It’s very common to see concrete companies doing tests that don’t make much sense. They test a mix for the slump at 50 psi. Then they get some numbers that come out low. So they add more water to the mix, call it 100 psi, and do another set of tests. They claim that their mixes are less likely to slump at 100 psi.
What’s going on here?
The standard slump test is entirely insensitive to whether you use water-based or cement-based mortar. It simply measures the force required to compress a block of concrete with a clamping bar at one end and two screeds on the other end (which are just free-standing blocks of wood). It does not measure how well the mortar holds up before it’s filled with concrete, nor does it measure how well the concrete holds up after it’s cured.
Here’s an example of what happens: You have a block that is 30 inches wide by 40 inches long and you fill it with mortar weighing 12 pounds per cubic foot (a good average). If you then clamp the block in such a way as to produce an applied force of 10 pounds at each end, you will find that when you let go, the block will sag down by 1/2 inch in 15
The concrete industry has relied on stories and anecdotes for decades, and it seems to be working just fine. Stories and anecdotes are not really bad as long as they are based on facts. But it seems that some concrete companies have changed the way they talk about concrete.
One example is the way concrete mixes are described. For years, concrete has been described in terms of “cement” and “aggregate.” Now some people are using “mix” instead of one of those two words. The idea is that a mix of cement and aggregate is superior to either alone. That sounds plausible, but what does it mean? What do test results show?
The answer depends on what you think is important: cement or aggregate? If you work with a concrete contractor, ask him or her to tell you the difference between cement and aggregate. Ask them if they have ever seen a batch of mortar that had no cement in it. Ask them if they have ever seen a batch of mortar that had no gravel in it. Ask them if they have ever seen a batch of mortar that had no sand in it.
Ask them if they know the answer to these questions: When was the last time anyone took a sample of their concrete outside, in the rain or at night? When
Concrete can be a very misunderstood product. Concrete is a mixture of cement and sand, and the concrete mix is made up of several different components. In the past, there were only two components, the cement itself and the aggregate. The aggregate was the ingredient that added strength to the concrete. It was not considered important because it was assumed that the aggregate would break down over time or would wash away with water.
The problem with aggregate is that it’s not stable—it does break down, and in fact it should be broken down over time. But for a long time no one knew how long it would take for this to happen. Most people assumed a fairly short amount of time, perhaps months or years, but no one could agree.
In order to settle this issue, concrete scientists did a lot of research on how long concrete needed to stand unexposed to moisture and water before it became weakened. They talked to engineers who used concrete in buildings, they did experiments on concrete mixtures they could control better themselves, they studied how various aggregates broke down over time under various conditions, and they even conducted controlled tests on concrete mixtures themselves by burying them underground under many different conditions (such as in basements or near water).
In order to settle these
The concrete industry is filled with horror stories. They are the worst kind of stories, because they tell us how not to do things. When a building collapses because of bad concrete, someone has probably saved you money by telling you how to avoid the same problem. When a building collapses because of bad concrete, you can be sure that every contractor in America has already heard the same story and done everything wrong that could possibly be wrong.
That’s why concrete is one of the most boring materials: it often tells us things we already know. But it’s also fascinating in its own way, because it reveals something about human nature. It is, in fact, a window into the human brain itself.
The purpose of this blog is to tell concrete stories in a different way: to see what happens when we tell different kinds of stories about concrete.
There is an obstacle to understanding concrete that doesn’t have a technical name, but it’s real. It’s more than just the fact that concrete is hard to understand; it’s also the fact that concrete can be very difficult to test.
Modern concrete contains dozens of different ingredients in all sorts of concentrations. Even for concrete, there are too many choices. There’s good evidence that if you’re not careful about your choice of ingredients, you’ll end up with something that isn’t strong enough for your project.
The problem is that when you start trying to figure out how much cement to use, you can’t rely on the same rules you used for testing other materials. For example, when you’re testing rock or clay in a lab, you can do things like drop some sample on a flat surface and see if it cracks. But when you’re testing concrete in a lab, there is no flat surface nearby to drop the sample on. So labs either have to make their own drop zone or test concrete in three dimensions—which means they get really expensive really fast.
The cement industry is divided into two main camps: the Portland Cement Association, which makes Portland cement and its allies, and the National Cement Manufacturers Association (NCMA), which makes other cements and its allies.
The NCMA has spent a great deal over the last several decades beating up on Portland cement and its allies. The NCMA has a large research budget, and it has used it to do things like what happened here in New York City. It sent people out to test old concrete against new concrete, knowing that there would be a difference but not knowing why. The goal was to confirm that there was something wrong with Portland cement that required regulation.
So far as I know, the NCMA has never published an article saying that Portland cement is safe. And yet this is the most common thing you hear about concrete and how it works: “Portland cement is no good.” Does anyone know what this means? Does anyone know why this is true?
A few years ago, when I was accepted to the Master of Fine Arts program at USC, I had no idea that making concrete would be my career. Yet in the course of my research for the program, I discovered that concrete was one of the most important building materials in the world.
I came to understand that it is not just what concrete does but what it represents: concrete is a material whose history stretches back to prehistory and whose future stretches forward as far as we can see. In fact, it is not enough to understand how concrete works. We have to understand how it symbolizes human aspirations and failures.
When Henry Ford built his first Model T automobile, he didn’t have a PhD in metallurgy; he couldn’t build an airplane or a skyscraper or a skyscraper-sized car engine. Or so we thought until recently–when research by UCLA materials science professor Steven Zemansky revealed that Ford’s genius lay not in his ability to convert aluminum into an automotive miracle but rather in his ability to turn metal into something else: steel.
Zemansky concluded that Ford was able to make steel because he understood how metals are made from scratch. He knew about steelmaking and understood the principles behind making steel; whether he could have built a