prestressing_manual.pdf

THE FREYSSI NET PRESTRESSED CONCRETE CO. LTD.6B, 6 t h Floor, St er ling Center , Dr. Annie Besant Road, Worli, Mumbai – 400018, Maharasht ra – I ndia. : + 91 22 43229500, 24921129, 24921141 Fax: + 91 22 24938461 THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 1 INTRODUCTION PRI NCPLES OF PRESTRESSI NG Prest ressing is a met hod of inducing known permanent st resses in a st r uct ure or member befor e t he full or live load is applied. These st resses are induced by t ensioning t he High Tensile St rands, wires or r ods, and t hen anchored t o t he member being Prest ressed, by mechanical means. The Pr estressing count eract s t he st resses, pr oduced by subsequent loading on t he st ruct ures, t hereby ext ending t he r ange of st r esses t o which a st r uct ur al member can safely be subj ect ed. This also improves t he behavior of t he mat erial of which t he member or st ruct ure is composed. For Example; The Concrete which has relat ively a low Tensile st rengt h, shall behave like a member having high t ensile st rengt h, aft er Prest ressing. The High Tensile wir es/ st rands, when bunched t oget her are called Cables. These cables are generally placed inside a cylindrical duct made out of eit her met allic or HDPE mat erial. The Anchorages, one of t he main component s of t he Prest ressing act ivit y, are used t o anchor t he H.T. Cable aft er inducing t he Load. The whole assembly of t he Anchorage and t he H.T. Cable is named as ‘TENDON’. APPLI CATI ON AND USE OF PRESTRESSI NG I n st r uct ural Member , wher e t he span lengt h is very high wit h low rises and low st ruct ural height , t he applicat ion of Reinfor ced Cement Concret e shall be virt ually impract ical. I n such a case, Pr est ressing is used t o achieve a light weight , elegant looking and much economical st r uct ure wit h high durabilit y. Prest ressing, t herefore, is widely used for long span beams and Br idges. I n building st ruct ur e also, prest ressing met hod is very effect ively used t o achieve light er beams and slabs; t hus reducing t heir dead load considerably as compared t o R.C.C. St r uct ur es. Applicat ion of Prest ressing in building const ruct ion also facilit at es a larger span bet ween t he columns, t hus reduces t he number of columns. This also makes t he str uct ure mor e ver sat ile for int erior planning. Prest ressing is also very widely used in t he const ruct ion of Mega St ruct ures like Cont ainment Wall of Nuclear React ors, LNG Stor age Tanks, Cement Silos, Chimneys, Dams and Rock Anchor s et c. SECTION - A THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 2 TYPE OF PRESTRESSI NG SYSTEMS Prest ressing System can be classified by two basic met hods, as under: - a. Pre- Tensioning b. Post - Tensioning. Pr e- Tensioning - is a met hod where Prest ressing St eels are pre- st ressed, prior t o concret ing, against t wo r igid abut ment s. This met hod is most widely used for mass pr oduct ion of shor t span str uct ures, where pre- st ressing is also a pr er equisit e, such as; Railway Sleeper s, Elect ric Polls, Fencing Polls, Pre- Tensioned Slabs and I - Sect ion Bridge Girders et c. I n t his syst em, a number of ident ical st ruct ural frames are placed in bet ween t he t wo rigid abut ment s or react ion bolst er. Prest ressing St eel is t hen placed longit udinally across t hese frames and abut ment s, in t he required or ientat ion, and st ressed. Aft er achieving required elongat ion and st r esses t hey are blocked at t wo abut ment s and t hen concret e is poured in t he frames wit h st ressed steels in posit ion. Post - Tensioning - is a met hod where Prest ressing Steels ar e st ressed after concret e at t ains it s pr eliminary st rengt h. Two ext reme ends of t he st r uct ure are considered as a react ion face, against which force is applied. Duct s are placed inside t he for mwork alongwit h reinfor cement and t he concret ing is completed. Aft er achieving required concret e st rengt h, a st ipulat ed number s of Prest ressing Steel is t hen inser ted in each duct for st ressing purpose. Aft er achieving required elongat ion and st resses t hey are blocked at two ends wit h t he help of Anchor Plat es and gr ip. The broad classificat ions are given as under SECTION - A PRESTRESSING SYSTEMS PRE-TENSIONING POST-TENSIONING BONDED TENDONS UNBONDED TENDONS MULTI STRESSING WIRES, BARS & STRANDS BARS STRANDS MONO STRESSING WIRES STRAND WIRES & STRANDS THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 3 THE FREYSSI NET PRESTRESSED CONCRETE COMPANY LTD. The FPCC Post - Tensioning syst ems have been successfully used t hr oughout t he count ry since 1954. They are t ime t est ed and have earned a reput at ion for t heir qualit y and reliabilit y. The FPCC Post -Tensioning syst em has been used in every sect or of pr est ressed concret e const r uct ions. While primarily used for Rail or Road Br idges, now ext ended it ’s applicat ion in various ot her str uct ures such as; Nuclear React ors, Cement Silos, Dams and buildings et c. FPCC Post -Tensioning is also used for Rock and Soil Anchors and for lift ing and shift ing of heavy loads. USERS OF THE MANUAL This manual present s det ailed t echnical infor mat ion about var ious FPCC Syst ems, and is int ended as a working guide for Consult ing Engineer s, Gover nment Aut hor it ies, Client s and t he I nspect ion Agencies, who ar e responsible for execut ion and super vision at sit e. We hope to give informat ion t o all t hose who have as yet lit t le or no exper ience in Prest ressed concret e and who wish t o acquire a wider knowledge of it s det ails and sit e usage. FPCC SYSTEM OF PRESTRESSI NG FPCC has developed a range of Anchorages, t o cat er t o a wide range of Prest ressing t endons. The fundament al concept of Mono- St r and and Mult i- St rand syst em remains t he same; regar dless of Prest ressing element s are used. i.e. - Wire/ St rand, St eel/ Concret e Anchorages, No. of Wires/ St rands in a t endon or met hod of st ressing. The Pr est ressing cables, when fit t ed wit h t heir Anchorages, are called ‘TENDONS’. They are made up of Wires / St rands gr ouped gener ally in a cylindrical duct made out of met al st rips or HDPE. FPCC Anchor ages, an import ant component of Tendon, cater t o a wide range of Prest ressing forces st art ing fr om an U.T.S. of 18.75 MT t o 700MT. Anchorages for larger capacit y t endons and for special pur poses are also available on request . The Tension member s of t he FPCC post - t ensioning syst ems are made up of high st rengt h st eel st rands, eit her in t he form of mono- st rand or as mult i- st rand t endons. The t erm “ I NTERNAL” and “ EXTERNAL” t endons are used, accor ding t o t he posit ion of t he cables wit h r espect t o cr oss sect ion of t he st r uct ures. SECTION - A THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 4 This Manual pr ovides t he det ails of t he FPCC post - t ensioning syst ems and cont ains t he most import ant infor mat ion for design and const ruct ion. Det ails about t he Pre- t ensioning syst em can be pr ovided on special request . Anchorages are categorised in t wo t ypes, given as under: - 1. Mono- Gr oup Anchor ages 2. Slab St ress Anchorages Mult i- St r and Anchor ages are recommended for Prest ressing of concret e st r uct ures where higher capacit y of Prest ressing for ces are required; such as Br idges, Silos, Dams and cont ainment walls et c. Slab St ress Anchorages are developed for using in flat concret e st ruct ure such as buildings et c. where Pr estressing for ces are considerably low. SECTION - A THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 5 PRESTRESSING MATERIALS PRESTRESSI NG STEELS Prest ressing St eels are best known as t he High Tensile St eel Wires, St rand or Bar s and are available in var ious sizes and configur at ions t o impart a range of UTS. A few of t hem are as list ed below: TYPES DI AMETER RANGE BREAKI NG STRENGTH SHAPE Plain Round Wire 2. 5 mm. - 8 mm. 9.87 kN - 69 kN I ndent ed Wire 4 mm. - 7 mm. 23 kN - 61 kN St rands - 3Ply 3. 0mm. x 3 Wire 38. 25 kN St rands - 7Ply 9. 5mm. - 15. 7mm. 89 kN - 265 kN Threaded Bar ( McALLOY) 20 mm. - 40 mm. 173 kN - 691 kN High st rengt h prestr essing st eels requires careful handling during t ranspor tat ion and st or age. They should neit her be dragged on hard rough surface nor laid unprot ect ed on naked soil. I t should be pr oper ly wrapped and covered wit h t arpaulin et c. t o prevent ingr ess of moist ure and dirt in a humid or cor rosive at mosphere. They should be st or ed at an elevat ed plat for m t o prevent t hem from r ising moist ure if any, fr om t he humid / wet gr ound condit ion. The St orage area must also have an adequat e vent ilat ion, t o pr event condensat ion. SECTION - B THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 6 ANCHORAGE SYSTEMS Prest ressing for ces of t he Tendons are t ransferred t o t he concret e st r uct ures t hrough Anchorages. Anchorage for t he Post Tensioning syst em normally comprises of a st eel plat es wit h a number of conical holes, t he conical Grips and t he Guide ( Trumpet ) . Trumpet or Guide is used t o connect t he duct s and provides a flat surface for locat ing t he Bear ing Plat e on it . As shown in t he figure below: PRESTRESSI NG ANCHORAGES a. Guide - This is specially formed component made out of graded Cast I ron, which generally get s embedded in t he concret e and used t o dist ribut e t he t endon forces t o t he concret e. b. Bearing Plat e - This is cylindr ical round shaped component made out of forging of graded Alloy St eel. This plat e cont ains a number of t apered holes for wedging t he Pr est ressing St eel wit h t he help of grips. c. Gr ips & Circlips - This is a t apered shaped component s made out of Alloy Car bon St eel and subsequent ly hardened & t empered. These gr ips are slit t ed in t hree equal par t s t o work like a split j aws and cont ains serrat ions inside. SECTION - B THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 7 FLAT ANCHORAGES The component s and t he funct ions of t he FLAT ANCHORAGES are similar t o t hat of Prest ressing Anchor ages, but t hey are specially designed in f lat shape for use in slab st ress applicat ions. BLI ND END ANCHORAGES: A Blind End Anchor s are used where t he end of t he Pr est ressing cable is buried in concret e and or t he locat ion is in- accessible for st r essing of t he t endons. A t ypical ar rangement is as shown below. BLI ND-END Anchor ages are also available for complete range. SECTION - B THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 8 ANCHORAGE COUPLERS: I n t he const ruct ion of cont inuous deck br idges, it is essent ial t o extend Prest ressing cables as t he const ruct ion pr oceeds. FPCC has a range of Mult i St rand Anchorage Couplers in configurat ion of 12, 19 and 27 st rands, for st rand diamet er of ½” ( 13mm.) & (0/ 6” ) 15.2mm. I n COUPLER arrangement , t he fir st st age of St ressing is carr ied out in t he same way as wit h t he st andard Anchorage, except t hat one COUPLER RI NG is placed between t he GUI DE and t he BEARI NG PLATE. The coupler Ring cont ains t he required number of slot t ed lugs t o accommodat e SWAGED GRI PS fit t ed on t he st rands for Second st age cable. Aft er t he complet ion of first st age of prest ressing and gr out ing, t he st rands fit t ed wit h swaged gr ips are inst alled for second st age st ressing. Swaged gr ips are prepared wit h t he help of a specially designed sleeves and locks, on an ext rusion press. The st rands are t hen deflected t hrough a conical shaped t r umpet called REDUCER CONE, which also prevent s ingress of concr ete and slurry during cast ing of second st age. This Reducer Cone pr ovides a gr out exit point , which should be placed at t he t op locat ion t o prevent any air being t rapped during grout ing. Small end of t he Reducer cone is securely t apped t o t he duct of second stage. SECTION - B SECOND STAGE 15. 