...can be detected by hammer testing the concrete and listening for hollow spots...
Professionals use hammers or heavy chains dragged across slabs to detect hollow spots.
http://tinyurl.com/o3udh2g

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Note added at 32 mins (2015-10-17 22:54:39 GMT)
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Bliżej oryginału: hollow-sounding areas areas/spots.

Using the hammer to tap around this spot, you identify more hollow-sounding areas.
http://tinyurl.com/qzjy3nf

...hollow-sounding spots were tapped with a hammer to verify that.
http://tinyurl.com/p3dbg8t

W związku z tym, że po upływie 3 - 4 tygodni, na posadzce pojawiły się liczne spękania (fot. 1) oraz wystąpiły masowe wręcz jej odspojenia od podłoża (głuche miejsca przy ostukiwaniu), wykonawca miesiąc później w ramach reklamacji na większości powierzchni usunął masę samopoziomującą i wykonał klasyczną nawierzchnię betonową. - http://www.bdb.com.pl/zdjecia/domye/20100519232134313232.pdf

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A delamination survey can be conducted by sounding - dragging a chain across the surface or tapping with a hammer and listening for hollow sounds. A hollow sound indicates delaminated areas, and a ringing sound indicates intact areas. - http://goo.gl/3fHrzL - page 5

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Delaminations in concrete slabs can be a serious problem, but often they are "heard" before they are seen. Here's an example. A concrete floor slab placed days ago has hardened and cured. As work from other trades continues in the area, someone drops a hammer from a ladder onto the floor, and an unexpected sound is heard. Instead of the clear ringing sound you would expect when metal hits solid concrete, a drummy sound or loud clack draws your attention. Using the hammer to tap around this spot, you identify more hollow-sounding areas. What's been discovered is a surface delamination in the concrete floor. - http://www.concretenetwork.com/concrete-delamination.html

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Delamination also occurs in reinforced concrete structures subject to reinforcement corrosion, in which case the oxidized metal of the reinforcement is greater in volume than the original metal. The oxidized metal therefore requires greater space than the original reinforcing bars, which causes a wedge-like stress on the concrete. This force eventually overcomes the relatively weak tensile strength of concrete, resulting in a separation (or delamination) of the concrete above and below the reinforcing bars. - https://en.wikipedia.org/wiki/Delamination

Profesjonalnie:
Visual Inspection of Concrete Structure
Table-2. Strength of concrete by tapping method:
http://www.engineeringcivil.com/visual-inspection-of-concret...

drum-like sound

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Note added at   9 godz. (2015-10-18 07:57:04 GMT)
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z popularnych określeń:
Sound concrete, when struck with metal hammer, will produce a clear ring. However, defective concrete will yield a dull thud, drum like sound.

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Note added at   9 godz. (2015-10-18 07:59:15 GMT)
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A dull sound is emitted when delaminated concrete is encountered.