pre stressed concrete

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Slide 1: 

PRESTRESSED CONCRETE # 1 ( # referances on page – 3) PRESTRESSING IS THE PROCESS BY WHICH COMPRESSION STRESSES ARE INDUCED IN SOME PART OF THE UNLOADED STRUCTURE, SO THAT TENSILE BENDING STRESSES WHICH ARE SUBSEQUENTLY CAUSED BY LOADING, MAY BE COUNTER BALANCED AND ELIMINATED OR REDUCED. WHEN REINFORCEMENT IS STRECHED BY PRESTRESSING AND ANCHORED TO THE CONCRETE AND THE PRESTRESS IS RELEASED, THE TENDENCY OF THE REINFORCEMENT TO RETURN TO ITS ORIGINAL LENGTH INDUCE, A COMPRESSIVE PRESTRESS IN CONCRETE. COMPRESSIVE PRESTRESS MAKES MORE ECONOMICAL USE OF THE CONCRETE BY ALLOWING ALL OF THE SECTION OF CONCRETE TO PLAY SOME PART IN SUPPORTING LOAD. IN ORDINARY REINFORCCED CONCRETE, CRACKS HAPPENE BECAUSE OF LOW TENSILE STRENGTH OF CONCRETE.THIS CAUSE SERIOUS PROBLEM IN LONG SPAN BEAM. IN PRESTRESSED REINFORCEMENT CONCRETE, THE OBJECT IS TO PRECOMPRESS THE UNIT TO PREVENT THE TENSILE STRESS WHICH CAUSE THE HAIR CRACK. PRESTRESSED CONCRETE B.M.C. – VIII PRARTHIT SUMIT HIREN NAITIK ZUBIN NISHIT 1 THE ADVANTAGE OF THE INDUCED COMPRESSIVE PRESTRESSED IS THAT UNDER THE LOAD THE TENSILE STRESS DEVELOPED BY BENDING IS ACTING AGAINST THE COMPRESSIVE STRESS INDUCED IN CONCRETE AND IN CONSEQUENCE CRACKING IS MINIMIZED. WHEN LOAD IS REDUCED OR REMOVED EVEN THE GENERATED CRACKS CLOSE UP DUE TO COMPRESSIVE PRESTRESS. ANOTHER ADVANTAGE OF PRESTRESSED IS THAT THE COMPRESSIVE STRENGTH OF THE WHOLE THE SECTION OF THE CONCRETE IS UTILISED AND THE RASISTANCE TO SHEAR IS CONSIDERABLY IMPROVED, SO OBVIATING THE NECESSITY FOR SHEAR REINFORCEMENT. HIGH TENSILE WIRE AND HIGH COMPRESSIVE STRENGTH CONCRETE IS NEEDED IN PRESTRESSED WORK TO GAIN THE MAXIMUM ADVANTAGE OF THE PRESTRESS. STRESSESD TENDONS BOTTOM PART IN HIGH COMPRESSION FINAL PRESTRESS TOP&BOTTOM BOTH IN COMPRESSION LIVE LOAD STRESSES TOP IN COMPRESSION BOTTOM IN TENTION COMBINED STRESSES WHOLE SECTION IS UNDER COMPRESSION ADVANTAGES OF PRESTRESSED CONCRETE # 2 1. IN PRESTRESSES SECTION ENTIRE CROSS SECTION COMES UNDER COMPRESSION, SO MORE EFFECTIVE USE CAN BE MADE OF MATRIAL.THUS, REQUIRED MATERIAL QUANTITY WILL REDUCE. AS PER R.BARRY, THE AUTHOR OF BOOK “THE CONSTRTION OF BUILDING”, STEEL WILL REDUCE UP TO 70%, AND CONCRETE WILL REDUCE 30 TO 40%. 2. AS SECTION BECOMES MORE EFFICIENT, FOR THE SAME SPAN, SMALLER SECTIONS CAN BE USED COMPARED TO REGULAR NON PRE-STRESSED MEMBER. 3. IT IS MORE RIGID, DUE TO USE OF HIGHER GRADES OF CONCRETE. 4. MEMBER CAN SPAN LONGER, SINCE CONCRETE IS PRE-STRESSED. 5. SECTION IS CRACK FREE, SO IT CAN BE USED TO STORE WATER, OIL OR SLURRIES. DISADVANTAGES OF PRESTRESSED CONCRETE # 3 1. DOES NOT WORK IN REVERSAL FORCES. 2. THE CONCRETE CASTING NEEDS SUPIRIOR QUALITY AND ACCURATE SUPERVISION. 3. SPECIAL EQUIPMENTS ARE NEEDED TO PROVIDE THE PRESTRESS. 4. ELASTIC SHORTENING. BECAUSE THE CONCRETE SHORTENS WHEN THE PRESTRESSING FORCE (IN FULL OR IN PART) IS APPLIED TO IT, THE TENDONS ALLREADY BONDED TO THE CONCRETE SHORTEN, SIMULTANEOUSLY LOSING PART OF THEIR STRESS. 5. IN CONSTANT-STRESS SITUATION DEFORMATION HAPPENS IN CONCRTE, WHICH IS CALLED ‘CREEP OF CONCRETE’ 6. SHRINKAGE OF CONCRETE.THE GRADUAL LOSS, WITH TIME, OF FREE WATER FROM THE CONCRETE. THE FORCES MUST BE CALCULATED VERY CAREFULLY SO THAT SUCH LOSSES WITH TIME ARE TAKEN IN ACCOUNT.

