9.3 Highway Materials

9.3 Highway Materials

Introduction to Highway Materials

The performance, durability, and cost-effectiveness of a highway pavement are fundamentally determined by the quality and properties of its constituent materials. Highway materials can be broadly classified into three categories: Aggregates (the skeleton), Binding Materials (the glue), and the Subgrade Soil (the foundation). This section details the types, essential tests, and design methodologies for these materials, providing the knowledge base for selecting and proportioning them to create pavements that can withstand traffic loads and environmental exposure over their design life.

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1. Types of Aggregates and Tests

Aggregates constitute 90-95% of asphalt concrete and 60-75% of Portland cement concrete by weight. They provide strength, stability, and durability to the pavement.

1.1 Types of Aggregates

1. Based on Size:

   • Coarse Aggregate: Material retained on the 4.75 mm IS sieve. Provides the main load-bearing skeleton.

   • Fine Aggregate: Material passing the 4.75 mm IS sieve and retained on the 75-micron sieve. Fills voids between coarse aggregate and contributes to workability.

   • Filler: Material passing the 75-micron sieve (e.g., stone dust, cement, fly ash). Fills very fine voids and improves stability in asphalt mixes.

2. Based on Source/Geology:

   • Natural: River gravel, pit sand, crushed stone.

   • Artificial: Blast furnace slag, broken bricks, recycled concrete aggregate.

3. Based on Shape (Critical for workability and strength):

   • Rounded (River gravel): Good workability, lower strength.

   • Irregular (Pit gravel): Moderate workability and strength.

   • Angular (Crushed stone): Best interlock and strength, may reduce workability.

   • Flaky and Elongated: Undesirable as they reduce strength and workability.

1.2 Aggregate Tests

Aggregates must satisfy requirements in three key areas: Gradation, Strength, and Durability.

A. Gradation (Particle Size Distribution)

1. Objective: To determine the distribution of particle sizes within an aggregate sample. This is the most important property as it affects density, strength, stability, workability, and permeability.

2. Test Method: Sieve Analysis (IS 2386 Part I).

   • A representative sample is passed through a nested set of sieves of progressively smaller openings.

   • The weight retained on each sieve is measured.

3. Presentation of Results:

   • Gradation Curve: Plot of % passing (or % retained) vs. sieve size (log scale).

   • Key Parameters:

     • Fineness Modulus (FM): An empirical index (sum of cumulative % retained on standard sieves divided by 100). Higher FM indicates coarser aggregate.

     • Effective Size (D10): The sieve size through which 10% of the material passes.

     • Uniformity Coefficient (Cu): $C_u = \frac{D_{60}}{D_{10}}$. Higher Cu indicates a well-graded (wide range of sizes) material.

     • Coefficient of Curvature (Cc): $C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}}$. For a well-graded gravel: 1 < Cc < 3; for sand: 1 < Cc < 3.

4. Types of Gradation:

   • Well-Graded: A good representation of all particle sizes, leading to maximum density and interlock.

   • Uniformly Graded (Poorly Graded): Most particles are of similar size, leading to high void content.

   • Gap-Graded: Some intermediate sizes are missing. Used in special asphalt mixes like Stone Matrix Asphalt (SMA).

B. Strength Tests

1. Aggregate Crushing Value (ACV) Test (IS 2386 Part IV):

   • Objective: To determine the resistance of aggregate to crushing under a gradually applied compressive load.

   • Significance: Measures the ability of aggregate to withstand the stresses imposed by roller compaction and traffic loads in the pavement. Lower ACV is desirable.

   • Specification: For bituminous surface courses, ACV should generally be <30%.

2. Aggregate Impact Value (AIV) Test (IS 2386 Part IV):

   • Objective: To determine the toughness or resistance of aggregate to sudden shock or impact (simulating traffic hammering).

   • Significance: Important for aggregates in surface courses subjected to pounding by heavy vehicles. Lower AIV is desirable (<30% for wearing courses).

