Technical field

The present invention relates to an asphalt surfacing material, particularly for use as road surface and comprising bitumen, steel slag material and stones.

Background Art

A number of different asphalt surfacing materials of the above mentioned type are known, the ratio between bitumen, stones or steel slag material as well as optionally aggregate and filler depending on whether the surfacing material is to be used for road surfaces, floors, swimming pools, joints, castings etc.

In its normal form asphalt is a flexible and durable material having a better ability to resist the stresses in a rigid system such as traditional a oad structure than other building materials such as concrete.

The deterioration of asphalt is partly caused by fatigue as described in the elasticity theory, partly by climatic and mechanic deterioration of the surface caused by effects such as abrasion, erosion, oxidative curing etc.

For particularly exposed areas, e.g. ψoad surfaces, square surfaces, swimming pools etc., presstire-stable asphalt is especially desirable, measured for instance by the so-called Marshall-test, said asphalt having an improved ability to distribute contact pressure on the surface to the subjacent layer. The latter occurs for instance in case of high shaft pressure or .static loads when depositing or parking heavy objects etc,- under normal load conditions and otherwise unchanged

conditions, an increased pressure stability results in a prolonged life of the surface.

The resistance of the asphalt surfacing material to climatic deterioration and mechanical influences depends largely on the cohesion and strength of the binder. A progressive loss of binder, i.e. an escape of the stones from the surface, is particularly characteristic for a deteriorating asphalt surface.

Especially suitable durability properties in the form of high pressure stability, high abrasion resistance and great strength of the asphalt binder are obtained by asphalt surfacing materials predominantly comprising steel slag material. These asphalt surfacing materials are, however, extremely expensive to manufacture, as the steel slag material used has to be of high quality to obtain the desired propitious properties, i.e. preferably steel slag material from the manufacture of steel in arc furnaces.

For many road surfaces, such as Danish country roads and side roads, where the requirements concerning abrasion resistance are not as high as mentioned above and where the asphalt surfacing material should be as inexpensive as possible, the most commonly used asphalt surfacing material comprises predominantly stones. Therefore, it is much- less expensive to manufacture than the above mentioned surfacing materials comprising steel slag material. One problem of these types of surfacing materials is the comparatively weak asphalt binder, as changing weather conditions such ^* as occur in Denmark quickly deteriorate the asphalt binder, resulting in erosion, oxidative curing and finally fatigue.

Description of the Invention

The object of the present invention is to provide asphalt surfacing material of particularly suitab durability properties, i.e. high pressure stability a great strength of the asphalt binder, said surfaci material being inexpensive to manufacture.

The asphalt surfacing material according to the prese invention is characterised in that the steel sl material comprises predominantly so-called steel sl powder having a grain size of less than 2 mm. T obtained asphalt surfacing material with asphalt bind has particularly propitious durability properties in t form of high pressure stability and great resistance climatic deterioration, said surfacing material being same time inexpensive to manufacture.

Due to the alcalinity of the steel slag powder it reac with the acidic components of bitumen, resulting in particularly strong asphalt binder, simultaneous improving the bonding ability of the binder to acidi stones.

According to the invention, the stones can have a grai size of predominantly more than 2 _tam. The asphal surfacing material is thus especially inexpensive t manufacture.

Moreover, the steel slag powder according to th invention can comprise crushed s-fceel slag from th manufacture of steel in arc furnaces. The resultin asphalt surfacing material has particularly propitiou durability properties and the- asphalt binder has ver high pressure stability and great strength.

In a particularly advantageous embodiment of the invention, predominantly 33% by volume of the granulate material in the surfacing material is steel slag powder.

Best Mode for Carrying Out the Invention

The invention is described in greater detail below and with reference to particularly preferred embodiments.

The asphalt surfacing material comprises bitumen, steel slag material and stones as well as optionally aggregate and fibres. A variety of bitumen types may be used and the asphalt surfacing material may be manufactured at elevated temperatures using hot as well as cold bitumen and bitumen emulsions or bitumen solutions.

