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Precast Technology in Infrastucture Projects
Kapilesh BhateDy Manager Tendering,
Planning & Control Precast India Infrastructures Pvt. Ltd. Pune
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AbstractIn a scenario of largely scarce and expensive labour force, need of fast track projects and high quality, construction industry is slowly moving towards Precast Construction. This paper intends to highlight the strides, the precast industry has taken in infrastructure projects through three case studies. The first case study explains various concepts in precast for Concourse Level of a Metro Station that were arrived at to bring about uniformity in design of metro stations and to achieve stipulated timeframes for such projects. The proposed solutions emphasized on addressing on-site execution issues like traffic congestion due to large manpower and extensive staging.The second case study showcases the award winning Multilevel car park building of one basement plus ten floors, measuring around half a million square feet, parking around 1300 cars is located in Pune, India and constructed as a combination of cast in situ and prefabricated structural elements, and later on treated with an envelope of façade elements, which includes the combination of a unique methodology of prefabricating façade elements to be fixed on spandrels specially designed and assimilated with the precast structural elements, in a simple but effective manner. The case study also explains evolution of the design and integration of façade elements, structurally and aesthetically, with the main structure, and deals with production, transport and installation techniques, and deciphers the achievement of safety standards of the building necessary as an additional aspect by annulling use of additional steel grills or meshes, which would have been needed in local conditions and normsThe final study speaks about the scope for precast technology in other infrastructure domains in the form of irrigation, bridges, Pipe Lines, urban and marine infrastructure that leads to faster, neater construction with much less site labour, better site control, greater accuracy and quality of finish.
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Case Study I: Precast In Metro Projects
To cater to urban India's needs of transportation, new
metro systems are currently under construction and
bringing about vast changes in 15+ cities all over the
country at a breathtaking pace. India is well on its way to
bring a shift from private modes of conveyance to public
transport. Attempts are being made to enhance
technology with low initial cost and avoiding high O & M
cost.
Precast construction can thus well serve these aspects
effectively.
The typical design of metro stations includes entry, exits,
concourse and platform. The scope is generally limited to
cast in place rib-slab arrangement with Precast I girders
at concourse and platform level parallel to the viaduct.
Sighting the primary objective of infrastructure projects of
delivering the facility to public within a stipulated
timeframe, an attempt to convert this conventional design
of metro station to Precast was made by us, at Precast
India IPL.
We tried to dwell into the feasibility of precast in Metro
Stations, through a case study of Nagpur Metro which is a
3 coach metro of two alignments running for 19km each.
The design of superstructure by the client is a simply
supported between the portal piers. The beams are
connected to the diaphragm on either ends which is
further seated on the bearings supported by the portal
piers.
General arrangement of the superstructure with
bearing arrangement and typical cross section of the
superstructure is shown in Fig 1.
Fig 1a: Cast in situ T shaped Rib beam and slab
arrangement of 26m long.
Fig 1b: Cross section of Cast in situ T shaped Rib beam
and slab arrangement
Concourse level consists of T shaped cast in situ rib
beam and slab arrangement (Fig 1a) spanning 26m for a
station length of 77m (Fig 1b). The staging for cast in situ
works is to be removed only after 48 - 72 days as per the
construction methodology laid down by the consultant.
This adds to the duration of the project.
Precast India in an attempt to overcome short-comings of
the above design and ease off the site execution,
proposed various precast alternatives for the concourse
level framing.
The primary aim was to cash-in the advantages of
production in controlled environment ensuring superior
quality, uniformity in element design, saving in formwork
due to repetitive production cycles and to limit on-site
activities.
In option -1, we proposed to split 26m long rib slab-beam
girder (fig 2) into 2 numbers of 13m long Precast T
shaped ,3.8m wide girders weighing 60T each which will
be connected together by post tensioning at site. Cast in
place stitch will render the girders monolithic. Precast
staircase of stringer beam–tread slab arrangement with
deep beam serving as a parapet was proposed in place of
25m long cast in situ staircase that connected concourse
to platform level.
