AJ 02.09.10 37

Technical & Practice

Visual concreteArchitects need to know exactly what they’relooking for in visual concrete and take control of its specification, says Felix Mara

The public’s response to exposed concrete can be so fickle that it’s easy for architects to forget it’s possible to achieve high-quality finishes by reinforcing artistic judgement with scientific principles. The result is referred to as ‘visual concrete’. The quality of recent visual concrete – exemplified in projects such as Foster + Partners’ 2003 Albion Riverside development in London – suggests a new strand in the material’s development, which retains brutalism’s conviction that concrete should be visible, but expands its repertoire to include less harsh finishes. Recent examples include exceptionally smooth finishes that are light in tone. This coincides with recent shifts in concrete’s economic viability

ArchitectArchitectArchitect

Above Flow diagram showing concrete specification sequence

AK

T

compared with that of steel and an awareness of the benefits of thermal mass and ways of controlling concrete’s environmental profile.

It’s sometimes said that the quality of concrete finishes in civil engineering projects is better than in buildings. According to Gerry O’Brien, a director of structural engineer consultancy Adams Kara Taylor (AKT), this is partly explained by the fact that higher-quality finishes improve concrete’s durability in structures such as bridges, where it is usually exposed. Exposed concrete isn’t the norm in architectural projects, which require special measures to control formwork, concrete materials, mix design and workmanship, which

should be the responsibility of the architect. ‘It makes sense for architects to take ownership’, says O’Brien.

Although other responsibilities are sometimes hot-potatoed between architects and structural engineers, O’Brien has a point here. He stresses that architects ‘should know what they’re after’ and commit to detailed specifications for visual concrete, which complement the engineer’s structural concrete specification. As shown on the flow chart above, it’s necessary to involve structural engineers, frame contractors, general contractors and, in particular, clients, who should be taken to see examples of visual concrete, especially if they’re apprehensive. >>

Architect Client

Yes Costing

Structural engineerSpecialist contractor

input

Decision to have visual concrete

Decide required locations and extent

Site visits to existing visual concrete buildings

Select benchmark projects

Define finish requirements inc. surface features

Select form face and layout

Select trial concrete mixes

Tender documents

Agree sample mixes with contractor and form-faces

Contractor produces small-scale samples

Construct full-size mock-up

Mixselection

Accept mock-up

Commence project works on site

Yes

No

Pre-tender

Post-tender

38 39

Technical & Practice

AJ 02.09.10AJ 02.09.10

Technical & Practice

Formwork typologies, release agents, panel sizes and rebates Form-face materials affect concrete finishes in several ways and their striking times must be consistent throughout the project to avoid colour variation. For floors, this is usually 24-36 hours after casting. Release agents, used with all form materials except controlled permeability formwork, influence colour uniformity of concrete surfaces if evenly applied. These are often darker where formwork is omitted. If the correct agent is specified, blow-hole retention

is minimised. Chemical release agents seem to be the most widely used.

Steel or timber soldiers and walings increase stiffness where formwork is assembled into larger panels and access sometimes determines panel size. Standardised panels, with minimal cutting, reduce waste. O’Brien says that ‘shuttering like cabinet work’ is sometimes necessary to achieve quality. Architects should produce drawings indicating board or panel arrangements if critical to concrete appearance. This enables contractors to produce formwork setting-out drawings, with

agreed approval procedures. Formwork panel sizes should correspond to those of concrete sections to be placed at given times to avoid leakage from inadequate temporary joints. This is the main advantage of large panels. Rebates, especially in columns and slabs, should be identified early as they may affect the structural design.

Formwork can now be fabricated in any shape, using modern technology such as five-axis routers. Recent developments in fabric formwork and double curvature steel have enormous potential.

Concrete materials and mix design In order to produce dense, well-compacted concrete, there must be enough cement paste with the necessary consistency to coat all aggregate particles, especially fine sand. The mortar formed by paste and sand must be cohesive as its richness largely determines concrete quality.

Cement colour depends on constituent materials and differences may be noticeable when cements from different works are juxtaposed, so it’s essential to specify that it comes from one source. White Portland cement (PC) contains materials that aren’t pigmented by firing and grinding.

