How to fill a joint in a concrete slab

[JacobZ]

Difficulty: Beginner

Gaps between concrete slabs are known as expansion joints. They help prevent cracks by allowing concrete to expand and contract.

These gaps might appear unsightly, so filling them with a flexible foam and sealant can improve appearance and help prevent water ingress, weeds and pests.

Steps

Step 1

Clean the crack.

Clean any dirt or debris from the crack using a brush, leaf blower, vacuum or a combination of these tools. Removing debris provides a clean surface that allows the sealant to form a solid bond.

 

Step 2

Cut and install the filler foam.

Lay out and cut the filler foam to length. Depending on the depth of the crack, you may need to cut the foam with a retractable knife or scissors to suit both length and height.

Compress and push the filler foam into the gap using a narrow tool such as a screwdriver or scraper. Ensure the foam sits 5-10mm below the surface of the concrete.

  

Step 3

Apply the adhesive sealant.

Cut the nozzle with your tin snips or retractable knife, so it is wide enough to apply a bead that will almost fully cover the width of the crack. Ensure you wear eye protection and cut-resistant gloves.

Cut the Sikaflex sausage end to open it. Insert the sausage into your caulking gun with the nozzle.

Apply the sealant to the gap, working backwards towards yourself. Once you have reached the end of the gap, look back along your sealant. If there are any low spots, apply more sealant to that area.

    

Step 4

Smooth off the sealant.

Place a soaking wet rag over your finger, then run along the sealant line to smooth the sealant into one uniform, unbroken bead covering the entire gap.

Repeat on any other expansion joints and admire your work.

   

Materials

• Abelflex joint filler foam
• Sikaflex 11FC adhesive sealant.

Tools

• Brush, blower or vacuum
• Screwdriver or scraper
• Caulking gun
• Retractable knife or scissors
• Tin snips
• Bucket
• Rag.

Images

[Nailbag]

Awesome tip there @JacobZ that will help a lot of people with this common issue

Nailbag

[HandymanDean]

If you run masking tape down either side of the gap, you can then take the gap filler all the way to the top. Once filled and smoothed off, peel off the tape and you have a nice neat edge

[JacobZ]

Hi @HandymanDean,

A warm welcome to the Bunnings Workshop community, it is fabulous to have you with us.

Thank you for the advice, it would certainly help to get a nice clean finish.

Jacob

[Yolzy]

I saw a YouTube video where a guy uses a foam backer rod…same thing right? Also he just leaves the silicone alone as it’s a self leveller..should we run our finger over it? Or does it just shrink when it dries? Another question , he adds sand in a narrow crack if the foam can’t fit, how can the sealant adhere to it if sand is a loose material/substance? Tube shapedDo we have this in Australia?

[JacobZ]

Hi @Yolzy,

Backer Rod and AbelFlex are pretty much the same thing. They are mould-resistant foam that will compress and wedge itself in the gap to minimise the amount of sealant required to cover/fill the gap. As they are foam, they will also allow expansion and contraction of the concrete in the same way the flexible sealant will.

It doesn't appear that self-levelling sealants are readily available in Australia. Sika, who are our main supplier for this kind of product, produces them, but they are only orderable in industrial quantities.

It's easy enough to just smooth the sealant out yourself. Just place a soaking wet cloth over your finger and run it along the sealant. 

Let me know if you have any further questions.

Jacob