Module 2: Materials & Testing
1. Material Selection
The selection of appropriate materials is critical for road durability and performance. Each layer of the pavement structure has specific requirements:
Subgrade (Natural Ground Layer)
Function: Supports the entire pavement structure.
Materials: Compacted native soil (clay, silt, sand, or gravel).
Key Properties: High California Bearing Ratio (CBR) (≥ 5% for highways), low plasticity, and good compaction.
Improvement Methods: Stabilization with lime, cement, or geotextiles for weak soils.
Sub-Base Layer
Function: Distributes loads and prevents water accumulation.
Materials: Crushed stone, gravel, or stabilized soil.
Thickness: Typically 100–300 mm, depending on traffic load.
Base Layer
Function: Primary load-bearing layer.
Materials: High-quality crushed aggregates (e.g., Graded Crushed Stone, Wet Mix Macadam).
Requirements: High strength (CBR ≥ 30%), good drainage, and resistance to deformation.
Wearing Course (Surface Layer)
Function: Provides a smooth, skid-resistant riding surface.
Materials:
Bituminous (Asphalt) Mixes: Hot Mix Asphalt (HMA), Stone Mastic Asphalt (SMA).
Concrete Mixes: Rigid pavements (PCC – Portland Cement Concrete).
Bitumen & Asphalt
Bitumen Grades: Penetration grade (e.g., 60/70), Viscosity grade (VG-30, VG-40).
Asphalt Mix Types:
Dense Graded Mix (DGM): Common for highways.
Open Graded Mix (OGM): Used for drainage layers.
Concrete for Rigid Pavements
Mix Design: M30, M40 grades (as per IRC/ACI standards).
Reinforcement: Steel dowels & tie bars for jointed concrete pavements.
2. Quality Control Tests
Ensuring material quality is essential for long-lasting roads. Key tests include:
California Bearing Ratio (CBR) Test
Purpose: Measures subgrade strength.
Procedure: Penetration test on soaked soil samples.
Acceptance Criteria: CBR ≥ 5% for subgrade, ≥ 30% for base.
Sieve Analysis (Gradation Test)
Purpose: Determines particle size distribution of aggregates.
Standards: ASTM D6913, IS 2386.
Importance: Ensures proper interlocking and density.
Compaction Tests (Proctor Test)
Purpose: Determines maximum dry density (MDD) and optimum moisture content (OMC).
Types: Standard Proctor (light compaction) vs. Modified Proctor (heavy compaction).
Marshall Stability Test (For Asphalt)
Purpose: Evaluates asphalt mix strength and resistance to deformation.
Parameters: Stability (kN), Flow (mm), Air Voids (%).
Standards: ASTM D6927, AASHTO T245.
3. Sustainability in Materials
With growing environmental concerns, sustainable materials are gaining importance:
Reclaimed Asphalt Pavement (RAP)
Process: Milled old asphalt is reused in new mixes (up to 30–40% RAP content).
Benefits: Reduces bitumen demand, lowers costs, and minimizes landfill waste.
Recycled Concrete Aggregate (RCA)
Use: Crushed old concrete as base/sub-base material.
Challenges: Requires removal of steel reinforcement.
Plastic & Rubber Modified Bitumen
Waste Plastic Roads: Shredded plastic enhances bitumen binding (e.g., India’s "Plastic Roads").
Crumb Rubber (From Tires): Improves elasticity and crack resistance.
Cold Mix Asphalt
Eco-Friendly Alternative: Uses bitumen emulsions, reducing energy consumption (no heating required).
Key Takeaways:
Subgrade must be stable, while base & sub-base require high-strength aggregates.
Bitumen/asphalt mixes vary based on traffic and climate; concrete is used for rigid pavements.
Quality tests (CBR, Marshall, Sieve Analysis) ensure compliance with standards.
Sustainable materials (RAP, RCA, plastic roads) reduce environmental impact and costs.
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