PRESTENSIONING & POST- TENSIONING. METHODS OF PRESTRESSING IN CONCRETE. PRESTRESSED CONCRETE. PRINCIPLE – Using high tensile strength steel alloys producing permanent pre-compression in areas subjected to Tension.
METHODS OF PRESTRESSING
STEEL BARS BEING STRETCHED BY JACKS
Post –Tensioned Structure
This innovative form is result of post tensioning.
since it allows a high degree of flexibility in the column layout,
span lengths and ramp configurations.
demanding geometry requirements, including complex
curves, and significant grade changes.
constructed without the use of temporary intermediate
supports. This minimizes the impact on the environment
and avoids disruption to water or road traffic below.
creative architecture. \
strength, usually between 3000 and 3500 psi (“pounds per
square inch”), the tendons are stressed and anchored.
(elongated) state causes a compressive force to act on the
counteracts the tensile forces created by subsequent applied loading (cars, people, the weight of the
beam itself when the shoring is removed).
Limitations of Prestressing
The limitations of prestressed concrete are few and really depend only upon the imagination of the designer and the terms of his brief. The only real limitation where prestressing is a possible solution may be the cost of providing moulds for runs of limited quantity of small numbers of non-standard units.
Wedges tensioned by jacks
DECK STEEL LAYING
Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
Unbonded post-tensioned concrete
The lower and upper terraces cantilever over the stream below. The temporary structural steel shoring was placed beneath the main level terrace.
Frank Lloyd Wright's Fallingwater Mill Run, Pennsylvania
Overlooking the 85th Street Bridge prior to concrete placement
The Seward Silo project involved the post-tensioning of three interconnected ash silos that are part of the Seward Re-Powering Project in Seward, Pennsylvania. The overall project involved the construction of a new, state-of-the-art 208 MW power plant designed to burn low-grade coal that can not be burned in ordinary coal plants. This is a design-build project with Drake-Fluor Daniel as the owner/construction manager until the completed plant is turned over to Reliant Energy, the ultimate owner.
T80 Post-Tensioning Jacks
T8Z-04 Torque Multiplier (4:1)
For use with T8Z Torque Wrench. Other sizes available
T1Z Deep Socket
2Z Regular Socket
K3F-26 Long Fitting Wrench Adapter
For applying torque to recessed anchor nuts that are under tension when using hydraulic jacks. Available in all anchor sizes.
Methods of Corrosion Protection
Fusion bonded epoxy coating of steel bars to help prevent corrosion has been successfully employed in many applications because of the chemical stability of epoxy resins. Epoxy coated bars and fasteners should be done in accordance with ASTM A-775 or ASTM 934. Coating thickness is generally specified between 7 to 12 mils. Epoxy coated bars and components are subject to damage if dragged on the ground or mishandled. Heavy plates and nuts are often galvanized even though the bar may be epoxy coated since they are difficult to protect against abrasion in the field. Epoxy coating patch kits are often used in the field for repairing nicked or scratched epoxy surfaces.
Cement Grout filled corrugated polyethylene tubing is often used to provide an additional barrier against corrosion attack in highly aggressive soils. These anchors are often referred to as MCP or Multiple Corrosion Protection anchors. The steel bars are wrapped with an internal centralizer then placed inside of the polyethylene tube where they are then factory pre-grouted. When specifying couplings with MCP ground anchors, verify coupling locations with a Williams representative.
Williams strand tendons contain an extruded high density polyethylene sheathing around each individual strand in the free-stressing portion of the anchorage. The sheathing is minimum 60 mils thick and applied once the 7-wire strand has been coated with a corrosion inhibiting compound. Extruded polyethylene sheathing provides a moisture tight barrier for corrosion protection and allows the strand to elongate freely throughout the free-stressing length during the prestressing operation
Williams corrosion inhibiting compounds can be placed in the free stressing sleeves, in the end caps, or in the trumpet areas. Often bars are greased/waxed and PVC is slipped over the greased/waxed bar prior to shipping. Each are of an organic compound with either a grease or wax base. They provide the appropriate polar moisture displacement and have corrosion inhibiting additives with self-healing properties. They can be pumped or applied manually. Corrosion inhibiting compounds stay permanently viscous, chemically stable and non-reactive with the prestressing steel, duct materials or grout. Both compounds meet PTI standards for Corrosion Inhibiting Coating.
Epoxy Coating Patch Kits
Epoxy Coating Patch Kits are available upon request.
Fiber Reinforced Nylon Cap
Steel Tube Welded on Flange with Threaded Screw Connections