Mb Hcc July05

HybridConcreteConstructionCOMBINING PRECAST AND IN-SITU CONCRETE FOR BETTER VALUE STRUCTURAL FRAMES

All advice or information from The Concrete Centre is intended for those who will evaluate the significance and limitationsof its contents and take responsibility for its use and application. No liability (including that for negligence) for any lossresulting from such advice or information is accepted. Readers should note that all Concrete Centre publications are subjectto revision from time to time and should therefore ensure that they are in possession of the latest version.

CPD presentations on concrete and hybrid concrete construction areavailable on request. Please call 0700 4 500 500 or 0700 4 CONCRETE,or email [email protected]

FURTHER READINGEcoconcrete: The contribution of cement and concreteto a more sustainable built environment.British Cement Association, 2001, Ref. 97.381.

Economic concrete frame elements A handbook forthe rapid sizing of concrete frames.British Cement Association, 1997, Ref. 97.358.

Concept An invaluable design tool for theconceptual design of reinforced concrete frames.The Concrete Centre, 2004, Ref. TCC/03/09.

Concrete and fire Using concrete to achieve safe,efficient buildings and structures.The Concrete Centre, 2004, Ref. TCC/05/01.

Altolusso Apartments, Cardiff, which used precast columns and in-situ floor slabs(Architect: Holder Mathias). Photo courtesy of Laing ORourke.

UDC

624.05.016-033.3

CI/SfB

The Concrete Centre, Riverside House,4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB

National Helpline 0700 4 CONCRETE or 0700 4 500 500

www.concretecentre.com

Ref. TCC/03/010

ISBN 1-904818-22-6

First published 2005

The Concrete Centre

1For a structural system that offers

BUILDABILITY AND SIMPLICITY OF CONSTRUCTION

GREATER SPEED AND LESS COST COMPAREDTO OTHER SYSTEMS

ALL THE BENEFITS OF CONCRETE

Hybrid concrete construction is the right choice.Front cover (left to right):Precast beam with exposed rebar for structural topping being craned into place at Home Office, London (Architect:Terry Farrell and Partners). Photo courtesy of Pell FrischmannOffice project, Homer Road, Solihull, showing the precast columns and floor units and cast in-situ beams (Architect: Foggo Associates). Photo courtesy of Foggo AssociatesHigh quality and durable finish of precast concrete, West Quay, Southampton (Architect: BDP).

Below (left to right):Gatwick office project, showing precast concrete floor beams onto in-situ beams and columns. Photo courtesy of John DoyleInsitu floor slabs and precast wall units were used as part of an HCC structure at Albion Riverside (Architect: Foster and Partners).Precast floor units at Ideas Store, Whitechapel, London (Architect: Adjaye/Associates). Photo courtesy of Hanson Building Products

WHAT IS HYBRID CONCRETECONSTRUCTION?Hybrid Concrete Construction (HCC) is a method of construction which integrates precast concreteand cast in-situ concrete to take best advantage of their different inherent qualities. The accuracy,speed and high-quality finish of precast components can be combined with the economy andflexibility of cast in-situ concrete. As this publication will demonstrate, hybrid concrete technologyembraces 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. The results areremarkable: faster construction and considerable cost savings in some cases of up to 30% comparedwith conventional structural frame systems.

Hybrid Concrete Construction produces simple, buildable and competitive structures. The client is given better value and the contractor benefits from increased off-site component manufacture,safer and faster construction and consistent performance. The featured case studies demonstratehow HCC can encompass a great variety of forms and building types: offices, schools, car parks andcommercial/retail buildings. Judging from the number of HCC projects at drawing board stage, todaysbuilding professionals are taking full advantage of this form of construction.

PRECAST CONCRETE

IN-SITU CONCRETE

HYBRID CONCRETECONSTRUCTION

+ =

Precast concrete for quality of finishand speed of erection.

Hybrid concrete construction for allthe benefits of concrete in theoptimum form.

