For prestressed tendons, apply the coating as an electrostatically
charged dry powder sprayed onto a grounded steel bar using an electrostatic
Criteria for Design. In the design of precast and prestressed
concrete items, fully detail and delineate the design of the member
connections, including the proper forming and construction, to ensure maximum
Connections and joints for exterior wall panels must be
watertight, and at the same time, allow shrinkage and temperature differential
movement. Do not use welded or bolted metal connectors where exposed to the
weather, unless such components are of stainless steel or fusion-bonded epoxy
coated steel. Where connectors are recessed, allow a minimum 2-1/2 inches of
concrete cover for epoxy resin filler.
Post-Tensioned Concrete. Give special attention to post-tension
strands when designing in tropical environments. The Post-tensioning
Institute has the following recommendations for unbonded tendons.
Tendons. For unbonded tendons use prestressing steel permanently
protected against corrosion by a properly applied coating of epoxy, grease,
wax, plastic, bitumastic, asphaltic mastic, or other approved material.
Ensure the coating remains ductile and free from cracks and does not become
fluid over the entire operating or anticipated range of temperatures. Ensure
that the coating is chemically stable, and be nonreactive to steel, concrete
and the material used for sheathing. Make sure the coating material adheres
to and be continuous over the entire tendon length to be unbonded. Where
coating material is applied before the tendons are pulled into the ducts or
casings, ensure the coating material is sufficiently tough to resist abrasion.
Sheathing. Ensure that the sheathing for unbonded tendons has
tensile strength and water-resistance sufficient to resist unrepairable damage
and deterioration during transport, storage at job site, and installation.
The sheathing prevents the intrusion of cement paste and the escape of coating
Protect the anchorage of unbonded tendons from
corrosion. Encase anchorage zones in concrete or grout and ensure the
encasement is free from any chlorides. Portland cement sand grout and epoxy
mortars have been used for this purpose. Detailing of the concrete or grout
encasement, design of the mix, and details of application are most important.
Include design features permitting a watertight connection of the sheathing to
the anchorage, and watertight closing of the wedge cavity in anchorages
intended for use in corrosive environments. In such cases, the film thickness
of the coating after curing shall be 5 to 12 mils inclusive.
Concrete. The use of quality concrete, adequate cover, good
concrete are all necessary to assure long-term durability, particularly in