2 mm. FREYSSINET EXTRUDED GRIPS FIRST STAGE FREYSSINET EXTRUDED GRIPS 15. 2 mm. SWAGED GRIPS SEATING WASHER REDUCER CONE STRAND DEFLECTOR GRIPS BEARING PLATE COUPLER RING GUIDE THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 9 TENDON DUCTS ( SHEATHI NG) : Sheat hing is used to creat e a void in t he concret e st r uct ure, t hr ough which t he st ressing st eels are inser ted and remain free t o st ret ch dur ing st ressing operat ion. Sheat hings are available in t wo t ypes: A) Met allic Duct s B) H.D.P.E. Duct s A. Met allic Duct s: Met allic Sheat hing ducts ar e manufact ured, by rolling CRCA St eel St rips wit h a spiral corrugat ion t hroughout it s lengt h. These corrugat ions pr ovide bet ter bonding wit h t he Concrete from out side and t he Cement Grout from inside. These spir al corr ugat ions also work as a helical t hr ead and facilit at e coupling of t wo duct s by screwing anot her oversize duct . This way a cont inuous lengt h of t he duct can be formed. These duct s are available in differ ent diameters and varying wall t hickness t o suit various configurat ions of Tendons. Since t hese duct s ar e made out of St eel and t he chances of cor rosion is very high and hence a lot of pr ecaut ions are needed t o st or e t hem at work sit e. However coat ed st eel st r ips are some t imes used t o pr ot ect t hem for corr osion for a longer period and enhance it s service life. However , t he need for using a perfect corr osion r esist ing mat erial, has int r oduced a non-met allic duct s in t he const ruct ion work. B. H. D.P. E. Duct s: Non- met allic duct s are made out of High Densit y Poly Pr opylene ( HDPE) or Poly- Propylene and have a number of advant ages over met allic duct s, such as- I . They are highly corr osion resist ant I I . They are effect ively r esist ant t o passage of chloride ions. I I I . They are very poor for elect rical conduct ivit y. I V. The duct mat er ials has a high co-efficient of t her mal expansion ( Typically - 140 x 10 -6 deg ˚ C) V. I t has a low Young’s Modulus ( Nearly 800 N/ mm 2 ) VI . Can be sealed against ingr ess of cont aminant s. VI I . Can be pressur e tested during const ruct ion t o demonst rat e int egr it y. SECTION – B THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 10 TENDON SUPPORTS: To ensure t he adequate t r ansfer of calculat ed for ces t o t he st r uct ur e, it is very import ant t hat t he t endon profile and t he locat ion of t he duct , in ‘X’ & ‘Y’ direct ions are maint ained as shown in t he drawing. For t his pur pose; t he duct s needs t o be pr operly support ed and secured at an intervals of 0.50 – 0.70 met er . The axis of t he duct is considered as t he line of Cent er of Gravit y of t he tendon. Examples shown above are two t ypical arr angement s of Tendon Suppor t s: A. St andard Solut ion: The duct s are insert ed thr ough t he Tendon Suppor t . B. Typical Ar rangement for Pre- assembled t endons placed from above. PROPER CARE TO BE TAKEN FOR I NSTALLI NG TENDON SUPPORT: 1. I n t he case of light ly cur ved t endons or duct s, which are not support ed by t he st rands dur ing concret ing, t he support s must be placed at short er dist ance t o prevent uplift . 2. Tendon suppor t s shall never be placed direct ly underneat h t he Guide Cast ings, Duct Joint s or Sleeve for Grout Vent . 3. Tendon Suppor t s shall never be welded near duct s or already placed t endons. SECTION – B WELDED WELDED WIRE TIED COVER BLOCK BLOCK COVER SHEATHINGS SHEATHINGS TENDON SUPPORT A B THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 11 ANCHORAGE DIMENSIONS AND SETTING-OUT DETAILS THE ANCHORAGES: The term Anchorage is r eferred t o t he assembly of t hree component s, used for pre- st ressing pur poses They are namely – a. GUI DE b. BEARI NG PLATE & c. GRI PS GUI DE is used as a pocket former and is made out of S.G. I r on Cast ing; in t he shape of conical t ubular for m. I t has a square flange wit h for not ches at four corners for securing it t o t he for mwork. The GUI DE is fixed in t he posit ion prior t o concret ing. The sheat hing duct is t hen fit t ed on t he smaller end of t he t ubular guide wit h t he help of wat erproof adhesive t ape or a heat shrink sleeve. The BEARI NG PLATE wit h t he GRI PS is mount ed over it at t he t ime of tensioning t he t endons. ANCHORAGE TYPE a c e f Ø g Ø h i j k 4 K 13 - 104 45 45 56 85 158 147 147 75 7 K 13 4 K 15 103 50 62 72 120 184 160 160 85 12 K 13 7 K 15 180 55 84 100 140 254 220 235 90 19 K 13 12 K 1 5 190 60 95 105 160 190 244 244 95 27 K 13 19 K 1 5 270 70 127 136 200 234 275 293 105 37 K 13 27 K 1 5 395 78 171 190 252 425 365 365 115 - 37 K 1 5 467 85 178 206 270 495 425 425 125 SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 12 Assumpt ions: a. The minimum dist ances bet ween anchorages “ a” and t he minimum dist ance “ b” from t he nearest free edge are given for various concr ete st rengt h levels. b. The minimum dist ance “ a” must be combined wit h a dist ance bet ween a’ > a in t he ort hogonal dir ect ion t o pr ovide for each anchorage a rect angular dist ribut ion area t hat sat isfies t he condit ions aa’ ≥ 3b 2 . c. M30 / M35 / M40 / .... ar e Char act er ist ic Breaking St rengt h of Concret e Cube aft er 28 days. d. Nominal Breaking St rengt h: for Ø 15.2mm. St r and – 260.7 kN. for Ø 12.7mm. St rand – 183.7 kN. e. Recess dimension d’ ( eg.220/ 270) are shown wit h basic Jack/ Opt ional Jack. f. All Dimensions shown here ar e in millimet ers. THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 13 SECTION – C REINFORCEMENT DETAILS OF THE ANCHORAGE ZONES The reinforcement of t he anchorage zones basically provides “ Burst ing Reinfor cement s” , and applies t o st ruct ures wit h a minimum number of bars such t hat all t he r einforcement in t he End- Block Zone, t o a dept h 2b, cor responds t o a minimum st eel percent age of 1% in each dir ect ion perpendicular t o t he Prest ressing axis. This t ype of reinforcement must be adapted t o par t icular cases whenever t here is a gr oup of anchor ages or a r isk of cracking at t he boundary of t he End-Block zone; independent small size reinfor cement cages must t hen be replaced by cont inuous reinfor cement of equivalent cr oss- sect ion. A select ion guide is given in t he t able below. GRI D TYPE REI NFORCEMENT SPI RAL REI NFORCEMENT UNI T TYPE Nb Dia b/ 2 s q q’ Nb Dia. D e f 7 K 13 4 K 15 6 10 80 100 350 260 6 10 200 50 300 12 K 13 7 K 15 6 12 100 120 420 312 6 12 280 50 300 19 K 13 12 K 15 6 14 120 150 500 374 6 14 360 60 360 27 K 13 19 K 15 6 16 150 170 570 426 6 16 440 65 390 37 K 13 27 K 15 2 X 6 14 180 210 560 434 7 18 540 70 490 55 K 13 37 K 15 2 X 6 16 200 240 640 496 8 20 640 70 560 SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 14 DETAI LS OF THE BLI ND END ANCHORAGES When a Prest ressing t endon can be st ressed from one end only, a dead end anchorage can be used at t he ot her end. When t he anchorage is accessible, a manually blocked type j acking anchorage ( Type A or B) can be used; t he anchor ing j aws are driven in by sledge hammer. I f t he Non- t ensioned end of t he tendon is not accessible during st ressing, a buried and fixed anchorage must be pr ovided and t he t endon is t hen put in place before concret ing. I n one t ype of Blind end Anchors; t he end of each st r and is provided wit h a swaged sleeve. The swaged sleeve ends ar e posit ioned on a dist r ibut ion plat e and ar e held in posit ion wit h t he help of a spr ing or a ret aining ring. I n anot her type of Blind end Anchors; t he end of t he st rands is unwound in t he form of bulbs or flower and can be laid out in square or rect angular or ient at ion. UNIT TYPE e f g h l 4 K 13 350 70 150 170 800 4 K 15 390 90 190 210 950 7 K 13 370 70 170 190 1280 7 K 15 450 90 210 230 1300 12 K 13 350 190 310 270 1280 12 T 13 S 350 190 310 270 1280 12 K 15 430 230 390 330 1300 19 K 13 470 190 310 390 1280 19 K 15 570 230 390 470 1300 27 K 13 670 220 470 430 1280 27 K 15 810 260 570 510 1700 37 K 13 870 310 570 430 1680 37 K 15 1050 370 690 510 2000 55 K 13 1170 350 670 550 1980 SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 15 FLAT- SLAB PRESTRESSI NG The F.P.C.C. Flat - slab Prest ressing System is used for all t ypes of slabs in buildings, br idge decks and similar ot her applicat ions. FPCC Flat - Slab Anchorages are available in t he range consist s of 2, 3, 4 & 5 st rands of 13mm. (0.5” ) & 15mm. ( 0.6” ) diamet er, placed in flat duct s, wit h r espect ive anchorages. Str ands are st ressed and locked- off individually. Tendon placement and grout ing pr ocedures are similar t o t hose of mult i- st rand syst ems. Flat Slab Prest ressing enables deflect ions and cracks under service condit ions t o be kept under cont r ol. This permit s t he use of a larger span wit h t hinner slab, and can effect a subst ant ial reduct ion in const r uct ion cost and t ime due t o r educt ion in mat er ial and labour cost . Applicat ion of Flat - Slab prest ressing also gives some impor tant indirect savings on foundat ions, column and beam sizes, walls and ver t ical services. TENDON FORCE A B C D E F DUCT SI ZE G X H ( I NNER) UNI T TYPE kN mm. mm. mm. mm. mm. mm. mm. X mm. 2 S 13 367. 40 145 78 80 165 95 90 40 X 20 3 S 13 551. 10 165 88 82 185 100 95 60 X 20 4 S 13 734. 80 192 96 88 215 105 98 70 X 20 5 S 13 918. 50 220 110 88 240 110 110 90 X 20 2 S 15 521. 40 160 85 82 180 100 95 45 X 25 3 S 15 782. 10 192 96 85 215 120 100 70 X 25 4 S 15 1042.80 230 110 100 250 120 110 90 X 25 5 S 15 1303.50 280 170 100 300 125 110 100 X 25 Assumpt ions: a. Nominal Breaking St rengt h: for Ø 12.7mm. St rand - 183.7 kN. for Ø 15.2mm. St rand - 260.7 kN. b. All Dimensions shown here are in millimet ers SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 16 STEEL CHARACTERI STI CS Prest ressing st eel st ressed and embedded in concret e losses a par t of it s init ial t ension, as t imes goes by. This loss is known as “ st ress relaxat ion” , and must be t aken care of in t he design of prest ressed st r uct ures. Any reduct ion in t he amount of relaxat ion losses will subst ant ially improve t he efficiency of t he pr estr essed t endons. “ Low Relaxat ion” st eels give a relaxat ion loss not gr eat er t han 2.5% ( aft er 1,000 hours, at 20 o C / 68 o F) , when init ially loaded at 70% of t he specified minimum breaking st rengt h. Nor mal st ress relieved st eel exhibit s, in comparison, losses of ar ound 7%. The I nt r oduct ion of Super St rand repr esent s a significant development for t he prest ressed concret e industr y. I t has higher breaking load t han nor mal st rand wit hout sacrificing any of t he ot her mechanical pr oper t ies. Higher st rengt h st rand means an init ial savings in mat er ial costs, and fewer st rands in a prest ressed member result s in a furt her savings due t o lower handling cost . NOM. DIA. NOM. SECTION NOM. WEIGHT SPECIFIED BREAKING LOAD SPECIFIED LOAD AT 1% ELONGATION STANDARD GRADE SIZE mm. mm. 2 Kg/m kN kips kN kips 1770 MPa ½” – T13 12.50 93. 