Maximising the potential of concrete by combining precast and in-situ concrete Hybrid Concrete Construction
Jan 03, 2016
Maximising the potential of concrete by combining precast and in-situ concrete
Hybrid Concrete Construction
ContentsBenefits of hybrid concrete construction 3
Hybrid options 8
Design and procurement 12
Case study 1: Jubilee Library, Brighton 13
Case study 2: Hilton Hotel Tower Bridge, London 14
Case study 3: West Quay car park, Southampton 14
Case study 4: Homer Road, London 15
References 15
IntroductionHybrid construction combines the most appropriate materials and methods of construction. The search for greater economy, in terms of material costs and reduced construction time, has resulted in innovative approaches that seek to combine construction materials and methods to optimum effect. Hybrid concrete construction (HCC) is one such development that combines in-situ and precast concrete to maximise the benefits of both forms of concrete construction.
Hybrid concrete construction embraces a number of different forms
of structural frame, but in all cases precast concrete and cast in situ
concrete elements are used where they are most appropriate for the
project. HCC produces simple, buildable and economic structures
which result in faster, safer construction and reduced costs. There are
many benefits of concrete which are shared by both precast and in-
situ concrete. Many of these are listed in Table 1 and described in the
Benefits of Hybrid Concrete Construction section (page 3).
Table 1: Benefits of hybrid concrete construction
Precast concrete Precast or in-situ concrete
In-situ concrete
Economic for
repetitive elements
Inherent fire
resistance
Economic for
bespoke areas
Long clear spans Durability Continuity
(structural efficiency)
Speed of erection Sustainability Inherent robustness
Buildability Acoustic
performance
Design flexibility
High-quality finishes
and consistency of
colour
Thermal mass Services coordination
later in programme
Accuracy Prestressing Locally sourced
materials
Reduced propping
on site
Mouldability Short lead-in times
Reduced skilled
labour on site
Low vibration
characteristics
The Ideas Store on Whitechapel Road, London is a hybrid precast and in-situ concrete structure. The project, which was completed in 24 weeks, was a combination of cast in situ beams and columns and precast ribbed soffits slabs (as shown above). The designers deliberately exposed the concrete to provide a high-quality visual interior finish, which also provides thermal mass efficiency. Courtesy of Adjaye Associates.
Cover imagesMain: Ideas Store, London, courtesy of Hanson
Top inset: Homer Road courtesy of Foggo AssociatesBottom inset: West Quay Car Park, Southampton
This page: Ideas Store, courtesy of Adjaye Associates
Hybrid Concrete Construction
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Hybrid Concrete Construction
Benefits of hybrid concrete constructionHybrid concrete construction produces simple, buildable and economic structures. It delivers increased prefabrication, faster construction and consistent performance. HCC can achieve very significant cost savings and can satisfy the requirements of the most demanding of clients.
Buildability
The key advantage of HCC is its buildability. Because precast and cast in
situ concrete are used where most appropriate, construction becomes
relatively simple and logical. The use of HCC encourages design and
construction decisions to be resolved at design stage. This means, for
example, that precast elements can be manufactured, stored at the
factory and delivered ‘just-in-time’ to site. They can then be lifted from
delivery truck to final position in a single crane movement, eliminating
the need for site storage and reducing crane hook time.
Traditional formwork typically accounts for 40 per cent of in-situ frame
costs and is dependent on weather and labour. The use of HCC means
that a percentage of the frame is manufactured in a weather-proof
factory, resulting in faster construction.
Cost
Although the structural frame of a building represents only 10 per
cent of the total construction cost, the choice of material has dramatic
consequences for subsequent processes. Hybrid construction can
reduce frame costs by using precast concrete for the repetitive elements,
or to act as permanent formwork. In-situ concrete is more cost-effective
for large volumes (due to reduced transport costs) for tying the frame
together and for bespoke areas. Using the two together maximises the
cost efficiency.
Speed
Speed of construction depends on designs which are easy to procure
and construct. HCC takes a proportion of work away from the site
and into the factory, reducing the duration of operations critical to
the building programme on site. The precast process takes place in a
controlled environment, unaffected by weather. Rigorous inspection
before installation removes causes of delay on site. Developments and
innovation in formwork systems and concrete technology mean that
in-situ elements of a HCC structure can also be completed within tight
programme constraints.
Some HCC techniques can reduce or eliminate following trades, e.g.
installing ceilings and finishes. This enables even faster programme
times but requires greater co-ordination and care in detailing and
protection on site.
