Exterior Grade Waterproof PVA or SBR?

[mlse]

Hi, I plan to make up some cement to repair a concrete garage floor (the floor is sound, it just has a few chunks knocked out of it).

I intend to use a 3:1 sharp sand/cement mix with either Waterproof PVA (the Unibond stuff) or SBR added to it for greater adhesion and strength. Which would you recommend, and how much should I use, in terms of ratio to the concrete mix?

Thanks in advance.