Coal fly ash, or Pulverised Fuel Ash (PFA) has been used for many years in road construction as a fill material, in concrete, lean mix sub-bases and in more recent years as a binder and aggregate in hydraulically bound materials. There is an excess of PFA produced annually within the UK and some 55,000,000 tonnes of material (UKQAA a, 2007) that are accessible from stockpiles. Its use reduces material being sent to landfill and preserves virgin aggregate stocks and is a major mineral resource for future generations. There are many environmental and sustainability reasons for using PFA in preference to virgin aggregates, reducing the overall greenhouses gas emissions
2. UTILISATION OF FLY ASH IN ROADS AND EMBANKMENT WORKS
Around 110 million tonnes of fly ash get accumulated every year at the thermal power stations in india. Internationally fly ash is considered as a by product which can be used for many applications. Fly ash mission was initiated in 1994 to promote gainful and environment friendly utilisation of the material. One of the areas identified for its bulk utilisation was in construction of roads and embankments. Central Road Research Institute (CRRI), new delhi, chosen as the ‘nodal agency’ for this activity, has undertaken many demonstration projects. Some of these are jointly with fly ash mission (presently fly ash utilisation programme). As a result of experiance gained through these projects, specifications for construction of road embankments and guidelines for use of fly ash for rural roads were compiled and have since been published by the indian roads congress. Fly ash utilisation in the country rose from 3 per cent (of 40 million tonnes) of fly ash produced annually in 1990s to about 32 per cent (of 110 million tonnes) of fly ash generated annually now. Out of this total utilisation, about 22 percent, amounting to 7.75 million tonnes, was used in the area of roads and embankments last year.
3. APPLICATIONS FOR PFA
PFA AS A FILL MATERIAL
PFA has been used for many years as an alternative to virgin aggregates for embankments, especially in road construction projects (UKQAA b, 2007). PFA is used for fill applications because:
· It is lightweight when compared to most materials, having a particle density of 2.10 to 2.3 and a dry bulk density ranging from 1100kg/m3 to 1450kg/m3. This leads to savings in material, transport costs and reduces settlement in underlying soils.
· When properly compacted, PFA settles less than 1% during the construction period with no long-term settlement. It is an easy material to compact with readily available equipment when used at the optimum moisture content.
· The self-hardening properties of some PFA’s offer considerable strength advantages over natural clay and granular materials. As PFA is pozzolanic, the small quantity of free lime normally present will enhance the strength of the resulting embankment. This reaction can be increased by mixing lime with the PFA when being placed. It is the pozzolanic reaction that is principally used in soil stabilisation and hydraulically bound sub-bases as the binder.
· PFA can exceed the design strength immediately after compaction. The design figures normally quoted are fresh saturated data which are conservative.
· The immediate strength of PFA means simple shallow trenches have a reduced need for shoring.
· With proper profiling PFA fill can be trafficked in all weathers.
PFA embankments should be constructed on a free draining layer that acts as a capillary break. Materials like concrete that may be attacked by sulfates should also isolated from PFA with a capillary break and metallic items should not be placed closer than 500mm from PFA. Reinforced earth embankments can also be constructed if an alkali resistant geo-grid polymer is used, see Figure 1. This allowed self supporting vertical faces to be constructed.
FIGURE 1 – A REINFORCED EARTH PFA EMBANKMENT
The PFA is normally placed in 150mm layers, compacted to 100mm, using standard vibrating rollers. As with all fine grained materials, the surface of the PFA embankment should be capped either with the construction, a capping layer, top soil, etc. It should be profiled to ensure adequate drainage, especially during the construction period. PFA, if properly profiled, will shed rain water easily, allowing construction to continue in poor weather conditions. If it should become saturated with water, it can be dug out, spread out and allowed to dry before being reused without any detriment.
It can be used as both a general fill, complying with class e, or as a structural fill complying with 7b of the specification for highway works. For general fill, 95% of maximum bulk density and moisture content are the critical control parameters, whereas for structural applications the effective angle of internal friction φ’ and effective cohesion c’ are required in addition to the 7b parameters.
PFA IN CONCRETE CONSTRUCTION
While PFA is the preferred UK term, in order to avoid confusion with other ashes from other furnaces, European Standards always refer to fly ash, , such as EN450-1 (BSI a, 2005). PFA has been used in UK concrete since the 1950’s. There are a number of benefits in using PFA based concrete as follows:
· Improves long term strength performance and durability.
