Carbon Fiber Repair Options for Historic Buildings, Woodwork-Part 1

cedars-jamb-extend        piney-point-treads

Continuing on with our early testing on the carbon fiber options I foresaw for Menokin, I had to explain to Tim Macfarlane more about what I had done in the past regarding timber strengthening and share my sketches for what I thought would be practical and minimally-intrusive means of strengthening the types of deterioration found at Menokin.

RSI beam veneer.017

Solving challenges of deterioration where woodwork remains unpainted and visible requires different skills than in-kind replacements of whole timbers.  I’ve spent decades finding different ways of addressing these challenges:

  • scribing wooden infill to rotted ends such as window trim,
  • or installing carbon fiber rods into the ends of rotted treads, such as at this lighthouse, and then casting the missing sections in filled epoxy to allow continued  use of weakened framing members and even smaller structural members,
  • whereas the infill of beam intersections in the Rising Sun basement allowed large timbers to remain in place during repairs where the deterioration was isolated to a discreet area.

But obviously Menokin’s large timbers were going to need a different approach.

SH_10

In preceding decades I had taken the remaining “veneer” of a timber  –such as this corner brace from the Stratford Hall Smokehouse – and wrapped it around a new wood core so that exposed wooden members would appear unchanged. This works well in cases where there is limited load on the timber such as with the corner brace or with the basement window lintels at Londtontowne Public House where I could put additional hidden support above the visible timbers. But what options might we have to strengthen the center of timbers like the second floor joists at Menokin that would be visible from all sides and yet needed to be able to return to their functional loads in the building? Could we vacuum-bag a custom-fit carbon fiber beam in the center of the beams that would allow us to achieve all of these criteria?

girder interior.028

girder interior.029

frass-removal de-frassed_timber

We did our mockup with a short section of non-Menokin timber. Split in half, we were able to dig out all of the termite frass back to sound wood. That left us with two halves that were relatively sound but left a large void at the center when the two portions were put back together. Into this center we would vacuum-bag carbon fiber cloth to each half.


vacuum-bag

I covered this half of the hollowed beam section with plastic and laid in three layers of unidirectional CF cloth with the center layer oriented at 90-degrees to the other two layers, then constructed a vacuum bag for this timber, hooking it up to the pump. Of course we also had a release fabric in between.

pop-out_CF

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By the next day I was able to remove the beam from the bag and hand an absolutely tight-fitting stiff liner for the center of the beam. We were then able to pop the halves out.

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2CFhalves_joined

Here you can see how the two halves of carbon fiber meet and overlap at the joint to create a stiff beam. This hollow beam can be further stiffened by filling with foam. This way we gain tremendous strength without adding weight.

re-strengthened_timber

The timber and its carbon fiber core assembled, leaving a nearly imperceptible repair from the top, bottom and sides of the timber’s length.

Finding data related to carbon fiber use for this type of use proved elusive, so we needed to gain relevant information about weaves from experience. We thus began a series of tests. The best information I found about the various weaves came from an unexpected source in the form of a textiles curator at the Cooper-Hewitt National Design Museum and her book “Extreme Textiles: Designing for High Performance” from 2005. Matilda McQuaid put very technical information into extremely clear layman’s terms.

CF-tube-making02   CF-tube-making01

Wanting to learn more about the structural capacity of various weaves of cloth and with various fillers to the beam such as expanding foams, we created a series of consistent hollow beams with this simple rig that pulled the clothes through a bath of epoxy and screeded off any excess while making a taut tube.

hollow-CF-tube   loading-CF-tubes

Here you can see the perfect hollow tubes that were created. Then we loaded each style of beam to determine deflection coefficients and failure points, both hollow and filled.

testing-overview.032

Finally, as we continue through these next few posts on carbon fiber repair options, it probably makes sense to discuss cost. Many may conclude that this is an expensive way to go about repairing a building – maybe even wasteful. My experience from the development of repairs that fit the artifact over the last four decades proves otherwise.

While the first few models of any new approach are expensive while the steps in the process are developed and refined, they provide new options in the conservator’s toolbox that quickly evolve to cheaper and faster repairs. Most importantly, these will be repairs that are better for the artifact because they fit to it, rather than cutting the artifact to make our repairs easy. By not compromising the original and by making the repairs reversible, the artifact can be left as the artifact.

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