Process of Manufacturing

The process of producing the pieces starts by designing the components in a CAD program and transferring the 3D model to the plotting stations in the manufacturing plant. Formwork is mostly fabricated from steel and is formed into pans or separators assembled as a negative shape of the final product. The formwork is robust because it is highly loaded then disassembled and interchanged for reuse in the factory.

At this stage, the cast-in items such as pipes, electrical boxes and conduit are set in the formwork which is sealed to keep the liquid mixture within. Also, other connecting devices such as anchorage, reinforcing steel bars and braces are laid inside the form to perform structural stability and to create connections to be used at later stages of installment. The reason that steel reinforcement bars are used together with concrete is to create strength and to prevent the positive and negative bending and cracking. (Fig. 3.33)

Meanwhile, the raw material consisted of aggregate (sand, rock, or gravel), water, and Portland cement are mixed and ready for pouring into the formwork. After applying a coat of oil to the inner surfaces of the formwork, the batch is poured into the forms and vibrated to release the air bubbles trapped in the mixture. Next, the surface is leveled and screeded to create a finished surface. To screed is to level a floor or layer of concrete with a straight edge using a back and forth motion while moving across the surface. The concrete is now ready to be cured where heat is applied in specific areas or chambers. After the curing time is complete, the piece is ready to be demolded and detached from the formwork.

The structural design requires the performance and behavior of the final elements. There is usually a testing procedure in the factory right after the offsite production is complete. However, the result of testing in that stage is not sufficient because there are other stresses that will be applied before final installation. These stresses include the handling of the material such as lifting, transportation and erection. A post-testing is additionally required for structural purposes which should be considered for the final stages in the process.

Attachment Methods for Precast Concrete Components

Connecting the precast concrete units together and to the main structure is a major design aspect of precast concrete assembly. The stability of the building as a whole and its safety, performance and economy of use in general are the outcome of such an assembly. Components such as wall panels, facings, interior partitions, columns, floors, hangers, and other precast elements are attached to each other and to other non-precast members by a combination of different connecting pieces such as fasteners, seats, brackets or outriggers, shear plates, tiebacks, bolts, welds, anchors, and inserts using different kinds of methods. The main 3 attachment methods are explained below.

1. Embeds
An anchor bolt or anchor with weld plate is cast in the factory within the precast element. Templates are used to place embeds while the concrete is wet. After the concrete is ready, the embedded surfaces will be welded together with other steel elements such as loose plates or angles. (Fig. 3.35)

3. Expansion anchor (Post-installment)
This method also requires predrilling. An expansion anchor is placed into the cavity. The bolt, screw, or hanger is then placed within the anchor and, when engaged, expands to restrict its movement. (Fig. 3.37)