Concrete: 5. Concrete Tests
Concrete: 5. Concrete Tests
Concrete: 5. Concrete Tests - Full TranscriptionLet's talk about concrete tests that are a very important thing to understand because of the quality of the concrete, its strength and its workability. So there are tests that are done on fresh concrete such as the Kelly ball test it's a certain weight hemisphere that is dropped from a graduated pole layer and the amount of penetration gives us an idea about the workability and its slump basically how well the concrete will flow into the formwork. The more common test is the slump test using a slump cone. We're going to look at that one in a little bit more detail because I have a lot of pictures about that. And also the air content test. These are all done on fresh concrete versus once the concrete has cured. That is typically to get the strength versus the workability. So there is a cylinder test. There's two sizes of cylinders, as we will see in the next slide. There is a core test. After the concrete has cured, they can go in there, take out a core, crush it, figure out the strength. This one also helps determine the thickness of a concrete slab, for example. There is also a non-destructive way of determining the strength of concrete using an impact rebound hammer. So there is a piston in here that is spring loaded. And once it's released, this is a graduated cylinder, and it tells you how much it rebound. Then on the back of this instrument, there is a chart that lets you know, based on the distance it traveled, rebounded, how much the approximate concrete strength is. So let's look at concrete tests on fresh concrete. We're going to start with the slump test, and again, it doesn't tell me anything about the strength. It's all about how fluid the mix is and how easy it is to go in the formwork. So here we can see a slump test being conducted. There is a slump cone, and this slump cone measures four inches on the top, eight inches on the bottom, and it's a 12-inch tall truncated cone. I write this because this is the same volume as a 6 by 12 concrete cylinder. So it's the same amount of concrete. So there are all kinds of rules. There's a 5 eighth inch rod, the cylinder, sorry, the cone has to be filled in three lifts. It has to be rotted 25 times, et cetera, et cetera, all kinds of ASTM conditions, but then the cone is raised and the concrete falls or it slumps. They'll put the rod on the top and they'll measure this dimension and that is the amount of slump. How much did the concrete fall in inches is the slump. So a typical slump for road beds, you want a stiff mix so less than one inch versus an inch to an inch and a half half an inch is when you have wide spacing of rebars it's not a tight space with a tight space you need a runnier mix so medium rebar spacing and it still needs vibration a slump of two to three and a half inches is good. And if it's really tight rebar spacing, you're going to need a slump of four inches or greater. Now, if there is any super plasticizer or plasticizer, an admixture in the mix that makes the water cement ratio adjusted, it gets runnier without losing strength, then that's going to give a high slump, clearly, but without additional water. So the strength is still the same, if not increased. So also, the air entraining or the void test, how much air is in the mix, is another one that is done with the air content test. There's this cylinder that is filled with concrete, same as the cone. It's in three lifts, it has to be rotted, etc. Then they clean it, they put the lid up, and then they inject a little bit of water to give it the pressure. And then they read how much air content and how much void is in there. Now, air bubbles act as a cushion that protects concrete from cracking when water freezes. So air and training is very helpful with freeze saw cycles. So on highway bridges, et cetera, all that has to be air-entrained concrete. So here is the process. It's being done, and then the lid is put on. They'll inject a little bit of water, and they'll read how much the water content in percent, how much that is. So looking at other tests on concrete once it's cured, not right on the site, there is a core test. Well, they take out the sample and they'll put it in this crushing machine. And it'll measure. It'll give you an idea of the stress or compressive stress that the concrete cracks at. Now, this instrument does not measure stress. It cannot measure stress. Stress cannot be measured. But instead, it calculates the amount of force that they place on the cylinder and divides it by the cross-sectional area of the cylinder to determine the stress. So let's see what the next slide is. So there is the standard 6-inch by 12-inch cylinders. It has the same volume as that cone, the slump cone. and more recently there's 4 inch by 8 inch cylinders that have to be cured. They're sent to the lab. Then the crushing machine crushes them. As we see here in this picture, that one is crushed. And they will report the amount of compressive stress that that specific concrete mix has. So it's typically crushed at 14 and 28 days after casting. Sometimes they want a finer reading or they need to know when they reach 40% or 60% of the strength. And so they might take additional cylinders and crush them at three and seven days. Typically, the American Concrete Institute says take two cylinders from every hundred cubic yards in a pour, which is basically 10 trucks. So a 2,500 PSI mix is for residential and foundations. A 4,000 PSI is for commercial. And then your precast, pre-stressed, has to be lifted and transported and whatnot. And so it's more typical at 5 to 8, even 10,000 PSI. And then there's high-rise construction. They have additional admixtures in there to give them a higher strength, like those pencil skyscrapers around Central Park in New York City. So at the end of the day, concrete is all about cracks.
