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Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test
Geotextiles and Geomembranes xxx (2015) 1e8
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Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test Sina Mirzapour Mounes a, *, Mohamed Rehan Karim a, Ali Khodaii b, Mohamad Hadi Almasi a a b
Centre for Transportation Research, Faculty of Engineering, Civil Engineering Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Civil and Environmental Engineering, Amirkabir University of Technology, 15914 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2014 Received in revised form 6 April 2015 Accepted 4 June 2015 Available online xxx
Permanent deformation (rutting) is one of the distresses that can adversely affect the bituminous surface of pavement structures, particularly in hot climates. The geosynthetics reinforcement of hot mix asphalt is one of the means to combat rutting. In this study, a dynamic creep test was performed on asphalt concrete samples reinforced with four different types of fiberglass grid as well as on unreinforced samples. The fiberglass grids used in this study contained two different sizes of grid openings and two tensile strengths, allowing us to test for the mesh size and tensile strength effects of the grids on the permanent deformation behavior of double layered asphalt concrete. In addition, we tested a recently developed creep curve model has been verified and used this to study the creep behavior of the samples in the primary and secondary regions of the creep curve, as well as determining the boundary point of the regions. The results suggest that not only grid tensile strength, but also grid mesh size is of great importance in combatting permanent deformation of fiberglass grid reinforced asphalt concrete within the conditions and grids used in this study. In a nutshell, higher tensile strength and/or smaller mesh size grids lead to overall better performance of grid reinforced samples. Moreover, great care must be taken when the creep curves are not reached in the tertiary region, and the creep rate must be taken into account to avoid any misinterpretation of the results. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Geosynthetics Permanent deformation Asphalt Dynamic creep test Creep curve model
1. Introduction 1.1. Overview A bituminous mixture applied to the surface or the base layer of a pavement structure serves to distribute the traffic load and prevent water from penetrating into underlying unbound layers (Epps et al., 2000). Due to applied traffic loading, there are many different types of distresses that can affect bituminous surface layers, including permanent deformation (rutting), and fatigue cracking. In recent years, because of increases in the volume of traffic and of heavy vehicles, rutting is one of the most frequent defects found in flexible pavements, particularly in hot climates. Rutting shows up as depressions formed in the wheel path in a pavement. It normally occurs when a permanent deformation of each layer in
* Corresponding author. Tel.: þ60 3 7967 5339; fax: þ60 3 7955 2182. E-mail address: [email protected] (S. Mirzapour Mounes).
the pavement structure accumulates under a repetitive traffic load (Tayfur et al., 2007). There are generally two modes of ruts that occur on pavements, compactive and plastic (Gabra and Horvli, 2006; Lee et al., 2010). Accumulation of residual strains in wearing course may cause serious problems, particularly through aquaplaning on wet pavements (Fwa et al., 2004; Sivilevi cius and Petkevicius, 2002; Verhaeghe et al., 2007). Thus, not only does pavement rutting lead to higher road maintenance costs, but it also increases the risk to human life through accidents caused by water accumulating in depressions (ruts) in pavements. Various laboratory testing methods have been developed to investigate the resistance to rutting of asphalt concrete. These include the static/dynamic creep test, wheel track test, and indirect tensile test. Monismith et al. (1975), quoted by Kalyoncuoglu and Tigdemir (2011), developed the dynamic creep test which is thought to be one the best methods to evaluate the resistance of asphalt concrete to permanent deformation. Furthermore, a report by the NCHRP (Cominsky et al., 1998), quoted by Kaloush and
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Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Witczak (2002), identified the dynamic creep test as having the potential to be utilized as a field quality control. One of the most important outputs of the dynamic creep test is the creep curve, which illustrates permanent deformation versus loading cycles. Since creep curves obtained by the dynamic creep test are used to assess the resistance to permanent deformation of asphalt concretes, not only is the behavior of each region of the creep curve of great importance, but the identification of the boundary points connecting the primary to the secondary region, and the secondary to the tertiary region, is also important. Analyzing the creep curves obtained from a dynamic creep test in this way can provide a better understanding of the resistance to permanent deformation of asphalt concrete. 1.2. Literature review During the past decade or so, there have been a lot of studies by various researchers into how to hinder rutting in pavements through geosynthetic reinforcement (Austin and Gilchrist, 1996; Collin et al., 1996; Laurinavi cius and Oginskas, 2006; Leshchinsky and Ling, 2012; Ling and Liu, 2001; Perkins, 1999; Thakur et al., 2012; Yang et al., 2012). Virgili et al. (2009) reported that the reinforcement of bound layers can be divided into three elements: fatigue life extension, the reduction of reflection cracking, and the reduction of permanent deformation. Geogrids are high-strength extruded sheets of polyethylene or polypropylene with holes punched to produce a regular, grid-like pattern. Geogrids are quite stiff compared to the fibers of geotextiles, and have a higher modulus (Appea, 1997). Various literature points to the fact that stiffer geogrids lead to better performance in pavements (Collin et al., 1996; Ling and Liu, 2001; Perkins, 1999). Past research further suggests that some geosynthetics, and particularly certain geogrids, have a positive influence on permanent deformation in asphaltic pavements. This influence is stronger when the geogrid reinforcement is laid at mid-depth of the asphalt concrete, rather than embedded at the bottom (Sobhan et al., 2005). Similarly Perkins (1999) reported that the closer the placement positions of the geosynthetic to the applied load, the higher the reinforcement effect. The elasticity modulus of asphalt concrete can be improved by incorporating materials with certain properties, such as grid reinforcement, within the asphalt mixture. There is evidence that the rutting depth of asphalt concrete depends on its modulus of elasticity, while the modulus of elasticity is in turn dependent on the type of geosynthetic material used (Laurinavi cius and Oginskas, 2006). Moreover, lateral tensile strains can be restricted through geosynthetics, which are stiff when under tension (Perkins and Ismeik, 1997). FEM calculations have shown that the mechanically restrained system generated by geogrids can hold back the movement of aggregates and increase the transverse binding force of an asphalt layer, which can contribute to increased resistance to rutting (Fei and Yang, 2009). Furthermore, the opportunity provided by grid-shaped geosynthetics for aggregates from the top and bottom layers to interlock should logically lead to greater friction between layers (Tutumluer et al., 2010). Researchers generally agree that geogrid reinforced asphalt concrete is more resistant to surface deformation than unreinforced concrete (Austin and Gilchrist, 1996; Bertuliene et al., 2011; Komatsu et al., 1998; Siriwardane et al., 2010; Sobhan et al., 2005). Yet very few researchers have provided a full-range comparison of the effects of the mesh size and tensile strength of such grids on the permanent deformation of bituminous systems. Tests of asphalt concrete plastic flow resistance suggest that durability increases when the size of grid openings (Komatsu et al., 1998). Similarly, Jenkins et al. (2004) observed slightly less rutting in grids
reinforced with smaller grid mesh sizes than those with larger mesh sizes. As quoted by Zhou et al. (2004), different mathematical models such as Barksdale's Semilog model (1972), Power-law models based on the Monismith model (1975), and Tseng and Lytton's model (1989), have been developed in order to fit the creep curve and estimate the flow number parameter in asphalt mixtures. For fitting and distinguishing between regions of the unmodified asphalt mixture creep curve, Zhou et al. (2004) proposed a three-stage model: a power model for the primary region, a linear model for the secondary, and an exponential model for the tertiary region. In other words, they modeled each region of the creep curve separately. At the same time, some other researchers believe that the logarithmic model simulates more accurately the primary region of the creep curves in SBS modified asphalt mixtures (Kalyoncuoglu and Tigdemir, 2011; Ahari et al., 2013). Ahari et al. (2013) developed a two-stage model for the primary and secondary regions of creep curves in SBS modified asphalt mixtures. They proposed two different approaches for modeling the primary and secondary regions of the creep curve as follows: Approach 1. Both the primary and secondary regions of the creep curve can be modeled simultaneously using the following logarithmic function:
3P ¼ a LnðXÞ þ b where: X: is loading cycle 3P: is accumulated permanent strain at loading cycle X a, b: are constants Then, in order to check if the developed logarithmic function fits well with both regions, the deviation errors of all the points, re calculated as below, must be less than or equal to 1%:
De ¼
3PðCalculatedÞ 3PðMeasuredÞ 3PðMeasuredÞ
100
where: De: is deviation error (%) Approach 2. In order to identify the boundary points of the primary and secondary regions of the creep curve, the following steps are taken: 1 Visual selection of loading cycles among the initial loading cycles of the secondary region. [It must be noted that this loading cycle is not necessarily the boundary point] 2 Removal of the loading cycles before the selected loading cycles, and plotting a new graph representing the approximate secondary linear region. 3 Fitting a linear model to the approximate secondary region and determining the model coefficients. 4 Calculating the accumulated permanent strain for all the loading cycles of the approximate secondary region, based on obtaining model coefficients. 5 Determining the deviation error (De) for all of the calculated accumulated permanent strains. 6 If simultaneously all the De(s) 1% / the criterion is met and the linear model is assumed to be representative.
