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46 ANNEX A-6 OBTAINING A UNIFORM DENSITY IN TYPE 3 SOILS 1. SCOPE 1.1 This method provides procedures for measuring and minimizing or eliminating density gradients in a speci- men of Type 3 soil for use in resilient modulus testing. 1.2 Specimens should be compacted in five lifts (layers) in a density gradient mold. Kneading compaction (Annex A-4) shall be used. The number of tamps per lift shall be adjusted for each lift to avoid imparting a density gradient to the specimen. 2. SIGNIFICANCE AND USE 2.1 A specimen fabricated in layers, with each lift receiving equal compactive effort, will typically exhibit a density gradient with the first lift compacted being more dense than the last. To measure this effect, the density gradi- ent mold is used to determine the density of each lift. A trial and error process is used to adjust the compactive effort for each lift until the desired specimen density is obtained with minimum gradient. 2.2 Density gradients can occur with vibratory (Annex A-2), impact (Annex A-3) or kneading (Annex A-4) compac- tion methods. The density gradient mold can be used in conjunction with any of the three compaction methods to measure density gradients from top to bottom within the specimen. 2.3 For the most accurate characterization of resilient modulus, the density gradient within the test specimen should be minimized. The test specimen should have a structure that closely approximates the one that will be obtained in field compaction. The use of kneading com- paction can help to achieve the proper structure. 3. APPARATUS 3.1 Density Gradient Mold â The density gradient mold is shown in Figure A-6-1. This is a split mold with the inside milled to receive five interchangeable solid rings that have been permanently numbered from 1 to 5. The ring I.D. shall be equal to the mold I.D. The mold height and diameter shall be the same as is used in preparing specimens for the resilient modulus testing (Annex A-4). 3.2 Compactor â To measure the density gradient, the same type of compactor shall be used as is used in preparing specimens for resilient modulus testing, as described in Annex A-2, A-3, or A-4. To minimize the density gra- dient, either a manual or mechanical kneading compac- tor, as described in Annex A-4, shall be used. 4. PROCEDURE FOR COMPACTING SPECIMENS TO MEASURE DENSITY GRADIENTS 4.1 Specimen material shall be prepared in accordance with Annex A-1. If the maximum particle size exceeds 25 percent of the mold inside diameter, the oversize par- ticles shall be scalped. The specimen will be fabricated 6 to 8 mm over height to allow trimming to a square end. Increase the quantity of material prepared to allow for the trimming. 4.2 Specimens shall be compacted to the same diameter, and using the same apparatus and procedure as is used in preparing specimens for resilient modulus testing, as described in Annex A-2, A-3, or A-4. 4.3 Remove the collar and carefully screed off the speci- men to the top of the mold. Small depressions in the screeded surface, caused by removal of larger particles, shall be filled with fines. Remove the split mold from the base and the mold halves from the specimen. Leave the five rings on the specimen. 4.4 Determine and record the net mass of the entire speci- men to the nearest gram. To do this, subtract the mass of the specimen with rings attached. 4.5 Determine and record the moisture content of the remaining soil according to AASHTO T 265. 4.6 Using a hacksaw or other abrasive device, carefully cut the specimen into five pieces. Each cut should be made midway between the rings. Screed off each piece to form square ends at the top and bottom of each ring. Small depressions in the screeded surface, caused by removal of larger particles, shall be filled with fines. 4.7 Determine and record the net mass of each numbered piece to the nearest gram. Use a tabular form, as in Figure A-6-2, to record the data. 4.8 Determine and record the moisture content of each num- bered piece. 4.9 Calculate and record the average bulk (wet) density of the entire specimen, γs, and densities of each five pieces, γs to γ5. 4.10 If the maximum difference between the density of each individual piece and the average density is 1.0 percent or less, report the density gradient as being uniform. If the maximum difference between the density of each individual piece and the average density is more than 1.0 percent, report the density gradient as being non- uniform. 5. PROCEDURE FOR COMPACTING TEST SPECIMENS TO ACHIEVE A UNIFORM DENSITY 5.1 Follow the procedure in Sections 4.1 through 4.9, using either the manual or mechanical kneading compaction procedure (Annex A-4).
47 Note 1 â The requirements in Annex A-4 that the number of tamps per lift be constant is waived. However, it is rec- ommended that the number of tamps per lift be constant for each lift in the first trial specimen in order to estab- lish that a density gradient does exist. 5.2 If the average bulk density differs from the target den- sity by more than the tolerance allowed in Section 7.3.2 or 7.3.3, then the compaction pressure shall be adjusted, holding the number of tamps per lift constant, to in- crease or decrease the average density toward the target density. Repeat 5.1 5.3 After the target average density is achieved, check the density uniformity according to Section 4. If the density Figure A-6-1. Density Gradient Mold is non-uniform, hold the compaction pressure constant and adjust the number of tamps per lift until a uniform density gradient is achieved. Note 2 â If a sufficient quantity of material is available, it is preferable to use new material for each subsequent specimen. If the old material is reused it will have an effect on the structure of subsequently compacted speci- mens. Use the compaction pressure and number of tamps per lift thus determined to prepare specimens for resilient modulus testing using the procedure in Annex A-4.
48 Figure A-6-2
