A Large-Scale Test Method for Mechanical Response of Pavement Structure

A test method for mechanical response of pavement structure with the large scale model was presented in the study..e strain was tested on three large scale models of pavement structure with three typical pavement materials of cement concrete (CC), cement stabilized macadam (CSM) and asphalt concrete (AC) under the different load levels. .eoretical calculations of the strain on the top surface of CC, CSM and AC pavement models were also developed by BISAR3.0, which is a software for mechanical analysis based on elastic layered system. .e research results indicate that the test method for mechanical response of pavement structure with large scale model presented in the study shows a low variability, and a good repeatability and reliability, which can be used as an effective way to study the mechanical response of pavement structure instead of full-scale test. .e research results can provide some references for theoretical calculation of pavement structure and determination of pavement material parameters.


Introduction
e vehicle load and the changes of ambient temperature can cause excessive stress, strain, and displacement within the pavement structure, which further lead to the structural failure of pavement.In recent years, authentic mechanical response of pavement under different loading has always been the focus point in the research area of basic theory and design of pavement structure.At present, theoretical analysis and experimental test are the main methods to obtain the mechanical response of pavement structure.ese two methods are usually used together to complement and verify each other.In comparison with the theoretical analysis, the experimental test can reflect the actual mechanical status of pavement structure.
e experimental test has gradually become one of the most important methods to study the mechanical response of pavement structure [1].Among the present researches, lots of experimental test were conducted to obtain the mechanical response of pavement structure on the accelerated loading facility, full-scale test track, and engineering test road.e representatives mainly include strain distribution of actual pavement.us, the experimental test has been widely recognized as an important research method for mechanical response of pavement by the researchers.However, the full-scale accelerated load test is extremely complicated, which is generally matched with a set of expensive test and operation management system.High cost is inevitable to ensure the normal operation of the system.In addition, the test period is too long due to the load e ciency.It usually takes about several months to several years to draw conclusions from the initial test design.us, the full-scale accelerated load test cannot be widely used in research of mechanical response of pavement, because of limitation by high cost and long test period.
Accordingly, in order to develop a common and applicable research method to obtain authentic mechanical response of pavement under di erent loading, a large-scale model of pavement structure was designed and prepared.
e test method of mechanical response with the larger scale model was proposed.e mechanical responses of typical pavement materials were studied on the large-scale model.

Objective and Scope
e primary objectives of this study are to develop a simple and reliable test method for mechanical response of pavement structure, which may be useful for obtaining the real mechanical response regularity of pavement under the action of vehicle load.ree large-scale models of pavement structure with cement concrete, cement-stabilized macadam, and asphalt concrete were prepared for the test.e in uences of the material property and load level on the mechanical response were analyzed.

Test Design
3.1.Test Load.According to the common method used in present research [25][26][27], the vehicle load is simpli ed to a circular uniform distributed load with a radius of δ 7.5 cm in this study, which is convenient not only for mechanical analyses but also for comparison with the conclusion of other researches.e load acts on the model center statically, as shown in Figure 1. e load magnitude is denoted by p, which is a variable value.ree large-scale models of pavement structure were made with the cement concrete, cement-stabilized macadam, and asphalt concrete.
e test temperature is 15 °C.For the research convenience, the load was acted on the large-scale model as follows.
First, to study the mechanical response of di erent pavement materials, the load magnitude is selected according to the stress level, which is the ratio of the test load to the strength of the material.e present researches indicate that the damage caused by the load to the material can be ignored, when the stress level is 0.2-0.4.Accordingly, the stress level was chosen as 0.3 in this research.e strengths of the cement concrete, cement-stabilized macadam, and asphalt concrete are 6.89 MPa, 5.02 MPa, and 2.33 MPa, respectively.e load magnitudes for cement concrete, cement-stabilized macadam, and asphalt concrete are determined as p 2.1 MPa, p 1.5 MPa, and p 0.7 MPa by calculation, respectively.
Second, to study the in uence of the load level on the mechanical response, three di erent load magnitudes were selected for each pavement material.For the cement concrete, the load intensity p is 1.3 MPa, 1.7 MPa, and 2.1 MPa.For the cement-stabilized macadam, the load intensity p is 1.1 MPa, 1.5 MPa, and 1.9 MPa.For the asphalt concrete, the load intensity p is 0.3 MPa, 0.7 MPa, and 1.1 MPa.