Slide 2: 

TYPES OF PRESTRESSED CONCRETE # 4 THE STRETCHING OF REINFORCEMENT BEFORE IT IS CAST INTO CONCRETE IS DESCRIBED AS PRE-TENSIONING AND STRETCHING AFTER THE CONCRETE HAS BEEN CAST IS DESCRIBED AS POST-TENSIONING. PRE-TENSIONING IN PRETENSIONING THE HIGH-STRENGTH STEEL TENDONS (WIRES, STRANDS) ARE STRETCHED BETWEEN TO PIERS TO A PREDETERMINED TENSION AND ANCHORED TO FIXED BULKHEADS OR MOLDS. THE CONCRETE IS CAST IN FORMWORK PLACED AROUND THE TENDONS, AND CURED. UPON HARDENING THE TENDONS ARE CUT. AS THE BOND BETWEEN THE TENDONS AND THE CONCRETE RESISTS THE SHORTENING OF THE TENDONS, AFTER CUTTING TENDONS, COMPRESSTION GETS TRNSFERED TO THE CONCRETE. THIS CAUSE THE MEMBER TO BOW UPWARD, AND THUS HELPS IN RESISTING LOAD. IT HAS TO BE NOTED THAT THE EXACT AMOUNT OF PRESTRESS FORCE MUST BE VERY CAREFULLY CONTROLLED. PRETENSIONING IS TYPICALLY DONE ON OFF-SITE FACTORY. THE METHOD IS MOSTLY USED FOR THE PRODUCTION OF PRECAST PRESTRESSED CONCRETE ELEMENTS BECAUSE IT OFFERS GREAT POTENTIAL FOR MECHANIZATION. POST-TENSIONING POST-TENSIONING IS A FORM OF PRESTRSSING DONE AT A LATER CONSTRUTION STAGE. IN THIS METHOD FORMWORK IS PUT IN PLACE, A TUBE CONTAINING UNSTRESSED CABLE IS SET IN PLACE, AND CONCRETE IS CAST AROUND THE TUBE. AFTER THE CONCRETE HAS CURED, THE CABLE ANCHORED ON ONE END AND IS TENSIONED BY JACKS FROM THE OTHER END. AT PREDETERMINED TENTION THE TENDONDS ARE ANCHORED ON THE JACKING END AND JACKS ARE REMOVED. THIS CAUSES BEAM TO RISE UPWARD. AFTER STRESSING AND ANCHORING, THE VOID BETWEEN EACH TENDON AND ITS DUCT OF THE TUBE IS FILLED WITH A MORTAR GROUT, WHICH SUBSEQUENTLY HARDENS. GROUTING ENSURES BONDING OF THE TENDON TO THE SURROUNDING CONCRETE, IMPROVES THE RESISTANCE OF THE MEMBER TO CRACKING, AND REDUCES THE RISKS OF CORROSION FOR THE STEEL TENDONS. ALTHOUGH POST-TENSIONING CAN BE USED IN PRECAST PRESTRESSED OPERATIONS, IT IS MOST USEFUL IN CAST-IN-PLACE CONSTRUCTION WHERE LARGE BUILDING AND BRIDGE GIRDERS CANNOT BE TRANSPORTED, AND FOR CUSTOMIZED STRUCTURES THAT NEED TENSIONING ON THE JOB SITE. IT’S APPLICATION IN UNUSUAL PROJECTS, SUCH AS NUCLEAR POWER VESSELS, TV TOWERS, AND OFF­SHORE STRUCTURES, HAS BECOME A MUST AND WILL CERTAINLY CONTINUE TO EXPAND. PRESTRESSED CONCRETE B.M.C. – VIII PRARTHIT SUMIT HIREN NAITIK ZUBIN NISHIT 2 # Pre-stressed concrete – I.I.GRADUCK # Prof. Dr. Azlan Abdul Rahman Universiti Teknologi Malaysia # Prof. Dr. Azlan Abdul Rahman Universiti Teknologi Malaysia # Pre-stressed concrete – I.I.GRADUCK

