Pouring Concrete Over Rebar in Washington State

This is a simple one, though I’m not sure why some people would confuse the two. Sure both are used in construction, and both are related building materials, but they aren’t the same and definitely shouldn’t be used interchangeably.

The rule of thumb is, if the choice is between concrete and cement, the structure in question is probably made of concrete. Generally, concrete comprises of cement, or cement is used to make concrete. (What you buy in home improvement stores is cement, not concrete!)

Cement is the dry powdery material that mixes with water and other additives to make concrete, though cement is also used to make materials like mortar, plaster, and stucco. The most common variety of cement is known as Portland cement, whose name is derivative of the natural stones from Isles of Portland in England. In 1824, a patent was issued for the manufacturing process of what would become known as Portland cement, which is generally gray or white. Cement is primarily made up of calcium and silicon from limestone, and smaller amounts of aluminum and iron from iron ore. The raw materials are mixed in proper proportions and then crushed and grounded to a powder. The powder is sintered in a kiln and clinker is produced. (Sintering is the process of fusing individual masses by heating rather than melting.) Clinker combined with small amounts of gypsum gives us cement.

Once we have cement, concrete is made by adding water and coarse aggregate, like granite, limestone, old concrete, or sand. Through the process of hydration (chemical reaction involving water), cement reacts and hardens when dry. (Follow proper instructions on the cement bag.) Concrete is the most common building material in the world, used to make roads, foundations, walls, bridges, etc.

As a material, concrete is extremely strong in compression (about 4000 psi or 28 MPa), but it is used at the expense of size and weight. It is important to remember, though, that concrete is weak in tension. If concrete is being pulled apart, the cement used to hold the aggregate together is stressed, and the structure is prone to cracking. Similarly, bending is an inherently big problem for concrete structures. Well, to counter this problem, we came up with the idea of reinforced concrete, which uses materials strong in tension to take the tensile stress that may develop. Often, steel bars, which are called rebar (reinforcement bar) are used. Grooves on the rebar help “stick” to the concrete to maximize effectiveness; slipage would defeat the purpose of the rebar.

Also, I found out why rebar is usually green. Apparently, rebar is coated with green epoxy to protect it from rust and corrosion. It’s funny how everything has an explanation in engineering; everything has a purpose.

Hopefully after reading this, there will be no more confusion between cement and concrete. As always, if you have any questions or comments, leave one below!

Sources:
http://www.buildeazy.com/newplans/eazylist/cement.html
http://en.wikipedia.org/wiki/Cement
http://en.wikipedia.org/wiki/Concrete

Categories: In-Depth Articles, What the Heck?
Tags: Cement, Compression, Concrete, Materials, Tension
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