# Presentation: 376169

## HMA MATERIALS

**Notes:**
There are two main components in asphalt concrete:  1) the asphalt cement, or glue, and 2) the aggregates.

## Background

**Asphalt**
** ****Soluble in petroleum products**
** ****Generally a by-product of petroleum distillation process**
**Can be naturally occurring**

**Tar**
**Resistant to petroleum products**
**Generally by-product of coke (from coal) production**

**Notes:**
Students commonly use the words “asphalt” and “tar” interchangably.  However, asphalt is actually a waste product from the refining of petroleum crude oil while tar is a coal by-product.  These chemical distinctions are important in a selecting the appropriate product for a given application.

For example, if protection from oil and fuel spills is desired, then a coating which will not be dissolved by these products is needed.  Since motor oils, fuel, and asphalt are all derived from petroleum, any spills on this pavement can damage the asphalt-based coating as well as the asphalt concrete.  The old chemistry rule-of-thumb applies: Like dissolves like.  Because tar is a coal rather than asphalt-based product, sealers from this material will not be damaged by petroleum  product spills.

## Background

**First US hot mix asphalt (HMA) constructed in 1870’s**
**Pennsylvania Ave.**
**Used naturally occurring asphalt from surface of lake on Island of Trinidad **
**Two sources**
**Island of Trinadad**
**Bermudez, Venezuela**

## Background

**Each lake asphalt source very consistent **
**Used solubility test to determine source **
** ****Insolubles differed substantially between sources**
**Demand for paved roads exceeded the supply of lake asphalts in late 1800’s**
**Led to use of petroleum asphalts**

**Notes:**
During the late 1800's and early 1900's the demand for paved roads increased so fast that the time-intensive processing and shipping of lake asphalt was limiting the miles of paved roads constructed each year.  This led to the increasing use of petroleum refinery-based asphalts.  It is interesting to note that these asphalts were originally considered inferior to lake asphalt because of their crude source-dependent material properties.    

## Petroleum-Based Asphalts

**Asphalt is waste product from refinery processing of crude oil**
**Sometimes called the “bottom of the barrel”**
**Properties depend on:**
**Refinery operations**
**Composition crude source-dependent**

**Gasoline**
**Kerosene**
**Lt. Gas Oil**
**Diesel**
**Motor Oils**
**Asphalt**

**Barrel of Crude Oil**

**Notes:**
A number of products are obtained from one barrel of crude.  Asphalt cement is what is left after the more volatile products are removed and it is commonly referred to as the “bottom of the barrel”.

## Asphalt Cement Components

**Asphaltenes**
**Large, discrete solid inclusions (black)**
**High viscosity component**
**Resins**
**Semi-solid or solid at room temperature**
**Fluid when heated**
**Brittle when cold**
**Oils**
**Colorless liquid**
**Soluble in most solvents**
**Allows asphalt to flow**

**Notes:**
The simplest general compositional model considers asphalt cements to be made up of asphaltenes, resins, and oils.  The combination of the resins and oils is also referred to as the maltenes.

Asphaltene contents vary considerably from asphalt to asphalt.  Low concentrations have been linked to tenderness in the HMA by some researchers.

Resins are credited with dispersing the solid asphaltene particles in the asphalt.  Oxidation of resins produce asphaltene-type molecules.

Oils are soluble in most solvents.   Upon oxidation, they produce both resin and asphaltene-type molecules.

## Refinery Operation

***Refinery Operation***

**FIELD STORAGE**

**PUMPING****STATION**

**LIGHT DISTILLATE**

**HEAVY DISTILLATE**

**PROCESS**
**UNIT**

**ASPHALT**
**CEMENTS**

**FOR PROCESSING INTO**
**EMULSIFIED AND**
**CUTBACK ASPHALTS**

**STILL**

**AIR**

**AIR**
**BLOWN**
**ASPHALT**

**STORAGE**

**TOWER**
**DISTILLATION**
**REFINERY**

**RESIDUUM**

**OR**

**GAS**

**PETROLEUM**

**SAND AND WATER**

**CONDENSERS **
**AND**
** ****COOLERS**

**TUBE**
**HEATER**

**MEDIUM DISTILLATE**

**Notes:**
Distillation is used to separate the various crude fractions by boiling point ranges.  The crude oil is heated in a large furnace to about 343 oC and partially vaporized.  The remaining material is transferred to a distillation tower.  Different fractions reach their boiling point ranges at various heights in the tower.  When the temperature is reduced, each material condenses on a tray and is drawn off at each level.

## Types

**Asphalt cements**
**Generally refinery produced material**
**Air blown asphalt cements **
**Cutbacks**
**Asphalt cements “cut” with petroleum solvents**
**Emulsions**
**Mixture of asphalt cement, water, and emulsifying agent**

**Notes:**
The residuum or “resid” from the distillation process is used to make various asphalt cement products.

## Cutbacks

**Rapid cure (RC) (Naphtha or Gasoline)**
**High volatility of solvent**
**Tack coats, surface treatments**
**Medium cure (MC) (Kerosene)**
**Moderate volatility**
**Stockpile patching mix**
**Slow cure (SC) (Low viscosity oil)**
**Low volatility**
**Prime coat, dust control**

**Notes:**
Cutbacks are liquid at room temperature and are manufactured by adding or “cutting” asphalt cements with petroleum solvents.  The volatility of the solvent dictates the rate at which the solvent evaporates.  How fast the solvent escapes into the atmosphere regulates how fast a mixture of cutback and aggregate will set.  Environmental regulations are increasingly limiting the use of these products.

