Chronological Trends and Source Types
Fig. 1 shows the yearly number of related publications in two decades. The publications are in the field of pavement and ash, which are briefly shown as “PAVE+ASH” in this research henceforth. Studies have increased from 17 in 2002 to 247 in 2021, which has grown significantly. A significant percentage (83.9%) is related to journal articles and conference papers. The 1778 publications studied involved 1032 journal articles (58.0%), 460 conference papers (25.9%), 238 series of books (13.4%), 37 trade journals (2.1%) and 11 books (0.6%).
Subject Categories
PAVE+ASH publications have been widely distributed in 21 areas. According to Fig. 2, the divisions of engineering (42%) and material science (17%) are conspicuous. Two 10%-portions are related to environmental science and earth and planetary sciences, and the following areas are social sciences (4%), physics and astronomy (3%), computer science (3%), and energy (3%). While the rest (Agricultural and Biological Sciences, Chemical Engineering, Business, Management and Accounting, Chemistry, Multidisciplinary, Economics, Econometrics and Finance, Medicine, Biochemistry, Genetics and Molecular Biology, Mathematics, Arts and Humanities, Immunology and Microbiology, Pharmacology, Toxicology and Pharmaceutics, and Decision Sciences) provided the residual 9%.
Countries
The studies have been published from 81 countries in the studied period, while twelve countries had only one, nine countries had two PAVE+ASH publications, and 109 publications had no author’s country information.
The top ten most effective countries in the field of PAVE+ASH are presented in Table 1. The United States, with 383 publications, ranked 1st, followed by India (370) and China (288). More than 75% (1338) of the PAVE+ASH publications are related to these ten countries.
Independent publications exhibit that all their authors are from the same country. In contrast, the cooperator publications identify individuals whose authors are not from a single country (Mostafa Hatami et al., 2021). According to Table 1, 76.8% of American’s, 93.0% of Indian’s, and 78.5% of Chinese’s were exclusively published from these countries. With 63.4% of Australian PAVE+ASH publications, Australia ranked 1st in cooperator publications.
Table 1 Top ten most productive countries
#
|
Country
|
Pavement-Ash publications
|
Independent publications
(%)
|
Cooperator publications
(%)
|
1
|
United States
|
383
|
294
|
76.8%
|
89
|
23.2%
|
2
|
India
|
370
|
344
|
93.0%
|
26
|
7.0%
|
3
|
China
|
288
|
226
|
78.5%
|
62
|
21.5%
|
4
|
Australia
|
82
|
30
|
36.6%
|
52
|
63.4%
|
5
|
Malaysia
|
71
|
33
|
46.5%
|
38
|
53.5%
|
6
|
Indonesia
|
61
|
46
|
75.4%
|
15
|
24.6%
|
7
|
Nigeria
|
61
|
39
|
63.9%
|
22
|
36.1%
|
8
|
United Kingdom
|
54
|
26
|
48.1%
|
28
|
51.9%
|
9
|
Canada
|
53
|
35
|
66.0%
|
18
|
34.0%
|
10
|
Thailand
|
47
|
21
|
44.7%
|
26
|
55.3%
|
As shown in Fig. 3, the nodes displayed in network visualization specified the countries (Wang et al. 2016b), while the nodes’ size demonstrated the activity of that country, and each link exposed a relation between two countries (Van Eck and Waltman 2019).
The United States, India, and China have contributed many publications in this field. The United States and China had the most significant number of cooperations with the other countries, with 32 and 24 links.
The United States was also the most effective country based on the number of citations (4890), followed by India (2507), China (2148), Australia (1920), Thailand (1389), Turkey (1237), and Canada (1036). Besides, regarding “total link strength” analysis, the United States (92), China (64), and Australia (52) have published the most number of PAVE+ASH publications.
Source Titles
Table 2 shows the ten most-productive sources with their respective impact factors, H-indexes, subject areas, countries, publishers, and the number of publications.
Impact Factor (IF) is the indicator to measure the journal's value (Amin and Mabe 2000). Also, H-index is specified as the h number of publications with h minimum number of citations each (Hirsch 2005). The specifications of the journals, conferences and book series shown in Table 2 have been extracted from “Scopus” and “SJR” (Scopus 2022; SCImago Journal Rank 2022).
As shown in Table 2, 575 publications (32.3% of all) are published in the ten sources, among which there are four journals, four book series, and two Conferences and Proceedings.
The United Kingdom with four, and the United States with three sources were the productive countries in this classification. All these ten sources involve engineering in their subject areas. At the same time, the American Society of Civil Engineers (ASCE), Trans Tech Publications, and Taylor and Francis Ltd. have been the most productive publisher in the field. In addition, the “Construction and Building Materials” ranked 1st in the number of publications (105), IF (6.141), and H-index (170), amongst the journals. Besides, “Geotechnical Special Publication” was the effective source in the conferences and Proceedings category, with 79 papers.
