max. rebar spacing

huangyhg

max. rebar spacing
according to aci 318, maximum spacing for rebars is 18" whether it's for flexure or shrinkage. however, i have seen in many places, including the residential bldg code, that wider spacings are specified. any reason why?  

aci released aci 332-04 requirements for residential concrete which calls for 3 #4 for walls to 8' and 4 #4 for taller.  obviously if plain concrete is acceptable, larger spacing should be, too.  commercial will still have to hold to aci 318.  the aci ad i got for the book recognized the loads and useage for residential were not as critical.
the reason why maximum spacings are specified is to prevent excessive shrinkage/flexural cracking.
the concrete at the reinforcing bars is restrained from cracking, but the reinforcing between the bars is not restrained and can therefore crack between the bars. the larger the spacing, the bigger this crack can become.
a small amount of reinforcement at large centres can sometimes be worse than no reinforcement due to this differential in shrinkage.
also worth noting is that the closer the spacing of the bars the better the shrinkage crack prevention as the less likely it is that cracks will form between the bars. welded wire reinforcing mesh is better for crack prevention than large bars.
18" seems high to me, i would normally use a maximum of 12" in walls and use smaller reinforcement if appropriate.
csd72, i agree. say you have a 10" antilever rw and your design calls for #5's at 16" oc. you place the rebars on the embankment side. what about the exposed face of the rw. i am also putting 2-#5's horizontally at the top of wall. i was thinking of placing #5's vertically at 32" or 48" on the front to hold the top horizontal rebars while pouring the concrete. do you think this will do more harm than good? would more horizontal rebars on the embankment side be necessary?
also, i was told that it is a good practice to put some rebars in the front face of the wall for shrinkage. any thoughts or suggestions.
thanks,
is the reinforcement for shrinkage only, or is it an area prone to frost? i am not experienced at frost, so my comments are on standard shrinkage.
for shrinkage only i cannot see how this can cause any cracking in the short and completely unrestrained vertical direction. also think of the fact that the front face will be under compresion.
in the horizontal, i would try and get cracking reinforcement at 16 to 18"cts if possible. you will need the vertical reinforcement to hold this in place. assuming the wall is at least 8" thick, i would put this both faces, if it is thinner, you can only fit one horizontal layer and i would put this on the external side of the vertical reinforcement.
i had a wall with 16mm (#5)bars at 12" cts and it still cracked due to soil restraint, so make sure you put in some control joints at reasonable centres.
csd72, were the cracks you mentioned vertical? my retaining wall is about 50' long and i am putting a construction joint at 25'. i could also place a 1/4" groove at 12.5' spacing for crack control.