lecture #5 highway construction

8/10/2019 Lecture #5 Highway Construction

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H22 IS3 : Transport Infrastructure

Topic #5 :Highway Construction (Road Pavement)


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Introduction

Unbound Layers

Asphalt Production

Batch Plant / Batch Type Mixer

Drum Mix Plant

Laying Asphalt Associated Processes

Concrete Paving

Modern Equipments & Procedures

Highway Construction (Road Pavement)

Contents


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Introduction

Theheart of the highway construction process is the building ofthe road pavement itself.

This lecture outlines some of the most important processesinvolved.



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Unbound Layers

The technology involved in placing the lower layers of apavement is much the same as that involved in earthworks.

The materials are better controlled than would be the case foran embankment fill material for instance.

The grading is carefully controlled, sometimes by blendingvarious sizes together at the quarry, often by sieving out someof the finest particles created during the quarrying process.

On site material is tipped from wagons, spread by means ofdozers & graders, & then compacted.

Moisture content controlis important for many materials as thiscan affect the compactibility of the material.



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Asphalt Production

Asphalt is the generic name given to stone / bitumen mixtures.

The commonly used term tarmac is actually short for tar

macadam, the standard tar bound material which to be usedbefore bitumen took over as the dominant binder.

The basic purpose of an HMA plant is to properly proportion,blend, & heat aggregate & asphalt to produce an Hot MixAsphalt (HMA) that meets the requirements of the Job MixFormula (JMF).

There are 2 basic types ofHMA plantscommonly in use today:

1) Batch plant

2)Drum mix plant(most popular)

Batch plants produce HMA in individual batches while drumplants produce HMA in a continuous operation.



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Asphalt Production


Figure 5.1 : Batch Plant Figure 5.2 : Drum Plant


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Asphalt Production

Batch plants, which produce HMA in individual batches, are theolder of the 2 types of HMA production facilities.

HMA was originally made in batches; it was not until the 1970sthat drum plants became a popular HMA production option.

The older batch plants are likely to be replaced by new drumplants, which can provide greater mobility & production capacity.

Typical batch quantities range from 1.5 to 5 tons of HMA.



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Drum Mix Plant



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Drum Mix Plant


Drum Mix Plant


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Drum Mix Plant


Control Center

The control center controls plantoperations from 1 central location.

Asphalt Cement Storage TanksAsphalt cement is stored in tanks

while awaiting delivery into thepugmill.

Asphalt cement in the tanks iskept heated between 150C

(300F) to 180

C (350

F)depending upon the grade & type

of asphalt.Asphalt is delivered from the

tanks to the pugmill for mixing

with aggregate.

Figure 5.5 : Control Center

Figure 5.6 : AC Storage Tanks


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Drum Mix Plant


Fines / Additive SiloSome plants may have one or more silos for storing mineral filler,

fines from the baghouse (secondary collector) or special

additives that are added to the mix.

Figure 5.7 : Fines / Additive Silo


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Drum Mix Plant


Cold Feed BinsStockpiled aggregate is

located into the cold feed

bins for delivery to theaggregate dryer.

Each bin usually holds aseparate aggregate sizeor gradation & has anadjustable gate thatmeters aggregate onto amoving conveyor belt.

The degree to which

these gates open & theconveyor belt speedcontrol the amount ofaggregate introduced intothe plant & thus, controls

mix gradation.

Figure 5.8 : Cold Feed Bins


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Drum Mix Plant


Cold Feed for Reclaimed Asphalt Pavement (RAP)Reclaimed asphalt pavement (RAP) can be used in hot mix at

ratios up to about 50%.

RAP is generally loaded into its own cold feed bin then typicallymoved by conveyor belt to be discharged directly into thepugmill or hot elevator where its heated by the already hotaggregate.

Figure 5.9 :RAP


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Drum Mix Plant


DrumThe rotating drum heats the aggregate then mixes the hot

aggregate with asphalt cement.

There are 2 basic types of drum mixes :(1) Parallel flow (where the aggregate enters the drum at the

same end as the burner & travels parallel to the hot air stream)(2) Counterflow (where the aggregate enters the drum at the

opposite end from the burner & travels counter to the hot airstream).