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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If at least one of the De(s) > 1% / go to the next loading cycle and repeat steps 2e6 until the former criterion is met. 7 Fitting the logarithmic function resulting from approach 1 to the primary region. 8 Solving the set of simultaneous equations, called logarithmic and linear respectively, for the results of the primary and secondary regions, in order to identify the accumulated permanent strain and its corresponding loading cycle, where the primary region is connected to the secondary region. In the present study, comparisons were carried out on the dynamic creep curves of asphalt concrete samples reinforced by four types of fiberglass grids with two different tensile strengths and two different grid opening sizes, as well as on unreinforced samples, in order to assess their respective resistance to pavement deformation. As the maximum number of cycles applied in this experiment was 10,000 due to time limitations, none of the samples reached the tertiary region. We first tested out a mathematical model by Ahari et al. (2013), recently developed to model the primary and secondary regions of SBS modified asphalt concrete creep curves, to see if could model the creep curves of the materials in the current study. Using this, we then investigated the effects of combined and separate variations in both grid tensile strength and opening size, applied at the mid-depth of asphalt concrete, on the samples' resistance to permanent deformation. Finally, the behavior of the primary and secondary regions and their boundary points in the creep curves obtained for, the various types of samples were analyzed and compared. 2. Experimental program 2.1. Materials and sample preparation Crushed granite supplied from the Kajang region of Selangor state in Malaysia was used as aggregates in this study. Fig. 1 shows the aggregate gradation for the dense graded mixture utilized in this research, with a nominal maximum aggregate size of 9.5 mm in accordance with ASTM D3515 (2000). Bearing in mind the opening size of the fiberglass grids used in this study, selecting this aggregate gradation should allow the grids to provide better interlocking with the asphalt concrete. The applied bitumen was 80/100 penetration grade, and the optimum asphalt content of the dense graded asphalt mixture was determined to be 5% by mass of the total mixture, using the Marshall Test. The asphalt concrete slabs were compacted using a roller compactor in accordance with EN 1269733 (2003) in two lifts to the target air void of 8%, in order to
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simulate compaction at the time of field construction (Kandhal and Chakraborty, 1996). The layer thickness of each lift of the slabs was 40-mm, resulting in 80 mm thick compacted slabs. Four types of fiberglass grid manufactured by a European corporation, with two different tensile strengths and two different opening sizes, were employed as the reinforcing material applied at mid-depth in the reinforced specimens. This study compares two levels (high and low) of grid tensile strength and two levels (large and small) of grid opening size. It should be noted that the dimensions of the girds with large opening sizes differed slightly from those with small opening sizes; however, since the difference was very small, this study assumes that they were of identical size. The basic properties of all these reinforcements are presented in Table 1. The specimens to be cored and trimmed into cylindrical shapes had dimensions of 150-mm diameter and 60-mm height as recommended by EN 12697-25 (2005) so that the applied fiberglass grid was placed at mid depth of the sample. The average volumetric properties of the testing samples are illustrated in Table 2. The code for each sample in Table 2 includes whether the sample was reinforced or unreinforced, as well as the type of glass grid used for reinforcement in that particular sample. 2.2. Dynamic creep test The creep test was conducted using a uniaxial cyclic compression test with the confinement method, as recommended by EN 12697-25 (2005). However, since only three cores were attainable from each slab, three test repetitions were carried out so as to minimize any variability among replicates of one type of specimen. For that purpose, UTM-5P from IPC was used to apply a constant dynamic load at a certain periodic rate onto the cylindrical asphalt samples, and vertical deformation was measured using a Linear Variable Displacement Transducer (LVDTs). The servo pneumatic UTM-5P machine has integrated software that allows the operator to select several input parameters such as loading function, stress, frequency and seating stress Static pre-loading fora certain period of time can also be applied to the samples before cyclic loading is started. The loading jig is moreover located in an environmental chamber so as to control the testing temperature. In the present study, in accordance with EN 12697-25 (2005), the test was performed for both reinforced and control samples at 40 C, at a cyclic stress level of 100 kPa and a frequency of 0.5 Hz, with 1000 ms allocated for each cycle width and the corresponding rest period. For all the samples, a constant stress of 100 kPa was applied for up to 10,000 cycles due to time limitations. Moreover, a static preloading stress of 10 kPa was applied to all the samples for a period of 10 min prior to initiating the dynamic load, in order to
Fig. 1. Grading curve for crushed aggregate.
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Table 1 Basic properties of fiber glass grid applied. Identification
Glass grid A
Glass grid B
Glass grid AA
Glass grid BB
Tensile strength (kN/m) (MD XD)
115 115 þ/15 12.5 12.5
115 115 þ/15 25 25
115 215 þ/15 12.5 12.5
115 215 þ/15 25 19
2.5 4600 4600
2.5 4600 4600
2.5 4600 8600
2.5 4600 8600
Grid size (mm) Center to center of strand Tensile elongation (%) Secant stiffness (N/mm)
Table 2 Physical properties of tested samples. Sample code
Applied glass grid
Bulk specific gravity
Maximum specific gravity
Air void (%)
C R1 R2 R3 R4
e Glass Glass Glass Glass
2.226 2.225 2.225 2.225 2.222
2.425 2.425 2.425 2.425 2.425
8.23 8.27 8.25 8.23 8.36
grid grid grid grid
A B AA BB
ensure proper contact between the core surface and loading platen. Moreover, all the samples were conditioned at 40 C for about 4 h in a temperature chamber to make sure that they had reached the testing temperature.
3. Test results and discussion 3.1. Permanent strain comparison The permanent deformation potentials of asphalt concrete reinforced with four different types of geosynthetics were compared with each other, as well as with unreinforced (control) samples in order to identify which type had the highest resistance to permanent deformation. Considering that there are three replicates for each type of sample, the diagrams are derived from the average amount of parameters. Fig. 2 illustrates the creep curves of the samples tested in this study. Thereafter, Ahari's stepwise model was verified for the materials used in this study and then used to fit the creep curves and determine the connecting points between the primary and secondary phases. This method consisted of eight steps as shown in Section 1 (Ahari et al., 2013).
Fig. 2. Creep curves of tested mixtures, including reinforced and control samples.
