Why does a concrete patio sweat? Is it just a normal phenomenon, or does it indicate a problem with groundwater?
While finding water droplets on your concrete patio could be alarming, it’s usually the result of condensation. When warm, humid air meets a smooth concrete slab with a temperature below the dew point, the water vapor in the air condenses, making the concrete surface “sweat.”
The explanation could be as simple as that, but sometimes, the issue is compounded by other reasons. So, let’s look at all the issues that could face your concrete patio as well as how to fix them.
A concrete slab is one of the most common foundation styles for houses nowadays. The surface is usually smooth and doesn’t have many large pores, which is an advantage from a finishing standpoint.
Sweaty Slab Syndrome (SSS) is what happens when you find the smooth surface covered in water drops, making it look like the concrete is sweating. This usually happens in areas with a hot, humid climate, like Florida, for example.
In the early morning, the water vapor in the air meets the cool concrete, which is usually below the air’s dew point. The concrete acts as a condensation surface for the water vapor, so it appears as little droplets on the patio.
That’s usually the most common cause of concrete sweat. However, in other cases, the water is the result of subsurface moisture making its way through the concrete pores.
If there’s not enough insulation between the ground soil and the concrete, it absorbs groundwater like a sponge through hydrostatic pressure. The result is water making it to the surface as sweat droplets.
Yes, it does. Although concrete might be slightly porous, the polished finish you’re looking for means it doesn’t absorb water as much as it pools it. That’s not entirely bad, though, since concrete can get weakened if it absorbs too much water.
The least damaging way to go about it is to establish good drainage when you’re building your concrete patio. Thankfully, it’s a relatively easy thing to do.
Pouring a slightly sloped, even surface on your concrete patio can make drainage a breeze. The smooth surface on an angle will simply let the water slide away and not pool up.
If your old concrete patio is level to the ground, you can cut out a channel where the water pools and make it slightly angled. Then, you can either line it with a metal channel drain, or for a more decorative approach, place gravel or stones in it.
Doing so prevents the water from stagnation, which is a huge problem if you live somewhere hot and humid. Mosquitoes love pools of standing water as a breeding ground!
Sealed concrete patios can be notoriously slippery when wet.
Any smooth, polished surface that doesn’t have large pores tends to be slick when covered in water. This is one of the dangers of having your concrete patio sweat since it can cause falls that lead to serious injuries, like broken bones.
The way to go about it differs depending on how your concrete patio is finished. The standard practice is to leave the concrete unsealed or strip the sealer that causes the slipperiness.
Another option would be to combine the sealer with something that would make it more grippy, called a grit additive. Silica sand used to be a popular choice, but it affected the clarity of clear sealers.
Currently, a clear plastic grit additive made from polyethylene is used with clear sealers to provide grit without affecting the aesthetic results. It also comes in different sizes you can choose from depending on how grippy you want the surface to be and how much foot traffic it gets.
Concrete chipping, also called spalling, can be caused by:
The moisture in the concrete expands when frozen and contracts when thawed. Over time, this causes internal stresses to build within the concrete, making it weaker in some areas.
When you use de-icing salts to melt ice and snow, the water mixed with the salts seeps into the concrete. Once the water evaporates, the salts recrystallize within the concrete pores, weakening it and making it chip or spall.
If the concrete mix had off ratios of water, sand, or cement, the resulting mix is usually weaker than well-proportioned concrete. Moreover, the finishing affects how well the concrete can withstand physical forces without failure.
If concrete pours were subjected to wind or hot weather, the surface tends to cure before the bottom layers. When the bottom parts cure, they expand, chipping the top layer away.
As for layering, the mechanical bond created between layers is what keeps them together. If that bond fails, the layers tend to separate or chip.
Concrete dusting is an issue where the top layer of your patio becomes friable and weak. This can happen due to:
Whether the water was added during the mixing, came from concrete sweat, or pooled after rainfall, it can significantly weaken your concrete patio.
Just like with the chipping issue, dusting can happen as a result of too many freeze-thaw cycles weakening the top layer of the concrete.
On the flip side, high heat and direct sunlight can also dry out the surface of the concrete, making it less stable and more likely to dust away.
Some contractors like to dust the wet surface of fresh concrete pours with cement. This is supposed to create nucleation points for the concrete to set.
That said, most of the time, the result is an imbalance in the concrete ratios that leads the surface to dust off when it’s completely dry. The same goes for a concrete mix that’s too heavy on the sand or the cement.
If your concrete patio sees a lot of foot traffic, it’s likely the top layer will erode much faster than other areas of your house. Concrete erosion usually leaves the surface looking pitted, dusty, and sometimes flaky.
The same goes for types of outdoor furniture that have rough legs that scrape away the concrete surface, causing it to become dusty.
Why does my concrete patio sweat?
The cause of concrete sweat or sweating slab syndrome (SSS) usually depends on the dew point. If you live in a hot, humid area, chances are the temperature of your concrete patio is cool, so the water vapor in the air condenses on it.
However, you shouldn’t exclude the issue being caused by subsurface moisture, which can seep through the concrete and cause it to sweat.
You shouldn’t ignore the problem, though, as good drainage prevents a variety of issues you could face with your concrete patio.
Concrete chipping or spalling, as well as concrete dusting, can be caused by water damage. If your patio is directed to rainfall or other sources of surface moisture, you should try to fix it before it compromises the patio’s structural integrity.
Ben has a bachelor’s degree in construction engineering. When not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies, you can find him at home with wife and two daughters. Outside of family, He loves grilling and barbequing on his Big Green Egg and Blackstone Griddle, as well as working on projects around the house.
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