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- Concrete repair: Repair methods and troubleshooting basics
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- Houston Foundation Issues: Answers to common questions about foundation problems in Houston
What is Slabjacking?An overview of how slabjacking fixes sunken concrete
Pictorial Overview of Slab Jacking
A grout mixture is pumped under the slab with our specialized equipment. Once any void is filled, the grout becomes pressurized, hydraulically raising the slab to the desired height.
The holes are patched using a concrete mixture.
Photos Courtesy of Concrete Slab Jacking, Inc. in Maryland USA
If your concrete is sinking, there is a very good possibility that the concrete slab was installed on poorly compacted fill dirt. Sub-surface erosion and shrinking soils are also possibilities.
If you are fortunate enough to have a slabjacker in your area, you should not have to replace the concrete. These individuals can float a slab back to its original position by pumping a mixture of sand, cement, fly ash, and other additives beneath your slab.
They simply drill strategically placed holes into the slab. Using a portable pump and flexible hoses, they fill these holes with the special mixture. Lifting a slab using this method can often be accomplished in a few hours.
Often the cost to perform this sort of concrete leveling is less than half that of replacing a new slab.
There are numerous benefits to slabjacking.
- It can be done in virtually any weather. The material injected beneath the slab provides a strong base.
- There is little or no disruption to landscaping.
- Nothing needs to be moved off the slab, as the pump can lift the weight of the slab and anything you have placed on it.
Why concrete sinks in the first place
Fill dirt is almost always placed along side of house and garage foundations after the foundation work is completed. This fills in voids created during the foundation construction process. Rarely does a builder take the time to compact this dirt.
Soils consist of solid particles and the spaces (voids) between these particles. However, void spaces in soil can cause big problems for buildings and concrete slabs. Concentrated loads, such as buildings or slabs can literally squeeze air and water from soils.
When this happens, the soil sinks and the buildings or slabs follow closely behind.
Avoiding the problem from the start
The problem could have been avoided. Instead of installing fill dirt, a builder should install granular fill such as sand or a sand and gravel mix. These materials can be compacted quite easily with a hand held vibratory compactor.
This material should also be used to fill trenches that cross sidewalks and driveways.
Proper compaction will remove air voids, which if not removed, will later settle and cause the concrete to crack and sink.
Advantages of Slabjacking
Slabjacking has many benefits:
- The grout used to raise the slab provides a stable base, thus strengthening the slab.
- Low cost: The cost to raise the concrete slab to its original position is approximately one third compared to replacement.
- Since slabs are not being removed, there is very little mess or inconvenience.
- No loss of use: Concrete leveling allows the client to use the slab immediately. If the slab is replaced the concrete has to cure for a minimum of 28 days.
- There is no waste involved, compared to slab replacement, where the removed material will end up in the landfill results are better environmentally.
- There is no noisy concrete breaking, with its associated dust and debris.
- Slabjacking can be performed in almost any weather conditions.
- Surrounding grass is not dug up, so there is no reseeding and waiting for new grass to grow.
- Because the old slabs are still there, the color of the concrete remains constant.
How Concrete Slabjacking is Done
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.
Problems/ Solutions When Repairing Concrete Foundations
Problem: Grout doesn't pump into hole, or squirts back out.
Solution: Blow out the hole with air or drill further into the sub-base.
Problem: New cracks form or old ones are enlarged.
Solution: Usually a problem with technique: pumping too much grout into one hole, using grout that is too stiff, or pumping holes in the wrong sequence. Rule of thumb: don't lift a slab more tha an inch while pumping at one hole.
Problem: Slab lifts in wrong place.
Solution: The grout may be too thin. Leave the hole for a while, and come back with a thicker grout.
Problem: Slab binds against concrete and won't rise.
Solution: Grout may be flowing beneath an adjoining slab, use a thicker grout. You can also relieve binding by chipping concrete the edge of the slab, sawing a relief slot all the way through the slab, make sure joints and cracks are clean before starting to pump grout.
Problem: Grout leaks through cracks or joints at the side of the slab.
Solution: Let it set up, then resume normal pumping. Leaks at the slab edge can be stopped by using a thicker grout.