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Aggregates for Concrete and Asphaltic Mixes



Concrete and asphalt are by far the most common construction materials used today. Aggregates make up a very significant volume of these materials. Coarse and fine aggregates are mixed with concrete paste or asphalt binder, providing surfaces for the material to bind to. Measuring and controlling particle size of these inexpensive fillers allows aggregates to occupy as much volume as possible.

Because aggregates are typically stored in the open, the way aggregates behave in contact with water must be tested as well. Aggregates should also be rigid, durable, strong, and chemically inert with respect to the concrete or asphalt they are used in.

In this video, we will examine the basic properties of aggregates that are needed to develop successful concrete mix designs. The primary characteristics that we will look at are size distribution or gradation, specific gravity, and moisture content and absorption capacity.

Aggregates are considered to be coarse if they are larger than about 4.75 millimeters, and fine if they are smaller particles. As they are mainly used as fillers in concrete and are relatively inexpensive, it is important that they occupy as much volume as possible.

When comparing a properly graded aggregate to one that has uniform distribution, less paste is needed to fill the voids. If there are too many fine particles, however, the increased surface area that needs to be coated results in a concrete mix that is too stiff.

Sieve tests are run to determine the amounts and distribution of particles. The smallest sieve number that all of the aggregate can pass through is the maximum size, while 95 percent can pass through the nominal size sieve. The sum of the cumulative weight percentages for the six standard sieve sizes, divided by 100, is the fineness modulus, FM. Smaller values indicate finer aggregates, and larger values indicate coarser aggregates.

In addition to size, the water condition of aggregate must be known. Because aggregate makes up so much of the mix, a small change in moisture content has an enormous impact on the water-to-cement ratio. Oven dry, which contains no water, and saturated surface dry, when the surface is dry but the pores are saturated, are two of the conditions studied. The saturated surface dry, or SSD condition, is assumed when designing mixes. In practice, water typically needs to be added or removed from aggregates to achieve the SSD condition prior to mixing.

The slump test is used to test for the SSD condition. In this test, a conical mold is packed with aggregate, and inverted; if the material slumps slightly when the mold is removed, it is in SSD condition. If the mold holds its shape, it is in the damp or wet condition.

Measurements of the weights of the sample that are oven dry and SSD can be used to calculate the absorption capacity and the moisture content, as well as the specific gravity in regards to both oven dry and SSD samples.

In the next section, we will measure moisture content, specific gravity, and perform sieve analysis for a fine aggregate sample.

Prepare roughly two kilograms of a fine aggregate such as sand, the day before testing, by drying it in an oven. Leave the aggregate in the oven for at least 24 hours with the temperature set above 220 degrees fahrenheit, so that all of the water evaporates. Add approximately one kilogram of the oven-dried aggregate to a flattened metal pan.

Finding the SSD condition is a trial-and-error procedure. Begin by adding a few drops of water to the aggregate, and then thoroughly mixing. Now, test the mixture by performing a slump test. To perform the test, hold a slump cone firmly on the flat metal pan with the large diameter down. Loosely fill the mold until the aggregate is heaping over the top, and then lightly tamp the aggregate into the mold with 25 light drops of the tamping rod. Start each drop about a quarter inch above the surface, and permit the rod to fall freely each time. As you are tamping, try to distribute the drops evenly over the surface.

Now, clear away any loose aggregate around the base, and then carefully lift the mold vertically. If the aggregate slumps slightly, it indicates that it has reached an SSD condition. However, if the cone retains its shape, the aggregate is still too dry, and if it collapses, the aggregate is too wet.

Adjust the mixture by adding more oven-dry aggregate or water as appropriate and thoroughly mixing. Continue adjusting and testing until SSD conditions have been achieved. Now, take approximately 400 grams of the SSD aggregate and record the exact weight as D.

Next, fill a flask with 500 milliliters of water and record the total weight of water and flask as B. Pour out the water and fill the now-empty flask with the SSD sample you just weighed. Add some additional water to the flask until the level is about half an inch above the aggregate.

Now, apply vacuum and a rolling action to the sample for at least five minutes to remove the air entrapped in the aggregate. After the sample is degassed, remove the vacuum and fill the flask with water up to the 500 milliliter mark. Record the total weight of the flask, water, and aggregate as C. Finally, pour the entire contents of the flask into a pan, and if necessary, use additional tap water to wash all of the aggregate out of the flask.

Place the pan in the oven and leave it to dry for at least 24 hours with the temperature set above 220 degrees fahrenheit. When the aggregate is dry, record the final weight as A. You now have four weight measurements that you can use to calculate the apparent specific gravity, bulk specific gravity, and absorption of the aggregate.

For this test, we will use a set of eight-inch diameter, standard sieves. Assemble sieve numbers 4, 8, 16, 30, 50, and 100 in an ordered stack, with the number 4 sieve on top, so that the clean opening is reduced in subsequent tiers, moving downward. Attach the emptied pan to the bottom of the stack.

Weigh out approximately 400 grams of fine, dry aggregate. After recording the final weight, pour the aggregate in the top sieve and cover the stack with the lid. When the lid is in place, secure the sieves in a mechanical shaker and shake the assembly for five minutes. Now remove the stack and carefully separate the sieves. Separately weigh and record the aggregate retained on each of the sieves and in the pan.

Confirm that the total weight of aggregate is less than 0.6 percent different than the original sample weight. If not, repeat the procedure. Adding the weight in each sieve to the cumulative weight in higher sieves computes the cumulative weight retained at each tier. Subsequently, dividing these results by the total weight gives us the cumulative percentages retained in each tier.

Finally, the fineness modulus is the summation of the cumulative percentages for the six standard sieve sizes, divided by 100. The fineness modulus for this test is 3.02, indicating a relatively coarse aggregate. The cumulative percent passing each sieve can be found by subtracting the percent retained from 100 percent. The sieve size opening can then be plotted against the cumulative percent passing each sieve, resulting in the gradation curve for the aggregate.

Now that you appreciate the importance of aggregate used in making concrete, let's see how it is used in the world around us.

Tall buildings are not the first thing that comes to mind when you think of structures made of concrete. But application-specific concrete mixes help the western hemisphere's tallest free-standing structure, the CN Tower in Toronto, Canada, soar to over 553 meters.

Concrete is commonly used for dam construction. The world's tallest concrete dam is the Grande Dixence, in Switzerland. The dam is 285 meters tall, and was finished in 1961 after eight years of construction, and six million cubic meters of concrete. Tests like those shown in this video are necessary for ensuring consistency between batches.

You've just watched JoVE's introduction to aggregates for concrete and asphaltic mixes. You should now understand the importance of water absorption slump testing, and size distribution of aggregates.

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