Types Of Road Pavements & Materials Used For Pavement Construction
In civil engineering, the term pavement refers to road or any surfaced paved with the requisite engineering materials. To pave means to cover with with bricks, stones, cement, asphalt or other engineering materials suitable for road construction. There are two main types of pavement known in civil engineering viz; flexible pavement and rigid pavement.
While rigid pavement is constructed using cement, reinforcement and aggregates, hence made of concrete slabs panels which are reinforced with steel bars and are usually very strong and rigid, flexible pavements are made of layers of different materials placed together, with the surfacing consisting of bituminous material which is mostly asphalt.
Rigid pavements are more durable and last longer than flexible pavements. Though the initial cost of flexible pavement is lower than rigid pavement, they require maintenance over time which makes the long term cost higher. Flexible pavements usually consist of bituminous surface or binder binder course which helps to bind the rest of the courses together, a coarse aggregate base, a granular sub-base and a well compacted sub-grade which may consist of cuts and fills of soil from borrow pits.
The main goal of road construction is to ensure the selection of materials that will require minimum haul or treatment for the improvement of their strengths, thereby enhancing economy which is one of the keys considerations of engineering aside safety. Different types of soils are generally used for the subgrade, sub-base and road base in road construction. Let us briefly look at the materials required for pavement construction:
1. Subgrade Materials: We have been talking about sub-grade. What exactly are they?
Sub-grade materials can be existing materials in the site of the road construction which may be cut sections or materials borrowed as fills from other locations to raise the level of the road in order to meet up the design specifications of the road.
Fill Materials are usually collected from borrow pits. This happens after the borrow pits have been tested and certified okay. For fill materials, the California Bearing Ratio (CBR) must not be less than 10. Additionally, the plasticity must not be less than 12 and greater than 20.
2. Sub-base Materials: As mentioned before, the sub-base materials are granular soil which comes on top of the subgrade materials. The strength and other characteristics of the sub-base must be suitable for the road. As a general rule of thumb, the California Bearing Ratio (CBR) of the sub-base should not be less than 30% after 24 hours of soaking. Similarly, the Liquid Limit (LL) should not exceed 30 and the Plasticity Index (PI) should not be greater than 12.
3. Road Base Materials: The road base is the main structural component of the roadway. It must be provided in all flexible pavement constructions. The minimum strength of the base material determined at the Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) must have a CBR value of 80% after 24 hours soaking.
4. Surfacing Materials: Surfacing is the last material applied to the pavement and it’s what is usually visible to the road users. Aside the aesthetic appeal, it helps to bind the other materials especially those closer to the them together. Roadway surfacing may consist of:
(i) Reinforced Concrete Slab (also known as Rigid Pavement)
(ii) Asphaltic Concrete which may act as the wearing course or base/regulating course
(iii) Hot Rolled Asphalt or
(iv) Surface Dressing
