Aggregate Combination and Separation

• Aggregate Combination and Separation is needed to meet Job mix Requirements

• First discard the oversize

• Separate into two or more portions on selected proper sieves

• Recombine using proper percentage for recombination with specification requirement (mid specification preferable)

• Add fine materials and filler if necessary

• This is one approach for correcting gradation to satisfy specifications, discard the oversize in the 3/4" sieve to comply with specifications

• By discarding retained materials on the 1" and 3/4" sieves ensures the remaining sample has 100% passing for both sieves

• Violation seen all other sieves starting from 3/8"

• Rectifying process requires separation at this sieve and calculating the remaining on the higher sieves

• Compute passing proportions to consider each part as a different sample

• Several blending proportion trials are needed until specifications are reached

• 50% of each part will make the sample compatible with the required specification

Example of blending proportions

• Step 1: Portion passing 3/4" and P(A): portion passing 3/8"
• Step 2: Retained 3/8", P(B): percent retained 3/8", and P(B):% passing 3/8"
• Step 3: 50% A + 50% B

Solution Blends

• 65% A + 30% B + 5% C
• 55% A + 35% B + 10% C
• 50% A + 35% B + 15% C

4 Bituminous Mixture Technologies

• Upper courses that distinguish asphalt pavement normally consist of specified mineral aggregates bound by bituminous material with or without additives
• These aggregate and bitumen blends are "Bituminous Mixtures"
• There are many different types of mixtures to satisfy specific requirements, such as:
• Crack resistance
• Resistance to permanent deformation
• Resistance to fatigue
• Resistance to wear
  • Resistance to water ingress
  • High skid resistance
• Main differences for these technologies are preparation and compaction temperatures

4.1 Hot mix asphalt (HMA)

• HMA is a bituminous mixture prepared and compacted at elevated temperatures, generally between 110-180 °C
• Elevated temperatures are necessary to:
• Dry the aggregates
• Coat the aggregates with the bitumen binder
• Get the required workability
• Grant enough time to compact the HMA course
• These temperatures are dependent on bitumen binder grade:
• Hard bitumen needs high temperatures
• Soft bitumen needs low temperatures
• Bitumen grade selection depends on ambient temperature in the construction region:
• Hard bitumen is suitable for hot climate regions
• Soft bitumen is more suitable for cold regions
• The bitumen should be solid enough during hot seasons to prevent bleeding
• At the same time, it should be relatively soft in cold seasons to prevent cracking
• HMA is prepared by adding hot bitumen to the hot aggregates (coarse, fine and filler)
• The blend is mixed to a stage where aggregates are fully coated with bituminous binder, then transported to the required site and spread by paver machine
• Material Laid is compacted under certain temperature ranges which are compatible to certain viscosity ranges, again depending on bitumen grade

Temperature Zones:

• Lower Temperature Zone: 90-70°C (195-160°F)
• Middle Temperature Zone: 115-90°C (240-195°F)
• High Temperature Zone: 160-140°C (320-285°F)
• Limited ranges avoids the mixture from being too soft with high temps, and prevents mixture from compacting correctly under lower limits
• Air void and density are limited with respect to the mixture's mechanical properties
• After finished road can be opened to traffic as soon as the mixture has cooled down to ambient temperature

4.2 Warm and half-warm mix asphalt (WMA/HWMA)

• Decreasing the HMA mix and compaction temperatures result in several economic, environmental, and even performance advantages
• Warm-mix asphalt (WMA) technology was introduced in 1995, lower HMA production temperature by 25 °C to 55 °C
• Jenkins et al. (1999) introduced "Half-Warm Foamed Bitumen Treatment".
• Involves applying the foamed bitumen on aggregate heated to less than 100 °C
• WMA technologies allow asphalt mixtures to be mixed and compacted at significantly lower temperatures
• WMA technology has become recently popular due to economic and environmental advantages
• WMA technologies are classified in three main groups:
  • By applying organic additives
  • Chemical additives
  • Water-based or water-containing foaming processes

6.4.3 Cold mix asphalt (CMA)

• Cold mix is defined as “a hydrocarbon mix made with aggregate, a hydrocarbon binder and possibly dopes or additives with characteristics such that the aggregate can be coated without drying or heating."
• Hydrocarbon binder could be cutback or emulsified bitumen
• Bitumen emulsion significantly covered all applications of cold mix asphalt, because regulations limit cutback use in pavement mixtures

