So you’re thinking of laying a new concrete patio? Read on because this article could prolong its life.
All concrete patios should be laid with a rebar or ‘reinforcing bar’ grid, to stop large cracks then areas of crumbling concrete forming over time. You will still get some small cracks with rebar, but these will not grow larger as the rebar effectively ‘knits’ the concrete together.
So let’s look in more detail at what rebar is, what happens when you don’t use it – and also consider if there are any alternatives available.
What is Rebar & How Does it Work?
Rebar is a builder’s term for “reinforcing bar”. Rebar is made from carbon steel: usually new steel, but it can also be made from recycled junk steel. Rebar comes in two forms: deformed and plain. Deformed rebar has distinctive and important lugs or ribs along the outside.
Concrete construction has been around for a long time. The Colosseum and the Pantheon in Rome are built with concrete. Ancient Greeks and Egyptians used concrete.
Rebar concrete construction has been around for 200 years. Some say the addition of rebar to concrete allowed the building industry to go “up” instead of “out”.
Concrete is used everywhere, so it must be strong, right? Yes – and no. Concrete withstands large amounts of weight on top of it, pressing down. Concrete is not so great holding up to pressures that pull on it.
What pulls on concrete? What does that even mean? Consider a concrete driveway, laid down in dirt and gravel. Now drive your car onto the driveway. What happens next depends on whether or not there are small holes, or even channels, in the dirt and gravel under the slab of driveway cement.
When first laid down, there will be very few divots in the dirt under the driveway. The driveway contractors were experts and did a good job tamping down the surface with something really heavy.
After several years, though, the ground under the driveway has settled in uneven ways, some places more than others. Maybe water has been channeling under your driveway. Where the ground under the driveway settled the most, it creates a gap between the ground and the bottom of your driveway slab.
Back to what happens after driving your car onto the driveway: two things are possible. The first, is that the ground under the cement, under the tires, has not settled and nothing happens to the driveway.
The cement in the driveway underneath your 2-ton vehicle is wholly supported by the ground below it. Your tires put the cement into compression but the cement doesn’t move – doesn’t sag even the smallest amount.
Scenario number two is that under the tires, under the cement, there is a gap. A hole. A divot. Maybe a channel. When you drive onto that spot, under the 2-ton trucks’ tires, the cement sags – bends – just a little. Under your tires, the car weight pushes on the cement and puts the surface of the cement into compression.
In this second scenario, the driveway slab has no support. Because the top of the slab is being pushed down, and cement doesn’t squish down very much, the bottom of the slab has to bend too. The bottom side of the slab is put under tension – it is stretched and pulled. Ultimately, the cement will fail, like in this video.
Cement has 10 times the compressive strength as it’s tensile strength. Rebar is just the opposite. Rebar can bend if you apply enough compression to either end – but it’s almost impossible to stretch it. Rebar is like a steel rope.
Rebar and cement: the perfect couple. A perfect marriage: a compressive strength superhero with a tensile strength one.
Rebar Versus Wire Mesh For Your Concrete Patio
Rebar is – a bar. There are lots of diameters and lengths, but they are all still just bars. That means, for a slab project, you will have to construct a grid of them, like this.
There is lots of engineering advice about how tight the grid needs to be for your slab. Once you have pieces cut, they have to be tied together in a sturdy grid so they don’t move when you dump a bunch of heavy concrete on top of them.
You can buy special rebar ties at a hardware store. For small projects, you can just use a pair of pliers to twist tie them. For giant projects, there are special twist-tie tools.
Rebar doesn’t do any good if the grid is positioned on the ground and cement poured on top. It needs to be somewhere in the middle of your slab. Opinions vary about where: in the bottom third of the slab, where tensile failure normally occurs? The top third of the slab where cracks get biggest? Or, hedging bets, in the middle of the slab?
Regardless of where you locate the rebar grid, you must support the grid off the ground with rebar “chairs” or “runners”.
Back to rebar versus mesh: to avoid the whole “construct your rebar grid” thing, some people just use a wire mesh. Wire mesh for concrete comes in flat sections or rolls. Some people recommend using left-over chain link fencing sections.
