When grinding hard concrete your cupwheel diamond segment bond needs to match the concrete hardness.

The confusing thing is that soft bonds are for hard concrete and hard bonds are for soft concrete.

The diamonds which are embedded in a  metal called the bond (or matrix) only cut well if they are exposed.

 

Diamond segment showing exposed diamonds and "rocket trails" behind each diamond

The final product is called a segment or diamond block and can be made in a variety of sizes and shapes then silver soldered  onto a plate, cupwheel, or cutting blade for final use.

The size of the diamonds can vary and so can the hardness of the metal bond around them which determines how fast they wear.

How diamonds cut concrete

When grinding concrete the diamonds cut into it and scratch it away breaking it down to sands and dust. During this process the diamonds fracture and wear down, but at the same time the bond metal is worn away by the sands of the concrete which exposes more of the embedded diamonds so that cutting can continue.

When grinding hard concrete the diamonds cut, blunt and break  with much less cutting action than medium hardness or soft concrete and as a consequence there is less sand and dust produced. The hard metal matrix is then not worn away sufficiently by the small amount of sands and dust so the new diamonds are not exposed. This is when the cupwheel stops working and rubs on the floor instead of cutting into it because the diamonds have worn down to the metal, but the metal matrix is not wearing away to expose new diamonds.

The diamond block is then flattened, polished and becomes very hot with continued use. At this stage you  need to press very hard and stay in the same spot longer for the cupwheel to grind any concrete.

 

Diamond segments where the diamonds have flattened after being used on hard concrete. Note the absence of "rocket trails".

To recognize all the signs of polished (glazed) cupwheels go to Glazed Diamond Segments.

Dry Grinding hard concrete

To dry grind hard concrete use a soft bond cupwheel with larger diamonds around 20 grit size if possible so that the bond can be worn away at a similar rate to the diamonds. Concrete grinding will still be slow, but at least grinding will be happening. When that fails to work there is another trick that often does the job.

Grinding trick

Wetting the concrete before grinding helps to expose the diamonds a little more. Try to scratch open the surface first, then apply water and give it a little time to soak in. Broom off the puddles and remaining water leaving the surface wet. This process makes the dust a little heavier which cuts the matrix more to expose more diamonds.

Don't be fooled

It will seem that the water has done nothing when you grind because the dust is the same as if you never wet the floor, but grinding will be more productive. Repeat this process for each new cut.

To find out how to renew your polished cupwheels go to Glazed Diamond Segments.

Sands and bonds are the key elements for grinding productivity

Sands produced by concrete grinding will wear away the metal matrix/bond which keeps the diamonds exposed. Without these sands the matrix will not be cut back to keep some diamonds exposed. When the diamonds are no longer exposed they will not cut the concrete.

The bond, or matrix is made of metal powders that are heated in a furnace until they melt into a hard block and these metals can be made to wear longer or faster when abraded by sands.

So for hard concrete you need a softer bond so that it wears away a little.

What about soft concrete?

Just the opposite.

You need a hard bond for soft concrete because it produces plenty of sands and they can wear the bond very quickly.

What makes concrete hard to grind?

This is the million dollar question.

Two main things contribute to concrete hardness - the ratio of cement & water in the mix which determines the hardness of the the concrete paste and the type of aggregate used.

 

Exposed aggregate showing the stones cut down level with the surrounding paste

Hard concrete means a hard paste (the grey matter that we recognize as concrete).

Hard aggregate means the hardness of the stones used in the concrete mix and there are very hard ones and quite soft ones used depending on the local supply.

Grinding aggregate

The unexpected variation comes when you have medium strength concrete (with very hard aggregate) which you grind easily at first, but as you get into the aggregate your speed slows down dramatically because there is possibly only 15% concrete paste now and 85% aggregate which may be considerably harder than the concrete.

So then you may have to change to a softer bond  (cupwheel) than the one you started with.

Diamond size

Generally speaking you use a larger diamond grit (low number) for faster removal of concrete and for harder concrete.

Because larger diamond grits are more aggressive they are not as smooth to grind with which you will notice at 30/40 grit and below.

Polished concrete

To polish concrete you start with a coarse diamond and approximately double the grit number (halve the diamond size) with each new pass until the floor is polished which may take several steps. ie. 30/40, 50/60, 80/100, 200, 500, 1000, 2000 and then 5000.

For example, with Situp N Grind use 30/40+40/50 Coarse for first cut then change to 100/80 Fine for second cut.

The concrete will not be polished but it will be very smooth, ready for resin bond pads to complete the polishing work using low speed hand grinders.

Resin bond polishing pads

Note that Situp N Grind fitted to a 9 in (230mm) angle grinder runs too fast for resin pads and grits above 80 mesh will quickly heat up the segments if the concrete is medium to hard strength. You then need to use a slow speed grinder or grind it wet.

Polished concrete look using a clear sealer

After using the fine cupwheel you can get a polished concrete look by applying a couple of coats of clear urethane or clear epoxy without the need to actually polish the surface. In between the grinding it will be necessary to fill in the thousands of tiny holes in the concrete to stop bubbles and holes appearing in you final coat.

Next page "Glazed Diamond Segments"