Construction of Cement Concrete Roads | Civil Engineering Optional Notes for UPSC PDF Download

 What is a Cement Concrete Road? 

• A cement concrete road, known as a Rigid pavement, is a robust type of road made from a solid layer of cement concrete.
• This road construction option is highly durable and long-lasting, making it a popular choice for infrastructure projects.
• During the construction process, a well-thought-out blend of cement, aggregates, water, and additives is laid on a prepared sub-base.
• Subsequently, the mixture undergoes compaction and curing to form a solid, resilient road surface.
• Cement concrete roads boast noteworthy benefits such as superior strength, the ability to withstand heavy traffic loads, minimal maintenance needs, and exceptional durability in diverse weather conditions. These roads are typically utilized for highways, major roads, and areas with significant traffic flow.

Structural Components of Cement Concrete Road 

• Subgrade
• Sub-base
• Concrete Slab

Subgrade

• The subgrade is a crucial element in constructing a cement concrete road. It serves as the natural or prepared surface where the road is built.
• This layer offers essential support to the layers above it, including the base, sub-base, and the pavement itself.
• The quality and characteristics of the subgrade play a significant role in determining the road's performance and longevity.
• Adequate strength, stability, and drainage properties in the subgrade are vital for ensuring the overall stability and durability of the road structure.

Sub-base

• The sub-base is a fundamental layer in cement concrete road construction, situated above the subgrade and beneath the concrete slab.
• It acts as a transition between the subgrade and the concrete slab, providing additional support and drainage capabilities.
• Proper compaction of the sub-base is essential to prevent settling and ensure uniform support for the road surface.
• Using suitable materials like aggregate and ensuring adequate thickness are crucial factors in constructing a stable and durable sub-base.

Sub-base Layer

• The sub-base layer is positioned above the subgrade and below the concrete pavement, serving as a crucial transition layer.
• It plays a vital role in distributing loads from the traffic and providing additional support to the road structure.
• Construction of the sub-base typically involves the use of granular materials such as crushed stone, gravel, or sand.
• This layer significantly boosts the road's load-bearing capacity and prevents the upward movement of moisture, essential for effective drainage.

Concrete Slab

• The concrete slab represents the topmost layer of the road and functions as the actual driving surface.
• Directly situated above the sub-base, it is responsible for delivering a smooth and durable driving experience.
• Comprised of a blend of cement, aggregates, water, and at times additives, the concrete slab offers exceptional strength and durability.
• It is designed to withstand heavy traffic loads, ensuring resilience against the stresses and strains imposed by vehicles.

Advantages of Cement Concrete Road

• The lifespan of these roads is extended.
• They provide a waterproof layer of cement.
• Cement concrete roads are robust and long-lasting, resisting the effects of weathering.
• They offer excellent visibility during nighttime.
• Cement concrete roads create clean and hygienic surfaces, free from dust.
• These roads remain smooth without developing ruts, resulting in a quieter ride.
• They can be engineered precisely for even weight distribution.
• Cement concrete roads are highly resistant to weather conditions and temperature changes.
• Old concrete roads can be repurposed as a base for new concrete roads or bituminous surfaces.

 Disadvantages of Cement Concrete Road 

• Initial construction expenses can be significant, particularly if suitable aggregate is not readily available locally.
• Prone to cracking, warping, and twisting over time.
• Construction necessitates skilled supervision and workmanship.
• Can generate considerable noise when subjected to traffic with iron tires.
• Requires an extended curing period before being opened to traffic.
• Identifying and fixing sewers and water mains beneath such pavements can be extremely challenging.
• Less flexible and durable compared to W.B.M roads or bituminous roads.

 Methods of Construction of Cement Concrete Roads 

• Alternate Bay Method
• Continuous Bay Method
• Expansion Joint and Strip Method

Alternate Bay Method:
When constructing a single lane road, it is divided into bays of 6 to 8 meters in length, with construction occurring in alternate bays. For double lane roads, construction is carried out in alternating bays of each lane, with work proceeding to the next bay once the concrete has dried.

For single lane roads, bays are divided into 6 to 8-meter lengths, where construction is done in alternate bays. In double lane roads, work is done in alternating bays of each lane, with curing bays serving as additional working platforms.

• Joints can be easily constructed with customizable widths.
• Cured bays can serve as extra working areas.
• During rainy periods, water collects on unfinished bays, requiring more time for completion.
• Numerous transverse joints are needed, causing traffic diversions due to the broad construction spread.

Continuous Bay Method

 Continuous Bay Method

• In the continuous bay method, construction joints are incorporated when the day's work is unfinished at the designated joint.
• Additionally, dummy joints are introduced at 5-meter intervals in the transverse direction, apart from construction joints. These are implemented to examine weak planes and regulate cracking.

Construction of Cement Concrete Road

1. Preparation of Subgrade:

• Subgrade refers to the natural soil on which the concrete slab is placed.
• It undergoes cleaning, shaping, and leveling processes.
• After cleaning, it is brought to the required grade and profile.
• The subgrade's uniform strength across its entire width is ensured.
• Local weak spots, if identified, are eliminated and reinforced by adding and compacting new material.
• Prior to concrete placement, the subgrade surface is saturated with water for 6 to 20 hours. This step prevents the subgrade from absorbing water from the concrete.