Safety
A high proportion of hybrid concrete construction is carried out in the
precast factory by experienced personnel. On site, the innovative use of
HCC and the improved buildability helps ensure that each safety plan is
prepared on the individual project’s merits.
HCC can reduce the potential for accidents by providing successive work
platforms and a tidier site. If precast spandrel beams are used they can
provide immediate edge protection.
Home Office Headquarters, London. The HCC frame was designed specifically for the project. This image shows the installation of the precast beams.
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Hybrid Concrete Construction
Benefits of both in-situ and precast concrete
Precast or in-situ concrete
Inherent fire resistance
Durability
Sustainability
Acoustic performance
Thermal mass
Prestressing
Mouldability
Low vibration characteristics
Fire resistance
Concrete has inherent fire resistance, which is present during all construction phases, and is achieved without the application of additional treatments. The fire resistance is also maintenance free. Concrete has the best European fire rating possible because it does not burn and has low heat conductance. Further information can be found in Concrete and Fire Safety [1] available from www.concretecentre.com/publications.
Durability
A well-detailed concrete frame is expected to have a long life and require very little maintenance. It should easily be able to achieve a 60-year design life and, with careful attention to the specification of the cover and concrete properties, should be able to achieve 100 years even in aggressive environments. BS 8500 [2] is the British Standard for durability and gives advice for various environments.
Sustainability
Concrete is a local product to the UK, manufactured from plentiful resources under strict regulations ensuring the highest environmental and social standards. Therefore the sector has been able to embrace responsible sourcing and manufacturers have gained accreditation at the highest level for their concrete products. This is recognised in sustainability assessment methods, enabling designers to gain maximum credits by choosing concrete.
Thermal mass
Buildings with concrete frames have embodied energy and CO2 of a
similar order to equivalent buildings constructed from other materials. For all buildings the operational energy consumption is far more significant than that during construction, but concrete buildings utilising thermal mass can reduce this impact on the environment by moderating building temperatures, delaying the peak temperatures to later in the day and thus minimising the need for air-conditioning. Use of thermal mass as part of passive solar designs can also reduce energy demands for heating during the winter, particularly in residential and education sectors. Further information is available from the document Utilisation of Thermal Mass in Non-Residential Buildings [3].
An award winning hybrid structure. Jubilee Library, Brighton. Courtesy of Bennetts Associates. For the full case study, see page 13.
Day
Internal temperaturewith high thermal mass
Internal temperaturewith low thermal mass
Peak temperaturedelayed by up to six hours
Up to 6-8oC differencebetween peak externaland internal temperature
External temperature
30oC
15oC
Night Day
Figure 1: Stabilising effect of thermal mass on internal temperature.
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Hybrid Concrete Construction
Homer Road speculative office development showing the tapered edge of the precast concrete perimeter unit. Courtesy of Foggo Associates. For the full case study, see page 15.
Acoustic performance
Concrete is a very good sound insulator, even when the source of
noise is an impact on the face of the concrete. For this reason concrete
floors and walls are often used in residential accommodation, including
flats, hotels and student residences, to prevent the passage of
sound between units.
Concrete can also be used to prevent the sound escaping into or out
of a building. A good example would be the use of concrete floors
beneath mechanical plant on the roof of a building to prevent the noise
penetrating to the habitable areas.
Prestressing
Prestressing concrete, using tensioned high-strength steel, reduces or
even eliminates tensile stresses and cracks. This gives rise to a range
of benefits that exceed those found in normally reinforced concrete
sections. Benefits include increased spans, stiffness and watertightness,
and reduced construction depths, self-weights and deflections.
Prestressing can be carried out before or after casting the concrete.
Tensioning the prestressing steel before casting (i.e. pre-tensioning)
tends to be carried out in factories e.g. in producing precast floor units.
Post-tensioning is more usually carried out on site using in-situ concrete.
Mouldability
Concrete can be formed into any shape and this can be achieved with
either precast or in-situ concrete. Concrete provides the opportunity to
create unusual shapes at a small cost premium. Repetition of elements
can make even complex shapes affordable for projects which are cost-
driven. This can be particularly beneficial if circular columns are required
for aesthetic reasons or where columns need to be contained in walls,
e.g. for apartments. Concrete can also be used for curved beams, unusual
plan shapes and shell structures. The layout of the vertical structure can
be arranged to suit the use of the building rather than having rigidly to
follow a structural grid.
