What is asphalt?
Asphalt is a building material that consists of a mixture of binding agent (bitumen), aggregate and filler (fine sand). Asphalt can be produced technically, but is also found in its natural form and is mined in places like Trinidad, Venezuela, Switzerland, California, Syria and Germany. Natural asphalt, also known as earth pitch, is currently only mined underground in Holzen in Lower Saxony.
What are the properties of asphalt?
Asphalt is chemically almost inert (sluggish reaction behavior) and exhibits thermoplastic behavior.
In road construction, asphalt consists of three layers, the thickness and position of which determine the load-bearing capacity of the road. They must be connected to form a compact structure.
Where is asphalt used?
The asphalt performance results from the properties of the building material. It is used in the following areas:
- Road construction for road surfaces or road surfaces
- in building construction for floor coverings
- in hydraulic engineering for sealing
- in landfill construction for sealing
- in garden and landscape architecture (with coloured asphalt) for the design of paths, squares and leisure facilities
However, not every asphalt recipe can be used in any layer or universally for different applications. Very coarse-grained mixes should be laid thicker, very fine-grained mixes can be laid thinner.
What asphalt layers are there and what are their functions?
Roads consist of three layers: the base layer, binder layer and surface layer. The asphalt base layer is the lowest layer, which consists of bitumen, filler and an aggregate mixture. The second layer is the binder layer. The top layer, which is visible to the driver, is the final layer.
Asphalt layers protect the foundation and the subsoil from precipitation in order to maintain the load-bearing capacity of the entire structure. The solid bond of asphalt layers carries the traffic load and distributes it across the subsoil. This system ensures that the entire road surface is not damaged.
Base layer:
The additional technical contract conditions and guidelines for base layers in road construction (ZTV T-StB) stipulate a minimum thickness of 8 cm. Further regulations for the asphalt base layer are laid down in the guidelines for the standardization of the superstructure of traffic areas (RStO).
Also known as an asphalt binder layer. Binder layers are used on roads subject to higher traffic, from construction class 3 onwards. The binder layer forms the transition between the coarse-grained base layer and the fine-grained top layer. This levels the surface of the base layer so that a flat top layer can be applied. This flatness enables the road to absorb the high shear stress of traffic and prevent deformation. The RStO recommends a layer thickness of 4 to 8 cm, depending on the construction class.
The composition of the asphalt binders is regulated by the “Additional Technical Contract Conditions and Guidelines for the Construction of Asphalt Road Surfaces” ZTV Asphalt-StB.
Is the top layer of the asphalt pavement that is directly subject to stress and should be 4 cm thick. It is subject to the direct effects of traffic, weather and de-icing agents. The layer thickness is regulated in the ZTV Asphalt-StB and RStO. The asphalt surface layer is explicitly designed to withstand wear and tear from daily traffic. However, this layer must be renewed as part of a regular surface construction program.
What types of asphalt are there?
The difference between the various types of mix lies in the composition of the aggregate and the binding agent content. In terms of the installation method, a distinction must be made between rolled asphalt and mastic asphalt. Rolled asphalt must always be compacted using road rollers, whereas mastic asphalt is processed in a liquid state and does not require compaction. The choice of mix type depends on the traffic load and the desired surface structure (fine to coarse-grain).
- Asphalt concrete (hot installation): Belongs to the surface layer and is suitable for roads of construction classes 2 to 6 and all types of paths. Asphalt concrete is characterized by its low-void composition and consists of fine chippings, fine crushed sand, natural sand, filler and bitumen. It is laid at around 120°C. Asphalt concrete gets its roughness from chippings, which are scattered on the hot surface layer and worked in with a roller.
- Asphalt concrete (warm installation): Belongs to the surface layers and is suitable for traffic areas with low loads in construction classes IV to VI. It is installed at approx. 60°C and only becomes completely watertight when exposed to traffic. The building material consists of a mineral mixture and flux bitumen. Due to environmental protection and flux bitumen, warm installation is rarely used anymore.
- Stone mastic asphalt: Special grade for surface layers with a higher bitumen and chippings content. In addition, it consists of road bitumen or polymer-modified bitumen (PwB) and stabilizing additives (e.g. cellulose or synthetic fibers). The modification is intended to increase durability, which is why stone mastic asphalt is suitable for high traffic loads (e.g. on motorways).
- Mastic asphalt: Has a high proportion of bitumen and filler, which creates a visually smooth surface. The cavities in the aggregate mixture are completely filled with bitumen. This means that it can be processed in a liquid form. It is used on roads (motorways) and bridges that are subject to heavy use due to its high level of water resistance. To ensure that the asphalt remains grippy, fine-grained chippings are spread and rolled in when it is laid.
- Asphalt mastic: Contains a low bitumen content and is mainly used for sealing structures and filling joints. Asphalt mastic is therefore used in road construction for repairs, improvements or to improve surfaces.
- Asphalt surface layers: Combines the functions of asphalt concrete and asphalt base layer in one. This asphalt is often used in agricultural road construction because it is relatively inexpensive.
- Open-pored asphalts: OpA, also known as whisper asphalt, is a special form of asphalt surface layer that consists of relatively large grains and therefore has a porous structure. Due to its special structure, the asphalt absorbs noise. It is established on motorways and expressways.
- Low temperature asphalt (NTA): Low temperature asphalt is regular asphalt that is manufactured using a different process. These different manufacturing methods mean that less energy is required during installation. The result is faster cooling, faster opening to traffic and faster implementation of maintenance measures.
Why does asphalt testing technology exist?
Roads connect not only cities, but also people. They cover our entire planet and are mainly made of asphalt. This building material is subject to different requirements and legal standards depending on its intended use (e.g. road construction, airport areas or cycle/pedestrian paths). Material tests must be carried out to ensure that quality specifications are met.
For example, asphalt mixing plants use asphalt extraction for self-control in order to guarantee a mix that complies with the asphalt recipe. Furthermore, external monitoring by institutes is necessary, which in turn monitor the products of the asphalt mixing plants.
In addition, the topics of recycled asphalt and cold asphalt are becoming increasingly important in view of climate change. Recycled asphalt or reclaimed asphalt is milled material that is taken from one road and, after testing, is to be reused in another road. During this process, a binding agent content test (bitumen test) must be carried out. Asphalt tests are also used for new developments or research. In this regard, cold asphalt is not yet established in the industry-specific regulations (ZTV Asphalt-StB 07, TL Asphalt-StB and TP Asphalt-StB) in Germany. In order to introduce it across the board, extensive studies must first be carried out.
What testing procedures and examination aspects are there for asphalt as a building material?
- Asphalt analysis/binder content determination: Breaking down the asphalt into its components mineral/rock, filler, bitumen and, if necessary, other additives
- Compactability: To determine the void content, compaction resistance or volume density, standard test specimens must first be produced
- Temperature behavior: Investigation of the properties of cold reaction behavior, stiffness, fatigue resistance or strain behavior at dynamic temperatures
- Deformation resistance