Sulphate-resisting Portland cement

(SRPC) is a darker form of PC, specified for enhanced durability. The term ‘cement’ includes PC and its composites with ground granulated blast furnace slag (GGBS) or pulverised fuel ash (PFA), forming a powder or, for more flexibility, blended in concrete mixers.

The addition of GGBS to PC lightens the colour of concrete surfaces. Newly exposed surfaces are often much darker, with a blue-green tinge which gradually fades. PFA or microsilica additions to PC generally make concrete darker, but lighter coloured microsilica, used in white concrete reduces this effect. Microsilica can improve fine detail concrete

Form-face materials (in order of permeability)

Steel • Completely impermeable• Blow-holes likely• Dark discolouration possible • Blast cleaning needed before first use

usually provides uniform light colour• Several hundred uses possible

MDO (medium-duty overlay) Douglas fir plywood with resin-impregnated film• Matt• More uniform colour, fewer blow-holes • Risk of dark lines in concrete • Inspect overlay to avoid grain pattern • Generally, consistently good •10-20 uses possible

GRP• Impermeability, longevity and finish

similar to steel • Double-curvature possible • Ideal for waffle and trough floors

Unsealed plywood and boards• Darker finishes in more absorbent areas • Grain sometimes pronounced• More uniform colour after first few uses • Few blow-holes • 10-20 uses possible with care

HDO (heavy duty overlay) all-birch plywood with bonded phenolic resin film • Hardwearing• Shiny surface can cause dark

discolouration • Blow-holes possible• More uniform colour after first few uses• 50+ uses possible

CPF with microporous polypropylene sheet lining• Requires structural backing• One-directional curves• Blow-holes and excess water from

concrete eliminated• Some types single-use• Finely textured• Normally darker

Top and aboveDavid Chipperfield

Architects used birch-faced plywood formwork

with three coats of

marine varnish on the Turner GalleryLeft On AHMM’s

Angel Building,

Portland cement with 30 per cent

PFA was self-

compacted, removing the

need to vibrate to

eliminate blow holes

surface definition and increase strength and durability. Sand particles (>600 microns) are available in various colours, but aren’t visible in as-struck surfaces, only affecting their colour if a thin outer layer of cement is removed. But finer particles (say <150 microns) act as pigments, especially in white or light-coloured cement.

Once requirements are specified, a specialist frame contractor or ready-mixed concrete supplier should help with final mix selection, usually producing 600 × 600 × 50-75mm-thick samples. Preliminary selections must be tested by inspecting full-size samples and the specification should provide for mix adjustments. >>

Technical & Practice

40 AJ 02.09.10

WorkmanshipManagement, supervision, delivery and placing should be agreed with the contractor. The key to quality is a detailed contractor’s method statement, confirming quality-checking procedures and showing that these can be achieved by a full-scale mock-up of key architectural features, including arises, recesses, construction joints and shadow gaps, ideally, to be included in the tender. It’s essential to have a frame contractor who understands visual concrete and is committed to quality, so the main contractor’s tender must show relevant experience.

The contractor should produce a full-scale mock-up for review by the

architect, who should only issue an instruction to proceed with permanent concrete when satisfied with its quality. Site supervision is crucial, especially at the start, when operatives first tackle agreed construction methods. To avoid disputes over finish standards, requirements must be clearly communicated to tenderers by a detailed specification and suggested examples of visual concrete that can be used as benchmarks. It’s prudent to include a clause prohibiting making good to visual concrete. Gerry O’Brien, director of AKT, worked with the AJ on this article. AKT was structural engineer on all projects

References and sources

Plain Formed Concrete Finishes Concrete Society Technical Report 52, by William Monks Visual Concrete: Design and Productionby William Monks, The Concrete and Cement Association, second edition 1988 The Control of Blemishes in Concrete by William Monks, The Concrete and Cement Association, 1981 Fabric Formwork edited by Alan Chandler and Remo Pedreschi, RIBA Publications, 2007

The three most common specifications for building works used in Britain have different terminology and approaches. These are:1. BS 8110: Structural use of concrete 2. National Building Specification 3. National Concrete Frame Specification for Building Construction (NCFS)

Above Mock-up

for the Sainsbury Laboratory,

University of Cambridge, by

Stanton Williams. GGBS was added

to white pigmented

Portland cement to lighten, reduce

shrinkage, add

durability and increase curing time