Model Size of Pavement Structure.
e aim of this research is to nd a method to simulate the pavement structure in lab between small size test and full-scale test.e more the model size is close to the actual pavement, the better the model can simulate the real situation, and the more di cult the mechanical responses of structure are tested.us, the model size of pavement structure was determined by considering the actual pavement structure and test di culty.

Model Shape.
Accordingly, the pavement structure model is designed as a cylinder.A circular uniform distributed load with the intensity of p and the radius of δ is placed at the central point on the top surface of the model.
e test model can be simpli ed to axisymmetric mechanical problem as shown in Figure 1.

Model ickness.
Currently in China, the thickness of asphalt surface course, single semirigid base course, and cement concrete surface course of highway are 15 cm-22 cm, 18-20 cm [28], and 24-26 cm, respectively.Accordingly, the thickness of the test model shown in Figure 1 is designed as 20 cm to better simulate the actual pavement structure.

Model Diameter.
In order to obtain the optimal diameter of the model, the pavement mechanical calculation software BISAR3.0 was adopted to calculate the stress and strain of the cylindrical pavement structure model under the load action.As the model can be simpli ed as axisymmetric mechanical problem, the 1/4 model was used for calculation in the research.e load intensity (p) is 2.1 MPa. e load radius (δ) is 7.5 cm. e cement concrete is selected as an example with the modulus of E 30,000 MPa and Poisson's ratio of μ 0.18.e diameter of the model (D) is 200 cm.e output parameters are the radial stress and strain along the diameter direction (σ r , ε r ) and circumferential stress and strain which are perpendicular to the diameter direction (σ θ , ε θ ).e calculation results are shown in Figures 2 and 3

Test Model Preparation.
For the preparation of the largescale model of pavement structure, rst, the materials are mixed through the large capacity of mixing equipment in the laboratory.Second, the mixtures are placed in the special moulds, which are 100 cm in diameter and 20 cm in thickness.Finally, the compaction was executed by a vibration roller.e surface of the sample is too rough to test the strain by electrical measurement.Hence, the specimen surface needs to be polished by sander and sandpaper to get a smooth surface before the test, as shown in Figure 4.

Test System
3.5.1.Loading System.In this research, the gantry MTS hydraulic servo system is used as the loading device.e supporting reaction frame is employed to provide the pavement structure model with random static test load in the range of 0-250 kN, as shown in Figure 5.

Data Acquisition System.
In this study, paper-based electrical resistance strain gauges are attached on the sample surface as shown in Figure 6.
e electrical measuring method is used to obtain the strain response of the sample  surface under the di erent loads.
e accuracy of strain gauge is 0.05 ( * 10 −6 ).e frequency of strain acquisition under the static load is 0.1 Hz.
In consideration of the characteristic of the axisymmetric structure, radial strain along the diameter direction and circumferential strain perpendicular to the diameter direction are the main test parameters.In this research, radial strain along the diameter direction is symmetrically arranged in 8 rows and circumferential strain perpendicular to the diameter direction is symmetrically arranged in 2 rows.e strain on the symmetric point of the 1/4 model was taken as average to ensure the accuracy.A certain distance is reserved between strain gauges in order to make every strain gauge work independently and has no in uence on the nearby strain gauges.
e distribution of strain gauges is shown in Figure 7.  e load boundary e ect is the main factor leading to this anomaly.Speci cally, although there is no direct load acting on the model surface nearby the load, the scope near the circular load edge (d 7.5-15 cm) still su ers the in uence of the

Parallel Test and Replicate
Test.Due to the impacts of instrument precision, signal acquisition mode, and environmental noise on the test method, inaccuracy and distortion occurred on the measured value sometimes.To ensure the reliability of test methods, parallel test and replicate test were developed.Taking AC pavement structure model as an example, the results of parallel tests and replicate tests were discussed.

