Method Statement for Prestressing I-Girder Prior to Casting
This Method Statement for Prestressing I-Girder explains each step related to the pre-tensioning work for constructing the pre-tensioned I-Girder. It also takes into consideration the general requirements of local authorities and the specifications provided by the designers and the consultants.
This scope is related to the Pre-Tensioning of Precast I girders to construct the flyover projects. It consists of the work on-site in order of:
2. The strands of HT are positioned
3. Cutting and putting in HDPE de-bonding tubes
5. The stressing of strands is caused by mono jacks/multi-pulling Jacks
6. De-stressing the strands and transfer pre-stressing to casting units
Description of the individual strands
Based on the specifications of the project, the strands must conform to the specifications of ASTM A 416, Grade 1860N/mm2, Low Relaxation, with the following characteristics:
Nominal steel area: 140.0 mm2.
Barrels that fit 15.2-millimeter diameter HT Strands
In this part, you will find the detailed method for carrying out Pre-Tensioning operations.
a. After receiving coils from the manufacturer and checking for the correct packing and identification tags in relation to the delivery of the challan.
c. The test certification for each coil provided by the company is inspected, and the records are kept in the on-site office.
d. A cut-off area of strands must be set close to the location, and the cutting bench must be erected. It will comprise a stationary Un-coiler. We will follow the following guidelines and safety measures when cutting strands.
ii) The entire length after cutting should be laid out on supports or sleepers made of wood.
iii) Safety measures must be employed during work, including the handrail being fixed in all areas should be considered necessary or restricting the movement of employees around the work site.
f. The required number of cut strands, as the case, maybe put together and tacked to each other with an elastic wire. A tag for identification is attached to each bundle and indicates the girder number/coil number for the reason this is created. The information is stored in a database in the office of the site. Tags are removed from the bundle at the moment of their insertion into the bed for casting. The tags removed from the bundle are stored securely in the office for a certain duration.
g. The strands that are bundled with tags are free of sand and dust and covered until they are transferred to the work area. If dirt or dust is found on the strands, they should be cleaned using cotton waste and a water-soluble solution in the event of need.
a. The strands are then placed in the cage of the re-bar casting bed according to the required dimensions and spacing.
b. The strands’ ends will be pulled into the anchor wall at one of their ends.
c. To de-bond lengths that are specified in the work diagrams, HDPE pipes are inserted at the ends of the girder.
a. The strands on either end of the wall anchor are secured to the frames for loading with wedges and barrels.
b. In one anchor wall, the strands are stretched (Initial tension) using a mono jack. The initial slackness of the strands is taken away.
c. The profiling process is inspected for accuracy; if required, the error discovered is fixed.
e. The consultant’s engineer grants clearance to proceed with the concreting of the structure.
f. The highest safety precautions should be taken before stressing, during, or after to avoid accidents.
h. Only certified personnel are permitted to perform the stressing activities and must safeguard themselves by wearing safety helmets or other devices.
i. The complete and exact documents of all operations associated with the prestressing are recorded. These records must include the extensions, pressure, and slip if any tendon is used for prestressing.
k. The required elongation and the equivalent force of prestressing must be shown in the illustrations.
Hydraulic jack- Mono Jack
Hydraulic Pumps
Introduction
Procedure
The frame made of steel is placed in the exact position, aligning with the holes for inserts in the frames as per the sketches.
At one end, macaloy bars are fitted with lock nuts.
On the surface of the jack (ram of the jack), the seating plate is fitted with a lock nut and is secured.
The rod stays in its position by the nuts.
The equipment that is used to stress the body is dismantled and removed from the position.
Before de-tensioning the string, the ends of strands are identified with a marker to gauge the strands’ slip. This is observed during the de-tensioning process, and the results are taken and recorded.
Release of the force in the jacks for prestressing gradually.
The engineer reviews the measurements recorded to get his approval This value is examined in relation to the figures in the drawings.
While concreting isn’t an integral component of stressing, it is crucial in successfully stressing structures. One technician who is a prestresser must be present during the concrete pouring process to ensure that the tendons’ ducts are not damaged in this process.
It is crucial to ensure the following steps are taken when making concrete:
It should be noted that needle vibration isn’t put directly on strands and HDPE de-bonding pipe, which could cause damage to the strand and HDPE de-bonding pipe.
Care must be taken when constructing the final block section. Proper compaction of concrete must be maintained to avoid honeycombing.
Attention is given to health and safety protection when working out.
A notice sign “Stressing Operations in Progress” should be displayed in areas where stressing operations are required to be performed.
Strand bundles that extend out of the plate bearing need to be covered with plastic or sheet pipe made of metal to prevent injuries caused by the sharp edges of the bundles.
Whenever stressing tendons is being done, it is imperative to ensure that no one is within the range of the apparatus that is prestressing.
The person in charge of the jack and pump must be located at the rear of the jacking device.
Any oil leaks should be cleaned.
Jack should not be handled with hoses.
Hogging of the girders during the pre-stressing process should be monitored and documented.
The quality control process and the assurance should be strictly observed in accordance with the checklist and the project’s specifications.