V ibration control
For some types of buildings the control of vibrations induced by people
walking across the floor plate are important. This is particularly the case
for hospitals and laboratories containing sensitive equipment, but even
in offices long slender spans can vibrate excessively. The inherent mass
of concrete means that concrete floors generally meet vibration criteria
at no extra cost as they do not require additional stiffening. For more
stringent criteria, such as for laboratories or hospital operating theatres,
the additional cost to meet vibration criteria is small compared with
other structural materials.
An independent study [4] into the vibration performance of different
structural forms in hospitals has confirmed that concrete can normally
be readily designed for the most complete control of vibration over
whole areas, without the need for significantly thicker floor slabs than
those used for a basic ‘office’ structure. This gives great flexibility for
change in use and avoids the cost penalties of providing this extra mass
and stiffness.
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Hybrid Concrete Construction
Benefits of precast concrete
Precast concrete
Economic for repetitive elements
Long clear spans
Speed of erection
Buildability
High-quality finishes and consistency of colour
Accuracy
Reduced propping on site
Reduced skilled labour on site
Economic for repetitive elements
Using precast elements reduces requirements for falsework; this saves
cost through reduced resources and by shortening the programme.
There is also less reliance on wet trades, which can be delayed by
unfavourable weather conditions. However, to maximise economy
the mould created to cast the concrete should be re-used as much
as possible, thus precast concrete is most economic where repetition
is maximised. Repetition does not mean the finished building will be
uninspiring; designers can produce aesthetically pleasing designs by
innovative use of repeat elements.
Long clear spans
Reducing the number of columns is often important in developments
such as offices, sports stadia and car parks. Prestressing the concrete can
deliver these longer spans or shallower construction depths.
Speed of erection
Speed of erection and tight construction programmes are primary
considerations in many building projects. To maximise the speed of
construction with precast elements two critical factors should be taken
into consideration:
• Thebuildinglayoutshouldbedesignedtomaximiserepetition
of precast units
• Constructiondetailsshouldbedesignedtomaximisethenumber
of standardised components.
Buildability
Precast elements are designed by specialist precast concrete designers.
Within their design they consider the erection sequence and process so
that the elements are engineered to be constructed easily. This planning
makes the frame highly buildable.
High-quality finishes and consistent colour
High-quality consistent finishes are generally achieved through the use
of robust, purpose-made formwork and dedicated concrete mix designs
in a factory environment. Sample finishes can be approved by the client
as a benchmark for the project requirements. For visual concrete that
is to be exposed to exploit the thermal mass of concrete construction,
consistency of tone and texture is important. Precast factories have
dedicated concrete supplies ensuring consistency of supply and giving
greater control of the constituent materials used.
Acceptability of finishes and consistency of tone can be confirmed
prior to leaving the factory. A wide choice of precast concrete cladding
finishes and facings is available, including:
• Surfaceretardingandwash-off
• Rubbing
• Abrasiveblasting
• Bushhammering
• Mechanicalgrindingandpolishing
• Acidetching.
More information on architectural finishes can be found in Precast
Concrete in Buildings [5].
Accuracy
Precast elements are cast to close tolerances, and checked in the factory
before delivery to site.
Reduced propping on site
Depending on the type of element used, there may be no temporary
propping or minimal propping required. This increases productivity and
reduces the temporary works.
Reduced skilled labour on site
The production of precast concrete takes place in a factory environment,
removing labour requirements from site. The factory work is carried out
in an internal environment at safe working heights.
Toyota UK Headquarters is an exposed precast and hidden in-situ reinforced concrete hybrid building. Courtesy of Trent Concrete.
Precast glazed insulated panels. These site-ready panels can reduce programme time on site. Courtesy of Roger Bullivant Ltd.
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Hybrid Concrete Construction
Benefits of in-situ concrete
In-situ concrete
Economic for bespoke areas
Continuity (structural efficiency)
Inherent robustness
Flexibility
Services coordination later in programme
Locally sourced materials
Short lead-in times
Economic for bespoke areas
In-situ concrete can be cost-effective for bespoke areas and can
therefore be combined effectively with precast concrete for more
unusual areas or elements of a building.
Continuity (structural efficiency)
In-situ concrete is generally designed to maximise the benefit of the
monolithic structure, by use of structural continuity which increases
spans and stiffness and reduces construction depths.
Inherent robustness
An in-situ concrete frame is generally very robust because of its
monolithic nature. Usually the tying requirements of the Building
Regulations to avoid disproportionate collapse are met with normal
detailing of concrete. In-situ concrete areas can be used with precast
concrete elements to provide the necessary tying without having to
introduce ties specifically for this role. How to Design Concrete Buildings
to Satisfy Disproportionate Collapse Requirements [6] is available at
www.concretecentre.com/publications.
