Some form of concrete has been used since early Roman days. Originally it was a mixture of lime, local clay and volcanic rock. Since the invention of Portland cement in 1824, it has been composed of cement, sand and some type of rock. It has great compressive strength, able to withstand loads placed directly on it. It has very little tensile strength, the ability to withstand stretching or bending. Steel is used to add tensile strength.
Reinforced concrete and prestressed concrete both have steel bars or wires embedded to bolster the material's weakness under tension, but the types of steel and the uses of the concrete are different. Concrete reinforcing steel is either solid bars with ribs on them, referred to as reinforcing bar or rebar, or a wire or metal mesh. Prestressed steel is solid bars or bundles of wire installed under tension.
Typical uses for reinforced concrete are street and highway paving, sidewalks, foundation walls and slabs and almost any other solid concrete form where most of the pressure will be compression. The reinforcing adds tensile strength to allow a concrete slab, for instance, to flex slightly without breaking apart. The type of reinforcing will vary. Patios and sidewalks may use wire mesh; highways and other large paving projects use heavy steel bars placed in a crisscross pattern to add tensile reinforcing.
Prestressed concrete is just that -- concrete formed under stress. Steel bars or wires are placed in a form and stretched or stressed with forces on each end pulling on them. When the tension is released, the steel will try to return to its original form. When concrete is poured around the stressing bonds before pressure is released, it creates tension; when it is released the steel's tendency to try to resume its original shape adds a compressive force to the concrete laterally, giving it strength to span distances.
Prestressing is used mainly to make beams and piers in such construction as highway overpasses and commercial buildings. It enables a concrete beam to support weight between piers on either side; without such reinforcing, concrete's lack of tensile strength would cause it to collapse without support in the middle.
Both Originated in Late 1800s
Both reinforced concrete and prestressed concrete date to the 1800s. European builders began experimenting with adding steel reinforcing to concrete to strengthen it in the mid-1800s and by the 1890s it was used to construct a building in the United States. A San Francisco engineer patented an idea for prestressed concrete in 1886 but it did not gain much acceptance in construction until after World War II when steel for beams was in short supply, and America's first prestressed concrete bridge was not completed until 1951.
- Portland Cement Association: Prestressed Concrete
- Britannica: Prestressed Concrete
- Childs Engineering: Prestressed Concrete
- Today's Concrete Technology: History of Reinforced Concrete
- Britannica: Reinforced Concrete
- Wire Reinforcement Institute; Why Steel Reinforcing Is Needed in Concrete Slabs; Roy Reiterman
Bob Haring has been a news writer and editor for more than 50 years, mostly with the Associated Press and then as executive editor of the Tulsa, Okla. "World." Since retiring he has written freelance stories and a weekly computer security column. Haring holds a Bachelor of Journalism from the University of Missouri.