Concreate, Inc. delivers concrete polishing and custom staining for both commercial and residential projects primarily in Virginia and Maryland (but also up and down the East Coast). We work hand in hand with with designers, architects, project managers, general contractors, tradesmen, and home owners alike from start to finish. We welcome the opportunity to serve your polished concrete needs in every way possible.

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How to Replace and Fill Joints in Concrete

How to Replace and Fill Joints in Concrete

Joints between slabs of concrete.

There’s a thin line between stressed-out concrete and stable slabs. Keeping this area firm but flexible means occasionally maintaining the joints.

Key Takeaways:

  • Joint filler replacements could be necessary as the existing material deteriorates
  • The process involves digging the old material out and replacing it with epoxy, urethane caulk, or polyurea
  • This job could save you from expensive concrete repairs
  • A concrete expert can complete the job quickly

If you’ve got a concrete surface indoors or outdoors, you’ll find some type of filler has been used on it. This is because concrete is a lot more flexible than most people think. It can expand and contract repeatedly over time due to environmental conditions cycling between hot and cold.

This cycle is managed by adding expansion joints between pieces of concrete and inserting some joint filler. These supple but strong materials supply a cushioning barrier between slabs so they can stretch and flex more easily.

Unfortunately, joint filler doesn’t last forever. It can get damaged, dry out, and lose its bounce, leaving the concrete vulnerable to cracking again. That’s the signal to add some new epoxy or urethane, so here’s our five-step guide to help you fill joints in concrete.

What you should be looking for first

The signs of failing joint filler are easy to spot, and the sooner you act, the longer and stronger life your concrete will have. You may see some spalling along the joint, which is a contractor’s term for flaky or chipping concrete. This must be removed and replaced with a good bonding filler to replace the weak spot.

You may also see parts of the filler itself peeling away, missing chunks, or getting pushed out of shape by dirt and debris. Sometimes, an entire line of filler can be yanked out like a toy rubber snake and used for any number of pranks. You paid for it, so why shouldn’t you get some fun out of it? Post jokes, though, it’s time to get the job done.

Step 1 – Clearing out the old filler

Concrete joints are narrow, so all that’s needed is an equally narrow, sharp-edged tool to dig out the failing filler. First, the area surrounding the joint must be swept clean because we don’t want any more stuff falling into the crack as it’s worked on.

Contractors may use a putty knife, a trowel, or even a screwdriver to wiggle out the old filler. Alternatively, they may pour on some caulk remover or mineral spirits and let that sit for a while. This all depends on the type of filler being removed. Using a right-angled grinder or specialized joint cleaning saw with a diamond blade are two other reliable cleaning methods.

Edged tools also help lift out any small pieces of debris that can get between the filler and the concrete. Compressed air and/or hosing are effective ways to literally get into the groove and blast out any unwanted bits and pieces. These can be easily vacuumed away to ensure the area stays clean as you fill joints in concrete. 

Step 2 – Pre-filler preparation steps

A quick-drying bonding adhesive is applied to the newly emptied and cleaned joint. This is necessary to give the new filler enough adhesion to stick to the new concrete.

A carefully cut piece of foam backing rod is then inserted into the joint and pushed down before the bonding adhesive dries entirely, with another layer of caulk added on top. This foam helps provide a shock-absorbing effect so the concrete can cope better with expanding and contracting.

Step 3 – Choosing the right kind of filler

Various materials like epoxy, urethane, and polyurethane can be used to fill joints in concrete. Our earlier blog on resinous floor coatings takes a deeper dive into these materials. The facts of their composition still apply to joint filling, so here’s a quick summary:

  • Epoxy – Bonds well to the substrate, is tough and durable, and resists degradation (so you can call it all the nasty names you want).
  • Urethane and polyurethane – Pretty much used as interchangeable terms. Also bond well to concrete. Stronger resistance to scratches, impacts, and abrasions. 

Both materials are about equal in terms of expansive capability. Polyurea can be put down faster if the team knows what they’re doing, saving on time and labor costs. Polyurea is also more resistant to UV light. Epoxies tend to be easier to remove when refilling joints.

Whichever filler is selected, it’s got to be tough! This means a minimum 80 hardness on the Shore A scale (jump to section 5.12), a grading system used to measure the firmness of materials like silicones and elastomers. 

Step 4 – Applying the filler

Gloves and goggles may be necessary depending on the type of joint filler being used. The filler is then poured into the joint and must be closely monitored as it settles. This is because filler can sometimes drain through the bottom of the joint, meaning more must be poured in to maintain the desired level.

Settling filler also releases air, which again can make the fill level drop and require a top-off. Contractors therefore must wait until the joint filler is cured before deciding if the correct amount has been added.

Step 5 – Smoothing and blocking off

The filler must now be smoothed off so it’s in line with the slab’s surface, a process known as flush grinding. Any overflow makes the filler vulnerable to getting kicked up by foot traffic or caught and torn away in the wheels of passing equipment.

Now that the joint filling has been done, the area must be clearly marked to block anyone from entering for a while. Again, shoe prints or wheel trails will only ruin the work before it dries, costing everyone more time, money, and trouble.

Contact Concreate if your joints are jiggling

Joint filling may be narrow work, but it requires deep knowledge of concrete and its related materials. Our highly skilled staff have years of experience filling joints in concrete and wrangling rubber snakes, so get in touch. You can contact us by query form, email, and telephone!