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For 4th Semester Polytechnic CE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic
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Subject: Transportation Engineering, CE 4006 (Same as CC 4006)
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Semester: 4th Semester 📚
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3. Construction of Road Pavements 🛣️
Pavement is a layer of material laid down on an area to provide a smooth surface for vehicles and pedestrians. It is constructed using different materials like bitumen, cement, and aggregates (stones). There are different types of pavements used for different purposes.
3.1 Types of Road Materials and Their Tests
The materials used for road construction include aggregates, bitumen, and cement. These materials must undergo tests to make sure they are of good quality and can withstand the weight of vehicles and weather conditions.
3.1.1 Test on Aggregates
Aggregates (stones, sand, gravel) are the main materials used in road construction. We need to test these aggregates to ensure they are durable and strong enough.
3.1.1.1 Flakiness and Elongation Index Tests 🧪
These tests measure the shape of the aggregates:
- Flakiness Index: Measures how thin and flat the stones are. Flat stones break easily.
- Elongation Index: Measures how long and stretched the stones are. Long stones can create weak spots in the road.
Example:
- Flaky stones = Less strength 💔
- Angular stones = Stronger road 🏋️
3.1.1.2 Angularity Number Test 🔶
This test measures how angular (sharp-edged) the aggregates are. The sharper and more angular the stones are, the better they will lock together and create a stable road surface.
Example:
- Angled Stones ➡️ Better stability 🛠️
- Round Stones ➡️ Less stability ⚠️
3.1.2 Test on Bitumen
Bitumen is a sticky substance used to bind the aggregates together in flexible pavements like asphalt roads. We need to test it to ensure it works well in different temperatures and under traffic loads.
3.1.2.1 Penetration Test 🧪
The Penetration test measures how soft or hard bitumen is. The test involves a needle that penetrates the bitumen at a certain temperature. A higher penetration indicates softer bitumen.
- Higher Penetration = Softer bitumen 🧴 (Good for colder climates)
- Lower Penetration = Harder bitumen ❄️ (Good for hotter climates)
3.1.2.2 Ductility Test 🌟
The Ductility test checks how flexible bitumen is. A high ductility means that bitumen can stretch without breaking, which is important for roads that experience temperature changes.
Example:
- Good for roads in areas with large temperature variations 🌞❄️
3.1.2.3 Flash and Fire Point Test 🔥
This test determines the temperature at which bitumen can catch fire:
- Flash Point: The temperature at which bitumen vapors start to burn.
- Fire Point: The temperature at which bitumen continues to burn.
3.1.2.4 Softening Point Test 🌡️
This test measures the temperature at which bitumen becomes soft. A higher softening point is better for roads in hot climates.
3.2 Pavement 🛤️
Pavement is a road structure designed to carry traffic. It is built in layers, each with a specific function. There are two main types of pavements:
- Flexible Pavements: These pavements bend under load, like bituminous roads.
- Rigid Pavements: These pavements do not bend, like concrete roads.
3.2.1 Definition
Pavement refers to the layer or layers of material placed on the ground to create a road surface.
3.2.2 Types of Pavements
Flexible Pavement:
- Made of bitumen and aggregates.
- Bends under the load of traffic.
- Common in urban roads and highways.
Rigid Pavement:
- Made of cement concrete.
- Rigid and does not bend under load.
- Used in areas with high traffic and heavy loads.
3.2.3 Structural Components of Pavement and Their Functions
A typical pavement has multiple layers, each serving a specific function:
- Subgrade (Foundation) 🌍: The natural ground beneath the pavement. It needs to be strong and well-compacted.
- Sub-base 🏗️: A layer that provides stability and prevents moisture from reaching the base.
- Base 🧱: A layer of crushed aggregates that provides strength to the pavement.
- Surface Course 🌟: The top layer, usually asphalt or concrete, which provides the smooth, durable surface for vehicles.
