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:
- 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;
- 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;
- The slab must be prepped with a “bonding agent” … this allows the new concrete to adhere to the slab … mistake #3;
- There very well might be the need for the bonding agent to also be mixed into the new concrete;
- The slab might possibly need to be wet down, before the bonding agent is applied;
- You should use a 5-sack mix (called 5000 lb concrete);
- 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;
- 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.