00 0.730 164 36. 9 139 31. 3 1860 MPa ½” – T13 12.50 93. 00 0.730 173 38. 9 147 33. 1 1670 MPa 0.6” – T15 15.20 139.0 1.090 232 52. 2 197 44. 3 Euronorm 138-6/ 79 St andard 1770 MPa 0.6” – T15 15.20 139.0 1.090 246 55. 3 209 47. 0 1860 MPa ½” – T13 12.90 100.0 0.785 186 41. 8 158 35. 5 Euronorm 138-6/ 79 Super 1770 MPa 5/ 8” – T13 15.70 150.0 1.180 265 59. 6 225 50. 6 250 kpsi ½” – T13 12.70 92. 90 0.730 160. 1 36. 0 144. 2 32. 4 270 kpsi ½” – T13 12.70 98. 71 0.775 183. 7 41. 3 165. 4 37. 2 250 kpsi 0.6” – T15 15.24 139. 35 1.094 240. 2 54. 0 216. 3 48. 6 A.S.T.M. A 416/ 80 250 kpsi 0.6” – T15 15.24 140. 00 1.102 260. 7 58. 6 234. 7 52. 8 Example of Elongation Curves Example of Relaxation Curves SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 17 TABLE FOR STANDARD PRESTRESSI NG TENDONS The ‘K’ Range of anchorages has been designed for use wit h all exist ing gr ades of st rand and in par t icular t hose possessing t he highest per for mance charact er ist ics, such as t hose complying wit h t he Euronorm Super Grade. Engineer s should of cour se refer t o t he specific manufact ure’s charact er ist ics ( e.g. – Nom. Breaking Str engt h, Cr oss- Sect ional Area, Relaxat ion et c.) for t he t ype of st rands specified for any part icular st r uct ure. For all anchorage syst ems list ed, individual or gr oups of st r ands may be omit t ed t o opt imize t he syst em. However , it should be bor ne in mind t hat t he t endons cont aining maximum possible number of st r ands for any part icular anchorage, represent s t he most efficient use of t hat anchor age. CABLE FORCES ( in kN) EURONORM 138 - 6/ 79 A.S.T. M. – A 416/ 80 STANDARD Gr ade 177 0 Grade 1860 Super Grade 1860 Grade 250 K Grade 2 70 K UNI TS 1 00% 8 0% 10 0% 80 % 100 % 80% 100 % 80% 100% 80% 4 K 13 656 525 692 554 744 595 640 512 735 588 7 K 13 1148 918 1211 969 1302 1042 1121 897 1286 1029 12 K 13 1968 1574 2076 1661 2232 1786 1921 1537 2204 1764 19 K 13 3116 2493 3287 2630 3534 2827 3042 2434 3490 2792 27 K 13 4428 3542 4671 3737 5022 4018 4323 3458 4960 3968 37 K 13 6068 4854 6401 5121 6882 5506 5924 4739 6797 5438 55 K 13 9020 7216 9515 7612 10230 8184 8806 7045 10104 8083 Gr ade 167 0 Grade 1770 Super Grade 1770 Grade 250 K Grade 2 70 K 4 K 15 928 742 984 787 1060 848 865 692 1043 834 7 K 15 1624 1299 1722 1378 1855 1484 1514 1211 1825 1460 12 K 15 2784 2227 2952 2362 3180 2544 2596 2077 3128 2502 19 K 15 4408 3526 4674 3739 5035 4028 4110 3288 4953 3962 27 K 15 6264 5011 6642 5314 7155 5724 5840 4672 7039 5631 37 K 15 8584 6867 9102 7282 9805 7844 8003 6402 9646 7717 SECTION – C THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 18 EQUI PMENT DI MENSI ON AND I NSTALLATI ON DETAI LS K- RANGE JACKS The FPCC is manufact ur ing a range of Jacking Unit s, t o cover t he ent ir e range of Prest ressing Syst ems; wit h Mono- St rand and t he Mult i- St ressing System. The K- Range Jacks are designat ed as Cent ral Hole, Twin Cylinder Double Act ing Hydraulic Jacks. The Main Jacking Unit has a to- n- fr o moving pist on and is used as St ressing Cylinder . St rands ar e being anchored at t he rear end of t he Pist on wit h t he help of specially designed Anchor Plat e and self releasing Conical Gr ips. Anot her Jacking Unit locat ed at t he front end, is a hollow cylinder Single Act ing Hydraulic Jack and is used for blocking pur poses. Depending on t he t endon t ype and t he available spaces; a complet e range of Jacks are available. The t able below can be referred for t he select ion of t he Jack for Prestr essing of Cables: - JACK TYPE ANCHORAGE TYPE e f g h i j k l 4 K 13 - 635 200 185 275 785 190 126 192 K 100 7 K 13 4 K 15 635 200 185 275 785 190 126 192 7 K 13 4 K 15 720 200 220 350 875 230 228 274 K 200 12 K 13 7 K 15 726 200 220 350 875 230 231 274 12 K 13 7 K 15 820 250 267 440 970 270 235 324 K 350 19 K 13 12 K 15 820 250 267 440 970 270 230 324 19 K 13 12 K 15 940 250 267 515 1090 310 230 410 K 500 27 K 13 19 K 15 933 250 267 515 1090 310 222 410 27 K 13 19 K 15 881 260 350 610 1030 360 142 478 K 700 37 K 13 27 K 15 973 260 350 610 1125 360 104 478 37 K 13 27 K 15 1062 220 400 710 1220 410 268 535 K 1000 55 K 13 37 K 15 1171 220 400 710 1320 410 279 535 SECTION – D THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 19 ELECTRI CALLY OPERATED HI GH PRESSURE PUMP: The FPCC Prest ressing Jacks ar e being operat ed using FPCC manufact ur ed E.O.H.P. Pumps. Three variant s of t hese power packs are used t o suit t he individual need of t he various j acks. The Pumping unit used in t hese power packs is of Mult i- Plunger , Axial Flow and Posit ive Displacement t ype. They are suit ably coupled wit h a TEFC Electr ic Mot or and are mounted on a special designed Oil Tank. The power pack also houses suit able D. C. Valve, Check Valve, Relief Valve and Shut - off Valve et c. mount ed on a manifold for effect ive cont rol of t he Hydraulic Jacks. Two dist inguished Pr essure Gauges are also mount ed on t he front panel of t he power pack t o read t he Str essing and Blocking Pressure; as t he case may be. Ot her t han t he EOHP, FPCC also manufact ures Hand Operat ed High Pressure Pump ( HOHP) . The t able below represent s t he suit abilit y of var ious models of t he power packs: E.O.H.P. Power Pack SALIENT FEATURES OF THE POWER PACK UNITS MK-I MK-II MK-III H.O.H.P. Pump Test Pressur e (Max. ) Kg.f / cm 2 600 650 650 640 Wor king Pressur e (Max. ) Kg.f / cm 2 450 580 580 550 Oil Del ivery ( Max. ) Lt r . / min . 1.50 1.50 2.50 0.20 ( S/ A) 1.50 ( D/ A) Oil Tank Capaci t y: Tot al Ltr . 25 60 - - - Do - - Useable Ltr . 10 25 - T.E.F.C. Elect r ic Mot or : 440 Volt s, 3 ph, 1440 r pm hp / KW 2.0 / 1.49 3.0 / 2.24 5.0 / 3.73 - Dimensions - LENGTH mm. 575 630 600 WI DTH mm. 335 950 350 HEI GHT mm. 705 800 980 Machine Weight : Dry kgs. 45.0 110.0 58.0 Wi th Oil kgs. 66.0 162.0 - Recommended Sizes of Prest r essing Cable - 4S13 - 5S13 4S15 - 5S15 4K13 - 7K13 4K13 - 12K13 4K15 - 7K15 or Equivalent 19K13 - 27K13 12K15 - 27K15 or Equivalent 4S13 - 5S13 4S15 - 5S15 4K13 - 7K13 JACK RECOMMENDATI ON - SC- 2 & K100 K-100 & K- 200 K- 500 & K-700 SC- 2 & K- 100 Ot her t han the Prest ressing Jack & Power pack, FPCC also manufact ures some ot her Special Applicat ion Jacks and Pumps t o suit cust omer’s specific needs, Such as - Heavy Lift ing Jacks, Lift ing & Lowering Jacks, Side Shift ing Jacks and Flat Jacks et c. A special power pack is also available t o operat e t hese j acks. SECTI ON – D THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 20 GROUTI NG SYSTEM: F.P.C.C. also manufact ures t he complet e machinery & syst em for Grout ing purpose. I t compr ises of Gr out Mixer & Agit at or and Gr out Pump. GROUT MI XER & AGI TATOR: I t is a combinat ion of t wo circular formed drum unit s, where t he first dr um is used as Cement Mixer and t he ot her dr um is used as an Agit at or for t he cement slurry. The fir st Drum ` MI XER’ is equipped wit h an elect r ically driven rot ary t urbulat ing blades at t he cent er and chur ning blades fixed on t he inner wall of t he dr um. I n t his drum Cement is mixed wit h wat er . The Mixing Drum is generally kept at a higher level t han t hat of Agit at or Unit , such t hat mor tar can flow dir ect ly int o t he second t ank, t he Agit at or, placed j ust beneat h t he mixing dr um. Before flowing int o t he Feed Dr um ( Agit at or ) , t he mort ar is passed t hrough a 2mm. mesh screen t o eliminat e impur it ies and lumps et c. Cement mort ar st ored in t he Feed Drum ( Agit at or ) , is cont inuously st ir red t o avoid set t ing or segr egat ion. A Schemat ic Diagr am of such an arrangement is shown above. AGI TATOR Model J- 700: Shaft Rotat ion : 120 r pm Elect r ic Power : 2 hp. / 440 volt s/ 1440 rpm Mot or Handling Weight : 38 kgs. Overall Dimension: Lengt h x Widt h x Height 125 x 25 x 48 mm. SECTI ON – D THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 21 GROUT PUMP - Model J- 600: To fill t he cement mort ar inside t he cable duct , FPCC Manufact ur es, a very compact electr ically driven Pump Model: J- 600. This is a reciprocat ing t ype, twin cylinder, posit ive displacement pump. The Salient feat ures of t his pump ar e: Delivery : 1200 lit ers/ hr . ( Maximum) Test ing Pressure : 30 kg.f/ cm 2 Delivery Pressure : 25 kg.f/ cm 2 ( Maximum) Elect r ic Power : 2 hp. / 440 volt s/ 1440 r pm Mot or Handling Weight : 214 kgs. Overall Dimension : Lengt h x Widt h x Height in mm. 980 x 480 x 940 GROUT PUMP - Model P- 2001: FPCC has also developed a higher version of Grout Pump Model: P- 2001, for grout ing of vert ical cables in t he cylindrical st ruct ures such as Cement Silos, Chimneys, Containment wall of Nuclear React ors et c. A represent at ive pict orial View is presented here for Reference. The Salient feat ures of t his pump are: Flow Rat e : 900 lit ers/ hr. t o ( Range) 3900 lit ers/ hr. Pressure : 10 - 40 kg.f/ cm 2 ( Range) Working Pressure : 30 bar ( Maximum) Power Supply : 7. 5 hp/ 440 volt s/ 3Ph., 1440 rpm Elect . Mot or For Cont rols : Solenoid Valve, Limit Swit ches, Pressure Swit ch & Cont rol Circuit s et c. Handling Weight : 580 kgs. Overall Dimensions : Lengt h x Widt h x Height in mm. 1460 x 1455 x 1540 SECTI ON – D THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 22 HAND OPERATED GROUT PUMP - Model HOGP: For special applicat ion in short er cables and small st ruct ures, FPCC has a unique Hand Oper at ed Gr out Pump. This pump is used where t he quant um of Grout ing Mort ar, t o be pumped, is relat ively low and t he cable cavit y is small. Since t he pump is hand operat ed one and does not require any elect ricit y, it has a very wide accept ance in t he r emot e areas where t he supply of elect r icit y is scares. A represent at ive pict or ial View is present ed here for Reference. The Salient feat ures of t his pump ar e: Flow Rat e : 8.50 lit ers/ minut e Delivery Pressure : 14 kg.f/ cm 2 Test ing Pressure : 17.5 kg.f/ cm 2 ( Maximum) Handling Weight : 55 kgs. Overall Dimensions : Lengt h x Widt h x Height in mm. 580 x 300 x 900 SECTI ON – D THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 23 PRESTRESSING OPERATION This chapt er shall deal wit h t he Handling and I nst allat ion of H.T. St r ands, St ressing Pr ocedur e and t he Gr out ing Oper at ions et c. HANDLI NG & I NSTALLATI ON OF H. T. STRANDS The H.T. St rands are used as medium for inducing t he st ressing for ces in t he st ruct ures and t hus become t he most import ant const it uent of t he Prest ressing Operat ion. The st rands are insert ed or placed int o t he duct s, provided in t he concr et e st ruct ures. Placement / I nser t ion of t he st r ands can be done eit her pr ior t o concret ing or aft er t he concret ing as well, but t he inser t ion before t he concret ing is more dependable and t herefore, should be given more preference. I n t his case, t he duct s/ sheat hings must be t ied fir mly t o t he t endon support s or reinfor cement bar s. When t he st rands ar e insert ed aft er concr et ing, necessary precaut ion should be t aken to prevent t he sheat hing duct , fr om get t ing clogged wit h cement slurry dur ing concret ing. The most commonly used met hod is t o inser t a dummy pipe or mandrel inside t he duct and regularly agit at e it during concret ing procedure. This prevent s any set t lement of cement slur ry in t he duct . NECESSARY PRECAUTI ONS – FOR I NSTALLI NG DUCTS & STRANDS 1. Fix t he Sheat hing duct firmly on t he smaller end of t he Guide or t rumpet 2. Test Cer t ificat e of t he Prest ressing Steel and I dent ificat ion Label from t he St rand Coil should be kept in safe cust ody. These dat a set s are very import ant and shall be used for Calculat ing t he modifi ed force and elongat ion at t he t ime of carrying out str essing operat ion. St r and dat a fr om t he “ I dent ificat ion label” and Tendon number , in which t he st rand has been used, should be recorded in t he Const r uct ion Book. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 24 3. St rand should be uncoiled in such a manner t hat it does not get t wist ed and should not be laid or dr agged on soiled or rough sur faces. A wooden palat e should ideally be erect ed in t he yard for dragging & laying t he st rands aft er de- coiling. 4. St rand should not be laid or dragged on soiled or rough surfaces. A wooden palat e should ideally be erected in t he yard for dragging & laying t he str ands aft er de- coiling. 5. St rands should be cut evenly by an abr asive cut t er in desired lengt h. 6. St rands ends should be ground smoot h, to prevent any damage t o t he duct dur ing insert ion by pushing. I f r equired a suit able bullet end can also be employed t o have a smoot h insert ion. 7. When t he str ands are pulled int o t he t endon duct s t oget her in bundle, special care should be t aken t o prevent t he damage t o t he duct or t he st rands. Roller cr adles can be used t o carry t he tendons on t he r ough gr ound. 8. I n case of t hreading of st rands aft er concret ing, it is recommended t o use a De- Coiler and t he Str and Pushing Machine for uncoiling and t hreading of st rands int o t he duct . 9. Before concret ing, check t he ent ir e lengt h of t he duct s for any opening or damages. Seal t hem fir mly wit h t he adhesive t ape or any suit able sealant . 10. Bot h t he ends of t he duct should be closed aft er inst allat ion, and t he st rand ends should be wrapped firmly wit h ant i moist ure wraps t o pr event t he ingress of any foreign part icles, cement slur ry, concrete or moist ure inside t he duct . They may damage t he duct and t he str ands in long run. 11. Precaut ion should be t aken to prevent t he sheat hing/ duct s from get t ing damaged due t o t he use of Needle vibrat or s during concrete. Damage or punct ure of Sheat hing may cause a severe ingress of cement slurry int o t he duct and will result in blocking t he St rands up t o cer t ain lengt h. 12. Clogging of st r ands inside t he duct will cause an unequal elongat ion dur ing st ressing and will obst r uct t he passage dur ing gr out ing. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 25 STRESSI NG OPERATI ON This chapter shall deal wit h t he St ressing Operat ion of Cables wit h Mult i- St rand Syst em. For St ressing Operat ions of Wir es/ Bar s/ Mono St rand, det ails can be furnished on request . PREPERATI ONS FOR STRESSI NG OPERATI ON Before st art ing wit h t he act ual Str essing Operat ions, cert ain preparat ions should be made; some are required t o be done pr ior t o concret ing. They are list ed out as below: - Basic Requirement s: 1. The St ressing Operat ion can be init iat ed, only aft er concret e at t ains it ’s st rengt h ( usually 60% t o 80% of it ’s ult imat e str engt h in 28 days) . This can be ascer t ained by test ing t he cube str engt h of t he concrete. 2. 3 or 4 set s of Cube Mould ( Size: 150 x 150 x 150 mm) should be poured/ prepared alongwit h t he concret ing of t he main st ruct ure, t o be St ressed. Each set should consist of minimum 3 numbers of moulds. 3. St ressing drawing should be t hor oughly st udied t o ascert ain following paramet er s, and if r equired, should be consult ed wit h t he Pr incipal/ Design consult ant . i. St ressing Schedule - Minimum St ay period aft er concr et ing, Minimum Concrete str engt h for St ressing, Sequence of Stressing for various cables et c. should be obt ained fr om t he drawing. ii. Anchorage/ Cable Type – The configurat ion and t ype of t he cables is also shown in t he dr awing ( i.e. No. of St rand x Dia. of St rand) . A suit able Guide/ Tr umpet and suit able size of Sheat hing duct should be inst alled in t he st r uct ure at t he t ime of fixing of reinfor cement cage and before t he act ual concret ing is done. iii. Assumpt ions – At t he t ime of designing any st r uct ure, designer assumes t he standar d values of cert ain parameters for deriving t he designed forces & elongat ions. These paramet er s are shown in t he dr awing as Assumpt ions and are mainly known as Cr oss- Sect ional Ar ea of Prest ressing st eel ( St rand) , Modulus of Elast icit y of Prest ressing St eel, Fr ict ion & Wobble Coefficient of Duct and t he Wedge Set et c. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 26 iv. St ressing Force - Stressing for ce t o be applied on each cable. Different cables may require t o be st ressed at differ ent for ce level. v. Elongat ion - Elongat ion t o be achieved, in each cable? Differ ent cable may require t o be st ressed for different elongat ion. vi. Type of St ressing - One End St ressing or Str essing from bot h t he ends. Local Condit ions: 1. The Prestressing Engineer should physically check t he access of t he Prest ressing point , for personnel and equipment s. 2. Jack Clearance from t he st ruct ure, dist ance bet ween t wo cables, Pocket dimensions of t he End block should be checked, wit h respect t o t he size and dimensions of our equipment and accessor ies. 3. I f r equir ed necessary plat form for t he per sonnel and equipment , and t he handling gant ry for t he equipment should be erect ed for easy and comfort able access t o t he St ressing point . 4. Our equipment s require elect ricit y as a source of energy, hence availabilit y of elect r icit y t o be checked t o operat e our equipment and t o illuminat e t he Job sit e for night work. 5. Availabilit y of pot able water ( calcium/ sulfur/ chlor ine free) is also required t o be checked. This is a most essent ial commodit y for car rying out t he grout ing operat ion Planning: 1. The Prest ressing operat ion should be carr ied out by a Qualified Engineer . He should be compet ent enough to calculat e t he requir ed forces and elongat ion dur ing st ressing and can t ake necessar y decision inst ant ly at t he work sit e. He should also be sufficient ly able t o lead t he team of experienced and skilled t echnicians and workforce. 2. Necessary planning should be made t o carryout t he str essing operat ion as per t he given schedule, by effect ively ut ilizi ng t he available work for ce and machinery and a reduced downt ime. 3. Each st r and carr ies a different cross- sect ional area and t hus a different Modulus of Elast icit y. These dat a has already been obtained from t he “ I dent ificat ion Label” and t he “ Test Cert ificat e” of t he Prest ressing st eel ( st r and) at t he t ime of prepar ing t he Cables. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 27 4. Designed For ces and Elongat ions, needs t o be modified from t he act ual values of t he Prest ressing st eel ( st rand) being used. Necessary for mulae for calculat ing t he same has been given in t he subsequent chapt er of “ GENERAL DESI GN & I NFORMATI ON” . 5. Fr ict ion & Wobble Coefficient of Duct s are t he st andar d dat a and does not necessar ily need any modificat ions in nor mal condit ions. These values are given in var ious approved cer t ificat es. Unless ment ioned ot herwise, t hey remain const ant ir respect ive of t he differ ent supplier or supplies. 6. WEDGE SET of t he Anchorages and equipment s are specified by t he manufact ur er . Necessary correct ion shoul d be applied, if dif fers fr om t he design assumpt ions. Equipment & Accessor ies: 1. Make a t horough visual inspect ion of t he equipments t o ascer t ain, if t hey are visibly not damaged, or having any of it s par t s tr uncated, dislocat ed or damaged. Check for any visual leakages et c. in t he hydraulic equipment s. 2. Check t he qualit y & quant it y of all t he accessor ies of t he Hydraulic Jacks and Accessor ies; such as Conversion Kit s, Mast er Grips, Terminal Adapt ors, Grip releasing Agent et c., for it ’s per fect ion. 3. Check t he condit ion of Pressure Gauges, it ’s fit t ings & connect ions et c. for correct ness. Obt ain necessary Gauge Calibrat ion Cer t ificat e from t he competent aut hor it y. The calibrat ion cert ificat es should not be older t han 6 mont hs or t he 100 r epet it ive uses ( whichever comes earlier) 4. I t is r ecommended t hat sit es should have Mast er Pressure Gauge or t he Pressure Gauge Calibr at or t o conduct , per iodical calibr at ion of t he Pressure Gauges. 5. Check t he Hose pipes and it ’s end fit t ings are int act and are connect ed correct ly wit h t he pump & Jack. 6. Obt ain Jack efficiency dat a from t he Fact ory or t he manufact urer/ supplier of t he Jack. I t should also be checked per iodically at sit e level t o updat e t he st ressing recor d. The det ail met hodology for “ Jack Efficiency Test ” is given in t he subsequent chapt er “ GENERAL DESI GN & I NFORMATI ON” . 7. Check t he adequacy of handling equipment s, in advance. They should not fail while lift ing, holding and posit ioning of t he j ack during st ressing. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 28 St r essing Dat a & Recor ds: Aft er complet e set up and planning; St ressing operat ion should be carr ied out , as descr ibed below: - 1. Check t he concret e st rengt h and if it at t ains 60% t o 80% st r engt h of it s prescribed value, St ressing operat ion may be st art ed. 2. Apply necessary correct ion t o t he ‘St r essing For ce’ and ‘Elongat ion’, wit h respect ive available ‘Cross- sect ional Area’ & ‘Modulus of Elast icit y’ of Prest ressing st eel, ‘Fr ict ion & Wobble Co- efficient ’, if any, and t he recommended ‘Wedge Set ’ for t he equipment et c. 3. St ressing operat ion is monit ored in t wo ways: a. By Pressure Gauge Reading: Pressure gauge reading on t he power pack is t ranslat ed int o t he for ce applied by t he j ack and ult imat ely t ransferred t o t he anchorages t hr ough Prest ressing Steel. The t endon force can be calculat ed by mult iplying t he pressure gauge reading t o t he ‘Ram Ar ea’ of t he Jack. Ram area of t he Prest ressing Jack is generally pr ovided by t he manufact urer . b. By Elongat ion Met hod: Act ual elongat ion can be compared wit h t he t heoret ical elongat ion of t he Prest ressing st eel. I t can be calculat ed wit h respect t o dat a available for t he Prest ressing st eel used. Necessary calculat ion det ail is given in t he subsequent chapt er “ GENERAL DESI GN & I NFORMATI ON” . 