SURVEY: How much T&S rebar do you provide in an upstand concrete parapet?

[Ingenuity]

Curious what amount/quantity of horizontal rebar you typically provide in a concrete upturn parapet that is poured AFTER placement of the roof slab, say 100 feet long, zero vertical joints?

I have a case of an existing (2 year old) 8+" wide x 15" high parapet (rather squat) with cracks every 4 feet. 3#4 horizontal bars in the center of wall were provided by the EoR. Amounts to 0.44% rebar.

Given the significant bottom slab restraint, the drying shrinkage cracks have telegraphed through the exterior finish (elastomeric-ish paint) and resulting in an upset building owner.

Curious what other engineers provide for such parapets (or a high curb in this instance)?

My guess is hokie66 will say "more than 0.6%" and I won't argue with that given this situation!

 

[hokie66]

Yes, but even with a lot of reinforcement, without joints, there have to be cracks. It is a bit surprising that they are that close together, as I normally see them at about 10' apart.

 

[JAE]

We usually try to reduce the spacing of joints in parapets as they get much higher thermal fluctuations than any other structural parts of buildings.
Parapets are typically the first to go bad over time.

Usually 12 ft. o.c. for parapets based on a "typical" wall joint spacing of 24 ft. for our region/weather.

Your situation is more like a small curb - the multitude of cracks suggests to me a concrete mix with too much water perhaps?

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[Ingenuity]

Thanks hokie66 and JAE for the reply.

Yes, vertical joints would have largely alleviated the issue.

The crack spacing of 4 feet also happen to align with the post-installed guardrail posts - core drilled right through the horizontal rebar I assume!

The concrete mix (or added water) may also have been an issue as you stated, compounding the situation.

 

[rapt]

Absolute minimum of .6% (I thought you would include me in the Hokie66 comment on this one). You will still get cracks but that should limit them to about .3mm wide!

The only way to avoid cracks would be to put full depth joints at a maximum of about 1.5 times parapet height.

 

[Ingenuity]

I thought you would include me in the Hokie66 comment on this one

I thought you would suggest a few UNbonded horizontal tendons

There are some "primary" restraint cracks that measure about 0.5mm at about 4' centres, and some "secondary" cracks between those cracks (so at 2' centres) and they are less than 0.25mm width, so right on 1.5H (1.5x15" = about 2').

 

[MrHershey]

We've started doing joints instead at every 20'-30' and seems to be going much better. The ones we've done this on are tropical regions so thermal fluctuations are much, much less than normal. Would probably need to decrease closer to what others in this thread are noting for nontropical regions.

 

[rapt]

Ingenuity,

I would have thought you would have lost your Aussie sarcasm after so long in the US, then I suppose Guam and possibly Hawaii are closer to Aus than the US and Canada may be North America but definitely not the US.

The secondary cracks are caused by the fact that the primary cracks have sufficient reinforcement to hold the cracks at .5mm, so sufficient tension develops between the cracks to cause another half way between. If there were expansion joints at the primary cracks, or even a little further apart at about 2 H, you probably would not have sufficient tension to create the secondary cracks.

Ignoring the "UNbonded" bit, I would never suggest using PT for crack control in a restrained concrete element