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
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
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
RE: Aluminum posts / cracked CIP wall cap
RE: Aluminum posts / cracked CIP wall cap
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
Linear shrinkage is going to be greater than lateral shrinkage, thus the orientation of the cracks.
RE: Aluminum posts / cracked CIP wall cap
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
RE: Aluminum posts / cracked CIP wall cap
Ralph
RE: Aluminum posts / cracked CIP wall cap
RE: Aluminum posts / cracked CIP wall cap
RE: Aluminum posts / cracked CIP wall cap
RE: Aluminum posts / cracked CIP wall cap
Long tubular railings typically have some kind of expansion mechanism. Does yours?
RE: Aluminum posts / cracked CIP wall cap
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
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
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
RE: Aluminum posts / cracked CIP wall cap
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
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
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
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
Ditto on Hokie's sentiments. Good discussion.
Ron
RE: Aluminum posts / cracked CIP wall cap
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
First group of phots (if they'll upload).
I think both shrinkage cracks and expansion cracks exist.
The aluminum posts at either 2"x4" or 4"x4".
Ralph
RE: Aluminum posts / cracked CIP wall cap
Second group of pics.
Most common core diameter is 6".
There is some serious distress in these walls that no one accounted for when they decided to grout the railing posts into place.
Ralph
RE: Aluminum posts / cracked CIP wall cap
Third group.
RE: Aluminum posts / cracked CIP wall cap
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
RE: Aluminum posts / cracked CIP wall cap
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
Steve
RE: Aluminum posts / cracked CIP wall cap
Steve
RE: Aluminum posts / cracked CIP wall cap
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