Failure of concrete block retaining wall due to starter bar corrosion

[demayeng]

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

 

[civeng80]

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

 

[kieran1]

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

 

[hokie66]

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.

 

[BigH]

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

 

[rscassar]

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.

 

[ron9876]

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

 

[HeavyCivil]

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.

 

[hokie66]

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.

 

[demayeng]

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..

 

[Ron]

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.

 

[msquared48]

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

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

 

[demayeng]

Thanks Ron,

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

 

[hokie66]

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.

 

[demayeng]

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..!

 

[hokie66]

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.

 

[Ron]

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.

 

[csd72]

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.

 

[hokie66]

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.

 

[Ron]

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.