huangyhg

thermal strains in concrete -concrete circular tanks
i would like to konw what is the recommended % of shrinkage reinforcent in circular concrete tanks t=450mm(specially nearby the base slab)when the tank walls are bottom fixed.
bs8007 calls for 0.35% ......aci350 valls for 0.45%. will it exceed 0.9%

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you cannot just pick a figure out of a code. you need to calculate it.
bs8007 appendix a does it like this:
[a] net effective strain ? = r?(t1+t2)
where r is your restraint factor (usually 0.5);
? is your coefficient of thermal expansion for the aggregate;
t1 is the fall in temperature between hydration peak and ambient (this is dependent on section thickness, type of formwork and type of cement used);
t2 is a further temperature fall due to seasonal variations;
[b] decide what the limitation on your crack width is (0.2 or 0.1mm to bs8007)
[c] smax is the estimate of maximum crack spacing
smax=(crack width/?)
and also:
smax =(fct/fb)*(?/2p)
(fct/fb) is the ratio of the tensile strength of the concrete to the bond strength between the concrete and steel. taken as 0.67 for high yield bars fy =460n/mm2.
? is the assumed bar diameter, p is the steel area based on areas of surface zones;
equating these two equations for smax you can calculate p. this will give you the required percentage of steel to resist the thermal strain. if it is less than the minimum, you use the minimum, else you take the higher value.
also the percentage is a percentage of the surface zone area, not the entire section thickness. for a wall up to 500mm thick the surface zone of h/2 is used, greater than 500mm, a constant surface zone of 250mm is used.
looks like i lost all my greek letters, strange as they showed up in the edit post board. ok, never mind,
e=ra(t1+t2)
smax=(crack width/e)
smax=(fct/fb)*(bar diameter/2p)

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