Comparative analysis between the magnesium sulfate soundness and micro-deval tests in the evaluation of bituminous aggregates

In the past ten years, several factors have led the Texas Department of Transportation (TxDOT) to reevaluate the way it designs bituminous pavements. Foremost has been the increase in the volume of heavy truck traffic and the permitted loads they are allowed to carry. Among the highway pavements designed and constructed by the Department, Asphaltic Concrete Pavement (ACP) is the most commonly specified type of bituminous pavement. Changes in the design of ACP have been seemingly small and incremental but if we step back and consider the wider view of what has evolved during this time, the changes are significant, particularly in the quality of aggregates needed to develop an acceptable bituminous mix. The quality of aggregates used in bituminous mixes is a primary factor that determines in the performance of these pavements.

New and innovative designs have raised the bituminous pavement construction process to higher levels. These designs provide us with pavements that are structurally superior to traditional ACP. CMHB, SMA, and Superpave designs are coarse, open graded mixes with fewer fines and higher asphalt contents. These bituminous mixes demand even higher quality in aggregates due to their rock on rock contact that exist in the mix. These changes will provide Texas with highways that will have longer design lives, require less maintenance and will be more cost effective in the long term.

Establishing specification requirements that meet the design criteria is of utmost importance. Having the means to insure compliance with the specifications is equally important. As the Department has modified both the procedures and the requirements for bituminous mixes, the contracting industry has had to adapt. This has introduced a level of uncertainty, which lends itself to an increased risk that the contractors must account for. One of the greatest risks that a contractor is faced with when bidding a job is the selection of an aggregate source. This is one of the factors that led to higher unit bid prices for bituminous mixes.

To meet this challenge, the Department must have quality control tests that are accurate, timely, and repeatable. The majority of the aggregate test methods used today were developed in the 1930's and 1940's. The relationship between the performance of aggregates in the laboratory and in the field was not well defined when these tests were developed and remains ambiguous today. One of the primary indicators of durability and resistance to degradation has been the Magnesium Sulfate Soundness (MSS) Test.