Flexibility
In-situ concrete is a flexible material to use; it can be cast into an infinite
number of shapes, and can be varied from floor to floor. It is available
throughout the UK from concrete suppliers and placed by experienced
contractors.
Services coordination later in programme
With in-situ concrete the location of services penetrations can occur
later in the programme. This is because the final design of the concrete
elements can occur later in the overall programme than for elements
fabricated off-site.
Locally sourced materials
In-situ concrete is available close to project sites, wherever they are in the
UK. Many ready-mix plants are located where the aggregate is extracted or,
where this is not possible, aggregate is often transported by rail or water.
Short lead-in times
The lead-in time for in-situ concrete can be considerably shorter than
other materials, this is because the materials are readily available and
assembled in position. This can result in in-situ concrete delivering
quickest overall construction times.
The average distance from a concrete plant to any building site in the UK is 8km, providing a sustainable solution to transportation.
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Hybrid Concrete Construction
Hybrid optionsThe ideal combination of precast and in-situ concrete is influenced by project requirements. There is a wide range of possible options, a selection of which is presented here as representative of current UK practice. It is not intended to be an exhaustive list.
Ease of services distribution
Minimises storey height
Suitability for holes
Clear spans Deflection control
Minimise materials
Soffit can be exposed
Maximises off-site construction
Temporary works minimised
Type 1
Type 2
Type 3
Type 4
Type 5
Type 6
Excellent Good Can be used
Type 1
Precast twin wall and lattice girder
slab with in-situ concrete
Type 2
Precast column and edge beam
with in-situ floor slab
Type 3
Precast column and floor units
with in-situ beams
HybridFinal versionOwen Brooker27.11.08
Type 4
In-situ columns or walls and beams
with precast floor units
Type 5
In-situ column and structural topping
with precast beams and floor units
Type 6
In-situ columns with lattice girder slabs
with optional spherical void formers
9
Hybrid Concrete Construction
Precast twin wall and lattice girder slab with in-situ concreteHybrid concrete wall panels are increasingly being specified on projects
throughout the UK and are often known as ‘twin wall’. They comprise
two skins of precast concrete connected by steel lattices, which are filled
with in-situ concrete on site.
The external skins of the twin wall system are factory made, typically
using steel moulds. This results in a high-quality finish. The panel surface
quality is suitable to receive a plaster finish or wallpaper. The panel
surface is not normally appropriate for visual concrete. Joints either have
to be expressed as a feature of the finish, or concealed. This type of HCC
offers advantages to the contractor in terms of speed of construction,
as well as reducing the number of skilled site staff required to construct
walls. Often the twin wall system is combined with the use of lattice
girder precast soffit slabs, with or without spherical void formers (Type
6, shown on page 8). These provide permanent shuttering for an in-
situ slab that can be relatively easily combined with the wall system.
Spans of up to 8m are common and spans up to 14m are possible. (The
manufacturer should be consulted early on to ensure the longer spans
are viable.)
Potential structural uses of the twin wall system include:
• Cellulartypestructuresforresidentialuse
• Wallscarryingverticalloadsonly
• Shearandcorewalls;thishassignificantimplicationsforthedesign
• Retainingwalls;thishassignificantimplicationsforthedesign
• ‘Singlesided’formworksituations,wherethereisnoaccessto
one side of the wall to erect formwork, for example wall
construction on a party wall line against neighbouring buildings.
The major advantage is that it is an ‘in-situ structure’, fully continuous
and tied together, but without the need for shuttering on site. Twin wall
can also be cast with fully trimmed openings and with ducts for cables
and other services.
Advantages:
• Qualityfinishforwallsandsoffitsenablinguseofthermalmass
• Noformworkforverticalstructureandhorizontalstructure
when lattice girder slabs are used
• Structuralconnectionbetweenwallandslabsreliesonin-situ
reinforced concrete detail and is inherently robust
• Reducedpropping
Disadvantages:
• Proppingoflatticegirderslabsisrequiredpriortosufficient
strength gain of in-situ concrete
• Thesmallerdimensionoftheprecastunitsistypicallyamaximum
of 3.6m, so joints in walls and soffits must be dealt with (expressed
or concealed)
• Reducedflexibilityoflayoutasthisoptionrequireswallsrather
than columns.
One Coleman Street, London. Inset: Off loading twin wall units. Courtesy of John Doyle Construction.
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Hybrid Concrete Construction
Precast column with in-situ floor slabThe combination of an in-situ slab, e.g. post-tensioned flat slab, with
precast columns can provide an economic and fast construction system.
