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Settlement or Shrinkage Cracks in Silo
6

Settlement or Shrinkage Cracks in Silo

Settlement or Shrinkage Cracks in Silo

(OP)
Have an issue with a heavily loaded mat foundation and the silos supported by it.  About 1 year after construction of the silos (4 silos in cloverleaf group on 1 mat...each silo about 40 feet in diameter by 180+/- feet tall)"caking" problem developed with cement near top of silo (upper 50 feet).  Problem obviously from water intrusion, so inspection of silo done and cracks found.  Mostly vertical to steep diagonal, with at least one circumferential horizontal crack.  Cracks are tight (0.006 inches,max).

Silo Design engineer says cracks are from differential settlement.  I disagree and say cracks are shrinkage, since no evidence of any significant settlement(max measured at construction of 1/4-inch), and no evidence of differential settlement.  Concrete at crack level is 10 or 12 inches thick and was slip-formed.  Mat foundation is 8 feet thick and extends only about 5 feet beyond the silo walls in size.  I contend this is too small for any significant bending to occur in the mat, particularly reverse bending as is claimed by the designer.

Does anyone have experience with cracks in cement silos and have any comments relevant to the claimed bending/differential settlement?

Posting this in ACI and Geotechnical also.

RE: Settlement or Shrinkage Cracks in Silo

Ron

With the water infiltration, could there be any expansion of the rebar due to oxidation? That could possibly initiate cracking.  Are the cracks just in the upper 50'?

This certainly doesn't sound like a foundation settlement problem.  Imagine the force required to distort a 180 foot long "shearwall" to initiate the cracks!  Per Hooke's Law I can't even imagine it.  It does sound too much like circumferential shrinkage.

RE: Settlement or Shrinkage Cracks in Silo

(OP)
JAE,
Thanks for the response.  The silo is only about a year old and there are no migratory rust stains, so I don't suspect rebar expansion or exfoliation at this point.

I agree with you...seems implausible that such a "chunk" of concrete would be affected at that level by differential settlement.  Even if there were an inch or so of differential, the strain attenuation over 180 feet would not produce cracks of this size (theoretically of course!).

Do you think there could be some "ovalling" of the silo walls due to differential filling or emptying stresses?

RE: Settlement or Shrinkage Cracks in Silo

Ron,

I do not believe that foundation settlement would create cracks so high up the silo, and agree with JAE's comments.

Do the silos have rigid (concrete) roofs or caps ?  If not, then ovalling effects, due to uneven emptying or due to wind turbulence, may be the cause.  Uneven emptying could perhaps be identified by observation while the silos are in use.  Wind turbulence effects (remember the cooling towers in the UK some years back) could require some sophisticated modelling to identify, as the group of four does not have simple aerodynamics.

Other possibilities :

Alkali-aggregate reaction produces bursting stresses which can typically cause a checkerboard pattern of cracking.  In your case perhaps the horizontal cracks are contained by vertical compressive stresses and you only see the vertical ones.  Indications for this reaction are high-alkali cement in the mix (cement-producer dependant) plus a susceptible aggregate (containing reactive quartz) plus water migration through the concrete to activate the reaction.  This last factor appears to occur in your case.

Alternatively, early-age shrinkage of the concrete could have left residual tensile stresses in the concrete, which have now produced cracks.  Difficulty of access may have caused them to go unnoticed after construction.  A indicator for this would perhaps be a change in the concrete mix design or in the curing method during the last 50 ft of construction.

Regards
Richard Beneke, South Africa.

RE: Settlement or Shrinkage Cracks in Silo

(OP)
Richard,
Thanks for the response.  I agree, there is no plausibility in the settlement issue.  I'm looking at simple volumetric shrinkage as the primary cause of the cracks.  Were it not for the water intrusion, the cracks would go unnoticed.  There is a nearby clinker silo with similar construction, but because water intrusion into a clinker silo is not a big issue, no problem.

There is a relatively high vacuum pulled on the silos to keep the dust down as this plant had to overcome great political opposition to be built.  The air pollution bird dogs are watching it carefully for the least sign of dust emissions, so they are very careful about the dusting problems.  Resultingly, the negative pressure pulls the water through the cracks and causes the caking problem on the walls.

We can solve their problem pretty handily...the real issue is who pays for the fix.  The construction is still under warranty, so currently they are just pointing fingers at each other (pretty typical!).  

After some consideration, I think the ovalling issue would be minor, but could contribute.  

Thanks to you and JAE for the contribution.  Both of you have reinforced my thoughts and provided excellent comments.

Ron

RE: Settlement or Shrinkage Cracks in Silo

Ron,

I had a further thought on the foundation settlement issue...

Obviously shear settlement is discounted, but what if the plate foundation were bending along a diagonal line from 2 corners.  This could result from 2 diagonally opposite silos empty and the other 2 full.  The settlement on the foundation may be once-off (plastic) or with each loading cycle (elastic).  Then the upper part of the structure would be pulled into an oval shape.  A fix to this would be to provide a roof that functions as a stiffening diaphragm across the tops.

Regards,
Richard Beneke

RE: Settlement or Shrinkage Cracks in Silo

One other thought as well, Ron.  I don't know the layout, geometry of this silo, but whenever I investigate cracks, I always ask the question, "Why here and not there?".  Is there a possibility that the "roof" of the silo changes the response of the tube of concrete in such a way to inititate cracks?....I don't know:  perhaps the shaft of the silo is shrinking/expanding thermally, or due to loading from material, and the diaphram roof is resisting any "uniform" displacement and initiating small tensile cracks.