The application of fiberglass grids at the mid-depth of the samples of asphalt concrete, notably increased their resistance to permanent deformation over that of the unreinforced samples. Moreover, as can be seen from Fig. 2, the samples reinforced by fiberglass grids with greater tensile strength and greater mesh size (R4) showed the lowest permanent deformation throughout all the cycles conducted in this study. Fig. 2 also clearly shows that the control (unreinforced) samples had substantially higher permanent deformation and accumulation rates of permanent deformation than the reinforced ones e due presumably to the tensile forces and lateral confinement provided by the grids. A further important finding is that the samples of identical mesh size reinforced by grids of lower tensile strength experienced more permanent deformation than those reinforced by grids with a higher tensile strength. Furthermore, the difference between the permanent deformations in samples reinforced with large mesh size was higher than for samples reinforced with smaller mesh size grids. In other words, increasing the tensile strength of grids with a large mesh size had a greater impact on their ability to resist permanent deformation than such increase in small mesh size grids within the test conditions performed in this study. In addition, by applying 10,000 loading cycles in this experiment, we found that larger permanent deformation occurred in samples with small rather than large mesh sizes, regardless of whether they had high or low tensile strength grids. Table 3 illustrates the results of a quantitative comparison between the measured permanent strain and grid tensile strength and grid opening size respectively, during the last loading cycle of the tests carried out in this study. In this table, reinforced samples with the same size of opening (mesh) are compared in terms of their grid tensile strength, and those with the same tensile strength in terms of their grid opening size. The improvements in resistance to permanent deformation shown in Table 3 were all determined based on the permanent deformation of the control samples. The clear conclusion from Table 3 is that, based on testing through 10,000 cycles, samples reinforced by grids with greater tensile strength and with larger mesh size achieve the best performance. In can also be seen from Table 3 that increasing the tensile strength in samples with small grid openings from R1 to R3 leads to a 4% improvement in permanent strain resistance by the last loading cycle. However, doing the same thing with grids with a large grid opening size from R2 to R4 leads to a 12% improvement, ie three times as much.
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Table 3 Comparison of permanent strains in the last cycle. Sample code
Tensile strength (kN/m) (MD XD)
Grid mesh size (mm) Center to center of strand
Measured last cycle permanent strain (m3)
Improved permanent strain resistance compared to control sample (%)
C R1
e 115 115*L þ/15 115 115*L þ/15 115 215*H þ/15 115 215*H þ/15
e 12.5 12.5**S
11,438 8756
0 31
8564
34
8495
35
7819
46
R2 R3 R4
25 25**OL 12.5 12.5**
S
25 19**OL
*L: Low level for grid tensile strength; *H: High level for grid tensile strength; **S: Small level for grid opening size; **OL: Large level for grid opening size.
Conversely, comparing the samples with grids of the same tensile strength, but different size of opening (mesh), it emerges that in samples with low tensile strength grids, an increase in grid opening size from small to large (R1 to R2) leads to a 3% improvement in permanent strain resistance: while doing the same (R3 to R4) with samples with grids of high tensile strength grids leads to a much larger, 11% increase in such resistance. In research studying the shear behavior of bi-layer asphalt concrete specimens, geogrid reinforced samples showed less interlayer shear resistance than unreinforced ones, even though some of the geogrid surface coatings were found to be able to maximize bonding between the interlayer and asphalt concrete (Ferrotti et al., 2012). It may be, therefore, that the effects of the smaller mesh size observed in the current testing condition of this study were due to reduced bonding between the lower and upper lift of the asphalt concrete, leading to the development of higher shear deformation. Comparing the samples reinforced by small mesh grids and the control ones, it should be noted that, although the bonding of two lifts was important in the reinforced samples, the reinforcing effect of the grid was much more significant than its effect on the bonding condition of the lifts; the upper and lower lifts in fact remained in full contact in the control samples. It can likewise be seen that the more the tensile strength increases, the greater the effect of mesh size on strength and resistance. In sum, if we look merely at the accumulated permanent strain up to the last cycle of the creep test conducted in this work, this leads to the conclusion can be drawn that not only increasing the tensile strength, but also enlarging the mesh size of glass grid reinforced asphalt concrete can increase its resistance to permanent deformation. However, it should be noted that the flow point was not reached in the performed test conditions, and that closer investigation of the creep curves after model fitting resulted in rather inferences from the ones drawn in this section.
3.2. Fitted models comparison Unfortunately, none of tested samples reached the third phase of the creep curve in the course of the 10,000 loading cycles conducted in this experiment. As a results, only the primary and secondary phases could be modeled; the two regions for which Ahari's model was developed. Table 4 presents the results of mathematical functions and estimated permanent strains at the boundary points at the last cycle for each phase of testing samples. Based on Figs. 3e5, and the coefficients of determination in Table 4 it can be seen that the fitted models, both for the logarithmic and linear regions, fit acceptably with the measured creep curves. Thus, it can be concluded that Ahari's model is suitable for modeling the primary and secondary regions of the creep curve for both the fiber glass grid reinforced samples and unreinforced hot mix asphalt samples. The slopes of both the primary and secondary regions are important, particularly the secondary region generally known as the creep rate. In Ahari's proposed model, a “linear logarithmic model” is utilized to model the primary region of the creep curve as shown bellow:
y ¼ a þ bðlnðxÞÞ b is the ratio of absolute change in y to the relative change in x. In other words, if x changes by 1%, then the absolute change in y is 0.01b unit (Thomas et al., 2001). However, the slope of the primary region is not as important as that of the secondary region. In our study, the slope of the fitted curve in the primary and secondary regions was determined for each individual sample type. The extend of improvement for each (in terms of smaller permanent strain accumulation rates) was then determined based on the control sample. These results are shown in Table 5. It is worth noting that the control samples in this table have their maximum slopes in both primary and secondary regions of the creep curves.
Table 4 Creep curve models based on Ahari's model and estimated critical values. Code First stage model
End of first stage
Second stage model
C R1 R2 R3 R4
3p ¼ 1523.945 Ln(N) 2653.456 R2 ¼ 0.9842 3p ¼ 1114.477 Ln(N) 1226.826 R2 ¼ 0.9927 3p ¼ 1075.835 Ln(N) 1099.156 R2 ¼ 0.9893 3p ¼ 1062.883 Ln(N) 843.671 R2 ¼ 0.9902 3p ¼ 954.627 Ln(N) 980.730 R2 ¼ 0.9918
7072
10,863
0
3945
8001
36
3223
7592
43
3001
7666
42
4847
7120
53
Last cycle Cycle (N) 3p(modeled) Improved 3p compared to control sample (%)
Cycle (N) 3p (modeled) Improved 3p compared to control sample (%) 3p ¼ 0.2155(N R2 ¼ 0.9913 3p ¼ 0.1336(N R2 ¼ 0.9864 3p ¼ 0.1515(N R2 ¼ 0.9957 3p ¼ 0.1291(N R2 ¼ 0.9824 3p ¼ 0.1436(N R2 ¼ 0.9868
7072) þ 9346.539 10,000
11,502
0
3945) þ 7474.168 10,000
8810
31
3223) þ 7103.268 10,000
8618
33
3001) þ 7278.914 10,000
8570
34
4847) þ 6424.536 10,000
7860
46
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Fig. 3. Measured and estimated permanent deformation for R1 sample.
estimated values were rather close to each other. We therefore, used the fitted curves from the measured values to find the turning point between the primary and secondary regions of the creep curves. When we only took into account the creep curves (as in Fig. 2), this pointed to the conclusion that enlarging the grid mesh size at the same level of tensile strength leads to better performance (resistance) within the used grids in this study. However, it can be seen from Table 5 that enlarging the mesh size has the effect of increasing the secondary region slope e something which does not emerge from just looking at the creep curves. In the secondary region, in which the mixture has reached to an optimum density level (Mehta et al., 2014), the presence of steeper slopes for grids with a larger mesh size but with the same tensile strength may be due to there being a lower number of grid junctions on the grid applied area. In other words, the number of stripes or threads of grid per unit area of the sample increases as the size of the opening (mesh) is reduced, leading to greater structural and dimensional stability through the higher number of grid junctions. This could possibly explain the smaller slope in the secondary region of the creep curve. In sum, looking at the effects of the mesh size of grids on a combination of permanent deformation and creep rate, the results suggest that larger mesh size grids performed better only in the initial stages of loading, whereas over the longer term, smaller mesh size grids will outperform large ones with same the tensile strength. These results showing small gird mesh sizes to perform better than large ones with the same tensile strengths are similar to those reported by some previous studies (Jenkins et al., 2004; Komatsu et al., 1998). 4. Conclusions
Fig. 4. Measured and estimated permanent deformation for R2 sample.