Precast concrete edge beams may also be used to avoid edge shutters
on site and to allow perimeter reinforcement, cladding fixings or
prestressing anchorages to be cast in. This reduces the time required for
reinforcement fixing and erecting the formwork.
The maximum span for this form of construction depends largely on
whether the in-situ slab is post-tensioned. For flat slabs with spans
greater than 10m punching shear is likely to be a critical design issue.
This form of construction relies on the structure being braced. This is
achieved by the lift core(s) or separate shear walls.
Advantages:
• Columnscanbeerectedquickly
• Qualityfinishforcolumns
• Precastedgebeamcontainspost-tensioninganchorages
(if required), slab edge reinforcement and cladding fixings,
and avoids need for slab edge shuttering
• Canbeusedwithavarietyofin-situslabs,selectedtosuit
individual project requirements
• Moreflexibleforlatechanges
Disadvantages:
• In-situslabrequiresfalsework,formworkandcuringtime
Precast column and floor units with in-situ beamsThis form of construction allows a high proportion of the structure to be
manufactured in quality controlled factory conditions off site leading to
fast construction on site.
A variety of precast floor products could be used with this type
of construction, including hollowcore units, double tees, lattice
girder slabs (with or without spherical void formers) or bespoke
coffered floor units. The latter have successfully been used in
high quality buildings designed for energy efficiency, where the
lighting, architectural features and cooling systems have all been
incorporated into the unit.
Advantages:
• Verticalstructurecanbeerectedquickly;noformworkrequired
• Precastfloorstructurecanbeerectedquickly;no
formwork required
• Qualityfinishforcolumnsandsoffits(althoughthisisnotalways
possible with hollowcore units)
• Structuralconnectionbetweenprecastelementsisviastandard
reinforced or post-tensioned concrete
Disadvantages:
• Precastflooringmustbetemporarilypropped
• Sealingbetweenprecastunitsisrequired
In-situ columns or walls and beams with precast floor unitsA variety of precast floor products could be used with this type of
construction, including hollowcore units, double tees, lattice girder slabs
(with or without spherical void formers) or bespoke coffered floor units.
Advantages:
• Precastfloorstructurecanbeerectedquickly;noformworkrequired.
• Qualityfinishforsoffits(althoughthisisnotalwayspossiblewith
hollowcore units)
• Shortleadtimeforstandardprecastproduct
Disadvantages:
• Precastflooringmustbetemporarilypropped
• Sealingbetweenprecastunitsisrequired
In-situ column and structural topping with precast beams and floor unitsIn this form of construction the floor consists entirely of precast
elements, which are tied together with an in-situ structural topping.
The column formwork can be designed as a temporary support for
the precast beams and slabs to reduce the requirement for propping
of the precast floor. The joint between the beam and columns and any
structural screed is concreted with the columns to form a monolithic,
robust structure.
This system requires particular attention to the connection details
between the precast beam and floor units. It should be ensured that
adequate structural ties are provided to achieve a robust structure.
Advantages:
• Precastfloorstructurecanbeerectedquickly
• Precastbeamssupportprecastfloorunits,minimisingfloor
propping
• Precastqualityfinishforsoffits(althoughthisisnotalways
possible with hollowcore units)
• Formworkforin-situcolumnscanbeusedtoprop
precast beams
• Structuralconnectionbetweenprecastelementsisvia
standard reinforced concrete
• In-situstructuraltoppingtobeampermitsbeamstobe
continuous over columns
Disadvantages:
• Downstandbeamsneedtobecoordinatedwiththeservices
distribution
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Hybrid Concrete Construction
Spherical void formers
In-situ columns with lattice girder slabs with optional spherical void formersThe main feature of this system is the use of the lattice girder panels
to act as permanent formwork for a flat slab. A variation is to include
spherical void formers. These reduce the self-weight of the slab for
only a small reduction in flexural strength and stiffness. Lattice girders
and void former cages are cast into concrete panels containing
reinforcement in two directions, providing a precast panel that acts as
the permanent formwork. If the spherical void formers are used, they are
removed in areas of high shear where a solid section provides greater
shear resistance. The slab may be designed as a flat slab to reduce the
overall floor zone of the building and to simplify installation of services.
Propping of the panels will be required. The quality of the factory
produced soffits provides the opportunity to take advantage of the
thermal mass properties of the concrete slab by exposing them.
