Aggregates and Bitumen Overview
Pavements are composed of a conglomeration of materials whose properties and interactions dictate the overall characteristics of the pavement. An understanding of these materials is fundamental to the construction of flexible pavements, which primarily utilize bitumen and aggregates.
Desirable and Undesirable Properties of Soil
Desirable Properties of Soil
- Stability: The ability of soil to withstand loading without significant deformation.
- Incompressibility: The characteristic that soil does not change volume under pressure.
- Permanency of Strength: The maintenance of strength over time.
- Minimum Changes in Volume and Stability: Limited fluctuation in volume and stability with changes in moisture.
- Good Drainage: The capacity to allow water to percolate through.
- Ease of Compaction: The ability to be compacted easily during construction.
Undesirable Types of Soil
- Highly Plastic Soils: These soils can undergo large deformations when moisture content is changed.
- Soils Which Favor Capillary Rise of Water: Such soils can hold water against the force of gravity.
- Frost Susceptible Soils: Soils that can freeze and lose integrity.
- Organic and Sulphatic Soils: These soils contain organic matter or sulfates that can affect stability.
Permissible Limits for Undesirable Soils
- Total Sulphate: Maximum of 0.5%.
- Organic Matter: Maximum of 1.0%.
Stabilized Soils
Inferior materials can be improved through soil stabilization techniques, including:
- Mechanical Stabilization: The physical alteration of soil to enhance properties.
- Lime Stabilization: The application of lime to improve soil characteristics.
- Cement Stabilization: The use of cement for soil improvement.
Important Tests on Soil as an Engineering Material
- Sieve Analysis (IS:2720-Part 4): A method to assess the size distribution of soil particles.
- Plasticity Index (IS:2720-Part 5): A measure of the plasticity of the soil, showing its ability to deform.
- Field Density and OMC (Optimum Moisture Content): Important for evaluating compaction properties.
- Proctor Compaction (IS:2720-Part 7): A test to determine the maximum dry density of soil.
- CBR (California Bearing Ratio) (IS:2720-Part 16): A test to evaluate the strength of subgrade soils.
- Permeability Test: To determine the ability of soil to transmit water.
- Swell Pressure: Measures the pressure to resist expansion in saturated conditions.
Indian Soil Classification System
- Coarse-Grained Soils: More than half of the total material by weight larger than 75 µm sieve size.
- Fine-Grained Soils: More than half of the total material by weight smaller than 75 µm sieve size.
- Highly Organic Soils: Soils containing high percentages of fibrous organic matter like peat.
Characteristics of Soil Based on Particle Size
- Gravel: 80 to 4.75 mm
- Sand:
- Coarse: 4.75 to 2.00 mm
- Medium: 2.00 to 0.475 mm
- Fine: 0.475 to 0.075 mm - Silt: 0.075 to 0.002 mm
- Clay: Less than 0.002 mm
Desirable Properties of Road Aggregates
Each property of aggregates must be tested to ensure they meet specific performance criteria:
- Resistance to Crushing or Strength: Assessed with crushing tests.
- Resistance to Abrasion or Hardness: Evaluated with Los Angeles abrasion value testing.
- Resistance to Impact or Toughness: Determined using aggregate impact tests.
- Good Shape Factors: Minimize flaky and elongated particles measured by combined flakiness and elongation index.
- Gradation and Size: Evaluated through sieve analysis.
- Resistance to Weathering: Tested through soundness tests.
- Good Adhesion with Bituminous Materials: Assessed with stripping tests.
- Resistance to Polishing Action Under Traffic: Measured by polished stone value.
Aggregate Testing Methods
Aggregate Crushing Test
- Purpose: To determine the crushing strength of aggregates.
- Testing Setup: Aggregates passing through the 12.5 mm IS sieve and retained on the 10 mm sieve are tested.
- Procedure: Subject the specimen to 40 tonnes of load at a rate of 4 tonnes/min on a Universal Testing Machine (UTM).
- Interpretation of Results: A low crushing value signifies strong aggregates. Acceptable limits are less than 20%.
Aggregate Crushing Value Specification
- Flexible Pavements:
- Soling: ≤ 50
- Water bound macadam: ≤ 40
- Bituminous macadam: ≤ 40
- Bituminous surface dressing or thin premix carpet: ≤ 30
- Dense mix carpet: ≤ 30 - Rigid Pavements:
- Other than wearing course: ≤ 45
- Surface or wearing course: ≤ 30
Los Angeles Abrasion Test
- Purpose: To determine the hardness of aggregates.
- Test Limits: A maximum value of 40% is allowed for Water Bound Macadam (WBM) surfacing and Bituminous Macadam (BM) binder courses in Indian conditions. For bituminous pavements, a maximum value of 35% is specified.
- Testing Process:
1. Weigh a specific amount of aggregate (5-10 kg) and place it in the machine along with the abrasive charge.
2. Rotate the machine at 30 to 33 rpm.
3. Sieve the abraded aggregate on a 1.7 mm IS sieve.
- Los Angeles Abrasion Value Calculation:
ext{Los Angeles Abrasion Value} = rac{w imes 100}{W}
where w = weight of fines passing through 1.7 mm; W = total weight of the sample.
Aggregate Impact Test
- Purpose: To measure the toughness of aggregates against sudden impact loads.
- Testing Equipment Description: The aggregate impact testing machine consists of a metal base and a cylindrical steel cup with an internal diameter of 10.2 cm and a depth of 5 cm. A metal hammer weighing 13.5-14.0 kg is used.
- Testing Procedure:
1. Fill the cylindrical measure in three layers tamping each with 25 blows.
2. Cause the hammer to drop freely from a height of 38 cm onto the specimen.
3. After applying 15 blows, sieve the crushed aggregate on a 2.36 mm sieve.
- Impact Value Calculation:
ext{Impact Value} = rac{w imes 100}{W}
where w = weight of fines passing through 2.36 mm; W = total weight of the sample.
