Concrete Balcony Slabs won't crack

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Author Message Jerome J. Lazar, RA, CCS, CSI, SCIP Senior Member Username: lazarcitec Post Number: 1212 Registered: 05-2003 Posted on Thursday, November 13, 2014 - 11:47 pm:    The structural engineer on a highrise condo project (post tensioned concrete)in South Florida has assured the Architect that he does not anticipate cracking in the balcony slabs at all as they will be in tension from the cables, not even hairline cracks. I have a concern because he is not specifying galv. steel nor is there waterproofing of the concrete, not by applying a traffic coating nor adding a corrosion inhibitor to the mix. I've always been told that concrete cracks as it cures as that is the nature of the beast. Any beast slayers out there? ken hercenberg Senior Member Username: khercenberg Post Number: 848 Registered: 12-2006 Posted on Friday, November 14, 2014 - 02:32 am:    Nice thought but not true. Had a post-tensioned project several years ago with balconies and corrosion inhibitor. Lots of cracks. Added traffic coating by change order to several hundred units. Thankfully the concrete sealer had not yet been applied or we'd have been talking removal first. Most cracks were hairline. Could have been a huge cost. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 702 Registered: 10-2007 Posted on Friday, November 14, 2014 - 01:37 pm:    Post tensioned concrete that is under net compression should not crack. The problem is that around the perimeter this condition may be difficult to satisfy. Cracks in the balcony concrete at its interface with the main slab should be minimal to non-existent if PT is used. Note that there could be some balconies that do not have tendons anchored at the edge of the balcony. This is a result of the layout of the tendons and is not something that can be dictated by the architect. These balconies should be treated the same as balconies in a non tensioned slab. You still need to protect the anchorages of the tendons. The tendons should also be specified as fully encapsulated. Galvanizing of the mild reinforcing steel delays the rusting of the steel but given enough time in adverse environment the reinforcing steel will still rust. Thus prudence suggests that this is not the only thing you consider. The PT and anchorages will not and should not be galvanized. Although specified in the concrete specification section the decision to use corrosion inhibitor should be that of the architect. While corrosion inhibitors may provide value they will not be effective where there are significant cracks. Jerome J. Lazar, RA, CCS, CSI, SCIP Senior Member Username: lazarcitec Post Number: 1213 Registered: 05-2003 Posted on Friday, November 14, 2014 - 01:47 pm:    Thanks Mark, appreciate your insight. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 703 Registered: 10-2007 Posted on Friday, November 14, 2014 - 07:32 pm:    Concrete only cracks when tension, above a certain amount, exists in the concrete. This tension can arise from internal or external effects. Differential shrinkage can cause tension to occur. Tension can also occur when the shrinkage is resisted by external restraint such as shear walls. Post tensioning attempts to apply compression forces to offset the tension. When this is successful the concrete will not crack. Properly designed PT will greatly reduce the cracking in the slab. Improperly designed it can make it worse. As stated above there are some, hopefully limited, regions where it is not feasible to apply the compressive forces.