🕑 Reading time: 1 minute
SLABJACKING
The purpose of slab jacking is to raise a slab in place permanently, prevent impact loading, correct faulty drainage, and prevent pumping at transverse joints by injection of a grout under the slab. The grout fills voids under the slab, thereby restoring uniform support when necessary; it can also be used to raise the slab. This work must be done by an experienced contractor due to work complexity and specialized equipment required.
Need for Slabjacking
Slabjacking should be considered for any condition that causes non uniform slab support, such as embankment settlement, settlement of approach slabs, settlement over culverts or utility cuts, voids under the pavements, differences in elevation of adjacent pavements, joints in concrete pavements that are moving or expelling water or soil fines, and pavement slabs that rock or teeter under traffic.
Location of Injection Holes
Location of injection holes must be determined in the field. The jacking crew superintendent normally locates the holes and must take into consideration the size or length of the pavement area to be raised, the elevation difference, sub grade and drainage conditions, location of joints or cracks, and the manner in which the slabs will be tilted or raised. As a general rule, holes should not be placed less than 12 inches or more than 18 inches from a transverse joint or slab edge. The holes should not be placed more than 6 feet center to center, so that not more than approximately 25 to 30 square feet of slab is raised by pumping any one hole. Additional holes may be required if the slab is cracked. The proper location of holes varies according to the defect to be corrected. For slab jacking a joint where faulting has not yet occurred, a minimum of two holes can be used. For slab jacking a joint where one corner of the slab has faulted, the hole at the low corner should be set back to avoid raising the adjacent slab. Where the pavement has settled and the slabs are in contact with the sub base, a single hole located in the middle of the panel may be sufficient. 
Drilling Holes
Holes that are 1-1/4 to 2 inches in diameter are drilled by pneumatic drills, core drills, or other devices which are capable of drilling grout injection holes through the concrete pavement and the base material. The equipment must be in good condition and operated in such a manner that the holes are vertical and round. The down feed pressure, whether by hand or mechanical means should not exceed 200 pounds per square inch (1,379 kilopascals). Where the concrete pavement is tight against the base material, the use of an airline or blow pipe may be necessary to form a cavity under the pavement slab for the grout pressure to take effect. Where the pavement is placed and bonded to cement treated or other stabilized base material, grout holes should be drilled completely through the base material. The grout should be injected below the base material rather than between the pavement and base material.
Grout Mixtures.
A variety of grout mixtures have been successfully used for slabjacking. They generally consist of three to seven parts fine aggregates or a mixture of aggregate and pozzolans or flyash to one part Portland cement with enough water to produce the desired consistency. Wetting agents or other additives may also be used to increase the flowability. The use of a wetting agent lubricates the grout and permits runs of up to 6 feet (, and it also tends to reduce “pyramiding” (a stiff grout may form a pyramid under the slab, leaving unfilled cavities). A definite method of proportioning the grout mixture should be used to ensure uniform consistency. The proper consistency to be used for any given condition is best determined by experience. Generally, a mix of stiff consistency is used to raise the pavement slabs and a more fluid mix is used for filling voids. The consistency should be checked by a flow faulted, the hole at the low corner should be set back to avoid raising the adjacent slab. Where the pavement has settled and the slabs are in contact with the sub base, a single hole located in the middle of the panel may be sufficient.
Drilling Holes
Holes that are 1-1/4 to 2 inches in diameter are drilled by pneumatic drills, core drills, or other devices which are capable of drilling grout injection holes through the concrete pavement and the base material. The equipment must be in good condition and operated in such a manner that the holes are vertical and round. The down feed pressure, whether by hand or mechanical means should not exceed 200 pounds per square inch (1,379 kilopascals). Where the concrete pavement is tight against the base material, the use of an airline or blow pipe may be necessary to form a cavity under the pavement slab for the grout pressure to take effect. Where the pavement is placed and bonded to cement treated or other stabilized base material, grout holes should be drilled completely through the base material. The grout should be injected below the base material rather than between the pavement and base material.
