Cleaning

• Before you can repair a broken concrete surface, you must clean off the surface to remove dust, dirt and other debris. You also want to get rid of small pieces of the concrete that have broken off completely. A scrub brush and bucket of water can accomplish this, but a pressure washer will do it quicker with less labor. Once the surface it clean, the concrete needs to dry out. The overlay won't work properly if too much moisture is in the old surface.
  
  

Framing

• Spreading an overlay on top of the old surface requires a frame just like the original patio needed when it was first poured. The frame keeps the new concrete from spilling out onto other surfaces and keeps it in the shape that you want. Generally the frame is made out of basic lumber such as 2-by-4s or 2-by-6s. It needs to be installed around the older concrete surface. To ensure the old and new bond well, it is best to build the frame slightly larger than the original concrete area, so that the overlay can cover the sides as well as the top.
  
  

Bonding Help

• New and old concrete do not bond together easily. In fact, they don't like to bond at all. To make this process work and create a new patio over cracked older concrete, you must use a bonding agent. You can find latex concrete bonding agents at home improvement stores, or you can make a bonder out of cement mixed to a consistency of paint. If you use the cement paint, it must be covered with the new cement before the paint dries. For this reason it is best for thicker overlays that can be applied slowly. Either way, the bonding agent is painted onto the surface of the old concrete.
  
  

Overlay

• You can buy overlay mixes such as self-leveling compounds at a home improvement store, or make up a cement mix yourself using Portland cement and sand. If mixing your own, a good overlay mix would use one part cement to 2 1/2 parts sand. Add water to get a consistency that is thick, but will slowly slide off a trowel. Store-bought mixes should be mixed according to package directions. Self-leveling compounds dry quickly and must be poured in an assembly line to keep the area from drying out before it is all laid down. For this reason the cement paint bonding agent won't work with this kind of overlay. However, if you are using a thicker mix that takes longer to dry, you can work with it easier and paint the surface as you are spreading on the new cement.