Mudjacking is the process of pumping a water, sandy loam soil, and cement mixture under a concrete slab in order to lift it. This mixture is called slurry. The exact slurry composition varies from company to company, and from job to job. Mudjacking can be the solution to many homeowners’ sunken concrete problems, as well as, stabilizing slabs that have voids (but have not sunk). It may also be called concrete leveling or slabjacking.
Critical Information – Voids:
The most important part of mudjacking is to make sure the voids are filled! Since mudjacking slurry can be pumped under the concrete in many different consistencies, it is critical to get the right thickness for each job. If the grout slurry is pumped in too thick, the slab will be lifted on columns that look like big ant piles (also called “collars”). This results in multiple voids underneath the slab after it is raised (i.e. it is not stabilized). Unfortunately for the industry, many mudjackers do pump their slurry too thick and raise the slab on these collars, leaving the slab unstabilized. Since the job is only half done, it will likely fail in the future. This, in turn, gives the industry a bad name. Be careful about choosing the lowest estimate, you may get what you pay for.
We use a portable pump, so we can custom make each batch of material for the thickness need to not only raise the slab but also fill all the voids. Our portable has a short hose, so we can change the thickness at a moment’s notice. Some contractors pump slurry from the street making it much more difficult to control the thickness of the slurry which makes it difficult to properly raise and then stabilize the slab (i.e. fill all the voids).
What Do the Experts Say about Raising Concrete with Poly-Urethane Foam?
Civil engineer James Warner (a leading expert in grout pumping, author of several textbooks on the subject, guest lecturer at the School of Mines) writes on page 371 of his textbook Practical Handbook of Grouting: Soil, Rock, and Structures,
“[There] are several limitations, the greatest of which is probably lack of complete filling of the cavity. Because the injected material [foam] reacts and reaches its maximum volume in increments shortly following injection, significant voids remains after treatment. This was found to be the case when a bridge approach slab near the Denver airport that had been so treated continued to settle and was subsequently removed. The foam grout was observed to have developed into pedestals at the drill holes, leaving a large portion of the total area untreated.” (James Warner, Practical Handbook of Grouting, 2004, page 371, italics added)
Reasons for Fallen Concrete:
Concrete can sink or settle for several reasons. If the original concrete was poured on dirt that had not been compacted properly, the slab will start to settle within a few years. Once the concrete does start to tilt or sink, it can cause trip hazards, unwanted water runoff, or major foundation issues. Additionally, slabs that do not have the dirt at a proper elevation around their edges are easily undermined by water, causing sinking.
Choosing a Contractor and Other Considerations:
There are several factors to consider when choosing a mudjacking contractor. The first step is to have a contractor come to your house and determine if the concrete slab is a good candidate. Be very skeptical of telephone estimates; it is very difficult for a contractor to get a correct idea of the problem on the phone. A concrete slab must also be intact, in good shape and at least 24” wide to be a possible candidate. If a slab is broken into small pieces it is best to have it replaced. A slab can be raised quite a distance – I, personally, have raised slabs up to 12” in elevation.
Concrete begins to get softer or weaker as it becomes old. There are some exceptions but usually concrete over 40-50 years old should be replaced. Most residential concrete is poured 2,500 PSI (pounds per square inch). The higher the P.S.I. the stronger the concrete is.
Interior floors that have heat vents in them are not good candidates for mudjacking. The mud slurry can get into the upper ground vents. Filling the heat vents with mud makes them ineffective.