Parallel Test Result.
In Tables 5 and 6, the parallel test results of circumferential strain and radial strain of the AC pavement structure model are listed, respectively.As shown, the di erences between the results of the 4 sets of parallel tests are very small for both circumferential strain and radial strain.e average variation coe cients are 2.1% and 2.3%, respectively, which are lower than 5% and belong to low variation level.e variation coe cients of the CC pavement structure model are 1.9% and 2.2%, and the variation    7 and 8, which indicate that the di erences between the results of the 3 sets of replicate tests are very small.e average variation coe cients of circumferential strain and radial strain are 3.3% and 3.8%, respectively, which belong to low variation level.At the same time, the replicate tests were conducted on CC and CSM pavement structure models.
e average variation coe cients of circumferential strain and radial strain are 3.9% and 3.1% for the CC pavement structure model and 3.2% and 4.0% for the CSM pavement structure model, which also belong to low variation level.All these results indicate that this test method has high repeatability.
us, the test methods can be used as an e ective way to study the mechanical response of pavement structure.

In uence of Pavement Materials on Strain (i) Test results
e test results of circumferential strain of the pavement structure models with three pavement materials are presented in Figure 14.It can be seen that, under the same test conditions, the trends of circumferential strain of the CC, CSM, and AC pavement structure model changing with the distance d from the test point to the center point of load are similar.e circumferential strain of the models with three kinds of materials is all compressive strain, which decreases as the distance from the test point to the center point of load increases.e strain data decrease to 0 at the edge of the model.
e strain at the same test point is in inverse proportion to the modulus of the pavement material.In the order of highest value of the circumferential strain, the sequence is AC, CSM, and CC.
e test results of radial strain of CC, CSM, and AC structure models under the stress level (the ratio of stress to strength) of 0.3 and temperature of 15 °C are shown in Figure 15.It reveals that, under the same stress level and test temperature, the trends of radial strain of the three kinds of pavement structure models are quite di erent.Radial strain of the CC structure model is tensile strain, which decreases

Advances in Materials Science and Engineering
as the distance d between the test point and the load central point increases and further decreases to 0 nearby the model boundary, whereas radial strain of CSM and AC structure models shows an alternation from compression to tensile strain.Radial strain nearby the load is the biggest.As the distance d from the test point to the load central point increases, the compression strain decreases to 0 and converts into the tensile strain.After reaching a peak value, the tensile strain decreases gradually and finally decreases to 0 nearby the boundary of the model.ere are maximum compressive strain point, compressive-tensile cutoff point, and tensile strain peak point for the radial strain of CSM and AC structure models obviously.
ese points can reflect the mechanical response characteristics of the pavement structure model under the load action.Accordingly, these points are defined as the mechanical response feature points.
e strain values at the feature points are defined as the feature values of mechanical response.Due to the influences of pavement material properties, the feature points and feature values of the radial strain of CSM and AC structure models are different.e compressive-tensile cutting point of CSM and AC pavement structure models lie in the position of d � 32 cm and d � 21 cm off the load central point.
e peak value of tensile strain is 4.1 με and 13.2 με, which appear in the position of d � 39 cm and d � 32 cm off the load central point, respectively.For the AC pavement, there are obvious compressive zone and tensile zone.Compared with CSM pavement, the tensile zone of AC pavement is wider and the peak value of tensile strain is higher.us, heavy load leads to large tensile strain, which may result in the topdown crack on the surface of AC pavement.
In conclusion, under the same test conditions, the mechanical response of CC, CSM, and AC are different with each other distinctly.