4. The Measurement of for ces ‘By Elongat ion Met hod’ provides t he measure of t he aver age forces t hroughout t he lengt h of t he Tendon, whereas t he Pressur e Gauge Met hod provides t he measure of for ces in t he t endon at Anchorages. At t he Str essing Ends: 1. I f t he Pr est ressing steel (St rands) are laid prior t o concret ing, Try t o move t he st rands t o & fr o, individually or in gr oup, t o ascert ain, t he cable is free fr om any incursions or clogging. I f t he cable is found t o be free, we can pr oceed wit h t he st ressing operat ion. Ot herwise some effort s are required t o make t he cables fr ee and st ress able. 2. Pull t he proj ect ed lengt h of t he st rands out side t he cable opening, up t o a lengt h equals t o t he “ St rand Gr ipping Lengt h of t he Prest ressing Jack” + “ Double of t he Elongat ion required” . Thoroughly clean t hem wit h some fluid cleaner and make t hem free fr om corr osion, dust and any short of inhibit ions et c. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 29 3. Repeat t he above procedure from t he ot her end of t he cable t oo. 4. Push t he extr a lengt h of t he st rands, inside t he cable and adj ust t he proj ect ed lengt h of t he st r ands equal t o t he prescribed “ St rand Gr ipping Lengt h” for t he Jack in use. Repeat t his procedure from other end of t he cable t oo. 5. Check t he Taper holes of t he ‘Bear ing Plat es’, and if necessary, remove r ust and clean t hem. Please consider unpacking and placing of ‘Bearing Plates’ and ‘Grips’ only shor t ly before t he st ressing. This wil l pr event t he ‘Bear ing Plat e’ holes and ‘Grips’ fr om get t ing exposed t o at mosphere as lit t le as possible. They should be free from r ust and cor rosion, unt i l fully st ressed. 6. Thread t he ‘Bearing Plat e’ over t he st rands and rot at e it for 5 t o 6 t imes in clockwise direct ion and t hen rever se t he rot at ion in ant i- clockwise direct ion for equal number of t imes. By doing t his a considerable lengt h of str and inside t he cable get s freed fr om ent anglement . 7. I nst al l t he ‘Grips’ over t he st rands and push t hem int o t he t aper holes of ‘Bear ing Plat e’ wit h t he help of a pipe. ‘Gr ips’ should be pressed int o t he ‘Bear ing Plat es’ simult aneously from bot h t he ends of t he t endons, t ill it get s fixed on t he surface of t he ‘Guide Cone’. Set - up and Equipment Connect ions: 1. Check for t he Oil level in t he r eservoir of t he power pack. Top up if necessary wit h required quant it y of ENKLO- 68 ( Hi ndust an Petr oleum) Hydraulic Oil. 2. Connect t he Jack wit h high- pressure hose pipes t o t he hydraulic power pack. Check t he connect ors/ coupler s for t he leakage, if any. Secure t hem firmly and keep t hem as clean and dry as possible. Leaking couplers at t ract more dust & dirt and can easily get inhibit ed in t he hydraulic syst em t hr ough oil. 3. Check t he hose pipes also and ensure t hat t hey are also clean dry and dirt free. 4. Check t he Pressure gauges ar e connect ed properly and t he end connect ions are leak- free, clean and dry. Leaking j oint s shall r eflect a cont inuous pressure dr op and will cr eat e hindrance in t he st ressing operat ion. I t is recommended t hat t he pressur e gauges fit t ed on t he power pack are pr operly calibr at ed wit h Mast er gauges or Dead Weight Calibr at or and an aut hent ic calibr at ion cert ificat e is t here in t he possession of t he engineer. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 30 6. Aft er connect ing all t he Hoses, bleed t he air fr om t he hydraulic syst em. For t his close t he hydraulic cir cuit and idle run t he Jack for full ext ension and retr act ion. Repeat t his for 3 t o 4 cycles. This way any air bubble pr esent in t he hydraulic cir cuit / system shall escape and will pr event any shor t of part ial pressure drop and j erky or r at t led movement of t he pist on. Mount ing of Jack & Conversion Kit : 1. I nser t t he ‘ Bearing Ring’ over t he ‘Bear ing Plat e’ at bot h ends of t he Tendon and press t o fully t o sit properly over t he ‘Guide Ring’. 2. I nser t t he ‘Pressure Plate’ as per t he or ient at ion of t he Bearing Plat e Hole. 3. Jack should be suspended t hrough a ‘Jack Handling Gantry’ wit h t he help of a ‘Chain Pulley Block’. The arrangement should be such t hat it gives a full flexibilit y of movement t o t he Jack in bot h tr ansverse & longit udinal direct ion. I n small proj ect s, a r igid t win Runway beam st ruct ure could be er ect ed at t he end of bridge girder , for easy maneuvering of t he Jack. 4. I nser t t he bunch of t he Prest ressing steel ( st r ands) in t he cent ral hole of t he j ack and push it down t owards t he ‘Bear ing Ring’ and ‘Anchorage’. Axis of t he Jack should coincide wit h axis of t he t endon. Raise, lower or t ilt t he j ack, as required, t he face of t he j ack should be perfect ly seat ing over t he face of t he ‘Bear ing Ring’. 5. I nser t t he ‘Rear Anchor Plat e’ on t he st rands as per t he orient at ion of t he Tendon and t hen firmly locate it over t he back seat of t he Jack Pist on. 6. Spr inkle ‘Molylube Spr ay’ or apply any ot her ‘Grip Releasing Agent ’ on t o t he ext er nal sur face of all t he ‘Master Grips’ and inside t he t aper hole of t he ‘Rear Anchor Plat e’ as well. Grip Releasing Agent works as a lubricat ion bet ween t he met als and prevent , met al t o met al locking of t he ‘Master Grips’ wit h ‘Anchor Plate’. While applying t hese agent s, be car eful not t o apply or spray t hem on t he ser rat ion / t hread por t ion of t he Master Grip. 7. I nser t t he 3 piece ‘Mast er Gr ips’ over t he st r ands coming out of t he back side of t he ‘Rear Anchor Plat e’ holes. Push ‘Mast er Grips’ int o t he taper holes of ‘Rear Anchor Plat e’ wit h t he help of a pipe, t o sit t ight ly inside t he Anchor plat e. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 31 8. Check all t he connect ion of j ack wit h power pack, giving flexibilit y t o hose pipes for movement of j acks at t he t ime of st r essing 9. I t is impor t ant t hat suppor t ing chains or hooks should be slackened off as soon as t he j ack st art s carrying load. Similarly they should also be r eady t o support t he j ack again while r et ract ing. 10. Now t he system is r eady for STRESSI NG OPERATI ON. 11. The Operat ing I nst r uct ions for t he Equipment and Accessor ies should be kept handy and available at t he working sit e during st ressing. STRESSI NG DATA & RECORDS Before st ar t ing Str essing Operat ions, following Dat a/ Record should be available wit h t he Engineer. 1. Tendon I dent ificat ions No. 2. St ressing Sequence 3. Lengt h of t he Tendon ( from Anchorage end t o Anchorage end) 4. Design Elongat ion of t he Tendon - ( lz ) 5. Elast ic Shor t ening of t he Concret e - ( lb ) 6. Wedge Set at Anchorage - ( ls ) 7. Elongat ion of t he Prest ressing St eel - ( le ) Wit hin t he lengt h of St ressing Jack ( From Bearing Plate Gr ip t o Mast er Gr ip) 8. Tot al Calculat ed Elongat ion lo = ( lz + lb + ls + le ) 9. Specified St ressing For ce - ( P ) 10. Specified Cr oss- Sect ional Ar ea of Pr estressing St eel 11. Specified Modulus of Elast icit y of Prestressing Steel 12. Act ual Cross- Sect ional Area of Prest ressing St eel 13. Act ual Modulus of Elast icit y of Pr estr essing St eel = ( Ea ) 14. Ram Area of t he Str essing Jack 15. Pressure Gauge Calibrat ion Report 16. Jack Efficiency Report 17. Elongat ion & St ressing For ce t o be modified/ est imat ed on t he basis of t he Act ual Jack Efficiency of t he j ack, Act ual Cross- Sect ional Area & Act ual Modulus of Elast icit y of The Prest ressing St eel under use. SECTI ON - E THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 32 APPLICATION OF PRESTRESSING FORCES AND ELONGATION Any load applied must cause an ext ension. Since t here must be some slack in t he t endon, it is very difficult t o est ablish t he dat um fr om which elongat ion should be measured. To overcome t his Pr oblem - Two met hods ar e being followed: : : : First Met hod: A Nominal for ce ‘µp’, suff icient t o get t he Jack t ight ened, is applied t o t he t endon. I t is assumed t hat j ack get s t ight ened only when t he slackness is r emoved. This level is t hen assumed as t he dat um for extension measurement . For all pract ical pur poses a nominal load of 40-50 kg.f/ cm 2 is nor mally considered as dat um level. Remaining forces are t hen applied in regular succession up t o t he desired level. Recor d corresponding extension at successive applied forces and plot t he r ecorded values on a st ress/ St rain diagr am. This is done by plot t ing a graph wit h t he Gauge Reading ( For ce) at abscissas and elongat ion as or dinat es. On ext ra- polat ion, t he plot t ed curve int er sect s at Y- axis, and t hus defines as t he effect ive elongat ion dur ing slackness or init ial t ensioning. This elongat ion dur ing init ial t ensioning should be added to t he measured elongat ion t o ar rive at t he act ual elongat ion. A represent at ive curve and dat a sheet is given in t he document sect ion. Second Met hod: I n t his met hod, a force equal t o P/ 3 is applied t o t he t endon t o est ablish t he dat um, and t he ext ension is measured bet ween t he ‘Force P/ 3’ and t he ‘Force P’. The measur ed ext ension is t hus cor responds t o ‘Force 2/ 3 P’ and t he full ext ension at ‘For ce P’ can be calculat ed by for mulae of propor t ion. Her e it is assumed t hat gr ip set is proport ional t o t he applied force. I n t his met hod, measured elongat ion should agree wit h calculat ed elongat ion wit hin ± 15% for individual t endons, and ± 5% for all t endons. These assumpt ions are based on t he t heory t hat t he calculat ions ar e done wit h act ual modulus of elast icit y and act ual cross- sect ional ar ea of st rand under use. Furt her , t he Co- efficient of Fr ict ion and t he Wobble Co-efficient are considered at an aver age value, and may vary slight ly from proj ect t o proj ect . SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 33 This met hod is very effect ively used for two st ages stressing where t he required t endon elongat ion is more t han t he st roke lengt h of t he j ack Met hodology: Go t o 1/ 3 rd of t he designed Load, Operate t he blocking Jack and block t he Live ‘Grips’ in ‘Bear ing Plate’ and ret ract t he j ack. No elongat ion Reading is required t o be t aken at t his st age. This forms t he dat um and slackness is t ot ally removed. Reset t he Jack and go back t o 1/ 3 rd Load again and measure t he Ram Extension. Now go t o Full Load and not e t he Ram Extension. Block t he Gr ips and ret ur n t o 1/ 3 rd Load. Not e t he reduct ion in elongat ion. This reduct ion in elongat ion represent s t he ‘Wedge Set ’ and ‘Elast ic Shor t ening of St rand’ for 2/ 3 rd Load. Proport ionat ely t he value of ‘Wedge Set ’ & ‘Elast ic Short ening’ for 1/ 3 rd Load can also be added t o get act ual ‘Wedge Set ’ and ‘Elast ic Short ening’. SOME TYPI CAL EXAMPLE FOR TWO END STRESSI NG: Ar e Given below- i. SI NGLE STAGE STRESSI NG – When Ext ension is less t han 200mm, in each j ack at each end. I st . JACK – At One End I I nd. JACK – At Ot her End * Go t o 1/ 3 rd Load ( 1/ 3 P) Go t o 1/ 3 rd Load ( 1/ 3 P) * Ext ension - Need not be recorded Ext ension - Need not be recorded * Block t he Gr ips and Ret ract t he Jack Block t he Gr ips and Ret ract t he Jack * Go t o 1/ 3 rd Load ( 1/ 3 P) again & Go t o 1/ 3 rd Load ( 1/ 3 P) again & Measure Elongat ion - Say 38 mm. Measure Elongat ion - Say 43 mm. * Go t o Full Load ( P) Go t o Full Load ( P) Measure Elongat ion - Say 165 mm. Measure Elongat ion - Say 176 mm. * Ext ension Measur ed = 165 – 38 Ext ension Measur ed = 176 – 43 = 127 mm. = 133 mm. * Mult ipl y t he Measur ed Ext n. by 1. 