Two kinds of asphalt pavement with thick asphalt layers were used to construct two samples. In structure I, a semirigid base and graded crushed stone subbase were used. In structure II, a granular base and semirigid subbase layer were used. Responses of the two structures under traffic loads were measured using optical fiber sensors, and the differences between theoretical model results and field measurements were analyzed. Field measurements show that vertical compressive stress in structure I is larger than that in structure II. The maximum tensile strain of the asphalt layer is located at the bottom of the AC-25C layer in structure I and at the bottom of the AC-25F layer in structure II. The latter is significantly larger than the former, indicating the possibility of fatigue cracking induced by traffic load is higher in structure II. The measured tensile horizontal strain at the bottom of the semirigid layer is relatively low (
Road paving materials and their mechanical properties are complicated. Ideal theory analysis cannot accurately reflect the true pavement response. So, theoretical calculation results should be revised to agree with pavement response data. In 1989, Mendez et al. proposed embedding fiber grating sensors (FBGs) in reinforced concrete to measure the status of the internal structure [
In recent years, the mechanics-empirical method has been widely accepted for its connection between field measurements, theoretical calculation results, and pavement distresses. In the theoretical method, road material mechanical parameters must be tested and input into the theoretical model. Mechanical parameters have an important effect on the accuracy of the results [
Real pavement responses can be obtained directly in the field, but a significant amount of manpower and cost is required. On the other hand, theoretical calculations can be carried out easily by computers but are less accurate due to assumptions in the theoretical model. In this study, two types of asphalt pavement with thick asphalt layers are proposed and constructed. Real stress and strain responses in the pavement layers are tested, and the difference between the real responses and theoretical simulation results is analyzed.
In many countries, the major materials used in flexible pavement include hot-mix asphalt (HMA), asphalt stabilized base, cement-treated base (CTB), other chemically treated materials (e.g., lime-fly ash, soil cement, lime-stabilized soils, etc.), and unbound aggregate base/subbase. In China, cement-stabilized material is widely used as the base and subbase layers. However, fatigue and reflective fracturing are inevitable if the chemically treated materials are used directly below asphalt layers. Unbound aggregate can restrain the reflective cracking progress, and thicker asphalt layers can also delay crack propagation within the asphalt layers.
Based on the above considerations, the pavement structures illustrated in Figures
Test road pavement structure I.
Test road pavement structure II.
FBGs have been used to measure pavement structure response in recent years [ According to the transverse distribution of vehicles, sensors should be placed underneath the wander of the traffic load to ensure the maximum pavement response is recorded. Two or more sensors are required at the same position to verify the data. If the data from the two sensors are significantly different, further analysis must be conducted. The distance between structure I and structure II should be as small as possible to link the transmission wire to a common point (Figure
FBG layout.
All sensors and transmission wires must be protected during pavement construction. Grooves should be cut in the pavement first; then the FBG and wires are placed in the groove and covered with road material to ensure that the sensors and wires are protected (Figures
The instrument and landfill.
Protecting the FBG.
Three different types of sensors are used: compressive stress sensors, strain sensors, and temperature sensors (Table
FBG layout.
