# Calculation Methods for Construction Material Stocks: A Systematic Review
## 4 Discussion
### Future Directions for Research  
Based on the review conducted, this study highlights potential directions for future research.  
* Diverse study areas: Based upon the articles reviewed, the study found that most of the studies were performed in certain countries such as China, Japan, and Europe. Considering the significant contribution of the construction industry in developing countries, the contribution from these countries is insignificant. Research in material stock from developing countries would ensure that the material intensity of different regions could be compared based upon economic and population numbers. Also, the research could be done on how much material stock is necessary to sustain economic growth in developing countries. Similarly, differences in waste generation and energy consumptions could also be investigated between different regions in the world.
* Simulation tools: There remains a need for investment in developing simulation tools that can proceed with data available from Google Earth. Such tools would be readily available and would reduce the extensive work of estimating material stock. Developing such tools would further encourage research in areas where such extensive research data is not available, as in most developing countries. Similarly, as discussed in the study, few studies research both buildings and infrastructure as it is data intensive. However, it is necessary to understand urban areas better. Therefore, appropriate software tools and databases should be researched in which both buildings and infrastructure could be researched simultaneously without making it complicated. It would help the researchers to compare different urban centers to understand the similarities and differences in material stock consumption.
* Integrating material passports: Research should also be done on incorporating the building's material passports into estimating the material stock of the regions. The material passport is a set of data that describes the characteristics of the material in products that give them value for recovery and reuse [52]. Therefore, the total material stock can be estimated by combining the data from the material passports. Moreover, the material passport could be used to calculate future waste and pollution generated. Such research would further enhance the prospects of research in the construction material circular economy.
* Infrastructure material stock: Material stock estimation of the allied infrastructure has not been researched thoroughly. For instance, based upon the articles reviewed only [PERSON] et al. [15] estimated the material stock of the sewage pipes. However, other allied infrastructures such as walkways need to be researched further. Similarly, the top-down method for estimation of the road material stock has only been done by [PERSON] et al. [22]. Thus, there remains a gap to further research road material stock using the top-down methodology as the cross-section of the same type of roads is similar, ensuring that the material intensity is more accurately measured.
* Uncertainty in models: Material stock is estimated by making models of the research area. However, there remains a question on the accuracy of material intensity. In building construction, the material intensity of structural elements could be accurately estimated as they follow the industry trends. However, the finished items depend upon user choice increasing its uncertainty [17]. Therefore, there is a need to research into improving models that would have higher accuracy for the building finishes.