Concrete dust and bonds are the key elements for grinding productivity. The dust produced by concrete grinding will wear away the metal matrix/bond which keeps the diamonds exposed. Without this dust the matrix will not wear 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 concrete dust. 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 course dust and they can wear the bond very quickly.
When grinding hard concrete your 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. 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, cup wheel, 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:
With concrete grinding 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 dust 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 concrete dust produced. The hard metal matrix is then not worn away sufficiently by the small amount of concrete dust so the new diamonds are not exposed. This is when the diamonds stop 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 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 diamond to grind any concrete.
SELECTING THE CORRECT DIAMOND TOOL:
This is the most important decision when beginning a diamond grinding project and often it is determined by trial and error. With experience you may often be able to make an informed guess as to which disc is going to be right for any different slab.
There are some simple statements that can be asserted before we go on:
- Even the best or biggest machine won’t work well if the correct tooling is not installed.
- There is really no such thing as a faulty diamond disc. It is always whether the disc is right for that particular concrete. There will always be the right slab for every disc.
- No two slabs are the same, even on the same project. Indeed, a slab can vary from one section to another on the same slab poured on the same day.
- There is no such thing as a “one diamond disc is right for every type of concrete floor”.
- The MPa of a concrete slab may have little to do with how hard it is to grind. (See para.1 below).
- Premium discs will operate satisfactorily over a broader range of different concrete hardness.
Budget discs, as a rule will work best over a narrower band of hardness. Premium discs have a higher concentration of diamond grit in them as well, and have different ratios of natural to synthetic diamond grit.
1. WHAT DO WE MEAN BY ‘HARD CONCRETE’ ?
When discussing how hard concrete is to grind, many people confuse HARDNESS and STRENGTH. Strength is measured in MPa and is a measure of compressive strength. It is true that high compressive strength can be harder, but not always. This is because the aggregate and sand in high MPa concrete is likely to be the same in low MPa concrete. The difference is in the amount of cement and water (water / cement ratio). This is also why 40MPa concrete from the batching plant may not be 40 MPa when placed, if the contractor adds water; it could just as easily be 25MPa.
When we talk about hardness (to grind) it really is all about the kind of dust that is produced during the process. Hard concrete tends to produce ultra fine, talcum powder dust. This dust is very un-abrasive; it does not wear the matrix of the diamond segment sufficiently. The result is that the diamond grit soon becomes hardly exposed so it grinds even finer, powdery dust; the segment ceases to grind, and the segments will get hot and glaze over. Simple isn’t it? (‘Exposed’ means; how much diamond grit is poking out of the segment. You can tell how exposed the grit is by rubbing your finger over the segment))
Concrete that is soft or water damaged (through addition of extra water to the mix before discharging from the truck or rain damaged) produces very soft, gritty, course dust; this is too aggressive for most discs and will wear them out very fast. THIS IS NOT A FAULT WITH THE DISC. It is simply a case of using the wrong tooling and/or not inspecting regularly.
So now we know that the AGGREGATE, and the SAND, and also the amount of WATER added to the mix, plus also the amount that it was power TROWELED, plus also whether the contractor VIBRATED the wet concrete all contribute to the final equation as to how easily the slab is going to grind.
2. WHAT DISC FOR HARD CONCRETE?
Grinding hard concrete means that the dust is soft and un-abrasive so the metal MATRIX that the segment is made of needs to be ‘soft bond’ and easily eroded to expose the diamonds. Often a coarser grit can be beneficial because it may produce coarser dust to help erode the matrix.
In addition, reducing the contact area with fewer segments makes a difference. This also has the effect of increasing the weight per square inch on the surface so that the diamond grit will occasionally crack a little bit off the peak to create a fresh sharp peak and start grinding again. Diamond grit will gradually round over if it does not crack off a little bit from time to time. Rounded grit won’t grind too well. This is why you will find that adding extra weight or ensuring all the weight of the machine possible is on the head; it helps to keep the diamonds working.
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.
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.
If all else fails, you can also try spreading a small amount of sand on the ground and lightly wetting it to help further break down the bond of the diamonds.
The Rule of Thumb:
- Use diamond tooling with a SOFT BOND and/or less segments for hard concrete and increase the weight on the diamond tooling. Using coarser grit diamond may increase the dust size and keep the tooling working.
- Equally important; do not use tooling for hard concrete on soft concrete; they will almost certainly wear out incredibly fast.
- If you turn down the vacuum so there is plenty of dust under the machine, this will help the diamonds to expose. Careful addition of sand may also help. Don’t overdo sand, it could cause premature wear!
- Watch out for the situation where there is a hard-to-grind top layer, and a soft layer below. You can wear out discs fast and you think that because the top is hard, the discs should last. If this occurs, use the soft bond only to, or nearly to the soft layer, then grind the soft layer completely separately with hard bond discs.
- Lastly inspect your tooling. If there is hardly any diamond exposed out of the matrix, and/or the tooling is getting hot, stop and change to a softer bond, or less segments.
3. WHAT DISC FOR SOFT CONCRETE?
Grinding Soft concrete requires ‘hard bond’ discs that resist the metal matrix being eroded away. The sandy, gritty, abrasive dust will erode many discs abnormally fast, so be certain that you are using the right disc. More than any other time, grinding soft concrete is when contractors need to be aware of the signs of rapid grinding and fast wear to stop work and rectify the problem.
Do not use the cheapest disc and expect to save money, it becomes an expensive exercise. A super premium disc is usually the best value for money.
In addition, it is also important that the weight on the head of the machine is reduced as much as possible to ensure that the sandy, gritty, abrasive dust that is rolling around under the segments does not erode the matrix any more than necessary. Once again, it can be water damaged concrete, the aggregate, sand, carborundum or metal fibres in the topping, all can be some of the things that require special attention to how you tackle each project.
The Rule of Thumb:
- If the disc begins to grind super well, STOP! You are almost certainly are going to wear your diamond tooling too fast. A good operator inspects his tooling regularly for unusually high diamond exposure. This means that when you run your finger over the surface the diamond grit is sticking out a lot. The diamond grit will be falling out of the matrix before you have had a chance to wear them out.
- Use diamond tooling with a HARD BOND and/or more segments.
- Be certain to use the machine weights to reduce the weight of the machine on the tooling ie, place towards the back.
- In addition, a vacuum that will extract as much dust away as quickly as possible will greatly improve the life of the tooling. If there is a lot of dust rolling between the floor and segments it will cause excessive wear.
Author – Peter O’Shannessy (Concrete Grinder Sales)
* The above information is to be used as a guide only and Con-Treat accepts no responsibility for any consequence resulting from the advice contained herein or verbally.