28.8. Pavement Composition

Sub-base materials comprise natural sand, gravel, laterite, brick metal, crushed stone or combinations thereof meeting the prescribed grading and physical requirements. The sub-base material should have a minimum CBR of 20% and 30% for traffic up to 2 msa and traffic exceeding 2 msa respectively. Sub-base usually consists of granular or WBM and the thickness should not be less than 150mm for design traffic less than 10 msa and 200 mm for design traffic of 10 msa and above.

The sub-base is the foundational layer beneath the main pavement structure, typically made from natural materials like sand and gravel or crushed stone. Its purpose is to provide support and distribute load to the underlying soil. The California Bearing Ratio (CBR) is a test that measures the strength of the subgrade soil; a higher CBR indicates better load-bearing capacity. For sub-bases, a CBR of 20% is required for lower traffic levels (up to 2 million standard axles or msa), and 30% for higher traffic. Additionally, the thickness of the sub-base must meet minimum standards depending on traffic levels: 150 mm for lower traffic and 200 mm for higher traffic.

Think of the sub-base as the foundation of a building. Just as a building needs a strong foundation to support its structure and prevent settling or collapsing, road pavements require a solid sub-base. If the foundation is weak (low CBR), the rest of the building (or road) will be unstable and more prone to damage.

The recommended designs are for unbounded granular bases which comprise conventional water bound macadam (WBM) or wet mix macadam (WMM) or equivalent conforming to MOST specifications. The materials should be of good quality with a minimum thickness of 225 mm for traffic up to 2 msa and 150 mm for traffic exceeding 2 msa.

The base layer of a pavement is critical for providing strength and stability to support the traffic loads. Typically constructed from high-quality materials like WBM or WMM, the base must be thick enough to handle the anticipated traffic. For example, a base thickness of 225 mm is recommended for roads carrying lower traffic (up to 2 msa), while 150 mm is adequate for roads with higher traffic levels. The thickness helps ensure that the layer can distribute loads effectively without deteriorating.

Imagine the base layer as the floor in a large warehouse. If the floor is too thin, it could crack or become damaged due to the weight of heavy equipment and goods. Similarly, the base layer of a road needs to be robust enough to handle the weight of vehicles crossing over it.

The surfacing consists of a wearing course or a binder course plus wearing course. The most commonly used wearing courses are surface dressing, open graded premix carpet, mix seal surfacing, semi-dense bituminous concrete and bituminous concrete. For binder course, MOST specifies, it is desirable to use bituminous macadam (BM) for traffic up to 5 msa and dense bituminous macadam (DBM) for traffic more than 5 msa.

The bituminous surfacing is the uppermost layer of the pavement that directly interacts with vehicle tires. It is designed to provide a smooth, weather-resistant surface, which enhances safety and comfort for vehicles. The surfacing typically involves a combination of a wearing course (which wears down over time) and a binder course that helps hold everything together. For different traffic levels, specific materials like BM and DBM are recommended; BM suffices for lower traffic loads, while DBM is more durable for heavier traffic.

Think of the bituminous surfacing like the paint and finish on a piece of furniture: it not only gives it a pleasing appearance but also protects the material underneath from wear and tear. Just as furniture paint needs to be chosen based on how much use the piece will get, the type of bituminous surfacing must be selected based on the expected traffic loads on the road.

--