30mpa parking garage concrete slab vs. 35mpa

huangyhg

30mpa parking garage concrete slab vs. 35mpa
hi guys,
what are the consequences of a well designed 30 mpa parking garage concrete slab (exposed condition) vs. 35mpa required as per code. as the slab in question is low by 5 mpa, as per engineering judgement, is it necessary to demolish the slab or are there any means to improve the durability/permability of the concrete to reduce the anticipated cracking due to freeze/thaw.
when was the testing done- is it possible to come up to 35 mpa over time?
we did coring but it gives max. 31mpa.
i don't know how much time it would take
to reach 35mpa, or is it wise to wait for that.
ali07 -
where are you located? that has big influence on the durability. parking or anything anywhere near a car must be looked at carefully. garages/ramps can be even worse because of the increased possibility of cycles and fresh "droppings" from cars coming and going. this gets compounded if you have a daily cycles and movement.
compressive strength is not a "magic number" or measure of durability, but one of the indicators.
usually, when strength is low or the gain is stubborn, there may be other factors - excess water, gradtions, etc., that contribute to other factors/indicators such as absorption and micro-cracking that can lead to early freeze/thaw deterioration.
just as a comparative situation, around here (minnesota), even for a common residential driveway some suppliers will not even deliver concrete less than 4000 psi (5%-7% air, 3"-4" slump) and some will give no guarantee unless it is a 4500 psi mix. since the cost differential is minimal there is not a big cost impact. - unfortunately, your concrete is in place.
if you allow non-spec concrete to remain, you could have liability down the road. your cores show or confirm the strength is low.
dick
we are located in toronto, canada and expecting maximum freeze/thaw cycles for sure.
we are having 4520 psi concrete. is it really weak for durability. i beleive the rapid chloride tests also not fulfilling csa requirements.
in this case what would be our next step.
demolish the slab and ask to rebuilt or are there any means to increase the durability of the existing concrete by sealers or some other method you know.
this would be a major decision for us.
toronto, ontario... one of the best places for deterioration!  lots of freeze-thaw cycles, and lots of salt.
you have a few options:
replace... costly and awkward with winter oncoming as well as possible disruption to local traffic.
where is the low strength concrete located within the structure?  location is critical... ramp, drive isle, or parking spot... in that order.  what concrete cover do you have?  what is your air?  strength is marginal, but not unworkable.
is it possible to provide a bonded coating/surface or has this been already specified.  there are a few excellent ones out there but none of them 'cheap'.
trow (if they're still around) used to have some real experts in parking garage deterioration.  have you checked with them or someone like them?
the owner may have an interest in the solution since he will be the one maintaining this.  i usually provide the owner with a maintenance manual when i do a parkade indicating what work to do seasonally, bi-anually, each five years and each decade.  in the event the owner is the contractor, then he can clearly be placed on the hook and fully responsible for actions taken.  if not, then, what is his interest in accepting lower quality material and assuming more responsibility?
dik
this location is parking space in front of new mall which is in construction too. it is about 40% of the whole parking deck. air content is less than specified too, it's about 3% as specified as 5%. strength is fine for all loadings for 30mpa. concrete cover is 1.5" to slab.
we can go with bonded treatment if it really works. can u specify some so we can check availability here.
what is trow, is that a company for repairs.
ali07
according to aci, concrete is acceptable if the value of a core is greater than 0.85(required strength).  your 35mpa
concrete is acceptable if the core breaks at 29.75mpa or greater. in addition , no single core can be below 75% of required strength. see aci 301  1.6.7.3  
ali07 -
you already know that the concrete job site tests showed an air content that was too low.
the concrete cylinders (that should indicate whether the mix was correct) shows the strength was too low.
the cored samples also were too low.
are you willing to accept 15% too low strength as accepatable when the strength is only an indicator of durability and a prime requirement in a northern climate is proper air entrainment that is also too low?
the concrete should not have been accepted and placed.
you know the concrete strength (from cylinders and cores) that as an indictor/factor that shows it is inadequate. - do you want to accept the liability in the future?
i would love to be an expert witness at the trial.
all too many structural engineers (which i am) think compressive strength is the answer to everything since they do not understand the behavior and principals of materials exposed to the real world.
durability of concrete is not a definite thing, but is based on experience with exposure to problems and examining reports/tests and looking at sliced samples after the problem is discovered.

concretemasonry
you are right. we will inform client not accepting that concrete. we believe it's hard to prevent it from cracking in future and we don't want either any liability issues at all.
trow are/were geotechnical consultants in the toronto area.
i can provide the names of a couple of protective wearing membranes that you can look into.  i normally use this type of product for ramps and drive isles.
the low air is problematic, but if the surface can be protected against water/chloride intrusion, then may not be a problem.
the 1-1/2" cover to the reinforcing steel is positive and will go along way to reducing the effects of chloride corrosion.
the lower strength and air content can/will lead to greater deterioration.  if properly treated and maintained, however, it may not end up court.  if it does, then the repair/replace decision may not be easy to defend... that's why i questioned if this was developer work...
how large is the parking deck? and what type of expansion joints have you used?  have you provided the owner with a maintenance manual?  can the deck be readily washed? are hose bibs installed?
dik