Fig 2: Cross section of Option-1 Precast I shaped girder
beam
In concourse floor slab, a 6.5 m escalator opening had
been a major constraint. Because of which, stitch at two
locations (between P4 P5& P5P6) would have been
required for a width 2.65 m.
This required introduction of temporary structural steel
members which will be supported at the ends in a manner
similar to typical precast units. Also, on P5, the precast
slab slab would be required to be supported on two
temporary steel trusses which will be supported at ends
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in a manner similar to typical precast units. The trusses
would only be removed once the slab is connected to
neighboring precast girders and main girders pre
stressed. The slab will have to be spanned for 8.25 m
because of escalator opening. Another challenge was to
maintain the thickness of this slab within 175mm.
Sighting transportation constraints due to large width of
the girder and to incorporate framing of elements around
the escalator opening in option 1, a girder-slab
arrangement was proposed with I girder weighing 80MT
each spanning 13m and 1.8 m width (Fig 3) .
Fig 3: Cross section of Option-2 Precast girder beam and
Inverted channel slab
The slabs were designed as inverted precast Channel
sections to rest on corbels of I girder. This arrangement of
channel slabs enabled flexibility to cater for escalator
opening.
In a step ahead to further minimize the complexity in
terms of weight of 13m long precast girders (fig 4) we
proposed to split the girder into two segments, each of
650mm width and total depth of 1750mm placed 700mm
apart so that they will be almost on grid line column,
perpendicular to the viaduct .These girders were
designed to support double Ts or channel sections or
Hollow Core slabs. This option would also take care of
escalator opening
Fig 4: Cross section of Option-3 Precast I girder beams
and Inverted channel slab or Hollow core slab
Precast elements were also suggested for Entry, Exit
buildings in the form of precast staircases (stringer beam-
treads arrangement), columns, beams, slabs (Hollow
core, solid slabs) parapet, walls panels, and closure
walls.
In a nutshell, a comparative can be derived based on the
following parameters.
Case II: Precast in Infrastructure for Car Parks
An Award winning project whose exposed surfaces
surpass all other material finishes for external facades,
while at the same time keeping sustainable factors of
unharmed air circulation and soft daylight factors for the
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the half million sqft spaces inside the building.
All the aspects of Architectural Design, Structural design
and detailing, façade design, integration of MEP
services, production, quality control, transportation, and
installation capabilities were tested to the limit in the
extremely challenging site conditions and the limiting
tolerances of +/- 20mm for each of the 4000 elements
forming the building, making it a truly challenging precast
structure, proving that “modularity” does not mean a
coarse repetitive sensation, but can be professionally
tweaked to generate a very interesting and eye catching
structure, which gives a constantly variable and different
visual sight as we pass by the structure.
The execution of this delicate but imposing project has
touched its life cycle at every point, beginning from the
experience of changing from conventional labour
oriented site executed projects to a mechanized /
industrialized path breaking technique. This session,
through this project will explain the thought process
behind how Precast India transformed itself into a leader
of the Indian Precast construction scenario including the
challenges of investing correctly into a precast facility by
knowing the existing market conditions and
requirements, while at the same time projecting future
real time scenarios, and finalizing factory layouts,
investments, site locations, type of automatic / semi-
automatic machineries, technology transfer etc. .
A short tour through the conception of the project, the
fundamental purpose of the structure, and the
justification of the cost of a stunningly simple but
articulate façade in prefabricated elements proves how
easily a professional Architect, Domingo Seminario;
transformed the “simple” and modular elements into a
disciplined and beautiful façade, attracting international
attention.
The challenges
The project originates from an International competition
of Architectural design managed by Precast India after
their appointment and requisite freedom granted by their
Client Infosys to create a “masterpiece” as a single
directive. After the winning entry was selected from 5
International and 3 Indian Architects by Infosys, started
the journey of changing mindsets of the Client, their
Consultants and Advisors, from going in for precast
technology instead of Conventional construction
methodologies.
