Polyurethane Foam vs. Mudjacking: Comparing Two Approaches to Lifting a Sunken Slab
When a patio or driveway settles, there are two well-established ways to lift it back into place — and the differences between them go well beyond just price.
Our erosion voids guide explains how the soil beneath a driveway, patio, or walkway can develop voids over time — pockets of unsupported space created when soil erodes, washes away, or consolidates beneath an otherwise intact slab. When that happens, the slab above the void eventually loses its support and settles, creating the sunken, uneven sections that are such a familiar sight on older Texas flatwork.
That guide introduces the general concept of slab lifting as a repair approach — injecting material beneath a settled slab to fill the void and raise the slab back toward its original position, as an alternative to demolition and replacement. What it doesn't get into is that there are actually two distinct approaches to this kind of repair, with meaningfully different materials, methods, and characteristics. This guide is a closer look at both: the traditional approach, often called mudjacking (or sometimes "mud-jacking" or "slabjacking"), and the newer approach using expanding polyurethane foam, sometimes called "polyjacking" or "foam jacking."
Both approaches share the same basic goal — get material under a settled slab and raise it back into position — but they go about it in quite different ways, and understanding those differences is useful for anyone evaluating options for a sunken section of concrete.
The Shared Starting Point: Why Lifting Works At All
Before getting into the differences, it's worth understanding what both methods have in common, because it explains why "lifting" is a viable repair concept in the first place.
When a slab settles because of a void beneath it, the slab itself is often still structurally sound — it's the support beneath it that's missing or insufficient, not necessarily the concrete itself. If material can be introduced beneath the slab in the void space, and that material can also exert enough lifting force to raise the slab back toward level, the result can be a slab that's back in its original position, sitting on (relatively) solid support again — without removing and replacing the concrete itself.
This is the underlying logic behind both mudjacking and polyurethane injection: each is, fundamentally, a method for getting material under a slab and using that material to both fill void space and lift the slab. Where they differ is in what that material is, how it's introduced, and what the resulting characteristics are.
Mudjacking: The Traditional Approach
Mudjacking has been used for slab-lifting repairs for many decades, and the basic concept is straightforward: a slurry mixture — generally based on a combination of soil, sand, cement, and water, though specific formulations vary — is pumped beneath the settled slab under pressure through holes drilled into the slab's surface.
The Material
The slurry used in mudjacking is a relatively dense, heavy mixture — generally in the range of comparable to a soil/cement mixture, which translates to a material that's considerably heavier than the polyurethane foam discussed below. As a rough point of reference, mudjacking slurry mixtures are sometimes described as weighing somewhere in the neighborhood of 80 to 100+ pounds per cubic foot, depending on the specific mix.
The Holes
Because the slurry is a relatively thick, particulate-containing mixture, it requires larger holes to pump through effectively — mudjacking holes are typically drilled at roughly 1 to 2 inches in diameter, spaced across the settled area in a pattern that allows the slurry to be distributed and to exert lifting pressure across the void space.
How Lifting Happens
As the slurry is pumped in under pressure, it fills the available void space — and as more material is pumped in, the building pressure beneath the slab provides the lifting force that raises the slab back toward level. The contractor performing the work monitors the slab's position (often using levels or similar measurement tools at multiple points) as material is pumped, stopping when the desired position is achieved.
General Characteristics
Mudjacking has a long track record — it's been used for slab-lifting repairs for a long time, and the basic materials involved (soil, cement, sand) are widely available and generally cost-effective relative to some alternatives. The larger hole sizes required mean the repair leaves more visible patching at each injection point, though these patches are generally not large and can often blend reasonably well with the surrounding concrete, especially with some weathering over time.
Polyurethane Foam Injection: The Newer Approach
Polyurethane injection (polyjacking) is a newer approach, relatively speaking, that's become increasingly common for slab-lifting repairs. Rather than pumping a heavy slurry beneath the slab, this method involves injecting a two-part polyurethane foam mixture that expands significantly after injection.
