The use of stone matrix asphalt (SMA) has increased significantly over the last few years. This mixture has been labeled the “Cadillac” of Hot Mix Asphalts in the state of Texas of its toughness and durability. Several of our members have placed a number of projects using SMA, while other members have not placed any SMA yet.

SMA is not to be confused with our conventional dense-graded mixtures, and I would like to offer a few tips associated with the design, production and placement procedures in dealing with SMA.

Design and Placement

During the initial mixture-design analysis, keep in mind that SMA mixtures can be difficult to place, so workability issues must be considered. The current design parameters call for a minimum of 75 gyrations (Ndes), which can be increased to 100 gyrations (Ndes) when allowed by the engineer. For those of you who have not had the pleasure of designing an SMA mixture to consider the workability aspect for placement, the number of gyrations (Ndes) needs to be lowered. Fifty gyrations (Ndes) have been used successfully on a number of projects in Texas. [Other states have reported a range of 65 gyrations (Ndes) to 35 gyrations (Ndes).] The liquid asphalt binder content for these mixtures must be at least 6.0 percent. Not only are we seeing more of this plan note (without a plan note the current specifications allow 6.0 percent + 0.3 percent), but any attempts to place SMA mixtures below a liquid asphalt binder content of 6.0 must assuredly will guarantee difficulty in the placement phase.

Production Phase

Controlling the - #200 in SMA mixtures is a critical aspect in the production phase. Adjustments to the plants to be able to have control of the “bottom end” of SMA mixtures have been successful in producing a workable mixture. Keeping the - #200 in the 8.5 percent to 9.5 percent range has proven successful for a number of producers. The key is being able to produce consistently a mixture in this range, and by being able to control the - #200 , liquid asphalt binder content likewise will be more consistent. The material used in our mixtures to achieve the needed - #200 must be a consideration as well. Being able to use a combination of material using “cold feed” bins has not only enabled better control of the “bottom end” of SMA mixtures, but has allowed the production rate to be increased by eliminating the need for a silo to introduce the mineral filler to the plant.

The current Texas Department of Transportation (TxDOT) specifications allow for gradation correlations that eliminate the need for washed sieve analyses. The use of this procedure has proven to be a viable process for conventional dense-graded mixtures. These correlations are determined in the mixture-design phase and are correlated to the original mixture design (JMF #1) prior to the production of the required trial batch. It is recommended highly for SMA mixtures that washed sieve analyses be performed at the trial batch and at a minimum for all sublots in Lot #1. Controlling SMA mixtures can be a tedious process, and even though washed sieves analyses can be a bit time consuming, we will have a true picture of what we are producing. This is another way to ensure that we have control of our SMA mixtures.

Placement Procedures

Compaction of SMA mixtures cannot be performed in the same manner as our conventional dense-graded mixtures. Roller operations must understand the difference and know that techniques used in the past for other types of Hot Mix Asphalt need to be modified. Constant monitoring of rolling patterns is a must. It is not uncommon to see rolling patterns established that require as many as five vibratory, six pneumatic, and two finish roller passes. But keep in mind that constant monitoring of our rolling patterns will ensure that we achieve or exceed the density requirements and not damage the SMA mixture. Over-rolling SMA can lead to undesirable ride-quality results.

A process that has been proven to be a success for compaction of SMA is to perform the breakdown process with tandem rollers. The size of the rollers as well as the amplitude and frequency settings must be the same. Intermediate rolling can be accomplished utilizing pneumatic rollers. The use of pneumatic rollers has proven successful for achieving and exceeding the density requirements for SMA. The key to using pneumatic rollers is to keep the tires properly inflated and hot. Skirts can be used to keep the tires on the pneumatic rollers from cooling off.

In the Balance

Balancing production, transportation and placement operations is a must for SMA mixtures. The goal should be to provide a paver speed of 50 feet per minute or less to eliminate placement-operation stops as much as possible. For nighttime placement operations and marginal temperature conditions, consider using a remixer to reduce thermal segregation.

TxDOT will be using more and more SMA on its projects, and it is imperative for us to understand the importance of good construction practices. SMA mixtures are presenting us with some new challenges, but our membership will meet and exceed these challenges by their attention to detail and their pursuit of quality.