Posted on December 9, 2020 by Tim Edland
This is a basic walkthrough of how precast concrete building components are made. We’ll focus on the daily plant cycle and go from preparing forms for casting to piece transportation to the jobsite. There are a few prerequisites to note before we begin casting precast. First, the various precast building components being used have already been determined and component manufacturing drawings have been created. Second, a precast plant is needed. Precast plants can vary quite a bit, and the age of precast plants can be from as far back as the 1950s, all the way up to present day. Casting areas can be indoors or outdoors, and the equipment and forming varies between plants. This will affect the available piece cross sections/shapes, colors and textures, maximum sizes, and efficiency/cost.
Plants contain reusable forms that will be filled with concrete to make pieces. Nearly all pieces are cast in a horizontal orientation, meaning their longest dimension will be parallel with the plant floor. This means floor and beam members will be cast in their final position, but walls and columns will be cast 90 degrees from their final vertical orientation and need to be rotated into that position at the jobsite. Forms are typically made from wood or steel with shapes varying depending on the intended shape of the precast. For rectangular shapes, such as walls or slabs, the form is a flat surface with side rails attached to confine the sides of the concrete.
A typical work cycle will cast a panel in a form every day. That cycle will include removing the previous day’s precast piece from the form, preparing the form for casting, casting the concrete, and curing the concrete. In preparing the form, all refuse concrete from the previous day is removed and the form is thoroughly cleaned as any imperfections will show through to the concrete surface.
If prestressing strands are used, forms are usually very long and multiple pieces will be put on a single form with a bulkhead form separating pieces. The bulkheads and rails are laid out and installed. Then prestressing, reinforcing, and embeds are installed. Prestressing refers to cables that stretch from one end of the form to the other, typically over 100 feet, and are stretched with a force typically in the tens of thousands of pounds. Prestressing’s basic function is to allow pieces to be longer. Embeds refer to the items cast into the concrete that will be used to connect the components together when made into a building, typically something to bolt or weld to.
Concrete is now needed for casting. A precast plant will usually have many forms and a batch plant system containing one or two mixers to provide concrete. The concrete needs to be delivered from the batch plant to the form. This is typically hundreds of feet and various systems are used, trucks and craneway buckets being the most common. The concrete is poured into the form, then a team vibrates and screeds the concrete. They may also install some embeds in fresh concrete after casting. The top exposed surface of the concrete may be finished with a broom, rake, or hard trowel depending on the desired finish.
Now the concrete needs to cure. This doesn’t require much labor, but it is important. To make a piece in a bed each day leaves about 16 hours of concrete cure time before it needs to be removed from the form. The concrete mixes and curing methods need to produce a concrete capable of withstanding the stresses in the stripping process. That can vary from 2500 psi to 4500 psi depending on piece length & shape, prestressing, and how the piece is stripped. The concrete may be heated to increase the curing rate.
Stripping is the first process occurring in the plant work day, but the last for the piece on the form. The piece needs to be lifted off the form, set onto a trailer, and moved out of the plant. Before stripping, some of the forming on the sides of the precast are usually removed to reduce the suction of the piece to the form. The piece is typically lifted by a plant craneway or a mobile gantry crane. The piece will have lifting attachment items cast into its concrete or specialty equipment will be used to grab it.
The plant now preforms the daily work-cycle again. The piece of precast may have a specialty finish applied to it, such as sandblasting, acid etching, or polishing. It’s then yarded for a longer curing duration and window installation. Next comes loading to a trailer for shipping to a jobsite where it will be erected onto the building.
Tim Edland, P.E.
Research & Development Director