Understanding Concrete Patio Sweating: Causes And Solutions

Concrete slabs are a common foundation style for houses, and they are usually smooth and non-porous. However, concrete sweating, or Sweaty Slab Syndrome (SSS), is a phenomenon where moisture forms on the surface of the slab, making it look like the concrete is sweating. This occurs when warm, humid air meets a cool concrete slab, causing water vapour to condense on the slab's surface. While this is often the cause, sometimes the issue is compounded by other factors, such as poor drainage or insufficient insulation.

What is concrete sweating?

Concrete sweating, also known as Sweating Slab Syndrome (SSS), is a phenomenon where moisture forms on the surface of an interior concrete slab, such as a patio or garage floor. It is often mistaken for a water problem due to the condensation effect that occurs.

Concrete sweating happens when warm, humid air comes into contact with a cooler concrete slab. The temperature difference causes condensation to build up on the surface of the slab, resulting in what appears to be the concrete "sweating". This typically occurs when warm, moist air flows into a building with relatively cool floors. The combination of temperature changes and increased humidity causes the concrete to sweat.

Concrete sweating is commonly observed in areas with hot and humid climates, such as Florida. In the early morning, water vapour in the air meets the cool concrete, which is usually below the dew point. The concrete then acts as a condensation surface for the water vapour, resulting in the formation of water droplets on the patio.

In some cases, the moisture on concrete may be due to subsurface water migrating through the concrete pores. If there is insufficient insulation between the ground soil and the concrete, it can absorb groundwater through hydrostatic pressure, leading to water seeping up to the surface.

Why is concrete sweating dangerous?

Concrete sweating, also known as sweating slab syndrome (SSS), is a phenomenon where moisture develops on the surface of an interior concrete slab. This is often mistaken for a water problem, as it is commonly seen as a condensation effect on the surface of the slab. However, concrete sweating can be dangerous and lead to several issues if left unchecked.

One of the biggest dangers of concrete sweating is the risk of slipping and falling. The moisture that accumulates on any smooth surface, especially sealed concrete, creates a dangerously slick surface. This is particularly true for larger concrete slabs with high foot traffic.

Another concern is the growth of mold and mildew, which are predominant factors in airborne allergies and pulmonary problems. A sweaty slab, if left untreated, provides the perfect environment for mold spores to multiply. Over time, this can lead to serious health issues for occupants.

In addition, concrete sweating can also result in structural damage to the concrete itself. The alkali-silica reaction (ASR), also known as "concrete cancer", is a swelling reaction that occurs between the highly alkaline cement paste and the reactive non-crystalline silica found in aggregates when there is sufficient moisture. ASR can cause serious cracking in concrete, resulting in critical structural problems that may even require the demolition of the structure.

To prevent these dangers, it is important to address concrete sweating promptly and take appropriate measures to control moisture levels and prevent condensation. This can include improving ventilation, installing vapor retarders, ensuring proper insulation, and using penetrating concrete sealers.

How to prevent concrete sweating

Concrete sweating, or sweating slab syndrome (SSS), is a phenomenon where moisture forms on the surface of interior concrete slabs, such as patios or garage floors. This happens when warm, moist air comes into contact with a cooler concrete slab, causing condensation to build up on the surface. Here are some ways to prevent concrete sweating:

Dry Out the Room by Increasing Air Movement:

Use a high-volume, low-speed fan to dry out the room and improve air circulation. This will help remove excess moisture from the environment and reduce the likelihood of condensation forming on the concrete surface.

Regulate Temperatures:

Keep the concrete floor temperature and the air temperature at similar levels to prevent condensation. This can be achieved by turning down the air conditioning or turning up the heat. An HVLS (High Volume, Low Speed) fan can also help regulate temperatures while keeping energy costs down.

Use a Dehumidifier:

Install a commercial dehumidifier to remove excess moisture from the air, reducing the likelihood of condensation. While this option can be costly, it effectively alters the interior humidity to minimise sweating slab syndrome.

Seal the Concrete:

Use a penetrating concrete sealer to keep moisture out. It is recommended to first apply a concrete densifier primer and then a penetrating stain repellent. This will help fill in pores and create a barrier that makes the concrete less susceptible to sweating.

Install Vapor Retarders:

Ensure vapor retarders or barriers are installed between the native soil and the concrete floor. This prevents moisture from seeping up through the floor, reducing the occurrence of sweating slab syndrome.

Maintain Good Housekeeping Practices:

Regularly scrub the floor with a strong cleaning agent to remove excess debris and slab moisture. This will help control moisture buildup and reduce the risk of slipping on wet concrete.

How to test for the source of concrete moisture

Testing for the source of concrete moisture is important to prevent issues such as mould, structural damage, and slippery surfaces. Here are some methods to identify and address moisture issues in concrete:

Plastic Sheet Method:

• Tape a 457 x 457 mm (18 x 18 inches) plastic sheet to the concrete floor. Ensure the concrete is dry before applying the sheet.
• Wait for at least 24 hours, and then remove the plastic sheet.
• Inspect the concrete surface for any signs of moisture, such as darkening or condensation.
• This test is simple but does not quantify the moisture level and only detects moisture in the upper portion of the slab.

Calcium Chloride Test:

• Place a calcium chloride disk under a sealed plastic sheet on the concrete surface.
• After 24 hours, retrieve the disk, weigh it, and compare it to its pre-test weight.
• The weight difference indicates the amount of moisture that has accumulated in that time frame.
• This test provides a quantifiable result and is more accurate than the plastic sheet method.

Relative Humidity (RH) Test:

• Drill holes into the concrete to a depth of 40% of the slab's thickness.
• Clear the holes of dust and debris using a wire brush and a vacuum.
• Insert specialised RH probes into the holes and seal them.
• Wait for 24 hours to allow the probes to equilibrate with the microenvironment.
• Take RH readings using a hygrometer.
• This method provides the most complete data by measuring moisture levels within the middle of the slab.

Additional Tips:

• Ensure the room is well-ventilated to reduce moisture build-up.
• Install vapor barriers or retarders between the native soil and the concrete floor to prevent moisture seepage.
• Maintain proper thermal insulation to regulate temperature differences between the external atmosphere and the room.
• Seal the concrete surface using penetrating concrete sealers to keep moisture out.
• Regularly clean the concrete surface to remove excess debris and moisture.
• Use fans or dehumidifiers to reduce moisture levels in the room.

Why is my concrete patio dusty?

Concrete dusting is an issue where the top layer of your patio becomes friable and weak. This can be caused by several factors:

• Water damage: Whether the water is from rainfall, concrete sweat, or pooling, it can significantly weaken your concrete patio over time.
• Extreme temperatures: Too many freeze-thaw cycles can weaken the top layer of the concrete. On the other hand, high heat and direct sunlight can dry out the surface, making it less stable and more prone to dusting.
• Cement dusting/Uneven mix: Some contractors dust the wet surface of fresh concrete pours with cement to create nucleation points for the concrete to set. However, this often results in an imbalance in the concrete ratios, leading to dusting once the surface is completely dry. A similar issue can occur with a concrete mix that's too heavy on sand or cement.
• Foot traffic: If your concrete patio sees a lot of foot traffic, the top layer will likely erode much faster than other areas. Concrete erosion usually leaves the surface looking pitted, dusty, and sometimes flaky.
• Outdoor furniture: Types of outdoor furniture with rough legs can scrape away the concrete surface, causing it to become dusty.

Frequently asked questions