Fig. 5. Measured and estimated permanent deformation for C, R3, & R4 samples.
Fig. 6 is a one-to-one graph of the measured versus estimated values of permanent strain in the last cycle, including the intercept, slope and correlation coefficients. Comparing the measured and estimated permanent strain in the last cycle for each type of sample (Fig. 6) and the improvements in the reinforced samples in Tables 3 and 4 as well as in Figs. 3e5, it can be seen that the measured and
The reinforcement of asphalt concrete with fiberglass grids is one of the means to combat permanent deformation. Fiberglass grids are manufactured with different tensile strengths and aperture (mesh) sizes. In this study, an attempt was made to study the effects of fiber glass grids with different tensile strengths and mesh sizes, applied at the mid-depth of bi-layer asphalt concrete samples, on the resistance of these samples to permanent deformation. Our results suggest that fiberglass grid reinforcement is remarkably effective in lowering the permanent deformation of asphalt concrete, probably due to the tensile forces and lateral confinement provided by such grids. Secondly, our study confirms that Ahari's creep curve model can be used with both fiberglass grid reinforced, and unreinforced hot mix asphalt. In the secondary region of the creep curves, in which the optimum density of the mixture is achieved, higher tensile strengths and smaller mesh size result in gentler slope (meaning a lower permanent strain accumulation rate). Another conclusion from our results is that increasing the tensile strength of a fiberglass grid can lead to a reduction in permanent strain, depending on the type of grid used. Moreover, not only is the tensile strength of a fiberglass gird effective in increasing the resistance of asphalt concrete to permanent deformation, but the mesh size of the grid is also of considerable importance. Our results suggests that, of the grid mesh sizes used in this study, larger mesh size fiberglass grids perform better than small ones only in the initial stages of loading. This is because enlarging the mesh size causes the creep rate to increase which eventually, in the longer terms, outweighs the smaller deformation achieved in the first stages of the creep curve. In conclusion, the range of experiments we carried out suggest that smaller mesh sizes should provide more resistance to permanent deformation in the long run. This could be due to the
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Table 5 Slope comparison of primary and secondary regions. Code
C R1 R2 R3 R4
Tensile strength (kN/m) (MD XD)
Grid mesh size (mm) Center to center of strand
Primary region Slope
Improved slope compared to control sample (%)
Secondary region Slope
Improved slope compared to control sample (%)
e 115 115*L þ/15 115 115*L þ/15 115 215*H þ/15 115 215*H þ/15
e 12.5 12.5**S
1523.9 1114.5
0 37
0.2155 0.1336
0 61
25 25**OL
1075.8
42
0.1515
42
12.5 12.5**S
1062.9
43
0.1291
67
25 19**OL
954.6
60
0.1436
50
*L: Low level for grid tensile strength; *H: High level for grid tensile strength; **S: Small level for grid opening size; **OL: Large level for grid opening size.
Fig. 6. One-to one graph of measured vs. estimated permanent strain of last cycle.
greater number of fibers per unit width in such smaller meshes than in the larger mesh geogrids. This finding that the best resistance to permanent deformation can be achieved by asphalt concrete reinforced grids with greater tensile strength but also with smaller mesh sizes is in line with what previous researchers have reported. Finally, our study shows that interpreting creep curves without creep rate consideration can be misleading when the tertiary region of creep curves is not achieved in tests. Further research on other types of asphalt mixtures, reinforced with other types of grids, at various depths and under other testing conditions, is recommended. Acknowledgment The authors would like to acknowledge the Ministry of Higher Education of Malaysia for their financial support under grant number FP021/2011A. References Ahari, A.S., Forough, S.A., Khodaii, A., Moghadas Nejad, F., 2013. Modeling the Primary and the Secondary Regions of Creep Curves for SBS Modified Asphalt Mixtures under Dry and Wet Conditions. J. Mater. Civil Eng. 26 (5), 904e911. Appea, A.K., 1997. In-Situ Behavior of Geosynthetically Stabilized Flexible Pavement. Polytechnic Institute and State University, Virginia. ASTM, D3515-00, 2000. Standard Specification for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures. ASTM International. Austin, R., Gilchrist, A., 1996. Enhanced performance of asphalt pavements using geocomposites. Geotext. Geomembr. 14 (3), 175e186.
Barksdale, R.D., 1972. Laboratory Evaluation of Rutting in Base Course Materials. Paper presented at the Third International Conference on the Structural Design of Asphalt Pavements, Grosvenor House, Park Lane, London, England, Sept. 11e15, 1972. Bertuliene, L., Oginskas, R., Bulevicius, M., 2011. Research of Rut Depth in Asphalt Pavements Reinforced with Geosynthetic Materials. Paper presented at the 8th International Conference Environmental Engineering, Lithuania. Collin, J., Kinney, T., Fu, X., 1996. Full scale highway load test of flexible pavement systems with geogrid reinforced base courses. Geosynth. Int. 3 (4), 537e549. Cominsky, R., Killingsworth, B., Anderson, R., Anderson, D., Crockford, W., 1998. Quality Control and Acceptance of Superpave-Designed Hot Mix Asphalt. NCHRP Report 409. EN, C., 12697-33, 2003. Bituminous Mixtures. Test Methods for Hot Mix Asphalt. Part 33: Specimen Prepared by Roller Compactor. EN, C., 12697-25, 2005. Bituminous Mixtures e Test Methods for Hot Mix Asphalt Part 25: Cyclic Compression Test. Epps, A., Harvey, J.T., Kim, Y.R., Roque, R., 2000. Structural Requirements of Bituminous Paving Mixtures. Transportation in the New Millennium. Fei, Y., Yang, Y., 2009. Fem analysis on geogrid reinforced asphalt concrete pavement. Geosynth. Civ. Environ. Eng. 677e682. Ferrotti, G., Canestrari, F., Pasquini, E., Virgili, A., 2012. Experimental evaluation of the influence of surface coating on fiberglass geogrid performance in asphalt pavements. Geotext. Geomembr. 34 (0), 11e18. http://dx.doi.org/10.1016/j. geotexmem.2012.02.011. Fwa, T., Tan, S., Zhu, L., 2004. Rutting prediction of asphalt pavement layer using cef model. J. Transp. Eng. 130 (5), 675e683. Gabra, R., Horvli, I., 2006. Simplified Testing Method for Evaluation of Asphalt Mixtures for Their Susceptibility to Permanent Deformation. Jenkins, K.J., Dennison, P., Ebels, L.J., Mullins, L.S., 2004. 3-D Polymer Grid Reinforcement of Asphalt for Rut Resistance. Paper presented at the Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04), Southern Africa. Kaloush, K., Witczak, M., 2002. Tertiary flow characteristics of asphalt mixtures (with discussion and closure). J. Assoc. Asphalt Paving Technol. 71, 248e280. Kalyoncuoglu, S., Tigdemir, M., 2011. A model for dynamic creep evaluation of SBS modified HMA mixtures. Constr. Build. Mater. 25 (2), 859e866. Kandhal, P.S., Chakraborty, S., 1996. Evaluation of Voids in the Mineral Aggregate for HMA Paving Mixtures. Paper presented at the Proceedings of the Annual Conference-Canadian Technical Asphalt Association. Komatsu, T., Kikuta, H., Tuji, Y., Muramatsu, E., 1998. Durability assessment of geogrid-reinforced asphalt concrete. Geotext. Geomembr. 16 (5), 257e271. Laurinavi cius, A., Oginskas, R., 2006. Experimental research on the development of rutting in asphalt concrete pavements reinforced with geosynthetic materials. J. Civ. Eng. Manag. 12 (4), 311e317. Lee, K.W., Gundapuneni, S.K., Singh, A., 2010. Effects of Asphalt Binder Grade on the Performance of Rhode Island Hot-Mix Asphalt. Leshchinsky, B., Ling, H., 2012. Effects of geocell confinement on strength and deformation behavior of gravel. J. Geotech. Geoenviron. Eng. 139 (2), 340e352. Ling, H.I., Liu, Z., 2001. Performance of geosynthetic-reinforced asphalt pavements. J. Geotech. Geoenviron. Eng. 127 (2), 177e184. Mehta, Y., Guercio, M.C., McCarthy, L., 2014. Determine Viscoelastic Mechanical Properties of Warm Mix Asphalt (WMA)-Reclaimed Asphalt Pavement (RAP) Mixes under High Stresses in Airfield Flexible Pavements and Its Impacts on Design Life. Monismith, C.L., Ogawa, N., Freeme, C.R., 1975. Permanent deformation characterization of subgrade soils due to repeated loading. Transp. Res. Rec. 537, 1e17. Perkins, S., 1999. Mechanical response of geosynthetic-reinforced flexible pavements. Geosynth. Int. 6 (5), 347e382. Perkins, S., Ismeik, M., 1997. A synthesis and evaluation of geosynthetic-reinforced base layers in flexible pavements e part I. Geosynth. Int. 4 (6), 549e604. Siriwardane, H., Gondle, R., Kutuk, B., 2010. Analysis of flexible pavements reinforced with geogrids. Geotech. Geol. Eng. 28 (3), 287e297. Sivilevi cius, H., Petkevi cius, K., 2002. Regularities of defect development in the asphalt concrete road pavements. J. Civ. Eng. Manag. 8 (3), 206e213.
