cracks on slab-on-grade : need your input
cracks on slab-on-grade : need your input
(OP)
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!!!
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!!!






RE: cracks on slab-on-grade : need your input
Techmaximus
RE: cracks on slab-on-grade : need your input
What type of operation occurs on the floor?
How long has the slab been in service?
RE: cracks on slab-on-grade : need your input
RE: cracks on slab-on-grade : need your input
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.
RE: cracks on slab-on-grade : need your input
RE: cracks on slab-on-grade : need your input
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.
RE: cracks on slab-on-grade : need your input
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?
RE: cracks on slab-on-grade : need your input
I'll be talking with the owner and the contractor later today.
I wil give you information on yur quiries.
Thanks.
RE: cracks on slab-on-grade : need your input
RE: cracks on slab-on-grade : need your input
In addition to Ron's excellent set of questions, is it possible that the perimeter walls settled?
RE: cracks on slab-on-grade : need your input
As the slab hydrates, the bleed water will go in all directions, including up to the surface and down to the bottom of the slab. The water at the surface will tend to evaporate unless the slab is cured correctly, ie., kept moist and at a good temperature (above 50 degreesF F). If the rate of evapoartion is faster than the rate of bleeding, the top of the slab will dry out. However, the bottom of the slab is still quite moist because the vapor barrier retains the bleed water. This unequal drying creates significant stresses and usually is the main reason for curling in a slab.
In the case of a floating slab, the slab will curl up, therefore it woll not be support by the sub-base, and any load will inevitably crack the slab. (this load will usually occurs when the Contractor grinds the slab to get rid of the curling.) When the slab is tied into the foundation, the reinforcing at the edges restrains the slab and the crack will form at the nearest point of least resistance.
SOLUTION = Either place vapor barrier directly below slab and CURE CORRECTLY, or place vapor barrier below 6-8" of pourous sub-base and to allow bleed water at bottom to move away and then CURE CORRECTLY.
Hope this helps.
RE: cracks on slab-on-grade : need your input
RE: cracks on slab-on-grade : need your input
CIP 19 at the bottom of the page explains curling due to curing difference between top/bottom of slab.
One can open these PDF files, but not print or cut/paste from them.
Techmaximus
RE: cracks on slab-on-grade : need your input
You can check out ACI 302.1R "Guide for Concrete Floor and Slab Construction" see section 9.2. ACI 212.4R, section 5.7, last paragraph, ACI 224R,"Control of Cracking in Concrete Structures" section 3.4.4.
Hope this helps.
RE: cracks on slab-on-grade : need your input