Precast concrete

One-line orientation

Precast concrete is plant-cast for quality and speed; prestressed precast is commonly pre-tensioned, but the pre-vs-post distinction should not be turned into an absolute rule.

Key points

• Plant-cast for quality control: members are cast in a controlled factory environment (casting bed), giving consistent strength/finish and faster on-site erection.
• Commonly pre-tensioned, not always: plant-cast prestressed precast members are often pre-tensioned (tendons stressed before the pour, released after cure), but precast members can also be post-tensioned and post-tensioning is not exclusive to cast-in-place work. Precast commonly uses Type III cement for high early strength to speed turnover.
• Common members:
  • Hollow-core plank — voids reduce weight (formed by extruder, embedded tubes, or removable tubes).
  • Double-tee — efficient long-span floor/roof member.
  • Beams and columns.
  • Wall / spandrel panels.
  • Tilt-up panels — cast on the slab and tilted up via cast-in lifting rings.
• Transport & erection limits govern member size: pieces must fit road transport and crane capacity. Typical limits are width ~12–14 ft and length ~59 ft.
• Connections: precast is joined and tied to foundations with welded or bolted steel connectors (weld/bolt plates), grouted connections, and extended stirrups that lap into a cast-in-place topping bed for continuity.

Confusions / comparison

	Precast	Cast-in-place
Where cast	Plant / casting bed	On site, in formwork
Prestress method	Commonly pre-tensioning; post-tensioned precast also exists	Often post-tensioning when prestressed
Typical cement	Type III (high early strength)	Varies by exposure
Quality control	High (controlled plant)	Field-dependent
Size limit	Transport + crane	Limited mainly by formwork/site

Related

→ Reinforcing & prestress (this topic): pre- vs post-tensioning mechanics · Cement types & admixtures (this topic): why Type III suits precast · Slab systems (this topic): hollow-core and double-tee as floor systems.