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Old 03-12-2013, 05:41 PM   #1
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Different colors of conc in the same sidewalk


not too long ago we had to dig up & replace a section of sidewalk,,, turns out the damn'd thing should've been called a ' grade beam ' as it was 8" thick

here's what i don't understand,,, pictures show the lower part is much darker than the top - why ? thought it might've been due to 2 different loads but, if that were the case, sidewalk would also be dark from 2nd trk to the curb,,, don't know how the rest of the 300' s/k is as we only open'd up 2' x 4.5'.

from what i can see, the aggregate load's the same on both strata,,, just mysterious how this occurred,,, flyash would change the color but there's no cold jnt - anyone want to hazard a wag ?

thanks in adv - always learn more from you guys than i expect
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Old 03-12-2013, 06:41 PM   #2
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Different colors of conc in the same sidewalk


What if they added calcium to the last load? Contractors often add or increase calcium as the day gets late, because their guys are about to go on overtime.
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Old 03-12-2013, 08:19 PM   #3
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Different colors of conc in the same sidewalk


I see that quite often, and more commonly with older concrete. THe bottom (earth) side is always darker, and generally has a "blue-ish" hue to it, while the top is obviously lighter. I suspect it's the same load of concrete through out the thickness. My best guess is that the lower part of the concrete is wicking up some kind of mineral content from the soil................
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Old 03-12-2013, 08:29 PM   #4
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Different colors of conc in the same sidewalk


There are many things that could have cause this.
How old is the sidewalk?
The sidewalk is 300ft. long x ??? wide x 8" thick, so it should be at least 20 yds. of concrete.
They could have poured the bottom 1/2 1st than poured the top 1/2 last with a higher w/c
They could have added a lot of water while finishing.
The top 1/2 cured at a different rate than the bottom.
The effects of de-icers over years.
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Old 03-13-2013, 04:03 PM   #5
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Different colors of conc in the same sidewalk


How old is the sidewalk? 6 or 7 yrs
The sidewalk is 300ft. long x ??? wide x 8" thick, so it should be at least 20 yds. of concrete. - yes, more than 1 trk load,,, but that's simple math - your point ?
They could have poured the bottom 1/2 1st than poured the top 1/2 last with a higher w/c - no apparent cold joint
They could have added a lot of water while finishing. if they added much wtr, there'd be crazing or dusting - none of the above
The top 1/2 cured at a different rate than the bottom. if that were the case, there'd be evidence of curling slabs - no curling
The effects of de-icers over years. this is south atlanta - we don't get ice worth a damn - by the time they call us for deicing salts, its melted

so far i like jomama's theory best - dick ? tscar ? canary ?
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Old 03-13-2013, 04:07 PM   #6
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Different colors of conc in the same sidewalk


mort, we often used ca cl ( up to 1% ) on hes hgwy pours where we had to open roadway to morning traffic,,, the ' last load ' would've displayed the color throughout the whole slab, no ?
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Old 03-14-2013, 09:45 AM   #7
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Different colors of conc in the same sidewalk


Possibly. However, often times when I'm not sure if I have enough left to finish a panel, the contractor will have me wet up what I have left and pour it low, so if the last load is delayed, they won't leave a visible cold joint. Perhaps the contractor did this, and then the next truck showed up in time to avoid a cold joint.

I dunno, just thinking out load (or typing, or whatever).
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Old 03-14-2013, 11:36 AM   #8
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Different colors of conc in the same sidewalk


jomama's answer makes sense,as doe's Mort's about spreading the remainder of a load,could also be a change of cement from another producer,on the balance of that particular pour.
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Old 03-14-2013, 01:36 PM   #9
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Different colors of conc in the same sidewalk


thought about cement supplier change + where it laid in the silo & stockpile stone moisture,,, thanks, ALL
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Old 03-15-2013, 07:29 PM   #10
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Different colors of conc in the same sidewalk


They could have poured the bottom 1/2 1st than poured the top 1/2 last with a higher w/c - no apparent cold joint I can pour the bottom 1/2 than the top half with 2 differents truck & have no cold joint.
They could have added a lot of water while finishing. if they added much wtr, there'd be crazing or dusting - none of the above This does not happen all the time
The top 1/2 cured at a different rate than the bottom. if that were the case, there'd be evidence of curling slabs - no curling Not always. Pour a slab & set a piece of Styrofoam on it. You will get a spot that is a different color & will never go away, because of the cure rate.
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Old 03-15-2013, 09:24 PM   #11
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Different colors of conc in the same sidewalk


[QUOTE=TOOL82;1138122]They could have poured the bottom 1/2 1st than poured the top 1/2 last with a higher w/c - no apparent cold joint I can pour the bottom 1/2 than the top half with 2 differents truck & have no cold joint. true but here in atl you'd have a hard time finding any poured like this + there's not enough room for the truck to fit

They could have added a lot of water while finishing. if they added much wtr, there'd be crazing or dusting - none of the above This does not happen all the time HERE it doesThe top 1/2 cured at a different rate than the bottom. if that were the case, there'd be evidence of curling slabs - no curling Not always. Pour a slab & set a piece of Styrofoam on it. You will get a spot that is a different color & will never go away, because of the cure rate. the same can be said for a 1x piece of wood or scotch tape - still think jomama's on the right track - but the color change is so constant - almost seems like leachate would show at different heights to me,,, evidently the mystery continues
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Old 06-20-2013, 08:49 AM   #12
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Different colors of conc in the same sidewalk


Quote:
Originally Posted by jomama45 View Post
I see that quite often, and more commonly with older concrete. THe bottom (earth) side is always darker, and generally has a "blue-ish" hue to it, while the top is obviously lighter. I suspect it's the same load of concrete through out the thickness. My best guess is that the lower part of the concrete is wicking up some kind of mineral content from the soil................
Tore this concrete out a few weeks ago, and had the same thing again. This concrete was only about 10 years old or so, but it was on relatively wet subgrade for most of it's life from what I can tell..........

Different colors of conc in the same sidewalk-dsc00006.jpg
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Old 06-20-2013, 04:47 PM   #13
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Different colors of conc in the same sidewalk


jo, you may have hit on the cause how's the baby ? thanks
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Old 06-20-2013, 05:08 PM   #14
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Different colors of conc in the same sidewalk


Wicking Iron out of clay would be my guess. Just a guess though.
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