Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands or bars, typically referred to as tendons.
Post-tensioning applications include:-
- office and apartment buildings,
- parking structures,
- sports stadiums,
- rock and soil anchors,
In many cases, post tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.
Examples of Post Tensioning
Although post-tensioning systems require specialized knowledge and expertise to fabricate, assemble and install, the concept is easy to explain.
Imagine a series of wooden blocks with holes drilled through them, into which a rubber band is threaded. If one holds the ends of the rubber band, the blocks will sag.
Post-tensioning can be demonstrated by placing wing nuts on either end of the rubber band and winding the rubber band so that the blocks are pushed tightly together. If one holds the wing nuts after winding, the blocks will remain straight. The tightened rubber band is comparable to a post-tensioning tendon that has been stretched by hydraulic jacks and is held in place by wedge-type anchoring devices.
Method of Post-tensioning
Post-tensioning is a method of producing prestressed concrete, masonry and other structural elements. The term prestressing is used to describe the process of introducing internal forces (or stress) into a concrete or masonry element during the construction process in order to counteract the external loads that will be applied when the structure is put into use (known as service loads). These internal forces are applied by tensioning high strength steel, which can be done either before or after the concrete is actually placed.
When the steel is tensioned before concrete placement the process is called pre-tensioning.
When the steel is tensioned after concrete placement the process is called posttensioning.
Benefits of Post-Tensioning
To fully appreciate the benefits of post-tensioning, it is helpful to know a little bit about concrete. Concrete is very strong in compression but weak in tension, i.e. it will crack when forces act to pull it apart.
In conventional concrete construction, if a load such as the cars in a parking garage is applied to a slab or beam, the beam will tend to deflect or sag. This deflection will cause the bottom of the beam to elongate slightly. Even a slight elongation is usually enough to cause cracking. Steel reinforcing bars (“rebar”) are typically embedded in the concrete as tensile reinforcement to limit the crack widths.
Rebar is what is called “passive” reinforcement however; it does not carry any force until the concrete has already deflected enough to crack. Post-tensioning tendons, on the other hand, are considered “active” reinforcing. Because it is prestressed, the steel is effective as reinforcement even though the concrete may not be cracked. Post-tensioned structures can be designed to have minimal deflection and cracking, even under full load.
Applications of Post-Tensioning
There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Posttensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas. A lower building height can also translate to considerable savings in mechanical systems and façade costs. Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.
Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. Post-tensioned parking garages can be either stand-alone structures or one or more floors in an office or residential building. In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement. Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex curves, variable super elevation and significant grade changes. Post-tensioning also allows extremely long span bridges to be constructed without the use of temporary intermediate supports. This minimizes the impact on the environment and avoids disruption to water or road traffic below. In stadiums, post-tensioning allows long clear spans and very creative architecture. Post-tensioned rock and soil anchors are used in tunneling and slope stabilization and as tie-backs for excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-tanks.
A post-tensioning \”tendon\” is defined as a complete assembly consisting of the anchorages, the prestressing strand or bar, the sheathing or duct and any grout or corrosion-inhibiting coating (grease) surrounding the prestressing steel. There are two main types of posttensioning: unbonded and bonded (grouted). An unbonded tendon is one in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchorages. The most common unbonded systems are monostrand (single strand) tendons, which are used in slabs and beams for buildings, parking structures and slabs-on-ground.
A monostrand tendon consists of a seven-wire strand that is coated with a corrosion-inhibiting grease and encased in an extruded plastic protectivesheathing. The anchorage consists of an iron casting and a conical, two-piece wedge which grips the strand. In bonded systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. T e strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct. Bonded systems are more commonly used in bridges, both in the superstructure (the roadway) and in cable-stayed bridges, the cable-stays. In buildings, they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical.
Rock and soil anchors are also bonded systems but the construction sequence is somewhat different. Typically, a cased hole is drilled into the side of the excavation, the hillside or the tunnel wall. A tendon is inserted into the casing and then the casing is grouted. Once the grout has reached sufficient strength, the tendon is stressed. In slope and tunnel wall stabilization, the anchors hold loose soil and rock together; in excavations they hold the wood lagging and steel piles in place.
There are several critical elements in a post-tensioning system. In unbonded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also provides protection against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand. The strand coating material reduces friction between the strand and the sheathing and provides additional corrosion protection.
Anchorages are another critical element, particularly in unbonded systems. After the concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.