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Sobhan, K., Genduso, M., Tandon, V., 2005. Effects of Geosynthetic Reinforcement on the Propagation of Reflection Cracking and Accumulation of Permanent Deformation in Asphalt Overlays. Paper presented at the Third LACCEI International Latin American and Caribbean Conference for Engineering and Technology (LACCET2005) “Advances in Engineering and Technology: a Global Perspective”, Cartegena, Columbia. Tayfur, S., Ozen, H., Aksoy, A., 2007. Investigation of rutting performance of asphalt mixtures containing polymer modifiers. Constr. Build. Mater. 21 (2), 328e337. Thakur, J.K., Han, J., Pokharel, S.K., Parsons, R.L., 2012. Performance of geocellreinforced recycled asphalt pavement (RAP) bases over weak subgrade under cyclic plate loading. Geotext. Geomembr. 35, 14e24. Thomas, G.B., Finney, R.L., Weir, M.D., Giordano, F.R., 2001. Thomas' Calculus. Addison-Wesley. Tseng, K.-H., Lytton, R.L., 1989. Prediction of permanent deformation in flexible pavement materials. In: Implication of Aggregates in the Design, Construction, and Performance of Flexible Pavements, pp. 154e172. ASTM STP, 1016.
Tutumluer, E., Huang, H., Bian, X., 2010. Geogrideaggregate interlock mechanism investigated through aggregate imaging-based discrete element modeling approach. Int. J. Geomech. 12 (4), 391e398. Verhaeghe, B., Myburgh, P., Denneman, E., 2007. Asphalt Rutting and Its Prevention. Paper presented at the Proceedings of the 9th Conference on Asphalt Pavements for Southern Africa (CAPSA'07), Gaborone, Botswana. Virgili, A., Canestrari, F., Grilli, A., Santagata, F., 2009. Repeated load test on bituminous systems reinforced by geosynthetics. Geotext. Geomembr. 27 (3), 187e195. Yang, X., Han, J., Pokharel, S.K., Manandhar, C., Parsons, R.L., Leshchinsky, D., Halahmi, I., 2012. Accelerated pavement testing of unpaved roads with geocellreinforced sand bases. Geotext. Geomembr. 32, 95e103. Zhou, F., Scullion, T., Sun, L., 2004. Verification and modeling of three-stage permanent deformation behavior of asphalt mixes. J. Transp. Eng. 130 (4), 486e494.
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Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test Sina Mirzapour Mounes a, *, Mohamed Rehan Karim a, Ali Khodaii b, Mohamad Hadi Almasi a a b
Centre for Transportation Research, Faculty of Engineering, Civil Engineering Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Civil and Environmental Engineering, Amirkabir University of Technology, 15914 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2014 Received in revised form 6 April 2015 Accepted 4 June 2015 Available online xxx
Permanent deformation (rutting) is one of the distresses that can adversely affect the bituminous surface of pavement structures, particularly in hot climates. The geosynthetics reinforcement of hot mix asphalt is one of the means to combat rutting. In this study, a dynamic creep test was performed on asphalt concrete samples reinforced with four different types of fiberglass grid as well as on unreinforced samples. The fiberglass grids used in this study contained two different sizes of grid openings and two tensile strengths, allowing us to test for the mesh size and tensile strength effects of the grids on the permanent deformation behavior of double layered asphalt concrete. In addition, we tested a recently developed creep curve model has been verified and used this to study the creep behavior of the samples in the primary and secondary regions of the creep curve, as well as determining the boundary point of the regions. The results suggest that not only grid tensile strength, but also grid mesh size is of great importance in combatting permanent deformation of fiberglass grid reinforced asphalt concrete within the conditions and grids used in this study. In a nutshell, higher tensile strength and/or smaller mesh size grids lead to overall better performance of grid reinforced samples. Moreover, great care must be taken when the creep curves are not reached in the tertiary region, and the creep rate must be taken into account to avoid any misinterpretation of the results. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Geosynthetics Permanent deformation Asphalt Dynamic creep test Creep curve model
1. Introduction 1.1. Overview A bituminous mixture applied to the surface or the base layer of a pavement structure serves to distribute the traffic load and prevent water from penetrating into underlying unbound layers (Epps et al., 2000). Due to applied traffic loading, there are many different types of distresses that can affect bituminous surface layers, including permanent deformation (rutting), and fatigue cracking. In recent years, because of increases in the volume of traffic and of heavy vehicles, rutting is one of the most frequent defects found in flexible pavements, particularly in hot climates. Rutting shows up as depressions formed in the wheel path in a pavement. It normally occurs when a permanent deformation of each layer in
* Corresponding author. Tel.: þ60 3 7967 5339; fax: þ60 3 7955 2182. E-mail address: [email protected] (S. Mirzapour Mounes).
