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Concrete slabjacking can restore support beneath a settled slab and, when the slab is sound enough, bring it closer to level. It is not a cure-all for weak soil, ongoing erosion, drainage problems or a badly damaged slab. The right repair starts with diagnosing why the concrete moved, deciding whether the goal is void filling, lifting or both, and choosing a contractor who can control the lift in small increments.
Grout and polyurethane foam use different equipment and behave differently underground. A proposal should explain the cause of settlement, the material and hole layout, how movement will be measured, and why lifting is preferable to replacement or another repair. The guidance below applies to residential walks, patios and driveways, while noting where official procedures come from transportation or pavement work rather than a universal homeowner specification.
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Slabjacking is the pressure injection of a material beneath settled concrete to fill voids, restore bearing support, raise the slab, or do more than one of those jobs. The American Concrete Institute’s RAP-11 procedure identifies settlement, erosion, flooding and soil shrinkage as possible contributors. A slab can look level after injection while poor soil conditions remain underneath, so leveling alone does not guarantee that movement has stopped.
Injection holes are drilled through the slab, material is placed in a planned sequence, and the crew monitors movement as support is rebuilt. Hole diameter and spacing depend on slab thickness, reinforcement, cracks, joints, the shape of the void and the selected material. A generic “one pattern fits all” layout is not reliable.
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Can slabjacking fix my sunken concrete?
It can be effective when the slab is structurally capable of being lifted and settlement resulted primarily from lost support or a manageable void. It is a poor fit when the slab is broken into unstable pieces, joints are faulted, the underlying soil will continue to consolidate, or water is still washing material away.
Before approving work, ask the contractor to identify the mechanism of movement and the evidence for that diagnosis. Look for explanations that address grading, downspouts, leaking water lines, erosion, expansive or shrinking soil, recently placed fill and the condition of the subgrade. If the cause cannot be controlled, the choices may include repeating the repair, rebuilding the base or removing and replacing the slab.
What signs suggest a void under a slab?
Common clues include a slab edge that has dropped, rocking or drumming when lightly tapped, an open gap at a step or curb, water collecting along a settled joint, and a visible change in slope. A crack by itself does not prove a void; cracks can result from shrinkage, structural overload, restraint or freeze-thaw damage.
Have the contractor map elevations and inspect the edges, joints and nearby drainage before injection. On larger pavement work, official guidance treats edge settlement and suspected voids as reasons for systematic inspection rather than pumping at the lowest-looking point until the surface rises.
What is the difference between grout and polyurethane foam?
Both systems can support concrete, but they are not interchangeable. Cementitious grout is a fluid, heavier fill that is commonly placed through larger holes and can spread over a broad area. Polymer foam is injected through smaller holes, reacts quickly and can reduce drilling and surface mess, but its expansion pattern may leave portions of a cavity unfilled if placement is not carefully planned.
| Consideration | Cementitious grout | Polyurethane foam |
|---|---|---|
| Typical objective | Void filling and stabilization; can also lift when controlled | Rapid, localized lifting and support; formulation and expansion matter |
| Access | Generally larger injection holes | Generally smaller holes |
| Spread beneath slab | Fluid grout may cover a wider area | Expansion is more localized and depends on resin and confinement |
| Main caution | Excessive pumping or thick grout can create a cone under a hole instead of filling the void | Fast reaction requires close monitoring, and a cavity may not be completely filled |
These are general tendencies, not a performance guarantee for a particular driveway. The ACI bulletin explains that material choice and hole layout must reflect the slab and the repair objective. A Missouri Department of Transportation guide, written for bridge approach slabs, distinguishes grout’s stabilization and void-filling role from polyurethane’s lifting role in that application and describes using different materials for different purposes. A residential contractor should explain how that principle applies to your slab rather than present it as a universal ranking.
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Should the job fill a void, lift the slab, or do both?
Define the desired outcome before comparing bids. Filling a void may stabilize concrete that is already close to grade. Lifting addresses an elevation or trip hazard. Some repairs require staged filling first and limited lifting afterward.
- Void filling: restores contact and reduces pumping or rocking without necessarily changing elevation.
- Lifting: raises a settled section toward a target elevation; it must be gradual to avoid cracking or breaking joints.
- Combined work: uses a planned sequence to fill lost support and then correct grade where the slab can tolerate movement.
