Concrete is a very basic material that has been around for a long time. If you stick with the basics, it's pretty hard to go wrong. There are three main ingredients that go into making concrete. Rock, sand and Portland Cement. The cement is the glue that holds it all together once you add water. The water creates a chemical reaction with the cement and the way I understand it, the amount that is needed, never actually leaves the concrete, it's converted into the concrete.
The biggest issue with pouring concrete is adding too much water. There is the ideal amount, and then there is extra. You will never have a foundation or pad poured with just the ideal amount, it will always have too much water. There is a slump test that you do to see how much is too much with water. Basically, you build a sand castle. Fill a small gallon or half gallon bucket with the mix and then try to make a sand castle. How well it stands on it's own tells you what your slump is. The only time you really need to do this is to win an argument with your contractor. Let him know before the pour starts that you will be there with your bucket and if you think he's adding too much water to the mix, that you will test it.
There are two problems with adding too much water. First, it lowers the PSI of the finished concrete. Second, and more common, it creates cracks.
There are several reasons that concrete cracks. First is when curing. The excess amount of water creates mass. When that water evaporates, the concrete still wants to fill the same amount of area, but there is now less mass, so it cracks. The more water you have in your pour, the bigger the cracks. Concrete is also affected by temperature and will swell and contract as the day heats up and cools off. This is why concrete roads are so hard to do because you have to all for expansion of the concrete so it doesn't crack. When pouring a house, you always do the living areas separate from the outside areas due to this movement. Porches and in some places, garages are done the next day or later in the build. Never on the same day. Settling from soil not compacted properly gets the most blame for cracking, but it's actually very rare. Just about every house out there with a concrete slab has a void under it somewhere. The concrete can easily support what's above it most of the time and nobody is aware of the issue. Poor roof framing is usually the cause of what most people call foundation issues. The weight of the house is on the footings, and it's extremely rare to have a footing move due to improper compaction. When it does happen, it's obvious because the concrete will break and be forced down or at an angle. Water cracks remain flat and smooth with the rest of the surface.
Rebar on a flat pad is there to hold the concrete together when it cracks. Most cracks are hairline that are impossible to see. To be effective, the rebar needs to be inside the concrete. Ideally, it has the most strength at the one third point up from the ground, but in real world work, just get it in the middle and you're fine. To do this, you HAVE to use chairs. One of the most common lies told is that they will lift it as they spread the concrete and that it will remain in the middle while they walk over it spreading the concrete. This is 100% impossible to do and where I walk away from hiring a contractor. If I'm told this, I will not use that person or company.
Wire should never be used on a big pad. It's find for sidewalks since you don't have to walk on it when you pour it. You can even pull up the wire and not put it on chairs for sidewalks, but eve if you use chairs, you cannot walk on wire and expect it to end up in the middle of the pad. It's pure junk and another reason to not hire a contractor that tells you that they will use it instead of rebar.
Fiber is a nice bonus to concrete that is proven to reduce cracking and add some strength to a pad. I know that some people will replace it for rebar, but I feel that this is a bad idea. In areas of the country that have severe weather, earthquakes and expansive soils, this is not allowed. To deal with bad conditions, more rebar is required!!! Rebar is the only thing that has a proven track record of success.
Most of the time 3/8's rebar, also called number 3, is used at 24 inches for the pad and then 1/2 inch rebar, also called number 4, is used in the footings. In some areas the size of the rebar is much larger to gain more strength, and sometimes it's installed closer together too. I personally like it at 18 inches for my projects. It's a small increase in money, but worth it in my opinion.
To increase the strength of your concrete, you can have them increase the amount of Portland Cement to the mix. I forget the actual numbers, but off the top of my head, it's something like 2,500 psi is the standard mix, and then if you add another sack to the mix, it goes up 500psi This adds to the cost, which isn't a huge increase, but around here, it's rarely done because there really isn't any need for it.
Making your pad thicker also increases the strength, but I don't know the specifics of what all is achieved for a pad. In a footing, it's done by increasing the overall mass to such a point that it can support a greater load, like the walls, or a load bearing beam. Since the weight is concentrated at those points, the type of soil you have is also very important to know how big to make the footing. For a load bearing post, here in East Texas, on red clay, you make it 24 inches square and deep.
Plastic doesn't hurt anything, but most of the time, it's doesn't accomplish anything either. Usually when it's done, it's because a client read online how important it is, so it's used. For my own projects, I don't use it. I also do not believe the moisture comes out of the ground and through the slab to create moisture, which I understand makes me a minority on here based on the discussions that I've been involved with. So far, nobody has changed my mind.
Cutting the concrete weakens it where it's cut, allowing it to crack easier in a place that you have decided that you want your cracks. Some people like the look of it, I personally don't like it and wont do it.
The crew that you hire to do your pad will install the forms, dig the footings and install the rebar. Your General Contractor will coordinate with your electrician and plumber so they can install their lines after the forms are done, and before the rebar is installed. If you are acting as the GC, then you need to get everyone on board with what they are going to be doing. Or are you going to install the utilities yourself? If so, that's where all the detailed planning and measuring comes into play and probably another topic to go into since there are some things to take into consideration that need to be done before the concrete is poured.
Try to pour on days that are above 50 degrees. If you do it colder, they have to add chemicals that cost more money and may affect the strength and overall finish.
Be sure to have an easy path and lots of open space for the cement trucks. They turn like buses and they are super heavy. The closer you can get the truck to the pad, and the more places that they have to pour from, the faster it will go, and that's really important since the concrete is getting harder every minute.
Once the pour starts, your main job is to make sure that they do not add more water to the mix. Otherwise, stay out of their way and take some pictures of what they are doing. If there is too much water, it's a lot easier to sue them if you have pictures that show it.
Water is your enemy with concrete. They will add more then you want, you will not be able to stop them, all you can do is keep it to a minimum and ignore their complaining on how hard it is to spread without adding more water to make it like soup. Every crack that happens in the slab after the pour will be from too much water. If there is any soil issues, that will take years to show up.
