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I have this aggregate gradation curve in hand, how can I say whether its gap, well or uniformly graded?

enter image description here

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    $\begingroup$ To get the distribution of particle sizes from that graph you need to take the derivative of it. $\endgroup$ – ratchet freak Apr 20 '15 at 8:08
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The flat "table" area in the middle of the curve indicates that particles of roughly 1–4 mm diameter are not well-represented in your aggregate. This gap in your particle size distribution is why it's called gap-graded.

Basically, you want to look at the slope of the curve in different regions to tell whether particles are accumulating in the sieves of approximately that size. Since there's a positive slope from about 0.2–1 mm, you know your aggregate contains a decent mix of sands of different grades. Since there's no significant slope from 1–4 mm, you know your aggregate doesn't contain much fine gravel. Then, since there's a positive slope after about 10 mm, you know your aggregate contains a decent mix of medium and coarse gravels.

A well-graded aggregate has a smoother gradation curve, indicating that there are some particles in the mix of every different size. For instance, take a look at these figures from the Pavement Design Guide of the Texas Dept. of Transportation:

dense graded mix figures

The figure on the right is a 0.45 power curve; the scale of the x-axis is the 0.45th power of the sieve opening, where your gradation curve uses a logarithmic scale of the actual particle diameter. It's a bit different way to see the data but the basic shape you're looking for is the same—a smooth curve that levels off at the tails but not in the middle.

There are also specific criteria for determining whether your mix is well-graded; see Wikipedia on soil gradation for an overview. Judging by your graph, $C_u \approx 19$ and $C_c \approx 8$, which also indicates your aggregate is not well-graded.

Uniformly-graded is pretty easy to identify on a size distribution graph because it's primarily composed of one size of particle; what you see on the graph is a flat tail on the left followed by a sudden jump to near 100% at or around the particle size that dominates the mix.

Take a look at Figure 2 in this article from Pavement Interactive for a visual comparison of the general shape of the curve for different classifications of aggregate. (Note that these are again 0.45 power curves.)

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