Asphalt is added to, & mixed with, the hot aggregate in thedrum at different location depending upon the design of thedrum mixer.


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Drum Mix Plant


Secondary Collector (Baghouse)The baghouse removes fine particulate matter from the dryer

exhaust gasses before the exhaust gasses are released to the

atmosphere.Its basically a series of cylindrical filter fabric bags through

which the exhaust gas travels.Some plants may use a wet scrubber as a secondary collector.This device sprays water on the exhaust gases & reduces the

velocity of the air flow to precipitate the fine particles form theexhaust gases.

Particulate laden water from the scrubber flows to a setting basinwhere the particles settle out so that the water can then be

reused.


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Drum Mix Plant


Figure 5.12 : Secondary Collector


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Drum Mix Plant


Storage SilosAfter the aggregate & asphalt cement are mixed in the pugmill

the resulting hot mix is either discharged directly to trucks for

transport or stored in silos for later delivery.Generally there are two type of silos: surge & storage.Surge silos are usually insulated but unheated & are designed to

hold hot mix for short periods of time (several hours) betweentruck arrivals.

Storage silos are well insulated, heated, near air-tight & aredesigned to hold mix for long periods of time (up to a week).

Coarse-graded mixes (e.g., friction course & stone matrix asphalt)should not be stored for long periods of time because the asphaltcement will tend to flow off the aggregate (drain down) & collectat the bottom of the silo.


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Drum Mix Plant


Figure 5.14 : Storage Silos


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Aggregate DryerIn the aggregate dryer, aggregate is heated by tumbling

aggregates through a hot gas stream.

The type of dryer is referred to as a counter flow dryer sincethe aggregate moves in a direction opposite that of the hot gasstream.

Aggregates enter the dryer at the upper end & tumble down therotating drum toward the burner at the opposite end of the dryer.

Hot ElevatorThe hot elevator takes the heated aggregate from the aggregatedryer & delivers it to the screen deck.


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Screen Deck & Hot BinsHot aggregate is discharged from the hot elevator over a series

of vibrating screens used to separate it into various sizes (the

screen deck).The various sizes are then temporarily stored in hot bins until

the predetermined amount of each aggregate size isaccumulated for a batch.

Once this occurs, the correctly proportioned aggregate from eachhot bin is discharged into the pugmill.

PugmillThe aggregate & asphalt cement are blended together in the

pugmill.Blending is done in batches, thus the name Batch Plant.Pugmills typically have a capacity of 2 to 5 tons per batch.Blending is typically quite short (e.g. short as 35 seconds per

batch).


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Laying Asphalt


Asphalt is a much more expensive material than the unboundlayers. It forms the upper part of the pavement & has to be laidto quite tight tolerances.

To achieve this, it has normally to be laid by a purpose builtpaving machine.

Figure 5.15 : Truck & Paver


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Laying Asphalt


Paving takes place continuously. The machine takes asphalt fromthe delivery wagons & feeds is at a controlled rate to the back ofthe paver, where it is spread mechanically to form an even mat

beneath the paving screed.

Figure 5.16 : Paver Schematic


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Laying Asphalt


The paving screed itself vibrates, helping to compact the asphaltmat but does not otherwise control the layer thickness.

This is done by the operator ensuring the feed of asphalt to the

back of the paver & the forward movement of the machine aresuch that the correct volume of material is fed onto thepavement.

Figure 5.17 : Asphalt Paving Machines


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Laying Asphalt


When it leaves the paver, theasphalt will probably be littleover 100C & will be partially

compacted. Final compaction is carried out

by rollers. There are 3 maintypes:Dead weight steel wheel -

traditional type, good atachieving a smooth surface;

Vibrating steel roller -achieves better compaction;

Dead weight pneumatic tired- gives a kneading action,excellent compaction.

Figure 5.19 : Pneumatic Tire Roller

Figure 5.18 : Steel Wheeled Roller


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Laying Asphalt


The asphalt has to be over about 80C in order to allow goodcompactionto be achieved.

