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Aluminum posts / cracked CIP wall cap

Aluminum posts / cracked CIP wall cap

Aluminum posts / cracked CIP wall cap

(OP)
I was reviewing a construction issue with a contractor this morning.  He constructed a cast-in-place (CIP) retaining wall - nominally 20" thick with form liner on both faces, approximately 4" relief on the liner, reinforcing was based on the "core" 12 inch wall, 2" cover on rebar, resulting in 8" out-to-out of rebar.  The original architectural detail for the railing posts called for embedded sleeves that would permit the posts to be grouted-in after the wall was complete.  There was no additional rebar called for to surround the sleeves.

Needless to say, the embedded sleeves were left out.  The rail installer core-drilled 4" diameter overlapping holes (2" c/c).  The architect changed the posts from the original round to 2" x 4" rectangular.  The aluminum posts were painted to avoid aluminum-to-concrete contact.  The posts were set in the core-drilling holes and grouted.

Several months after the installation was complete, the wall cap cracked at each post, both sides, perpendicular to the wall face.  In fact, the posts at the ends of the wall completely spalled off the cap.

I suspect that the grout used was either non-shrink or an anchoring grout (trade name avoided) that has an expansive property.  Most non-shrink grouts exhibit a +4% volume change early on, shrinking back to about a 0% to +0.3% change at full cure.  Had the embedded sleeves been used the slightly expansive nature of a grout would have been better contained and perhaps had less of an impact on the wall's cap.  I do not suspect that a reaction between the aluminum posts and the grout caused an expansive corrosion product.

Anyone care to chime in?  Can a petrographic analysis of the grout used determine its expansive nature?

Ralph

RE: Aluminum posts / cracked CIP wall cap

If expansive grout is the culprit, I would have expected splitting at the top, a crack parallel to the wall.

Without seeing the situation, I would think the transverse cracks are more likely restraint shrinkage cracks, occurring at the stress risers created by the drilled holes.  This is assuming that the cap was cast after the wall, in which case the wall would have already done some shrinking, and the later cast cap would then have experienced differential shrinkage with the restraining wall.

RE: Aluminum posts / cracked CIP wall cap

Can you provide a sketch to show orientation of the overlapping holes and the 2"x4" post, I think it (the orientation) has something to do with this situation.  

RE: Aluminum posts / cracked CIP wall cap

(OP)

Forgot to add:

The cap was cast monolithic with the wall.  The core drilling occurred some time after the wall was completed.
 

RE: Aluminum posts / cracked CIP wall cap

I agree with Hokie66...the top of the wall, being exposed, will shrink more than the mass of the wall and the holes have created a reduction in cross section at the top (thus a stress riser on either side of the hole).

Linear shrinkage is going to be greater than lateral shrinkage, thus the orientation of the cracks.

RE: Aluminum posts / cracked CIP wall cap

I agree with both.
Also, it seems there is more grout space at the long sides ([4'-2"]/2 =1"). The grout expansion in the direction of wall obviously has facilitated the forming of cracks, which have already been weakened by effect of drying shrinkage (likely be hair line cracks for 12" thick wall). Check the location without posts, it might clear the air (shrinkage cracks should be observed in everywhere at an approx. regular interval).  

RE: Aluminum posts / cracked CIP wall cap

I guess those top bars were in there for a reason, and they just found out what it was.

RE: Aluminum posts / cracked CIP wall cap

I doubt that grout expansion had much to do with it.  You originally described the balustrade on the wall as a "railing", but there is a great big bottom member on this thing which would restrain the concrete shrinkage of the top of the wall.  Differential thermal movement may be involved as well, but the jacking force on the end of the wall by the rail was too much for the concrete to handle.  By your photos, I would think there are no U-bars around the end posts which might have controlled the cracking to an acceptable width.

RE: Aluminum posts / cracked CIP wall cap

hokie66 is right...not only do you have the restraint of the rail in the primary direction of shrinkage, but some possible linear expansion of the rail as well.

