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Heading: 1.2.1.1.1Concrete roping during trowel finishing
Message:
I would appreciate any suggestions. We are experiencing some problems with our concrete "roping" during finishing. We have no air entrainment. The concrete is poured on plastic both inside and outside pours. It occurs when the trowel machine goes one way the length of the slab and then turns to come back. A ridge is created with the paste(cement/sand) and it is difficult to level/flatten out. What can be done by the finisher and is there anything that the ready mix plant can do with mix to reduce/eliminate problem. I have been told that tilting the blades will help. Now blades are being kept flat. We have tried changing our mix several times and still have problem. We are using limestone but have also tried a blended creek rock/limestone mix. Any suggestions would be appreciated. We continue to get great breaks with our concrete. Our sand may have some gap grading and some extra sand on the 50/100 seive. Our sand is natural sand not manufactured.


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Sarah Wittenauer


Hi Sarah

May be should try to reduce the paste in your mix (cement +sand).

A too rich mix in paste can lead to such problems.

Rgds

Vikram
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Vikram Gunnoo


Sarah:

Here's a couple of things to look into regarding your issue.

1. Sand to Aggregate Ratio - might want to consider running the sand around 42% and the coarse agg. around 58%.

2. Might consider using a little accelerator to assure more of an even set throughout the slab.

3. Look at your w/c ratio. Pouring over poly tends to fool the finishers as it takes longer for the bleed water to come to the surface. Shoot for a low w/c ratio.

4. Are you on the edge of plastic shrinkage cracking? Fall weather?

Best,
Ryan
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Ryan Morman


Sarah,

There are admixtures designed to help the finishing process. It sounds like your mix has too much cementitious material in it. Perhaps just a touch of air entrainment 3 -3.5%) might help eat up some of the bleed water and not affect your finishers.

Let us know what you end up with.

Bob Banka
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Bob Banka


Please remember to be very careful if you start entraining air in flatwork that is getting a hard finish. There is the possibility of blistering and delamination from the finishing action on the air voids. We can avoid the whole "whose fault is the blistering" if we just don't put the air in there in the first place.

As to cause, my first instinct is the check the mortar fraction.
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Matthew Sherman
 
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