Durable Pavements

Soheil Nazarian, PhD, PE, Professor, Director, Cesar Tirado, PhD, Research Associate, Center for Transportation Infrastructure Systems, The University of Texas at El Paso, Mehran Mazari, PhD, Assistant Professor of Civil Engineering, California State University, Los Angeles, and Anjan Siddagangaiah, PhD, Assistant Professor of Civil Engineering, Indian Institute of Technology Guwahati

Build right to minimize maintenance and rehabilitation

Satisfactory pavement performance can be assured only with an appropriate design with a well-calibrated algorithm, material selection based on the appropriate mix design, proper processing of the materials for uniformity, and adequate compaction equipment to ensure proper density and stiffness.

The primary tool for performance management is currently the nuclear density gauge (NDG) to ensure that appropriate density is achieved. Measurement of density, even though quite practical and straightforward, does not directly tie the construction quality with the anticipated performance. In-situ non-destructive testing (NDT) devices that estimate the stiffness parameters of a constructed pavement structure are emerging. Such stiffness parameters are more representative of the pavement performance predicted during the mechanistic design process. A transformation from density-based to modulus-based quality assurance (QA) approach involves technical and organizational challenges that must be recognized and addressed in order to develop an efficient and practical specification.

This article will cover the different concepts and definitions for a performance management program that may yield to a long-lasting highway network with optimal maintenance and minimal rehabilitation.

Durable-PavementsFigure 1: Harmonization of Construction with Design and Material Selection
What is Performance Management?

Performance management should supplant the traditional quality control/quality acceptance in order to implement a rational and quantifiable definition of quality. Performance management is a comprehensive paradigm that ensures performance of pavement structure with defined risks for owner agency and contractor. In that sense, performance management is defined as carrying out tests that their results can be used to ensure that a certain pavement lasts for a pre-defined life. To achieve this goal, the pavement design, material selection and construction quality control should be harmonized and integrated as shown in Fig. 1.

Risk Management

Given the inherent variability in the materials and construction, and time and funding limitations, it is impossible to construct a perfect road, and as such there is always the risk of premature failure in some sections of the road. This risk has to be balanced between the owner agency and the contractor so that the owner agency receives what it paid for while ensuring reasonable profit for the contractor who is working hard to build a high-quality road. To minimize the premature rehabilitation and maintenance, the frequency of acceptance tests should be based on the type of contract (design-build-maintain vs. design-bid-build), importance of layer to performance of the pavement, and to some extent, the historical and present contractor performance.

Performance Indicators

Since most design programs require modulus or strength of each layer as input, modulus along with the thickness of the pavement layers are the major performance indicators. Adequate in-place density, moisture content and their uniformity are vital to the success of compacted materials. However, satisfying these criteria may not necessarily yield adequate modulus. To provide continuity among the design, construction, and laboratory testing, it is desirable to migrate from the traditional specifications to a rigorous modulus-based approach.

Major Steps in Performance Management

Durable PavementsFigure 2: Steps in Performance Management
Several inter-related parameters have to be considered in such a specification. A mechanistic-empirical structural design software should be considered from the beginning so that the level of sophistication of the pavement design, laboratory testing, and field testing can be balanced (Figure 2). The construction specification should be ideally tied to a mechanistic-empirical (ME) design algorithm.

Achieving adequate modulus will not assure the durability of a compacted geomaterial. The source of the material for each layer, either in-place or imported, should be identified, and its suitability for a durable layer should be ascertained. This step can only be achieved by taking advantage of the local know-how and solid engineering judgment.

A design parameter (usually modulus) of each layer should be estimated. Depending on the level of sophistication of the analysis and budgetary constraints, layer modulus can be estimated from either empirical relations, or presumptive default values, or a catalog of values established for common local geomaterials. One advantage of the modulus-based inspection is that the designer will have access to the actual moduli that were achievable at nearby sites built with similar materials.

Target field modulus should be set in conjunction with establishing the design modulus. The target modulus should be set considering the following parameters:

  • Thickness of the layer being tested and the subsequent layers below it
  • Design modulus of the layer being tested and the layers below it.
Field moduli should be measured during construction with an appropriate device to ensure that the target modulus has been achieved in a uniform manner. The appropriate equipment for this purpose should have the following four attributes:
  • able to measure fundamental properties of materials (i.e., modulus)
  • sensitive enough so that poor and high-quality materials can be readily delineated
  • accurate enough to provide feedback to the pavement designer
  • precise enough so that it can be confidently used in the acceptance process.
Durable Pavements

A fair and equitable acceptance process requires appropriate tolerances based on the uncertainties in establishing the target modulus and the measuring devices to minimize any disputes between the contractor and the highway agency. Appropriate statistical analyses should be carried out to ensure that the modulus and its variability along the project are in control.

