Aluminum posts / cracked CIP wall cap 1

[RHTPE]

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

 

[hokie66]

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.

 

[Ron]

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

Ron

 

[RHTPE]

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

 

[RHTPE]

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

 

[RHTPE]

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

 

[RHTPE]

Third group.

 

[RHTPE]

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

 

[civilperson]

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

 

[RHTPE]

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

 

[SteveWag]

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

 

[SteveWag]

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

 

[RHTPE]

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