×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Shrinkage in existing slab?

Shrinkage in existing slab?

Shrinkage in existing slab?

(OP)
I got called from the contractor on an adaptive re-use project we have under way to review some supposedly new cracks found in an existing elevated slab. The cracks were discovered when someone on the upper (2nd) floor knocked over a bucket of water, which subsequently poured through the cracks to the ground level below.
We have the original building drawings from 1935, which show a 4" concrete slab over 18" wide flange steel beams, which are encased in concrete. 4 approximately equal continuous spans (~8') with 1/2" bars continuous along the bottom and 4' bars at the top centered on the steel beams. 3/8" bars transverse at 18" o.c. for T&S. (3) 18WF70 beams spanning 31'-6" between 33WF200 girders, also encased in concrete.
The "new" cracks run parallel to the span of the slab, at approximately the 1/3 points of the 31'-6" main beam span. Almost identical in each of the 4 bays, and all but one of the cracks run roughly perpendicular to the support beams. The cracks propogate down and around the encasement on the 18WF's. Standard shrinkage crack, no? There are also small cracks approximately mid-span of the 33WF girders at the bottom, which could be explained by beam deflection (the drawings do not even mention the beam encasement let along any T&S control).
My issue is that the cracks were not noted prior, which doesn't mean they were there, just that they didn't stand out as an issue. However the underside of the slab was recently painted and the cracks appear to be through the paint. I just want to make sure I can't be missing something before I tell them to fill in the cracks and don't worry about it any more. Could the recent enclosure / finishing / HVAC installation of the renovated space (which had been originally used as a partially enclosed loading dock area and had been vacant for several years) bring about "new" or at least expanded shrinkage cracking? Last week was also the first "cold" overnight temps of the year (<40 counts as cold around here).
Thanks for any opinions.

RE: Shrinkage in existing slab?

Cracks across slab are almost certainly classic shrinkage cracks. Though shrinkage of concrete continues for long time, it approaches zero after several years (at most)......PCA, ACI have various graphs depicting shrinkage over time.

"New" cracks are quite doubtful unless some new heavy loading condition has occurred.......or corrosion of steel beams or rebar has been occurring.

Bending of steel beams might contribute to cracks at bottom of slab acting as "T-beam" type assembly......though sounds like just classic shrinkage.

John F Mann, PE
www.structural101.com

RE: Shrinkage in existing slab?

I agree that these are drying shrinkage cracks, which have been there for a long time. In the situation as described, these cracks are inevitable. And just because there is also a crack in the paint doesn't mean that the concrete is moving.

RE: Shrinkage in existing slab?

This classic question is answered difinitively with an Otoscope. Go look *into* the cracks where they were painted... If you see paint inside the crack, even the least little bit inside, they are old. Where the cracks are new, it is extremely obvious.

This recent paint job is your best friend...

RE: Shrinkage in existing slab?

(OP)
thank you all for your input.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - The Criticality of the E/E Architecture
Modern vehicles are highly sophisticated systems incorporating electrical, electronic, software and mechanical components. Mechanical systems are giving way to advanced software and electronic devices, driving automakers to innovate and differentiate their vehicles via the electric and electronic (E/E) architecture. As the pace of change accelerates, automotive companies need to evolve their development processes to deliver and maximize the value of these architectures. Download Now
White Paper - Model Based Engineering for Wire Harness Manufacturing
Modern cars, trucks, and other vehicles feature an ever-increasing number of sophisticated electrical and electronic features, placing a larger burden on the wiring harness that enables these new features. As complexity rises, current harness manufacturing methods are struggling to keep pace due to manual data exchanges and the inability to capture tribal knowledge. A model-based wire harness manufacturing engineering flow automates data exchange and captures tribal knowledge through design rules to help harness manufacturers improve harness quality and boost efficiency. Download Now
White Paper - Modeling and Optimizing Wire Harness Costs for Variation Complexity
This paper will focus on the quantification of the complexity related costs in harness variations in order to model them, allowing automated algorithms to optimize for these costs. A number of real world examples will be provided as well. Since no two businesses are alike, it is the aim of this paper to provide the foundational knowledge and methodology so the reader can assess their own business to model how variation complexity costs affect their business. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close