[arch dude]

I am struggling to read all the way through the "big thread." It's fun to re-live the excitement and energy of the early months, but it's hard to see where it's all going, and I have not found any definitive summary of results.

Much effort was spent on analyzing beams and whether or not rebar was as useful addition as a tension member. I have not seen any discussion of using Kevlar tape for this purpose. Have I missed it? Kevlar tape is used to provide strength, and especially strengh in tension, to fiberglass boat hulls. It is not too expensive, and being a woven product it has a large penetrable surface area bond and embed in epoxy. A strip of this stuff along the bottom of a gantry beam and perhaps one along the back of a column or other areas in tension might dramatically improve the rigidity of an E/G mill.

[TechGuy5002]

I am struggling to read all the way through the "big thread." It's fun to re-live the excitement and energy of the early months, but it's hard to see where it's all going, and I have not found any definitive summary of results.

Much effort was spent on analyzing beams and whether or not rebar was as useful addition as a tension member. I have not seen any discussion of using Kevlar tape for this purpose. Have I missed it? Kevlar tape is used to provide strength, and especially strengh in tension, to fiberglass boat hulls. It is not too expensive, and being a woven product it has a large penetrable surface area bond and embed in epoxy. A strip of this stuff along the bottom of a gantry beam and perhaps one along the back of a column or other areas in tension might dramatically improve the rigidity of an E/G mill.

My guess is doubtful. You would need to apply tension to the kevlar tape to create stiffness. I think internal slack without tension will not provide any additional strength. I an not sure if epoxy will wick into the kevlar need to take advantage of kevlar's surface area. Tyically for fluids to fully wick into a material with large internal surface area, it would require vaccuum impregnation to get rid of air bubbles inside. The high visocity of epoxy (plus granite) likely makes vacuum impregnation inffective for materials like kelvar. since the vacuum (at 15 psi) probably isn't strong enough to pull out the air.

FWIW: In some new construction, Concrete slabs for buildings use steel cables instead of rebar. But the cables are first clamped and tensioned (about 30K to 50K psi is applied to the cables), then they pour in the concrete and let it hardened. The cable clamps are removed and they cut off the excess cables outside of the slabs. The cables apply internal compression to the concrete slab to increase stiffness. You would need to do something like that for the kevlar tape.

[arch dude]

My guess is doubtful. You would need to apply tension to the kevlar tape to create stiffness. I think internal slack without tension will not provide any additional strength. I an not sure if epoxy will wick into the kevlar need to take advantage of kevlar's surface area. Tyically for fluids to fully wick into a material with large internal surface area, it would require vaccuum impregnation to get rid of air bubbles inside. The high visocity of epoxy (plus granite) likely makes vacuum impregnation inffective for materials like kelvar. since the vacuum (at 15 psi) probably isn't strong enough to pull out the air.

FWIW: In some new construction, Concrete slabs for buildings use steel cables instead of rebar. But the cables are first clamped and tensioned (about 30K to 50K psi is applied to the cables), then they pour in the concrete and let it hardened. The cable clamps are removed and they cut off the excess cables outside of the slabs. The cables apply internal compression to the concrete slab to increase stiffness. You would need to do something like that for the kevlar tape.

It should be possible to apply a modest amount of tension to the tape. We are not attempting to create a pre-stressed structure, just take out the slack, since kevlar does not stretch much at all. Perhaps a couple of tons of tension, applied using a turnbuckle and a frame of 4"x4"s outside of the mold. As to wetting, I would use a pair of rollers in opposition to force epoxy (with no filler) into the tape, driving the air out, prior to placing the tape into the mold. Pure low-viscosity epoxy should wick into the tape very well indeed.
I envision connecting the tape to a plate at each end and forming the E/G between the plates, with the plates becoming part of the mold and remaining as part of the resulting beam. The plates can themselves be E/G or they can be steel. In either case the plates would attach firmly and permanently to the tape, and would also have attachments (bolt holes) for the tensioners.

[CaptainVee]

I spoke to an owner of a pultrusion plant once.
He said that all aramids have a waxy coating. If that is true than the epoxy will not stick to the aramidto a level where stretching is an option.

[arch dude]

I spoke to an owner of a pultrusion plant once.
He said that all aramids have a waxy coating. If that is true than the epoxy will not stick to the aramidto a level where stretching is an option.

That's odd. I purchased some for another project. I got if from a firm that sells it specifically to reinforce castings:
Kevlar Tape