· Reduces permeability, which reduces shrinkage, creep and gives greater resistance to chloride ingress and sulfate attack.
· Minimises the risk of alkali silica reaction.
· Reduces the temperature rise in thick sections.
· Makes more cohesive concrete that has a reduced rate of bleeding, is easier to compact, gives better pumping properties and improves the surface finish of the finished structure, e.g. When used in Self Compacting Concrete.
FABM can be specified and formulated to meet capping, sub-base and base requirements of all classes of road, airfield, port, residential and commercial pavements.
In FABM, fly ash is the main constituent of the binder with lime, quick or hydrated, usually the other constituent. Cement can substitute for lime but is not as effective in mobilising the full pozzolanic and thus cementing potential of the fly ash, see Table 1.
Compared to mixtures based on cement, FABM based on lime are slow-setting, slow-hardening, self-healing mixtures. This more protracted rate of hardening has distinct advantages in pavement construction. In the short term, FABM have extended handling times and thus the versatility of unbound granular pavement materials. In the medium term, FABM are autogenous, in that they possess a pozzolanic reserve which allows them to re-heal should say cracking occur under differential settlement. In the long term, FABM develop significant stiffness and strength with the performance and durability of bituminous and cement bound mixtures.
Where quicker hardening is required, say in cold weather, the addition of gypsum or the partial or complete replacement of lime with cement can be employed. FABM based on cement however, behave like cement bound mixtures (CBM) and do not have the advantages of laying flexibility and autogenous healing described above.
Table 1: Compressive strength in mpa of treated fly ash Age of 1:1 sealed cylindrical specimens cured @ 20C
Fly ash with 2.5% cao
Fly ash with 5% cao
Fly ash with
7% CEM 1
Fly ash with
9% CEM 1
4. ADVANTAGES OF USING FLY ASH FOR ROAD CONSTRUCTION
· Fly ash is a lightweight material, as compared to commonly used fillMaterial (local soils), therefore, causes lesser settlements. It isEspecially attractive for embankment construction over weakSubgrade such as alluvial clay or silt where excessive weight could Cause failure.
· Fly ash embankments can be compacted over a wide range of Moisture content, and therefore, results in less variation in density With changes in moisture content. Easy to handle and compact because the material is light and there are no large lumps to be broken down. Can be compacted using either vibratory or static rollers.
· High permeability ensures free and efficient drainage. After rainfall, Water gets drained out freely ensuring better workability than soil.Work on fly ash fills/ embankments can be restarted within a few hours after rainfall, while in case of soil it requires much longer period.
· Considerable low compressibility results in negligible subsequent settlement within the fill.
· Conserves good earth, which is precious topsoil, thereby protecting the environment.
· Higher value of California Bearing Ratio as compared to soil provides for a more efficient design of road pavement.
· Pozzolanic hardening property imparts additional strength to the road pavements/ embankments and decreases the post construction horizontal pressure on retaining walls.
· Amenable to stabilisation with lime and cement.
· Can replace a part of cement and sand in concrete pavements thus Making them more economical than roads constructed using Conventional materials.
· Fly ash admixed concrete can be prepared with zero slump making it amenable for use as roller compacted concrete.
· Considering all these advantages, it is extremely essential to promote use of fly ash for construction of roads and embankments.
b) ECONOMY IN USE OF FLY ASH
Use of fly ash in road works results in reduction in construction cost by about 10 to 20 per cent. Typically cost of borrow soil varies from about Rs.100 to 200 per cubic metre. Fly ash is available free of cost at the power plant and hence only transportation cost, laying and rolling cost are there in case of fly ash. Hence, when fly ash is used as a fill material, the economy achieved is directly related to transportation cost of fly ash. If the lead distance is less, considerable savings in construction cost can be achieved.Similarly, the use of fly ash in pavement construction results in significant savings due to savings in cost of road aggregates. If environmental degradation costs due to use of precious top soil and aggregates from
borrow areas quarry sources and loss of fertile agricultural land due to ash deposition etc. The actual savings achieved will be much higher and fly ash use will be justified even for lead distances up to say 100 km.
