Pavement overlay is one of the most basic methods of the road major maintenance. However, applying of a new asphalt concrete overlay on the old pavement with cracks will lead to road cracking with a pattern similar to the existing ones [1–3]. These types of cracks are called reflective cracking. Considering the constant environmental conditions and the loading pattern, it can be stated that generally after implementing a new asphalt concrete overlay on the previous pavement, the new cracks will appear on the new overlay after a short time. Therefore, these structural defects will damage the new overlay [4–7]. According to the Texas Department of Transportation (TxDOT), the reflective crack is one of the most serious damages affecting the asphalt overlay and concrete pavements [8].
Regarding the nature of such defects, various opinions have been stated about their origin and conditions. These defects are also likely in asphalt overlays placed on the cement concrete slabs. These cracks are mainly created due to heat- or humidity-induced displacements in the concrete slab under the asphalt surface. Although the origin of cracking is not always the traffic loading, it may cause failures and cracks near the joints [9–11]. According to “Asphalt concrete modeling book” by Kim, when a pavement overlay is applied on the existing concrete pavement (semi-rigid structure), some cracks appear due to concrete pavement shrinkage, thermal displacements, or lateral displacements in concrete pavement. Generally, the asphalt overlay layers cannot tolerate high displacements without the damage and cracking. Therefore, the cracks from the joints existing in the concrete pavement or in the old asphalt layer spread out gradually with the destruction of the new asphalt overlay and reach to the overlay surface along its thickness (reflective cracking) [12]. Todays, various methods are available to prevent or delay the occurrence of reflective cracks. Overall, these methods can be classified into the following three categories: 1) Reinforcing the overlay, 2) Reducing the stress through installing stress absorbing membrane interlayer (SAMI), and 3) Repairing and restoring the resistance of the underlying pavement to the time before the overlay was applied.
Prevention of the stress concentration in the newly applied pavement (top layer) can significantly reduce the occurrence of reflective cracks. Therefore, the researchers have considered several methods to deal with the reflective cracking phenomenon; for example, using the asphalt with wide and non-uniform granulation, warm mix asphalt (WMA) with the low viscosity, embedding a modified SAMI, and fabrics interlayer that can reduce strains [13].
In this regard, the results of an extensive study on the different stress-absorbing interlayers by Zhang et al [Ref.???]. showed that the rubber asphalt can be considered as a suitable mixture for the stress-absorbing layer due to its high elasticity and resistance to fatigue. Also, the results showed that the interlayer shear resistance curve reaches to its maximum value when the asphalt application rate is 2.2 kg/m. According to the cracking data, the fatigue and rupture life of asphalt overlays with rubber stress absorbing interlayer on the old concrete pavement increased by 30 percent [14].
The pavement reinforcement and stress reduction on the pavement layers are the key functions of the geosynthetic materials in road construction projects. The most important advantages of using the geosynthetic materials in asphalt overlay are the stress and displacement reduction on the pavement structure. So, using the geosynthetic materials on the pavement can delay or control some types of cracks including reflective cracks as a stress-reducing layer [15]. Researchers have evaluated the feasibility of the geosynthetic materials’ effect in reducing and delaying the reflective cracks [16]. In addition, some geocomposites have been analyzed as an interlayer in finite element modeling (FEM) for the different designs of asphalt overlay, and their ability to reduce the reflective cracks has been evaluated. Based on the observations, as long as using the geocomposite as an intermediate layer, the asphalt pavement remains healthy without sustaining any damage. In other words, the crack does not pass through the pavement, and the energy generated at the crack tip is dissipated. This membrane serves as a protective shield against the crack tip [17]. In another research on the efficiency of geosynthetics using the 4-point bending test at 20℃, it was found that the composite samples with modified and reinforced asphalt overlays present the higher fatigue life performance than other samples even in heavier traffic loads [18]. The results of an experimental study by a group of transportation researchers showed that in all geocomposite-reinforced specimens, the permanent deformation resistance increased compared to the control samples. Besides, geocomposites reduce the energy required for the crack propagation in thin layer overlays by 3 times [19].
Another role of geosynthetics used in road maintenance is the pavement reinforcing. The geosynthetics, especially geogrids, increase the resistance of the pavement by changing the load distribution pattern and aggregate’s behavior against the load after being implemented in asphalt overlays. Using geogrids with the high tensile strength in pavements can be assumed as a network of rebars in reinforced concrete that prevent the crack propagation in the asphalt overlay [20, 21]. In an advanced laboratory investigation, Çelik et al. (2021) found that using asphalt overlay reduces the vertical displacement in concrete surfaces by 2 to 75%. Furthermore, applying geogrid reinforcement on the developed cracks reduces the strain in the bottom of the asphalt layer from 29.5–92.5%. Moreover, using geogrid on the joints instead of increasing asphalt layer thickness from 50 to 80 mm, decreases the strain, displacement, and the overall cost by 57.9 percent [22].
Furthermore, the deformation of geogrids in the pavement causes energy dissipation in the crack tip area, which means preventing the progressive destruction in the pavement properly [20, 21]. Regarding the excellent performance of geogrids in controlling the reflective pavement cracks, many studies have been recently conducted on the effect of different parameters on the pavement performance [23–26]. The geogrid overlay reinforcement can significantly reduce the crack propagation from the bottom to the top layer. It is noteworthy that the pavement reinforcement would be effective only when the geosynthetic modulus is more than the asphalt layer modulus. Also, it should sufficiently bond to the surrounding asphalt material to strengthen the asphalt overlay [4, 15]. In this regard, Kikwata and Muramatsu (1989) investigated the degree of connection and continuity of geosynthetics with the surrounding asphalt pavement. The results revealed that the geosynthetics hardness indicates the greatest effect in achieving proper overlay performance [27]. The pavement reinforcement can be accomplished using the geosynthetic layer either in the pavement’s lower layers (i.e., aggregate pavement layers such as the base and sub-base) or in its asphalt layer. In any case, to control the reflective cracks, it is necessary to implement the geosynthetic layer in the overlay layer [28]. The type of old cracked pavement (either asphalt or concrete), the location of the geogrid layer in the pavement, and the weather temperature of the pavement site are the most critical factors affecting the cracking rate [4].
Another important parameter of geosynthetics in controlling reflective cracks is the type of loading applied to the pavement complex. In this respect, some experiments have been conducted with the static and dynamic loadings by constant strain on the asphalt overlay reinforced with geogrid. Based on the obtained results, the hardness and bearing capacity of the asphalt overlay before cracking increase with geogrid installation. It should be mentioned that this increase for the sample under dynamic loading is much higher than the similar sample under the static loading [29]. Therefore, it can be concluded that the effect of using geosynthetics in cracking rate and increasing the hardness of the pavement layer under static loading (e.g., parking lots) is far less than areas under dynamic loading (e.g., highways and high traffic areas) [29]. Gonzalez-Torre et al. (2015) investigated the role of type-in geosynthetic in cracking behavior and reported that the geosynthetic elastic modulus is not the only determining factor in controlling the reflective cracks [30]. Finally, it can be said that the anti-reflective cracking systems can be developed by geosynthetics with a high elastic modulus (e.g., geogrid) between asphalt overlays.
1.1. Objective
In this research, the cyclic bending fatigue tests were performed to study the cracking rate in the asphalt overlay by considering the effect of temperature, loading frequency, and different geocomposites. Also, models for predicting the cracking rate based on the mentioned parameters were established using the statistical analyses. Furthermore, to simulate an old crack, a 1.2-mm wide crack was created in the bottom layer in all samples.