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Research Article
 

Field Evaluation of Nondestructive Tests in Measuring the Pavement Layers Density



H. Ziari, H. Behbahani, A. Izadi and H. Divandary
 
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ABSTRACT

The aim of this study was field evaluation of nondestructive devices in measuring the density of road layers. Density may be considered as a reliable criterion for evaluating pavement quality and has a high level of importance in that it can ensure the proper performance of pavement at least prior the period of life-time. At present time, the most current and also accurate method to determine in-situ density of asphalt mixture is core sample method, however it has some disadvantages. As well as being costly and time consuming, core sample method causes some distress on the pavement surface and it is not possible to repeat the test for a specific location. In view of this, some attempts were made to develop new methods as alternatives for core sample method. And as such, nondestructive tests have grown into a huge area over the last few years. These tests including nuclear and nonnuclear nondestructive tests do not have the limitations of core sample method. In this study, field evaluation in a new constructed part of a highway was conducted using pavement quality indicator (PQI301) and Troxler nuclear gauge (Model HS-5001EZ). According to the results of validation tests for nondestructive devices (PQI and Nuclear devices), PQI device has sufficient reliability to determine density of asphalt mixture layer but Nuclear device is not reliable to determine density. The obtained results from validation of nuclear device revealed that it has sufficient reliability to determine density of soil layers. Also it was found that the role of calibration procedure in obtaining correct readings From PQI device is highly critical.

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  How to cite this article:

H. Ziari, H. Behbahani, A. Izadi and H. Divandary, 2010. Field Evaluation of Nondestructive Tests in Measuring the Pavement Layers Density. Journal of Applied Sciences, 10: 3091-3097.

DOI: 10.3923/jas.2010.3091.3097

URL: https://scialert.net/abstract/?doi=jas.2010.3091.3097
 
Received: May 11, 2010; Accepted: September 28, 2010; Published: October 14, 2010



INTRODUCTION

Density can be characterized as one of the most important parameters in the design and implementation of asphalt pavement. Selection of an appropriate compaction level is highly critical to achieve the desired performance of pavement.

After spreading asphalt mixtures on the road bed, it should be compacted by rollers to achieve appropriate level of density. Before coming the nonnuclear nondestructive devices, there were only two methods to determine the density of fresh asphalt. The first, core sample method, a destructive method, is on the basis of coring and the second method employs a portable nuclear nondestructive device. Although, the first method is the most accurate method for determination of density, it is costly and time-consuming and is preferred to be applied as the last option. In this method, samples, first, should be sent to laboratory and since, it is likely that there is a notable distance between site and laboratory, it may take from 1 h to 1 day. In addition to the above, when the asphalt is still warm, the method does not provide the possibility to control the density during different stages of rolling process (Shane et al., 2004).

In recent years, the application of nondestructive tests has been increased, particularly to evaluate pavement quality and control the compaction process of pavement layers. Nuclear density gauge can be used to determine in-situ specific gravity and moisture content in accordance with ASTM D2922-D3017. For this purpose, this device employs emission of gamma-ray and neutron particles to compute specific gravity and moisture content, respectively. It offers two advantages over the constructive methods: the high speed and not being destructive. A significant amount of studies have been conducted on the results obtained from nuclear density gauge by different manufacturers. Site location, materials type and device model may have appreciable effects on the results (Khani, 2004). Nuclear density gauge is much faster and extremely cost-effective compared to core sample method, however, it is subjected to the following disadvantages: (1) use of a radioactive source requiring high adjustment and special training of experienced operators, (2) getting permit, renovation of equipment, technicians training and complicated operation. In order to overcome many of the disadvantages of nuclear density gauge, nonnuclear nondestructive device has been developed as a safer, lighter and more cost-effective alternative (Karlsson, 2002). Also Karlsson (2002) found that water content (more than 15%) in asphalt layer effects on the instrument’s reliability. Pedro Romero found in his study that, to measure density in the field, a calibration procedure that is mixture specific should be carried out and also indicated that it is necessary to correct for changes in moisture and temperature (Pedro Romero, 2002). Robert Schmitt found in his study that non-nuclear devices could be used for quality control, however they were not recommended for quality assurance or acceptance testing (Schmitt, 2006). This study is aimed at evaluating the performance of nondestructive devices using conducting field tests and statistical analysis of obtained results.

MATERIALS AND METHODS

In order to perform field studies and evaluate the performance of two nondestructive devices, namely PQI and nuclear, some tests were performed in 2008 at the specified segments of newly established Tehran-Pardis Freeway.

Calibration of PQI device before taking measurement operation: The role of calibration procedure in obtaining correct readings is highly critical. It should be noted that the calibration of the device for each project is necessary; therefore, Manufacture Company of PQI has presented different calibration method. In this study, normal method has been employed to calibrate the device. This is mainly due to the fact that other methods require density of unrolled asphalt mixture and prediction of the density of implemented mixture by experts as input data.

Five core samples as reference data were taken from asphalt mat and the densities of core samples were calculated. The results are shown in Table 1. The readings from PQI are represented in Table 2. From the results in Table 1 and 2 calibration process was done.

