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Materials
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Durability and Soundness

Overview

The soundness test determines an aggregate’s resistance to disintegration by weathering and, in particular, freeze-thaw cycles. Aggregates that are durable (resistant to weathering) are less likely to degrade in the field and cause premature HMA pavement distress and potentially, failure.

The soundness test repeatedly submerges an aggregate sample in a sodium sulfate or magnesium sulfate solution. This process causes salt crystals to form in the aggregate’s water permeable pores. The formation of these crystals creates internal forces that apply pressure on aggregate pores and tend to break the aggregate (Figure 1). After a specified number of submerging and drying repetitions, the aggregate is sieved to determine the percent loss of material.

Figure 1: Aggregate before (top) & after (bottom) the soundness test.

The formation of salt crystals is supposed to mimic the formation of ice crystals in the field and could therefore be used as a surrogate to predict an aggregate’s freeze-thaw performance.

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[1]) 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[2]). 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[1]).
  • 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[3]).
  • 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[2]). 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[3]).

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[1]) 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[4]). 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[1]).

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.

Figure 2: Aggregate basket surrounded by sulfate bottles.

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

  1. 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.

WARNING

Either sodium sulfate or or magnesium sulfate can be used however, results from the two solutions will be significantly different.

  1. Prepare the fine aggregate (Figure 3).

Figure 3: Sieving the aggregate sample.

    • 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).

NOTE

If the sample contains less than 5 percent of any of the above sizes then that size will not be tested.

    • 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).
  1. Prepare the coarse aggregate.
    • Obtain enough material to yield at least the weights listed in Table 1.
Table 1: Sample Mass Requirements
Sieve Size Minimum Mass of Specimen
U.S. Metric (mm)
2 inches 50 mm 3000 g
1.5 inches 37.5 mm 2000 g
1.0 inches 25.0 mm 1000 g
0.75 inches 19.0 mm 500 g
0.5 inches 12.5 mm 670 g
0.375 inches 9.5 mm 330 g
No. 4 4.75 mm 300 g

NOTE

If the sample contains less than 5 percent of any of the above sizes then that size will not be tested.

    • 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).

Figure 4: Washing the aggregate sample.

    • 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.
  1. Place each sample in separate containers for the test (the aggregate can be placed in 3 to 5 layers of cheesecloth)
  2. 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
  3. Remove the samples and allow them to drain for 15 minutes.
  4. Place the samples into an oven set at 230°F (110°C).
  5. 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).
  6. 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.
  7. Repeat the immersion process (steps 4 through 8) until the specified number of cycles is obtained (five cycles are normally performed).
  8. After the final cycle is complete and the sample has cooled, wash the sample.

WARNING

Do not impact or abrade the sample while washing because this could break particles and cause inaccurate results.

  1. 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.

WARNING

Many water supplies will react with barium chloride even before washing the sample; in these cases the barium chloride check will not work and other analytical methods should be used to assure thorough washing.

  1. After washing is complete, dry each fraction of the sample to a constant mass in an oven at 230°F (110°C).
  2. 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.
  1. Examine the coarse aggregate.
    • Use Table 2 to determine which sieve to use to sieve the material when determining loss.
Table 2: Sieve Used to Determine Loss Based on Aggregate Size
Aggregate Size Sieve Used
U.S. Metric U.S. Metric
≥ 1.5 inches ≥ 37.5 mm 1.25 inches 31.5 mm
1.5 to 0.75 inches 37.5 to 19.0 mm 5/8 inch 16.0 mm
0.75 to 0.375 inches 19.0 to 9.5 mm 5/16 inch 8.0 mm
0.375 inches to No. 4 9.5 to 4.75 mm No. 5 4.0 mm
    • Sieve the coarse material by hand with agitation sufficient only to assure that all undersize material passes the designated sieve.

WARNING

Do not try to break up any particles and do not manipulate in a way to force the particles to pass the 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.

Calculations (Interactive Equation)

For each aggregate size reported, determine the mass loss (in percent) using the following equation:

Where:
MB = mass before the test
MA = mass after the test

A weighted average (by mass) of each aggregate size tested should be calculated and reported as the overall mass loss of the sample.



Footnotes    (↵ returns to text)
  1. Wu, Y.; Parker, F. and Kandhal, K. (1998). Aggregate Toughness/Abrasion Resistance and Durability/Soundness Tests Related to Asphalt Concrete Performance in Pavements. NCAT Report 98-4. National Center for Asphalt Technology. Auburn, AL.http://www.eng.auburn.edu/center/ncat/reports/rep98-4.pdf. Accessed 23 June 2004.
  2. Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.
  3. Meininger, R. (2002). Validity of the Sufate Soundness Test. Rock Products, 1 August 2002 issue.http://rockproducts.com/mag/rock_validity_sulfate_soundness.
  4. American Association of State Highway and Transportation Officials (AASHTO). (2000b). Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Twentieth Edition: Part II – Tests. American Association of State Highway and Transportation Officials. Washington, D.C.

 

 

 

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