Post-tensioned slabs are almost unheard of in residential work in the northeast. We’ve used them for parking structures, though.
It is the same around here in Michigan.
Post tension here in Cruces is not common. I would have not known if the builder would have not said something. The city here requires styrofoam on the stem walls which would cover the wedges and bolts used at the stem walls. I am going to call for a mark or sign. I need to talk with the city slab guy because this could be dangerous in the future if someone decides to cut into this slab for some reason.
I didn’t think so, as I said I’ve not seen one marked here. It’s a state req to have the insulation around the slab, so some may be pt slabs, but I wouldn’t know unless told like you were on this one.
I asked both concrete co. and the local architects, and neither seems to see them frequently up here. I was curious if you had seen something different there, but it sounds similar.
I am one of those that have not seen this type of tensioning and therefore looking into finding more information like all else.
I thought it would be appropriate to share what I find so we can all learn.
What is Post - Tensioning?
Simply put, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tensioning is a method of prestressing. Prestressed concrete or masonry has internal stresses (forces) induced into it during the construction phase for the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle.
There are two methods of prestressing. One is called pre-tensioning. This method consists of stressing the reinforcing inside of large steel buttresses, and then casting the concrete around the reinforcing. This method can only be done at a precast manufacturing facility and requires the completed prestressed concrete members to be trucked out to the job site and then assembled.
The other method of prestressing is called post-tensioning. Instead of stressing the reinforcing inside of large steel buttresses at a manufacturing plant, the reinforcing is simply installed on the job site after the contractor forms up the slabs or constructs the walls. The reinforcing steel is housed in a sheathing or duct that prevents the steel from bonding to the concrete so that it can be stressed after the concrete cures (hardens). Using the post-tensioning method of prestressing enables a builder to get all the advantages of prestressed concrete or masonry (described below) while still enabling the freedom to construct the member (slab, wall, column, etc,) on the job site.
**WHY DOES CONCRETE AND MASONRY NEED TO BE REINFORCED **
Concrete, masonry, and most cement based products are very strong in compression, or, in other words, they have a high capacity to resist compressive forces. Compressive forces can be described as crushing forces. Concrete has a very high compressive strength. It can be anywhere from 2,500 pounds per square inch, in most residential foundations, to 4,000 psi in suspended slabs and walls in buildings, to even higher strengths in bridges. However, concrete is relatively weak in tension, i.e. it doesn’t resist tensile forces very well. Tensile forces are the forces that pull an element apart.
***Tensile forces pull apart the bottom of this concrete slab when it bends ***
Conversely, steel is very strong in tension. It has a high capacity for resisting the forces that pull apart or bend it. Therefore, combining reinforcing steel with concrete or masonry results in a product that can resist both compressive forces and tensile forces. Additional, substantial benefits can be obtained by using the reinforcing steel to “squeeze the concrete together”, or place it in compression. Compressing the concrete increases it tensile (bending) strength. By increasing the tensile strength of the concrete itself (making the concrete slab or masonry wall stiffer), a designer can achieve longer spans with thinner concrete sections.
Putting the concrete into compression also helps to resist the development of shrinkage cracks. Shrinkage cracks, while typically not detrimental to the performance of the structure, can be unsightly, and can allow the passage of moisture or termites. Shrinkage cracks will develop in most cement based products as the water combines with the cement and the concrete cures (hardens). The more the concrete is “squeezed together”, the less likely it is that shrinkage cracks will develop or open.
**WHAT KIND OF MATERIALS ARE USED IN POST-TENSIONING **
Post-Tensioned reinforcing consists of very high strength steel strands or bars. Typically, strands are used in horizontal applications like foundations, slabs, beams, and bridges; and bars are used in vertical applications like walls and columns. A typical steel strand used for post-tensioning has a tensile strength of 270,000 pounds per square inch. In comparison, a typical non-prestressed piece of reinforcing (rebar) has a tensile strength of 60,000 psi . Strands typically have a diameter of ½ in., and are stressed to a force of 33,000 pounds using a hydraulic jack.
The prestressing steel is housed in a sheathing or duct to allow it move as the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned, and it is locked into place using an anchoring component that forms a mechanical connection and keeps the force in the strand for the life of the structure.
**USES AND ADVANTAGES **
Post-Tensioned reinforcing has been used for many decades in bridges, elevated slabs (parking garages and residential or commercial buildings), residential foundations, walls, and columns. The use of post-tensioned reinforcing can result in thinner concrete sections, longer spans between supports, stiffer walls to resist lateral loads, and stiffer foundations to resist the effects of shrinking and swelling soils. The additional advantage of putting the concrete into compression can be used to construct slabs and walls that have fewer visible cracks that can allow the passage of moisture and termites.
