What is prestressed concrete and why is it beneficial for bridge decks?

• A. Prestressing is used to increase compressive stresses only in compression zones; not relevant for bending.
• B. Prestressing introduces compressive stresses to counteract tensile stresses from loads, reducing cracking, increasing stiffness, and improving durability and span capability.
• C. Prestressing increases cracking and reduces durability.
• D. Prestressed concrete is only used for walls, not decks.

Answer: (B)

Prestressed concrete means introducing compressive forces into the concrete member before service loads are applied, by tensioning steel tendons embedded in the concrete. This precompression is achieved either by pretensioning the tendons before the concrete is poured or by post-tensioning after the concrete has gained strength, with the tendons anchored so they compress the member.

In a bridge deck, loads cause bending that puts the bottom of the slab in tension. By applying prestress, the entire section carries a compressive state that counteracts those tensile stresses, so cracks are delayed, narrowed, or even prevented. This improves durability because water and aggressive agents have fewer routes to reach reinforcing steel. The precompression also stiffens the deck, reducing deflections and allowing longer spans or thinner slabs, which translates to lighter, more economical bridges.

Some might think prestressing only affects compression in limited zones, or that it increases cracking or is only for walls. In reality, the goal is to keep critical tension zones in compression under service loads, which is why prestressed decks perform better in cracking control, stiffness, durability, and span capability.