Purpose of Subsealing
The purpose of subsealing is to stabilize the pavement slab by the pressurized injection of a cement grout through holes drilled in the slab. The cement grout will, without raising the slab, fill the voids under it, displace water from the voids, and reduce the damaging pumping action caused by excessive pavement deflections. To ensure that the slab is not raised, straightedges with gauges attached should be placed over the slab to measure any upward movement of the slab. At the first indication of movement, the grout injecting procedure should be stopped.
A thorough survey should determine the void locations beneath the concrete pavement. Void detection measurements should be taken during the preliminary evaluation and during the repair process. Void detection can be a complicated process as natural wetting and drying cycles and thermal variations can cause slab curling. Several suggested methods follow, but interpretation of field conditions by experienced personnel is always desirable. Several methods of void detection are in use. Perhaps the simplest is a visual inspection of the pavement to locate areas of distress. The presence of ejected subgrade or base material, staining of pavement surfaces adjacent to joints, vertical movement at joints or cracks, and faulting of joints are evidence of possible voids under the slab. The most common method of determining the presence of voids is called “proof rolling.” This is the procedure of slowly driving a heavily loaded vehicle (minimum 18,000-pound (80-kilonewton) axle load) over a transverse joint while observing deflection of the slabs. If deflection can be visually observed, the joint should be undersealed. Deflection can also be measured by devices equipped with sensitive dial gauges which contact the pavement and are attached to a firm base located off the pavement. The dial gauges can be read visually or recorded electrically. When deflection is measured in this way, any slab showing deflection in excess of 0.015 inch should be undersealed. Other methods for measuring deflection to locate voids include non destructive equipment such as the Falling Weight Deflectometer, which measures the deflection response of the pavement under a dynamic load
Need for Subsealing
For jointed concrete, pavement subsealing should be accomplished as soon as significant loss of support is detected at slab corners. Symptoms of loss of support include increased deflections, transverse joint faulting, corner breaks, and the accumulation of fines in or near joints or cracks on traffic lanes or shoulders. Subsealing should also be considered at all existing repairs that show evidence of pumping or settlement. To be effective, subsealing should be performed before the voids become so large that they cause pavement failure.
Hole patterns for effective distribution of cement grout under the pavement are not easily determined in advance. Some preliminary testing is often necessary in advance to locate holes in a way that will ensure good grout distribution. Where a hole pattern is selected for repetition, it should provide sufficient holes to permit grout to reach all voids beneath the pavement. The most common hole pattern is a four hole pattern with two holes on each side of a transverse joint. The holes are located in the wheel tracks, with the approach slab holes nearer the joint than the leave slab holes. Typical distances from the joint are 12 to 18 inches for the approach slab and 18 to 24 inches for the leave Slab. Additional holes may be required for voids under the longitudinal joints or at the shoulder.
Usually, one hole that is 24 to 36 inches from the shoulder and 4 to 6 feet from the transverse joint is adequate. Ideally, the hole should be placed as far from the adjacent joints and cracks as possible but within the void area, so the grout can flow from the injection hole toward the joint or crack.
Grout holes may be drilled with pneumatic, hydraulic, or diamond core drills. An important factor is hole size. Holes should not be larger than 2 inches in diameter. The downward pressure, whether by hand or mechanical means, should be less than 200 pounds (91 kilograms), particularly at the bottom portion of the slab. Excessive down pressure can cause breakout of the concrete adjacent to the injection hole. This breakout can seriously weaken the slab and may result in premature cracking. Usually, the breakout material drops in such a way that it seals the hole thereby prevent the grout from reaching the void. The grout holes should be drilled vertically and round and to a depth completely through the base material. Grout holes should not be left ungrouted overnight and preferably should be grouted within 4 hours.
Grout mixtures for subsealing are typically cement grouts consisting of approximately one part Portland cement to three parts pozzolan, either natural or artificial, or three parts limestone dust with enough water to achieve the required consistency. Other additives may include super plasticizers, water reducers, fluidifiers, expanding agents, and calcium chloride. Each must be tested and evaluated in the laboratory to ensure compatibility of the materials. Consistency should be checked by a flow cone at least twice each day. Flow cone time varies between 9 and 20 seconds depending on the types of materials used in the grout mix. Typically flow cone times for limestone grouts are 16 to 22 seconds. Flyash grouts generally have flow times from 10 to 16 seconds. Strength requirements of the grout mixtures should be specified.
Grout injection proceeds by lowering into successive holes a pipe connected to the discharge hose of the grout pump. The grout hole is sealed by a device called a packer. Two types are commonly used:
1. The drive packer consists of a tapered pipe tapped into and out of the grout hole. Drive packers are used with 1-inch- diameter holes.
2. The expanding rubber packer consists of a threaded inner pipe, a thin-walled steel outer sleeve, and a short rubber sleeve at the bottom. This type of packer is used with 1.5-inch- diameter and larger holes.
Movement of the slabs must be monitored during the grouting operation. To properly monitor movement of the slabs, gauges capable of reading movement of 0.001 inch have to be used. The base for the gauge should be 3 to 4 feet off the slab being monitored. The gauges are set up at the outside edge of the slabs at the joints and are not moved until grouting of the joint is completed. Typical pumping pressure should be in the 40- to 60-pounds per square inch (275- to 413-kilopascals) range. Grout injection should always start with a low pumping rate and pressure. Pumping should stop if the slab begins to rise or when no material is being injected at the maximum allowable pressure of 100 pounds per square inch
Pumping of short surges up to 200 pounds per square inch (1,378 kilopascals) are allowable for the grout to penetrate the void structure. If grout returns through an adjacent hole, pumping should stop, and the packer should be inserted into another hole. If grout is observed flowing from joints or cracks in the pavement, pumping should continue until undiluted grout is observed. Generally when pumping the four-hole pattern, pumping should begin at the centreline holes in each slab first and then continue with the holes closest to the shoulder. This sequence will drive any trapped water to the outside of the slab and through the transverse and shoulder joints. Where there is an additional shoulder void and extra holes are required, the sequence becomes More complicated. Usually, the shoulder joint is pumped last. If, however, the transverse joint is Wider than the shoulder joint, it may be necessary to pump the shoulder hole first.
Retesting Slab Corners.
After a minimum of 24 hours has elapsed following completion of subsealing, testing of the grouted slabs for stability should be accomplished at the same points as previously tested. This testing should also include other joints that were not grouted for use as control. If loss of support still exists after grouting, the slab should be regrouted. In each regrouting, new holes will be needed. It is recommended that if, after three attempts to stabilize the slab, voids are still present, no further regrouting should be attempted. Other methods of repair should then be considered, such as full-depth repair.
Plugging and Cleanup
After grouting has been completed at any one hole, the packer is removed and the hole is plugged with tapered wooden plugs to permit the grout to set, thus preventing back pressure from forcing the grout back through the hole. The plugs are removed and the hole is filled with a cement grout and finished to match the existing pavement. Surfaces of the pavement adjacent to the holes should be kept clean of excess grout and other materials. Grout and cement slurry on the pavement should be broomed and washed to avoid unsightly discoloration and to remove the grout and slurry before it bonds to the pavement.