How to Seal an Expansion Joint in a Floor, Slab or Wall
To seal an expansion joint in a floor, slab or wall, clean the joint back to sound, dry material, push in a backer rod so the sealant depth is about half the joint width, mask the edges, then gun in a low-modulus polyurethane sealant such as Akfix P635, tool it against both sides and pull the tape while the sealant is still soft. Work between +5 °C and +40 °C. The backer rod matters as much as the sealant: it sets the depth and makes the sealant bond to two sides only.
Which sealant for which joint
The joint opens and closes as concrete heats, cools and shrinks, so choose by how much it moves.
| Joint | Sealant | Movement (Akfix figure) | Price |
|---|---|---|---|
| Floor and slab expansion joints, precast panel joints, joints between different materials, inside or out | Akfix P635 PU sealant, 600 ml | 25% | KSh 1,450 each, KSh 16,200 for 12 |
| Window and door perimeters, facade and cladding joints, marble and stone | Akfix 915N weatherseal silicone, 600 ml, black or transparent | Low modulus, high elasticity | KSh 950 each, KSh 10,800 for 12 |
| Non-moving interior joints and cracks that will be painted | Akfix AS606 siliconized acrylic | Up to 15% | KSh 450 each |
For the wider comparison of the three families, see silicone vs PU vs acrylic sealant.
Akfix P635: the default for floors and slabs
Akfix describe P635 as a one-component, low-modulus polyurethane that cures with moisture from the air. Akfix publish it for expansion joints between different construction materials, movement and connection joints in floors, pedestrian and traffic areas indoors and outdoors, and joints between precast concrete panels. The data sheet lists adhesion to cement-based materials, brick, ceramic, glass, wood, galvanised and painted sheet iron and various plastics, with no sag and no shrinkage.
When 915N is the better choice
Akfix make 915N specifically for weather sealing and glazing. For window, door frame and facade joints that face sun and rain all day, use 915N rather than P635. The P635 data sheet warns that prolonged direct sunlight can discolour it. Akfix recommend 915N for joints wider than 10 mm and narrower than 35 mm, at about 2:1 width to depth. It is not over-paintable or for swimming pool joints.
When AS606 is enough
For non-moving interior joints that will be painted, such as plaster cracks, skirting and cornice joints, AS606 does the job at a fraction of the cost. Akfix limit it to joints 5 to 25 mm wide and say it should not be used where the joint is permanently exposed to water. With that 15% movement limit, it is not the product for floor or slab expansion joints.
Step by step
1. Clean and dry the joint
Akfix's instruction for P635 is that joint surfaces must be clean, dry and free of all contamination. For concrete their data sheet says brush and remove dust. Rake out old sealant and loose material, then brush and blow out the dust. The P635 data sheet also says do not apply on wet or frozen surfaces or through standing water, so after rain or washing, wait until the joint faces are properly dry.
2. Set the depth with backer rod
Akfix give a 2:1 width-to-depth ratio for P635: a 20 mm joint is filled about 10 mm deep. Push a foam backer rod into the joint to that depth. As general practice, choose a rod slightly wider than the joint so it grips the sides, and press it in evenly with a blunt tool. Akfix name backer rods and bond breakers as the way to make the sealant adhere to only two surfaces; where the joint is too shallow for a rod, a bond breaker in the bottom of the joint does that job.
3. Prime only if the data sheet says so
The P635 data sheet calls for clean, dust-free concrete and does not call for a primer. AS606 is different: on porous surfaces such as concrete, stone, cement and plaster, Akfix say a primer of one part acrylic sealant to four or five parts water can be applied.
4. Mask the edges
Run masking tape along both edges, as Akfix suggest where necessary. Our guide to painting tools lists the tape sizes we stock.
5. Gun the sealant
Cut the nozzle to suit the joint and push the bead forward, filling from the backer rod up so no air is trapped. Akfix describe P635 as thixotropic with no sagging, which helps on vertical wall and panel joints.
6. Tool and de-mask
As general practice, tool the bead straight away, pressing the sealant against both joint faces. Then pull the tape off while the sealant is still soft, as Akfix instruct. Akfix give a tack-free time of 20 to 70 minutes at 23 °C and 50% relative humidity; as a general rule, keep traffic off until it has cured through.
How many 600 ml sausages do you need?
Akfix's consumption table for P635 gives these approximate lengths per 600 ml sausage:
| Joint width x depth | Metres per 600 ml | Sausages for 10 m | Sausages for 50 m |
|---|---|---|---|
| 15 x 8 mm | About 5 m | 2 | 10 |
| 20 x 10 mm | About 3 m | 4 | 17 |
| 25 x 12 mm | About 2 m | 5 | 25 |
The sum is joint length divided by metres per sausage, rounded up. For example, 36 m of 20 x 10 mm joint is 36 ÷ 3 = 12 sausages, which is one box of 12 at KSh 16,200 rather than 12 singles at KSh 17,400. These are approximate figures before waste; as a general rule, add a sausage or two for uneven joints.
The three failures behind most failed joints
Three-sided bonding
If the sealant sticks to the bottom of the joint as well as the two sides, it cannot stretch freely when the joint opens. Akfix are explicit that sealant should adhere to two surfaces only. In general, a three-sided bead tears early.
No backer rod
In general, without a rod the sealant runs to the bottom, bonds to it and uses far more material.
Filled too deep
As general guidance, a deep, thick bead resists stretching and puts more stress on the bond line, so it is more likely to pull away from the concrete. It also cures slowly: Akfix give a minimum of 2.5 mm a day, so a deep bead stays soft for longer.
Limits to know
- Akfix say P635 is not for joints under continuous water immersion, and not for totally confined spaces where it cannot get moisture from the air to cure.
- Waterproofing a flat roof is a different job; see how to waterproof a flat roof in Kenya.
Where to buy
All three sealants are in our Akfix collection, with P635 and AS606 also in PU, acrylic and waterproofing. Collect from our Nairobi trade counter at Royal Business Park, opposite Complast, Old Mombasa Road, Monday to Saturday, 8:00 to 17:30, with free pickup, or order for countrywide delivery at a flat KSh 350, free on orders over KSh 15,000. Contractors buying by the box can request a trade quote on our trade page. To check quantities for a job, send us the joint widths and lengths on WhatsApp.
Common questions
What is the best sealant for concrete expansion joints in Kenya?
For floors, slabs and precast panels, a low-modulus polyurethane such as Akfix P635, which Akfix publish for movement joints in floors and for pedestrian and traffic areas, with a stated 25% movement capability.
Can I seal an expansion joint without backer rod?
Only if the joint is already shallow enough for the 2:1 ratio and you use a bond breaker in the bottom; otherwise it bonds to three sides.
Can I paint over the joint?
The P635 data sheet lists it as paintable and AS606 is over-paintable. Akfix state 915N is not over-paintable.