Profile Corrective Course (DBM) Construction Methodology

This work involves preparing an existing road surface (granular or bituminous) and laying a bituminous mix, typically 50 mm to 100 mm thick, to correct the pavement profile and strengthen the surface.

1.0 Scope and Preparation

The Profile Corrective Course (PCC) is a layer of varying thickness designed to correct the existing pavement profile (e.g., local depressions, sags, or incorrect camber/super-elevation) before the main overlay.

Existing Bituminous Surface

• Repair: Existing potholes and cracks must be repaired and sealed according to relevant specifications (MoRT&H Clauses 3004.2 and 3004.3).
• Scarifying: Where specified, the existing bituminous layer is carefully removed without disturbing the underlying layers. The exposed base is reworked, compacted, and primed (Clause 502) if needed.
• Tack Coat: After preparation, a Tack Coat (bituminous emulsion) is applied to the surface before laying the new DBM.

Existing Granular Surface

• All loose materials are removed, and the surface is prepared to be firm and clean.
• If the PCC is bituminous, the granular surface must be primed (Clause 502) after removing loose material.
• The surface must be free from dust and capable of being swept by a mechanical broom.

Pre-Laying Checks

Before and after scarifying/preparation, the existing road top levels must be checked and recorded by the Engineer's representative.

2.0 Resource Arrangement: Plant & Equipment

3.0 Materials and Mix Production

3.1 Materials

• Aggregates: Coarse and fine aggregates from approved quarries.
• Bitumen: VG-30 or VG-40 grade from approved sources.
• Tack Coat: Rapid setting Bituminous Emulsion from approved sources.

3.2 Job Mix Formula (JMF)

The JMF determines the precise proportions of coarse aggregates, fine aggregates, bitumen, and mineral filler/additives. It is designed in the site laboratory and must be approved by the Independent Engineer (AE/IE)/PMC. The process is the same as we do for DBM & BC mix designs.

3.3 Hot Mix Plant Operation

• Storage: Bitumen (VG-30/VG-40) stored in automatic temperature-controlled tanks (150°C to 165°C).
• Aggregates: Separate cold bins and feeders proportion the aggregates and dust into the dryer.
• Drying: Dryer continuously heats and dries aggregates (150°C to 170°C) minimizing fuel consumption and dust release.
• Mixing: Aggregates and binder combined in an inert, steam-laden atmosphere. Bitumen Temp: 150°C–165°C, Aggregate Temp: 150°C–170°C, Mixture Max Temp: 165°C.

4.0 Laying and Compaction

4.1 Transportation of Mix

• Mix loaded into tipping trucks (up to 20 MT). Beds sprayed with release agent. Covered with tarpaulin to maintain temperature.
• Temperature at loading: 155°C to 165°C.

4.2 Application of Tack Coat

• Applied using a mechanical sprayer. Rate: 0.25–0.30 Kg/m². Emulsion Temp: 20°C–70°C.
• Allow tack coat to cure before DBM layer is laid.

4.3 Spreading and Finishing

• Sensor wire fixed over brackets at 10m intervals. Paver guided to achieve designed finished levels.
• Laying temperature (Min): 125°C. Paver uses fully activated screed with tampers/vibrators. Loose thickness allowance: 25%. Manual methods for irregular areas.

4.4 Compaction

• Equipment: Vibratory Tandem Roller + PTR.
• Rolling sequence: From lower end to higher side. Passes determined on-site.
• Density: Rolling until specified density achieved or no further movement. Transverse joints cut full thickness; edges painted with hot bitumen/tack coat.

5.0 Quality Control and Traffic

• Core Cutting: After 24 hours to check density.
• Bitumen Testing: Softening Point, Penetration, Ductility for each consignment.
• Surface Finish: Must conform to Clause 902 of MoRT&H.
• Opening to Traffic: Minimum 24 hours after laying. Traffic diversion with cones, caution boards, flagmen.