Limestone, Clinker, Cement, Concrete, Cure, Transit-Mix Truck, Batch Plant, Slump Test and Cylinder Compression Test

Posted by PITHOCRATES - June 20th, 2012

Technology 101

Cement is basically Dehydrated Limestone Rock

Limestone is a sedimentary rock.  Made from the sediment of dead sea life.  Skeletal remains that settle to the bottom of the sea.  And shells.  Made largely of calcium carbonate (CaCO3).  Which gives them strength.  (We even urge people to drink milk for the calcium to build stronger bones.)  Calcium carbonate makes strong shells and bones.  And over time they make a strong rock called limestone.  Which has many uses.  One of which has changed the world.

If we crush limestone and mix it with some other materials and then heat it up to about 2642°F (1450°C) we will remove all the water from it.  We do this in a very large cylindrical kiln that slopes downward.  A fuel-fed fire heats the lower end.  The hot exhaust gases travel up the kiln.  The crushed rock and other materials are loaded in the higher end of the kiln.  As the kiln rotates slowly the rocks tumble down the kiln as they heat up to the melting point.  But not quite.  They become gooey globules as they exit the kiln.  They then cool into hard round lumps.  Or clinker.  Then this clinker is ground into a powder.   In another large round rotating cylinder.  Only this one is not heated.  It’s full of steel balls.  The end product is a fine powder free of all water we call cement.

So, basically, cement is dehydrated rock.  And it can be as strong as rock again once we rehydrate it.  Which brings us to how this fine powder has changed the world.  When you add water to cement it starts a chemical reaction.  The water molecules join with the other chemicals in the cement and they start to combine into new molecules.  Add some sand and we get mortar.  The stuff that binds bricks and blocks together.  And tiles in our kitchens and bathrooms.  Add some crushed stone and we get concrete.  The stuff we pave our roads with.  Build bridges out of.  Buildings.  Sewer pipes.  Runways.  Dams.  Canals.  Reservoirs.  Aqueducts.  The Romans even used it to build an empire.

Transit-Mix Trucks have about 90 Minutes to Deliver their Concrete before it Cures

This chemical reaction takes time.  And when it’s done curing the concrete sets.  That is, it becomes rock again.  So the clock is running once we mix cement, aggregate and water together.  Concrete doesn’t cure because the water evaporates.  If it did we could ship concrete cross-county in unit trains with all the concrete needed for one project.  But we can’t.  Because once the water starts rehydrating the cement you only have so much time to pour it.  Shape it.  And trowel it.  Before it becomes rock again.  Which is why you see a lot of concrete trucks racing through the city from different companies.  A concrete plant can only service a given radius.  So the larger the city the more concrete plants you’ll see.  And the more concrete trucks plying the streets.

These concrete trucks, or transit-mix trucks, actually mix the concrete.  At the concrete plant, or batch plant, the materials that make concrete come together in the transit-mix truck.  The truck pulls under a hopper and the cement, aggregate and water pour into the drum on the truck.  The drum turns slowly in one direction to mix these materials.  The truck then has about 90 minutes to get to the construction site.  Once there they position the truck near the area of the placement and add chutes to transport the concrete from the truck to the concrete forms.  When ready the drum rotates in the opposite direction at a higher speed. And the concrete rides up a screw inside the drum and out onto the chute.  If a truck arrives too late on site the concrete cures inside the rotating drum.  Requiring someone to crawl inside with a jack hammer to break it apart back at the batch plant.

Because of this approximate 90 minute limitation large projects don’t use these concrete plants.  Instead they’ll build a batch plant on site.  Or near the site.  And ship cement, aggregate and water to the project batch plant.  Or connect it to a local water source.  When they schedule a larger pour (or placement) a caravan of trucks will line up at the batch plant.  Load up.  And deliver their concrete.  Then wash out the residual concrete from their trucks at a designated location.  So they won’t have to use a jack hammer later to chip it out.  You’ll see a large sloppy pile of concrete there.  Which they can bust up later and recycle into aggregate.

A Cylinder Compression Test determines the Strength of Placed Concrete

Depending on the concrete mix you can get different properties.  Increasing one property, though, often reduces another property.  For example, to increase workability you can add more water.  This will allow the concrete mix to flow easier and fill in all voids in a concrete form.  But the tradeoff is strength.  If you watch a concrete placement on a construction site you may see the concrete workers using tools to help get rid of any trapped air to help the concrete fill all the voids in the forms.  Such as a vibrating hose placed all around the concrete in a building foundation form or basement wall form.  And you may see them fill a cone with each batch delivered to the site.  For a slump test.  Concrete specifications will call for a slump range.  Which is basically how much the concrete will slump when that cone is removed.  If the slump fails the test they reject the batch.

Concrete strength is so important there is a test for that.  Especially for roads and runways.  Which are very costly and very inconvenient to replace.  So the concrete specification calls for a specific strength to get a predetermined number of years of use out of these surfaces.  But the higher the strength the higher the cost.  Which is why roads break apart far sooner than an airport runway.  A road is much easier to close.  Because you can reroute traffic elsewhere.  Not quite that easy with a runway.  There aren’t many places a 747-400 with a full fuel load can take off from.  So they build runways to last.  Which makes them very expensive.  They could make roads as strong and long lasting as runways.  But that would leave little funding available for anything else.

A contractor could under bid a new runway project with the intention of cheating the specifications.  And put in a weaker runway than specified.  And pocket a nice profit.  But they don’t.  Because of those strength tests.  With every batch of concrete they place they also have to fill cylinders with that concrete.  Which they send to an independent testing lab.  After the concrete cures they place these cylinders in a press.  For a cylinder compression test.  Which compresses these cylinders until they break.  If they break below the specified strength they will reject the concrete.  Which means the contractor will have to replace it.  Or the owner may discount their final payment to the contractor to factor in the inferior product they delivered.  Which is a powerful incentive not to cheat the specification.  And few do.  A pothole strewn road may not be the fault of the contractor.  Over-weight trucks may have caused that damage.  By not having enough axles under their load to disperse that weight over the surface of the road.  Putting higher than allowed weights per axle on the pavement.  Greatly increasing the stress each axle applies.  Exceeding the design strength of the road.  And breaking the pavement apart as these axles roll and bounce across it.

All of this from the little sea creatures that died so long ago.  Forming the limestone that we use to make cement.  Which we mix with aggregate and water to make concrete.  That lets us enjoy the modern world concrete gives us.


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