Garage Slab Contraction & Expansion Joints : Helpful “How To Fill” Guide!

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Should I Fill Garage Slab Contraction & Expansion Joints?

Unless you are extremely familiar with concrete slabs, you may have wondered why your garage floor has joints. What is the purpose of the + shaped joints that bifurcate your 2-car garage floor into 4 rectangles? Why is there a gap between the garage floor and the driveway? And most importantly, should you fill the garage slab contraction & expansion joints in the garage?

You should fill the garage slab contraction & expansion joints to get a seamless garage floor that is easy to clean & maintain. Filling of gaps prevents moisture & water from seeping into the concrete floor and weakening it.

The filling must, however, be flexible; else it will defeat the purpose of putting the joint in the first place.

It is important & necessary to understand the role of contraction & expansion joints in a concrete slab and the difference between the two.

How to Fill Expansion Joints in Garage Floor?
GMS Intro
GMS Intro

Are Contraction Joints the Same as Expansion Joints?

Both contraction (aka control) and expansion (aka isolation) joints are generally required in a concrete slab. They appear to be similar and can sometimes be confused with each other. Garage slab contraction & expansion joints are necessary to allow space for the concrete slab to expand and move, when the temperature rises.

According to EMSEAL (a Sika Company):

In building materials, a control joint is used to control cracking while an expansion joint is designed to handle structural movement. 

EMSEAL a SIKA Company

NOTE: Henceforth, I shall only use the terms “contraction joints” and “expansion joints” as favored by the American Concrete Institute.

Garage Slab Expansion Joints

Garage Slab Expansion Joints are used to separate (isolate) two concrete slabs or structures. Expansion joints are created before the concrete is poured. Moreover, expansion joints are typically at least ½” wide. 

Some locations that require expansion joints are:

  • Where the horizontal floor slab meets a vertical column or wall
  • Where two different horizontal slabs (such as garage floor & driveway) meet

Garage Slab Contraction Joints 

Garage Slab Contraction Joints are made into the concrete slab shortly after the slab has been poured. As part of the curing process, water evaporates from the concrete causing it to shrink. 

Contraction joints are created to accommodate that shrinkage. Without contraction joints, concrete will shrink and crack randomly. With contraction joints, we can induce the cracks to happen within the space of the contraction joint.

Contraction joints can be made by:

  • Saw Cutting
  • Grooving Tool
  • Inserting a Plastic Strip

Contraction joints are always made after pouring the concrete but before it has fully cured and set. Contraction joints should be no farther apart than 2 – 3 times (in feet) in relation with the concrete slab thickness (in inches). 

Minimum Slab Thickness
for Garage (in inches)
Recommended Gap between
Contraction Joints (in feet)
4”8’ – 12’
6”12’ – 18’
8”16’ – 24’

Should You Fill Contraction & Expansion Joints Before Epoxy?

Irrespective of whether your garage has been built on T-shaped wall foundation or a slab-on-grade foundation, the garage floor itself will certainly have contraction & expansion joints.

The question that may be bothering you is what do you do with these joints? Should you fill the garage slab contraction & expansion joints before or after the epoxy application? Or should you not fill them at all?

Filling up the contraction & expansion joints before epoxy application will result in a seamless, visually appealing and easy to maintain epoxy garage floor.

The most critical decision at this stage is the material that you will use to fill the joints. Use of a non-flexible joint filler defeats the very purpose of the contraction & expansion joints.

If you were to use a non-flexible filler in a contraction joint, the garage floor epoxy coating will look seamless. To begin with!

However, sooner rather than later the cracks in the concrete will telegraph themselves into the epoxy coating. The epoxy coating will certainly crack as it is rigid, whereas the slab is expanding and contracting along the contraction joint.

You don’t want that to happen to your garage floor epoxy coating. Experienced contractors will warn you about this possibility in advance and will not guarantee the work, if you insist on contraction joints being filled with non-flexible fillers.

The best fillers for contraction & expansion concrete garage floor joints are either a 100% solids, epoxy filler or an elastomeric polyurea filler. They will cure to a hard finish but will have the flexibility to move along with the concrete. 

This will prevent the epoxy topcoat from cracking.

Recommended Product:

ElastiPoxy Joint & Crack Filler Kit is a commercial-grade, self-leveling, 100% solids, hybrid, two-component flexible epoxy. Used for a strong and long-lasting repair of contraction control joints, saw cuts, defects, mortar joints, and large stress cracks in poured concrete, concrete blocks, and masonry surfaces. 

ElastiPoxy is widely used in the building industry, but easy enough for DIY homeowners to use around the home.

ElastiPoxy is impact resistant and bonds tenaciously with concrete. Virtually indestructible under high water pressure and heavy vehicular, machinery, and foot traffic. 

It is chemically resistant to perform under the harshest of conditions. Alternatively, when mixed with construction sand, ElastiPoxy provides a strong, rigid repair for filling large cracks, mortar, and surface defects in poured concrete walls, floor, and concrete blocks

Should You Fill Contraction & Expansion Joints After Epoxy?

Have you chosen to epoxy your garage floor without filling in the contraction & expansion joints? If so you may be facing a situation where dirt & debris collect in those joints and are difficult to clean. But can you fill garage slab contraction & expansion joints after epoxy coatings or sealers?

Yes, you can fill the garage slab contraction & expansion joints even after sealer or epoxy application. You have to use the right product as well as the right application method, such that the joints blend into the coating, both visually and physically..

The most suitable product for filling the contraction & expansion joints, after epoxy application, is 100% solids, polyurethane (PU) joint sealers / fillers. They are usually gray in color and can be applied with a caulking gun.

