Utilisation of Fly Ash in Road Construction

 

Fly ash is an industrial waste from thermal power stations using pulverised coal as boiler fuel. Disposal of huge quantity of fly ash produced in country which gets easily air borne (thats why the name 😌 ) and thus is a health hazard to the community, has grown into a pressing problem. On the other hand both laboratory and field studies have confimed that this waste material can be used profitably in road construction by taking advantage of its pozzolanic action with lime leading to economy in cost.

Despite its known merits, utilisation of fly ash for road construction in this country has been very limited due to Storage issue and and High O.M.C. requirement almost 25%. This could be attributed to two main reasons namely -

i) Non availability of fly ash of assured quality, among other factors due to the mixing of fly ash with coarse bottom ash at the extraction stage in power plants and

ii) virtual absence of a marketing system assuring to the consumers supply of fly ash as per I.S. specifications.

For overcoming these deficiencies simultaneous action is being taken to collect fly ash in an unmixed form and to market it with assurance about its quality. with these measures coming into operation, it is felt that there should be no difficulty in greater utilisation of fly ash for road works.

Some specifications using flyash:

i) Lime-fly ash soil stabilisation. 

ii) Lime-fly ash concrete.

iii) Lean cement-fly ash concrete and

Specifications:- 

Indian Standards Institution has already laid down quality requirements of fly ash vide 

IS: 3812 (Part-I) <-- Specification for Fly Ash for Use as Pozzolana" 

IS: 3812 (P.II) <-- "Specification for Fly Ash for use as Admixture for Concrete" 

IS: 3812 (Part-III) <-- "Specifications for fly ash for Use as Fine Aggregate for Mortar and Concrete"

Out of the above specifications one that can be put into practice immediately is stabilisation of alluvial soil with lime and fly ash. This is a kin to lime stabilisation except that a percentage of fly ash is incorporated alongwith lime. Laboratory studies and field trials have shown that this specification can be profitably employed for construction of sub-base on all classes of roads and even as a base on lightly trafficked roads. As a positive step towards exploiting the benefits of this specification, it has been decided by the Committee of Economic Secretaries that for construstion of sub-base on National Highway works, where these jobs falls within economic orbit of the thermal power stations, lime-fly ash stabilisation should be increasingly used in place of conventional granular matorials like brick soling and oversize W.B.M. Proposals for National Highway Works should in future be framed keeping this requirement in view. Similarly, it has been decided that lime fly ash stabilisation should be used as much as feasible for the construction of sub-base/base courses on roads under the control of State Governments.

For actual application of the above specification on specific works, it will be necessary to detemine mix proportions using local soil and available lime/fly ach. This could be done through the State Laboratory/Central Road Research Institute, taking design factors into consideration.

Practice for Lime fly ash stabilised soil as Sub-base In Pavement Construction. 

In many parts of the country fly ash is available as a waste products from the Thermal Power Plants. This recommended practice describes a method of utilisation of fly ash for stabilisation of soils with the help of lime so as to replace the soling or oversize metalling in sub-base course of the pavement.

Applicable Layer

Stabilising soil with a mixture of lime and fly ash for use as a sub-base course in road pavements.

Design.

The thickness of the sub-base should be designed as per IRC- 37: 2018 entitled "Guidelines for the Design of Flexible Pavement. The C.B.R.value of the stabilised soil is to be determined in the laboratory for pavement design purposes. It has been observed that soils stabilised with lime fly ash mixture give a soaked laboratory C.B.R. value of 40-70 which should yield a field soaked C.B.R., value of 20-35. 

Materials

    Soil:-

Normally soils with a Plasticity Index (PI) between 5 and 20 are suitable. This technique may also be applied to soils having PI value beyond these limits provided detailed investigations show that these will be suitable.

    Lime:-

Normally lime used in the stabilised works should have purity (CaO plus MgO content) of not less than 60% when testud in accordance with IS:1514.

    Fly Ash:-

 Fly ash should conform to IS: 3812 (Part III).

