Cement Concrete Roads

The construction of a cement concrete road involves several crucial operations:

1. Preparation of Sub-grade:

- The sub-grade is the natural soil where the concrete slab will be laid.

- It needs to be cleaned, shaped, and leveled.

- After cleaning, the sub-grade is prepared to the required grade and profile, ensuring it has uniform strength across its entire width.

- Any weak spots identified should be removed and reinforced by compacting new material in their place.

- When concrete is directly placed on the sub-grade, it is essential to saturate the surface with water for 6 to 20 hours before the concrete is poured. This process prevents the sub-grade from absorbing water from the concrete mix.

2. Provision of Sub-base:

- In cases where the natural sub-grade is not firm, a sub-base is provided over it. The decision to add a sub-base depends on soil type, design load, traffic intensity, and economic considerations. The sub-base serves three primary purposes:

  - It acts as a capillary cut-off, preventing damage due to mud pumping.

  - It provides a strong supporting layer.

  - It reduces the thickness of the concrete slab, leading to lower construction costs.

- The sub-base may consist of any of the following layers:

  - A 15 cm thick layer of well-graded soil-gravel mixture.

  - Brick soling with one layer of W.B.M. totaling a maximum thickness of 10 cm.

  - Two layers of W.B.M. with a maximum total thickness of 15 cm.

  - A lean cement concrete layer not exceeding 10 cm thick.

- For very poor sub-grade soil, the sub-base should be placed over a blanket of granular material or stabilized soil. 

Also Read: Earth Road: Types and Construction Procedure

3. Placing the Forms in Cement Concrete Road:

- Forms used in construction may be made from steel or timber. Mild steel channel sections are typically used, designed to match the thickness of the pavement.

- Forms need to be properly braced and fixed to the ground using stakes, with three stakes fixing every 3 m length. 

- Once secured, forms must be checked for trueness, with maximum allowable deviations of 3 mm in the vertical plane and 5 mm in the horizontal plane for every 3 m length.

- Prior to placing concrete, forms are oiled to facilitate smooth pouring.

4. Watering the Prepared Sub-grade or Sub-base:

- After the forms are installed, the surface prepared for concrete must be adequately moist. 

- If the sub-grade is dry, it should be sprinkled with water in quantities that allow for absorption. 

- It is advisable to wet the surface at least 12 hours before placing the concrete unless a waterproof concrete floor is being implemented, in which case, surface moistening is unnecessary.

5. Batching of Materials and Mixing:

- After determining the appropriate proportions for the cement concrete road mix, fine and coarse aggregates are correctly proportioned by weight at a batching plant. 

- They are then fed into a hopper along with the required quantity of cement, also measured by weight. 

- Ingredients for the concrete are mixed in dry state, ideally using a concrete mixer. 

- The measured water is added to achieve the desired water-cement ratio.

6. Transporting and Placing of Concrete:

- Once mixed, the concrete is transported to the site using wheelbarrows or manually carried pans. 

- The mixed concrete is deposited rapidly onto the sub-grade in layers not exceeding 50 mm to 80 mm, about two or three times the size of the aggregates. 

- Concrete should be placed across the entire bay width in successive batches, maintaining a continuous operation. The topmost layer should be laid approximately 10 mm higher than the intended profile for further tamping.

- Proper camber and gradient must be ensured during placement, using suitable tools to eliminate any voids. 

- To avoid segregation, care must be taken during transportation and placing. If reinforcement is included in the road slab, concrete placement occurs in two stages. In the first stage, concrete is compacted to the reinforcement level indicated in the design, and reinforcement is laid over the compacted concrete before completing the slab thickness in the second stage.

7. Compaction of Cement Concrete Road:

- After placement, the concrete must be compacted into the proper position using a heavy screed or tamper equipped with suitable handles. 

- A wooden tamper should be at least 75 mm wide, with an underside shaped to the slab's finished cross-section, weighing around 10 kg/m to maintain its form under working conditions. 

- The tamper's length should equal the bay length plus 60 mm, with operators gripping the handles during use.

- Concrete can also be compacted using a power-driven finishing machine or vibrating hand screed; screed vibrators are effective for slabs up to 12.5 cm thick, while immersion vibrators are suitable for thicker slabs.

8. Floating:

- Following compaction, the slab surface is floated longitudinally using a wooden float board to ensure an even surface free from corrugations.

9. Belting:

- After floating, the surface undergoes belting just before the concrete hardens, creating a non-slippery and skid-resistant road surface. This step may sometimes be omitted.

10. Brooming:

- Brooming proceeds after belting, where brushes are drawn at right angles to the center line of the road from edge to edge, preferably performed just before the concrete sets.

- This step may also be omitted.

11. Edging:

- After brooming, the edges of the slab are meticulously finished with an edging tool before the concrete sets completely.

12. Curing Process of Cement Concrete Road:

- Curing is essential to prevent water loss from the concrete slab while keeping it moist during the hardening period. 

- Initial curing lasts for 24 hours, during which time the concrete becomes firm enough to walk on, followed by removing wet mats and performing final curing for 2 to 3 weeks using one of the following methods:

  1. Ponding Method.

  2. Covering the slab with a 4 to 8 cm thick layer of wet sand or earth.

  3. Spraying with a suitable chemical such as sodium or calcium chloride.

13. Fillings of Joints and Edging:

- Once curing is complete, the surface must be cleaned and washed, and joints filled using an appropriate sealing compound.

14. Opening to Traffic:

- The cement concrete road can be opened to traffic once it achieves the required strength, typically after 28 days of curing.