The structural design of the project as conceived was a
super challenge, met very efficiently by the in-house
Structural engineers of Precast India, involving the
challenges of maintaining speed, quality and logistical
constraints of the site within city limits and for elements
ranging from 2 tons up to 28 tons for heights up to 45
meters, involving tower crane capacities of 50 tons and
moving on rails.
The detail of each structural / nonstructural precast
element, speaks volumes of the innovations and cutting
edge technologies that have been used for structural
design, production planning, production, transportation,
quality control, testing of mock ups, installation, and
handing over the completed structure.
There were special calculations of sustainability resulting
into excellent natural lighting and fresh ventilation
factors. There was a tolerance of only +/- 15 mm in the
structural arrangements of all the dowels / bolts of the
element and the sleeves provided in the other. The whole
structure was designed and analysed for Zone 3 seismic
conditions, and therefore connections and detailing in
structural design was a special consideration for raft to
basement walls, raft to columns, column to column,
column to beam, beam to spandrel, beam to slab, façade
to spandrel, and from façade to façade.
With a reputation for extensive integration and use of IT
systems into all processes of design, execution, and
installation, all project stages, Precast India created
accurate
production drawings through 3D BIM Tekla softwares, in-
house production Protrak software, integrated mould
booking systems, and tied up logistic software with bar
coded elements using mobile Android phones, to
address the challenging volume in a short time.
Therefore reducing the complexity of the project through
proper and systematic use of precast technology coupled
with latest IT systems and 3D BIM software, Precast India
IPL has opened its horizons to tackle further challenging
projects, and the many national / international
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recognitions plus awards won for this project in the
various Architectural, Structural, façade, and
prefabrication categories, have bolstered the morale.
Case Study III: Precast in other Infrastructure
Domains
Precast technology in bridges and flyovers is common all
over the world and consequently India has also leaped
into it in the form of segmental construction for bridges,
culverts, deck slab and segments for viaducts for metros.
Precast technology ventured into Pre-stressed bridge
planks, pre- stressed Double T Slabs(Fig 5) and I
girders, post tensioned girders, piles, modular deck
units, railway box girders, bridge piers, headstocks,
abutments, etc. and other heavy constructions.
3.1 Bridges and Flyovers
Most common prefabricated elements for bridge
connections are pre stressed elements such as I beam
(Fig 8a), box beam (Fig 8b), channel/U shaped elements
(Fig 8c) and other segments.
Fig: 6a, I beam Element for Bridge Construction
Fig: 6b Pre stressed box beam for bridges
Fig: 6c Pre stressed box beam for bridge construction
3.2 Irrigation:
Irrigation infrastructure generally comprises of water
retaining and regulating structures. The conventional
methodology at times proves uneconomical, with the use
of rapid strengthening admixtures and temporary water
barriers until the structure attains stability.
On the contrary, where Precast elements can be put to
use in Canal lining, culverts, channels, river intake
cabins, spillways, walkover bridges, sewage drains,
water storage tanks, ground irrigation , it can minimize
wastage and ensure quality and delivery of infrastructure
projects in stipulated time-frame .
Precast in infrastructure can also find its use in culverts.
They may be three sided or four sided. Three sided units
allow for stream crossings maintaining the natural
stream-bed. A four sided box culvert unit is generally
used as a permanent service tunnel or a carrier tunnel for
underground piping .Precast box culverts are superior
quality, require low maintenance and have large number
of applications including storm sewers, service tunnels,
or small bridges for stream crossings.
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Fig 7: Precast box culverts
As a part of external development works of the multilevel
car park, Precast India produced culverts to cater for
future provision of underground services of the state
Industrial Development Corporation, without the need to
excavate the newly laid concrete road at the main gate of
the Car Park.
The 250mm thick base was first cast, after which the
shuttering and reinforcement was in place to cast the side
walls. The walls were cast after the base attained
required strength.
The whole U shaped assembly measured 4.4m in length,
2.5m in width and heights varying from 1.53m to 1.8m
thus weighing 16.1 MT which was transported on the
trailer to the site.
The culvert was installed overnight so as to not disrupt
the employee traffic during the peak working hours of the
day. The hollow core slabs were then placed over the
culverts and cast in place screed was done
Fig 8: Precast culverts Cast in lengths up to 2.5m
weighing 16.1MT..
Fig: 9 precast elements in irrigation infrastructure
3.3 Pipe line infrastructure: Pipe lines are laid
across states for conveyance of different resources.
Where the alignment passes through the adverse
locations, the use of precast elements
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of precast elements simplifies the execution. Precast
pipes, precast pipe holders, pipe chambers are some of
such elements.
Fig: 10 precast elements in pipe line infrastructures
3.4 Urban infrastructure: As the concept of Smart
cities is being materialized in India, infrastructure is
expected in short time period with minimal maintenance
in the form of water drains, pathways, walkovers, service
ducts, underground chambers, Precast electrical poles &
their foundation, road dividers, Traffic barrier, etc.
Fig: 11 precast elements in pipe line infrastructures
3.5 Marine infrastructures: India having the longest
coastline, the requirements in marine infrastructure
cannot be ignored. Commonly used precast elements
are Tripods, tetra pods, and other break water elements.
In recent past for stability and longer life span quay walls
are also built using precast elements.
Fig: 12 precast elements in marine infrastructures
3.6 Tunnel infrastructure: Interlocking precast
segments has changed the face Tunnel construction
methodology which has achieved standardization by use
of Tunnel Boring machine
Fig: 13. Precast elements in marine infrastructures
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3.7 Building Infrastructure : It comprises of Pre-
stressed Kerbs,Tactile Tiles , Bollards, Tree Guards,
P r e c a s t P a v i n g S l a b , G r e a s e
interceptors,Headwalls,Road Medians, Precast Bicycle
Tracks, Precast Bicycle Tracks with storm water drainage
underneath, Precast Slabs with finishes, Storm Water
Channels,Signage,Precast Benches, Precast Steps,
Precast Reinforced Ramp System, Precast Traffic
Islands, Precast Planter Boxes. Precast elements like
trenches, road elements can be re-used in case of
change in service alignment with minimal damage to the
elements.
Fig: 14. Precast Kerb Stone, Electrical Trench, Road
Median, Storm water drain
CONCLUSION
Precast Construction has a wide scope to establish itself
in infrastructure as is evident from the case studies.
Presently, Metro rail infrastructure is of highest
importance in India. However, the construction is a
challenge, as the cities are highly populated and traffic is
increasing day by day. Precast is the best suitable option,
as it does not hinder the traffic flow with construction
taking place at a faster pace. The Governing authorities
must thereby encourage precast in infrastructure
tenders.
As the efforts to transform cities to Smart cities are
gaining momentum in the country, Precast satisfies the
requirements like Underground services, Subways,
Tunnels, Flyovers, and Stations etc. Airports as well as
well high-rise utilities. Precast facade elements of various
geometries can add to the aesthetic value of the
infrastructure projects
The conventional construction methodologies are highly
un-standardized even today. In case of complex projects,
probability of human error and accidents is high. Precast
Construction standardizes various construction
methodologies and work procedures both on site and off
site. Cost and Time, the two driving factors of every
project can be controlled efficiently in Precast
Construction. Need for huge construction labor, Cost and
Time overruns, accidents at site, time consumption,
resource wastage are alleviated to a greater extent in
Precast Construction.
The un-paralleled advantages of Precast in infrastructure
projects viz. superior quality due to controlled
environment , reduction in project duration, minimized
material wastage ,standardization of methodologies,
effective pre-engineering by use of BIM software for
modelling the structure at element level, reduction in on-
site activities definitely supersedes the additional cost
incurred in Precast in terms of machinery and
transportation of elements
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