Whether you are just thinking about a new concrete garage floor or ready to start planning a new one, you probably have a lot of questions. Sure, you might be satisfied to call a concrete contractor and let him tell you what you need. Call several contractors, however, and you are likely to get several different ideas about what you need with just as many different estimates for what it will cost you.
Not all surfaces need concrete rebar reinforcement, but adding it makes concrete stronger and more resistant to large cracks. Concrete surfaces expected to carry heavy trucks or machinery do need concrete rebar reinforcement. Rebar reinforcement has been used to provide concrete with the support necessary to withstand tension forces for over 150 years.
Concrete is incredibly strong in terms of compression, but it doesn’t have much tension strength. That’s where concrete rebar reinforcement comes in handy. Without rebar reinforcement, concrete is highly prone to cracks due to tension forces. Rebar helps prevent cracks from growing wider largely by preventing cracked slabs from moving apart.
What Is Rebar?
Rebar is shorthand for reinforcing bar, also known as reinforcement/reinforcing steel. No matter what you call it, it’s a steel bar or mesh of steel wires formed to create tension in reinforced concrete as well as masonry structures. Rebar helps uphold a building’s strength and compression by holding it in a compressed state.
Reinforcing bars are made from a variety of hot-rolled steel materials. Most are made of new steel billets, but materials can also be derived from steel debris or even old railroad tracks. Rebar is marked with an identifying symbol to indicate where it was produced. Another number you’ll find on concrete rebar reinforcement is rebar yield strength, which is either a 60 or 75 or metric 420 or 520.
Do You Need Concrete Rebar Reinforcement?
Not all projects require the use of concrete rebar reinforcement, but adding it will greatly reduce the number of cracks that appear in concrete surfaces over time. Concrete surfaces required to uphold large trucks, heavy machinery or nonstop traffic need concrete rebar reinforcement. On the other hand, if your driveway is only expected to uphold the family minivan, you might not need rebar reinforcement.
There’s also the option for welded wire mesh made into galvanized panels that offer added reinforcement. This is a common solution for everyday residential driveways that are not regularly driven on by heavy trucks. Mesh is thinner than rebar reinforcement, but it’s also less expensive.
Garage slabs take a lot of abuse. They have to support heavy cars and trucks and resist ice, dirt, salty water, de-icers, gasoline, motor oil, antifreeze, and other contaminants. Plus, since few garages are heated, they have to tolerate freeze/thaw conditions. But a garage slab that has been carefully planned, installed, and cured should have no trouble providing years of crack-free performance in any climate, even under the toughest conditions. Commercial concrete contractors do this kind of work every day; with attention to detail and high standard of quality.
For Better Garage Floors, Start With A Good Base
Don’t worry too much about your soil’s bearing capacity. Even poor soils like silt and soft clay have an allowable soil pressure of around 400 pounds per square foot (psf). A 6-inch-thick slab weighs only about 75 psf, and live loads — anything that is not part of the building itself, including vehicles—typically don’t exceed 50 psf in a garage. That means that the soil under a typical garage slab only has to be able to support 125 psf.
Far more important than bearing capacity is the base’s ability to provide consistent support. If one part settles more than another, the slab will bend and potentially crack. To avoid this problem, you need to know which areas have been cut and filled, and then you need to make sure that filled areas have been well-compacted. Any soil that’s been disturbed during excavation must also be compacted. Keep in mind that it’s hard to get good compaction with soil that is too dry or too wet. To test for moisture content, squeeze a handful of soil. If you can squeeze out water, it’s too wet, and if it falls apart when you open your hand, it’s too dry. If it holds its shape, it’s about right.
The safest approach is to remove the topsoil and place a minimum 4-inch layer of compactible gravel or crushed stone as a base over the undisturbed subsoil. Use unwashed material that has a top size of 11/4 inches and includes smaller sizes right down to the fines; the irregular shapes and sizes interlock nicely when compacted. Gravel or stone provides a layer for the installation of under-slab conduit and pipes, allows for water and radon to escape, and helps keep the slab thickness uniform, which saves money on concrete. It also helps spread the load over the underlying soil, so that the slab is supported more evenly. Plus, it’s easy to compact and hand-grade.
Probably the most difficult soil you will have to deal with is expansive clay, which swells when it’s wet and shrinks when it dries, and can’t be compacted easily. It’s best to remove this clay and replace it with a compatible fill. If that’s impractical, you should consult with a foundation engineer. In some cases, the engineer may recommend structural slabs or post-tensioned slabs that don’t rely on the soil for structural support.
Install A Vapour Barrier For Better Garage Floors
Better garage floors most building codes say that an unheated detached garage doesn’t require a vapour barrier, but that doesn’t mean you shouldn’t install one anyway. Moisture in the ground can wick up via capillary action, and water vapour is always present beneath slabs; any air in the subbase is almost always at 100 per cent relative humidity. Without a vapour barrier, moisture will move through the concrete and condense beneath anything stored on the slab surface, leaving telltale dark spots. In extreme cases, the slab will even “sweat.” If the slab is ever covered with flooring or a finish coating, the moisture could cause delamination. A vapour barrier is a cheap insurance.
