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
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
RE: Failure of concrete block retaining wall due to starter bar corrosion
RE: Failure of concrete block retaining wall due to starter bar corrosion
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
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
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
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
RE: Failure of concrete block retaining wall due to starter bar corrosion
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
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
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...