     Water:-

Water used for both mixing and curing the stabilised soil should be clean and free from injurious amount of deleterious matter. Potable water is generally considered satisfactory for mixing and curing lime fly ash stabilised soil.

    Mix Proportions:-

The mix proportion should be determined through laboratory tests meeting the strength requirements. 

Typical mix proportion of soil, lime, fly ash
Soil	85 parts by weight
Lime	3 part (based on 80% purity of lime)
Fly ash	12 parts by weight

    Water

The quantity of water should be as per the O.M.C. requirements determined on soil, lime fly ash mix- ture by Proctor density method.

Construction

        Fly ash stabilised soil utilizes the same construction technique as used for lime or coment stabilisation.. It is preferable that mixing is done by mechanical plants either of single pass or multiple pass type. Where such plants are not available, manual method may be adopted with rigorous control over quality of construction. 

        In the manual method, the soil is pulverised by means of crowbars, pick-axes, bullock-drawn plough, Tractor etc., and deposited on the road bed in stacks of suitable size about 300 mm height. Water in requisite quantities may be sprinkled on the soil for aiding pulverisation. The degree of pulverisation should be as given in the below table :

Sieve Designation	% by weight passing the sieve
25 mm	100
4.75 mm	60

        On the pulverised soil stacks, lime and fly ash in a thoroughly mixed form and in requisite quantities should be spread uniformly and mixed by cutting with spades till the whole mass is uniform. The mixed soil should then be spread over the prepared sub-grade to the required thickness and rolled. Before rolling the moisture content should be adjusted to be within + 1% and - 2% of O.M.C.

Rolling:-

        Rolling should be done with 8-10 tonne rollor. Rolling is continued till the required density (100% of Lab. Proctor density) and a smooth surface is obtained without leaving any roller marks on the surface.

Curing:-

        The compacted surface should be cured for a mini- mum period of 7-days before the next layer is placed. Curing is done by sprinkling water over the surface 5 or 6 times a day. The surface should not be allowed to dry during the curing period. Curing by ponding should not be adopted.

Practice for Lime-Fly Ash Concrete

        Lime-flyash concrete can be used in pavement as base/sub-base in flexible and rigid pavement construction. This is particularly useful as a base course in heavy rainfall areas or in black cotton soil areas when laid over lime stabilisod black cotton soil.

Applications:-

This work consists of providing lime-flyash concrete as sub-base or base course in flexible or rigid pavement construction.

Design:-

    Thickness:-

            The thickness of the base/sub-base for flexible pavements is to be designed as per C.B.R. method of design (Indian Roads Congress: 37) "Guidelines for Design of Flexible Pavements", with the adoption of an equivalency factor of 1.25 to 1.5 depending on the discretion of the designer for lime fly-ash concrete and then if need be, checking the load factor of lime fly ash concrete slab through Meyerhof equation for ultimate load which would take care of borth flexible and somi- rigid pavement design considerations. For sub-base under rigid pavements, the pavement is to be designed as per Indian Roads Congress: 56-1974 "Guidelines for the Design of Rigid Pavements for Highways". In any case the thickness of the layer should not be less than 10 cm.

    Meterials:-

        Lime:- 

            Lime used in these works should have a purity of not less than 60% when tested in accordance with IS: 1514.

        Fly Ash:-

                Flyash should conform to IS: 3812 (Part-II)

        Coarse and Fine Aggregates:-

                Should conform to 15: 383

        Water:-

                Water used for both mixing and curing the lime fly ash concrete shall be clean and free from injurious amount of deleterious matter. Potable weter is generally considered satisfactory.

   Strength and Mix Proportions:-

        The 28 day compressive strength of lime-fly ash concrete should be in the range of 40-60 kg/cm². The actual mix to be used should be designed in the laboratory by trial and error.

    Guidance about the suitable mix proportion to bo used in preparation of lime fly ash concrete utilising good quality crushed stone, medium coarse sand and fly ash can be had from the following typical mix.

