Slabjacking is a specialized technique used to restore and stabilize concrete pavements. It is employed to raise concrete slabs that have settled or become uneven, preventing further deterioration and ensuring long-term pavement integrity. The primary purpose of slabjacking is to permanently raise a slab in place, correct faulty drainage, prevent impact loading, and eliminate issues at transverse joints by injecting grout underneath the slab. This article delves into the importance of slabjacking, the methods used, and the procedures followed during this process.
The Need for Slabjacking
Slabjacking is required when concrete slabs experience uneven support or settlement, which can result in structural damage, tripping hazards, and further deterioration if left unaddressed. Some of the most common conditions that necessitate slabjacking include:
- Embankment Settlement: Areas where the ground beneath the concrete has shifted or settled, causing uneven support for the slab.
- Settlement Over Culverts or Utility Cuts: Excavations beneath or near pavements, such as utility work or culverts, can lead to voids and settlement.
- Voids Under Pavements: Gaps or hollow spaces beneath the slab that cause it to sink or become unstable.
- Elevation Differences: Disparities in the height of adjacent pavement slabs, resulting in an uneven surface.
- Faulty Joints: Concrete joints that move or expel water or soil fines can also lead to unevenness.
- Slabs that Rock or Teeter: Slabs that are not properly supported and move under traffic, potentially causing safety concerns.
In each of these cases, slabjacking helps restore the uniformity and stability of the pavement, improving both functionality and safety.
Location of Injection Holes
The success of slabjacking heavily depends on the proper placement of injection holes for the grout. These holes allow the grout to be injected underneath the slab, filling voids and lifting the pavement. The location of the injection holes is determined in the field by the jacking crew superintendent, who takes several factors into consideration:
- Size and Length of the Pavement Area: Larger areas may require more injection holes.
- Elevation Differences: Slabs with significant height variations may need additional holes to ensure proper lifting.
- Subgrade and Drainage Conditions: The soil conditions beneath the slab influence how the grout will flow and distribute.
- Joints or Cracks: The location of joints and cracks must be considered to avoid exacerbating the problem during the jacking process.
As a general guideline, injection holes should not be placed less than 12 inches or more than 18 inches from a transverse joint or the edge of the slab. Additionally, the distance between holes should not exceed 6 feet, ensuring that no more than approximately 25 to 30 square feet of the slab is raised by the grout from any single hole. In cases where the slab is cracked, additional holes may be necessary. The location of holes may vary depending on the specific defect being corrected, with more targeted approaches used for faulted joints or severe settlement.
Drilling Holes for Grout Injection
Once the locations for the injection holes are determined, the next step is drilling. The holes, typically 1-1/4 to 2 inches in diameter, are drilled through the concrete pavement using pneumatic drills, core drills, or other specialized equipment. The goal is to ensure that the holes are vertical and round, providing a suitable entry point for the grout.
The equipment used must be in good condition, and the pressure applied during drilling should not exceed 200 psi (1,379 kilopascals) to avoid damaging the slab or base material. In certain situations, such as when the pavement is tightly bonded to the base, an airline or blowpipe may be used to create a cavity under the slab. This allows the grout pressure to effectively lift the slab.
Grout Mixtures
The grout used for slabjacking plays a crucial role in the success of the operation. A variety of grout mixtures have been found effective, typically consisting of a blend of fine aggregates (or a mix of aggregates and pozzolans or fly ash), Portland cement, and water. The specific proportions and consistency of the grout are vital to achieving the desired results.
A stiff grout mixture is generally used to lift the pavement slabs, while a more fluid mixture is employed for filling voids. Additives such as wetting agents can improve the flowability of the grout, reducing issues like “pyramiding” — a phenomenon where stiff grout forms a pyramid shape under the slab, leaving unfilled gaps. Wetting agents also enable grout to travel longer distances, up to 6 feet in some cases.
Achieving the right consistency requires experience. The mixture should be tested for flowability and adjusted as needed to match the specific conditions of the slabjacking job. Stiffer mixtures are ideal for raising slabs, while more fluid mixtures are better for filling voids and gaps.
Grout Pumping and Jacking Operations
The process of lifting the slab begins with grout pumping. This operation typically starts at the lowest point of a depressed area and progresses outward. The grout is injected through the drilled holes using an injection pipe connected to a grout pump. The pipe is sealed with an expanding rubber packer to ensure that the grout flows only under the slab and does not leak out of the hole.
During the jacking process, the slab is lifted in increments, usually about 1/4 inch at a time. The grout injection should be gradual and uniform, with frequent changes in injection locations to avoid excessive stress on the slab, which could lead to cracking. The rate of grout injection is typically between 1/2 and 2 cubic feet per minute. Initially, the injection rate is lower and increases as lifting progresses. As the slab nears its desired elevation, the rate of grout injection should be reduced.
Monitoring the pressure during pumping is critical. Normal pressure ranges are between 75 and 200 psi, with occasional surges up to 600 psi to initiate lifting in bonded slabs. Rapid increases in pressure may indicate a blockage or the potential for excessive lifting, while sudden drops in pressure could signal subsurface leakage or a loss of lift.
Elevation Control During Jacking
Maintaining precise control over the elevation of the lifted slab is vital to the success of slabjacking. Prior to beginning the operation, methods for monitoring and controlling the slab’s elevation must be determined. For smaller dips (up to about 50 feet in length), a stringline or straight edge may be sufficient to ensure proper leveling. For larger dips, an engineer’s level and rod should be used to check the profile beyond the affected area, ensuring that a bulge is not created in the pavement.
Plugging and Cleanup
After the slab has been successfully raised to the desired elevation, the injection holes must be sealed. Temporary wooden plugs are inserted into the holes to retain the grout pressure and prevent any backflow of the mixture. Once the jacking operation is complete, the plugs are removed, and the holes are filled with a stiff grout mixture (one part water to three parts cement) or an approved concrete mixture. The surface is then finished to blend with the surrounding pavement.
To ensure a clean, professional finish, any excess grout or slurry on the pavement must be promptly cleaned. Grout that has spilled onto the surface should be broomed away and washed off to prevent unsightly discoloration and ensure that the material does not bond to the pavement.
Conclusion
Slabjacking is an effective and efficient method for repairing and stabilizing concrete pavements that have settled or become uneven. By injecting grout beneath the slabs, it fills voids, corrects drainage issues, and restores uniform support. The process involves careful planning, precise hole placement, and thorough monitoring of grout injection and elevation control. When performed by an experienced contractor, slabjacking offers a long-term solution to pavement settlement and can significantly extend the lifespan of concrete surfaces, reducing the need for costly replacements or extensive repairs.