Prestressed And Precast Concrete
Chris Richard
What is the purpose of prestressing concrete if it is already subject to significant stress when supporting a load? Imagine building a concrete beam but not including any reinforcing bars or stirrups. It would have a very limited capacity for supporting any load. Before too long as the loading is increased the plain concrete beam will start to crack in the bottom as shown in the sketch below because concrete is inherently weak in tension. The cracks will propagate upward until they reach the top of the beam, and it collapses suddenly as graphically depicted in Figures 1 & 2 below.
But prestressed concrete takes the design of flexural members like beams to a whole new level. This is accomplished by pre-compressing the concrete in the bottom of the beam by pre-tensioning the tensile reinforcing such that it tries to retract much like a rubber band around some wooden blocks. This is graphically depicted in Figure 4 below.
Concrete is a strong material with a long history. It has uses in forming siding, foundations, driveways, and columns. Its versatility, including possible reflectivity, and its relatively low cost, are the reasons why this construction component became and has remained so popular.
Regardless of its application, concrete has traditionally been poured onsite as needed. Newer methods of concrete component construction have been developed, however, such as precast and prestressed concrete, that allow for the offsite prefabrication of concrete slabs and products. These prefabricated products can be used in commercial, industrial, infrastructure, and residential projects and have nearly limitless possibilities.
Precast concrete is a product cast somewhere other than where constructors will use it. Most are cast in a factory through a wet-cast method, but some sites opt to create them like tilt-up panels. The wet-cast process involves pouring concrete into a mold and then vibrating it.
The concrete stays in the mold until it hardens and someone removes it. To create the concrete, contractors mix water, cement and aggregates like sand. Aggregates usually make up about 60% to 75% of the mix.
Builders often use precast concrete in larger, multi-resident structures such as apartment buildings, hotels, and nursing homes. The material offers excellent fire resistance and sound control for individual units. Also, precast concrete is popular for office buildings due to its durability. Its smooth surface spans long distances making it ideal for storage structures.
One of the benefits of precast concrete is it's manufactured in a controlled environment. This makes it easier to work with the mix and find the proper placement. The quality of the product can be controlled, creating more durable material. Since laborers can buy materials for multiple projects, this leads to lower costs.
One of the downsides of this material is the high initial investment. In fact, one cubic yard may cost between $300-$420. Heavy and advanced machinery is necessary for production plants. Another concern is transportation. The offsite construction site may be further away from the manufacturing area, increasing the risk of damaged pieces and fuel overuse.
Precast concrete is helpful for offsite construction projects, like building prefabricated homes. Workers can produce the material in tight spaces, which is ideal in cities. Since the product isn't shipped to the job site until ready, the area has less clutter.
In addition, the quality is better since the process takes place in a factory with strict guidelines. Technology can also minimize errors common in onsite construction. The job requires fewer workers, which lowers the risk of injury. The precast concrete process is also quicker, speeding up the project schedule.
Prestressed concrete is where contractors use initial compression before applying the external load. Workers insert high-strength steel wires into the beam and stretch and anchor them. They then pour concrete into the formwork, allowing it to harden around the steel strands. Constructors then remove the formwork and cut the steel strands. This compressed material is less likely to crack under external forces.
Due to its high tensile length, builders use prestressed concrete in commercial construction projects. These include things like parking garages and shopping centers. In addition, they can utilize it in auditoriums, gymnasiums and cafeterias because of its acoustic properties and ability to provide open spaces.
A high degree of artistry and control is necessary to create the product. This can entail more pressure to find qualified candidates for the job. Also, these steel materials can be more expensive than traditional building elements. Other costs include special equipment and safety regulations.
One of the benefits of prestressed concrete in offsite construction is removing limitations. It reduces the barriers placed on spans and loads for roofs, floors and bridges with longer sections. That way, contractors can build lighter and shallower structures without sacrificing strength.
Precast concrete is made from molds, usually offsite. The prestressed concrete is similar but gest reinforced with steel compression. Both have multiple applications, from commercial spaces to home features like countertops.
