Although working with concrete is quite an exact science, the reality is that sometimes things can and do go wrong. One issue that might occur when you pour a concrete slab is that there ends up being a void underneath.
A void under a concrete slab is of course not a good thing. Not at all. Therefore, if you notice that there is one, you need to take immediate action to correct the issue as soon as possible. Right now, we are going to teach you what exactly you should do.
Why Is It Important to Not Leave Voids Under Concrete Slabs?
The simple reality is that having voids under concrete slabs is very dangerous. At the very least, such a void can lead to uneven floors as well as bowls and dips in the floor. However, that is only the tip of the iceberg, because if things go really wrong, it can lead to tilted or leaning building walls, it can lead to total foundation disruption, and it can even lead to the walls buckling.
In worst-case scenarios, large voids under large concrete slabs can lead to full buildings completely crumbling. If you see any signs that there may be voids under your concrete slab, you need to take immediate action to correct the issue.
How to Fill a Void Under a Concrete Slab
Let’s get right to it and go through an in-depth step-by-step process on how to fill a void under a concrete slab. Keep in mind that in this case, we are talking about small or medium-sized slabs. If you find a void under your house foundation or a similar structural slab, you should contact professionals immediately.
Step 1: Gather Your Materials
To start, you will need to gather materials including a drill, a masonry drill bit, a slab jacking pump (typically rented), cement, fly ash, sand, and concrete mortar. You will need all of these materials and items to complete this task.
Step 2: Drill Holes in the Concrete
Once you have determined exactly where the void under the concrete slab is, what you need to do is to drill four to six holes that are centered around that void. Using a drill and a specialized masonry bit that is able to drill through concrete, you need to drill holes that are 1.5-inch in diameter. Make sure that all of these holes allow for easy access to the void under the concrete slab. This is where you will be pouring the filler material through.
Step 3: Prepare the Slab Jacking Pump
Keep in mind that a slab jacking pump is a machine designed specifically for this job, to deliver filler material to voids underneath concrete slabs. What you need to do now is to plug the slab jacking pump into a power source, then connect a hose to the slab jacking pump, and then lead the hose to the holes that you drilled in the previous step.
Make sure that the hose is as straight as can be. The filler material is quite thick, so there needs to be a clear line of sight from the pump to the holes, so to speak.
Step 4: Prepare the Void Filler
Now you need to prepare the material that you will use to actually fill the void. First of all, you will need to determine roughly how large that void is. Once you have determined that, you can then figure out how much filler material you will need.
Once you know how much filler material will need, using a wheelbarrow, mix one part cement with one part fly ash and two parts clean sand. This is your filler mixture. You will also want to add a special expander additive that will help prevent the mixture from shrinking once it starts curing.
Of course, you will also need to add water to the mix. Keep adding water until the consistency is that of thick grout. You can always just buy some specialized concrete void filler as well, although these tend to be fairly pricey.
Step 5: Fill and Pump
All you really have to do now is to fill the slab jacking pump with the filler material. Set the slab jacking pump to a level of ten pounds per square inch (10 PSI). Use the slab jacking pump to pump the filler material through the connected hose into the voids under the slab.
Keep turning the slab jacking pump off and switching the hose between the various holes that you drilled in the second step. You want to fill the void as evenly as possible, so keep alternating between the various drilled holes.
Once the filler material reaches the bottom of the slab, the job is more or less done. You know that the void is full when the filler material starts to come through to the bottom of the drilled holes.
Step 6: Finish It Off
To finish it off, you just have to mix up a small amount of concrete mortar to fill the drilled holes with. Use a trowel to level out the surface to make it look like new. The job should now be done.
Mistakes to Avoid, Tips & Tricks
While the above should give you a good idea of what the job involves, below are a couple of additional tips:
- If this job sounds like too much work, something you can try doing is using expanding polymer foam, just like the foam sprayed into houses for insulation (albeit a little different). This works well for hard-to-reach areas and uneven voids, because the foam expands as it enters the void, thus filling all sorts of gaps.
- If the slab has a void underneath, and the slab is damaged, you will need to remove the damaged section. You will then need to fill the area with compacted sand, and then pour new concrete overtop.
Summary
There you have it people, everything you need to know about how to fill a void under a concrete slab. Once again, voids under concrete slabs are very dangerous, particularly if there are walls and more floors above, so you need to make sure to correct the issues ASAP.