Understanding Concrete Before You Start Grinding

Before choosing a floor grinder, diamond grit or bond hardness, it helps to understand one simple thing:

What are we actually grinding?

A concrete slab may look like one solid material, but as you grind through the surface, what the diamond tooling encounters changes.

Understanding these changes can help explain why a diamond segment may cut perfectly at the beginning of a job and behave differently once the floor starts to open.

How a Concrete Floor Is Made

Concrete is primarily a mixture of cement, water, sand and aggregate.

When concrete is poured, it is placed and compacted to remove trapped air and help the concrete settle properly. Depending on the job, this may involve vibration. The concreters then screed the slab to the required level and finish the surface.

During this process, the heavier aggregate generally remains within the body of the concrete while water can migrate toward the surface. This is known as bleeding.

In more severe cases, aggregate can settle while a cement-and-sand-rich layer remains toward the top. Excessive working of concrete while bleed water is present can also contribute to a weaker surface.

Once the slab has cured, the surface we see isn't necessarily representative of everything underneath it.

And that's important when we start grinding.

A Simple Way to Think About Concrete When Grinding

For someone learning concrete grinding, we can simplify what happens into three stages of exposure.

1. The Surface — Cement Paste and Fine Material

The first thing the grinding diamonds encounter is the finished surface.

At this stage, little or no coarse aggregate may be visible.

The behaviour of this surface depends on many factors: the concrete mix, how it was placed, how much it bled, how it was finished, whether it was power-trowelled and how it cured.

This means you cannot judge exactly how a floor will grind simply by knowing its MPa.

For example, a smooth, heavily trowelled concrete surface can behave like very hard concrete when grinding and may require a softer-bond diamond.

This is one of the reasons experienced concrete grinders pay close attention to what the diamonds are doing rather than relying only on the concrete specification.

2. Opening the Surface — Fine Aggregate Begins to Appear

As we remove the upper surface, the concrete begins to open.

Small particles of sand and fine aggregate become visible.

You may notice that the grinder begins behaving differently.

Perhaps the machine was cutting quickly at the beginning, but now the removal rate has slowed. Or the diamond segments start polishing rather than cutting.

This is where understanding concrete hardness and diamond bonds becomes important.

If the concrete you're encountering is very hard, a softer bond may be required to allow the metal matrix to wear and continually expose fresh diamond.

If the concrete is soft and abrasive, the opposite applies: a harder bond can help prevent the segment from wearing away too quickly.

We'll cover this properly in a later article because bond selection deserves a topic of its own.

3. Grinding Deeper — Larger Aggregate Is Exposed

Continue removing material, and progressively larger aggregate can become visible.

At this point, we're doing more than simply opening the surface.

Depending on the required finish, exposing aggregate may be intentional. This is particularly relevant when preparing concrete for an exposed-aggregate polished finish.

The deeper we grind, the greater the aggregate exposure can become.

This is why the amount of concrete removed at the beginning of a grinding or polishing job can have such a large effect on the final appearance of the floor.

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Does Concrete Get Stronger the Deeper You Grind?

This is where two terms are commonly confused:

Concrete compressive strength (MPa) and surface hardness.

MPa tells us the concrete's compressive strength. Generally, higher-strength concrete tends to be harder, but that doesn't mean a slab consists of neat layers with progressively increasing MPa as you grind downward.

For grinding, we're particularly interested in how hard and abrasive the surface we're actually cutting is.

MPa and surface hardness are not the same thing. Higher-strength concrete will often be harder, but the way a floor grinds is also heavily influenced by how the slab was finished, cured and treated at the surface. This is why two concrete floors with a similar MPa rating can behave very differently under the same diamond tooling.

That's why two floors with similar specified MPa can still grind very differently.

Why This Matters When Choosing Diamond Segments

Metal-bond diamond segments need the correct balance between cutting performance and bond wear.

A useful basic rule is:

Hard concrete → softer bond

Soft, abrasive concrete → harder bond

On extremely hard concrete, a softer bond allows the metal matrix to wear at the correct rate, helping expose fresh diamond and maintain cutting performance. On softer, more abrasive concrete, a harder bond helps slow down segment wear and extend tooling life.

On softer, more abrasive concrete, the segment naturally wears faster, so a more wear-resistant bond is needed.

This is something we'll explore much further as we work through this series.

Don't Assume the Whole Floor Will Grind the Same

One of the most important lessons for someone starting in concrete grinding is:

Watch the floor and watch your diamonds.

Concrete can vary across the same slab.

Finishing, curing, weather exposure, repairs, previous coatings and differences in the original placement can all affect how the surface behaves.

A diamond that performs well in one area may behave differently somewhere else.

That's why understanding the concrete comes before understanding the grinder.

The Three Stages to Remember

For now, keep it simple:

1. Finished surface
Cement paste and fine material are what the diamonds initially encounter.

↓ GRIND

2. Fine aggregate exposure
The surface opens and sand/smaller aggregate becomes visible.

↓ GRIND DEEPER

3. Larger aggregate exposure
More material is removed, and larger aggregate becomes visible.

That is the basic picture we want someone to understand before we start talking about grinders, diamond segments, grit or polishing systems.

What's Next?

Now that we understand what we're grinding, the next question is:

How do we know whether the concrete is hard or soft?

In the next article in the Paddle Concrete Grinding Basics series, we'll look at concrete hardness, MPa, the Mohs scratch test and some practical signs you can watch for while grinding.

Once we understand hardness, we can start choosing the correct diamond bond.