ADHESION TESTS

The Shell Bitumen Handbook places adhesion testing into a number of categories. There may be several different tests within each type but, in most cases, the individual tests of one type differ in detail rather than in principle (Shell Bitumen, 2003) .These categories can be summarised as follows:

 Static immersion & Dynamic immersion tests

 Chemical immersion tests

 Immersion Mechanical & Immersion Trafficking tests

 Coating Tests

 Absorption tests

 Impact tests

7.2.1

This is the simplest type of test and consists of aggregate being coated with bitumen that is then immersed in water. The fundamental problem with this method is its subjective nature and the fact that it is completely operator dependant and has poor repeatability. It must also be noted that in some cases, an aggregate with good laboratory performance may perform poorly on some occasions and those with poor static immersion test results may perform satisfactory in practise. An example of a static immersion test is the British Standard EN 13614:2011, Bitumen and bituminous binders. Determination of adhesively of bituminous emulsions by water immersion test.

7.2.2

This type of test is very similar to the static immersion test with the only difference being that the sample is agitated mechanically by shaking or kneading. The degree of stripping is again estimated visually and repeatability is very poor and results are operator dependant. An example of a dynamic immersion test is the British Standard EN 12274-7:2005 Slurry surfacing. Test methods. Shaking abrasion test.

7.2.3

In this type of test, aggregate coated in bitumen is boiled in solutions containing various concentration of sodium carbonate. The strength of sodium carbonate solution in which stripping is first observed is used as a measure of adhesion. However the artificial condition of the test is unlikely to predict the likely performance on the road. An example of a chemical immersion test is the TMH 1 - Method B11 -The determination of Adhesion of Bituminous Binder to Stone Aggregate by means of the Chemical Immersion test also known as the Riedel & Weber test.

7.2.4

Immersion mechanical tests and Immersion trafficking tests is mostly relevant to Asphalt and include test like the Retained Marshall Stability Test, the Retained stiffness test, retained Cantabro test and the Immersion wheel tracking test. These tests are not relevant for chip seals and will therefore not be discussed in more detail.

7.2.5

This type of testing aims to assess the adhesion between aggregate and bitumen in the presence of water. In the Immersion Tray Test from the British Road Note 39 (Roberts & Nicolls, 2008), aggregate chips are applied to a tray of bitumen covered with a thin layer of water. By careful examination of the chippings, it may be possible to determine whether surface active agents improve adhesion under wet conditions.

7.2.6

As part of the Strategic Highways Research Program (SHRP) in the USA, the Net Absorption Test method was developed to evaluate moisture damage in asphalts (Curtis, et al., 1993) .This test is an extremely complicated test and it requires a ( not so readily available) spectrophotometer to perform measurement. It is not a commonly used test in South Africa and will not be discussed in further detail.

7.2.7

The Vialit pendulum Test and the Vialit Plate Test is generally the only two test available to measure bitumen adhesion with impact tests. Both these methods are easily adapted to measure a wide range of conditions. The test are relevant in situations where aggregate is in direct contact with traffic stresses and it is therefore ideal for test related to chip seals.

The Vialit Pendulum Test is shown in the picture below. The procedure involves placing a thin film of binder between two cubes and measuring the energy required to remove the upper block.

For the Vialit Plate Test, aggregate particles are pressed onto a steel plate of bitumen. The plate is the turned upside down and a steel ball is dropped onto the reserve side. The impact of the ball may cause detachment of the aggregate particles depending on the test conditions. The number of detached aggregate chips vs. the number of impacts can be used as an indication of performance. Visual inspection of the detached aggregate can usually determine the type of failure. In South Africa a modified version of the Vialit Plate test is used as per Method MB-7 of the TG1 (Asphalt Academy-TG1, 2007)

7.2.8

The Shell Bitumen Handbook list two pull-off tests used to asses bitumen adhesion. The first is the Instron pull-off test which uses an Instron Tensile apparatus to extract aggregate test specimens from containers of bitumen under controlled laboratory conditions. The second is the limpet pull-off test that was developed to measure, the bond strength between the aggregate of a chip seal and the base course. The test consists of a 50mm diameter steel plate that is fixed to the road surface and the maximum load to achieve pull off is achieved.

The forum of European National Highway Research Laboratories in the BiTVal Phase 1 Report published in 2004 (Forum of European National Highway Research Laboratories, 2004) notes the pneumatic adhesion tensile testing instrument (PATTI) as a possible adhesion tester. This method was originally developed to test the pull-off strength of a coating on rigid substrates such as metal, concrete or wood as per ASTM D 4541-02 and was used as early as the middle 1990’s to evaluate the adhesive loss of bitumen binder-aggregate systems exposed to water (Youtcheff & Aurilio, 1999).

The sketch on the next page shows a schematic cross section of the original PATTI setup consisting of a piston connected to a pull out stub. To perform a test, air pressure is transmitted to the piston which is placed over the pull stub and screwed on the reaction plate. The air pressure induces an airtight seal formed between the piston gasket and the aggregate surface. When the pressure in the piston exceeds the cohesive strength of the binder or the adhesive strength of the binder-aggregate interface, a failure occurs. The pressure at failure is recorded and converted into the pull-off tensile strength (kPa).

Figure 7-2- Cross section schematic of PATTI piston attached to a pull stub (Youtcheff & Aurilio, 1999)

The BiTVal report notes the following concerns related to the original Patti Test:

“

 Results could be significantly improved by including a temperature controlling system and a standard methodology for detecting the type of failure of the specimens.

 Attention needs to be paid to examining the entire soak-time/pull-off strength curve while evaluating binder.

 Little attention is given to the preparation and characterization of substrates other that glass.

 Questions concerning the surface chemistry or roughness of substrates remain unanswered. “

In South Africa two types of pull-out test is specified in the (Asphalt Academy-TG1, 2007)

 Method MB-8: Pull out test method for surfacing aggregate

 Method MB-9: Pliers test for assessment of adhesion properties

Both these tests are extremely complicated and difficult to complete and the results are almost entirely operator dependant. The test has low repeatability and is therefore not often used in practise.