Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete. Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.
Pre-tensioned concrete is cast around steel tendons—cables or bars—while they are under tension. The concrete bonds to the tendons as it cures, and when the tension is released it is transferred to the concrete as compression by static friction. Tension subsequently imposed on the concrete is transferred directly to the tendons.
Pre-tensioning requires strong, stable anchoring points between which the tendons are to be stretched. Thus, most pre-tensioned concrete elements are prefabricated and transported to the construction site, which may limit their size. Pre-tensioned elements may be incorporated into beams, balconies, lintels, floor slabs or piles. An innovative bridge design using pre-stressing is the stressed ribbon bridge.
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 a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.
A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react (push) against the concrete member itself.
When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is 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 (such as adobe clay) 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.
Among the advantages of this system over unbonded post-tensioning are:
- Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
- Tendons can be easily "woven" allowing a more efficient design approach.
- Higher ultimate strength due to bond generated between the strand and concrete.
- No long term issues with maintaining the integrity of the anchor/dead end.
History of problems with bonded post-tensioned bridges
The popularity of this form of prestressing for bridge construction in Europe increased significantly around the 1950s and 60s. However, a history of problems have been encountered that has cast doubt over the long-term durability of such structures.
Due to poor workmanship of quality control during construction, sometimes the ducts containing the prestressing tendons are not fully filled, leaving voids in the grout where the steel is not protected from corrosion. The situation is exacerbated if water and chloride (from de-icing salts) from the highway are able to penetrate into these voids.
Notable events are listed below:
- The Ynys-y-Gwas bridge in West Glamorgan, Wales – a segmental post-tensioned structure, particularly vulnerable to defects in the post-tensioning system – collapsed without warning in 1985.
- The Melle bridge, constructed in Belgium during the 1950s, collapsed in 1992 due to failure of post-tensioned tie down members following tendon corrosion.
- Following discovery of tendon corrosion in several bridges in England, the Highways Agency issued a moratorium on the construction of new internal grouted post-tensioned bridges and embarked on a 5-year programme of inspections on its existing post-tensioned bridge stock.
- In 2000, a large number of people were injured when a section of a footbridge at the Charlotte Motor Speedway, USA, gave way and dropped to the ground. In this case, corrosion was exacerbated by calcium chloride that had been used as a concrete admixture, rather than sodium chloride from de-icing salts.
- In 2011, the Hammersmith Flyover in London, England, was subject to an emergency closure after defects in the post-tensioning system were discovered.
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 a grease (generally lithium based) 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 main 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 (For example: shifting a group of 4 cables around an opening by placing 2 to either side).
- The procedure of post-stress grouting is eliminated.
- The ability to de-stress the tendons before attempting repair work.
Picture number one (below) shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack.
Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable. Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made.
The advantages of prestressed concrete include crack control and lower construction costs; thinner slabs - especially important in high rise buildings in which floor thickness savings can translate into additional floors for the same (or lower) cost and fewer joints, since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced constructions with the same thickness. Increasing span lengths increases the usable unencumbered floorspace in buildings; diminishing the number of joints leads to lower maintenance costs over the design life of a building, since joints are the major focus of weakness in concrete buildings.
The first prestressed concrete bridge in North America was the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951. Prestressing can also be accomplished on circular concrete pipes used for water transmission. High tensile strength steel wire is helically-wrapped around the outside of the pipe under controlled tension and spacing which induces a circumferential compressive stress in the core concrete. This enables the pipe to handle high internal pressures and the effects of external earth and traffic loads.
Design agencies and regulations
In the United States, pre-stressed concrete design and construction is aided by organizations such as Post-Tensioning Institute (PTI) and Precast/Prestressed Concrete Institute (PCI). In Canada the Canadian Precast/Prestressed Concrete Institute (CPCI) assumes this role for both post-tensioned and pre-tensioned concrete structures.
Europe also has its own associations and institutes. It is important to note that these organizations are not the authorities of building codes or standards, but rather exist to promote the understanding and development of pre-stressed design, codes and best practices. In the UK, the Post-Tensioning Association fulfills this role.
Rules for the detailing of reinforcement and prestressing tendons are provided in Section 8 of the European standard EN 1992-2:2005 - Eurocode 2: Design of concrete structures - Concrete bridges: design and detailing rules.
In Australia the code of practice used to design reinforced and prestressed concrete is AS 3600-2009.
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- Ed Davey and Rebecca Cafe (3 December 2012). "TfL report warned of Hammersmith Flyover collapse risk". BBC News, London. Retrieved 3 December 2012.
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