Do It Yourself Rural Roads Construction Using Fly Ash
2015 Edition

Physical Properties

• Bulk density ranges from 0.9 to 1.3 gm/cc, lighter than typical soil (1.3-1.8 gm/cc), which effectively reduces settlements.
• Specific gravity lies between 1.6 and 2.6, compared to soil at 2.55-2.75.
• Exhibits low to no plasticity, enhancing stability.
• Consists mainly of fine sand and silt with minimal clay content.
• Water retention capability is between 40% and 60%, higher than that of soil.
• Porosity varies from 30% to 65%, ensuring efficient drainage.
• Fineness as per Blaine test is at least 250 m²/kg.
• Suitable for compaction over a broad moisture range using vibratory or static rollers.

Chemical Properties (per IS:1727)

Advantages for Rural Road Construction

• Lightweight, which minimizes settlement over weak substrates.
• High California Bearing Ratio (CBR), leading to reduced pavement thickness.
• Pozzolanic hardening effect, resulting in strength gain over curing time.
• Can be stabilized effectively with cement or lime, enhancing unconfined compressive strength.
• High permeability ensures rapid drainage.
• Low compressibility, thereby minimizing post-construction deformation.

Construction and Erosion Control for Fly Ash Embankments

• Preferred materials include pond ash or bottom ash; fly ash itself is non-plastic with zero cohesion, making it susceptible to erosion.
• Prior to embankment construction, compact the existing ground thoroughly.
• Employ vibratory rollers preferably, otherwise static rollers for compaction.
• Loose layer thickness should be limited to 200 mm when using static rollers and 400 mm with vibratory rollers.
• Achieve a minimum compaction of 98% of the maximum dry density as per Standard Proctor test.
• Side protection with soil cover is essential to prevent erosion caused by wind and rainfall.
• Soil cover thickness should be 30 cm for embankments under 1 m height and increase to 50 cm for taller embankments.
• For embankments exceeding 3 meters in height, side soil cover thickness and intermediate soil layers should be increased accordingly.
• Use soil with maximum dry density over 1.52 g/cc for embankments up to 3 m in non-flood areas and 1.60 g/cc elsewhere, with plasticity index between 5% and 9%.
• Subgrade or shoulder soils should have dry density not less than 1.75 g/cc.
• Turfing on slopes is recommended to further reduce erosion.
• Fly ash should be kept moist during handling and transported in covered vehicles.
• The top embankment shall have 500 mm earth cover with at least 300 mm subgrade thickness.
• When subgrade CBR is 5% or more, the reverse trench method is recommended to minimize borrowing earth.

The design approach and detailed specifications are available in IRC SP:58 and Clause 306 of MORD Specifications 2014.

Stabilization Techniques for Fly Ash with Cement or Lime

1. Material Selection:

   • Prefer dry fly ash for optimal reactivity; pond ash can be stabilized but generally requires more cement.
   • Lime used should contain at least 50% calcium oxide, either quicklime (CaO) or hydrated lime (Ca(OH)2).
   • Cement types include Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), and Portland Slag Cement (PSC) conforming to IS:269, IS:455, and IS:1489 respectively.

2. Mixing Process:

   • Spread fly ash evenly over the working surface.
   • Apply cement or lime uniformly at specified intervals on uncompacted fly ash.
   • Use tractor-towed rotavators for thorough mixing.
   • Add water and remix to achieve uniform moisture content.
   • Compact the stabilized layer as per MORD rural roads specifications.

3. Strength Requirements:

   • Minimum 7-day unconfined compressive strength (UCS) for base course: 2.76 MPa.
   • Minimum 7-day UCS for sub-base: 1.70 MPa.
   • Cement content typically varies between 3% and 9%.
   • Incorporation of coarse aggregates may be necessary if UCS values are inadequate.

4. Curing:

   • Proper curing for 7 to 14 days significantly enhances strength.

Typical UCS Values for Cement Stabilized Fly Ash (Kahalgaon Fly Ash)

Chemical Requirements for Fly Ash as Pozzolanic Material

Environmental Impact and Safety Aspects of Fly Ash in Rural Roads

• Dust emissions are controlled by maintaining moisture content in fly ash during construction activities.
• Erosion is mitigated by applying soil covers and establishing grass turf on embankment slopes to prevent runoff damage.
• The presence of topsoil and pavement layers significantly reduces rainwater infiltration, limiting leaching of contaminants.
• Indian fly ash contains low concentrations of heavy metals due to the high ash content in local coal; the typically alkaline conditions of fly ash-water mixtures inhibit heavy metal mobility.
• Water quality assessments reveal that leachate from fly ash is within permissible limits as per Indian drinking water standards (IS:10500) and industrial effluent norms (IS:2490).
• Regulatory bodies such as the EPA classify fly ash as non-hazardous waste when used in road construction, affirming its environmental safety.

Summary of Environmental Safeguards

With appropriate construction controls, fly ash usage in rural road projects poses negligible environmental risks.

Quality Assurance Tests and Standards for Fly Ash in Roadworks

1. Chemical Composition (per IS:1727 and IS:4032):

   • Combined SiO2, Al2O3, and Fe2O3 content: ≥ 70% for anthracitic fly ash, ≥ 50% for lignitic fly ash.
   • Silicon dioxide (SiO2): ≥ 35% (anthracitic), ≥ 25% (lignitic).
   • Magnesium oxide (MgO): ≤ 25% (anthracitic), ≤ 5% (lignitic).
   • Sulfate (SO3): ≤ 2.75% (anthracitic), ≤ 3.5% (lignitic).
   • Alkalis (Na2O + K2O): ≤ 1.5%.
   • Chloride content: ≤ 0.05%.
   • Loss on ignition (LOI): ≤ 5%.

2. Physical Properties:

   • Fineness as measured by Blaine test: minimum 250 m²/kg.
   • Maximum 40% particles retained on 45-micron sieve.
   • Lime reactivity: minimum 3.5 N/mm².
   • Soundness tests (autoclave expansion ≤ 0.8%, Lechatelier expansion ≤ 10 mm).

3. Mechanical Strength:

   • Cement stabilized fly ash sub-base should achieve at least 1.70 MPa UCS after 7 days.
   • Base layers require a minimum UCS of 2.76 MPa at 7 days.
   • Cement content typically ranges from 3% to 9%, with strength improving on curing periods of 3, 7, and 14 days.

4. Construction and Testing Procedures:

   • Mixing of fly ash with cement or lime should be conducted using tractor-drawn rotavators.
   • Embankments should be compacted to a minimum of 98% of the maximum dry density as determined by the Standard Proctor test.
   • A minimum 500 mm earth cover, including 300 mm subgrade, should be provided atop the embankment.
   • Follow MORD clauses 306, 404, and 409 along with IRC SP:89 for durability and quality tests.

Summary Table: Critical Fly Ash Quality Parameters