Are shear connectors strictly required between the precast and cast-in-situ concrete?

Not always. For low shear forces, a properly roughened interface may provide sufficient bond. For higher shear, projecting steel ties or stirrups crossing the interface are required.

When is propping required for precast units?

Propping is required during the pouring and curing of the in-situ topping unless the precast units are explicitly designed for 'unpropped' construction to handle the wet concrete load.

How does this code relate to IS 456?

This code acts as a supplementary guide specifically for the composite action interface and staging; general reinforced concrete design principles of the members must still comply with IS 456.

IS 3935

: 1966

Code of practice for composite construction

CurrentSpecializedCode of PracticeBIMStructural Engineering · Special Structures

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This code covers the design and construction of composite concrete structures made up of prefabricated structural units (reinforced or prestressed) and cast-in-situ concrete. It focuses on ensuring monolithic action between the distinct concrete pours.

Code of practice for composite construction

Quick Reference Values

Minimum bearing on masonry supports100 mm

Minimum bearing on concrete or steel supports75 mm

Typical minimum thickness of cast-in-situ structural topping40 mm

Key Formulas

v = V / (b * d) — Horizontal shear stress at the interface between precast and cast-in-situ concrete

Practical Notes

Surface preparation is vital: the precast concrete surface must be suitably roughened, cleaned of laitance, and wetted before placing the cast-in-situ concrete to ensure proper shear transfer.

Unpropped construction requires the precast unit to be designed to independently carry its self-weight, the wet weight of the in-situ concrete, and all construction live loads.

Always account for differential shrinkage and creep in the design, as the precast member is typically much older than the cast-in-situ topping when monolithic action begins.