[Racinmason]

I would mix 2.5-3 parts fine sand to 1 part Portland cement. Mix it rich and thick.
That step looks very smooth, does it get slippery when wet? If it is slippery, you can add some anti-slip to some sealer, it will help a lot.

 

[Racinmason]

If anything is loose, take it out. It looks like it’s ready for some high strength concrete. Coat it with some bonding adhesive, and use a 50/50 mix of bonding agent and water when mixing. A parging trowel would work good in that size of a repair.

 

[Racinmason]

You're on the right track except for the concrete. That stuff has gravel in it, you will need concrete made with sand, not gravel. I believe Home Depot has sand topping mix. Or....try what Chandler said, Sika concrete repair mix. They also have stuff in a 2 gallon pail made for concrete repair. I always just make my own with Portland cement and mason sand.

 

[Racinmason]

Home Depot calls them margin trowels. They are a wide tuck pointer.

 

[Racinmason]

I just looked up the Sika concrete repair mix, they have a 1/4" maximum thickness. Probably not the best product for your application. Home Depot has Sakrete Sand Topping mix, cement and fine sand in a pre-mix. I will say that I never use the pre-mix stuff because I think their cement ratio is too low. I always just make my own formula. I would get a bag of Portland cement and a bag of play sand or mason sand. Mix 2.5 parts sand to 1 part of Portland, add the water/bonding agent. Use any trowel that is steel, not magnesium

 

[Racinmason]

Chandler's idea of the Sika stuff would work, even though it looks like you might have a thicker depth than 1/4".
It would sure be easier than the Portland/sand route like I mentioned. You only need half of a shovel full. A bag of Portland would do your job 25 times! I use it because I will use what's left over on another project.

That Sika comes premixed I believe, but you could still apply the bonding agent to the repair.

 

[Racinmason]

That's a good question. I think I would apply all of it in one coat instead of multiple coats, even though it might be thicker than a 1/4". I think that stuff is stronger than any of the premix stuff. I think it will match the color well.

 

[Racinmason]

I think that Sika stuff is already mixed.

 

[Racinmason]

I think blue painters tape would work. Might cover the step areas also.

 

[Racinmason]

Same approach. That gap will probably help you strength wise with the new concrete. You will force concrete in that gap, that will help stabilize that step if it needs it. Those might be made that way to add any amounts of steps you need, what's on the ends that holds it together, another seam?

 

[Racinmason]

Those are designed to allow any amount of steps needed, they are modular. You can stack that lower style on top of each other, very cool idea, I’ve never seen that before. The upper style is different and ends the steps.

 

[Racinmason]

I wouldn't worry about that, they are designed that way. It it was a solid chunk of concrete, it would weigh 2000 lbs. It's actually a great design. All of the ones I've seen were all built together, and in different heights. You have to find the exact size you need.