The finished stairway.
The finished stairway.
Pouring form made with AOC Altek resin.
Pouring form made with AOC Altek resin.

MFG-CP, Independence, Kansas, USA, has more than four decades of experience in the manufacture of FRP forms for concrete. This project required three forms, one for each floor. At the job site, concrete was poured into the forms, which remained in place until the concrete cured.

A design feature was engineered into the form so that the top riser could accommodate an overlaid marble cover that matched the surface of the floor at the top. In addition, MFG-CP engineered a solution on site that allowed one form to be modified so it could form the staircase around a large column.

The architectural firm was GS&C Group, and structural engineering was handled by Structure + Haynes Whaley. Both are headquartered in Austin, Texas.

How the FRP forms were made

Before the composite forms could be made, MFG-CP used engineering drawings to create replicas of finished stairway sections out of wood and masonite. The replicas served as precise dimensional models for building the FRP forms that would give the poured concrete its final shape.

To keep the FRP forms from adhering when removed from the model, the surface of the model is coated with a special wax. Workers then apply the first layer of the form itself – a sandable gel-coat that will create the finished surface of the concrete. Next comes the critical structural laminate that keeps the form rigid for shipping, on-site handling, and the pour and cure cycles. The laminate is formed by lay-up of resin-impregnated glass fibre mat and woven roving.

To achieve the required rigidity, select areas of the laminate are internally cored with balsa wood and externally braced with hardwood pieces. The coring and braces are encapsulated into the laminate to result in a monolithic unit when the resin reactively cures into a crosslinked solid.

MFG-CP used Altek® H864, a low-shrink unsaturated polyester from AOC, to mould the forms.

“This is a very good all around resin that wets the glass fibres well and rolls out easily,” reports MFG-CP Sales Manager Jim Williams. “And by showing very little entrapment of air, Altek H864 also ensures a more solid laminate construction.”