Avaliação in loco das perdas de protensão de cordoalhas engraxadas em lajes planas

Abstract

This dissertation presents the results of in situ measurements of stress losses in unbonded tendons of post tensioned flat slabs. These flats slabs have approximately 350 m2 in area and are 17 centimeter thick. They are part of a 17 story high hotel building in Belo Horizonte. Load cells were employed to measure the forces in the tendons; they were placed in the vicinity of the active and the passive anchorages. The forces in the tendons were monitored from the time of their straining up to 170 days afterwards. The measurements reveal a frictional coefficient equal to 0.0528; this value is smaller than 0.07 prescribed by the manufacture but very close to the value suggested by the Brazilian Code NBR 6118 of 0.05. The stress loss due to the anchorage of the tendons was of 5.17% of the initial stress. This value corresponds to an average tendon return of 3,21 mm, lower than 7 mm prescribed by the manufacturer and commonly used in design. The results indicate also that the remaining force near the passive anchorage is larger in relation to the value measured in the vicinity of the active one. The total stress loss measured so far (170 days) corresponds to 14 % of the initial post-tensioning stress.