The Sweating Slab Syndrome (SSS) Story
Sweating Slabs Syndrome
Sweating Slabs Syndrome
Sweating Slabs Syndrome
Sweating slab events are common in Humid Subtropical (Texas through South East USA), Humid Continental (Missouri through New Jersey) and in rare cases Marine Westcoast climates (Seattle). In all cases, the buildings operate as open-air warehouses or manufacturing facilities, typically with winter heat, make-up air units, or exhaust fans a
Sweating slab events are common in Humid Subtropical (Texas through South East USA), Humid Continental (Missouri through New Jersey) and in rare cases Marine Westcoast climates (Seattle). In all cases, the buildings operate as open-air warehouses or manufacturing facilities, typically with winter heat, make-up air units, or exhaust fans and louvers. Sweating slab events are very common within the first 3-years of concrete placement and more importantly, the first 3-years after protection from natural or man-made re-wetting events. The severity of sweating decreases gradually over time as the concrete loses excess moisture from cement hydration, curing, uncontrolled rewetting events (rain, broken sprinkler lines, excessive scrubbing, high humidity, etc.) and with appropriate housecleaning.
Sweating events primarily occur as a result of rapid changes in internal humidity, typically a 20% increase in humidity in 24-HRS. Depending on the project location, the daily external humidity can fluctuate between 15 to 90%. In a closed environment, the internal humidity will fluctuate in open-air environments (non-conditioned) due to air infiltration around loading docks, man doors, fixed or power actuated louvers. Infiltration by itself is normally not enough to trigger sweating events unless there are other contributing factors, such as:
- wet bases
- insulated wall panels and ceilings
- tenant move-in activities
- lack of an effective vapor retarder
- excessive salts on the surface
- dirty slabs
- chemical cocktails on the surface
When exhaust fans are interlinked with the louvers, make-up air units are paired with barometric louvers, or multiple dock doors or man doors are opened in cross dock facilities during periods of high humidity, unregulated highly saturated air can easily be introduced over a fairly fixed temperature slab. Once the dew point of the air approaches the slab surface temperature, sweating begins to develop.
The Players
Sweating Slabs Syndrome
Sweating Slabs Syndrome
In an attempt to improve employee comfort, many Owners, Developers and Tenants increased the number of air exchanges in their properties beyond Code minimums, which simply decreases the time it takes for the sweating events to develop. Many facilities trigger the exhaust fans with timers at night when its cool or when the internal tempera
In an attempt to improve employee comfort, many Owners, Developers and Tenants increased the number of air exchanges in their properties beyond Code minimums, which simply decreases the time it takes for the sweating events to develop. Many facilities trigger the exhaust fans with timers at night when its cool or when the internal temperature exceeds a program setpoint, unfortunately, these strategies are independent of the exterior humidity, which can be significantly higher than 65%. Given moisture moves much faster than temperature, a 30 minute run of exhaust fans can easily increase the internal humidity 10% or more with less than a 1-degree change in temperature. The introduction of highly saturated air has a negative impact on worker comfort, box stacking strength, metal corrosion and mechanical damage, as well as life safety concerns in the form of slip-falls and safe material handling equipment operation.
After repeated sweating events, hydration biproducts such as potassium and calcium hydroxides (salts) that migrate through the concrete in the form of vapor will dry to a white powdery residue. If the salts are not removed with thorough, regular scrubbing, the presence of the salt attracts moisture where it eventually forms a black slippery substance known as deliquescence.
In the absence of an ASTM E1745 compliant vapor retarder under the slab, salts beyond those that commonly occur as part of the cement hydration process, such as hygroscopic salts, minerals and fertilizers that reside in the insitu soils that support the slab, will ultimately migrate from a higher humidity under the slab to a lower humidity above the slab surface where they will dry and act as moisture attractants. Unlike a concrete slab where there is a limited supply of salts from the cement hydration process, there is an unlimited amount of salts that can migrate through the soils if a vapor retarder is not present.
Take the ground out of play with an ASTM E1745 compliant vapor retarder as part of your overall building strategy. Your floor will stay cleaner with lower sweating potential and your building is ready for floor finishes and air conditioning.
The Solution
Sweating Slabs Syndrome
The Solution
- Regularly clean the slab to remove salts, box dust, oils, tire and other hydrocarbon debris (propane exhaust) from the surface as they develop. We suggest scrubbers based on electrostatically precipitated water (ecH20) to reduce chemical (surfactants) buildup. If detergents are used, residual detergent must be rinsed from the surface wit
- Regularly clean the slab to remove salts, box dust, oils, tire and other hydrocarbon debris (propane exhaust) from the surface as they develop. We suggest scrubbers based on electrostatically precipitated water (ecH20) to reduce chemical (surfactants) buildup. If detergents are used, residual detergent must be rinsed from the surface with clean water before the surface dries.
