Most concrete professionals understand that in just about any job, even one that’s considered successful, their concrete is likely to crack in some capacity. However, not all cracks are created equal. The main goal of most concrete structures is to try and minimize the amount of movement going on under load, like a bridge swaying under heavy wind. This is why reinforced concrete is so popular. However, when you’re combining a substance that is rigid, like steel, with something inherently brittle, like concrete, cracks are likely to happen. Read more to learn about prestressed concrete.
A small, aesthetic crack is one thing. But deep structural cracks create a variety of issues, from creating an opening for water to get into, compromising the integrity of the entire structure. However, there is a method to introduce stress into the concrete before combined with steel, to create something that’s more versatile and sturdy as a final reinforced concrete product. That would be the practice of prestressed concrete. So, from prestressed concrete slabs to prestressed concrete beams, here’s all you need to know about this essential construction component and how it works.
Table of Contents
Understanding Prestressed Concrete Design
So, what exactly is prestressed concrete?? To explain that, we first need to talk about conventional reinforced concrete as a comparison. Normally, all the stresses of the weight on a reinforced concrete structure are held by the steel reinforcements. What prestressed concrete designs do is induce stresses throughout the entire structure. The end result is a product that better handles vibrations and shocks than conventional concrete. In addition, this makes it possible to form longer, thinner structures that can still handle those heavier loads.
We see this most commonly in longer structures, including some with a larger beam span. Examples include:
— Floor beams
— Water tanks
— Railroad ties
What’s the best way to illustrate the value of prestressing?
Let’s say you have a stack of books you need to move across a room. If you try to hold them from the bottom only, chances are the books are going to sway around and be hard to keep steady. However, if you hold them from the bottom and use your arms to hold the sides, it’s going to be a lot easier to keep things in place. We’ve used this basic principle for much of our history. For example, centuries ago, metal bands were applied around wooden barrels to better hold their contents in place. Prestressing concrete is taking this humble concept and applying it to our modern structures and superstructures.
Prestressing concrete does mean additional costs for contractors. This includes the cost of additional materials and the act of prestressing itself. It’s also more complex overall than using reinforced concrete. However, with all those caveats comes major benefits, including:
— More material efficiency overall
— The ability to work for spans over 35 meters
— Better durability, shear strength, and fatigue resistance
— No cracks even at top load conditions
As a result, the deciding factor for whether you use this is generally whether you need the added load capability, or are working with a longer span.
How To Implement Prestressed Concrete
So, now we understand the need and value for prestressing. What about how it’s actually done? The compressive stresses that give prestressed concrete its stability are generally introduced in one of two ways: pre-tensioning or post-tensioning.
When using pre-tensioning, steel is stretched out before the placing of the concrete. This entails steel tendons set between two abutments, then getting stretched to roughly 80% of their strength. The concrete then gets poured into molds around them and cured. When the concrete is cured and at the right strength, the steel is released. The steel will try to get back to its original length, creating the tensile stress that becomes a compressive strength in the concrete.
Post-tensioning is similar, but the steel doesn’t get stretched until after the concrete is hardened. Here, the concrete gets cast around the unstretched steel, but not in direct contact with it. Generally, ducts get formed inside the unit via thin-walled steel forms. After the concrete stretches to the proper length, those same steel tendons get inserted and stretched against the unit. This creates compression in the concrete. This is the preferred method for cast-in-place installation as well as some of the larger projects out there, like bridges and floor slabs.
With post-tensioning, there is also generally an additional step that needs to be done, generally related to the ducts. This is either bonded construction or unbonded construction. Bonded construction involves filling the space between the tendon and duct with cement grout. This is generally done to help the steel minimize corrosion, but also generally increases the overall strength of the structure as well. This particular grout is made with cement, water, and sometimes admixture, with no sand.
Unbonded construction is when no grout is used between that space. In this case, to cut down on corrosion, a waterproof galvanization method is used instead.
When it comes to adding a new component to your concrete company’s suite of services, you’re ideally appealing to a whole new set of customers. However, at the same time, you’re also substantially driving up the potential expenses for your business. This can range from professional development costs to actually learning how to use things to accommodate the costs of material/equipment like prestressed concrete cylinder pipes. Under these circumstances, it’s essential that concrete contractors have the clearest picture of their finances possible. This will help determine whether it makes sense to branch out now or later.
To do this, you need the top project management software around and that means using eSub. Our cloud storage capabilities mean that your entire team can take a look at relevant financial data to see how much operations currently cost and how much adding prestressed concrete to the mix can benefit you. We also can help with tracking equipment and employees after you begin.