Carbon fiber masonry restraint goes big

tower introSoon after the Menokin chimney, we had a project to repair an 1870s brick building. This era of construction meant ⅛” black mortar joints that had suffered deferred maintenance for at least the last 70 years. Nearly all mortar from the decorative projecting bands of masonry was gone where water had worked down through the skyward-facing joints. Failing internal drain leaders at the building corners had also caused major deterioration of mortar through the depth of the walls. Some of this drain failure appeared to be related to bad design that put dissimilar metals in contact with the anodic cast iron drains downstream of large copper (cathodic) gutters.

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The dark decorative terra cotta band typifies skyward facing joints

Had the repointing project occurred two years earlier, it is likely the damage the building and tower suffered from the 2011 earthquake would not have been so severe. At the tower in particular, the shaking loosened bricks at the round windows and there was corner cracking as well. Some of this was no doubt increased by the shifting cantilevered balconies above. But these windows also span between the cast iron stair landings inside which meant they were moving freely between two hard points inside that were pushing on the building above and below during the earthquake. The metal band holding the polycarbonate window also created a hard point.

Rather than do a typical repair with a series of criss-crossing through-tower ties with ugly pattress plates on the exterior, we decided that carbon fiber buried in the joints would make a more elegant restraint to future outward thrust around these windows. We opted to wrap the tower in epoxy-impregnated folded CF tape plus filled the center of this “C” with epoxy to create a custom built-in-place rigid beam.

Tower Spec

In addition to drawings, we did a simple mockup in the shop to 1) help us work out any bugs and 2) to help describe the process to the contractor so they could bid this uncommon repair. We were lucky that on the tower the joints were closer to 1/4″ high.

Brushing epoxy into CF weave

Brushing epoxy into CF weave

Screeding excess epoxy off surface

Screeding excess epoxy off surface

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Opening the folded CF into a “C”

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Injecting epoxy into the CF fold to form a “beam”

Peeling back the masking to reveal clean brick

Peeling back the masking to reveal clean brick

The bricks were masked at each joint to be pointed on this project. Remnants of 1960s brick waterproofing turned white in contact with high pH materials, so we had to isolate the brick faces when repointing. Because this step eliminated staining of the brick faces, there was essentially no masonry cleaning required, except local brushing with a vinegar/water solution in a few spots.

Obviously when repointing with lime mortar which is slower to set than portland mortars, it is not an option to hose the entire building down with muriatic acid after the fact because the masonry work was done sloppily. This crew was pretty neat, but with the masking, they could actually afford to be messier with their pointing because the masking would remove any debris and stains. We were very appreciative to have this “Rubber Mask” from American Building Restoration to protect the waterproofed bricks from turning white, but it also took away the cleaning step which is such an integral (and pricey) part of most masonry project budgets. This means we also eliminated the efflorescence risk that comes from leftover acids and the risk of biological growth that plagues so many projects after cleaning due to residual surfactants that become food sources.

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You’ll notice that because repairs on this building were handled on an as-needed basis inch by inch, the conservation team determined the areas needing repair and chalked them for the masons. In order to agree on what all these smaller repairs added up to, the conservator transferred the marks to the drawings and totaled the lineal inches in concert with the masonry crew so the amount of billable work was agreed upon in advance of repointing. You’ll note the CF annotation for the carbon fiber course. Although this meant more time for the conservators, it stretched the budget for repairs. In fact when this project started there was no expectation of being able to complete repairs to the tower or the “moats,” but because we only repaired joints that needed it, these projects were able to be completed for the same budget that originally was only intended for repairing the main building out at the public corner where falling bricks had become a life safety hazard.

In spite of the unique nature of the tower’s carbon fiber stabilization, we had a very agreeable masonry team from Atlantic Refinishing & Restoration that worked with us on this one-off installation. The tape was installed two courses above and two courses below the round windows, wrapping the exterior and then the ends overlapped for a foot before being turned up and locked around a brick above and below into the adjoining courses.

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It took several guys to keep the tape uncoiling evenly into a C and not have it snag on the bricks. They quickly learned how to inject the bedding epoxy, press and fold the CF open into a “C” and keep it open with wood wedges. Then when it came time to inject the epoxy that would fill the “C” to make it rigid, this was a quick and seamless process. We inserted this carbon fiber tension “beam” recessed from the brick faces by 2” so that the front of the joint could be properly pointed afterwards. Superintendent JD Stinner (black shirt) and Masons Mike Manning (white suit) and Mike Barnett were the crew.

Exacting work

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Wrapping the corners was the biggest challenge we faced.

In order to impregnate the tape as it went into the joint without wearing the epoxy, a board was hung over the scaffold rail and up to the wall that allowed the tape to be draped across it where epoxy was brushed on and the excess wiped off with a screed right before placing the CF in the joint.

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When the entire perimeter CF was installed into its C formation at the appropriate depth, the crew came through with tubes of the epoxy to fill the center of the carbon fiber. This was then left to set overnight before the rest of the joint was pointed the next morning.

One of the other last-minute additions to the first phase of this project that became possible due to stretching the budget was repairs to two of the retaining walls that held back the earth around the basement light wells which were bowing. (These retaining walls were humorously referred to as “moats.”) These 10′ tall retaining walls were unbuttressed and stretched between 80’ and 150’ in length without internal restraint to counteract the thrust of the earth. Bracing inside the light well would have been unattractive and would have potentially hampered the movement of equipment in and out of the building. Likewise, digging out the earth behind them and installing proper buttressing was not an option since they were either under public sidewalks and entangled in electrical substations below the street or under the elaborate (and in daily use) childcare playground for the building’s occupants. We needed to find a way to resist the thrust of these light well walls from inside the “moat” that was relatively unobtrusive.

At the sidewalk moat where only the top few feet had bowed above where some earlier bracing had been installed, we opted to relay the top several courses of the wall and reset the stone cap, laying carbon fiber into the top few courses as it was relaid and ensuring the ends of the CF “beams” were pinned into the return walls.

In the 150’ long moat against the childcare area, we integrated a single continuous course of carbon fiber. This was aligned to bear against the back of star plates set over threaded rod that tied the wall back to concrete deadmen strategically located in the playground so that the holes could be dug, poured, and the padding relaid over a holiday weekend.

Moat Spec

 

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