the pavement structure accumulates under a repetitive traffic load (Tayfur et al., 2007). There are generally two modes of ruts that occur on pavements, compactive and plastic (Gabra and Horvli, 2006; Lee et al., 2010). Accumulation of residual strains in wearing course may cause serious problems, particularly through aquaplaning on wet pavements (Fwa et al., 2004; Sivilevi cius and Petkevicius, 2002; Verhaeghe et al., 2007). Thus, not only does pavement rutting lead to higher road maintenance costs, but it also increases the risk to human life through accidents caused by water accumulating in depressions (ruts) in pavements. Various laboratory testing methods have been developed to investigate the resistance to rutting of asphalt concrete. These include the static/dynamic creep test, wheel track test, and indirect tensile test. Monismith et al. (1975), quoted by Kalyoncuoglu and Tigdemir (2011), developed the dynamic creep test which is thought to be one the best methods to evaluate the resistance of asphalt concrete to permanent deformation. Furthermore, a report by the NCHRP (Cominsky et al., 1998), quoted by Kaloush and
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Witczak (2002), identified the dynamic creep test as having the potential to be utilized as a field quality control. One of the most important outputs of the dynamic creep test is the creep curve, which illustrates permanent deformation versus loading cycles. Since creep curves obtained by the dynamic creep test are used to assess the resistance to permanent deformation of asphalt concretes, not only is the behavior of each region of the creep curve of great importance, but the identification of the boundary points connecting the primary to the secondary region, and the secondary to the tertiary region, is also important. Analyzing the creep curves obtained from a dynamic creep test in this way can provide a better understanding of the resistance to permanent deformation of asphalt concrete. 1.2. Literature review During the past decade or so, there have been a lot of studies by various researchers into how to hinder rutting in pavements through geosynthetic reinforcement (Austin and Gilchrist, 1996; Collin et al., 1996; Laurinavi cius and Oginskas, 2006; Leshchinsky and Ling, 2012; Ling and Liu, 2001; Perkins, 1999; Thakur et al., 2012; Yang et al., 2012). Virgili et al. (2009) reported that the reinforcement of bound layers can be divided into three elements: fatigue life extension, the reduction of reflection cracking, and the reduction of permanent deformation. Geogrids are high-strength extruded sheets of polyethylene or polypropylene with holes punched to produce a regular, grid-like pattern. Geogrids are quite stiff compared to the fibers of geotextiles, and have a higher modulus (Appea, 1997). Various literature points to the fact that stiffer geogrids lead to better performance in pavements (Collin et al., 1996; Ling and Liu, 2001; Perkins, 1999). Past research further suggests that some geosynthetics, and particularly certain geogrids, have a positive influence on permanent deformation in asphaltic pavements. This influence is stronger when the geogrid reinforcement is laid at mid-depth of the asphalt concrete, rather than embedded at the bottom (Sobhan et al., 2005). Similarly Perkins (1999) reported that the closer the placement positions of the geosynthetic to the applied load, the higher the reinforcement effect. The elasticity modulus of asphalt concrete can be improved by incorporating materials with certain properties, such as grid reinforcement, within the asphalt mixture. There is evidence that the rutting depth of asphalt concrete depends on its modulus of elasticity, while the modulus of elasticity is in turn dependent on the type of geosynthetic material used (Laurinavi cius and Oginskas, 2006). Moreover, lateral tensile strains can be restricted through geosynthetics, which are stiff when under tension (Perkins and Ismeik, 1997). FEM calculations have shown that the mechanically restrained system generated by geogrids can hold back the movement of aggregates and increase the transverse binding force of an asphalt layer, which can contribute to increased resistance to rutting (Fei and Yang, 2009). Furthermore, the opportunity provided by grid-shaped geosynthetics for aggregates from the top and bottom layers to interlock should logically lead to greater friction between layers (Tutumluer et al., 2010). Researchers generally agree that geogrid reinforced asphalt concrete is more resistant to surface deformation than unreinforced concrete (Austin and Gilchrist, 1996; Bertuliene et al., 2011; Komatsu et al., 1998; Siriwardane et al., 2010; Sobhan et al., 2005). Yet very few researchers have provided a full-range comparison of the effects of the mesh size and tensile strength of such grids on the permanent deformation of bituminous systems. Tests of asphalt concrete plastic flow resistance suggest that durability increases when the size of grid openings (Komatsu et al., 1998). Similarly, Jenkins et al. (2004) observed slightly less rutting in grids
reinforced with smaller grid mesh sizes than those with larger mesh sizes. As quoted by Zhou et al. (2004), different mathematical models such as Barksdale's Semilog model (1972), Power-law models based on the Monismith model (1975), and Tseng and Lytton's model (1989), have been developed in order to fit the creep curve and estimate the flow number parameter in asphalt mixtures. For fitting and distinguishing between regions of the unmodified asphalt mixture creep curve, Zhou et al. (2004) proposed a three-stage model: a power model for the primary region, a linear model for the secondary, and an exponential model for the tertiary region. In other words, they modeled each region of the creep curve separately. At the same time, some other researchers believe that the logarithmic model simulates more accurately the primary region of the creep curves in SBS modified asphalt mixtures (Kalyoncuoglu and Tigdemir, 2011; Ahari et al., 2013). Ahari et al. (2013) developed a two-stage model for the primary and secondary regions of creep curves in SBS modified asphalt mixtures. They proposed two different approaches for modeling the primary and secondary regions of the creep curve as follows: Approach 1. Both the primary and secondary regions of the creep curve can be modeled simultaneously using the following logarithmic function:
3P ¼ a LnðXÞ þ b where: X: is loading cycle 3P: is accumulated permanent strain at loading cycle X a, b: are constants Then, in order to check if the developed logarithmic function fits well with both regions, the deviation errors of all the points, re calculated as below, must be less than or equal to 1%:
De ¼
3PðCalculatedÞ 3PðMeasuredÞ 3PðMeasuredÞ
100
where: De: is deviation error (%) Approach 2. In order to identify the boundary points of the primary and secondary regions of the creep curve, the following steps are taken: 1 Visual selection of loading cycles among the initial loading cycles of the secondary region. [It must be noted that this loading cycle is not necessarily the boundary point] 2 Removal of the loading cycles before the selected loading cycles, and plotting a new graph representing the approximate secondary linear region. 3 Fitting a linear model to the approximate secondary region and determining the model coefficients. 4 Calculating the accumulated permanent strain for all the loading cycles of the approximate secondary region, based on obtaining model coefficients. 5 Determining the deviation error (De) for all of the calculated accumulated permanent strains. 6 If simultaneously all the De(s) 1% / the criterion is met and the linear model is assumed to be representative.
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If at least one of the De(s) > 1% / go to the next loading cycle and repeat steps 2e6 until the former criterion is met. 7 Fitting the logarithmic function resulting from approach 1 to the primary region. 8 Solving the set of simultaneous equations, called logarithmic and linear respectively, for the results of the primary and secondary regions, in order to identify the accumulated permanent strain and its corresponding loading cycle, where the primary region is connected to the secondary region. In the present study, comparisons were carried out on the dynamic creep curves of asphalt concrete samples reinforced by four types of fiberglass grids with two different tensile strengths and two different grid opening sizes, as well as on unreinforced samples, in order to assess their respective resistance to pavement deformation. As the maximum number of cycles applied in this experiment was 10,000 due to time limitations, none of the samples reached the tertiary region. We first tested out a mathematical model by Ahari et al. (2013), recently developed to model the primary and secondary regions of SBS modified asphalt concrete creep curves, to see if could model the creep curves of the materials in the current study. Using this, we then investigated the effects of combined and separate variations in both grid tensile strength and opening size, applied at the mid-depth of asphalt concrete, on the samples' resistance to permanent deformation. Finally, the behavior of the primary and secondary regions and their boundary points in the creep curves obtained for, the various types of samples were analyzed and compared. 2. Experimental program 2.1. Materials and sample preparation Crushed granite supplied from the Kajang region of Selangor state in Malaysia was used as aggregates in this study. Fig. 1 shows the aggregate gradation for the dense graded mixture utilized in this research, with a nominal maximum aggregate size of 9.5 mm in accordance with ASTM D3515 (2000). Bearing in mind the opening size of the fiberglass grids used in this study, selecting this aggregate gradation should allow the grids to provide better interlocking with the asphalt concrete. The applied bitumen was 80/100 penetration grade, and the optimum asphalt content of the dense graded asphalt mixture was determined to be 5% by mass of the total mixture, using the Marshall Test. The asphalt concrete slabs were compacted using a roller compactor in accordance with EN 1269733 (2003) in two lifts to the target air void of 8%, in order to
3
simulate compaction at the time of field construction (Kandhal and Chakraborty, 1996). The layer thickness of each lift of the slabs was 40-mm, resulting in 80 mm thick compacted slabs. Four types of fiberglass grid manufactured by a European corporation, with two different tensile strengths and two different opening sizes, were employed as the reinforcing material applied at mid-depth in the reinforced specimens. This study compares two levels (high and low) of grid tensile strength and two levels (large and small) of grid opening size. It should be noted that the dimensions of the girds with large opening sizes differed slightly from those with small opening sizes; however, since the difference was very small, this study assumes that they were of identical size. The basic properties of all these reinforcements are presented in Table 1. The specimens to be cored and trimmed into cylindrical shapes had dimensions of 150-mm diameter and 60-mm height as recommended by EN 12697-25 (2005) so that the applied fiberglass grid was placed at mid depth of the sample. The average volumetric properties of the testing samples are illustrated in Table 2. The code for each sample in Table 2 includes whether the sample was reinforced or unreinforced, as well as the type of glass grid used for reinforcement in that particular sample. 2.2. Dynamic creep test The creep test was conducted using a uniaxial cyclic compression test with the confinement method, as recommended by EN 12697-25 (2005). However, since only three cores were attainable from each slab, three test repetitions were carried out so as to minimize any variability among replicates of one type of specimen. For that purpose, UTM-5P from IPC was used to apply a constant dynamic load at a certain periodic rate onto the cylindrical asphalt samples, and vertical deformation was measured using a Linear Variable Displacement Transducer (LVDTs). The servo pneumatic UTM-5P machine has integrated software that allows the operator to select several input parameters such as loading function, stress, frequency and seating stress Static pre-loading fora certain period of time can also be applied to the samples before cyclic loading is started. The loading jig is moreover located in an environmental chamber so as to control the testing temperature. In the present study, in accordance with EN 12697-25 (2005), the test was performed for both reinforced and control samples at 40 C, at a cyclic stress level of 100 kPa and a frequency of 0.5 Hz, with 1000 ms allocated for each cycle width and the corresponding rest period. For all the samples, a constant stress of 100 kPa was applied for up to 10,000 cycles due to time limitations. Moreover, a static preloading stress of 10 kPa was applied to all the samples for a period of 10 min prior to initiating the dynamic load, in order to
Fig. 1. Grading curve for crushed aggregate.