100% solids, PU fillers are flexible, moisture proof and quite inexpensive. Once the filler has dried, it feels like synthetic rubber but is strong enough for foot & vehicle traffic.

Recommended Product:

  • Premium quality sealant, concrete self leveling caulk. No tooling required, levels itself. It is Water immersible & waterproof after cure
  • PERFORMANCE: Self-leveling, pourable with permanently elastic and can be painted with water-based, oil-based or rubber-based paints. Able to bridge gaps up to 1.5 in. wide
  • SUPPORTS: Excellent adhesion on concrete and cementitious slabs.
  • DURABILITY: High durability and resistant to ageing and weathering.
  • USES: Can be used on the driveways, garages, sidewalks, balconies, pavements, terraces, warehouses, factories, civil structures and plazas.

How To Fill Garage Slab Expansion Joints?

To fill the garage slab expansion joints you need to

  1. Use a broom to clean out the joints. Make sure that all dirt, grit, pebbles and other debris are removed. Use a shopvac to get the last bits out.
  2. Use a stiff brush dipped in a concrete cleaner to degrease the joints.
  3. Wash away the cleaner with a garden hose and wait for the joint to completely dry.
  4. Press a backer rod along the length of the joint. Use the correct size. The top of the backer rod should be at least ⅛” and not more than ½” below the floor surface.
  5. Cut the tip of the sealant tube such that the diameter of the cut nozzle matches the width of the joint.
  6. Insert the tube into a caulking gun and gently press to fill the joint as you move the gun along it.
  7. Clean up any excess immediately with a putty knife to get a smooth level finish.

NOTE: Always wear safety eyeglasses & waterproof gloves when working with PU sealants.

How To Fill & Seal the Gap Between Garage Floor & Driveway?

Filling & sealing the gap between the garage floor & the driveway is possibly even more important than filling & sealing contraction and expansion joints within the garage.

Rainwater and snow melt, almost always, will fill this gap and then permeate down into the concrete. Water is concrete’s worst enemy. Water seepage destroys concrete from within.

The only way to protect the concrete garage slab and the concrete driveway is to fill & seal the gap!

You can do this in three simple steps:

Prep the gap by cleaning it thoroughly and removing all dirt & debris

Due to the exposure to the elements the gap between the garage and driveway is usually full of dust, mud, dried leaves, dead insects, etc. Take a screwdriver and dig deep into the gap, pulling out everything that you can. 

Next use a wire brush to remove any remaining debris. You can also use a pressure washer to clean out the gap. Of course then you need to let it completely dry out.

Fill the gap with fine sand, followed by backer rod

The gap between the garage floor and the driveway is wide and deep. Without filler you will spend just too much on the sealant. And the end result will not even be good!

Start off by using fine sand. Sand will go into the cracks and crevices (something a backer rod can not do). This will prevent the sealant from dripping through the cracks and getting wasted. 

Next insert the right sized backer rod, pushing it to the bottom of the joint. However, the top of the backer rod should remain reasonably below the garage slab and driveway. 

This is where the sealant will go

Seal the joint

Finally use a self leveling, flexible sealant to fill the gap. The sealant will quickly sink into the deep spots and level out. Try not to overfill the joint. Wipe out any overfill with a putty knife.

Recommended Product:

Thank you very much for reading the post. I do hope you found it informative and useful.

FAQ

Can You Fill Expansion Joints with Epoxy?

You can fill expansion joints with epoxy provided it is 100% solid and flexible. Do not use standard epoxy coatings as epoxy is not flexible.

A 100% solids, epoxy is self leveling. As such it flows into the expansion joints, every nook & corner, and fills it up completely.

The key word here is “flexible”. Anything used to fill an expansion joint that is not flexible, defeats the very purpose of an expansion joint. 

A non flexible filling will simply crack when the expansion joint expands or contracts with changes in temperature.

Elastomeric polyurea is also a good choice for filling expansion joints in a garage floor.

Can You Fill Expansion Joints with Sand?

Contraction joints are typically 3/16” wide and 1” deep. A 2-car garage will have 2 of them running in a + shape, right across the floor. 

That is a lot of volume to fill up with flexible sealant. Flexible sealants do not come cheap!

To reduce the cost you can insert flexible backer rods or silica sand before applying the flexible sealant. 

Silica sand works better in V-shaped joints as the fine particles reach deeper. Sand will also prevent sealant from going through the cracks at the bottom of the contraction joints.

Sand is both waterproof and weatherproof. So that helps too!

Can You Use Polymeric Sand in Concrete Expansion Joints?

Polymeric sand is an upgraded version of sand. Fine sand is mixed with additives & polymers that harden and lock the sand together, when water is added. Polymeric sand allows joints to expand and contract with temperature variations.

Using polymeric sand as a filler, especially for large outdoor concrete slabs such as driveways & patios is that the filling is more durable and unlikely to be washed away by rain. 

Polymeric sand is also an insect and weed deterrent. On the flip side polymeric sand is expensive and must be applied quickly & carefully. A mistake and you will have hardened gritty concrete surfaces around the joint!

What Do Backer Rods Do?

Backer rods are made of flexible foam and are available in many lengths & diameters. They can be inserted in garage slab contraction & expansion joints or concrete cracks to reduce the consumption of the sealant, which is expensive.

Backer rods also have following other benefits:

* Reducing the effective joint depth & sealant thickness
* Increasing sealant elasticity
* Increasing contact area between sealant & joint by creating an hour-glass shape
* Making it easier to clean out the joint later, if required

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