New article in this months STRUCTURE Magazine focusing on Post-Tensioned Slab On Grade. We are looking forward to reading the forthcoming followups mentioned at the end of the article. Below is the link.
http://www.structuremag.org/article.aspx?articleID=1007
Wednesday, January 6, 2010
Tuesday, December 1, 2009
CRSI Parking Publication
We found a good source of information regarding Post-Tensioned Parking Structures on the CRSI website recently. This publication discusses the pros and cons relating to Post-Tensioned Parking Structures as well as designs, costs and maintenance of the system. Check it out at
http://www.crsi.org/pdf/ParkingStructures.pdf
Copyright © 2009 by KLG Corporation. All rights reserved.
http://www.
Copyright © 2009 by KLG Corporation. All rights reserved.
Sunday, October 25, 2009
Post-Tensioned Concrete Dead End Anchor Repair
Nobody enjoys repairs – on or off the job, during or after construction, fixing something is always a hassle. Unfortunately, post-tensioning is no different, but with the help of your post-tensioning material supplier, the aggravation associated with repairs can be minimized.
Everybody who has worked on a post-tensioned job knows that tendons break. Somebody core-drills without looking at the shop drawings, or uses a power-driven fastener, or a rock-pocket collapses . . . or the tendon assembly wasn’t quite up to specifications, and pulls free of the dead-end. Once the cable is broken, how it broke is less important than fixing it. The focus of this article is repair via dead-end replacement.
There are three typical situations that are best resolved by removing and the dead-end anchor.
Situation one is a tendon breaking in the end bay of a slab. With only a few feet to the edge-of-slab, there may not be a suitable place for a coupler, and opening the slab to couple the one broken tendon may require de-tensioning several others.
Situation two occurs when the cable quite simply pulls out of the anchor, generally attributed to wedges not gripping properly. With nothing remaining to couple to, the cable may need to be replaced in its entirety.
Situation three occurs when the concrete gives way during the stressing operation and the dead ends move abruptly toward the stressing anchor (also known as a “blowout”). The primary cause for this is under-strength concrete, most often from poor consolidation (but, in rare circumstances, from under-mature or frozen concrete as well).
Blowouts are sometimes an "easy" fix. If the anchors have not kinked the cables and are not damaged themselves, you can simply pull the tendon back into its original position. Dowels should then be drilled into the existing slab to help tie the new concrete to the old. Care should be taken not to damage any of the other existing post-tensioning cables during the dowel installation process. Before re-pouring the concrete, it is wise to double-check the anchor to verify that it is still attached to the cable correctly. Remember that any repair requires the approval and direction of the Engineer of Record!
However, if the anchor did kink the tendon, or if the anchor cracked (and inside an encapsulated anchor, how do you know?), it will be necessary to replace the dead end. There are three ways to do this.
First, in the case of a blowout, a pull-out failure identified immediately, or a break literally at the dead end, it may be possible to field-stress on a new anchor. Simply re-set the new head and proceed as usual. Field-seating a dead end can be done effectively and correctly by any knowledgeable and certified installer; consult your post-tensioning material supplier and the PTI Field Procedures manual for details. Please note that, while a poorly-stressed dead end will appear secure, concrete paste between the wedges will prevent them from moving during the stressing operation – generally preventing full force transfer or allowing the cable to pull free.
Second, your post-tensioning material supplier can provide a new tendon, with a new dead-end installed. Fishing a new tendon through an existing sheath is never fun, but it will get the job done.
Third, you can reset the tendon as a double-live stress. This has many benefits – the replacement tendon can be borrowed from material on-site, there’s no question about the quality of the field-stressing a dead end, and it’s possible to visually verify that the cable is holding at the dead end. Also, if something does go wrong, it isn’t necessary to re-remove the concrete and re-replace the dead end.
Re-setting the tendon as a double-live is fairly easy. Nail a new anchor and pocket former to the repair form, leaving about 6” of strand sticking out of the “dead” end (if your cable simply pulled out of the dead end, you may be able to get by with just exposing the outside face of the anchor). Repair the slab per the Engineer of Record’s direction. Once the repaired concrete has cured, remove the pocket former and use a hand-seating tool (or “wedge-setter”) to set the wedges. Stress normally from the live end. If the dead-end wedges are tightly fitted, the cable shouldn't move, but if it does, it will seat the wedges properly. You should mark and measure both tails and subtract the "dead" end movement to make sure it doesn’t get included as part of your elongation
Once the elongations have been approved, burn off both ends, install grease caps if necessary, and grout both pockets.
