Resilient Modulus Lime-Treated Subgrade CBR: Evaluation of Pavement Performance under Repeated Loadi

The material quality of granular base and subbase layers is characterized in the AASHTO flexible pavement design procedures in terms of structural layer coefficients ai (see Section 3.5.2). These coefficients were entirely empirical through the 1972 version of the Guide. Beginning with the 1986 Guide, the recommended procedure for estimating structural layer coefficients is through correlations with resilient modulus.

It must be emphasized that structural layer coefficients are not fundamental engineering properties for a material. There are no laboratory or field procedures for measuring structural layer coefficients directly. The structural layer coefficients were originally defined as simple substitution ratios - i.e., how much additional thickness of granular base at a given reference stiffness must be added if a unit thickness of asphalt concrete of a given stiffness is removed in order to maintain the same surface deflection under a standardized load? These substitution ratios were evaluated in the 1986 AASHTO Guide1 via a parametric analytical study for a limited range of flexible pavement geometries and layer stiffnesses. In this approach, the value of the structural layer coefficient for a given material also depends not only on its inherent stiffness, but also upon the material's location within the pavement structure (e.g., the a2 value for a given material when used in a base layer is different from the a3 value for that same material when used as a subbase). Subsequent correlations between structural layer coefficients and other engineering properties such as resilient modulus and CBR are entirely empirical. Structural layer coefficients are not used in mechanistic-empirical design procedures like the NCHRP 1-37A Design Guide.

resilient modulus lime-treated subgrade cbr

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The values of EBS from the base layers in the original AASHO Road Test are summarized in Table 5-40. Note that the EBS values are not only functions of moisture, but also of stress state θ, which in turn is a function of the pavement structure - i.e., subgrade modulus and thickness of the surface layer. Typical values of θ recommended in the 1993 AASHTO Guide for use in base design are summarized in Table 5-41.

Figure 5-19 summarizes correlations between the a2 structural layer coefficient for nonstabilized granular base layers and corresponding values of CBR, R-Value, Texas triaxial strength, and resilient modulus. Similar correlations between a2 and various strength and stiffness measures for cement- and bituminous-treated granular bases are given in Figure 5-20 and Figure 5-21.

The resilient modulus ESB for granular subbase layers is influenced by stress state in a manner similar to that for the base layer, as given in Eq. (5.17). Typical values for the k1 and k2 material properties for granular subbases are:

The values of ESB from subbase layers in the original AASHO Road Test are summarized in Table 5-42. Note that the ESB values are not only functions of moisture, but also of stress state θ, which in turn is a function of the pavement structure - i.e., thickness of the asphalt concrete surface layer. Typical values of θ recommended in the 1993 AASHTO Guide for use in subbase design are summarized in Table 5-43. Figure 5-22 summarizes relationships between the a3 structural layer coefficient for granular subbase layers and corresponding values of CBR, R-Value, Texas Triaxial strength, and resilient modulus.

Mechanistic solutions for the stresses and strains in rigid pavements have historically characterized the stiffness of the foundation soil in terms of the modulus of subgrade reaction k (Figure 5-23). However, the modulus of subgrade reaction is not a true engineering property for the foundation soil because it depends not only upon the soil stiffness, but also upon the slab (or footing) size and stiffness. For an example of a square footing on a homogeneous isotropic elastic foundation soil, k can be expressed as:

The effective modulus of subgrade reaction is a direct input in the AASHTO design procedures for rigid pavements (see Section 3.5.2). The modulus of subgrade reaction was first introduced in the 1972 version of the Guide, with the recommendation that its value be determined from plate loading tests. Beginning with the 1986 Guide, the recommended procedure for estimating k for new/reconstruction designs is through correlations with subgrade MR plus various adjustments for base layer stiffness and thickness, presence of shallow rock, potential loss of slab support due to erosion, and seasonal variations2. The recommended procedure for determining k for rehabilitation designs is backcalculation from FWD test results.

The subgrade, base, and subbase resilient moduli values are the direct inputs in the NCHRP 1-37A design methodology. These values are adjusted internally within the NCHRP 1-37A Design Guide software for environmental effects and then converted into an average monthly effective k-value for structural response calculation and damage analysis.

