Epoxy application - cold joints and concrete trenches

Dealing with old concrete can be very different to new concrete, but what do you do if you've got both on the same job?

Now, I’m not talking about a new slab being located somewhere in the same building as an old slab, because when separated nice and cleanly like that you can tackle each one individually without too much extra to think about. No, what I am talking about here is entirely different. Something that often represents a bigger challenge; something called a cold joint.

A cold joint is a discontinuity in the slab where one section of concrete has hardened before another is laid right up against it (without a flexible joint installed in between). This delay could be a matter of hours, days, or even years, but it can cause problems regardless.

While these joints can simply be a case of two slabs side-by-side, the tricky, worst-case scenario often comes when a new section of concrete gets plonked into an old slab, i.e. a trench. Trenches are typically cut to allow pipe work, drains or electrical gear to be installed and there are a few important things I want to highlight that don’t always get considered when going over the top with a coating.

Trenches and hydrostatic pressure

Ideally the cuts made when forming a trench don’t go right through to dirt as this can breach waterproofing measures and result in large cracks or blisters in the coating through hydrostatic pressure. As a flooring contractor, it pays to work with builders closely to ensure waterproofing is kept intact or at least know what was done so that any necessary precautions or corrections can be made before coating.

Trenches and concrete porosity

More often than not you'll find the concrete used to fill a trench isn't the same as the surrounding material and this can lead to a few problems as well. Cheaper concrete mixes and different finishing techniques can lead to a softer, more porous surface. Not only does this raise doubts about how well the trench material will bond to the sides of the existing slab, it also means you have to be very careful when you coat it.

The solution to both of these issues is the use of primers and sealers. Priming the sides of the trench immediately before the fresh batch is poured can address the concrete-to-concrete bond concerns, while sealing the new batch after it has cured may be necessary to stop the coating from soaking in too much. A quality low-viscosity, clear epoxy can be a good fit for both jobs in most cases.

Trenches and green concrete

Cold joints will rarely be given the full 28 days to cure like standard concrete because most renovation jobs don't have the luxury of shutting down for that long. With that in mind, it’s a big advantage to use products that can handle the moisture and alkalinity of green concrete. Some solventless epoxies are capable of this, with other options including water-based epoxies and possibly colloidal silica solutions.

Alternatively, some trenches use “deep fill” products rather than concrete that are suitable for coating within 24 hours. If that’s the case and there are no compatibility issues to speak of, you can get on with coating the next day and side-step the fresh concrete concern. Just like everything though, if there’s any doubt on how your product will go over the top then you should test it first!

If you keep on top of the considerations above, applying over a cold joint or trench needn't be a major headache. I personally believe the best thing you can do is communicate with the builder at every opportunity so that you know exactly what’s going on and can make informed decisions on how to go about your task. This, unfortunately, is sometimes easier said than done though!

Take care and keep smiling,

Jack

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Want to learn more about this and related topics? We suggest the following Epoxy School courses: