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MCM 511
CONSTRUCTION EQUIPMENT
MANAGEMENT
Concreting Equipments
Prof. Debopam Roy
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Concreting Methods & Equipments
Batching & Mixing
Transportation
Placing
Compaction
Finishing &Curing
Weigh Batcher
Mixer
Batching Plant
Builders Hoist
Bogey/Barrow
Transit Mixer
Chute
Crane Bucket
Pump
Boom Placer
Slip Form
Needle Vibrator
Form Vibrator
Surface Vibrator
Aggregate
Cement
Water
Batching
Mixing
Handling and
transporting
Placing
Finishing
Curing
Compaction
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Batching & Mixing
Batching is weighing or volumetricallymeasuring and introducing into themixer the ingredients for a batch ofconcrete
Manual / Volumetric Batching Cement added by bags,
Aggregate & Water added
volumetrically using forma. Nominal mix design used.
For small jobs
Semi Automatic / Weigh Batching Cement added by bags.
Aggregate added by weight.
Water added volumetrically.
Mix Design can be used. For medium jobs
Automatic Batching Cement added in bulk from silo by
weight
Aggregate & Water added by weight
Automatic plant operating as per mix
design For large jobs
The concrete should be mixedthoroughly until it is uniform inappearance, with all ingredientsevenly distributed.
Portable Stationary Mixer Low capacity mixers
Mixing executed near the placement
location, and concrete transported byhead-load/ bogey/ hoist
For small jobs
Ready Mix Plant Large capacity
Automatic Batching
Transportation by transit mixers
For large jobs May cater to multiple projects
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Batching & Mixing
Weigh Batcher weighs different
aggregates one at a timeand loaded on top of thosepreviously weighed
May have one or morebuckets for unloading ofdifferent aggregates
Weigh beam Scales or dialtype scales may be used
Weighed materialtransferred to mixerhopper
Local Brands are mostcommon
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Batching & Mixing
Portable Stationary mixers Small machines for mixing concrete near the structure
Components added manually
After mixing concrete is transported by manual/semi-mechanicalmeans
Sequence: Add 10% water, Add cement; Add aggregate along with 80%water; Add balance 10% water
Specifications usually require minimum of one minute mixing forstationary mixers of upto 1 yd3 with an increase of 15 seconds for eachadditional 1 yd3 capacity
The mixer drum speed is usually 18 to 20 r.p.m
Some times the total volume of the unmixed ingredients is given as aprefix. Thus, a total 10/7 mixer takes 10 cu.ft. of unmixed material andgives 7 cu.ft. of mixed concrete per batch
Mixers are specified by the volume of mixed concrete discharged aftermixing of each batch, expressed in m3 or ft3
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Batching & Mixing
10/7 ConcreteMixer
Non Tilting
Concrete Mixer
Pan Mixer
Hand Feed
Concrete Mixer
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Batching & Mixing
Ready Mix (RMC) Plant /Batching Plant Centrally located
Automatic/Semi Automaticplant
Produce concrete as per setrecipe
High capacity of production;better level of quality control
Preferred over on-site concretemixing because of the precisionof the mixture and reduced
work site confusion Plant capacity defined by
production in cum/hr. Rangesfrom CP 30 to CP 120
Leading Manufacturer:SCHWING STETTER
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Batching & Mixing
Parts of RMC plant :
Aggregate Feeding System Radial Boom scraper
Ramp Cold Bins Conveyor
Radial Conveyor Cold Bins Conveyor
Cement Feeding System Bag Splitting Cement Feed hopper & Screw Conveyor
Cement Silo & Screw Conveyor
Water & Admixture Tank
Pan Mixer
Discharge Unit
Control Cabin
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Batching & Mixing
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Batching & Mixing
Advantages of Ready Mix Concrete A centralised concrete batching plant can serve a wide area.
The plants are located in areas zoned for industrial use, and yet the delivery trucks can serviceresidential districts or inner cities.
Better quality concrete is produced.
Elimination of storage space for basic materials at site.
Elimination of procurement / hiring of plant and machinery
Wastage of basic materials is avoided.
