Feature

Facing Facts

Elaine Toogood explores the design and manufacturing process of brick-faced concrete cladding

You can be forgiven for not always recognising when a building is clad in concrete, especially when the panel incorporates a facing of brick. Brick-faced concrete panels can be a very effective means of combining the aesthetic of brick with the benefits of offsite manufacture. They are also a cost-effective way of achieving intricate brick detailing, which is otherwise challenging without the appropriate skills. As with any finishing technique for architectural precast concrete, it is worth getting a grasp of the manufacturing techniques at the outset to optimise the design.
 

The basics

Brick facing can be used for most bespoke precast-concrete applications, from arches and columns to wall panels, balconies, soffits and retaining walls. They can be used in combination with other precast-concrete finishes, within the same or adjacent units, and smaller elements can be used in combination with traditional brickwork, such as brick-faced lintels or underslung soffits.

The same design principles can be adopted as for architectural precast, with similar overall sizes, fixings and installation. The exact sizes, however, should correlate with the brick dimensions – just as with ordinary brickwork, small offcuts should be avoided. Detailing of reinforcement, thickness and specification of concrete will also typically be designed by the precast supplier to suit the structural requirements and other performance specification given by the design team.

As a guide, facade panels are typically around 200mm thick, incorporating 50mm facing brick (half-brick) and 150mm concrete. As with all precast cladding, insulation, windows and other facade features can be factory-fitted if required.
 

Brick selection

The selection of brick type is a key decision, not just for aesthetic impact but for manufacture and overall cost. Purpose-made precast brick slips or virtually any cut traditional brick material can be used, but not all brick types are appropriate or as easy to bond into concrete. A positive key at the back of the brick anchors it to the concrete.

For brick slips, this profile of well-formed grooves is created when manufactured. For cut bricks the original perforations can provide the anchor once cut, otherwise a dovetailed slot can be cut in to the back. Pull-off tests will demonstrate if the key is sufficient.

Colour uniformity must be considered, and pallets should be mixed. It is common practice to mix pallets of bricks to avoid obvious variations on a facade, and this needs to be stated in the specification. This is particularly important for large expanses of brickwork, or where adjacent panels are manufactured at different times, increasing the risk of colour changes between panels.

At window reveals, corners, corbelling and other three-dimensional details, the bricks will require multiple finished faces and specials may be required. Significant cost and resource efficiencies can be made by using bricks with four usable faces, reducing the overall number of bricks required, with minimal wastage. Such bricks are often perceived as expensive, and while that may be true when set by hand, in precast cladding panels they are far more economically viable.

Ideally the brick manufacturer should be able to supply all of the bricks pre-cut, including any specials, within a specific delivery period to avoid delays in panel manufacture. Bricks that vary significantly in size are difficult to place and tighter manufacturing tolerances are required when casting in bricks compared to normal bricklaying.

Garden Halls, London

TP Bennett’s £140m Garden Halls student residence for the University of London in Bloomsbury shows how brick-faced concrete panels can be used to create a highly detailed, crafted facade.

The challenge that confronted the facade architect, Macreanor Lavington, was to develop a design that could grace a massive, contemporary nine-storey block of student rooms – a typology that is repetitive by nature
– while also responding sensitively to its setting in a historic garden square opposite a crescent of listed four storey Georgian townhouses.

Macreanor Lavington began to develop a highly articulated facade dominated by a sequence of stepped window reveals. The rationale behind this motif was that by revealing the depth of the front elevation it would give the building an imposing stature while animating the fa├žade with light and shade. Buff-coloured Petersen bricks were chosen for their association with domestic Georgian architecture, thereby creating a relationship with
other buildings in the square.

It quickly became clear that the stepped soffits would be extremely difficult to make on-site, requiring a high level of craftsmanship and taking a great deal of time. Off-site manufacture, on the other hand, offered a degree of control over the complex brick detailing. The use of Petersen bricks offered a cost advantage here too: unlike most bricks, they have a good face on both sides, so both halves could be used to face the concrete panels.

The different precast elements of the facade were made by Thorp Precast in Staffordshire before being transported to the site. The building can be read as three distinct horizontal sections, and this is reflected in the process of precast manufacture.

The ground and first floor form a base to the building, with each bay framed by a massive brick-faced precast-concrete pier. These piers, which are recessed every fifth course, were the first element to be craned into position; the brick-faced spandrels and concrete sills were then bolted on to either side.

