INTRODUCTIONThere are several standard mix design procedures, which involve asphalt and aggregate mixes of varying proportions, compacting the mix samples using standard equipment and measuring the volumetric properties and strength of the samples. The properties of the mix are directly related to that of its constituents. The Hot Mix Asphalt (HMA) mix design process is a laboratory simulation used to determine what aggregate and binder should be used, and the best combination of the two. Asphalt content is selected to give optimum density, resistance to load and air voids ratio. Air voids in the sample are the pockets of air left after the aggregates have absorbed as much asphalt binder as possible during compaction. It is therefore dependent on the volume of asphalt content being used. All mix design methods use density and voids to determine basic hot mix asphalt (HMA) physical characteristics. Two different measures of densities are typically taken which are, bulk specific gravity and the theoretical maximum specific gravity. The densities are then used to calculate the volumetric parameters of the HMA. The specific gravity of the mix is found in accordance with AASHTO T209 and ASTM D 204 and the HMA design is carried out in accordance with the Marshall Mix Design method. The Bulk Specific Gravity of a compacted asphalt mixture is the ratio of the mass in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature. The bulk specific gravity is needed to determine weight-volume relationships and to calculate various volume-related quantities such as air voids and voids in mineral aggregate.The Marshall stability of mix is defined as the maximum load carried by a compaction specimen at a standard test temperature of 600C, and the flow value is the deformation the Marshall Test specimen undergoes during the loading up to the maximum load 0.25mm units. The Marshall Stability and flow test provides the performance prediction measures for the Marshall Mix Design method.

The Marshall Mix design process involves the three basic tasks of aggregate selection, asphalt binder selection and optimum asphalt binder content determination. In this experiment, selected aggregate and asphalt binder were provided and the asphalt content was altered for five trial mixes. The mixes were compacted and tested and the optimum ratio was determined based on the test performance and properties of each mix.

AIMThe aim of this laboratory experiment is to understand and implement the method of evaluation used in the Marshall Mix Design to determine the optimum asphalt content required to satisfy the strength parameters of a pavement.APPARATUSThe apparatus used to conduct the experiment include the following:

Apparatus used in the determination of the theoretical maximum specific gravityName: Yanik LubinCVNG 3009 Marshall Mix Design LabI.D.# : 809100024

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1) Vacuum container2) 2-2,000 mL Volumetric flasks3) Vacuum gauge4) Vacuum pump5) Thermometers6) Water bath7) Orbital shaker8) Pan9) Glass cover plate10) BalanceName: Yanik LubinCVNG 3009 Marshall Mix Design LabI.D.# : 809100024

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Apparatus used in the compaction and determination of flow and stability

1) Components for the mixture (1/2, 3/4 ,sand and dust aggregate along with bitumen)2) Specimen mold assembly3) Specimen extractor4) Compaction hammer5) Compaction pedestal6) Specimen mold holder7) Metal bowls for mixing8) Circulating water bath9) Ovens and hot plates10) Marshall compression and testing machine11) Spatula12) Bowls13) Spoons14) Flow meter

PROCEDUREFour aggregate samples were provided , , sand and dust with a target gradation of 12:30:11:47 respectively for the aggregate mix. The weight of each aggregate was calculated and the weight of the asphalt binder needed to produce 4.5, 5.0, 5.5, 6.0 and 6.5 percent by weight were calculated and recorded. The following procedure was then repeated for each asphalt content, to produce 1200 g of the mix:

1. Four samples with these aggregate ratios were prepared and heated in the oven.2. The amount of each aggregate calculated was weighed out into a bowl. The calculated weight of asphalt binder was added to the aggregate at the mixing temperature. 3. The aggregate and bitumen were transferred to a hot plate, to maintain the high temperature. It was mixed thoroughly until a uniformly covered sample was obtained, that is, aggregate must be completely coated with the binder. 4. The mixture was then left to heat to a temperature of 140oC.5. The uniformly coated 140C sample was then carefully poured into a mould assembly, containing rounds of paper on the top and bottom, to assist in easy removal of the sample. While in the mould, the sample was stuck with a spatula 15times around the edges and 10times at the centre. 6. The sample was then placed in the compaction machine and hammered with 75 blows on each face of the sample with a 10lb hammer.7. Steps 2 6 were repeated to obtain three compacted samples8. Each sample was weighed in air, in water, and then towel dried and weighed to obtain its saturated weight.9. Each sample was then placed in water at 60 C for 30 minutes, after which they were removed and tested with the Marshall Stability Test while simultaneously measuring the flow. 10. The stability gauge was zeroed and an initial flow gauge reading was recorded. Load was then placed until the sample failed (an abrupt, negative deflection in the stability gauge). The maximum value reached by the stability gauge up to failure was recorded as the stability reading.11. The procedure was repeated for the remaining asphalt contents (5.0%, 5.5%, 6.0% and 6.5%).Another mix was prepared using each binder percentage and allowed to cool in air. The following procedure was followed to obtain the specific gravity for each mixture. 1. 500g of the sample was weighed and placed in a conical flask containing water. The flask was then fastened to a vacuum and the air contained in the sample was extracted. 2. The flask was agitated periodically until no more air bubbles were visible. 3. The vacuum seal was removed and the flask was topped off to the brim with cold water, to reach a temperature of 25C. A glass plate was used to seal the flask by sliding it over the brim of the flask, ensuring that no air bubbles were present.4. It was then weighed and the temperature was recorded 5. The bottle was then emptied and filled with water at 25 C and weighed.