Test channel | Sensor number | Initial wavelength (nm) | Sensor type | Position | Structure type |
---|---|---|---|---|---|
Channel one | 1-YB-1/1-YB-2 | 1561.28/1562.65 | Strain | Bottom of AC-25F | Structure I |
Channel two | 2-YB-1/2-YB-2 | 1557.507/1558.91 | Strain | Bottom of AC-25C | |
2-T-1 | 1529.092 | Temperature | Bottom of SMA | ||
Channel three | 3-CS-1/3-CS-2 | 1550.05/1548.63 | Pressure | Top of CTB layer | |
3-CS-3/3-CS-4 | 1562.994/1564.31 | Pressure | Top of granular base | ||
3-CS-5/1-CS-6 | 1557.768/1555.923 | Pressure | Bottom of AC-25C | ||
Channel four | 4-YB-1/4-YB-2 | 1539.382/1538.102 | Strain | Bottom of CTB | |
Channel five | 5-T-1 | 1552.834 | Temperature | Bottom of AC-25F | |
5-YB-1/5-YB-2 | 1557.569/1554.43 | Strain | Bottom of AC-25F | ||
5-T-2 | 1544.588 | Temperature | Bottom of AC-20C of AC25-C | ||
|
|||||
Channel six | 6-YB-1/6-YB-2 | 1541.914/1542.723 | Strain | Bottom of CTB | Structure II |
6-YB-3/6-YB-4 | 1545.33/1547.62 | Strain | Bottom of AC-25F | ||
6-YB-5/6-YB-6 | 1550.016/1548.012 | Strain | Bottom of AC-25F | ||
Channel seven | 7-CS-1/7-CS-2 | 1545.966/1544.827 | Pressure | Bottom of AC-25C | |
7-YB-1/7-YB-1 | 1557.584/1556.791 | Strain | Bottom of AC-25C | ||
7-T-1 | 1553.012 | Temperature | Bottom of SMA | ||
7-T-2 | 1544.609 | Temperature | Bottom of AC-25F | ||
Channel eight | 8-T-1 | 1544.693 | Temperature | Bottom of AC-20C | |
8-CS-1/8-CS-2 | 1537.951/1539.832 | Pressure | Top of granular base |
A heavily loaded truck was used as a test vehicle. The weight of the front and rear axles was 7.46 tons and 23.7 tons, respectively. Before the field test begins, a mark should be made on the road surface over the FBGs to ensure that the test vehicle passes directly over the FBGs. The test truck had a uniform speed of 60 km/h. A demodulator with a frequency of 100 Hz was used to collect wavelength data every 0.01 s. Pavement response can be calculated using the FBG wavelength variation using formulas (
The vertical compressive stress vs. time for structures I and II are plotted in Figures
Vertical compressive stress at the top of AC-25F. (a) Structure I. (b) Structure II.
Vertical compressive stress at the top of the granular layer. (a) Structure I. (b) Structure II.
The compressive vertical stress in structure I is larger than that in structure II at the same position. For instance, the peak vertical compressive stress at the top of the AC-25F layer is 338.0 kPa in structure I; however, the peak vertical compressive stress is 206.3 kPa in structure II. The former is 1.64 times larger than the latter.
In Figure
Horizontal strain in the pavement layers is plotted in Figures The horizontal strain response in structures I and II are similar when the traffic load passes over the test sensors. Horizontal strains are compressive at the bottom of the AC-25C and AC-25F layers when the test vehicle is not directly over an FBG; however, when the test vehicle is directly over an FBG, the horizontal strain is extensional. The tensile strain in structure II is significantly larger than that in structure I. For example, the maximum tensile strain at the bottom of the AC-25C layer in structure I is The position of the maximum strain is different in structure I and structure II. The point of maximum tensile strain is at the bottom of the AC-25C layer in structure I but is located at the bottom of the AC-25F layer in structure II. In structure I and structure II, tensile strain exists beneath the cement stabilized layer; however, the tensile strain magnitude at the bottom of cement stabilized layer is small. Thus, fatigue cracking is less likely in CTB with a thick asphalt layer and thick CTB base layer.
Horizontal strain at the bottom of AC-25C.
Horizontal strain at the bottom of AC-25F.
Horizontal strain at the bottom of the cement-stabilized base.
Field tests were conducted from 10 am to 12 am on a sunny day without wind and an atmospheric temperature of 29.2°C. Temperature in the asphalt layers during the test period is plotted in Figure
Measured temperatures in asphalt layers. (a)
Although field tests can help to obtain the real inner responses of pavement structures, field tests have some limitations. For instance, the subgrade condition, pavement structure, and testing method can all have an effect on test data, which results in difficulty directly applying test results to pavement design. Field tests are also expensive and time consuming. By comparison, theoretical analysis is cheaper and faster. The following paragraphs focus on comparing theoretical results and field test results in order to improve the accuracy of the theoretic results.
In the theoretic model, road pavement is assumed to be composed of multiple elastic layers characterized by an elastic modulus and Poisson's ratio. In this section, the effects of material properties including static modulus, dynamic modulus, and interface contact conditions between layers are analyzed.
The contact parameters between the test truck’s tire and road surface are calculated listed in Table
Parameter of test vehicle.
