Concrete Sweating – Phenomenon, Causes and Prevention

Sweating concrete refers to the accumulation of water droplets on the surface of concrete caused by temperature disparities between the concrete and its surrounding environment. This occurrence can be quite bothersome, as it leads to a damp floor that becomes slippery.

Concrete sweating is the focus of our research in this article. We delve into the causes behind this phenomenon as well as explore various methods for prevention.

Phenomenon of Concrete Sweating

When warm and humid air enters a structure through openings like doors, windows, and ventilators, it diffuses throughout the interior. As the air encounters surfaces within the structure that are at or below the dew point temperature, which is the temperature at which air becomes saturated with moisture, condensation occurs. This results in the formation of water droplets on concrete surfaces.

Let’s take an example to understand this phenomenon. Imagine that the ambient conditions within a structure have remained stable at around 70ºF with a relative humidity of 50% for several weeks. The temperature of the concrete slab surface is also close to 70ºF. However, a weather front moves in, bringing in warmer air at 80ºF with a relative humidity of 90%. As this warmer and moisture-laden air infiltrates the building, any surface that is at or below 77ºF, which is the dew point in this case, will reach the dew point temperature.

Since the concrete surface is at the dew point temperature, the moisture in the air tends to condense on it. Concrete is not readily absorbent, so the condensed moisture forms water droplets on the surface, resulting in what is commonly known as Sweating Slab Syndrome.

To provide a practical example, think of the same conditions that cause the outside of a cold beverage container to become wet on a warm and humid day, or the surface of a car to get wet when it is left outside on a cool and damp night. These situations demonstrate the phenomenon of condensation occurring when warmer, moisture-laden air encounters cooler surfaces.

Causes of Concrete Sweating

Concrete sweating is primarily attributed to temperature differentials, although there are additional factors that contribute to this phenomenon. While the variation in temperature plays a significant role, other causes of concrete sweating have been identified and discussed in the following paragraphs.

1. Dew Point

Concrete sweating occurs when warm, moist air encounters a significantly cooler concrete slab. The concrete’s low temperature causes the air in contact with it to cool. If the air’s temperature drops to or below the dew point, condensation will form on the concrete surface. Consequently, the concrete appears wet, sometimes even misty in appearance.

2. Due to Wet Adjacent soil

The structure in question is a retaining concrete wall, specifically a breast wall, that is faced with a wet soil on its opposite side. Despite its solid, hard, and dense composition, the concrete contains tiny capillaries that enable the transfer of water droplets between the two sides. This characteristic leads to the phenomenon known as “sweating” of the concrete, as moisture is able to permeate through the capillaries and appear as droplets on the wall’s surface.

When the temperature of the air is higher than that of the concrete, a phenomenon occurs where the moisture present in the air moves towards the surface of the concrete and undergoes evaporation. If there is a sufficient amount of moisture in the air, this process can result in the concrete surface becoming wet. Additionally, this moisture movement can lead to the formation of efflorescence, which is a white crystalline deposit on the surface of the concrete. In summary, when the air above the concrete is warmer, it can cause the concrete surface to become wet due to the moisture rising and evaporating, and it can also contribute to the occurrence of efflorescence.

3. Salts Deposits

Salts possess a hygroscopic nature, enabling them to attract moisture from both the surrounding air and potentially from within the concrete. These salts can be deposited for various reasons, whether originating from the concrete itself or through external application.

4. Density of Concrete Slab

Concrete sweating is typically not observed in the main concrete slab due to meticulous compaction and surface finishing procedures. However, it is commonly encountered in the pour back strips located within the perimeter of tilt-wall constructions. Porosity tests frequently reveal that pour back strips possess higher porosity compared to the main floor. This discrepancy can be attributed to the fact that pour back strips are often not finished using the same techniques as the main floor and may not have undergone the application of sealers or densifiers that were used on the rest of the slab.

5. Cleanliness of Concrete Slab

Slab cleanliness is a factor that can affect permeability. If the surface of the slab is covered in black rubber tire marks or exhaust deposits, it can reduce its permeability. As the slab becomes less permeable, it absorbs less moisture from above. However, if there is no effective vapor retarder beneath the slab, moisture levels within the concrete and near the surface can increase over time.

Prevention Measures for Concrete Sweating

Preventive measures for concrete sweating include the following:

1. Adequate ventilation: Ensure proper air circulation in the area where the concrete is placed. Good ventilation helps in reducing humidity levels and preventing condensation on the concrete surface.
2. Control moisture sources: Identify and address any sources of excess moisture in the vicinity of the concrete. This can include water leaks, high groundwater levels, or poor drainage systems. By eliminating or minimizing these moisture sources, the likelihood of sweating on the concrete can be reduced.
3. Insulation: Apply insulation materials to the concrete surface or surrounding areas to minimize temperature differentials. Insulation helps in maintaining a more consistent temperature, reducing the chances of condensation.
4. Waterproofing: Apply a suitable waterproofing coating or membrane to the concrete surface. This acts as a barrier, preventing moisture from penetrating into the concrete and reducing the likelihood of sweating.
5. Surface treatments: Consider using surface treatments such as sealants or coatings that help in reducing moisture absorption by the concrete. These treatments can create a protective layer, minimizing the contact between moisture and the concrete surface.
6. Proper curing: Ensure that the concrete is adequately cured after placement. Proper curing techniques help in reducing the internal moisture content of the concrete, decreasing the chances of sweating.

By implementing these preventive measures, the occurrence of concrete sweating can be minimized, leading to better aesthetics and improved durability of the concrete structure.

1. Good Housekeeping Practices

To maintain a clean concrete surface, it is important to remove salts and deposits regularly. This can be achieved by utilizing commercial cleaning agents for floor scrubbing and vacuuming machines. Ensuring proper execution of these cleaning practices is crucial for employee safety and asset protection.

2. Good Air Movement Within the Facility

Sweating slabs often happen when the air inside a structure cannot flow out easily. Inadequate air movement within the building requires a solution to de-stratify the air, which can be achieved through the use of an HVLS fan.

3. Use HVLS Ceiling Fans

HVLS fans minimize the temperature difference between the ceiling and floor while enhancing surface evaporation. They offer an affordable solution to maintain cool structures and prevent floor moisture accumulation.

4. De-Humidification Units

Dehumidification units can modify the building’s indoor environment to alleviate sweating slab syndrome. However, they can be expensive and may increase energy consumption, resulting in higher bills.

5. Use Low-Permeance and Low-Slab Retarders

Placing products directly on the floor can lead to moisture rising and condensing underneath them. However, using these measures can effectively reduce that risk.