Technical report Draft 1
Introduction
Background Information
This proposal has been
initiated in response to the request for proposals on developing engineering
solutions for engineering problems.
As demands for
electricity continue to rise, coal-powered and incineration plants are on
double shifts to meet electricity demands and as a result, fly ash have been
produced in abundance. This byproduct, which ends up consuming landfill space,
has environmental concerns that need to be addressed. These ashes generate leachate,
where coal ash and water precipitation react to become harmful solid wastes
that contaminate underground water. Turrentine (2019) reported that people
suffered from respiratory illness and thyroid problems, just by inhaling these
ashes. Also, within the Singapore context, land scarcity is a problem. Our only
landfill, Pulau Semakau, will reach its peak come the year 2035. 1,500 tonnes
of incinerated fly ash has been actively dumped into this landfill and thus, minimising this action
is a bright initiative to save the environment and create
land-use opportunities.
At the same time, concrete is the most widely used man-made material in existence. It is second only to water as the most-consumed resource on the planet. But while cement, the key ingredient in concrete, has shaped much of our built environment, it also has a massive carbon footprint. Cement is the source of about 8% of the world’s carbon dioxide (CO2) emissions, according to Think Thank Chatham House.
At the same time, concrete is the most widely used man-made material in existence. It is second only to water as the most-consumed resource on the planet. But while cement, the key ingredient in concrete, has shaped much of our built environment, it also has a massive carbon footprint. Cement is the source of about 8% of the world’s carbon dioxide (CO2) emissions, according to Think Thank Chatham House.
Taking a quick glance
along Singapore road systems, “There are an average of 150 road works happening
each day.” (E. Ang, personal
communication, March 05, 2020.) .These underground
pipe-laying works require excessive use of cement which can be channelled to
other practical uses in a building such as preparing foundation and pillars.
Looking at the two
environmental concerns discussed, the team has proposed to integrate fly ash as
a substitute for cement into flowable concrete for non-structural works
(flowable fills). Flowable concrete is a self-compacting cementitious slurry
consisting of a mixture of cement, sand and water which is used as a fill or
backfill. This mixture is capable of filling all voids in irregular excavations
and hard to reach places. According to the controlled low strength material
(CLSM), the strength for flowable fill only requires 0.3-0.7 megapascal (MPa).
However, most of the flowable fills used today have a compressive strength of
2.1 MPa (U.S. Department of Transportation, 2016) and this is an excessive use
of cement. Moreover, the bulk of cement can be replaced by fly ash without
compromising its strength and durability.
In doing so, fly ash
can be channelled to a beneficial use while cement can be massively reduced.
This would minimise the environmental concerns raised without compromising any
quality nor content issues.
Problem Statement
The ideal flowable concrete for non-structural should contain 95% fly ash and 5%
cement. However, the current state of flowable concrete for non-structural
works contains 100% cement which is not environmentally sustainable and ignores
the availability of free fly ash from coal-powered plants as a cheap viable
component. Cement contributes to high emission of carbon dioxide while fly ash
is a byproduct dumped into landfill. The goal is to integrate fly ash into
concrete mix to reduce fly ash dumping into landfill and decrease cement use to
reduce carbon footprint. This integration should result in cost effectiveness and
potential sustainability.
Purpose Statement
The purpose of this
statement is to replace 95% of cement with fly ash to reduce the overuse of
cement.
Proposed Solutions
Currently, the mix of
flowable fill for non-structural works is cement, water and sand. The proposed
solution is to reduce the amount of cement used, by replacing it with fly ash.
Fly ash is generated from coal powered and incineration plants. Currently, tons
of fly ash are being wasted and dumped into landfills. This could be improved by
integrating fly ash as such into flowable fill mix before they are being
disposed. These allows Singapore landfill, Pulau Semakau, to sustain a longer
lifespan.
According to Concrete
in Practice, flowable fills should not exceed an ultimate strength of 1.4 MPa
for easy excavation. Most of the flowable fill used today has a compressive
strength of 2.1 MPa (U.S. Department of Transportation, 2016). Thus, the plan
is to incorporate fly ash into flowable concrete that will provide a strength
of 0.3 - 0.7 MPa which is more sustainable and equally functional. This
flowable concrete will contain 95% fly ash and 5% cement which saves up to
500kg of cement per cubic metre. The mix can also achieve the strength required
according to CLSM. At the same time, a lower amount of cement content will lead
to an exponential drop in the contribution of carbon emissions.
Flowable Fill Mix
Design
Flowable Fill Mix
Design (28 days strength of 0.3 - 0.7MPa)
|
||
Type
|
OPC Flowable
Concrete
|
Fly Ash Flowable
Concrete
|
Cement Content
(kg/m3)
|
550
|
21
|
Fly Ash (kg/m3)
|
0
|
514*
|
Sand (kg/m3)
|
1070
|
1070
|
Water (kg/m3)
|
412
|
412
|
*the amount of fly ash varies due to
different composition.
