For construction of WBM roads, which of the following is the correct sequence of operation after spreading the crushed aggregate?
- AFilling voids with screening, application of binding material, dry rolling, wet rolling
- BDry rolling, filling voids with screening, wet rolling, application of binding materialCorrect
- CDry rolling, wet rolling, application of binding material, filling voids with screening
- DWet rolling, application of binding material, dry rolling, filling voids with screening
Solution
Concept
In WBM construction, the coarse aggregate is first dry rolled, then voids are filled with screenings, then wet rolling is done, then binding material is applied.
Step-by-step solution
1. Spread the coarse crushed aggregate to the correct camber and thickness. 2. Do dry rolling to interlock the aggregate. 3. Fill the voids with screenings and roll lightly. 4. Sprinkle water and do wet rolling. 5. Apply binding material (like stone dust or moorum), sprinkle water and roll again. 6. Allow the layer to set and dry. 7. This order matches option B.
Explanation
Dry rolling locks the stones together first. Screenings then fill the gaps, and wet rolling makes the layer dense. Binding material is added last to fill the remaining fine voids and bind the surface.
Why the other options are wrong
A: Screenings and binder cannot go before dry rolling, because the stones are not yet interlocked. C: Screenings must fill the voids before wet rolling and binder, not at the end. D: Wet rolling cannot be the first step, and dry rolling cannot come after the binder.
Shortcut
Remember the order 'Dry – Screen – Wet – Bind'.
Common mistake
Putting binding material before screenings. Screenings (coarser) always come before binder (finer).
Correct answer: B) Dry rolling, filling voids with screening, wet rolling, application of binding material