Avoiding a Pilaster Disaster

A PILASTER IS “an upright architectural member that is rectangular in plan and… usually projects one third of its width or less from the wall, and that may be load-bearing” (Merriam-Webster’s). I.e., it’s a rectangular pillar or post that sticks out from a wall. 

Our sanctuary’s south wall has 7 load-bearing pilasters. And in July I learned that the base of one of them was visibly damaged, as described HERE and HERE. Because of the structural importance of the pilaster, this damage is turning out to be a significant problem.

Damaged base of pilaster, as seen in July
(As usual, click/tap on a photo to enlarge it.)

This blog post discusses the challenges involved in repairing this pilaster base.

Executive Summary

The base of this pilaster is falling apart. It needs to be rebuilt. However, it may be helping to support the wooden post inside the pilaster’s shaft — and if it is, the integrity of the building is at stake. Certain measures are needed to ensure that the pilaster is stable enough to be repaired. These measures are not trivial, and their full extent is not yet known.

Problem Statement

Dismantling the base (in order to rebuild it) risks unsettling the pilaster shaft above it — which could be disastrous for the integrity of the building. Consequently, our structural engineering consultants have become very cautious about ensuring that the post will remain well supported as the pilaster base is dismantled.

Damaged pilaster after stucco has been chipped away,
revealing apparent rebar corrosion (which causes concrete to spall off).

What Went Wrong in the First Place?

This pilaster appears to be different from the other ones in only one respect: as we learned last week from the discovery project, there is a break in the adjacent vapor barrier (a break that was apparently created during the original construction). Moisture that elsewhere has been passing through the stucco downward to the ground has apparently instead been channeled into the concrete pilaster base, soaking it. 

In turn, that moisture has caused the rebar (reinforcing steel rods) to rust. The rust naturally expands. As it does, it cracks the surrounding concrete.

On the left (western) side, the rebar is lined with a visible layer of rust that separates the rebar core from the nearby concrete. See the dark lines and cracking alongside the rust-colored metal.

Bottom portion, after brief application of fine-grade steel wool

Top portion

Top portion from another angle

On the right (eastern) side, the rebar seems to be much more badly corroded.

The iron bars that were supposed to add strength have instead created weakness. In other words, the projecting portion of the pilaster base should not be thought of as contributing much (if anything) to the support of the post. The base needs to be replaced. Hopefully the concrete stem (foundation) wall is, on its own, capable of supporting the post’s weight and its load.

A Hopeful Sign?

Judging from the position of the steel anchor plates at the base of the wooden post, the post does not appear to protrude beyond the stem wall. This suggests that the stem wall is supporting the load of the pilaster, such that the protruding base may not be structurally crucial.

Left (west) side

Right (east) side

What Next?

Just as we reached the expenditure limit of our consulting contract, the structural engineer advised the following next steps.

1. Remove the stucco that faces the concrete stem wall, on 2 feet (from center) on each side of the pilaster, to expose the concrete, and see whether cracks have propagated on this side of the stem wall. (Hopefully not, but we owe it to ourselves to check.)
2. To know that we can maintain support of the post while making the repair, we will need to run some calculations in order to figure out what the loads coming down on that column are.

Damaged pilaster after stucco has been chipped away,
revealing apparent rebar corrosion (which causes concrete to spall off).

Because the stucco is still firmly attached to the stem wall on either side of the pilaster, removing it does not seem to be a trivial step. At least, it’s more than I myself can do! Meanwhile, the load calculations and follow-up are estimated to take about 16 hours of the consultant’s time, or about $3,000. That’s not including the actual repair work, or the possible cost of shoring up the post.

It seems to be a touchy situation. Presumably the repair task needs to be completed before the wall can be successfully waterproofed. And because the waterproofing project is urgent, the pilaster-base repair is now doubly so. (Both the wall and the base are in such poor condition that they will continue to degrade markedly, and be more expensive to fix, with every rainfall.)