Asked on Mar 16, 2014

Newly poured concrete has dark spots and streaks throughout..HELP!

by Sunnyca
We had a new concrete patio installed this week and after top cast was removed we noticed there was dark concrete streaks in some areas. It looked like hardened cement that had dried on top and was darker in color than the rest of concrete.
The day they installed the concrete it was hot about 80 degrees and very windy. They applied a top cast per our request to expose some aggregate and left that to dry overnight. In the morning, they used a power washing and broom to clean the top cast off. After asking about the dark streaks in some area they told me they would clean them with acid, but I believe they applied acid and power washed the entire patio, which ended up creating more streaks and newly formed brush marks.
We still have money on the table that we have not paid them. The licensed contractors suggestion to remove the dark spots shown in the picture is to use a sand blaster. He said another round of acid would be too strong. Does anyone have experience with sand blasting? Will this help? Any other methods you would suggest?
  22 answers
  • If it helps, it appears to be a nice looking patio from the pictures - good forming and jointing. What I can't tell from the pictures, is how consistent the finish is. I'm guessing this is a Topcast 03 or 05..?? Regardless, with a micro-finish the surface should be like sand paper - without any sort of residual concrete paste above the sands in the concrete mix. If some paste is left, we have had luck with straight acid washes or with HD 24 from Surface Gel Tek. The HD 24 is an acid gel that seems to burn better. The sandblasting would also work, but the blaster needs to have good control with it to get a good consistent result. After all, the idea behind these chemicals is to produce a sandblast finish without the sandblast mess. If there is not residual concrete above the sands and color is the bigger complaint, you might want to let the concrete cure a bit more as the colors will change and may blend better after a few weeks. I'm not sure what sealers are allowed in your area, or if it is common to use a sealer there (CA is very strict), but a light acrylic seal sometimes helps pull it all together. You can get a sense of how it might looked sealed by wetting the entire area. Hope that helps.
    • Sunnyca Sunnyca on Mar 17, 2014
      @Schneider Contracting Corporation Thanks so much for the reply and insight! They used Topcast 15 and what is left on top of the concrete is exactly as you described residual concrete paste. The acid wash they used (not sure what brand) seemed to make the problem look worse and created inconsistency in color across the entire patio. In your opinion, would the sand blaster be the best option to remove the cement paste that still exists? Will sand blasting create more inconsistency in color across the patio? Do they sand blast the entire patio or just the areas that have the cement paste on them? Appreciate your help!
  • If they sandblast, do the whole thing. The sandblasting result is slightly different from Topcast, so it would need to be consistently done across the entire patio. The concrete color should be more consistent when complete, but the hazard in sandblasting is exposing too deeply in areas that will then expose more of the larger aggregates (rock/gravel) in the concrete mix (rather than just the sands). Depending on the color of those aggregates, it can really change the look.
  • Cyndi Moore Tippett Cyndi Moore Tippett on Mar 18, 2014
    I actually like the look you received, it seems to give it a dimension that is attractive. By the time you decorate it and it "weathers" some I think you will be happy with it. If it was mine, I would leave it because if they keep working on it, they could make it ugly.
  • Mak Meyers Mak Meyers on Mar 18, 2014
    I agree with Cyndi--love the look as is...
  • Mags Mags on Mar 18, 2014
    My contractor told me one time, I guarantee you the 30 day rule......I looked at him, thinking he was crazy and what did he mean? Ma'mm 30 days guarantee means you won't even notice it in 30 days. Well he was right, by the time I placed flower pots, outdoor rugs & furniture you never notice the concrete. So over the years as my husband & I done projects we look at each shake our heads and say remember the 30 day guarantee rule! Then we burst out laughing..........
  • Debbi W Debbi W on Mar 18, 2014
    as a concrete contractor myself, it takes 30 days for it to cure, It is still wet even when dry to the touch, It looks like a fine job to me, relax it will be fine.
    • See 3 previous
    • Mike Mike on Mar 29, 2021

      I just had mine done 3 days ago and have these same spots. Please tell me they went away.