Labor associated with production of concrete is eliminated.
Time required is greatly reduced.
Noise and dust pollution at site is reduced.
Reduce cost.
Disadvantages of Ready-Mix Concrete The materials are batched at a central plant, and the mixing begins at that plant, so the
traveling time from the plant to the site is critical over longer distances
Generation of additional road traffic; furthermore, access roads, and site access have to beable to carry the weight of the truck and load. Concrete is approx
Concrete's limited timespan between mixing and going-off means that readymix should beplaced within 120 minutes of batching at the plant.
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Handling & Transportation
Methods used to handle and place concrete should be based onseveral objectives, including
Economy
The prevention of segregation
Final placing before concrete attains initial set
Proper handling and transporting of concrete is as important tothe end result as are the other factors of water - cement ratio,mix design, correct batching and thorough mixing
The mode of transportation depends on the distance betweenthe mixing & placing
Part of mixing can be done during transportation Method of transportation can be manual / semi manual /
mechanical
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Handling & Transportation
Manual
For small jobs concrete
may be transported by
human chain through
head-load
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Handling & Transportation
Builders Hoist Can be used for vertical lifts
Popular in building projects
Concrete Unloaded on a Bucketwhich is lifted along a tower bywinch-pulley arrangement
Buggies & Barrows For horizontal transportation/along
ramp, for maximum distance of70m
Equipped with pneumatic tyres toprevent segregation and easiermovement
Hand buggies superior to wheelbarrows because the two wheelsprovide a better balance for theload
Power driven buggies of 0.5 cumcapacity & 25kmph speed are alsoavailable
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Handling & Transportation
Transit Mixers RMC is mixed partially in the plant,
and completed in the truck mixer.Water is added partially in theplant, and balance added from thetank of the TM
Maintains the concrete in workablestate through agitation, or turningof the drum, until delivery
The interior of the drum on aconcrete mixing truck is fitted witha spiral rotating blade whichagitates the mix
On reaching site, the concrete is
unloaded from the rear Capacity : 6 cum. May need
periodic maintenance to retaincapacity
Leading Manufacturer: SCHWINGSTETTER
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Handling & Transportation
Operation of Transit Mixer The inside of a transit mixer uses a
simple Archimedes' screw to mix and tolift the concrete to the delivery chute
In one rotational direction, the concreteis pushed deeper into the drum(Charging). When the drum rotates inthe other direction, the Archimedes'screw-type arrangement "discharges",or forces the concrete out of the drum
70 to 100 revolutions of the drum or blades atthe rate of rotation specified by themanufacturer, required to produce the specifieduniformity of concrete.
Not more than 100 revolutions at mixing speed
are allowed. After 100 should be done at a ratedesigned for agitation
Agitating speed is usually 2 to 6 rpm, whilemixing speed is generally about 6 to 18 rpm
Concrete should be delivered with in one andhalf hour or before the drum has revolved 300times.
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Placing
Chutes & Drop pipes Can be used for placing concrete to lower
elevation
Chutes should have a round bottom &slope should be steep enough forconcrete to flow continuously withoutsegregation
Drop pipes should have a diameter of atleast 8 times the maximum aggregatesize at top, and 6 times at bottom
Crane Bucket Bottom dump cylindrical buckets with
central discharge gates can be used
Using a crane to lift the bucket to desiredheight, this method can be used to placeconcrete at a higher elevation
Gates on the smaller buckets areoperated manually, while the gates onthe larger buckets are operated bycompressed air or other mechanicalmethods
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Placing
Pump / Boom Placer Pressure applied to a continuous column of
fresh concrete in a pipe - Rigid or flexible
Mix should be properly proportioned
Outer layer should provide lubrication
Pump must be fed concrete of uniform
workability and consistency Concrete should be supplied steadily
Effective pumping ranges vary from 80 to300 m horizontally and 20 to 100 m vertically
Capacities varies from 10 m3/hr to 150 m3 /hr pipe diameters from 4 to 6 inch
Maximum size of the aggregate is usually 20to 25 mm
Pumps may be mounted on trucks, trailers orskids. Truck mounted pump and boomcombination (Boom Placer) particularlyefficient and cost effective in saving laborand eliminating need for pipelines to carryconcrete
Leading Manufacturer: SCHWING STETTER
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Compaction
Purpose of Compaction is toremove entrapped air
Normally mechanical vibratorsare inserted into the concrete forcompaction
In case of heavily reinforcedsection, form-vibrators may beused, and in thin slabs, surfacevibrators may be used
All types of compactors aremostly available locally
Needle Vibrator Internal vibrators have a vibrating
casing or head which is immersedinto the concrete and vibrates at ahigh frequency against theconcrete
Powered by electric motors orcompressed air or gasoline
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Compaction
Needle Vibrator Whenever possible, the vibrator should be
lowered vertically into the concrete atregular interval and allowed to descend bygravity
In thin slabs vibrator should be inserted at
an angle, so that the head is fullyimmersed.
