Describe the structure, properties and applications of reinforced and pre-stressed concrete, identify why steel and concrete are used in combination, and apply this knowledge to Australian civil engineering examples including dams and bridges

What this dot point is asking

NESA wants you to explain why concrete and steel are used in combination, distinguish reinforced concrete from pre-stressed concrete, describe the manufacturing process for each, and link the technique to Australian civil engineering applications.

The answer

Why concrete needs reinforcement

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.

Mild steel has a yield strength of about $250$ MPa in both tension and compression. Embedding steel where the tensile stresses occur lets the composite member carry bending without the concrete cracking under service load.

Reinforced concrete

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

• Mechanical interlock from the ribs on the rebar
• Adhesion between cement paste and steel
• Friction
• Similar coefficients of thermal expansion (steel $12 \times 10^{-6} \text{ K}^{-1}$, concrete $10 \text{ to } 12 \times 10^{-6} \text{ K}^{-1}$)

Australian practice uses N-grade (normal ductility, 500 MPa yield) bars in 12, 16, 20, 24, 28 mm diameters.

Pre-stressed concrete

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.

Pre-tensioned concrete (used for factory-cast bridge girders, sleepers, floor planks): high-tensile tendons are stretched between abutments. Concrete is cast around the tendons. After curing, the tendons are released. The tendons try to shorten and so compress the concrete by bond stress.

Post-tensioned concrete (used for in-situ floors, bridge decks, dam structures): ducts are cast into the concrete. After curing, tendons are threaded through and stretched against external anchors, then locked off. The reaction force at the anchors compresses the concrete.

Australian examples

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.