Copyright © 2009 by KLG Corporation. All rights reserved.
Everybody who has worked on a post-tensioned job knows that tendons break. Somebody core-drills without looking at the shop drawings, or uses a power-driven fastener, or a rock-pocket collapses . . . or the tendon assembly wasn’t quite up to specifications, and pulls free of the dead-end. Once the cable is broken, how it broke is less important than fixing it. The focus of this article is repair via dead-end replacement.
There are three typical situations that are best resolved by removing and the dead-end anchor.
Situation one is a tendon breaking in the end bay of a slab. With only a few feet to the edge-of-slab, there may not be a suitable place for a coupler, and opening the slab to couple the one broken tendon may require de-tensioning several others.
Situation two occurs when the cable quite simply pulls out of the anchor, generally attributed to wedges not gripping properly. With nothing remaining to couple to, the cable may need to be replaced in its entirety.
Situation three occurs when the concrete gives way during the stressing operation and the dead ends move abruptly toward the stressing anchor (also known as a “blowout”). The primary cause for this is under-strength concrete, most often from poor consolidation (but, in rare circumstances, from under-mature or frozen concrete as well).
Blowouts are sometimes an "easy" fix. If the anchors have not kinked the cables and are not damaged themselves, you can simply pull the tendon back into its original position. Dowels should then be drilled into the existing slab to help tie the new concrete to the old. Care should be taken not to damage any of the other existing post-tensioning cables during the dowel installation process. Before re-pouring the concrete, it is wise to double-check the anchor to verify that it is still attached to the cable correctly. Remember that any repair requires the approval and direction of the Engineer of Record!
However, if the anchor did kink the tendon, or if the anchor cracked (and inside an encapsulated anchor, how do you know?), it will be necessary to replace the dead end. There are three ways to do this.
First, in the case of a blowout, a pull-out failure identified immediately, or a break literally at the dead end, it may be possible to field-stress on a new anchor. Simply re-set the new head and proceed as usual. Field-seating a dead end can be done effectively and correctly by any knowledgeable and certified installer; consult your post-tensioning material supplier and the PTI Field Procedures manual for details. Please note that, while a poorly-stressed dead end will appear secure, concrete paste between the wedges will prevent them from moving during the stressing operation – generally preventing full force transfer or allowing the cable to pull free.
Second, your post-tensioning material supplier can provide a new tendon, with a new dead-end installed. Fishing a new tendon through an existing sheath is never fun, but it will get the job done.
Third, you can reset the tendon as a double-live stress. This has many benefits – the replacement tendon can be borrowed from material on-site, there’s no question about the quality of the field-stressing a dead end, and it’s possible to visually verify that the cable is holding at the dead end. Also, if something does go wrong, it isn’t necessary to re-remove the concrete and re-replace the dead end.
Re-setting the tendon as a double-live is fairly easy. Nail a new anchor and pocket former to the repair form, leaving about 6” of strand sticking out of the “dead” end (if your cable simply pulled out of the dead end, you may be able to get by with just exposing the outside face of the anchor). Repair the slab per the Engineer of Record’s direction. Once the repaired concrete has cured, remove the pocket former and use a hand-seating tool (or “wedge-setter”) to set the wedges. Stress normally from the live end. If the dead-end wedges are tightly fitted, the cable shouldn't move, but if it does, it will seat the wedges properly. You should mark and measure both tails and subtract the "dead" end movement to make sure it doesn’t get included as part of your elongation
Once the elongations have been approved, burn off both ends, install grease caps if necessary, and grout both pockets.
Copyright © 2009 by KLG Corporation. All rights reserved.
Minimum and Maximum Compression Forces In Post-Tensioned Slabs & Beams
Experience has taught me that when engineering post-tensioned structures utilizing the psi ranges specified by the Post-Tensioning Institute are good indicators of a design that is on the right track.
