Category:625 Slab Stabilization
625.1 Undersealing
Undersealing Portland cement concrete (PCC) pavements with either high density polyurethane or hot asphalt cement is a preservation treatment that prolongs pavement life by filling subsurface voids and eliminating slab movement. Undersealing may also correct slight faulting (≤ 1/8 in.) at transverse joints and working cracks, but is not intended to correct moderate to severe faulting. Diamond grinding, or if the condition warrants, an asphalt overlay, is the proper treatment to restore surface ride and should normally be combined with undersealing in any contract. Undersealing is usually reserved for bare PCC pavements. In unusual circumstances, a composite pavement (HMA overlay on PCC) may be undersealed if an analysis by the Pavement Section determines it is the optimum treatment for restoration. Care is taken to provide uniform material coverage under the slab so that larger voids are not created and the problem is instead made worse.
625.1.1 Project Scoping and Design
The designer should coordinate with the Pavement Section to determine the need for undersealing. Telltale signs of subsurface voids include faulting, pumping of water and fines, and visually discernable movement under truck loads. The optimum time to perform this evaluation is when the pavement surface dries after a heavy rain. A fractured slab, with more than two discrete pieces comprising the original monolithic panel, is normally not an undersealing candidate and should instead be replaced.
When undersealing locations are difficult to ascertain visually, quantities can be estimated with the falling weight deflectometer (FWD). The FWD simulates a truck axle and measures load transfer between slabs and maximum edge deflections. FWD testing should normally be conducted during cooler weather (≤ 70° F), when slab lengths have contracted enough to avoid joint lockup, but also when the subgrade is still thawed. The designer can schedule the FWD through the Pavement Section.
Estimating the number of gallons of asphalt or pounds of high density polyurethane per PCC slab is not an exact science; however, through the experience of many previous projects in Missouri and other States, a reliable formula under normal conditions for both materials has been developed. The designer should assume 66 pounds of high density polyurethane and 64 gallons of asphalt will be required per transverse joint or working crack. Drilled holes are not a pay item and are incidental to the cost of the undersealing material.
Deflection testing can be included in the contract as a pay item at the discretion of the designer under advisement from the Pavement Section. Deflection testing before undersealing establishes the need at a slab location (assuming it had not been predetermined with the FWD) and after undersealing verifies the improvement. The testing is performed in accordance with MoDOT TM-64, which requires an 18K pound truck axle placed adjacent to a joint or crack and then again on the opposite side of the joint or crack. During each load a dial gauge resting on the shoulder measures the deflections on both sides of the joint or crack. The inspector should be careful to note that the shoulder itself is not moving, which would nullify the slab measurements.
The contract should be set up with alternate sections for bid items allowing the bidder to choose between "Undersealing Material High Density Polyurethane" and "Undersealing Material Asphalt Cement". The Job Special Provision Alternates for Slab Stabilization JSP-09-05 must also be used when the contract has alternate bid items for slab stabilization.
625.1.2 Construction Inspection for Sec 625.10 Slab Undersealing
The current undersealing specification is driven by performance results, i.e. increased slab support, therefore the contractor is responsible for determining and providing the appropriate personnel and equipment. The MoDOT inspector should be primarily concerned with the following concerns:
1. Ensuring safe conditions are maintained, especially with hot asphalt.
2. The drilling pattern submitted by the contractor (See Sec 625) for approval being followed, unless sufficient reason is provided for deviation.
3. No undersealing material being allowed to enter drains and utilities or come within the specified distance of standing or running water resources.
4. Passing traffic not being splattered.
5. The undersealing operations at each slab location ceasing when seepage is detected at observation holes and around the slab perimeter, and before too much slab movement (> 1/8 in.) occurs.
6. Holes being plugged or cleaned after completion of undersealing.
The drill patterns for polyurethane and asphalt are different – typically asphalt requires a few large diameter (approximately 1 - 1/2 in.) holes, while high density polyurethane uses more frequent small diameter (approximately 5/8 in.) holes. The patterns will also vary depending on whether just the slab end is being stabilized or the intent is to underseal the entire slab. Not all holes need necessarily be filled; some may simply be used to observe seepage and confirm material coverage.
625.2 Slab Jacking
The process of raising PCC pavement slabs and filling subsurface voids with cementitious grout or high density polyurethane is known as "slab jacking". Slab jacking is typically not used for mainline pavement, but rather at bridge approach slabs which have experienced significant settlement (see Mud Jacking Bridge Approach.
625.2.1 Project Scoping and Design
This treatment should only be used only in select cases. Typically, it should only be used to correct moderate to severe faulting (> 1/4") on mainline PCC pavement which cannot be diamond ground without compromising the pavement’s structural integrity, or in the case of adjoining slab ends that have settled and formed an unpleasant dip in the riding surface. Most pavements in this condition would be overlaid. The designer should first consult with the Pavement Section about the appropriateness of this treatment. As with undersealing, fractured slabs are not slab jacking candidates.
In past decades, MoDOT maintenance forces have logged many miles of slab jacking; however, the treatment has become something of a lost art for most employees, either because of retirement turnover or bad experiences with slab failures from overjacking. Therefore, in nearly all cases the designer should depend on contract work to perform a slab jacking operation. Both cementitious grout, which is split out into Portland cement and flyash pay items, and high density polyurethane are paid for by the pound. Drilled holes are incidental and included in the material prices.
The contract should be set up with alternate sections for bid items allowing the bidder to choose between "Slab Jacking Material High Density Polyurethane" and "Slab Jacking Material Cementitious Grout". The Job Special Provision Alternates for Slab Stabilization JSP-09-05 must also be used when the contract has alternate bid items for slab stabilization.
625.2.2 Construction Inspection for Sec 625.20 Slab Jacking
As with undersealing, MoDOT is concerned primarily about the performance of slab jacking and allows the contractor to determine and provide the right work crew and equipment, which would vary significantly between the two types of materials. The MoDOT inspector should be primarily concerned with:
1. Checking that the hole pattern, which the contractor should have submitted to the Engineer at least seven days prior to the start of operations, is laid out in a way to avoid damaging the structural integrity of the existing pavement. A correctly spaced hole pattern will have overlapping zones of material.
2. Observing the extent of material coverage in observation holes or at adjacent joints and cracks, at the beginning of the slab jacking work, to verify that the hole pattern is sufficiently tight. This will help ensure, especially with cementitious grout, that columns don’t form under drilled holes to create even larger voids under the pavement that lead to slab cracking.
3. No undersealing material being allowed to enter drains and utilities or come within the specified distance of standing or running water resources
4. Checking the rate of lift. Generally, a slab should not be raised more than 1/4" during any single time at a hole location, or else the risk of cracking becomes too great.
5. Checking each joint with a straight edge after dip or faulting corrections. Laser levels or dial indicator devices should also be used by the contractor to monitor and verify that the pavement is raised to an even plane and to the required elevation.
6. Holes being plugged or cleaned after completion of undersealing.