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Epoxy Floor Coatings: What about the Cracks? What if I have a Joint? (Part 2)

Again, sorry if the title threw you off.  This is not a blog poking fun at plumbers’ beltlines or the new Cannabis laws in Colorado.    It deals with common questions we receive about epoxy floor coatings.     Many of these questions come from residential homeowners concerning their garage or basement floors.   So, I’ll concentrate my answers to garage and basement coatings.    Much of the information is applicable to the commercial environment as well.

concrete-jointsIn part 1 of this blog, I discuss some of the common questions concerning cracking of the concrete surface. Here in part 2, I’m discussing the types of joints used in residential construction and how they impact the installation and performance of an epoxy or other polymer floor coating system.

Question:  Why do the cracks in my garage floor have white plastic strip in them?

Answer:   Those cracks are actually a type of control joint and the white plastic is what’s referred to as a Zip Strip (also known as a point of weakness or POW strip).   These are very common in garage and basement slab construction. The plastic strip is inserted while the concrete is poured and is usually hidden during the initial, early cure of the concrete slab.   Within a few days, the fairly straight cracks appear directly above the plastic strips.   Below the strip, the concrete slab cracks down through the remainder of the slab.    Since the concrete would have cracked anyway due to shrinkage, this zip strip causes an intentional, controlled cracking.

Question:   Why does my floor have a joint in it?

Answer:   As mentioned in the prior question & answer, newly placed concrete will typically crack during the curing process, as the entire mass shrinks.   Control joints are placed into the overall pour as a way to engineer the “weak” spots into the entire slab and control where the cracks will appear.  Typically, garage and basement floors that have too few or no control joints end up with a lot of 3 point pattern cracks (see Part 1 of this blog).

Question:   Why does my floor have straight cuts in it?

Answer:   A concrete floor that has very straight cuts contains a type of control joint called a saw cut (see picture above).  When the concrete floor was still very new, a circular saw with a special diamond blade is used to cut straight, predetermined lines at approximately 1/3 of the total slab’s depth.   Like the earlier mentioned zip strips, these cuts will force the slab to crack at the bottom of these cuts during the early curing process. 

Question:   Does my floor’s joint help with expansion and contraction?

Answer:   An expansion joint is different than the control joints discussed so far.   An expansion joint fully separates the concrete pours with some form of a joint material – often a felt or fibrous board.   Most of us have seen these joints every so often in a sidewalk.   They allow for thermal expansion and contraction of the slabs with causing structural damage to the slabs.   Garage and basement floors rarely contain these types of joints in the main floor.  Expansion joints may be present on the outer perimeter of the slabs where they meet concrete or block walls. 

Question:   Why does my garage floor have a different joint than my sidewalks?

Answer:   Sidewalks often have several tool joints (a type of decorative control joint) in a row and then an expansion joint.   This allows for cracking during the early cure of the concrete and the more rapid expansion and contraction due to temperature changes and direct heating from sunlight.   While tool joints can be used in garage and basement floors, it’s very rare in central Pennsylvania, greater Maryland or Northern Virginia.   We do occasionally see tool joints used in garages in the Brandywine area (southeastern PA), but there use seems to be fading.

Question:  Will the coatings fill the joint?

Answer:   The actual epoxy and polyaspartic coatings do not fill the joints.   Here are the normal approaches we take based upon the joint type:

  1. Zip Strips – these narrow joints are filled with a type of thixotropic epoxy paste that the coating can go over immediately after the paste is placed
  2. Saw Cuts – these joints may be left open or can be filled with an epoxy joint filler that must setup overnight; in very short timeline projects the saw cuts may be filled with an injectable polyuria and coated over once the polyurea has been shaved flush
  3. Tool Joints – the small cracks in the bottom of the joint are filled with an epoxy paste and the coating traverse into and back out of the joint so that the tool joints appear as decorative lines in the garage floor finish

epoxy-garageWith a garage floor that contains saw cuts, some homeowners prefer to leave the joints open for the distinctive look the joints take on once the floor is coated.   

If saw cuts were left open during the coating process, they can be later filled by the homeowner with a caulk designed for use in concrete floors.

For more examples of how the garage floors look after coating, see our Portfolios.

Question:  If your crews fill the joints, will the floor crack somewhere else?

Answer:   It’s highly unlikely that a garage or basement floor will crack somewhere else in the slabs.   If any crack was to appear, it would normally follow the original joint.  But this is rare and would be caused by slab vibration, rapid slab expansion/contraction or hydrostatic vapor pressure.   Typically, if it’s likely the joints will crack after floor coating are applied, our staff works with the customer to design an alternative solution for the joints.

Until Next Week

If I didn’t answer your question about concrete floor joints or cracking, feel free to contact us by replying to this blog or give us a call at 888.577.0452 (PA) or 443.299.2963 (MD & VA).


Guide to Garage Coatings


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Mike Mincemoyer has 10 years of experience with the design and execution of repairs to concrete slabs.    His installation teams have helped thousands of homeowners in Pennsylvania, Maryland and Virginia with the surface preparation, crack repairs and coatings installations required to give them a beautiful garage and basement floor.