C) ENVIRONMENTAL IMPACT OF FLY ASH USE
Utilisation of fly ash will not only minimize the disposal problem but will also help in utilizing precious land in a better way. Construction of road embankments using fly ash, involves encapsulation of fly ash in earthen core or with RCC facing panels. Since there is no seepage of rain water into the fly ash core, leaching of heavy metals is also prevented. When fly ash is used in concrete, it chemically reacts with cement and reduces any leaching effect. Even when it is used in stabilisation work, a similar chemical reaction takes place which binds fly ash particles. Hence chances of pollution due to use of fly ash in road works are negligible.
D) DEMONSTRATION ROAD PROJECTS USING FLY ASH
In the last decade, CRRI had undertaken many R&D and field studies to promote utilisation of fly ash in road embankment and pavement construction. Of the different types of ash available in a power plant, pond ash is available in abundance, it needs no processing and its moisture content happens to be nearer to OMC after having been taken out from pond and stored for one or two days. The major activities in such field
· Investigation/collection of data on existi ng ground/sub-soil conditions
· Characterisation of engineering and physical properties of identified source of fly ash
· Design of embankment structure incorporating ash as a constructionaterial
· Premparation of specifications for construction using fly ash
· Field visits during construction and advising the implementing agency regarding quality control measures to be adopted during construction
· Instrumentation and monitoring
· Preparation of project reports and dissemination of information among the user agencies after completion of the project
E) CHARACTERISATION OF FLY ASH
Engineering and chemical properties of Indian ashes of various power plants tested at CRRI have been found to be favourable to construction of roads and embankments. Properties of fly ash from different power plants vary and therefore it is recommended that characterisation of ash proposed to be used should be conducted to establish the design parameters. The properties of ash depend primarily on type of coal and its pulverisation, burning rate and temperature, method of collection, etc. The significant
properties of fly ash that must be considered when it is used for construction of road embankments are gradation, compaction characteristics, shear strength, compressibility and permeability properties. Individual fly ash particles are spherical in shape, generally solid, though some times hollow. Fly ash possesses a silty texture and its specific gravity would be in the range of 2.2 to 2.4, which is less than natural soils. Fly ash is a non-plastic material.
5. ENVIRONMENTAL IMPACTS
OVERVIEW OF CO2 EMISSIONS
In order to estimate the reductions in environmental impact of using PFA/fly ash in road construction, we need some reasonable estimates of the impacts of the primary materials it is replacing. As fly ash is a by-product of the production of electricity from coal fired generation, it is usual to consider the environmental impacts wholly assigned to the electricity. Therefore, the fly ash would be considered to have a zero impact at the power station gate.
One issue is transportation which will vary from contract to contract. Comparisons between differing sources of materials are difficult to make accurately if there are significantly differing distances and modes of transport being used. However, depending on the location of the contract in relation to quarries, power stations, cement works, railway terminals, etc can all have a significant bearing on the environmental impact associated with material use. In respect of PFA, there has been increasing interest in transporting the material to site using trains for the larger contracts. For example, some 1,100,000 tonnes of PFA were sent to a single grouting contract in Northwich during 2006/7 using trains hauling 2,000 tonnes per day. While these modes of transport are appropriate for larger contracts, the reliable tipping vehicle will be the main mode of transportation for PFA in the foreseeable future. For the following comparisons of Environmental impacts, transportation has been left from the equations, e.g. Materials are taken as from cradle to production facility gate.
In order to make comparative estimates knowledge of the impacts of producing the various materials is needed, whether it is for making concrete, an embankment, etc is needed. The Environment Agency V.2 (EA, 2007) carbon calculator uses CO2 for a variety of materials ranging from concrete, aggregates, timber, metal, plastics, etc. Additionally there is the Waste & Resources Action Programme carbon calculator for recycled aggregates (WRAP, 2006) which is aimed at road construction. For the purposes ,concentrate on CO2 emissions as the important parameter.
The impact of producing aggregates from quarries varies greatly depending on the data source. The Environment Agency V.2 carbon calculator uses CO2 data ranging from 21kg/tonne forcrushed stone, through to 5.3kg/tonne for sand. The Quarry Products Association annual report for 2006 (QPA, May 2007) reports a value of 9.98kg of CO2 per tonne for all aggregates for 2006. Other papers, (Flower, 2007) quote values of 45.9 to 13.9 for crushed rock and sand aggregates respectively. For the purposes of this paper an average CO2 figure of 21kg/tonne of aggregates has been used.