3. Los Angeles Abrasion Test (IS 2386 Part IV):

   • Objective: To determine the hardness, abrasion resistance, and durability of aggregate.

   • Principle: A sample with steel spheres is rotated in a drum; the wear due to rubbing and impact is measured.

   • Significance: Predicts resistance to wear under traffic. Max. allowable loss is typically 40% for surface courses and 50% for base courses.

C. Durability Tests

1. Soundness Test (IS 2386 Part V):

   • Objective: To determine the resistance of aggregate to weathering (disintegration) due to repeated cycles of wetting and drying, or freezing and thawing.

   • Principle: Aggregates are subjected to cycles of immersion in a saturated solution of sodium or magnesium sulphate and oven drying. The weight loss indicates susceptibility to weathering.

2. Water Absorption Test (IS 2386 Part III):

   • Objective: To determine the porosity of aggregate.

   • Significance: High absorption indicates high porosity, which can make the aggregate susceptible to frost damage and require more binder in asphalt mixes. For good quality aggregate, absorption should be <2%.

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2. Binding Materials and Their Tests

Binding materials hold the aggregate particles together. The primary binders in highway engineering are Bitumen (for flexible pavements) and Cement (for rigid pavements).

2.1 Bitumen

• Definition: A viscous, black hydrocarbon material derived from the distillation of crude petroleum. It is thermoplastic (softens with heat, hardens on cooling) and is waterproof and adhesive.

• Types:

  • Paving Grade Bitumen: Used in road construction (grades like 60/70, 80/100 based on penetration).

  • Modified Bitumen: Bitumen modified with polymers (SBS, EVA) to improve elasticity, temperature susceptibility, and fatigue resistance.

  • Cutback Bitumen: Bitumen dissolved in a solvent (kerosene, gasoline) to reduce viscosity for cold-weather spraying.

  • Bitumen Emulsion: A suspension of bitumen droplets in water stabilized by an emulsifier. Used for tack coats, prime coats, and cold mixes.

Key Tests for Bitumen (IS 73, IS 1201-1208)

1. Penetration Test (IS 1203):

   • Objective: To measure the consistency or hardness of bitumen.

   • Procedure: A standard needle under a 100g load is allowed to penetrate a bitumen sample for 5 seconds at 25°C. The depth penetrated in units of 0.1 mm is the penetration value.

   • Significance: Softer bitumen has higher penetration. It is used for grading (e.g., 60/70 pen means penetration is between 60-70).

2. Softening Point Test (Ring and Ball) (IS 1205):

   • Objective: To determine the temperature at which bitumen attains a particular degree of softening.

   • Procedure: A steel ball is placed on a disk of bitumen in a ring, and the assembly is heated in a water or glycerin bath. The temperature at which the bitumen softens enough to allow the ball to fall 25 mm is recorded.

   • Significance: Indicates temperature susceptibility. Higher softening point bitumen is less prone to rutting in hot weather.

3. Ductility Test (IS 1208):

   • Objective: To measure the ability of bitumen to stretch without breaking.

   • Procedure: A briquette of bitumen is pulled apart at a specified speed and temperature (27°C). The distance in cm at which the thread breaks is the ductility.

   • Significance: Bitumen with low ductility is brittle and cracks easily. Minimum ductility is 75 cm for paving grades.

4. Viscosity Test (Using Saybolt Furol or Kinematic Viscometer):

   • Objective: To measure the resistance to flow, which is critical for mixing and compaction temperatures.

5. Flash and Fire Point Test (IS 1209):

   • Objective: To determine the safe heating temperature for bitumen. Flash point is the temperature at which vapors ignite momentarily; fire point is when sustained burning occurs.

2.2 Cement

• Role in Highways: Used in Cement Concrete Pavements (Rigid Pavement) and Cement-Treated Bases/Sub-bases.

