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Concrete Plant Operations

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Purpose of Concrete Plants

• To produce a concrete mixture that:

 – is cost effective

 – contains the desired proportions of concrete aggregate, andadmixtures

 – meets all specifications

 – is consistent

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Common Plant Operations

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Materials

• Water

• Cement

• Aggregates

 –

Coarse – Fine

 – Recycled- slag, fly ash

• Admixtures

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Cement

• Production – http://www.cement.org/basics/images/flashtour.html 

• Delivery –

Rail – Truck

 – conveyor

• Storage – silos

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Stockpiling

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Aggregate Bins and Feeds

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Liquid Admixtures

• Storage

• Dosing

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Batching Operations

• Batch Materials

 – Charge order- water, aea/wr, aggregates, cement, water

• Agitation- avoid headpack, cement balls

•

Truck Cleanliness – Wash off- after loading

 – Wash down- at site or end of discharge

 – Wash out- end of day

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Central Mix Plant – High Volumes

 – Improved QC/QA

 – Reduced Wear on trucks

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Batch Control

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Transit or Shrink

Mix Plant• Truck mix separately

mixes all ingredients intruck

• Shrink Mix partiallymixes in plant

• Small batch production

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Mobile Mixing Plant

• Intermittent or smallproduction quantities

• Advantages: Combinedmaterials transporter andbatching and mixingsystem. One-manoperation.

• Good preventivemaintenance program.

•

Materials must beidentical to those inoriginal mix design.

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Transportation- Agitating Trucks

• Front Discharge • Rear Discharge

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Non-agitating Trucks

• Transport concrete onshort hauls over smoothroadways.

• Cost of nonagitatingequipment is lower thanthat of truck agitators ormixers.

• Slump should be limited.

Possibility of segregation.Height upon discharge isneeded.

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Transportation- Non-Agitating Trucks

• End Dump • Belly Dump

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Unloading/Discharge

• Direct Discharge

• Pumping Operations

• Crane/Bucket Delivery

• Conveyor

 – Truck mount

 – Belt

• Buggy

• Armstrong Method

•

Concerns – Adding Water

 – Minimize segregation

 – Minimize contamination

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Adding Water

• ASTM C 94 (AASHTO M 157)- Remixing of PCC

• Water may be added at jobsite provided:

 – Spec. w/c-ratio is not exceeded

 – Measured slump less than specified

 – Allowable mixing and agitating time (or drum revolutions) are notexceeded

 – Concrete is remixed

• min. 30 rev. at mixing speed

•until uniformity is within limits

• If workability is a concern, consider a water reducing orsuper plasticizing admixture instead of water.

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Direct Discharge

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Pumping Operations

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Pump Discharge

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Crane/Bucket Delivery

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Truck Mount Conveyor

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Belt Conveyor

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Buggy and Armstrong Method

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Environmental Concerns

• Water Runoff

 – washout areas at jobsites

 – plant operations

• Dust Control

 –

dust collection – plant washing

 – truck washing

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Recycling Operations

• Returned Loads

 – coarse aggregate

 – fine Aggregate

 – slurry

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Recycling Flow Diagram