Don’t place a sand or gravel blotter layer on top of the vapour barrier. There was a time when that practice was recommended to reduce curling in the slab, but if you’re using the proper low-water-content concrete mix, a blotter layer is unnecessary. It can trap moisture, which will then keep rising through the slab.
The vapour barrier’s seams should overlap at least 6 inches and be sealed with tape. To prevent concrete from pushing into them and tearing the material during placement, try to orient the seams so that they will be parallel to the direction of concrete placement.
Don’t Add Water To The Mix
The IRC requires that slabs be built with concrete with compressive strengths from 2500 to 3500 psi, depending on the climate. ACI goes further and recommends 4500-psi concrete for garages in the northern half of the country. To achieve this strength, the water-cement ratio should be kept at 0.5 or less, typically about a 5-inch slump concrete. Because this mix is slightly dry and stiff, it’s tempting to add water to make the concrete easier to place. But be careful: There’s an inverse relationship between the eventual compressive strength of concrete and the amount of water used in the mix—the higher the water-cement ratio, the lower the strength. The best way to get concrete that flows well enough to consolidate in the forms and around the reinforcement is to use a high-range water reducer or super-plasticizer.
Super-plasticizers work by pushing the cement grains apart, so that the mix flows more easily.
For Better Garage Floors, Use Air-entrained Concrete
Most garage floors are not made with air-entrained concrete because finishers don’t like its sticky consistency. However, this type of concrete is essential in any climate that experiences freezing temperatures. Entrained air is accomplished by adding a soaplike admixture that froths to produce billions of microscopic air bubbles. These bubbles relieve internal pressure in the concrete by providing tiny chambers for water to expand into when it freezes. Without air-entrainment, concrete exposed to freeze-thaw cycles will scale, or flake off, at the surface, and may eventually disintegrate.
The amount of entrained air required depends on the maximum aggregate size; less air is needed with a larger aggregate. Climate is also a consideration, though there are only a few regions across the southern U.S. that fall outside ACI’s moderate and severe weathering zones (see map, page 26). With 3/4-inch top-size aggregate, for example, ACI recommends 5 per cent air entrainment in moderate regions and 7 per cent in severe regions.
How do you know you’re getting concrete with the proper water-cement ratio and strength and air content? Unless you learn to do some tests or hire a testing company, you’re going to have to trust your ready-mix provider. You can check the batch ticket, but your best insurance is a good relationship with the concrete producer. Be clear about what you want and what the concrete will be used for, and the company will adjust the mix accordingly.
Isolate The Slab
All slabs settle to some extent, while foundation walls and columns are usually more stable. Slabs also shrink as they dry, and they continue to expand and contract with changes in temperature. Using isolation joints around the perimeter of the slab and any penetrations allows it to move independently and help prevent cracks.
Isolation joints should be made from 1/2-inch-thick materials like asphalt-impregnated fiberboard, neoprene, and even cork. Some builders use sill seal to make isolation joints, but this material is only 1/4 inch thick. Though it may serve as a bond break, it might not create a big enough gap to allow for differential movement between the slab and the foundation walls. Regardless of the material you use, the joint must be set at the proper grade and be at least as wide as the thickness of the slab, so the slab won’t partially bond to the walls.
Rebar And Reinforcing Mesh Options
When it comes to reinforcing concrete, there are two main options. One option is the rebar, or a reinforcing bar, which is a steel rod that comes in different thicknesses. These are usually ribbed for better grip. Rebars typically encompass the edges of the slabs.
The wire mesh is often paired with rebars, but not always. The steel mesh is made up of wires melded together into a flat sheet. The wires form a square, grid-like pattern. Like the rebars, mesh comes in varying sizes and thicknesses. The most common size is 1/8″ thick (4 mm) and 6″ long by 6″ wide (150 mm x 150 mm).
Neither rebar nor mesh rust inside the concrete like one might suspect. The curing of concrete seals it off from oxygen, which is what causes the rusting of steel. That’s why thicker slabs of concrete are better suited to steel reinforcement.
Do All Concrete Projects Need Reinforcement?
No, they do not. Larger projects or slabs may need steel reinforcement to provide support or extra strength. The wired mesh can also help resist cracking. However, not every piece of concrete necessarily requires that extra boost.
When you’re working on a major project, it’s important to look over plans and building requirements carefully. Some locations require reinforcement no matter the size or scale of the concrete. This is especially true of certain public buildings.
The main slabs that require the steel reinforcing mesh will be those that expect a heavy workload. Anything from a lot of foot traffic, walls, or other foundational pieces will need that extra strength.