The results obtained from reading of density by PQI and coring devices are as follows:

Mean of coring results: 2181 kg m-3
Mean of PQI reading: 2010 kg m-3
Difference between coring results and PQI reading: 171 kg m-3

This modification value means that 171 should be added to Mean of PQI reading (Fig. 1).

Calibration of nuclear device before taking measurement (in asphalt layer): The nuclear device utilized in this study is HS-5001EZ and produced by TROXLER Ltd.

Table 1: The obtained data from coring at five stations for device calibration

Table 2: Readings of PQI device at five stations and different local positions (kg m-3)

Fig. 1: Reading location layout and PQI measurement pattern

If the difference between the results of successive measurement is minor, the device is calibrated and ready to be used.

Field measurements on the asphalt layer: In this study, 10 sections with the width of 3.65 m that have been placed 7 m apart from each other were selected. Afterwards, 6 points were specified at each section and finally the density was computed for 60 points using two methods, namely nuclear and nonnuclear non-destructive methods (Fig. 2). The reading from each method is shown in Table 3 that in each section, 6 readings were done by both nuclear and PQI devices and one reading was done by coring device. Finally the density of asphalt mat was computed for 60 points to be enough for proper statistical analysis.

Field measurements on the soil surface: The density of subgrade layer was determined at the segment of Tehran-Pardis Freeway and the results are presented in Table 4, then, the density values obtained from sand bottle test have been utilized as a criterion for validation of outputs of nuclear device (Fig. 3).

Fig. 2: Test section layer for asphalt layer density measuring

Table 3: Results of field measurements by three methods

Fig. 3: Taking measurement for 20 points in soil sub grade layer using nuclear device

Table 4: Determination of density of soil layer using sand bottle test

Table 5: Measures of density for 20 points in soil subgrade layer using nuclear device

Afterwards, determination of density by nuclear device has been made at 20 points with the interval of 50 cm and the results are shown Table 5.

RESULTS

Analysis of obtained results from different methods and presenting some practical approaches for the optimal use and reduction of error: The obtained results from measurement of PQI and nuclear devices at the edges of segments were lower than that of at the medium of segments. The same result was observed in the conducted research by North Carolina Department (Shane et al., 2004). SPSS Software has been applied to analyze data for the purpose of validation of non-destructive devices.

The function used for conducting statistical test is called test statistic and varies in different conditions. In what follows, the relevant test statistic to each test is introduced. In parametric tests like T-student test, some assumptions are made concerning population distribution. For instance, T-student test is on the basis of this assumption that the societies will follow normal distribution. T-student test aimed to obtain a better understanding of unknown parameters of statistical society is normally employed to evaluate the differences between means of two societies which are under analysing. Since, the distribution of considered population is not known in nonparametric tests, accordingly, t-student test can not be applied, because it is based on the normal distribution. In such conditions, nonparametric Mann-Whitney test may be used instead. The other difference between parametric and nonparametric test is that nonparametric tests employ median in lieu of mean so as to compare societies. The T-test used to compare means of normal societies is strongly dependent on the hypothesis of equality of variances and normal distribution of statistical society. Therefore, different solutions to this problem have been proposed in the literature. One way to solve this problem is to use equivalent nonparametric tests. It should be mentioned that nonparametric tests investigate the differences between two societies through evaluation of differences between median of two societies.

Analysis for validation of PQI and nuclear devices in the asphalt layer: Comparison of two statistical societies in the parametric tests is based on the means of these two societies. Since, the considered societies in the current research follow normal distribution, thus, parametric tests (T-test in this research) was employed to compare two societies.

If the amount of significant level (sig) in the test output is less than 0.05, it can be concluded that the assumption of equality of two societies is not valid with the probability of 95%. On the other hand, if amount of significant level (sig) is more than 0.05, then with the probability of 95%, the two societies are close to each other.

In accordance with the above results in Table 6, the measured values by PQI and coring devices did not represent significant difference with the probability of 95%. However, the measured values by nuclear and coring devices showed sensible difference with the probability of 99%.

Table 6: Output of SPSS software for validation of PQI and nuclear devices using core sampling results as precise values

Practical hints for reduction of error and optimal application of PQI device:

It should be noted that PQI has appropriate capability to identify points with improper compaction such as edges or places with insufficient compaction
It is recommended to avoid taking measurement at the edges. This is due to the fact that density at these points is lower than other points and accordingly it causes some error in the calculations
Do not contact with device during measurement, because it causes some errors in the reading of the device
Rainy weather and high moisture content of pavement may result in error in the reading of PQI device
PQI device should be calibrated according to the type of asphalt before starting operation
Although, there are different types of calibration methods including empirical methods, it may be advisable based on the experiences of executive team to employ normal which is on the basis of comparison with coring results (the most precise method for determination of density). In other words, since other methods require density of unrolled asphalt mixture and prediction of density of implemented mixture by experts as input data and considered sites often do not meet the above requirements, thus, normal method is proffered

Validation of nuclear device to measure density of soil layers: The sand bottle test has been run five times to properly determine density of subgrade layer. Then, measurement was made by nuclear device for 20 times through using direct distribution method. According to the results of sand bottle test (Table 4), it can be concluded that these data do not follow normal distribution (five observations is not sufficient for judging whether the society is normal or not), whereas, nuclear data follows normal distribution with the possibility of 95%.

