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Concrete Casting News from the Hill and Griffith Company

3D-Printed Tooling Offers Durability for Precast Concrete

Posted by Hill and Griffith Company on Oct 8, 2020 4:24:36 PM

As an alternative to wooden tooling, 3D-printed forms for precast concrete are proving to be more durable and better able to support a large-scale renovation project.

Article excerpt from the May 2018 issue of Additive Manufacturing by Stephanie Hendrixson

There are certain applications today where 3D printing makes sense. An injection molder might choose to print a small batch of plastic parts that would be cost-prohibitive to mold. A machine shop might invest in a 3D printer to make jigs to aid in inspecting short runs of parts. A service bureau might rely on 3D printing for product development work, as a way to make prototypes quickly and easily.

What these scenarios have in common is that 3D printing makes it easier to produce a small, custom quantity: A few dozen parts, a couple of temporary fixtures, one prototype. There are exceptions, but manufacturers today don’t necessarily see 3D printing as the solution for repeatable, high-volume jobs.

Precast Concrete Made with 3D-Printed Forms

One of those exceptions is proving to be the precast concrete industry. Gate Precast, a supplier of precast structural and architectural concrete, is finding that 3D-printed tooling is exactly the right solution for a job requiring high repeatability over many concrete pours: manufacturing hundreds of punched windows for the façade of a 42-story building in New York City. For this large-scale project, 3D-printed forms have proved their worth in terms of faster lead times, increased durability and better quality in the end product.

Precast Concrete 3D Printed Form

Enter 3D Printing

Gate Precast manufactures precast concrete in nine locations nationwide. Some of these facilities focus on structural concrete—weight-bearing items like the beams and columns that make up parking garages—which is produced with metal forms, often in very large quantities. Others, like its Winchester, Kentucky, plant, specialize in architectural pieces that are typically made in smaller batches.

For most jobs, the Winchester plant builds its own concrete forms from plywood and fiberglass through an in-house carpentry department. These forms are not highly durable, but they don’t need to be. A wood form will start to break down, typically after 15 to 20 castings. But for a typical job where only 5 to 10 castings might be needed, this is no problem. It’s larger jobs that are the challenge when multiple forms must be built to support many concrete pours—which is where 3D printing comes in.

Pouring Concrete with a 3D-Printed Form

In practice, pouring concrete with a 3D-printed form is not much different from pouring with a wooden one. The windows are typically cast three together on top of a large platform in Gate Precast’s Winchester facility. The 3D-printed forms actually represent the front and inner sides of the window cavities, with the outer edges formed by removable plywood walls. Both the forms and sides are treated with a form release oil to help with the unmolding process.

Although they are not structural features, the punched windows still require reinforcement in the form of rebar, which helps to support the windows during stripping and shipping. The rebar is set into the empty form before the pour, along with lifting inserts and tiebacks that will serve to enable shipping, lifting and eventually hold the windows onto the building.

An overhead crane carries buckets of concrete to the platform; it takes about three buckets of concrete to fill one of the three-window forms, Schweitzer says, with each concrete cast weighing 20,000 to 30,000 lbs. depending on the profile. The pour stops periodically to allow the platform to vibrate and consolidate the concrete around the rebar, removing any air pockets from the face. Once the form is filled, workers manually smooth the top surface of the concrete (which will be the back of the windows) and clear the areas around the lifting and tieback inserts.

Inspecting Precast Concrete

Concrete pours take place each day in the afternoon, and then cure for 12 to 14 hours. The plywood walls are stripped from the cast first, and then the concrete pieces are lifted from the forms around 3 a.m. each morning. Each window frame is manually cleaned with an acid wash spray that exposes the sand in the mix. Then, some of the front faces are polished to expose the sand and aggregate. The resulting finish sparkles—quite intentionally—like sugar cubes.

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