Utilization of Fly Ash in Rural Road Construction in India and its Cost Effectiveness
Lecturer in Applied Mechanics, B.E College (D.U), Howrah, India
Amartya Kumar Bhattacharya
Senior Lecturer in Applied Mechanics, B.E College (D.U), Howrah, India
Sri L. K. Paira
Assistant Engineer, P & R.D. Department, Govt. of West Bengal, India
In construction of rural roads in low lying or flood prone areas, fly ash should have to be considered as normal choice in near future. The physical and chemical properties of fly ash are reviewed first. Then the utilisation of fly ash in road works is discussed. The cost effectiveness of the utilization, especially in rigid pavement construction, is studied.
From the illustrations and discussions, it is found that fly ash can be used economically for embankment construction in the vicinity of thermal power stations when lead distances are appropriate. In case of rigid pavements, usage of fly ash leads to considerable savings even if fly ash is to be transported over large distances. For rigid pavement construction in a large scale, part replacement of cement by dry fly ash is found acceptable. In case of bridge and other hydraulic structures fly ash may be used as a backfill material with necessary earth cover at approach to reduce surcharge, as cinders are not easy available now-a-days. Recommendations are made on the attention to be paid to characterization of fly ash and quality control during construction of road pavement and embankment for better performance of such road sections so that fly ash can be turned from a liability to an asset.
Keywords: Fly ash, rigid pavement construction, embankment construction, bridge backfill
Power generation is the most vulnerable criterion of modern civilization, where thermal process takes lead in comparison with hydro-electricity and others, owing to its easiness and availability of main ingredient that is coal. Nearly 70% of India’s total installed power generation capacity is ’thermal’ of which coal based generation is about 90%. But at the same time, disposal of huge quantity of fly ash generated from the power plants is a burning problem. This is detrimental to animal and plant life, since it pollutes the environment as well as its requires large area for its disposal, when availability of land is getting scarce day by day. Most of the plants now are facing shortage of dumping space for these waste materials. According to report of concerned authority, the accumulated fly ash in the last year over the country was about 100 million tons which is expected to be raised beyond 150 million tones by the year 2010. This necessitates effective utilization of this accumulated fly ash is being felt by the engineers and scientists.
In the meanwhile a good many utilization patterns have been suggested at different levels. But utilization in the field of Civil Engineering extends ample scope for consuming bulk volume efficiently and economically. Precisely to mention, construction of road embankments along with hard crust construction comes in front.
One more thing if mentioned will not be out of context that not only in urban area but also in rural area earth is not an easy available material now-a-days. In construction of rural roads in low lying or flood prone areas, fly ash should have to be considered as normal choice in near future.
In surrounding area of Delhi, use of fly ash for road construction has already been examined and taken over. But in other parts of country this practice is yet lacking. The entire area of Midnapore district of the state of West Bengal has the scope of utilization of fly ash, which will not be too distant from Kolaghat Thermal Power Plant (KTPP). Similarly, the other regions of India which are nearer to thermal power stations should also start this technique.
PHYSICAL AND CHEMICAL PROPERTIES OF FLY ASH
The three major ingradients in fly ash are Silicon, Aluminum and Iron. Fly ash to be used as a filling material should not have soluble sulphate content exceeding 1.90 gm per litre. Otherwise it should not be deposited within 50 cm from concrete or metallic surfaces due to its corroding effects..
Depending upon type of coal, its degree of pulverization and combustion techniques, their collection and disposal systems, etc., the properties of fly ash vary. Ash from two different power plants would be having significantly different properties. However, typical properties of fly ash are given below:
Table 1. Physical and Chemical Properties of Fly Ash
|Sl No||Description||Observed values|
|1||Specific gravity||1.90- 2.50|
|3||Maximum dry density||0.95-1.60 gm/cm3|
|4||Optimum moisture content||19%-38%|
|5||Permeability||8x10-6 to 7x10-4 cm/sec|
|9||Angle of shearing resistance||300- 400|
|10||Coefficient of consolidation||1.75x10-5 to 2.00 x10-3 cm2/sec|
|16||Sulpher Content (SO3)||Traces|
UTILISATION OF FLY ASH IN ROAD WORKS
Fly ash can be used for constructing different layers of road pavement. It is being utilised mostly for filling works and sub grades of road works. An effective interaction between fly ash and sub grade soil will allow scope for stabilization of sub grade. As per specification of Ministry of Road Transport & Highway, Government of India, clayey soil having plasticity index more than 8 are required to be treated and stabilised before road is constructed. If the type of soil available is responsive to pozzolanic action with fly ash, strength parameters of the soil would be improved. Clayey soil when mixed with fly ash exerts cementing property because of existence of puzzolanic compound.
Though silty soil reacts with admixture of fly ash to a little extent but it can be develop by adding lime, which will reduce leaching action. Moreover mixing of lime helps to attain adequate strength and will harness durability.
