712 - Bridge Rehabilitation, Concrete
GENERAL
Rehabilitation of bridge concrete includes deck removal, deck patching, post-installed adhesive and mechanical anchors, mechanical reinforcement splicing, rigid overlays, epoxy overlays, substructure patching, joint repair or replacement, crack sealing, crack injection, grouting, and carbon fiber wrapping. These rehabilitation techniques, or any combination of them, are used to improve the existing condition, and increase the service of the bridge. The MDOT Project Scoping Manual offers excellent guidance on the method of determining the rehabilitation techniques.
Weather Limitations
See Section 712 of the Standard Specifications for Construction for Cold and Hot Weather Limitations.
MATERIALS
-Reserved-
CONSTRUCTION
Bridge Deck Overlays
Existing deck elevations will be taken to establish grades for the overlay course.
Preparation of new decks for the overlay involves abrasive or water blasting to remove any laitence or foreign substance and to promote a good bond to the concrete. Placing and curing of the overlay is similar to that described below for existing decks.
Deck Removal and Deck Patching
Deck patching is recommended when less than 10 percent of the bridge deck area is indicative of spalling or delaminations, and is typically not shown on the plans, but rather quantified in contract pay items. Deck patching involves sounding and marking areas of concrete removal, saw cutting the boundaries, hand chipping shallow or hand chipping deep, patch preparation and placing of the new concrete. Deck patching must be conducted in accordance with section 712 of the Standard Specifications for Construction.
- Saw Cutting
The Engineer will determine the areas to be patched in accordance with the plans. Ensure the saw cut perimeter extends to sound concrete to ensure all delaminated concrete is removed. Patches spaced at less than 1 foot must be combined into larger patches. After the locations to be patched are marked the perimeter of the patch should be saw cut to a minimum depth as shown on the plans, typically 1”. Extend saw cuts 1” beyond intersecting lines. Ensure the saw cutting operation does not cut the deck reinforcement. The saw cutting should be performed to “square” the patch as best as practicable. A square or rectangular patch is less susceptible to shrinkage cracking than patches with sharp angles or many corners. See Figure 712.1 for an example of an acceptable saw cut and chipping operation.
Saw cutting is also used during deck removal operations to free the deck from the shear connectors in the top flange of the beams. The beam locations should be marked on the top of the deck. When saw cutting over steel or prestressed concrete I-beams, the depth of saw cutting must be no greater than 90 percent of the deck thickness. Do not saw cut over spread box beams or 1800 girders, rather saw cut in between beams and use a pneumatic hammer for concrete removal over tops of beams. Carefully inspect the beams or girders after deck removal, and notify the Engineer of any saw cuts in the top flanges. Saw cuts equal to or less than 1/32 inch deep in steel beams must be repaired by grinding, to a surface roughness no greater than 125 micro-inches per inch rms, and tapering to the original surface using a 1:10 slope. Saw cuts in excess of 1/32 inch deep in steel beams require a welded repair to be submitted to the Engineer for approval. Ensure all welds are in accordance with subsection 707.03.D.8 of the Standard Specifications for Construction and that adequate notice is provided to allow the Engineer to witness the repair work. Inspect and test all saw cut repairs (including grinding repairs) using ultrasonic testing in accordance with subsection 707.03.D.8.c of the Standard Specifications for Construction. This should be done at no additional cost to the Department.
- Hand Chipping
Hand chipping of bridge decks must be done in accordance with section 712.03.F.1 of the Standard Specifications for Construction. Hand chipping is conducted with pneumatic hammers, air chisels, scarifying machines, milling machines, or other methods approved by the Engineer. Machine mounted hydraulic or pneumatic equipment may be used for deck patching on structures with steel beams, or on prestressed concrete beam structures, between the beams only. All patches that lie directly above prestressed beam top flanges must be chipped with 60 lbs or less hammers to avoid damaging the beams. For structures consisting of adjacent box beams, spread box beams or 1800 girders, machine mounted equipment is not permitted for deck patch chipping. Ensure all unsound concrete is removed as part of the chipping operation. The chipped area is to be blast cleaned prior to forming. Shallow hand chipping involves chipping concrete from the deck surface to the mid-point of the top mat of steel (approximately 1.5”). This type of patch will utilize a Latex Modified Concrete (LMC) for repair. The LMC does not rely on the mechanical bond around the reinforcing steel like other concrete mixes, rather bond is generated by horizontal friction between the proposed and existing surfaces. Deep hand chipping involves chipping to ¾” below top mat or more (approximately 4” to full depth). This type of patch may be repaired with the LMC; however the special provision for Bridge Deck Surface Repair (7 sack) mix or Grade D may be substituted for the LMC with the Engineer’s approval. For hand chipping concrete other than deck concrete, the concrete should be removed with air hammers or other methods as approved by the Engineer. The perimeters of repairs are to be saw cut to an edge depth of ½ inch, and the concrete is to be removed to a depth of 3 inches measured from the edge of the concrete surface. Ensure concrete is removed to at least ¾ inch below exposed reinforcing steel.
- Patch Preperation
The patch must be free from unsound concrete, loose material and debris. Inspect the edges of the patch for indications that the delaminated area extended beyond the proposed patch area. If necessary extend the size of the patch. See Figure 712.2 for an example of delaminated areas within a patch, meaning the patch should be extended.
Any exposed structural steel (beam tops, diaphragms, etc.) must be cleaned the SSPC-SP3, Power Tool Cleaning standards.
