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Photograph of a concrete slab control joint How to Seal or Repair Cracks in Concrete Floors & Walls

  • SEAL CRACKS in CONCRETE, How To - CONTENTS: Recommended methods for sealing cracks in concrete floors & slabs - How to seal control joint & expansion joint cracks in concrete slabs.
  • POST a QUESTION or READ FAQs about sealing floor or foundation or wall cracks, water entry through cracks, crack sealing approaches, costs, reliability
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How to seal cracks in concrete floors, foundations, walls, or other masonry: this article how to seal and repair of cracks in poured concrete slabs, floors, or walls.

We list all of the current methods used to seal control joints or cracks that occur in those building surfaces, giving the properties, general procedure, and pros and cons of each method: control joint inserts, masonry caulks, radon crack sealants, semi rigid epoxy resin crack fillers, special poly urea caulks designed as a joint filler - polyurethane foam injection to seal cracks, grouts including portland cement, latex-modified, epoxy, modified epoxy-supported, and furan grouts or other products used to fill or seal cracks in concrete or other masonry surfaces & structures.



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How to seal control joint & expansion joint cracks in concrete slabs

Photograph of a sketch of typical crack patterns in a poured concrete slabHere we discuss how to choose among and apply the alternative methods for repairing or sealing cracks in masonry surfaces such as poured concrete floor slabs, concrete foundation walls, or brick or masonry block foundation walls. We describe use of epoxy sealants, polyurethane foam sealants, and hydraulic cement crack repair sealants.

This article series describes how to recognize and diagnose various types of foundation failure or damage, such as foundation cracks, masonry foundation crack patterns, and moving, leaning, bulging, or bowing building foundation walls.

Types of foundation cracks, crack patterns, differences in the meaning of cracks in different foundation materials, site conditions, building history, and other evidence of building movement and damage are described to assist in recognizing foundation defects and to help the inspector separate cosmetic or low-risk conditions from those likely to be important and potentially costly to repair.

Here we discuss: a list of materials used to fill poured concrete slab control joints. How to seal a cracked masonry foundation wall or floor slab.Typical concrete crack preparation for sealant with an epoxy product. Use of Polyurea as a control joint filler or crack sealant in concrete slabs (Polyurethane Foam Injection Method). Use of Epoxy-supported Grouts as a control joint or crack filler in concrete slabs or walls. List of the Types of Tile Grouts, their Bonding Agents and other Properties. Concrete expansion joint sealant products, epoxies, caulks. Use of Hydraulic Cement for Repairs in Concrete Slabs, Foundations, or Masonry Block/Brick Walls. When should we not seal a foundation wall or floor slab crack?

Warning: some crack sealing operations may make crack diagnosis or monitoring more difficult

First of all do not just seal a crack if the crack size, shape, pattern, location, or other evidence indicate that something important is going on with the foundation.

If for example we think that there may be active foundation movement or settlement going on and if that condition is going to be monitored for evidence of further movement, just pushing a flexible sealant or caulk into a crack is leaves some worries unattended:

We may decide to seal a cracked wall or floor slab anyway, to try to reduce water entry in a building, but remember the implications of sealing we've just listed.

How to seal a cracked masonry foundation wall or floor slab

To seal a shrinkage crack or a control joint crack in a poured concrete slab, regardless of whether or not it has occurred at an expansion or control joint, it may be appropriate to seal the surface to resist water entry and radon gas entry. Control joints are also sealed to provide a smooth and clean concrete surface which does not collect surface moisture or debris. Keeping surface water from entering at a control joint might in some cases also help avoid uneven soil settlement below the slab and tipped or heaved slab sections.

To seal a control joint or expansion joint crack in a concrete slab use a flexible sealant designed for foundation crack repair or sealing. The radon mitigation industry offers special foundation caulks and sealants for that purpose as well. (If water is coming up through a concrete floor or slab, sealing cracks is probably not going to be enough - you need to address the cause of water below the floor.)

