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.

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

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. https://www.todayonline.com/singapore/republic-polytechnic-team-finds-way-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