Purpose of Subsealing
Subsealing is a technique for stabilising a pavement slab by injecting a pressurised cement grout via holes punched in the slab. Without lifting the slab, the cement grout will fill voids beneath it, displace water from the voids, and lessen the harmful pumping action caused by severe pavement deflections. Straightedges with gauges connected should be placed over the slab to measure any upward movement of the slab to ensure it is not elevated. The grout injecting technique should be terminated at the first sign of movement.
Void Detection
The positions of the voids beneath the concrete pavement should be determined by a detailed survey. Measurements for void detection should be made both during the preliminary evaluation and the repair process. Natural wetting and drying cycles, as well as thermal changes, can produce slab curling, making void detection a difficult task. Several strategies are given, however it is usually preferable to have experienced staff interpret field conditions. There are several void detecting technologies in use. A visual evaluation of the pavement to find areas of concern is perhaps the simplest method. Evidence of probable voids under the slab includes ejected subgrade or base material, staining of pavement surfaces near to joints, vertical movement at joints or fissures, and joint faulting. “Proof rolling” is the most popular way of determining the presence of voids. This is the process of carefully driving a heavily laden vehicle (minimum 18,000 pound (80 kilonewton) over a transverse junction while watching the slabs deflect. The joint should be undersealed if deflection can be seen visibly. Devices with sensitive dial gauges that contact the pavement and are attached to a firm base positioned off the pavement can also be used to measure Deflection. The dial gauges can be visually read or electrically recorded. When deflection is measured this way, any slab with more than 0.015 inch of deflection should be undersealed. Non-destructive equipment like the Falling Weight Deflectometer can also be used to measure deflection to find voids.
Need for Subsealing
Pavement subsealing for jointed concrete should be completed as soon as considerable loss of support is observed at slab corners. Increased deflections, transverse joint faulting, corner breaks, and the accumulation of fines in or near joints or cracks on traffic lanes or shoulders are all signs of loss of support. All current repairs that show evidence of pumping or settlement should be examined for subsealing. Subsealing should be done before the voids get large enough to cause pavement failure for it to be successful.
Hole Patterns
Hole patterns for appropriate cement grout distribution under the pavement are difficult to predict ahead of time. In order to position holes in a way that would assure good grout distribution, some preliminary testing is typically required ahead of time. When choosing a hole pattern for repetition, make sure there are enough holes for grout to reach all voids beneath the pavement. A four-hole pattern with two holes on either side of a transverse joint is the most prevalent hole layout. The approach slab holes are closer to the joint than the leave slab holes, and they are positioned in the wheel tracks. The approach slab is typically 12 to 18 inches from the junction, while the leave slab is 18 to 24 inches. For vacancies under the longitudinal joints or at the shoulder, additional holes may be necessary.
One hole, 24 to 36 inches from the shoulder and 4 to 6 feet from the transverse joint, is typically sufficient. In order for the grout to flow from the injection hole toward the joint or crack, it should be located as far away from the neighbouring joints and cracks as possible yet within the void region.
Drilling Holes
Pneumatic, hydraulic, or diamond core drills can be used to drill grout holes. The size of the hole is a significant consideration. The diameter of the hole should not exceed 2 inches. The downward pressure, whether applied by hand or by machine, should be less than 200 pounds (91 kilogrammes), especially near the slab’s bottom. Excessive down pressure might cause concrete near the injection hole to split up. This cracking can substantially weaken the slab and cause it to crack prematurely. The breakout material usually falls in such a way that it plugs the hole, preventing the grout from reaching the vacuum. The grout holes should be drilled vertically, spherical, and deep enough to penetrate the foundation material fully. Grout holes should not be filled in. Grout holes should not be left uncovered overnight and should be grouted as soon as possible.
Grout Mixtures
Cement grouts for subsealing are commonly made up of one part Portland cement to three parts pozzolan, either natural or manufactured, or three parts limestone dust, mixed with enough water to achieve the desired consistency. Superplasticizers, water reducers, fluidifiers, expanding agents, and calcium chloride are examples of other additions. To ensure material compatibility, each must be examined and analysed in the laboratory. A flow cone should be examined for consistency at least twice a day. The time it takes for the flow cone to form ranges between 9 and 20 seconds, depending on the components used in the grout mix. Limestone grouts typically have flow cone durations of 16 to 22 seconds. Flow times for flyash grouts typically range from 10 to 16 seconds. The grout mixtures’ strength criteria should be mentioned.
Grout Injection
The grout is injected by lowering a pipe attached to the grout pump’s output hose into successive holes. A device known as a packer is used to close the grout hole. There are two varieties that are often used:
1. A tapered pipe is tapped into and out of the grout hole to form the drive packer. With 1-inch-diameter holes, drive packers are employed.
2. A threaded inner pipe, a thin-walled steel outer sleeve, and a short rubber sleeve at the bottom make up the expanding rubber packer. With 1.5-inch-diameter and bigger holes, this type of packer is employed.
During the grouting process, the movement of the slabs must be monitored. Gauges capable of reading movement of 0.001 inch must be utilised to effectively monitor slab movement. The gauge’s base should be 3 to 4 feet away from the slab to be monitored. The gauges are placed around the outside edge of the slabs at the joints and are not moved until the junction has been grouted. Pumping pressure should be in the range of 40 to 60 pounds per square inch. Always start with a low pumping rate and pressure while injecting grout. If the slab begins to rise or no material is being injected at the maximum permissible pressure of 100 pounds per square inch, the pumping should be turned off.
The grout can enter the void structure with brief surges of up to 200 pounds per square inch (1,378 kilopascals). Pumping should be stopped if grout returns via a neighbouring hole, and the packer should be fitted into another hole. Pumping should be continued until undiluted grout can be seen oozing from joints or fissures in the pavement. Pumping the four-hole pattern should generally begin at the centreline holes in each slab and then progress to the holes closest to the shoulder. Any trapped water will be forced to the exterior of the slab and through the transverse and shoulder joints by this sequence. Extra holes are necessary where there is an additional shoulder void. As the series progresses, it becomes more difficult. The shoulder joint is usually the last to be pumped. If the transverse joint is wider than the shoulder joint, however, the shoulder hole may need to be pumped first.
Retesting Slab Corners
After a minimum of 24 hours has passed since the subsealing was completed, the grouted slabs should be evaluated for stability at the same locations as before. Other joints that were not grouted for use as controls should also be included in this testing. If there is still a loss of support after grouting, the slab should be rerouted. New holes will be required for each regrouting. If voids remain after three attempts to stabilise the slab, it is suggested that no further regrouting be tried. Other repair options should be considered.
Plugging and Cleanup
The packer is removed after each hole has been grouted, and the hole is closed with tapered wooden plugs to allow the grout to set and prevent back pressure from pulling the grout back through the hole. After removing the plugs, the hole is filled with cement grout and finished to match the existing pavement. Excess grout and other materials should be removed from the surfaces of the pavement near to the holes. To avoid unattractive discoloration and to remove the grout and slurry before it adheres to the pavement, broomed and cleaned the grout and slurry on the pavement.
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