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Table 1 Basic properties of fiber glass grid applied. Identification
Glass grid A
Glass grid B
Glass grid AA
Glass grid BB
Tensile strength (kN/m) (MD XD)
115 115 þ/15 12.5 12.5
115 115 þ/15 25 25
115 215 þ/15 12.5 12.5
115 215 þ/15 25 19
2.5 4600 4600
2.5 4600 4600
2.5 4600 8600
2.5 4600 8600
Grid size (mm) Center to center of strand Tensile elongation (%) Secant stiffness (N/mm)
Table 2 Physical properties of tested samples. Sample code
Applied glass grid
Bulk specific gravity
Maximum specific gravity
Air void (%)
C R1 R2 R3 R4
e Glass Glass Glass Glass
2.226 2.225 2.225 2.225 2.222
2.425 2.425 2.425 2.425 2.425
8.23 8.27 8.25 8.23 8.36
grid grid grid grid
A B AA BB
ensure proper contact between the core surface and loading platen. Moreover, all the samples were conditioned at 40 C for about 4 h in a temperature chamber to make sure that they had reached the testing temperature.
3. Test results and discussion 3.1. Permanent strain comparison The permanent deformation potentials of asphalt concrete reinforced with four different types of geosynthetics were compared with each other, as well as with unreinforced (control) samples in order to identify which type had the highest resistance to permanent deformation. Considering that there are three replicates for each type of sample, the diagrams are derived from the average amount of parameters. Fig. 2 illustrates the creep curves of the samples tested in this study. Thereafter, Ahari's stepwise model was verified for the materials used in this study and then used to fit the creep curves and determine the connecting points between the primary and secondary phases. This method consisted of eight steps as shown in Section 1 (Ahari et al., 2013).
Fig. 2. Creep curves of tested mixtures, including reinforced and control samples.
The application of fiberglass grids at the mid-depth of the samples of asphalt concrete, notably increased their resistance to permanent deformation over that of the unreinforced samples. Moreover, as can be seen from Fig. 2, the samples reinforced by fiberglass grids with greater tensile strength and greater mesh size (R4) showed the lowest permanent deformation throughout all the cycles conducted in this study. Fig. 2 also clearly shows that the control (unreinforced) samples had substantially higher permanent deformation and accumulation rates of permanent deformation than the reinforced ones e due presumably to the tensile forces and lateral confinement provided by the grids. A further important finding is that the samples of identical mesh size reinforced by grids of lower tensile strength experienced more permanent deformation than those reinforced by grids with a higher tensile strength. Furthermore, the difference between the permanent deformations in samples reinforced with large mesh size was higher than for samples reinforced with smaller mesh size grids. In other words, increasing the tensile strength of grids with a large mesh size had a greater impact on their ability to resist permanent deformation than such increase in small mesh size grids within the test conditions performed in this study. In addition, by applying 10,000 loading cycles in this experiment, we found that larger permanent deformation occurred in samples with small rather than large mesh sizes, regardless of whether they had high or low tensile strength grids. Table 3 illustrates the results of a quantitative comparison between the measured permanent strain and grid tensile strength and grid opening size respectively, during the last loading cycle of the tests carried out in this study. In this table, reinforced samples with the same size of opening (mesh) are compared in terms of their grid tensile strength, and those with the same tensile strength in terms of their grid opening size. The improvements in resistance to permanent deformation shown in Table 3 were all determined based on the permanent deformation of the control samples. The clear conclusion from Table 3 is that, based on testing through 10,000 cycles, samples reinforced by grids with greater tensile strength and with larger mesh size achieve the best performance. In can also be seen from Table 3 that increasing the tensile strength in samples with small grid openings from R1 to R3 leads to a 4% improvement in permanent strain resistance by the last loading cycle. However, doing the same thing with grids with a large grid opening size from R2 to R4 leads to a 12% improvement, ie three times as much.
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Table 3 Comparison of permanent strains in the last cycle. Sample code
Tensile strength (kN/m) (MD XD)
Grid mesh size (mm) Center to center of strand
Measured last cycle permanent strain (m3)
Improved permanent strain resistance compared to control sample (%)
C R1
e 115 115*L þ/15 115 115*L þ/15 115 215*H þ/15 115 215*H þ/15
e 12.5 12.5**S
11,438 8756
0 31
8564
34
8495
35
7819
46
R2 R3 R4
25 25**OL 12.5 12.5**
S
25 19**OL
*L: Low level for grid tensile strength; *H: High level for grid tensile strength; **S: Small level for grid opening size; **OL: Large level for grid opening size.
Conversely, comparing the samples with grids of the same tensile strength, but different size of opening (mesh), it emerges that in samples with low tensile strength grids, an increase in grid opening size from small to large (R1 to R2) leads to a 3% improvement in permanent strain resistance: while doing the same (R3 to R4) with samples with grids of high tensile strength grids leads to a much larger, 11% increase in such resistance. In research studying the shear behavior of bi-layer asphalt concrete specimens, geogrid reinforced samples showed less interlayer shear resistance than unreinforced ones, even though some of the geogrid surface coatings were found to be able to maximize bonding between the interlayer and asphalt concrete (Ferrotti et al., 2012). It may be, therefore, that the effects of the smaller mesh size observed in the current testing condition of this study were due to reduced bonding between the lower and upper lift of the asphalt concrete, leading to the development of higher shear deformation. Comparing the samples reinforced by small mesh grids and the control ones, it should be noted that, although the bonding of two lifts was important in the reinforced samples, the reinforcing effect of the grid was much more significant than its effect on the bonding condition of the lifts; the upper and lower lifts in fact remained in full contact in the control samples. It can likewise be seen that the more the tensile strength increases, the greater the effect of mesh size on strength and resistance. In sum, if we look merely at the accumulated permanent strain up to the last cycle of the creep test conducted in this work, this leads to the conclusion can be drawn that not only increasing the tensile strength, but also enlarging the mesh size of glass grid reinforced asphalt concrete can increase its resistance to permanent deformation. However, it should be noted that the flow point was not reached in the performed test conditions, and that closer investigation of the creep curves after model fitting resulted in rather inferences from the ones drawn in this section.
