Tag Archives: Density

Loss of Unit Density Zero-Slump Concrete CMUs

Hang in there to the end, I’ll get to the point, promise.

Every CMU starts with a mold that is loosely filled with zero-slump concrete.  The loose concrete is tamped down with much pressure, noise, and vibration until loose concrete becomes very dense concrete.  After curing you have a very dense, high quality, concrete paver, or retaining wall, or other CMU.  High density makes for strong CMUs.

How much compaction and density you achieve depends on several things.  Given the tamper head/shoe will try to press the unit to the same height (and therefore the same volume) every time it is critical that you put enough concrete in the mold.

What factors make it possible to put enough concrete in the mold to get a good dense unit?  What factors could interfere with filling the mold properly?

The wonderful machines that make CMUs can perform the exact same operations over and over.  So they fill the mold with the same motions time after time.  To a certain extent they can adjust the mold filling in response to changes in the height of the units.  What they can not do is adjust in response to changes in the density of the unit.  I should add, “to my knowledge”, they can not.

If our CMU machine is consistently performing the same mold fill motions every time the biggest variable becomes the zero-slump concrete itself.  How it flows through the various hoppers of the machine, how it drops out of the fill box, how it responds to vibration during fill and compaction.

There is much literature, many mix design tools, and a zilliion opinions about how to design a mix to flow, fill, compact, and look good as a CMU.   Lets not tackle that bottomless subject.  This is about time.

This is a reminder.  A reminder that when you get water, cement, and aggregates together you have clicked a stop watch.  You are under the gun.  Barring any extraordinary chemical intervention, those components are reacting, hydrating, and with time will hardened into concrete that will not run through your machine.  This is so basic, so obvious, that we forget to talk about it.  Why talk about such tacit information?

Why?  Because this hydration process is not like flipping a switch.  Here flowable, “CLICK”, here hard as a rock.  No big “BOOM”, oh, too late, hard concrete.  As the curing clock ticks there are periods of time where the concrete is fresh, it flows and fills great.  Later there are periods of time where the concrete just starts to change, the flow and fill are being negatively impacted, but it still goes through the machine.  Then it gets just plain hard, we can all recognize that and it’s not going anywhere fast.

The dangerous period for the CMU guy is the time after all is well and the units are great, but before the concrete is obviously too hard.  Usually this period of bad fill, poor compaction, and sub-par unit density starts at around 20 minutes and goes down hill from there.

This is not theory.  A producer was having trouble with the compression strength of some SRWs.  Some were good, some were, shall we say, not so good.  I saw cut the units as part of normal testing.  Being concrete curious I was looking closely at the matrix exposed by  the cut.  The units were a two color mix.  The charcoal parts were visibly less dense.  No petrographic analysis, just naked eye and you could see the difference.

When alerted to this the producer discovered that the units were about 1/3 charcoal and 2/3 base color.  The charcoal batch was sitting in the hopper much longer than the base color. Units made when the charcoal batch was fresh were OK.    Units that happened to be from the tail end of the charcoal batch were less dense and weak.  Switching to smaller charcoal batch size cured the problem.  Like magic!

I’ve found this “old concrete” thing more than just this once.  Dodo happens and everything else gets scrutinized first.  Aggregates, order of addition, mix time, cement, water content, and (God forbid) admix.  Pay attention to time.  It can run through the machine OK but still come out bad.  Operators at my example producer had no clue there was an issue. Keeping concrete fresh can save you many headaches.

Gentleman, start your stopwatches!


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