The Material
The two liquid components of the foam mixture react chemically when combined, and that reaction causes the mixture to expand dramatically in volume — the foam that ends up filling the void space is a rigid, closed-cell structure that's dramatically lighter than mudjacking slurry. As a rough point of reference, the cured foam used in these applications is sometimes described as weighing somewhere in the range of 2 to 4 pounds per cubic foot — for context, that's something on the order of 20 to 40 times lighter than typical mudjacking slurry, by weight per unit volume.
The Holes
Because the foam is injected as a liquid (before it expands) through a relatively narrow injection port, and because the expansion process itself does much of the work of filling the void space, the holes required for foam injection are generally much smaller than those needed for mudjacking — often described as being in the range of roughly 3/8 to 5/8 inch in diameter. This is a small enough hole that, once patched, it's often barely noticeable.
How Lifting Happens
The injected liquid mixture begins reacting and expanding almost immediately after the two components combine — the expansion process itself generates the pressure that lifts the slab, as the expanding foam fills the void space and, once the void is filled, continues to exert upward pressure against the underside of the slab. Similar to mudjacking, the contractor monitors the slab's position during injection and controls the process to achieve the desired lift.
General Characteristics
Because the reaction and expansion happen quickly — often within minutes — polyurethane injection repairs are generally associated with faster completion times compared to mudjacking, where the slurry mixture (being a cement-based material) involves its own curing process. The small hole sizes mean less visible patching. And the dramatically lighter weight of the cured foam, compared to mudjacking slurry, is often cited as a meaningful long-term consideration — discussed in more detail below.
Why Weight Matters: The Secondary Settlement Question
Here's where the comparison gets particularly relevant to the broader themes of this series — because the weight difference between these two materials connects directly back to the soil dynamics discussed throughout our other guides.
What Created the Void in the First Place
In many cases, the void beneath a settled slab developed because the soil beneath the slab eroded, washed away, consolidated, or otherwise lost volume — meaning the soil in that area may already be in a condition that's more prone to further settlement than soil that hasn't experienced this kind of void formation.
The Concern With Heavy Fill Material
If the material used to fill that void and lift the slab is itself quite heavy — as mudjacking slurry generally is, relative to foam — that material is now resting on (and adding load to) soil that may already be in a weakened or settlement-prone condition. The concern some raise about this is whether the added weight of a heavy fill material could, over time, contribute to renewed settlement in the same area — essentially, whether the repair material itself becomes a new load that the underlying soil has to support, on top of whatever loads (the slab itself, vehicle traffic, etc.) were already present.
The Argument for Lightweight Foam
This is the basis for one of the most commonly cited advantages of polyurethane foam in this context: because the cured foam is dramatically lighter than mudjacking slurry — by a factor of roughly 20 to 40 times, based on the general weight ranges discussed above — it adds substantially less load to the soil beneath the repaired slab. The reasoning is that a lighter fill material is less likely to contribute to the kind of renewed soil consolidation that could lead to secondary settlement — the slab settling again, in the same area, sometime after the repair.
A Caveat Worth Noting
It's worth being clear that the underlying cause of the original void — whatever drainage, erosion, or moisture issue led to soil loss in the first place — isn't necessarily addressed by either lifting method on its own. Both mudjacking and polyurethane injection are, fundamentally, methods for addressing the symptom (a settled slab) by filling the void and restoring the slab's position; neither method inherently corrects an ongoing drainage problem, an erosion source, or other root cause that contributed to void formation in the first place. This is why, as our erosion voids guide discusses, addressing drainage and grading issues that may have contributed to void formation is often a worthwhile part of a complete repair approach — regardless of which lifting method is used for the slab itself.
With that said, to the extent that the weight of the fill material itself is a variable in whether secondary settlement occurs, a lighter material is generally considered to place less demand on the surrounding soil than a heavier one — which is the core of the argument often made in favor of polyurethane foam for this application.