Scarifying Existing Deck
After closing the deck to traffic, all HMA overlays and patches will be removed and cleaned up. The entire deck surface will be scarified to a depth of 1/4 inch (7 mm) or as specified on the plans. The scarification depth should be controlled to prevent deep cutting which will result in an overrun of the overlay material. Areas that cannot be machine scarified are to be hand chipped to remove the required scarification depth. Again, the entire deck surface will be cleaned up.
Areas that are cracked, undercut, or suspicious looking should be checked. Hand chipping hammers may be used for the second pass removal. The deck will be abrasive or water blasted.
Equipment for Hydro-Demolishing
Equipment Description - see Section 712 of the Standard Specifications for Construction.
Placing Latex Modified Concrete (LMC)
The deck must be thoroughly wetted one hour before the pour. All freestanding water should be blown off the deck with oil-free compressed air. Water blasting may be substituted at this time. The deck will be kept damp during the pour.
After a small amount of LMC has been discharged, the material is broomed into the deck leaving a creamy surface. Excess stone from the LMC that has the mortar used up will be removed.
Placement of the overlay should follow the brooming so it does not dry out. A bridge finishing machine will be used. The overlay will be consolidated by the pan vibrator and/or hand-held vibrators. Pay particular attention to consolidation along bulkheads and deep removal areas. After the machine has moved ahead, the surface will be tined and covered with a layer of damp burlap, and then plastic sheeting.
The overlay will be wet cured for 48 hours, then air cured for an additional 48 hours. High-early LMC will be wet cured for 24 hours.
The mobile concrete units will be calibrated every three months by C&T personnel. A copy of the calibration will be carried by each truck. File copies are available from the C&T Bridge Construction Section.
Mixing, Placing, Finishing, and Curing SFMC Concrete Overlays
See Section 712 of the Standard Specifications for Construction for trial batches.
Proportioning and mixing the SFMC overlay shall be by either a ready mixed or central mixed batch plant meeting the requirements of subsection 701.03. Trucks delivering SFMC to the project site must be fully discharged within one hour of charging at the plant.
One hour prior to placing the SFMC overlay, the prepared deck will be wetted with a uniform spray application of water. Water collecting in depressions will be blown out with clean, oil-free compressed air.
SFMC mixture placed in localized areas deeper than 3 inches (75 mm) is to be hand vibrated in front of the finishing machine by drawing a probe vibrator horizontally through the concrete parallel to the transverse reinforcing bars at intervals not greater than 18 inches (450 mm) on center. Should the operator have trouble locating the deep removal areas, the Contractor will devise a system for use such as a deck diagram or markings on the barriers. The Contractor will coordinate this system with the Engineer.
A continuous fog spray of water will be applied to the screeded and finished concrete to prevent net evaporation from the surface of the unhardened concrete. Fogging equipment will be capable of placing a fine mist over the concrete surface without ponding water. Fogging will continue behind the final floating operation until the wet cure system is in place and activated. Fogging shall not be used to apply water to the surface of the concrete to facilitate finishing.
The silica fume concrete overlay will be wet cured for 7 days according to Section 712 of the Standard Specifications for Construction.
Mixing, Placing, Finishing, and Curing Concrete Patches
See Section 712 of the Standard Specifications for Construction.
Substructure Repair
Areas to be patched or filled will be saw cut to a minimum edge depth of 1/2 inch (15 mm), and the concrete unsound or sound, will be removed to a minimum depth of 3 inches (75 mm). If resteel is exposed, chip 3/4 inch (20 mm) beneath the bar. Make sure the area is outlined with a saw cut to insure neat lines and no spall areas. After the concrete removal, blast clean the area to be patched. Forms must be cleaned, have a bond breaker applied, be tight and form neat lines. Place and cure patching material according to specifications.
MEASUREMENT AND PAYMENT
Measuring and Calibration
Prior to casting any overlay mixture, the finishing machine will be passed over the deck to check for plan thickness and the depths recorded on Form 1131 - Bridge Decks - Concrete Depth Measurements (see Figure 706-1).
Concrete Bridge Deck Overlay can be either a Silica Fume Modified Concrete or a Latex Modified Concrete.
Silica Fume Modified Concrete is required for Deep Bridge Deck Overlay Concrete.
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Find an area of good concrete approximately 30 square feet (3 m2). |
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Hydrodemolish approximately 10 square feet (1 m2). |
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Using a straightedge, set it across the area, measure to the mortar line, and adjust depth as necessary. |
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After the proper depth is obtained, the hydrodemolisher is ready for production cutting. |
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After the machine has moved approximately 20 linear feet (6 m), recheck the cut depth and make any necessary adjustments. |
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Now production can continue with spot checking the depth. |
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Record the settings (water pressure, cutting head speed, forward advancement) on the IDR for future reference. |
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During the first pass, the material will be cleaned up. |
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No runoff water will leave the deck until it has been filtered clean. |
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Allow the deck to air dry for one day before sounding the concrete for a second pass. |
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Areas of exposed resteel and deep removal should be sounded for good concrete. Where resteel is debonded, the concrete will be chipped 3/4 inch (20mm) below the steel. |
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Runoff water, filtered or unfiltered, shall not be allowed to enter storm sewers, bridge drainage downspouts or bridge approach downspouts, nor be discharged into surface water floodplains or wetlands. |
Testing of Latex Modified Concrete (LMC)
Slump, air and yield tests will be run on all trucks the first time around. Random testing will be allowed after this. The air test will be run immediately; the slump test after four to five minutes has elapsed. Yield tests will be adjusted by opening or closing the aggregate bins accordingly. New settings will be recorded for further use. No additives can be added for high or low air. If either of these conditions exists, a reblending of the latex is necessary. Out of specification material will be rejected.
Testing of Silca Fume Concrete
Slump and air tests will be taken on the mixture throughout the pour.