Adequacy of internal vibrator is judged byexperience and by changes in the surfaceappearance of each layer. Usually takes 5to 15 sec.
Changes to watch are embedment of largeaggregates, the leveling of surface, theappearance of thin film of glistering pasteand cessation of large bubbles ofentrapped air escaping at the surface
The length of time that a vibrator can beleft in the concrete will depend on theslump of concrete, the power of thevibrator, and the nature of the section
being vibrated
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Compaction
Form Vibrators Attached to the outside of the form or
mould
They vibrate the form which in turnvibrates the concrete
Generally used in large pre-cast
concrete plant Also useful for consolidating concrete
in members that are very thin orcongested with reinforcement, Tosupplement the internal vibration, Forstiff mixes where internal vibratorscannot be used
Should be equipped with controls sothat frequency and amplitude can becontrolled according to size of elementand consistency of the cement.
Increasing the frequency anddecreasing the amplitude as vibratingprogresses will improve consolidation
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Compaction
Surface Vibrators Exert their effect at the top
surface of the concrete andconsolidate the concrete from thetop down
Used mainly in the slabconstruction
Operate in the range of 3,000 to6,000 vibrations per minute
Vibratory screeds give positivecontrol of the strike off operationand save a great deal of labour
However, it should not be used onconcrete with slumps in excess of75 mm. Surface vibration of suchconcrete will result in an excessaccumulation of moisture and finematerial on the surface and thusreduce wear resistance
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Slipform
Slip forming, continuous poured, continuouslyformed, or slipform construction is aconstruction method in which concrete ispoured into a continuously moving form
Used for tall structures (chimneys, towers), aswell as horizontal structures (PQC, Kerb,Gutter)
Slipforming enables continuous, non-interrupted, cast-in-place "flawless" (i.e. no
joints) concrete structures which have superiorperformance characteristics to piecewiseconstruction using discrete form elements
Concrete needs to be workable enough to be
placed into the form and packed, yet quick-setting enough to emerge from the form withstrength
The slip forming technique was in use by theearly 20th century for building silos, but wasintroduced to buildings & industrial structuresin the late 1960s
Leading brands: Slipform Intl (Vertical), PowerCurber/Arrow (Horizontal), Wirtgen (Paving)
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Slipform
Vertical Slip Form The concrete form may be surrounded by a platform
on which workers stand, placing steel reinforcing rodsinto the concrete and ensuring a smooth pour.
Together, the concrete form and working platform areraised by means of hydraulic jacks.
Generally, the slipform rises at a rate which permits
the concrete to harden by the time it emerges fromthe bottom of the form
Horizontal Slip Form For kerbs, gutters, concrete is laid down, vibrated,
worked, and settled in place while the form itselfslowly moves ahead.
For concrete pavements, a slipform paver with largescreed is used, and the concrete is laid continuously
The slip form machine (Power Curber / Kerb Castingmachine) consist of a bucket, a conveyor, and a mouldfabricated as per desired section , mounted on acrawler track / wheel axles
After the concrete sets, expansion joints are cut intothe kerb/drain power saws
In addition to curb and gutter work, the machine pourshighway safety barrier, bridge parapet, sidewalk, golf
cart paths, agricultural foundations, irrigation ditchesand stadium risers
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Slipform
Slipform Paving The paver usually performs screeding, consolidation
and initial finishing.
