Concrete is made up of three main ingredients: water,
Portland cement, and aggregates. The ratio of the ingredients changes the
properties of the final product, which allows the engineer to design concrete
that meets their specific needs. Admixtures are added to adjust the concrete
mixture for specific performance criteria.
Water
The water in the concrete mix should be clean and free of
impurities. The amount of water relative to the amount of cement changes how
easily the concrete flows, but also affects the final strength of the concrete.
More water makes for easier flowing concrete, but also makes for lower strength
concrete upon curing.
Portland cement
Cement hardens when it is mixed with water, which binds all
of the ingredients together. Portland cement is the most common cement used. It
is composed of alumina, silica, lime, iron, and gypsum. Small amounts of other
ingredients are also included.
Aggregates
The majority of a concrete mixture is made up of coarse and
fine aggregates, which helps to increase the strength of the concrete beyond
what cement can provide on its own. Sand, gravel, and crushed stone are used as
aggregates. Recycled materials like blast furnace slag, glass (mostly used for
decorative purpose), and ground-up concrete are starting to be used as concrete
aggregates.
Admixtures
Admixtures accomplish wide number of goals. It can be as
simple as adding a pigment to color the concrete. Other admixtures are used for
faster curing times in cold weather, creating extremely high-strength concrete,
or for increasing the flowable nature of concrete without compromising the
strength. Unfortunately, admixtures can generate unwanted results such as poor
adhesion of finish-flooring. For this reason, many structural engineers and
architects are hesitant to use admixtutres.
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