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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Energy Absorption and Conversion, Vibration Isolation, Dampening, Oscillation, Advanced Building Technologies, Building Codes and Code Enforcement

Posted by PITHOCRATES - March 7th, 2012

Technology 101

Springs and Shock Absorbers on a Car provides Vibration Isolation from the Shocks of the Road

Roads aren’t perfect.  They have their bumps.  And their potholes.  Especially in the north.  Where they use salt to melt snow and ice.  Which get to the reinforcing steel within the concrete.  Causing it to corrode.  Further stressing and cracking the concrete.  Allowing water to get underneath the concrete.  Where it expands as it freezes, heaving and cracking the road.  Then there’s the normal heating and cooling.  That can buckle and crack blacktop.  Heavy truck traffic that stresses and hammers our roads.  Even sinking slightly into our asphalt roads making tire ruts.  And then there are railroad crossings.  Sewers and manholes that aren’t flush with the surface.  There’s a lot out there to make for a rough ride.  Yet in a new car you barely feel any of this.  And you can drink a cup of coffee while driving without it splashing out of the cup.  Why?

Because the shocks from the rode are isolated from the passenger compartment.  Air-inflated rubber tires smooth out much of that rough ride.  By compressing to absorb some bumps.  Then expanding back to their original shape.  Springs handle the larger bumps.  Which compress underneath the car as the tires hit a large bump.  Absorbing the energy from that impact before it reaches the passenger compartment.  By using it to compress a spring.  Then the energy in that compressed spring releases and the spring expands until it can expand no longer.  Placing the stretched spring into tension.  The stored energy in the tensioned spring compresses it again.  And this continues back and forth until the energy fully dissipates.  Or is absorbed in a shock absorber.  That dampens the oscillation of the spring.  Bringing it back to a steady-state quickly.  Further smoothing out the ride.

A car is a magnificent piece of engineering.  From converting a fuel into motive power.  To brakes slowing a car down by converting kinetic energy into heat via the friction of brake pads or shoes on rotors or drums.  To the isolation and dampening of the road forces imparted to the car.  It’s a remarkable control and conversion of energy.  That provides for a comfortable ride.  And a smooth ride.  Smooth enough to enjoy a cup of coffee while driving.  Without being too distracted from the business of driving.

Tuned Mass Dampers prevent Dangerous Oscillations in Buildings that can lead to Structural Failures

But a car moving over a road is not the only way energy transfers between the earth and something manmade.  Sometimes the earth moves.  And energy is transferred into something stationary.  Manmade structures like buildings and bridges.  During earthquakes.  And some of these stationary things get damaged.  Some even collapse.  Depending on how we constructed them.  And how similar they are to a car.

Tectonic plates are trying to move.  But the friction between these plates as they jam into each other holds them in place.  Until the pressure builds so much that the plates shift.  Causing an earthquake.  Sending seismic waves through the earth.  In active seismic regions structures need to be like cars.  They need isolation and dampening from the shockwaves caused by shifting tectonic plates.  For during a seismic event these shockwaves ‘grab’ these structures by their foundations and shake them.  This energy applying great forces on these buildings.  Energy that needs to go somewhere.  Because of the conservation of energy principle.  We can’t create it.  Nor we can destroy it.  At best we can redirect it.  Absorb it.  Or convert it.  Like converting the forward movement of a car (kinetic energy) into heat (created during braking).  Or the conversion of kinetic and potential energy of moving springs into heat (via shock absorbers). 

Waves have an amplitude and a frequency.  They oscillate.  That is, they vibrate.  And have energy.  Which is why we build buildings and bridges to move.  To bend and sway.  To dissipate this energy.  For if they were too rigid the forces could instead lead to a structural failure.  However, if they move too much and the external force is in ‘resonance’ with the building’s natural frequency of movement, this oscillation can grow.  Producing great vibrations.  (Like a car driving without any shock absorbers.)  And great forces on the structural integrity of the building.  Itself leading to a structural failure.  That’s why high rises include dampening systems.  Such as tuned mass dampers.  A great mass suspended within a building and restrained by hydraulic cylinders.  Such as the tuned mass damper atop Taipei 101 in Taiwan.  So when the building sways in one direction the mass swings in the opposite direction.  Thus dampening the oscillation of the building.

Free Market Capitalism allows a Higher Standard of Living and Creates the Kind of Wealth that can build Safe Houses and Buildings

Smaller buildings may use springs-with-damper base isolators.  Which does the same thing springs and shocks do for a car.  Isolates the structure from vibrations.  But using the proper building materials to allow a building to move or withstand destructive forces without structural failure provides most seismic protection.  And this is nothing new.  The Machu Picchu Temple of the Sun in Peru is an early example of good seismic engineering.  Peru sits on the Ring of Fire.  A highly seismic region that circles the Pacific Ocean.  The Inca were highly skilled stone cutters.  They built the Machu Picchu Temple of the Sun without mortar.  Because of this the stone can move during seismic events.  Which has let it stand through the millennia.  Today we use mortar.  And reinforcing steel to strengthen our masonry construction (these blocks can’t move but when the walls they make crack the steel inside keeps them from collapsing).  As well as other advanced building technologies.  And ever evolving building codes and code enforcement to make sure builders meet the exacting standards of these technologies.  To keep these buildings from collapsing and killing hundreds of thousands of people.  Which is why in the most modern and advanced cities in seismic regions survive some of the worst seismic events with minimal loss of life.  Where they count deaths in the hundreds instead of the hundreds of thousands.  As they did before we used these advanced building technologies.

The countries and regions sitting on the Ring of Fire (New Zealand, Indonesia, the Philippines, Japan, Alaska, California, Mexico, Peru and Chile) use some of the most advanced building technologies.  And can withstand some of the most severe earthquakes.  With little loss of life.  Now compare that to the impoverished country of Haiti.  Their 2010 earthquake was devastating, claiming 230,000 lives.  Because they have no such building codes or code enforcement.  Or advanced building technology.  Because Haiti is not a nation of free market capitalism.  Or the rule of law.  But one of political corruption.  And abject poverty.  Are they predisposed to be impoverished?  No.  Because countries can change.  If they embrace free market capitalism.  And the rule of law.

Chile was one such country at one time.  Corrupt.  And anti-capitalistic.  During the heyday of Keynesian economics.  Where nations said goodbye to the gold standard.  And ramped up their printing presses.  Igniting hyperinflation.  Including the Chileans.  But they changed.  Thanks to the Chicago Boys.  Chilean economists schooled in the Chicago school of economics.  With a little help from Milton Friedman.  Perhaps the most esteemed member of the Chicago school. Economic reforms produced solid economic growth.  A prosperous middle class.  And advanced building technologies, building codes and code enforcement.  So when Chile suffered a more powerful earthquake than Haiti did that same year Chile measured their death toll in the hundreds.  Not the hundreds of thousands as they did in Haiti.  And the major difference between these two nations?  Chile has a higher standard of living than Haiti.  And has less poverty.  Because Chile embraces free market capitalism.  Which creates the kind of wealth that can build safe houses.  And safe buildings.  For everyone.  Not just the ruling elite.

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