- Stop using air exchanges as a strategy for employee comfort! Utilize a combination of HVLS (high volume low speed) and ZOO (zone of occupancy) fans for operator comfort. HVLS and ZOO fans can provide the same evaporative cooling effect employee's experience standing in a man door while the exhaust fan is running or the wind is blowing. HVLS and ZOO fans constantly mix the air, which prevents “hot and cold, moist and dry” zones from developing in portions of the building. HVLS and ZOO fans force warm air trapped near the roof towards the slab, which can increase the concrete surface temperature and differential delta between the dew point temperature.
- Monitor the internal and external environment for temperature, relative humidity, dew point and concrete temperature. Interlink the monitoring system with the make up air units or exhaust fans to replace the higher internal humidity air with lower external humidity air when the opportunity presents itself. More importantly, keep the outer doors closed and lock-out the exhaust fans or positive air units during periods of high relative humidity to prevent the rapid introduction of highly saturated air.
- Most importantly, educate the tenant with data about what causes the issue, how to properly maintain the cleanliness of the slab surface and the importance of keeping the outer doors closed at all times and especially during periods of high humidity.
I Can Help
Open Your Mind
Instrument, Continuously Monitor and Clean Up Your Act
Instrument, Continuously Monitor and Clean Up Your Act
I have troubleshooted dozens of sweating slab environments over the past 23 years and have heard various opinions regarding the cause:
- "This is good concrete and that is bad concrete. Remove and replace the bad concrete!"
- "My other building down the street doesn't sweat. You did something wrong."
I have troubleshooted dozens of sweating slab environments over the past 23 years and have heard various opinions regarding the cause:
- "This is good concrete and that is bad concrete. Remove and replace the bad concrete!"
- "My other building down the street doesn't sweat. You did something wrong."
- "The building is facing the wrong direction."
- "There's an aquifer under our building."
- "It's too hot and humid in here. You need to air condition the space and I'm not paying for it."
- "Yes we scrub the floor. We scrubbed it when we moved in last year!"
- "It can't be the exhaust fans. We run them 24/7 in the summer or only when its hot inside."
If we are honest with each other, we can resolve the problem together.
Instrument, Continuously Monitor and Clean Up Your Act
Instrument, Continuously Monitor and Clean Up Your Act
Instrument, Continuously Monitor and Clean Up Your Act
The first thing I do is instrument and continuously monitor the environment. Sweating does not occur unless you have environmental factors that specifically contribute to a problem. In most cases, simple adjustments to how we operate the building can mitigate the problem.
Second, I recommend appropriate cleaning equipment, chemicals (if
The first thing I do is instrument and continuously monitor the environment. Sweating does not occur unless you have environmental factors that specifically contribute to a problem. In most cases, simple adjustments to how we operate the building can mitigate the problem.
Second, I recommend appropriate cleaning equipment, chemicals (if required), procedures and frequency. Owners, Developers and Property Management companies need to verify the concrete is being cleaned on a regular basis, adjusted as required for the tenant's environmental processes. Sweeping alone is insufficient and don't even think of throwing oil based floor sweep agents on the surface to control your dust.
Once the building is cleaned, we use the recorded measurements to study the building's response to stimulus, educate the players, lock-out the exhaust fans during periods of high external humidity and run the exhaust fans when the conditions outside the building are better than the conditions inside the building.
Study, Make Slight Adjustments and Provide Comfort
Instrument, Continuously Monitor and Clean Up Your Act
Study, Make Slight Adjustments and Provide Comfort
Within a few weeks of continuous monitoring, we'll have enough data to revise and fine tune program thresholds. In many moist and humid markets, there may be very short periods of time (less than 30 or 60-minutes each day) when the exterior conditions are favorable to exchange the air. The rest of the day will increase the humidity of the
Within a few weeks of continuous monitoring, we'll have enough data to revise and fine tune program thresholds. In many moist and humid markets, there may be very short periods of time (less than 30 or 60-minutes each day) when the exterior conditions are favorable to exchange the air. The rest of the day will increase the humidity of the air space, leading the workers to open the outer doors to provide evaporative cooling. We lose! In my experience, the manual operation of the exhaust fans will eventually lead to an accidental sweating event.
Finally, stop punching so many holes in your roof and wall panels! Comfort is not achieved by increasing air exchanges. If you desire improved comfort, control the internal humidity, add high volume low speed fans and zone of occupancy fans or air condition the space.
If your building is setup with 0.6 to 1 air exchange per hour, I can work with that. If your building has 2, 3 or 4 air exchanges per hour, the good news is we might be able to turn off 50 to 75% of those units to extend their service life and lower your utility bills.