While concrete is durable, it wears down rather quickly for stone, as it is comprised of granules that were mixed together and allowed to dry when they were first poured. So as the concrete steps you use start to wear down and have uneven places of cracks and holes which will only get bigger with time and weather, it's important to know how to repair them. With a few supplies and tools and a small amount of knowledge as to how concrete works, steps can be repaired quite easily.

Clean the Stairs

Spray down the stairs with a hose or, preferably, a power washer, so they are clean of all dirt and loose pieces. If this is not done, the new layer of concrete won't adhere to the old layer and will bubble or crack under old loose pieces, even after it dries. Allow the steps to dry in the sun.

Prepare the Concrete Mix

Use the right kind of concrete mix: one that can withstand both the elements and people running or walking over it for the next few years. Choose a mix with at least 3,000 PSI and good adhesion. The concrete will come with its own bonding agent. Add all of these to a pail or wheelbarrow that you can easily access and dip into. Mix only enough concrete that you will use within 20 to 30 minutes. Mix it with enough water that it makes a peanut butter consistency, but not too much, or it won't adhere properly.

Apply the concrete

Tape the edges of the stairs where you don't want new concrete so you can have a visual guide when spreading the mix. Then spread your first layer of concrete, filling all the holes and pores with concrete spreading tools. You can use two trowels, one to spread and the other to lean on. You will need knee pads as you kneel on the old concrete or the hard ground so you can comfortably concentrate on your work. Use the edge of the trowels to smooth out the surface. Once you have added a layer or used all the concrete, let that first layer dry and then apply the next one after mixing more concrete. Repeat as necessary until the concrete steps are smooth and new.

Source:

Concrete Repairs 101

DIY Repair Concrete