Reinforced and Prestressed Concrete Structures- Types, Styles, Benefits, and Designs
Out of the many building materials used today, concrete claims the title of "most commonly used." There are a variety of reasons why, but it is largely due to its versatility, accessibility, and overall strength and capacity as a structural component. Furthermore, advancements in precast concrete (concrete that is produced in a plant environment and shipped to a job site) have increased its popularity and structural & architectural capabilities. There are many advantages concrete structures have, as well as various ways to reinforce them - all of which cement the claim of being the most commonly used building material.
Precast concrete construction will be the focus of this article, considering that is where my expertise lies. However, cast-in-place concrete structures can accomplish some of the same goals with different methods and design techniques. Precast concrete is often preferred due to its construction speed! General contractors and owners are very cognizant of their project schedules, and precasts can often meet or exceed what they are looking for. What makes precast concrete so speedy? Firstly, precast concrete, as mentioned, is produced in a controlled environment (a manufacturing plant). Because of this, precast components can be cast while groundwork is being done on-site and other preliminary tasks that would typically hold up cast-in-place concrete. Another reason precast may be preferred is the types of architectural finishes and features that can be provided: cast-in brick, cast-in designs and logos, various openings and protrusions, different color finishes, and different concrete textures. Speed and quality are just two of the many reasons a contractor or owner may select a precast concrete structure over a traditional cast-in-place option. But if they do select precast, what other options remain?
2. Combining Building Materials
Combining building materials to create one homogenous building is common. For example, a steel frame may hold the building up, but a concrete exterior welded to the steel frame may provide architectural features and thermal advantages. However, it is becoming more popular to have a structure completely comprised of precast concrete: this is commonly referred to as "total precast."
3. Thermal and Architectural Benefits
Total precast structures have the thermal and architectural benefits of concrete cladding while also providing lateral and gravity structural systems. Concrete columns and walls bring gravity loads down to the foundation, while shear walls or concrete frame systems transfer lateral loads from a wind or seismic event. Concrete beams and floor slabs tie the diaphragm (floor system that creates a load path for lateral loads) together, while spandrels provide large openings while supporting floor levels. From the outside, the visible walls, columns, and spandrels can have a variety of finishes, textures, features, and colors to create the showstopping structures clients are looking for.
4. More Beneficial Properties
Speed, thermal abilities, and architectural features/finishes are not the only reasons for a concrete structure: design flexibility, fire and corrosion resistance, strength, and durability are all excellent properties concrete possesses. Concrete can be formed into all sorts of shapes and sizes: curves, hollow, pointed, rounded, with openings, sloped, with protrusions and extrusions, and much more. Because of this, concrete can meet and exceed many of an owner's desires. Concrete is also fire resistant so long as the reinforcing within is placed properly. Concrete structures easily hit a 2-hour fire rating but can sustain a much higher rating if code requirements are met for the region it is being constructed in. Concrete's durability has been showcased in many of the structures in our history books! Today, admixtures, reinforcement detailing, connection design, and concrete mix designs can cater to extreme durability: time, fatigue, and weather.
5. Common Concrete Structures
As you can imagine, with a building material so common and with so many advantageous properties, many different types of structures utilize the homogenous mixture of sand, water, stone, and cement! Commonly seen concrete structures include warehouses, storm shelters, parking garages, stadiums, bridges, piles, poles, retaining walls, and sound barriers. However, many other types of structures like malls, hospitals, libraries, churches, museums, hotels, airports, offices, and correctional facilities all are commonly constructed with concrete (and many times precast)! Schools can be erected in a matter of weeks with precast and even feature a storm shelter within the building footprint! Because concrete can include insulation between concrete wythes, structures for residential and commercial, including data centers, fridges/freezers, and manufacturing facilities, are increasingly becoming more common. There is almost no limit to what type of structures can be concrete: the design and casting flexibility drives the industry to new, creative solutions.
6. Reinforced Concrete
One creative solution that has been implemented for well over a century is to reinforce concrete with various materials: the most common being steel. Advancements in concrete reinforcement led to the use of prestressing concrete to achieve span lengths that were once unattainable. Mild steel (rebar, welded wire mesh, etc.) is used in almost all concrete structures. In a precast plant, pre-tensioned cables are run continuously from one end of the form to the other (typically over 100ft and beyond can even range beyond 500ft). Once the cables are secured with high-strength chucks on both ends, a stressing machine grips the "live" end and pulls the tendons to their designed tension. Typical cables are made from 270ksi steel and have an area of steel less than 1in^2; the design tension is typically about 30,000 pounds per cable! Working together, the mild steel and pre-tensioned cables provide the strength and requirements for a concrete component. There is another type of prestressing that occurs in the field: this is referred to as post-tensioning. Concrete components are cast with pipes running the length, once the concrete has cured, cables are pulled through the pipes, chucked at either end and tensioned in a similar fashion to those in a plant.
What exactly does prestressing do? Prestressing aids in the stress mitigation and deflection resistance of a concrete member. If the pre-tensioned cables are tensioned below the center of gravity of the member, eccentric is introduced. This eccentricity will cause the concrete to bow upward and redistribute the tension and compression stresses. The added compression (without eccentricity) also subtracts from the tension stresses once a load is introduced. Concrete is known to be extraordinary in compression whilst relatively poor in tension; reducing tension wherever you can in concrete leads to a great design! Deflections can be thought of similarly: if a concrete beam is bowed upwards and load is applied, it will bow back downwards less than it would if it was flat, to begin with. Figure 1 shows this simply.
On the left is a prestressed concrete beam:
- Eccentricity is introduced, causing an upward bow.
- This upward bow introduces tension in the top of the beam, potentially causing minor cracking.
- Load is applied, the beam is now flat, and a potential crack has been compressed together.
On the right is a non-prestressed concrete beam:
- The beam is flat, to begin with.
- Load is applied, and the beam bows downward, introducing a potential crack at the bottom of the beam.
This figure is a simplified version of what structural concrete designers are looking at and designing against. Often, the load is so great that there is a downward bow for a prestressed beam after the load is applied - however, that typically only occurs during periodic maximum loading (full live load and other transitory loads). There are various stress limits for both tension and compression, as well as allowable deflection values that must be maintained per the code requirements.
8. Designing Concrete
Concrete structures all typically follow the same design process, whether they are plain concrete, mild reinforced, prestressed, or a combination of styles. A lateral analysis is performed, and the LFRS (Lateral Force Resisting System) is developed. Once the LFRS is determined, the lateral loads must follow a load path to those systems: through concrete, connections, across joints, and through reinforcement. Connection and concrete member design typically happen after the lateral analysis in an order that makes sense to the global design and/or project schedule. Designing concrete components has many variables since the material is so versatile: concrete strength, steel type, steel size, concrete depth and with, and reinforcement placement. Engineers, like myself, enjoy designing concrete structures because there are typically many solutions to the same problem, and we are tasked with discovering the most practical and economical option.
Being the "most commonly used construction material" means that most of us (or all of us) know what concrete is but may not fully understand what concrete can do or make. Often, concrete buildings look so incredible, but you may not know that behind that beautiful facade (cast-in or post-applied) lies a concrete wall or beam. Similarly, a building may have a pigmented surface color you never imagined could be concrete. Who knew concrete was such a solid building material?
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