**The above details are common for post-tensioned foundations. These are located and noted on the foundation plan below. In normal post-tension construction, footings are 12 inches wide, but when deep brick shelves are inserted, footings must be at least 18 inches wide. **
http://www.posttensionslabs.com/images/foundation_plan_sm.gifTypical Foundation Plan Showing Effective Bearing Width
**The interior footings and depth of slab are all noted on these details. Since details change, following the submitted engineering plan provided to you for your particular project is advised. **
**This plan and details are typical post- tensioned. If you notice, the footings (ribs) run from the front to the back and from the left to the right of the foundations. The current building code and PTI (Post-Tension Institute) recommend that footing spacing for one and two family dwellings be at approximately 17 feet on center each way. **
**A typical post-tensioned slab contains very little conventional steel. There may be sections that are too short to put our tendons in, so there may be instances where some conventional rebar reinforcement may be utilized. There are inside corners where concrete cracking would possible occur, so as you will see on the plan layout, there are some corners that will have three #4 (1/2") rebar 10 feet long laid in the corners to minimize cracking. The x pattern indicates an elevation change. Reference the architectural details for all drops, offsets, elevation changes, etc. **
One also notices, that our tendons run from the front to the back and also left to right throughout the slab thickness and footing areas. Arrowheads denote the tendons. One arrowhead indicates one tendon and two arrowheads indicates two tendons, one below the other. Normal post-tensioned footings are 12 inches wide, but when deep brick shelves are inserted, footings must be at least 18 inches wide.
**Never seen this in Maine and most likely will not. Sure is fun learning though. **
**Well, this help me a little, so I hope it dose the same to some. ha. ha. **
These are what I get to deal with daily
Hope searching this site helps you and others
http://www.houston-slab-foundations.info/ they’re real helpful if you have questions
Although Houston, TX is about 250-300 miles away we have the same and even worse soil conditions right here in my backyard (work radius)
Thanks for the information Barry, it really adds to my knowledge of what I never experieced before.
Oh, and thanks for the Basketball commentator presentation, I don’t think I heard a word that was said. :mrgreen:
Glad you enjoyed
She does a great presentation
I’ve learned to LIP read
Very helpful for just these occasions
Thanks Mark, and what kind of machine is used to stress the cables, and how is it attached permanently to the slab?
What compressive strength of concrete do they use?
Noticed what looks like a control joint or tooled joint, is that required?
A hydraulic ram is used to stress the cables and steel wedges (grippers) are inserted at the end to secure them.
Minimum psi for prestressed concrete is 3000.
Joints are added at the discretion of the engineer. Due to the compressive nature of the tendons, large slabs can be poured without the need for any control joints.
Thanks Jeff, and Merry Christmas
Thats what I was going to suggest. Ask Jeff he is an expert in the PT field… He has helped me before.
I have yet to see a stamp in my neck of the woods. If it was there and I didn’t see it I know my buyers would and ask about it. Do any of the other TX inspectors see them?
sorry answered my ?'s after reading the earlier posts.
Hi. I am considering installing an drop-in anchor in or garage (Baldwin Park, FL), but don’t know if our house has post tension garage slab. Based on your expertise, do houses in Baldwin Park have the post tension cables built (the house constructed around 2005.
thanks in advance.
To all builders, home inspectors, etc. let’s be very clear - under NO circumstances should post tension cables be used for any slab- residential or commercial- in a seismic area like Southern California or North Texas, Oklahoma, Missouri, etc. This is dangerous and in an earthquake could cause fatal injury to the owner or even rescue workers, etc. Quite frankly(despite) what a Structural Engineer may tell you the experience of time outweighs the post tension engineering theory (I am a trained engineer by the way) in other words good old fashion rebar was used (and still is by many) for many years with virtually no problems whatsoever with fantastic structural performance. Yes unfortunately builders in Texas have used this technique more and more with little or no rebar,which is simply unacceptable from my standpoint. Houston builders use it a lot and although the local geological classification is not that of a seismic zone nevertheless Houston has a high water table AND “elastic” soil the foundations can still shift although not nearly dramatically as in an earthquake, however, I have heard reports of post tension foundations cracking in Houston (significantly). I have never seen or heard of a slab with significant rebar cracking in my entire life. Period. Mark my words! If you build a house without rebar in the foundation you are ignoring literally over a hundred years of existing success! Ironically installing rebar is even cheaper than using (and stressing) steel cables! Why would anyone use this technique? Answer: Just don’t do it! Think Rebar. Rebar!! (And leave the stress to someone else, ha ha)
Licensed Home Inspector- State of Texas