Asphalt Content of HMA by Ignition Method

Introduction:

This test method specify the equipment and procedures needed to determine the asphalt binder content of hot mix asphalt / bituminous mixtures by removing the asphalt cement at 5400 C by ignition in a furnace.

This method replaces the traditional solvent procedure and thereby not only avoids the health and environmental concerns associated with chlorinated solvents, but also the related expense of such chemicals.

The asphalt binder analyzer combines a sophisticated furnace and weighing system to continuously measure the weight loss of bituminous mixture during combustion, and automatically calculates its binder content at the end of the test. This test method can be used for quantitative determination of asphalt content in hot-mix asphalt (HMA) paving mixtures and pavement samples for quality control, specification acceptance, and mixture evaluation studies.

Objectives:

Objective of this test is to determine the asphalt binder content of hot mix asphalt / bituminous mixtures by removing the asphalt cement at 5400 C by ignition in a furnace.

Apparatus:

The following apparatus are required.

1) Balance Readable to 0.1 g and capable of measuring the mass of sample, sample trays, and catch pan.

2) Sample Tray(s) Appropriate size that allows the samples to be spread thinly and allows air to flow up through and around the sample particles. The sample shall be enclosed completely with screen mesh or perforated stainless steel plate or other suitable material.

Note: Screen mesh or other suitable material with maximum and minimum openings of 3.35 mm and 600 microns, respectively, has been found to perform well.

Figure 1 Furnace.

3) Catch Pan Appropriate size to hold the sample trays so that aggregate particles and melting asphalt binder falling through the screen mesh are caught.

4) Catch Pan / Sample Tray(s) Handling Apparatus Suitable for inserting catch pan and sample tray(s) into furnace and removing hot catch pan and sample tray(s) from furnace.

5) Assorted Spatulas, Pans, Bowls and Wire Brushes For preparing hot-mix asphalt mixtures and removing aggregate from sample tray(s) and catch pan.

6) Protective Gloves Gloves be well insulated and capable of withstanding 5800 C.

7) Ovens Mechanical ovens, convection or forced draft, shall be provided for drying aggregates and HMA mixtures, and preheating HMA mixtures prior to ignition testing.

8) Furnace (Figure 1) Having a minimum temperature capability of 5800 C and having an internal weighing system capable of measuring the mass of sample sizes of at least 2500 g. The furnace chamber shall be of sufficient size to accommodate sample sizes of at least 2500 g. A data collection system also shall be included so that the sample mass loss can be determined automatically to an accuracy of 0.1 g and displayed during a test. The test is deemed complete when the difference between consecutive measured mass losses does not exceed 0.01 % of the sample mass for three consecutive 1-min intervals. The equipment shall provide a printout of the test results. A system capable of reducing furnace emissions to an acceptable level also shall be incorporated in the furnace. The furnace shall be vented into a hood or to the outside and when set-up properly will have no noticeable odors escaping into the laboratory. Furnace will have fan with capability to pull air through the furnace to expedite the test and to reduce escape of smoke into laboratory. The furnace shall be equipped so that the door cannot be opened during the ignition test.

Figure 2 ABA Control Panel

Procedure:

Sampling and Preparation of test specimen

1) Samples of asphalt cement were obtained in accordance with Practice D 140.

2) The size of the test sample was governed by the nominal maximum aggregate size of the mixture and was conformed to the mass requirement shown in Table 1 (See Note below).

Note: When the mass of the test specimen exceeds the capacity of the equipment used (for a particular method), the test specimen may be divided into suitable increments, tested, and the results combined for calculation of asphalt content based on the weighted average of the masses used in the increments.

Table 1: Size of the Sample.

Nominal maximum Aggregate size standard (mm)Minimum mass of sample (kg)

4.750.5

9.51

12.51.5

19.02

25.03

37.54

3) If the mixture was not soft enough to separate with a spatula or trowel, it was placed in a large, flat pan and warmed in an oven set at 110 50 C until it could be separated or mixed.

4) The material was split or quartered until the mass of material required for the test was obtained.

Calibration Procedure:

1) The results of this test method may be affected by the type of aggregate in the mixture because different aggregates lose mass on ignition to varying degrees. The results also may be affected by the presence of additives and modifiers in the HMA sample. Accordingly, to optimize accuracy, a calibration factor was established by testing three calibration samples for each mix type.

2) The calibration was performed on a prepared sample of asphalt mixture, which also additives and modifiers were included, if any, to be used.

