## 3 Results and Discussion
### Keywords and Trending Topics of the Research on Pavement Management  
As previously said, author keywords generalize the major attention of a research. Bibliometric method, through author keyword analysis in a specific period, has been developed in the last decade, and has proved to be a helpful method in revealing the research hotspots and discovering scientific research trends [5]. This time, in order to analyze, exclusively, the research trends in pavement management in the last years of studied period, where more papers were published, author keywords in the publications during the years 2012 and 2013 only have been assessed. The limitation about the years of publication was imposed to consider the state of art of the first years of the second decade of the 21 st century. This way, the focus on the trends of 2012 and 2013 can give clues for the research directions at present, providing a review less biased by previous references of the first decade of the century.  
Examination of author keyword in this study period revealed that 1.509 keywords were used. Among them, 1.126 (75%) keywords appeared only once, and 212 (14%) keywords appeared twice. Only 11% of all author keywords (171) were used more than three times. Table 7 shows the top 30 most frequently used author keywords appeared in articles of pavement management field during 2012 and 2013, with combinations of their plural forms, abbreviations and other semantic relations. For example, as all the keywords are related to pavements; keywords like \"deterioration\" and \"pavement deterioration\" have been considered together. Furthermore, expressions like \"deterioration models\" and \"deterioration modelling\" have been associated to the previous expressions, since all of them underline the same idea.  
Figure 2: The top 30 most frequently used keywords from 2000 to 2013.  
As was expected after using \"pavement management\" as keywords in the article title, abstract, and keywords fields of the search engine, \"pavement\" (and its plural form) was the most relevant keyword, as it appeared 161 times in 178 papers (90.4% of the total amount of articles), and \"pavement management\" (and \"management\") was selected as a keyword by 100 papers (56.2%). Between them, the expressions \"pavement management systems\" (and its singular and its abbreviation, PMS) occupied the 2 nd position in this rank, highlighting the importance of this set of systematic tools for assisting engineers in effectively and efficiently managing the road pavements. Nevertheless, other used keywords that were not in the top three helped to identify the research trends. Expressions like \"maintenance\" (4 th position, 63 times), \"optimization\" and \"optimization models\" (ranked in 6 th position), and \"deterioration\" (8 th) support the idea of the improvement of classical pavement management systems by incorporating new optimization strategies to ensure a better planning of the maintenance and administration of the network. The employment of \"pavement performance\" and related words (7 th position), and \"mathematical models\" (13 th), as keywords, shows that efforts were made to improve the modeling of the pavement deterioration process. Finally, despite the data collection advances, pavement indices still attracted research attention, due to the 5 th place of \"pavement condition\", or the presence of \"International Roughness Index\", a worldwide-utilized index in 20 th position.  
Based on these analyses, the research trend in the initial years of the 2010s in the field of pavement management can be divided in three categories: pavement data collection, pavement performance models, and pavement management systems. As stated before, with the aim of reviewing the last research tendencies in the mentioned period, publications from 2012 and 2013, which have oriented present research, are mainly highlighted. Therefore, the state-of-art of the last years of the studied time interval is more objectively described.