Originally posted by: MrDudeMan
Originally posted by: rahvin
Originally posted by: MrDudeMan
so yield is the wrong word. good. next, concrete has tensile strength, but it is only 10% of its compressive strength. you can increase the tensile strength by incorporating polyproylene fibers into the concrete matrix, which also act as crack arresters. what if this wall were in compression? are you forgetting that concrete in compression has much higher resistance to flexure, even in the lateral direction? or are you reading this from a book? at its maximum ductility, concrete with poly fibers and steel mats would hold back a dumptruck while sustaining maybe a few cracks. i doubt it would fracture so badly that there would be a gaping hole in the wall. of course, that all depends on the size, depth, and type of concrete, but most types (if reinforced correctly) would resist this truck.
I realize that you think you know what you are talking about because if I remember correctly you are recently graduated and think you know everything (or even worse are still in college and really think you know everything). For all intensive purposes concrete has no tensile strength and that's a near direct quote from the PCA design book. As you should be well aware those fiberglass fibers you talking about using in the mix drasticly alter the concrete, and mix designs such as that are highly controversial. If I remember correctly the fibers result in an increase in tensile strength and a corresponding drop in compressive strength. They also have a nasty tendency to reduce the fire resistance of the mud (and who the hell knows what they do to air entrainment and whether superplastisizers have to be used to even make it finishable)
Any wall can be designed to sustain an impact with any load and I admit I have not watched the video. Chances are though that unless the wall is similar to those designed to house nuclear reactors the concrete will crack and cracks are failure. The amount of force generated by the impact of an HS-20 load stoped near instantly is immense. A 10 wheeler hit a local bridge near where I work (I don't know it's load status). It did 45mph to 0mph in 1 foot. (the deflection of the I-beam it hit). Concrete can take the impacts but it will crack under the impact (I've seen plenty of Precast I-beams hit) unless it has enough mass to sustain the impact without exposing too much of the section to tensile forces. I've seen cast in place single slope barrier with 6 bars on each face (1meter high, half a meter bottom width and 0.100m top width) and hooks at the top busted all to hell from a 20^ impact by what was likely an HS-20 load.
One other thing you should keep in mind, there are other Civil Engineers on this board and some probably have more experience than you do.
its kind of funny actually, because i dont think i know everything and i did just graduate. first, its all intents and purposes, not all intensive purposes
, and 2nd, before i posted, i asked our PE some questions regarding this and mixed in his answers with what i already know...i just added in some sarcasm
im sorry that you think i dont know what im talking about, and personally i dont really, but most of that wasnt my material. and for the record, while i am at work, i have access to the code books and design parameters which are updated yearly (sometimes biyearly) for any type of engineering you wish to read about. concrete does have tensile strength, just not much, and while it isnt typically accounted for in design, it isnt totally ignored either. i design slabs everyday, and even though i havent been doing it for 30 years, i have learned a thing or two about concrete and i have had the opportunity to actually do some on the job work. obviously i dont have a degree in it so i dont know exactly how an elevated slab routes load or piers can resist 20kips of uplift, but i know how it is designed since that is my job.
i really didnt mean to get in an arguement with anyone, and when i look back at it, i see that i was pretty much taunting you. however, for the record, i dont think i am a post-HS-graduate/pre-degree know-it-all and i will actually admit i know very little. i just know what i have learned this year from my experience at work and i remember a little bit from my materials class i took my senior year in HS at a local college.
p.s. poly fiber in concrete only makes up for like 1.5% of volume, so it doesnt actually hurt the compressive strength that much. post-tension slabs with poly fiber are pre-stressed twice but at half the force, and then they have the full force final stress. i think that is to counter any loss in the compressive performance of that particular mix. of course, they never get 1.5% exactly, so i would say it is anywhere between .75-2.25% (accuracy down here is highly dependant upon the concrete company).