In part one of this two part series on concrete floor cracks and joints, we discussed the different types of cracks and how they affect our epoxy floor coatings. In this article, we’ll talk about the common joints used in residential concrete construction and how they impact the installation and performance of an epoxy or other polymer floor coatings system.
These questions, while geared towards residential coatings customers, are also applicable to commercial environments.
The different types of control joints used in residential concrete construction.
Why do the cracks in my garage floor have a white plastic strip in them?
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).
Example of a zip strip control joint. Zip strips create controlled cracking in the concrete slab.
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. Because the concrete would have cracked anyway due to shrinkage, this zip strip causes an intentional, controlled cracking.
Another example of a zip strip.
Why does my floor have a joint in it?
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.
Tool joints (one of the control joints) featured in a decorative chip, epoxy garage floor coating in Camp Hill, PA. These joints control where the cracking occurs in concrete. While we do fill in some types of joints, we never fill tool joints.
Why does my floor have straight cuts in it?
A concrete floor that has very straight cuts contains a type of control joint called a saw cut (see picture below). When the concrete floor is 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.
Saw cuts in a garage floor. Saw cuts are an example of a control joint.
Does my floor’s joint help with expansion and contraction?
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 without 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. They aren't typically present in the middle of a garage floor like control joints and are usually between 1/2'' to 1'' in width.
Expansion joints, as pictured above, usually appear near the edges of the slab, close to the walls or driveway.
Why does my garage floor have a different joint than my sidewalks?
Sidewalks often have several tool joints (a type of decorative control joint) in a row and then an expansion joint. The tool joints allows for cracking during the early cure of the concrete. The expansion joints allows the concrete to expand and contract from 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 (our service areas). We do occasionally see tool joints used in garages in the Brandywine area (southeastern PA), but their use seems to be fading.
A tool joint used in a new construction.
Will the coatings fill the joint?
The actual epoxy and polyaspartic coatings don't fill the joints. Here are the normal approaches we take based upon the joint type:
- 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 applied.
- Saw Cuts: These joints may be left open or can be filled with a joint filler that must setup overnight. In very short timeline projects, the saw cuts may be filled with an injectable polyurea and coated over once the polyurea has been shaved flush.
- Tool Joints: The small cracks in the bottom of the joint are filled with an epoxy paste. The coatings traverse into and back out of the joint so that the tool joints appear as decorative lines in the garage floor finish
Saw cuts featured in a before and after photo. While we usually recommend leaving these types of control joints open (as pictured), there is an option to fill them. Filling them will require an extra day to complete the entire project.
If saw cuts were left open during the coatings process, they can be later filled by the homeowner with a caulk designed for use in concrete floors.
For more examples of our garage floor coatings projects, check out our project portfolios page. There you will find plentiful examples of all the different types of joints in action.
If your crews fill the joints, will the floor crack somewhere else?
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.
If cracks did appear, it 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 coatings are applied, our staff works with the customer to design an alternative solution for the joints. This is rare.
Can't seem to figure out what type of joint your garage floor uses? That's okay! By requesting a free quote online, our experienced staff will arrange a date to visit and give you all the information you need about your garage floor and our coatings.
Mike Mincemoyer has decades 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.
