Polymer vs. Portland

Polymer vs. Portland

We’ve heard that architects sometimes confuse GFRP (glass fiber reinforced polymer) with GRFC (glass fiber reinforced concrete).  GFRC is familiar, and GFRP isn’t really on their radar, certainly not by that acronym, so the confusion is understandable: they see the acronym and assume concrete… and proceed designing based on that assumption. Considering the enormous weight difference between the two materials, and the different performance properties, it’s also easy to understand how this confusion can wreak havoc, until it’s caught.

(GFRP is sometimes just called FRP, but that could also mean a carbon-fiber, or Kevlar, etc. GFRP is the more precise term.)

This got us thinking about the broader phenomenon of polymers being used where portland cement used to be used. It seemed like it might be a good idea to talk about the two materials, their similarities, and differences.

Concrete – or more properly “portland cement concrete” to distinguish it from asphalt concrete and other concretions – is a composite of fine aggregate (usually sand), coarse aggregate (usually crushed rock), and a binder made by mixing portland cement powder with water. The cement locks together the rock and sand, which bear on each other and produce high compressive strength.

Concrete is used as a structural building material primarily because of that high compressive strength, its resistance to crushing. But with that compressive strength comes great density, great weight: concrete does its compressive-strength job well because of heavy materials.

Concrete has relatively low tensile or flexural strength. Thin sections of it are brittle, and subject to breaking easily. Concrete is often reinforced with steel or with glass fiber to increase tensile strength.

Concrete is also used as a decorative building material, largely because of its ability to reproduce a shape and a surface texture accurately (its plastic quality), and its durability and imperviousness to the elements. In this application, concrete’s compressive strength is not nearly as important, and its great weight is often its curse. Concrete weighs approximately 140-145 lbs./cu. ft. That’s about 12 lbs. per square foot of 1-inch thick concrete (but it’s hardly ever that thin), 24 lbs. for 2-inch thick, 36 pounds for 3-inch think…

In GFRC, glass fiber is mixed into the concrete – along with a healthy dose of polymer – to increase tensile strength, and that makes possible thinner sections. But GFRC is still concrete, and it’s still heavy.

Glass Fiber Reinforced Polymer (GFRP) is also used in decorative applications because of its ability to reproduce a shape and a surface texture accurately (its plastic quality), and its durability and imperviousness to the elements.

GFRC is all about tensile strength. It is composed of glass fiber – which has high tensile strength – and a binder made of a polymer – yes, plastic. While it has moderate compressive strength, in decorative applications, it is usually designed in ways that take advantage of its tensile strength to achieve very thin sections that are very light-weight.

Considering the two materials without prejudice, GFRP would seem better suited to the job of providing a durable decorative surface that is not a load-bearing structure.

So why haven’t architects jumped all over GFRP? Why is its acceptance, while continually increasing, still very slow?

Is there, possibly, a deep-seated societal prejudice, that anything plastic cannot be strong? The adjective ‘plasticky,’ after all, is a common way to convey that something is either inauthentic or flimsy. Are architects haunted by a fear that polymer is flimsy?

Consider the tale of the Monsanto House of the Future at Disneyland. It was originally designed in 1954 by visionary engineer Frank Heger and a fellow MIT professor, Albert Deitz. It opened at Disneyland in 1957, a solar-heated house made entirely of GFRP. A decade later, when Disneyland wanted to scrap the house, it is said that the wrecking ball bounced off it, and it eventually had to be cut apart by hand.

But GFRP remains off the radar of many architects. Is it difficult to believe in the performance of polymer as a binder in place of portland cement? Consider the case of terrazzo: It was originally composed of marble chips in a portland cement matrix, but most terrazzo changed to an epoxy matrix several decades ago. Few people were (or are) aware they’re walking on plastic, so they still think of terrazzo as a very strong, hard surface, basically as hard as stone.

(BTW, a terrazzo-like surface can even be made with crushed glass, instead of marble chips, bound by an epoxy matrix. This product, Glamento is about 95% glass, and can be polished similarly to terrazzo. It’s essentially the same raw materials as GFRP, yet a very different composite.)

One of the principles of this blog is the proposition that there are some applications where GFRP is better-suited to the job than traditional materials. But GFRP, apparently, needs to get better known, and perhaps overcome a few prejudices. It is our hope that the projects and applications presented here help make the case for considering alternative architectural materials.