In building and slab-on-ground construction, unbonded tendons are typically prefabricated at a plant and delivered to the construction site, ready to install. The tendons are laid out in the forms in accordance with installation drawings that indicate how they are to be spaced, what their profile (height above the form) should be, and where they are to be stressed. After the concrete is placed and has reached its required strength, usually between 3000 and 3500 psi (“pounds per square inch”), the tendons are stressed and anchored. The tendons, like rubber bands, want to return to their original length but are prevented from doing so by the anchorages. The fact the tendons are kept in a permanently stressed (elongated) state causes a compressive force to act on the concrete. The compression that results from the posttensioning counteracts the tensile forces created by subsequent applied loading (cars, people, the weight of the beam itself when the shoring is removed). This significantly increases the load-carrying capacity of the concrete. Since post-tensioned concrete is cast in place at the job site, there is almost no limit to the shapes that can be formed. Curved facades, arches and complicated slab edge layouts are often a trademark of post-tensioned concrete structures. Post-tensioning has been used to advantage in a number of very aesthetically designed bridges.
Post-tensioning ni njia ya kuimarisha (kuimarisha) saruji au vifaa vingine na nyuzi za chuma zenye nguvu nyingi au baa, kwa kawaida huitwa tendons. Post-tensioning maombi ni pamoja na: ofisi na majengo ya ghorofa, vituo vya maegesho, viwanja vya michezo, miamba na ardhi, katika kesi nyingi, baada ya tensioning inaruhusu ujenzi ambayo ingekuwa vinginevyo haiwezekani kutokana na ama tovuti vikwazo au mahitaji ya usanifu. Mifano ya Post Tensioning Ingawa mifumo ya baada ya tensing inahitaji maarifa maalum na utaalam wa kutengeneza, kukusanya na kufunga, dhana ni rahisi kuelezea. Hebu wazia mfululizo wa mawe yenye mashimo yaliyochimbwa, ambayo yana kamba ya mpira. Ikiwa mtu atashika ncha za mkanda wa mpira, vitalu hivyo vitashuka. Post-tensioning inaweza kuonyeshwa kwa kuweka nuts mrengo juu ya pande zote mbili za bendi ya mpira na winding bendi ya mpira hivyo kwamba vitalu ni kusukumwa tightly pamoja. Ikiwa mtu atashikilia karanga za mrengo baada ya kuzunguka, vitalu vitabaki sawa. Rubber band imefungwa ni sawa na tendon ya baada ya kuimarisha ambayo imepanuliwa na jacks hydraulic na imefungwa mahali na vifaa vya anchoring aina ya mviringo. Post-tensioning ni mbinu ya kuzalisha precasted saruji, masonry na vipengele vingine vya muundo. "Jina la ""prestressing"" linatumiwa kuelezea mchakato wa kuingiza nguvu za ndani (au mkazo) katika sehemu ya saruji au masonry wakati wa mchakato wa ujenzi ili kukabiliana na mizigo ya nje ambayo itatumika wakati muundo unatumiwa (unajulikana kama mizigo ya huduma)." Nguvu hizi za ndani hutumiwa kwa kushinikiza chuma cha nguvu ya juu, ambayo inaweza kufanywa kabla au baada ya saruji kuwekwa. Wakati chuma ni tensed kabla ya kuweka saruji mchakato inaitwa pre-tensioning. Wakati chuma ni tensed baada ya kuweka saruji mchakato inaitwa posttensioning. Faida za baada ya tensioning Kwa kufahamu kikamilifu faida za baada ya tensioning, ni muhimu kujua kidogo kuhusu saruji. Zege ni nguvu sana katika compression lakini dhaifu katika mvuto, yaani. Itapasuka wakati nguvu zinapofanya kazi ili kuivunja. Katika ujenzi wa kawaida wa saruji, ikiwa mzigo kama vile magari katika karakana ya maegesho hutumiwa kwa slab au beam, beam itaelekea kupotosha au kuanguka. Kupotoka huko kutasababisha