Fine Aggregate Angularity: FAA Test, Formula and Specs

Fine aggregate angularity, or FAA, is how asphalt labs judge whether the sand in a mix is sharp and gritty or smooth and round. You cannot easily measure the shape of millions of tiny grains, so the test measures something that follows from shape: how much empty space is left when the sand is poured loosely into a small cylinder. Angular grains prop each other up and leave more gaps. Rounded grains settle into a tighter pack.

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The short version

What it measures: uncompacted void content of fine aggregate, used as a stand-in for angularity and texture.
The standards: AASHTO T 304 and ASTM C1252 (Method A for Superpave).
Higher is more angular. Typical results run about 38 to 52 percent.
Why it matters: rut resistance in hot mix asphalt.

Why sand shape matters

In a dense-graded asphalt mix, the fine aggregate makes up a big share of the stone skeleton. When those grains are angular and rough, they interlock and resist sliding past each other under heavy, slow traffic. When they are smooth and rounded, as in many natural river sands, the mix can feel tender during paving and can rut or shove in service. Many agencies cap how much natural sand can go into a heavy-traffic mix for exactly that reason.

Superpave made FAA one of its "consensus" aggregate properties, a short list of tests that every Superpave agency agreed to use, along with coarse aggregate crushed faces, flat and elongated particles and clay content. Our Superpave guide covers the wider system.

Equipment

A funnel held in a stand at a fixed height.
A calibrated cylinder of about 100 mL.
A spatula with a straight edge, a small brush and a pan to catch spills.
A scale and the fine aggregate's bulk dry specific gravity, usually from AASHTO T 84.

How the test is run (Method A)

Build the sample. Wash and dry the sand, sieve it into sizes ranging from No. 8 (2.36 mm) down to No. 100, and recombine set amounts into a standard 190 gram sample. Using a fixed grading means the test reflects shape and texture rather than gradation.
Set up. Put the stand on a level surface with no vibration and center the cylinder under the funnel.
Pour. Hold a finger over the funnel outlet, pour the sample in and level it. Remove the finger and let the sand fall freely into the cylinder.
Strike off. After the funnel empties, level the heaped sand with one smooth pass of the spatula held on edge. Sawing back and forth would shake the sand down and spoil the result.
Weigh. Brush stray grains off the outside and weigh the cylinder and contents.
Repeat. Recombine the sand and run it again. The results are averaged.

The test itself takes about an hour, plus drying and sieving time.

The math

U = (V − F ÷ G) ÷ V × 100

U = uncompacted voids, percent (the FAA value)
V = cylinder volume, mL
F = mass of sand in the cylinder, g
G = the sand's bulk dry specific gravity

F ÷ G is the solid volume of the sand grains. Subtracting that from the cylinder volume leaves the empty space.

Example (made-up numbers): a 100.0 mL cylinder holds 147.0 g of sand with a bulk dry specific gravity of 2.650. Solid volume = 147.0 ÷ 2.650 = 55.5 mL. Voids = 100.0 − 55.5 = 44.5 mL. FAA = 44.5 percent. That would meet a 40 percent minimum but fall just short of 45.

The specific gravity has to be right. If G is off by 0.03, this example shifts by about six tenths of a percentage point, enough to flip a borderline result. Our bulk specific gravity page explains how specific gravity values carry through asphalt math.

Superpave minimums

20-year design ESALs (millions)	Within 4 in of surface	Deeper than 4 in
Under 0.3	No requirement	No requirement
0.3 to under 3	40	40
3 to under 10	45	40
10 to under 30	45	40
30 and up	45	45

A minimum of 45 percent for the blended fine aggregate is the usual target for high-traffic surface mixes. Many agencies tweak these limits, so check your state's spec.

Typical values

Crushed manufactured sands: about 43 to 52 percent.
Natural, uncrushed sands: about 38 to 46 percent.

The overlap shows the limits of the test. Some well-shaped natural sands score like crushed material, and some crushed sands from smooth rock score low.

Criticisms and alternatives

FAA mixes angularity, surface texture and particle form into one number, and research has questioned how well it alone predicts rutting. Alternatives that have been studied include the compacted aggregate resistance test, which pushes a piston into confined sand much like a California bearing ratio test, and digital imaging systems that measure grain shape directly. The loose-void test remains the standard because it is simple and cheap.

What this means when you buy sand

Sharp, angular sand locks up and compacts firmly, which is why manufactured or concrete sand is often chosen under pavers. Round sand flows and drains but shifts more easily. Our mason sand vs concrete sand guide compares the common types, and the sand calculator turns your area and depth into tons.

Questions people ask

What is fine aggregate angularity?

It is a measure of how angular and rough the grains in a sand or crushed fine aggregate are. It is found indirectly by measuring the uncompacted void content of a standard sample. Sharp, rough grains pack loosely and leave more voids than smooth, rounded ones.

How is FAA calculated?

U = (V − F ÷ G) ÷ V × 100, where V is the volume of the cylinder in mL, F is the mass of the sample in the cylinder in grams and G is the sand's bulk dry specific gravity.

What FAA value does Superpave require?

For mix within 4 inches of the surface, the minimum is 40 percent for lighter traffic and 45 percent for 3 million design ESALs and up. Deeper layers require 40 percent until traffic reaches 30 million ESALs, where 45 applies.

What are typical FAA values?

Results commonly run from about 38 to 52 percent. Crushed manufactured sands tend to fall between 43 and 52, and natural rounded sands between 38 and 46.

Why does angularity matter in asphalt?

Angular grains lock together and resist sliding, which helps a mix resist rutting. Mixes made mostly with smooth, rounded sand can be tender and rut under heavy traffic.

What standard covers the test?

AASHTO T 304 and ASTM C1252, both titled for the uncompacted void content of fine aggregate. Method A, which uses a fixed standard grading, is the one Superpave relies on.

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