Statistics on the benefit of using fly ash
For every 500kg/m3 of
ash used, an equivalent amount of landfill space can be conserved and 500kg of
cement being saved for structural purposes. According to the Environmental
Protection Agency (EPA), between 900 - 1100 kg of CO2 is emitted for every 1000kg
of cement produced.
For instance,
Singapore disposed of 1,500,000 kg fly ash daily; a volume of 3000m3 of fly ash
can be used for flowable fill purpose. (Tang, 2017)
Applications of
Controlled Low-Strength Material
Backfill
CLSM can be easily placed into a trench,
hole or other cavity which do not require any forms of compaction, thus, the
trench width of excavation can be reduced.
Structural fill
Sometimes, CLSM can even be used for
foundation support, depending on the strength requirements. It can provide a
uniform and level surface.
Void filling
When filling abandoned tunnels and sewers, it is
important to use a flowable mixture. A constant supply of CLSM mixture will
help to keep the material flowing a greater distance. CLSM was used to fill an
abandoned tunnel that passed under the Menomonee River in downtown
Milwaukee,Wis. The self-leveling material flowed over 71.6m.
Benefits of replacing
Cement with Fly Ash
Reduce carbon
footprint
Using Fly ash as a cement replacement
reduces the overall CO2 footprint of the concrete. (how?) The environmental
savings can equate to 20% reduction in overall CO2 emissions for 30% fly ash
content.
Better mix
Fly ash and cement mixes have better strength,
durability, chloride and sulphate resistance of the concrete than Portland
cement.
Finishing
Fly ash integrates well with other admixtures and
thus allows the concrete to have a smooth and dense finishing.
Improves Workability
Fly ash improves the workability of concrete. Fly
ash mixed cement produces a more cohesive concrete with a lesser segregation
and a reduced rate of bleeding hence making it easier for compaction as well as
giving better pumping properties to the concrete.
Reduces
Permeability
Fly ash reduces permeability, which reduces
shrinkage and creep. Fly ash, when mixed with water and lime, reacts with lime
to form chemical bonds such as stable calcium silicates and calcium aluminate
hydrates. These then fill the voids in the concrete and some of the lime is
removed as reaction progress and the permeability of the concrete is reduced.
Reduces Temperature
Fly ash reduces the temperature that rises in
thick concrete sections. The heat that is produced through hydration is greatly
reduced with the addition of less cement in a concrete mix which hydrates
easily and rapidly when exposed to water.Tyson (2017) pointed out that
integrating coal ash minimised 60% to 80% of the heat generated, without
compromising its strength and durability properties.
Minimises Time
In application, the trenching works complete in
less time as the time taken to manually backfill the cement has been replaced
with pouring flowable concrete. Not only does it save time in backfill, it also
saves time as there is no need to compact the material. Generally, time saved
is crucial as contractors have to adhere to the non-working periods and peak
hours.
Lesser
costs, more gains
The integration of fly ash would result in
decrease of water-cement ratio, which means lesser costs in purchasing them.
With fly ash virtually free, there will be further decrease in operational
costs. Coal plant owners can also sell fly ash with a profit, and mitigate
imposed costs of dumping the ashes into landfills.
Reduce
pollution, increase strength
As concerns arose regarding dumping of fly ash
resulting in pollution, Turrentine (2019) explained that the mixture is safe to
use as there is no water precipitation present in the concrete to form
leachate. Horwitz-Bennet (2015) reported that the reaction also produces
calcium silicate hydrate, the exact product that increases concrete strength.
Benefits of Controlled
Low Strength Materials (CLSM)
- CLSM makes use of coal combustion products and turn
waste materials into useful materials and save cost for disposal.
- CLSM mixtures are versatile as they can be adjusted to
meet specific fill requirements.
- CLSM mixtures are strong and durable.
- CLSM can be placed quickly and support traffic loads
within several hours.
- CLSM does not form voids during placement and it will
not settle, this advantage is especially significant if the backfill is to
be covered by pavement patch.
- CLSM reduces excavation costs as it allows narrower
trenches to eliminate having widen trenches to accommodate compaction
requirement.
- CLSM improves worker safety as workers can place CLSM in
a trench without entering the trench.
- CLSM allows all-weather construction.
- CLSM allows easy excavation of having compressive
strength of 0.3 to 0.7 MPa, yet it is strong enough for most backfilling
needs.
- Reduces equipment needs such as loaders, rollers and
tampers.
Proposal
Evaluation
In this section,
limitations and challenges for this proposed solution will be evaluated and
discussed.
Environmental Consideration
Despite having
enormous benefits for using fly ash as a construction material in construction
industry, some have questioned the negative environmental impact of using fly
ash such as presence of alkaline in fly ash. Hence, the environmental
consideration will be further discussed under this section, in addition, some
potential recommendations will also be discussed in response to the concern
raised on the negative environmental impacts.
