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

Pressure Formula

Stress, it turns out, is exactly the same. It has the same units, and it conceptually and intuitively has the same behavior. They go hand in hand, so I’ll refer the pair interchangeably. While stress isn’t derived from engineering, it is a fundamental science tool that nearly all engineers use.

So, what is it exactly? In every day life, we deal with pressure in our car and bike tires. We have tire gauges to tell us the air pressure inside the tire, in “psi”. The metric equivalent is the Pascal (or Pa), where the force is in Newtons, and the area is in squared meters. I guess to follow the “psi” format and for purposes of demonstration only, it’ll supposedly be Npm, or Newton per square meter.

For now, we’ll simplify the whole deal by looking only at positive axial normal stresses. It’s positive because we’re stretching and not compressing. (It is also called tensile stress.) It’s axial and normal because the area we look at is perpendicular to the direction we’re pulling; for now, we’re not pulling at some angle to the cross-sectional area in the general formula.

In everyday engineering, we tend to deal instead with ksi (kilopounds per square inch) and MPa or GPa (megapascals or gigapascals, respectively). One psi or one Pa is far too small to deal with because a pound-force is too small to act over a square inch and a square meter is too large for a Newton to act upon. Don’t worry. This’ll make more sense when we discuss really strong bars and beams made out of steel and aluminum, as opposed to rulers made out of plastic.

The next time to go walking or driving over a bridge, for example, you can hopefully realize that all the metal cables and beams are working in tandem with huge amounts of tensile stress to support you, the other cars, and itself. The cabling of a suspension bridge, for example, is completely engineered with the fundamental principle of tensile stress of force divided by area.

That’s it for now.

But check out the video below. It shows what happens to a steel bar when there is too much tensile stress in the bar from too much pulling force.

If you notice at 0:27, the steel actually appears to stretch, a process called necking. At 0:30, there is too much force and stress to the point that the steel broke in two. In future posts, we’ll discuss these magical stretching and breaking points, and how we can calculate how much force it took to fracture this piece of steel.

Take care.