Aggregate is one of the key constituents of both asphalt and concrete paving. These chunks of crushed stone give the pavement its strength, durability, and longevity. Aggregates vary widely in terms of maximum size, overall size distribution, shape, and texture. In addition, different aggregates possess different levels of soundness.
The concept of soundness encompasses how well a particular aggregate can resist disintegration and wear caused by environmental exposure. The greater the soundness, the better the aggregate will be able to maintain its internal stability as time goes on. Pavement created with sound aggregate suffers a much lower rate of wear and damage.
In order to provide actionable information, an aggregate's soundness must be quantified. Contractors use three main tests to measure soundness. This article takes a closer look at each of these three tests, outlining the precise types of information that each test yields about how an aggregate will perform over time.
1. Sulfate Soundness Test
As noted above, soundness tests act to measure an aggregate's resistance to disintegration caused by environmental factors. Seasonal cycles of freezing and thawing tend to be one of the most destructive influences on aggregate. The sulfate soundness test simulates an aggregate's resistance to the freeze-thaw cycle.
In this test, workers place a sample of the aggregate in a high-concentration solution of either sodium or magnesium sulfate, the latter of which goes by the common name of Epsom salt. As these salts soak into the aggregate's pores, the salts cause salt crystals to form inside of the aggregate. These crystals closely mimic the effects of ice formation.
In other words, as the salt crystals form, they increase the amount of internal stress acting on the aggregate. Less sound aggregates won't have the strength to resist these stresses; instead these aggregates will break apart along weak areas. More sound aggregates, on the other hand, will be able to resist the crystal formation to a much greater extent.
The aggregate soaks in the salt solution for a fixed amount of time, then dries out completely. Usually testers repeat this process at least five times before passing the aggregate particles through various sieves used to measure their sizes.
Comparing these results to the original values allows the tests to determine the maximum loss value. The greater the maximum loss value, the less sound the aggregate.
2. Freezing and Thawing Soundness Test
The sulfate soundness test owes its popularity to the fact that it can be conducted in a relatively short period of time while still yielding relatively accurate information. Yet the stresses generated by sulfate soundness differ somewhat from the stresses that pavement experiences as the result of freezing and thawing.
For one thing, the freeze-thaw cycle also involves temperature changes which the sulfate soundness test cannot account for. Likewise, the pressure of the sulfate solution migrating through the aggregate affects the results in ways that water alone would not. Simply put, the disruption mechanism of the sodium sulfate test differs in small yet potentially key ways from freezing and thawing.
For this reason, many aggregate testers choose to also perform what is known as a freezing and thawing soundness test. As its name suggests, this test involves exposing the aggregate to water and then subjecting it to freezing temperatures. The cycle repeats several times before tests measure the resulting aggregate sizes in the same manner as with a sulfate soundness test.
Different states have different requirements regarding the ways in which aggregate soundness must be tested. While most permit the use of sulfate tests, a smaller number - around 10 percent - require that freezing and thawing soundness tests also be conducted.
For more information about how aggregates are tested in your area, please contact the paving industry experts at Black Jack Asphalt and Concrete.