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 jobsite 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 which are 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 being 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 (which are 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 which is used to ensure the concrete releases easily from the form after it’s cured. 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.
Mixing and Pouring the Concrete
Precast concrete is made up of coarse and fine aggregates, cement, water, and admixtures. It is mixed in the factory’s batch plant according to the concrete mix design specified by the engineer.
Each Oldcastle Infrastructure plant has a concrete laboratory used to conduct routine raw material testing and control exact quantities of each material in a given batch. This is especially important when using high-flow self-consolidating concrete, or SCC, which is used so the concrete flows quickly and easily, filling the mold completely and minimizing the possibility of air pockets.
The lab collects sample cylinders to check the compressive strength and verify the quality of every batch. The concrete also undergoes a variety of additional tests including a spread test to verify the mix has the proper flow and that there is no segregation of the aggregate.
The approved batch of concrete is transported in a dispensing machine to the molds using an overhead crane. When the crane is positioned over the mold, the production team fills each form with the required amount of concrete, taking care to fill the mold completely without trapping air voids. Many plants use vibrating tables to ensure the concrete is completely settled into the form. Once the form is filled, the top is leveled off and the specified finish is applied.
Curing the Concrete
Immediately after the concrete is poured, the curing process begins. This may include putting a tarp over the product overnight to capture the heat generated as part of the hydration process which speeds up curing. The controlled environment of a precast factory enables the product to properly cure and reach the full design strength much quicker than in the field.
Stripping the Forms and Inspecting the Product
Once the QC technician confirms that the product has reached the desired strength required (2500 – 3000 psi) to remove the concrete from its form, the stripping process can begin. This involves opening the outer jacket of the mold and collapsing the inner core, attaching the crane’s hooks to the lifting hardware, and moving the product via crate to the post-pour inspection and finishing area.
There, the structure is reviewed by the QC technician to ensure the final product matches the production drawings and that there are no visual defects. Once it passes the final QC inspection and the technician signs off, the product is labeled and transported to the yard, awaiting delivery to the project site.
PRECAST ON YOUR JOB SITE
Will that precast be delivered to your job site? Using precast can save time and money while improving the overall quality of your construction, but how do you know if precast is right for your project? With all the variables and options, it can be tricky to figure out. Read our article on how you can build smarter with precast, including the benefits.