CEE 395- Materials for Constructed Facilities

Civil Engineering Materials

Asphalt Concrete Design

Week 8, Lecture 14

I. Classifications of Asphalt

There are 4 methods of classifying asphalt binders.

1. Performance Grading (PG)(Superpave)


  • Traffic volume and speed

The maximum temperature is shifted for traffic speed and traffic volume. For example a PG 64 is replaced with PG 70 is traffic speed is slow or volume is high

  • PG 76 and 82 are for slow transient or standing loads

(i.e. intersections)

  • Reliability

Temperature values vary from year to year and are represented by a normal distribution.

Reliability of temperature is determined by the area under the normal distribution.

2. Penetration Grading

  • ASTM 946 Penetration Graded Asphalt
  • Penetration numbers are in 1/10 of a millimeter
  • Pen 40-50, 60-70, 85-100, 120-150, 200-300
  • Check flash point and ductility

3. Viscosity Grading

  • ASTM 3381 Viscosity Graded Asphalt
  • Classified with an AC followed by a number which is 1/100 of the middle of the viscosity range measured at 60 C (Table 9.5)
  • AC-2.5, AC-5, AC-10, AC-20, AC-30, AC-40
  • Also check kinematics viscosity at 135 C, penetration at 25 C, and flash point
  • Viscosity test: Measures the time for a fixed volume of asphalt to be drawn through a capillary tube

Asphalt Concrete Design

Week 8, Lecture 14 (Con’t)

4. Viscosity of Aged Residue

Classified with an AR followed by the middle viscosity value after RTFO aging (Table 9.6)

Same as viscosity grading except the asphalt has been conditioned

It is conditioned to simulate the effects of aging when asphalt cement is heated to make asphalt concrete

II. Asphalt Cutbacks
Different types and different grades

There are 3 types of cutbacks produced

Rapid Curing Cutbacks - RC

hard residue in a highly volatile solvent (solvent ex. gasoline)

Curing is complete in 5 to 10 minutes

  • Medium Curing Cutbacks - MC

medium hardness residue in a less volatile solvent (solvent ex. kerosene).

Curing in a few days

  • Slow Curing Cutbacks - SC

soft residue in nonvolatile solvent or low-volatile fuel or oil or by stopping the refining process before all the fuel is removed from the stock.

Curing in a few months.

Grades are determined by kinematics viscosity at 60 C

The different grades (30, 70, 250, 800, 3000)

i.e. MC-800 is medium curing cutbacks with grade 800

III. Classification of Asphalt Emulsions

By Rate of Set (Break):

the separation between the asphalt residue and water allowing asphalt globs to come together forming the binder

  • Rapid Setting (RS) - sets in 5-10 minutes
  • Medium Setting (MS) - sets in several hours
  • Slow Setting (SS) - sets in up to a few months

By Charge:

  • Cationic - The electrostatic charge at the head portion of the emulsifying molecule is positive

(For cationic emulsions, use the letter C before the abbreviation for rate of set)

  • Anionic - The electrostatic charge at the head portion of the emulsifying molecule is negative

(For anionic emulsions, use no additional letters before the abbreviation for rate of set)

By Saybolt Furol viscosity at 60 C (140 F)

  • Grade 1 - less viscous
  • Grade 2 - more viscous

Asphalt Concrete Design

Week 8, Lecture 14 (Con’t)

Examples:

  • CMS-1 would be cationic medium-setting emulsion with low viscosity
  • SS-2 would be anionic slow-setting emulsion with high viscosity
  • Specifications of various asphalt emulsions are standardized by ASTM D977
Asphalt Concrete
  • Also known as hot-mix asphalt (HMA)
  • Consists of asphalt cement and aggregates
  • Cover 93% of paved roads in the United States
  • Placed and compacted on road while still hot
Desired Properties

The objective of asphalt concrete mix design process is to provide

  • Resistance to permanent deformation under the action of traffic loads, especially at high temperatures
  • Fatigue resistance to prevent fatigue cracking under repeated loadings
  • Resistance to thermal cracking that might occur due to contraction at low temperatures
  • Resistance to hardening or aging during production in the mixing plant and in service
  • Resistance to moisture-induced damage that might result in stripping of asphalt from aggregate particles
  • Skid resistance by providing enough texture at the pavement surface
  • Workability to reduce the effort needed during mixing, placing, and compaction
Asphalt Concrete Production
  • Batch Plant

Aggregates are dried first then proportioned and mixed with asphalt in a pug mill

  • Continuous (Drum) Plant (Figure 9.17)

