Our front steps are concrete and exposed to the elements. Even worse than being exposed to the elements, the front porch roof gets ice dams in the gutters and for several days after a snow, we get melting water dripping down directly onto the steps, where it freezes at night. So we throw down salt to melt it. That salt water works its way into the concrete and I guess refreezes or something. This takes a toll on the concrete, and it gets all cracked up and crumbling
So about a month ago, I got a sack of fast setting concrete patching material and started with the bottom step, hitting the concrete with a regular hammer to break up anything that was loose. I'd smack it with a hammer and then brush it with a steel brush to get anything loose to come out. Then suck it all up with a shop vac. It's tricky to decide when to stop. You could demolish the entire set of stairs with the hammer if you wanted to. With this bottom step, thee were actually tree roots working their way up into the concrete from below and helping to break it all apart.
It took the entire sack of concrete to repair this one step. So I stopped there for the day.
Next day, I got two bags of concrete for the upper steps and started beating the crap out of them with the hammer. They just crumbled in spots.
In this picture you can see a plastic trash bag on the lower step. That's to keep the water from evaporating from the concrete as it cures. It will be stronger if the water stays in there as it cures.
I used these chisels to try to undercut the edge of the concrete as much as possible so the patch would want to stay securely in place. It's tricky because sometimes as I was undercutting the edge, a big chunk would just pop off, and I'd need to form a new edge.
I didn't get any pictures of me mixing the wet concrete and applying it, because it was fast setting and I had to work quickly.
In this picture you can see I took the plastic off the bottom step and the top steps have now been patched and have plastic on them.
The stairs are solid here, but look like hell. They need some paint.