Insitu concrete for flexibilityand economy.

Photo courtesy of Trent Concrete Photo courtesy of Trent Concrete

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THE BENEFITS OF USINGHYBRID CONCRETECONSTRUCTIONHybrid Concrete Construction produces simple, buildable and competitive structures.It delivers increased prefabrication, faster construction and consistent performance.HCC can achieve very significant cost savings and can satisfy the requirements of themost demanding of clients.

COSTAlthough the structural frame of a building represents only 10% of the totalconstruction cost, the choice of material for the frame has dramatic consequences for subsequent processes. Until recently cast in-situ concrete was commonly viewedas the most economic frame material, but hybrid construction can offer greaterspeed, quality and overall economy. Although the elemental cost of a hybrid concreteframe may be higher, total project costs are usually lower due to time savings on site.Faster programmes mean an earlier return on investment, lower interest charges andreduced construction preliminaries. For example, the use of a hybrid concrete frameinstead of a composite steel frame on a shell-and-core office project in London ledto a saving of 29% and a 13% increase in lettable floor area.

The use of concrete has additional benefits in assessing whole-life costs a factorimportant to owner-occupiers and PFI operators. For example the thermal mass ofconcrete can moderate energy demands in cooling and heating buildings.

SPEEDSpeed of construction depends on designs which are easy to procure and construct.Essentially 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. Theprecast process takes place in a controlled environment, unaffected by weather.Rigorous inspection before installation removes causes of delay on site.

Some HCC techniques can reduce or eliminate follow-on trades, eg installing ceilingsand finishes. This enables even faster programme times but requires greater co-ordination and care in detailing and protection on site.

BUILDABILITY AND CONSTRUCTIONThe key advantage of HCC is its buildability. Because precast and cast in-situ concrete are used where most appropriate, construction becomes relatively simpleand logical. The use of HCC encourages design and construction decisions to beresolved at design stage. This means, for example, that precast elements can bemanufactured, stored at the factory and delivered just-in-time to site; they can belifted 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% of cast in-situ frame costs. It isdependent on weather and labour. The use of HCC means that a percentage of theframe is manufactured by a skilled workforce in a weatherproof factory, resulting infaster construction and better quality.

SAFETYA high proportion of the work for a hybrid concrete construction is carried out in theprecast factory by experienced personnel. On site, the innovative use of HCC and thefact that buildability is a key concern helps ensure that each safety plan is drafted onthe individual projects merits.

HCC can reduce the potential for accidents by providing successive work platformson a generally less cluttered site. If precast spandrel beams are used they can provideimmediate edge protection.

SUSTAINABILITYHCC also offers the opportunity to exploit theinherent thermal mass of concrete by exposing thesoffit of precast concrete floor slabs. This fabric energystorage (FES) of the structure can help to controltemperatures in the context of a naturally ventilatedlow-energy building. The finish and shape of exposedprecast concrete units can also be used to help withthe even distribution of lighting and to reduce noise levels.

Concrete structures have equivalent or less embodiedenergy and CO2 than structures constructed from othermaterials. For all buildings the operational energyconsumption is far more significant than that duringconstruction, but concrete buildings utilising thermalmass can reduce this impact on the environment byminimising the need for air-conditioning.

OTHER BENEFITSConcrete is a uniquely versatile material. It is a cast material of high strength which can be shaped to produce a vast variety of structural elements.Concrete structures are stable, robust, fire resistant and adaptable, as well as, solid, quiet and essentiallyvibration-free. The composition of concrete can bevaried to produce a variety of colours, textures andfinishes, some of which closely match natural stone.Both the structural and the aesthetic advantages ofconcrete can be incorporated into the design of an HCC structure.

The structural versatility of concrete can be developedwith HCC. Long spans can be achieved by using largeunits, or by pre-stressing or post-tensioning. Precastunits can also be welded together using modern ultra high-strength concretes.

