What is "Steel Reinforced Epoxy" No, it's not epoxy for reinforcing steel. It's "steel reinforced epoxy." 

http://www.jbweld.com/products/j-b-weld-twin-tube

How in the world does steel reinforce epoxy? 

I can understand how carbon nanotubes dispersed in epoxy can help strength it, quote from paper below:


  The mechanical behavior of multiwalled carbon nanotube/epoxy
  composites was studied in both tension and compression. It was found
  that the compression modulus is higher than the tensile modulus,
  indicating that load transfer to the nanotubes in the composite is
  much higher in compression. In addition, it was found that the Raman
  peak position, indicating the strain in the carbon bonds under
  loading, shifts significantly under compression but not in tension. It
  is proposed that during load transfer to multiwalled nanotubes, only
  the outer layers are stressed in tension whereas all the layers
  respond in compression.


But back to steel - as far as I know, steel is a crystalline material and it doesn't possess any property akin to CNTs. Can someone explain how this particular epoxy works?
There's no &quot;steel&quot; in that stuff. It is simply filled with inorganic particles, probably (partly?) iron dust. I'm using jbweld too. Quick test with a bar magnet showed the tube with the &quot;steel&quot; component as well as the cured epoxy are indeed slightly ferromagnetic.
Most importantly, the filler reduces macroscopic shrinking during curing. That's a general principle: The density of the filler particles stays constant, while that if the glue component goes down, remarkably. The filler material type is not really important, only its surface must be compatible with the epoxy.
Plus the filler particles impede the propagation of cracks, and reduce the permeability to water, oxygen, etc. Again, that's a general property of fillers, not specific to this one.
It's hard to tell what the specific advantage of using iron/steel powder is, instead of some inorganic oxide (vulgo &quot;sand&quot;). A metal might catch environmental oxygen that diffuses in, increasing the temperature stability of the epoxy. If that's the case I don't know, but this stuff is remarkably temperature-resistant.