Road Surface Treatments: Bituminous Layers and Irrigation
Road Surface Treatments Explained
Surface treatments involve applying a thin bituminous layer (1-4 cm). Their primary purposes are:
- To provide specific surface features like texture and waterproofing.
- To improve adhesion.
However, they generally do not increase the structural strength capacity or improve the evenness of the underlying pavement structure.
Gravel-Free Irrigation Methods
Black Irrigation
This involves surface irrigation on low-intensity traffic routes. It provides waterproofing, aging protection, and rejuvenation of the pavement surface. It may be further sealed with sand.
- Products: Fluid binders, slow-breaking (L) or medium-breaking (M) anionic emulsions (EA) diluted in water.
- Application Rate: 0.2 to 0.4 kg/m² of residual bitumen.
- Execution: Sweeping, wetting, emulsion irrigation.
Dust Control Irrigation
This is the application of a binder on an unpaved road. Its function is to prevent dust generation, limit erosion, and provide waterproofing, although its effect is temporary.
Primer Irrigation (Between Pavement Layers)
Application of a fluid binder onto an untreated surface, such as a granular layer or subgrade. The material must penetrate the surface.
- Function: To prepare the surface and ensure a bond with subsequent bituminous mixture layers.
- Materials: Special primer emulsions (EAI, ECI) and slow-breaking emulsions (EAL-1, ECL-1).
- Application Rate: Aim for absorption within 24 hours, approximately 1 kg/m² of residual bitumen (minimum 0.5 kg/m²).
- Execution: Sweeping, wetting, irrigation, and potentially covering with aggregate.
Adhesion Irrigation (Tack Coat)
Application of a small amount of asphalt emulsion onto an existing treated surface (asphalt or hydraulic binder).
- Function: To ensure adhesion between layers.
- Materials: Fast-breaking emulsion.
- Application Rate: Low, typically less than 0.3 kg/m².
Curing Irrigation
Ensures moisture retention during the setting and hardening of cement-treated layers.
- Materials: Similar to adhesion irrigation materials, or special film-forming concrete curing compounds.
Note: After sweeping and watering, a new tack coat (adhesion irrigation) is applied before extending the asphalt mix.
Gravel Irrigation Methods (Chip Seal)
This involves applying bituminous irrigation followed by a uniform layer of gravel, which is then compacted (tamped).
Types:
- Monolayer: One application of binder and one layer of aggregate.
- Bilayer or Multilayer: Several successive applications of binder and aggregate, typically with decreasing aggregate sizes.
- Monolayer Double Graveling: Irrigation + thick grit + fine gravel.
- Sandwich Seal: Coarse gravel + irrigation + fine gravel.
Benefits Provided:
- A rough, non-polished rolling surface.
- Waterproofing.
- Resistance to horizontal forces (braking, turning).
Use: Primarily for surfacing low-traffic roads.
Gravel Irrigation: Features and Materials
- Aggregates: Must be crushed, clean, tough, and cubic (not flaky or elongated). They should have low polishing characteristics and uniform particle size. Good adhesion with the binder is crucial (may require activators, suitable emulsions, or thorough cleaning).
- Bitumen: Needs to be fluid enough for uniform irrigation (low viscosity). It should allow quick coating of the aggregate and provide good adhesiveness. The residual binder must be viscous enough to hold the aggregate against traffic forces.
Gravel Irrigation Dosage
- Monolayer Irrigation: One application of binder and aggregate. The aggregate should be embedded to about 2/3 of its height.
- Bilayer Irrigation: Two successive applications of binder and aggregate.
Gravel Irrigation Execution
- Weather: Best performed in warm, dry, and windless conditions.
- Surface Preparation: Requires sweeping and potentially primer irrigation on the existing surface.
- Binder Application: Use a sprinkler tank for uniform application. Hand spraying with a lance can be used for small areas. Ensure continuous, uniform application at the specified rate.
- Aggregate Spreading: Use a paver or chip spreader immediately after applying hot bitumen, ensuring even coverage.
- Compaction: Use a high-pressure pneumatic roller before the emulsion breaks. Compact carefully to avoid crushing the aggregate.
- Repeat Process & Opening to Traffic: For multilayer systems, repeat the process. Wait for the binder viscosity to increase before opening to traffic, initially at low speeds (e.g., 30 km/h).
- Major Faults: Watch for black streaks (“combing”) if the spreader skips, poor aggregate retention due to incorrect binder application rate, or issues with aggregate/bitumen compatibility.
Slurry Seal Application
This involves the application of one or two layers of a bituminous mortar, mixed cold. The mortar consists of aggregates, bitumen emulsion, water, and potentially mineral fillers or additives. It has sufficient consistency at room temperature to be spread using a squeegee box.
Uses:
- Sealing pavement surfaces.
- Providing non-slip surfaces.
- Changing surface color.
Slurry Seal: Features and Materials
- Aggregates: High percentage of crushed particles, resistant to wear and polishing (CPA > 50 for T00, T0, T1, T2 traffic; CPA > 45 for T3, T4 traffic). Must be very clean and meet specific gradation requirements (spindle test).
- Binder: Slow-setting bituminous emulsions, often specially formulated with additives.
Slurry Seal Project Considerations
- Factors to Consider: Type of slurry (fine or coarse), application rate, number of layers, desired final texture, and required slip resistance.
- Slurry Type Examples: Fine aggregate slurry for sealing (approx. 5mm thickness); coarse aggregate slurry for slip resistance (up to 15mm thickness, potentially in 2 layers).
Slurry Seal Execution
Manufacturing (mixing) and spreading are often done with the same specialized machine. Typical production rates can exceed 10,000 m²/day. Slurry seals are generally not compacted after application.