A lot of clients I speak to ask about strengthening their concrete installation by making their concrete thicker throughout, but this method isn’t always the best option for making your concrete stronger.
There are more things you need to consider rather than just thickening the depth of concrete.
Now this isn’t to say that your concrete won’t be stronger by thickening it, but you need to consider the weight of the concrete upon the ground that it is sitting on. Concrete is extremely strong in compression and relatively weak in tension. Compression obviously being the sheer weight pressing straight down on the concrete, as opposed to tension being allowing the concrete to flex along it’s axis.
If you are considering making your concrete thicker simply because you need to drive heavy vehicles over it, then the difference between say 100mm of concrete and 125mm of concrete is negligible, as typically 100mm of concrete will handle the same weight in compression as 125mm will handle.
The main areas to attack firstly in strengthening your concrete I will explain now in better detail below. The trick is to initially strengthen the concrete in tension, then worry about the compression factor lastly.
Increasing Thickness of Steel
Steel Reinforcing acts as the backbone for the concrete under tension. It is used primarily to control cracking if/when it occurs by not allowing the cracks to pull apart and become structural. On a standard driveway installation for example, you can expect a contractor to install 6.2mm steel mesh throughout (hopefully chaired into the concrete). A good first step to improving the strength of your concrete under tension, is by installing a thicker version of reinforcing anywhere from 7.2mm to 9.2mm. A house slab for instance will usually have 8.2mm reinforcing installed. Thicker steel has less “bend” if you will throughout it’s horizontal line, which helps holding the concrete from being allowed to move along it’s horizontal line as much, therefor limiting the tension.
Thickening Beams / Footings
As you can probably imagine by looking at a slab of concrete, it is along the edges that the concrete is most at risk from moving up and down along the horizon (tension). Concrete, as a general rule of thumb as well, tends to want to tension crack roughly every 2-6m of laid concrete. It used to be if you’re using 6.2mm mesh that you need controls every 2.4m, 7.2mm mesh you need controls every 3.0m, 8.2mm mesh with controls every 4.0m then 9.2mm mesh controls every 6.0m. This I put into memory a very long time ago as per Australian Standards, so don’t check me up on this as they’ve most likely changed since then, although they are still a good reference point to work from. When I talk about “controls” they can either be saw or tool cut control joints, but also a thickening beam or footing can do the job just as effectively. By installing thickening beams or footings along your perimeter, as well as throughout the middle of your slab, you are effectively adding more strength in base compression whilst restricting the concrete from being able to “tension”.
Maintaining Course Aggregate Size
The course (or large) aggregate within your concrete is kind of the backbone of your concrete. It acts as the main property of your concrete that all the other properties (cement, fine aggregates etc), bond to to form your concrete. Standard concrete is installed with 20mm course aggregate, this is considered to be the strongest standard for the size of the aggregate. From here, contractors can install smaller aggregate of 10mm depending on the installation, usually though when this happens, the MPA rating is also increased to assist with de-strengthening your concrete. There is also an option getting around of “reduced line” concrete, which has approximately 30% less aggregate through your concrete, plus also a maximum size of 7-10mm course aggregate….if you ever have a contractor looking to install reduced line concrete, sack him/her immediately and find someone who is serious about supplying you a proper installation.
Ground Compaction / Subgrade
Typically the most common reason for your concrete cracking under tension is by the ground underneath the concrete moving under the weight of the installation. Upon request, an contractor can bring in a better sub-grade material than what is already on your site, typically being good fill sand, crusher dust or even 20mm road base depending on the depth of fill that is needed. Then from there, have this sub-grade compactor with a plate compactor making sure the ground is as solid as it can be to assist with holding that new concrete in one place.
Further, typically when a professional contractor is out to install your project, there are base standards for your new concrete installation. The contractor is usually seeing what the underlying ground is like and endeavoring to install your new concrete to a good, solid standard for strength. Concrete has been around since the Romans, and for instance, having your concrete installed by Walker Concreting & Resurfacing, you will receive what is considered “the norm” for a good installation.
The factors I have listed above are considered above and beyond the standard requirements and are for clients who require that little bit of extra piece of mind with their concrete installation.
After the above factors have been met, you can then start talking about thickening your concrete installation, to assist with “compression” of the concrete. A good rule of thumb when thinking about “compression”, is taking a look at Hoover Dam in America, it was specifically designed in an arc against the force of the Colorado River because the compression strength with the water constantly pressing against it is when it’s at it’s strongest.
As always, by using these guidelines to strengthen your concrete, you can be assured of a structurally solid installation. This is not a guarantee against cracking, as cracking and concrete go hand in hand. But, these processes’s will assist in making sure your installation is there to last a lifetime.