How To Design An Asphalt Concrete Mix

Transcription

1 CEEN 3144 Construction Materials Asphalt concrete mix design Francisco Aguíñiga Assistant Professor Civil Engineering Program Texas A&M University Kingsville Page 1 Asphalt concrete Hot-mix asphalt Asphalt cement + aggregates mixed at high temperature placed and compacted while still hot 93% of the US roads (2 M miles) Page 2 1

2 Objectives of asphalt concrete mix design Provide Stability avoid permanent deformation under traffic loads Fatigue resistance avoid fatigue cracking Resistance to thermal cracking Resistance to hardening Resistance to moisture damage Provide skid resistance Provide workability Page 3 Fundamentals of mix design Figure 1 Stable structure with considerable internal friction High resistance to deformation under load Figure 2 Less stable structure with less internal friction Less resistance to deformation under load Page 4 2

3 Fundamentals of mix design Figure 3 Aggregate skeleton with asphalt binder Some air Aggregate internal friction available + asphalt binding Too little air brittle pavement Figure 4 No air rutting, bleeding Too much asphalt Aggregate floats on asphalt Easily deforms under load since there is no internal Page 5 friction Asphalt concrete mix design 1. Select the grading max aggregate size 2. Select the aggregate to use 3. Select the proportion of each aggregate to use 4. Find specific gravity of aggregate combination and asphalt cement 5. Make trial specimens 6. Find specific gravity of each specimen 7. Conduct stability tests 8. Compute percentage of voids, VMA, and percent voids filled with asphalt 9. Select optimum asphalt content Page 6 3

4 1. Select the grading Page 7 2. Select the aggregate to use Aggregates almost always selected by the contractor Select most economical materials Have as few aggregate piles as possible (usually 2 to 5) $ transportation > $ aggregate Uncrushed river gravel can be inexpensive, but yield a poor performance Select crushed stone whenever possible Page 8 4

5 3. Proportioning of aggregates In the lab trial and error Mathematically aa + bb = T When b is zero A = T/a Page 9 4. Specific gravities Asphalt specific gravity Usually provided by manufacturer Aggregate specific gravity Obtained in the laboratory Average specific gravity = 100 g a W1 + W2 g1 g + 2 g a = Specific gravity of combined aggregate W 1, W 2, W 3 = Respective percents by weight of aggregate 1, 2, and 3 g 1, g 2, g 3 = respective specific gravities of aggregate 1, 2, and 3 W 3 g 3 Page 10 5

6 4. Specific gravities Page Specific gravities Page 12 6

7 4. Specific gravities Page Specific gravities Page 14 7

8 4. Specific gravities Page Specific gravities Page 16 8

9 4. Specific gravities Page Preparation of compacted specimens Marshall Method Objective Optimize the asphalt content to obtain the highest stability value (load) and highest density Try asphalt contents in increments of 0.5 % Preparation of mix Optimize the asphalt content to obtain the highest stability value (load) and highest density Heat aggregates (350 to 375 F) Heat asphalt (250 to 280 F) stir constantly Discard hot asphalt sitting longer than 1 hour Weight size fractions and add aggregate, mix Temperature of mix should not be less than 225 F before compaction No reheating of mix Page 18 9

10 5. Preparation of compacted specimens Compaction of specimens Clean and heat compaction mold and hammer to 200 to 300 F in a bath of boiling water Place mold on base and insert paper filter, place 1200 g of hot batch mix and spade the mixture 15 times on perimeter and 10 times in the middle Remove collar and smooth surface Replace collar and apply 50 blows Reverse specimen and apply another 50 blows Release specimen from mold and allow to cool at room temperature over night Page Specific gravity of compacted specimens Page 20 10

11 6. Specific gravity of compacted specimens Page Specific gravity of compacted specimens Page 22 11

12 7. Stability test of compacted specimens Marshall flow test Weigh and measure specimens Heat specimen to 140 F in a water bath for 30 to 40 min Clean and lubricate guide rods Remove specimen from bath and place in lower segment of head, place top segment of head Place and zero flow meter Load specimen at 50.8 mm (2 in.) per minute until the load decreases (max reached) Record the flow in twenty-five hundredths of a millimeter (hundredths of an inch) Total elapsed time not to exceed 30 sec Page Compute percentage of voids Page 24 12

13 8. Compute percentage of voids Page Select optimum asphalt content Page 26 13

14 9. Select optimum asphalt content Page Select optimum asphalt content Page 28 14

15 9. Select optimum asphalt content Page Select optimum asphalt content Page 30 15

16 9. Select optimum asphalt content Page Select optimum asphalt content Page 32 16