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Failure of concrete block retaining wall due to starter bar corrosion

Failure of concrete block retaining wall due to starter bar corrosion

Failure of concrete block retaining wall due to starter bar corrosion

(OP)
Hi,

We have recently seen a cantilevered concrete block retaining wall fail due to corrosion of the starter bars at the base of the stem where it joins the footing.

We have considered galvanized starter bars for these types of walls as a result, however predictably most builders have a problem with this.

I assume that the wall that failed had poor contact of the corefill at the base - that clean-out blocks were not used or the process was not done correctly.

Do you think doing the clean-out process is sufficient, as long as cover requirements from masonry codes are adhered to?

What are other peoples' thoughts on this?

Thanks

RE: Failure of concrete block retaining wall due to starter bar corrosion

I think for a retaining wall I would consider a reinforced concrete wall rather than a block reinforced wall.

RE: Failure of concrete block retaining wall due to starter bar corrosion

What about waterproofing the wall to prevent water ingress to the rebar?

However I would also recommend the RC wall rather than the core filled block wall

Kieran
 

RE: Failure of concrete block retaining wall due to starter bar corrosion

Core filled block walls have a reliable record of longevity, as long as the grout cover is correct and the grout is well consolidated.  Too little grout cover, or in some cases too much (which means the "d" is too small) can lead to premature corrosion.  In severe conditions or where moisture staining on the exposed face is to be avoided, a membrane should be used on the soil side.  Galvanised starters would help.  

RE: Failure of concrete block retaining wall due to starter bar corrosion

How much sacrificial steel did you give knowing that the starter bars would be subject to corrosion?   

RE: Failure of concrete block retaining wall due to starter bar corrosion

I use corefilled cmu all the time. Specify a waterproofing membrane to the earth retaining side (a good architect will provide a detail for this). I have never galvanized the starter bars.

RE: Failure of concrete block retaining wall due to starter bar corrosion

Use lots of masonry retaining walls. Clean outs and inspections before and after grouting should do it.  

RE: Failure of concrete block retaining wall due to starter bar corrosion

Are you in the United State? In taking a road trip through 3 Canadian provinces last summer, I noticed a lot of bridge construction, all of which used galvanized bar.

I have only seen galvanized reinforcing stateside in the field one time, and it was welded wire mesh. Epoxy bar is much more expensive than black, and fabrications have significant lead times. The reality is that the coating is often nicked, either leaving it vulnerable to corrosion or causing additional labor in repair-painting. Galvanized bar would appear to be more sensible, yet I still don't see it. Is anyone specifying it in the States? Any thoughts as to why Epoxy bar is the norm here?

Didn't mean to hijack your thread, just curious about this.

RE: Failure of concrete block retaining wall due to starter bar corrosion

If that is the case in the US, it must be because the epoxy salesmen have done a good job of marketing their product, which is inherently faulty in protecting reinforcement steel.

RE: Failure of concrete block retaining wall due to starter bar corrosion

(OP)
Thanks all

I am in Australia, where concrete block retaining walls are common.

It was not our design, however I have taken notice because it was a wall next to a kid's play area (!).

I think I will just be putting more attention on the clean-out process - emphasise the notes on the drawing and require an inspection.

Another question regarding these walls - does anyone know the reason for building the blocks off a formed hob on the footing? a lot of standard details (eg CMAA) show a hob however I've never seen one built. I've heard it's to properly locate starter bars, but it seems like a hassle to me..

RE: Failure of concrete block retaining wall due to starter bar corrosion

Using a formed hob (curb in the US)upon which to place the masonry will aid drainage, provided the lateral depth of the hob is the same as the masonry.  Having the masonry applied directly to the footing allows water to collect and not drain away from the mortar bedding, which is more pervious than the concrete, thus keeping water on the bond joint of the filled cells and footing.

A hob can be sloped slightly, yet maintain level block with the mortar bed.  This will help drain water away from the cold joint between the filled cells and the footing.

RE: Failure of concrete block retaining wall due to starter bar corrosion

Just spec deformed stainless bars.  That'll get their attention.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Failure of concrete block retaining wall due to starter bar corrosion

(OP)
Thanks Ron,

That makes a lot of sense. Do you specify this? it seems like forming it would be difficult

RE: Failure of concrete block retaining wall due to starter bar corrosion

I've never seen a hob for a cantilever block retaining wall, and would see no benefit.  If there is moisture behind the wall, it won't be confined to one level.  Through wall drainage will help, as will back face waterproofing.

