Prestressed Concrete vs. Precast Concrete

Prior to giving specifics, it would be best to describe the process of prestressing concrete. Prestressed concrete is achieved by stretching high strength steel cables, or strands, between two fixed abutments, then casting concrete on a form that is placed between the abutments. The concrete is allowed to cure, which creates a bond between the cables and the concrete. After the curing process, the cables are cut free of the abutments, causing the cables to try to relax, thus compressing the concrete.

• Concrete is much stronger in compression than in tension. Concrete is able to handle very high compressive loads. (Up to 8,000 psi in some instances.) However it is only capable of 550 to 650 psi in tension. The idea behind prestressing concrete is to never let any portion of the concrete member go into tension under full design loads.
• Mild steel reinforced concrete is unable to recover if it is loaded beyond capacity. When mild steel is used it does not react the same way as prestressing cables because it is cast in the concrete in a relaxed state. Mild steel simply does not start to react until it is stretched. This means that the concrete has to begin to go into tension before the rebar becomes useful and contains the tension. By utilizing prestressing in the casting of concrete, the concrete member becomes elastic rather than rigid. This is because the cable has already been stretched, and is working even before the concrete member is loaded, to keep it in compression. An example would be to take our largest cross section double tee (8'-0" wide with 48" deep stems) and assume a span of 105'-0" with a 125 psf uniform load.
  1. At the time the tee is erected, with only self weight considered, the camber would be + 6.15"
  2. With full dead load (60 psf) the camber would be + 0.45"
  3. With full dead load and full live load (125 psf total) the camber would be - 4.75"

  It is important to note that the double tee described above is designed to vary between a substantial positive camber and a substantial negative camber, as conditions require, for the life of the concrete member. The prestressing truly allows great flexibility in a concrete member.

Mild Steel Reinforced Unit	Prestressed Unit

• The tensile strength of a prestressing strand is 4 ½ times greater than a 60 grade rebar. The prestressing strands are typically ½" diameter, 7 wire strands that are rated at 270,000 psi. Each strand is tensioned to 31,000 pounds of force, and multiple strands are used in each concrete member. (We have used as many as 40 strands in an element, which would give a total of 1,240,000 pounds of compressive force.)