U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-052
Date: September 2007
The procedure described in this section applies to all unbound granular base and subbase materials. This can include specimens classified as Type 1 (152-mm [6-inch] diameter specimens) or Type 2 (71-mm [2.8-inch] diameter specimens) material.
8.2.1 Assembly of the Triaxial Chamber Follow Steps 220.127.116.11 through 18.104.22.168. When compaction is completed, place the paper filter, dry porous bronze disc and sample cap on the top surface of the specimen. Roll the rubber membrane off the rim of the mold and over the sample cap. If the sample cap projects above the rim of the mold, the membrane should be sealed tightly against the cap with the O ring seal. If it does not, the seal can be applied later. Install the sample in the triaxial chamber as in steps 22.214.171.124 through 126.96.36.199.
188.8.131.52 Connect the chamber pressure supply line and apply a confining pressure of 103.4 kPa (15 psi).
184.108.40.206 Remove the vacuum supply from the vacuum saturation inlet and open the top and bottom head drainage ports to atmospheric pressure.
8.2.2 Conduct the Resilient Modulus Test After the test specimen has been prepared and placed in the loading device as described in 8.2.1, the following steps are necessary to conduct the resilient modulus testing:
220.127.116.11 If not already done, adjust the position of the axial loading device or triaxial chamber base support as necessary to couple the load generation device piston and the triaxial chamber piston. The triaxial chamber piston should bear firmly on the load cell. A contact stress of 10 percent ± 0.7 kPa (± 0.1 psi) of the maximum applied axial stress during each sequence number shall be maintained.
18.104.22.168 Adjust the recording devices for the LVDTs and load cell as needed.
22.214.171.124 Conditioning - Set the confining pressure to 103.4 kPa (15 psi) and apply a minimum of 500 repetitions of a load equivalent to a maximum axial stress of 103.4 kPa (15 psi) and corresponding cyclic axial stress of 93.1 kPa (13.5 psi) using a haversine shaped load pulse consisting of a 0.1 second load followed by a 0.9 second rest period. If the sample is still decreasing in height at the end of the conditioning period, stress cycling shall be continued up to 1000 repetitions prior to testing.
The foregoing stress sequence constitutes sample conditioning, that is, the elimination of the effects of the interval between compaction and loading and the elimination of initial loading versus reloading. This conditioning also aids in minimizing the effects of initially imperfect contact between the sample cap and base plate and the test specimen. The drainage valves should be open throughout the resilient testing.
If the total vertical permanent strain reaches 5 percent during conditioning, the conditioning process shall be terminated. A review shall be conducted of the compaction process to identify any reason(s) why the sample did not attain adequate compaction. If this review does not provide an explanation, the material shall be re-fabricated and tested a second time. If the sample again reaches 5 percent total vertical permanent strain during preconditioning, then the test shall be terminated and the appropriate item on the data sheet shall be completed. No further testing of this material is necessary.
NOTE 16: The operator/technician shall conduct appropriate QC/QA comparative checks of the individual deformation output from the two vertical transducers during the conditioning phase of each Mr test in order to recognize specimen misplacement and misalignment. During the preconditioning phase, the two vertical deformation curves should be viewed to ensure that acceptable vertical deformation ratios are being measured. Desired vertical deformation ratios (Rv) are defined as Rv = Ymax/Ymin < 1.10, where Ymax equals the larger of the two vertical deformations and Ymin equals the smaller of the two vertical deformations. Unacceptable vertical deformations are obtained when Rv > 1.30. In this case, the test should be discontinued and specimen placement/alignment difficulties alleviated. Once acceptable vertical deformation values are obtained, (Rv < 1.30) then the test should be continued to completion. It is emphasized that specimen alignment is critical for proper Mr results.
126.96.36.199 Testing Specimen - The testing is performed following the loading sequences in Table 2 using a haversine shaped load pulse as described above. Decrease the maximum axial stress to 21.0 kPa (3 psi) and set the confining pressure to 21.0 kPa (3 psi) (Sequence No. 1, Table 2).
188.8.131.52 Apply 100 repetitions of the corresponding cyclic stress using a haversine shaped load pulse consisting of a 0.1 second load followed by a 0.9 second rest period. Record the average recovered deformations for each LVDT separately for the last five cycles on Worksheet T46.
NOTE 17: The contact stresses shown in Table 2 should be adjusted to compensate for the resultant force created by the chamber pressure (upward force) and the weight of the chamber piston rod, including the LVDT holder, (downward force). Instructions for adjusting the contact load are given in Appendix D of this procedure.
184.108.40.206 Continue with Sequence No. 2 increasing the maximum axial stress to 41.0 kPa (6 psi) and repeat 220.127.116.11 at this new stress level.
18.104.22.168 Continue the test for the remaining load sequences in Table 2 (sequences 3 to 15) recording the vertical recovered deformation. If, at any time the total vertical permanent strain (after preconditioning) exceeds 5 percent, stop the test and report the results on Worksheet T46.
22.214.171.124 After completion of the resilient modulus test procedure, check the total vertical permanent strain that the specimen was subjected to during the resilient modulus (after preconditioning) portion of the test procedure. If the total vertical permanent strain did not exceed 5 percent, continue with the quick shear test procedure (Section 126.96.36.199). If the total vertical permanent strain exceeds 5 percent, the test is completed. No additional testing is to be conducted on the specimen.
188.8.131.52 Apply a confining pressure of 34.5 kPa (5 psi) to the specimen. Apply a load so as to produce an axial strain at a rate of 1 percent per minute under a strain controlled loading procedure. Continue loading until either (1) the load values decrease with increasing strain, (2) 5 percent strain is reached (from the initiation of the quick shear test) or (3) the capacity of the load cell is reached. Data from the internally mounted deformation transducer in the actuator shaft and from the load cell shall be used to record specimen deformation and loads at a maximum of 3 second intervals.
NOTE 18: It has been noted that even though some samples visually bulge and appear to have failed, they do not achieve the above definition of failure at the maximum strain value (5 percent). In some cases, the stress-strain curves "level out" and the load values remain at, or near, constant and do not decrease with increasing strain. If a sample appears to fail without achieving the aforementioned criteria, a comment note should be added to the test data reporting sheet to document this occurrence.
184.108.40.206 At the completion of the triaxial shear test, reduce the confining pressure to zero and remove the sample from the triaxial cell.
220.127.116.11 Remove the membrane from the specimen and use the entire sample to determine the moisture content in accordance with LTPP Protocol P49. Record this value on the appropriate form (See Worksheet T46).
18.104.22.168 Plot the stress-strain curve for the specimen for the triaxial shear test procedure.
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