We came across a situation during our study. A few weeks ago I found a Company proposing a new method of concrete mixing (High strength concrete, details attached).
Now the company might have made the procedure having in mind that the silica fume pellet ought to be properly crushed. However this is an excellent method to enhance the strength of the transition zone by replacing Ca(OH)2 with C-S-H, which they might it might not have intended for. Point is that we are concentrating the silica fume on the surface of the aggregate particle. This will no doubt increase our concrete Strength to let say 10,000 psi. But what about the bond between steel and concrete that develops later. We can't expect the same kind of replacement behavior and enchantment there right.
This is where we're stuck right now. This will make the steel concrete bond the limit state which is a far more severe problem then early concrete crushing since we can have steel slipping in joints that defeats the entire purpose of proposing high strength concrete.
My question is what could be potential solutions for this problem.
Development length fc' to be limited to a value?
Abandoning the procedure by the company?
Or any others please comment. Thanks
In such cases it would be prudent to conduct pull out test and ascertain the bond stress applicable so that appropriate development length can be used and dia of bars can be decided to take care of bond property.
In such a case in order to improve bond stress in steel bar and high strength concrete following options can be tried.
1. Increase anchorage length , either by increasing embedded length
2. Provide suitable type of end hook or end bolt for the reinforcement
3. For the same area of steel use smaller bar diameter instead of larger bar diameter
4. Use fibre reinforced concrete at joint location or end section or section where higher bond stress is desirable
5 Improve surface roughness of bond at desired location.
6 Provide more confinement of reinforcement in the region of high bond stress.
7. use end plate anchorage.
Ishwar Sonar sir could you please elaborate points 3 and 6, I mean we can't make things too complicated for normal laborers (I presume you understand the general situation of the Indian subcontinent) so that (for the time being) rules out point 4,7 and 5( thats too much of a hassle)
But 3 and 6 seem to be quite alluring, please explain how smaller bar diameter could bring a change and how can confinement in joints be a solution.
I'm not too familiar with the development length concepts since we were recently focused on making our concrete strength higher
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