Airports - Unpaved Runways

Although not the focus of this Blog, from time to time I'm going to voice my opinion on subjects that I feel strong about.  The subject of pervious concrete & airports is one of them.

It seems that when this subject comes up, inevitably comments turn to loose stone and jet intakes.  Yes, large jet aircraft land on unpaved airstrips in third world countries, and yes they do have to put stone guards on the engines, and no we don't want the liability, and yes there's the jet fuel, and yes, and yes, and no, and no......

These concerns of some are valid as they apply to commercial aviation, but let me be very clear...what we are talking about is general aviation.  General aviation covers a large range of activities, both commercial and non-commercial, including private flying, flight training, air ambulance, police aircraft, aerial firefighting, air charter, bush flying, gliding, and many others.  Experimental aircraft, light-sport aircraft and very light jets have emerged in recent years as new trends in general aviation.

I still remember my first experience flying.  I was five years old and I sat in the back seat of a J-Cub behind my dad while we did touch-n-goes on a grass airstrip/cow pasture with the side windows folded down.  I was traumatized for years.  Years later, after I flew the coup, he bought a 200+ MPH Mooney and we were able to fly all over in style.  I grew up in Lakeland, FL, home of "Sun-N-Fun" the southeast's premier fly-in, so flying was a large part of my early years.  My dad no longer flies due to medical reasons and long ago my wife clipped my wings.  Perhaps one day......

Anyway, my point is, there are many general aviators out there, taking off and landing on, not only concrete and asphalt, but also on grass, dirt, sand, and gravel.

In fact here are some statistics on general aviation in the US:

Number of:
Student Pilots........................84,339 (2007)
Private Pilots.......................228,475 (2007)
Piston Engine Aircraft.............163,743 (2007)
Airports w/Paved Runways..........5,174 (2009)
Airports w/Unpaved Runways.......9,921 (2009)

Don't be quick to close your mind to this market.  As you can see, most runways in the US are unpaved.  They are grass, dirt or gravel, with the majority being gravel.  Averaging 2000' in length, these unpaved runways represent approximately 600 million square feet!  So if you come across anyone in general aviation asking about pervious concrete, please don't shut the door on the opportunity, it could be huge.

Here is an example of a project that is an ideal fit for pervious concrete:
http://www.workingwaterfront.com/articles/Improvements-planned-for-island-airstrips/13728

By the way, concerning last week's poll, the majority (75%) likes tooled joints.

Cleaning Pervious Concrete

Kentucky Ready Mix Concrete Association just uploaded this video of the cleaning of their parking lot.

Minimum Cement Content

Question of the Week (5/31/2010): What should be the minimum cement content for pervious concrete?

Poll Results:
700 lbs. ld/yd3 …………… 0%
600 lbs. ld/yd3 …………… 38%
500 lbs. ld/yd3 …………… 12%
None of the Above………. 50%

The NRMCA Pervious Concrete Certification program correctly states that “500 to 600 ld/yd3 would be a good starting point…..and further fine tuning can be accomplished by the concrete producer”. 

In the early days of pervious concrete promotion, statements were made about proportioning mixes, specifically 600 lbs/CY as a good starting point. This idea seems to have carried itself along and wound up in many documents about pervious concrete and ultimately in the minds of specifiers.  And what started as a "starting point" became a "minimum" in the conservative mind.  Not surprisingly, the poll result reflects this mindset.....38% chose 600 ld/yd3.

You need cement to make pervious concrete, however, caution should be taken when specifying minimum cement content. The producer needs the freedom to develop his mix design based on locally available materials and its' available gradations. Although I typically work with mix designs containing 500-600 ld/yd3, some aggregate gradations I’ve worked with required less than 500 ld/yd3. In fact there are some mixes (high sand) that require less than 400 ld/yd3!

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Be sure and take part in the new Poll: What is Your Preferred Joint for Pervious Concrete?

Yield....the dirty little secret

It's somewhat of a balancing act to develop a pervious concrete mix that places easily, performs well, and has good durability.  Finding this balance requires the producer to develop mix designs for each of his aggregates and also have the ability to adjust the mixes on-the-fly based on field density tests.  Unfortunately, a concrete producer or contractor that doesn't have the experience or ability to make these adjustments, will have problems on the job site as the aggregate gradations will typically change during the project.  There may be yield issues, mixes that wont drain, mixes that ravel, and the list goes on.

As for yield issues in pervious concrete, understand that changes in aggregate gradation always affect the yield.  I'll say it again for emphasis....changes in gradation  always affect the yield.  If the gradation becomes more uniform in the mix, the yield increases...if more fines are introduced, the yield decreases (it could be as much as 5% and still meet common specs).  I should note that pervious concrete mixes rarely become more uniform during the course of the job, but typically mixes increase in density as the gradation changes.  

This is really a lose/lose scenario from the producer and contractor's perspective.  Not only will the producer waste material like cement and admixtures (I'll explain later in another post), but the contractor will work harder to place a mix that will under-yield (I'll explain this later too).  This is why I encourage good communication between the producer and the contractor, good quality control, and do your density tests.

Remember pervious concrete's dirty little secret......keep the gradation under control or your yield will be out of control!