All concrete is HARD if you fall and hit the back of your head on it whether it is 2,500 PSI in your home or 8,000 PSI on a bridge or military installation! But, In the construction world where a lot of us live it is oftentimes critical to have and know the properties of concrete so we can select the correct tooling. One important property to know is how hard the concrete is. This is referred to as its PSI or compression strength in pounds per square inch, this information is critical in helping you know if it is acceptable for the project it is to be used on. Or, if your cutting or grinding concrete you need to know the PSI so you can select the correct diamond matrix to be used.
Just a little FYI here, we in the USA use PSI when determining concrete hardness, it is an Imperial measurement, pounds per square inch. But most of the rest of the world uses a metric rating known as MPa or Megapascals (whatever the heck a Megapascal is, I looked it up and my eyes just kinda glazed over, something to do with a Newton being a measurement of force and pushing a gram of matter at a certain velocity, stuff like that….I’m sticking with PSI!)
Here is a general comparison between the two ways of measuring concrete hardness as well as what is considered hard and soft concrete.
2500 psi = 18 MPa soft concrete
3000 psi = 20 MPa soft concrete
3500 psi = 25 MPa medium concrete
4000 psi = 30 MPa medium concrete
5000 psi = 35 MPa medium-hard concrete
6000 psi = 40 MPa hard concrete
This comparison is not exact but if you need exact comparison use 0.00698915 to convert PSI to MPa
CONCRETE, THE FACTORS INVOLVED
So then, what is it that makes concrete hard or soft? Well, there are a number of things affecting concrete hardness. First there is this,
AGGREGATE-(the rocks in concrete)
This portion accounts for about 70% of a concrete mixture. So, it is very important and varies greatly across the country, where the aggregate is made up of very hard rock like granite, chert, quartzite you tend to get harder concrete, and places with aggregate like shale, sandstone, and limestone you get softer. The following chart gives you a general idea of hardness and softness across the country as influenced by aggregate composition but as we will see there are other factors that determine the hardness of concrete as well.
MIX DESIGN - Ends up you don't just add water after all!
Concrete mix design is the process of developing the correct proportions of cement, water, range of aggregate from sand to larger aggregate usually up to 1.5 in. to achieve the desired hardness for both performance and durability, there is also a wide range of both chemical and mineral additives, their uses will depend on what the contractor wants to achieve, weather shrinkage reducers to accelerating agents helping in the curing process and a host of others. often included in the witches brew are materials like Fly Ash, originally used as a filler in concrete to cut down on all the millions of pounds of fly ash, a residue of the coal power industry being deposited in landfills every year but now is now found to actually aid in the hardness of concrete.
THE CURING PROCESS
Growing up I just assumed like probably everyone else that concrete cured by drying and we were all totally wrong. Concrete cures by a chemical process called “hydration” where the components in cement form a chemical bond with water molecules and become hydrates, thus the name.
It is important then to not let the concrete “dry” before the hydration process has taken place as this makes for a weaker, more prone to cracking slab. so, as the concrete hardens it is sometimes necessary to spray the surface, keeping it wet to allow for the necessary time for hydration to take place. In fact, concrete will harden just as well if not better underwater if you can keep from “washing out” the concrete mix.
A personal example, I once mortared rock onto a wall in the direct summer sunlight. the next day I came out to observe my quality installation and found 95% of the rock on the ground. After accusing my supply house of selling me defective mortar the salesman kindly explained the importance of hydration process to me, I had let the mortar dry way too fast!
OTHER THINGS THAT MAKE A DIFFERENCE
Time of year, a slab poured in cold, wet conditions will make for a harder slab and the reverse is true when placed in hot dry conditions. Power troweling can give a very hard surface even if the concrete is soft underneath.
HOW DO I TELL IF I HAVE HARD OR SOFT CONCRETE?
There are two tools commonly used for testing concrete hardness, they are the rebound hammer and the Moh’s scratch test.
Rebound Hammer Moh’s scratch test
The rebound hammer has a piston that is “fired” by the user and gives a PSI rating for the slab. I like this one as it tells you about the whole slab and not just the very surface.
The Mohs’ test is a more commonly used tool to test for hardness. The test has a rating of #1-#10 but for testing concrete you usually have #2-#9 as #1 is baby powder hardness and #10 is diamond. The kit will have 8-9 “pencil tips” these tips have progressively harder and harder tips, so, you would start with maybe a #4 and make a 2 in. scratch then if the concrete scratches you go to your #5 and so on until your pin does not scratch the surface so, if it scratches at #6 but not #7 you look at the chart and that gives you your concrete hardness of CSP, this is a good test and less costly that the rebound hammer that can range up to $600.00 bucks opposed to the Mohs’ kit around $120.00 bucks. The only thing I don’t like about the Mohs’ kit is it is only telling you about the very surface of the slab where the hammer tells you about the slab deeper into it.
TO SUM UP
So, we can start to see that you can get an idea of concrete hardness by looking at the aggregate used as well as the geographic location, but there is a lot more to making hard or soft concrete. I live in the Northwest, Washington state on the Puget Sound and we have some of the hardest concrete on the continent but I will find very soft concrete here at times and it is because of all the other factors that go into making hard-medium-soft concrete.