All About Abelflex (A Story about Joints)

Abelflex is a compressible foam strip cast into concrete to create a gap, known as an isolation joint, between new concrete and something else: a house wall, another slab, a column or a pipe. It is often spelt “Ableflex”, but the brand name is Abelflex.

It is a small, cheap product, but whether it is installed and finished properly can decide whether water ends up in your footings.

What Is Abelflex?

The technical answer is that Abelflex is a closed-cell, cross-linked polyethylene foam expansion joint filler.

In plain English, it is a plastic foam full of tiny bubbles that are not connected to each other, so water cannot soak through it. It is similar to the foam used in pool noodles. The bubbles make it soft and compressible, so the gap it creates can close up slightly if the concrete or the structure next to it moves.

Abelflex is a proprietary name. Other manufacturers make equivalent foam joint fillers, and those are generally fine to use where they have the same features.

Abelflex Sizes

Abelflex is sold in rolls, commonly 10 mm thick in heights such as 100 mm and 150 mm, to suit typical slab and path thicknesses. Other thicknesses, such as 20 mm, are available for wider joints. A self-adhesive “sticky back” version can be stuck to a wall before the concrete is poured.

The height should match the full depth of the slab, so the new concrete is separated over its full thickness. Check the manufacturer’s current range for exact sizes and roll lengths.

What Is Abelflex Used For?

Abelflex is used to make gaps in concrete. It is cast in place and left there.

Engineers specify that gap when they do not want new concrete to bond to, or push against, something next to it. Common uses include:

• Driveways, paths and patios against a house. The new slab can expand, shrink and move slightly without pushing on or cracking the house.
• New slabs against existing slabs, where the two may move differently. Steel dowels are sometimes added across the joint so the surfaces stay level and do not create a trip hazard.
• Around columns and posts that pass through a slab.
• Around pipes and penetrations, so the pipe is not broken if the slab heaves or settles.

Isolation Joints: The Joints in the Title

The gap Abelflex creates is called an isolation joint. It isolates the new concrete from whatever it sits against.

Isolation joints are different from control joints, which are saw cuts or grooves that encourage shrinkage cracks to form in a straight line. Abelflex is for isolation, not crack control.

The Part Everyone Forgets: Seal the Top

Abelflex does not seal the joint. On its own, water can run down the gap between the slab and the house and into the ground beside the footings.

That is why many Abelflex products have a tear-off strip along the top. Once the concrete has cured, the strip is pulled out and the gap is filled with a flexible sealant, such as a polyurethane joint sealant like Sikaflex. In a wider joint, a foam backer rod may be used under the sealant to control its depth.

This step is often missed. A concreter pours a carport slab or path, the concrete cures, and nobody comes back to seal the joint. If you see Abelflex with the tear-off strip still in place and no sealant, the job is only half done.

It matters because water running down an unsealed joint lands right at the edge of the house footing. On reactive clay sites, that concentrated moisture can contribute to slab heave and cracking. A tube of sealant is a very cheap way to avoid it.

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Where to Buy Abelflex

Abelflex and equivalent foam joint fillers are stocked by building and concrete supply outlets and some hardware stores. Equivalent products from other manufacturers are generally acceptable where they match the features described above: closed-cell foam, the right thickness and height, and ideally a tear-off strip.

Water, Joints and Footings

Unsealed joints are one of several small details that let water into the ground beside a house. Others include paving that falls towards the house, garden beds against the slab edge and badly installed ag drains. Each looks minor, but together they are behind a lot of the moisture-related cracking we investigate.

If your house is cracking and you suspect water around the footings, Cornell Engineers can investigate the cause and advise on what needs to change. See our drainage and stormwater advice and forensic structural engineering pages.

Send us the property address, photographs, a short description of what you have noticed and any reports you have. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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