Hi @Anniep2480,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
If you are looking to replace the material in the expansion joint that is cut in your concrete driveway, you would first have to remove what is currently there. If it is a polyurethane sealant, which is what is typically used for this kind of joint, you will be able to run a utility knife along each edge of the concrete to cut it away from the concrete, and then it should peel out relatively easily. If this is the case, please remember to wear gloves, long protective clothing and enclosed shoes to reduce the risk of lacerations.
If it is a harder material, which would be less common, you would need to use an angle grinder with a cut-off disc or masonry cutting disc to remove it. If you use this method, please wear the same protective equipment as you would when using a utility knife, plus ear and eye protection.
Once the sealant has been removed, clean out the groove to a depth of around 50mm. This could be done with a vacuum and a flathead screwdriver to agitate any dirt within the crack.
When it is clean, start by inserting some Sika 10 x 50mm 6m Abelflex Expansion Joint Filler Foam into the crack so that the top sits around 10mm below the surface of the driveway. This will reduce the amount of sealant required while maintaining the ability of the concrete to expand and contract.
You can then cover the gap using Sikaflex 11FC Purform. If you'd like a straight, clean application, run a length of painter's tape along each side of the crack, then apply a bead of your sealant that is wide enough to fully span from the concrete on one side of the crack to the other. It is important that it touches each side of the gap so that the gap is fully sealed.
You can then use a soaking wet rag to smooth off and remove any excess sealant before pulling your tape to reveal a nice, smooth expansion joint. Keep a bucket of water and a few spare rags handy as it can be a bit of a messy job.
Check out How to fill a joint in a concrete slab for an example of this process in application. I did not tape off the crack as it wasn't in a visible location, so aesthetics weren't too important to me. In your situation, it would be worth adding this additional step to get a nice-looking joint.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
