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Concrete Casting News from the Hill and Griffith Company

Precast Concrete Overview

Posted by Hill and Griffith Company on Dec 17, 2020 4:55:46 PM

An overview for understanding precast concrete types and techniques

Article excerpt from the ConcreteNetwork.com by Bill Palmer

Precast concrete is simply concrete that is cast somewhere other than where it will be used. Most precast products are cast in a factory using a wet-cast method, but others are cast on site—such as tilt-up panels. There are lots of reasons—mostly advantages—why one would precast, and we'll get into those, but the biggest negative of precasting is that the resultant concrete item must be moved. Concrete is heavy—typically about 150 pounds per cubic foot—so concrete elements don't have to be very big before moving them becomes unrealistic.

Some decorative contractors, such as those that precast concrete countertops, stretch the boundaries on what's too big or heavy to move, developing special rigs to transport massive pieces of their concrete work. Other times, it's just simpler to cast the concrete in place as the precast advantages are outweighed by convenience, such as with concrete slabs and floors.

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Advantages of Precast Concrete

As long as there has been concrete, it has been precast—going clear back to the Romans. And there are lots of good reasons for that. The National Precast Concrete Association has information on its website outlining the value of precast concrete. Most of the advantages they cite are really advantages of concrete in general rather than specific to precast, but when compared to site-cast concrete, precast does have lots of advantages:

  • Since precast is manufactured in a controlled casting environment, it is easier to control the mix, placement, and curing
  • Quality can be controlled and monitored much more easily
  • Since a precaster can buy materials for multiple projects, quantity discounts can lower costs
  • Weather is eliminated as a factor—you can cast in any weather and get the same results, which allows you to perfect mixes and methods
  • Less labor is required, and that labor can be less skilled
  • On site, precast can be installed immediately, there is no waiting for it to gain strength and the modularity of precast products makes installation go quickly
  • Repeatability—it's easy to make many copies of the same precast product; by maximizing repetition, you can get plenty of value from a mold and a set-up
  • Accelerated curing, by heating the precast parts, greatly increases strength gain, reducing the time between casting the part and putting it into service
  • With the ability to so tightly control the process, from materials to consolidation to curing, you can get extremely durable concrete

Types of Precast Concrete

There are lots of applications for precast concrete across the industry, including:

    • Bridge beams
    • Double Ts
    • Hollow-core slabs
    • Septic tanks/manholes
    • Pipes/culverts
    • Foundation walls
    • Architectural panels
    • Traffic barriers and retaining/sound walls
    • Steps
    • Fences
    • Pool coping

Precast Structures

Another popular use for precast concrete is large-scale commercial buildings such as apartments, hotels, warehouses or office buildings. However, it is also possible to build smaller-scale structures, such as single-family residences using precast concrete.

Some of these buildings are built using a tilt-up construction method where concrete panels are cast flat, cured and then raised into position. Others are built with a prefab or modular approach where more complete units are created, delivered to the site and set in place.

Tips on Starting a Precasting Operation

Precasting offers contractors an opportunity to make more money—to increase the scope of the types of concrete work they can offer their customers. To begin precasting, you will need a shop large enough to accommodate the size of the pieces you intend to precast (or a yard if you intend to precast outside), material storage areas or bins, molds, a mixer sized for the precast products you are making, a way to consolidate the concrete in the molds, and a material handling system.

Quality Control for Precast

If you are going to manufacture precast products in your shop, you should consider doing your own quality control. Large precast operations have big labs and develop intricate quality procedures, testing all aspects of the concrete both before and after casting and developing extensive QC tools like fishbone charts and scatter diagrams and Pareto charts. You may not need to get to that level, but some basic testing and analysis can save you money both in production costs and higher quality precast products.

Starting with good materials is the first step. All aggregate is not acceptable for use in concrete. If there are organic materials, shale, chert, or other soft materials in your aggregate, you can't produce good concrete. Reactive aggregates can lead to alkali-silica reaction that can destroy your concrete. The best defense may be simply to require that the aggregate meets ASTM C 33, "Standard Specification for Concrete Aggregates." It's your aggregate supplier's responsibility to assure that you are getting good aggregate.

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