Thin Wearing Courses using the Stone Mastic Asphalt Method

In the future the maintenance and expansion of available road networks worldwide will increasingly play an important role.
In Germany alone this network is made up of 626,820 kilometers (Ebner 2006) of roads.

Here as in other places, funding is often lacking to adequately maintain this network. That is why it is especially important to have and use economically advantageous, and at the same time, technically satisfactory construction methods for road maintenance.
In Germany the appropriate construction methods for this maintenance for the various conditions of the roads are described in the "ZTV BEA-StB" (Supplementary Technical Conditions of Contract, Specifications, and Guidelines for Structural Maintenance of Traffic Areas" Asphalt Construction Methods). Without going into the various possibilities contained therein, it has been determined from extensive, available practical experience that thin surface courses paved using hot application, have proven to be outstanding when allowances are made for the technical and economic aspects.

Here thin wearing courses paved using the Stone Mastic Asphalt principle play a special role.

Then with this construction method, it is possible "unlike with any other comparable method" to span very different paved thicknesses without resorting to additional measures (e.g. profiling). It is also possible to stretch thin wearing courses of Stone Mastic Asphalt 0/5 mm very thinly if needed, for example, when connecting pavement with existing paved structures such as manhole covers, gullies, etc.

A mix design test is usually used to check the right composition for a Stone Mastic Asphalt. A pointer for a typical Stone Mastic Asphalt 0/5 mm mixture composition is given below:

Aggregate mixture:

• 66.0 wgt.% high quality chippings 2/5 mm
• 11.0 wgt.%. crushed sand
• 11.0 wgt.%. natural sand
• 11.7 wgt.%. filler < 0.09 mm
• 0.3 wgt.%. cellulose fiber

It is important here to point out that the homogeneous addition of loose cellulose fibers can be problematic due to the low frictional forces in the aggregate mixture during mixing. For this reason it is absolutely necessary to use the right cellulose fiber granulate and bitumen" the most advantageous is VIATOP 66 in an amount of 0.45 to 0.5 wgt.% or respectively VIATOP premium in an amount of 0.3 wgt %.

Binder content:

7.2 to 8.0 wgt.%. road bitumen 70/100

The following properties of this mixture should be targeted using a mix design test, determined by a sample according to Marshall:

• Void content (calculated): 2.5 vol.%.

• Water absorption in vacuum: 2.0 vol.%.

The use of cellulose fibers as binder carriers also makes possible - and especially with these thin wearing courses - the highest possible binder content without damage.

In this way each aggregate chipping is covered with the desired thick mastic film.

This especially effects the adhesion between mineral and binder / mastic mortar in a beneficial way. Signs of detachment, e.g. through the penetration of water, have not been seen in practical experience. The well-known long useful life of these wearing courses can be attributed for the most part to the high binder content.

In Germany numerous thin wearing courses made with Stone Mastic Asphalt - as mentioned at the beginning under the brand name MASTIMAC - have been in use now for over 30 years.

The calculated void content as well as the water absorption in vacuum is given with the properties deliberately. Long years of experience have shown that the method of volume density determination for asphalt is problematic with Stone Mastic Asphalt, i.e. largely single sized mixtures with coarse-pored voids as laid down in the relevant German standards DIN 1996 Part 7 (in the future DIN EN 12697). Since, however, for the assessment of an asphalt of this type only the accessible voids from the surface are relevant when going on past experience, the test of water absorption in vacuum is recommended as an - additional - assessment criterion.

The difficulties which occur with the volume density determination on samples from this type of material, affect by definition the test for degree of compaction. Here too experience has shown that despite excellent, consistently manufactured and compacted Stone Mastic Asphalt wearing courses, large variances of coefficient occur in the test for degree of compaction. This is especially true for thin wearing courses of the type described.

With wearing courses that are 1.0 to maximal 1.5 cm thick - and that is the rule for Stone Mastic Asphalt wearing courses 0/5 mm - a degree of compaction test of the conventional sort does not appear to make sense. The targeted test value with these tests is always a random variable which has nothing to say about the targeted compaction during paving or the void content of the finished surface course and least of all about its useful life.

The requirements for a pavement, which in the following table are clearly arranged once more, are optimally met by the thin wearing courses made with Stone Mastic Asphalt.

From the drivers point of view	From the environments point of view	From the developers point of view
Traffic	Environmental compatibility	Economy
Skid resistance Levelness Brightness Water drainage ability	Noise absorption Recycling ability	Construction costs Construction time Traffic management Can be paved over

For questions about the economy, a cost-benefit analysis of the different construction methods for road maintenance should be made:

Measure	Material and paving costs (DM/m²)	Useful life time (years)	Costs (DM/m² and year)
Thin asphalt course in cold application (approx. 12 kg/m²) Thin asphalt course in cold application (approx. 12 kg/m²)	3,20 bis 3,60	4	0,80 bis 0,90
Thin asphalt course in cold application (approx. 18 kg/m²)	4,40 bis 5,40	6	0,73 bis 0,90
Thin asphalt course in hot application (35 to 50 kg/m²) asphalt concrete	3,60 bis 5,20	8	0,45 bis 0,65
Thin asphalt course in hot application (35 to 50 kg/m²) Stone Mastic Asphalt	4,40 bis 6,50	12	0,38 bis 0,54