For rehabilitation projects, the modulus of subgrade reaction k can be determined from FWD deflection testing of the existing PCC pavement. An FWD with a load plate radius of 5.9 inches and a load magnitude of 9000 pounds is recommended, with deflections measured at sensors located at 0, 12, 24, and 36 inches from the center of the load along the outer wheel path. For each slab tested, a dynamic kdynamic value (pci) can be determined from Figure 5-28 based on the deflection at the center of the loading plate, d0 (inches) and the AREA of the deflection basin as computed by4:

All subgrade and unbound pavement layers for all pavement types are characterized using MR in the NCHRP 1-37A design methodology. The pavement response model for rigid pavement design, however, is based on a Winkler-spring foundation model that requires a value for the modulus of subgrade reaction kdynamic (see Appendix D for more details on the rigid pavement response model). The modulus of subgrade reaction is obtained from the subgrade and subbase MR values and the subbase thickness through a conversion process that transforms the actual multilayer pavement structure into an equivalent three-layer structure consisting of the PCC slab, base, and an effective dynamic k, as shown in Figure 5-29. This conversion is performed internally in the NCHRP 1-37A Design Guide software as a part of input processing.

The modulus of subgrade reaction is a direct input for rigid pavement rehabilitation designs in the NCHRP 1-37A procedure. Measured surface deflections from FWD testing are used to backcalculate a kdynamic for design. The mean backcalculated kdynamic for a given month is input to the NCHRP 1-37A Design Guide software, and the kdynamic values for the remaining months of the year are seasonal adjustment factors computed by the EICM.

The permanent deformation characteristics of unbound materials are used in the empirical rutting distress models in the NCHRP 1-37A design methodology. This information is not required for rigid pavement design in the NCHRP 1-37A Design Guide or at all in the 1993 AASHTO design procedure. Permanent deformation characteristics are measured via triaxial repeated load tests conducted for many cycles of loading; Figure 5-30 shows schematically the typical behavior measured in this type of test. The repeated load permanent deformation tests are very similar to the cyclic triaxial tests used to measure resilient modulus (see 5.4.3), except that the cyclic deviator stress magnitude is kept constant throughout the test. There are at present no ASTM or AASHTO test specifications for repeated load permanent deformation testing. However, the first 1000 conditioning cycles of the AASHTO T307-99 resilient modulus testing procedure are often used for permanent deformation modeling.

In Eq. (5.28) through Eq. (5.30), MR is the resilient modulus in psi, and Wc is an estimate of the average in-situ gravimetric water content in percent. The NCHRP 1-37A procedure proposes the following equation for determining Wc in the absence of measured values:

Several types of chemically stabilized materials are usedunder pavements as base courses, subbase courses, or treated subgrade. Theseinclude lean concrete, cement stabilized or treated aggregate, soil cement,lime-cement flyash, and lime-stabilized materials. Typically, the compressivestrength of these materials is used for construction QA and modulus andflexural strength for pavement design. However, compressive strength testing ismore common than resilient/elastic modulus testing and flexural strengthtesting. Industry groups and individual researchers have published severalcorrelations to estimate chemically stabilized base elastic modulus andflexural strength from the compressive strength as shown in figure 29 through figure 32. The more common or feasible of these correlations are presented in figure 32. Resilient modulus (Mr) can be estimated conservatively as20 percent of the unconfined compressive strength (qu).(95)

As a result of extensive research into thecharacterization of resilient modulus characterization conducted over the pastfour decades, it is now widely recognized that resilient modulus exhibitsstress-state dependency, material dependency, and moisture and temperaturedependency. About 54 percent of State transportation departments use resilientmodulus in routine pavement design.(102) Ideally, resilientmodulus should be obtained from laboratory measurements; however,standard test procedures such as AASHTO T 307 and NCHRP 1-28A requiresubstantial time and resources and are not used in routine engineeringpractice, especially beyond the design phase of the project.(28,103)

Of the several approaches put forth to estimate resilientmodulus in the laboratory for design purposes, the one that has gainedconsiderable traction over time is using a universal constitutive model asproposed in NCHRP 1-28A (see figure 33).(103) The strength of thisapproach is that two of the resilient modulus dependencies, stress-state and material type, canbe handled by this model form, which is an improvement over previously useddiscrete models for coarse- and fine-grained soils which require knowledge ofmaterial behavior prior to applying a function to characterize it. 2ff7e9595c