(ii) Comparison between test results and theoretical calculations
In order to study the deviation between real mechanical response and theoretical calculations, software for mechanical analysis based on the elastic layered system, BISAR3.0, was employed for theoretical calculations of the strain on the top surface of CC, CSM, and AC pavement models.e calculation parameters of BISAR are displayed in Table 9.
e test results and theoretical calculations of the circumferential strain and radial strain of CC, CSM, and AC pavement structure models are shown in Figure 16.It indicates that the theoretical calculations of circumferential strain present a coincident trend with the test results.e circumferential strain decreases with the increase of the distance d from the test point to the load central point.e theoretical calculation values and test results of radial strain of the CC pavement structure model also display a good match with each other, whereas the theoretical calculation results of the CSM and AC pavement structure models reveal the total different regulations with the test results.e radial strain of theoretical calculation of CSM and AC pavement structure models is tensile strain, which decreases with the increase in the distance d off the load central point.However, the test results of the radial strain of CSM and AC pavement structure models include the compressive zone and tensile zone with an obvious compressive-tensile cutting point.
ere is no correlation between theoretical calculation values and test results.As listed in Table 9, the theoretical calculation models of three pavement structure are distinguished only by changing the modulus and Poisson's ratio of the pavement materials without considering the influence of constitutive relations on the mechanical response of the model, which may be the reason resulting in the different trends of theoretical calculation values with the test results.Comparison results between the theoretical calculation values and test results indicate that the elastic layered system is suitable for the CC pavement, whereas appropriate modification is needed on the elastic layered system for the CSM and AC pavement.
In conclusion, because of the different material properties, the mechanical response is different obviously under the same conditions.Hence, the material property should be fully considered in the theoretical calculation model in order to obtain the more accurate calculation values to reflect the mechanical response of actual pavement structure.

Influence of Load Level on Test Results.
e test results of circumferential strain and radial strain of the CC pavement structure model under the different load levels of p � 1.3 MPa, 1.7 MPa, and 2.1 MPa are displayed in Figure 17.It reveals that all the circumferential strains are compressive and all the radial strains are tensile.For the same distance d off the load central point, the strain value of the CC model increases with the increasing load level.Under the different load levels, the strain displays the same trend which decreases with the increasing distance d off the load central point and becomes 0 at the boundary of the model.Especially for the circumferential strain, when distance d ranges from 10 cm to 35 cm, the strain at same test point increases with the increase of the load level.When distance d > 35 cm, there is little difference among the circumferential strains under three load levels.
e test results of circumferential strain and radial strain for the CSM model under different load levels of p � 1.1 MPa, 1.5 MPa, and 1.9 MPa are displayed in Figure 18.It can be seen that the strain of the CSM model increases with the increasing load level.Under the di erent load levels, the strain shows the same trend.When distance d ranges from 10 cm to 30 cm, there are obvious di erences among circumferential strains under the di erent load levels at the same test point.However, when d > 30 cm, the di erences become tiny and approach to 0 at the boundary of the model.
ere are mechanical response feature points for the radial strain of the CMS pavement structure model under the di erent load levels.e positions of the feature points are the same for the di erent load levels that the compressivetensile cuto points appear at d 32 cm and peak points of tension strain appears at d 39 cm.However, the maximum compressive strain and the peak tension strain increase with the increasing load level.
e test results of circumferential strain and radial strain for the AC pavement model under di erent load levels of p 0.3 MPa, 0.7 MPa, and 1.1 MPa are shown in Figure 19.As displayed, the strain of the AC pavement structure model increases with the raise of load level and shows basically the same trend under the di erent load levels.For the circumferential strain of the AC pavement model, the greater the load level, the steeper the curve and vice versa.Similar to the CSM model, mechanical response feature points remain in original position without any change when the load level increases so that the compression-tension cuto points appear at d 21 cm and peak points of tensile strain appears at d 32 cm.Similar to the CSM model,  Advances in Materials Science and Engineering the maximum compressive strain and peak tensile strain increase with the increasing load level.