Fly ash material
together with its disposal procedures include holding ponds, lagoons and
landfills and slag heaps – unsightly and environmentally undesirable and a
non-productive use of land resources. This will lead to financial burdens
through long-term maintenance. Fly ash contains toxic elements into
groundwater, decreased germination rates of some crops due to high levels of
fly ash application including uptake of heavy metals, toxic elements by plants
in the surroundings. The uptake of heavy metals, toxic elements by plants was
demonstrated when fly ash was applied to the soil. Thus, when fly ash is used
in construction, such as roads, it will lead to water contamination and hence
its environmental surroundings.
Limitations of Fly Ash
Fly ash and Portland
cement mixes tend to be slower to hydrate than Portland cement only mixes. The
chemical composition of fly ash differs from Portland cement in a way that fly
ash when mixed with water, does not moisturise directly but needs a mixture of
lime and water to moisten.
Usually, based on the
type of application, fly ash is mixed with Portland cement in range of 80%
Portland cement + 20% fly ash to 60% Portland cement + 40% fly ash. Longer
curing time may be required for casting. Fly ash has heavy toxic metals
and may cause negative environmental impacts.
Methodology and Procedure
The topic sparked
research interests due to concerns over how fly ash is affecting the capacity
of landfills, and the wide use of cement which results in carbon footprints.
While these two concerns are separate, they stamp onto environmental concerns
which the team feels should be mitigated. The team then leveraged on two of the
team members’ experiences in dealing with recycling of fly ash and
observing how cement is excessively used in road-trenching works, and
discussed the possibility of integrating fly ash into cement mix to reduce
cement content.
Primary Research
One of the members deepened his knowledge
on the use of fly ash during his polytechnic days, participating in a 18 month
research to understand fly ash properties and how it affects cement mix. His
team also collaborated with one of the major companies in Singapore, Samwoh
Innovation Centre, that deals with research and development in using recycled
aggregates, to study the integrations of fly ash into cement. He completed more
than 108 different mix designs and extended the research to complete 90-day
strength tests for the mixes. His experience in dealing with fly ash brought
about a better understanding in dealing with the byproduct.
Another member, who worked for the Land
Transport Authority (LTA), was involved in ensuring that the composition of
underground materials adhered to the Code of Practice for Road Works upon
successful laying of service pipes. In doing so, he had observed the complete
procedure of excavation, pipe-laying and backfilling works. Upon further
reading, he noticed the excessive use of cement in the underground material
compositions and thus, were interested in the proposal to integrate fly ash
into cement.
Secondary Research
The team gathered researches from books, online
articles and online videos to understand the different aspects in tackling a
wide-angled discussion about integration of fly ash. Several components were
carefully studied, from the different types of fly ash available in different
countries to the discussions over experimental results of different cement
trial mixes obtained, to ensure that the proposal was sufficiently
relevant and significant to address the concerns brought forward.
Conclusion
The exponential
increase of fly ash, produced from coal-powered and incinerated plants in
Singapore due to increasing amount of garbage will be fully occupied Pulau
Semakau landfills by 2035. At the same time, Singapore is a country with
limited natural resources. Thus it is all the more important to continually be
in search for ways to find replacements for material of high demand such as
cement.
Fly ash found overseas
is found to possess significant cementitious properties to the extent of it
being used in structural concrete elements. Before adapting such applications,
it is essential that local fly ash undergoes a series of thorough physical and
chemical tests to understand how relevant the existing applications are to
Singapore. Though the study of local fly ash is still in its primary stages,
this study has shown that local fly ash possesses satisfactory strengths that
may be used for controlled low strength material (CLSM) for backfilling in
trenching works.
Not only it fulfills
the strength, it also reduces the overuse of cement, cost effective,
environmentally-friendly, conserves landfills and thereby effectively reducing
the carbon footprints.
References
National Ready Mixed
Concrete Association. (2008, June). Concrete CO2 fact sheet. http://www.nrmca.org/greenconcrete/concrete%20co2%20fact%20sheet%20june%202008.pdf
National Ready Mixed
Concrete Association. (n.d.). Concrete in practice: CIP 17- Flowable fill
materials. https://www.nrmca.org/aboutconcrete/cips/17p.pdf
Federal Highway
Administration. (2016, August 03). User guidelines for waste and byproduct
materials in pavement construction: Flowable fill. Retrieved March 6, 2020 from
https://www.fhwa.dot.gov/publications/research/infrastructure/structures/97148/app6.cfm
Rodgers, L. (2018,
December 17). Climate change: The massive CO2 emitter you may not know about. BBC
News. https://www.bbc.com/news/science-environment-46455844
Tang, L. (2017,
September 09). Republic Polytechnic team finds way to ‘clean’ incineration ash.
Turrentine, J. (2019).
Coal ash is hazardous. Coal ash is waste. But according to the EPA, coal ash is
not “hazardous waste.” Natural Resources Defense Council. Retrieved
September 06, 2019, from https://www.nrdc.org/onearth/coal-ash-hazardous-coal-ash-waste-according-epa-coal-ash-not-hazardous-wast
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