  • Sunnyca Sunnyca on Mar 19, 2014
    I appreciate the advice and agree that it may be best to just leave it as is instead of fixing and making it worse. To confirm, the darker streaks are not moisture they are cured cement residue that was not removed after TopCast was taken off. If anyone has experience removing dried cement paste would love to hear what has worked for you. Thanks!!!
  • Moxie Moxie on Apr 15, 2014
    From what I have read, I am thinking you were expecting a solid color with the casting process? Sounds like there was a miscommunication between you and the contractor. Concrete has so many variables that can flux the finish when you start adding colorant products it is almost evil. I agree that trying to fix it may end up a worse and it really doesn't look bad. If I am correct, perhaps you should express the dissatisfaction of communication and either be clear what you what and have them do what they need to to get it there with the understanding that if it doesn't work they need to remove and replace it all...or settle that you are a bit satisfied but are willing to accept it for maybe a small percentage discount without complaint? I don't think you will like the results of sandblasting it. I would wait a year to seal it and let nature mellow out the differences in color. Just my thoughts. Understand that decorative concrete is not easy and can be unpredictable. The fact you requested a certain cast to be used kinda puts the results on you even if application was partially to blame (which I am not sure it was)
  • Krazy K Krazy K on Apr 21, 2014
    There r products at home depot and lowes to "color" ur concrete. My husband had poured a slab outside our side door and I hated it looked soooo drab. U can pick from a variety of colors .I was lucky I found mine in the clearance section and only paid $5 for mine and went home and painted my concrete. It seals and also makes it skidproof. I used Olympic" Rescue it"...the only drawback is, it takes quite a bit to cover. Its thick and doesn't go very far. Hope this helps...Good Luck!
  • Ponce Construction Ponce Construction on Jun 17, 2014
    We also are a licensed concrete contractor in OC, CA for over 25+ years, and we state to the homeowner and in our contracts that "WE CANNOT GUARANTEE THE UNIFORMITY OF COLORED CONCRETE". Concrete is a natural product just like stone, and depending on the day of the pour and the weather conditions will determine many things in the finished product. I'm in agreement with the previous postings that you need to allow time for the concrete to cure. It will change in 30 days and then again in three months and again in three years, etc. Also was it a cloudy day when your contractor poured and finished the concrete? If it was, then there may be areas where the sun hit the concrete versus where the clouds shaded the drying concrete. This can bring about a mottled look. From your pictures, it appears to us that your concrete contractor did a heck of a good job! I would not worry about it for awhile. Just enjoy for now.
    • Dan Smith Dan Smith on Jan 04, 2021

      What is your opinion of what happened to my concrete. This is 40 days after pour. Thank you.

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  • AL AL on Oct 01, 2015
    We just had our driveway redone and are not happy with the end result. The new concrete was poured 6 weeks ago. There already is a crack and now there is yellow discoloration throughout the driveway. The contractor has agreed to come out and fix the crack but have yet to see him. Also, not sure how to remove the yellow. They have already been paid. Suggestions please!!
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  • AL AL on Oct 01, 2015
    Also, when they poured, the base soil was very wet...
  • I have a question for readers. We had a concrete slab poured, and it looked fine. Approx.. 1 year later, black splotches formed. No color was used, nothing poured onto slab, etc. I heard that this can occur due to the calcium levels of the soild. I would appreciate comments or suggestions.
  • sorry for mistype above: "calcium levels in the soil."
  • Carla Lord Carla Lord on Jul 02, 2020

    I know this post is very old but if you see this, I’m wondering if you ended up trying anything else? We have the same situation with black spots and I believe it’s from the top cast not being applied evenly. Places with very little top cast didn’t bring the sand to the surface so the spotted look is from some areas still having the smooth darker concrete and the lighter areas have the sand exposed.

  • Based on the photos, I wonder if it just needs more time to cure.

  • Chloe Crabtree Chloe Crabtree on Jun 16, 2021

    The most common ways to remove dark spots include repeatedly flushing and scrubbing the surface with warm water or acid washing the concrete surface

  • Maura White Maura White on Jun 17, 2021

    If they are the ones recommending sandblasting, then they are the ones that need to come and do it!

  • To me, it looks really great but if you're dissatisfied, that's definitely something to keep pursuing. Luckily, you hadn't paid in full. It sounds like a lot of factors are at play here but it's also important to remember that it takes a while for concrete to fully cure and its appearance will change daily in that time. Keep in communication with them and after the cure time is up, if you're still unhappy, pursue further options.

  • Holly Lengner - Lost Mom Holly Lengner - Lost Mom on Sep 30, 2021

    If you aren't happy with the work or are concerned about it in any way, call the company that installed it and see if they have any solutions.

  • Cory Weissman Cory Weissman on Aug 03, 2023

    Hi all! So…I’m sure plenty of new people checking out this post. I just had a slab poured in my backyard. It was a rainy day and also had the same dark grey spots. Don’t worry this will fade with time. It takes 28 days to pour, plus if it rained there may be additional moisture in the concrete. Be patient, they will go away! 😀

  • Mogie Mogie on Aug 05, 2023

    You should wait about a month so the concrete can dry or cure. Just like paint needs to cure so does cement.