Rear axle load of test vehicle |
237 |
Weight of single wheel (kN) | 59.25 |
Contact pressure of tire (MPa) | 0.948 |
Contact area (cm2) | 626 |
Equivalent diameter of single wheel |
28.2 |
Center-to-center distance of two wheels (cm) | 1.5 |
Static modulus is widely used in theoretic modeling. Asphalt layer modulus is determined using the real temperature of the test road in Figure
Static modulus.
Pavement layers | Modulus (MPa) combination I | Modulus (MPa) combination II | Modulus (MPa) combination III | Poisson’s ratio |
---|---|---|---|---|
SMA-13 | 450 | 450 | 450 | 0.25 |
AC-20 | 700 | 700 | 700 | 0.25 |
AC-25C, AC-25F |
|
|
|
0.25 |
CTB | 1400 | 1400 | 1400 | 0.20 |
Granular | 400 | 400 | 400 | 0.25 |
Subgrade | 40 | 40 | 40 | 0.3 |
Variations in asphalt layer modulus have a small effect on the theoretical vertical compressive stress (Figure
Comparison of theoretical vertical stress and measured vertical stress. (a) Structure I. (b) Structure II.
The theoretical tensile strains at the bottom of the AC-25C and AC-25F layers are significantly smaller than the measured values (Figure
Comparison of theoretical horizontal tensile strain and measured tensile strain. (a) Structure I. (b) Structure II.
In order to analyze the influence of semirigid layer modulus on pavement response, the modulus of all asphalt layers in the theoretical model were fixed, and only the semirigid layer modulus was varied. The modulus of the AC-25C and AC-25F layers were fixed at 1000 MPa, and the modulus of CTB layer was varied from 1000 MPa to 1800 MPa. The parameters of the other layers are listed in Table
Variation in the CTB base or subbase modulus has little effect on the compressive vertical stress of pavement layers (Table
Compressive vertical stress.
Position | Compressive vertical stress (kPa) | ||||
---|---|---|---|---|---|
Modulus (MPa) combination I | Modulus (MPa) combination II | Modulus (MPa) combination III | Field measured (MPa) | ||
Structure I | AC-25C | 524.5 | 539.7 | 549.8 | 338.0 |
AC-25F | 325.1 | 344.5 | 357.8 | 199.6 | |
CTB | 42.3 | 38.7 | 35.9 | 39.6 | |
Structure II | AC-25C | 446.2 | 450.0 | 451.9 | 206.6 |
AC-25F | 237.0 | 243.3 | 247.6 | 165.4 |
Tensile horizontal strain.
Position | Tensile horizontal strain ( | ||||
---|---|---|---|---|---|
Modulus (MPa) combination I | Modulus (MPa) combination II | Modulus (MPa) combination III | Measured values | ||
Structure I | AC-25C bottom | 56.0 | 49.8 | 47.6 | 115.9 |
AC-25F | 62.1 | 24.9 | 3.8 | 56.8 | |
CTB | 145.7 | 133.6 | 123.5 | 10.9 | |
Structure II | AC-25C bottom | 73.1 | 72.9 | 72.5 | 156.3 |
AC-25F | 157.5 | 149.0 | 144.3 | 280.6 | |
CTB | 167.9 | 142.5 | 125.7 | 24.4 |
The dynamic modulus of asphalt mixtures can be tested using laboratory methods, and results were related to loading rate and temperature [
Asphalt dynamic modulus.
Real temperature (°C) | Dynamic modulus (MPa) | Poisson’s ratio | |
---|---|---|---|
SMA-13 | 39.4 | 1900 | 0.25 |
AC-20 | 34.5 | 5500 | 0.25 |
AC-25C | 25.5 | 10000 | 0.25 |
AC-25F | 24.6 | 10500 | 0.25 |
Theoretical results.
Structure type | Position | Compressive vertical stress (kPa) | Tensile horizontal strain ( | ||
---|---|---|---|---|---|
Theoretical | Field test | Theoretical | Field test | ||
Structure I | AC-25C | 437.7 | 338.0 | 8.0 | 115.9 |
AC-25F | 190.3 | 199.6 | 54.1 | 56.8 | |
CTB | 23.2 | 39.6 | 83.5 | 10.9 | |
Structure II | AC-25C | 380.5 | 206.6 | 10.1 | 156.3 |
AC-25F | 150.5 | 165.4 | 66.0 | 280.6 | |
CTB | 85.7 | 24.4 |
Theoretical results show that if dynamic modulus is used, the compressive vertical stresses in the asphalt layers are clearly less when using the static modulus. The difference between theoretical compressive stress and measured data decreases when the dynamic modulus is adopted in the theoretical model.
