Repeated immersions of aggregate samples in sodium or magnesium sulfate solutions and alternating with oven drying to precipitate the salt in the permeable pore spaces is a test deeply rooted in aggregate technology. But is it worthwhile?

The test was initially published in ASTM in 1931. Its use, variability and interpretation is discussed by Stephen W. Forster in ASTM STP 169C. After each drying cycle, the sodium or magnesium sulfate salt re-hydration precipitated in aggregate pores causes expansion during soaking cycles. This expansion is said to simulate the expansion of water upon freezing. Soundness is a general descriptor for the ability of an aggregate to resist weathering. Freezing and thawing in a wet environment is the most severe action, he writes.

The ASTM and AASHTO methods include separate procedures for fine and coarse aggregates and options for using sodium sulfate (used by about ⅔ of the states) or magnesium sulfate in the test. Take care to use the appropriate test, and verify that specified limits are correctly applied. The five-cycle sulfate test with magnesium is more severe and often causes a higher loss percentage than sodium. ASTM Specification C 33 recognizes this by allowing a higher limit for magnesium.

The sulfate soundness test is designed to simulate the physical effects of freezing and thawing. There is little reason to impose a strict sulfate soundness limit in climates absent of hard freezes. The test provides an initial quality evaluation because of the mechanical actions of sieving, the wetting and drying, and the heating and cooling cycles. It provides a means to rule out obviously inferior material; but for others, performance-related evaluation should precede rejection.

Where freezing and thawing is a factor and where a history of aggregate performance has been related to tests in one central laboratory, the sulfate test can be a valuable first evaluation. However, performance tests or service records should be considered. For use in concrete, the aggregate can be tested in concrete freezing and thawing in the laboratory and ranked with other known aggregates. For use in unbound base or asphalt mixtures, the aggregate can be tested in unconfined freeze-thaw tests. When sulfate soundness testing is done in many different labs, the high variability in the test is a significant problem.

For ASTM and AASHTO, the precision statements are based on AASHTO Materials Reference Laboratory samples sent periodically to various labs. (It is good to know if a laboratory participates in the AMRL sample program.) The variability is higher for the sodium sulfate test, but poor for both. The coefficient of variation for the sodium sulfate soundness test is 41% for tests done in different labs. Differences between two labs can be as much as 116% of the average. For example, if the two labs average 10% loss (10 ∞ 1.16 = 11.2 %), one lab might get 4.4% loss and the other 15.6% loss. It is best to do all the sulfate soundness testing in one central laboratory that participates in the AMRL program. Changes in test procedures, particularly temperature and timing controls, and types of containers and sieving methods in different laboratories can change the percent loss obtained. Also, some aggregates containing carbonates of calcium or magnesium are attacked chemically by fresh sulfate solution, resulting in erroneously high measured losses.

As Forster writes, “In view of the precision of the test … and the uncertainty as to what performance characteristics the test actually relates, it can easily be seen that interpretation of the results must proceed with caution. One suggestion was to accept aggregates that passed the test, but not reject those that fail it.” A low result will usually indicate a durable aggregate, but results of other tests and performance in service also should be considered.

RICHARD MEININGER, PE has been active in developing aggregate standards for decades. He is past vice president of engineering for both the former National Aggregates Association and the National Ready Mixed Concrete Association.

TEST METHODS

• ASTM C 88 and AASHTO T 104, soundness of aggregates by use of sodium sulfate or magnesium sulfate.
• Other state DOT specific test procedures for sulfate soundness using sodium or magnesium sulfate are also used. These may differ substantially from the ASTM and AASHTO procedures.

ASTM C 33 SULFATE SOUNDNESS LIMITS — PERCENTAGE LOSS

• Coarse Aggregate — 12% loss for sodium sulfate and 18% loss for magnesium sulfate.
• Fine Aggregate — 10% loss for sodium sulfate and 15% loss for magnesium sulfate.