Tips For A Better Concrete Garage Floor Slab
Do Your Research
That’s when it pays to do a little research. It always makes sense to understand as much as you can about a home remodelling project, whether you plan to do it yourself or pay someone else to do the work. When searching for contractors, you need to be able to ask good questions and to recognize when the answers you’re receiving don’t add up.
Lousy concrete slabs are all too common. Slabs that develop cracks are probably the biggest headache, and the folks who pour such slabs are prone to say things like “concrete is always going to crack.” Don’t believe it. Good concrete slabs in garages will hold up to all of the weight they need to bear, through hot and cold weather, and for many long years with little if any evidence of cracks. People have garage slabs like that, as some that were poured more than 50 years ago are still as solid today as it was at the advent of the television age.
If you haven’t been quite as lucky, it’s important to note that both small and large cracks in concrete can be repaired. But if you want to know what differentiates good from not-so-good concrete slabs, here are some things you need to know.
Old habits die hard. Building codes used to stipulate that the floor of an attached garage be four inches lower than the floor level of the house. The rationale for requiring this little step was that it would prevent spilled gasoline, gasoline vapours, and carbon monoxide from getting inside the house. Today’s residential building code does not include this requirement (presumably, cars are less likely to leak these days), which means that an entire house (garage included) can be set on a concrete slab poured at one height.
But there are still a lot of contractors and building inspectors who are stuck in the past, suggesting or insisting on a four-inch step, which costs more than a level slab. If you run into one of these folks, feel free to question their wisdom and request that they produce evidence that this is a legal necessity. It still could be legally required by local building codes, but it is not a requirement listed in the International Residential Code, which serves as the model for most local and state codes.
The biggest threat to a concrete slab in the garage comes not from what gets built or parked on top, but what might move below. If the soil or base beneath the slab shifts or settle, the result can be a cracked slab.
Good ground preparation starts with removing the topsoil. Then, if the soil has not previously been “disturbed” (i.e., dug up), a four-inch layer (minimum) of gravel or stone should be added. (Soil that has been dug up before should be compacted.) The gravel or stone also needs to be compacted.
Vapour barriers (essentially, thick sheets of plastic) are often not required, but you really should insist on one. It’s an inexpensive bit of insurance against water moving up through the porous concrete, condensing on the surface and damaging items placed on the slab. Vapour barrier products manufactured specifically for use under concrete are the best choice.
The Right Mix
The concrete slab should be four-inches thick at least; it needs to be thicker if heavy equipment will rest on it. Building codes offer requirements for the concrete mix, which vary by region. The standards are expressed in terms of “compressive strength,” which boils down to the amount of water used in the mix.
More water makes for easier pouring, which is why some contractors try to water down the mix that arrives in the truck. The problem is that adding water can also make for a weaker slab. Contractors can add plasticizers to the mix that create an easier flowing mix without compromising the compressive strength.
If you live in a cold climate, your concrete mix should probably include air-entraining agents, which limit damage to the slab through seasonal freeze-thaw cycles. Fibre reinforcement is another inexpensive product that you should insist on. The fibres are mixed in with the concrete and produce a stronger slab.
Wire mesh or thick reinforcement bars (“rebar”) has long been a component of concrete slabs. However, with proper ground preparation, a good concrete mix, and sufficient expansion joints, it isn’t necessary. Still, many contractors continue to add the cheap insurance offered by reinforcement. If the reinforcement is to do its intended job, it needs to rest in the middle of the slab, not on the bottom. That means that it needs to be anchored in the ground well enough to stay put when the concrete is poured.
Many people understand that wood shrinks and expand as temperature and humidity ebb and flow through the year. But the same thing happens with concrete. That’s why it is necessary to include expansion joints along the edges of the concrete garage floor as well as around posts or other protrusions in the slab. The resilient material used in expansion joints absorbs any expansion, thus reducing stress, yet fills that joint when it contracts.
Finishing And Curing
Once the concrete has been poured, it needs to be levelled and smoothed. Expansion grooves should be cut into the wet concrete to provide additional crack resistance. Then, the new slab needs to be left alone.
Concrete doesn’t dry out; it undergoes a chemical curing process that creates a dry, solid mass. The top surface needs to remain wet while the curing process takes place. Contractors often spray curing compound on top of the slab or cover it with sheeting to reduce evaporation. You can also offer to spray a little water on the slab every day while it cures.
It’s worth noting, however, that although some bigger jobs (such as driveways which take heavy traffic, carport floors and larger shed floors) might not require a permit, it could well be a good idea to include steel reinforcing. And sometimes smaller jobs also benefit from reinforcing steel, especially if the sub-ground is less firm than it should be or there are pockets of spongy sub-ground. Even for a smaller shed floor, it does not hurt to throw in a steel rod (rebar) around the perimeter of the floor to give great added strength.