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Re: cracking in waffle slab

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About the query from Jerry King regarding: 

an existing waffle slab that has
>numerous cracks visible from below.  

>The cracks appear to emanate from the corners of
>the solid heads, and mostly run transversely across
>the width of the structure.  There are also numerous
>cracks at the interface between the joist and the slab
>when viewed from below.  Unfortunately, most of the
>top slab is covered with asbestos tile, so we cannot
>see the crack patterns from above.

My investigation of cracks which may be similar, in a slab band type
platform supporting an apartment building in Vancouver, is focussing on the
top mat of steel being placed too low. (I had asked the group a question
about cracking of this slab, around new year's).

Today, I re-visited the building with a material testing technician to take
rebar depth readings from the top of the slab, through the carpet of the
suites above. Sure enough, the top bars are lower than they were supposed to
be over the columns. Next I'll adjust our concrete beam analysis to see how
much the strength is affected. I think it can explain why the cracks are
where they are.

The "black box" the technician uses is called a "Micro Cover Meter". You
program in the rebar size, and it tells you how deep the bar is below the
surface. It looks like a big calculator with a plastic brick shaped sensor
on the end of a wire.

I think the information is valuable for checking rebar locations in a
completed concrete structure. I think you have to know either the bar size
or the depth to determine the other parameter, but they say there are ways
of judging both if you take enough time and care with it.

                                                Jim Warne, Vancouver BC Canada