- Aggregates of different gradations are placed in cold bins

- The gradation proportions transferred to first part of drum where they are heated and dried

  • Hot asphalt cement is introduced in the last 1/3 of drum

- Aggregates and asphalt are mixed and stored until transported to job site

- Greater energy efficiency than batch plant

Asphalt Concrete Design

Week 8, Lecture 14 (Con’t)

Asphalt Concrete Mix Design
  • Purpose:

Determine the design asphalt content using the available aggregates and asphalt

  • Design AC% (by weight)

Give optimum stability and durability

  • Typically 4% to 7%

Varies depending on aggregate gradation, pavement temperature, and traffic volume

  • Design procedures include

- Marshall (impact hammer)

- Hveem (kneading compactor)

- Superpave (gyratory compactor)

Density and Voids Analysis
  • Determination of the proper ratios of binder and aggregates for desired properties
  • Three parameters are used to calculate the percent of air voids in the mix

VTM - Voids in Total Mix (Total Air Voids)

VTM = (Vv / Vt) (100)

VMA - Voids in Mineral Aggregate

VMA = (Vv + Vb) / Vt (100)

VFA - Voids Filled with Asphalt

VFA = Vb / (Vb + Vv) (100)

Vv = volume of air voids

Vb = volume of effective asphalt binder

Vt = total volume of the mixture

  • Specific gravity is used to convert weight to volume
  • The theoretical max specific gravity (Rice Gravity, Gmm) is found according to ASTM D2041
Superpave Mix Design

Step 1:

Selection of aggregate source. Based on:

- Coarse aggregate angularity measured by the percentage of fractured faces

- Fine aggregate angularity (AASHTO TP 33)

- Flat and elongated particles (ASTM D4791)

- Clay content (ASTM D2419)

Specification limits depend on traffic level and depth under pavement surface the materials will be used

Step 2:

Selection of aggregate gradation. Use the 0.45 power chart gradation.

Asphalt Concrete Design

Week 8, Lecture 14 (Con’t)

Step 3:

Binder is selected based on maximum and minimum pavement temperatures.

In addition these are needed:

- Specific gravity

- The rotational viscosity versus temperature relationship of the selected asphalt binder

Mixture design system varies depending on traffic design level; three levels of mix design are available

- Volumetric

- Intermediate

- Complete

Step 4:

Volumetric Mix Design

Three main steps used in testing and analysis:

1.Selection of design aggregate structure(s)

2.Selection of design asphalt content

3.Evaluation of moisture sensitivity of design mixture

  • A total of 20 specimens, 6 inch diameter and 3.75-4.5 inch high are needed (Figure 9.24)
  • Specimens are compacted using the Superpave Gyratory Compactor at an angle of 1.25 degrees and a constant vertical pressure of 600 kPa (87 psi)
  • Three important numbers of gyration are determined; N_initial, N_design, and N_maximum numbers of gyration
  • Bulk Specific Gravity can be estimated throughout the gyration process, this is done by dividing the mass of the specimen by it's volume (height x cross section)
  • When test is complete, bulk specific gravity is determined according to ASTM D2726

A correction factor is then determined which is the ratio of the measured bulk specific gravity to the estimated bulk specific gravity

  • Two specimens are prepared for each trial blend and their data averaged
  • Design Aggregate Structure and design asphalt content is selected as the one that is closest to design criteria is used. Density at

- Ndes = 96% Gmm (Required 4% voids)

- Nini < 89% Gmm

- Nmax < 98% Gmm

Asphalt Concrete Design

Week 8, Lecture 14 (Con’t)

  • Four binder content are tested for each trial aggregate blend

- Estimated optimum binder content

- Optimum - 0.5%

- Optimum + 0.5%

- Optimum + 1.0%

Binder content that meets design criteria is selected

Moisture Sensitivity of Design Mixture (AASHTO T283)

  • Samples produced at design binder content and 7% air voids
  • Three samples are conditioned by vacuum-saturation and freezing and thawing, another three that are not
  • Tensile strength ratio is determined by ratio of average tensile strength of conditioned samples to unconditioned samples
  • Minimum criterion is a ratio of 80%
oIntermediate Mix Design

Some performance-based properties are used

Required 23 specimens

oComplete Mix Design

Uses fundamental material models to predict the amount of distress and in what time frame it occurs

  • Samples are 6 inch diameter and 2 inch high

Figure 9.26 shows the test for the intermediate and complete Superpave Mix Design

Superpave Software Support System

Used to perform calculations in the mix design and analysis system Includes weather database files and other subroutines