Applications of
CFRP-Sika Carbodur
for CPCI girder
strengthening
Hwy 416-Dilworth
Bridge, 2004
Wojciech Remisz, M.Sc., P.Eng., FCSCE
REMISZ Consulting Engineers Ltd., Ottawa, Ontario, Canada
Abstract
Strengthening of a structure may be necessary if increases in loads, changes in structural articulation or intended use occur, and also, after accidental damages. Depending on the location of the structure, access to the repair area, or the time allowed for the repair, several materials and techniques have to be considered. Here we're discussing how carbon fiber reinforced polymers (CFRP) were used to reinforce a severely damaged CPCI 1600 girder of the Highway Bridge, with minimal traffic interruption. Procedures of cutting and removing the girder, estimating the residual strength and the required strengthening are presented based on a very practical, hands-on experience.
1. Introduction
When an oversized load on a transport truck hit the 37m long pre-stressed girder of the two-span Highway Bridge, concrete at the bottom flange broke off, several strands were ruptured, and the web cracked in a parabolic shape.
This girder carried the monolithic barrier wall, which as it was believed, helped to carry the dead loads by shifting up of the neutral axis the whole overhang portion of the deck, and increased the lever arm to the stressing strands in this composite section. The girder was structurally condemned by the Ministry of Transportation Ontario (MTO), and its removal required special consideration. Due to the extent of concrete loss, presumed loss of pre-stress in torn strands and the amount of cracking, the project engineers hired by the MTO considered removing the girder by cutting it into smaller sections. In this scenario, scaffolding frames, used to support the girder during its cutting, would take too much of HWY 416's road clearance, and other options were to be investigated. Also, it was important for the road under this bridge not to be closed to traffic for extended periods of time.
2. A need for repair
We were retained by the general contractor to prepare procedures for the
strengthening and removal of this unsafe girder. Our project criteria were
agreed to as:
1. Reinforce the girder so it will not fall apart during the removal.
2. Remove a piece as long as possible, even a full length using two cranes.
3. Leave as much of the concrete deck on the top flange as possible, as to reduce the cost and time of saw-cutting.
4. Stage the work to absolute minimum road closures.
The author carried out a close examination of the girder and identified seven
pre-stressing strands damaged at 90% (the number of wires cut to the total
number of wires), the bottom flange disintegrated full width and depth over the
length of almost three metres, the parabolic crack of
the web, and the girder's longitudinal axis that bowed from the impact, as
shown in Fig. 1 (before repairs) and Fig. 2 (during repairs).
Following the review of the site conditions and discussions with the contractor regarding the available equipment and crane capacities, we suggested the following procedures:
1. Reinforce the girder so it can carry its self-weight.
2. Remove the barrier wall by saw-cutting.
3. Remove the asphalt and saw-cut the concrete deck slab flush with the top flange, leaving an I-section.
4. Remove the girder in one long piece.
For general arrangement of the damaged girder and its cross section details, see Fig.3.
Figure 3
In order to reduce the number of saw cuts and crane set ups, the general contractor asked
us further to investigate the other steps:
1. Reinforce the girder.
2. Remove the barrier wall.
3. Remove the asphalt but leave the deck slab on the girder, forming a T-section.
4. Remove the girder and deck slab in one long piece.
The initial choice of steel beams or steel plates to be externally applied to the girder, proved too be to cumbersome to handle over the traffic, and required scaffolding with lane closures. Our attention concentrated then on new technology and the use of much lighter high strength materials, known as carbon fiber reinforced polymers. The carbon fiber laminates are of unidirectional composition, have a very high strength in the longitudinal direction of fibers, do not corrode, and are very light. Our initial calculations of girder strengthening for bending and shear were done for SikaWrap. However, since SikaWrap was not readily available in required quantities, and we selected as an alternative Sika Carbodur S1012 plates.
3. Calculations
To get a better feeling of the girder bending moments' capacity under various
removal scenarios, we performed calculations for the following conditions:
1. Girder alone, undamaged.
2. Girder composite with the deck slab, undamaged, as a T-section.
3. Girder damaged, alone, with only the bottom-most layer of pre-stressing strands considered cut and ineffective.
4. Girder damaged, alone, with two bottom layers of pre-stressing strands considered cut and ineffective.
5. Girder damaged, but acting as a composite T-section with 1500 m deck, again with some strands cut and ineffective.
The analysis was necessary in order to determine whether the damaged girder can support its own dead load, and over what span, since it would dictate pick-up points and the whole demolition procedure. Cutting the girder into smaller sections was considered before, but was not viable. Understanding the MTO and contractor's objectives, we tried to find a way to remove the damaged girder in one piece, possibly the full span, and even with the deck slab. Obviously, each case analyzed above would result in a different dead load, different crane setups, scaffolding systems, types and locations of saw cuts, and the number of road closures.