3.3 Construction of WBM Road (Water Bound Macadam) 🚜
WBM roads are made by compacting crushed stone aggregates together with water. These roads are cheaper but less durable than bituminous roads.
Steps in Construction:
- Spread Crushed Stone: Place a layer of crushed stone on the ground.
- Add Water: Sprinkle water to bind the stones together.
- Compact: Use a roller to compact the stones into a solid surface.
Merit: Low-cost construction 💸
Demerit: Not as durable or smooth as bituminous roads 😕
3.4 Merits and Demerits of WBM & WMM Roads
WBM Road:
- Merits:
- Affordable 🏷️
- Easy to construct 🚜
- Demerits:
- Not very durable 🔧
- Prone to damage in wet conditions 🌧️
WMM Road:
- Merits:
- Stronger and more durable 💪
- Better load distribution 🚛
- Demerits:
- More expensive 💰
- Requires skilled workers 👷♂️
3.5 Construction of Flexible Pavement / Bituminous Road 🛣️
Bituminous roads are flexible and can adjust to traffic and weather conditions.
Steps in Construction:
- Prepare Subgrade: Ensure it is level and compacted.
- Add Sub-base Layer: A layer of crushed stone for support.
- Base Layer: Crushed stone with bitumen mixed for strength.
- Surface Layer: A layer of bitumen with aggregates to form the top surface.
3.6 Types of Bitumen and Its Properties 🛢️
Bitumen is essential for flexible pavements. Different types of bitumen are used depending on the climate and purpose.
- Emulsion: Bitumen mixed with water; used for patchwork repairs.
- Cutback: Bitumen mixed with solvents; used for quick-setting roads.
- Tar: A substance from coal, now less common but still used in some roads.
3.7 Terms Used in Bituminous Roads 📝
3.7.1 Prime Coat 🛠️
A layer of diluted bitumen applied to the base layer to improve adhesion between layers.
3.7.2 Tack Coat 🧰
A thin layer of bitumen applied between layers of asphalt to ensure they stick together.
3.7.3 Seal Coat 🌟
The topmost layer that seals the surface to prevent water penetration and wear.
3.8 Merits and Demerits of Bituminous Roads 🚗
Merits:
- Smooth Surface 🌟
- Flexible 💪
- Good Durability ⏳
Demerits:
- Can Wear Out 💔
- Expensive 💰
3.9 Cement Concrete Road 🏢
Concrete roads are more durable but also more expensive.
3.9.1 Methods of Construction 🏗️
- Alternate Bay Method: Concrete is poured in sections or “bays” and allowed to cure separately.
- Continuous Bay Method: Concrete is poured continuously for faster construction.
3.9.2 Alternate and Continuous Bay Method 🧱
- Alternate Bay Method: This method helps in controlling cracking and works well in small sections.
- Continuous Bay Method: Faster, but can lead to cracks if not properly managed.
3.9.3 Construction Joints 🔩
Joints are used in concrete roads to allow expansion and contraction without cracking.
3.9.4 Filler and Sealers 🛡️
Filler is used to fill gaps between joints, and sealers are applied to protect the surface from water damage.
3.9.5 Merits and Demerits of Concrete Roads 🏆
Merits:
- Highly durable 💪
- Good for heavy traffic 🚚
Demerits:
- Expensive to construct 💰
- Requires regular maintenance 🛠️
3.9.6 Types of Joints 🏗️
- Construction Joints: Joints made between sections of concrete laid on different days.
- Expansion Joints: Allow concrete to expand due to heat.
- Contraction Joints: Allow concrete to shrink without cracking.
Diagrams 🖼️
Pavement Structure 🌍
Bituminous Road Layers 🛣️
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| Surface Coat 🌟 |
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| Seal Coat 🌈 |
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| Tack Coat 🧰 |
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| Prime Coat 🛠️ |
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| Base and Sub-base 🏗️ |
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| Subgrade 🌍 |
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