I was at a job the other day and one of the other flooring sub-contractors asked me to come down and take a look at a piece of floor he was working on. There was a spot in the floor he was trying to grind down, but was having a hard time. He had been grinding on this spot for quite some time with no results. The concrete he was grinding on was very hard and started to look very shiny.
I took a look at the diamond wheel and noticed the problem right away. He was using the wrong kind of metal bonds for the hardness of concrete he was trying to grind down. I explained it the best I could, which I will attempt again here. This is a good lesson for anyone thinking of grinding your own concrete.
First of all, take note of the type of concrete you need to grind down. Does it seem extremely hard, does it look soft like it could almost chip away with a hammer easily, or is it somewhere in between. Most concrete that we run into here in Oklahoma is somewhere in between hard and soft. Once you think you have an idea of the hardness, then you can choose which metal bond you want your diamond wheel to be.
In the case of this sub-contractors concrete being very hard, he was using very hard metal bond diamonds. Therefore, once the actual diamonds wore down, the metal bond was to hard to continue to wear down, thus exposing more diamonds. He wanted to be using a softer metal bond. So once the diamonds wear down, the softer metal wears down on the hard concrete, which will continuously expose more diamonds.
Now then, just on the opposite side of this picture, if the concrete is soft, you want to use harder metal bond diamonds. So it sounds weird, but think of this process in opposites. Hard concrete, use soft metals. Soft concrete, use hard metals.
The pictures below show exposed diamonds in the metal bond. If the diamonds are cutting correctly, this is how it should look. You always want some of the diamonds exposed in the metal. Do however take note that not all diamond wheels are created equal and have as much diamonds as the one pictured here. But you should still always see some amount of diamonds exposed to get a good cut in the concrete.
If when you are attempting to grind a floor and nothing seems to be happening, check your diamond wheel. If it is looking smooth and shiny (as well as the floor), you may be using the wrong grit.