6.5 Requirements for Bituminous Mixes

• To properly design an asphalt paving mixture, consideration must be given to the following desirable mix properties:

1. Stability

• Ability of the asphalt paving mixture to resist deformation from imposed loads

• Unstable pavements are marked by channeling (ruts), and corrugation

• Depends on both:

  • Internal friction depends on:
  • Surface texture
  • Gradation of aggregate
  • Particle shape
  • Density of mix
  • Quantity of asphalt

• It is a combination of the frictional and interlocking resistance of the aggregate in the mix

  • Cohesion is the binding force that is inherent in the asphalt paving mixture
  • The asphalt maintains contact pressures developed between aggregate particle Cohesion is dependent upon:
  • Rate of loading
  • Loaded area
  • Viscosity of the asphalt
  • Temperature
  • Asphalt content

• If aggregate interlock mechanism predominates, stability is achieved by using coarse aggregate with:

• High crushing strength

• Angular in shape, to achieve good packing

• Rough surface

• If mortar mechanism predominates, stability is achieved by using:

• High viscosity binder

• High filler content

• Rough texture sand

2. Durability

• Ability of asphalt paving mixture to resist disintegration by weathering and traffic
• Weathering includes changes in asphalt and pavement, aggregate oxidation, volatilization, and reaction to water action, including freezing and thawing
• Durability is generally enhanced by:
• High asphalt contents
• Dense aggregate gradations
• Well-compacted
• Impervious mixtures
• A mixture having a high asphalt content and voids completely filled with asphalt, may provide the ultimate in durability
• However, undesirable from the standpoint of stability
• The pavement would channel and creep under traffic with bleeding/flushing of asphalt to surface.
• Compromise by keeping the asphalt content is as high as possible while maintaining adequate stability

3. Flexibility

• The ability of an asphalt paving mixture to conform to gradual settlements and movements of the base and subgrade
• Differential settlements in the fill embankment occasionally occur
• Impossible to develop uniform density in the subgrade during construction because section or portions of the pavement compress and settle under traffic
• Asphalt pavement must have the ability to conform to localized and differential settlements without cracking
• Flexibility of the asphalt mixture is enhanced by high asphalt content and relatively open-graded

4. Fatigue Resistance

• The ability of asphalt pavement to withstand repeated flexing caused by wheel loads
• Tests indicate that dense-graded asphalt mixes have more fatigue resistance than open-graded mixes

Well-graded aggregates that

• Well-graded aggregates that permit higher asphalt content prevent flushing or bleeding in compacted pavement should be incorporated in the mix

5. Skid Resistance

• This is the ability of asphalt paving surface, particularly when wet, to offer resistance to slipping or skidding
• Proper asphalt contents and aggregates with a rough surface texture are the greatest contributors to skid resistance and stability
• Must resist polishing
• Mixes rich in asphalt to fill voids in compacted pavement cases asphalt to flush to surface, or bleeding
• Free asphalt on the pavement surface can also cause slippery conditions when the pavement is wet

6. Impermeability

• The asphalt pavement's resistance to the passage of air and water into or through it
• More significance is the interconnection of the void and their surface with the void content indicating compacted susceptibility
• Imperviousness to air and water is extremely important from durability's standpoint in asphalt mixes

7. Workability

• The ease with which paving mixtures may be placed and compacted
• Workability is not a problem with careful attention to proper design machine spreading, however properties that promote high stability can make it difficult to spread or compact
• Mix design adjustments should be quick during paving operations to resolve workability problems efficiently

Design of Bituminous Mixes

• The main objectives of mix design are developing an economical blend of aggregates and asphalt that meets design requirements and covers all performance requirements of Hot Mix Asphalt (HMA)
• Includes mix design in the lab, preparation of the mixtures in the plant, construction in the suite and monitoring the performance during the service life.
• Marshall mix design method first made by Bruce Marshall of Mississippi Highway Department around 1939 and then refined by the U.S. Army.
• Seeks to select the asphalt binder content at a desired density that satisfies minimum stability and range of flow values
• Limited to be used for a maximum aggregate size of 25.4 mm (1 in)
• Used for design and evaluating asphalt concrete mixture
• The Marshall test is a simple and inexpensive (apparatus and test procedure)

Marshall Apparatus Components

• Mold Assembly: cylindrical moulds of 101.6 mm (4 in) diameter and 76.2 mm (3 in) height consisting of a base plate and collar extension
• Marshall Compactor
• Sample Extractor: for extruding the compacted specimen from the mould
• Water bath (60 °C, 30-40 min)
• Loading machine (Marshall Apparatus)
• Breaking head
• Flow meter, thermometers