This means that compaction has to be carried out within about8

minutes from the time it is laid. However, if it is too hot, it willmove around too much rather than densifying properly.

Thus it should be appreciatedthat control of temperatureis criticalto achieving a good final product.

The weather of course has a part to play here & very cold or windy

weather is not suitable for laying asphalt.


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Associated Processes


Figure 5.20 : AC Paving Process

Surface textureis one of the important safety requirements for aroad surface.

One of the ways of achieving this is byspreading coated chippings,

that is single sized pieces of stone coated with bitumen, over thesurface of the road.

This has to be carried out during the 8 minutes before the asphaltis too cold to compact.


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Associated Processes


In many cases it is necessaryto spray a film of bitumen onto asurface in order to ensure that the next layer of asphalt sticksproperly.

This is a tack coat & the form of bitumen used is an emulsionasphalt. In this form (rather like an emulsion paint) the bitumen isworkable & can be sprayed through a nozzle.

An emulsion sprayed onto the surface combined with a layer ofcoated chippings forms a very useful means of restoring the

texture of the surface without the need for a full asphalt layer. Thisprocess is surface dressing.

Often a new layer of asphalt has to be applied to an existingsurface. In this case the existing surface will probably requireshaping in some way &the top surface will have to be planned outto leave a rough texture for the new asphalt to adhere to.


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Concrete Paving


Concrete paving is a specialised activity requiring a number ofpurpose built pieces of equipment.

Together, these pieces of equipment are referred to as a paving

train. The traditional paving train runs on fixed rails either side ofthe pavement, the rails also forming the side shutters to theconcrete.

1) Fix tie bars for longitudinal joint2) Spread first layer of concrete

3) Place dowel bars for transverse joints4) Spread second layer of concrete5) Form longitudinal joint recess6) Form transverse joint recess7) Diagonal finisher gives even surface8) Texture surface using brush9) Spray on curing compound10)Cover during early stages of curing


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Concrete Paving


In modern concrete paving,slipform pavingis used & its mostappropriate for larger jobs that require high production rates.

Slipform paving is defined as a process used to consolidate, form

into geometric shape & surface finish a PCC mass by pulling theforms continuously through & surrounding the plastic concretemass.

Slipform paving is most appropriate for larger jobs that requirehigh production rates.


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Concrete Paving :Modern Equipments & Procedures


1) Placer / SpreadersAlthough not always used,

placer/spreaders are quitecommon.

They place a metered supply ofPCC in front of the slipform

paver using a series of conveyor belts, augers, plows &strikeoff device.

Using a placer/spreader allowsthe contractor to receive

material from transportvehicles & place a uniformamount of PCC in front of theentire paver width, whileminimizing segregation.

Figure 5.21 : Smaller Placer/Spreaders

Figure 5.22 : Large Placer/Spreader


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1) An auger spreads the PCC in front of the strike off plate.2) The strike off plate (screed) removes excess portions of the auger-placed

PCC & brings the slab near its final elevation.3) The PCC is consolidated by a group of vibrators.4) A tamper (if present), pushes large aggregate particles below the slab

surface.5) The profile pans level off the slab at the right elevation & provide initial

finishing.

Figure 5.24 : Basic Slipform Paving process as it occurs underneath the Paver


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Concrete Paving :Modern Equipment & Procedures


Figure 5.25 : PCC Vibrators on theUnderside of a Paving Machine

Figure 5.26 : Sideforms & Profile Pan


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3) Texturing & Curing MachineThis machine follows the paver & is used to impart

macrotexture (usually by dragging a tined instrumentacross the fresh pavement) & apply a curing membraneover the pavement.

Sometimes the paver is equipped with a tining machine,while a separate machine is used for applying the curing

membrane.Curing is typically done once finishing of an area is

complete & the original wet sheen has nearly disappeared.On tined pavements, curing is usually specified to occur

in 2 passes, 1 forward & 1 in reverse, to ensure both

sides of the texture ridges are coated with curingmembrane.


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lecture #5 highway construction

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