9.6 Road Construction & Maintenance

9.6 Road Construction & Maintenance

Introduction to Construction & Maintenance

The successful realization of a road project hinges on meticulous construction practices and a robust maintenance regime. Construction transforms design concepts into physical infrastructure using specialized techniques, tools, and equipment. Maintenance is the ongoing process of preserving this infrastructure in its designed condition, ensuring safety, serviceability, and extending its service life. This section details the activities, methodologies, and materials involved in building and sustaining road pavements.

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1. Activities, Techniques, Tools, Equipment and Plants Used in Road Construction

Road construction is a systematic sequence of activities requiring specialized resources.

1.1 Key Construction Activities & Sequence

1. Clearing and Grubbing: Removal of vegetation, trees, stumps, and debris from the right-of-way.

2. Earthwork:

   • Excavation (Cut): Removal of soil/rock to achieve the required formation level.

   • Embankment Construction (Fill): Placement and compaction of soil to raise the road level.

   • Subgrade Preparation: Shaping and final compaction of the natural or improved soil foundation.

3. Construction of Drainage Structures: Building side drains, culverts, and bridges concurrently with earthwork.

4. Pavement Construction: Building the sub-base, base, binder, and wearing courses.

5. Construction of Shoulders and Sidewalks.

6. Installation of Traffic Control Devices: Signs, markings, signals.

7. Landscaping and Slope Protection: Seeding, turfing, pitching, and retaining walls.

1.2 Major Equipment and Plants

• For Earthwork:

  • Excavators: For digging and loading.

  • Dozers: For spreading, grading, and pushing material.

  • Motor Graders: For precise final grading and sloping of the subgrade and base layers.

  • Scrapers: For hauling soil over medium distances.

  • Dump Trucks: For material haulage.

  • Compactors:

    • Smooth Wheeled Rollers: For finishing and sealing layers.

    • Sheepsfoot/Padfoot Rollers: For compacting cohesive soils.

    • Vibratory Rollers: Most common for granular and asphalt materials.

• For Pavement Construction:

  • Crusher Plants: For producing graded aggregates.

  • Hot Mix Plant (Batch/Continuous/Drum Mix): For producing asphalt concrete.

  • Asphalt Paver (Finisher): Lays the asphalt mix in a uniform layer.

  • Concrete Batching Plant: For producing PCC.

  • Concrete Mixer Transit Trucks: For transporting fresh concrete.

  • Concrete Pavers (Slipform/Fixed Form): For laying concrete slabs.

  • Water Tankers: For curing and dust control.

• For Quality Control:

  • Nuclear Density Gauge: For rapid field measurement of density and moisture content.

  • Profilograph/Roughometer: Measures pavement smoothness (ride quality).

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2. Preparation of Road Subgrade

The subgrade is the foundation. Its proper preparation is critical for pavement performance.

2.1 Steps in Subgrade Preparation

1. Establishing Final Formation Level: Using grade stakes and string lines, the final level and cross-slope are marked as per design drawings.

2. Compaction:

   • The soil is brought to near its Optimum Moisture Content (OMC) by sprinkling water or aerating.

   • Compaction is done in layers (lifts, typically 150-200mm loose thickness) using appropriate rollers to achieve the specified Minimum Dry Density (usually 95-100% of Maximum Dry Density from Modified Proctor Test).

3. Proof Rolling: After final compaction, a heavily loaded truck or roller is driven over the subgrade to detect any soft, yielding spots that need correction.

4. Testing for Acceptance: Field density tests (Sand Replacement Method, Nuclear Gauge) are conducted to ensure compaction meets specifications.

2.2 Key Requirements of Finished Subgrade

• Uniform density and strength.

• Proper camber/cross-slope for drainage.

• Free of organic matter, debris, and loose material.

• Adequate width and alignment as per design.

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3. Field Compaction Control and Soil Stabilization

3.1 Field Compaction Control

1. Objective: To ensure the constructed earthwork (embankment, subgrade, granular layers) achieves the density and strength assumed in the design.

2. Key Parameter: Degree of Compaction, expressed as a percentage of the laboratory-determined Maximum Dry Density (MDD). $\text{Degree of Compaction (\%)} = \frac{\text{Field Dry Density}}{\text{Lab MDD}} \times 100$

3. Field Density Tests:

   • Sand Replacement Method (Core Cutter for cohesive soils): A small hole is dug, the excavated soil is weighed, and its volume is determined by filling with dry sand of known density. It is the standard but slow method.

