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Publication Number: FHWA-HRT-07-052
Date: September 2007

Long Term Pavement Performance Project Laboratory Materials Testing and Handling Guide

Protocol P46
Test Method for Resilient Modulusof Unbound Granular Base/Subbase Materials and Subgrade Soils (UG07, SS07)

3. DEFINITIONS

The following definitions are used throughout this protocol:

(a) Layer: That part of the pavement produced with similar material and placed with similar equipment and techniques. The material within a particular layer is assumed to be homogeneous. The layer thickness of unbound granular base and subbase materials is determined from field exploration logs (borehole logs and/or test pit log).

(b) Sample: A representative portion of material from one or more pavement layers received from the field. A sample can be a core, block, chunk, pieces, bulk, thin walled tube or jar sample.

(c) Bulk Sample: That part of the pavement material that is removed from an unbound base or subbase layer or from the subgrade. Bulk samples are retrieved from the borehole(s) or a test pit at the designated locations. The bulk sample of each layer is shipped in one or more bag(s) to the Laboratory Material Testing Contractor. The material from one layer should never be mixed with the material from another layer—even if there is less than the desired amount to perform the specified tests.

(d) Test Sample: That part of the bulk sample of an unbound base or subbase layer or subgrade which is prepared and used for the specified test. The quantity of the test sample may be the same but will usually be less than the bulk sample.

(e) Test Specimen: For the purpose of this protocol, a test specimen is defined as, (i) that part of the thin walled tube sample of the subgrade which is used for the specified tests and (ii) that part of the test sample of unbound granular base or subbase materials or untreated subgrade soils which is remolded to the specified moisture and density condition by recompaction in the laboratory.

(f) Unbound Granular Base and Subbase Materials: These include soil aggregate mixtures and naturally occurring materials used in each layer of base or subbase. No binding or stabilizing agent is used to prepare unbound granular base or subbase layers. These materials may be classified as either Type 1 or Type 2 as subsequently defined in articles (h) and (i).

(g) Subgrade: Subgrade soils are prepared and compacted before the placement of subbase and/or base layers. These materials may be classified as either Type 1 or Type 2 as subsequently defined in articles (h) and (i).

For the LTPP material sampling and testing program: the thin walled tube sample of the subgrade is considered to be representative of the subgrade soils within the top 1.5 meters (five feet) of the subgrade; and the bulk sample of the subgrade retrieved from 305-mm (12-inch) diameter boreholes or the test pit is considered to be representative of the subgrade soils within 305 mm (12 inches) below the top of the subgrade, unless otherwise indicated on field exploration logs (borehole logs and/or test pit logs).

(h) Material Type 1: For the purposes of this protocol (resilient modulus tests), Material Type 1 includes all unbound granular base and subbase material and all untreated subgrade soils which meet the criteria of less than 70% passing the 2.00-mm (No. 10) sieve and less than 20% passing the 75-µm (No. 200) sieve, and which have a PI < 10. Soils classified as Type 1 will be molded in a 152-mm (6-inch) diameter mold. NOTE 1: If 10 percent or less of a Type 1 sample is retained on the 37.5-mm (1.5-inch) sieve, the material greater than the 37.5-mm (1.5-inch) sieve shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 37.5 mm (1.5 inch) sieve, the material shall not be tested and the material shall be stored until further notice and the COTR shall be notified. Instructions concerning the testing of these materials will be issued at a later date.

(i) Material Type 2: For the purpose of this protocol (resilient modulus tests), Material Type 2 includes all unbound granular base/subbase and untreated subgrade soils not meeting the criteria for material Type 1 given above in (h). Generally, thin walled tube samples of untreated subgrade soils fall in this Type 2 category. Remolded Type 2 specimens will be compacted in a 71-mm (2.8-inch) diameter mold.

NOTE 2: If 10 percent or less of a Type 2 sample is retained on the 12.5-mm (½-inch) sieve, the material greater than the 12.5-mm (½-inch) shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 12.5-mm (½-inch) sieve, the material shall not be tested and the material shall be stored until further notice and the COTR shall be notified. Instructions concerning the testing of these materials will be issued at a later date.

