Gaziantep Şahinbey Millet Mosque — Fibreglass Formwork Manufacture

The muqarnas, capitals and border motifs of classical architecture are today mostly stuck on afterwards in plaster or GRC panel. That solution looks well for the first year; but joints, thermal movement and damp bring the seams out within a few years.

At Şahinbey Millet Mosque we moved the motif surfaces into the formwork. The negatives were worked into fibreglass shells, the concrete was poured, and when the formwork was struck the motif was the concrete. There is no joint and no layer that can separate.

How a mould negative is made

A positive model of the motif is produced first, then a fibreglass negative is taken from that model. The sequence runs like this:

• Model — the motif is worked at full scale in timber or foam.
• Negative — gel coat and layers of glass fibre are laid over the model to take off the shell.
• Frame — the shell is braced with steel section against the pressure of fresh concrete.
• Trial pour — the first piece is cast in the workshop; draft angle and surface are checked.

Draft angle in architectural concrete

The commonest mistake in a motif mould is working the negative too steeply for it to release. A mould that cannot be struck damages both itself and the concrete. Every negative is given the smallest draft that does not compromise how the motif reads — a decision belonging to manufacture, not to design.