Most cement slab experts say that using deformed rebar is better than using mesh, for the simple reason that deformed rebar has ribs. Deformed rebar has ridges, while cement wire mesh, horse fencing and left-over chain link fencing are all slick – no ridges, just smooth wire.
Deformed rebar ridges, are designed to make cement stick to them – besides the heavily raised ridges, the ridges sort of spiral up and down the length of the bar. Once the cement sets up around the rebar ridges, it’s not going to let go.
Mesh or rebar? Some experts say – use both! Use both rebar and mesh for a super strong, long-lasting patio.
Do You Need To Use Rebar For Paving Slabs?
To avoid confusion – cement pavers are not paving slabs. Cement pavers are small, pre-formed concrete pieces. They come in lots of styles and sizes: you buy them by the piece and set them side by side to form a patio. They don’t need rebar.
Paving slabs are patios from poured concrete. They also vary in size and in how they are finished, but they are made from concrete that pours in wet and then dries hard.
Concrete patio experts agree that you can pour a patio without rebar. But why take the risk?
Studies show that the most common reason patio slabs crack is ground settling. In the back yard, watering happens all the time. Watering lawns. Flower beds. Kids playing in the sprinkler.
Water washes soil away. It can create channels under your patio. Water can cause clays in the soil to expand. During the building process, if you or your contractor did not compress the soil and gravel sufficiently before pouring the concrete, the soil can shift. Ground settling is always uneven and unpredictable.
To add rebar to your project will be slightly costlier and it does take more time, but cement’s strengths and failures are well-known. Sure, your backyard patio doesn’t have to be as strong as a high-rise building or even a driveway – but it should be durable and beautiful for decades.
What Will Happen To My Concrete Patio if I Don’t Use Rebar?
Answer: small cracks at first, then big, unsightly cracks, then whole areas of crumbly concrete. Construction of a cement slab patio is not simple. It requires some precision and mixing accuracy.
Small cracks in concrete are unavoidable. They can be caused by plastic (before hardening) shrinkage, expansion, freezing/thawing cycles, overloading and premature drying. Some cracks are the result of common cement-pouring mistakes and some are thanks to Mother Nature.
Rebar can prevent some small cracks from happening in the first place. But the reason the building industry uses rebar is – rebar prevents small cracks from becoming giant cracks. Giant cracks become crumbly areas and then the patio will have to be replaced.
What Can I Use Instead of Rebar?
If you don’t want to use rebar or wire mesh, there are lots of products on the market to add to cement mixes. The other ways to increase cement strength are: fiber reinforcing, admixtures, adjusting the cement water to cement ratio, changing the cement quality, using cementitious materials (SCM’s) and changing the aggregate ratio.
Fibers can reinforce concrete. Synthetic fibers used in concrete are called structural fibers. Fibers can be glass, nylon, polypropylene or some other plastic.
Generally, a small amount of fibers can lessen plastic shrinkage issues but opinions vary about crack prevention. Some fiber manufacturers claim that the addition of fibers can strengthen tensile strength in all directions, unlike rebar.
Admixtures are chemical products, usually liquids, that you can buy to change cement properties. Chemicals are introduced to the cement mix to overcome potential issues related to the site, like low or high temperature climates, or the need for a faster setting process.
Cementitious materials are solid materials: ash, slag, silica flume, and calcium carbonate (limestone). These materials can improve the overall hardening properties. The addition of a limestone powder can also lighten the color of cement.
So before you lay that concrete – or have a professional do it for you – make sure the plan is to use rebar within the structure of the patio itself.
A rebar grid will help prolong the life of your patio and stop those small cracks you always get in concrete from turning into larger ones. It will then also stop those large cracks from becoming entire areas of rubble.
Although there are some alternatives to rebar, we think there’s nothing quite as good as the original and best concrete reinforcement method. Experts have been using it in concrete construction for two centuries, so who are we to argue?
Enjoy a cold one on your new rebar strengthened patio when that concrete sets 🙂