The Superpave mix design system is a comprehensive method of designing paving mixes tolerated to unique performance requirements dedicated by traffic, environment(climate), and structural section at particular environment site. It facilitates selecting and combing Asphalt binder, Aggregate and any necessary modifier to achieve the required level of pavement performance.
Under the strategic highway research program (SHRP) an initiative was undertaken to improve material selection and mixture design by developing:1. A mix design method that accounts for traffic loading and environmental conditions.
2. A new method of Asphalt Binder evaluation.
3. New method of Mixture Analysis
The objective of Superpave mix design system is to define an economical blend of Asphalt binder and aggregate that yields a paving mix having
1. Sufficient Asphalt binder
2. Sufficient Voids in Mineral Aggregate (VMA) and Air voids.
3. Sufficent Workability and
4. Satisfactory performance characteristics over the service life of pavementSuperpave Mix
The Important Features of Mix Design.
1. Performance based
2. It is specification based i.e every part of this is in under control points.
3. The use of formal aggregate depends on consensus requirement
4. The use of P G Asphalt binder grading system and its associated asphalt binder selection system.5. The use of Gyractory compactor to know field compaction. So we easily know what will be the field compaction
6. Its all depend on traffic and environment of project
7. Full volumetric approach to mix design
MIX DESIGN STEPS
SELECTION OF MATERIAL
SELECTION OIF DESIGN AGREGATE STRUCTURE
SELECTION OIF DESIGN ASPHALT CONENT
STEP 1.SELECTION OF MATERIAL
1A. ASPHALT BINDER
STEP 1 A SELECTION OF MATERIAL (BINDER)ASPHALT BINDER
Asphalt is a principle constituent of HMA. It work as a inexpensive, water proof, thermoplastic, viscoelastic adhesive its define as "A dark brown or black cementitious material in which the predominating constituents are bitumen, which occur in nature re obtained in petroleum processing".
SELECTION OF ASPHALT BINDER GRADE
For Asphalt binder grade selection the following information’s are required.
1. Determine project weather data for 30 years in past
2. Determine design temperature
3. Determine the performance grade In country
Selection of Asphalt Binder Grade is based on
1. Country temperature zoning map.
2. Verify Asphalt PG3.
3. Determine temperature viscosity relationship
The grading system is based on climate.
Example PG = 70 - 10
PG= Performance grade 70 = Average 7 day pavement Design Temperature 10= minimum pavement design Temperature
This selection of Asphalt Binder is based on Country temperature Zoning map.GRADE SELECTION (2) BUMPING.Grade Bumping for speed and loading of traffic: increase the selected PG based on the map by the grade equivalent as shown below
TRAFFIC CLASS DESIGNATION
ASPHALT BINDER MIXING & COMPACTION
We can determine Laboratory mixing and Compaction temperature using rotational viscometer
The method which will we use for this
Mixing Temperature at 0.17 Pa.s(±)
Compaction Temperature at 0.28 Pa.s(±)
we need to check the consensus requirements and source requirement of aggregate which will we use for superpave design
Consensus Requirements are
1. Course Aggregate Angularity(CAA)
AASHTO T 335
2. Fine Aggregate angularity.(FAA)
3. Flate and elongated particles(F & E)
ASTM D 4791
4. Clay Content,(SE)
The Source Requirement
3. Clay lumps and Friable particles
1 B. AGGREGATE SELECTION SUPERPAVE AGGREGATE REQUIREMENT
Second we need to see the superpave aggregate requirement .which depends upon traffic and depth of layer
It is the relative distribution of the weight of different sizes to the total weight of sample.It is determined by sieve analysis of sample.
Gradation which we obtain it has three
The type of gradation that is near the FHWA’s 0.45 power curve for maximum density
The gradation that contains only a small percentage of aggregate particles in the mid size rang. The curve is flate in the mid size range.
The gradation that only contains small percentage of aggregate particles in the small range. This result in more air voids because there are not enough small particles to fill in voids between the larger particles. The curve obtained in this case is near vertical in the mid size range and flat and near zero in small size range.
A gradation that contains most of particle in a very narrow size rang. In essence all particle in the same size. The curve is steep and only occupies the narrow size range specified.
A gradation that when plotted on 0.45 power gradation graph fall mostly above the 0.45 maximum density line.