Sika Carbodur
plates S1012 were available, and therefore the calculations were done as per
the new CSA 806 and some provisions of the OHBDC/CSA S6. MathCad
worksheets for I-girder reinforced with Carbodur
plates were prepared, and several cases were then analyzed, as per Table 1,
which illustrates calculated moment resistance.
Table 1: Moment resistance
|
Number of good strands left |
Number of RFP SikaDur S1012 plates And resulting Factored Moment Resistance in kN-m |
|||||
|
2 |
3 |
4 |
5 |
6 |
None |
|
|
25 |
4951 |
|
|
6304 |
|
4037 |
|
30 |
5729 |
|
|
7069 |
|
4825 |
|
32 |
6038 |
|
6956- 7157 |
7372 |
|
5138 |
|
34 |
6346 |
|
7267 |
7672 |
|
5447 |
|
36 |
6650 |
|
|
7973 |
|
5757 |
Notes:
1. Bending Moment Resistance was 6205 kN-m for 1 plate and 36 good strands
2. 1 plate gives 448-477 kN-m extra capacity, on average
3. Required abs. minimum was 4137 kN-m for dead load
The following is an example of the last steps in calculations, where we analyzed the position of the neutral axis of the damaged girder, and strains in concrete after reinforcing with Carbodur plates, following MathCad notations:
(3)
Similarly, we calculated the bond
length as demonstrated below:
(2)
It should be noted that the web was severely cracked, and the arching crack was reaching the bottom flange. The part of the bottom flange in between the cracks was considered as ineffective in developing the bond strength for the Carbodur plates; therefore the bond of Carbodur plates had to be developed beyond these cracks, more towards the abutment and the pier. Given that Ld=required development length, thus the total length of a single plate, Lplate, was equal to:
Lplate = Ld LEFT + distance between the outermost cracks + Ld RIGHT (4)
Finally it was concluded that:
1. The girder could be reinforced and removed in one long piece, using Sika Carbodur S1012 plates, 100mm wide and 1.2mm thick, 25m long, 200 m total
2. The deck slab could be left to form a composite T-section, and help to stabilize the girder
3. The web cracks had to be epoxy injected, with low viscosity resin Kemko 038 Regular IR by Chemco Systems Inc.
4. The bottom flange be repaired around the damaged strands using King cementitious grout.
The results were tabulated and allowed us to make an informed decision on site as to how many strands were absolutely necessary, and how many of them were required to be compensated by Carbodur plates.
4. Preparation and removal procedures
Due consideration was also given to the balancing of the damaged girder cross section. It was observed on site that the web was visibly bowing about the longitudinal girder axis. The front strands were torn, while the backside strands were partially damaged, with a few wires from a few strands cut. This created an eccentric force acting along the bottom flange, and resulted in bowing in the direction of the traffic. We had quite a challenging task to perform: balance the girder cross sectional forces so that during the cutting and removal, the girder does not snap and self-destroy. It is worth mentioning that based on our observations with demolition of similar girders of the Bytown Bridges and on LeBreton Flats bridge in Ottawa, I-girders have relatively little resistance about the vertical Y-Y axis, and must be lifted and transported in upright position.
Therefore, the author with the assistance of one worker using a metal grinder, operating on a scissors lift accessed the damaged girder area and used the following procedure:
· Take a damaged strand to the left hand, and identify corresponding strand on the other side of the vertical axis of symmetry by grabbing it with the right hand.
· Direct the worker to cut so many wires in the right hand strand, as the number of damaged wires in the left hand strand.
· Repeat the same for all damaged strands, wire after wire.
As the wires were cut with a grinder one by one, the effective pre-stressing force was gradually reduced. The resulting gap between strand ends over the damaged exposed and unbonded length close to three meters was approx 25mm.
After the first few strands were completely cut, the other strands were undercut only, leaving only two or three good wires out of seven wires per strand. The objective was to have the same number of continuous wires on the left and on the right hand side of the bottom flange. Prudent engineering judgement and safety on site was exercised during this cutting, and the strands at the two bottom layers were therefore successfully balanced. It was reported that by the next day the girder had already straightened up.
