Repairing the concrete retaining wall of the stairwell below the school building (rev. 5-16-24)

TO THE WEST of the school building is a below-grade, concrete retaining wall, which forms one side of the emergency-exit stairwell that leads from the Social Hall up to our “Ruben Alley” and then to the public alley. There is a large (6-foot-wide) cement patch on this wall—evincing a previous problem in that spot. That patch has some rough edges that I find troubling, for the concrete there is actively degrading. And the longer that we wait to repair it, the more costly it will be.

November 2023
(As usual, click/tap on an image to enlarge it.)

I mention this now because we are contemplating concrete repair work elsewhere on the premises. Handling all of it at the same time would be prudent. So below the jump, this post discusses the stairwell situation. It closes with a suggested remedy.…

I cannot find a record of this patch. (The digitized expense ledgers go back to 2012.)

The patch is lighter in color and smoother in texture than the rest of the wall. It has thin expansion cracks, which suggest that it is no longer keeping water out, if indeed it contained waterproofing material. 

More troubling still is the spalling in the wall: to the left of the patch, and along much of its lower edge. In those areas, the deep cracks and the uneven surface protrusions are sure signs that rusting steel rebar is weakening the concrete. The gaping cracks bring more rainwater to the rebar, hastening its rusting. Unsurprisingly, those two areas seem to be getting worse over time.

November 2018

This patch was noted in the Property Condition Assessment (2019), which gives us a baseline for visual comparison.

March 2024

This image is out of sequence because it most closely matches the baseline photo.

November 2023

I looked at this section of wall most carefully last November.…

May 2024

Some of the concrete has crumbled due to spalling — revealing the horizontal corroded rebar underneath.

Historical perspective

In the 1940s, standard construction methods for concrete placed the rebar too close to the surface, and they did not coat the steel with a rust inhibitor. Nor did they know of the need to waterproof the surface. Consequently, it’s not surprising that spalling occurs here and there on our concrete walls. 

Judging from the Historic Structure Report (2003: IIC-22), this issue should be considered a matter of seismic safety.

Remedy

Jim Farasatpour, the structural engineer, looked at some of November photos in January 2024. He was inclined to agree that we need to adopt the same sequence of measures here as we did last year on the foundation wall on the south side: remove all loose concrete, clean the rust off of the rebar, apply a rust-inhibitor, and cover the metal with epoxy cement. He said that epoxy might make a better finish coat than cement.

Surely it would make economic and logistical sense to take care of the degrading concrete here at the same time as the planned repair work is being done on the front façade’s foundation wall. (Concrete contractors charge a relatively high overhead to get their equipment and crew to the job site.)

Finally, should we be thinking in terms of applying some kind of waterproof coating (such as Xypex) to the entire retaining wall, to prevent future rebar corrosion? Probably not, in my view. If there haven’t been other such problems during the past 75 years on the remainder of the wall, they are probably not going to start now.