5 ( 3/ 2) Mul t ipl y t he Measur ed Ext n. by 1. 5 ( 3/ 2) . . . 127 x 1. 5 = 190.5 mm. . . . 133 x 1. 5 = 199.5 mm. * Tot al Elongat ion would be: 190.5 + 199. 5 = 390.0 mm. * Block t he Gr ips and Ret ract t he Jack Block t he Gr ips and Ret ract t he Jack * I f t he Losses due t o Blocking ( Wedge Set ) have not been considered in design load, t hen subt ract t he same from bot h t he ends, fr om above Elongati on. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 34 ii. TWO STAGE STRESSI NG – When Ext ension is more t han 200mm, in each j ack at each end. I st . JACK – At One End I I nd. JACK – At Ot her End * Go up t o 1/ 3 rd Load ( 1/ 3 P) Go up t o 1/ 3 rd Load ( 1/ 3 P) * Ext ension - Need not be r ecorded Ext ension - Need not be r ecorded * Block t he Gr ips and Ret ract t he Jack Block t he Gr ips and Ret ract t he Jack * Go up t o 1/ 3 rd Load ( 1/ 3 P) & Go up t o 1/ 3 rd Load ( 1/ 3 P) & Measure Elongat ion - Say 40 mm. Measure Elongat ion - Say 47 mm. * Go up t o 2/ 3 rd Load ( 2/ 3 P) & Go up t o 2/ 3 rd Load ( 2/ 3 P) & Measure Elongat ion - Say 183 mm. Measure Elongat ion - Say 195 mm. * Ext n. Measur ed ( I st . St age) = 183 – 40 Ext n. Measur ed ( I st . St age) = 195 – 47 = 14 3 mm. = 1 48 mm. * Block t he Gr ips and Ret ract t he Jack Block t he Gr ips and Ret ract t he Jack * Go back t o 2/ 3 rd Load ( 2/ 3 P) & Go back t o 2/ 3 rd Load ( 2/ 3 P) & Measure Elongat ion - Say 23 mm. Measure Elongat ion - Say 32 mm. * Go up t o Full Load ( P) Go t o Ful l Load ( P) Measure Elongat ion - Say 170 mm. Measure Elongat ion - Say 177 mm. * Ext n. Measur ed ( I I nd. St age) = 170 – 23 Ext n. Measur ed ( I I nd. St age) = 177 – 32 = 1 47 mm. = 145 mm. * Total Ext ension Measured= 143 + 147 Total Ext ension Measured= 148+ 145 = 2 90 mm. = 2 93 mm. * Mult ipl y t he Measured Ext n. by 1.5 ( 3/ 2) Mult ipl y t he Measured Ext n. by 1.5 ( 3/ 2) . . . 290 x 1.5 = 435 mm. . . . 293 x 1.5 = 439.5 mm. * Tot al Elongat ion would be: 435.0 + 439. 5 = 874.5 mm. Block t he Gr ips and Ret ract t he Jack Block t he Gr ips and Ret ract t he Jack * I f t he Losses due t o Blocki ng ( Wedge Set ) have not been consider ed in design load, t hen subt ract t he same fr om bot h t he ends, from above Elongati on. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 35 COMMON I RREGULARI TI ES DURI NG STRESSI NG & THEI R CORRECTI ONS Dur ing St ressing Operat ion, we nor mally observe t hat t ar get ed ext ension ar e met at t he designated level of for ce/ pressure, but some t ime, some common irr egular it ies in t he t endon or t he abnormal behaviour s of t he equipment arises in between; and forces us t o st op t he work. These ir regularit ies, needs t o be diagnosed and cor rected properly. Some of t hese irregularit ies and t heir correct ive measures are described below: - A. Desired Force is achieved but t he Ext ension is very Low. PROBABLE CAUSES CORRECTI VE MEASURES 1. St roke Lengt h of t he Pi st on has r eached t o t he fi nal st age. Temporari ly block t he Grips and, Ret ract t he Pi st on and st ar t wi th new st r oke. 2. Faul t y Pressure Gauge Calibrat e t he Pressure Gauge or connect a Mast er Gauge in Pressur e Line. 3. Er ror i n comput ing t he St r essing Values: I ncorrect assumpt ions or er ror i n calculat ions Not i fy t he Design Engineer, corr ect error and recalculat e st r essing values. 4. Cross- Sect i on Ar ea of Prest r essing St eel i s larger t han assumed. ( i. e. St rand Dia. is Ø15. 7 inst ead of Ø15.2 or Ø12. 9 inst ead of Ø12.5 mm. Not i fy t he Design Engineer, and r ecalculat e t he Elongat ion value. 5. Tendon i s clogged due t o i ngress of cement sl urry in t he duct . Repeat edl y over st ress t he t endon up t o 80% of UTS and Relax. This may br eak t he loose slur ry, if any, and can fr ee t he t endon. 6. Higher fr i ct ion i n t he Sheat hi ng/ Duct . This may be caused due t o r ust , dent s, depression or t he deviat i on angl e larger t han assumed Repeat edl y over st ress t he t endon up t o 80% of UTS and Relax. This may reli eve t he t endon. I f not , t hen not i fy t he designer. 7. Higher fri ct ion in t he St ressing Jack Check for fr ee t ravel for t he Jack Pist on. I f t ight , not i fy t he j ack manufact ur er . 8. Higher fr ict ion i n t he St r essing Jack due t o deviati on of st r and, inside t he j ack. Check for abnor mal st rand deviat ion, i f any, inside t he Jack. Take Corr ect i ve act ion NOTE: 1. I n t he case of (5) & ( 6) , over st ressing limit should not exceed 80% of t he UTS, or t he 90% of t he Yield St ress of t he St rand, as specified in t he FI P regulat ions. 2. Dur ing Over st ressing Procedure, Don’t put ‘Gr ips’ in t he ‘Bearing Plate’. Fix up t he Jack over t he cable and t ravel out t he Pist on for at least 100- 150mm. befor e fixing ‘Rear Anchor Plat e’ and ‘Mast er Grips’. Load should be applied direct ly t hr’u. Rear Anchor Plat e. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 36 B. Elongat ion achieved but t he Gauge Pr essur e is very Low. PROBABLE CAUSES CORRECTI VE MEASURES 1. Some of t he Mast er Gr ips are Mi ssing or slipping De- st r ess t he t endon, check t he Mast er grips and replace t he defect ive ones. 2. Faul t y Pressure Gauge Calibrat e t he Pressure Gauge or connect a Mast er Gauge in Pressur e Line. 3. Er ror i n comput ing t he St r essing Values: I ncorrect assumpt ions or er ror i n calculat ions Not i fy t he Design Engineer, corr ect error and recalculat e st r essing values. 4. Cross- Sect i on Ar ea of Prest r essing St eel i s small er t han assumed. ( i. e. St rand Dia. is Ø15. 2 inst ead of Ø15.7 or Ø12. 5 inst ead of Ø12.9 mm. Not i fy t he Design Engineer, and r ecalculat e t he Elongat ion value. 5. Anchorage is yielding: I nsuffi cient concret e st r engt h or honeycombs in t he concr et e in Anchorage zone. Essent ial Judgement by the Engineer may st op mishap. St op St ressing and r epair t he Anchorage Zone. 6. Some st rands may be sl ipping at t he Dead End Anchor, or t he Coupler Joi nt or t he Tendon Spli ce et c. Replace t he faul t y Anchors, Couplers or Splices et c. , i f possibl e. Ot herwi se not i fy t he engineer t o check t he design for r eserves. 7. Small er fri ct ion in t he Sheat hing or duct . Not i fy t he Design Engi neer t o r evi ew and r ecalculat e t he Elongat ion value. 8. Fri ct ional Losses i n t he Jack are considerabl y less t han assumed. Check t he Jack effi ci ency and r e- evaluat e t he fri ct ional losses. And make necessary cor rect ion in applicable forces. C. WEDGE SET: Wedge Set is a t erm used t o denot e t he losses/ gains caused due t o set t ing of t he ‘Gr ips’, dur ing t ransfer of for ces. Set t ing of Grips in t he Dead End Anchorage and in t he Mast er Gr ips of St ressing Jack causes an appar ent increase in extension, but does not lose any Prest ressing force. Whereas t he set t ing of Gr ips in t he Bearing Plat e cert ainly causes a loss of Prestr essing Force, during t r ansfer of for ces from Jack t o t he anchorage. Average values of such Wedge set , in normal condit ions ar e as under: - a) For Mast er Gr ips - 7 t o 8 mm. b) For Live Grips - 5 t o 6 mm. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 37 METHODOLOGY FOR JACK EFFI CI ENCY TEST OBJECTI VE: To check t he Efficiency ( ) of t he syst em, at work sit e, compr ising of 2 ( t wo) Prestr essing Jacks & 2 ( t wo) Power Packs. EQUI PMENT: i) Two Prest ressing Jacks wit h dist inct ive I dent ificat ion Nos. USED ii) Two Power Packs suit able t o operat e above j acks, wit h dist inct ive I dent ificat ion Nos. iii) Two Pressure Gauges for STRESS line of above Power packs, wit h dist inct ive I dent ificat ion Nos. PROCESS: Calibrat e t he Pressure gauges on Dead Weight Pressure Gauge Test er, and connect t hem on t o t he STRESS line of t he t wo Power packs ( under t est ) . Place bot h t he Prest ressing Jacks ( under t est ) facing each ot her in horizont al condit ion as shown in Dr g. No. - A4: FPCC: MI SC: 012 & A4: FPCC: MI SC: 013 wit h a React ion Ring in bet ween so t hat t he concent ricit y of bot h t he Jacks could be maint ained. Connect t he St ressing Cylinder of bot h t he j acks t o t he r espect ive Power packs wit h t wo set of hose pipes. Ensure t hat t he STRESS port of t he Jack is connect ed t o t he STRESS por t of t he Power pack and t he RETURN por t of t he j ack is connect ed t o t he RETURN por t of t he Power Pack. Operat e bot h t he j acks independent ly wit h t heir respect ive Power packs for full ext ension and ret ract ion, at no load. Repeat t his procedure for couple of t imes t o perform t he Air Bleeding oper at ion. Aft er complet ing t he Air Bleeding operat ion, ext end t he Ram of bot h t he j acks by about 100 mm. and t ighten t he Shut - Off Valves of bot h t he Power packs t o close t he hydr aulic circuit of bot h t he syst em. I nser t requisit e number of st rands ( Maximum Capacit y of t he Jack- Under Test ) t hrough t he cent er hole acr oss t he t wo j acks and lock t hem wit h t he help of Master Gr ips at Rear Anchor Block located at t he r ear end of t he j acks. