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]

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.

 

[kslee1000]

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.

 

[RHTPE]

Please see the attached for the wall & post condition.

 

[RHTPE]

Forgot to add:

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

 

[Ron]

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.

 

[kslee1000]

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).

 

[csd72]

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

 

[RHTPE]

Here are some pictures of the end of the wall. These look like grout expansion to me.

Ralph

 

[hokie66]

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.

 

[Ron]

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.

 

[kslee1000]

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.

 

[miecz]

RHTPE

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

 

[RHTPE]

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

 

[hokie66]

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?

 

[RHTPE]

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

 

[hokie66]

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

 

[RHTPE]

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.

 

[Ron]

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.

 

[RHTPE]

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