Aggregate Soundness and Duranbility

The soundness test certifies a conglomerate's safety to crumbling by weathering and, specifically, stop-thaw cycles. Totals that are solid (insusceptible to weathering) are less presumable to debase in the field and create rash HMA asphalt misery and reasonably, washout.

The soundness test more than once submerges a total specimen in a sodium sulfate or magnesium sulfate answer. This procedure creates salt precious stones to shape in the total's water penetrable pores. The structuring of these gems makes inner drives that connect force on conglomerate pores and will usually break the conglomerate (Figure 1). When a specified number of submerging and drying reiterations, the total is sieved to certify the percent misfortune of material

The shaping of salt gems is supposed to copy the creation of ice precious stones in the field and would be able to accordingly be utilized as a surrogate to expect a conglomerate's solidify-thaw display.

The standard soundness test is:

AASHTO T 104 and ASTM C 88: Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate

Background

Aggregates must be resistant to breakdown and disintegration from weathering (wetting/drying and freezing/thawing) or they may break apart and cause premature pavement distress. Durability and soundness are terms typically given to an aggregate’s weathering resistance characteristic.

Typical Soundness Tests

There are several tests for aggregate soundness used in the U.S. These tests, described in decending order of popularity, are briefly described here.

Sulfate Soundness (Figure 1)

This test, described in the Test Description section, subjects aggregate samples to repeated imersion in either sodium sulfate or magnesium sulfate solution. Salt crystals that form during this test are intended to mimic ice crystals formed in the normal water freeze-thaw process in the field. Wu, Parker and Kandhal (1998) report that just over half of the states have a sodium sulfate soundness requirement, while about one-fifth have a magnesium sulfate soundness requirement.

Issues with Sufate Soundness Tests

In general, sulfate soundness tests can serve as a useful first evaulation of a particular aggregate or as a confirming test for an aggregate with a long test and performance record. However, issues with applicability, variability and relation to actual performance prevent the test from being meaningful beyond this:

Applicability. While aggregates used in unbound base courses usually contain small amounts of water (usually 0.1 to 3 percent by weight), aggregates used in HMA usually do not contain appreciable water because they are dried during production and are, if incorporated in a proper HMA mix design, coated with a waterproofing film of asphalt binder (Roberts et al., 1996). Therefore, while freezing and thawing conditions may be important to unbound base course aggregate, they may not be of concern to aggregate used in HMA. There is some question as to whether or not soundness tests are applicable to HMA. Despite this, they seem to be at least somewhat capable of differentiating between poor performing and well performing HMA pavement aggregate (Wu, Parker and Kandhal, 1998).
Variability. Testing variability is poor for both the sodium and magnesium sulfate tests (it is worse for the sodium sulfate test). The coefficient of variation for the sodium sulfate soundness test is 41% for tests done in different laboratories meaning that it is unlikely two different laboratories will closely agree on test results. The implication is that soundness testing should be done in one central laboratory where close control over testing elements like temperature, timing controls, types of containers and sieving methods can be maintained (Meininger, 2002).
Relation to HMA Pavement Performance. The sodium and magnesium sulfate tests have been widely criticized and many reports exist which describe their inability to accurately predict field performance for specific aggregates (Roberts, et al., 1996). For instance, some aggregates containing carbonates of calcium or magnesium are chemically attacked by the sulfate solution resulting in erroneously high measured losses (Meininger, 2002).

Freezing and Thawing Soundness

This test, specified in AASHTO T 103, is similar to the sulfate soundess tests, however it uses actual freeze-thaw cycles with water or a weak ethyl alcohol – water solution. Wu, Parker and Kandhal (1998) report that about 10 percent of states have a freeze-thaw soundness requirement.

Aggregate Durability Index

This test, specified in AASHTO T 210, measures the relative resistance of an aggregate to produce detrimental clay-like fines when subjected to mechanical methods of degradation (AASHTO, 2000b). It is not widely used; its chief use has been by western states to identify weathered basalt containing interstitial montmorillonite, which will not maintain strength when used as unbound aggregate base (Wu, Parker and Kandhal, 1998

Test Description

The following is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete test procedure can be found in:

AASHTO T 104 and ASTM C 88: Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate

Summary

An aggregate sample is subjected to a number of cycles (usually 5 cycles) of submergence in a sulfate solution (either sodium sulfate, Na₂SO₄, or magnesium sulfate, MgSO₄) followed by drying in air. This process causes salt crystals to form in the aggregate’s water permable pores. Crystal formation tends to create internal pressure and break the aggregate. After a specified number of cycles, aggregate samples are washed and sieved to determine their mass loss. Aggregates are separated into several size ranges and tested independently during the test. The final reported loss value (reported as a percentage of total aggregate mass) is a weighted average of the mass loss of each size range. Figure 2 shows the sulfate chemical bottles and basket used to suspend the aggregate in the sulfate solution.

Approximate Test Time

About 5 days. Each cycle involves between 16 and 18 hours of submergence in the sulfate solution followed by 4 or more hours of drying. Therefore, about one cycle can be done per day with a typical number of cycles being 5.

Basic Procedure

Prepare the sulfate solution. When used, the sodium sulfate solution’s specific gravity should be between 1.154 to 1.171 and the magnesium sulfate solution’s specific gravity should be between 1.297 and 1.306.
Prepare the fine aggregate (Figure 3).

