slab-on-grade

huangyhg

slab-on-grade
i'm looking at a current problem.
has anyone encountered a reinforced slab-on-grade/ground where the reinforcing steel continues through the sawcut and the reinforcing area is sufficient to interfere with normal shrinkage cracking?

dik,
i am familiar that kinds of slabs-on grade but, we are cutting the upper reinforcing bars just under the steel angles 2 days later after concreting...the main problem i think not using any formwork two sides of the slab-on grade. the second one is whether one of the dummy joints or working joints can serve as an expansion joint where the weakest cross section.
see you,
erkan
erkan:
two days is too late for sawcutting.  sawcutting should occur about 8 or 10 hours after the slab is finished.  after this period, the microcracking has likely determined the location of the cracks and sawcutting just puts more lines on the slab.
can you empty your mailbox?
dik
we have placed slabs with continuous reinforcing placed without any regard for where the sawcuts occur.  concrete will usually crack prior to fully engaging the reinforcing.  we've had pretty good results doing this.
you often hear the concept of reinforcing not eliminating cracks, just keeping them tight.  
i would think the best detail would be to terminate some or all the reinforcing at the joint, but this involves extra cost and layout and i'm not convinced that it really makes a huge difference.
there's little doubt that the addition of reinforcing 'holds' the concrete together.  cracking is generally finer and more distributed. for placing reinforcing, i've staggered bars that are about 95% of the length of 2 panels and the end panels are the only ones that have 'short' bars.  with reinforcing steel, the time that sawcutting occurs is still critical; most of the badly cracked slabs i've encountered are a result of this.  the other main cause is being restrained.
i was wondering if it is possible to have 'too much' reinforcing that it affects the cracking pattern (minimizes or negates the effects of sawcutting).
most slab on grade construction i use has a single layer of rebar located a sawcut depth below the top of the slab.  this keeps it near the top (you can't see the cracks on the bottom <g>) where it is most effective and keeps it from being cut.
about ten years ago the state of wisconsin replaced major sections of the i-90 interstate using "continuously reinforced pavement".  no joints whatever (except at the end of very long pours), but the pavement was heavily reinforced.  i drive the road regularly and don't notice any cracking (but at 70 mph it might be kinda hard to notice <g>).  you might try searching the web for research information on this method, if you have further interest.
most of the slab-on-grade floors i see use mesh reinforcing not rebar.  if the subgrade is good soil and prepared well there is little reason to heavily reinforce a slab-on-grade. as dik and jae have noted above, the key is to saw the joints as soon as possible after finishing the slab. the joint spacing is also critical, roughly square sections in 12'-20' lengths depending on thickness of the slab.  the "soft cut" saw method is really the way to go.
jheidt2543:
how do they accommodate thermal movement?  even if the road is extruded using 0 slump to reduce shrinkage.  maybe they use the rebar to minimize crack widths and the weight of the road to hold it down in the summer <g>.  wisconsin could have temperatures ranging from -40? to +100 (f scale).  i know they do that with welded rail, but i still don't know how it works.  i have a picture of a bridge in winnipeg where the concrete curb and median went over an expansion joint.  the thermal movement lifted the median by approx 6".
6m or about 20' is ok for a 10" slab.
is the soft cut saw the one with the preloading plate behind the blade?
dik...the area of reinforcing will almost always interfere with shrinkage.  the key, as jae noted, is causing the cracks to occur where you want them.  the key to this is timing.  for reinforced slabs on grade, the stress distribution gets a bit wacky during initial curing and shrinkage.  as the rebar engages (again, noted by jae), the shrinkage is restrained.  if you examine closely, you'll see more cracking in reinforced slabs than non-reinforced, but the cracks will be closer together and much tighter.  sometimes even difficult to see.
make your sawcuts even sooner than you would with unreinforced as the rebar engagement issue is already occurring.  this is where the sof-cut saw is useful.  you should also make the joints closer (yes, closer, not farther apart!), again accommodating the shrinkage stress distribution difference.
thanks, ron
and jae...<bsg>
dik,
i don't understand the theory behind the continuously reinforced pavement either.  i'm sure that there have been studies of it and there may be information available on the wisconsin dot website.  
the soft cut saw is a small saw about the size of a hand circlular saw with a 5" (i think) diamond blade.  as soon as the slab is finished, the saw cuts are made.  thus, the control joints are in place before most of the shrinkage stresses have built up.  in past "normal" practice, we would use a large walk behind paving saw with a 16" - 36" steel or fiber blade and make the cuts 8 - 24 hours after finishing.