Grout Mixtures
A variety of grout mixtures have been successfully used for slabjacking. They generally consist of three to seven parts fine aggregates or a mixture of aggregate and pozzolans or flyash to one part Portland cement with enough water to produce the desired consistency. Wetting agents or other additives may also be used to increase the flowability. The use of a wetting agent lubricates the grout and permits runs of up to 6 feet (1.8 meters), and it also tends to reduce “pyramiding” (a stiff grout may form a pyramid under the slab, leaving unfilled cavities). A definite method of proportioning the grout mixture should be used to ensure uniform consistency. The proper consistency to be used for any given condition is best determined by experience. Generally, a mix of stiff consistency is used to raise the pavement slabs and a more fluid mix is used for filling voids. The consistency should be checked by a flow other elevation differences. If the temperature is high, the concrete may be in compression at the slab ends and may not be free to move. This may require freeing the joints by sawing to complete the lifting process.
Grout Pumping
Pumping and jacking operations should normally start at the lowest point in a depressed area and work outward in both directions. Pumping progresses by lowering into successive holes an injection pipe connected to the discharge hose of the grout pump. An expanding rubber packer is used to seal the open space between the pipe and the drill hole. The injection pipe must not extend below the bottom of the pavement and it must be equipped with a return line to circulate the grout while no grout is being placed. Lifting should be done in increments of about 1/4 inch with frequent changes in injection locations to keep slab stresses at a minimum and avoid cracking. The rate of grout injection should be uniform and as slow as possible, usually a minimum of 1/2 cubic foot (0.014 cubic meter) per minute to a maximum of 2 cubic feet (0.056 cubic meter) per minute. Initial pumping is normally at the lower rate and is increased as lifting progresses. As the desired elevation is approached, the lifting rate should be reduced. When grout is extruded from joints, cracks, or from the pavement edge before the target elevation is reached, regrouting in new drill holes and additional slabjacking will be necessary. Gauge pressures for slabjacking are normally in the range of 75 to 200 pounds per square inch with short pressure surges up to 600 pounds per square inch (4,134 kilopascals) to initiate lifting of bonded slabs. Constant observation and analysis of pressure behavior is the most important single factor affecting good Slabjacking. A rapid increase could signal a stoppage of flow that could be followed by a buildup of pressure and excessive lift and cracking, if pumping continues. A sudden reduction of pressure could indicate a loss of lift due to subsurface leakage. In slabjacking operations, the temperature is important when raising slabs to correct faulted joints or other elevation differences. If the temperature is high, the concrete may be in compression at the slab ends and may not be free to move. This may require freeing the joints by sawing to complete lifting
process. 
Elevation Control During Jacking
Before slabjacking operations are started, some method of controlling the amount the slab is to be raised and the finished elevation of the pavement should be determined. For correcting faulted slabs, a straight edge may be used. For short dips up to approximately 50 feet (18.3 meters) in length, a tight stringline is adequate provided the joints are true and plane with those of the adjacent pavement. For dips in excess of 50 feet (18.3 meters) in length, an engineer’s level and rod should be used to check the profile well beyond the dip. This will avoid building a bulge into the pavement. 
Plugging and Cleanup
After slabjacking has been completed in a hole and the discharge pipe removed, the hole should be plugged immediately. Tapered wooden plugs are temporarily placed into the injection hole to retain the pressure of the grout and stop any return flow of the mixture. When slabjacking to the desired elevation has been accomplished, the temporary plugs are removed and the injection holes are filled with a stiff one-part-water, three-part-cement grout or approved concrete mixture. These areas are then finished to an approximate match with the existing pavement. Surfaces adjacent to the grouting operation should be kept clean of excess grout and other materials. Grout and cement slurry on the pavement should be broomed and washed off to avoid unsightly discoloration and to remove the grout slurry before it bonds to the surface.
