Making a Concrete Mold Using Fiberglass

Turn on Closed Captions (CC) for detailed step by step info.

We were hired to make "the latest in concrete road obstructions" for a new development here in St. Louis. These bollards will prevent traffic from colliding with pedestrians in a new area made only for people and not cars.

In the video I begin by making an MDF master, a full sized model around which we will make the mold. After surface prep and paint, a metal divider is temporarily installed in order create a flange and establish a parting line on the fiberglass mold. Partall, a mold release is sprayed to create a film barrier between the master and the gelcoat. The white gelcoat then is applied, forming what will be the hard, slick, interior of the mold. Many coats of fiberglass and resin are applied to reinforce the mold.

When the first side is done, the metal divider is removed and more partall is applied and the process repeats for the second side. When all the structural elements like rebar and plywood are installed, it's time to trim the loose fibers and pop the mold from the master.

Pouring 3500 lbs of concrete into a mold was sketchy the first time but the sand-filled bin worked to stabilize the mold as it increased in weight. Vibrating periodically helps to loosen bubbles that may be stuck in the concrete but too much vibrating can weaken concrete by causing separation between the cement and aggregate.