Table 2 The top ten most productive sources and specifications
#
|
Source
|
Number of Publications
|
Impact Factor
|
H-index
|
Subject area
|
Country
|
Publisher
|
1
|
Construction And Building Materials
|
105
|
6.141
|
170
|
Engineering Materials Science
|
United Kingdom
|
Elsevier Ltd.
|
2
|
Geotechnical Special Publication
|
79
|
Conferences and Proceedings
|
38
|
Earth and Planetary Sciences Engineering
|
United States
|
American Society of Civil Engineers (ASCE)
|
3
|
Journal Of Materials In Civil Engineering
|
75
|
2.169
|
103
|
Engineering Materials Science
|
United States
|
American Society of Civil Engineers (ASCE)
|
4
|
Transportation Research Record
|
68
|
Book Series
|
119
|
Engineering
|
United States
|
US National Research Council
|
5
|
Iop Conference Series Materials Science And Engineering
|
52
|
Conferences and Proceedings
|
44
|
Engineering Materials Science
|
United Kingdom
|
IOP Publishing Ltd.
|
6
|
Advanced Materials Research
|
49
|
Book Series
|
38
|
Engineering
|
Germany
|
Trans Tech Publications
|
7
|
Lecture Notes In Civil Engineering
|
49
|
Book Series
|
7
|
Engineering
|
Switzerland
|
Springer Singapore
|
8
|
Road Materials And Pavement Design
|
38
|
3.792
|
46
|
Engineering
|
United Kingdom
|
Taylor and Francis Ltd.
|
9
|
Applied Mechanics And Materials
|
30
|
Book Series
|
33
|
Engineering
|
Switzerland
|
Trans Tech Publications
|
10
|
International Journal Of Pavement Engineering
|
30
|
4.139
|
46
|
Engineering
|
United Kingdom
|
Taylor and Francis Ltd.
|
Affiliations
Table 3 shows the top ten most productive affiliations in the field of PAVE+ASH. More than 17% (307) of the PAVE+ASH publications are related to these ten affiliations. “Ministry of Education China” with 57 publications, was the most productive organization in the field of “PAVE+ASH,” followed by the University of Wisconsin-Madison (United States), Chang'an University (China), and Swinburne University of Technology (Australia), with 44, 40, and 32 publications, respectively. It was worth noting that there were four from China among the top ten affiliations, and three were from the United States.
Table 3 Top ten productive institutes
#
|
Affiliation
|
Number of publications
|
Country
|
1
|
Ministry of Education China
|
57
|
China
|
2
|
University of Wisconsin-Madison
|
44
|
United States
|
3
|
Chang'an University
|
40
|
China
|
4
|
Swinburne University of Technology
|
32
|
Australia
|
5
|
Universiti Teknologi Malaysia
|
24
|
Malaysia
|
6
|
Suranaree University of Technology
|
24
|
Thailand
|
7
|
Tongji University
|
24
|
China
|
8
|
Purdue University
|
21
|
United States
|
9
|
Wuhan University of Technology
|
21
|
China
|
10
|
Iowa State University
|
20
|
United States
|
Author Keywords
A total of 1797 author keywords during 2002-2021 were surveyed in this bibliometric research. 1362 (75.8%) keywords appeared only once, and 202 (11.2%) keywords occurred twice. Each node represents a specified author keyword. Also, the bigger the size of the nodes, the more the occurrence of the keywords (Wang et al. 2016b).
Depending on the software settings, each number of related keywords form a cluster that was characterized with an indicated color. Fig. 4 shows the studied keywords which are classified into six clusters. Each cluster accommodates at least 200 keywords, while the most significant cluster was cluster 1 with 494, and the smallest one was cluster 6 with 218 keywords.
Cluster 1 (red) was approximately related to the publications pointing strength and stiffness characteristics by covering keywords such as “resilient modulus,” “unconfined compressive strength,” “CBR,” and “California bearing ratio.” Cluster 2 (green) generally belonged to recycling studies, by keywords “recycling,” “cold recycling,” and “recycled concrete.” Some strength characteristics related keywords can be seen in cluster 3 (blue), such as “compressive strength,” “flexural strength,” “elastic modulus,” and “stiffness modulus.” “Durability,” “strength,” “mechanical properties,” “water absorption,” “porosity,” and “waste materials” could be found obviously among the most-repeated keywords of cluster 4 (yellow). Cluster 5 (Purple) involves “fly ash” with the most occurrence among the existing keywords of all groups, “pavement,” and “sustainability.” Cluster 6 (Pale blue) belongs to soil-related publications by covering keywords “black cotton soil,” “lateritic soil,” and “soil stabilization.”
The network visualization map of frequently used keywords in three time periods of 10 years (2002-2011 and 2012-2021) is shown in Fig. 5. Keywords with at least one occurrence are displayed on the maps in both periods. The keywords “pavement,” “pavements,” and “fly ash” were eliminated manually to improve the clarity of the maps. The minimum number of words in each cluster was considered to be 100 in the first period and 250 in the second period. 682 keywords are visible in four clusters in the first period, while 2334 keywords are visible in six clusters in the second period. In addition to the increase in the number of keywords, the increase in the number of clusters could also be considered. The keyword “compressive strength” ranks first in the second period with 67 repetitions, while “durability” ranks first in the first period with 19 repetitions. In contrast, “durability” ranks second in the second period with 56 repetitions, and “compressive strength” ranks second in the first period with 17 repetitions. Comparing the trend of using keywords in the two periods, the increase of “Stabilization,” “Sustainability,” and “microstructure” could be mentioned, which increased from 7, 3, and 2 occurrences in the period 2002-2011 to 54 and 29 and 24 occurrences in the period 2012-2021.
Regarding the growth of emerging keywords, “geopolymer,” “pervious concrete,” “reclaimed asphalt pavement,” and “lateritic soil” are noteworthy that none of which existed in the first period; while have reached 49, 25, 24, and 17 repetitions in the second period, respectively.
Pavement testing keywords have also grown significantly. The keywords “CBR,” “unconfined compressive strength,” “California bearing ratio,” and “UCS” were repeated 6, 4, 1, and 0 in the first period, and 44, 43, 38, and 19 repetitions in the second period. Finally, the reduction in the keyword “recycling” is considerable, from 19 to 15 repetitions.
On the other hand, by investigating the CSV output of VOSviewer software, the growth of some specific keywords could be significantly remarkable, representing the growth of related research in recent years. Among these keywords, “sustainability,” “environmental sustainability,” “life cycle assessment,” and “circular economy” could be mentioned strictly.