Obtain a sample that is large enough to yield at least 100 g of material on each of the following sieves: No. 50 (0.300 mm), No. 30 (0.600 mm), No. 16 (1.18 mm), No. 8 (2.36 mm) and No. 4 (4.75 mm).

Thoroughly wash the sample on a No. 50 (0.300 mm) sieve and dry it in an oven at 230°F (110°C).
Separate the different sizes using a nest of sieves. Sieve each size individually until no more material passes through the sieve.
Obtain 100 g of each size, record the weight, and place in separate containers for the test (the aggregate can be placed in 3 to 5 layers of cheesecloth).

Prepare the coarse aggregate.

Obtain enough material to yield at least the weights listed in Table 1.

Thoroughly wash the sample on a No. 50 (0.300 mm) sieve (Figure 4) and dry it in an oven at 230°F (110°C).

Separate the different sizes using a nest of sieves. Sieve each size individually until no more material passes through the sieve.
Weigh out quantities of the different sizes.
Combine the 2 inch (50 mm) and 1.5 inch (37.5 mm) material to yield a 5000 g sample.
Combine the 1 inch (25.0 mm) and 0.75 inch (19.0 mm) material to yield a 1500 g sample.
Combine the 0.5 inch (12.5 mm) and 0.375 inch (9.5 mm) material to yield a 1000 g sample.
Record the masses of each fractional component and the masses of each combined test sample. In the case of sizes larger than 0.75 inches (19.0 mm), record the number of particles in the test samples.

Place each sample in separate containers for the test (the aggregate can be placed in 3 to 5 layers of cheesecloth)
Immerse the samples in the prepared solution of sodium sulfate or magnesium sulfate for 16 to 18 hours. Cover the containers to reduce evaporation and prevent contamination and maintain the temperature between 20.3 to 21.9°C for the immersion period
Remove the samples and allow them to drain for 15 minutes.
Place the samples into an oven set at 230°F (110°C).
Allow the samples to dry until the change in mass is less than 0.1 percent over a 4 hour period (the weight is checked on four hour intervals without letting the sample cool).
After the samples reach constant mass allow the samples to cool to 68 to 77°F (20 to 25°C); cooling may be aided by the use of an air conditioner or fan.
Repeat the immersion process (steps 4 through 8) until the specified number of cycles is obtained (five cycles are normally performed).
After the final cycle is complete and the sample has cooled, wash the sample.
Check the thoroughness of washing by obtaining a sample of rinse water after it has passed through the samples and adding 0.2 M barium chloride. If the sample water becomes cloudy when the barium chloride is added then continue to wash the sample.
After washing is complete, dry each fraction of the sample to a constant mass in an oven at 230°F (110°C).
Examine the fine aggregate.

Sieve the fine aggregate over the same sieve on which it was retained before the test and in the same method as was used in preparing the test samples
Determine the mass of the material retained on each sieve.
The difference between the amount retained at the end of the test and the initial amount of the samples is the loss in the test and is expressed as a percentage of the initial mass.

Examine the coarse aggregate.

Sieve the coarse material by hand with agitation sufficient only to assure that all undersize material passes the designated sieve.
Determine the mass of the material retained on the sieve.
The difference between the amount retained at the end of the test and the initial amount of the samples is the loss in the test and is expressed as a percentage of the initial mass (this is the quantitative examination).
For samples coarser than 0.75 inches (19.0 mm), separate the particles of each test sample into groups according to the action produced by the test (i.e. disintegration, splitting, crumbling, cracking, flaking, etc). Record the number of particles showing each type of distress (this is the qualitative examination).

Results

Parameters Measured

The percentage loss by weight for each sample fraction. For material that was coarser than 0.75 inches (19 mm) before test, the number of particles in each fraction before test and the number of particles affected, classified as to number disintegrating, splitting, crumbling, cracking and flaking is also measured. Additionally:

For samples containing significant amounts of both fine and coarse materials, get separate results for the fine and coarse material as if they were from separate supplies.
For fine material with more than 90 percent passing the 0.375 inch (9.5 mm) sieve, assume sizes finer than the No. 50 (0.300 mm) sieve to have zero percent loss and sizes coarser than the 0.375 inch (9.5 mm) to have the same loss as the next smaller size for which test data are available.
For coarse material with less then 10 percent passing the No. 4 (4.75 mm) sieve, assume sizes finer than the No. 4 (4.75 mm) sieve to have the same loss as the next larger size for which test data is available.
For any size that was not tested due to the material containing less than 5 percent of that size, consider the loss to be the average of the next smaller and the next larger size, or if one of these sizes is not available, consider the loss to be equal to the next larger or next smaller size, whichever is available.

Specifications

Table 3: Source Property Soundness Specifications
Material	Value	Specification	HMA Distress of Concern
Aggregate	% Loss	Varies1	Deformation, rutting

Note 1 Percent loss specifications vary from agency to agency but typically specify a number of cycles (most often 5) and a maximum percent loss (e.g., 12 percent). For example, ASTM D 692, Coarse Aggregate for Bituminous Paving Mixtures, specifies 5 cycles and a maximum percent loss of 12 percent when the sodium sulfate is used or 18 percent when the magnesium sulfate is used.

Typical Values

Typical values depend upon the type of soundness test used. The sodium sulfate loss is typically between about 0 and 15 percent, while the magnesium sulfate loss is typically between about 0 and 30 percent. For a particular aggregate sample, the sodium sulfate loss will tend to be less than the magnesium sulfate loss by 0 to 20 percent with 5 to 10 percent being most typical.