Here are some concrete basics that may be helpful for you to know.
The base for your concrete
Whether you are pouring sidewalks, a driveway, or a pad on an industrial site, a proper base is essential because it separates the concrete from the ground (clay or dirt) thus minimizing the effects of freeze and thaw. The base also adds strength and support to the concrete.
The thickness of your base depends on the strength you need in your concrete and how heavy the concrete will be. To give one example, on a 4-inch thick driveway, a well-compacted base would be composed of road crush (sand and 20mm rock) and compacted to a 2-inch thickness.
DO NOT allow a contractor to use ONLY sand in your base for exterior projects because sand must be contained in order to remain compacted. If you have only sand under a driveway, for example, rainwater or snow melt can make it displace and wash away from under your concrete. The result will be heaving, cracking, a void underneath the slab, loss of strength etc.
Certain concrete projects will be completely flat, but when concrete is being poured around a house, it should always be sloping away from the house to prevent water draining toward the house.
The reinforcement of your concrete
The reinforcement of your concrete is important for the longevity and durability of your project. There are different types of reinforcements, notably, rebar, wire mesh, and metal or nylon fibre.
– Rebar is the most commonly used reinforcement as it can be used to reinforce almost any concrete structure.
– Wire mesh is generally used in addition to rebar (for example on parking lot ramps) to give the foundation extra strength. On a garage driveway, a person could choose to use only wire mesh.
– Metal fibre is normally used in areas where there will be constant pressure from heavy equipment – ie. at a bus stop. The disadvantage of working with metal fibre is that it is extremely heavy and difficult to move in the placement stage.
– Nylon fibre is generally used as a cheaper alternative to rebar and wire mesh. It is strong enough to be used on things like driveways. The nylon fibre gets mixed right into the concrete so there are also cost savings, as a contractor does not have to spend time cutting, placing and tying rebar.
How to get the right concrete mix and thickness for your project?
If concrete will be on the inside of a building, the mix will not contain air; if the concrete will be outside and/or exposed to temperature fluctuations, air will be necessary in the mix so expansion and shrinking can happen when thawing and freezing make the concrete shift; this flexibility makes the concrete more resistant to environmental factors.
MPA (megapascals) is the measurement used to determine the strength of concrete. Concrete is meant to be strong as far as the pressure put on it and this is determined by the MPA. For example, a garage pad that will just bear the load of a small car might be 27-30 MPA, while a parking pad for a tractor/trailer would be around 35 MPA. The use of the concrete (ie. sidewalk, garage pad etc) will determine what MPA needs to be used.
The thickness of the pad will also be increased based on the use the pad will be put to, as the strength comes from the thickness of the pad and the concrete mix. An engineer will normally determine the thickness and MPA of any concrete structure. If you want to make sure the concrete you are getting is best suited to your needs, hire an engineer to tell you exactly what thickness and what MPA is needed. In the long run, this will save you money because you will not pay for more than you need; alternatively, if you go the least expensive route, you may end up needing to redo or repair your concrete.
Concrete has limited workability time, meaning you need to work with the concrete while it is still shapable. For a driveway, for example, concrete will give you about 4 hours of working time. In the first two hours, the concrete will need to be placed and shaped (ie. elevation set, and edging and control joints done). In the last two hours, the concrete can be “finished” – ie. broom finish, stamping etc.
It is important to know that concrete can “flash”, meaning weather and/or the age of the concrete can make it dry very fast, giving you very limited time to work with it.
The importance of consolidating concrete
Concrete needs to be consolidated, meaning that on bigger pours the concrete has to be vibrated. If concrete is not properly consolidated, it becomes “honeycombed”, which means there are voids in the concrete (like air pockets) that reduce the structural integrity of the project. On small projects, if a vibrator is not being used, the concrete can be consolidated by a person walking on it as they’re placing it with a rake, and by lightly tapping the forms/footing of the pad. If concrete is over-vibrated segregation happens, which means the cement separates from the rocks in the concrete mix; this affects the structural integrity of the finished product.
There are different finishes to concrete which are mainly used for sidewalks, driveways, patios, restaurant floors etc. These finishes include:
– Coloured concrete
– Exposed aggregate
– Broom finish
The lines placed in concrete are not just for appearance; they are called control joints and they give a concrete pad a “weak point” so any crack that is going to appear migrates to the control joint rather than forming in the pad.
Cracks in concrete
There are two types of cracks that can appear in concrete:
– Hairline cracks appear naturally in concrete because it shrinks as it cures. To limit hairline cracks during the curing process, the concrete can be saturated with water or a curing compound can be applied which will slow down the drying process and limit the number of cracks that appear.
– Larger cracks are cracks bigger than a ¼ inch gap.
■ Anything over a ¼ inch gap is a substantial crack; it will need to be repaired in order to prevent further damage.
There are many ways to repair concrete, however there are always disclaimers on concrete repair products because many factors will affect how well the repairs work.
There are ways to repair concrete that work well, but the first thing to determine is that the concrete needing repair is still sound. If the concrete is scraped with a sharp tool and it crumbles or turns to dust, it is time to replace it. If it remains intact, repairs can be done.
Repairing sound concrete:
– It is important to make sure the concrete is clean before performing any repairs – free of dirt, oils, contaminants, standing water etc. If there are any contaminants or oils on the surface of the concrete, the concrete will need to be ground down to the level of clean concrete.
– Once that’s done, it is important to “saturate dry” the existing concrete, meaning the area to be repaired is soaked with water until it looks wet without having any standing water in it. This will prevent the existing concrete from drawing in the water in your repair mix.
– If the area being repaired is very smooth, it will need to be roughed up using a grinder with a diamond disc or grinding cup. The texture the grinding creates in the concrete is called “tooth” and it is what allows the new concrete to bond to the old.
Things to pay attention to when hiring a concrete company
When you meet with a concrete company representative, ask questions related to the above material, for example:
– What kind of base and what thickness do you recommend?
– What kind of reinforcement do you recommend, and why?
– What type of concrete will you use – what MPA?
– What kind of curing process will you use to minimize hairline cracks?
The representative should be knowledgeable and able to answer your questions well.