A case study in reinforced concrete
Precast concrete structures are manufactured in a factory then delivered to a job site, ready to be installed. But, have you ever wondered how precast concrete products are made? In this video, you’ll get a behind-the-scenes look at one of our Oldcastle Infrastructure, a CRH Company, plants and gain first-hand experience on the manufacturing process.
Precast concrete structures are used in many types of construction and for many different purposes, including electrical and communication utilities, stormwater storage and conveyance, wastewater applications, bridges, building structures, and more.
There’s good reason for using precast concrete – it provides many benefits on a project, including quick and easy installation since there are no on-site forms to construct or waiting for the concrete to cure in the field, improving job site safety by decreasing the amount of time an excavation is open, and providing a high-quality, higher-strength product since it is produced in a controlled environment.
Engineering & Design
The process starts with engineering. On each project, a design engineer or owner (such as a Department of Transportation, DOT) sets requirements for their precast components. When we get the drawings and requirements, every product is engineered in-house according to the design engineer and owner’s specifications.
The engineers ensure each precast structure has the appropriate steel reinforcement (rebar) and meets the structural requirements for the area in which it will be installed. Some important considerations include the soil type, whether the precast structure will be adjacent to a building or other structure, and the water table of the area.
Once the calculations are complete, the drafting team creates detailed drawings. These drawings, called submittals, are then sent to the design engineer or owner for approval. When the submittal drawings are approved, the engineering and drafting teams create production drawings sent to the factory floor and used to manufacture the product. The production drawing set includes a bill of materials, or BOM, which includes all the components that go into the product, including sizes and lengths of each piece of rebar and how much concrete (measured in cubic yards) will be used.
Prepping the Rebar Cage
When the production team receives the drawings, the first step is to assemble the rebar cage. To do so, they must cut all rebar to the appropriate lengths according to the BOM and then bend and tie them together. Rebar wheelchairs, sometimes called wagon wheels, are round plastic components that hook onto the rebar and ensure it is properly positioned inside the walls of the precast product – not too close to either side of the wall – matching the engineer’s design and meeting the structural requirements.
Prepping the Form
While the rebar cage is made, another team preps the forms. This team reviews the drawings to see if the structure has any openings or knockouts and places foam inserts (removed after the concrete cures) into the form.
Openings are used where pipes connect or where other junctions are needed. Knockouts are thinner wall sections which allow openings to be “knocked out” in the field once the subcontractor knows where electrical conduit or communication lines would enter the vault. These inserts, along with the proper lifting hardware, are embedded and secured to the form so they don’t move when the concrete is being poured.
Next, the team applies a form oil used to ensure the concrete releases easily from the form after curing. Finally, the rebar cage is lifted using a crane and is lowered down into the form. Before concrete can be poured, each product undergoes a pre-pour inspection by a certified quality control technician to ensure it conforms to the production drawings. Once approved, the technician signs off and flags the form, indicating approval to pour the concrete.
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