Reinforcement placement in large-diameter sewers: For large circular sewers subjected to external pressure only (no internal pressure), where should the primary reinforcement be placed?

Introduction / Context:
Reinforced concrete pipes and large-diameter sewers experience different stress distributions under internal and external loads. Correct placement of steel ensures that tensile zones are reinforced, preventing cracking and extending service life.

Given Data / Assumptions:

• Loading case: external pressure only (e.g., soil and surcharge), negligible internal pressure.
• Concrete is strong in compression, weak in tension; steel should be located where tensile stress develops.
• Ring action of a pipe section under uniform external pressure.

Concept / Approach:
Under internal pressure, hoop tension is maximum at the inner surface; reinforcement is therefore placed near the inner face. Conversely, under external pressure alone, tensile hoop stress develops near the outer surface, so primary reinforcement should be concentrated there. This aligns steel with the tensile zone, counteracting crack formation.

Step-by-Step Reasoning:
Identify loading: external pressure (soil, groundwater head, traffic surcharge) on the outside.Determine tensile zone under ring compression: tension occurs closer to the outer surface.Place main reinforcement accordingly near the outer face to resist tensile hoop stresses.

Verification / Alternative check:
Standard R.C.C. pipe design practices differentiate reinforcement cages for internal vs external pressure cases, with outer-face steel specified for external-load-only design.

Why Other Options Are Wrong:
Inner-face steel suits internal pressure, not external-only loading.Both faces or elliptical cages may be used for combined or special loads but are not required for external-only case here.None of these: incorrect as a clear correct placement exists.

Common Pitfalls:

• Assuming symmetric steel regardless of load case, which wastes material or misplaces capacity.
• Ignoring construction tolerances that can shift steel cover—outer-face placement must still meet cover requirements.

Final Answer:
Near the outer surface of the pipe