Are you asking how to prevent making those marks, or how to "fix" the marks so they're not so obvious?
The rings are caused by either pulling the trigger to the surface cleaner while the surface cleaner is in one spot, or just letting the surface cleaner sit in one spot for too long. When you first pull the trigger, there is usually a surge - so moving the surface cleaner while you pull the trigger helps. If you're using hot water, you won't have to get anywhere close enough to the gum to damage the concrete. If you don't have hot water, get hot water.
As far as "fixing" these marks, a good cleaning should make them less obvious. You may have to use a little oxalic to remove the "rings' tottally, but usually just a good cleaning will remove them, unless the concrete was severely damaged, like pitted. Where the top surface of concrete was removed along with the gum, these marks will usually stay until the concrete is replaced. These marks are usually not only cleaner then the rest of the concrete, but also gouged. So if you clean the rest of the concrete well, you only see the gouged part, but theres not much you can do about that. If there are many spots like these, exp. around the entrance, make sure you are not accused or blamed by pointing them out before you actually clean.
Are you sure 0.05 is the best price you can get for concrete? I hope you clean 20,000 sg foot a day.
