Quick questions on Reinforced and pre-stressed concrete: HSC Engineering Studies Civil Structures

Plain concrete is strong in compression but brittle in tension. A typical structural concrete (grade N32) has compressive strength $f'_c \approx 32$ MPa and tensile strength of only about $3$ MPa. Any beam that bends will develop tensile stresses on the convex face and crack the concrete.

Reinforced concrete uses deformed bars (rebar) placed in the tensile zone of a member. The bond between concrete and steel relies on:

Pre-stressing applies a compressive force to the concrete before service loads arrive. Under service load, the imposed tensile stress only partially cancels the pre-compression, so the concrete never enters tension and never cracks. There are two production techniques.

The Snowy Mountains Hydro-Electric Scheme (1949 to 1974) used mass concrete for the gravity dams (Tumut Pond, Eucumbene) and pre-stressed concrete for the more recent additions. Snowy Hydro 2.0 uses post-tensioned concrete in surge shafts and powerhouses. The Sydney Harbour Tunnel approach spans, many sections of the M4 Smart Motorway, and bridges along the Pacific Motorway all use pre-stressed concrete bridge girders.

Pre-tensioning is done before the concrete is cast (used in factory precast). Post-tensioning is done after curing (used on site).

The reason concrete and steel work together is partly that they expand at similar rates. Without that, temperature changes would crack the bond. :::