Influence of the Course Adhesion

The preservation and thus the useful life of asphalt pavement are among other things, dependent on a good adhesion of the individual asphalt courses.

It is known from statics that 3 boards, each 3 cm thick lying on one another do not have the stability of a beam with a thickness of 9 cm - although both, have, indeed, the same (total) thickness.

But this simple knowledge is often inexcusably ignored in asphalt road construction.

Numerous trade publications and guides point out this connection, mostly without the necessary acknowledgement.
In his article "Course design and course adhesion in road pavements made of asphalt" in Strasse + Autobahn, Issue 5/99, Dr. Walther Glet wrote in detail about this topic.

A schematic representation shows the stress on road pavements with relation to trucks' wheels and the coordination of the requirements of the properties with regard to the shearing stress and pressure stress dependent on the course depth.

For problems in the courses' adhesion, four different faults are responsible:

• no adhesion: courses lie loosely on each other,
• too soft a bond: courses move,
• too thick an adhesion film: courses are not stable,
• too little mortar : courses only meet at certain spots, stone framework breaks down.

Precisely this last case is what happens to wearing courses made with Stone Mastic Asphalt, especially when they are paved on coarse-grained asphalt binder courses.

That is why it is absolutely necessary to take steps to prevent this phenomenon from happening.

To this end Glet gives the following advice:

• the best: hot on hot,
• good: hot on new, undriven-on pavement sprayed with adhesive,
• very good: hot or fresh or on cleaned old pavement with the right tack coat,
• passable: hot on new without tack coat,
• poor: hot on new, driven-on pavement or hot on wet,
• impossible: hot on new with contamination or hot on old without tack coat.

Suggestions for the dosing of the primer binder dependent on the type and design of the underlying structure are given in the following table.

In an article in "Strasse + Autobahn, Issue 5/79", as far back as 20 years ago, Dr.Eng. Metelmann and Eng. (grad.) Becken calculated how the load-bearing capacity of asphalt structures change when an inadequate course joint exists. With the help of the BISAR program from German Shell AG, the variations in the useful life of the course dependent on the adhesive were determined.

The 4 alternative systems of adhesion examined according to the (at that time responsible) RStO 75 (Guidelines for road construction) were:

• A) Both joints bonded
• B) Upper joint not bonded / lower joint bonded
• C) Lower joint not bonded / upper joint bonded
• D) Both joints not bonded.

The results are given in the following Tables (2 and 3).

Wie dramatisch sich die ertragbare Beanspruchung einer Asphaltbefestigung verringert, bevor ein Ermüdungsbruch eintritt, ist in der nachfolgenden Darstellung für einen Asphalt-Oberbau nach Bauklasse I Zeile 1 der RStO 75 aufgezeigt.

If both joints are not bonded, the useful life is reduced to only 3 % of that of a well bonded pavement.

Glet in his article illustrated the possibility to technologically prove a well-bonded course adhesion.
A tried and true test is the shearing test by Leutner. Here core samples are pushed parallel into the bonded area and the force and distance are measured.

A system outline of the shearing device by Leutner follows.
From the existing evidence on Leutner's shearing test, the following (preliminary ) requirements have been suggested in the literature:

Shearing force

on the course edge wearing / binder course, a minimum value of 14.0 kNon the course edge binder / base course, a minimum value of 10.0 kN
on the course edge wearing / base course, a minimum value of 13.0 kN.

Shearing distance

• on the course edge wearing / binder course, a minimum value between 2.0 and 5.0 mm
• on the course edge binder / base course, a minimum value between 1.0 and 4.5 mm
• on the course edge wearing / base course, a minimum value between 1.5 and 4.5 mm.

A sufficient amount of background experience is not available at present. It is essential to make sure that we have as much testing information and results at our disposal as soon as possible.