Conclusions
Based on the results from this study, the following conclusions can be drawn.
A test method for mechanical response of pavement structure with a large-scale model was presented in the research, which proves to be a reliable method to study the mechanical response of pavement structure while the fullscale accelerated loading test is not available.e research results can provide some references for theoretical calculation of pavement structure and determination of pavement material parameters.e mechanical responses of three typical pavement materials, CSM, AC, and CC, were studied by large-scale test, which show great di erences with each other under the same test conditions.ere are obvious mechanical response feature points for radial strain of CSM and AC, which are maximum compression strain point, compressive-tensile cuto point, and tensile strain peak point, while the phenomenon does not exist on the CC pavement structure, which su ers tensile radial strain on all the test points.
ere is a large di erence between the test results and theoretical calculation values of radial strain for both CSM and AC pavement structures, which indicates that the elastic layered system need to be modi ed for theoretical calculation of mechanical response of CSM and AC pavement structures.
All the strains of CC, CSM, and AC pavement structures increase as the load level increases.However, for each pavement structure, under the di erent load levels, the trends of strain changing with the distance from the test point to the load central point are similar.e mechanical response feature points of CSM and AC pavement structures stay at the same position under the di erent load levels, but the feature values change.Advances in Materials Science and Engineering For the AC pavement structure, there are obvious compressive zone and tensile zone.
e top-down cracks may appear on the surface due to the maximum tensile strain beyond the tensile strength when heavy load is applied.

Further Research
is is a preliminary test method for mechanical response of pavement structure with the large-scale model, in which the pavement structure is simplified to a single layer.e largescale model with multilayers which is more similar to the real pavement structure should be prepared in the further researches.Furthermore, the test results of the mechanical responses of the large-scale model will be compared with the results of the full-scale test to verify the accuracy of the test method with the large-scale model. ,

Figure 1 :
Figure 1: Sketch of the test model of pavement structure.

Figure 2 :Figure 3 :
Figure 2: Calculation results of surface stress of the CC pavement structure model.

4. 1 .
Data Processing Method.Taking CSM pavement structure model for an example, the laws of test data and the data processing method were discussed as follows.e test results of circumferential strain on the top of the CSM pavement structure model under the static uniform load of p 1.0 MPa are shown in Figure 8.It can be seen that as the distance d from the test point to the center point of load increases, the circumferential strain increases at rst and then decreases, which is a kind of abnormal phenomenon obviously.e peak value appears at the point of d 14 cm.e test results indicate that the same abnormal phenomenon exists in the measured circumferential strain of CC and AC pavement structure models.Peak value usually emerges at the point of d 12-15 cm, as shown in Figures 9 and 10 .

Figure 4 :
Figure 4: Specimen of the large-scale model of pavement structure.

Figure 5 :
Figure 5: Loading system for test.

Figure 9 :Figure 10 :Figure 11 :Figure 12 :Figure 13 :
Figure 9: e test result of circumferential strain on the top of the CC pavement structure model.

Figure 14 :Figure 15 :
Figure 14: Circumferential strain test results of models with different pavement materials.
radial strain Test value of radial strain Calculation value of circumferential strain Test value of circumferential strain (c)

Figure 18 :
Figure 18: Strain test results of the CSM pavement structure model under the di erent load levels.(a) Circumferential strain.(b) Radial strain.

Figure 19 :
Figure 19: Strain test results of the AC pavement structure model under the di erent load levels.(a) Circumferential strain.(b) Radial strain.

Table 2 :
Composition of coarse aggregate for cement concrete.

Table 3 :
Composition of macadam for cement-stabilized macadam.

Table 4 :
Aggregate composition of asphalt mixture.
average value of parallel test results is used as measured strain for compared analyses.4.2.2.Replicate Test Result.e replicate tests were developed on the AC pavement structure model by three researchers individually under the load intensity of p � 0.7 MPa and the temperature of 25 °C on different dates which were 7 December 2013 (first time), 21 February 2014 (second time), and 1 March 2014 (third time).e test results Advances in Materials Science and Engineering coefficients of the CSM pavement structure model are 2.1% and 2.2%, which also belong to low variation level.However, to ensure the accuracy of test results, more than 3 sets of parallel tests are carried out each time for the test.e

Table 5 :
Parallel test results of circumferential strain of the AC pavement structure model.

Table 6 :
Parallel test results of radial strain of the AC pavement structure model.

Table 7 :
Replicate test results of circumferential strain of the AC pavement structure model.

Table 8 :
Replicate test results of radial strain of the AC pavement structure model.

Table 9 :
Calculation parameters of BISAR.Type of material Stress level Load, p (MPa) Radius of the load circle, δ (cm) Structural layer modulus, E (MPa) Poisson's ratio, μ