Your finished product has to be delivered to your customer in new condition. Nobody would accept your goods if they weren’t in top condition. That’s why you spend time, effort, and lots of your budget on protecting the condition of your product prior to shipping. The logical location to store this valuable product is in proximity to where it’s produced. But that can lead to issues that might pose real problems.
Think about it: you’re making paper within a large building that is very warm, and the air is full of moisture. These are simply givens due to the process of making paper. If your finished product or roll storage is connected to this process, it could be absorbing a lot of this moisture. And to complicate matters, you are continually loading trucks or rail cars to get the product to your customers. This means a fresh supply of cooler air comes into contact with all that warm air from your process. You might see it as wet spots on the floor or maybe a “sweat” on the slab.
Concrete slab sweating, technically known as Sweating Slab Syndrome (SSS), is primarily caused by moisture accumulation on the surface due to atmospheric conditions or ground-level moisture migration. So, what are the main causes of concrete slab sweating?
Main Causes of Slab Sweating
1. Atmospheric Condensation (Leading Cause)
The most frequent cause is dew point condensation. This happens when warm, humid air enters a building and comes into contact with a concrete slab that is significantly cooler.
The Physics: When the temperature of the concrete surface is at or below the dew point of the ambient air, the air’s moisture condenses into liquid water on the floor.
Contributing Factors:
- Poor Air Circulation: Stagnant air allows temperature differentials to persist between the ceiling and floor.
- Rapid Weather Changes: Sudden spikes in outside temperature and humidity, common in spring and fall, often outpace the slab’s ability to warm up.
- Building Openings: Propped-open bay doors or malfunctioning vents allow humid air to rush in.
2. Moisture Migration from the Ground
If sweating appears even without high humidity, the moisture may be wicking upward from the earth.
- Inadequate Vapor Barrier: A missing, torn, or low-quality vapor retarder beneath the slab allows soil moisture to penetrate the concrete’s porous structure.
- Hydrostatic Pressure: High water tables or poor site drainage can force liquid water through the concrete’s capillaries under pressure.
3. Surface Contaminants and Chemistry
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- Salt Deposits (Efflorescence): Water-soluble salts (hygroscopic substances) on or in the slab naturally attract moisture from the air, exacerbating dampness.
- Surface Densifiers: Some chemical treatments (like sodium or potassium silicate) can increase the pH at the surface, which attracts atmospheric moisture.
- Unclean Floors: Dust, grit, and oil buildup can trap moisture and reduce the slab’s natural permeability, making it harder for the surface to dry.
✨ Pro Tip: The Plastic Sheet Test
To determine the source, tape a 3′ x 3′ square of clear plastic to a dry section of the floor for 24 to 48 hours.
- Moisture under the plastic indicates water is rising from the ground.
- Moisture on top of the plastic indicates an atmospheric condensation problem.
Solutions for Each Slab Sweating Cause
To fix Sweating Slab Syndrome (SSS), the solution must match the specific source of the moisture. Here is how to address each of the three main causes:
1. Solutions for Atmospheric Condensation
The goal here is to equalize the temperature and humidity levels within the space.
- Improve Air Circulation: Install HVLS (High-Volume, Low-Speed) fans. These large fans move massive amounts of air, breaking up the “stratification” (layers of air) and ensuring the slab temperature stays closer to the air temperature.
- Dehumidification: In smaller or climate-controlled spaces, use commercial-grade dehumidifiers to keep the indoor humidity level below the dew point.
- Climate Control Management:
- Avoid turning off the HVAC system at night during humid seasons.
- Keep bay doors and windows closed when outdoor humidity is high.
- Heating the Slab: In some high-end or industrial settings, radiant floor heating can keep the slab temperature consistently above the dew point.
2. Solutions for Ground Moisture Migration
Since you cannot easily install a vapor barrier after the concrete is poured, these solutions focus on blocking the moisture path or redirecting it.
- Penetrating Lithium Sealers: Apply a high-quality penetrating sealer. These soak into the concrete and react with the free lime to form a crystalline structure, effectively “clogging” the pores and stopping capillary action.
- Topical Moisture Vapor Barriers: For extreme cases (often before laying carpet or tile), you can apply a specialized epoxy moisture mitigation system. This creates a waterproof “skin” on top of the slab.
- Improve Exterior Drainage: Ensure gutters, downspouts, and soil grading move water away from the building foundation to reduce hydrostatic pressure.
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3. Solutions for Surface Chemistry and Cleanliness
These solutions focus on restoring the slab’s surface integrity.
- Deep Scrubbing: Use an automatic floor scrubber with a product that cleans and neutralizes to remove salts, oils, and hygroscopic (moisture-attracting) dust.
- Neutralize Efflorescence: If white, powdery salt deposits are present, use a cleaner and neutralizer specifically designed for concrete to remove the salts, followed by a thorough rinse.
- Reduce Alkalinity: Apply a treatment that reduces the pH of the concrete while also densifying the surface to reduce vapor drive that would bring salts to the surface of your concrete slab.
If you are struggling with slab sweating and looking for a solution that could be performed by your own maintenance personnel or an in-house contractor, give us a call to discuss your specific issue.