BEST PRACTICE GUIDANCEHCC offers great advantages to the constructionindustry. In return it demands a high level ofcommitment from all parties at all stages of the designand construction process. To make the most of itsadvantages, HCC should be considered at the verybeginning of the design process.

HCC offers many advantages over conventionalconstruction methods. A new publication, Best PracticeGuidance for Hybrid Concrete Construction, is availableto help construction teams. Based on comprehensiveresearch, it explains the principles of HCC and describes,with detailed how it should be process maps, how toachieve best practice. Four case studies, illustrated withdetailed construction photographs and drawings, showhow hybrid concrete construction is used in practice.An appendix gives guidance on structural design.

Published by The Concrete Centre Best PracticeGuidance for Hybrid Concrete Construction byC H Goodchild and J Glass is available from TheConcrete Bookshop. Telephone 0700 4 500 500or visit www.concretebookshop.com

The exposed soffits of this officebuilding help to control temperatureas part of the ventilation strategy.Homer Road, Solihull(Architect: Foggo Associates).Photo courtesy of Foggo Associates

High quality precast concrete floor slabs with exposed soffits wereused as part of an HCC structure for the Toyota headquarters, Epsom(Architect: Sheppard Robson). Photo courtesy of Trent Concrete

Precast concrete floor slab units with vaulted soffits and castin-situ beams and columns. Office building, Paternoster Square,London. Photo courtesy of John Doyle

Hybrid concreteconstruction can provide:

Faster construction

Improved safety on site

Cost effective construction

Simple, buildable structures

Excellent fire performance

Sustainability benefitsassociated with high thermal mass

Exceptional acousticperformance

Advantages:

Precast quality finish for walls and soffits

No formwork for vertical structure

Structural connection between wall and slabs is by standard reinforced concrete detail:inherently robust and, for basements, watertight

No permanent sealing at connections between precast units

Flexible for casting-in items

Disadvantages:

Propping of precast required prior to sufficient strength gain of in-situ concrete

The smaller dimension of the precast units can be a maximum of 2.8m, so joints in walls and soffits must be dealt with: expressed or concealed

Reduced flexibility of layout as there are walls rather than columns

Proponents of this system claim:

Twice as fast as in-situ concrete

Cost neutral compared to in-situ and cheaper than fully precast

Project example:

Albion Riverside, Battersea, London

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OPTIONS FOR HYBRIDCONCRETE CONSTRUCTIONThe ideal combination of precast and in-situ 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 exhaustive, but toreflect the spectrum of possibilities.

Precast twin wall and lattice girder soffit slab with in-situ infill and topping

Advantages:

Columns can be erected quickly

Precast quality finish for columns

Precast edge beam houses post tension anchorages, slab edge reinforcement and claddingfixings and avoids need for slab edge shuttering

Post tensioning minimises materials handling, steel fixing and striking times

More flexibility for late changes

Disadvantages:

In-situ slab requires falsework, formwork and curing time


Programme benefits compared with 100% in-situ

Cost savings due to speed, less edge formwork and better quality

Project example:

Chiswick Park office project, London

Artillery Mansions, London

Precast floor unit being craned into place at Ideas Store, Whitechapel.A hybrid concrete construction that used cast in-situ columns and beamswith precast floor units (Architect: Adjaye/Associates).Photo courtesy of Hanson Building Products

Precast columns and edge beams with cast in-situ floor slab

Precast columns and floor units with cast in-situ beams

Advantages:

Vertical structure can be erected quickly, no formwork required

Precast floor structure can be erected quickly, no formwork required

Precast quality finish for columns and soffits

Structural connection between precast elements is via standard reinforced concrete

Disadvantages:

Precast flooring must be temporarily propped

Sealing between precast units is required


Programme benefits compared with 100% in-situ concrete

Advantages of precast quality soffits and columns

Project example:

Office building, Homer Road, Solihull

Cast in-situ columns and beams with precast floor units

Advantages:

