A horizontal main structural member with a T-shaped cross-section that works as main support for vertical loads is known as a T-girder in the civil engineering sector. It is one of the most common and used structural members in bridge construction works all over the world. T-girder can be both Pre-tensioned and Post-tensioned. In this article, we’ll discuss the overall construction method of Pre-stressed T-Girder
Reinforcement work for the Construction method of Pre-stressed T-Girder:
The first step of T-girder manufacturing is to start reinforcement work. The characteristics of reinforcement will be as per the Contract Documents and Construction Specifications.
Yield strength of Steel Reinforcement for manufacturing a T-shape girder should be of minimum 500 Mpa and shall conform to British Standard BS4449:2005 Grade B500C or as per Specification provided and approved by the concerned authority.
The storage yard for reinforcement shall be cleaned and leveled according to the construction layout. Bars of different diameters and grades of steel are to be kept separate.
The quality of the reinforcement does not fall below the acceptable limit before its placement at the site. To ensure this we’ve to maintain the following two things:
-Use of timber/concrete pad under reinforcement as ground support to store it as we shouldn’t store reinforcement directly on the ground.
-Use of Proper covering to store reinforcement.
After ensuring the proper quality and storage facility for rebar, the contractor can start cutting and bending steel reinforcement for the structural element. This work will commence as per the approved shop drawings and bar bending schedule.
First step formwork and rebar fixing for the Construction method of Pre-stressed T-Girder:
The steel formwork consists of 4 parts, as follows:
a1. bottom formwork
a2. side formwork
a3. fixing device
a4. flange slope adjusting devices
There is a sequence for installing this steel cage. At first, The contractor should Clean up the bottom formwork by steel wire brush and apply form oil or other approved release agent on the f/w surface.
Then they can install the rebar of the T-Girder as per the approved construction drawings and install concrete cover blocks. The concrete cover to the reinforcement shall be a minimum of 35mm or as per the project specification. The concrete cover blocks should be of a minimum class 45 Concrete and proper curing is mandatory before utilizing them. The face of blocks in contact with forms shall have a color and texture that will match the concrete surface and per square meter shouldn’t contain more than 4 concrete blocks.
The consultant engineer should inspect the rebar quantity, distance, size, shape, etc. as per the approved shop drawing before the contractor continue to further works.
The rest of the F/W fixing works again can continue after completing Pre-stressing works.
Pre-stressing Work for the Construction method of Pre-stressed T-Girder:
Pre-stressing Steel (Strand) :
The strand shall be delivered in coils or reels which shall be firmly secured and well-protected from any kind of damage during transit or shipping. Each coil or reel shall have a sufficient diameter to ensure that the strand pays out straight and shall be tagged by proper requirements. We are going to take AASHTO M203-90 (ASTM A416-86 Grade 270) as standard strand quality in this article though it may vary from project to project for design requirements. Some tests should be carried out by the contractor before using these strands at the site in front of the proper authority. They shall cut out a sample (approximately 1.0m long) for every 25 tons of steel, or as instructed by the approval authority. The breaking load, 0.2% proof load, and elongation tests shall be done according to ASTM A 370, by an approved independent testing laboratory.
⦁Pre-stressing steel can be a Seven-wire uncoated low-relaxation Strand conforming to AASHTO M203-90 (ASTM A416-86 Grade 270) or as per the Project specification or approved drawings/documents.
⦁The diameter of Pre-stressing steel should be specified on the project specification.
⦁Storage facility should be flat on elevated floors or racks under such conditions as are necessary to ensure that the quality of the material is not adversely affected. They shall be protected from damage, oil, corrosion, or any deleterious matter. Protective coverings are not to be removed before the strand is required for use. Before being fabricated into pre-stressing tendons, the strands or bars shall be cleaned of loose rust and any deleterious matter and inspected by the Employer/Employer’s Representative for approval.
⦁Pre-stressing steel reinforcement that shows signs of pitting or has any surface defects such as splits, roughness or necking is not to be used, and any lengths of wire or strand so affected shall be cut off and rejected.
⦁All proposed pre-stressing steel shall be approved by the Employer/Employer’s Representative.
