State-of-the-Art Report: Criteria for Cathodic Protection of Prestressed Concrete Structures
|Publication Date:||8 March 2018|
Types and Principles of Prestressed Concrete
Prestressed concrete has evolved during the past five-plus decades to the point that it is now widely employed for transportation structures, buildings, pipelines, and other applications because of its technical viability and economic competitiveness. While concrete per se normally exhibits acceptable compressive strength, it is relatively weak in tension. Therefore, embedded steel is added to accommodate tensile stresses. For structural applications, concrete is either reinforced or prestressed (or a combination of the two). For the former, bars are positioned in the formwork; and the concrete is poured and sets such that, neglecting dead weight and service loadings, no stresses are imparted by either component (steel or concrete) to the other. The principle of prestressed concrete is based on tensioning of the steel in such a manner that it ultimately places the concrete in a state of residual compression. Consequently, service tensile loadings on the concrete, up to a certain level, act to reduce this compression; and tensile stresses are either less than would otherwise be the case or are avoided altogether. A basic introductory discussion of the two types of prestressed concrete, pretensioned and post-tensioned, is provided in this Introduction. For more detailed information, a standard text in the field may be consulted.4
Types and properties of prestressing steel. Imposition of adequate residual compression to a concrete member via prestressing steel uses steel of high strength, because its cross-section is generally small compared with that of the concrete and the net force in each component (steel and concrete) balances. The specification for prestressing steel strand is provided by ASTM(2) A416.5 Currently, most prestressing steel is in the form of spiral, seven-wire strand that is designated as either Grade 250 or Grade 270, in which the number refers to minimum ultimate strength in kilo pounds per square inch (ksi) units (1 ksi = 1,000 psi [6.895 MPa]). Historically, bar as well as strand has been employed. Prestressing for concrete pipe is in the form of wire and is addressed by ASTM A648.6 In either case (strand or wire), strengthening is achieved by a carbon concentration near the eutectoid composition (0.77 w%) combined with cold drawing. Heat-treated (quenched and tempered) steel is not used because of its greater susceptibility to brittle fracture and environmental cracking. In the past, however, quenched and tempered material has been used in some countries. Otherwise, the above standards primarily address dimensions and strength, with the means by which the requisite strength is achieved being left to the producer. Steel composition is typically considered to be important, because this influences the strengthening that is derived from cold drawing. Either plain carbon or micro-alloyed steel, with small amounts of either chromium, vanadium, or chromium plus vanadium, is commonly employed.
Pretensioned concrete. Components in this class are normally produced in a prefabrication yard and then transported to the construction site. Consequently, there are practical limits on member size. Beams, columns, and pilings are examples of components that are routinely pretensioned. Fabrication involves placement and pretensioning of the tendons(3) in a form bed, pouring the concrete, allowing the concrete to set, and, finally, removing the applied tensioning force on the tendon. The tendency for the tendon within the hardened concrete to contract, once this force is removed, places the concrete in a state of residual compression.
Posttensioned concrete. Components in this class are normally produced in place at the job site (an exception to this, segmental construction, is described below). Slabs for buildings and decks for parking garages are examples in which posttensioning is commonly employed. For fabrication, tendons are contained in ducts that are, in turn, positioned in the pouring forms. Consequently, only the duct and not the tendon is in direct contact with and is bonded to the concrete. Once the concrete has achieved a prescribed strength, the tendons are tensioned and the loaded ends secured by collets in anchors. As is the case for pretensioning, compressive stresses are imparted to the concrete. In some cases grout slurry is then pumped into the duct pore space (bonded posttensioning), while no such measure is used in others (unbonded posttensioning). Tendons in unbonded posttensioned concrete are typically surface treated with grease that contains a corrosion inhibitor.
Reinforcing steel has invariably been present in addition to the prestressing strands within prestressed concrete members. This is done either to ensure integrity in areas of locally high stress or to provide strength in the transverse direction, or both.
Segmental construction is a special case of prestressed concrete in which individual pretensioned members are secured together into a larger assembly by posttensioning.