For buildings, a minimum F/A should be 125 psi and a maximum around 300 psi.
For parking structures, a minimum F/A should be 200 psi with a maximum of 500 psi in beams. During the design phase, the engineer should occasionally check the F/A in both slabs AND beams.
When engineering a standard two-way system for a building and the F/A is significantly above the minimum 125 psi, the floor slab may need to be thicker. For example, if the F/A is say 235 psi for interior bays of a flat plate, it would appear that the overall design needs looking into. If your design is approaching high a psi, check the following:
Slab thickness? Use L over 45 as a starting point for two-way slabs.
Skip live loading?
A wrong load entry?
Columns too small? Drop panels may be required.
Verify minimum cover at the high and low points
Check how much dead load you are balancing.
Make sure bending, shear and deflections are in line.
Was there an allowable live load reduction that was not utilized?
Normally, my rule of thumb for compression over the 125 psi would be no more than 20% for a 40 or 50 psf loaded slab. Anything over 20% should throw up a red flag. However for podium or transfer slabs, psi ranges are usually around the 300 mark. These types of slabs require special details to account for the higher compressive forces.
The correct F/A is extremely important for the following reasons:
Large compressive forces can increase the shortening of the slabs. This may cause cracking in core walls and columns.
Columns can be pulled out of plumb.
The floor slabs may camber upward at mid-span beyond acceptable tolerances.
Floor cracks may appear due to various restraints.
Engineering judgment is a critical factor in Post-Tensioning design. Do not over or under design.
Contact KLG for free design consultation.
Copyright © 2009 by KLG Corporation. All rights reserved.
Sunday, October 11, 2009
Double End Stressing At Construction Joints in Post-Tensioned Concrete Slabs
When strands must be pulled at the building exterior as well as at the intermediate construction joint of the first pour on a multiple pour level, the post-tension cable installer must not leave an excessive length of strand protruding from the building exterior side. We at KLG have had success with one foot of strand protruding.
When the time comes to stress the cables in the first post-tensioned concrete pour, it is absolutely imperative that the cable installer pull each strand from the intermediate joint prior to pulling the ends at the building exterior. This procedure ensures that the elongations from pull number one are added to the overall length of the post-tensioned slab strands in pour number two. This procedure helps ensure that the length of the cable remaining in pour number two will be sufficient enough to extend past the bullhead and be stressed by a standard ram.
If too much cable is protruding from the slab edge in pour number one and the stressing sequence above is not followed the potential exists that there will not be sufficient length of strand extending past the slab edge in pour two. If the previous scenario exists the post-tension strands may have to be de-tensioned, pulled back into pour number two and then re-tensioned. The bare strand now exposed at the intermediate construction joint must be wrapped by an approved method in order to prevent the strand from bonding to the concrete. If the cable is not wrapped, the elongations will most likely be short do to the reduction in length of the unbonded strand. The de-tensioning process is dangerous and very time consuming... it's a lot easier to do it right the first time.
For additional information and details see page 15 of the 3rd edition of the Post-Tensioning Institutes Field Procedures Manual.
As a rule of thumb, the first stressing procedure for the cables passing through the construction joint in a two pour post-tensioned concrete slab should be done at the intermediate joint. This holds true for single end stresses and double end stresses.
Always ask your post-tension cable supplier if you have any questions regarding this stressing procedure.
Copyright © 2009 by KLG Corporation. All rights reserved.
When the time comes to stress the cables in the first post-tensioned concrete pour, it is absolutely imperative that the cable installer pull each strand from the intermediate joint prior to pulling the ends at the building exterior. This procedure ensures that the elongations from pull number one are added to the overall length of the post-tensioned slab strands in pour number two. This procedure helps ensure that the length of the cable remaining in pour number two will be sufficient enough to extend past the bullhead and be stressed by a standard ram.
If too much cable is protruding from the slab edge in pour number one and the stressing sequence above is not followed the potential exists that there will not be sufficient length of strand extending past the slab edge in pour two. If the previous scenario exists the post-tension strands may have to be de-tensioned, pulled back into pour number two and then re-tensioned. The bare strand now exposed at the intermediate construction joint must be wrapped by an approved method in order to prevent the strand from bonding to the concrete. If the cable is not wrapped, the elongations will most likely be short do to the reduction in length of the unbonded strand. The de-tensioning process is dangerous and very time consuming... it's a lot easier to do it right the first time.
