This article needs more links to other articles to help integrate it into the encyclopedia. (December 2013) (Learn how and when to remove this template message)
In civil engineering, concrete leveling is a procedure that attempts to correct an uneven concrete surface by altering the foundation that the surface sits upon. It is a cheaper alternative to having replacement concrete poured and is commonly performed at small businesses and private homes as well as at factories, warehouses, airports and on roads, highways and other infrastructure.
Causes of settlement
Concrete slabs can be susceptible to settlement from a wide variety of factors. In some areas of the United States, naturally occurring soils can consolidate over time, including areas ranging from Texas up through to Wisconsin. Soil erosion also contributes to concrete settlement, which is common for locations with improper drainage. Concrete slabs built upon filled-in land can excessively settle as well. This is common for homes with basement levels since the backfill on the outside of the foundation frequently is not compacted properly. In some cases, sidewalk or patio slabs pitch down, directing water down towards the basement level. Tree roots can actually heave a slab upwards; this is common along public roadways, especially within metropolitan areas.
Concrete settlement, uneven concrete surfaces, and uneven footings can also be caused by seismic activity especially in earthquake-prone countries including Japan, New Zealand, Turkey and United States.
Slabjacking is a specialty concrete repair technology. In essence, slabjacking attempts to lift a sunken concrete slab by pumping a grout through the concrete, effectively pushing it up from below. The process is also commonly referred to as "mudjacking" and "pressure grouting" as well.
Accounts of raising large concrete slabs through the use of hydraulic pressure date back to the early 20th century. Early contractors used a mixture of locally available soils (sometimes including crushed limestone and/or cement for strength), producing a "mud-like" substance and thus the term "mudjacking." Modern slabjacking contractors can now use an expanding polyurethane foam, providing a multitude of benefits when compared to traditional slabjacking materials.
The slabjacking process generally starts with drilling small diameter access holes in the concrete, strategically located to maximize lift. These holes are generally 3/8" or 5/8" diameter for urethane slabjacking and can be over 2" in diameter for traditional mudjacking applications.
Initial material injections will fill any under slab void space. Once the void space is filled, subsequent injections will start lifting the concrete within minutes. After the slabs are lifted, the access holes are patched and the work is complete. The process is rapid when compared to traditional remove and replace applications and is minimally disturbing to the surrounding areas.
Slabjacking technology has several limitations. Most importantly, poor quality concrete may crack and/or deteriorate when being lifted. This is especially true for slabs that are less than 4 inches in thickness. Cracks can also develop in the concrete during the lifting process. Slabs built over filled-in land and/or poor subsoils can also be susceptible to further settlement.
Expanding structural foam leveling
Foam leveling, also known as polyjacking, uses closed cell polymer expanding foam in an injection process. A two part polymer is injected through a hole less than one inch in diameter. Although the material is injected at a higher pressure than traditional cementitious grouts, the pressure is not what causes the lifting. The expansion of the air bubbles in the injected material below the slab surface performs the actual lifting action as the liquid resin reacts and becomes a structural foam. Material injected below a slab to be lifted will first find weak soils, expanding into them in such a manner as to consolidate and cause sub-soils to become denser and fill any voids below the slab. One inherent property of expanding foams is that they will follow the path of least resistance, expanding in all directions. Another inherent property includes reaching a hydro-insensitive or hydrophobic state when cured with 100% cure times as little as 30 minutes. Closed-cell injections will not retain moisture, which in northern climates can cause frost heaving. They are not subject to erosion once in place. Some closed cell polymer foams have baseline lifting capabilities of 6,000 lbs per sq. ft. and leveling procedures have been performed in which loads as high as 125 tons have been lifted and stabilized in a surface area of less than 900 sq. ft. Some foams are even stronger, with compressive strengths of 50 psi and 100 psi in a free rise state. That’s equal to 7,200 to 14,000 lbs per square ft of support.
There is some controversy regarding whether polyjacking is the best technique to use for concrete leveling. Some professionals in the industry say that professional mudjacking is the most preferred long-term solution for concrete leveling. There have been several studies on the toxicity of polyjacking as well as videos illustrating the differences between mudjacking and polyjacking. Polyurethane is also highly flammable. Mudjacking repairs require fewer holes being drilled. However, both techniques are still used to address concrete leveling issues. Also, while the drilled holes are relatively small, the holes in the concrete must be filled. Most professional mudjacking companies will attempt to match the hole filing color with the color of the concrete, but minor color variations are expected. 
- Sivertsen, Katrine (Spring 2007). "Polymer Foams, 3.063 Polymer Physics" (PDF). Retrieved 14 Feb 2013.
- "Technical Data Sheet, Precision Lift 4.0# - Components A and B" (PDF). Prime Resins, Inc. 2015-03-31. Archived from the original (PDF) on 2016-04-23. Retrieved 13 April 2016.
- "Slab Jacking With Polyurethane Foam – How Strong is Strong Enough?". Alchemy Polymers. 2013-06-12. Retrieved 9 September 2016.
- EPA, OCSPP, OPPT, EETD, US (2015-08-29). "Health Concerns about Spray Polyurethane Foam | US EPA". US EPA. Retrieved 2018-09-29.CS1 maint: Multiple names: authors list (link)
- Thyssen, J.; Kimmerle, G.; Dickhaus, S.; Emminger, E.; Mohr, U. (1978). "Inhalation studies with polyurethane foam dust in relation to respiratory tract carcinogenesis". Journal of Environmental Pathology and Toxicology. 1 (4): 501–508. ISSN 0146-4779. PMID 722200.