For Portland cement (CEM I) a wide array of figures for CO2 are quoted in many publications Flower quotes a range from 700 kg/tonne to 1,000kg/tonne. Other figures are 740kg.tonne to 970kg/tonne within the Environment Agency CThe depletion of natural aggregate sources has not been considered within these calculations. However, it is arguable that readily available by-products like PFA should be used in preference to the quarrying of virgin materials, which should remain for the use of future generationsarbon Calculator and 801kg from WRAP calculator. For the purposes of this paper a CO2 figure of 860kg/tonne has been used. For hydraulic lime WRAP quotes a CO2 figure of 800kg/tone.
Construction of embankment with fly
Ash at Second Nizamuddin Bridge
Construction of embankment with fly ash
at Kalindi Bypass in Delhi
THE CURRENT ENVIRONMENT BENEFITS OF USING PFA
Some 3,000,000 tonnes per annum are landfilled in the UK and in addition there are some 53,000,000 tonnes of available material on stock. The UK power industry is able to supply large quantities of materials suitable for many applications with ease. These stockpiles represent a major mineral source for future generations. However, it is beholden to this generation to minimise the use of high quality virgin aggregates and cement to leave sufficient resources for future generations and reduce global warming effects.
Currently about 191,000 tonnes of PFA are used annually in embankment construction, representing an overall CO2 emissions saving of ~4,100 tonnes per annum. As all PFA produced, plus the stockpiled ash, is suitable for use as a fill material, this represents a major potential mineral resource for such applications.
When PFA is used as a cementitious binder, considerably greater environmental benefits are achievable than the simple displacement of virgin aggregates. Portland cement inherently produces a large quantity of CO2 during its manufacture as it involves the calcination of calcium carbonate. This releases ~550kg of CO2 for each tonne of cement made. In addition to this chemical release, the raw materials and resulting cement clinker have to be ground to a fine powder, which in itself is an energy intensive process. Even with the most efficient cement work, figures of 700kg/tonne of CO2 for CEM I are only just possible
As well as a cementitious binder, PFA can be used as a raw material within the cement manufacture process. It is used as a source of silica and alumina replacing the clays and sands traditionally used. This market is increasingly significant in the PFA marketing industry.
Replacing some of this Portland cement with pozzolanic materials like fly ash has considerable environmental benefits without compromising technical aspects, e.g. Strength and durability. In fact fly ash enhances many durability aspects of the resulting concrete, e.g. Improved sulfate resistance, prevention of alkali silica reaction, reduced permeability to chloride ions, etc.
The degree of benefit varies depending on the exact specification for the concrete mix and its application. Using BS8500 criteria, Table 3 are some estimates of the relative benefits of various mixes
Table 3: Comparison of CO2 emissions associated with some concrete mix types (excludes aggregates, which are considered constant for mixes) Mix designation
Portland cement (CEM I)
Fly ash equivalent mix
Overall CO2 savings
C30/37 design strength concrete
280 kg/m3 of CEM I ≡ 241 kg/m3 CO2
320 kg/m3 of CEM I + 30% PFA ≡ 193 kg/m3 CO2
-48 kg/m3 (-20%)
RC25/30 MCC260 W/C 0.65
270 kg/m3 of CEM I ≡
232 kg/m3 CO2
290 kg/m3 of CEM I + 30% PFA ≡
175 kg/m3 CO2
-57 kg/m3 (-25%)
XS1 50mm cover
C40/50 MCC380 W/C0.40
395 kg/m3 CEM I ≡
330 kg/m3 CO2
C25/30 MCC320 W/C0.55
320 kg/m3 of CEM I + 30% PFA≡
193 kg/m3 CO2
-137 kg/m3 (-42%)
PFA/fly ash has considerable benefits when used in road construction, whether it is for embankment construction, for concrete in roads and bridges or for sub-base materials as in Fly Ash Bound Mixtures. Where PFA replaces virgin aggregates, or acts as a cementitious binder, significant reductions in overall CO2 emissions are possible to the benefit of the environment. In addition the existing stocks of material represent a large mineral reserve for future generations ensuring the sustainable construction of our road infrastructure. However, we are all responsible for the future of this planet and by maximising the use of by-products materials, such as PFA, this will reduce virgin aggregate depletion and leave resources for the future.