## 5 Discussion and Conclusions  
### Contribution to Research and Practice  
This article provided insight into an under-represented public procurement topic. While certain scientific insight in successful SPP, GPP, and partly CPP is already available, in-depth knowledge is still missing in various market areas. In this context, the management of tyres in a way that is fully in line with CE goals is an important task. This article analysed suitable strategies to implement circular public tyre procurement and aimed to enrich CPP research by in-depth product-specific analyses and to promote the further development of a tyre CE. Due to GT principles, this article's GT result is already empirically proved by the interdependence of empiricism and theory, cf. [49; 50]. This proof means that further variations of the GT discovered later can be inserted as further developments of this original theory, cf. [47]. This article identified five barrier types of sustainable tyre CPP which are of regulatory, technical, social, market, and ecological nature. With a broader scope on the CE, [14] discovered four barrier types: cultural, regulatory, market, and technological barriers. Their work and the social and cultural barriers identified in this paper have similarities, highlighting the relevance of [14]'s findings also on a specific product level. This observation applies in particular to the awareness aspect. However, while [14] focus on consumers' awareness, this paper refers to the awareness of CE tyre issues by public procurement professionals. Likewise, [14]'s market barrier and this article's counterpart's features have similarities, e.g., concerning limited funding. However, our work shows the specific barriers of public procurement professionals in the tyre context while the [14] market barrier has a slightly stronger focus on the market's supply side. In contrast to [14], this article identified not only missing standards as a CE barrier, but also highlighted the need for appropriate labelling. In addition, it provides solutions for both issues. As an additional contribution compared with [14], this article identified also an ecological barrier of a tyre CE. Fourteen recommendations were derived from this research, also encouraging the development and usage of standards and certificates. In this context, four specific standardisation topics and eight labelling topics were mentioned and will be further pursued by the ConCirMy project. The standardisation needs described by the interviewees were discussed with a standardisation expert. The specification of the tyres' recycled content includes two aspects. The specification of minimum percentages could only be enforced by legislation. Open questions, which may be addressed by standardisation are, however, e.g., how the quality'recycled' is defined, how the recycled content is declared and whether it refers to the tyre or parts of it. Important standards addressing the needs 'CO\({}_{2}\) footprint [...] according to uniform measurement procedures' and 'procedure for comparison to conventional tyres on CO\({}_{2}\), etc.' are the ISO standard 14000 and the ISO 14000 standards series as a whole. However, more product-specific specifications may be necessary to specify LCAs and the carbon footprint of tyres. A PCR (product category rule for type III environmental declarations) has been developed for tyres by the International EPD system [60] and two tyre EPDs have been published to date (28 September 2021). These standards may also provide a suitable foundation for comparisons with conventional types. If additional sustainability characteristics shall be compared, however, additional specification may become necessary. Regarding comparisons between retreaded tyres and conventional tyres, safety is an important aspect. Both types of tyres can be evaluated according to these standards and compared afterwards. However, more guidance may be necessary to decide which parameters shall be used and how the comparisons take place.