Indicators of High Quality Products

A high-quality product should have the following two characteristics:
  • the modulus of each layer should be close to the value specified by the designer
  • each layer should be uniform.
A modulus-based spot testing device can provide the information necessary to ensure that the required modulus is achieved. Proof rolling with an intelligent compaction (IC) roller, as described below, can be an effective way of determining the uniformity of given project.

Durable-PavementsFigure 5: Variations of Modulus of a Subgrade Layer as a function of Placement Moisture Content
Available Tools for Spot Tests

The most common portable devices available in the market are the Plate Load Test (PLT), Falling Weight Deflectometer (FWD), Dynamic Cone Penetrometer (DCP), Lightweight Deflectometer (LWD) and Portable Seismic Property Analyzer (PSPA). The Plate Load Test (PLT) is a field test for determining the ultimate bearing capacity of soil and the likely settlement under a given load. PLT consists of loading a steel plate placed on the compacted layer surface and recording the settlements corresponding to a given load increment.

Many Departments of Transportation (DOTs) have moved away from the Plate Load Test because of the time-consuming process for the test. The FWD is a nondestructive field test to assess the material properties under simulated traffic loads when performed on top of the finished pavement. The pavement layer parameters, FWD load and measured deflections, at different offsets from the loading plate, are used to back calculate the modulus of each pavement layer. Since the logistics of the full implementation of FWD may be problematic, use of faster and more portable devices is on the rise.

Table 1 - Comparison of Tools for Measuring Modulus

Device DCP LWD PSPA
table-pic table-pic table-pic
Parameter Reported Penetration Rate Deflection/ Modulus Modulus
ASTM Standard D-6951 E-2583 None
Expertise needed for data collection and interpretation Minimal Moderate Moderate but more than other devices
User-friendliness Easy Easy Easy
Speed 10 minutes 2 Minutes 1 Minutes
Initial Costs $3,000 $6,000-10,000 $20,000
Advantages Ability to estimate the in-place strength of compacted materials. Does not require extensive support software for evaluating test results. Can test multi-layers State of stress is closer to vehicular stresses than any other device. Pavement community is familiar with concept of deflection-based testing Measures layer-specific modulus independent of thickness of layer. Results can be calibrated to specific material being tested prior to construction
Disadvantages akes time to perform test. Test results are more dependent on aggregate size than other NDT devices. Penetration rate has to go through two levels of empirical correlations to estimate modulus Moduli can be influenced by the underlying layers. Any error in thickness of the layer being tested can result in large errors and more variability in modulus Requires more sophisticated training of technicians

Table 1 compares the cost, speed and ease of use of the more portable devices. The DCP test involves driving a cone shaped probe into the compacted geomaterial layer using a dynamic load and measuring the advancement of the probe for each applied blow or interval of blows. The LWD is a portable FWD that has been developed as an alternative in-situ testing device to the plate load test. Generally, the LWD consists of a loading device that produces a defined load pulse, a loading plate, one center displacement sensor (and up to two optional additional sensors) to measure the center deflection or a deflection bowl. Similar to FWD, LWD determines the stiffness of pavement system by measuring the material’s response under the impact of a load with a known magnitude and dropped from a known height.

Durable PavementsFigure 6: Pavement Structure of Site, and Mapping of Subgrade (ICMV, Coefficient of Variation of ICMV, and LWD Deflection)

Intelligent Compaction (IC) Roller

An IC roller is a vibratory roller equipped with a data acquisition (DAQ) system that processes and displays compaction data in real time. DAQ can be either factory-installed/original equipment manufacturer (OEM) or a retrofit system. The roller drum acceleration (i.e. ICMV) is measured during proof-mapping of the compacted geomaterials using roller-mounted accelerometers. Proof mapping, also known as final coverage, is used to evaluate the compaction uniformity, often by generating a color-coded map of the ICMVs. DAQ consists of an accelerometer that is mounted on the roller (drum), a data acquisition box, a global positioning system and a computer to monitor the data collection process.

Consequences of not incorporating Performance Management

The best way to discuss the reason for incorporating the modulus-based testing is through a case study. Three test sections (see Figure 4) were constructed with full-scale construction equipment to simulate normal highway construction. The difference among the three sections was their moisture contents ─ dry (2% below optimum moisture content), optimum (at optimum) and wet (2% wet of optimum) ─ before compaction. All sections were compacted until they achieved the required 95% relative density.

