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Hard Concrete, Soft Concrete and Grinding Disc Selection

By Jack Josephsen

In this blog I’ve regularly touched on the importance of getting surface preparation right, however haven’t really honed in on the finer points of this task for solvent-free, two-pack epoxies.

Regular readers will know that, at the very least, you’ve got to be looking to remove the top layer of laitance and surface contamination - but, what’s the best way of doing that and how do you know you’re doing it well?

I’ll start of by saying that while there are other effective means of preparation, the average epoxy contractor in the residential, retail and commercial fields should be able to tackle most jobs with only a diamond grinder at their disposal. While this may sound like good news in a way, just putting any old grinder with any old discs onto concrete unfortunately does not guarantee a successful outcome. The performance of your grinder and the quality of your surface preparation is largely controlled by two concepts you need to develop a basic understanding of: “hard” and “soft” concrete.

Grinding hard concrete

To explain these terms and the impact they have, I’ll cite some material published by Floorex - a specialist surface preparation company I’ve had a bit do with over the years and always respected for the educational approach they bring to their field. They describe hard concrete as follows:

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 fine 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 may even get hot and glaze over.

They go on to list some handy rules of thumb you should keep in mind when dealing with this type of concrete:

  • 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 be exposed. 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.

Even big grinders like the one shown in this image will suffer with performance if the right discs aren't used.

Grinding soft concrete

On the other side of the coin, soft concrete is a very different proposition to hard concrete because of the extremely abrasive dust it generates and Floorex offer the following words of advice:

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 fail be aware of the signs of rapid grinding and fast wear to stop work and rectify the problem.

With the main threat of soft concrete clearly being excessive wear, the rules of thumb they provide are understandably focussed on awareness and preservation:

  • 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.

  • 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.

More words of grinding wisdom

With the main topics of hard and soft concrete covered, I’ll finish off by throwing in some general words of wisdom Floorex have for contractors on grinding and disc selection.

Perhaps the biggest message they try to get across is that every slab is different, and you can’t fall into the trap of thinking that one disc will be good enough for every job you do. Even the “best” or “biggest” machine won’t get satisfactory results if incorrect discs are used and you’ll benefit greatly if you’re willing to go through a little trial and error at the start of a project.

The other point they make is a personal favourite of mine because I see big similarities with the use of cheap resins. In this case, contractors tempted by the lower price of budget discs will have to put up with discs that work over a relatively narrow band of hardness and have less concentration of diamond grit in them.

You can see more about the different types of surface preparation for epoxy flooring in a separate know how article. 

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