1. Site Preparation: Compact and level the subgrade or base.
2. Mix Design: Select aggregates and bitumen, perform mix design to determine the optimum binder content.
3. Mix Preparation: Heat aggregates and bitumen, mix thoroughly in the asphalt plant.
4. Transportation and Paving: Transport the mix and lay it using a paver.
5. Compaction: Compact the mix using a combination of breakdown, intermediate, and finish rollers.
6. Surface Finishing: Ensure the surface is smooth and meets the required specifications for skid resistance and texture.
7. Quality Control: Perform routine checks for binder content, aggregate gradation, and compaction.
8. Maintenance: Allow the surface to cool and perform routine inspections.

Step-by-step procedure for laying Dense Bituminous Macadam (DBM):

1. Site Preparation

• 1.1 Subgrade Preparation
  • Ensure that the subgrade (the underlying layer) is properly compacted and prepared before laying the DBM.
  • Perform compaction using a vibratory roller to achieve the required density (typically around 95-98% of the maximum dry density as per the compaction test).
  • Level the subgrade to the required profile, ensuring proper drainage and slope.
• 1.2 Base Layer Check
  • If the DBM is being used as a binder layer over a WMM (Water Bound Macadam) or other base layer, ensure that the base layer is leveled and compacted.

2. Mix Design

• 2.1 Aggregate Selection
  • Choose the appropriate aggregates (both coarse and fine aggregates) that meet the required gradation for the DBM.
  • The coarse aggregates should be angular and durable to provide strong interlocking in the mix.
  • The fine aggregates (usually sand) should fill the voids between the coarse aggregates.
• 2.2 Binder Selection
  • Select the bitumen grade according to the design specifications. The typical grades used are VG-30 or VG-40, but the grade may vary based on climatic conditions, traffic loads, and other factors.
• 2.3 Design Mix
  • Optimum Binder Content (OBC): Perform mix design to determine the optimum binder content for the DBM, usually ranging between 4% to 6% by weight of the total mix.
  • Use the Marshall Method or Superpave method for mix design, conducting tests to determine properties like Marshall Stability, Flow, and Density.
  • Perform tests to check for air voids, voids in mineral aggregate (VMA), and voids filled with bitumen (VFB).

3. Preparation of the Mix

• 3.1 Heating of Materials
  • Aggregate Heating: Heat the aggregates to a temperature between 150°C to 170°C. Ensure that the aggregates do not overheat, as it may cause degradation or damage to the particles.
  • Bitumen Heating: Heat the bitumen to a temperature of 160°C to 180°C. Ensure that the bitumen is not overheated as it may lose its effectiveness or become too viscous for proper mixing.
• 3.2 Mixing
  • In the asphalt mixing plant, combine the heated aggregates and bitumen. The mix should be thoroughly blended to ensure that each particle of aggregate is uniformly coated with bitumen.
  • Mixing Time: The mix should be thoroughly mixed for the correct amount of time to ensure that all aggregates are coated uniformly, and the bitumen is uniformly distributed.

4. Transportation and Paving

• 4.1 Transporting the Mix
  • After mixing, the DBM mix should be transported in asphalt transport vehicles to the site. Maintain the mix at an appropriate temperature (around 150°C to 160°C) during transport to prevent segregation or cooling.
  • Use thermal covers to maintain the temperature and avoid loss of heat.
• 4.2 Laying the Mix
  • Paver Setup: Use an asphalt paver for laying the mix. The paver should be set to achieve the desired thickness of the DBM layer, typically between 40mm to 75mm, depending on the design.
  • Paving Width: Ensure that the paving width matches the project requirements.
  • Continuous Paving: The paver should lay the material in continuous passes without any gaps or overlaps.
• 4.3 Tack Coat
  • If the DBM is being laid on an existing surface, apply a tack coat of diluted bitumen to promote adhesion between the new layer and the old surface. The tack coat should be applied at a rate of 0.3 to 0.5 kg/m², depending on the surface condition.

5. Compaction of the DBM Mix

• 5.1 Breakdown Rolling
  • Immediately after the DBM is laid, start compacting the mix using a vibratory roller (static roller can also be used depending on the thickness of the layer).
  • Perform breakdown rolling to reduce the air voids and improve the interlock between the aggregates.
  • Ensure that the initial compaction is done while the mix temperature is between 130°C to 160°C.
• 5.2 Intermediate Rolling
  • After the initial breakdown, use a rubber-tired roller for intermediate compaction. This will help further densify the mix and provide the desired surface texture.
• 5.3 Finish Rolling
  • Finally, perform finish rolling with a steel-wheel roller to achieve a smooth, dense, and uniform surface.
  • Aim for a final density of approximately 95%-98% of the theoretical maximum density (Gmm). Regularly monitor the in-place density using a nuclear densometer.
• 5.4 Compaction Temperature
  • Avoid over-compaction, and ensure the mix temperature does not drop below 120°C before finishing compaction. If the mix cools too much, it may result in a non-compactable surface.

6. Surface Finishing

• 6.1 Surface Texture and Smoothness
  • Ensure that the surface has a uniform texture and meets the required smoothness specifications. This can be checked using a profilograph or stringline to ensure the surface is within the specified tolerance.
• 6.2 Skid Resistance
  • Ensure that the surface texture provides adequate skid resistance for the intended traffic conditions. If necessary, apply a skid-resistant treatment to the surface to improve safety.

7. Quality Control and Testing

• 7.1 Quality Control During Construction
  • Perform regular quality control (QC) checks during the construction process. This includes:
    • Binder content checks to ensure it matches the designed content.
    • Aggregate gradation to verify it meets the specifications for DBM.
    • Compaction tests to ensure the layer achieves the required density.
• 7.2 Laboratory Testing
  • After laying, sample the DBM mix for further laboratory testing to ensure it meets the mix design properties, including Marshall Stability, Flow, Density, and Bitumen Content.
• 7.3 Field Testing
  • Conduct field tests such as core sampling to check the in-place density and confirm the final quality of the pavement.

8. Post-Construction Maintenance

• 8.1 Allow the Surface to Cool
  • After laying and compacting the DBM, allow the surface to cool naturally. The mix should be protected from traffic until it has cooled down sufficiently (typically to 40°C to 50°C).
• 8.2 Inspection
  • Regularly inspect the surface for deformation, cracking, or any other signs of premature failure. Conduct timely maintenance such as crack sealing or patching as required.