Figure 5 summarizes the acceptance scenarios based on the field measurements. The target modulus of the layer is shown in the figures. Also shown is a line corresponding to 80% of target to allow for the inevitable variability in the materials and tolerance with moisture content allowed in the specifications. Half of the test spots in dry section passed the acceptance limit. None of the two sections placed at optimum and wet of optimum, achieved the acceptance limit. This case study shows that relying on field density for quality management may be the reason for premature failure of a number of pavements, since the moduli estimated by the designer may not be achievable in the field.

Durable PavementsFigure 7: Backcalculation of Base Modulus for the Prediction of Service Life Reduction

A Case Study

Durable PavementsFigure 8: Mapped Reduction in Service Life and Reduction in Service Life per Station
The significance of performance management by means of a tool that evaluates stiffness and uniformity, and provides full coverage of the compacted area, is demonstrated with the following example. A site consisting of 7.5in-thick hot mix asphalt, over 12in of unbound aggregate base, and 8in. of lime-stabilized clayey subgrade with the design moduli (shown in Figure 6) was evaluated. Proof-mapping of the site with an IC roller was performed on top of the lime-stabilized subgrade and the base layer. For mapping the ICMV measurements, the section was divided into blocks with the width equal to the width of the roller and the length equal to the minimum length of the compacted section that is practical to rework (e.g. 25ft, 7.5m). The ICMV measurements within each rectangular area were averaged to obtain a representative ICMV characterizing the stiffness of that block. This approach addresses the inherent uncertainties related to the moving of the roller and the accuracy of GPS devices. To address the uniformity of the test section, a second color-coded map representing the coefficient of variation of the data within each block is also provided. Figure 6 shows the mapping process on the construction site, the distribution of ICMVs, and the coefficient of variation of the ICMV measurements. LWD testing was performed at the center of each block and the LWD deflection values visualized in the same format (Fig. 6). Proof-mapping of the site on top of the lime-stabilized layer with an IC roller provided very low ICMVs (below 5) while LWD testing yielded deflection measurements above 30 mils (0.75 mm).

Figure 7 shows the LWD moduli as calculated from the deflection measurements on subgrade and on top of base layer at the same locations. The back-calculated modulus of the base layer was determined using a multi-layered equivalent-linear algorithm by means of an iterative process that adjusts the base modulus until the LWD deflection on top of the base layer is converged. Once the base moduli were back-calculated, the design structure was simulated using a finite element analysis program suited for the analysis of flexible pavements subjected under truck traffic to determine the number of passes of 34 kip (150 kN) tandem axles to reach a rutting failure (defined as 0.5 in., 12.7 mm). This number was then compared to the number of passes to reach failure for each of the individual blocks using their respective back-calculated properties to determine the reduction in service life.

Highway

Figure 8 shows the predicted reduction in service life in terms of rutting as obtained for each of the rectangular blocks. The average values per block are also shown. The mapped life reduction shows that some areas will be far from meeting their designed service life. This example shows the need to identify compaction uniformity by means of a tool that can provide full coverage of the site that relates to stiffness.

This case study shows the importance of improving the quality control/quality management practices, especially for concession projects, where subsequent maintenance, rehabilitation and reconstruction are necessary.

For more information, the reader is referred to:
  • Nazarian, S., M. Mazari, I. Abdallah, A. J. Puppala, L. N. Mohammad, and M. Abu-Farsakh (2014). Modulus-Based Construction Specification for Compaction of Earthwork and Unbound Aggregate. NCHRP Project 10-84 Final Report, Transportation Research Board of the National Academics.
  • Tirado, C., Rocha, S., Fathi, A., Mazari, M. and Nazarian, S. (2018). Deflection-Based Field Testing for Quality Management of Earthwork. Research Report FHWA/TX-17/0-6903-1, Center for Transportation Infrastructure Systems, The University of Texas at El Paso. Same concept can also be applied to hot mix asphalt pavement layers as reflected in:
  • Celaya M., Nazarian S., Zea M. and Tandon V. (2006). Use of NDT Equipment for Construction Quality Control of Hot Mix Asphalt Pavements. Research Report FHWA/AZ- 2006-574, Center for Transportation Infrastructure Systems, The University of Texas at El Paso. For Portland cement concrete, please review:
  • Nazarian S., Yuan, D. Smith, K., Ansari, F. and Gonzalez, C. (2006). Acceptance Criteria of Airfield Concrete Pavement Using Seismic and Maturity Concepts. Research Report IPRF-01-G-002-02-2, Innovative Pavement Research Foundation, Skokie, IL.
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