What is prestressed concrete, how is it made and what are its advantages?

Prestressed concrete has steel rods or cables inside it to put the concrete under high compressionbefore the concrete is put under external load. This is done so that when additional tension is added by the load, the beam is still in compression.

There are several ways to produce prestressed concrete beams; here is one of them. First, using wood or steel forms, a concrete beam is poured with PVC pipes cast inside near the bottom face of the beam, Figure 7–22 (a). Beams that fit on a truck are factory-made, larger ones are cast in place. The PCV pipes provides channels for threading steel rods (or steel cables), Figure 7–22 (b). When the concrete hardens, hydraulic puller jacks tension the rods, and the rods are secured so they remain under tension to compress the beam along its lower part, Figure 7–22 (c). Load on the beam when in service reduces the compression on the beam somewhat, but there is enough tension in the rods to keep the beam in tension, Figure 7–22 (d).

Prestressing concrete with rods or cables makes concrete beams significantly stronger than just adding rebar. Prestressing is widely used for bridges today.

Figure 7–22. Prestressing a concrete beam.

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