Contd. from Part-I ….
The main equipments used in post tensioning operationing are:
a) Stressing Equipmemts: These include jack & pump of suitable capacities. Proper calibration certificates for these equipments are quite necessary. For general idea, calibration tolerances for these max be +/- 2%. Calibrations are checked periodically in order to ensure accuracy throughout. Scrutiny of friction loss calculations for the jack is also important in addition to other documents related to these items. Physical inspection is done periodically.
b) Threading Equipment: This consists of Strand Dispenser & a Pushing Machine. Physical inspection and scrutiny of supplier’s documents are part of quality control measures.
c) Grout Mixer: As the name suggests, this one is used for mixing grout to be used for grouting the steel cables inside concrete. Large quantity of high quality grout mixes are required for the purpose and ordinary methods of mixing are usually not deployed. Physical inspection of equipment, scrutinies of supplier’s documents, calibration certificate etc. are important aspects of quality control measures. For general idea, the grouting equipment should have a constant delivery pressure not exceeding 1 MPa.
Execution of post-tensioning operation in a Reinforced Cement Concrete (RCC) bridge or similar structure primarily include the following steps:
Cutting & Handling of Strands: The same is done as per approved Job Procedure, shop drawings in conjunction with relevant specifications. Cutting of strands are best done by friction saw or high-speed abrasive cutting wheel. Proper inspection is necessary in order to ensure compliance to specifications & job procedures.
Ducts & Anchorages Installation: The same is done as per approved Job Procedure & shop drawings in conjunction with relevant specifications. Proper inspection is necessary in order to ensure compliance to specifications, job procedures and drawings. For general idea, placement tolerances for ducts and anchorages are normally kept within +/- 5mm and anchorages are kept perpendicular to the tendons within +/- 1¤.
Anchor cones, Plates & Blocks: These provide the necessary anchorage to the cables while applying tensile stresses. Proper inspection is necessary in order to ensure compliance to specifications, drawings etc.
Threading of Tendons: Proper inspection is necessary in order to ensure compliance to specifications, job procedure etc. For general idea – tendons to be cut at least 3 days after completion of grouting. Friction test on tendons are also conducted as per specifications. Total friction losses estimated from friction test on tendon should not exceed theoritical friction losses. Otherwised all wires are lubricated and re-tensioned.
Tensioning: Tensioning of cables or tendons are continued until required tendon load and extension are achieved. For general idea – forces on tendons should not exceed 75% of the prescribed minimum ultimate tensile strength of tendon material except during application of stress when the same should not exceed 80%. Proper inspection is necessary in order to ensure compliance to specifications, job procedure, drawings etc. and tensioning record sheets are kept.
Extension Measurement: Proper inspection is necessary in order to ensure compliance to specifications and job procedure. For general idea — measured extension is maintained within +/- 5% of estimated extension. Extension record sheets are maintained as part of QC measure.
Grouting: Proper inspection is necessary in order to ensure compliance to specifications, job procedure etc. For general idea — grout delivery pressure & w/c ratio should not exceed 1 MPa & 0.45 respectively. Grout temperature is also kept under control, say, within 32¤C. The entire quantity of grout planned for a day or so needs to be injected in one continuous operation. Grout should be placed within 30 minutes of completion of mixing. Grout record sheets are maintained as part of QC measures.
The above steps have been discussed very briefly in order to provide a quick practical idea on the subject which otherwise is an extensive one. For more details, one needs to refer to specifications, job procedures, drawings etc.