Should Concrete Blocks Be Filled? [Answered]


Concrete blocks are hollow so that they’re lighter and easier to transport and handle. Their hollow cores also make them simple to align.

But should you fill concrete blocks to make them stronger and more solid?

Concrete blocks can be filled to enhance their structural strength, stability, adhesion, water resistance, and durability; however, they do not have to be filled. You can fill hollow concrete blocks with rebar and concrete, soil, sand, gravel, crushed rock, or foam. When filling concrete blocks with concrete, select a commercial-grade pre-blended mix designed for structural masonry applications.

In this article, we’ll discuss why you should fill concrete blocks and what you should fill them with.

We’ll also talk about how to fill blocks with concrete and why they have holes in the first place.

Do Hollow Concrete Blocks Need To Be Filled?

Whether you need to fill hollow concrete blocks depends on their purpose, but generally, you should fill them.

Filling hollow concrete blocks comes with many benefits. First, it enhances the blocks’ structural strength and stability. 

If you’re using concrete blocks for a retaining wall, filling them will also increase their adhesion. This is because a solid core (in comparison to a hollow core) has more surface area for each subsequent layer of concrete blocks to adhere to.

Plus, filling concrete blocks means better water resistance as well. When the cores of the concrete blocks are filled rather than hollow, there’s much less space for water to make its way in and wreak havoc.

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As a result, filled concrete blocks are also more durable.

What Do You Fill Concrete Blocks With?

When looking for the right material with which to fill your concrete blocks, you have several options.

Rebar And Concrete

One option is to fill the cores with rebar and additional concrete. This strengthens the concrete blocks quite a bit.

However, you’ll need to make sure the concrete you’re pouring inside of the blocks is able to cure fully. Otherwise, you could run into issues with moisture and cracking.

You can also skip the rebar and simply fill the blocks with concrete if you’d like. However, the rebar does add more strength and stability.


Some people choose to fill their concrete blocks with soil, but this isn’t always the best choice, especially if you’re in an area with a freeze/thaw cycle.

Any moisture within the soil will freeze and then thaw as temperatures fluctuate, potentially causing damage to the concrete blocks.


Sand is another option to fill concrete blocks, and an advantage to choosing sand is that it’s very inexpensive. If you choose sand, then make sure to compact it as much as you can. 

Fill all the voids and continue adding sand until you can add no more. This way, the bottom of the concrete block will be totally filled.


Although filling your concrete blocks with gravel is sure to strengthen them, it doesn’t do much for their insulation capabilities. In warm climates, this may not be an issue. 

To make sure your concrete blocks are well-insulated, try mixing gravel with sand rather than using gravel on its own.

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Crushed Rock

Crushed rock is very similar to gravel and can also be mixed with sand to fully fill in the hollow cores of the concrete blocks.


You can also fill your concrete blocks with foam if you want to maximize their insulation capabilities. Although this can be quite expensive, it’s easy to do.

It’s best to fill concrete blocks with foam in the summer and allow plenty of time for them to dry completely.

What Type Of Concrete Do You Use To Fill A Block Wall?

The best type of concrete to use when filling a concrete block wall is a commercial-grade pre-blended mix that’s designed for use in structural masonry applications.

Of course, you don’t have to choose concrete as your filling material. Other options, like foam, crushed rock, gravel, sand, and soil, can also be used. 

How To Fill A Block Wall With Concrete

Your first step is to calculate the amount of concrete you’ll need in cubic feet. You can do this by determining the area of the core for one concrete block and multiplying by the number of concrete blocks you need to fill.

Read the directions on the concrete you’ve purchased so that you know how to prepare the concrete. Don’t prepare it until you’re ready to use it. 

Once the concrete is ready to go, carefully pour it into the holes in the first row of concrete blocks. Give the concrete enough time to completely set and dry before you continue.

After the concrete has set, you can lay the next row of concrete blocks and repeat the process of filling them with concrete and letting it dry.

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Alternatively, you can lay all of the rows of concrete blocks first, ensuring that the hollow cores align. Then, you can pour concrete into all of the blocks at once.

Why Do Concrete Blocks Have Holes?

Since many people choose to fill their concrete blocks, you might wonder why they have holes in the middle to begin with.

Concrete blocks have holes in the middle primarily for alignment purposes. With their hollow cores, it’s simple to lay several rows of concrete blocks and ensure they’re aligned properly.

Another reason for the holes is to make the concrete blocks lighter and easier to transport and handle. Solid concrete blocks are much heavier and more challenging to work with.


Although it’s not required to fill concrete blocks, there are many advantages to doing so. Filling concrete blocks makes them stronger and more stable. It also increases their durability, adhesion, and water resistance.

When filling concrete blocks, you have plenty of filler materials to choose from. You can select foam, gravel, sand, crushed rock, soil, or rebar and concrete. A mixture of gravel and sand is also a good choice.

If you decide to fill your concrete blocks with concrete, look for a commercial-grade pre-blended mix that is specifically designed for applications in structural masonry.