RE: Failure of concrete block retaining wall due to starter bar corrosion

(OP)
Hokie,

My boss is of the same opinion as you. I just found this extract out of the CMAA manual:

3.10 HOB
Reinforced concrete footings for retaining
walls should include a means of positively
locating the steel starter bars accurately
and a means of providing drainage
through the wall at the level of the base.
Both requirements may be achieved by
including a concrete hob (or up-stand),
through which vertical starter bars are
placed and on which the masonry is
built. Horizontal 50-mm diameter weep
holes may pass through the hob at 1.2 m
maximum centres.


I've also attached the standard detail.

It's going to be difficult to get builders to agree to this..!

RE: Failure of concrete block retaining wall due to starter bar corrosion

The starters can be accurately located without a hob.  And you don't need a hob for the drains, just punch them through the block.  The CMAA section shows a hob which projects beyond the earth face of the block.  That goes against the only reason I could see for the usefulness of a hob, which is to allow vertical waterproofing, with an unimpeded resting place for sheet protection over the waterproofing.  So I'm not a fan of the CMAA detail.   

RE: Failure of concrete block retaining wall due to starter bar corrosion

demayeng...no, as with others, I see no particular reason other than the vertical waterproofing potential.  More effort than benefit.

Cleanout, consolidation, bonding and weep holes are more important.  You might consider at least one weep hole on either side of a filled cell, not just every few feet.

RE: Failure of concrete block retaining wall due to starter bar corrosion

demayeng,

I would think that these failures are more due to construction issues that design. Starter bars are never placed as accurately as they should be and therefore cover is compromised.

Though I would be surprised if a properly designed wall would fail simultaneously due to corrosion, I would expect one part to fail first exhibit signns of cracking e.t.c and the followed later on with a full failure.

If the whole thing failed at once then I would expect it is something more fundamental such as incorrect asessment of soil properties or a lack of drainage to alleviate pore pressure. The wall amy not even have been properly designed.

RE: Failure of concrete block retaining wall due to starter bar corrosion

One thing Ron said caught my eye..."each side of a filled cell".  I would never build a reinforced retaining wall without grouting all the cells.  Using H-blocks helps to assure complete filling, along with base cleanouts.

RE: Failure of concrete block retaining wall due to starter bar corrosion

hokie66...true and I agree, but some do.  No reason for cell weeps in a filled wall, obviously, just soil drainage weeps, which should be routed through the wall, not into the wall.

I don't particularly like masonry retaining walls....for a variety of reasons.   

RE: Failure of concrete block retaining wall due to starter bar corrosion

(OP)
It seems to me that having a hob would mean that any small amount of water that didn't fully get out the drainage holes would not be sitting against the pervious mortar joint. So even though I think it's too hard to build, i can see the benefit.

We've decided to eliminate the hob and/or galvanised bars, and just go with bitumen and polythene sheeting as waterproofing (unless a proper waterproof membrane is required).


csd72: i was exaggerating when i said the wall 'failed' - it looked dodgy so it was decided to replace it. The excavator driver said he barely touched the top of it and the whole length of wall fell over. All the starter bars along the length were apparently corroded.

RE: Failure of concrete block retaining wall due to starter bar corrosion

Another thing that I think makes the CMAA detail unrealistic is that the top of the footing would rarely be right at the external grade level.  The drains need to exit at grade, not on the top of the footing.

RE: Failure of concrete block retaining wall due to starter bar corrosion

Anything going on with the soil, drainage, etc. that may have made the water/moisture more caustic than would be anticipated? Is this near the coast? Have you been able to inspect the corroded rebar?

CMU retaining walls, especially up to 6ft or so, are extremely common here in Florida, and I have designed many without thinking about this problem. I have also never heard of this being a problem, other than maybe near the coast. Ron- have you ever seen retaining wall problems because of wall bar corrosion?

This is why I asked about the soil/site conditions, seems like a rare occurrence, but that has only been my experience.

RE: Failure of concrete block retaining wall due to starter bar corrosion

Yes,

i am also surprised that they are all corroded. Could it be possibly due to the use of ocean sand or seawater in the grout mix. I have heard of such stupidity before.

RE: Failure of concrete block retaining wall due to starter bar corrosion

(OP)
It was not near the coast (probably 40 kms away)

I haven't inspected it myself - I would say it is more that the core fill did not reach the footing junction properly and/or drainage was not done properly...

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