The Remarkable Qualities and Uses of Prestressed Concrete
Prestressed concrete is a type of concrete that has been strategically pre-loaded with compressive forces to improve its structural strength and performance. Prestressed concrete is one of the most innovative construction materials in recent years, and it has revolutionized modern building and design technology.
The techniques involved in creating prestressed concrete involve the use of high-strength steel tendons that are tensioned within the concrete. This method increases the concrete’s load-bearing capacity and improves its resistance to cracking and deflection, making it a versatile and popular solution in modern construction and design.
What is Prestressed Concrete?
The creation of prestressed concrete involves compressing it, or “prestressing” it during production so that it will better counteract external forces when it is put into place. The compression of the concrete happens with steel tendons that manufacturers place within or adjacent to the concrete to improve its performance. Manufacturers use two main types of steel tendons: pre-tensioned tendons and post-tensioned tendons.
With pre-tensioned tendons, steel strands or wires used as tendons are stressed before the concrete is cast. The manufacturers stretch the tendons between anchorages and then apply tension to them. Once they apply force to the tendons, they pour the concrete around them. As the concrete dries, it adheres to the tendons. Pre-tensioned concrete is common for precast concrete elements like beams, walls, and slabs.
Post-tensioned tendons are stressed after the concrete has hardened. Once the concrete has dried, the tendons are added in a duct within the concrete and are tensioned with hydraulic jacks. The tendons are anchored to maintain the tension. Post-tensioned tendons are more common for cast-in-place concrete structures and offer more design flexibility.
Innovations in Prestressed Concrete
Early techniques for strengthening concrete with steel occurred throughout the late 19th and early 20th centuries. Some of the earliest prestressed designs came through the work of French civil engineer Eugene Freysinnet who did much to advance the ideas of prestressing concrete. The Belgian engineer, Gustave Magnel and German engineer, Eugen Morsch, later independently introduced and developed the concept of prestressed concrete in the 1930s.
The Prestressed Concrete Institute was founded in the United States in 1954. This organization played an important role in advancing knowledge and techniques in the prestressed concrete industry. Today, similar organizations exist all over the world in Canada, the UK, Australia, South Africa, and Europe.
Over the years, important advancements in materials and production have created new innovations in prestressed concrete technology. The development of high-strength steel tendons and improved anchorage systems have increased the strength of prestressed concrete. Computer-aided design and enhanced technology has also improved the accuracy and optimization of prestressed structures.
Uses of Prestressed Concrete
There are numerous and wide-ranging applications for prestressed concrete because of its advantages.
Bridges
Prestressed concrete is popular in bridge construction because of its ability to span long distances and its immense strength. Prestressed bridges can span long distances while maintaining structural integrity and minimizing the number of needed support piers.
Buildings
Architects use prestressed concrete extensively in high-rise building design, particularly in elements like floor slabs, columns, and beams. The enhanced carrying load of prestressed concrete means that it allows for longer spans and reduces the need for intermediate support which also represents a significant cost savings. It is popular for both residential and commercial buildings.
Nuclear and Blast Facilities
Engineers have come to rely on the strength of prestressed concrete in high-pressure containment structures like nuclear plants and petrochemical blast tank structures. For these uses, manufacturers create specialized prestressed components to increase their resistance to cracking and leakage.
Parking Structures and Pavement
Parking garages and pavements are subject to constant load stress, so engineers regularly use prestressed concrete to minimize cracking and ensure long-term performance. This crack resistance allows builders to pour larger slabs than they could with regular reinforced concrete. This allows for wider joint spacing to reduce joining costs and joint maintenance.
Water-Related Structures
Water exerts enormous pressure and friction, so water applications are ideal for prestressed concrete. Prestressed concrete is common for use in dams and off-shore structures like oil rigs, wind farms, and off-shore infrastructure. It is vital in these applications that corrosion protection is applied to any exposed steel tendons to ensure long-term durability.
Stadiums and Sports Facilities
The long span and high-strength capacity of prestressed concrete mean that it is ideal for arenas, stadiums, and other sports facilities. Because prestressed concrete does not require as much support, architects can allow more open space for unobstructed viewing and open layouts for flexible seating arrangements.
Qualities of Prestressed Concrete
Prestressed concrete has many valuable qualities that make it an ideal building material for particular designs, but there are also drawbacks that builders must also consider when deciding the optimal building material to use.
Strength
Prestressed concrete offers enhanced strength compared to other types of reinforced concrete. The prestressing introduces compressed forces that counteract external forces allowing for construction with longer spans and higher load capacities.
Improved Durability
The prestressing of this type of concrete helps to minimize cracking, buckling, and deformation because of heavy loads. This means enhanced durability and resistance to external factors that would deteriorate its structure integrity, including shrinkage, creep, and fatigue. Properly maintained, prestressed concrete has a longer lifespan than other concrete.
Structural Efficiency
The strength of prestressed concrete means that builders can use less of this material to accomplish the same tasks as other concrete. The strength of prestressed concrete also allows for less structural support. This quality saves builders both time and money and allows architects more flexibility in their designs.
Design Versatility
Architects and builders use prestressed concrete in a wide range of applications. It is versatile enough for designers to use it in the construction of bridges, buildings, parking structures, water-related applications, and industrial facilities. It accommodates many design styles and allows for creative and innovative designs.
Sustainability
The production of concrete is indirectly related to the build-up of greenhouse gases as the production of clinker in cement is energy-intensive, but of all types of concrete, prestressed concrete is one of the most sustainable. Builders can use less of this material and its durability means that they do not have to rebuild these structures as often as with normal concrete.
Fire Resistance
In general, concrete is a fire-resistant material, but the steel tendons in prestressed concrete can be susceptible to fire if the concrete does not protect them. Fire or even elevated temperatures can weaken the pre-stress in the tendons compromising their strength. Fire-resistant coatings or enclosures can help improve fire resistance performance.
Complex Design and Construction
Designs using prestressed concrete are more complex than other concrete designs. This type of concrete requires specialized knowledge for design and building to ensure proper tendon placement, tensioning, and anchoring.
Tendon Protection
The key to the durability and strength of prestressed concrete is the steel tendons. Concrete companies are making constant improvements in ways that they protect the steel tendons including tendon grouting, tendon coating, double-layer encapsulation, and anchorage protection.
Limited Design Modification
Prestressed concrete designs are complex. Once the tendons have been tensioned and anchored, there is little option for changes. Any modifications must be structurally analyzed and carried out by trained engineers and highly-skilled builders.