What are prestressed concrete ducts?

Question

I need help on info for a small section of an engineering assignment on prestressed concrete ducts including:
-what it is
-where and when used
-techniques used
-costs
-problems in usage
-maintenance requirements
or anything similar. Thanx.

Answer 1

bd gave a very good answer. I would like to add to it by showing you the prestressing manual that Caltrans uses to do their concrete box girder bridges. Take a look at it, it is very good to see what is involved in the operation.



http://www.dot.ca.gov/hq/esc/construction/manuals/OSCCompleteManuals/PrestressManual_Rev1.pdf

Answer 2

I don't know how far along in your engineering program you are... so forgive me if my answers are a bit simplified. I just want to make sure I'm actually helping you, so I'll be somewhat basic.



Simple answer, they are used in the process of strengthening concrete for structural purposes.



Let's begin by saying that concrete alone is very strong when compressed, but very weak in tension. When you make a beam out of concrete and load it from the top with floor, roof, people, or snow, it causes bending in the beam. When you bend a beam it causes compression in the top of the beam and tension in the bottom. To keep the concrete from cracking, since it is weak in tension, it is common to place reinforcing steel near the bottom of the section to provide strength to the concrete beam. These are the general principals for reinforced concrete design.



Prestressed concrete design takes these same ideas a step farther. Why wait for the concrete to crack before the steel begins taking significant load? Why not stretch (stress) the steel ahead of time, before the beam is loaded?



One style of prestressing concrete is called post-tension. In this method, the concrete will be poured, partial cure will be allowed and then the tendons (steel cables) will be stressed to induce the prestressing forces. Post, because it it stressed after the pour. When you have large sections or loads, it is common to need several tendons to be bundled together. This is typically accomplished by creating a void in the concrete with a prestressed concrete duct.



I have not designed with these types of systems before, but have seen design examples in bridge work to be more common.



Unfortunately I am unaware of the cost of this type of system.



One part of the method which I am uncertain about is the transfer of load to the concrete. It is possible that there are two methods, but there may be one that is more industry standard. One method is through end anchorage. The ends of the strands are anchored to the face of the concrete after they are tensioned. The other is to grout the duct solid once the tendons are stressed. The first method transfers stress through bearing, the second method transfers stress through bond.



I would guess the main concern, especially with bridge applications, would be corrosion to the strands due to infiltration of water. Likely one of the benefits of fully grouting the duct once the tendons have been pulled is reducing water infiltration.



Hope that helps.

Answer 3

Prestressed concrete is that one in which the concrete pile or gurder are make with tensioning to reinforcement in laboratory or site at the time of it built. and post tension concrete is that one in that the tension are gives after placing