02-27-2014 Report

Meeting Recap – February 27, 2014

Post Tension Inspection & Repair in Concrete02-27-2014mtg

Keith Batchelor, Georgia Construction Products

We had an enthusiastic crowd of nearly 50 attend the February meeting of the ICRI GA Chapter at Maggianno’s Italian restaurant at Perimeter Mall.  Keith Batchelor, Vice-President welcomed everyone and thanked them for the support of our organization.  There were over 10 new attendees at the meeting, many of which were guest of current members.  The meeting was sponsored by Georgia Construction Products (GCP). Todd Harris briefly described the company’s line of services and recently expanded locations of distribution.  The meeting began with an informative notice regarding weather alerts and the use of smart phone applications to keep us aware of our surroundings at all times.  An invocation was given by Nathaniel Jones just before the guest were served a delicious lunch.   All past presidents & board members were recognized and visiting guests were introduced to everyone.  The entire group self introduced themselves and their roles with the company they represent, as we started our lunch program.  We continued our scholarship fundraising efforts with a cash raffle generating $70.00.

Technical Presentation:
Today’s meeting presented by Bud Wingate from STRUCTURAL Group focused on post tension inspection and repair techniques.  Mr. Wingate has over 40 years of experience with post tension placement and repairs in the concrete industry.  He noted the importance of fully understanding the placement of this PT system prior to any cutting or drilling into the concrete slab.  Without this knowledge there can be catastrophic results.

When looking at repair strategies he stressed that we want to avoid the low point and high point areas of the concrete.  Most of the repairs are completed in the mid-depth of the slab.  The PT cables are stressed to 33,000 pounds of force. The final strength of the cables are 27 KIPS (Kilo Pounds) and they normally do not fail without outside forces impacting them, typically outside penetrations.  Moisture can also cause corrosion in the PT system normally on the outside edges, which can lead to failure.

He stressed the importance of having highly qualified technicians when investigating these types of problems in concrete.   He noted the Post Tension Institute (PTI) has a valuable training program for persons intending to complete these types of repairs.  The contractor must establish a safety plan from the beginning of the scheduled work and follow this throughout the repair process in order to avoid the hazards.  During the construction phase ONLY knowledgeable people should be allowed in the repair zone.   All PT repairs are unique to themselves and so should the approaches be for completing the scope of work.

Identifying the cause of the failure begins with first asking building owners and property managers if any drilling, cutting, coring or digging has gone on lately, as well as understanding if there has been a weather or seismic event.  The grout pockets of concrete located at the end of the PT Cable system are some of the most dangerous parts of these failures as they can be projected long distances.  Because these cables run the entire length of the slab, once a cable is damaged or fails in a localized area the entire cable has been compromised.

Once you know the point of break within the cable you can then address the repair method.  He discussed using different types of PT cable repair techniques including center-splicing coupler; splice coupler, trouble shooting anchors, split anchors, lock-off anchors & wedge plates surrounded by end grout pockets.  He noted that proper surface preparation, including the removal of all grease and metal slag prior to placing the grout is critical for long-term durability.  It is necessary to provide a clean & profiled substrate where a good mechanical bond between the slab and grout will seal off any moisture from entering this critical end point of the system.

Many of the PT failures occur due to “sweep” blowouts as the cables move within the concrete.  Once a sweep failure occurs, using a repair material hand placed within the blowout will never achieve the original sheer transfer through the repair mortar.  This is due to variance between the compositions the concrete repair materials & the originally placed concrete.  He noted that some of these sweep failures occur within five (5) years of the concrete being place and finished; noting the importance of correct engineering and proceeding with proper construction methods from the beginning of the project.

It is important to know the cable lengths to ensure the repairs, including re-tensioning of the PT cables, have obtained the desired forces.  Once you identify the problem area it is important to sound out the surrounding concrete in order to remove all compromised concrete surfaces.  This will likely mean removing additional areas of concrete outside the original zone of failure.  Any overhead repairs should be completed using a form and pour method.  This is very important to get a good mechanical interlock and sheer transfer through the repair area.  You can only achieve this by placing the concrete under pressure, typically using self-consolidating concrete (SCC) materials.  You also want to maintain a minimum of four (4) inches of concrete in slab repairs.

He discussed several case studies including an airport taxiway where the original design was a bonded system with reverse curve in the concrete.  When they originally stressed the PT cables it split sheered the concrete without full blowout, but resulted in lots of delamination.  They removed two (2) inches above the existing steel to get the bonding capacity of the original steel.  Then using a form and pour method with SCC they completed the repairs.  This was not enough to gain the original capacity back so they installed a carbon fiber over-layment system in two (2) directions to regain the needed capacity.  A final coating with an acrylic enamel paint to match the surrounding concrete provided a consistent finish to the system.

In summary he noted the importance of using a certified professional engineer for these types of projects.  He cautioned rushing out and immediately shoring up problem areas.  When this is done improperly or in the wrong area it can actually cause more damage to entire structure.  Improper shoring can transfer the problem from one level to multiple levels of the building.

He also noted the difference in repair methods when addressing cables in a beam system versus a slab system.  This is due to the minimum amount of concrete within the design of a beam.  Evaluate the depth of these repairs and consider other options than concrete removal, including, but not limited to exterior post tensioning cable repairs.  If possible DO NOT ever abandon any cable with damage.  Locating defective cables with ground penetrating radar (GPR) is useful, but only as effective as the technicians using the equipment.  Do not completely rely on this method alone, physically chip down and identify the true cause of the failure.

When approaching these repairs establish a plan, use logic, get an engineer involved and follow protocol.  Your safety and the safety of everyone are too important not to do so.