Pre-stressing Procedure for the Construction method of Pre-stressed T-Girder (Step by step):
a. Calibration of Hydraulic Jack, pressure gauge, and strand test
b. Pull and install strands through rebar case and install de-bonding PVC pipe as per approved shop drawing, and clean up the dust on the strands.
c. Tensioning the strands as per the correct sequence
d. Apply initial load (25% of full load) to strand with the correct sequence by a hydraulic jack
e. Apply load (50% of full load) to strand and measure the elongation length (La) of the jack, La is the elongation of the strand from 25% to 50% of full load. Since 0% to 25% elongation cannot be measured directly, La is used instead.
f. Apply full load to the strand (hold for 5 minutes) and measure the total elongation length (Lb) of the jack (When the jack reaches the tensioning stroke, it needs to be reset), Lb is the elongation of the strand from 25% to 100% of full load.
g. Wedge Slip value (Lc) is considered to be 5mm
h. After each strand is tensioned, the Draw-in value (Ld) of the sandbox is measured (specific measurement method: just measure the stretch length of sandbox before and after tensioning, and its difference is the Draw-in value (Ld)). The actual elongation “Le” of Pre-stressing Screw-thread Steel Bar is replaced by the theoretical elongation.
where,
ΔL: Elongation of per meter length Pre-stressing Screw-thread Steel Bar ;
P: Tension stress of Pre-stressing Screw-thread Steel Bar ;
L: Length of Pre-stressing Screw-thread Steel Bar ;
A: Sectional area of Pre-stressing Screw-thread Steel Bar ;
E: Elasticity modulus of Pre-stressing Screw-thread Steel Bar ;
i. The total measured elongation of the strand is ΔL=La+ Lb- Lc- Ld- Le.
j. Repeat steps “d” to “i” until all strands are properly tensioned.
k. If sandbox Draw-in happens after finishing all strands tensioning, all strands should be re-tensioned to full load with the correct sequence again. If the sandbox has no Draw-in, there is no need to add this step
l. Compare the calculated theory elongation with the measured elongation, and the deviation shall be within ±5%.
m. Stressing Work Report form should be completed during pre-stressing work
Final Formwork fixing for the Construction method of Pre-stressed T-Girder:
⦁Cleaning up the strands, rebar, and side formwork.
⦁When assembling the formwork, the sponge strip or foam double-sided adhesive or suitable materials shall be used for gaps between formwork to avoid concrete mortar leakage
⦁Formworks shall be treated with form oil or other approved release agent. Material that will adhere to or discolor the concrete shall not be used.
⦁ After complete formwork fabrication and before the commencement of the first cast, the formwork shall be inspected and verified that all dimensions are conformed to the approved drawing.
Concrete Work for the Construction Method of Pre-stressed T-Girder:
There are 3 steps of concrete placement, as follows:
Step 1: Concreting for the bottom of the ribbed slab:
（1）Concrete will be poured to the bottom Level
（2）Compaction will be carried out by external Vibrators at every 2.0m. spacing along the F/w.
（3）The vibration duration for each position is about 5-10 seconds.
Step 2: Pour the upper concrete of the ribbed slab:
（1）Concrete will be poured to intermediate Level
（2）Compaction will be carried out by external vibrator and internal vibrator with poker head dia. 35-45 mm. at every 0.5m. spacing.
（3）The vibration duration for each position is about 10-15 seconds.
Step 3: Pour the concrete of the wing plate:
（1）Concrete will be poured until reaching the Top Level of formwork
（2）Compaction will be carried out by internal vibrator with poker head dia. 35-45mm. at every 0.5m.
（3）The vibration duration for each position is about 10-15 seconds.
Contractor work doesn’t finish immediately after pouring concrete. Curing is another important factor to finish the work properly. Proper curing helps the concrete to gain its actual strength with time. Curing serves the main purpose of retaining moisture in the concrete to continue gaining its strength. It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking. Water is one of the most common natural curing materials. But there are a lot of options nowadays to serve the purpose of curing with chemical compounds. Sika Antisol, Ereducer-201, etc. are mostly used curing compound around the world.
we can divide curing a girder in three parts:
a) Top Surface
On the initial setting, contractor should cover the concrete surface by canvas.
b) Side Surface
Contractor should apply concrete curing agent immediately after removing side formwork.
c) Bottom surface
Once the girder is lifted from bottom formwork, contractor should apply curing compound immediately at the bottom part of the girder.
De-tensioning:
After the concrete gain minimum 85% of its actual strength, contractor can start de-tensioning work.
Pre-stressing end: use 2 sandboxes for De-tensioning.
Dead end: use pre-stressing Screw-thread steel bars and nuts to De-tensioning
After De-tensioning, the contractor should use grinding machine to cut the strand along the end face of the T-girder from pre-stressing end to a dead-end. For cutting sequence of strands, follow the below fig.
End sealing:
After De-tensioning, contractor should seal the both ends of the girder with approved repair material and proper repairing method.
Finishing and Marking:
The Contractor should print the casting date, reference number, span length, and project name on both ends of the T-Girder.
Reference:
- British Standard BS4449:2005 for reinforcement grade
- AASHTO M203-90 (ASTM A416-86 Grade 270) for pre-stressing strand quality
- ASTM A 370 for strand elongation test
- DEEP/MS-016