For additional information and details see page 15 of the 3rd edition of the Post-Tensioning Institutes Field Procedures Manual.
As a rule of thumb, the first stressing procedure for the cables passing through the construction joint in a two pour post-tensioned concrete slab should be done at the intermediate joint. This holds true for single end stresses and double end stresses.
Always ask your post-tension cable supplier if you have any questions regarding this stressing procedure.
Copyright © 2009 by KLG Corporation. All rights reserved.
Friday, September 25, 2009
Post-Tension Cable Repair? KLG Does The Work!
Have you run into a situation similar to the one in the pic at the top left? Did the plumber forget a sleeve before a concrete pour, then decide to fire up the core drilling machine and inadvertently slice through a few post-tensioning cables? If so, rest assured that KLG Corporation can help. KLG Corporation has experienced staff members with the know how to get your repair designed, detailed and constructed. With well over four decades of combined engineering and construction experience specific to the post-tension cable industry, we are well prepared to tackle even the most sophisticated repair or tenant finish... i.e. adding an elevator core or stair penetration to an existing post-tensioned concrete slab. Let our team of experienced professionals handle your repair from start to finish.
Copyright © 2009 by KLG Corporation. All rights reserved.
Tuesday, September 22, 2009
Post-tensioned Cable In Concrete Slabs Stressed At Low Air Temperatures
One of the most common issues with post-tension strand installation is the elongation report. As a check to ensure that proper forces have been achieved within the strands, the ACI mandates that field-measured elongations match the calculated elongations within ±7%.
Everyone realizes that low temperatures are a significant headache in concrete construction. Aside from the danger of concrete freezing, there are several post-tensioning issues that can be aggravated by extreme cold.
As we mentioned in our post on equipment usage, low temperatures aren't good for the post-tensioning ram. The pressure gauge tends to drift away from calibration, the gauge doesn't always register correctly, and, of course, the hydraulic fluid has the consistency of molasses. A cold ram can perform very strangely in the field, and then work perfectly in the (warm) shop.
In addition to this, elongations may consistently come up low. This can be attributed to the post-tensioning coating having increased viscosity at low temperatures. This increased viscosity will create more friction between the strand and the sheathing, thus resulting in lower field elongations.
While cold (but not frozen) post-tensioning grease can shorten elongations, the elongations generally fall within the ±7% tolerance allowed by the ACI. However, in situations where there are other factors, this may become the proverbial straw that broke the camel's back. Forewarned is forearmed -- at low temperatures, make sure your strand is well marked and your pockets are clean and your equipment is warm!
Related Posts:
Suggested Post-Tension Cable Marking & Measuring
Clean Post-Tensioning Anchor Pockets
Copyright © 2009 by KLG Corporation. All rights reserved.
Everyone realizes that low temperatures are a significant headache in concrete construction. Aside from the danger of concrete freezing, there are several post-tensioning issues that can be aggravated by extreme cold.
As we mentioned in our post on equipment usage, low temperatures aren't good for the post-tensioning ram. The pressure gauge tends to drift away from calibration, the gauge doesn't always register correctly, and, of course, the hydraulic fluid has the consistency of molasses. A cold ram can perform very strangely in the field, and then work perfectly in the (warm) shop.
In addition to this, elongations may consistently come up low. This can be attributed to the post-tensioning coating having increased viscosity at low temperatures. This increased viscosity will create more friction between the strand and the sheathing, thus resulting in lower field elongations.
While cold (but not frozen) post-tensioning grease can shorten elongations, the elongations generally fall within the ±7% tolerance allowed by the ACI. However, in situations where there are other factors, this may become the proverbial straw that broke the camel's back. Forewarned is forearmed -- at low temperatures, make sure your strand is well marked and your pockets are clean and your equipment is warm!
Related Posts:
Suggested Post-Tension Cable Marking & Measuring
Clean Post-Tensioning Anchor Pockets
Copyright © 2009 by KLG Corporation. All rights reserved.
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