In order to clarify the differences between precast concrete and traditional concrete construction, it is important to define the types of concrete construction.
Traditional concrete construction is often called cast-in-place concrete. This is when steel or wood forms are assembled on the project site and at the building component’s final location. Steel reinforcing bars are placed inside the forms, and then wet concrete is placed into these forms, encasing the steel reinforcing bars. Once the concrete hardens and has sufficient strength, the forms are then removed, and the hardened concrete is exposed. The hardened concrete is in the geometric shape of the forms. The time from placing the wet concrete until the forms can be removed depends on factors such as the type of concrete used, ambient temperatures, etc. Usually, this time period ranges from seven to twenty-eight days.
Precast concrete construction is when a concrete member is cast in a place other than where it is used. Normally, precast concrete members are made in a factory and later transported by trucks to the project site. High-performance, early-strength concrete is used in the manufacture of precast concrete. This, along with advanced curing methods, allows the concrete members to be removed from their forms very quickly. Usually, the precast members can be removed from the forms within twelve to sixteen hours after the wet concrete has been placed. The types of forms vary but are usually long steel forms in which several members can be cast end-to-end in the same pour. Wood and/or fiberglass are often used to make the forms for odd-shaped or unusual members.
Prestressed Concrete Explained
Prestressed concrete is a concrete section or material that has been “stressed” by steel tendons inside of the concrete member. Prestressing is accomplished by two basic methods. These are pre-tensioning and post-tensioning. The “pre” in pretensioning indicates the steel tendons are stressed prior to the wet concrete being placed in the forms. The “post” in post-tensioning means the tendons are stressed after the concrete has hardened and reached ample strength. Nearly all prestressing for precast members is accomplished using the pretensioning method.
Prestressing is used to enhance the tensile strength of the concrete member. This is beneficial because concrete is very strong in compression (a pushing type of force) but not nearly as strong in tension (a pulling type of force). The concrete compressive strength is approximately ten times the tensile strength. Prestressing “clamps” the concrete member with a compressive force that enhances the tensile strength of the member. This pre-loaded compression must be overcome before the concrete experiences any tension that may cause cracking. The result of prestressing is that for a given member shape or geometry, larger loads can be supported over a greater distance than similar members that are not prestressed.
A simplified, basic explanation of prestressing is described below.
- Imagine lifting a number of books by placing your hands on either side.
- To prevent the books in the middle from falling as you raise the books, you press inward on either end of the stack with your hands.
- If the pressure your hands exert on the books is sufficient, the entire stack can be lifted as a single unit. This is “prestressing” the line of books.
- Furthermore, if the force exerted on the books is sufficient, then you can place an object on top of the stack without it falling apart.
How is Pretensioning Accomplished in the Precast Facility?
The main component used in pretensioning is a high-strength steel 7-wire prestressing strand. These strands are made up of 7-individual wires with six wires wound around a central wire. These strands have a tensile strength of more than four times the tensile strength of normal reinforcing bars used in cast-in-place concrete. These strands are strategically located within the cross-section of the precast members. The strand locations are determined by evaluating where the resulting precompression force will be most beneficial. The strands are manufactured in different diameters, the most common being 3/8”, 1/2”, 9/16”, and 0.6.
Usually, pretensioned concrete members are made using a long-line steel form. The strands are delivered to the precast factory in circular bundles. The strands are pulled from the bundle and along the full length of the form.
These strands are anchored at one end of the form and then stretched by a hydraulic jack to a specified load at the other end of the form. After the pulling is finished, the strands are then anchored at each end to lock the force into the strands. The stretching or elongation of the strands is significant. For example, for a 400-foot-long form, the elongation of each strand during stressing is almost three feet!
The strands are anchored using “chucks”. These chucks consist of conical steel wedges, which are encased in a steel cylinder. As the strand force is released into the chuck, the toothed wedges are engaged and grab the strand, locking it into the chuck assembly.
Once the strands are locked, any additional reinforcing, embedded items, etc., are placed within the form. After Quality Control inspectors check all items, the form is ready for placement of the concrete. After all the concreting is done, the form is covered, and curing heat is applied as needed.
Approximately twelve to sixteen hours later, the form is uncovered. Using an oxyacetylene cutting torch, the prestressing strands are cut at the ends of the form and in between the individual members. At this point, chemical and mechanical bonds have formed between the strands and the surrounding hardened concrete. Once a strand is cut, its internal tension force is transferred from the strand into the hardened concrete, which results in the desired precompression force in the members. The members can now be removed (stripped) from the form.
The steps described are shown below.
An example of a pretensioned double tee member being stripped is shown below.
Why are Precast Concrete Members Prestressed?
Since precast members are made in one location and used in another, they must be picked up and moved several times during their life cycle. The products are first removed from the forms, transported to a touch-up area, moved into storage, picked up again and loaded onto a truck, transported to the project site, and finally picked up and moved into their final location on the structure. Each of these activities puts stress on the precast member, which could cause damage. Using prestressing enhances the resistance to the applied stresses and thus limits or precludes cracking in the concrete during these movements.
Other benefits of prestressing the precast concrete components are:
- Because of the high resistance to stress, larger individual pieces can be made. Often, two pieces can be combined into one piece. This results in fewer joints between pieces, fewer connections needed, and fewer pieces to be handled and transported. Pieces can be made longer, higher, and wider.
- Shallower beams, smaller columns, and thinner walls than those used in normal concrete construction can be used to resist project-specified loads. This leads to very efficient use of materials and economic structures.
- Prestressing allows longer floor and roof spans and greater clear heights, which improves the interior use of space and enhances the user experience.
- Because of this efficient material use precast, prestressed concrete construction is very environmentally friendly.
- Very little, if any, member cracking occurs in precast, prestressed members. Visible cracks cause concern to the general public.
- Because precast, prestressed concrete components have little, or at most, minor cracking, there is no path for the intrusion of water into the concrete, which can cause further damage to the concrete or result in unsightly staining. Durability and long-structured life are the results.
Summary
The use of pretensioning in precast, prestressed concrete components improves the performance of the individual members and the overall structure. Tindall Corporation has been at the forefront of state-of-the-art pretensioning use and procedures for many decades and continues to strive to improve the use of pretensioning to benefit its clients and the public. For more information concerning this topic, please contact your nearest Tindall manufacturing facility.