Advantages:
• Precastfloorstructurecanbeerectedquickly;noformwork
required
• Structuralconnectionbetweenprecastelementsisvia
standard reinforced concrete
• Qualityfinishforsoffits
• Moreflexibleforlatechanges
Disadvantages:
• Precastflooringmustbetemporarilypropped
INSTALL FORMWORK & PRECAST BEAM
Lifting with the crane
Precast beamSafety Safety
Level +1
Moving formwork
Steelformwork
Back-propping (if necessary)
2
1
AA
POURING COLUMNS
Precast beamSafety
Reinforcement
Propping
Level +2
Concreting (column & stitch)
Level +1
Steelformwork
3
4
PLACING HOLLOWCORE PLANKS
Level +1
Level +2
Lifting
Propping
Hollowcore
4
5
Props
POURING TOPPING
Level +1
Level +2
ToppingFinishing
Concreting
Slab Reinforcement
FREE AREA
67
Stage 1:
Column formwork erected to provide temporary support for the precast beams. Precast beams positioned on the column formwork
with beam rebars projecting into the column stitch.
INSTALL FORMWORK & PRECAST BEAM
Lifting with the crane
Precast beamSafety Safety
Level +1
Moving formwork
Steelformwork
Back-propping (if necessary)
2
1
AA
POURING COLUMNS
Precast beamSafety
Reinforcement
Propping
Level +2
Concreting (column & stitch)
Level +1
Steelformwork
3
4
PLACING HOLLOWCORE PLANKS
Level +1
Level +2
Lifting
Propping
Hollowcore
4
5
Props
POURING TOPPING
Level +1
Level +2
ToppingFinishing
Concreting
Slab Reinforcement
FREE AREA
67
Stage 3:
Hollowcore slabs placed between the beams.
INSTALL FORMWORK & PRECAST BEAM
Lifting with the crane
Precast beamSafety Safety
Level +1
Moving formwork
Steelformwork
Back-propping (if necessary)
2
1
AA
POURING COLUMNS
Precast beamSafety
Reinforcement
Propping
Level +2
Concreting (column & stitch)
Level +1
Steelformwork
3
4
PLACING HOLLOWCORE PLANKS
Level +1
Level +2
Lifting
Propping
Hollowcore
4
5
Props
POURING TOPPING
Level +1
Level +2
ToppingFinishing
Concreting
Slab Reinforcement
FREE AREA
67
Stage 4:
Slabs topped with 50mm cast in situ concrete to achieve a monolithic structural unit.
INSTALL FORMWORK & PRECAST BEAM
Lifting with the crane
Precast beamSafety Safety
Level +1
Moving formwork
Steelformwork
Back-propping (if necessary)
2
1
AA
POURING COLUMNS
Precast beamSafety
Reinforcement
Propping
Level +2
Concreting (column & stitch)
Level +1
Steelformwork
3
4
PLACING HOLLOWCORE PLANKS
Level +1
Level +2
Lifting
Propping
Hollowcore
4
5
Props
POURING TOPPING
Level +1
Level +2
ToppingFinishing
Concreting
Slab Reinforcement
FREE AREA
67
Stage 2:
Cast in situ columns poured to the top of the precast beams: stitching together the beam/column joint.
The Home Office headquarters hybrid concrete structure was constructed using the above four stage sequence.
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Hybrid Concrete Construction
Design and procurementDesign
Hybrid concrete construction can be designed as a normal reinforced
concrete building, with full composite action between in-situ and
precast elements. The design should also consider the construction
phase, as one of the load cases is normally precast concrete elements
supporting the weight of wet in-situ concrete. An additional stage
may be considered if de-propping happens before the in-situ concrete
reaches its design strength.
The interface between precast and in-situ concrete elements
should be considered in the design process and a detailed
guide, Design of Hybrid Concrete Buildings [7] is available from
www.concretecentre.com/publications. This gives essential
guidance on the key considerations.
Initial sizing
The initial sizing of the elements for HCC can be carried out using
normal methods, for example The Concrete Centre publications
Economic Concrete Frame Elements [8] and Concrete Buildings Scheme
Design Manual [9] both give guidance on sizing concrete frames.
Procurement
Many UK engineers are experienced in using in-situ concrete, but may
feel less confident specifying precast concrete. To obtain the maximum
benefit, it is advisable to involve the precast concrete manufacturer at
the earliest opportunity. The precast industry is able to give initial advice.
The publication Best Practice Guidance for Hybrid Concrete Construction
[10] looks at the procurement process from concept stage through to
design and construction, suggesting processes that allow the capture of
best practice. It is supported by a number of case studies. The guidance
explains the benefits that result from:
• Earlyinvolvementofspecialistcontractors
• Usingaleadframecontractor
• Usingbestvaluephilosophy
• Holdingplannedworkshops
• Measuringperformance
• Trust
• Closecooperation–withanemphasisonpartnering.