2. Provision of Sub-base:

Sub-base provision involves...

• When the natural subgrade lacks firmness, a sub-base is added above it.
• Factors influencing the decision to include a sub-base are soil type, design load, traffic intensity, and cost considerations.
• The sub-base fulfills three main functions:
• 
  
  • Acts as a capillary cut-off, preventing damage from mud pumping.
  • Provides a robust support layer.
  • Reduces the concrete slab thickness, thereby lowering construction costs.
• The sub-base options may include:
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  • A layer of well-graded soil-gravel mix up to 15 cm thick.
  • Brick soling with a single layer of Water Bound Macadam, max 10 cm thick.
  • Two layers of W.B.M. up to 15 cm thick.
  • A layer of lean cement concrete, max 10 cm thick.
• For very poor subgrade soils, the sub-base should rest on a blanket of granular material or stabilized soil.

3.Placing the Forms in Cement Concrete Road:

• Forms can be constructed using steel or timber materials.
• Steel forms typically consist of mild steel channel sections with a depth matching the pavement thickness.
• Proper bracing and fixation of forms to the ground are essential using stakes.
• Three stakes are used to secure forms at the back of every 3-meter length.
• Ensure forms are accurately positioned by checking their alignment.
• Permissible maximum deviations are 3 mm vertically and 5 mm horizontally in a 3-meter length.
• Forms should be oiled before pouring concrete into them.

4. Watering the Prepared subgrade or Sub-base:

Watering the prepared subgrade or sub-base is a crucial step in the construction process. It involves...

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• After the forms are set in place, it is important to dampen the prepared surface where the concrete will be poured.
• If the ground beneath the concrete is dry, it should be watered adequately so that it can absorb moisture. It is recommended to moisten the surface at least 12 hours before pouring the concrete.
• When a waterproof layer of paper is used as insulation, pre-dampening the surface before concrete placement becomes unnecessary.

5. Batching of Materials and Mixing:

• Once the correct proportions of materials for the concrete mixture are determined, the fine and coarse aggregates are proportioned by weight in a batching plant.
• These aggregates are then placed into the hopper along with the necessary amount of cement, also measured by weight.
• The components of the concrete are mixed in appropriate ratios while in a dry state, ideally within a concrete mixer.
• The measured water quantity is added to achieve the desired water-cement ratio.

6. Transporting and Placing of Concrete:

• Concrete is transported and placed following the mixing process, ensuring it reaches the designated location efficiently.
• Proper care is taken to avoid segregation during transportation, maintaining the quality of the concrete mix.
• Upon reaching the site, concrete is placed evenly using tools like shovels and rakes to prevent air voids and achieve the desired finish.
• It's essential to make sure that the concrete is compacted well to remove any entrapped air and ensure structural integrity.

7.Compaction of Cement Concrete Road:

• Compaction of cement concrete roads is a critical step in the construction process, ensuring durability and longevity of the road surface.
• Proper compaction reduces voids within the concrete, enhancing its strength and resistance to wear and tear.
• Compaction is typically achieved using heavy rollers or compactors that apply pressure evenly across the surface.
• It is essential to compact the concrete uniformly to avoid uneven settling, which could lead to cracks and other structural issues.
• Compaction also aids in achieving the desired density of the concrete, which is crucial for the road's overall performance.

Floating is a crucial step in concrete finishing where a smooth, dense surface is achieved. It involves the following key points:

• Freshly laid concrete is floated using a tool called a float to level and smooth the surface.
• Floats come in different materials like wood, magnesium, or aluminum, each serving specific purposes.
• Wooden floats are ideal for decorative finishes while magnesium or aluminum floats are used for a smoother finish.
• Floats are pushed and pulled across the concrete surface in a sweeping motion to eliminate imperfections.
• Floats help bring the cream of the concrete to the surface, enhancing durability and aesthetics.

• After floating, the surface is further finished by belting just before the concrete becomes hard.
• The purpose of belting is to make the road surface non-slippery and skid resistant. This operation is sometimes omitted.

• After belting, brooming is done by drawing brushes at right angles to the centerline of the road from edge to edge.
• Brooming is done just before the concrete becomes non-plastic.
• This operation is also sometimes omitted.

• After brooming, the edges of the slab are carefully finished with an edging tool before the concrete is finally set.

• Curing involves monitoring water loss from the concrete slab and maintaining moisture in the fresh concrete slab during the hardening phase.
• Initial curing lasts for 24 hours.
• After this period, the concrete is sufficiently hardened for walking, and then the wet mats are removed, followed by final curing for 2 to 3 weeks.
• Final curing can be accomplished through various methods such as Ponding Method, covering the slab with a 4 to 8 cm thick layer of wet sand or earth, or spraying a suitable chemical like sodium or calcium chloride on the concrete surface.