Steel slag from a large number of steel manufacturing processes may be used for the manufacture of the asphalt surfacing material according to the invention. Steel slag from arc furnaces is particularly suitable because of its unusually good homogeneity, the chemical composition of said steel slag being especially advantageous for obtaining high binder strength due to the high alcalinity of the material.

Other types of steel slag such as slag from the manufacture of steel according to the LD process is of lesser homogeneity and chemical/physical stability due to a high content of CaO, free MgO etc. These types of slag become brittle due to weathering effects and are therefore unsuitable as aggregate in the abrasion layer of the asphalt. These types of steel slag can, however, be used for the manufacture of asphalt surfacing materials according to the present invention, as they are ground to steel slag powder instead of being aggregate.

The present invention thus allows an increa-sed use of by-products from the manufacture of steel, resulting i the quality of certain asphalt materials i>eing improved while simultaneously solving a waste removal problem of steel mills.

The chemical composition of steel slag from the manufacture of steel in arc furnaces is typically:

The pH value of the slag in an aqueous slurry is > 10.

As is common in the manufacture of asphalt surfacing materials, the stones comprised therein may be granite, quartzite, flint or the like. The asphalt surfacing material may moreover be manufactured according to usual procedures for the manufacture of such materials, i.e. it is not necessary to alter the commonly used production equipment. In other words, the stones and steel slag material are selected first, whereupon the steel slag material is crushed to powder. The latter is heated together with the stones and dried, whereupon adjuvants such as bitumen are optionally added, the substances being mixed either in a positive mixer or a drum mixer to form a homogenous mass. The optimum bitumen percentage and the type of asphalt surfacing material to be manufactured such as asphalt concrete for the abrasive layer or the bearer layer of road surfaces is determined by means of the usual procedures for the composition of asphalt surfacing materials, for instance the

Marshall-method.

The following example describes asphalt concrete composed according to the Marsha11-method, the different grain size fractions being determined according to the Fuller-curve. Two commonly known types of asphalt concrete are compared with the corresponding asphalt concrete manufactured according to the present invention, the portion of the granulate material of less than 2 mm volumewise being replaced by a corresponding amount of steel slag powder so that the original grain curve mentioned above remains unchanged.

Asphalt concrete Asphalt concrete 8t granite 8t granite with steel slag

0/6 gravel 15% by vol. 15% by vol.

0/2 stone powder 33% by vol. 0/2 steel slag 33% by vol. 2/5 granite 17% by vol. 17% by vol. 5/8 granite 35% by vol. 35% by vol. 8/12 granite lime filler 3% by weight 3% by weight bitumen 5,9% by weight 5,5% by weight

Marshall-stability 7,6 kN 11,1 kN

Asphalt concrete Asphalt concrete 12t granite Y2-t granite with steel slag

0/6 gravel 10% by vol. 10% by vol.

0/2 stone powder 34% by vol.

0/2 steel slag 34% by vol.

2/5 granite 6% by vol. 6% by vol.

5/8 granite 10% by vol. 10% by vol.

8/12 granite 39% by vol. 39% by vol. lime filler 2% by weight 2% by weight bitumen 5,5% by weight 5,2% by weight

Marshall-stability 9,5 kN 13,8 kN

As is apparent, the bottom line of the table states the Marshall-stabilities of the asphalt concrete materials in question, i.e. an expression of the pressure stability and strength of the asphalt binder according to the so-called Marshall-method. It is apparent that tlhe pressure stability and strength of the asphalt binder is increased by approx. 50% in both asphalt concrete materials according to the present invention, resulting, of course, in the asphalt binder and thus the asphalt surfacing material according to the present invention having a considerably prolonged durability compared to corresponding known asphalt concrete materials used for reference.

The invention may be varied in many ways without thereby deviating from the scope of the invention. The asphalt surfacing material may, for instance, comprise a number of different types of known additives, such as gravel and lime filler. The asphalt surfacing material is not restricted to use as road surfacing material only, as the composition of the surfacing material may be varied so that it can be adapted to the requirements of the material being used as floor surfacing, for swimming

pools, joints, castings etc,