It is experienced that the compacted fly ash attains sufficient shear strength so that the embankment can be constructed with 2:1 (Horizontal:Vertical) side slope, when the factor of safety for embankment constructed using fly ash should not be less than 1.25 under normal serviceability condition. Special care is required to be taken in respect to provide earthen membrane over the slope of embankment since ash is highly erodible.
An admixture of soil and fly ash improves plasticity index, liquid limit, plastic limit and C.B.R. values to acceptable limits.
The transportation cost of fly ash, limits the average distance upto which fly ash can be economically utilized in place of borrow soil. The guidelines issued by Ministry of Environment & Forest, Government of India, is now essentially to take up road works specially embankment construction using fly ash within 100 KM radius of thermal power station.
It is apprehended that fly ash for supplementing or alternating earth work and also for sub-grade improvement. Strictly following the guideline of Indian Roads Congress - SP:58 (2001), use of fly ash should be considered mandatory in road embankment construction in areas where fly ash is available in adequate quantities unless rejected for any technical reason by the Engineer-in-charge. Also, if fly ash is utilized as subgrade material having higher C.B.R. values, the required pavement thickness of Road pavement would go down substantially resulting into strong durable and economical roads.
A comparative picture is worked out taking a typical cross section of pavement (Figure 1), where fly ash is used in sub grade. This will show an acute cost effectiveness. The design data are assumed reasonably as follows:
Initial traffic in the year of completion of construction = 42.
Design life = 10 years.
Growth rate factor = 6% (= 0.06).
Projected traffic = 24(1+0.06)10 = 75
C.B.R. value of sub grade = 2.8
As per the design specification of “Rural Road Manual” special publication, Indian Roads Congress: SP-20 (2002), pavement thickness comes = 500 mm.
Top width of embankment = 7.5 m
Height of embankment = 2.0 m.
Side slope – 2H : 1V
Bottom width of embankment = 15.5 m
Side slope earth over thickness = 1.0 m
Length of embankment = 1000 m.
Figure 1. A typical cross section of pavement without fly ash.
In case fly ash is used in subgrade in place of soil, C.B.R. value of subgrade will be very easily increased to 7 from 2.8. Hence, the required pavement thickness work out 300 mm in place of 500 mm.
Figure 2. A typical cross section of pavement with fly ash.
The saving in cost for 3.75 m wide 1000 m long road where average lead of sub-base material and drainage layer within 20 km has been estimated in the Table-2.
Table 2. Saving of cost due to adoption of fly ash
|(a) Sand moorum||0.15 x 8.0 x 1000 = 1200||@ Rs. 400.00 per m3 = 4.80 lakhs|
|(b) Laterite||0.075 x 3.75 x 1000 = 281.25||@ Rs. 500.00 per m3 = 1.40 lakhs|
|Total saving: 6.20 lakhs|
Hence saving per km of road = 6.20 lakhs.
COST EFFECTIVENESS IN RIGID PAVEMENT CONSTRUCTION
In rigid pavements fly ash can be used to replace a part of cement or sand or both. Considering a typical case of rural road construction, where roller compacted concrete is proposed to be used. Approximately 25 % of the volume of cement of actual consumption is expected to be saved. Thereby 15% of the cost of construction will be saved if fly ash is collected with a lead of 10 to 15 km.
Onward some more discussion regarding FaL-G concrete may open a new horizon in road construction process. Fal-G concrete is an admixture of Fly ash, lime and Gypsum. Corresponding to the maximum compressive strength, a mix proportion of 60 : 30 : 10 (Fa : L : G) may be selected. The compressive and flexure strength of the Fal-G concrete has been presented in Table 3.
A blend of Fal-G concrete in ratio 60:30:10 will take super-hand over ordinary portland cement (O.P.C.) in consideration of cost effectiveness. Cost is expected to be reduced by 30-60%.
Table 3. Compressive and flexural strength of Fal-G concrete. [Water Cement ratio = 0.50]
|FaL-G mix proportion||7 day average compressive strength (MPA)||14 days average compressive strength (MPA)||28 days average compressive strength (MPA)||28 days average flexural strength MPA|
|60 : 30 : 10||6.40||8.50||9.00||0.80|
From the above illustrations and discussions, it is clear that fly ash can be used economically for embankment construction in the vicinity of thermal power stations when lead distance are less than 10 to 15 km. In case of rigid pavements, usage of fly ash leads to considerable savings even if fly ash is to be transported more than 50 km or perhaps 100 km. For rigid pavement construction in a large scale, part replacement of cement by dry fly ash is acceptable. Thermal power plants should be directed to provide dry fly ash in bags (polythelene) for convenience where a reasonable cost may be charged.
In case of bridge and other hydraulic structures fly ash may be used as a backfill material with necessary earth cover at approach to reduce surcharge, as cinders are not easy available now-a-days.
Adequate attention should be paid to characterization of fly ash and quality control during construction of road pavement and embankment for better performance of such road sections so that fly ash can be turned from a liability to an asset.
The authors acknowledge their debt to Mr. S.K. Konar, Technical Consultant, Paschim Medinipur Zilla Parishad for assisting with requisite data essential for the scheme.
|© 2004 ejge|