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Concrete is an amazing thing; I have had a great deal of experience with it even before my recent employment change. There are four main ingredients to concrete: water, sand, stone (called the aggregate), and Portland cement (the glue).
It starts out in a "fluid" (not liquid) state and becomes a solid. Concrete will continue to cure for a considerable amount of time. Concrete has a life expectancy of 75-100 years, depending on the mix and whatever add-mixtures are used. Typically, concrete reaches 98% of its designed strength after 28 days and there are several factors governing the designed strength of a particular batch of concrete. These factors are the amount of water in the mix, the amount of Portland cement in the mix, the size of the aggregate, along with whatever add-mixtures are introduced to hasten or slow the cure rate. The more water in the mix, the weaker the concrete; the more Portland cement the stronger the concrete; the larger the aggregate the stronger the mix as well; and there are add-mixtures that will also increase the final strength.
Many people want to talk about conservation of natural resources, like water. Consider these facts: Water used for washing or sanitation gets recycled; either by nature or man-made processes. The concrete industry uses huge amounts of water. The truck I drive can deliver 10 cubic yards of concrete per load, any more and the truck is overloaded. One cubic yard of concrete initially contains 30 gallons of water, give or take, when the batch is first mixed. Most people think that when concrete dries, the water evaporates, not so. In the fluid state when it is freshly mixed, concrete weighs 4000 pounds per cubic yard, give or take depending on the mix. In the solid state when fully cured, concrete weighs 4000 pounds per yard. The water stays in the concrete! Less than 2% actually evaporates when the concrete "dries". Furthermore, for every load of concrete I deliver, I use at least half of the 150 gallons of water on board the truck in the water tank for washing out purposes and adjusting the slump. (Slump is a term used to indicate how stiff or watery a particular batch of concrete is when first mixed; the lower the number, the stiffer the mix.) A typical slump of “4” is usually specified for flat work such as sidewalks and driveways. Once the concrete is hard, the water used for that batch is forever in that concrete; it will never again be part of the environment. In the fluid state, the water in the concrete is in the form of water droplets; when it hardens, the water droplets break up into individual water molecules and bond with the calcium molecules in the limestone; the main ingredient of the Portland cement. This is part of what gives the concrete its strength.
The next time you see a concrete truck on the road understand that it is a unique vehicle; the load, which can be more than half the overall weight of the vehicle, is in constant motion and unlike a tank truck carrying a liquid, the load does not always seek the lowest point. My truck weighs 35,450 pounds empty; with 10 yards it weighs 75,000+ pounds. And finally, concrete trucks don't stop on a dime. I have determined that a minimum of 500 yards is necessary to be able to safely stop my truck without calamity if I am traveling at 55mph. When someone cuts in front of a concrete truck and the driver has to slam on the brakes with a full load, there is a significant possibility of concrete puking out the front of the truck onto the car or whatever is immediately in front of the truck!! Most people are unaware of these facts including many construction workers who work with concrete. I would like to admonish everyone to be aware of these things.
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