huangyhg

the sound of concrete?
we have a building retrofit project under construction. it consists of 3 levels of new steel floor framing constructed inside an existing masonry shell. the floors are concrete placed on composite metal deck. concrete placement was completed 2-1/2 months ago in november. contractor now reports hearing "thumping and cracking" sounds (in january) that he thinks are coming from the concrete cracking. there are some hairline cracks visible, but nothing out of the ordinary.
have any of you experienced a situation like this? any thoughts or insight would be appreciated.
jcali.
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if it was steel framing, it could be that slip-critical connections were made by the erector and the connections are slipping into a bearing configuration. sometimes when a connection "goes", it sounds like a small gunshot. this is ussually not a problem, just a little unnerving until you find out.
hopefully, it is not slip critical connections that are slipping! banging bolts - when steel is erected and tc bolts tightened, it creates a clamping effect (load on the beam is usually just its own dead weight). as load on the structure increases (concrete pour, live loads), the clamping effect is overcome, and the beam slips into a bearing type connection and the bolts are now in a bearing connection as designed. unnerving, yes. dangerous? no.
i have heard of concrete cracking causing loud noises. as the concrete cures and dries, it tries to shrink but is restrained from freely shrinking because is is mechanically bonded to the composite metal deck. internal stresses build up in the concrete until they overcome the tensile strength of the concrete and the energy is released with a loud crack.
lppe,
the operative words are "bearing connection as designed." sometimes slip critical connectors are installed because it is easier and faster. but, the number of connectors typically required for a truly slip critical connection are more than for a bearing connection. thus, the slip resistance is overcome and the connections enter into a bearing connection, what they were originally designed for.
tensile failure in concrete does not produce loud cracks, though on a steel form deck it is a bit like beating a drum. popping and cracking are somewhat normal as structures reach equilibrium from temperature changes. acoustic emission monitoring will tell you if the sounds are problematic or not.
what is acoustic emission testing?
-jcali
acoustic emission testing is done by using an acoustic transducer attached to an amplification device that records "sound" events. commonly used for checking for cracking in large welds or tanks.

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