Ask for the target elevation and the tolerance, not merely a promise to “make it level.” A driveway may need a deliberate slope for drainage, and a walkway may need a transition that avoids creating a new lip at a step or threshold.
How much can a slab be lifted at one time?
Controlled, incremental movement is essential. The U.S. Department of Defense UFM 3-270-01 cautions that a slab should not be raised more than 0.25 inch (6 mm) per lift increment, with close monitoring. That figure comes from a pavement maintenance manual, not a universal residential guarantee, but it illustrates why a crew should not keep injecting at one point until a large visible jump occurs.
Missouri’s bridge-approach guidance likewise calls for gradual lifting through multiple holes and moving among injection points. Continuing to pump in one location can concentrate stress and crack the pavement. The DoD manual states: “Use only experienced contractors to perform the slab jacking work due to the complexity and specialized equipment required for the work.”
How should a contractor measure movement?
Visual judgment alone is inadequate for precise lifting. A contractor may use a laser level, stringlines, dial indicators or comparable references to track the slab and nearby fixed points. A polyurethane lifting procedure published by Prime Resins lists these types of monitoring equipment and calls for checking support conditions, injecting in intervals and watching surrounding movement.
A laser level is a measurement aid, not a repair system. The crew should establish reference points before injection, observe adjacent slabs and structures, and stop or change holes when movement is uneven. Ask how measurements will be recorded and what happens if the slab begins to crack, bind at a joint or move a neighboring section.
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What should happen before injection?
- Inspect the slab and surroundings. Document cracks, joint offsets, spalling, reinforcement indicators, edges, utility covers, steps and structures that could restrict movement.
- Trace water and soil causes. Check roof drainage, surface grading, irrigation, plumbing leaks, erosion paths and signs of shrinking or recently placed soil.
- Confirm the repair objective. Decide whether the scope is stabilization, elevation correction or both, and establish acceptable transitions and drainage slopes.
- Evaluate alternatives. If the slab is extensively cracked or movement is expected to continue, compare replacement, localized removal, grinding or other repairs.
- Plan access and protection. The work plan should protect walls, siding, landscaping, utilities and adjacent concrete, and should explain where holes will be drilled.
The Specialty Foam and Polymers Association guide SPFA-157 emphasizes preplanning, a hole layout based on measured settlement and voids, protection of surrounding property, movement monitoring, hole filling and job documentation. It also notes that no two geotechnical projects are identical and that manufacturer instructions matter for the specified foam.
What happens during the slabjacking process?
- Mark the layout. Hole positions are selected for the slab geometry and suspected void, not copied from a standard diagram.
- Drill access holes. The diameter depends on grout or foam equipment and the contractor’s method. Reinforcement, joints and utilities must be considered.
- Set monitoring references. Establish elevation checks before material is pumped.
- Inject in short intervals. The crew alternates holes and observes the slab, joints and adjacent surfaces rather than forcing one point upward.
- Verify support and grade. The contractor checks for rocking, remaining voids, drainage slope and the agreed elevation.
- Finish the surface. Injection holes are filled with a compatible patch, and the scope should state how visible cracks and joints will be treated.
- Document the work. Request the material name and formulation, hole locations, approximate quantities, final measurements, photos and maintenance or warranty terms.
Infrastructure procedures should not be copied as a do-it-yourself recipe. For example, Missouri’s instructions address bridge approach slabs, and product-specific foam procedures state that they supplement hands-on training. Resin handling can expose workers to isocyanate respiratory irritation; trained crews need the manufacturer’s safety controls and protective equipment.
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When is slabjacking the wrong repair?
Slabjacking is not a dependable way to rebuild concrete that is already structurally fragmented. The DoD manual says it is not recommended for repairing faulted joints and discusses subsealing and diamond grinding for that condition. ACI advises evaluating removal and replacement according to cracking, distress and anticipated movement.
Be cautious when slabs are heavily cracked, locked against foundations or steps, undermined over a broad area, or supported by soil that is still settling. Lifting a section that cannot act as a coherent plate can widen cracks or transfer stress to adjacent construction. A contractor who immediately proposes injection without discussing these limits is not providing a complete diagnosis.
How can I compare slabjacking proposals?
Compare the scope and reasoning, not just the injection price. Each proposal should answer these questions:
- Cause: What caused settlement, and what evidence supports that conclusion?
