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Applications & Innovations
Posted By admin On January 9, 2013 @ 3:58 pm In Applications & Innovations,In the Magazine | No Comments
By Lauren Heartsill Dowdle
Whether it’s patching a hairline crack or completely rebuilding an area with a crater-sized pothole, repairing concrete roads early on can minimize future damage and the time required for the job.
The repairer’s success with the work depends not only on the quality of the patching work itself but also on his pre-patching assessment and choice of solution. Figuring out the cause of the pavement deterioration can help determine what type of repair is best suited for the project.
“Know what kind of repair you need to make and then follow through with it,” says Dale Harrington, senior principal engineer, Snyder and Associates/National Concrete Pavement Technology Center. “You can’t just jump into it.”
If a crack occurs while working on a current project, it may be easier for the crew to diagnose the problem by retracing their steps. Was there a sudden change in the temperature? Did it rain? Did they forget to cover the surface? Or, was there a change in materials, saw timing or subgrade with the cracked section? Volume changing, restraint, curling, warping, strength, stiffness, base condition and early loadings are also factors that can cause early stage cracking.
It’s possible for concrete to go 20 to 30 years without needing any renovation, so for existing pavement, the direct cause may be unknown – it could, for example, range from load-associated issues to long-term drying shrinkage. But no matter the reason, the road deterioration symptoms are universal. Once diagnosed, the repairer can set a plan of action.
In many jobs, the deterioration will require a combination of techniques to become a smooth, sturdy pavement. Knowing what signs to look for and how to best treat the area will help road contractors achieve the best long-term results. “The types of repairs range from slab stabilization to joint-crack resealing,” Harrington says. “If multiple repairs are required, the sequence of applications needs to be understood and followed.”
Let’s dicuss eight popular types of concrete repair.
1. Full-Depth Repair
For major distress pavement areas, full-depth repairs can provide quality performance for 10 to 15 years if crews use the right materials. These repairs normally consist of cast-in-place concrete that replaces the full depth of the existing slab, or at least a portion of a slab to the bottom of the concrete.
This technique can be used where utility cuts are needed in the roadway, to restore problem areas at airports or at deteriorated cracks and joints on highways.
One of the most common types, full-depth repairs are usually a full lane-width wide, say12 feet, and 4 to 6 feet long, says the American Concrete Pavement Association.
Key techniques for effective full-depth repairs include having adequate working room (a minimum of 6 feet longitudinally), a uniform/stable platform, full-depth perimeter saw cuts, a longitudinal strike-off and being prepared for changes. The success of this repair also depends on installing it at the proper time with the right design.
If the slab could disintegrate when lifting it, crews should remove the patch area with a backhoe, angle the lift pins, position the cables and keep the lift height to a minimum.
2. Partial-Depth Repair
When only a small area of deteriorated concrete needs to be removed and replaced, partial-depth repair may be the best option. This method is traditionally used where joint or crack deterioration is in the top third of the slab (if it extends below this, usually a full-depth repair is necessary), Harrington says.
There are three types of partial-depth repair: spot repair, long joint/crack repair and bottom-half repair. Ranging from 10 inches to 6 feet in length, spot repairs can be used where existing load-transfer devices are still functional. The saw-and-chip method is the most common, but milling is rising in popularity in some areas, according to Daniel Frentress, Frentress Enterprises.
With cracks longer than 6 feet, a long joint/crack repair would be best and is usually done with milling. Workers can use a small jackhammer for the rest of the patch where the mill can’t reach. If the deterioration spreads to the full depth at a short distance, a bottom-half repair can be used on the edges or cross-joint locations that are not longer than 18 inches.
Steps for partial-depth repair include determining the repair boundaries, removing the concrete, preparing the area by sandblasting or air blasting, placing a strip of polystyrene or polyethylene compressible material in the joint, coating it with grout, adding patch material, applying curing compound and, finally, sealing the joint.
Longitudinal cracks that are in a moderate or fair condition can be repaired with cross-stitching or slot-stitching. With these methods, crews use tie bars to stop the crack from widening vertically and horizontally. Cross-stitching creates a less-exposed surface area and is less intrusive to the slab, compared to slot-stitching. Neither method is intended for severely deteriorated or transverse cracks.
If contractors stitch a transverse crack that serves as an adjacent joint, the stitching will not allow the transverse joint to move. This can cause a new crack to form near the stitched working crack. The concrete may also spall over the reinforcing bars. For less-deteriorated cracks, sealing and maintaining the crack may be enough to repair the road.
4. Joint Sealing
Well-constructed joints can prevent random cracks and prevent faulting, so repairing them is important for the overall road success. “Concrete repairs have a lot to do with joints,” Harrington says.
Sealing joints leads to proper joint expansion and contraction and can reduce moisture amounts. This technique also prevents the intrusion of incompressible materials that can cause pressure-related distress such as spalling, buckling and slab shattering.
What material the road contractor selects should be based on their experience level, the traffic level, crack characteristics, climate conditions and cost concerns.
Crews should only place joint sealer when the surface is dry. The sealer is sensitive to temperature, so crews should place it when the pavement and temperature is 40 degrees Fahrenheit or higher.
