huangyhg

concrete cracking stress vs. fatigue
i didn't get any response in the concrete forum so i'll try here:
in the aci, there is the value given for modulus of rupture, fr = 7.5 x sqrt(f'c). this is the theoretical stress at which normal weight concrete will crack. for 4000 psi concrete this is about 474 psi.
my question is this. when concrete is stressed to something less than that value, whether it be 100 psi or 300 psi, it isn't supposed to crack. but if that concrete is subjected to repetitions of stress at these lower values, does the concrete fatigue (like steel) and crack at the lower stress levels after so many cycles?
if so, is there any documentation of this anywhere that would suggest acceptable stress levels with repeated loading? like the aisc fatigue tables for steel stress ranges?

my nilson text, in section 2.8, gives a little information (a paragraph) about fatigue in concrete. you can also search "endurance limit". the text states that the fatigue limit is generally in the 50%-60% range. some research i found suggests about 55%, which agrees. other research may show a little bit different numbers. this numbers are only meant for general guidance, since the actual fatigue strength will depend on many factors. references sited are a.m. neville properties of concrete, 4e john wiley and sons 1996 and "fatigue of concrete structures" sp-75 aci 1972.
macgregor and wight give similar numbers, about 55% of the static strength.
the %'s above are also noted in aci 215 "considerations for design of concrete structures subjected to fatigue loading". aci 215 is also cited by macgregor and contains some further information, s/n curves and shows the use of the goodman diagram mentioned in nilson.
are you referring to plain concrete or prestressed concrete. if it is prestressed concrete, then fatigue should not be a problem as long as the tension and compression stresses are within code prescribed limits. in fact this type of structures can go thru million cycles without any distress. you can check with north carolina state university (ncsu) site. i know dr. paul zia has run lot of research in this area.
handbook of concrete egineering
edited by mark fintel
van nostrand reinhold company
1974
section 6.9, page 170, fatigue of concrete
section 22.4.5, page 646, fatigue
thees sections discuss fatigue. they include s-n curves, goodman diagrams and also recommend a limit of 50% of the modulus of rupture as the limiting value for fatigue. table 22-4 on page 648 shows the % of modulus of rupture and the number of allowable repetitions. below 50%, unlimited repetitions. the table goes from 51% (400,000 repeitions) to 85% (30 rpetitions).
shin25 - this is a concrete floor slab with edge curling exposed to wheel loads. trying to find a rationale for how much curling could be acceptable. this led me to look at what magnitude of curling would not crack, and what magnitude of curling would allow some repetititions without cracking, etc.
i was looking for information on stress levels with various repetitions (like what eureka indicated).
non-prestressed.
eureka, does it indicate what repetitions are suggested for 66%?
66% (6,000 repetitions)

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