Several research projects on circular materials at VTI

A radical transformation that will fundamentally change road construction in Sweden? Or diligent and unglamorous work with lab tests and computer calculations? Or maybe both at the same time? The on-going work with alternative road materials can be described in many ways.

The aim of the project “Secondary ballast materials” is ambitious – to make recycled materials a natural part of the tender process for urban infrastructure projects and to promote circular and sustainable construction. A paradigm shift, according to the project description.

The project covers the use of three alternative materials – slag gravel, crushed concrete, and recycled asphalt – that have been tested in VTI’s road surface material lab followed by more real-life conditions in the Heavy Vehicle Simulator, HVS. Measurements of the material’s characteristics are then entered into ERAPave – a dimensioning tool developed by VTI – to estimate track depth, deformation and lifespan during different conditions.

The results so far have been promising:

“Most of all I’m impressed with the crushed concrete. So far, it is as good as the usual bedrock material”, says Shafiqur Rahman who predicts that even the other two materials will be used in future road construction.

“By combining a layer of slag gravel (ash from waste incineration) and bedrock material we can reduce the need for new bedrock material. By dimensioning in a smart way, it is possible to achieve roughly the same characteristics as with conventional road construction”.

Amongst the challenges to be faced is the need to examine the characteristics of the alternative materials over a longer period of time. Some materials, such as slag and concrete, have so called cement-based characteristics that mean that they become more rigid over time. These characteristics need to be described in more detail in order to be able to predict how they will function in the long-term.

Even the project ”Use of circular materials in road construction” examines how recycled materials can, at least partially, replace virgin raw materials in the future. The Project Leader Håkan Arvidsson describes the work as “hands-on” and limited in scope.

“I was working in this field already in the 1990’s, although then we talked about different types of cycles. The same ideas have resurfaced, but now the terms circularity or circular economy are used”, says Håkan Arvidsson.

The project will develop methods for testing the rigidity and stability of circular materials, characteristics that determine a material’s load-bearing capacity. The tests are conducted as so called triaxial tests, that are also used in Shafiqur Rahman’s project, where the material is placed in a cylinder and exposed to pressure from three directions in order to simulate real-life traffic loads at the laboratory-scale.

Rigidity and stability are functional requirements that can, over time, replace the descriptive requirements (distribution of grain size) that are currently used for road materials. This, however, assumes changes to the current regulations, that is in its own right an important component of the transition to a greater degree of circularity. Håkan Arvidsson says that there are secondary materials available that work well, but that they are not covered by the current regulations and therefore cannot be used.

“Instead, these materials are deposited in large piles on landfill sites. It would of course be immensely positive if they could rather be used to build infrastructure”, says Håkan Arvidsson.

The project will also develop methods for monitoring the production of alternative materials. The tests need to be fewer and simpler while at the same time ensuring that the materials will function in the long-term.

A third current project at VTI demonstrates the breadth of research within circular materials. The research with “Alternative chemical de-icing agents” concerns, amongst other things, the testing and evaluation of so-called circular salt – a fly ash from waste incineration that has been collected and cleaned.

Göran Blomqvist, Senior Researcher, holds up a small jar with a white fine-grained content. It is finer grained than normal table salt, but otherwise it is pure sodium chloride, that is to say, salt.

“The fact that it is fine-grained is an advantage when it is used on pavements where there are no vehicles that could otherwise help to waft up and spread the salt. At the same time, it contains more water than normal salt and this is a disadvantage”, says Göran Blomqvist and spreads out small piles to show the recycled and normal salt.

“Part of our work is focused on finding a technical solution to enable the use of this type of salt”.

The project also compares three other de-icing agents – sodium chloride, calcium chloride, and potassium formate as well as mixtures of these such as the so-called Karlstadslake – all of which have different advantages and disadvantages. The end product of the project will be guidance and instructions regarding how and when different types of products should be used.

Text: Mikael Sönne

Translation: CBG