RANDOM THOUGHTS #293: CONCRETE AND CEMENT

  Steve Cole, a registered engineer, explains that not everything gray is the same.

        

Most people use the words concrete and cement interchangeably, but they aren't the same. Point to one of those big trucks with that huge barrel on the back spinning around and most people will say it is a cement truck, when in fact it is a ready-mix concrete truck. Cement trucks are actually much bigger, don't spin, and are rarely seen as they only go to concrete plants, not job sites.
    

Cement, more properly Portland cement, is the glue that holds concrete together. Concrete consists of cement, water, sand, and gravel (I will spare you precise numbers). Cement is actually the smallest element because it is the most expensive. If you have to fill a cubic foot of space with artificial stone (that is, concrete) you can save a lot of money by replacing most of the cement with rocks and sand. Without them, the space is entirely filled with expensive glue; with them, a bit of glue holds one rock to another to another until the space is filled. Water activates the glue, and when the water dries and evaporates, the glue remains stuck to the rocks and sand. There is Roman concrete all over Italy that is 2,000 years old.
      

The trick is that the amount of water needed to activate the glue is fairly small, but the resulting mix is unworkable for most cases. That gray muddy stuff you see being poured in foundations and patios has about twice as much water as it needs. The extra water makes the concrete flow and spread through the area surrounded by concrete forms. We can call that Standard Concrete or Structural Concrete if you want. It's usually got a strength around 2500 pounds per square inch (more if you include extra cement), far above any load a household of people and furniture would contain. The surface and walls of the Oroville dam spillway in California (if you recall the recent near disaster) are made of this kind of concrete.
   

If you mix in some more water, you get a very soupy mix of concrete called Footer Concrete or Leveling Concrete. Strength drops to 2000 pounds per square inch, but the mix is so liquid it will seek its own level like tomato soup. This is often used in the footings of foundations (the part around the edge that is two feet down into the ground) or to fill the deepest holes in the rocks at the Oroville dam spillway. While weaker, this is more than adequate strength, and some of those holes, nooks, and crannies of the fractured rock at Oroville aren't going to get filled up any other way.
        

Then we can go back to that original discussion of very dry concrete that has just enough water to activate the cement. This was first applied to construction of dams (and was called Roller Compacted Concrete or RCC) after I graduated from engineering school. This stuff is very strong (or you can use less of the expensive cement) and doesn't generate heat as it cures. (Hoover Dam is all structural concrete and they had to run ice water through pipes to deal with the massive heat that developed.) The thing is that RCC won't flow. You don't bring it to the job site in mixer trucks with those big spinning barrels; you bring it into the site in dump trucks. When dumped out, it just sits there in a pile as high as it is wide, not spreading at all. You have to use a bulldozer to push it around, usually into layers 15 inches thick. Then big heavy rollers are run over it, compacting it to 12 inches per layer. Some dams have hundreds of feet of this stuff; the deepest holes at the Oroville spillway look to be 20 or 30 feet (plus the depth of the leveling concrete filling the deepest holes). When you're dealing with massive amounts of concrete this kind of material is easier to get into place, much stronger, and doesn't have the heat problem. It also cures more quickly as there is less water to evaporate. It's also about the only place you could use this kind of concrete if you wanted to.
       

Then there's brick mortar and grout, which is just cement and sand and water, but that's another subject, really.

        

What is it that's gray, made out of Portland cement, lives in the woods, and howls at the moon? Give up? A timber wolf. Oh, the cement, it's just in the riddle to make it harder.