The Subtle Might of Civil Engineering: Prestressed Concrete

Learn everything there is to know about prestressed concrete, an essential construction material used for building an array of structures. Understanding its use, benefits, as well as its downsides, means better ascertaining how it differs from reinforced concrete. By way of this article, you will come to grasp the concepts of stressing, pretensioning, and post-tensioning, as well as the importance of anchorage systems as part of the process. 

What Is Prestressed Concrete?

Prestressed concrete can be defined as a sort of improved version of reinforced concrete, or rather a material designed and suited for specific stresses structures must withstand once in service. To achieve this goal, axial compression is used to counteract or limit tensile stresses that structures must endure. 

As such, the term “prestressed” comes from “pretensioning," which is done by way of steel tendons before pouring the concrete, or afterward (“post-tensioning”) when the concrete is curing.   

How to Make Prestressed Concrete

Prestressing Implementation

To prestress concrete, various steel components must be used and stretched to create the necessary tension. It can be: 

• Steel wires

• Steel strands (2 or 3 twisted strands of steel)

• Tendons (bundles of twisted steel strands)

• Cables (a bundle of steel cables)

• Steel bars (wider diameter than steel wires)

The above-mentioned steel components will be stretched, either pre- or post-concrete hardening (known as pretensioning or post-tensioning), with the hope of counteracting the process by which concrete naturally shifts, whether that means creep, shrinkage, or instantaneous deformations. Why? Because such strains can reduce the steel cables' elongation, thereby diminishing or potentially eliminating the prestressing effect. 

To face it, head-on, the method’s inventor, Eugène Freyssinet, quickly took notice of concrete’s limitations. Hence, to make quality prestressed concrete, one has to use high-strength concrete and tension the cables used at higher values than is normally necessary. 

As such, throughout the concrete’s curing process to its subsequent load-bearing phase, its movements will not result in prestressing failure.  

The Purpose of Prestressing Anchorage Systems

Anchoring refers to the process of sealing the ends of the reinforcements. It is absolutely indispensable every time concrete is post-tensioned. On the other hand, when proceeding with pretensioning, anchors are solely necessary if the adherence between the steel and concrete is insufficient or if the concrete’s strength isn’t as expected. 

Anchors transfer tensioned steel loads over to concrete. They can be: 

• Loop anchorages

• Tube anchorages

• Screw collars or tightening bolts

However, concrete also has to be able to withstand the anchorages and inherent stresses when under pressure by the steel cables. To do so, it’s typically reinforced, ensuring the component’s structural integrity. 

Post-Tensioning: A Prestressing Method

This technique consists of having tendons or steel cables that are stretched once the concrete is poured. To do so, one has to include framework reinforcements or ducts. The steel strands will be snaked inside the ducts after the concrete is poured and tensioned, using a hydraulic jack and by way of the anchorage system. 

Once the jack is removed, the stretched steel strands transfer a part of their stored energy over to the concrete, resulting in compressive force. The latter will balance the load borne by the concrete within the larger structure of which it is a part. 

As a result, tendons aren’t always positioned along a straight line inside the concrete. Therefore, prestressing can be: 

• Centered

• Off-center 

• Parabolic 

Choosing the positioning of the tendons is directly dependent on the zones to be reinforced, meaning those affecting the structure’s tensile strength. Post-tensioning is typically employed when building segmented structures as part of an extensive civil engineering project. 

What Is Prestressed vs Reinforced Concrete?

Reinforced concrete and prestressed concrete are both reinforced by steel components, improving their tensile strength and ductility. However, when it comes to reinforced concrete, the rebar used is passive, while with prestressed concrete, active cables are used. This means that the tension imposed on the steel tendons actively involves said reinforcement in enhancing the concrete’s tensile strength. 

Ergo reinforced concrete and prestressed concrete aren’t used to the same ends. While both types of concrete are used for large-scale construction projects, such as bridges or dams, prestressed concrete replaces reinforced concrete as soon as its strength peaks. 

This especially applies during the building process of highway overpasses or certain commercial structures. Without prestressing reinforcement, the structure would collapse under tensile strength.  

Hence, reinforced concrete is used for smaller structures like a building’s footings, walls of a house, certain dams, and some bridges. In such constructions, the use of prestressed concrete just can’t be justified.   