Also....we always (but will little positive luck) reinforce the idea with the owner that all concrete cracks.

RE: Settlement or Shrinkage Cracks in Silo

I can't see that this is a foundation flexure problem as there would be cracks evisent in the base slab  if it were flexing sufficiently to cause shear cracking in the top of the silo. An 8' thick raft is quite a stiff structure, and assuming it is designed as a raft it should be designed to cope with the worst case combination loading of full/empty silos.

With the cracks being vertical or sub-vertical then it is cracking across the primary hoop reinforcement and I agree that these are indicative of shrinkage cracking.  This suggests either a poor mix or inappropriate curing for shrinkage cracks to occur.  A secondary thought is that the main circumferential rebar may have been reduced to correspond with reduced hoop load from the retained clinker, and that this reduction in rebar content has not been checked against cracking.
Regards

Andy Machon
Andy@machona.freeserve.co.uk
 
 

RE: Settlement or Shrinkage Cracks in Silo

(OP)
Andy,
Thanks.  Good point about the reduction in rebar at that location.  Would be a logical drop near top.  I checked the top of the mat foundation and found only typical shrinkage cracks.  About 1/4 of the mat is exposed...the rest is covered by pavement concrete, which shows no distress consistent with settlement or flexure of the foundation.  

We have not received records yet, but suspect that no active curing procedure was used..thus initial drying shrinkage was likely high.  

Ron

RE: Settlement or Shrinkage Cracks in Silo

Circular structures, such as silos, are notorious for developing vertical cracking from the inside to the outside - shrinkage cracking almost certainly.  Normally this doesn't have much effect on the usability of the structure, but as you're operating under strong negative pressure, it become an issue.

At this early point in the life of these silos, stresses due to unequal loadings are not likely to have caused the cracking.

The issue could very well be the distribution of the hoop steel in the wall - a layer should be close to the outer surface, another near the inner surface.  Even if the drawing shows these layers, they are hard to maintain during slipforming.  Many silo builders do not adequately tie the steel, but "float it" during the slipping operation.  How was the inspection?

You stated the silos were in a grouping of four, a "cloverleaf" formation.  Are the four silos joined, or are they seperate?  If they are separate, vortex induced vibrations such as those that caused the natural draft cooling towers to collapse at Ferrybridge in Great Britain (as mentioned above) could possibly be a contributing factor.  I don't believe it would have an effect if they are joined, but then problems with unequal stresses during silo filling and drawdown are multiplied, and are not well understood.

As to who pays to seal up the silo, that is a contractual problem which may or may not be determined from the wording of the contract.

Good Luck.

RE: Settlement or Shrinkage Cracks in Silo

Ron,

With respect to your silo cracking problem...  I think there is a very high probability that the cracking is related to an ambient air moisture differential between the inside and outside of the silos.  The vacuum or negative pressure on the inside of the silos will have a dramatic drying effect on the concrete located on the interior of the silos.   This water content differential within the concrete can cause the interior of the silo to shrink substantially in relation to the exterior of the silo, especially in wet weather.

One question comes to mind...  Are there cracks opening up in the upper 50 feet of all the silos, or just one or two?   If only one silo is exhibiting this phenomenon, then it is very likely that the fault is the cause of a bad mix design that went into the construction of that particular silo or a lack of proper curing during construction.   Was the ambient air temperature and relative humidity different during the construction of the several silos?   This too can also explain why one silo may exhibit cracks while the others do not.

I too do not believe the problem is related to foundation settlement.   I think a clue to the answer lies in the location of the cracking in relation to the structure.   The uppermost portion of the structure has a relatively small vertical compressive stress that would tend to permit larger crack widths than at lower portions of the structure.  A vertical compressive stress will also tend to place a horizontal compressive stress in the concrete ring due to Poisson volumetric changes (anisotropic compression).   Although the cracks may not be visible, there is a very high likelihood that the cracking extends far below the point at which they are visible to the naked eye.

Lets not forget that all concrete cracks.   The problem and the solution has always been to limit the width of the crack that is developed.   One solution has been to widely disperse the reinforcing steel (i.e., more smaller diameter bars as opposed to fewer large diameter bars).   Another solution has been to place the concrete under a compression stress thereby minimizing or eliminating crack development (pre- or post-tensioning).   There is a relatively expansive volume of material available from the American Society of Civil Engineers with respect to silo design and construction.

A well-respected engineering firm that specializes in silo design and rehab is CH2M-Hill located in Corvallis, Oregon, USA.   You may want to talk to these folks to see if they might be able to assist you in determining the cause of the cracks.

Unfortunately, concrete is one of the construction materials whose performance is highly dependent on the skill of the workmen.   It seems Murphy's Law was specifically written for concrete design, manufacture, and construction.

Best Regards and Good Luck.

RE: Settlement or Shrinkage Cracks in Silo

Ron,

there is an interesting paper just published by Prof. Burland of Imperial College in the Journal of the Institution of Structural Engineers (UK)  5th December which describes failure of some silos. Although it certainly seems that drying shrinkage is the cause of the cracks in your case, this paper discusses the effect of eccentric flow within the silos, and may be worth a look.

Regards,

David.

RE: Settlement or Shrinkage Cracks in Silo

Ron,
can u please let me know on the final outcome for my record purposes. The issue seems to be very interesting.
Riz

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