Measuring the Bitumen Content of Asphalt - The Loss On Ignition Technique

Topics Covered

Background

Loss On Ignition Testing

How The Loss On Ignition Method Works

Acceptance of the Loss On Ignition System

Advantages of the Loss on Ignition System

Performing the Loss On Ignition Test

Background

In the past, the most common method for measuring the binder content of asphalt has involved removing the bitumen from a weighed sample using methylene chloride. The process then follows either the directly determined (Aliquot) procedure or the indirectly determined (Pressure Filter) procedure, in accordance with the relevant British Standard. However, methylene chloride requires special equipment and health and safety procedures for handling, use, storage and disposal.

Loss On Ignition Testing

To avoid using this hazardous substance, some years ago the National Centre for Asphalt Technology in the USA piloted the development of a method to determine binder content by burning off the bitumen from the aggregate, a process known as ‘loss-on-ignition’.

Lafarge Aggregates Ltd, a leading manufacturer of asphalt in the UK, had been evaluating alternatives to the solvent method for several years. Keen to try the loss-on-ignition method, it turned to Carbolite. The UK-based manufacturer of electric ovens and furnaces had developed the ABA furnace, a unit that uses the loss-on-ignition method to determine the binder content of asphalt. Lafarge wanted to replace the solvent method at all its main plants and decided to test the Carbolite equipment at two quarries in tandem with the conventional solvent system before making a decision.

How The Loss On Ignition Method Works

Carbolite’s ABA analyser works by burning off the bitumen from a sample in an electric furnace and comparing the weight before and after ignition. The equipment incorporates a balance linked to microprocessor instrumentation, so the whole process is automatic. Emissions from the samples go through an afterburner before passing into the atmosphere, ensuring minimal emissions. The loss-on-ignition method can also shorten the time taken to perform a test on certain product types, particularly where polymer modified bitumens are involved, which require extra time to dissolve in the solvent.

Acceptance of the Loss On Ignition System

Once Lafarge was confident of the reliability of the system and its ability to conform with the various industry standards, it equipped 11 more sites with ABA analysers and removed the solvent method from all area laboratories.

One of the sites with the new equipment is the Mountsorrel Quarry in Leicestershire, UK, which also tests material from two other Lafarge plants. This unit carried out more than 2,000 tests last year, according to laboratory manager Stephen Mee, and two analysers have been installed to meet the work load.

Advantages of the Loss on Ignition System

Mee says that one of the main advantages of the new system is its simplicity and the greatly reduced time required to produce a binder result. The Aliquot method previously used a number of separate operations and could take up to two hours, with a technician involved most of the time.

Performing the Loss On Ignition Test

Using the Carbolite ABA analyser, the technician weighs the sample on a standard laboratory balance, puts it into the furnace, keys in the weight of the sample and tray, enters a calibration factor for the type of material and then starts the furnace. The whole process is then automatic, with a display on the furnace showing the progress of the test - current sample weight, current weight loss, the rate of change of sample weight and the duration of the test.

At the end of the programme the binder content of the sample is shown on the display, and a full print-out is also produced automatically. The remainder of the test on the sample - measuring particle size distribution - is then carried out in the normal way.

 

Source: Materials World, Vol. 11, no. 6, pg. 25, June 2003.

 

For more information on this source please visit The Institute of Materials, Minerals and Mining.

 

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