RE: Long cracks in top slab[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Long cracks in top slab
- From: Nicholas Blackburn <nblackburn(--nospam--at)fdgoak.com>
- Date: Mon, 8 Jan 2001 12:57:06 -0800
It is typical in concrete slabs that are long and narrow to have pour strips provided that break the slab into separate pieces that are more square than rectangular (preferably less than 1:1.5). The slab reinforcing is not continuous through the pour strip to allow the separate segments to shrink without undue distress. Per PTI, at least one pour strip should be provided for buildings of this length (whether the slab is post-tensioned or not). We usually use two to break the slab up into two segments about 100' in length and then one segment that is the balance. The pour strip is cast at least 28 days after the last adjacent slab pour was made. Based on shrinkage/shortening analysis, you would expect about 1/4-1/3 of the total long term shortening to occur over the first 28 days. It is your decision and the client's schedule that drives the actual length of time the pour strips remain open (the longer the better). I like to leave them open as long as possible to minimize cracking, with 28 days as a minimum (56 days preferred and longer if I can get it).
For a typical parking structure, you can expect total shortening something on the order of 3/4" per 100 feet of slab in each direction. For your building this would work out to about 2.75" in the long direction (one half at each end) and 1.25" in the short direction. The actual shortening is dependant on thickness, shape etc... and can be calculated per ACI 209 methods. If the slab was poured as one monolithic pour I would expect to see the cracking that you are seeing. Cracking can be exacerbated by poor curing techniques, sudden large changes in temperature, pour quality concrete (frequently the case) as well as locations of construction joints/pour strips, shearwalls etc.
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