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.
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,
lower production of cement 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
