement.Recycled
aggregate concrete can be safely used as plain concrete. With proper
corrections in mix
design, it can be used for R.C.C. works also.
Recycling Crusher Dust
Crusher
dust is the by-product of crushing of rock to obtain coarse aggregates/jelly
for concrete.The
disposal of this dust is a serious environmental problem. If it is possible to
use this crusher dust in
production of concrete and mortar by partial or full replacement of natural
sand, then this will not
only save the cost of construction but at the same time it will solve the
problem of disposal
of the crushed dust.The
researchers have reported that replacement of natural sand with crusher dust
results into
reduction in workability which can be compensated by using chemical admixtures.
The test results
reveal:
1.
Replacement of natural sand by crusher dust is not detrimental upto 50%.
2.
Concrete with 100% crusher dust can be used with greater precaution. In such
concrete durability
requirement is not met in stipulated 28 days. However, incorporating fly ash
can enhance
the durability characteristics.
3. Combination of crusher
dust and fly ash is beneficial in cost and durability aspects.
Use of Fly Ash
Fly ash
is the by-product from thermal plants. The thermal plant owners take all care
to prevent it from
flying in the air and dump it in wet ponds. Till 15-20 years ago it was treated
as a waste and
disposal as a serious problem. This gave rise to research to utilise fly ash.
Now it is found that it
is a useful material in the following works:
1. It is
a dependable resource material for brick production. 40 to 50 per cent fly ash
can be used
with 25 to 30 per cent sand, 10 to 15 per cent lime and 10-15 per cent gypsum
to produce strong
and durable bricks.
2.
Adding 15 to 30 per cent fly ash in cement, blended cement is produced. The
blended cement
has got very good weather resisting capacity and hence it is more durable than
ordinary portland
cement. Hence use of blended cement is becoming more popular than ordinary
portland cement.
3. In
recent years high performance high volume fly ash concrete has emerged that incorporates
a large volume of fly ash (HVFA) into conventional portland cement. Fly ash
used is 40 to
50% . Use of fly ash reduces heat generation during setting of cement. Use of
HVFA makes
concrete pavements cost effective. This concrete takes more time to gain full
strength (120 days
compared to 28 days by ordinary concrete) and needs small quantity of
superplasticizer (chemical
to improve workability).Nowadays
demand for fly ash has increased so much that there is no problem of disposing but
thermal power stations can sell it to nominal price.
Silica Fume
Silica
fume is a by-product from the silicon, ferrow-silicon manufacturing process
from quartz and
carbon in electric furnace. It is in the form of extremely fine spherical
particles. Before 1990 it was
viewed as factory waste. But in 1987, silica fume concrete was used for Deepak Fertilizers It was
discovered that it has beneficiary effect on concrete, including increase in
strength and durability.
Now nearly all major projects are using High Performance Silica Fume concrete.
Some of the
important projects where silica fume is used are Tehri Dam, Bandra Worli Sea
Link, Nuclear Power
Station, Kaiga, Karwar, Mumbai to Poona express way. Replacing cement by 12.5
per cent silica
fume has the maximum advantage in increasing the strength.As a
result of increase in its demand, now silica fume is expensive. However when
analysed against
the cost of an alternative concrete of similar performance it is found to be
economical also.This
conclusion has been drawn from the cost analysis of Mumbai-Poona express way.
Spent Foundry Sand
The most
common type of casting process in the foundry industry is known as sand
casting. The sand
used for preparing the moulds is known as foundry sand. Molten metal is passed
into the mould
and allowed to cool. After cooling, the mould is broken away from the metal
piece in a process
called shake out. These broken pieces of mould which consists of slag, wastes
etc. is called spent
foundry sand. This spent foundry sand is posing a threat to the environment and
needs safe disposal.
To reduce this environmental pollution, this spent foundry sand can be used as
part of concrete.It has
been found that if 10% of natural sand is replaced with spent foundry sand
compressive strength
of concrete increases by 3%. More than 10% replacement result into reduction in
concrete strength.
Blast Furnace Slag
The
blast furnace slag which is a by-product in the manufacture of pig iron is
converted into foamed/expanded
slag by tripping steam while cooling process is on. Industries have come up near the
steel mills to manufacture readymade building blocks and partition wall panels
using blast
furnace slag.Ground
Granulated Blast Furnace Slag (GGBFS) from cement industries are used as replacement
of cement. It is found that only 5% of strength is lost, if 40% of cement is
replaced by
GGBFS. However, in this case strength gained is slow. It takes about 60 days to
get almost full strength
compared to 28 days taken by ordinary cement concrete. However it is the best
means of
recycling this industrial waste.
Red Mud
Red mud
is a waste material obtained from aluminium plants causing environmental
pollution and
disposal problems. For every tonne of alumina produced equal quantity of hazardous,
highly alkaline
red mud is produced, which mainly consists of hydrous hydrated oxides of
alumina,titanium,
silica and alkalies.A
process has been developed by CBRI Roorkee utilising red mud and fly ash to
manufacture bricks
of various colours and texture. Compressive strength achieved is 40 N/mm2
(minimum compressive
strength required being 3.1 N/mm2).
Water absorption of such bricks is only 12%.It has
been reported that in mortars 10% and in concrete 5% of cement may be replaced
to get better strength.
In Taiwan red mud is used with PVC to produce red mud plastics (RMP). The corrugated
roofing sheets manufactured have shown highly improved properties over PVC in terms of
weather resistance.
Polythene
Polythene
is the environmental hazardous materials, since it is not getting dissolved in
nature.While
chemical engineers are busy in finding methods of dissolving it using chemical
technology and
reusing the polythene, civil engineers are trying to recycle it as building
materials.Some
success has been reported in using polythene as damp proof material in building
low cost
houses. Major breakthrough is likely in using it as a useful material in road
works. Polythene cleaned
and cut into small pieces is mixed with tar and is used in about 600 km of
roads in and around
Bangalore. Results are encouraging. Roads laid in 2002 are showing good
performance.It is expected that
durability of such roads is twice that of tar roads without using polythene.
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