        Typical Mix Proportion by weight:-

Typical proportion mix by weight
Lime	1 part (based on 80% purity of lime)
Fly ash	2 parts
Fine aggregates	2.7 parts
Coarse aggregates	6.3 parts
Water	11% by weight on dry mix

    Construction:-

        The construction technique for lime-fly ash concrete bases is similar to cement concrete construction in respect of batching, mixing and placement except compaction. The compaction of the mix is done by rolling with 8 -10 tonne roller.

    Curing:-

        The compacted surface should be cured for a minimum period of 7 days before the next layer is placed. Curing is done by sprinkling water 5 to 6 times a day, over the surface. The surface should not be allowed to dry during the curing period. Curing by ponding should not be adopted.

    Surfacing:-

        Rigid pavement can be laid directly over the Lime Fly ash concrete bases. In case of flexible pavements, a minimum cover 100 mm, thick consisting of either W.B.M. or (Grading-I and Grading-II) or any bitumen bound layer should be provided before laying the wearing surface.

Practice for Lean Cement FlyAsh Concrete

        Lean cement fly ash concrete can be used in all constructions where lean cement concrete is used. It can be used as sub-base/base course in flexible and rigid pavement construction. Addition of fly ash in lean cement concrete mixes, decreases bleeding and segregation, improves plasticity and cohesiveness and permits casier placing & finishing of concrete.

    Application:-

        This work consists of providing lean cement fly ash concrete as sub-base or base course in flexible or rigid pavement construction. With suitably designed mixes of such concrete, it is possible to achieve substantial saving in cement to the extent of 35% - 40% compared to conventional lean concrete mixes.

    Design:-

        Thickness:-

            The layer thickness for flexible pavements is to be designed as per  IRC-37 "Guidelines for the design of Flexible Pavement" taking the equivalency factor of lean-cement fly ash concrete in terms of granular material such as W.B.M. or (Grading-I and Grading-II) in the range of 1.25 to 1.5, depending on the designer's discretion and then if need be, chocking the load factor of the loan concrete slab through Meyerhof equation for ultimate load which would take care of both flexible and semi-rigid pavement design considerations. For sub- base under rigid pavements, the pavement is to be designed as per IRC-58 " Guidelines for the design of Rigid Pavements for Highways ". In any case, the thickness of the layer should not be less than 100 mm.

    Cement:- 

            Cement used in these works should conform to IS: 269.

        Fly Ash:-

                Flyash should conform to IS: 3812 (Part-II)

        Coarse and Fine Aggregates:-

                Should conform to 15: 383

        Water:-

                Water used for both mixing and curing the lime fly ash concrete shall be clean and free from injurious amount of deleterious matter. Potable weter is generally considered satisfactory.

   Strength and Mix Proportions:-

        The 28 day compressive strength of lime-fly ash concrete should be in the range of 40-60 kg/cm². The actual mix to be used should be designed in the laboratory by trial and error.

    Guidance about the suitable mix proportion to bo used in preparation of lime fly ash concrete utilising good quality crushed stone, medium coarse sand and fly ash can be had from the following typical mix.

        Typical Mix Proportion by weight:-

Typical proportion mix by weight
Cement	1 part
Fly ash	3.5 parts
Sand	3.5 N parts (Where N = ratio of Specific gravities of Fly ash and Sand) Usually range of N = 0.8 to 0.85
Coarse aggregates	14 parts

    Water Requirements:-

        Because of higher specific surface of the fly ash the water content required will be substantially more than ordinary Lean Coment Concrete, in order to keep the same workability. A water cement ratio of the order of 2.2 is usually found suitablo.

    Construction:-

        Lean cement fly ash concrete is required to be mixed in a concrete mixer and compacted by rolling with 8 - 10 tonne roller as is done in the case of Lean Cement concrete.

    Curing:-

        The compacted surface should be cured for a minimum period of 7 days before the next layer is placed. curing is done by sprinkling water 5 or 6 times a day, over the surface. The surface should not be allowed to dry during the curing period.

    Surfacing:-

        Rigid pavement can be laid directly over the lean cament fly ash concrete bases. In case of flexible pavements, a minimum cover of 100 mm. thick consisting of either W.B.M. or (Grading-I and Grading-II) or any bitumen bound layer should be provided before laying the wearing surface.