Why Is Using Prestressed Concrete Advantageous?

Simply because it can offset the weaknesses of standard reinforced concrete, meaning its tensile strength. Indeed, with reinforced concrete, tensile strength only amounts to 8-14% of its compressive strength. As such, cracks can appear on reinforced concrete surfaces, whereas, under similar circumstances, prestressed concrete would remain unaffected.

Prestressing serves as to improve concrete’s strength when exposed to: 

• Flexion

• Shear stress

• Torsion 

• Corrosion 

Prestressed concrete is better suited for large-scale worksites. Yet, despite its numerous upsides, said material still has its limitations. 

The Benefits of Prestressed Concrete

Prestressed concrete is more flexible compared to reinforced concrete, and it isn’t restricted when it comes to the span of its components. Therefore, it renders the process of building precast segments a possibility, which, in turn, makes the whole construction process that much faster.

Furthermore, given its great compressive strength and tensile strength, the structural component designed can be optimized, limiting the use of materials, and, in turn, overall project costs. 

Prestressing through post-tensioning also means benefiting from the ability to adjust the structural element’s load-bearing capacity post-concrete pouring with precision. As we already mentioned, the positioning of the steel tendons allows for reinforcing the concrete structure in clear-cut areas.

Since it makes concrete more resilient, it limits the risk of cracking, which, in turn, makes the concrete structure more durable and cheaper to maintain. 

Disadvantages of Prestressed Concrete

Prestressed concrete is more expensive than traditional concrete, most especially reinforced concrete. Why is it more expensive when fewer materials are needed for the structural composition? Because additional materials, such as must-have equipment used to stretch the steel tendons, are necessary. 

Also, prestressed concrete isn’t as flexible as reinforced concrete, in the sense that the framework’s complexity makes building one-of-a-kind shapes almost impossible. 

The worst of it probably lies in its preparation since the margin for error is a lot smaller for prestressed concrete than it is for other materials. A calculation error, or worse, mispositioning the steel strands during tensioning can quickly result in a disastrous affair.

Consequently, prestressed concrete isn’t for all contractors, and its use is simply not in the cards for inexperienced homeowners and DIYers. 

Why It's Used: Applications of Prestressed Concrete

Building Bridges

The interest in using prestressed concrete to build bridges lies in the material’s ability to span long distances by way of precast segments. Not only will these long-spanning sections be stronger than those made of reinforced concrete, but they’re also faster to put in place on worksites. 

Without prestressed concrete, there wouldn’t be the Louis-Hippolyte-La Fontaine bridge-tunnel connecting Montréal with its south shore. If reinforced concrete had been used, it would have meant inching the tunnel’s roof by another metre, which would not comply with the Port’s height standards. In other words, using prestressed concrete made optimizing the structure’s dimensions possible.  

Used in Slabs and Floors

By slabs and floors, we mean those constructed on prestressing beds that span over 328 feet (100 metres). The goal is to build prestressed hollow-core floors with spans of 39 feet (12 metres) that can also withstand office loads. 

In such structures, the prestressed tendons are often done in two orthogonal directions to maximize the floors’ load-bearing capacities. Alternatively, they can be aligned along a preferential direction and reinforced perpendicularly with passive rebar.

Used to Make Beams

A prestressed concrete beam can be compared to a pile of books carried horizontally. To hold the pile together, pressure must be applied on both ends of the pile simultaneously. This same concept applies to prestressed concrete beams. 

The span of the beam results in gravity exerting tensile stress on its structure. To prevent it from collapsing under its own weight, a significant amount of tension is applied to the steel cables. Consequently, the resulting compression from the steel tendons balances the tensile stress exerted by the beam, thereby maintaining the structure’s integrity.

Structural Revolution: The Result of Prestressed Concrete

Prestressed concrete is a major innovation in civil engineering, offering robust and durable solutions for structures mandating strength and longevity. Prestressing allows concrete to surpass its natural compressive limits, improving its ability to withstand tensile strains. Using said technique translates into narrower and lighter structures, capable of spanning longer distances without intermediate supports. Furthermore, prestressed concrete mitigates the risks of cracking, prolonging the service life of structures, and minimizing maintenance needs. Employing prestressed concrete in construction projects means tackling complex architectural challenges while ensuring structural safety and reliability.