The construction of the base or sub-base for the pavement construction is dependent on the type of base chosen and the equipment available. In this article, some of the top base and subbase layers used for pavement construction are explained. They are:
- Cement Treated Base
- Unstabilised Base
- Asphalt Treated Base
- Lean Concrete Base
1. Cement Treated Base for Pavement Construction
A cement treated base involves the process of laying a cement spread on the grade and compacted by rollers. Best practices to follow for cement base construction for pavement are:
- Once the cement mixture is mixed, it must be placed, compacted and trimmed within 4 hours. This working time is limited depending on the temperature and wind condition at the site.
- Minus or plus 1/2 inch of the design grade has to be trimmed.
- Once the base is completed, the surface must be cured by applying a frog spray of water or a bituminous curing agent.
- When the surface has to be trimmed and it is an unavoidable process, then it is recommended to use a bond breaker before the PCC layer is applied.
2. Unstabilized Base for Pavement Construction
The construction of an unstabilised base for pavement involves mixing, placing, compacting and grading the material. The recommendations followed while constructing an unstabilized base of pavement are:
- Proper mixing must be performed to obtain a homogeneous mixture.
- The materials used must be properly conditioned with water before and after the compaction. This would help in maintaining optimum moisture content.
- As per AASHTO T99, a minimum of 95 percent of standard proctor density must be obtained in the field. For a pavement to carry large volumes, a minimum of 98 percent density must be obtained.
- Once the compaction is complete, the base must be trimmed to plus or minus 1/2 inches of the design profile grade.
- The segregation of the aggregates can be avoided by the following consistency in placing, compacting and trimming.
- The unstabilized base has to be wetted prior to the paving so that dry base condition is avoided.
3. Asphalt -Treated Base
The construction procedure of asphalt treated base is similar to that of the conventional asphalt surface. Important recommendations are:
- A smooth asphalt treated base works better
- A rough surface results in friction with the PCC layer.
- Before placing the PCC layer, the asphalt treated base must be sprayed with water or concrete curing compound or water lime solution in order to reduce the surface temperature. As the surface temperature can go up to 140-degree celsius. In risk situations, night construction is recommended.
- Asphalt treated base also demand control of the surface grade. The finished surface of asphalt treated base must be with plus or minus 1/4th inch of the design profile grade.
4. Lean Concrete Base for Pavement Construction
The construction of the base by lean concrete is performed in a similar manner to that of conventional concrete. The equipment used for conventional base construction can be used for a lean concrete base by the contractor. This helps to distribute the mobilization cost to a greater scope of the work.
Other base types like asphalt treated base require the specialized contractor and hence lean concrete base is very much economical and versatile. A successful lean concrete base must follow the following specifications:
- The compressive strength of the lean concrete base must be between 750 and 1200 psi. A lean concrete base with this strength value does not demand joints. There are chances for shrinkage cracks but won't be reflected through the PCC slabs.
- Control over the surface base is achieved by having a lean concrete base. The finished surface of the lean concrete base must be with plus or minus 1/4th of the design profile grade.
- In order to prevent the bonding with the PCC slabs, the lean concrete base must not be textured. A bond breaker like a wax-based curing compound helps to prevent bonding.
- The lean concrete gains strength with time. The strength development of this concrete type must be hence factored in the project sequencing process. This helps the lean concrete to prevent excessive stiffness during the placement of the PCC layer.
Also Read: Bituminous Mix Design for Pavement