Above this base level is the three-storey central portion of the building, which introduces the deep window reveals. These were manufactured as full-height T-sections, comprising brick-faced precast- concrete piers topped by the stepped soffit. Again, these were craned into position (see below), with the spandrel panels and architectural precast-concrete sills slotted in afterwards. Finally, the upper section of the building is formed of two-storey T-sections, topped by a mansard roof. The roof was also factory-built, with glazed terracotta tiles cast onto the face of concrete panels.

Because of its depth, the cladding system can bear all of its weight back to the ground, with the building’s reinforced-concrete frame only taking the horizontal loads from the facade. The piers and T-sections were simply stacked on top of each other and restrained back to the building’s reinforced-concrete frame at the top of each section. This also meant that the designers could keep movement joints to a minimum, reinforcing the impression that this thoroughly modern facade is in fact a traditional monolithic brick structure.

Manufacturing process

Bricks are placed facing-side down in the concrete mould, arranged in the required bonding or pattern. Proprietary templates are commonly used to hold the bricks in position, spaced apart by 10mm for the mortar joints. Non-standard patterns or brick sizes will require proprietary spacing templates.

Three-dimensional elements may need an additional means of supporting the bricks until the concrete has cured. For complex shapes, this may require some ingenuity and it is worth discussing such details and their implications with a specialist supplier at the early stages of design.

Once the bricks are in place, the joints may need additional fill to prevent the concrete from seeping through to the face of the brickwork. This is traditionally done with sand. The reinforcement and any lifting eyes and fixing supports are then put into position and the concrete placed into the mould, vibrated appropriately as it goes. When self-compacting concrete is used, vibration is not required. The exposed top of the concrete is usually a hidden surface so is simply levelled.

Once the concrete has reached its required strength, a day or so later, the formwork is struck and the panels turned over. The brickwork is then cleaned and, where required, mortar joints pointed before preparation for site delivery. Lifting eyes are typically located on hidden facings but one advantage of brick-faced concrete panels is that they can be placed on the surface if required to help with installation. The missing bricks are then fixed over the lifting positions once the panel is in place.

Mortar joints

Mortar joints may be pointed in the factory or on site depending on the project requirements and service offered by the precast manufacturer. Pointing on site can take place when the panels are in place, or on the ground before fitting, with obvious differing implications for installation and the need for scaffolding. The choice of mortar joint profiles will be determined by the same factors as traditional brickwork.

Brick slips offer the time-efficient option of pre-filling the mortar joints before the concrete is poured. The mortar is simply placed from behind, effectively cast in place rather than pointed. The profile of the mortar joint is created by the inverse profile of the brick spacers. Joint profiles available using this process create either a recessed or bucket-handle finish. Flush-filled and weatherstruck mortar joints are not recommended using this technique.
 

Joints between panels

As with any visible precast component, the location and treatment of the joints is a key design decision and should be considered very early in the facade design, especially if there is the desire to minimise their visual appearance. Joint widths are typically larger than the 10mm mortar joints but may be comparable with the movement joints required of contemporary brickwork. They are determined by the tolerances necessary for manufacture, anticipated natural shrinkage and thermal expansion and the tolerance needed for installation. Panels should be designed to prevent them exceeding 20mm.

Early consultation with the precast supplier can help to reduce the size of joints. For example, tighter  manufacturing tolerances are possible with smaller sizes of elements (see BS 8297:2017, Table 6) and installation tolerances are typically reduced where wall panels are self-stacking.

Joints are filled with sealant after installation and, to better coordinate and approve standards of quality, it is good practice to incorporate this into the precast manufacturer and installer’s package of works. The sealant should be carefully specified to ensure the appropriate thermal expansion and to avoid colour-staining. A “dusted seal” is worth considering. This is where the joint is rubbed with mortar dust when newly applied, to take the shine off of the sealant and provide better visual consistency with the mortar joints.


Code of practice

Brick-faced precast concrete is included in BS 8297:2017 Design, manufacture and installation of architectural precast concrete — Code of practice, a new version of which was published at the end of 2017. This edition includes more detail on samples, testing and inspection procedures for brick-faced cladding than the previous version, and incorporates current methodology and practice.

There are no other significant changes that would affect architectural design decisions related to brick-faced panels. The standard states that typically “the first in line production panel forms the basis of an agreed quality sample”, so designers should be clear if other arrangements are required. Viewing distances are stated as 5m for areas intended to be seen at close range and 10m for all other faces.