Friday, May 11, 2018

Concrete, revisited

The post on Concrete generated a flurry of comments.  That motivated me to find an expert on concrete and ask him "What is new in concrete?"  The resulting brain-dump was wide ranging.

Concrete's compressive strength ranges from 3500 psi (which anybody can achieve) to 10,000 psi.

The higher the compressive strength, the lower the impact strength.  High strength concrete is not usable without steel reinforcement because of the impact issues and because concrete isn't worth spit in tension...can't exploit the concrete's increased strength for loads that create bending or flexing.

The "good old days" of concrete were before 1989.  In 1989 the EPA banned the dumping of kiln dust into landfills.  That resulted in EVERYTHING being ground up and shipped out as cement powder.

Much of the "klinker" is now produced over-seas and shipped to the US where it is ground up into cement powder.  Many of the overseas producers simply dump waste in the ocean...the same oceans that touch our shores, incidentally.

Some of the over-seas producers produce cement klinker as a way to monetize natural gas produced as a byproduct of oil production.  Cement klinker is a value added product that uses natural gas that would otherwise be flared off.
Michigan has a butt-load of bridge beams that look exactly like this.  The concrete is flaking off and you can see the steel re-rod that is supposed to be embedded within it.  The flaking is caused by alkali-silica reaction.

One of the issues with the post-EPA cement is that it is much more alkaline than before.  That makes it more likely to burn the skin of the cement worker AND it provokes Alkali-silica reaction in the finished product.

Much of the usable advancements in concrete involve aggregate.  Some aggregate is less likely to cause ASR. 

Concrete is a preferred way to dispose of fly-ash.  My expert was grumbling under-his-breath "More EPA bullshit."  Not all fly-ash is created equal.  You cannot just add fly-ash to concrete willy-nilly, you have to make other adjustments.  Unfortunately, the concrete business is a mature business.  Profit margins are tight and the industry often accepts inputs, like fly-ash, as being exactly what the originator claims they are. That can result in non-optimal concrete.

One finding out of Canada determined that "gap graded" aggregate was much worse for durability than "bank run" aggregate.  Gap graded means that the concrete was made with sand and graded gravel.  Bank run means that the aggregate varies from fine sand to sand to cracked pea gravel to pea sized gravel to broken marble size fist sized rocks.  In retrospect, the reasons were clear.  The bank run material settles more compactly.  The finer particles can fill the interstices and cause the aggregate to lock up, even sans cement.  It is a bit like the Steven Covey story "Is the jar full?"

Finally, the $3 a bag concrete mix most of us use for home projects does conform to the curve shown in the earlier post.  The fancy stuff costs more money than most of us want to spend.

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