 

RE: Aluminum posts / cracked CIP wall cap

In my humble opinion, as I am not an material person, there is more than T & S effect had occurred. I don't know could it be caused by expansion, bending, or the combination of all 3 factors. But I do know this is a structural failure, as all 3 photos shown deep, wide diagonal cracks, and the last one shown the concrete has been chipped away. Looks like you are facing difficult repair works.

RE: Aluminum posts / cracked CIP wall cap

RHTPE

Long tubular railings typically have some kind of expansion mechanism.  Does yours?

RE: Aluminum posts / cracked CIP wall cap

(OP)

Thanks for all of the input folks.

I will be visiting the site tomorrow to get a first-hand close-up look at the conditions.  I will report back on what I find.  My client is the contractor who built the wall.  The CM maintains that it is his responsibility to fix the wall's cap, as he should have provided the means to prevent the cracks from occurring (e.g. providing additional rebar or something).

Our position is that the Design Professional who signed-off on the alternate method of rail installation (core-drilled holes vs embeds) has a responsibility to know the effects of the change.  We believe that it is not a construction defect attributable to the contractor who built the wall in accordance with the contract drawings in effect at the time and who was intructed by the CM to omit the embeds.

Anyone care to comment?

Ralph
 

RE: Aluminum posts / cracked CIP wall cap

Miecz is correct in that the balustrade system should have provision for movement.  Without that, the ends of the cap were always doomed to fail due to the proximity of the last post to the end.

The CM is just a paper pusher, he wants to push it all your client's way.  Your client's liability is strictly from having cut the reinforcing.  Did anyone authorise him to do so?  

RE: Aluminum posts / cracked CIP wall cap

(OP)

Just for further clarity - the railing contractor cored the holes, not the concrete contractor.  My client had completed the walls before he started.

 

RE: Aluminum posts / cracked CIP wall cap

Then it is a slam dunk.  The railing contractor and the CM did it, they pay.

RE: Aluminum posts / cracked CIP wall cap

(OP)

hokie66 - If everything came about as it has been related to me, then I agree whole-heartedly.  I also know that folks have a tendency to shade the chain of events so as to put themselves in the better light.  AND, when the CM is withholding your final payment as a bargaining chip, being right ain't always the most pragmatic stand to take.

The final outcome will be interesting.

 

RE: Aluminum posts / cracked CIP wall cap

Careful here guys...what's the contract chain?  Did the rail installer do so under a subcontract to the contractor?  Did the CM contract with the rail installer?  Did the owner contract directly?

Was there any provision for linear expansion of the railing system prior to the change from blockouts to cored holes?  Was the railing installer informed of the location of the rebar?  Was the rebar placed as required by the plans?

Would the same failure have occurred had the blockouts been used instead of the cored holes?

Liability is rarely a slam dunk.  Numerous questions have to be answered first.

 

RE: Aluminum posts / cracked CIP wall cap

(OP)

Ron - I'm very much aware of the points you raise.  The planned site visit will enlighten as far as the structure in question goes.  It should also shed light on the manner in which thermal expansion of the rail is accommodated.

From there, I would be exploring the chain of contracts and their language.

The underlying reasons for the change from embeds to core-drilled holes is also relevant.

I suspect the same cracks may have occurred using embeds, but at least the top horizontal bars would not have been cut, thus the cracks would have been smaller.

As I stated early, "If everything came about as it has been related to me ...".  While I have an obligation to my client, I also have an obligation to myself - to ferret out all contributing factors and known facts and only then formalize a conclusion.

Ralph
 

RE: Aluminum posts / cracked CIP wall cap

Ralph,

Best wishes in resolving this amicably.  I think your approach is sensible and measured.

As an aside, this thread is an example of what eng-tips is about, a good question, with advice freely given from a range of perspectives, all after thoughtful consideration.

RE: Aluminum posts / cracked CIP wall cap

Ralph,
Ditto on Hokie's sentiments. Good discussion.

Ron

RE: Aluminum posts / cracked CIP wall cap

(OP)

My site visit today was enlightening.

We broke off a portion of the wall's cap at the end of a wall where it was severely cracked.  The crack patterns I saw were of 2 varieties - very narrow cracks of uniform width essentially perpendicular to the line of the wall (shrinkage) and multiple cracks radiating from the posts (I believe attributable to the grout used), wider at the post tapering to almost nothing at the cap edge.