Precast floor structure can be erected quickly

Precast quality finish for soffits

In-situ can account for site irregularities

Disadvantages:

Precast flooring must be temporarily propped

Sealing between precast units is required


Programme benefits compared with 100% in-situ concrete

Project examples:

Ideas Store, Whitechapel Road, London

Whitefriars, Canterbury

Car Park, West Quay, Southampton

Cast in-situ columns and floor topping with precast beams and floor units

Advantages:

Precast flooring can be erected quickly

Precast beams support precast floor planks minimising floor propping

Precast quality finish for soffits

Formwork for in-situ columns can be used to prop precast beams

Structural connection between precast elements is via standard reinforced concrete

In-situ topping to beam permits beams to be continuous over columns

Disadvantages:

Downstand beams need to be co-ordinated with services distribution


Twice as fast as conventional in-situ

Project example:

Home Office, Marsham Street, London

Key: Precast In-situ

Hybrid concrete

construction offers

a wide range

of options to suit

specific projects.

A top down

HCC strategy for

a town centre

redevelopment.

CASE STUDY 2:WHITEFRIARS, CANTERBURYWhitefriars is a new 5-hectare retail development in the heart of Canterbury. Thescheme, which is due for completion in 2006, will be entirely pedestrianised andcomprise of 38 shops, a department store, two other major stores and a 520-spacemulti-storey car park.

Why hybrid concrete construction was chosen

An extensive basement access and delivery area, 4 to 5 m deep, runs below pedestrianlevel and extends over the whole site. Its completion was critical to the programme.Originally the basement and ground floor slab was designed as a fully cast in-situconcrete frame with two-way solid slabs and downstand beams. During the tenderperiod the contractor investigated alternative methods of construction which woulddeliver a faster and more buildable project at a price which would win the job. Theadvantages of a hybrid construction, using precast hollowcore floor slabs, quicklybecame apparent. The number of cast in-situ concrete beams was almost halved due to the greater spans achieved by the hollowcore floor slabs. Less formwork and propping was needed and the frame construction programme was reduced by a month. Because the floor slabs were lighter and precast, the amount of cast in-situconcrete was reduced; this in turn reduced the number of truck deliveries to the site,a sensitive issue in this historic city.

Construction

The use of HCC allowed the contractor to adopt a form of top down constructionwhich brought additional time savings. A series of secant piled retaining walls wasinstalled to create the basement level and then propped by the precast hollowcorefloor slabs which formed the ground floor pedestrian level. The walls and floor wereconnected with cast in-situ beams. Once complete, this allowed excavation andconstruction to take place in the basement while the superstructure was beinginstalled above.

What HCC brought to the project

The HCC solution helped to win the job by saving time and money. It made possible the key decision to construct the basement from top down a decisionwhich proved to be a turning point in winning the contract. The use of HCC reducedthe number of beams and columns required and saved 15 weeks on initial programme estimates.

In the multi-storey car park area the use of hollowcore floor slabs allowed bay sizes to be increased from 8 x 9.6 m to 16 x 9.6 m, a huge saving on column and beam construction.

Prefabricated staircase components reduced the need for scaffolding and gaveconstruction workers easy access to all parts of the building at an early stage.

Project team

Architect, Chapman Taylor Architects

Engineer, Upton McGougan Consulting Engineers

D & B Contractor, HBG Construction

D & B Engineer, HBG Construction

Frame Contractor, Whelan & Grant

Precast Floor, Bison Concrete Products

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An innovative

composite HCC

frame creates a

low energy

office building.

CASE STUDY 1:32 HOMER ROAD, SOLIHULLThe new three storey office building in Solihull, now occupied by National GridTransco, is an elegant, glazed pavilion standing in a landscaped corner site. A glass-walled atrium at the heart of the building acts as a ventilation chimney and its glazedroof brings daylight into inner office spaces. Services are contained in two cores atthe opposite ends of each floor.