List of Types of Materials Used to Fill or Seal Poured Concrete Slab Control Joints

Goodbye Crack Filler at InspectApedia.com

Any sealant that is going to be used to fill a control joint in a slab needs to have the ability to bond to the sides of the cut or opening of the joint, to remain flexible over temperature and moisture changes, and to withstand both compression and expansion as the concrete moves in response to curing and in response to temperature and moisture changes.

Epoxy, for example, is not generally used to fill the expansion joint in newly-poured concrete because the concrete is just too wet and has too much movement for the epoxy to bond and perform acceptably. Similarly, a special product would be needed to fill a control joint in concrete in very cold weather.

Watch out: select the right product for your application. Some crack fillers like the spray injection product Good-Bye Cracks elastic crack "cover" product shown in our photo are not suitable for concrete.

The manufacturer says this product can be used to fill cracks in plaster, drywall, and wood and that it dries to a flexible, paintable finish.

Typical concrete crack preparation for sealant with an epoxy product

Concrete crack patching methods (C) Carson Dunlop Associates Carson Dunlop Associates' sketch shows three common methods used to seal cracks in masonry walls in an effort to stop foundation leaks.

[Click to enlarge any image]

  1. Cleaning a concrete crack for patching and sealing: The crack or expansion joint must be cleaned of all debris; power washing and vacuuming may work but inspect the crack or joint to be sure it is clean along its entire length.

    Otherwise the sealant may fail to adhere and the crack will leak. Wire-brushing or chipping the crack sides is not normally required when using an epoxy sealant.

    (Traditional concrete or masonry patching (and plaster repairs), on the other hand, was traditionally applied after cracks were chipped and widened into an inverted "vee" shape to help keep the patch material in the crack.)
  2. Mix the epoxy sealant according to the manufacturer's directions. Some low volume epoxy sealants are supplied in a dispenser which mixes the two parts together in the proper ratio automatically during dispensing from a tube (see your dentist for an example.) Other crack sealants such as prepared caulks, may come pre-mixed in a tube.
  3. Install the backer rod specified by the sealer manufacturer if you are sealing an expansion joint. Sand (clean, dry) may be used as a filler for cracks - an easier approach if you are repairing an irregular crack in a floor. Sand should stop about 3/4" from the top of the floor surface to give adequate volume of sealer epoxy in the crack.

    The backer rod provides a flexible filler that keeps the sealant in the joint during curing and also reduces the total volume of concrete crack sealer epoxy needed. We recommend using backer rods also if sealing wide cracks in concrete walls or floors, but first you need to have those cracks evaluated to determine the cause as other important structural repairs could be needed first.
  4. Epoxy crack patch cure time: Keep traffic off of the sealed crack until the sealant has cured. Typically a thin film will form on a crack filler epoxy in 8 hours, the sealant will be hard enough to withstand traffic in 24 hours, and it will fully cure in about a week.

These notes are based on epoxy product application information available from Lone Star Epoxies.

Use of Polyurea as a control joint filler or crack sealant in concrete slabs (Polyurethane Foam Injection Method)

Wall crack repair by polyurea injection (C) Carson Dunlop Associates

Polyurea joint filler products for concrete control joints have received attention for filling concrete control joints and is increasingly used in that application since this material is resistant to moisture, has high adhesion properties when used with concrete, and will cure in very cold weather.

Polyurea sealant products are also reported to be useful in sealing control joints in "green" concrete which has not yet had its full 28 day period of initial curing.

An advantage of polyurea or polyurethane used as a crack sealant is that the flexibility of the material will accommodate slight seasonal or temperature-related movement that might otherwise cause new cracks in an epoxy-repaired structure or reopening of repaired cracks in a mortar or concrete-repaired crack.

Also in our experience, because the injected polyurethane foam expands after it is injected into a crack, you may find it easier to fill the crack through the building floor slab or wall than when using alternative repair methods such as epoxy or masonry repair kits.