RESULTSMaterial3/4"1/2"SandDustBitumen

%12%30%11%47%-

Specific Gravity (s)2.6402.6802.6452.6851.020

Combined Bulk SG Aggregate2.674-

Table 1: Showing combined bulk specific gravity%Bitumen4.5%5.0%5.5%6.0%6.5%AVG

WO (g)500500500500500-

WW (g)1828.81828.81828.81828.81828.8-

WS (g)2130.72129.22128.72126.62125.5-

Max Theor. SG2.5242.5052.4992.4732.459-

Effective Agg. SG2.7122.7132.7292.7202.7272.720

Absorption Rate (%)0.5460.5530.7750.6520.7460.65

Table 2: Showing Max theoretical, effective aggregate specific gravity , and absorption rate per asphalt content

Table 3: Showing final calculations

SamplePlastic FlowStability

BeforeAfterFlow (in)Flow (mm)ReadingStability LoadFactorAdjusted Stability

10.3350.4480.1132.8339813.2361.0413.765

20.3270.4550.1283.2039913.2691.0413.800

30.3300.4570.1273.1836612.1891.0412.677

40.3220.4520.1303.2530910.3161.0410.729

50.3360.4640.1283.2034711.5621.0412.024

60.3370.4730.1363.4034611.5301.0411.991

70.3360.4960.1604.0030610.2171.0410.626

80.3350.4970.1624.052899.6591.0410.045

90.3290.4900.1614.032789.2981.049.670

100.3460.5400.1944.852668.9051.049.261

110.3410.5360.1954.882548.5111.048.851

120.3390.5250.1864.652819.3971.049.773

130.3440.5520.2085.202087.0021.047.282

140.3480.5920.2446.102327.7891.048.101

150.3420.5990.2576.432588.6421.099.420

Table 4: Showing flow and stability values from the readingsAsphalt Content (%)Max Theor. SGAir Voids(%)VMA (%)VFA (%)Effective Asphalt Content (%)Avg Asphalt Absorption Rate(%)Stability (kN)Flow (mm)

4.52.5244.8014.1766.129.370.65513.413.07

5.02.5053.8714.4373.2010.560.65511.583.28

5.52.4993.4914.7676.3611.270.65510.114.03

6.02.4732.5315.2683.4412.730.6559.304.79

6.52.4592.4016.0685.0313.650.6558.275.91

Table 5: Summary of values used in the graphs

SAMPLE CALCULATIONSMaximum theoretical density, Gmm (for 4.5%)

Combined Bulk Aggregate SG, Gsb

Effective SG of Aggregate. Gse

Asphalt Absorption rate, Pba

Effective Asphalt Content, Pbe (for 4.5%)

VMA (for 4.5%) = 100 - Volume of aggregate = 100 85.83 = 14.17 %

Air Voids, Pa(for 4.5%) = VMA Pbe = 14.17 9.4 = 4.8 %

The stability load is obtained from a Calibration Table, based on the stability gauge reading.

GRAPHS

Graph 1: Illustrating AC vs Air Voids

Graph 2: Illustrating AC vs Max Theoretical Specific Gravity

Graph 3: Illustrating AC vs VMA

Graph 4: Illustrating AC vs VFA

Graph 5: Illustrating AC vs Effective Asphalt Content

Graph 6: Illustrating AC vs Avg Absorption Rate

Graph 7: Illustrating AC vs Flow

Graph 8: Illustrating AC vs Stability

DISCUSSIONThe Marshall Mix Design method seeks to obtain the optimum asphalt binder to aggregate ratio for use in the surface course of a HMA. The properties of the aggregate and asphalt binder as well as their combination greatly affect the performance of an HMA. The Voids in mineral aggregates (VMA), the voids filled with asphalt (VFA), Stability and Flow properties are used throughout the design to ensure the suitability of a typical asphalt mix.Stability is the maximum load developed before the sample fails. Stability typically decreases with increasing asphalt content. The minimum stability value is vital to the performance of the pavement; high stability values for pavements not requiring them can be detrimental to its performance. Using the wrong stability mixes can produce a mixture that is difficult to compact in the field and not very durable. The mix must produce enough stability to satisfy the demands of traffic without displacement.Flow is the deformation that occurs at the maximum load which the specimen can withstand before failure. Flow just like stability is dependent on asphalt content; it increases with increasing asphalt content. From the graphs developed for the tests, errors occurred during the experiment and it should be noted that these tests should be repeated so that a better analysis may be performed. The percent air voids chosen for the mix design is 4% as stated in the laboratory manual.The Graph of Air voids vs. Asphalt content (AC) shows that as AC or binder increases the air voids decreases, due to the binder filling the spaces between the aggregate. At 4% air voids the binder was 4.90%. A small change in asphalt content produces a relatively larger change in air voids. The air voids in a mix can be altered by changing the compactive effort used during compaction. Flow increases with increasing asphalt content it also increases due to the viscosity of the AC when it is subjected to heat. The hotter an asphalt sample is, the more susceptible it is to deflection and deformation. Hence, the more asphalt content there is the more vulnerable the sample will be to deflection under a loading. Low flow values are indicative of a too strong aggregate structure while high flow values suggest a too weak aggregate structure. The stability is inversely related to the flow and is the resistance to deformation of the concrete, it decreases with increasing asphalt content, showing a peak in the lower AC range.

The purpose of the VMA is to provide adequate adhesion and durability without bleeding of the asphalt at high temperatures, by providing enough space for sufficient asphalt binder in the matrix. VMA is adjusted by changing aggregate properties. Voids in Mineral Aggregate (VMA) typically produces a flat curve in the area of the design asphalt content. This was not the case in the experiment and could be as a result of sources of errors throughout the experiment. The Max theoretical specific gravity as shown from the graph decreases with increase in asphalt content. Voids Filled with Asphalt (VFA) increases with increasing asphalt content and is simply the percentage of the VMA filled with asphalt. Effective asphalt content increases with increasing asphalt content and is the amount of asphalt remaining in the mixture after asphalt has been absorbed into the aggregate. Asphalt absorption remains constant with increasing asphalt content as it is a property of the aggregate and is not affected by the amount of asphalt binder present.Graph 1 shows that 4% air voids corresponds with an asphalt content of 4.9% by volume. This optimum asphalt content was used (as shown in Graphs 2-8) to identify the other optimum properties for a mix using the specific aggregates and asphalt binder provided. The optimum properties are outlined in the table below.

Mix PropertyOptimum Value

Air Voids (%)4

Voids in Mineral Aggregate - VMA (%)14.35

Max Theoretical Specific gravity 2.509

Voids Filled with Asphalt VFA (%)71.5

Effective Asphalt Content ACeff (%)10.3

Stability (kN)12.0

Flow (mm)3.2

Asphalt Absorption Rate0.650

Table 6 - Properties of the Design Mix corresponding to 4% Air Voids.

The advantages of the Marshall Design Method are: Attention to voids, strength, durability Use of inexpensive equipment Easy to use in process control/acceptanceDisadvantages: Impact method of compaction Does not consider shear strength Load perpendicular to compaction axis

SOURCES OF ERROR The mixing was done manually which could affect the quality of the sample mixed. Parallax error in reading off the flow and stability gauges.

PRECAUTIONS To allow for faster mixing the asphalt and aggregate were placed in the oven and heated prior to the experiment. This was also to ensure that there was good binding between the asphalt and aggregate. While the sample was mixing the sample was kept on a hot plate which was at a constant temperature. When mixing the sample, it was ensured that the mixing was done such that all the aggregate particles were fully covered with asphalt. This is necessary so that during compaction, there is proper bonding between the aggregate particles, thus improving the stability of the mix. Filter paper was placed both at the top and bottom faces when compacting the sample, before compaction began. This was done to prevent the hot mix from sticking to the base plate and to the hammer. The sample was also placed into a water bath and then towel dried before testing in the Marshall testing machine. This was done to simulate the worst conditions that could be present in the pavement in real situation (the pavement is waterlogged and heated).CONCLUSIONThe optimum asphalt content for the 12:30:11:47 graded aggregate provided is 4.9%, producing and asphalt concrete with 4% air voids, 14.35% VMA, 71.5% VFA, Stability of 12.0kN and Flow of 3.2mm.REFERENCES1. CVNG 3009. Highway Engineering Laboratory Scripts. Civil Engineering Department, University of the West Indies, St Augustine, Trinidad and Tobago.2. Murphy, Tim, and Ross Bentsen. 2001. Marshall Mix Design- Getting the Most out of your Marshall Mixes. IL: Humboldt Mfg. Co.3. http://pavementinteractive.org/index.php?title=Marshall_Mix_Design4. http://pavementinteractive.org/index.php?title=HMA_Mix_Design_Fundamentals