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- Slabjacking: What is slabjacking?
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How Concrete Slabjacking is DoneA description of the three steps involved in slabjacking
First, a pattern of holes typically between 1-1/2 and 2 inches are drilled or cored through the sunken slab.
Next, a grout mixture is pumped under low pressure (about 10 PSI) under the slab using a 2-inch hose with a nozzle that fits into the holes. Once any cavities or voids are filled, the grout becomes pressurized, hydraulically raising the slab to the desired height. The grout is typically comprised of water, Portland cement, Bentonite or flyash and sand. Additives are used to prevent shrinkage.
Then, the drilled holes are patched using a concrete mixture.
Step 1: Making Drill Holes
The number of drill-holes needed depends on the size of the slab. A smaller slab, such as a sidewalk slab no more than four feet on a side, may only require pumping grout through one or two holes in the middle of the slab.
Larger slabs may need three holes, arranged in a triangular pattern. Spacing, roughly related to slab thickness, determines the total number of holes. The thicker the slab, the farther apart you can drill the holes.
In most operations, the holes are drilled three to eight feet apart, but no closer than 1 foot from the edge of the slab. Unless there's a reason not to, holes should be drilled at distances approximately equal from each other.
Some experienced contractors are adamant that the size of the hole is very important, others it may matter very little as long as it gets the job done so the smaller the better. The usual range is 1 inch in diameter at the low end of the spectrum, and a maximum of 2 inches in diameter at the upper end.
Step 2: Pumping Grout
Pumping the grout should begin at the lowest point of the slab, on most jobs. At the areas in which the grout will do the most lifting, a heavier grout is used. The operator moves from hole to hole as the slab rises about inch to inch. He then moves back and repeats the process.
Additional holes may have been drilled as "support" holes, to be filled in order to support the slab as it rises. These holes usually are filled with a less dense, more fluid grout, so it flows easily into the smaller voids.
Step 3: Hole Patching
This is the final step in the slabjacking process. First, any remaining grout in the drilled holes is removed. The holes are then filled with a stiff mortar mix, striking the surface off cleanly.
Remember that patched holes are often the most noticeable feature of a lifted slab, but they can be made less noticeable by drilling cores instead of holes, coding the cores, and gluing then back into the same holes after slabjacking. However, this process is much slower, and thus more expensive.