Carson Dunlop Associates' sketch (left) illustrates foundation wall crack repair using epoxy or polyurethane injection.

List of materials, product sources, how-to for polyurethane foam crack sealant

Use of Epoxy-supported Grouts as a control joint or crack filler in concrete slabs or walls

Epoxy grouts are available which can be mixed and troweled into a concrete crack or joint. These products are less flexible than the control joint fillers described above. In our opinion, an epoxy grout filler may work fine to provide a well-bonded repair to a stable wall or floor crack in an area where there is no anticipated further movement such as from temperature or moisture variations.

Watch out: while epoxy, a material stronger than concrete alone, is sometimes used for structural repairs, if your building masonry (concrete, brick, masonry block) is cracking due to ongoing movement, settlement, frost, or other active or recurrent problems, unless those conditions are fixed, sealing a crack with epoxy will not prevent new cracks from forming.

But read the manufacturer's intended application before buying an epoxy reinforced grout for building crack repair. Do not use the product where it was not intended unless a call to the manufacturer provides you with good information that it's acceptable to do so.

On the one hand we like using a rigid material to fix a supposedly stable crack in a residential building since the fact that the patch is inelastic means it will be easy to see if there is ongoing or new building movement.

On the other hand, a small amount of moment in a foundation wall or floor slab due to changes in temperature will leave a flexible crack filler un-damaged and sealed where a rigid material may fail.

List & Sources of Epoxy Grout Products

Quoting: Emecole Epoxies are recommended for structural cracks.  Unlike polyurethane foams, epoxies will not expand.  However, they cure to an incredibly hard solid, making them ideal for structural repairs.  Our two-component epoxies are strong, durable and reliable. After curing, Emecole Epoxies create a more powerful bond than concrete, resulting in an extremely strong repair.

Use of Peel-and Stick Surface Sealant Kits for Epoxy-Repair Cracks in Concrete Walls

For building surfaces that will remain exposed and where cosmetic appearance is a concern, RadonSeal (and perhaps others) offers a stick-on injection port system that allows the crack material to be injected into a wall or floor crack to leave a smooth surface even with the existing wall or floor surfaces on either side of the crack. RadonSeal's Easy-Peel Sealer kit uses a surface seal that includes injection ports. After the injected sealant has cured, the surface seal and injection ports are peeled away from the repaired surface.

Quoting: RadonSeal Crack Seal and Port Adhesive 1:1 is a high modulus epoxy gel designed for surface sealing of cracks prior to injection and for attaching surface ports. It can also be used for bonding miscellaneous materials to concrete.As with any epoxy adhesive, surface preparation is critical. Concrete surfaces should be cleaned by wire brushing or other mechanical means. All loose or unsound material must be removed. Surfaces should be dry and dust free to insure a superior bond. Application onto wet surfaces is not recommended.

How to Choose Between Using an Epoxy Crack Sealant & a Polyurethane Crack Sealant

High Pressure or Low Pressure Slab or Foundation Wall Crack Sealant Injection

Most foundation wall and floor slab cracks can be successfully filled and sealed using low-pressure injection such as that afforded by a caulking gun and cartridge of sealant, or similar device for both epoxy and polyurethane foam sealants.

Professional polyurethane foam injection crack sealing applicators may use a high-pressure injection system using equipment that includes a pump or a device similar to an automotive grease gun and pressures up to 250 psi. Drilling to prepare injection ports along the crack may be required.

Reasons to use an epoxy-based crack repair or sealant method

Reasons to use a polyurethane foam injection crack sealant

Use of Hydraulic Cement for Repairs in Concrete Slabs, Foundations, or Masonry Block/Brick Walls

Hydraulic cement used for crack repairs is a waterproof cement product that is sold dry and mixed with water on the job to prepare a slurry that is painted (or troweled) over a crack in a foundation wall or in some cases (DryLok or UGL foundation wall sealants) painted over the wall surface in a paint mixture.