sehemu ya chini ya mhimili huo kupanuka kidogo. Hata kunyoosha kidogo kwa kawaida kunatosha kusababisha kupasuka. Bar za kuimarisha chuma (rebar) kwa kawaida huingizwa katika saruji kama kuimarisha mvuto ili kupunguza upana wa ufa. Rebar ni kile kinachoitwa <unk>passive<unk> kuimarisha hata hivyo; haina kubeba nguvu yoyote mpaka saruji tayari deflected kutosha kuvunja. Post-tensioning tendons, kwa upande mwingine, ni kuchukuliwa <unk>active<unk> kuimarisha. Kwa sababu imefungwa kimbele, chuma hicho kinaweza kutumiwa kama kiunganishi hata ingawa saruji hiyo haiwezi kuvunjika. Miundo ya baada ya mvutano inaweza kubuniwa kuwa na deflection ndogo na kupasuka, hata chini ya mzigo kamili. Maombi ya Post-Tensioning Kuna maombi ya baada ya tensing katika karibu nyanja zote za ujenzi. Katika ujenzi, baada ya tensing inaruhusu span muda mrefu wazi, slab nyembamba, beams chache na zaidi nyembamba, vipengele vya kuvutia. Vipande vyepesi vinamaanisha kwamba saruji ndogo inahitajika. Kwa kuongezea, inamaanisha urefu wa chini wa jumla wa jengo kwa urefu sawa wa sakafu hadi sakafu. Posttensioning hivyo inaweza kuruhusu kupunguzwa kwa kiasi kikubwa katika uzito wa ujenzi dhidi ya jengo la kawaida la saruji na idadi sawa ya ghorofa. Hii hupunguza mzigo wa msingi na inaweza kuwa faida kubwa katika maeneo ya seismic. Urefu wa chini wa jengo pia unaweza kutafsiri kwa akiba kubwa katika mifumo ya mitambo na gharama za facade. Faida nyingine ya baada ya tensing ni kwamba beams na slabs inaweza kuwa kuendelea, yaani. Mti mmoja unaweza kukimbia kwa kuendelea kutoka mwisho mmoja wa jengo hadi mwingine. Kwa muundo, hii ni ufanisi zaidi kuliko kuwa na beam ambayo tu huenda kutoka safu moja hadi nyingine. Post-tensioning ni mfumo wa uchaguzi kwa ajili ya miundo ya maegesho tangu inaruhusu kiwango cha juu cha kubadilika katika mpangilio wa nguzo, urefu span na mipangilio ramp. Baada ya tensed maegesho ya maegesho inaweza kuwa ama kusimama peke yake miundo au ghorofa moja au zaidi katika ofisi au jengo la makazi. Katika maeneo ambapo kuna udongo wa kupanua au udongo na uwezo wa chini wa kubeba, post-tensioned slabs-on-ground na mat misingi kupunguza matatizo na cracking na makazi tofauti. Post-tensioning inaruhusu madaraja kuwa kujengwa kwa mahitaji ya geometry sana kudai, ikiwa ni pamoja na curves tata, kutofautiana super elevation na mabadiliko makubwa ya daraja. Baada ya tensing pia inaruhusu daraja ya muda mrefu sana span kuwa kujengwa bila ya matumizi ya msaada wa muda wa kati. Hii hupunguza athari kwa mazingira na kuepuka usumbufu kwa maji au trafiki ya barabara chini. Katika viwanja vya michezo, baada ya tensing inaruhusu span muda mrefu wazi na usanifu sana ubunifu. Post-tensioned mwamba na udongo nanga ni kutumika katika kuchimba handaki na mteremko utulivu na kama tie-nyuma kwa ajili ya uchimbaji. Baada ya tensing pia inaweza kutumika kuzalisha karibu crack-free saruji kwa ajili ya maji-tank. Post-tensioning <unk>tendon <unk> ni kufafanuliwa kama mkutano kamili inayojumuisha anchorages, pretensioning strand au bar, sheathing au duct na yoyote grout au kutu-kuzuia mipako (mafuta) kuzunguka pretensioning chuma. Kuna aina mbili kuu za posttensioning: unbonded na bonded (grouted). Tendon unbonded ni moja ambayo chuma pretensing si kweli amefungwa kwa saruji ambayo inazunguka ni isipokuwa katika anchorages. Mifumo ya kawaida unbonded ni monostrand (single strand) tendons, ambayo ni kutumika katika slabs na beams kwa ajili ya majengo, majumba ya maegesho na slabs-on-ardhi. Monostrand tendon ni pamoja na tambo saba waya ambayo ni coated na kutu-kuzuia mafuta na encased katika extruded plastiki