The Mac arts centre by Hackett Hall McKnight

Hackett Hall McKnight’s building, composed of three monolithic elements, two clad in brick, the other in stone, is the £12.9 million new home for the Old Museum Arts Centre, now called the Mac.

Two seven-storey brick-clad “boxes” contain two auditoriums and 1,000sq m of gallery and dance studios. A third tower structure, also seven storeys but clad in stone, addresses a new city square, and contains a foyer and offices.

The site is in the oldest unredeveloped part of Belfast, still largely derelict but now being regenerated. The arts centre is surrounded by a new commercial development and square, St Anne’s Cathedral and the University of Ulster.

The project deploys an architectural language derived from the city’s traditional brick warehouse and mill buildings. Hackett Hall McKnight’s design, won in an RIBA international competition in 2007, reflects the methods of construction and simplicity of details used in the historic industrial buildings. “In-situ concrete was fundamental to the way we thought of building from the start,” says practice partner Ian McKnight.

“The plastic quality of concrete lent itself to creating the complex stacked arrangement of large spaces that would have been very difficult to achieve using another structural approach.”

The project is largely defined by the two boxes, with the foyer and bars occupying the space between. Although both are clad in brick, the expression of each is distinct through the use of different details. One is a regular cuboid while the other is less
regular, with large, individual openings.

The foyer is a top-lit space defined by internal elevations of brick and concrete, including a five-storey high wall in board-marked in-situ concrete.

“The approach to the pattern of the foyer wall has developed like a process of drawing on the concrete,” says McKnight. “The structural material has been developed into an element of surface relief, texture and character within the foyer.”

Concrete boxes

As the site is highly constrained, the internal arrangements of the two concrete boxes are by necessity volumetrically complex. The main gallery is placed above the larger auditorium with the rehearsal studios above this again – a structurally and acoustically demanding arrangement best achieved using structural in-situ concrete.

Apart from the stone-clad tower, internal and external wall finishes are mostly brick and concrete, ceilings are fair-faced (smooth) concrete and floors are terrazzo.

Pre-cast concrete is used for string-courses to the elevations of the larger block, resonating with the stone string-courses of Belfast’s traditional brick buildings. These, combined with load-bearing pre-cast lintels, create depth and modelling that also defines a series of independent brick panels.

Where the concrete is not exposed as a finish, GGBS (ground granulated blast furnace slag) is used to replace a significant proportion of the cement. The mix for the fair-faced concrete was developed through the production of samples using various aggregate and sand mixes, aggregate type and water/cement ratio. These samples were then cured over different time scales and a preferred mix that uses a 10mm aggregate was established.

The installation was supervised by the experienced contractor team and pre-pour and post-pour inspections formed part of the process for the sample panels and the eventual construction protocol for the final pours. The fair-faced in-situ concrete is poured with the establishment of a number of basic joinery rules for the form-work layout and construction.

Board-marked foyer wall

The brick cladding to one of the concrete boxes has been peeled away to reveal a five-storey- high board-marked concrete wall to the foyer.

The tender documents defined an extensive run of sample wall that would be used as a control sample to agree the particular details and quality standards for the board-marked wall. This involved face-fixing timbers to the surface of a standard concrete form and checking the degree of chamfering required to the end of the applied boards to allow the removal of the form work without damaging the arrises.

Samples were carried out using redwood and Douglas fir, and a number of treatments were tested, such as plain sawn timber and various degrees of sand-blasted boards.

A review of the sample wall led to the decision to heavily sandblast the Douglas fir. Options were also considered for the formation of the day joint at the top and bottom of each pour, and a rebate was introduced to control the joint.

Construction of the wall progressed using coated formwork plywood and steel formwork supports with standard bolt-through arrangement. The timber boards were applied to the ply forms with nails, using a 6mm chamfer at the end of each board to allow clean striking of the form. The timber boards were used a maximum of four times, and the variation in the pattern was created using different arrangements of the same panels.

Rules were established during the sampling process, such as that the bolt-holes occur only in the smooth panels between the board-marked sections and definition of the maximum and minimum width of board-marked recesses and smooth sections of concrete.