Hill and Griffith
Video: Precast Prestressed T Form Construction - Including Concrete Form Oil Application
Learn how a double T structural prestressed concrete member is plant produced.
Thanks to the Florida Precast Concrete Association.
This video will explain the entire process of producing a double T prestressed concrete unit at a prestressing plant. A double T is used primarily in parking structures, but can be used in virtually any type of building including office buildings, industrial buildings, and schools to name a few.
Randy Neumeyer: Double Ts are kind of a unique product with having the capability of having a very long span. When we start talking long spans, you're talking 60 feet plus, and sometimes go as high as 120 feet.
Narrator: The typical day of a prestressing plant usually starts very early in the morning. The product from the previous days' casting has cured by now, and the pieces need to be stripped from the form to get ready for today's casting.
For any prestressing plant, it is very important to maintain a clean plant. Maintaining neatness is not only positive from an image standpoint, but also from a safety standpoint, since any debris that might be leftover from the day before could create a tripping hazard or other safety problem.
The first part of the process is setting up of the form. A thorough cleaning of the entire form is done with basic labor and appropriate cleaning equipment. Concrete form oil is applied. Setting the side forms, or in the case of a typical double T, the forms may be fixed forms with an appropriate draft to allow for easy stripping. Once the bed is cleaned, it's ready for setting up today's cast.
This particular bed is set up for five double Ts, each about 60 feet long. The bed has a total length of 400 feet. This is a self stressing form, where the form itself is designed to take the total prestress force in compression, and also to handle the eccentricity of the strands. Sometimes the producer may use a form that is set between bulkheads that are built right into the ground. The prestressing force is then resisted entirely by the bulkheads and their foundations.
Next, the strands are placed in the form and extended from end to end. Crew members thread the strands through each of the dividers, and then run them through the stressing plates at each end. They typical spacing of the strands is approximately two inches. This divider is a steel device that separates the individual double T units from one another. A gap of about a foot is typical between the ends of the adjacent pieces. This gap is required so the strands can be accessed and cut, and that each double T can be stripped from the forms.
The next step is the tensioning of the strands. In this case, we have a strand pattern with straight strands of 6 strands in each stem, as shown here. The strands are first tensioned to about two to five kips based on the gauge pressure reading. In this case, a mark is made on the strand after the preload is applied. The strand is then tensioned to the specified level and is measured using the gauge pressure on the hydraulic pump.
The QC technician then checks the elongation against the theoretical elongation. The requirement that is the force on the gauge and the elongation must match within five percent. If that tolerance is not achieved, the reason must be established, and the strand my have to be de-tensioned and re-tensioned.
After the strands of this double T are all stressed, the remaining embeds are set in place. These include stem reinforcing, lifting devices that will be used to strip and handle the piece, forms for blockouts in the flange, and forms for blockouts in the stems. Flange reinforcing and flange connectors or vectors along the edges.
In this case, one of the double Ts in the bed has typical flange connectors applied along one edge, and special flat plate connectors applied along the other edge, to accommodate connections across an expansion joint.
The form is now ready to have concrete poured, but only after a pre-pour quality control check has been made, and the QC person signs off on it. This QC check can also be done by the bed superintendent and depends on the particular plant's practices. Either way, the check is critical to ensure an end product with the highest quality.
Russell D.: There's always things you look for. The mesh. You look for anything that could be touching the stems on the bottom, any wire ties that might've fell in there, 'cause that would develop some type of little rust spot.
You check the length. You wanna make sure that the damps are correct. Do we have the strand stripped properly. So there are a lot of critical areas that you need to verify.
Narrator: Concrete is mixed at the batch plant under tightly controlled conditions. The cement is supplied using tanker trucks. Here, the cement is being blown from the tanker, through piping, to the cement silos.
When needed, aggregates are delivered to bins in the batch plant. The aggregates are stored outside of the batch plant in separate bins. In this case, an enclosed vertical bin system is utilized. During winter, in northern climates, the aggregates must be heated to get them to float properly through the batching system and to maintain proper concrete mix temperatures.
Payloaders are used to shuffle materials to the conveyor belt. Computers are used to proportion the materials for the desired concrete mix. Cement, sand, stone, water, and add mixtures are measure according to signals from the computer.
Resources from the Florida Precast Concrete Association
Discover the Freedom of Precast Concrete