• Key Properties and Tests: (Same as detailed in Section 1.2 of the curriculum).

  • Consistency & Setting Time (for workability control).

  • Compressive Strength (most critical for structural capacity).

  • Soundness (for long-term durability).

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3. Design of Asphalt Mixes

Asphalt mix design is the process of determining the optimum proportion of aggregate, bitumen, and filler to produce a mix with sufficient stability (to resist deformation under traffic), durability (to resist weathering and aging), workability (for ease of placement and compaction), and skid resistance.

3.1 Types of Asphalt Mixes

1. Dense Graded Mixes (Bituminous Concrete):

   • Well-graded aggregates designed for maximum density.

   • Used for surface, binder, and base courses. Provides a smooth, impervious surface.

2. Open-Graded Mixes:

   • Contains little or no fine aggregate, resulting in high air voids (15-25%).

   • Used for surface courses to improve surface drainage (Permeable Friction Courses) and reduce spray and hydroplaning.

3. Stone Matrix Asphalt (SMA):

   • Gap-graded mix with a coarse aggregate skeleton, high binder content, and cellulose fibers.

   • Provides excellent rut resistance and durability for heavy traffic.

3.2 Marshall Method of Mix Design (Most Common Method)

A standardized empirical method (ASTM D6927) involving the preparation and testing of laboratory specimens.

Step-by-Step Process:

1. Selection of Materials: Suitable aggregates and bitumen are chosen.

2. Preparation of Trial Blends: Several aggregate gradations (blends) are prepared, aiming for the target specification band.

3. Preparation of Specimens:

   • For each trial blend, specimens are prepared at multiple bitumen contents (e.g., 4.0%, 4.5%, 5.0%, 5.5%, 6.0% by weight of total mix).

   • Aggregates and bitumen are heated, mixed, and compacted in a standard mold (101.6 mm diameter, approx. 63.5 mm height) with a Marshall hammer (75 blows on each side).

4. Testing of Specimens:

   • Bulk Specific Gravity (Gmb) of the compacted specimen is determined.

   • Specimen is submerged in a 60°C water bath for 30-40 minutes.

   • Stability Test: The specimen is placed in the Marshall testing machine, and the maximum load (in Newtons) it withstands before failure is recorded as Marshall Stability.

   • Flow Test: The total deformation (in 0.25 mm units) at the point of maximum load is recorded as Marshall Flow.

5. Analysis and Determination of Optimum Bitumen Content (OBC):

   • For each bitumen content, calculate: Stability, Flow, Bulk Density, Air Voids (%), Voids in Mineral Aggregate (VMA %), Voids Filled with Bitumen (VFB %).

   • Plot these properties against bitumen content.

   • OBC is selected as the bitumen content corresponding to:

     • Maximum Bulk Density, and/or

     • 4% Air Voids (typical for surface course), and/or

     • Meeting minimum stability and flow requirements.

   • OBC must also satisfy requirements for VMA and VFB.

3.3 Key Mix Parameters and Their Significance

• Air Voids (Va): Small air pockets in the compacted mix. Too low (<3%) causes flushing/bleeding; too high (>8%) allows air/water ingress, leading to oxidation and stripping. Target: 3-5% for surface courses.

• Voids in Mineral Aggregate (VMA): The volume of intergranular void space between aggregate particles in a compacted mix. Must be sufficient to accommodate enough bitumen for durability. Minimum VMA depends on nominal aggregate size.

• Voids Filled with Bitumen (VFB): The percentage of VMA filled with bitumen. Indicates the relative bitumen content. Range: 65-75% for surface courses.

• Marshall Stability: A measure of the mix's resistance to deformation (shear stress) under load. Higher is better.

• Marshall Flow: A measure of the mix's plasticity and ability to adjust to settlement. Too low (<2mm) indicates brittle mix; too high (>4mm) indicates plastic mix prone to rutting.