A typical track-mounted, self-propelled paveroperates at speeds between 1 and 2.5 m/minute
Some pavers are equipped to place reinforcing steel(if needed), dowel bars and tie rods as well
First, an auger spreads the concrete in front of thestrike off plate. Second, the strike off plate (screed)removes excess portions of the auger-placedconcrete and brings the slab near its final elevation.Third, the concrete is consolidated by a group ofvibrators. Fourth, a tamper pushes large aggregateparticles below the slab surface. Finally, the profilepans level off the slab at the right elevation andprovide initial finishing
Transverse joints across the paving lanes are saw-cutinto the pavement
Longitudinal center joints are commonly formed byuse of a continuous polyethylene strip the paverdisperses as it moves along
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Problem
To execute a 500 cum pour, using a 90 cum/hr
boom placer, at a distance of 50kms (at
medium traffic) from the central batching
plant CP -110, assuming 80% productivity ofplant & boom placer, what is the optimum no.
of 6cum transit mixers to be deployed? How
much time will it take to finish the concretingusing 10 TMs?
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Dos & Donts - Batching
DOSUse consistent quality of rawmaterialsUse central batching plants &design mix for important jobsCalibrate weigh batcherregularly.In volume batching havedifferent sized farmas readilyavailableDesign size of batch so that1bag cement is required.Prevent dusting of cementusing canvas tremies for fall.Make allowance for bulking ofsand &moisture absorption ofcoarse aggregate.
DONTSDump head loads ontoweighing pans.Dump different raw materialssimultaneously.Gauge cement by volumebatching.Make farmas larger than 50L.Compact aggregates intofarmas.Allow intermingling ofbatches, loss of materialsduring transfer
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Dos & Donts - Mixing
DOSFeed ingredients simultaneouslywith same period of flow. Aportion of water should precede&follow the introduction of otheringredients.Butter the drum before use&remove hardened concrete fromdrum &blade.Use skip hopper for charging&chute for discharging.Add final part of water to transitmixer just prior to delivery&rotate at a slower speed whenused as agitator.Test slump at regular intervals&use a slump slightly greaterthan that required for placing.
DONTSAllow loss of material duringcharging/mixing/discharging.Allow incomplete/over mixing.Allow considerable variations inproperties of the mix in different
portions of a batch.Load mixers in excess of 10%more than rated capacity.Allow accumulation of hugeamount concrete at site due tounscheduled movement of transitmixers.
Use transit mixers for concretewith low slump/large aggregates.
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Dos & Donts - Transportation
DOSUse agitator trucks ifbatching-mixing plant isdistant from site/RMC isused.
Provide properwalkways&staging, largebuckets if concretetransported manually.Provide sloped covered
chute, downpipes, etc tounload concrete fromhoist/conveyer ontoplatform.
DONTSAllow delay intransportation/split inbatch.Use manual means for
vertical transportation.For pump concrete use ano of bends &interruptions, small/Alpipes, stiff mix.
Allowsegregation/bleedingduring discharge due tolack of end-control
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DOSUse light hoppers, roundedchutes, vertical falls (
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Dos & Donts - Compaction
DOSUse the largest possible diameter ofpoker that the formwork&reinforcement layout can permit.Insert the poker quickly and allow itto penetrate by its own weight to thebottom of the layer as fast aspossible, Extend poker about 100mmto the previous layer.Slowly withdraw poker after airbubbles stop emerging and pitchbecomes constant..Poker insertion must be staggered&made at regular intervals.Use external vibrators if poker cannotprovide sufficient compaction.Mount the form vibrator on a steel
plate attached to a ]/I beam whichrun along formwork touching formfasteners. Use a suitable spacing sothat intensity of vibration is uniform.
DONTSTouch formwork/reinforcement withpoker.Use poker to make concrete flowthrough long distance.Insert poker at the tip of the heap.Use sharp bends in flexible drive.Proceed in downward direction.Use form vibrators on top of verticalformwork.
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