- Future movement: What drainage, soil or erosion correction is required, and what movement remains possible?
- Objective: Is the work filling, lifting or both? What final elevations and slopes are specified?
- Material: Why is grout or foam appropriate for this slab, and what formulation or mix is proposed?
- Layout: How will hole locations respond to slab size, cracks, joints, reinforcement and void geometry?
- Control: What instruments and incremental procedure will limit uneven lifting?
- Alternatives: Why is injection preferable to replacement, grinding or another repair?
- Finish: How will holes, cracks, joints, landscaping and adjacent surfaces be restored?
- Records: Will you receive written scope, measurements, material details, photos and warranty conditions?
A qualified contractor is important because the work combines structural judgment, specialized pumps and careful movement control. Do not select a company solely because it promises a dramatic lift or quotes a fixed number of holes without inspecting the site.
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How do I reduce the chance of settlement returning?
Correct the water and soil condition identified during diagnosis. Keep roof downspouts and surface runoff from discharging beside the slab, repair leaking supply or drainage lines, and maintain positive grading where practical. Do not assume injection has solved a continuing erosion path; the slab may settle again if water keeps removing support.
After repair, watch for renewed rocking, widening cracks, new edge gaps, ponding or a change in slope. Keep the contractor’s measurements and material documentation so a future inspector can distinguish a new movement pattern from the original defect.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How we selected these slabjacking tips
The recommendations above are based on the ACI slabjacking procedure, the U.S. Department of Defense pavement manual, Missouri’s bridge-approach stabilization guidance, a product-specific polyurethane procedure and the SPFA contractor guide. They prioritize cause diagnosis, a clear repair objective, material-specific planning, incremental lift control, measurement, safety and consideration of alternatives. Transportation and product documents are identified as such rather than treated as universal residential specifications.
Frequently asked questions
Can slabjacking stop a concrete slab from cracking again?
Not necessarily. Injection can restore support, but it does not remove stresses from expansive soil, ongoing erosion, poor drainage or a slab that is already badly distressed. The contractor should explain what cause is being corrected and what residual movement risk remains.
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No. Smaller access holes and rapid reaction can be advantages, but foam’s expansion is localized and may not fill every part of a complex cavity. Material selection should follow the void geometry, slab condition and objective rather than hole size alone.
Can a contractor pump continuously from one hole to finish faster?
That is risky. Official guidance advises alternating locations and monitoring movement because concentrated pumping can create uneven stress, a grout cone or cracking. A controlled sequence may take longer but gives the crew opportunities to stop and redirect injection.
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What should I do if the slab is level but still rocks?
Have the contractor reassess support rather than adding material automatically. Rocking can indicate an unfilled void, a broken slab, a faulted joint or movement at an adjacent section, and each condition may require a different repair.
Is slabjacking a DIY project?
It is generally unsuitable for do-it-yourself work. Pumps, injection materials, monitoring equipment and resin safety controls require training, and an incorrect lift can crack the slab or damage nearby construction. Obtain a written diagnosis and method from an experienced contractor instead.
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Frequently Asked Questions
Can slabjacking stop a concrete slab from cracking again?
Not necessarily. Injection can restore support, but it does not remove stresses from expansive soil, ongoing erosion, poor drainage or a slab that is already badly distressed. The contractor should explain what cause is being corrected and what residual movement risk remains.
Are smaller foam holes proof that foam is the better choice?
No. Smaller access holes and rapid reaction can be advantages, but foam’s expansion is localized and may not fill every part of a complex cavity. Material selection should follow the void geometry, slab condition and objective rather than hole size alone.
Can a contractor pump continuously from one hole to finish faster?
That is risky. Official guidance advises alternating locations and monitoring movement because concentrated pumping can create uneven stress, a grout cone or cracking. A controlled sequence may take longer but gives the crew opportunities to stop and redirect injection.
What should I do if the slab is level but still rocks?
Have the contractor reassess support rather than adding material automatically. Rocking can indicate an unfilled void, a broken slab, a faulted joint or movement at an adjacent section, and each condition may require a different repair.
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Is slabjacking a DIY project?
It is generally unsuitable for do-it-yourself work. Pumps, injection materials, monitoring equipment and resin safety controls require training, and an incorrect lift can crack the slab or damage nearby construction. Obtain a written diagnosis and method from an experienced contractor instead.