Steps to find procedures to do test

Step (1):

• Prepare a series of test specimens for a range of different binder content
• The difference in binder content is 0.5% or 1%

Step (2):

• Determine the bulk density of each specimen with the following formula:
  • d = MA/V = MA/(MA-MW)
• Where:
• d = bulk density of the compacted mix (g/cm³)
• MA = mass of the specimen in air (gm)
• MW = mass of the specimen in water (gm)
• V = volume of the specimen (cm³)

Step (3):

• Calculate the percentage of air voids in each compacted specimen
• Formula: Ψ=WA/(Wb/Gb + WC/Gc+Wf/Gf+Wmf/Gmf)
  • Ψ = maximum theoretical density (gm/cm³)
  • WA = weight of specimen
  • Wb, WC, WF, Wmf = weight of binder, coarse agg., fine agg., and filler respectively
  • Gb, Gc, Gf, Gmf = specific gravity of binder, coarse agg., fine agg., and filler respectively
• %V.T.M. = (ψ-d)/Ψ
• The percent of voids in total mix
• Ψ = maximum theoretical density (gm/cm³)
• d = bulk density (gm/cm³)

Step (4):

• Calculate the percentage of voids in the compacted mineral and percentage of voids filled with binder for each specimen
• V.M.A. = V-VC-VF-Vmf
• V.M.A. = (W/d) - (Wc/Gc)-(Wf/Gf)-(Wmf/Gmf)
• V.M.A. = volume of voids in compacted mineral aggregate (cm³)
• V = volume of specimen (cm³)
• Vc, Vf, Vmf = volume of coarse agg., fine agg., and filler respectively (cm³)
• W = weight of specimen
• d = bulk density of specimen
• Wc, Wf, Wmf = weight of coarse agg., fine agg., and filler respectively
• Gc, Gf, Gmf = specific gravity of coarse agg., fine agg., and filler respectively
• %V.M.A. = (V.M.A./V)*100, The percent of voids in mineral agg.
• % V.F.B. = (VBN/V.M.A.)*100, The percent of voids filled with binder

Step (5):

• Determine the Marshall stability and flow of each specimen
• Stability = maximum load required to produce failure (N)
• Flow = deformation at failure (mm)

Step (6):

• Correct the measured stability values based to what had been obtained from the specimens
• Corrected stability is the measured stability x correlation ratio.

Step (7):

• Prepare separate graphical plots for binder content versus each of:
  • (a) Corrected Marshall stability
  • (b) Marshall flow
  • (c) Unit weight (density)
  • (d) % V.T.M.
  • (e) % V.F.B.
  • (f) % V.M.A.

Step (8):

• Determine the optimum binder content as an average of the binder content at:
  • Maximum stability
  • Maximum density
  • Median of the % V.T.M. specification or desired % V.T.M, and/or % V.F.B.

Step (9):

• Check the optimum binder content with design specification

Step (10):

• Reject the original mix. Adjust the grading of the original aggregate blend and carry out step 1-9 again, if the optimum binder content does not meet the allowable limits of specification

The Iraqi Roads Design Specification Includes,

• Property
• Wearing coarse
• Binder coarse
• Base

Stability satisfactory but voids too low:

• Reduce the filler and/or the binder
• Change the proportions of the coarse and fine aggregates to produce higher voids in the mineral aggregates

Stability satisfactory but voids too high:

• Increase the amount of filler and/or binder
• Porous aggregate absorbs binder and requires a higher binder content
• Change the proportion of the coarse aggregate to the fine aggregate to produce lower voids in the mineral aggregate

Stability too low and voids too low:

• Increase the filler and reduce the binder
• Increase the proportion of coarse aggregate

Stability too low and voids too high:

• Increase the percentage of filler
• Change the proportion of coarse to fine aggregate to produce lower voids in the mineral aggregate

Stability too low but voids within the specified limits:

• It may be necessary to change the source of aggregate, if the percentage of binder is near the lower limit, and the aggregate is inherently unstable
• Change the fine aggregate if the coarse aggregate is a crashed stone, or the coarse aggregate if it is a round gravel to the fine aggregate; use a crushed stone or increase the amount of the coarse aggregate; use a round gravel

Solution Calculations:

• Calculated using specific gravities and volumes of mix content