The maximum tensile horizontal strain positions appear at the bottom of the AC-25F layer in both structure I and structure II when the dynamic modulus is used in the theoretical model. The maximum tensile strain of asphalt layers in structure II decreased significantly compared with the calculated results using the static modulus model. Thus, the dynamic modulus has a greater effect on the theoretical strain response of structure II.
On the whole, using the dynamic modulus leads to a decreased theoretical compressive stress and tensile strain in asphalt layers. As a result, the difference between theoretical vertical compressive stress and field measurements is reduced and the difference in tensile strains is increased.
When pavement structures are constructed, the interface between layers may be not fully bonded because the adjacent materials are not same. Bonding conditions are considered in the theoretical model to analyze the effect of interface characteristics on pavement response. Only the bonding status between the lowest asphalt layer and base layer is considered, including a fully bonded boundary and a sliding boundary (Figures
Effect of bonding condition on compressive vertical stress. (a) Structure I. (b) Structure II.
Effect of bonding condition on tensile horizontal strain. (a) Structure I. (b) Structure II.
The bonding conditions between the asphalt and base layers have the same effect on the compressive vertical stress in structure I and structure II. The sliding boundary led to an increased compressive vertical stress in the AC-25F layer and decreased compressive vertical stress in the AC-25C layer. Overall, the change in compressive vertical stress magnitude is not significant. In contrast, the effects of interface condition on tensile horizontal strain are significant. For a sliding boundary, the theoretical tensile horizontal strains of the AC-25C and AC-25F layers in structure 1 are 3.9 times and 16.5 times larger, respectively, compared to a fully bonded layer. In structure 2, the increase ratio of tensile horizontal strain is 1.9 times and 2.1 times, respectively.
The theoretical horizontal strain in asphalt layers is larger than field measurements for a sliding boundary. However, if the boundary is fully bonded, the theoretical results will be less than field measurements. Thus, the interface type has an important effect on the theoretical results. It is necessary to reasonably evaluate the actual interface type in order to decrease the difference between theoretical and real responses.
In this study, two kinds of pavements with thick asphalt layers are designed and used to construct test road samples. The responses of pavement layers under traffic loading are measured. The differences between the theoretical results and field measurements are analyzed, and several conclusions are drawn as follows: According to field data, the compressive vertical stress in the asphalt layers of structure I is larger than in structure II at the same depth, indicating that the asphalt material in structure I undergoes more severe compression under traffic load. The position of the maximum horizontal tensile strain in structure I is located at the bottom of the AC-25C layer. The same point is located at the bottom of the AC-25F layer in structure II, and the magnitude of maximum horizontal tensile strain is 2.42 times larger in structure II than in structure I based on field data, indicating fatigue cracking is more likely to occur in structure II with a granular base layer. The measured tensile strains of a semirigid base layer are relatively low (less than Pavement responses calculated using the theoretical model are distinctly different from field measurements. Theoretical compressive vertical stresses are significantly larger than field measurements, and the theoretical tensile strain in asphalt layers is significantly less than field measurements if the interface condition is good. Thus, the predicted service life of pavement will deviate from the actual service life if the theoretical response is directly used to design the pavement. Material properties and interface bonding condition have significant effects on theoretical pavement response. It is difficult to reduce the difference between theoretical results and field measurements only by varying the material modulus of certain layers or one interface bonding condition. Numerous material tests and field tests must be conducted in order to establish a reasonable relationship between theoretical results and field measurements.
The authors declare that all the data supporting the conclusions of the present study can be obtained from the corresponding author.
There are no conflicts of interest regarding the publication of this article.
Jingsong Shan and Hongmei Shao designed the field test. QiuZhong Li and Peili sun performed the field test. Jingsong Shan conducted the theoretical analysis. Hongmei Shao analyzed the field test data and wrote the paper.
This research was funded by the Project of Shandong Province Higher Educational Science and Technology Program, grant no. J17KA213.