The Importance of Water
All of concrete's ingredients are important to its quality. Your supplier, not you, has to worry about providing good quality cement, sand, and gravel. But one big factor is in your hands - the amount of water in the mix. When you holler out "Add water!" during a pour, you really change the concrete. Restrain yourself: Limiting the water content is crucial for strong, hard, durable concrete, and to reduce shrinkage and cracking.
As engineers will tell you, the water/cement ratio is the weight of the water per yard in the mix divided by the weight of cement per yard. Concrete's water/cement ratio is usually somewhere between .4 and .6.
As an example, let's take an average yard of concrete, a typical five-bag residential mix. If we start with 30 gallons of water per yard, which weigh 8.33 pounds per gallon, the water weighs about 250 pounds per yard. Five bags of cement per yard, at 94 pounds to the bag, works out to about 470 pounds of cement per yard. Divide 250 by 470, and we find that this typical residential concrete mix has a water/cement ratio of .53 - that is, before the contractor yells out, "Add 20 gallons!"
If you add 20 gallons to a 10-yard load, that's 2 gallons per yard, or 16.66 pounds. Your water/cement ratio will go up to .56 or .57 - and your concrete will be weaker.
As a rule of thumb, for every gallon per yard of water added on the job, you lose at least 200 psi in strength. If you add 20 gallons to a 10-yard load - 2 gallons per yard - you are probably losing 400 to 500 psi.
Shrinkage. Besides reducing the structure's ultimate strength, adding water on site will also increase the total amount of shrinkage.
Of the water in a concrete mix, just 25% to 30% is necessary for the hydration reaction. The rest of the water is only there to make the concrete workable. Any water that isn't involved in the hydration reaction either evaporates (25% to 30%), or remains indefinitely within the pore spaces in the concrete (40% to 50%). Most of the extra water you may add on site will quickly bleed and evaporate out. As the water leaves, the concrete shrinks; as the concrete shrinks, it cracks.
Some cracking of concrete is unavoidable, as we all know. But cracking is directly related to the water content of the mix. More added water means more and bigger cracks. That's another reason to add as little water as you can.
Reduced density, increased porosity. Not all the water that evaporates out of concrete results in shrinkage. If the concrete sets up and hardens before the water comes out, it won't typically shrink and crack as much; instead, it will be less dense and more porous. That's because when you add a lot of water to concrete, you increase the volume; but when the water dries out, all it leaves behind is air.
Figure 6 shows a series of test cylinders made with concrete of differing water/cement ratios, ranging from a .25 ratio (about the minimum you can do) up to a .7 ratio. There is the same amount of cement, sand, and rock in each of these samples - the only difference is the amount of water.
After oven drying, all of these samples weigh the same amount; but as you can see, the ones with more water are larger. The difference is air - and air doesn't have a lot of strength. The sample on the left is probably 8,000-psi concrete, but the low-density sample at the right is probably 1,800 psi. The center samples are where most residential concrete is, probably somewhere between 3,000 and 4,000 psi.
Besides being weaker, porous concrete made with a high water/cement ratio will end up being more permeable. As concrete is hardening, the cement and the aggregate sink as the bleed water rises. Water flows around the particles and through the sand and cement paste, leaving behind a lot of little voids and capillaries in the hardened concrete. After the concrete cures, those capillary voids let water and vapor move through finished slabs and walls, which can add to moisture problems.
Most basements experience a certain amount of dampness: at first from water evaporating out of the concrete itself, and later from moisture in the ground moving through the concrete via capillary action. Both moisture loads are directly related to the proportion of water in the mix.
If an average yard of concrete contains 32 gallons of water, and 28% of that evaporates, 9 gallons of water per yard is going to come out of a typical foundation. From the footings, walls, and slab in a big basement, something like 400 to 700 gallons of water vapor will enter the house. That's a lot of moisture - and the wetter the concrete you pour, the more moisture there will be.
It takes about three years for all of that vapor to come out of the concrete, but the bulk of the evaporation is a first-year concern. About a third comes out in the first 30 days - probably before the house is even finished. The next third comes out in the first year after the pour, so by the end of the first year two-thirds of the water is out. Right when a house is built, if you stud out the basement walls and slap up insulation and poly, you'll get moisture within the wall. You'd be wiser to let the basement dry out for a while first.
Ground moisture. Unfortunately, water in the poured concrete isn't the only source of moisture in basements and slabs. All over the country, ground moisture moving through foundation slabs is a common problem.
As the water moves through the slab, it brings up various chemical salts from the unhydrated cement. The water evaporates and leaves the salt behind as a white fuzzy dust. This material can build up behind a membrane and debond many types of coatings. If rising moisture exceeds about 3 pounds of water per 1,000 square feet over a 24-hour period, it exceeds the breathing ability of many floor coverings: Such moisture can build up and ruin the adhesion of vinyl flooring and tile.
If you pour a stiff mix, the concrete should be watertight. But vapor permeability, though reduced, will still exist. So good concrete alone isn't the total solution to ground moisture problems: Proper grading, surface drainage, waterproofing, and perimeter drainage are also important (see the related article).