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We often want to reinforce some operational processes, precisely because of the high construction safety performance requirements for reinforcement, so Anchorage has been used. In prestressed tension device, the most important thing is post-tensioned. Post-tensioned, what does? Loss how generated?
Pre-tensioned
In accordance with the provisions of the first pedestal design requirements will be reinforced sheets pulled control stress → and temporary fixed anchorage → → pouring concrete to meet the design strength of concrete to be more than 75% cut relax rely on bond strength of reinforced steel and concrete to pass the pre-stress. Construction is simple, relying on bond strength of self-anchored, no need to spend a special anchorage, temporary anchorage can be reused (generally referred instrumental anchorage or fixtures), when mass production economy, quality and stability. Small components used in industrial production.
Post-tensioned
Buried control holes for pouring concrete → → → conservation wear bars pumping pipe tensioning → Anchorage → filling (to prevent rusting steel). Anchorage stop relying on elastic rebound of steel, so get pre-stress concrete cross-section. This method is simple equipment, no tension pedestal, production flexibility, but requires reliable anchorage and jack, anchor steel capacity, on-site construction applicable to large components.
Measures:
① Select the deformation is small or narrow anchor prestressing steel to minimize the number plate; ② for pre-tensioned members, choose a long pedestal.
2 friction loss s12: Reason: In the process of tensioning the tendons, the friction-tensioned tendons and pore wall between between pre-tensioned tendons and anchorages break point and friction, but also tensile stress will cause losses.
Measures: ① for longer member can be tensioned at both ends; ② ultra-tensioned.
3 the temperature difference between the loss of s13: Reason: loss measures the temperature difference between the pre-tensioned caused by heat conservation: ① using secondary heating maintenance. ② on prestressed steel reinforcement.
4 Stress relaxation losses σl4 reasons: length constant tendons in the long-term effects of high stress will produce relaxation, can cause loss of prestress.
Measures: ultra small tension or relaxation prestressed reinforcement.
5 concrete shrinkage and creep damage caused and σl5 reasons: to produce concrete harden the volume contraction in the pre-pressure effect, concrete creep will occur, which will cause the member to shorten, member of the prestressing steel followed by retraction , resulting in loss of prestress.
Measures: ① use of high-grade cement, reduce the amount of cement, reducing the water-cement ratio; ② the use of well-graded aggregate, strengthen vibration, increase the density of concrete; ③ strengthen conservation in order to reduce shrinkage of concrete, ④ control concrete stress σpc, require in order to prevent the occurrence of nonlinear creep.
6 ring reinforcement loss σl6 reasons: an annular member with spiral prestressing steel for reinforcement of concrete due to prestressing losses caused partial extrusion.