3.2. Fitted models comparison Unfortunately, none of tested samples reached the third phase of the creep curve in the course of the 10,000 loading cycles conducted in this experiment. As a results, only the primary and secondary phases could be modeled; the two regions for which Ahari's model was developed. Table 4 presents the results of mathematical functions and estimated permanent strains at the boundary points at the last cycle for each phase of testing samples. Based on Figs. 3e5, and the coefficients of determination in Table 4 it can be seen that the fitted models, both for the logarithmic and linear regions, fit acceptably with the measured creep curves. Thus, it can be concluded that Ahari's model is suitable for modeling the primary and secondary regions of the creep curve for both the fiber glass grid reinforced samples and unreinforced hot mix asphalt samples. The slopes of both the primary and secondary regions are important, particularly the secondary region generally known as the creep rate. In Ahari's proposed model, a “linear logarithmic model” is utilized to model the primary region of the creep curve as shown bellow:
y ¼ a þ bðlnðxÞÞ b is the ratio of absolute change in y to the relative change in x. In other words, if x changes by 1%, then the absolute change in y is 0.01b unit (Thomas et al., 2001). However, the slope of the primary region is not as important as that of the secondary region. In our study, the slope of the fitted curve in the primary and secondary regions was determined for each individual sample type. The extend of improvement for each (in terms of smaller permanent strain accumulation rates) was then determined based on the control sample. These results are shown in Table 5. It is worth noting that the control samples in this table have their maximum slopes in both primary and secondary regions of the creep curves.
Table 4 Creep curve models based on Ahari's model and estimated critical values. Code First stage model
End of first stage
Second stage model
C R1 R2 R3 R4
3p ¼ 1523.945 Ln(N) 2653.456 R2 ¼ 0.9842 3p ¼ 1114.477 Ln(N) 1226.826 R2 ¼ 0.9927 3p ¼ 1075.835 Ln(N) 1099.156 R2 ¼ 0.9893 3p ¼ 1062.883 Ln(N) 843.671 R2 ¼ 0.9902 3p ¼ 954.627 Ln(N) 980.730 R2 ¼ 0.9918
7072
10,863
0
3945
8001
36
3223
7592
43
3001
7666
42
4847
7120
53
Last cycle Cycle (N) 3p(modeled) Improved 3p compared to control sample (%)
Cycle (N) 3p (modeled) Improved 3p compared to control sample (%) 3p ¼ 0.2155(N R2 ¼ 0.9913 3p ¼ 0.1336(N R2 ¼ 0.9864 3p ¼ 0.1515(N R2 ¼ 0.9957 3p ¼ 0.1291(N R2 ¼ 0.9824 3p ¼ 0.1436(N R2 ¼ 0.9868
7072) þ 9346.539 10,000
11,502
0
3945) þ 7474.168 10,000
8810
31
3223) þ 7103.268 10,000
8618
33
3001) þ 7278.914 10,000
8570
34
4847) þ 6424.536 10,000
7860
46
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Fig. 3. Measured and estimated permanent deformation for R1 sample.
estimated values were rather close to each other. We therefore, used the fitted curves from the measured values to find the turning point between the primary and secondary regions of the creep curves. When we only took into account the creep curves (as in Fig. 2), this pointed to the conclusion that enlarging the grid mesh size at the same level of tensile strength leads to better performance (resistance) within the used grids in this study. However, it can be seen from Table 5 that enlarging the mesh size has the effect of increasing the secondary region slope e something which does not emerge from just looking at the creep curves. In the secondary region, in which the mixture has reached to an optimum density level (Mehta et al., 2014), the presence of steeper slopes for grids with a larger mesh size but with the same tensile strength may be due to there being a lower number of grid junctions on the grid applied area. In other words, the number of stripes or threads of grid per unit area of the sample increases as the size of the opening (mesh) is reduced, leading to greater structural and dimensional stability through the higher number of grid junctions. This could possibly explain the smaller slope in the secondary region of the creep curve. In sum, looking at the effects of the mesh size of grids on a combination of permanent deformation and creep rate, the results suggest that larger mesh size grids performed better only in the initial stages of loading, whereas over the longer term, smaller mesh size grids will outperform large ones with same the tensile strength. These results showing small gird mesh sizes to perform better than large ones with the same tensile strengths are similar to those reported by some previous studies (Jenkins et al., 2004; Komatsu et al., 1998). 4. Conclusions
Fig. 4. Measured and estimated permanent deformation for R2 sample.
Fig. 5. Measured and estimated permanent deformation for C, R3, & R4 samples.
Fig. 6 is a one-to-one graph of the measured versus estimated values of permanent strain in the last cycle, including the intercept, slope and correlation coefficients. Comparing the measured and estimated permanent strain in the last cycle for each type of sample (Fig. 6) and the improvements in the reinforced samples in Tables 3 and 4 as well as in Figs. 3e5, it can be seen that the measured and
The reinforcement of asphalt concrete with fiberglass grids is one of the means to combat permanent deformation. Fiberglass grids are manufactured with different tensile strengths and aperture (mesh) sizes. In this study, an attempt was made to study the effects of fiber glass grids with different tensile strengths and mesh sizes, applied at the mid-depth of bi-layer asphalt concrete samples, on the resistance of these samples to permanent deformation. Our results suggest that fiberglass grid reinforcement is remarkably effective in lowering the permanent deformation of asphalt concrete, probably due to the tensile forces and lateral confinement provided by such grids. Secondly, our study confirms that Ahari's creep curve model can be used with both fiberglass grid reinforced, and unreinforced hot mix asphalt. In the secondary region of the creep curves, in which the optimum density of the mixture is achieved, higher tensile strengths and smaller mesh size result in gentler slope (meaning a lower permanent strain accumulation rate). Another conclusion from our results is that increasing the tensile strength of a fiberglass grid can lead to a reduction in permanent strain, depending on the type of grid used. Moreover, not only is the tensile strength of a fiberglass gird effective in increasing the resistance of asphalt concrete to permanent deformation, but the mesh size of the grid is also of considerable importance. Our results suggests that, of the grid mesh sizes used in this study, larger mesh size fiberglass grids perform better than small ones only in the initial stages of loading. This is because enlarging the mesh size causes the creep rate to increase which eventually, in the longer terms, outweighs the smaller deformation achieved in the first stages of the creep curve. In conclusion, the range of experiments we carried out suggest that smaller mesh sizes should provide more resistance to permanent deformation in the long run. This could be due to the
Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003
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Table 5 Slope comparison of primary and secondary regions. Code
C R1 R2 R3 R4
Tensile strength (kN/m) (MD XD)
Grid mesh size (mm) Center to center of strand
Primary region Slope
Improved slope compared to control sample (%)
Secondary region Slope
Improved slope compared to control sample (%)
e 115 115*L þ/15 115 115*L þ/15 115 215*H þ/15 115 215*H þ/15
e 12.5 12.5**S
1523.9 1114.5
0 37
0.2155 0.1336
0 61
25 25**OL
1075.8
42
0.1515
42
12.5 12.5**S
1062.9
43
0.1291
67
25 19**OL
954.6
60
0.1436
50
*L: Low level for grid tensile strength; *H: High level for grid tensile strength; **S: Small level for grid opening size; **OL: Large level for grid opening size.
Fig. 6. One-to one graph of measured vs. estimated permanent strain of last cycle.
greater number of fibers per unit width in such smaller meshes than in the larger mesh geogrids. This finding that the best resistance to permanent deformation can be achieved by asphalt concrete reinforced grids with greater tensile strength but also with smaller mesh sizes is in line with what previous researchers have reported. Finally, our study shows that interpreting creep curves without creep rate consideration can be misleading when the tertiary region of creep curves is not achieved in tests. Further research on other types of asphalt mixtures, reinforced with other types of grids, at various depths and under other testing conditions, is recommended. Acknowledgment The authors would like to acknowledge the Ministry of Higher Education of Malaysia for their financial support under grant number FP021/2011A. References Ahari, A.S., Forough, S.A., Khodaii, A., Moghadas Nejad, F., 2013. Modeling the Primary and the Secondary Regions of Creep Curves for SBS Modified Asphalt Mixtures under Dry and Wet Conditions. J. Mater. Civil Eng. 26 (5), 904e911. Appea, A.K., 1997. In-Situ Behavior of Geosynthetically Stabilized Flexible Pavement. Polytechnic Institute and State University, Virginia. ASTM, D3515-00, 2000. Standard Specification for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures. ASTM International. Austin, R., Gilchrist, A., 1996. Enhanced performance of asphalt pavements using geocomposites. Geotext. Geomembr. 14 (3), 175e186.
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Please cite this article in press as: Mirzapour Mounes, S., et al., Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.06.003