Hydraulic cement is often lower in cost than the epoxy and polyurethane products described here. However, the repair is at risk of re-cracking due to product shrinkage or due to even slight movements in the structure from settlement, earth or frost pressures, or thermal changes.

A principal advantage of hydraulic cement for repairing & sealing cracks in masonry floors or walls is that it can handle and effectively seal a crack even when there is significant active water leakage through the crack. The cement, properly mixed (as stiff as possible) cures and expands to seal the crack in the presence of water.

[As early as 1969 we used this approach to successfully seal holes in a masonry block foundation wall through which groundwater was squirting several feet into the basement. We mixed a plug of hydraulic cement and forced it into and against each hole until it set firmly enough to remain in place - about 3 minutes. - Ed.]

Watch out: any cement product is caustic and can cause skin burns or eye or lung damage if you do not handle it properly and wear the proper protective gear for eyes, lungs, skin.

For an effective repair of a wall or floor crack using hydraulic cement you will need to clean out and widen the crack to approximately 1" wide x 1-1/2" deep, ideally with the crack wider at its innermost and more narrow at the wall or floor surface (a "V" shape) to give a mechanical bond to the cured concrete patch.

For a wall crack through which water is seeping, seal the crack progressively beginning at its highest point on the building wall. When all of the crack has been sealed except a single pressure relief point, that final leak is sealed with a molded plug of cement.

List of the Types of Tile Grouts, their Bonding Agents and other Properties

Questions and answers about sealing floor cracks in Slabs

Question: should we seal floor cracks exposed when we pulled up basement carpeting? Our house had a radon problem

We have had our home mitigated for radon. The latest reading is 2.9. Therefore, we feel we are controlling the radon successfully. We are in the process of removing carpet from a 8' X 12" bathroom in the basement. In preparation for installing an overlay linoleum we found a irregular crack in the concrete floor.

Our question is - should we seal this crack with some time of sealer that could be painted on and would this be effective? If so, what type of product would you recommend? - F.H.

Reply: In general, sealing exposed floor slab cracks is a good idea where there was a radon concern, though if your mitigation system is working it might not be critical.

Hairline concrete slab cracks (C) Daniel Friedman

A competent onsite inspection by an expert might find other cracks, openings, or sources for radon gas leakage that you'd want to address, but usually that would have been done when your radon mitigation system was installed.

Following the installation of a radon mitigation system the installer is expected to make a follow-up radon gas level measurement to be sure that the mitigation is working as expected, and s/he also inspects the home to see if you have exhaust fans or other features that could accidentally interfere with proper operation of the radon mitigation system itself.

If all of that was done, the added radon gas leakage into your home from pulling up carpet to expose a floor crack should not be significant. Nevertheless it would be good practice to seal any floor slab cracks that you expose. Here are some things to consider:

If your floor slab cracks are hairline (less than 1/8" across - see our photograph above left) they may be normal concrete shrinkage and they may be too fine to seal with a polyurethane sealant (see our polyurethane sealant link below). In that case a sealant paint or a combination of mesh tape and sealant used to cover cracks in concrete before installing tile would still work well.

Concrete slab crack (C) Daniel Friedman

For larger floor slab cracks (such as shown in our photo at left) there are some sealants ("caulks") widely used by radon mitigators that work well in floor or wall cracks, typically polyurethanes. The crack is vacuumed or cleaned, and when dry, sealed.

More sophisticated floor slab crack sealing is done if you are going to put down ceramic tile: the repair includes a strengthening mesh glued to the floor over the crack to resist the transmission of the crack upwards through the ceramic tiles. Under carpet you don't need that step.

At CONCRETE SLAB CRACK REPAIR and Seal Cracks by Polyurethane Foam Injection at we discuss methods to seal cracks in floors.

At How to Remove Indoor Radon we include more details on how to get the radon level down in homes. You'll see that sealing and caulking to stop radon gas from entering at floor or wall cracks or joints is important.

While you're at it, don't forget to check for gaps that may have opened between the floor slab and the foundation wall - seal those too.

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