Portland Cement Problems After Skim Coating A Patio Slab

Let’s Learn About the Right vs. Wrong Way to Resurface
Existing Concrete Patio Surfaces

I received this email from Trisha asking me:

I have encountered a problem on my patio. After much research your website was the only one that made it clear there was a difference between concrete and cement.

We have a slab in our back yard (like a river rock slab), we wanted to skim it over with a smooth layer. We did so with what the hardware store told us to do it with, which was Portland cement. But it came out with lots of cracks and you can scratch your fingernail in it.

Is it because we used cement and not concrete?

My reply:

Hi Trisha:
First, thank you for telling me how you found my website. Glad my concrete-vs-cement page helped you.

Next … the easiest way to explain cement vs. concrete: cement is the dry, powdery product; once it is mixed with water and aggregate, it is called “concrete”. So no, your problem wasn’t in the terminology … your problem was a very ill-informed salesperson. 😡

What a shame! You were given horribly wrong information!!! Even I know that you can’t put pure Portland cement onto a concrete (or rock) slab! And I’m no concrete expert by any means.

**First … I seriously suggest that you go back and confront the store manager about what happened because you’re not going to like my answer, meaning this isn’t going to be an easy “fix”, nor cheap. (Unless you clean off the crumbling Portland and forget about doing anything further.)

Let me tell you that the husband of a very close friend of mine was a professional concrete finisher and I picked his brain for you … and in a nutshell, here are some guidelines I can offer you:

  1. Portland cement CANNOT be used all by itself in this situation! it needs some kind of aggregate added to it … so this was mistake #1;
  2. You cannot just put a “skim coat” onto the slab … you need at the minimum a 2″ thick “layer” of concrete … so here was mistake #2;
  3. The slab must be prepped with a “bonding agent” … this allows the new concrete to adhere to the slab … mistake #3;
  4. There very well might be the need for the bonding agent to also be mixed into the new concrete;
  5. The slab might possibly need to be wet down, before the bonding agent is applied;
  6. You should use a 5-sack mix (called 5000 lb concrete);
  7. The new concrete needs to be kept moist for 28-days – it will reach about 90% of it’s full cure (hardness) in that time frame – spray it down with your hose every day & night to dampen – this will help it to not crack and cure properly;
  8. And then seal with a good quality concrete sealer after the 28 days.

And before any new concrete can be laid, you are going to have to THOROUGHLY clean/scrape off all the flaking Portland cement.

PLEASE NOTE: There could be other things to take into consideration before attempting this concrete project. I DO NOT know the particulars. There are many variables when it comes to concrete and the specific project.

***Bottom line … I suggest you call a reputable professional concrete company to add the smooth “layer” for you. Forms have to be built along the outside edges to hold in the concrete; expansion joints may need to be added into the wet concrete; depending on the size of the slab, interior wire “mesh” might need to be laid to help it not crack; the top must be finished off correctly to make it smooth, etc.

Does this overview of things-to-do help you? I hate to hear of problems like this. If nothing else, you deserve all your money back for whatever supplies and such that you bought for this project!

~~~

Trisha wrote back:

Thank you very much Claudia. Your info is extremely helpful!

We must be on the right track because yesterday we bought that same 5000 concrete mix and blended it with the Portland cement. It made a smooth finish like we wanted. It does not have the best appearance but it bonded and it is concrete. HA!

It is just a small 16 x 20 fenced in back patio, so anything done to the slab is better than what it looked like before.

Again thank you for all your info.

Comments

  1. Just for future reference, when over-coating stone or concrete, after a thorough cleaning, I suggest you should etch it using a mix of muratic acid and water, followed with a wash-down of ammonia to neutralize the PH.

    This is the proper way to “prep” an existing project for an overlay or even for a longer lasting paint job!

    To get that smooth finish with concrete, use a wooden float over the surface to sink the small rock and bring up the cement.

    Oh, and I’m glad to hear your project turned out okay.

  2. I don’t know If I said it already but …This blog rocks! I gotta say, that I read a lot of blogs on a daily basis and for the most part, people lack substance but, I just wanted to make a quick comment to say I’m glad I found your garden art projects blog. Thanks, 🙂

    A definite great read..

  3. Nice one, I thoroughly enjoyed reading your article. I really appreciate your wonderful knowledge and the time you put into educating the rest of us and thank you for this special read.

  4. Grant Harper says:

    I never had any idea that there was a difference between cement and concrete! It is really helpful to learn this, since I want to get some concrete repair done, but kept saying cement repair. This definitely wouldn’t have meant the same thing, so it is great to learn this. It also sounds like it might be best for me to look for a professional business to help me get this concrete put in so that it is done properly. I want to be sure that the concrete ends up being smooth and is sealed properly, which sounds like a job for the pros! Thanks for the great post!

  5. Nice info for her. I did my own before I read this research and ( i am no expert) novice . But have been reading and doing some projects. It seems the answers were correct given on site. You need to clean it, add a bond agent, mist the floor concrete before laying down the concrete paint. My old school italian grandfather did this stuff and it always looked great. I was 7 years old so not sure how he did it.

  6. Benny Schiller says:

    Great ideas! I was really impressed. It reminded me of how I got into the tedious job of trying to figure out what I was to do with my small patio. That’s when I came up with some ideas that you could use for a small patio like ours. I wrote a blog post about it right here if you want to check it out:

  7. Tommy Eagle says:

    Hello,

    I just wanted to make some comments and corrections on some of the things I read, and to give you a less expensive way of doing it. The first is , in your case Trish you can use the portland cement but first you have to apply a bonding agent (Octoweld), it’s basically glue for cement. the portland cement must be mixed with sand, 2-3 ( a heavy 2) parts per sand to 1 part cement. Now this would only be used to level off your pad, the low area’s, you lay a 2′ x 4′ on top of the stone patio and get it as flat and level as you can only filling in the low area’s and no more than an inch depth at a time. Depending on the weather you will need at least 24 hours for the cement to dry it doesn’t have to look great and it won’t but that’s ok. Now, you can go to home depot and get what’s called resurfacing cement, I don’t know what state you’re from but the materials they carry vary from state to state. ask someone in the department and they will guide you. NOTHING has to be added to this material and it is very easy to work with. a bag generally covers approx (100 10’10’ area) square feet on a very flat surface. In your case it would require a little more because of what you’re resurfacing. Its about $18 a bag. You can also color it many different ways with colored grout simply fan it out and trowel it in. It goes on on the wet side. First wet the existing pad lightly. After you mix it up start at one end you get your trowel and go down each side to the height you want your pad to be, then do the other side, get your 2 x 4 and set it down on top of each (pad) you made down the sides, now you are going to fill in the middle, dump the material and holding your 2 x 4 down on the pads you made on the sides you simply screed it back ( holding it down on the pads but not pushing into them you want to glide it back and as you come back move it side to side. You have to move quickly cement gets hard get your trowel I would use whats called a pool trowel they sell them at home depot also and start troweling it down until it’s smoothed out then when it is almost dry you take a sponge, ( big yellow sponge some are square shaped others are shaped like a big dog bone) you will find them in the paint section of home depot) and in a circular motion rub the sponge on the pad, you will see how easy it is. Good luck! p.s. You do NOT need to use 5000 psi concrete for a patio the 300 mix is more than enough I don’t even use 5000 psi on driveways. What that means is this 5000 psi means that concrete can hold 5000 lbs per square inch and unless you’re planning on putting a bldg on there 3000 psi concrete is more than adequate. I answer any questions you have have a great day! 🙂

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