i. Introduction Post Tensioning

I.

INTRODUCTION

POST TENSIONING -“Post”,

a prefx, meaning “behind,” “ater,” “ ater,” “later,” “subsequent to,” “posterior to,” occurring originally in loanwords rom Latin ( postscript), but now used reely in the ormation o compound words ( post-li!abethan" post-li!abethan" postfx" postgraduate" http$%%www#thereedictionary#com%post postorbital)#

-“Tensioning”, an act or process o stretching something tight or the condition o so being stretched" tautness# http$%%www#thereedictionary#com%tensioning &ost tensioning is a technique or reinorcing concrete# &ost-tensioning tendons, which are prestressing steel cables inside plastic ducts or slee'es, are positioned in the orms beore the concrete is placed# terwards, once the concrete has gained strength but beore the ser'ice loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges o the concrete# &ost-tensioning is a orm o prestressing# &restressing simply means that the steel is stressed (pulled or tensioned) beore the concrete has to support the ser'ice loads# ost precast, prestressed concrete is actually pre-tensioned-the steel is pulled beore the concrete is poured# &ost-tensioned concrete means that the concrete is poured and then the tension is applied-but it is still stressed beore the loads are applied so it is still prestressed# https$%%www#concretenetwor*#com%post-tension% &ost-+ensioned concrete was originally de'eloped during the ./0s by a 1renchman, ugene 1reyssinet, who reali!ed that placing concrete under compression greatly increased its strength# &ost-tensioning consists o laying sheathed cables, pouring concrete around them and allowing it to set up, then stretching stretching the cables and loc*ing them into place# ter 2orld 2ar 33, post-tensioned concrete became a popular building method because o the steel shortage and the need to replace damaged and bombed-out bridges# +he design and use o this method waned until the mid-4/0s, when much o its utili!ation was in the area o ground-supported slabs or warehouse, apartment, and residential 5oors# 3n 64, the &ost-+ensioning 3nstitute, which recogni!ed the expanding interest in the feld, was ormed# +he use o post-tensioned concrete slabs or tennis courts, howe'er, is airly new# +he 7nited 8tates +ennis 9ourt and +rac* :uilders ssociation did not issue a general specifcation or post-tensioned construction construction until ;compressi'e? strength @ meaning that it happily bears its own weight within a

structure# s soon as you introduce the >li'e? loads o e'eryday usage, such as 'ehicles in a car par* or on a bridge, the concrete tends to de5ect or sag which leads to crac*ing, thus wea*ening the structure# 9oncrete lac*s >tensile? strength# lone, it does not always o=er the 5exibility needed# +hat?s why steel reinorcing bars @ >rebar? @ are oten embedded in the concrete to limit the width o crac*s# Aowe'er, rebar pro'ides only passi'e reinorcement @ that is, it does not bear any load or orce until the concrete has already crac*ed#  +his is where post-tensioning comes in# &+ systems pro'ide acti'e reinorcement#  +he unction o post-tensioning is to place the concrete structure under compression in those regions where load causes tensile stress# &ost-tensioning applies a compressi'e stress on the material, which o=sets the tensile stress the concrete might ace under loading# &+ is applied by the use o post-tensioning >tendons? @ a complete assembly including the 'ery high strength prestressing steel strands or bar, the sheathing or protecti'e ducting, plus any grout or corrosion-inhibiting coating surrounding the steel strands or bar and the anchorages needed at both ends#  typical steel strand used or post-tensioning has a diameter o B#6mm and a tensile strength around our times higher than an a'erage non-prestressed piece o rebar# 8heathing or ducting houses the prestressing steel# +his allows it to mo'e as necessary when the tensioning orce is applied ater the concrete cures# +he steel stretches as it is tensioned and it is loc*ed into place using an anchoring component, thus maintaining the orce in the strand or the lie o the structure # http$%%www#bbrnetwor*#com%technologies%what-is-pt#html

Top: Post-tensioning tendons are draped within the slab cross secon. Boom: Signifcant over-balancing o the weight o the concrete with post-tensioning can actuall li! and crac" the slab.