protective sheathing. Anchorage inajumuisha chuma kutupwa na conical, mbili-sehemu mviringo ambayo grabs strand. Katika mifumo ya kushikamana, nyuzi mbili au zaidi huingizwa katika mfereji wa chuma au plastiki ulioingizwa katika saruji. T e strands ni alisisitiza na kubwa, multi-strand jack na anchored katika kifaa kawaida anchorage. Kisha mfereji huo hujazwa na mchanganyiko wa saruji ambao hutoa ulinzi dhidi ya kutu kwa kamba na kuunganisha tendon na saruji inayozunguka mfereji huo. Mifumo ya bonded ni zaidi ya kawaida kutumika katika madaraja, wote katika superstructure (barabara) na katika cable-kushikilia madaraja, cable-kushikilia. Katika majengo, wao ni kawaida tu kutumika katika beams heavily kubeba kama vile uhamisho beams na landscaped plaza decks ambapo idadi kubwa ya nyuzi zinahitajika hufanya yao zaidi ya kiuchumi. Anchers mwamba na udongo pia ni mifumo bonded lakini utaratibu wa ujenzi ni tofauti kidogo. Kwa kawaida, shimo lenye kifuniko huchimbwa upande wa uchimbaji, mteremko wa mlima au ukuta wa handaki. Tendon ni kuingizwa katika casing na kisha casing ni grouted. Mara tu mchanganyiko huo unapopata nguvu ya kutosha, tendon hufadhaishwa. Katika mteremko na ukuta wa handaki, nanga huhifadhi udongo na miamba pamoja; katika uchimbaji wao huhifadhi mbao na mizizi ya chuma mahali pake. Kuna vipengele kadhaa muhimu katika mfumo wa baada ya tensing. Katika ujenzi unbonded, plastiki sheathing vitendo kama bond breaker kati ya saruji na strands pretensing. Pia huhifadhiwa dhidi ya uharibifu unaosababishwa na vifaa vya kuendesha gari na huzuia unyevu na kemikali zisipite kwenye kamba. nyuzi coating nyuzi hupunguza msuguano kati ya nyuzi na sheathing na hutoa ziada ya kutu ulinzi. Anchorages ni kipengele kingine muhimu, hasa katika mifumo unbonded. Baada ya saruji kuwa imepona na kupata nguvu zinazohitajika, vijiti huingizwa ndani ya kutupwa kwa nanga na kamba hutiwa mkazo. Wakati jack inapoachilia kamba, kamba hujipinda kidogo na kuvuta miiba ndani ya nanga. Hilo hufanya kamba hiyo iwe na kifuniko kigumu. Hivyo, miiba hiyo huhifadhi nguvu inayotumiwa kwenye tendon na kuipeleka kwenye saruji inayoizunguka. Katika mazingira corrosive, anchorages na tails strand wazi ni kawaida kufunikwa na nyumba na cap kwa ajili ya ulinzi wa ziada. Katika ujenzi wa ujenzi na ujenzi wa slab-on-ground, tendons zisizounganishwa kwa kawaida hutengenezwa kwenye kiwanda na kusafirishwa kwenye tovuti ya ujenzi, tayari kusanikishwa. Tendons ni kuweka katika fomu kulingana na michoro ya ufungaji ambayo kuonyesha jinsi ni kuwa spaced, nini profile yao (urefu juu ya fomu) lazima kuwa, na ambapo ni kuwa alisisitiza. Baada ya saruji imewekwa na kufikia nguvu yake inayohitajika, kwa kawaida kati ya 3000 na 3500 psi, tendons ni alisisitiza na anchored. Kama vile bendi za mpira, mishipa ya mwili hutaka kurudi kwa urefu wake wa awali lakini imezuiwa kufanya hivyo na vifungo. Ukweli kwamba tendons ni kushikiliwa katika hali ya kudumu alisisitiza (elongated) husababisha nguvu compressive kutenda juu ya saruji. Mfumo wa mkazo unaotokana na posttensioning hupinga nguvu za kuvuta zinazozalishwa na mzigo uliofanywa baadaye (magari, watu, uzito wa beam yenyewe wakati shoring inaondolewa). Hilo huongeza sana uwezo wa kubeba mzigo wa saruji. Kwa kuwa saruji iliyochanganywa na msongamano hutengenezwa mahali pake mahali pa kazi, kuna karibu hakuna kikomo kwa maumbo yanayoweza kufanyizwa. Curved facades, arches na tata slab makali mipangilio ni mara nyingi alama ya biashara ya baada ya tensed saruji miundo. Baada ya tensing imekuwa kutumika kwa faida katika idadi ya madaraja sana aesthetically iliyoundwa.