Carbon and Concrete
by Jake on Jun.19, 2008, under Composite Works, Miscellaneous
Before I get too deep into this one, no, I’m not crazy and yes, this will really work! The engineering behind it is sound and there’s evidence of it’s success elsewhere!
The problem: A good friend of mine recently purchased a fixer-upper house and knew that there was some problems with the 60′ front basement wall….the front yard was trying to get into the basement and had cracked the (what we later found out) incredibly inadequate 4 inch block that was used to build the foundation. The wall had a long horizontal crack, nearly the length of the wall, and in some places had bulged out 2 inches. Before he signed the deed, he had estimates. Jacking up the house and completely replaceing the wall = $30,000 and he had two estimates in the same range. What was interesting, however, was a third company that proposed an $11,000 solution by gluing carbon fiber strips to the existing wall. This wouldn’t straighten it but would stabilize it to keep it from shifting further. Naturally, this caught my attention and investigation into the process caught my imagination. I said to Mark, I have all the tools to do this and we can do it for less than $1,000 and do it better! (Mark seems to have a lot of faith).
I don’t have much of a “before” shot – but here’s an angle looking down the wall. Remember, this is only 4″wide block where it should have really been, at the very least, 8″ (better to be 12″).
So why carbon and not something less expensive like Kevlar or even fiberglass? The answer is because the carbon, for all practical purposes, will not stretch. The Kevlar, while strong, will stretch a little more than carbon and any movement in the wall can degrade it over time.
We decided to do one better and instead of using a pre-cured strip (which would be much easier to work with), we decided to vacuum bag wet uni-directional carbon directly to the wall. Before we did this, however, we needed to seal the block and even out some of the major irregularities (like grout lines). After masking off a vertical 6″ area every 4 to 5 feet, we rolled West System epoxy with slow hardner onto the wall with a 4″ paint roller. We made sure to get it really wet so the epoxy would soak deeply into the concrete…and soak it did. It sucked it up like a sponge. Then we came back with epoxy thickened with milled fiberglass (in hindsight, cabosil would have been easier to work with), and filled in the grout lines and squeegied it on the whole surface to fill in most of the pock-marks in the wall. Then a layer of peel-ply went ontop of that would smooth out the filler even more and give us a really good surface for secondary bonding once removed. There was still some minor irregularity in the wall – but it wasn’t critical. The epoxy at this stage did a couple of things for us. 1) it made the wall smoother so the carbon will be mostly straight and setup to handle loading in tension (what it’s best suited for) and 2) sealed the block so we could pull a vacuum over each strip.
Using 10 rolls of mastic vacuum bagging tape, we sealed the perimeter of the prepared areas and wetted out an 8′ long 4″ strip of uni-directional carbon fiber tape. A skim coat of unthickened fast cure epoxy was brushed onto the wall. The carbon strip went up followed by a bleeder ply of plastic (with tiny holes). Then a long thin piece of felt was applied to transfer the vacuum evenly followed by a top layer of vacuum bagging film. The amount of vacuum we were able to achieve was incredible and with the high flow of a compressed air venturi vacuum generator, we were able to vacuum up to five strips at a time.
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June 23rd, 2008 on 2:18 PM
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