Handling concrete is both an art and a science! In this article we will focus more on the science part and the required steps that should be followed no matter what your budget or time frame is. In other words, don’t mess with the basic recipe or your cake will suffer!
PREPARATION: before any concrete is poured, you must make sure a few things are in place:
1) Know if the base is ready
Starting with a solid base that has been properly drained is essential. Skipping this important step could cause soft areas to later produce cracks over time and under the pressure of heavy vehicles. The best way to do this is pour between 2-4 inches of sand or small gravel and pack it down. This is especially important if the area is known to not have good drainage.
2) Use reinforcement
Once the forms are built, you should reinforce the concrete with rebars (3/8 or ½ inch). Avoid steel mesh or even plastic fibers because they do not provide additional strength. They are helpful in reducing cracking.
3) Know how much water to add
While it may be tempting to add more water to the mix to ease its flow down the chute and speed the pouring process into forms, this will also result in a weaker concrete.
So resist the temptation and instead keep the cement mix as dry as possible. It may be a little more work to push it into the form, in the corners and the edges but this will pay in the short, medium and long-term.
There are a few things to keep in mind after the concrete has been poured:
1) Know when to begin smoothing the surface
This step requires precise timing, having everyone ready and all the required tools nearby. What happens if you start trowelling too soon? You risk bringing the cement paste to the surface and weakening it. One easy way to know when it is time to start troweling is when the water starts to disappear from the surface. This signals the perfect time to start smoothing quickly as concrete is known to harden fast. Depending on the job you are doing, you may need a rebar cutter, a plate compactor, a bull float (for large slabs), a square-edge shovel, one or more finishing trowels. For the finishing details, you will need as a minimum, a groover, an edger and a stiff square broom.
2) Cutting down control joints
Once the concrete has been poured, it is necessary to cut the control joints down to 25% of the depth of the slab. So if you poured a 4” slab, your joints should be 1” deep. This is done to accommodate for the shrinkage and cracking that occur during the curing process (about 1/8″ per 10 feet of length). The cutting can be done while the cement is still fresh using grooving tools or with an early-entry dry-cut lightweight saw (1” to 3” deep). On hot days, you may need to cut joints within 6-12 hours or risk cracks. The important thing, if you decide to wait and use a saw blade, is that the edges of concrete won’t chip during the cut. So proceed carefully.
3) Slow down the curing process
Good curing means evaporation should be prevented or reduced. Slowing down the drying process is the key to strong concrete. The strength is the direct result of the lime hydrating during the curing process. One simple way to do this is to cover the slab with wet burlap, wet blankets or a plastic sheet for a week or more. In hot conditions, above 80 degrees, watering the slab frequently with a garden hose or a light sprinkler is necessary to keep it moist and let it reach it maximum strength. Concrete will reach approximately 80% of its strength in 7 days and almost 100% after 28 days. Using curing compounds (retarders, air entraining agents, plasticizers etc.) will also help sealing the moisture. Find out more about ad mixers in this article http://excavatorheavyequipment.com/blog/2013/07/01/why-use-concrete-ad-mixers/
5. Wait to build
Concrete reaches a high percentage of its final strength after 28 days, so construction should wait for at least 5 days to start.
By following these important steps you will avoid some of the most common mistakes that are learned the hard way. They will save you time, money, reputation and give you more time to focus on the art of concrete and its many forms.
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