Since, the assumption of equality of variance of two societies can not be met with the probability of 95%, therefore employing parametric t-student test is not justified. Hence, equivalent nonparametric t-student test, namely Mann-Whitney was utilized. The main difference between parametric and nonparametric tests lies in that the assumption of normal distribution of data is not required in the nonparametric tests. In recent years, these methods have been dramatically widespread and this is because of type of society distribution which is not often given. It should be mentioned that nonparametric tests evaluate the differences between two societies through assessing the difference between the median of two societies.

The output of Mann-Whitney test which is aimed at evaluating the difference between median of two statistical societies (in the current study: sand bottle method and nuclear device) revealed that the value of sig (0.126) is greater than 0.05 and accordingly the assumption of median equality of two statistical societies is reasonable with the probability of 95%. Moreover, other qualified nonparametric tests also confirmed the above finding.

In Table 7, the validation of the obtained results from nuclear device using nonparametric Mann-Whitney, Moses and Kolmogorove tests are illustrated. In all tests, it is assumed that null hypothesis implies equality of medians of two statistical societies (the results of sand bottle and nuclear methods) and the opposite hypothesis represents non-equality of medians.

Table 7: Output results relevant to nonparametric mann-whitney, moses and kolmogorove tests as alternatives for t-student test

Hence, the obtained results indicated that nuclear device can be considered a reliable method to determine density of soil layers.

DISCUSSION

Based on the results of statistical hypothesis testing in this study, use of the PQI model 301 for both Quality Control (QC) and Quality Acceptance (QA) testing is sufficient but it found that calibration should be done to achieve accurate results. Numerous studies on the suitability of non-nuclear pavement density gauges for QC and QA testing of pavements were reviewed. Most of these studies have concluded that non-nuclear gauges are suitable for contractor QC but not recommended for QA. Also It was found that the PQI appears to be an acceptable alternative to the nuclear gauge for density profiling, even if not calibrated (Sargand et al., 2005). A study prepared by the University of Utah indicated that, Calibration of these devices to local materials and conditions is critical to obtain accurate results. Whenever practical, the calibration should be done using a test section (Pedro Romero, 2002).

The following conclusions are drawn from the results of this study:

According to the results of validation tests for PQI and Nuclear devices, indicated that PQI device has sufficient reliability to determine density of asphalt mixture but nuclear device is not reliable to determine density
Due to the negative difference between mean measurements of nuclear and coring devices, it can be concluded that the measured values by nuclear method are lower than that of by core sample method with the probability of 99 % and include zero. While, this difference for nuclear and core sample method does not include zero and is negative. Hence, the validation results of PQI and nuclear devices indicated that contrary to nuclear device, PQI device has sufficient reliability to determine density of asphalt mixture
The obtained results from validation of nuclear device revealed that it has sufficient reliability to determine density of soil layers
It should be noted that PQI has appropriate capability to identify points with improper compaction such as edges. It is recommended to avoid taking measurement at the edges. This is because the density at these points is lower than other points and accordingly it causes some error in the calculations
PQI device should be calibrated according to the type of asphalt before starting operation
Although, there are different types of calibration methods including empirical methods, it may be advisable based on the experiences of executive team to employ normal which is on the basis of comparison with coring results (the most precise method for determination of density). In other words, since other methods require density of unrolled asphalt mixture and prediction of density of implemented mixture by experts as input data and considered sites often do not meet the above requirements, thus, normal method is proffered
The required time for calibrating the Nuclear device is much affected by type of project and used materials (asphalt, soil)
Finally, it is also noteworthy that the most precise and reliable method to determine the pavement layers density is core sample method
REFERENCES
1:  Shane, M., J. Turner and J. Barton, 2004. In place density evaluation of Stone Matrix Asphalt (SMA) mixes in Alabama. Asphalt Paving Technol., 73: 195-228.
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2:  Khani, H., 2004. Evaluation of nuclear nondestructive methods for determination of pavement density. M.Sc. Thesis, Civil Department Library, Iran University of Science and Technology.

3:  Karlsson, T., 2002. Evaluation of the Pavement Quality Indicator (PQI). Skanska Asphalt and Concrete Technology Region-VTO South380 30 ROCKNEBY, An SSIJF-Financed Development Project No. 1040. http://www.transtechsys.com/pdf/Swedish%20Roads%20Report.pdf.

4:  Pedro Romero, P.E., 2002. Evaluation of non-nuclear gauges to measure density of hot-mix asphalt pavements. The University of Utah, Department of Civil and Environmental Engineering.

5:  Schmitt, R.L., 2006. Non-nuclear density testing devices and systems to evaluate in-place asphalt pavement density. University of Wisconsin-Platteville Chetana Rao and Harold Von Quintus Applied Research Associates.

6:  Sargand, S.M., S.S. Kim and S.P. Farrington, 2005. Non-nuclear density gauge comparative study. Ohio Research Institute for Transportation and the Environment.

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