There are various additives that give cement products better adhesion,
the most common being some form of acrylic. Milk was even sometimes used in
the past. You can buy the
acrylic additives in quarts or gallons at Home Depot, but they'll be in
the ceramic tile section.
Another way to improve concrete adhesion is simply to increase the ratio
of the cement (within reason) relative to the sand and aggregate. I'd
also keep the water content to the minimum amount necessary for whatever
plastic flow you need ... that keeps the shrinkage (which puts stress on
the interface) to a minimum. It's a good idea to make sure the surface of the
concrete is nice and damp first, though ... soak it with water for a
while and then wipe it semi-dry.
Concrete resurfacing is fairly common, and if that's all you're
interested in, you can usually find various versions online or at Home
Depot, Lowe's, etc. They typically are referred to as thinsets ... they
aren't structurally strong but they trowel on smoothly and the stuff
sticks pretty well. I'm not sure how well some of it holds up outdoors.
By the way, several folks have recommended an acid wash on the old
surface. That's good, but it basically just eats away at the cement in
the old concrete. You can literally blast away the surface (cement,
sand, etc) with a pressure washer if you're patient and not trying to do too
an area. I was amazed at what even a low end model could do. In any
case, you're not really making much of a chemical bond ... the cement
cures in a crystalline fashion and you're just trying to interlock the
new crystals with the old surface at a microscopic level, so rough is
Rick Karlquist wrote:
> My neighbors poured a new concrete floor over their
> old one and told me that they used some sort of special
> adhesive concrete that was designed to stick to the old
> work. They didn't do any rebar pinning and the new
> concrete had to be a fairly thin layer that was not
> able to support the weight on its own without the help
> of the existing concrete. This job worked perfectly.
> Has anyone heard of this magic concrete?
> Rick N6RK
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