   • Nuclear Density Moisture Gauge: Uses gamma radiation to measure wet density and neutron moderation to measure moisture content instantly. It is fast and non-destructive, used for rapid control.

4. Control Process: If the tested density is below specification, the area is reworked (re-watered and re-compacted).

3.2 Soil Stabilization

The process of improving the engineering properties of soil (strength, durability, volume stability) to make it suitable for construction.

A. Mechanical Stabilization

• Principle: Improving gradation by blending different soils or adding aggregates to achieve a well-graded mix.

• Method: Proportioning and mixing in place or in a plant.

B. Chemical Stabilization

1. Cement Stabilization:

   • Adding 3-10% Portland cement by weight to soil/aggregate.

   • Process: Pulverizing soil, spreading cement, mixing, compacting at OMC, and curing.

   • Result: Cement-treated base (CTB) – a strong, rigid layer.

2. Lime Stabilization:

   • Effective for expansive clays. Lime (3-8%) reduces plasticity, increases workability, and provides strength gain through pozzolanic reaction.

3. Bituminous Stabilization:

   • Adding bitumen emulsion or cutback (2-6%) to granular soils to provide cohesion and water resistance.

C. Geosynthetics Stabilization

• Using geotextiles (for separation and filtration) and geogrids (for reinforcement) within soil layers to improve performance.

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4. Construction of Asphalt Concrete Layers

Asphalt concrete (bituminous concrete) is a hot-mix, plant-produced material.

4.1 Production: Hot Mix Plant

1. Drying and Heating: Aggregates are dried and heated to 150-170°C in a rotating drum.

2. Mixing: Heated aggregates are mixed with hot bitumen (155-165°C) in a pugmill for a few seconds to coat all particles uniformly.

3. Storage/Discharge: The hot mix is stored in a silo or loaded directly into trucks.

4.2 Transportation and Placement

1. Transport: In insulated dump trucks to minimize heat loss.

2. Placing: Using an asphalt paver.

   • The truck dumps mix into the paver's hopper.

   • The paver spreads the mix in a uniform layer using a screed. The screed can be vibrated/tamped for initial compaction.

   • Temperature at placement is critical: typically 135-150°C.

4.3 Compaction

• Objective: To achieve the design density (typically 92-97% of theoretical maximum density) for durability and stability.

• Rolling Sequence:

  1. Breakdown Rolling: Done immediately behind the paver with a vibratory roller (2-4 passes) at high temperature to achieve most of the density.

  2. Intermediate Rolling: With a vibratory or pneumatic roller to further densify.

  3. Finish Rolling: With a static smooth-wheeled roller to remove roller marks and provide a smooth surface.

• Control: Core samples are taken after cooling to check in-situ density and thickness.

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5. Construction Procedure for Specific Pavement Types

5.1 Penetration Macadam

A low-cost, early form of flexible pavement where binder is sprayed over compacted aggregates.

1. Base Preparation: A prepared and compacted granular base.

2. Spreading Aggregates: A single layer of coarse, uniform-sized aggregate (50-75mm) is spread and rolled dry.

3. Grouting: Smaller chipping stones are rolled into the voids.

4. Binder Application: Hot bitumen (of specified grade) is sprayed in two or more applications until the voids are filled.

5. Seal Coat: A final layer of key aggregate and binder is applied.

6. Limitations: Low strength, not suitable for heavy traffic. Primarily used for rural roads or shoulders.

5.2 Bituminous Bound Macadam (Wet Mix Macadam)

A premixed bituminous base/binder course.

1. Mixing: Aggregates (coarse and some fines) are mixed with a controlled amount of water (for workability) and a low percentage of bitumen emulsion or cutback bitumen in a plant or mixer.

2. Transportation & Spreading: The mix is transported, spread by a paver or manually, and shaped.

3. Compaction: Compacted with vibratory rollers. The emulsion breaks (water evaporates, leaving bitumen) or the cutback cures (solvent evaporates).

4. Curing: Requires a curing period (24-48 hours) before the next layer is placed.

5.3 Plain Cement Concrete (PCC) Pavements

A rigid pavement constructed as jointed slabs.

