Sweating concrete, also known as efflorescence, occurs when moisture builds up under concrete surfaces and evaporates, leaving behind white deposits that can stain and damage outdoor concrete surfaces. Preventing outdoor concrete sweating requires controlling moisture and promoting proper drying of the concrete. With some diligent maintenance and preventative steps, you can keep outdoor concrete looking its best.

Why Concrete Sweats

Concrete is porous, allowing water vapor to pass from underneath the slab through to the surface. When this moisture evaporates, it leaves behind calcium carbonate deposits that appear as whitish blotches or powdery residues on the concrete.

Several factors can contribute to excessive moisture under concrete that leads to sweating:

Poor drainage – Improper grading or compacted soil under the slab doesn’t allow water to drain away properly. Puddles or saturated soil keep concrete damp.
No vapor barrier – Plastic sheeting under the concrete prevents moisture from wicking upwards. Missing or inadequate vapor barriers allow moisture migration.
Fast drying conditions – Hot, dry, sunny conditions cause concrete to dry rapidly on the surface sealing in moisture underneath that continues to evaporate.
Cool, humid climates – These conditions slow concrete curing and moisture evaporation leading to increased sweating.
Fresh concrete – New concrete slabs contain a lot of residual moisture from the mixing and curing process. This moisture needs to fully evaporate before residues stop forming.
Hydrostatic pressure – High groundwater levels or moisture in the soil underneath can “push” moisture up through concrete with force.

Best Practices for Preventing Concrete Sweating

While concrete sweating can’t always be prevented fully, especially on new slabs, you can take steps to minimize unsightly efflorescence.

Install Proper Base Preparation and Drainage

The base under a concrete slab plays a huge role in minimizing moisture issues.

Install a vapor barrier like thick plastic sheeting under all new concrete work. This blocks rising moisture.
Grade and slope soil to direct water away from the slab. Use sand and gravel bases that won’t retain water.
Allow for proper drainage with drain tiles, French drains or sump pumps if high groundwater is an issue.
Compact and stabilize soil with geotextile fabric to prevent erosion and moisture retention.
In colder climates, use insulation under outdoor slabs to prevent frost heaving.

Proper base preparation creates ideal conditions for concrete to cure evenly and remain stable and dry.

Allow Adequate Curing Time

Curing keeps concrete moist and allows hydration for proper strength gain. But moisture gets trapped under fast drying surfaces.

Cure slabs like sidewalks, porches and patios for 5-7 days using plastic sheets. Mist and re-wet the slab as needed.
For driveways, garage slabs and basement floors, cure 10-14 days before removing plastic.
Prevent rapid surface drying by misting concrete as needed for 1 month.

Adequate curing allows concrete to dry evenly and completely, reducing sweating.

Use Concrete Sealers

Applying sealers to outdoor concrete helps control moisture evaporation from the surface.

On new concrete, wait a minimum of 30 days for initial curing before applying any sealer.
Use silicone, acrylics or polyurethanes designed for exterior concrete that won’t degrade from sun exposure or freezing.
Reapply sealers every 1-2 years as needed to renew protection on horizontal surfaces.

Sealers prevent surface moisture loss that can trap water under concrete and lead to sweating.

Promote Fast Drying

While allowing proper curing, you also want concrete to dry thoroughly to prevent trapped moisture.

Remove any pooled water or puddles on the slab with a squeegee. Don’t allow water to penetrate cracks and joints.
Use dehumidifiers and fans to actively dry out basements or enclosed slab areas.
When conditions allow, fold back plastic sheeting to promote drying after the initial cure period.

Accelerated drying removes excess moisture from the slab so it can’t migrate upwards later on.

Maintain and Monitor the Slab

Ongoing maintenance helps minimize moisture available to create efflorescence.

Reseal concrete every 1-2 years to maintain the moisture barrier. Look for worn or peeled areas.
Repair cracks, spalls or deteriorated joints with hydraulic cement which resists water.
Keep drain tiles clear of debris and test sump pumps yearly to prevent moisture backups.
Clean up spills like oil or chemicals quickly to avoid penetration and moisture retention.

