In highway materials and construction (water-bound macadam), which material is primarily used as the binding agent during the finishing and compaction stages of WBM roads?

Introduction / Context:
Water-bound macadam (WBM) is a classic flexible pavement base/sub-base construction in highway engineering. It relies on graded aggregates, mechanical interlock, and a binder-like fine material called screenings (often stone dust) that fills voids under wet rolling to form a dense, stable layer. Knowing the correct binding material is essential for durability and performance.

Given Data / Assumptions:

• Pavement type: WBM layer (base or sub-base).
• Construction method: hand or machine spreading, rolling, application of screenings and water, reshaping, and final compaction.
• Material choices listed: sand, stone dust, cement, brick dust, fly ash.

Concept / Approach:
In WBM, the coarse aggregate is compacted in layers. Fine screenings (stone dust from the same or compatible rock) are broomed into surface voids while water is sprinkled. Under continued rolling, the slurry migrates downward, filling voids and “binding” the coarse pieces through mechanical interlock and suction as the layer densifies. Cement is not used in WBM (that would form a cement-treated base). Brick dust and fly ash are not standard screenings for WBM and may be incompatible or too weak. Sand can be used in certain cases but does not produce the same interlock and is not the standard binding material in WBM specifications—stone dust is.

Step-by-Step Solution:
Identify the WBM construction sequence: spread graded metal → dry roll → apply screenings + water → wet rolling → fill voids → finish. Select a fine, angular material compatible with aggregate mineralogy: stone dust (screenings). Reject cement (would change to stabilized base), brick dust/fly ash (non-standard, weaker), and sand (less effective void filling).

Verification / Alternative check:
Standard practice prescribes screenings of the same or approved rock type to ensure good adhesion, compatible hardness, and durability. Field performance confirms that angular stone dust enhances stability far better than rounded sands.

Why Other Options Are Wrong:

• Sand: rounded texture; inferior interlock and retention compared to screenings.
• Cement: converts the layer into a stabilized base, not WBM.
• Brick dust/fly ash: not standard screenings; variability and strength concerns.

Common Pitfalls:

• Using excessive water leading to slurry pumping and loss of fines.
• Adopting screenings of dissimilar mineralogy causing debonding or polish.

Final Answer:
Stone dust (screenings).