Flakiness and Elongation Index Tests
Flakiness Index
- Defined as the percentage by weight of particles whose least dimension (thickness) is less than 0.6 times their mean dimension.
- Applicable to aggregate sizes larger than 6.3 mm.
- Flakiness Index Calculation:
ext{Flakiness Index} = rac{w}{W} imes 100
Where w = weight of flaky material, W = total weight of the sample. - Maximum Values: Ideally, the flakiness index should be less than 15% and typically does not exceed 25%.
Elongation Index
- Defined as the percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension.
- Elongation Index Calculation:
ext{Elongation Index} = rac{w}{W} imes 100
Where w = weight of elongated aggregates, W = total weight of the sample. - Acceptable Limits: An elongation index in excess of 15% is considered undesirable.
Combined Flakiness and Elongation Index
- Evaluation involves separating flaky stones, then calculating each index's contribution before summing them.
- The maximum combined index for WMM, WBM, and BM is capped at 30%.
Specific Gravity & Water Absorption Tests
- Purpose: To aid in the weight-volume conversions for aggregates.
- Definitions:
- Apparent Specific Gravity (Gsa): Includes only the volume of aggregate particles, excluding void volume filled with water during testing.
- Bulk Specific Gravity (Gsb or BSG): Includes both the volume of aggregate particles and the void volume.
Specific Gravity Calculation
ext{Specific Gravity} = rac{C}{B-A}
where A = weight of the sample in water, B = weight of saturated surface dry sample, C = weight of oven dry sample.
Water Absorption
Specific gravity for aggregates utilized in road construction typically ranges from 2.5 to 2.9, while water absorption values range from 0.1% to 2.0%.
Grain Size Analysis
- Involves determining the percentage of individual grain sizes using a sieve analysis, expressing weight retained on each sieve as a percentage of the total sample.
- Uniformity Coefficient (Cu) is defined as
C_u = rac{D_{60}}{D_{10}} - Coefficient of Curvature (Cc) is defined as
C_c = rac{(D_{30})^2}{(D_{10} imes D_{60})}
where D30, D60, and D10 are particle sizes corresponding to 30%, 60%, and 10% finer, respectively. - Desired ranges: For uniformly graded soil, Cu ≈ 1; for well-graded soil, 1 < Cc < 3 and Cu > 4 for gravels and Cu > 6 for sands.
Bitumen Overview
- Bitumen is a principal component in the composition of pavements, characterized as an inexpensive, waterproof, thermoplastic, and viscoelastic adhesive.
- It is obtained from the fractional distillation of petroleum crude and is identified by its black color.
Requirements for a Good Bituminous Binder
- Adequate viscosity during mixing and compaction.
- Low temperature susceptibility.
- Resistance to stripping from aggregates when exposed to water.
Types of Bituminous Materials
- Standard Types:
- Tar
- Bitumen
- Cutback Bitumen
- Emulsion
- Foamed Bitumen: Hot bitumen binder with small amounts of cold water
- Modified Bitumen: Includes polymer, crumb rubber, natural rubber, and waste plastic modified types.
Grading Systems for Bitumen
Grading Types:
- Penetration Grading (PG):
- Penetration test
- Flash point
- Ductility
- Solubility
- Thin-film oven test - Viscosity Grading (VG):
- Viscosity at 60°C
- Viscosity at 135°C
- Penetration test
- Flash point
- Ductility
- Solubility
- Thin-film oven test - Performance Grade (PG):
- Rolling thin film oven (RTFO)
- Pressure aging vessel (PAV)
- Rotational viscometer (RV)
- Dynamic shear rheometer (DSR)
- Bending beam rheometer (BBR)
- Direct tension tester (DTT)
Standard Tests on Bitumen
Viscosity Test
- Characterizes flow behavior, typically measured at temperatures of 60°C and 135°C.
- Reference: IS:1206 - Part II - 1978.
Penetration Test
- Measures the hardness or consistency of bituminous material; reference IS: 1203-1978.
Softening Point Test
- Defines the temperature at which the substance reaches a particular degree of softening; reference IS: 1205-1978.
Ductility Test
- Measures the adhesiveness and ability to stretch of bitumen; reference IS:1208-1978.
Loss on Heating Test
- Assesses the mass loss of water, oil, and other constituents when bitumen is heated at 163°C for 5 hours; reference IS: 1212-1978.
Specific Gravity Test
- Used for making volume corrections based on temperature; reference IS: 1202-1978.
Penetration Test Overview
- Involves measuring the penetration of a standard weight into bitumen at 25°C over 5 seconds.
Softening Point Test Apparatus
- Determines the temperature at which softening occurs during bitumen testing.
Viscosity Testing at Various Temperatures
- Kinematic Viscosity: Calculated as the absolute viscosity divided by density. This is tested at 135°C, simulating typical mixing and laying temperatures.
Rotational (Brookfield) Viscometer (RV)
- Measures high-temperature viscosities at 135°C. This test evaluates the torque to maintain constant speed of a spindle in the binder. This simulates binder workability during mixing and laying.
Ductility Test Apparatus
- Measures bitumen's ability to stretch, identified by the distance at which the sample can be drawn before breaking.
Flash and Fire Point Test Procedures
- Conducted with Pensky-Martins closed cup or open cup apparatus. The bitumen sample is heated gradually while observing for flash and fire points.
Dynamic Shear Rheometer (DSR)
- Tests medium to high-temperature viscosities, measuring G* and phase angle to determine resistance to deformation under load. It can predict rutting and fatigue resistance in pavements by assessing the elastic and viscous components of the bituminous binder.