Feature

Back to Black and White

Before committing to an aesthetic of black or white concrete, it is essential to ensure that the desired outcome is achievable using the chosen method of manufacture. White concrete is created using white cement but since sand and even coarse aggregate contain some fine material that can migrate to the surface of the concrete, to create pure white concrete it is usually necessary to use white fine and coarse aggregate as well, in order to avoid any hint of colour. White aggregate is essential where it is to be exposed on the surface. Spanish dolomite is often used to create a good white concrete surface, particularly with an acid-etched finish. It should be noted that as aggregates are a natural material, polishing the surface can occasionally reveal off-white stone.

Two St Peter’s Square, Manchester by SimpsonHaugh (2017)

Two St Peter’s Square is a 12-storey office development in the heart of Manchester’s revitalised civic quarter. The polished white precast-concrete facade establishes an elegant presence in a historic location among listed buildings including Manchester Town Hall.

Overlooking St Peter’s Square, the facade forms a sculpted grid of precast concrete with deep vertical reveals that frame and shade full-height glazing. SimpsonHaugh says the aim was to reflect the weight of the Town Hall directly opposite, and reinterpret the Town Hall extension’s monolithic facade and deliberately repetitive rhythm. The reveals are angled in three directions across the elevation, directing views out to the historic buildings around. The facades have been created using Techrete C195, a Portland Stone mix with neutral aggregates, which has been polished around the window reveals at the gable ends, ornate precast-concrete tracery screens offer both decoration and shading.

On the St Peter’s Square elevation, three motifs reinterpret stone details found in the Town Hall and extension, based on local references such as the Lancashire Rose. Overlooking Princess Street, just one motif is repeated: the cotton bud, a symbol of Manchester’s industrial heritage and a reference to the nearby cotton warehouse buildings. For the tracery panels, an acid-etched finish with fewer exposed aggregates was specified to give a more understated effect. Mica contained in the mix brings a slight sparkle, while hand finishing has ensured a consistently smooth surface.

In the UK, white cement and aggregates are not stocked as standard in ready-mixed concrete plants, so white concrete is generally an architectural precast product, produced in the factory using specifically sourced white materials. Ready-mixed concrete can be made “whiter” using a cement containing ground granulated blast-furnace slag (GGBS), a near-white cementitious material that lowers the carbon footprint of concrete and improves its durability. BS 8500, the British standard for concrete specification, allows a maximum of 70% cement replacement with GGBS, but up to 50% is more common for practical reasons. Greater quantities are unlikely to achieve a noticeably lighter surface due to the fact that GGBS delays strength gain, and longer periods in formwork tend to have a darkening effect on concrete.

WHITE CONCRETE IS GENERALLY AN ARCHITECTURAL PRECAST PRODUCT, PRODUCED IN THE FACTORY USING SPECIFICALLY SOURCED WHITE MATERIALS

Small quantities of white pigment can also be added for extra whiteness. Titanium dioxide, for example, is widely used in products such as ceramics, paints and toothpaste. According to BS EN 12878, the grade most appropriate for colouring concrete is anatase. For ready-mixed concrete, carefully measured doses are usually added directly to the concrete mixer or agitator, ideally in liquid or granulated form to aid colour distribution. But just as with GGBS, the concrete is only made lighter, not white, due to the presence of non-white aggregates and grey cement.

Achieving black concrete comes with a different set of considerations. Black cement does not exist, and since cement is essential for the creation of concrete it will always have some degree of greyness. Concrete can be made darker using fly ash, a dark grey by-product of coal-fired power stations. BS 8500 allows a maximum 55% replacement with fly ash, but this is typically limited to around 30% or less. Adding the black inorganic pigment iron oxide is an effective way to create very dark grey concrete, the tone of which can be further darkened using sealant. Architectural precast manufacturers often use pigment in combination with white cement to create greater consistency and repetition. A practical maximum dosage is 5%. Designers may be tempted by pigments that are blacker than iron oxide, such as carbon black, but this is not advised as it is more prone to fading.

A more common way to achieve long-lasting near-black concrete is to specify architectural precast concrete with naturally black fine and coarse aggregate, such as black basalt, exposed on the surface – for example in the highly polished facade panels of the Dyson Building at the Royal College of Art in London by Haworth Tompkins. At Lanterna in east London (overleaf), the black aggregate was exposed using a surface retardant, applied to the inside of the formwork. After striking, the grey surface laitance was washed away to reveal the black stones. A post-finishing process is recommended when trying to achieve a dark colour, not only to remove the lighter grey cement on the surface, and therefore expose the dark aggregates, but also to reduce any impact of efflorescence. Using a permeable formliner can also deepen the tone slightly, as water from the concrete is permitted to freely drain from its entire formed face.

The darkening effect of water loss, normally evident at formwork junctions, is therefore evident across the whole surface. A combination of all three techniques – fly ash, pigment and permeable formliners – was used to create the near-black in-situ concrete for the curved facades of V&A Dundee (CQ 265, Autumn 2018). Creating solid colours in black or white polished concrete floors requires an altogether different technique. Pigmented surface hardeners, available in a wide range of RAL colours including various shades of near-black and white, are trowelled into the surface of the fresh concrete floor. This creates a durable 2-3mm depth of solid colour, which can again be enhanced with the right choice of sealant.

Finally, it is possible to create concrete that is both black and white, by exposing black aggregate on the surface of otherwise white concrete. To achieve a two-tone effect at distance, however, requires a sufficient size of each area of colour, exposing the aggregates in some areas and not others. This can be done using stencils or by masking the surface prior to treatment, or by lining the formwork with paper on which the pattern has been printed in retardant to control the location of colour, precisely and in advance. This process was used to create concrete at the Danish National Archives in Viborg.

Lanterna, Fish Island Village, east London by Lyndon Goode (2018)

Lanterna is a striking black-concrete building in a mixed-use canal-side development on Fish Island in Hackney Wick. “We specified textured black concrete to bring a strong sense of drama to the building, which acts as a backdrop to a new public square, says architect Simon Goode. “The colour also references the industrial heritage of this area of east London – a centre of tar processing in the 19th century.”

The concrete’s through-colour was achieved using fine and course black basalt aggregate and a black powder dye in the mix. “Inspired by local street art, we wanted to imprint the facade with herringbone grooves wrapping in and out of the reveals and around all elevations. We designed the facade as a series of standardised components, just small enough to be made offsite at Cornish Concrete Products’ workshop in Truro and craned into position on site.”

The biggest technical challenge was ensuring the intricate herringbone pattern was aligned across panels, particularly in light of tolerances in concrete construction. To achieve this, the entire cladding structure, including every groove, was modelled in 3D software. Cornish Concrete referred to this 3D model to hand-build timber moulds for each component.

“We wanted a rough, textured finish to the faces of the precast panels, with a contrasting smooth finish inside the grooves,” says Goode. To achieve this, Cornish hand-painted retardant onto the flat planes of the mould, missing out the triangular extrusions that form the grooves.

Photos: Alessandro Crinari, Daniel Hopkinson, Rory Gardiner