Slide 3: 

PRE-TENSIONING AND POST-TENSIONING ONE ADVANTAGE OF POST-TENSIONING OVER PRESTESSING IS THAT THE CABLE CAN BE SET FAIRLY EASILY SO THAT THE ECCENTRICITY OF THE CABLE FORCE IS VARIABLE WHICH CAN BE ADJUSTED TO ACCOUNT FOR THIS PHENOMENON. ADJUSTING PRE-TENSIONED STEEL IN THIS WAY IS MORE DIFFICULT. REINFORCEMENT IN CONTINUOUS MEMBER CLERIFICATION AS PER DENIAL SCHODEK’S BOOK “STRUCTURE”, AS LOADS INCREASE BEYOND DESIGN LOADS, THE BEAM BEGINS BEHAVING MUCH LIKE A STANDARD REINFORCED CONCRETE BEAM (i.e., cracks develop etc.). THE ULTIMATE STRENGTH OF A PRESTRSSES BEAM IS NOT OVERLY SUPERIOR TO THAT OF A SIMILARLY PROPORTIONED PLAIN REINFORCED CONCRETE BEAM. THE PRIMARY VALUE OF THE TENSIONING OPERATION IS THUS TO IMPROVE THE PERFOMANCE OF THE BEAM AT DESIGN OR SERVICE LOADS. THIS MAY BE TAKEN AS ONLY SCHODEK’S OPINION, BUT IT IS CLEAR THAT FOR BETTER PRFOMANCE OF PRESTRESSES BEAM, DESIGN LOADS SHOULD BE CALCULATED WITH ACCURACY. CONCLUSION PRESTRESSING IS A WAY OF MAKING BEAMS MORE EFFICIENT,THAT IS PARTICULARLY SUITABLE FOR CONCRETE STRUTURES. BASICALLY, THIS IS A TECHNIQUE FOR PERMANENTLY LOADING A BEAM IN A CONTROLLED WAY WITH THE INTENT OF BUILDING UP STRESSES IN THE MEMBER OPPOSITE TO THOSE DEVELOPED BY THE EXTERNAL SERVICE LOAD. IN A PLAIN REINFORCED CONCRETE BEAMS CRACKS DEVELOP IN THE TENSION REGION AND PROPAGATE UNTILL THEY REACH THE COMPRESSIVE ZONE. IN A PRESTRESSED BEAM, NO TENSILE STRESSES ARE DEVELOPED AND HENCE NO CRACKS DEVELOP. THUS, THE ENTIRE CROSS SECTION OF A BEAM CONTRIBUTES TO THE STIFFNESS OF THE BEAM. SO, A VERY SIGNIFICANT REASON FOR PRESTRESSING IS TO CREATE A LARGER EFFECTIVE SECTION FOR RESISTING DEFLETIONS. PRESTRESSED CONCRETE B.M.C. – VIII PRARTHIT SUMIT HIREN NAITIK ZUBIN NISHIT 3 # D.k.ching “Constrution of building” # Denial Schodek “Structure” References # 1 R.Barry “Constrution of Building” # 2 & # 3 R.Barry “Constrution of Building” & Pre-stressed concrete – I.I.GRADUCK # 4 Denial Schodek “Structure” & Pre-stressed concrete – I.I.GRADUCK