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 38 Now t he syst em is ready for st art ing t he calibrat ion pr ocess. One unit of Jack and Power Pack is made ` ACTI VE UNI T’ and the ot her one is made ` PASSI VE UNI T’, and vise versa. Load is applied t o t he ` ACTI VE UNI T’ in small int ervals and t he pressure gauge reading is not ed. Since bot h t he j acks are connect ed t o each ot her wit h t he help of st rands, load of ` ACTI VE UNI T’ shall be t ransferr ed t o t he ` PASSI VE UNI T’, t hrough t he st rands; and hence t he pressure gauge of t he ` PASSI VE UNI T’ will also st ar t indicat ing pressure. Theoret ically, t he load indicat ed in t he ` PASSI VE UNI T’ should be equal t o t he load applied on t he ` ACTI VE UNI T’, but pract ically, due t o some losses in t he system, caused by t he frict ion or ot her t ransmission hindrances, t he load indicat ed in t he ` PASSI VE UNI T’ is always less t han t he ` ACTI VE UNI T’, and t his var iat ion in t he load is known as ` LOSS OF EFFI CI ENCY’. Now apply t he pressure t o t he ` ACTI VE UNI T’ upt o 75% of t he tot al cable Load, in 10 int ervals, in succession of 50 kg.f/ cm² . Not e down t he pressure indicat ed on t he ` PASSI VE UNI T’ and calculat e t he EFFI CI ENCY LOSS at each int ervals of First St age. I n t he Second St age, Process is rever sed. The ` ACTI VE UNI T’ is now kept ( idle) closed and made a ` PASSI VE UNI T’ and where t he pressure is applied t o t he ot her unit . Not e down t he pressure readings of bot h t he UNI TS ( reversed) and calculat e t he EFFI CI ENCY LOSS at each int ervals of t he Second Stage, as described in Para 8. The aver age ` LOSS OF EFFI CI ENCY’ is t hen calculat ed from t he data of t he bot h t he st ages, which is denot ed as t he ` AVERAGE EFFI CI ENCY LOSS’ of t he syst em. Deduct t his Efficiency Loss fr om t he Ult imat e value of 100%, shall comput e as Net Jack Efficiency ( ) Refer t he st andard format , on next page, for calculat ing t he efficiency of t he syst em. SECTI ON - F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 39 EFFI CI ENCY TEST ON PRESTRESSI NG JACK TEST REPORT No. :_________________ DATE :______________ TYPE OF JACK : Mult ipull Pr est r essing Jack MODEL : ` FPCC’ K- 500 MODEL JACK JACK Sl. No. : K 500 - 94: X1X1 ( Under Test) K 500 - 94: X2X2 POWER PACK Sl. No. : MK- I I I / 94: Y1Y1 ( Under Test) MK- I I I / 94: Y2Y2 PRESSURE GAUGE No. : AA/ BBBB for Powerpack No. - MK- I I I / 94: Y1Y1 ( Under Test) CC/ DDDD for Powerpack No. - MK- I I I / 94: Y2Y2 STRESSI NG RAM AREA : 765. 70 cm 2 ( As shown i n t he at t ached Drawing) ACTI VE PASSI VE ACTI VE PASSI VE Jack No. 94: X1X1 Jack No. 94: X2X2 Jack No. 94: X2X2 Jack No. 94: X1X1 Pump No. 94: Y1Y1 Pump No. 94: Y2Y2 Pump No. 94: Y2Y2 Pump No. 94: Y1Y1 Gage No. AA/ BBBB Gage No. CC/ DDDD Gage No. CC/ DDDD Gage No. AA/ BBBB Sl. No. Pressure Applied Pressure Observed % LOSS Pressure Applied Pressure Observed % LOSS 1. 50 kg.f/ cm 2 50 kg.f/ cm 2 2. 100 kg.f/ cm 2 100 kg.f/ cm 2 3. 150 kg.f/ cm 2 150 kg.f/ cm 2 4. 200 kg.f/ cm 2 200 kg.f/ cm 2 5. 250 kg.f/ cm 2 250 kg.f/ cm 2 6. 300 kg.f/ cm 2 300 kg.f/ cm 2 7. 350 kg.f/ cm 2 350 kg.f/ cm 2 8. 400 kg.f/ cm 2 400 kg.f/ cm 2 9. 450 kg.f/ cm 2 450 kg.f/ cm 2 10. 500 kg.f/ cm 2 500 kg.f/ cm 2 Avg. Efficiency Loss in STAGE – I Avg. Efficiency Loss in STAGE – I I ( Effi ci ency Loss i n STAGE – I ) + ( Effici ency Loss in STAGE – I I ) Avg. Loss of Efficiency = 2 = ( ) Net Jack Efficiency = 100% - ( Avg. Loss of Effi ci ency) = TESTED BY WI TNESSED BY APPROVED BY Sd. Sd. Sd. SECTI ON – F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 40 Schemat ic Layout for ‘Jack Eff iciency Test ’ is shown her e r e y s s i n e t I n d i a SECTI ON – F THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 41 GENERAL DESIGN DATA & INFORMATIONS The dat a pr ovided in t his chapt er are in compliance wit h FI P- CEB code for concret e st ruct ures. For more precise and t horough calculat ions, t he user must r efer t o t he informat ion bullet ins issued by COMI TE EURO- I NTERNATI ONAL DU- BETON and t heir appendices or nat ional design codes. Det erminat ion of t he final Pr est ressing force in a t endon must allow for t he losses of t ension r esult ing from t he following fact ors: -  Losses occurred due t o frict ion of t he tendon in it s duct  Losses occurred due t o inst ant aneous deformat ion of t he concrete; result ing out of non- simult aneous tensioning of several t endons ( elast ic loss) .  Losses occurred due t o Anchorage pull- in.  Losses occurred due t o deferr ed concr ete shrinkage.  Losses occurred due t o creep failure of concrete.  Losses occurred due t o r elaxat ion of Prest ressing st eel. I n most cases, simply mark t he losses at t ime ‘t o ’ ( st art of t ensioning) and ‘t ∞ ’ ( long t erm) . But in some par t icular case of const ruct ion, where par t ial pr estr essing is induced init ially and t he final prestr essing is done in phases, consider at ion must be given t o losses at a given t ime‘t ’. Appendix (e) of t he FI P- CEB code gives inst r uct ions for calculat ion. 1. Losses due t o Fr ict ion in Duct : At a point on t he t endon dist ance (x) fr om the Jacking anchor age, t he st ress in t he prestr essing st eel is given by COOLEY’s Law:  X =  o . e - µ (  + Kx) Where:  o - St ress at point of anchor age ( x = 0) µ - Coefficient of Frict ion of t endon in duct ( in Radian - 1 )  - Sum of angular deviat ions over t he dist ance ( x) ( in Radian) K - Unwanted angle of deviat ion fr om t he t heoret i cal profile, expressed per uni t of l engt h ( i n Radian/ met er) X - Dist ance fr om Anchorage t o calcul at ion poi nt ( in met er) SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 42 Following is a t able for µ in t he absence of t he informat ion r elat ing t o t he par t icular proj ect or t o t he r egulat ions purposes. Values of µ ( in Radi an - 1 ) Recommended Values Type of Duct Range of St andard values Dry Non- Corroded St r ands Lubr icat ed St r ands Passage t hrough concret e holes 0. 40 - 0. 60 0.50 - Non Gal vanised Rigid Pipe 0. 20 - 0. 30 0.25 0. 23 Non Gal vanised Flexible Sheat hing 0. 18 - 0. 26 0.22 0. 20 Galvanised Rigid Pipe 0. 16 - 0. 24 0.20 0. 18 Galvanised Fl exible Sheat hing 0. 14 - 0. 22 0.18 0. 16 Gr eased & Wrapped Tubing 0. 05 – 0. 15 0.09 0. 05 Wobble coefficient ( K) basically depends on t he accuracy wit h which t he t heoret ical profile is achieved. The accuracy is dir ect ly proport ional t o t he rigidit y of t he duct s and t he simplicit y of t he t endon pr ofile. The average value of K can generally be t aken as 0.01 r ad/ m. This figur e must be adapt ed to t he diamet er of t he duct s, t o t he dist ance between t heir suppor t s and workmanship. 2. Losses due t o I nst ant aneous Concret e Deformat ion: A t endon t hat is alr eady st ressed and anchored in t he concret e is affect ed by t he elast ic shor tening of t he concret e due t o t he lat er st ressing of ot her t endons. The result ant loss in prestr essing forces in such cases is very lit t le and is difficult t o calculat e. I t can be compared t o a uniform loss cor responding t o t he fract ion ( n- 1) / 2n of t he t otal inst ant aneous deformat ion of t he concret e. Following formulae give t he loss of st r ess in t he prest ressing steel: n- 1  ci ∆ і = E s . є ci є ci = 2n E ci Where: n - Number of t ensioning St ages. є ci - I nst ant aneous st rain of concret e due t o t he t otal prest ressing forces E s - Modulus of elast icit y of steel ( Average Value is 195x10 9 .Pa)  ci - Permanent prest ressing force in concrete est imat ed at cent re of gr avit y of prest ressing tendons ( in Pascal) . E ci - I nst ant aneous longit udinal modulus of elast icit y of concret e at t he age, when st ressed ( in Pascal) . SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 43 3. Losses due t o Anchorage Pull- in: Dur ing blocking oper at ion of Str ands int o t he anchorages, t he j aws move an average dist ance ( є) given in t he t able below. On account of t he loss of elongat ion, t he st ress in t he t endon at t he anchorage drops from  0 t o  1. Due t o reverse fr ict ion effect s, t he lengt h of t endon affect ed by t his loss is limit ed t o t he dist ance ( a) I t can be assumed in most cases t hat t he loss due t o frict ion is linear. On t his assumpt ion, t he under ment ioned formula gives t he dist ance ( a) . Є. E s ∆ a =  ∆ = 2a ∆  Where: a - Maximum dist ance of effect of t he anchorage pull- in ( in met er ) . є - Anchorage pull- in ( in met er) . E s - Modulus of elast icit y of steel ( in Pascal) ∆  - Loss of Prestress by frict ion over t he dist ance ( in Pascal) . 4. Losses due t o Creep and concret e shrinkage: I t is difficult t o r esolve t his problem precisely and in most cases, appr oximat e solut ions ar e acceptable. Assuming t hat deformat ion and str ess are direct ly pr opor t ional, t he for mulae below indicat e t he loss of st ress in t he st eel.  c∞ ∆ c+ s = E s ( Є c ∞ + Є s ∞) Є s∞ =  ∞  E c28 SECTI ON – G Type of Anchorage Uni t s Wi thout Hydrauli c Blocking Syst em Wit h Hydraulic Blocking Syst em T 13 7 mm. 5 mm. T 15 8 mm. 6 mm. THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 44 Where: Є c ∞ - Concrete st rain due t o Creep. Є s ∞ - Concrete st rain due t o shr inkage.  c∞ - Permanent compression st ress in concret e at center of gr avit y of t he Prest ressing Tendons ( in Pascal) .  ∞ - Coefficient of Creep. E c28 - Deferred longit udinal modulus of deformat ion of concret e at 28 days ( in Pascal) . Charact erist ic St rengt h of concrete at 28 days, fck ( MPa) 12 16 20 25 30 35 40 45 50 Longit udinal Deformat ion Modul us, E c28 ( 10 3 MPa) 26 27. 5 29 30. 5 32 33.5 35 36 37 Humid At mosphere Out side ( Rel . Hum. 75%) Dr y At mospher e I nside ( Rel. Hum. 55%) U = Peri met er i n contact wi t h At mospher e ( m) Ac = Sect i on of Concret e ( m 2 ) 2Ac = Art i fi cial Di mension of t he U St ruct ure Small < 0.2 m Large > 0.6 m Smal l < 0.2 m Large > 0.6 m COEFFI CI ENT OF CREEP:  ∞ Age of concret e when loaded: Young ( 3 – 7 days) Middle ( 7 - 60 days) Old ( > 60 days) 2.7 2.2 1.4 2. 1 1. 9 1. 7 3. 8 3. 0 1. 7 2.9 2.5 2.0 SHRI NKAGE: Є s∞ Age of concret e at t ime t o when effect of shr inkage is consider ed: Young ( 1 – 7 days) Middle ( 7 - 60 days) Old ( > 60 days) 10 - 3 0. 26 0. 23 0. 16 10 -3 0. 21 0. 21 0. 20 10 - 3 0.43 0.32 0.19 10 - 3 0. 31 0. 30 0. 28 5. Losses due t o Relaxat ion in Prest ressing St eel: Relaxat ion is t he loss of t ension in highly t ensioned st eel t hat is maint ained at const ant lengt h. The suppliers give figures for relaxat ion of t he st rand (see page 16) aft er 1000 hours, for 60%, 70% and 80% of t he charact erist ic br eaking load. I t is possible t o int erpolate int ermediat e values proport ionat ely, wit h accept able accuracy SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 45 The Table below gives a few aver age figures for relaxat ion ( ) of prest ressing st rands aft er 1000 hours: Values of  Fract ion of I nit ial Tension 60% 70% 8 0% Non St abilised St rand ( %) 4.5 8 12 St abilised St rands ( %) 1 2 4. 5 I n order t o t ake account of t he int eract ion of losses due t o shr inkage and creep of t he concret e and relaxat ion of st eel, a reducing coefficient is simply applied t o t he losses due t o r elaxat ion alone. The under ment ioned formulae indicates t he loss of st ress in st eel, in simplif ied form: ∆ c+ s ∆ r =  o .  1 – 2 - - - - - - - -  o Where: ∆ r - Effect ive loss due t o r elaxat ion ( in Pascal) .  o - I nit ial t ension of pr est ressing tendons at t he r elevant point ( in Pascal) .  - Nominal relaxat ion ( %) ∆ c+ s - St ress losses due t o creep and shr inkage alr eady calculat ed ( in Pascal) . I t is possible t o approach t he pr oblem of int eract ion bet ween shrinkage, creep and relaxat ion more precisely. The reader may refer t o t he appendix of t he FI P- CEB Code, but t he above simplified formula is on t he safe side. The loss at t ime ( t ) greater t han 1000hour s can be calculat ed fr om t he following formula: t  ∆ r ( t ) = ∆ r ( 1000t h) . - - - - - - - 1000 Where:  - 0.20, failing pr ecise infor mat ion. NOTE: Based on t he above formulae, some t ypical calculat ions are shown, as example, in t he subsequent pages. SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 46 EXAMPLE OF CALCULATI ONS: Assuming a rect angular Beam of size 0.40m X 1.20m, lengt h 34 met ers, was prest ressed by 5 parabolic t endons t hrough t he beam, from one end t o t he ot her. Paramet er s: Angular deviat ion at anchorages = 9° = 0.157 r adian. I nit ial st resses   = 1330 MPa Charact erist ic Concr ete St rengt h f ck = 35 MPa Permanent compression at t he Prest ressing Level  c = 10.5 MPa 1. LOSSES DUE TO FRI CTI ON: Calculat ion for t he mid- span cross- sect ion at x = 17m., Oi l lubricat ed st rand was used wit h Non- Galvanised flexible sheat hing. = 0.20 r ad. -1 and K = 0.01 rad/ m  17 = 1330 . e - 0. 