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Hammers, Chains, & Magic Boxes

By Eric R. Kizak, P.E.

Engineers are often described as techno-geeks.  And while this title may apply to some, for those involved in concrete repair the tools are a bit more rustic. 

One of the first work items in all concrete restoration or repair projects is to define the extent of the problem.  (The first step is to acknowledge there is a problem – my apologies if that sounded like a 12 step self-help program.)  To do this a process called “sounding” is used. 

Sounding is essentially a methodical process of annoying the neighbors by tapping a hammer on the concrete surface or dragging a heavy metal chain over a slab.  While this may sound crazy, with some experience a detailed assessment of the condition can be made.  The process requires a bit of focus and visual, auditory, and kinesthetic attention.

Visual:  Throughout the process you are looking for surface defects such as cracks, bumps, holes, spalls (missing chunks) and discoloration. 

Auditory:  Sound concrete, when struck with metal hammer, will produce a clear ring.  However, defective concrete will yield a dull thud or hollow, drum like sound.  The difference between the two is a reflection of the defect and the soundness of the concrete above the defect.  It indicates that the concrete below is not solid but delaminated.

Kinesthetic:  Sound concrete is hard.  An obvious statement, but this also means that sound concrete will not absorb much kinetic energy, such as hammer strike.  As a result, a hammer striking sound concrete will rebound like a drumstick of a tight drum.  Cracked or delaminated concrete on the other hand will absorb energy along the cracks, reducing the kinetic rebound.

Collectively, these three sensatory responses can determine the size, type, and approximate depth of a defect.  For example, an area without surface cracking, a drum-like sound and good kinesthetic would likely indicate a deep (greater that 1”) delamination, where as a location yielding cracking sound and little rebound is likely a shallow delamination or spall. 

Sounding with a chain can also be done by dragging a heavy metal chain over the surface of a slab.  With this method you trade the kinesthetic feedback for the ability to examine a large area quickly.  The auditory feedback differs slightly than described above.  Solid concrete yields what sounds like the ringing of a bunch of small bells or triangles (remember elementary music class?).  Defective areas are located by a change in tone or volume (broken bells).  This is good for locating defects but if a more refined assessment is necessary, supplementary sounding with a hammer should be performed. 

So what about the magic boxes?  Several technological devices such as impact and ultrasonic echo locators exist to help locate deficiencies but their cost and ease of use do not compare to the lowly hammer and chain.