It is recommended that this guidance is used to maximise the
advantages of using HCC.
Inland Revenue, Nottingham, interior of building. The design fully exploited the potential of precast concrete and prefabrication of other major structural elements to achieve realbuildability.Image:MartineHamilton-Knight/BuiltVision.
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Hybrid Concrete Construction
Case study 1: Jubilee LibraryThe Jubilee Library, Brighton has been lauded for its design values
and sustainability performance. It has won numerous accolades and
achieved a BREEAM rating of ‘Excellent’. A mixture of precast and in-situ
concrete was used to meet these high standards.
Construction
The building consists of four storeys, with reading rooms, meeting rooms
and staff accommodation situated either side of a central, double-height
atrium, itself built on two floors. The central space was constructed using
an in-situ concrete slab supported by a series of eight tree-like
in-situ concrete columns with fins. The thermal mass of the concrete
assists with moderating the temperature fluctuations within the building.
Elsewhere, 260mm thick precast hollowcore units have been used as
part of a Termodeck system. Air is pumped through the cores in the
units to heat or cool the building as necessary; again the thermal mass
of the concrete is used to minimise the energy required for heating and
cooling.
What HCC brought to the project
Concrete was a key component of the buildings heating and cooling
systems. A variety of concrete elements were used to suit specific
situations. The hollowcore units provided the ducts for the air flow.
Precast was also used where a high quality finish was required. In-situ
concrete was used for larger floor areas to avoid visible joints, and for the
feature fins.
Project team:
Architect: Bennetts Associates with Lomax, Cassidy and Edwards Architects
Structural engineer: SKM Anthony Hunt
Contractor: Llewellyn
Concrete frame contractor: Gallaghers
14
Hybrid Concrete Construction
The Hilton Hotel, Tower Bridge is located on the south bank overlooking
the river Thames. It is 13-storeys high and contains 255 bedrooms. The
lower three storeys contain public spaces and a 500-seat conference
centre.
Why hybrid concrete construction was chosen
The twin wall solution, with lattice girder slabs was proposed as an
alternative to fully cast in situ walls and slabs. This proposal allowed
the contractor to reduce the frame construction programme enabling
earlier opening of the hotel.
Construction
The building has a double storey height basement over part of the area
with a conventional concrete frame for the lower storeys. Above the
public spaces the vertical structure consists of twin wall precast units
and floors that use lattice girder slabs. The lattice girder slabs were lifted
into position with the edge protection already in place.
What HCC brought to the project
Theuseofhybridconcretegaveafastconstructionprogramme–each
floor was completed in just five days, including placing the bathroom
pods. The precast walls, which were used for all the dividing walls and
soffits, gave a high-quality, accurate finish and minimised following trades.
The use of precast lattice girder slabs gave a safe working platform for
fixing reinforcement and pouring the topping concrete. The lattice girder
slabs also reduced the falsework and propping requirements allowing the
bathroom pods to be lifted into position before placing the floor above.
Overall, compared to other construction methods, the site was cleaner
and there was less construction noise.
The West Quay car park is one of the largest multi-storey car parks in the
UK.Thestructureis95mlong,95mwideand20mhigh–eightstoreys
comprising 15 split levels with a 2m clear headroom throughout. Access
to the car park is by means of seven staircases and two double lifts.
Why hybrid concrete construction was chosen
At scheme design stage the design team considered various options
for the structural frame, before selecting a HCC structure based on
precast concrete double-tee floor slabs on to cast in situ concrete beam-
and-column frames. The decision to use HCC was based on a ‘value
engineering’ exercise. By combining the cost advantages of cast in situ
concrete with the speed of assembly of precast, meant that the structure
could be completed on time and within budget.
Construction
The precast concrete double-tee floor slabs span 15.8m and are 2.4m
wide, matching the width of a standard car parking bay and fitting
neatly into the 7.2m grid in the east-west direction. The cast in situ
concrete beams were cast with nibs projecting at both sides and the
ends of the slabs were cast with extended scarf joints; they rest on
the nibs and create a 300mm wide channel for service trunking. The
east wall of the car park takes the form of a sloping buttress clad with
precast concrete panels with a reconstructed stone mix and knapped
flint aggregate inserts. At upper levels the car park is clad with precast
spandrel panels of reconstructed stone. The panels were doweled to the
cast in situ concrete structure with cast-in sockets.
