Concrete Installation Guidelines
Concrete technology is, in fact, a science. There are scientists who devote their entire life to working with this material. They research methods to improve it and its use.
My guess is that just about everyone has seen concrete being "poured." This wonderful material begins its life as four basic ingredients (sand, gravel, cement and water) which are mixed into a plastic liquid. Depending upon a variety of conditions, this "liquid" transforms into a solid material. I must tell you that each and every time I work with concrete I marvel at this transformation.
If you know how and why concrete transforms from the liquid to the solid state, you will more easily understand why concrete fails or experiences surface defects.
The cement powder, which is one of the four basic ingredients, is the "glue" that holds the sand and gravel together. When water is added to the dry powder, an irreversible chemical reaction begins to happen. This reaction causes tiny crystals to begin to grow. This reaction continues for months after the concrete is initially mixed!
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The needles of the crystals begin to interlock with one another and begin to penetrate microscopic holes and cracks in the sand and gravel. They "cement" everything together. No pun intended!
The growth of the crystals can be monitored. If you have ever had the pleasure of working with and finishing concrete you can "feel" the crystals growing. As each minute or hour passes after the concrete truck has left the jobsite, the concrete becomes progressively stiffer. It soon gets to a point where it will support the weight of a person using an automatic finishing machine. The growth of the crystals allows this to happen.
Things can go wrong if the crystals grow too fast. For example, you may not be able to achieve the desired finish because the concrete "set" up. "Set" up is just another way of saying that the crystals grew faster than you or the finisher would have preferred.
Surface Defects - Some Causes
A multitude of concrete surface defects are often caused as the crystals begin to grow. One of the biggest reasons for surface defects is the addition of extra water to the mix and/or the use of water on the surface of concrete as a part of the finishing process. Both of these practices are frowned upon by concrete scientists. The reasons are numerous, however, they all have to do with those silly cement paste crystals.
Water - The Labor Saver
Water is frequently added to concrete after it arrives at a jobsite. Concrete finishers often will look at the batch as it begins to discharge from the concrete truck. They may think that it is too stiff . The addition of extra water makes the concrete flow easier. There will be less work. They think they will have more time to finish the concrete.
Believe it or not, when the concrete was initially mixed at the concrete plant, it had plenty of water. In fact, as long as accepted work practices are used in placing and finishing the product, the concrete has extra water. You can actually observe this fact if you pour concrete on top of a plastic vapor barrier. Frequently, after the finishers have bull-floated the concrete to smooth it initially, water migrates to the surface of the slab. This is called bleed water. It comes to the surface, because it is actually the lightest component of the entire batch!
The addition of extra water at the jobsite alters the chemistry of the crystal reaction. The cement paste is severely diluted. The cement particles are farther apart from one another and not as many crystal needles interlock.
Because the strength of the concrete is in direct relation to how many crystals interlock, the concrete with extra added water will be weaker. The addition of extra water during the finishing process produces the same results, however just the top layer of concrete is usually affected. Water is often added by finishers because the concrete is 'setting' (crystals are growing.) The addition of water allows the finisher to more easily work the surface of the concrete. It is a mistake to allow this to happen. There should be enough man(woman)power on the job to stay ahead of the growing crystals.
The problem is really very simple. Since the entire batch of concrete, in many instances, is mixed at the same time, the crystals are growing at the same rate throughout the mix. Remember, if you are finishing a large slab alone, the crystals are growing everywhere across the slab!