Rapid Set Concrete: Myths 

Rapid Set concrete. It sounds great: it’s quick and you can get on with finishing the job in under one hour. Right? Well, not if you’re using aluminium posts!

Even on the packaging and technical specification sheets, Cement Australia warns against using Rapid Set for securing aluminium posts.

But why? Surely, it’ll still do the job and much quicker than normal concrete?  No, don’t do it!

There are a host of reasons, but simply put: Rapid Set concrete eats the aluminium, and in as little as six months your new aluminium fence or Hideaway Cover (www.Hideaway-covers.com.au) could be falling over. Then you have to re-do the job all over again with some sort of fix-up to reinstate the structural integrity that you had originally.

The chemical additives and alkalise in Rapid Set – the very compounds that allow it to set so quickly, are the demons of aluminium which cause it to corrode – and very quickly! It also does not have the same strength as normal aggregate concrete.  Let’s face it: if you’re going to the trouble of cementing a post into a hole…. you want it to stay there for a long time.

But – maybe you’re still thinking that there must be a work-around that allows you use it, right? No.

Myth 1: The aluminium post is powder-coated so it’s protected from the concrete! Yes, it’s protected but no, it won’t stop the corrosiveness of the chemicals in rapid set. The powder-coating itself does not provide adequate protection from these chemicals.

Myth 2: I’ll wrap it in plastic or even thick builder’s plastic – that will fix it! No, it wont. As the rapid Set cures, it generates amazing heat which is part of the quick setting aspect of the product. This heat can soften and melt the plastic around the post. It’s also not just the heat, recrystallisation can be highly significant at around 200 degrees centigrade which can cause a greater than 50% reduction in the strength of cold worked alloys such as aluminium.

Further, how can you be sure that the sharp ends of the post have not pierced the plastic when it was put into the hole?  Can you guarantee that the tools you used to tamp down the concrete didn’t cut the plastic sleeve or the concrete got inside the post? Finally, you have to cut the bright orange plastic around the post once the concrete has set – with the plastic still most likely visible.  

Myth 3: OK, I’ll coat the posts in a bitumen-based product like Ormonoid. Really? Think about the cost and the additional drying time (24 hours) required to do all this just so you might be able to use Rapid Set. Plus, you will have to coat above where the concrete finishes to protect it from concrete splashes. This is going to detract from the lovely powder coated aluminium project you’ve just spent money and effort on. Remember the heat effect on bitumen. It’s just not worth it – and again, what if you scrape-off or put a nick in the coating when you’re tamping down the concrete with tools. It only takes one ingress point for the corrosion to start.

Myth 4: I’ll have plenty of time for adjusting the post and to finish off the concrete. Wrong. Rapid Set is deigned to be placed directly into the hole with water added and the once mixed in the hole – it goes off very quickly (15 mins). There is very little time for post or structure adjustments, finishing-off, and smoothing the surface to give a professional finish as there is is with traditional concrete.

Rapid Set concrete is undoubtedly a great product, but only if it is used as specified. 

It is designed for non-structural, light-footing projects such as letter boxes, timber and Colorbond fences, clothes lines, small concrete repair jobs and other non-load bearing minor projects requiring a quick (usually 1 hour) curing time to allow you to get on with finishing your project.

Rapid Set is fine for steel posts, it’s just not the right product for your aluminium posts! The only exception is if it is fully protected in accordance with specifications from the aluminium manufacturer.

Finally, be sure to check with Dial Before You Dig: (https://www.infrastructure.gov.au/department/media/publications/before-you-dig) to find out where electricity, water, gas and telecommunications cables and pipes are on your property.  This is free Government service; you just need to register on-line.

Summary: Aluminium posts should not be set in Rapid Set concrete as the aluminium reacts with the chemicals and alkalis producing hydrogen gas and a large amount of heat, which cause corrosion of the aluminium post. Instead:

• use normal concrete mix, or
• consider creating a concrete base and use surface-mount flanged posts fixed with Dynabolts (or similar).

Hideaway Covers recommends post holes should be 600mm deep (min 400mm) and 300mm round.  Each hole will require approximately 1.5 bags of normal concrete mix.

Keep it simple: use normal concrete mix and wait for the proper time for it to set.  Your job will thank you and you won’t be worried about it falling over in 12 months’ time! 

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