The low-energy office building is a type with which the architect and engineer, FoggoAssociates, has long been associated. Previous office designs utilised the fabric energystorage potential of an exposed concrete structure and the new building uses HCC todevelop these principles.

Hybrid concrete was also used to create a structure which allows full continuity tooccur between the vertical and horizontal structural elements, thus providing a stiffsway framework. The combination of elements allowed the whole frame to act as acomposite structure without relying on expensive mechanical fixings. This method ofconstruction produces a rigid frame which is inherently stable without the need forshear walls or bracing. HCC was the natural choice of material. It fulfilled the designcriteria for a visible expression of the structure; behind the delicate glazed facades theprecast column and beam structure is clearly visible, needing no further treatmentsuch as cladding for fire protection. In addition, by exposing the painted soffits of theconcrete floor slabs in the offices, the ventilation strategy could exploit the fabricenergy storage potential of the concrete.

Construction

The hybrid concrete structure consists of 430 mm diameter precast columns andprecast floor units connected together by means of cast in-situ concrete spine beams.Each floor unit takes the form of a double T-section with end plates to each trough. Ateach column connection the end plates are cast with a curved cut-out to follow partof the column profile.

Once the precast columns were fixed on site, the double T-section floor units wereconnected to them, positioned so that the curved edge profiles trimmed the outeredge 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 unitstogether. 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 spineedge beam was cast between the final row of end plates (which ran up to the innerside of each perimeter column) on one side and a special precast perimeter unit onthe other side, which creates a tapered edge to the ceiling soffit. The perimeter unithas a row of precast holes which allow warm buoyant air rising up the faade to beeffectively captured and cooled by the passive chilled beam elements above theceiling 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 thecolumn heads. They were also used at the atrium and core perimeters, cast in thesame moulds with minor adaptations.


HCC was used to create a composite rigid frame structure which is inherently stablewithout the need for shear walls or bracing.

The precast column and beam structure is clearly visible; it was factory-finished andneeded no further treatment such as cladding for fire protection.

The soffits of the concrete floor slabs are exposed so that the temperature andventilation strategy can exploit the potential of the concrete for fabric energy storage.

Project team

Architect, Engineer and Cost Consultant, Foggo Associates

Construction Manager, Bovis Lend Lease

Precaster, SCC (Structural Concrete Contractors)

Car park loading

Shop unitloading

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32 Homer Road, Solihull

Exploded detail of the precast concreteelements at the atrium perimeterbeam and column junction.

Dotted detail indicates original scheme downstand beams.The revised scheme doubled floor spans by using precastunits (in-situ concrete topping not shown for clarity).

Typical section through in-situ concrete downstand beam,showing connection with precast concrete floor units.

Key: Precast In-situ

An HCC structure

adds value to

an innovative

community school.

CASE STUDY 4:BARKING SCHOOLThe Jo Richardson Community School in Barking provides state-of-the-art teachingspaces for its 1500 pupils together with performing arts spaces, resource areas, diningand sports halls. The building is primarily a secondary school but it is also a place forthe whole community, housing a nursery, community one-stop shop, public libraryand resource centre. The schools facilities can also be used by the public when theschool day is over.

The building occupies a large green-field site next to a busy dual carriageway. It isdivided into two by a central street, with teaching areas in four 2 and 3-storey blocksfacing west on to the main outside play area. The large spaces, main hall, dramarooms, library and sports facilities, are set along the street to the east.


The school was designed by architecture plb which is part of the Bouygues UK PFIconsortium. As a contractor, Bouygues has extensive experience of concreteconstruction on the continent, and it was the companys preferred material, offeringthe advantages of fast-track construction and the creation of a weatherproofenvelope for following trades at an early stage.

For a school the hybrid concrete construction had specific advantages: the cast in-situconcrete walls and precast floors are inherently robust and virtually vandal-proof andgive excellent sound-reduction between classrooms.

Construction

Over 90% of the building is a hybrid concrete structure. The load-bearing externalwalls and corridor walls and the partition walls between classrooms are all of cast in-situ concrete. The walls, mainly 180mm thick, were cast using storey-high steel-faced formwork which produced a high quality surface. This only neededa paint finish, reducing the need for other trades.

The main floors and roof are formed of precast hollowcore concrete planks whichspan between the cast in-situ walls. Corridor floors are of precast reinforced concreteplanks. All floors are finished with 50mm cast in-situ structural concrete topping.Services, including sprinkler supply lines, run in a suspended ceiling system. Theexternal walls are clad with a combination of metal panels and insulated acrylicrender. The roofs are covered with a single ply membrane.


The contractors decision to use a hybrid concrete structure was viewed verypositively by the client and users. The inherent robustness, fire integrity, acoustic andthermal properties of concrete were all seen as desirable in a new school. The use ofHCC was crucial to complete the construction on time and within budget.

The use of HCC for school buildings raised one important issue: does a classroom withsolid concrete walls have less flexibility than a conventional block-walled classroomto deal with future and potentially different educational needs? In the new schoolflexibility is unlikely to be a problem because the general teaching classrooms aredesigned to accommodate innovative teaching methods and are 20% larger thanstandard DfES classrooms. This means that the general teaching rooms are similar in size to the specialist spaces, which allows for future teaching flexibility.

Project team

Architect, architecture plb

Structural Engineer, Terrell International

M&E Engineer, Battle MacCarthy

PFI Contractor, Bouygues UK

8

Only HCC met the

time and budget

constraints for this

multi-storey

car park.

CASE STUDY 3:WEST QUAY CAR PARK,SOUTHAMPTONThe West Car Park, West Quay, Southampton, is one of the largest multi-storey carparks in the UK and the larger of two car parks which serve the West Quay shoppingcentre, Southampton, home to more than 150 shops, restaurants, bars and cafes.

The structure is 95 m long, 95 m wide and 20 m high eight storeys comprising 15 split levels with a 2 m clear headroom throughout. Access to the car park is bymeans of seven staircases and two double lifts.

Although it took only 7% of the cost, completion of the car park became absolutelycritical to the whole development, in order for it to open in time for Christmas.


At scheme design stage the design team considered various options for the structuralframe structural steel, precast concrete, in-situ concrete and post-tensionedconcrete before selecting a composite HCC structure based on precast concretedouble-tee floor slabs spanning 15.8 m on to cast in-situ concrete beam-and-columnframes. The decision to use HCC was based on a value engineering exercise. Bycombining the cost advantages of cast in-situ concrete with the speed of assembly of precast, the design-and-build contractor concluded that the structure could becompleted on time and within budget.

Construction

The precast concrete double-tee floor slabs span 15.8 m and are 2.4 m wide,matching the width of a standard car parking bay and fitting neatly into the 7.2 mgrid in the east-west direction. The cast in-situ concrete beams were cast with nibsprojecting at both sides and the ends of the slabs were cast with extended scarfs;they rest on the nibs and create a 300mm wide channel for service trunking. The eastwall of the car park takes the form of a sloping buttress clad with precast concretepanels with a reconstructed stone mix and knapped flint aggregate inserts. At upperlevels the car park is clad with precast spandrel panels of reconstructed stone. Thepanels were doweled to the cast in-situ concrete structure with cast-in sockets.


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 precastconcrete double-tee floor slabs were fully realised; they proved to be a positive wayto create large areas of floor very quickly.

Delivery and construction went smoothly. The relatively small number of types offloor slab gave flexibility in supply; deliveries could be delayed or brought forward.

The precast floor slabs had a high quality finish good enough to be used as a ceilingsoffit. Pre-finished precast elements minimise, and in some cases eliminate, on-sitewet trades.

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

Precast Cladding, The Marble Mosaic Company

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In-situ concrete walls and precast floor units produce a robustand low maintenance school for this PFI project.Photo courtesy of BouyguesEast elevation of West Quay car park

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