THE CONCRETE SURGERY
Using additional fibres to ‘Dry up’ the concrete
Dr. Concrete, PICS UK.
29th AUGUST 2018
Ok, so Dr Concrete #10, ‘Adapt your Concrete Mix for the Winter’, was about changing the concrete mix design in the winter to reduce the time hanging around on site waiting for the concrete to ‘go off’, and in the process the additional benefit of getting a better quality job.
The three factors that could help towards achieving this goal were to have more cement, possibly use larger aggregate, and take a lower slump (drier) concrete. Bingo! Sorted!
Well not quite.
Although the concrete companies normally get the main parts of the concrete mix design as per the order, for various reasons, the slump of the concrete can and does vary, regardless of what has been ordered. The concrete companies have a degree of tolerance as to what they deem to be acceptable variations from the ordered slump and if the delivered concrete falls within those tolerances, there really isn’t a whole lot that can be done about it if it turns up on site drier or wetter than you ordered.
In the winter, it’s unlikely to be ‘too dry’ and if it is, simply add some water to the ready mixed concrete, just like you would do in the summer.
If it turns up a ‘bit wetter’ than you’d have liked, this isn’t normally a great problem. Assuming you have more than the bare minimum of CSH on site, you can dry the surface up using some extra CSH.
If the concrete is delivered significantly wetter than you’d have liked, this can be a problem, regardless of how much extra CSH that is available on site. This scenario creates three potential problems.
- Poor water / cement ratio, resulting in a weakened surface
- Untrue colour, resulting in a colour difference, particularly problematic on multi pour
- Poor quality print
- A very late finish on site
You order a target slump of between 50 → 90 and the concrete company are within their tolerances to supply it to you -40 →+50.
Order 50, and you could get 10 → 100
Order 60, and you could get 20 → 110
Order 70, and you could get 30 → 120
Order 80, and you could get 40 → 130
Order 90, and you could get 50 → 140
These are VERY BIG variations and you are legally obliged to accept the concrete if it falls within these ranges!
Fortunately these big differences are rare, and they don’t normally happen, especially if you have a good working relationship with your local concrete supplier. However, they do still happen and if the concrete turns up too wet, in an extreme case, it could be a midnight finish on site.
So, what can be done?
Either accept that it’s going to be a long torturous day on site, or, when you start unloading the concrete, and you identify it’s ‘miles too wet’, add some extra fibres to the concrete, and get the driver to spin it on full revs for at least 5 minutes. Add one bag per cubic metre. So, a 6m3 load add 6 bags of fibres.
It’s amazing how much it dries up the concrete! And for £30, the team ends up going home hours earlier, plus, almost always, it will result in a better quality job and a happier workforce!
Where do I get fibres from?
Dr Concrete isn’t on the sales side of things, but in this instance, as some of you may not know where to get hold of them, I do have to let you know that PICS are able to supply fibres to you should you require some.
Red Shute Hill Industrial Estate