what concrete strength to assume

huangyhg

what concrete strength to assume
when we only have a mix design indicating the ratio of components, such as 1:2:3 or 1:2:4 and no further information?  i intend to require material testing of the existing structure to confirm, but what should i assume to start with?
i have typically taken 1:2:3 mix as being between 2500 and 3500 psi, and testing has usually found something in this range, however this project is one a tight deadline and i'm just a bit worried that i've been "getting lucky".  
any guidance anyone would care to offer?  the structure in this case is a reinforced concrete bridge built in late 1930s in rural new zealand.
thanks,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...

1930's rural concrete batching anwhere can be extremely variable. even in the 1940's somce was a combination of weight (94# bags of cement) and approximate volume catching of aggregates. wet sand can bulk, cusing a higher than normal material content, that can give higher strngths.
i would keep expectations low in the 2500 to 3000 psi range, but would not be surprised to see cores over 4000 psi if all the batching variables, placement methods and inplace curing was good. - time does wonders on some concrete.
a bridge may have had better ot tighter tighter controls, but can only go so far if there are not options.
only testing will determine what kind of animal your have to deal with. on a lighter note, are there wambats in new zealand?
if it were me, i wouldn't assume anything...or just assume "something" with a wide range simply for the sake of assuming realizing that you will probably not be correct. it doesn't take long to core and break...
this might help (the website has a lot of free documentation too that might help). just keep in mind that you're dealing with 70+ year old concrete.
i don't have a copy in front of me, but the only thing i can think of off the top of my head is that you look at the w/c ratio for that mixture and see what expected f'c you have according to the pca concrete mixture design book. i'd bump up the water content to account for moisture in the sand. by how much? use some judgment for that.
could you do some "non-destructive" testing to get a ballpark?
thanks for the advice all; i'm not too concerned regarding the concrete compressive strength for the beams as the steel strength is much more critical.  the compressive strength is critical for the piles, and i can test this as there are original pile cut-offs still on site (surprising after 70 odd years, but also very convenient).
i was hoping someone would have a resource for an indication regarding proportionate design to reliable assumed concrete strength, even though i'm experienced enough to know better than to think a design-reliable assumption is possible not knowing w/c ratio or anything about the aggregates (both fine and course).
i own a copy of the mix design text from cpca, but unfortunately it's in canada...
oh, and no, new zealand does not have wombats.  that's australia.
cheers all,
i'll update when i know the core results,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...