Rigid pavements – principal function of a base (sub-base) layer In cement concrete pavements, what is the primary reason for providing a base/sub-base course beneath the slab?

Introduction:
Rigid pavements transfer loads through slab action, but interface conditions at slab–subgrade are critical. Repetitive wheel loads and the presence of free water can lead to fines being ejected from joints or cracks, a damaging process known as pumping. A well-designed base/sub-base mitigates this risk and enhances performance and durability.

Given Data / Assumptions:

• Cement concrete slab resting on an underlying base or sub-base.
• Traffic repetitions causing joint deflections and pore-water pressure.
• Drainage and erosion resistance are considered in the base material.

Concept / Approach:

Pumping is the expulsion of water and fine particles through joints/edges under traffic-induced deflections. Eroded support leads to faulting, loss of load transfer, and cracking. A dense, erosion-resistant, and well-drained base limits pore pressures and fine migration, thus preventing pumping. While bases can indirectly influence cracking by improving support uniformity, the principal design intent is to control erosion and drainage rather than thermal expansion (which the slab accommodates via joints).

Step-by-Step Solution:

Verification / Alternative check:

Pavement design guides specify stabilized or dense-graded bases with drainage to minimize pumping, aligning with field performance observations.

Why Other Options Are Wrong:

Subgrade settlement is governed by subgrade strength and compaction; bases cannot eliminate it entirely. Cracking prevention is mainly a function of slab thickness, joints, and load transfer. Thermal expansion is managed by joints and dowels, not by base. Skid resistance pertains to the wearing surface, not the base.

Common Pitfalls:

Assuming a very thick base can replace good joint design; neglecting drainage leading to pumping even with a strong base.

Final Answer:

prevention of pumping