Other Practical Differences Worth Knowing
Beyond the weight and hole-size differences discussed above, a few other practical distinctions tend to come up in comparisons between these methods:
Cure Time and Return to Use
Polyurethane foam's rapid reaction and expansion generally means the repaired area can often be returned to use relatively quickly after the work is complete — sometimes within a short window. Mudjacking, involving a cement-based slurry, generally involves more of a curing period before the repaired area should be subjected to normal traffic, similar in concept to how freshly poured concrete needs time to cure before use.
Cost Considerations
Mudjacking has historically been positioned as a lower-cost option relative to polyurethane injection, reflecting differences in material costs (the foam materials used in polyurethane injection are generally more expensive per unit of finished volume than the soil/cement/sand materials used in mudjacking) and, in some cases, differences in the specialized equipment involved. That said, cost comparisons can be affected by the scope and specifics of a given project, and getting quotes for a specific situation is the most reliable way to understand the cost picture for that project.
Aesthetic Considerations
The smaller injection holes used for polyurethane foam generally mean less visible evidence of the repair afterward, compared to the larger holes required for mudjacking — though, as noted earlier, mudjacking patches are generally not large and can blend reasonably well over time.
Application Range
Both methods are used across a range of residential flatwork applications — driveways, patios, sidewalks, pool decks, and similar — though the specific suitability of either method for a given situation (the size of the void, the amount of lift needed, the condition of the existing slab, access considerations) is something a contractor experienced with both methods should be able to evaluate.
Is One Method "Better"? A Few Honest Caveats
Given how the comparison above is structured — with polyurethane foam coming out favorably on several dimensions (weight, hole size, cure time, secondary settlement considerations) — it's worth being direct about a few things:
This isn't a case where one method is universally obsolete. Mudjacking remains a widely used and viable repair method, with a long track record, and there are situations and budgets where it remains a reasonable choice. The comparison above reflects general characteristics of each method, not a verdict that one is simply "better" in all circumstances.
Contractor experience matters as much as material choice. A polyurethane injection performed without appropriate attention to injection patterns, lift monitoring, and void-filling technique may not outperform a well-executed mudjacking repair — and vice versa. The general material characteristics discussed in this guide are real, but they're not a substitute for the skill and experience of whoever is performing the repair.
Neither method is appropriate for every situation. As our erosion voids guide discusses, some settlement situations — particularly those involving very large voids, structural concerns beyond the slab itself, or underlying issues like karst features — may not be good candidates for either lifting method, and may warrant a different repair approach (including, in some cases, removal and replacement of the affected slab section).
What This Means If You're Evaluating a Repair
If you're getting quotes for a settled section of driveway, patio, or walkway, and contractors are proposing different lifting methods (or only one), a few questions can help you understand the comparison for your specific situation:
- What method is being proposed, and why — does the contractor have a reason for recommending one approach over the other for your specific situation, or is it simply what they're set up to do?
- What's the general size and apparent cause of the void, to the extent it can be assessed — this can inform whether either lifting method is appropriate, or whether a different repair approach might be warranted.
- Has the underlying cause of the settlement been addressed, or is the lifting repair addressing the symptom while a drainage or grading issue that contributed to the void remains unresolved?
A contractor familiar with both methods, and willing to discuss the trade-offs honestly for your specific situation, is generally a good sign — the "right" method often depends on specifics of the project rather than a one-size-fits-all answer.
The Bottom Line
Mudjacking and polyurethane foam injection represent two different technical approaches to the same basic repair goal — lifting a settled slab back into position by filling the void beneath it. The materials involved differ substantially in weight (heavy cement-based slurry versus lightweight expanding foam), which translates into differences in hole sizes, cure times, and — notably — the amount of additional load placed on soil that may already be in a settlement-prone condition. Polyurethane foam's lighter weight is often cited as an advantage from a secondary-settlement perspective, but neither method is a substitute for addressing whatever underlying drainage or erosion issue contributed to the original void — and the experience of whoever performs the repair matters as much as which material is used.
Have a sunken section of driveway, patio, or walkway and want to talk through repair options?