What Can Precast do that Cast-in-Place Cannot?

Posted on November 18, 2020

Precast concrete is simply concrete that is not cast in its final position, hence, pre-cast. It is cast in a reusable mold and is typically cured in a controlled environment. The last step is moving it to its final, or in-service, position. Cast-in-place concrete is simply concrete that is cast in its final position. So, while both are made of concrete, and contain rebar and steel, they are a bit different in what they can get us, and how we get to the final product. Figure 1 shows precast being swung into position while Figure 2 shows poured-in-place concrete being pumped into position.

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The table below compares precast and cast in place construction on a variety of items:

  Precast Cast-in-Place
Preparation Excavate land and pour foundations Excavate land and pour foundations
Schedule
  • Eliminate on-site logistical concerns
  • Time to produce is off-site, pieces arrive ready to install
  • Pieces held until needed on site, in any order
  • Minimal onsite space needed
  • Can combine structural and architectural
  • Fully cured upon arrival
  • Onsite formwork and rebar cages may be difficult
  • Must setup and pour onsite, time intensive
  • Created as needed
  • Space needed for staging of cages, forming, etc.
  • Typically structural
  • May need to wait until specified curing strength
Produce
  • Efficient assembly line techniques
  • Custom equipment readily available at location produced
  • Less Labor
  • Must create in final position
  • Must get equipment on site, or perform by hand
  • More labor
Curing Time
  • Cured to in-service strength prior to arrival
  • Shrinkage has occurred prior to arrival
  • Accelerated curing with infra-red or steam
  • Wait for specified cure strength
  • Extra detailing to mitigate shrinkage forces
  • Curing dependent upon weather
Weather
  • Climate controlled
  • Ideal conditions maintained constantly
  • Winter construction
  • Unexpected weather delays
  • Variables like temperature, wind, humity, etc.
  • Cold weather tenting needed
Quality Control
  • Controlled environment testing
  • Tighter installation and cover tolerances
  • More precision
  • On-site testing
  • Looser tolerances
Economy
  • Uses standard reusable forms
  • All materials flow towards forming
  • Build forms and cages, brace, pour
  • Forming must move as pouring
Design
  • Usually prestressed prior to pouring, anchor to reusable forming
  • Prestressing can increase span-to-depth ratios
  • Prestressing allows for uncracked members, increasing durability, especially in corrosive environments
  • Prestressing replaces rebar
  • Design may emulate cast-in-place if needed
  • Post-tensioning requires cure time prior to apply internal stresses
  • Generally thicker members
  • Unsightly cracking may occur
  • Rebar required for ultimate strength
Future Expansion Non-loadbearing walls allow for precast to be 'taken off' the structure, as well as re-used as a future wall

Demolition needed
Architectural Limitless finishes and features using aggregates, colors, textures, reveals, finished formliners, thin brick, and other inset products Usually structural, can use formliners
Joints Precast requires more joints, and is connected across joints to mimic cast-in-place Minimal jointing
Versatility
  • Aesthetic flexibility achieved by a variety of textures and colors
  • Can mimic brick, wood, large blocks, via custom moldings
  • Can inset granite, terracotta, or other materials prior to arrival
  • Can add certain elements to product, like insulation
  • Added labor and time to add isulation

Another significant advantage of precast is the available architectural finishes and features. They seem limitless, especially considering that each finishes look is dependent upon the type of aggregate specified:

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Figure 3 shows an interesting precast architectural ramp with precast cladding. The custom shaped cornice has a “finger” to hide lighting accents. It has inset terracotta; a clay product that was placed in the form prior to pouring to become a part of the member. It also has some interesting arches that would be labor intensive to produce on as cast in place.

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So, while precast concrete and cast in place concrete are made of the same materials, there are numerous differences between the two. Precast can work for you and provide countless finishes and features; all while shortening the timeline to a final building!

Dustin Jones P.E.
Project Engineer