void under concrete patio

How to Fix Erosion, Voids, or the Hollow Sound Under Your Concrete

You’ve got a piece of concrete around the outside of your home that is showing signs of erosion and washout under it. Perhaps you can clearly see a void forming underneath, or maybe you can’t see anything; it just feels hollow when you walk on it. That sounds like a void that needs filling . . . but with what??

Anchor Foundation Repair Bryan College Station

At Anchor Foundation Repair, we are concrete raising experts and consequently void filling experts too. Every concrete surface we have raised in the last 35+ years has a resulting space under it that we fill in to provide extra support and minimize future issues. We can tell you how we fill in these gaps because we do it every day and that’s by mud-pumping.

This article will review the method and process of mud-pumping to fill voids under concrete surfaces and its potential benefits. We also cover what kind of concrete and what kind of problems can be solved using this technique. We discuss estimated costs for void filling and the factors that affect the pricing too.

What Kind of Problems Cause Voids Under Concrete

Soil erosion under concrete surfaces is typically due to water intrusion. This unwanted water causes voids to form under patios, sidewalks, driveways, and pool decks. The erosion might be caused by water from gutters and downspouts flowing in the wrong places. You might have a leaking sprinkler head creating this issue or just some other undesirable drainage situation. 

Pool decks are especially prone to washout due to recurring splashing and water runoff.

You start to see evidence of erosion, holes forming around the edges of concrete, or hollowed-out low spots in your yard that continually catch water. The telltale hollow sound and feel are also indications of unseen erosional voids underneath these exterior concrete surfaces.

erosion under concrete patio

Luckily, your concrete hasn’t cracked, moved, sunk, or become uneven, it’s just got this space under it that is starting to concern you. As long as your concrete is otherwise in good condition and not crumbling or severely broken, it’s a good candidate for some void filling using mud-pumping.

We’ve got a good video here showing a common void under a driveway that Anchor can fill with mud-pumping.

Why Use Mud-Pumping to Fill Voids Under Concrete?

Rather than leaving these gaps open and doing nothing, filling the erosional voids is a preventative measure to avoid more costly problems down the road. If the erosion continues unchecked, the voids can continue to get bigger. 

tripping hazard
Mega tripping hazard

The larger the void, the more likely your concrete surface is to crack in one or more places or settle and become uneven. Settlement and cracking concrete create tripping hazards and pooling water. Tripping and slipping hazards also increase the potential for injuries and liability.

Settled concrete might later require mud jacking (or some other method of concrete leveling) to correct the settlement. If the issue cannot be corrected by raising, then a full replacement of your concrete could be needed.

*Disclaimer Alert* We do want to be clear here that mud-pumping and filling voids under concrete is not a guarantee that settlement won’t happen. Even though it’s not a 100% prevention solution, it’s your best option to minimize the chances of these other bad things happening to your concrete.

Also, some homeowners just want to fill in the voids because it looks nicer than leaving an unsightly gap. This is especially true if the void affects your curb appeal or patio ambiance.

How Mud-Pumping Works to Fill Voids Under Concrete

fill voids with slurry
Mixing slurry

We often raise and level concrete that has sunken using a method called mud jacking. That’s where we simultaneously raise the concrete AND fill the created void using a thick slurry mixture made from topsoil, cement, and water. 

Mud-pumping is very similar to mud jacking only there is no *raising* involved, only filling in the existing void. Because there is no raising needed, the slurry mixture is created with a thinner consistency but using the same ingredients.

The process of mud-pumping a void under your concrete flatwork is comparable to mud jacking as well:

  1. Hard pack any visible and accessible voids with topsoil,
  2. Drill holes in the concrete surface if needed (or access the void from the side),
  3. Mix a watered-down slurry used for filling that flows well under pressure,
  4. Pump as much slurry into the void as possible until it starts to outflow,
  5. Patch the holes and clean up the excess.

Before starting the work of filling the void under your concrete, you also need to determine the cause of the erosion. Take any possible steps to correct the source issue. 

So be sure you also fix that sprinkler head, improve your downspouts, or do whatever can be done to stop the water from going where it shouldn’t in the future. Otherwise, you might not be able to *avoid* another void — sorry it had to be done . . . 

How Much Does It Cost to Fill Voids Under Concrete?

The cost of filling voids under concrete is difficult to estimate because we often don’t know the true size and scope of the space that needs filling. You know, because we don’t have x-ray vision and can’t see through concrete . . . We can only pump in slurry until it’s full and that amount cannot be precisely calculated in advance.

Foundation Repair Solutions for the Brazos Valley

For a general ballpark figure, you should plan on spending somewhere between $1,000 to $3,000 to fill a void under one concrete surface or panel. The number of surfaces needing void fill will increase the costs. Filling voids and even raising concrete costs far less than fully replacing these concrete surfaces.

Ultimately, pricing depends heavily on how long the mud–pumping job takes. For mud-pumping projects, we can only estimate how much we *think* it *might* cost, but we don’t know exactly how long it’s going to take until we’re done. Then we give you a final invoice with actual labor and material costs.

More About the Mud-Pumping Process

mud-pumping and mud jacking

Now you know that *Yes, we can fill voids under your concrete without raising it.* And by we, I mean Anchor. 

At Anchor Foundation Repair, we have been handling voids under foundations and other concrete surfaces since 1985. So if you’ve got concrete with a growing gap under it, we can take care of it for you in Bryan, College Station, and the surrounding Brazos Valley communities like Navasota and Madisonville.

We mentioned that mud-pumping under concrete is very similar to mud jacking. The process will look almost identical at your home. Check out this article on our mud jacking process for a more detailed rundown of what happens during a project like this. 

We don’t have one on mud-pumping flatwork voids just yet, so it’s the best we’ve got for now!