Method Statement for Prestressing I-Girder Prior to Casting
Purpose
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.
Table of Contents
Scope of Work
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:
1. Cutting of the High Tensioned (HT) hair strands
2. The strands of HT are positioned
3. Cutting and putting in HDPE de-bonding tubes
4. Initial adjustment, checking the profile
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
7. Cut the extra length of strands on the ends of the girder and apply epoxy sealant within the HDPE to the de-bonding tubing.
Resources
Strands
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 diameter: 15.2 millimeters
Nominal steel area: 140.0 mm2.
Barrels that fit 15.2-millimeter diameter HT Strands
Grips/Wedges Re-usable
Construction Methodology
General
In this part, you will find the detailed method for carrying out Pre-Tensioning operations.
Cuts in HT Strands and Identification
a. After receiving coils from the manufacturer and checking for the correct packing and identification tags in relation to the delivery of the challan.
b. Each coil is removed from the store close to the store and kept on the ground under cover.
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.
i) Strands coiling and Un-coiler should be hung on wooden planks or supports covered with a polythene sheet.
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.
e. The coil is then tucked away. It is then loaded onto the stationary un-coiler using a crane; the packing strips are cut. The strand of the bundle is manually pulled out until the desired length is cut using abrasive cutting tools.
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.
Profiling
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.
Stressing
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.
d. Common pumps operate multi-pulling jacks on the other end of frames. The stressing process is performed according to the force required in increments, and the lengthenings are recorded using the format. This tension locks tendons.
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.
g. Sandbags or timber shields should be placed on top of pre-stressing steel and in front of the jacks so as to safeguard personnel.
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.
j. The de-tensioning of the steel shall be permitted before the concrete has reached the strength specified as determined through tests of cylinders that have been dried and hardened in similar conditions to the concrete on the member.
k. The required elongation and the equivalent force of prestressing must be shown in the illustrations.
4.1.4 Equipment for stressing
Hydraulic jack- Mono Jack
Multi-pulling jack that includes a Jacking Unit including Wedges, Seating Devices, and Ancillary Components
Hydraulic Pumps
The Stressing Operation for Strands
Introduction
The steel frames are placed against the anchor wall using appropriate support. Macaloy bars are passed through the hole in the anchor frame to where the strands are fixed. Another end of the rod goes across the frame of the reaction, which sits against the wall of the anchor. The overall arrangement is tensioned using multiple-pulling jacks.
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.
The macaloy bars are threaded through the hole of the steel frame.
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 tendon is strained to the force required. In the meantime, the steel frame holding the girder’s strands is moved along the rail as the strands lengthen. The measurements are taken.
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.
De -Tensioning of Strands
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.
Cut the ends of the strands that are prestressing at the end of the girders, apply an epoxy coating to the surface of the strands, and seal the epoxy inside the HDPE de-bonding tube.
Controls During Concrete Pouring
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:
Make sure to use suitable vibration devices passing through the gap in which they are located.
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.
Safety Requirements and Health Protection
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.
Only sealed and intact pressure hoses are allowed to be used.
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.
Operators must wear protective helmets when they are stressed.
Any oil leaks should be cleaned.
Jack should not be handled with hoses.
Do not allow welding in proximity to high-tensile pre-stressing steel. A drop of molten steel, when applied to the strand, can alter the mechanical properties of the strand and increase the likelihood that it will fail prematurely.
Hogging of the girders during the pre-stressing process should be monitored and documented.
Quality Assurance and Quality Control
The quality control process and the assurance should be strictly observed in accordance with the checklist and the project’s specifications.
tag: Method Statement for Prestressing I-Girder Prior to Casting