If the poured joint sealant does not adhere, water or materials can enter the joint and cause poor performance. Problems can also arise if there are poor sealant properties due to over/under heating, the sealant is not sized properly or the joint is too wide.
5. Slab Stabilization
If repairers choose slab stabilization (undersealing) as the mode for repair, they should make sure the joints and working cracks have lost support. If this is not the case, the stabilization is not only wasteful; it could also damage the pavement’s performance.
For the best results, the stabilization must be performed prior to the onset of pavement damage due to support loss. However, it is important to address the underlying mechanisms, as well, to provide long-term results. Crews should use materials that can penetrate into thin voids, while being strong and durable enough to withstand external pressures.
Another issue to be on the watch out for with this type of repair is the ability to identify voids underneath the slab. If crews perform stabilization where there is no void, pumping material beneath the slab can cause stress points and increase the concrete’s deterioration.
6. Slab Jacking
Slab jacking differs in that it consists of the pressure insertion of a grout material beneath the slab to slowly raise the slab until it has a smooth profile.
With this method, it is important for the crew to monitor the amount of lift being performed. The slab should not be lifted more than 1/4 inch at a time to cut down on excessive stress. Materials used for slab jacking, such as cement grout and polyurethanes, are normally stiffer than those for stabilization.
7. Dowel Bar Retrofit
A mode of reinforcing cracks in pavement, dowel bar retrofit (DBR) consists of inserting steel dowel bars in slots that are cut across the cracks. It is important that the slots do not cross over any parallel cracks.
To prevent corner cracking from slots cut too deep or patching-material failure, crews should measure the dowel bar retrofit assembly height to guarantee proper saw-cut depth. The dowel-bar-slot width must also be checked to make sure the chairs and foam core board fit comfortably within the sides of the slot.
Crews should remove the existing concrete from the slot using jackhammers weighing 30 pounds or less, and the jackhammer should be operated at a 45-degree angle or less, says the Washington State Department of Transportation. If held in a vertical position, the jackhammer could go through the bottom of the slot.
When the dowel bar slots are finished, workers need to make sure all of the water and debris are removed before they add the patching materials. Transverse joints and cracks must be caulked with the slot, and the caulking material must extend more than 1/2 inch on either side of the joint or crack.
The dowel bars should be coated with a parting compound before being placed in the slot. To prevent the dowel bar from moving, crews should place the patching material adjacent to the bar, instead of dumping it directly, and spread it toward the slot.
Overworking the patching material during the finishing project can cause segregation and floating of the fine material to the surface of the slot, so crews should leave the material 1/8- to 1/4-inch high. The excess material will be removed during the diamond-grinding process.
8. Diamond Grinding
To smooth bumps, restore friction and reduce tire-pavement noise, workers can turn to diamond grinding. This technique produces grooves and lands in the pavement to correct irregularities such as roughness or faulting.
Structurally deficient pavements should not be ground without having major-distressed sections repaired, according to the Federal Highway Administration (FHWA). It needs to begin and end at lines normal to the pavement centerline and be longitudinal.
Road repair teams should consider the concrete aggregate type, grinding head and equipment size, reducing excessive vibrations and making sure the grinder bogie wheels are true (round). It is also important for them to make sure the match line between passes of the grinder doesn’t coincide with the wheel path, which could cause irregular grinding patterns.
Another aspect of diamond grinding that should be avoided is “holidays” – an unground area on the pavement surface.
To avoid these, crews should not lower the isolated low spots (holidays) that are less than 2.7 square feet, according to the ACPA. If the operation causes an unacceptable amount of holidays, they should lower the grinding head and make another pass.
The probability of a diamond-ground surface lasting eight years before needing another rehabilitation is 98 percent, according to the study The Longevity and Performance of Diamond-Ground Pavements by Sreenath Rao, Thomas Yu and Michael I. Darter in the Portland Cement Association Research and Development Bulletin RD 118.
What caused the cracks?
The problem: Road crews pour concrete pavement on a warm day in October. The temperature drops substantially in the afternoon, and the concrete is not covered. The next day, they see a 600-foot section of roadway has transverse cracks every 12 feet. The cracks occur about every second or third joint. There were no changes in materials, saw timing, the base or subgrade with this section.
What happened: Concrete expands as the temperature rises and contracts as it falls, which can cause early-age cracking when the top of the slab is considerably colder than the bottom of the slab. In this case, there was a sharp drop in the temperature when the concrete was at its warmest. The pavement was expanding during the concrete hydration and heat of the day. After the cold front moved in during the night, the top of the newly poured section was cooling following the final setting. The differential temperature in the concrete from the top to bottom caused movement. This resulted in stress building up in the new section that was greater than its strength, causing the cracks.
The solution: They should have kept the pavement covered for the first night to prevent substantial temperature change throughout the depth of the new pavement. It is also important to saw the joints as soon as possible with an early-entry saw to relieve the stress. When tying lanes, road contractors may also need to start paving later in the day when it is not as hot.
Source: Iowa Department of Transportation, Dale Harrington
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