3) Samples of blended aggregate to be used were obtained in HMA in accordance with sampling. The sample should be approximately the same mass and gradation as that to be used for the HMA test sample.

4) Three calibration samples were prepared at the design asphalt cement content (P). Additives and modifiers were incorporated, if any, to be used.

5) The mass of the sample tray(s) and catch pan were determined and recorded to the nearest 0.1 g.

6) The sample was evenly distributed in the sample tray(s).

7) The mass of the sample, sample tray(s) and catch pan were determined to the nearest 0.1g. The initial mass of the sample (M1) was calculated and recorded.

8) The calibration sample was heated in the furnace at 540 50 C until the change in mass of the sample during three consecutive 1-min intervals did not exceed 0.01 % of the sample mass (M1).

9) The mass (ML) of the sample was measured and recorded after ignition to the nearest 0.1g. The mass could be obtained immediately upon completion of the test from printout or display.

10) The calibration factor (CF) was calibrated as follows:

CF = M1 ML x 100 P

M1

Where;

M1 = total mass of the mixture calibration sample prior to ignition.

ML = total mass of the mixture calibration sample after ignition.

P = percentage of actual asphalt cement in the mix by mass of the total mix expressed as a percentage.

11) These steps were repeated for two additional calibration samples. The average calibration factor (CF) was calculated by averaging the three CF values. The temperature for testing HMA samples should be the same temperature selected for testing mixture calibration samples.

Test Procedure:

1) A HMA sample was obtained in accordance with the Table 1. The sample mass should be approximately the same mass as that used for calibration.

2) The HMA sample was oven-dried to constant mass at a temperature of 110 50 C.

3) The furnace temperature was set to 540 50 C. Samples can be placed in the furnace at significantly lower temperatures since the furnace will quickly heat to the desired temperature once the sample begins to burn. The furnace temperature is likely to increase during the ignition phase of the test.

4) The mass of the sample tray(s) and catch pan were determined and recorded to the nearest 0.1 g.

5) The sample was evenly distributed in the sample tray(s).

6) The mass of the sample, sample tray(s) and catch pan were determined to the nearest 0.1g. The initial mass of the sample (MB) was calculated and recorded.

7) The sample in the furnace was heated at the specified temperature until the difference between consecutive measured mass loss did not exceed 0.01 % of the sample mass (MB) for three consecutive 1-min intervals. This point was determined automatically by the furnaces data collection system.

8) The furnaces data collection system was measured and recorded automatically the aggregate mass (MA) of the sample after ignition to the nearest 0.1 g.

9) The mass was obtained immediately upon completion of tests by subtracting the mass loss measured by the furnace from the initial mass of the mix (MB).

10) The corrected asphalt content was calculated automatically by the furnaces data collection system.

Observation:

Initial wt of container

= 4000.8 g

Total mass of the HMA sample prior to ignition (MB) = 2044.5 g

Total mass of the container with aggregate sample

remaining after ignition = 5947.2 g

Calculations and result:

The corrected asphalt content was calculated automatically by the furnaces data collection system as follows:

% AC = MB MA x 100 CF

MB

Where;

AC = measured asphalt content percent by mass of the oven-dry HMA sample.

MA = total mass of aggregate remaining after ignition.

MB = total mass of the HMA sample prior to ignition.

CF = obtained calibration factor should be entered into the furnaces data collection system.

Specimen Calculation:

MA = 5947.2 4000.8 = 1946.4 g

MB = 2044.5 g

CF is assumed as zero.

% AC = 2044.5 1946.4 x 100 0

2044.5

= 4.8 %

Discussion:

1) List out advantages and disadvantages between asphalt content determine by HMA ignition method and centrifugal extractor method.

Centrifugal extractor method

Advantages

Quantitative determination of bitumen and aggregates, gradation of bitumen paving materials can be done.

It is a low cost method.

Disadvantages

An older method.

Required skilled persons.

The used solvents are hazarded.

Take longer test time.

Accuracy is lower.

Ignition method

Advantages

Quick quantitatively method.

Completely automated test cycle.

Test time reduces to 30-40 minutes.

No solvents are required.

Disadvantages

Test method may costly.

Prior calibration is required.

2) Why its important to calibrate asphalt binder analyzer before the test? State special requirements of the calibration samples.

In ignition method of extraction asphault binder from HMA, the whole sample is ignited. A certain amount of aggregate fines may also be burned off during the ignition process. Therefore, a calibration mast be done by placing a sample of known asphault binder content in the furnace and comparing the test result with the known asphault binder content.