Introduction

Highway engineering plays a vital role in establishing and maintaining efficient transportation networks that facilitate the safe and smooth movement of people and goods. When it comes to constructing roads, choosing the appropriate techniques and materials is essential to meet the diverse requirements of different environments and traffic conditions. In this article, we will explore the practices of WBM (Water Bound Macadam) and WMM (Wet Mix Macadam) road construction, providing valuable insights into highway engineering principles. By conducting a comprehensive comparison, we will evaluate the construction techniques, materials, and characteristics of WBM and WMM roads. This analysis aims to assist decision-makers in selecting the most suitable road construction method for their specific projects, ensuring optimal performance and longevity. Join us as we delve into the world of highway engineering and explore the similarities, differences, and benefits of WBM and WMM road construction practices.

(I) – WBM (Water Bound Macadam) Roads Construction:

The water-bound macadam (WBM) road construction technique was introduced by John Macadam. It is still widely used in modern times. Let’s delve into the process:

During WBM construction, three primary materials are utilized:

• Aggregates
• Screeners
• Binders

(1) Aggregates

In WBM road construction, aggregates of different grades are employed, following the guidelines set by the Indian Roads Congress (IRC). The IRC classifies coarse aggregates into nine grades based on their size. For WBM roads, the aggregates are functionally divided into three grades:

• Grade 1: Particles ranging from 90 mm to 40 mm in size
• Grade 2: Particles ranging from 63 mm to 40 mm in size
• Grade 3: Particles ranging from 50 mm to 20 mm in size

The preferred usage of these grades varies based on the road layer. Grade 1 aggregates are suitable for the sub-base, grade 2 for the base, and grade 1 for the surface course. However, to enhance durability and prevent rapid deterioration due to traffic abrasion, it is common practice to apply bituminous surfacing over the WBM surface course.

(2) Screeners

Screeners are aggregates of smaller sizes, typically 12.5 mm or 10 mm, categorized as grade A and grade B. They possess the same chemical composition as coarse aggregates and are incorporated into the WBM construction.

For cost-effective purposes, the IRC recommends using non-plastic materials such as crushed over burnt bricks, moorum, gravels, etc., provided they meet specific criteria. These criteria include a liquid limit (LL) of less than 20%, a plasticity index (PI) of less than 6.0%, and a fines content passing through a 0.075 mm sieve of less than 10%. However, if crushable aggregates are employed, the use of screeners may be unnecessary.

(3) Binders

Binders are the layers of materials which are laid after the compaction of the aggregates and the screening materials one after the another. Kankar dust or limestone dust may be utilized if locally available. The binding material, with a Plasticity Index(PI) value of 4% to 9%, is used in surface course construction. PI of binding course material should be less than 6% in the case of the WBM layers, used as a base course or sub-base course, with the bituminous surfacing. However, if the screening used are of crushable material like moorum or soft gravel, there is no need to apply binding material, unless the plasticity index value is low.