Plastic shrinkage cracks 2

[Thecapn007]

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!

 

[Ron]

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?

 

[BigH]

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.

http://www.sieu.edu/CCRU

Hit on "research" and they have programme called "curing".

 

[Thecapn007]

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!

 

[Ron]

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?

 

[JConc]

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.