When it comes to the Crunch
Oh that delicious chocolate bar with the light and airy honeycomb centre…Although I never liked it at all I have a lasting memory of my Grandmother syphoning boxes of the stuff from the working men’s club for me to ‘enjoy’.
Fast forward to today and the airy honeycomb centre is reserved for cast concrete walls and contractors scratching their heads for a solution. I’m writing this blog because recently there has been a spate of Honeycombing problems that have been compounded by carrying out knee jerk reaction repairs. In panic mode swathes of decent concrete surfaces have been lost in refacing walls and columns. Blowholes obliterated, joint and board lines lost and surface flatness and tone compromised! In effect the concrete has been lost to a bag and a half of full cream fairing coats.
However…Honeycombing might look and taste awful but it’s no big deal to fix so in the wise words of the Cadbury’s Caramel Bunny: “take it easy.”
What is Honeycombing?
Honeycombing is voids (sometimes very large) in the concrete surface.
It is normally present at the base of a pour, edges or around knockouts. This is important as it makes it easier to fix and remove from view.
Honeycombing is nearly always accompanied by a dark ‘cloud’ of cement rich concrete. This cloud cannot be removed and rubbing it will make it worse. I can be hidden though (more later).
Honeycombing looks awful and very often there is a knee jerk reaction to fix it quickly.
Honeycombing is rarely a structural defect but if it is extensive it would be wise to consult a structural engineer.
Rebar may be exposed so it would be wise to fill with a proprietary repair mortar soon after discovery (more later).
Why is it caused?
Under compacted concrete. Essentially the matrix has not completely filled the formwork.
Grout loss
Incorrect mix design (ratio of fine material to coarse material)
The Concrete Society have a page on Honeycombing here: http://www.concrete.org.uk/fingertips_nuggets.asp?cmd=display&id=454
Another useful brief link is: http://www.concreteconstruction.net/Images/Troubleshooting%20Common%20Defects%20in%20Vertical%20Cast-in%20Place%20Concrete_tcm45-342496.pdf
To further clarify, under compaction may arise from:
Dropping concrete from a height within the form causing aggregate bridging*
Congested reinforcement causing aggregate bridging*
Pumping concrete and not using a Tremie line to place the concrete.
Pour rate is too fast.
Low slump concrete.
Difficulty in reaching the base and corners with a vibrator.
Unlit formwork interior.
Unplanned compaction points.
Human error.
*aggregate bridging is where the concrete gets clogged up on the reinforcement creating a void below.
How can it be prevented or reduced?
Light the interior of the form really well so you can see what’s happening and where to place the line.
Assign someone to be the ‘eyes’ of the pour within the formwork.
Mark on the top of the formwork compaction positions of equal distance so you know where to vibrate.
Mark on the top of the formwork the position and width of any knockouts within the formwork.
Mark depth points on the poker with tape every 300-500mm and number them or have someone remove the tape after each poker pass.
Calculate the pour rate so it is as slow as possible.
Get the Tremie line in as far down as possible. “The gap between the reinforcement was too small so we designed the wall to be thicker” [quote from recent CIRIA ‘Issues with concrete’ workshop] http://www.ciria.org
Always be methodical and poker with equal measure.
Ensure grout tight formwork
Use a cohesive well balance mix design
What if I get Honeycombing?
If it is deep and the rebar is exposed check with an engineer that it is ok to fill with a proprietary repair mortar such as Fosroc and an associated primer. See these links for details: http://www.fosroc.com/product/show/nitoprime-zincrich-plus and http://www.fosroc.com/product/show/patchroc
Leave the repair 3-5mm shy the final surface for cosmetic repair keeping the surround wall clean.
Take some images of the repair and send it to jonathan@greymatterconcrete.co.uk and I can give some advice.
In conclusion… try to avoid it but don’t panic as honeycombing really is not a major problem to sort out even with the crumbliest flakiest concrete. Control any patchwork repair. Always keep any intervention to a minimum from the outset so as to respect the ‘concreteness’ of concrete.
