Minimum reinforcement for mass/plain concrete 1

[canwesteng]

Curious how others approach this - I have a block of concrete used largely as a chunk of mass to prevent overturning/uplift. The design works for internal forces as plain concrete. I would never just pour a block of concrete without bar, since it could crack and fall apart, but if I add 0.2% bar it becomes a tremendous amount of bar on each face. I usually reinforce say the outer 6" at 0.2% and call it ok if it's looks reasonable, or possibly assume a crack occurs somewhere and check that the bar works as interface shear reinforcement. Are there any other reasonable approaches?

 

[BAretired]

I would probably arbitrarily use a cage of #5 @ 12"o/c (15M @ 300) on each face, top, bottom and 4 sides, without even bothering to consider percentage of steel. In fact, there seems to be no theoretical justification for selecting a percentage of steel in a block of mass concrete.

 

[Tomfh]

If you’re concerned about a crack forming then add enough bars to carry the tensile force.

There is no point trying to meet arbitrary steel percentages. That’s not what minimum steel is for.

 

[OldDawgNewTricks]

ACI 350-20 12.13.2.7 might be a reasonable approach:

 

[Lomarandil]

I think the news (risk) of a block of monolithic concrete meeting its demise due to ethereal "cracking" has been greatly exaggerated. Lots of plain concrete deadman blocks in good service around the world.

(assuming the ratios of the major dimensions are less than roughly 2)

 

[canwesteng]

Thanks for the input all - ACI 350-20 is great reference, and I guess aligns very similarly to my guesstimating. I agree that the risk of the block cracking and falling apart can't be that high - plenty of stuff is made of lock blocks and holds together fine.

 

[Tomfh]

I think the news (risk) of a block of monolithic concrete meeting its demise due to ethereal "cracking" has been greatly exaggerated.

Yes it’s all a bit silly isn’t it. We happily anchor into concrete using design methods that rely fundamentally on concrete tensile forces. Yet when it’s a cubic yard of concrete all of a sudden we lose our nerve. “It could crack!!“