Friday, January 14, 2011

MIC pt 3: Additives vs Coatings

To continue talk on MIC Corrosion, the next topic is the addition of additives and coatings along with quality concrete.  I want to make this known upfront, I am a firm believer in the addition of additives to concrete in contrast to post applied concrete.  My  reason being is that I think additives take out as much of the human element as possible.  With coatings you have a greater chance of not applying or not mixing chemicals correctly and thus leading to problems.  That being said, I do believe there is a time and place for coatings and if used properly they can be effective in extending the life of concrete products.  Now, I will have the opportunity to listen to a gentleman speak about a poly urea coating he uses effectively; what interests me is to see how this process works and how effective it is.  Poly urea is a coating similarly used in truck bedliners and I have really started to see this technology catch on quickly.  My only other question to ponder with regards to coatings is, will the coatings degrade? and if they do will they get mixed into the groundwater? and what are the repercussions of that?  The water table is under threat already and compromising it more with chemicals that soil can't break down is a bad news for everyone. 

Additives can post their own problems, but also have a very well define solution to the problem as well.  Most additive agents can be separated into two categories densifiers and anti-microbial. Densifiers are by far the most common as any precaster anywhere has access to tens of hundreds densifiers ranging from liquid to powder.  Some common densifiers  include Granulated Blast Furnace Slag, Fly Ash, and crystalline additives.  What these additives aim to do is form crystalline structures internally in the mix and close the pours making the concrete more dense and thus more resistant to chemical attack.  These products have been used for years and have been proven successful in many different situations.  The only problem with these additives is the effect on fresh concrete.  A lot of times these additives can change water demand, admix demand, workability, and finishability of your mix.  These properties can be compensated for easily, but should be taken into account before mixing.  Another disadvantage is cost.  These additives can add  $50-$100 more per yard, but I tend to believe you would be paying for a higher end mix as well.  A lot of people will see increases in compressive strength and durability that will easily justify the extra cost.  The other additive that I am not too familiar with is anti-microbial additive.  This is what I call microbiological concrete, "living concrete".  What essentially happens is the anti-microbial admix is internal in the concrete and kills the Thiobicillus bacteria upon contact of the concrete.  Again, I am not familiar with this additive, but I have heard there has been some success stories coming from using this product.  I am starting to see it speced more and more, especially in wastewater treatment plant applications.  The only disadvantage I have heard of is that it was ineffective.  This is hard to justify though because this could have been a result of poor mixing, or bad starting concrete.  Again the best place to start with MIC abating is a good mix design and good quality control.  I think given a good strong base mix, this additive can have a lot of benefits to your design.

As of late there has been a lot of discussion on the topic of revamping the oldest concrete known to man.  The ancient Romans found that if they used the ash from the volcanoes it had a good glue like property that made concrete.  This technology has been little used over the centuries, but as of late has come back with talks of corroding products and degrading infrastructure.  If the Romans built structures hundreds of thousands of years ago and they are still standing, it must have some good mix properties. Essentially the concrete has a different chemistry that makes it more resistant to corrosion and aging.  I know of two companies having great success using this product in very corrosive environments.  It is interesting to find that the technologies of old are having successes today. 

So all being said, there are many advantages and disadvantages to either an industrial coating or  additives to the concrete.  Again I am firm believe in additives over coatings, but there are some applications in which coatings can perform well and keep the concrete protected for years on end.  One should be careful and follow the mix designers parameters when experimenting with additives and coatings.  Used properly they can effectively help in extending the life of concrete longer than if it were just mixed plain.  Again, but one should start with a good mix design and good quality control before even considering these additives. NO SUBSTITUTE FOR GOOD QUALITY CONCRETE!