Proper maintenance and repair minimizes concrete damage that allows moisture intrusion leading to sweating.

How to Remove Efflorescence

While prevention is ideal, concrete sweating still happens, leaving behind unsightly white staining. Removing efflorescence involves both drying and surface cleaning.

Allow Concrete to Dry

Don’t attempt removal until sweating has stopped and the slab is fully dry.

Wait until efflorescence formations have stopped reappearing after cleaning them away. This takes 1-3 months on new concrete.
Allow the slab to dry thoroughly following rain or snowmelt so all free moisture has evaporated.

Trying to clean off efflorescence while the concrete is still sweaty and damp simply causes more to form again quickly. Patience pays off.

Clean and Scrub

Once dry, efflorescence needs abrasive removal using acidic cleaners or minerals.

Use a concrete degreaser or mild acid cleaner diluted with water. Wear protective gear.
Scrub vigorously with a stiff brush to dissolve and remove white deposits. Focus on textured areas.
For heavy efflorescence, use poultice scrubbers made with minerals like talc or clay.
Rinse thoroughly. Repeat cleaning if some staining remains. Allow to dry again.

Thorough cleaning dissolves efflorescence and removes the white residues before resealing the concrete.

Apply New Sealant

Closing up the concrete surface prevents future moisture passing through.

Allow the cleaned slab to dry fully, typically a day or two.
Apply a fresh concrete sealant like silicone or acrylic. Penetrating sealers may offer better efflorescence resistance.
Cover all areas of the slab, focusing on joints, cracks or transitions that are vulnerable.

Resealing protects cleaned concrete from new moisture penetration that could start the sweating process again. Maintain sealants regularly.

When to Call a Professional

In some cases, excessive concrete sweating and efflorescence requires professional help to address:

If moisture keeps migrating upward despite sealing and drying efforts. This indicates a high water table or improper base preparation.
For cleaning and removal from textured or stamped concrete, which requires pressure washing and specialty chemicals to penetrate textured surfaces.
If sweating and efflorescence reoccur quickly after removing, indicating a persistent moisture issue that needs correction.
For evaluation of suspected structural issues like faulty vapor barriers, cracked foundations, or poor site drainage causing excessive sweating.

Preventing and managing concrete sweating does require diligence and persistence. But taking the right approaches can keep outdoor concrete bright, clean and attractive for years without ugly white staining.

FAQs About Preventing Concrete Sweating

What’s the fastest way to stop new concrete from sweating?

Allow for adequate curing time, at least 5-7 days. Use plastic sheets to prevent surface drying. Mist the slab to keep the surface moist. Allow the concrete to dry thoroughly over the next several weeks before sealing.

What is the best sealer for outdoor concrete?

Silicone or acrylic-based sealers are best for outdoor use. Look for penetrating sealers formulated to resist UV exposure, freezing temperatures and salt. Reapply yearly.

How long does it take for concrete to dry after rain?

Allow 1-3 days of dry, sunny weather for concrete to fully dry out after heavy rain or snowmelt. Test moisture levels with plastic sheeting.

What causes powdery white spots on cured concrete?

This is most likely efflorescence, which is caused by moisture evaporating through the surface of the concrete, leaving mineral deposits behind. It indicates moisture is passing through the slab.

How can I permanently get rid of efflorescence?

Removing it fully may take several cycles of cleaning, drying, and resealing. Address the underlying cause of excess moisture to prevent recurrence long-term. Some efflorescence may return seasonally.

Is efflorescence harmful?

It doesn’t compromise the concrete structurally but can be an eyesore. Preventing moisture wicking up through the slab is the best solution. Maintaining sealers helps control surface efflorescence.

Conclusion

Outdoor concrete will look its best when protected from excess moisture passing through the slab surface. Paying attention to proper base preparation, concrete curing, moisture control, drying, sealing and maintenance goes a long way in preventing unsightly efflorescence from forming.

While efflorescence can’t always be prevented completely, following these guidelines provides the best protection against white, powdery deposits on outdoor concrete. With some diligence, your concrete surfaces can stay clean, attractive and uniform in appearance for maximum curb appeal and enjoyment.

How to Keep Outdoor Concrete from Sweating