Tips to Measure AC Viscosity Inline
BY Robert McGregor
Asphalt labs around the world use rotational viscosity to measure binder viscosity. Highway departments in each country have practiced this method of testing even before the Strategic Highway Research Program in the United States began the practice more than 25 years ago. While the benchtop viscometer is a popular instrument of choice for testing, there is another way to obtain viscosity data in a timely fashion.
Inline process viscometers have been in existence since the 1980s, and offer this alternative. Measurement of binder viscosity in pipes or tanks during manufacturing can assure that the binder satisfies established QC parameters and that the production plant is performing to spec. For example, in automatic blending control systems, viscosity measurement can be used as the process feedback variable to regulate temperature, mixing and heating times, and addition of performance enhancing additives.
Perform Proper Technique in the Lab
The American Society of Testing and Materials recommends method D-4402 for testing asphalt binder in the lab: “Standard Test Method for Viscosity Determinations of Unfilled Asphalts Using the Brookfield Thermosel Apparatus.” Figure 1 shows the Brookfield thermosel system. D-4402 defines the methodology for placing the sample in a chamber, which is then heated and measured for viscosity using a rotating spindle—normally Brookfield number 27—immersed in the binder. The rotational speed is either 10 or 20 rpm and the viscosity reading is recorded in units of centipoise.
This test evaluates the pumpability of the asphalt. Manufacturers have defined target values for viscosity based on experience with different formulations. When the viscosity exceeds the target value, the consequence may be difficulty with pumping and subsequent problems with placement of finished product. It is important to set viscosity limits based on desired flow behavior properties that have been gained from field experience.
A potential improvement in the equipment to expedite testing is choice of disposable chambers. The 13R chamber is traditionally used and requires 16mL of sample or less, depending on the spindle that is immersed in the binder. As mentioned above, common choice of spindle is Brookfield number 27, in which case the sample size is slightly over 10mL. Both are now available as disposable items, which saves the operator time by avoiding the messy cleanup after the test.
The jacketed container into which the chamber is placed for testing has built-in temperature control. This brings the sample to temperature quickly, which can vary anywhere from 100oF (38oC) to 500oF (260oC), depending on the specific test. The normal temperature range for most testing is between 192oF (135oC) and 227oF (170oC).
A new time-saving feature is the EZ-Lock method for attaching the spindle to the viscometer. Traditional rotational viscometers use the screw-on method for connecting the spindle, which can be problematic when busy fingers are dirty, etc. The EZ-Lock allows the spindle coupling to be inserted directly into the spindle chuck, which is built into the instrument.
Operators record the viscosity value displayed on the face of the instrument during the binder test. Most labs require documented records so the question is whether to write down the number or have the instrument send the data to a printer or a PC. The obvious advantage of today’s digital viscometers is that the results can be automatically captured electronically.
How to Use a Process Viscometer
The process viscometer is significantly more expensive than its lab counterpart. Considering its advantages, the actual cost-to-operate may be substantially less than the benchtop viscometer. Whether you use the term inline, online or online process viscometer, the viscosity measurement instrument used to measure material in a tank or pipe during the manufacturing process offers advantages beginning with time saved over the lab procedure.
A popular candidate for inline viscosity measurement is the vibrational viscometer, which is an instrument with no moving parts in the fluid stream. Although there is no defined shear rate—rotational spindle speed—with this type of viscometer, it delivers a kinematic viscosity reading in scientific units of centistokes. The primary advantage of this approach is the continuous viscosity reading obtained at operating process temperature. While sensitive to changes in viscosity, this type of instrument is insensitive to changes in binder flow rate and process vibrations, because it is designed to operate in the harsh environment of an asphalt plant.
Viscosity measurement is a requirement for all asphalt plants. The real opportunity, however, is to go beyond the requirements and use viscosity data in a timely way to make sure that asphalt quality is consistent. This guarantees reliable performance of the binder when used by field construction crews who are placing asphalt. Producers have the opportunity to pursue inline measurement of viscosity for reasons stated above while using standard benchtop measurement that meets regulation in the lab.
Robert G. McGregor is the general manager of global marketing and high-end lab instrument sales for Brookfield Engineering Laboratories, Inc., Middleboro, Massachusetts. For more information, contact him at (508) 946-6200 x7143 or email@example.com.