5 overstress

huangyhg

5% overstress
i am working on a project that has a floor slab that was poured in the 1930's. i know the reinforcing size and spacing, but not the fc'.  this is in a shower where the the slab slopes to a central drain (2 1/2" elev. change). the owner wants to use light weight concrete as a topping material. my question is: is it applicable to use the 5% overstress method? or do i make an assumption on the fc'? (coring a sample and breaking it, is out-of the question. i already asked).  
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what do you mean about the 5% overstress method? how can you know the overstress if you do not know the f'c? politely tell your owner, light-weight fill is out of the question if we can't do some cores.
what was the design ll of the slab? what is the fy of the rebar?
some owners just don't understand how we do things i guess.
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

the windsor probe method is a non-destructive method (uses an impact hammer) to determine compressive strength.  its' not as accurate as a core, but is better than nothing.
also keep in mind that concrete continually gains compressive strength over time--and it's been a long time since the 1930's!
but i would guess the reinforcing is the weak link--probably 40 ksi steel, and maybe even less.  and in the 1930's, they may have been using square bars, twisted bars, etc.
and yes, if you do not increase the loads by more than 5 percent, the code allows you to say ok without checking.
daveatkins
dave,
if there is a change in occupancy would the 5% still be allowed to apply? that is why i asked what the design ll was.
if it is not going to be a shower anymore, then the ll might be higher. also, if it was a shower for the last 75 years, i'd worry about corrosion of my steel.
just some thoughts.
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke