Why Your Concrete Pots Crack -And The Fix

Do your DIY concrete planter pots keep cracking? There are 5 reasons why that can happen, and how you can prevent it!

So your concrete pots crack, and you’re pulling your hair out trying to figure out why.

Cracking is typically the biggest issue when making anything with concrete and cement.

Whether it’s countertops, patios, planters or concrete crafts. But I have really good news, hairline cracks in cement planters can be prevented.

5 Reasons Your Cement Pots Are Cracking

Reason 1. Cracking Caused From Too Much Moisture

The first and most common reason your concrete planter pot cracked is because the mix contained too much water.

The more water that is in the mix, the better the chance that as it evaporates, the concrete will shrink. 

Concrete by nature will shrink, but the extra water can cause it to shrink too much or quickly.

When it has excessive shrinkage, it has nowhere to go and the pressure from cement contracting against itself will cause cracks. 

Typically these types of cracks are hairlines, but they can also crack right through.

They can sometimes be repaired by making a thin mix of cement and filling in the crack. With a little bit of sanding, you may be able to conceal the crack.

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The solution for crack prevention is reducing the amount of water in the mix, which can sometimes pose a challenge. This can happen if your mold has narrow cavities, then it may require a thinner and more fluid mix.

You may want to read this post for more on concrete and cement crafts techniques.

You can avoid adding extra water by using an additive. These additives provide more flow, or fluidity without the need to add more water. 

Many concrete and cement manufacturer’s sell additives for this, that work with their mixes.

The one I’m familiar with is Cement All’s cement mix and the additive is called Flow Control.

Reason 2. Climate Conditions During Curing Causes Cracking In Concrete Pots

You can end up with cracking if the weather conditions are windy or very dry when your concrete pot is curing. 

This is essentially the same problem as too much water in the mix. The wind and lack of humidity are causing the concrete to shrink quickly, forcing it to crack.

The easiest solution for preventing the concrete from losing too much water is to wrap it in plastic while it’s curing.

You can even mist it with water first and then wrap it if you want to play it a bit safer. 

Using plastic to keep in moisture is what is referred to as wet curing.

Please see my *Note below about my projects and wet curing.

By the way, if you are looking for some ideas for plants that you can put into pots and keep outside during winter, I found 17 cold hardy plants that are great for this.

Reason 3. The Type Of Mix Used Caused Deep Cracks Or Completely Broken Pieces 

It’s important to choose the right mix for your project because not all concrete mixes are suitable for every size or shape of a planter. 

Some mixes are smoother, but some have quite a bit of aggregate. Some can be quite desne and some are very lightweight.  

Not all mixes work the same across the board.

To understand more about mixes, I recommend reading my article about the differences between cement and concrete mixes. 

An example of a poor choice of a mix based on the mold is when I made the concrete pot pictured above. 

I had made a silicone mold for a planter and cast a few of them without any problems. 

Some types of silicone molds are more durable than others, which also means that demolding them may be tougher.

The photo above shows a concrete pot that cracked due to the choice of mix. I wanted my concrete planter to be white and there was only one white concrete mix that I could find locally. This was a mortar mix called Glass Block Mortar mix– by Quikrete. 

The mix wasn’t dense enough to withstand the pressure it underwent when I removed it from the mold. 

However, this mortar mix did prove to be a great fit for this Green Marbled Planter. 

This is because this planter mold was larger with thicker walls, as well as wide and shallow, so  it didn’t cause problems when demolding.

Make sure you choose the right cement mix for your project.

Reason 4. Old Expired Mix Causing Crumbling In Concrete Pots

If a concrete planter crumbles and breaks into multiple pieces when you demold it, then the likely reason is a bad bag of mix. 

Concrete mixes are made of chemicals that have reactive properties. These reacting agents that allow them to ‘glue’ together, will only last so long before they lose their binding ability.

If the package has not been opened, then the concrete mix is good for about 6 months to a year.

Once the bag is opened, even with proper storage (cool dry conditions and sealed airtight), the mix may last less than six months. 

If it’s not properly sealed, then it will lose its ability to bind in only a few months.

Reason 5. Issues With The Concrete Pot Mold 

This one is usually pretty obvious, but it’s the reason I’ve experienced cracking with concrete pots. 

The inner mold is usually what is likely to cause this. If your mold is deep and not wide, then trying to remove an inner mold piece can apply a great deal of pressure on the cement.

I’ve started making my own inner molds to combat this problem, which can add to the making time. But I can be very much assured that I won’t accidentally crack my pots when removing them from the molds. 

Note: It’s important to know that for the use of smaller concrete crafts, you shouldn’t need to water cure your pieces very often. 

As of the date of my writing this article, I have not experienced cracking in my planters due to shrinkage. Nor have I needed to water cure any of my concrete planters.  

That being said, I live in the southern U.S., and days with super low humidity are not common.

In addition, my pieces are cured in a garage and not exposed to wind.

Manufacturers formulate their mixes to meet specific demands.

While some mixes may experience a higher chance of shrinking, there are compounds in them that meet other requirements that a low-shrinkage mix may not have. 

Which again, is why it’s important to understand the concrete mix and which is the best choice for your concrete planter project.

You can find detailed information on how to make concrete planters here.

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