A. Preparation and Formwork

1. Sub-base: A well-compacted granular or DLC sub-base is prepared.

2. Formwork: Steel or wooden forms are set to the exact grade, alignment, and slab thickness. Forms must be rigid and well-braced.

B. Batching, Mixing, and Transport

1. Concrete: Designed for high flexural strength (4-5 MPa) and workability (low slump ~25mm).

2. Batching/Mixing: Done in a central batching plant for quality control.

3. Transport: Using transit mixer trucks to the site.

C. Placing, Compacting, and Finishing

1. Placing: Concrete is placed between forms, either manually or using a concrete paver.

   • Slipform Paver: Continuously extrudes the concrete slab without side forms, used for long stretches.

   • Fixed Form Paver: Places concrete within fixed forms.

2. Compaction: Internal vibration using poker vibrators is essential to remove air voids and achieve full compaction.

3. Screeding and Finishing: A straight edge (screed) levels the surface. Floating and troweling follow to create a smooth, dense surface.

4. Texturing (Brooming): A broom is dragged to create a micro-texture for skid resistance.

D. Joint Construction

• Contraction Joints: Formed by inserting a metal plate or by saw-cutting (within 4-12 hours of placement) to a depth of 1/3 the slab thickness. Dowel bars are placed at mid-depth across the joint.

• Expansion Joints: A preformed compressible filler is placed full depth, with dowel bars in sleeves.

• Longitudinal Joints: Formed with a plate or by saw-cutting, with tie bars.

E. Curing

• Critical Process: To prevent plastic shrinkage cracks and allow full strength development.

• Methods: Ponding, wet hessian/mats, spraying curing compounds (membrane-forming).

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6. Road Maintenance, Repair and Rehabilitation

Maintenance is a planned, cyclical activity to preserve the asset. It is classified based on the level of work required.

6.1 Routine Maintenance (Frequent, Low Cost)

• Activities: Cleaning drains and culverts, repairing potholes (<0.1m²), filling edge breaks, grass cutting, sign cleaning/repair.

• Objective: To address minor defects before they worsen.

6.2 Periodic Maintenance (Recurring, Planned)

• Activities: Resealing the surface, regravelling shoulders, overlay of thin asphalt layers (20-40mm), and repainting markings.

• Trigger: Based on pre-defined condition indicators or a fixed time cycle (e.g., every 5-7 years).

6.3 Special Repairs (Reactive, Unplanned)

• Activities: Repairing damage from landslides, floods, or major failures. Includes pothole patching (larger areas) and structural patch repairs.

6.4 Rehabilitation (Major Intervention)

Undertaken when the pavement is structurally deficient but the subgrade is sound. It restores structural capacity and ride quality.

1. For Flexible Pavements:

   • Overlays: Laying a new bituminous layer (40-100mm+) over the existing pavement after necessary repairs.

   • Recycling:

     • Cold In-Place Recycling (CIR): The existing pavement is pulverized, mixed with a stabilizing agent (emulsion/foamed bitumen), and relaid.

     • Hot In-Place Recycling (HIR): The surface is heated, scarified, mixed with rejuvenator/new mix, and re-compacted.

2. For Rigid Pavements:

   • Slab Stabilization (Mudjacking): Pumping a cement slurry under a sunken slab to restore profile.

   • Load Transfer Restoration: At joints with faulting.

   • Diamond Grinding: To restore smoothness and skid resistance.

   • Concrete Overlay: Bonded or unbonded PCC overlay on the existing slab.

6.5 Reconstruction

• Definition: Complete removal and replacement of the pavement structure, undertaken when the pavement and subgrade have failed.

• Most expensive option, involving full-depth excavation.

6.6 Pavement Management System (PMS)

• A systematic, data-driven approach to planning and prioritizing maintenance and rehabilitation activities across a road network to optimize the use of limited funds. It involves:

  • Condition Surveys: Regular inspection and rating of pavement distress (cracking, rutting, roughness).

  • Performance Prediction: Modeling future condition.

  • Decision Making: Selecting the right treatment at the right time for each road section.

Conclusion: Construction is the disciplined execution of design, where quality control is paramount. Maintenance is the diligent stewardship of the asset, where timely, appropriate interventions are cost-effective. Together, they ensure that the significant investment in a road yields maximum returns in terms of safety, service, and longevity.