Detailed repair and demolition procedures were prepared considering the sequence of operation, minimum curing times, staging, access for the equipment, placing cranes and dolly. Finally, it was agreed to proceed as follows:
1. Remove the asphalt.
2. Remove the diaphragm at pier and abutment.
3. Place early strength, 30 MPa grout in the form, encasing the damaged area in width and depth, but flush with the bottom flange outline, to provide a smooth and continuous surface for the bonding of Sikadur plates.
4. Remove the forms after min. 12 hrs and allow additional 12 hours min. to air dry.
5. Clean and grind the girder underside, sides and top part of the bottom flange to enhance the bonding with the epoxy paste.
6. Do the epoxy injection of the web-to-top-flange crack above the damaged area.
7. Apply primer and Sikadur 30 epoxy paste resin to the bottom flange and Sika CarboDur S1012 plates, for bending moment enhancement, applying carbon fiber plates over one half of the girder span, leaving the other half installed and supported, but left unbonded. See Fig. 4 and Fig.5. Switch the traffic lanes under the bridge, and the next day bond the other half.
7.1. Four plates were installed on the bottom.
7.2. Two plates on the top of the bottom flange.
7.3. Two plates on the sides of the bottom flange, symmetrically about the damaged and regrouted area.
8.
After the curing of the flexural CFRP strengthening, saw-cut the deck on the
outside, just in front of the barrier wall, and remove barrier wall in
sections.
9. Saw cut the inside part of the deck between girders. While saw-cutting, install stabilizing angle irons crossing the cut and bolted to the girders to provide lateral stability to the outside girder at all times. Total deck width within limits of saw cut didn't exceed 1500mm, and was symmetrical about girder CL. See Fig.6 and Fig.7.
10. Break the deck slab and top flange close to the girder's ends, to allow passage of the lifting chains and slings. See Fig.7.
11. Position the cranes at the controlled and designated places, so that the front outriggers are resting on the solid section-diaphragm
12. Install chains and slings, cranes to take up the initial slack to snug condition, but without lifting effort, "feel the load".
13. Remove the stabilizing angle irons connecting the girders, and complete cutting of diaphragms
14. Lift girder as a large T-section, 37 m long, and place it on a truck-dolly for transportation off site. See Fig.8 and Fig.9.
5. Placing the CFRP-Carbodur plates
The installation of the reinforcing material required surface preparation: grinding small irregularities, priming, and gluing. Light duty access scaffolding was provided, and the unglued parts of the plates were placed inside a protective wooden cover to prevent them from flapping in the wind during the curing period. We arranged the plates at the bottom of the flange for bending moment strengthening, and on the sides, for lateral strengthening and stability, as an extra precaution. During the gluing of the second stage, one plate on the bottom flange was not parallel to the others, but overlapped the next plate over the last three meters. However, it was concluded with confidence that there was sufficient bond length developed by the straight plates beyond the damaged area of the bottom flange, and we left the situation 'as-is'. SikaDur 30 epoxy resin was used for bonding CFRP plates to the girder, and was expected to develop adhesive strength to concrete at more than 2 MPa.
6. Lifting and removing
The two-crane setup was analyzed very carefully, since the total lifting load for the full 37m length of the CPCI-1600 girder with the deck slab, was approaching 97 metric tonnes. The positioning of cranes was checked so that the outrigger point loads were placed over the solid pier or abutment diaphragm. The lifting chain slings in a basket arrangement were placed at three meters from the girder ends, effectively reducing the girder simple span from 37m to 31m. Both crane operators were in constant eye and radio contact, fully synchronized. When the cranes were in a ready position, the last operation was to complete saw cutting of the concrete diaphragms at pier and abutment end, thus freeing the girder. At that time Hwy 416 was closed to traffic and the actual lifting started. See Fig.10. It took only 13 minutes to lift, swing, and place the girder on the truck with dollies. Only 13 minutes of road closure! Before moving off the bridge, the author inspected the girder on the dolly and found it to be nice and straight, with all the Carbodur plates fully bonded, and no new cracks. See Fig.9.
The big T-section was transported safely to the demolition site, in its upright
position, and with substantial time and cost savings.
7. Conclusion
This project has demonstrated that the use of carbon fiber
external reinforcement plates is not excessively complex, it is practical and
safe to work with, and can be installed with minimal disruption to
traffic. It was the first application of
this kind to be carried through in Ontario. The new design code S806 gives
adequate directions, however the design procedures and field application
inspections should be done by experienced engineers. There is quite a potential in using this
innovative technology in bridge rehabilitation, which allows to restore or to
increase its load carrying capacity.