What makes steel awesome is that the properties can be tuned across a wide range.
Sure, cheap structural steel has a tensile strength of about 0.4 to 0.5 GPa and the most common types (i.e. cheapest) of "high strength" steel can reach 0.8 GPa.
But... some specialty steels can reach well over 2+ GPa. The steel for some cables have tensile strengths of 2.0 GPa, maraging steel for can reach 2.7 GPa.
There's a good reason steel is still so widely used. It's awesome!
My high school physics understanding of reinforced concrete is that the coefficients of expansion for steel and concrete are similar to a degree that it is very resilient to heat stress. I doubt that's the case for silk.
Also, steel, while more expensive than concrete, is still cheap compared to many other solutions for tensile strength. Also it tensile strength is sufficient for steel reinforced concrete. Concrete itself has performance limits for compression strength.
The problem for such an application is not the strength, but the chemical stability in time.
For it to be competitive, it would be necessary for the rate of hydrolysis of the protein fibers (which breaks the fibers) in an alkaline environment to be less than the rate at which rusting diminishes the strength of steel bars.
This is very unlikely. At most there might be a chance for such fibers to be used for reinforcing some other kind of cement, less alkaline than the common cements.
Also, for reinforcement applications, good adhesion between the reinforcing material and the reinforced material is necessary, which for silk vs. a mineral cement would need some kind of extra adhesive coating of the fibers, with adequate properties, which might be hard to discover.
These are of course, good points; cement probably being more porous than many people perceive. Just a little (hopefully interesting) speculation: adhesion might be less important, or entirely unnecessary if the reinforcing material isn't arranged like rebar. I.e. if it were just a tangle in the concrete.
Elastic stretching, the one that reverses completely on taking the load off, is the regime where there is a linear relationship between the stress load and the stretching. This should be seen as different from yielding which in mechanical testing is an irreversible change usually signifying a destructive change in the material. So, yes, silk stretches but is not damaged while doing so and the resistance it provides at the time relates to its strength.
