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Hello @pravinpk
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about footings.
Can you please provide us with a little bit more information in regards to what these footings are holding and how big they are. If you can share a picture with the community, we can get a better idea of the footings current condition. We can then make recommendations on what product to use.
If you need a hand posting the pictures, please let us know.
Eric
Hi @pravinpk,
Thanks for the extra detail. For light-duty steel post footings like the ones you're working on, using properly embedded reinforcement bars and ensuring a clean, roughened surface will already give you a decent mechanical bond between the old and new concrete. That said, it makes sense to want to go a step further and use a bonding agent, especially with the added moisture risk from clay soil and being outdoors.
You’re right that BondCrete isn’t suitable here. Being PVA-based, it doesn’t perform well when exposed to constant moisture or structural stress. While epoxy bonding agents like SikaDur 32 would give you the best result, they’re understandably expensive for a small job. However, if you're looking for a product that will glue the two sections together, epoxies are really the only option. Anything else improves the bonding capabilities. I think it's important to note that we really shouldn't have to glue the sections together or even increase the bonding characteristics. Your new section should be tied in with reo and large enough to self support itself in the ground. So, you're 200mm section might need to be 400mm deep.
A reliable method is to mix a cement slurry with a bonding agent such as Bondcrete and brush it directly onto the old concrete just before pouring the new mix. You want to pour while it’s still tacky, not dry. That way, you’re getting both chemical and mechanical adhesion. Boncrete is used extensively on concrete slabs and screeds exposed to water, and I haven't heard of moisture excluding its use.
If the existing footing is solid and you’ve done a proper job tying in the reo and roughening the surface, it’s possible you could skip the bonding agent altogether and still get a strong result, especially since you're only extending by about 200mm and the structure isn’t particularly heavy. Still, adding a bonding agent is a good call if you want extra reassurance without the cost of epoxy.
Please let me know if you have any questions.
Mitchell
BondCrete is fine where it's sealed within concrete (like when used as a primer or admixture) @pravinpk, because once it's covered and cured, it's protected from moisture. However, for a direct concrete-to-concrete bond, it is not ideal. Being PVA-based, BondCrete can slowly break down if exposed to constant moisture over time, especially in structural joins.
For this kind of work, a structural epoxy like Sikadur 32 would be much better. Epoxies are waterproof, extremely strong, and made for bonding old and new concrete securely.
Mitchell
I think the previous sentence added some context "BondCrete is fine where it's sealed within concrete (like when used as a primer or admixture) @pravinpk, because once it's covered and cured, it's protected from moisture.".
So, if the Bondcrete is within concrete, it's fine. If it's mixed into the concrete, it's fine. If it's mixed into a slurry, it's fine. But in a direct concrete-to-concrete bond, if you paint it onto the existing concrete, then water can penetrate this line and break down the bondcrete as it's not mixed into the concrete, but rather just painted onto it.
I hope that makes sense.
Mitchell