From the location where we removed a corner of the cap we were able to carefully examine the grout used by the rail installer.  It appears that 2 (or more) types of grout were used - the major portion of grout (up to within an inch of the top of wall) was white in color with a redish-brown tinge.  Over that (say the finish course) was another another layer of grout that appeared more like concrete with a blue-gray fine aggregate.  I suspect the intial grout to be a product with gypsum, which would have had a fast set property.

What little rebar exposed by removing the corner of the cap indicates nothing that would have restrained the concrete shrinkage OR the expansion of the grout.

I will compile the photos taken and post them tomorrow.

Ralph
 

RE: Aluminum posts / cracked CIP wall cap

(OP)

To all who commented - thank you for your input.  My report has been submitted - the conclusion:

The core-drilled holes provided a "natural" control joint to provide a place for shrinkage cracks to occur.  Careful examination of the grout used to anchor the aluminum posts revealed that 2 different grouts were used, one very white in color and very likely containing gypsum.  The expansive nature of most cementitious grouts when mixed to a highly fluid state, combined with the initial shrinkage crack, caused further cracking of the wall's cap.  The lack of additional reinforcing steel to counteract the concentration of shrinkage at each post was also a contributing factor.

Several of the 2"x4" posts had bulged on the 4" face, suggesting the perhaps the inside of the aluminum tube was not coated, reacted with the grout after being dropped into the grout-filled core-drilled holes, producing hydrogen gas, and essentially "inflated" the tube like a balloon.  This is the most plausible conclusion, since it is highly unlikely that enough condensation water existed to fill the post to the same height as the bulged surface would suggest (considering that the railing had been in place less than a year).

The were no thermal expansion/contraction joints in the aluminum railing system, so the sun's heating effect and subsequent expansion of the rails further exacerbated the cracking.  One could hear the sounds eminating from the railing as they were warmed by the sun.

Again, I appreciate everyone's input.

 

Ralph
Structures Consulting
 

RE: Aluminum posts / cracked CIP wall cap

The original design was suspect but it was not used and the contractor can repair his design at his cost.

RE: Aluminum posts / cracked CIP wall cap

(OP)

civilperson:  The orginial design indicated only the reinforcing required for the structural intent of a retaining wall.  The means to install the railing system was not evident on the structural detail and simply referenced the Architecturals.  The concrete contractor was not in the loop regarding the change to core-drilled holes - that was a decision between the CM and the railing contractor.  No additional reinforcing was required by those who made the change to core-drilled holes, nor was any specific type of grout specified.
 

Ralph
Structures Consulting
Northeast USA

RE: Aluminum posts / cracked CIP wall cap

If the 4-inch side has "bulged", I think that water entered and froze. Freezing water exerts a powerful force.
Steve
 

RE: Aluminum posts / cracked CIP wall cap

Looking at the second set of photos, photo 6, I think I can see expansion on all sides of the rectangular member well above the embedment level. This expansion cannot be explained by "expanding grout". I'll experiment with beer cans in the freezer tonight to verify my freezing/expanding theory, but I'll remove the beer and replace it with water.
Steve
 

RE: Aluminum posts / cracked CIP wall cap

(OP)

SteveWag:  Look carefully at some of the posts and you'll see that the drainage holes were drilled after painting (you can see raw aluminum around the edge).  The posts & railing were painted in the field after installation, as evidenced by the section of concrete cap that we removed.  Not having the fabrication drawings makes it difficult to know how the post & rail sections were fabricated.  But then, my charge was to evaluated the cracking of the retaining wall's cap, not the railing's shortcomings.

Concrete reacts with aluminum to produce a fair amount of hydrogen, which is why all aluminum forms have to be "seasoned".  I have seen many pictures of walls formed by new aluminum-faced forms showing the bubble trails left in the surface of the concrete wall.  If the posts were simply dropped into grout-filled holes as I suspect, where can the gas go that is trapped inside of the post?

I would drink the beer, not simply remove it from the cans.  You have beer in square cans?
 

Ralph
Structures Consulting
Northeast USA

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