Monday, December 13, 2010

MIC part 2: Quality Concrete

Continuing on with discussion of MIC in septic tanks we have first identified what the problem is, now it is time to talk about solutions.  Although many additive solutions are present at this time, none have been properly tested in these situations as this is a time dependant problem.  Therefore, only time will tell if newer additives in concrete will help aid the concrete's resistance to corrosion.  The one definitive we have as precasters, is the ability to produce a quality product for your customers.  Above all others, producing quality concrete is a good starting place for fighting MIC.  With an inadequate mix, the Thiobacillus bacteria essentially has less work to do to destroy your customer's septic tank. 

Now what is meant by an inadequate mix?  Inadequate mixes are those that have reduced strength and improper water to cementitious ratios.  Right now, most codes specify either a 4000 or 5000 psi concrete compressive strength and a water to cementitious ratio no greater than 0.45.  That being said, why are we still finding concrete with water to cementitious ratios in the 0.50's and strengths less than 4000 psi?  It is my opinion that this is largely due to inadequate eduction, lack of care in what we produce, and given the times a reduction in product costs to keep prices down.  I will say, though, that now is not the time to save pennies by making less quality products.  Now is the time to get educated and make the best possible product for customers!!! 

Cement requires very little water to become hydrated enough to start the process of forming paste.  The rest of the water goes toward aiding in workabilty and making the concrete easy to form.  However most precasters should have access to admixtures that can aid in workability without the addition of adding water.  Using modern superplastizers there is no reason why a precaster cannot get his/her water to cementitious ratio down around the .40 region or lower.  And the lower the water to cementitious ratio, the lower the amount of pores in concrete, thus making a very durable product.  Modern self compacting concrete admixtures can make concrete almost like water while still maintaining high strength and low porosity.

In addition strength is something we have great control over.  Cement is typically the number cost in a concrete mix design.  The more cement the greater the cost in your mix.  However, the more cement the higher the strength.  Given the recent economic scale, it is easy to try and cut cost by making a less cement rich mix.  This, however, is not the right move!!  I would be the first to caution, cutting cost by making a more inferior product is not good practice.  Most codes specify a 4000 psi concrete for tanks.  Recent studies have found that concrete around 5000 psi or higher will stand up to more adverse conditions than a 4000 psi mix.  Some municipalities even spec a 6000-7000 psi mix for wastewater applications. Again, given the advances in concrete admixtures a 5000 psi batch is not hard to attain by today's standards.  Setting the bar at a 5000 psi minimum is a great starting point. 

In addition to mixing adequate concrete, checking and testing is also just as important.  One should keep constant monitor on what is coming out of the mixer.  Check your air, check your strengths, check your yields.  Keep tabs on everything.  That is the only way to learn!  The days of mixing on the fly are over.  Engineers and GC's are demanding more and more of us everyday, keep yourself in check.  Also, use your resources.  Talk with your cement salesman, talk with your admix salesman find out what is going on in the industry.  Put them to the test, tell them what you want out of your concrete.  Keep yourself educated!!!

To summarize this I'm going keep this short and sweet.....THERE IS NO SUBSTITUTE FOR GOOD QUALITY CONCRETE...Learn it, live it!

Sunday, December 12, 2010

Microbially Induced Corrosion in Septic Tanks pt 1

Microbially Induced Corrosion or MIC has been a problem with the concrete industry for years; however, it's impact on the precast industry has really been brought to the forefront recently with the discovery of rapid corrosion in septic tanks.  Septic tanks that normally have a service life 30-50 years are failing in a matter of years.  There are many aspects of this problem from how does the MIC happen to quality of concrete to actual relevance in an already struggling infrastructure.  Although it is my intent to cover most of these issues in most parts I would first like to start with an introduction.
MIC is a form of corrosion that is very dependant on bacteria.  Naturally occuring bacteria in septic tanks exrete hydrogen sulfide gas (rotten egg smell in sewerage).  Now this gas naturally stays dormant in septic tanks, until a form of turbulence is introduced.  When the turbulence is introduced the gas is released and floats in the air space of the tank.  Another occuring bacteria the thiobacillus bacteria converts the hyrogen sulfide gas into hydrosulfuric acid which eats concrete.  So looking at this from the outside you say ok eliminate the sulfur production or the thiobacillus bacteria and everything should be ok.  However it is those two aspects that make this problem one of the hardest to figure out.  Where does all the extra sulfur come from? Medications? Foods? Cleaners? Fertilizers?  How do we kill the thiobacillus bacteria?  There are modern additives that can be put in concrete to kill the bacteria, however I have not had the chance or heard from a first person perspective how effective the additive is.  Another approach is to realize that the thiobacillus bacteria cannot strive on surfaces with pH's greater than 9.  Fortunately or unfortunately, concrete is very basic pH 12-14.  It is the progression of the wastwater becoming more and more acidic that allows the thiobacillus to grow more prevalent on the concrete surface.  So as you can see, this problem just from the beginning is really complex.  In future blogs I plan on touching on topics such as quality concrete, coatings, additives in concrete, impact on infrastructure, the future of MIC, and other topics so stay tuned.   I leave you with a few pics to see how bad this problem is.  Thanks...Aaron