cracks on slab-on-grade need your inpu

huangyhg

cracks on slab-on-grade : need your input
we are currently approached to comment on cracks found on slab-on-grade and would like to ask for your idea on its causes and prevention. cracks are found on corners especially near the stiffer retaining wall ( 250mm thk others are 150mm thk only )and also at the center of the slab. building dimension is around 70m x 70m
history :
  1.) sub-grade has passed fdt tests done per layer of     
        300mm. fdt is at least 96%.
  2.) slab is resting on a loose gravel base ( 100mm thk )
        with polythelene vapor barrier just below the slab
        on top of the gravel base coarse.
  3.) the slab thickness is 200mm ( 8 inches ) doubly
        reinforced ( two layers )with 16mm dia spaced
        500mm oc.
  4.) concrete cover is 25mm ( 1 inch )thus saw-cutted
        contraction joints are limited to 25mm deep.
  5.) contraction joint is at around 7.0m x 9.0m.
  6.) slab reinforcements are anchored to the perimeter
        retaining walls.
initially, we have identified as probable causes the following items:
  1.) the contraction joint is too shallow thus not able
        to give enough weak plane for the crack to follow
        once it shrinks. usual depth is at 1/4 the depth
  2.) the slab is fully restrained by the retaining wall
        thus creating the cracks.
  3.) contraction joints are spaced farther than usual
        ( usual  is sround 16 to 24 feet for an 8-in slab)
we are also looking as a probable reason the loose gravel base ( no binders ) under the slab but could not find an article on this matter.
measures to prevent farther cracks from occuring we are looking into :
  1.) deepening the saw cut
  2.) deep saw cutting the perimeter to free up the slab
kindly give your comments.
best regards and more knowledge!!!
        
  
        
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the timing of sawing is widely considered to be far more important than depth of sawing.  sawing should be performed as soon as practicable. more sawing now more may be ineffectual in preventing additional cracking.  if you wanted to epoxy inject the existing cracks and make deeper, more closely spaced contraction joints, well that's a whole different story.
  
techmaximus
that's a lot of steel in an 8" slab.  
what type of operation occurs on the floor?  
how long has the slab been in service?
with that thick of a slab, and if the slab is "restained" at its perimeter, what you have is a huge mass of concrete generating a lot of heat of hydration during curing, and a large amount of shrinkage as the concrete hardens over a period of a about 2 months. could be the cause - shrinkage cracking.
if your reinforcing continues through your contraction/control joints, then the joints probably are ineffective.  i would have used about 30% more steel in the slab to prevent shrinkage cracks, based on the formula that appears on page 21 of the pca publication "concrete floors on ground (2nd ed.).  you'll see about half the final shrinkage after about 12 months.
if you can live with them, just leave them alone.  whenever i've seen repairs to cracks, the repair has only highlighted them and made them really ugly.
descibe the cracking. how wide and deep are the cracks? do you have a picture you can post on a web page?
i agree with techmaximus on the timing...it is critical.
jmiec also correctly points out that the quantity of steel is likely making your sawcut joints less effective.
the list of variables causing cracking is quite long.  we do need more info to help solve this, such as:
how long after placement were the joints cut?
what was the slump of the concrete at placement?
how well was the grade of the gravel controlled?
when did the cracks first appear?
what are the shape, size, and depth of the cracks?
have you done a crack map on the surface of the concrete?
is there any faulting of the cracks?
what were the weather conditions at time of placement and for the first 36 or so hours afterward?
was there a curing procedure and for how long?
most likely your cracks are related to drying shrinkage, though you could have some plastic shrinkage influence as well.  the cracks near the wall are likely from the wall restraint condition.
thank you for your responses.
for the inquiries :
the slab is quite new, just over a month.
it will be used for some processing of products watches and its components also for conveying raw materials on a racking system. the reinforcing bars was adapted all throughout the slab in case big loads will be used.         ( reinforcing bars : 16mm dia @ 500mm oc, two layers ).
could the uncompacted gravel base coarse without binders ( fine sand ) have any effect on the cracking?
ron,
i'll be talking with the owner and the contractor later today.
i wil give you information on yur quiries.
thanks.
the gravel base can have an effect if it is uneven or was displaced during placement.  that's one of the problems with using a gravel base...i don't usually allow it unless there is a need for capillary barrier.  i prefer a clean, compacted, fine sand subbase.
if you're seeing cracks after one month, then disregard my previous post, as the cracks are probably not due to drying shrinkage.  concrete at one month is only beginning to shrink.
in addition to ron's excellent set of questions, is it possible that the perimeter walls settled?