20 ( 0.157 + 0. 01 x 17) = 1246 MPa 2. LOSSES DUE TO I NSTANTANEOUS DEFORMATI ON: Calculat ion for t he mid- span cross- sect ion at x = 17m., Oi l lubricat ed st rand was used wit h Non- Galvanised flexible sheat hing. E s = 195 x 10 9 Pa E ci = 33.5 x 10 9 Pa 10.5 є ci = - - - - - 10 -3 = 0.31 x 10 -3 33.5 5 - 1 ∆ r = 195 x 10 9 - - - - - - - - 0.31 = 2.5 MPa 2 x 5 3. LOSSES DUE TO ACHORAGE PULL- I N: Anchorage Pull- in є = 6 mm.  =  17 -  0 = 1330 – 1246 = 84 MPa Dist ance at which loss is nil: 6 x 10 -3 x 195 x 10 9 a = - - - - - - - - - -- - - - -- - - - - - - X 17 a = 15.40 met er 84 x 10 6 I nit ial pr essure at mid- span is unaffected by t he anchor age pull- in. Loss at Anchor age  0 = 2 x 15.4 x 84/ 17 = 152 MPa SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 47 4. LOSSES DUE TO CREEP & SHRI NKAGE: Tensioning aft er 28 days of concret ing Humid Out side at mosphere Cr oss-Sect ion of Beam: A c = 0.4 x 1.2 = 0.48 m 2 Perimeter in cont act : u = 3.2 m Coefficient : 2A/ u = 0.3 I nt erpolat ion bet ween 0.2 and 0.6 for t he values  ∞, Є s ∞  (0.2) = 2.20   (0.6) = 1.90   (0.3) = 2.13  Є s ( 0.2) = 0.230 x 10 - 3 Є s ( 0.6) = 0.210 x 10 - 3 Є s ( 0.3) = 0.225 x 10 - 3 10.5 x 10 6 Є s ∞ = - - - - - - - -- - - - - x 2.13 = 0.67 x 10 -3 Pa 33.5 x 10 9  c+ s = 195 x 10 9 ( 0.67 x 10 - 3 + 0.225 x 10 -3 )  c+ s = 174.5 MPa 5. LOSSES DUE TO RELAXATI ON OF PRESTRESSI NG STEEL: Nominal Str ess  k = 1770MPa I nit ial St resses x = 0  0 = 75% of  k = 1330 MPa I nit ial St resses x = 17  17 = 70% of  k = 1240 MPa St abilised st rands relaxat ion at 1000 hrs. = 2% 174.5   r ( 1000h) = 1246 x0.02 ( 1 – 2 x - - - -- - - ) 1246  r ( 1000h) = 18 MPa Loss at 10 5 hours ( about 12 Year s) 10 5 0.2   r ( 10 5 h) = 18 x - - - -- = 45 MPa 10 3 6. FI NAL STRESSES AT MI DSPAN:  17.∞ = ( 1246 - 2.5 – 174.5 – 45) MPa = 1024 MPa SECTI ON – G THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 48 CHECKLIST FOR INSPECTION OF POST-TENSIONING OPERATION AT MATERI AL MANUFACTURI NG STAGE: For BONDED TENDONS For UN- BONDED TENDONS Are t he Anchor Plat e ( Bear ing Plat e) pr oper ly machined, cleaned and pr otect ed fr om corr osion? Are t he Gr ips of Dead End Anchorage; seat ing evenly and adequat ely on t he Anchor Plat e? Are t he Grips free from Cor rosion? Are t he Back relief provided uniformly? I s excessive sheat hing/ duct st r ipped & t rimmed at Dead End ( Fix) Anchorage? Are t he Gr ips of uniform lengt h and serrat ions of consist ent qualit y? I s t he plast ic sheat hing of sufficient and uniform t hickness? Are t he duct s manufact ured fr om qualit y st eel str ip and specified str ip t hickness? I s t he qualit y & t ext ure of t he filling grease is consist ent? Are t he Seam Joint s int act , wat er t ight & unifor m? I s t he gr ease applied evenly all over and packed? Does t he bare pr est ressing st eels appear t o be new and are free fr om cor rosion? Does t he st rand appear t o be new and free fr om corr osion, when sheat hing & grease ar e r emoved? Are t hey adequately prot ect ed in st ore? Are t he Anchors proper ly machined wit h smoot h Taper Holes for Grips? Are t he Anchors proper ly machined wit h smoot h Taper Holes for Grips? Are t he Grips of uniform lengt h and serrat ions of consist ent qualit y? - Are t he Grips free from Corrosion? Are t he Back relief pr ovided unifor mly? Are all Test Report s and Cer t ificat e available for t he Prest ressing St eel and all t he component s of Anchorage, as required by t he specificat ion Are all Test Report s and Cer t ificat e available for t he Prest ressing Steel and all t he component s of Anchorage, as required by t he specificat ion SECTI ON – H THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 49 AT TENDON I NSTALLATI ON STAGE: For BONDED TENDONS For UN- BONDED TENDONS Are t he high & low point s of t he Sheat hing duct at t he corr ect elevat ion? Are t he high & low point s of t he Tendons, at t he correct elevat ion? Are duct pr ofiles smoot h and correct ly shaped (parabolic, circular or st raight ) bet ween reference point s? Are t he tendon profiles smoot h and correct ly shaped (parabolic, circular or st r aight ) bet ween reference point s? Are all duct j oint s pr oper ly mat ched and sealed wit h adhesive t ape? Do t he t endons have excessive horizontal wobble? Are t here any holes in t he ent ire lengt h of t he duct , and if so, have t hey been repaired t o prevent concret e ingress? Are t here any holes in t he ent ire lengt h of t he sheat hing, and if so, have t hey been r epaired? Are t here any kinks in t he ent ire lengt h of t he duct , which may prevent t he inst allat ion of prest ressing st eel? Does t he chairs or Suppor t - bar syst em are adequat e and confir m t o t he design requirement? I s t he Cable Suppor t s are adequat ely t ied t o prevent displacement or float ing of t he duct dur ing concret ing? Are t he St ressing Anchors secured and fast ened wit h t he appropr iat e pocket former? Are t he Guides firmly secur ed and fast ened wit h t he End Block shut t ering? I s bur st ing r einforcement inst alled behind t he anchorages are adequat e and sat isfying t he design requir ement ? I s burst ing reinfor cement inst alled behind t he anchor ages are adequat e and sat isfying t he design requirement? - Has t he met hod of concrete placement been reviewed as t o it s effect on duct st abilit y dur ing placement ? Has t he met hod of concr ete placement been r eviewed as t o it s effect on duct st abilit y during placement ? Has t he convent ional st eels placement been reviewed? Has t he convent ional st eels placement been reviewed? SECTI ON – H THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 50 AT TENDON STRESSI NG STAGE: For BONDED TENDONS For UN- BONDED TENDONS Are t he Bear ing Plat es holes & Gr ips, fr ee from rust dirt and grease? I f not , clean t hem. Are t he St ressing Anchors holes & Gr ips, free from r ust dir t and foreign part icle? I f not , clean t hem. Has t he elongat ion dat um been marked for t he init ial and final reading? Are t hey logically and clear ly locat ed? Has a consist ent dimension been used, as t he elongat ion dat um mark on t he st r and? I s t he st ressing equipments well maint ained, and are all t he calibr at ion chart s & report s available? I s t he st r essing equipment s well maint ained, and ar e all t he calibr at ion char t s & repor t s available? I s t he Oper at ing Technician, t rained and careful enough about t he equipment and consist ent from t endon t o t endons? Are t he t endons st ressed slowly enough t o allow t he st r ands t o over come as much frict ion as possible prior t o locking? Are t he Bearing Plat e and Grips seat ing pr oper ly aft er st ressing? Are Gr ips seat ed evenly, proper ly under pressur e? Aft er elongat ion appr oval, is t he t endon t ails properly cut well inside and cleaned t o fix Grout Cap for gr out ing? Aft er elongat ion approval, is t he t endon t ails pr operly cleaned and greased well inside & secured wit h Gr out Cap? Are t he str essing pocket s proper ly cleaned and prepared t o allow good gr out bond dur ing and aft er pat ching? - SECTI ON – H THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 51 SAFETY PRECAUTION DURING POST-TENSIONING OPERATION Dur ing Prestr essing oper at ion, a huge forces are been induced int o t he t endons by t he Jacking Equipment s. These j acking equipment s are working under t remendous hydraulic pressure, and t hus a car eful and compet ent workmanship can avoid accident s and damage t o t he equipment . Some essent ial precaut ions are being list ed here, t o be adhered t o:  St ressing oper at ion should be carr ied out by t he t rained and experienced personnel, under a competent engineer or supervisor.  The equipment s, especially t he high pressure hose pipes, and t he end fit t ings should be in perfect condit ions. Damaged hose pipes must be replaced immediat ely. Any lack of decision in t his regar d may cause a serious accident .  Pr ot ect ion caps must be placed over t he Hose pipes nipples and t he end fit t ings of t he equipment s. Always avoid put t ing t he gausses or cot t on wast e int o t he nipple hole, as t his may cause a severe ingress of dust & dir t int o t he hydraulic syst em.  Jacking equipment s and power- packs shall never be lift ed, using hose pipes as a lift ing devise.  St ressing shall be done according t o t he specified dat a provided by t he designer . I n no case t he pressure should be exceeded beyond 5% of t he allowable limit .  Dur ing st ressing, nobody should be allowed t o st and behind or underneat h t he Jacking unit , since failure of a t endon can cause a sever inj ur ies or even deat h.  I n t he st ressing sit e; which are close to public t r affic areas, A st rong pr ot ect ive shield should be erect ed and t he j ack must be secured by r opes or chain pulley block t o pr event failed st rands from shoot ing out . SECTI ON – H THE FREYSSINET PRESTRESSED CONCRETE COMPANY LTD. Page: 52  The Prestr essing st eel ( i.e. bar s, wires or st rands) should be st ored carefully t o ensure t hat t hey are not damaged in any way and should be checked for rust or cor rosion before use.  Care should be t aken while handling and unwinding of prestr essing st eel coils, as t hey may ‘whip- back’ wit h force, if not securely bound.  All t he t echnicians should wear hand gloves and ot her safet y har nesses while working at over hanging/ r aised st ressing plat form. Technicians ar e prone t o fall down from a height in such cases.  Ensur e t hat unaut hor ized people should not gat her in and around St ressing Sit e as sight seeing locat ion. Er ect a safety war ning sign ar ound.  Wear safety helmet s at sit e dur ing st ressing oper at ion.  Do not permit welding operat ion near H.T. Prest ressing st eel. Molt en sparks ar ises fr om welding, if falls on t he t endons, will change it s mechanical pr opert ies and promot es t he possibilit ies of premat ure failure. The damages caused by inadvert ent heat ing, from welding, may not be det ect able by visual inspect ion.  Do not use Acet ylene gas Torch heat ing apparat us t o cut or t r im t he st rand befor e st ressing. Only abrasive cut t ing wheels should be used.  Prest ressing st eel should not be used for providing eart hing t o t he elect rically operated equipment s. An elect r ic spar k, when j umps t hrough t he prest ressing st eel, may effect or alt er t he molecular st r uct ur e and a loss of st rengt h may occur.  Light ening conduct ors should be r out ed clear of tendons and t he Anchor ages et c. SECTI ON – H