×
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

Plastic shrinkage cracks
2

Plastic shrinkage cracks

Plastic shrinkage cracks

(OP)
I was called out to a job for what a customer called "spoingey" concrete. He stated that the job called for a 9.5 yard order and would have no order back. Well the job goes as planned, up until the finisher calls complaining that the concrete just "took off" and then cracked all to hell. The mix was a 1" mix,5 sac with water reducer. No water was added at the job site, and the weather was warm with a bit of a breeze blowing. Now warm l mean it was in the 90's and the breeze about 10 mph. When l went to the job site l noticed the cracking had appear where they should be, in the control joints, but that most would jump out of them and come back in. Typical plastic shrinkage cracks were about but not terribly frequent, and on all inverted corners they were right there too! Tell me this, l think l know what happened, but is the finisher trying to pull a fast one over on my company. He thinks we over dosed the product with water reducer. l think not, not even close. Give me a professional way to tell this guy, we aren't buying this load for him! Yours truly, Q.C. man!

RE: Plastic shrinkage cracks

You didn't give one important piece of info.  How long did it take them to place the concrete and how old was it when they started the placement?

From your description of the control joints, it sounds like they didn't saw them quickly enough or not deep enough, but got close.

The temperature and wind conditions were right to cause some plastic shrinkage cracking, particularly since you had so little water to begin.

Have you had issues with dwell time for your HRWRA?  If they went long on the placement, his description that the concrete "took off" is an indication of exceeding the dwell time of the admixture.  That used to be a significant issue, but not so much in the past 10 years as the admixtures have gotten progressively better.  Another cause of sharp dwell reduction is overworking the concrete.

Are you sure you saw plastic shrinkage cracking?  What was the pattern and what did it look like?   What was the finish condition?  Was it hard troweled or broomed?

RE: Plastic shrinkage cracks

I've seen, in the last years, a lot of plastic shrinkage cracking and they were always diagonal to the jointing system - unless they were plastic settlement cracking (i.e., over close to surface rebar).  Ron, as usual, gives pretty good points.  I ran across the following website today that might help you all with respect to potential for plastic shrinkage cracking.

www.sieu.edu/CCRU    Hit on "research" and they have programme called "curing".
cheers

RE: Plastic shrinkage cracks

(OP)
Looking over the ticket, the truck arrived @ 10:15 and started unloading @ 10:23. They finished the job @ 10:50, so they had plenty of time left on the load. As for the plastice shrinkage cracks, they seem to have run  perpendicular with the control joints. They start in the middle of no place in particular and end in no place in particular, start no where and end no where. The finish they did was a hard troweled finish and did have some curing compound sprayed on top. So far on this school job, they have done excellent work and our one of our top customers. Thanks again!

RE: Plastic shrinkage cracks

Thecapn007...Yup...sounds like plastic shrinkage cracking.  Probably a combination of the low slump/wind/humidity conditions.

Plastic shrinkage cracks as you have described will tend to connect with time, resulting in "random" cracking.

Did they by any chance dry shake the surface?

RE: Plastic shrinkage cracks

In the future, providing your letter to the contractor with an  NRMCA "Concrete In Practice" Technical Info sheet on Plastic Shrinkage [CIP 5]would easily back you up. These bulletins are easy to read, to the point and provide preventative measures for all [educational]. In my experience these CIP's do not tend to get the contractors "backs up" too much.

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!


Resources