BDCS Notes - Stressing Concrete

PRESTRESSED AND POST-TENSIONED CONCRETE

CONCRETE FLOOR CONSTRUCTION

• Stressing Concrete:
• Concrete by itself is inherently strong in tension and weak in compression.
• There are two procedures used for placing concrete in compression.
• PRESTRESSING of the reinforcing steel occurs prior to placement of concrete and is used almost exclusively with precast concrete.
• POST-TENSIONING is the permanent tensioning of reinforcing steel for cast-in-place concrete.
• Concrete strength is usually 5000 psi at 28 days, and at least 3000 psi at the time of post-tensioning.
• Use hard-rock aggregate or lightweight concrete.
• Low-slump-controlled mix concrete is required to reduce shrinkage.
• Concrete shrinkage after post-tensioning decreases strength gains.
• Post-tensioning systems can be divided into three categories depending on whether the tendon is wire, strand, or bar.
• Wire systems use 0.25-in. diameter wires that have a minimum strength of 240,000 psi, and re usually cut to length in the shop.
• Strand systems use tendons (made of seven wires wrapped together) that have a minimum strength of 270,000 psi, and are cut in the field.
• Bar systems use bars ranging from 5/8- to 13/8 inches in diameter, with a minimum strength of 145,000 psi, and may be smooth or deformed.
• The system used determines the type of anchorage used, which in turn affects the size of blockout required (in the edge of slab or beam) for the anchorage to be recessed.
• Grease and wrap tendons, or place in conduits, to reduce frictional losses during stressing operations.
• Limit the length of continuous tendons to about 10 ft. if stressed from one end.
• Long tendons require simultaneous stressing from both ends to reduce friction loss.
• Tendons may be grouted after stressing, or left unbonded.
• Bonded tendons have structural advantages that are more important for beams and primary structural members.
• Minimum average post-tensioning (net force per area of concrete) equals 150 to 250 psi for flat plates and 200 to 500 psi for beams.
• Exceeding these values by much causes excessive post-tension loss because of creep.
• Field inspection of post-tensioned concrete is critical to ensure proper size and location of tendons, and to monitor the tendon stress.
• Check tendon stress by measuring the elongation of the tendon and by monitoring gauge pressures on the stressing jack.
• Make provisions for the shortening of post-tensioned beams and slabs caused by elastic compression, shrinkage, and creep.
• After the post-tensioning is complete, build shear walls, curtain walls, or other stiff elements that adjoin post-tensioned members and isolate them with an expansion joint.
• Otherwise, additional post-tensioning force will be required to overcome the stiffness of the walls and prevent cracking.
• Fire tests have been conducted on prestressed beams and slab assemblies according to ASTM E119, “Standard Test Methods for Fire Tests of Building Construction.”
• They compare favorably with reinforced cast-in-place concrete.
• There is little difference between beams using grouted tendons and those using ungrouted tendons.
• When working with a prestressed or post-tensioned beam, keep the following in mind:
• Prestressing force compressed the entire cross section of the beam, thereby reducing unwanted tension cracks.
• Permanent tension is introduced into the tendon and “locked in” with the stressing anchorage in one of two ways, though the principle in both cases is the same.
• In prestressed concrete, the tendon is elongated after concrete has been poured and allowed to cure be means of hydraulic jacks pushing against the beam itself.
• Post-tensioned beams permit casting at the site for members too large or heavy for transporting from the factory to the site.
• Internal vertical forces within the beam are created by applying tension on the tendon, making the tendon begin to “straighten out.”
• The tension reduces downward beam deflection and allows for shallower beams and longer spans than in conventionally reinforced beams.
• Auxiliary reinforcing steel provides additional strength, and controls cracking and produces more ductile behavior.
• Use stirrups to provide additional shear strength in the beam and to support the tendons and longitudinal reinforcing steel.
• Stirrups should be open at the top to allow the reinforcing to be placed before the tendon is installed.
• After the tendons are placed, “hairpins” that close the stirrups may be used, when required.