Three- or seven-wire pretensioning strand should be bonded beyond the critical
section for a development length, in, of at least
(The expression in parentheses is used as a constant without units.) Investigations for bond integrity may be limited to those cross sections nearest each end of the member that are required to develop their full strength under factored load. When bonding does not extend to the end of the member, the bonded development length given by Eq. (9.134) should be doubled.
Minimum Bonded Reinforcement. When prestressing steel is unbonded, the ACI 318 Building Code requires that some bonded reinforcement be placed in the precompressed tensile zone of flexural members and distributed uniformly over the tension zone near the extreme tension surface. The amount of bonded reinforcement that should be furnished for beams, one-way slabs, and two-way slabs, except for two-way flat plates, is
The Code requires the bonded reinforcement computed by Eq. (9.137) to be distributed between lines that are 1.5h outside opposite faces of the column support, and that at least four bars be provided in each direction spaced not over 12 in, where h is the depth of the flat plate. Requirements are included in the Code for minimum lengths and extensions of the bonded reinforcement computed by Eqs. (9.135), (9.136) and (9.137).
Grouting of Tendons. When posttensioned tendons are to be bonded, a cement grout is usually injected under pressure (80 to 100 psi) into the space between the tendon and the sheathing material of the duct. The grout can be inserted in holes in the anchorage heads and cones, or through buried pipes. To ensure filling of the space, the grout can be injected under pressure at one end of the member until it is forced out the other end. For long members, it can be injected at each end until it is forced out a vent between the ends.
Grout provides bond between the posttensioning tendons and the concrete member and protects the tendons against corrosion.
Members should be above 35F in temperature at the time of grouting. This minimum temperature should be maintained until field-cured 2-in cubes of grout reach a minimum compressive strength of 800 psi.
Tendon Sheaths. Ducts for grouted or unbonded tendons should be mortar-tight and nonreactive with concrete, tendons, or filler material. To facilitate injection of the grout, the duct should be at least 1â„4 in larger than the diameter of a single posttensioning tendon. For multiple strand, bar or wire tendons, the duct should have an internal area at least twice the gross area of the prestressing steel.
