The short, salesperson’s answer is “it’s a superior building material.” Now I’m sure long before you reach the end of this you will have figured out that I’m not a salesperson. Whether it be because of the fascination I have with the manipulation of material properties, because of the lack of wordsmithing prowess that engineers are generally known for, or because I just told you, either way, when you read “Design Engineer” at the bottom it should make sense.
So what makes it a superior building material? Because it’s precast? Because it’s prestressed? Is there a difference between the two? I mean, even the trade organization for these building products felt the need to differentiate them in their name, Precast/Prestressed Concrete Institute (PCI). It can be a little confusing sometimes.
Precast refers to any piece of concrete that is cast at a different location from the final building site and the cured (hardened) piece is transported to site when the site is ready for delivery, whether it is prestressed or reinforced with conventional reinforcement bar. This is compared to the alternate method of cast-in-place where forms are assembled at the building site and ready mix concrete trucks deliver the wet concrete to be placed and once the concrete is cured the forms are removed. Since both are working with the same material, there are bound to be some similarities in the processes. Both require formwork to hold the wet concrete in the desired shape until it is cured and both have some type of steel reinforcement to help concrete meet the design requirements, but that’s about where it ends. Beyond that the different aspects of the processes start to really determine what kind of product you’re going to end up with. The largest of these aspects would have to be the controlled environment throughout production that precast concrete can offer. Because of the ability to strictly control the environment in which the concrete cures, the tolerances, and therefore quality, you are able to achieve with precast outpaces cast-in-place by leaps and bounds.
Now on to the really fun stuff, prestressed. Prestressed concrete is primarily a form of precast concrete. I have to say primarily because there is technology available to prestress on site, but the feasibility of the application is rare. Generally, if you have the ability and infrastructure to produce precast concrete, you are set up pretty well to produce prestressed concrete. The basic idea of prestress plays on the desirable attributes of concrete and steel. Anyone that works with concrete will know compression is good, bending (which results in tension forces) is bad. And anyone that works with steel will know that compression isn’t necessarily bad, but it is mostly used in situations where tension will be present.
To prestress a piece of concrete:
- Steel strand made up of individual cables woven together are stretched and held in that position (imagine when you pull on a rubber band) inside of the form.
- Concrete is then cast around these steel cables and left to cure and bond to the steel cables.
- Once the concrete reaches sufficient strength (is hard enough), the steel cables are cut away from the supports holding them in their stretched position (letting go of the rubber band).
Since the steel cables are determined to return to their original position just like the imaginary rubber band did, this allows the tension force that was required to stretch the steel cables to squish the concrete together, also known as compression. Not only that, but when you concentrate the steel cables towards the bottom of the member you can create an upward force from the prestressing, resulting in compression on the bottom and tension on the top, or better known as camber. Now when this member is loaded up and wants to bend and deflect, it wants to put the top into compression and the bottom into tension. So the applied load must overcome the counteracting forces already in the member from the prestressing. Pretty slick, huh? I think so. That right there is the ultimate difference between cast-in-place and prestressed, because for a cast-in-place beam it is immediately put into tension on the bottom face because it is only reinforced with conventional reinforcement bar. This method uses the tension properties of steel, but not to take advantage of the compressive properties of the concrete, only to resist the load. This leads to earlier cracking and lower design strength. That’s why I’m all about prestressing, because inch-for-inch and pound-for-pound it is simply not possible for cast-in-place to compete. You would either have to shorten the span or deepen the beam, and we all know how well that typically goes over.
Now when you hear the buzz terms of “better crack control”, “better durability”, “higher span-to-depth ratio”, or any of the many others when compared to cast-in-place, you’ll know that it is first and foremost because it is precast. We control our enviroment so we can better control our product. On top of that, all of those advantages are made possible by prestressing the precast concrete. Using the natural advantage of steel to make concrete even more advantageous. This is why prestressed is so often referred to specifically instead of under the umbrella term of precast, because in the words of one of my past professors, Dr. Reza, “it’s a game changer.”
Chase Radue, EIT