Rebar spacing requirements for exterior exposure

[ali07]

Just curious, are we strictly follow the rebar spacing requirements for exterior exposure requirements as per code.

We have an open parking garage flat slab structure and need to know, whether i have to check this structure for rebar spacing requirements for crack control.

I am well above the cracking moment, but does it meant i have no need to check cracking. Is that requirement for interior or exterior or just basic crack control requirements. If i go for exterior exposure, i can't place my rebars farther than 6" spacing, but normally 12" spacing works well for us.

 

[kslee1000]

Yes, for parking garage it is good practice to check ACI crack control using Z factor for exterior exposure. Better result can be reached by reducing both bar size and spacing, for example - instead of #8 at 12" oc, you may use #6 st 6". "As" has increased a little for this case, but it is much better from crack control point of view.

 

[ali07]

I also heard from practical point of view for exterior exposure like structures prone to aggressive exposures, don't use smaller rebars, as they lost the x-sectional area due to rust faster than large x-sectional area rebars.

Although from calculations, it is ideal to have smaller rebars for better crack control.

 

[kslee1000]

ali07:

Closer spacing minimizes the number and width of crack, in turn, it helps to provent corrosion due to leakage. I am not expert on concrete paving subjects to aggressive weather & chemical assults, but I think better concrete mix design, in conjunction with surface protective covering should be in your interest list. Also, how about increase concrete cover, however, ignore the extra thickness in crack control cal, otherwise you would be penalized for it, which does not make sense.

 

[ali07]

we are looking for an option to increase 1 in concrete cover, but it attracts a tremendous amount of steel. I think it is not better to provide new concrete topping over existing concrete, as the bonding issue, future spalling would be the issues. But what i just found that if i increase the cover above rebar, for our parking structure, we have to pay 200 tons of extra reinforcing just for crack width satisfaction. To keep z < 25000 for exterior exposure.

 

[kslee1000]

ali07:

I have explained the phenonmenon in one of the thread. From our past dealing with USCOE for mass concrete structures, for which 4" or more cover was common place, if follow ACI without adjustment, yes, the reinforcing would be a killer of the projects. It was finally agreed/accepted, that any cover beyond the 2" used in the development of the ACI method was considered EXTRA (non-structural), thus ignored in the calculation.

 

[kslee1000]

Another important facts I have neglected to point out, 1)ACI crack control provisions applies to beam and one way slab only. 2) ACI crack control provisions has changed since 1999, please read the entire commentary of 10.6, pay direct attention to R10.6.5. Hope this helps.