A gradation that when plotted on the 0.45 power gradation graph fall mostly below the 0.45 maximum line Graph shows all types
SELECTION OF DESIGN AGGREGATE STRUCTURE (DAS)
Selection of DAS based on four steps
1. Develop Trial Blends
2. Compact blends
3. Evaluate blends
4. Select confirming blend
Develop Trial BlendsThis depends on three points
1. For DAS minimum three blends selected
2. Gradation of Samples from hot bin
3. It must vary three blend so it fulfill Superpave criteria
Calculate combine propertied
Estimate mathematically the combined aggregate properties based on blending percentage and the individual aggregate properties. For the selected blend verify the combined properties by testing
2. COMPACT BLENDS
Select % trial Asphalt binder Pbi one of the following methods AASHTO R-35
A)Estimate effective specifc gravity of each blend using Gse using
Gsb= Apparent bulk Specific Gravity
Gsa= Apparent specific Gravity
C= Aggregate absorption factor assumed but for absorptive aggregate use 0.6 or 0.5
B) Estimate the volume of Asphalt binder (Vba) absorbed into aggregate for each blend
Vba = Ps*(1-Va) * (1/Gsb-1/Gse)
Vba = Volume of absorbed binder
Va = Volume of Air Voids
Pb= Percent of binder
Ps= Percent of Aggregate
Gb= Specific Gravity of binder
C) Estimate Volume of effective binder (Vbe for trial Blend)
Sn= The Nominal Maximum size in inches
D)Finally estimate the Initial AB% (Pbi) for each trial blend using
Ws = Ps*(1-Va) (Pb/Gb+Ps/Gse)
Compact 2 specimen from each blend to Ndes then for each compacted specimen
Measure Gmb AASHTO T166
3 . Get the specimen height at Nini and Ndes in mm from Gyractory compactor data
1. For each blend mesure Gmm method AASHTO t 209
C. Evaluate Trial Blends
Evaluate trail blend from the following equation
% Gmm at any Nx Gmm Nx=Gmm Ndes*Hnx Hnx
% of Air Voids = 100-% Gmm at Ndes
% VMA = 100-(%Gmm@ Ndes*Gmm*Ps)/Gsb
% Gmm at Nini = % Gmm at Ndesx * HNdes HNini
HNdes= Height of specimen at Ndes
HNini= Height of specimen at Nini
2. Reculculate 4 % air voids(96% Gmm at Ndes)
Asphalt content percent using
Pb estimated =Pbi-(0.4+(4-Va)
VMA using = % VMAinitial+C(4-Va)
%VFa estimated =100*(%VMA estimated – 4.0)
%Gmm @ Nini=%Gmm@ Nini-(4.0-Va)
Calculate DP ratio
Compute Pbe using
SELECTION OF DESIGN ASPHALT CONTENT
Prepare HMA using four Asphalt binder
Here X= Estimated binder content from step 2 at 0.40 % air voids for the selected blend·
Compact 2 samples at each AB% to Ndes
· Measure specimen height and gmb @ Ndes
· Measure 2 Gmm for each mix
1. Analyse the Result
Use the same formula in the selection of DAS to compute
% of Gmm at Nini and Ndes
Volumetric properties( Air Voids,VMA,VFA)
DP ratio Generate graph for each property vs AB%
C. SELECT DAC
From the graph at 4.0 % airvoids determine DAC and check
VMA and VFA and % Gmm @ N ini
Compute DP at the effective DAC
Compare with super pave criteria and adjust your design as needed
D. Ndes & Nmax verification
Prepare Hot mix at selected DAC & DAS
Gyractory compactor 2 specimen to Ndes and 2 specimen for Nmax
Compare the result with Superpave criteria and adjust as required
It consists of four parts
1. Select Nx
2. Compact samples
3. Evaluate Strength
4. Evaluate Moisture sensitivity
1. Select Nx
Use the densification data of the Nmax verification step to determine the number of gyration to achieve 7 % air voidsi.e % Gmm 93%%
Gmm @ Nx = %Gmm @ N Hn Hnx
Prepare HAM using DAS AND DAC
Gyractory compactor 6 specimen to Nx
Three specimen are control (non conditioning)
The other three is conditioned for 24 hrs at 60 degree centigrade
Test all specimen to determine maximum indirect tensile load and failure
Now to Calculate indirect tensile strength (ITS)Maximum allowable Strength.
ITS loss is 20 %Compare with Superpave criteria and adjust design In case
Note.ITS is failing use cement dust, lime and Portland cement or anti striping agent as MOT specifications.