(j) Resilient Modulus of Unbound Materials: The modulus of an unbound material is determined by repeated load triaxial compression tests on test specimens of the unbound material samples. Mr is the ratio of the amplitude of the repeated axial stress to the amplitude of the resultant recoverable axial strain. Figure 1 illustrates a typical load (stress) and deformation (strain) versus time relationship for P46 testing.

The necessary input values for the calculation of resilient modulus are determined from the load-time and deformation-time plots as illustrated in Figure 2 and described herein. The loads/deformations are established by using the maximum load/deformation value minus the minimum load/deformation value for a given cycle. The minimum load/deformation value is determined by taking the average load/deformation values from the last 75 percent (nominally 0.75 second) of the cycle. The average value is used to negate the impact of possible "overshooting" of the load on the rest period cycle. Otherwise, if a strict maximum minus minimum algorithm is used, the overshoot values would become the minimum value and thus this would bias the resulting load/deformation value.

(k) Haversine Shaped Load Form - the required load pulse form for the P46 test. The load pulse is of the form (1-COSθ°)/2 and the cyclic load (Pcyclic) is varied from 10 to 100 percent of the maximum load (Pmax) as shown in Figure 3.

(l) Maximum Applied Axial Load (Pmax) - the total load applied to the sample including the contact and cyclic (resilient) loads.

Pmax = Pcontact + Pcyclic

Figure 1 of protocol P46 shows a typical plot of load versus time and deformation versus time.  The plot illustrates the definitions of the 0.1 sec load pulse and the 0.9 second recovery cycle.  It also provides an illustration of the definition of repeated load, P sub cyclic and the total vertical deformation, delta T

Figure 1. Typical load and deformation versus time relationships

Figure 2 of Protocol P46 illustrates the theoretical means for determining the maximum and minimum load and deformation.  The plot shows a theoretical load or deformation versus time.  It shows the point to be considered as the maximum deformation or load and the data used to establish the minimum load or deformation which is to come from the last 75 percent of the cycle

Figure 2. Theoretical determination of maximum/minimum load and deformation

Figure 3 of Protocol P46 illustrates the various terms associated with the loading.  The curve shown in the picture depicts the change in load over time which is described by the haversine waveform as defined by the equation 1 minus cosine of theta over 2.  The load pulse lasts for 0.1 seconds.  The contact load, P sub contact, is the minimum applied load to the sample that remains constant throughout the test sequence.  The maximum applied load is the P sub max.  The difference between P sub max and P sub contact is the cyclic load or P sub cyclic

Figure 3. Definition of resilient modulus terms

(m) Contact Load (Pcontact) - vertical load placed on the specimen to maintain a positive contact between the specimen cap and the specimen.

(n) Cyclic Axial Load (Resilient Vertical Load, Pcyclic) - repetitive load applied to a test specimen which is used to calculate resilient modulus.

(o) Maximum Applied Axial Stress (Smax) - the total stress applied to the sample including the contact stress and the cyclic (resilient) stress.

(p) Cyclic Axial Stress (Resilient Stress, Scyclic) - cyclic (resilient) applied axial stress.

(q) Contact Stress (Scontact) - axial stress applied to a test specimen to maintain a positive contact between the specimen cap and the specimen.

Also:

(r) S3 is the total radial stress; that is, the applied confining pressure in the triaxial chamber (minor principal stress).

(s) er is the resilient (recovered) axial deformation due to Scyclic.

(t) εr is the resilient (recovered) axial strain due to Scyclic.

NOTE 3: "L" is considered to be the original test specimen length. This calculation of strain is only valid for testing equipment with linear voltage displacement transducers (LVDTs) positioned outside of the triaxial chamber. If measurement devices are mounted on the specimen, then the value of "L" in the strain calculation becomes equal to the gauge length of the transducers.

(u) Resilient Modulus (Mr) is defined as Scyclicr.

(v) Load duration is the time interval the specimen is subjected to a cyclic stress (nominally 0.1 sec.).

(w) Cycle duration is the time interval between the successive applications of a cyclic stress (nominally 1.0 sec.).


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