reginaldb
Civil/Environmental
- Jan 5, 2015
- 1
Hi folks. Question(s) about UHPC, mainly for my own education as opposed to anything related to a project-specific inquiry.
In every application I have seen, UHPC mixes are proportioned to include steel fiber "reinforcement". Per FHWA, these steel fibers help resist tensile stresses in the member both before and after tensile cracking of the matrix. Often times you'll see this notion taken a step further and characterized as a selling point predicated on the elimination of discrete mild steel shear reinforcement (stirrups, ties, etc.).
What I'm trying to wrap my head around is the following:
Is this a situation where (A) discrete shear reinforcement is being "replaced" by steel fibers, or is it more accurate to say that, (B) given the unique composition of UHPC (very low W/C ratio, high cement and fineness), steel fibers are actually a necessity in order to maintain the integrity of the matrix to the degree necessary for realization of an ultra-high Vc? Given I've never really seen UHPC literature discussing a mix proportioned WITHOUT steel fibers, I'm inclined to subscribe to (B). But maybe I'm oversimplifying?
Conventional reinforced concrete design procedures acknowledge that the concrete itself has some shear capacity that is essentially independent of participation of discrete reinforcement that might be present. But could the same be said for UHPC without steel fibers? Can you even have a reliable UHPC - with compressive strengths upwards of 20 ksi - if you don't have steel fibers as one of your ingredients? If the answer to this is "no", then this seems less like a case of steel fibers replacing discrete mild steel shear reinforcement and more like a case of discrete shear reinforcement being precluded because the threshold of need for Vs has been pushed way up.
Thanks in advance for your insights.
In every application I have seen, UHPC mixes are proportioned to include steel fiber "reinforcement". Per FHWA, these steel fibers help resist tensile stresses in the member both before and after tensile cracking of the matrix. Often times you'll see this notion taken a step further and characterized as a selling point predicated on the elimination of discrete mild steel shear reinforcement (stirrups, ties, etc.).
What I'm trying to wrap my head around is the following:
Is this a situation where (A) discrete shear reinforcement is being "replaced" by steel fibers, or is it more accurate to say that, (B) given the unique composition of UHPC (very low W/C ratio, high cement and fineness), steel fibers are actually a necessity in order to maintain the integrity of the matrix to the degree necessary for realization of an ultra-high Vc? Given I've never really seen UHPC literature discussing a mix proportioned WITHOUT steel fibers, I'm inclined to subscribe to (B). But maybe I'm oversimplifying?
Conventional reinforced concrete design procedures acknowledge that the concrete itself has some shear capacity that is essentially independent of participation of discrete reinforcement that might be present. But could the same be said for UHPC without steel fibers? Can you even have a reliable UHPC - with compressive strengths upwards of 20 ksi - if you don't have steel fibers as one of your ingredients? If the answer to this is "no", then this seems less like a case of steel fibers replacing discrete mild steel shear reinforcement and more like a case of discrete shear reinforcement being precluded because the threshold of need for Vs has been pushed way up.
Thanks in advance for your insights.