What HCC brought to the project
The use of HCC allowed the project to be completed on time and within
budget, with a remarkable lack of interface problems. In particular the
advantages of precast concrete double-tee floor slabs were fully realised;
they proved to be a positive way to create large areas of floor very
quickly, whilst maintaining a high quality finish.
Case study 2: Hilton Hotel, Tower Bridge
Case study 3: West Quay car park
Project team
Architect: Jestico & Whites
Structural engineer: Adams Kara Taylor
Cost consultant: EC Harris
Construction manager: Bovis Lend Lease
Concrete frame contractor: John Doyle Construction
Project team
Architect: BDP
Structural engineer: Pell Frischmann
D & B contractor: Sir Robert McAlpine
D & B engineer: Sir Robert McAlpine Design Group
Precast floors: Tarmac
Precast concrete cladding: The Marble Mosaic Company
15
Hybrid Concrete Construction
Why hybrid concrete construction was chosen
With the Homer Road office building, hybrid concrete construction was
used to create a structure which allows full continuity to occur between
the vertical and horizontal structural elements, thus providing a stiff
sway framework.
The combination of elements allowed the whole frame to act as a
composite structure without relying on expensive mechanical fixings.
This method of construction produces a rigid frame which is inherently
stable without the need for shear walls or bracing.
HCC was the natural choice of material. It fulfilled the design criteria for
a visible expression of the structure; behind the delicate glazed facades
the precast column and beam structure is clearly visible, needing no
further treatment such as cladding for fire protection. In addition, by
exposing the painted soffits of the concrete floor slabs in the offices, the
temperature and ventilation strategy could exploit the thermal mass
potential of the concrete.
Construction
The hybrid concrete structure consists of 430mm diameter precast
columns and precast floor units connected together by means of cast
in situ concrete spine beams. Each floor unit takes the form of a double
tee-section with end plates to each trough. At each column connection
the end plates are cast with a curved ‘cut-out’ to follow part of the
column profile.
Once the precast columns were fixed on site, the double tee-section
floor units were connected to them, positioned so that the curved edge
profiles trimmed the outer edge of the columns. The cast in situ concrete
spine beam was then cast between two rows of end plates, stitching
lower and upper columns and adjacent units together. Between the
longitudinal joints, loop connectors were cast into the units and a
continuous cast in situ beam joined the units together. The floor units
are self-finished and no screed or topping was required.
At the perimeter the same principle was used with a slightly different
detail. The spine edge beam was cast between the final row of end
plates (which ran up to the inner side of each perimeter column) on one
side and a special precast perimeter unit on the other side, which creates
a tapered edge to the ceiling soffit. The perimeter unit has a row of
precast holes which allows warm buoyant air rising up the facade to be
effectively captured and cooled by the passive chilled beam elements
above the ceiling panels. Similar precast holes connect each trough and
provide return air paths to the central atrium. The precast perimeter
units were cast with a sculpted feature where they meet the column
heads. They were also used at the atrium and core perimeters, cast in the
same moulds with minor adaptations.
Project team
Architect, engineer and cost consultant: Foggo Associates
Construction manager: Bovis Lend Lease
Precaster: SCC (Structural Concrete Contractors)
Case study 4: Homer Road
References1. THE CONCRETE CENTRE. Concrete and Fire Safety. The Concrete Centre, 2008.
2. BRITISH STANDARDS INSTITUTION. BS 8500 Concrete – Complementary British Standard to BS EN 206-1. BSI, 2006.
3. THE CONCRETE CENTRE, Utilisation of Thermal Mass in Non-Residential Buildings, TCC, 2007
4. ARUP. Hospital floor vibration study. Comparison of possible floor structures with respect to NHS vibration criteria . Research Report, Arup, 2004.
5. THE CONCRETE CENTRE. Precast Concrete in Buildings. The Concrete Centre, 2007.
6. BROOKER, O. How to Design Concrete Buildings to Satisfy Disproportionate Collapse Requirements. The Concrete Centre, 2009.
7. WHITTLE, R & TAYLOR, H. Design of Hybrid Concrete Buildings. The Concrete Centre, 2009.
8. GOODCHILD, C H, WEBSTER & R M, ELLIOTT, K S. Economic Concrete Frame Elements. The Concrete Centre, 2009.
9. BROOKER, O. Concrete Buildings Scheme Design Manual. The Concrete Centre, 2009.
10. GOODCHILD, C H & GLASS, J. Best Practice Guidance for Hybrid Concrete Construction, The Concrete Centre 2004.
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