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4. Evaluation of Subgrade Soil

The subgrade is the natural soil or improved soil layer upon which the pavement structure is built. It is the foundation of the road. Its evaluation is critical for determining pavement thickness (through design methods like the California Bearing Ratio method).

4.1 Functions of Subgrade

• To provide a stable platform for constructing pavement layers.

• To support the loads transmitted from the pavement structure without excessive deformation (settlement or shear failure).

4.2 Key Properties for Evaluation

1. Strength and Stiffness:

   • The ability to resist deformation under load. This is the primary design parameter.

2. Compaction Characteristics:

   • The relationship between soil dry density and moisture content for a given compactive effort (Proctor Test).

3. Swelling and Shrinkage Potential:

   • Expansive clays change volume with moisture variation, causing pavement heave and cracking.

4. Frost Susceptibility:

   • In cold climates, soils that retain water can heave upon freezing.

4.3 Tests for Subgrade Evaluation

A. California Bearing Ratio (CBR) Test (IS 2720 Part XVI)

• This is the most widely used test for flexible pavement design worldwide.

• Objective: To evaluate the strength of subgrade, sub-base, and base course materials by comparing their bearing capacity to that of a standard crushed rock aggregate.

• Procedure:

  1. Soil is compacted in a standard mold (152mm dia.) at its optimum moisture content (OMC) to maximum dry density (MDD).

  2. The sample is soaked in water for 96 hours (to simulate worst-case moisture condition).

  3. A cylindrical plunger (50mm dia.) is penetrated into the soil at a rate of 1.25 mm/min.

  4. The load is recorded at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 5.0, 7.5, 10.0, and 12.5 mm.

• Calculation: $CBR (\%) = \frac{\text{Load on Soil at chosen penetration}}{\text{Standard Load for Crushed Stone at same penetration}} \times 100$

  • Standard loads: 1370 kg at 2.5 mm penetration, 2055 kg at 5.0 mm penetration.

  • The CBR value is usually taken at 2.5 mm penetration, unless the 5.0 mm value is higher.

• Significance in Design:

  • Higher CBR → Stronger Subgrade → Thinner Pavement Required.

  • CBR values typically range from 2% (very poor clay) to 50%+ (excellent granular material).

  • Design charts (like the one in the IRC:37 guidelines) use CBR and traffic volume to determine total pavement thickness.

B. Plate Bearing Test (Modulus of Subgrade Reaction, k)

• Objective: Primarily used for rigid pavement design. It measures the pressure required to produce a unit deflection in the soil.

• Procedure: A rigid plate (usually 750 mm dia.) is placed on the subgrade and loaded. The settlement is measured. The modulus of subgrade reaction $k$ is calculated as: $k = \frac{p}{\Delta}$ Where $p$ is the pressure (kN/m²) and $\Delta$ is the settlement (m). Units are MN/m³ or kg/cm³.

• Significance: A key input for rigid pavement thickness design using Westergaard's theory.

C. Triaxial Shear Test

• Objective: To determine the shear strength parameters of the soil: Cohesion ($c$) and Angle of Internal Friction ($\phi$).

• Significance: Used in the analysis of slope stability of embankments and deep cutting failures.

D. Compaction Tests (Proctor Test - IS 2720 Part VII & VIII)

• Objective: To determine the Optimum Moisture Content (OMC) at which a soil achieves its Maximum Dry Density (MDD) for a given compactive effort.

• Types: Standard Proctor (lower energy) and Modified Proctor (higher energy, simulating heavy compaction).

• Importance: Field compaction of subgrade and embankments must be done near OMC to achieve MDD, ensuring maximum strength and minimum future settlement.

Conclusion: The science of highway materials is applied through rigorous testing and systematic design. Selecting the right aggregates, designing an optimal asphalt mix, and accurately evaluating the subgrade soil are non-negotiable steps in constructing a pavement that is not only strong on the day it opens but remains durable and serviceable for its entire design life under the relentless dual assault of traffic and climate.