Blowholes: Are they REALLY such a problem?
Blowholes seem to have been the flavour of the month this January causing all sorts of debate and dilemma. We recognise Blowholes as a category of remedial works but within a framework that also sees them as friendly helpful nuances of fair faced concrete, giving it a sense of uncompromising honesty. Blowholes also perform a superbly useful role of helping to detract from other more urgent visual defects that warrant greater attention. Luckily they guide the eye away from nearby remedial works by being punctuated points of interest.
Blowholes occur when air gets trapped between the formwork face and the concrete surface. The more permeable the formwork face is, the less likely air is going to get trapped and the fewer blowholes there are going to be. Max Frank for example, produces a controlled permeability formliner called Zemdrain: http://www.maxfrank.co.uk/uk/products/formwork-technologies/zemdrain-formwork-liners.php
This is designed specifically to absorb excess moisture and entrapped air decreasing the final porosity of the concrete surface. This leads to (as they claim) a denser more resistant concrete. The principle take up of this formliner is not for visual concrete works but they mention that it can be used. It makes perfect sense for example, that for water treatment works or similar there are great advantages to reducing blowholes on the surface of concrete.
But what of Architectural Concrete per se? Are Blowholes really such a problem? In my experience when discussing Blowholes the biggest problem is the discussion. In all cases where Blowholes are concerned the focus of conversation lies within aesthetic appearance and not physical performance of the concrete itself. Given that in our opinion Blowholes allude to a sense of ‘concreteness’ and that evaluating Visual Concrete works is such a subjective activity anyway, why bother with trying to minimise them via the specification in the first place? Secondly, does giving Blowholes such focus of attention in the specification really lead to better overall Visual Concrete production?
The answer to why there is a defining quantity and size of Blowholes per m2 may come from using entrapped air as a means to ensure or measure correct compaction of the mix. Compaction may be an influencing factor for the presence of Blowholes but not the sole factor as stated earlier. ‘Concrete a studio guide’ by Michael Stacey has a useful list of formface materials and their relationship to permeability and Blowholes on page 69. If concrete is poorly compacted other issues such as honeycombing will prevail and problems with placement will be obvious. It may not therefore follow that good compaction equals concrete within the specification.
Furthermore what if there is paranoia or over focus on reducing blowholes and the mix is over compacted or subsequently over repaired (bagged up)? This could clearly cause more problems with segregation or with ‘bagged up unconcreteness’ than the existence of the Blowholes themselves.
I think a sensible view needs to be taken which may not necessarily follow the specification. Perhaps instead of picking up the specification, pick up on a few ‘in your face’ Blowholes (if really necessary) to be remedied. Do this in the knowledge that colour matching spread may be 40 times the size of the Blowhole itself. Perhaps then the informed choice and decision making will leave the remaining to hide other more probelmatic remedial works and keep concrete concrete.
Less is most definitely more and bearing this in mind Blowholes really will not be such a problem after all.