The types, advantages and history of prestressed concrete.
By Abdul Khan
Bigger is better, as many in the building product and bridge industries would say, and it’s certainly true of precast concrete products. Steel reinforcement bars add a great deal of strength to large concrete products, but rebar alone can’t provide the tensile strength necessary for precast products that stretch for extended lengths. There is a bit of wizardry that imparts sufficient strength to these enormous products, and it’s called prestressing.
In order to capture the idea of how prestressing works, imagine a barrel made of wooden staves and metal bands. At least, this is how T.Y. Lin, a civil engineering professor with the University of California, described it the introductory chapter of his book “Design of Prestressed Concrete Structures.”
Lin says the basic principle of prestressing was applied to construction, perhaps centuries ago, when ropes or metal bands were wound around the wooden staves to form a barrel (see Figure 1). When the bands were tightened, they were under tensile prestress, which in turn created compressive prestress between the staves and enabled them to resist hoop tension produced by internal liquid pressure. In other words, the bands and the staves were prestressed before they were subjected to any service loads.
In more formal terms, prestressing means the intentional creation of permanent stresses in a structure or assembly to improve its behavior and strength under various service conditions.
Prestressing tendons (generally of high tensile steel cables or rods) are used to provide a clamping load, which produces a compressive stress to offset the tensile stress that the concrete compression member would otherwise experience due to a bending load (see Figure 2).
Classification and types
Prestressed concrete structures can be classified in a number of ways, depending upon their features of design and construction. The following types of prestressing can be accomplished in three ways: pretensioned concrete, and bonded and unbonded post-tensioned concrete.
Pretensioned concrete. Pretensioned concrete is cast around already-tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars, and when tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched, and so the tendon usually forms a straight line.
Most pretensioned concrete products are prefabricated in a factory and must be transported to the construction site, which limits their size. Examples of pretensioned products are balcony elements, lintels, columns, solid slabs, hollow core slabs, tees, walls, sandwich panels, ledger beams, I-beams, bulb-T beams and foundation piles.
Bonded post-tensioned concrete. Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around curved ducts made of plastic, steel or aluminum that are placed in the area where tension would occur in the concrete element. A set of tendons is fished through the ducts before the concrete is poured. Once the concrete hardens, the tendons are tensioned by hydraulic jacks that react against the concrete member. When the tendons have stretched sufficiently, according to design specifications, they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct openings are then grouted to protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for house construction in locations where expansive soils create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure.
Post-stressing is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge. The advantages of this system over unbonded post-tensioning are:
- Large reduction in traditional reinforcement requirements
- Tendons can be easily ‘weaved,’ allowing a more efficient design approach
- Higher ultimate strength due to the bond generated between the strand and concrete
- No long-term issues with maintaining the integrity of the anchor/dead end
Unbonded post-tensioned concrete. Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with grease and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab.
The disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
- The ability to individually adjust cables based on poor field conditions
- Post-stress grouting is eliminated
- The ability to destress the tendons before attempting repair work
According to AASHTO, prestressing reinforcement must be high-strength seven-wire strand, high-strength steel wire, or high-strength alloy bars of the grade and type as specified by the design engineer. Uncoated seven-wire strand must conform to the requirements of AASHTO M 203 (ASTM A 416). Supplement S1 (Low-Relaxation) applies when specified.
Stronger concrete is usually required for prestressed than for reinforced work. Present practice calls for a minimum 28-day cylinder strength of 5,000 psi. High strength is necessary in prestressed concrete for several reasons. First, in order to minimize cost, commercial anchorages for prestressing steel are always designed for high-strength concrete. Hence weaker concrete either will require special anchorages or may fail under the application of prestress. Also, concrete of high compressive strength offers high resistance in tension and shear as well as in bond and bearing, and is desirable for prestressed concrete elements whose various portions are under higher stresses than ordinary reinforced concrete.
Another factor is that high-strength concrete is less prone to shrinkage cracks. It also has a higher modulus of elasticity and smaller creep strain, resulting in smaller loss of prestressing in the steel.
Advantages of prestressed concrete
Prestressed concrete is one of the most reliable, durable and widely used construction materials in building and bridge projects around the world. It has made significant contributions to the construction industry, the precast manufacturing industry and the cement industry as a whole. It has led to an enormous array of structural applications, including buildings, bridges, foundations, parking garages, water towers, nuclear reactors, TV towers and offshore drilling platforms.
The advantages of prestressed concrete include:
- Lower construction cost
- Thinner slabs, which are especially important in high-rise buildings where floor thickness savings can translate into additional floors for the same or lower cost
- Fewer joints since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced construction with the same thickness
- Longer span lengths increase the usable unencumbered floorspace in buildings and parking structures
- Fewer joints lead to lower maintenance costs over the design life of the structure, since joints are the major locus of weakness in concrete buildings.
History of Prestressing
The art of prestressing concrete evolved over many decades and from many sources, but we can point to a few select instances in history that brought about this technology.
In the United States, engineer John Roebling established a factory in 1841 for making rope out of iron wire, which he initially sold to replace the hempen rope used for hoisting cars over the portage railway in central Pennsylvania. Later, Roebling used wire ropes as suspension cables for bridges, and he developed the technique for spinning the cables in place.
During the 19th century, low-cost production of iron and steel, when added to the invention of portland cement in 1824, led to the development of reinforced concrete. In 1867, Joseph Monier , a French gardener, patented a method of strengthening thin concrete flowerpots by embedding iron wire mesh into the concrete. Monier later applied his ideas to patents for buildings and bridges.
Swiss engineer Robert Maillart’s use of reinforced concrete, beginning in 1901, effected a revolution in structural art. Maillart, all of whose main bridges are located in Switzerland , was the first designer to break completely with the masonry tradition by putting concrete into forms technically appropriate to its properties – yet visually surprising. His radical use of reinforced concrete revolutionized masonry arch bridge design.
The idea of prestressing concrete was first applied by Eugene Freyssinet, a French structural and civil engineer, in 1928 as a method for overcoming concrete’s natural weakness in tension. Prestressed concrete can now be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
Abdul Kahn is NPCA’s director of Technical Services and past president of ASCE – Illinois Section 2006.