Each is helpful in offsite construction because they offer a quicker turnaround and more efficiency. Plus, controlled settings can reduce waste from manufacturing processes. The one thing to keep in mind is the initial cost may be higher and it requires more extensive equipment.
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Background:
Driving prestressed-precast concrete piles (PPCP) is one of the options among various types of piles and installation methods that conforms to the principals of Accelerated Bridge Construction since it employs pile segments prefabricated in precast plants and delivered to the site for installation. This option provides in many cases an economic and rapid alternative to other types, therefore reducing the construction time overall in line with the benefits promised by ABC methods. However, traditional prestressed piles that use carbon steel strands and bars are prone to corrosion, especially when they are in marine environment. There have been attempts and investigations into the use of alternative prestressing strand and reinforcing material that are corrosion resistant. The use of Carbon Fiber Reinforced Polymers (CRFP) and High Strength Stainless Steel (HSSS) for strands and other reinforcement in concrete piles have shown great improvements in the resistance against corrosion.
It often happens that shipping and transportation constraints or other reasons limit the length of PPCP segments that can be delivered to the bridge site. Variable and unforeseen soil conditions may also require longer piles than anticipated. Hence, splicing of pile segments has to be performed at the site to achieve longer lengths using various types of joints. FDOT has used splicing methods and has undertaken efforts to develop more effective and corrosion resistant joints for their marine environment. Because of lack of understanding of the structural behavior and sometime associate complexity and cost, their use has been very limited and scarce. On the other hand, much has been done in relation with ABC connections and details for sub- and super-structure joints and connections, and a variety of new and effective joints have been developed and are in use. The aim of the proposed study is to build upon the experiences gathered in general for ABC connections and develop an effective yet simple splice connection for PPCP using alternative configuration and new materials. The focus will be on connection types that are easy to implement, provide adequate strength, and reduce interruption to operation.
Objective:
The objective of this project is to explore alternative pile splice connection configurations and materials, and to investigate the feasibility of these connections in comparison with the existing epoxy dowel splice for prestressed-precast concrete piles.
This research project focuses on the use of analytical modeling and computational means for investigation on the structural behavior for newly developed conceptual designs. Future activities, within a separate project, will include performing experimental verification of the newly developed details.
This paper describes a three-dimensional approach to modeling the nonlinear behavior of partial-depth precast prestressed concrete bridge decks under increasing static loading. Six full-size panels were analyzed with this approach where the damage plasticity constitutive model was used to model concrete. Numerical results were compared and validated with the experimental data and showed reasonable agreement. The discrepancy between numerical and experimental values of load capacities was within six while the discrepancy of mid-span displacement was within 10 %. Parametric study was also conducted to show that higher accuracy could be achieved with lower values of the viscosity parameter but with an increase in the calculation effort.
The most common problem reported with the use of partial-depth deck panels is reflective cracking on the top surface of the deck. Cracks in the transverse direction of the bridge may form at locations at which adjacent panels are placed (panel edges), while cracks in the longitudinal direction may form at the locations at which the panels are supported on the girders (panel ends).
The cause of the transverse reflective cracks is attributed primarily to the concentration of shrinkage and stress of CIP concrete at the joints between the precast panels (Hieber et al. 2005) (Fig. 1a). Transverse reflective cracks generally raise a deterioration concern because they permit the ingress of moisture and corrosion agents of steel reinforcement in the deck (Fig. 1b). When reflective cracks extend the full thickness of the CIP concrete layer, the ingress of moisture and corrosion agents can be concentrated at the panel edges (Fig. 1c), which has been observed to cause corrosion of steel prestressing tendons at the panel edges and spalling of concrete along the panel edges (Fig. 2) (Wieberg 2010; Sneed et al. 2010). Critical combinations of panel geometry, material properties, and reinforcement details can lead to long-term serviceability problems (Young et al. 2012; Wenzlick 2008).
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