## Emulsions

**Emulsifier gives surface charge to asphalt droplets suspended in water medium**
**Anionic**
**Negative charge**
**Alkaline**
**Good with limestones (positive charge)**
**Cationic **
**Positive charge**
**Acid**
**Good with silica gravels (negative charge)**

**Notes:**
Emulsions are produced by blending asphalt cements, water and an emulsifier (e.g. soap)  in a high shear colloid mill.  The resulting asphalt droplet suspended in the water has a charge which reflects the chemistry of the emusifier.  The charge of the emulsion is usually selected so that it is opposite that of the aggregate to promote adhesion of the asphalt to the aggregate surface.

Emulsions are an asphalt-based product which is also liquid at room temperature.  

## Purchasing of Asphalt Cements

**Need to be able to specify desirable characteristics**
**“****Desirable characteristics” have evolved over time and with increasing technological advances**
**Purchasing requires *****specifications***

**Notes:**
In order to purchase a product, the desirable characteristics and important material properties must be known.  When these parameters are presented in written form, they are called specifications.

## Early Specifications

**Lake Asphalts**
**Appearance****	**
**Solubility in carbon disulfide**
**Petroleum asphalts (early 1900’s)**
**Consistency**
**Chewing**
**Penetration machine**
**Measure  consistency**

**Notes:**
The first approach to characterizing the stiffness of an asphalt cement was to chew on it.  An engineer would take a small sample of asphalt, roll it into a ball, and place it between his teeth.  The pressure required to deform the ball was considered an indication of the stiffness of the asphalt.  However, this method was very subjective and could not be used to routinely specify the particular requirements of an asphalt cement for a given construction project.  These drawbacks eventually led to the development of the penetration test.

## Penetration Testing

**Sewing machine needle**
**Specified load, time, temperature**

**Notes:**
The penetration test started out using a No. 2 sewing machine needle mounted on a shaft for a total mass of 100 g.  This needle was allowed to sink into (penetrate) a container of asphalt cement at room temperature (25 oC) for 5 seconds.  The consistency (stiffness) of a given asphalt was reported as the depth in tenths of a millimeter (dmm) that the needle penetrated the asphalt.

## Penetration Specification

**  ****Five Grades**
** ****40 - 50**
** ****60 - 70**
** ****85 - 100**
** ****120 - 150**
** ****200 - 300**

**Notes:**
This test was used to standardize the penetration grading system approach for specifying asphalt cements.   This specification uses the penetration of the original asphalt cement in the grade names.  That is, a 120-150 penetration grade asphalt will have a penetration value for the original asphalt of between 120 and 150 tenths of a millimeter.

## Viscosity Graded Specifications

**Notes:**
These disadvantages led to the development of the viscosity grading system in the 1970's which is detailed in ASTM D3381.  This one ASTM standard actually contains three separate specifications designated as: Table 1, Table 2, and Table 3.  The first two specifications are based on the original properties of the asphalt while the last table is based on the properties of the asphalt after rolling thin film oven aging.  Each of these tables and differences between them will be discussed in the following slides.

## Definition

***Definition***

**Notes:**
Viscosity is shear stress divided by the shear strain rate.  These units of measurement provide at least a limited estimate of fundamental material properties.  

The easiest way to picture shear is to visualize a deck of cards which have a vertical line marked on the side.  As pressure is applied to the top of the deck, the cards slide over one another and the marked dots on the individual cards start to separate.  The shear rate is the rate at which these dots separate.

## Types of Viscosity Tubes

**Asphalt Institute Tube**

**Zietfuchs Cross-Arm Tube**

**Notes:**
Two viscosity measurements are used in this specification: Absolute viscosity (60 oC) and kinematic viscosity (135 oC).  Both use the principle of the rate of flow through a known area to measure viscosity.  Because asphalt is still very thick (stiff) at 60 oC, a vacuum is needed to move the asphalt through the tube in a reasonable time.  At 135 oC, gravity and a falling head pressure is sufficient to get the asphalt to flow.

## Testing

**Absolute viscosity**
**U-shaped tube with timing marks & filled with asphalt**
**Placed in 60C bath**
**Vacuum used to pull asphalt through tube**
**Time to pass marks**
**Viscosity in Pa s  (Poise)**

**Notes:**
At the 60 oC test temperature, the tube is charged at 135 oC and then placed in the test temperature bath.  The tube temperature is allowed to equalize  with the bath temperature, a vacuum line is attached to the top of the small diameter tube, and the flow is started.  The time it takes the asphalt to flow past the timing marks times the tube calibration constant gives the viscosity of the asphalt in Poise.

## Table 1 Example

**	****	****	****       ****AC 2.5****	****	****AC 40**

**Visc, 60C****	****     250 **_**+**_** 50    4,000 **_**+**_** 800**

**Visc, 135C****	****          80+****	****	****  300+**

**Penetration****	****	**** 200+****	****	****   20+**

**Visc, 60C****	****	****<1,250****	****    <20,000**

**Ductility****	****	****	**** 100+ ****	****	****   10+**

**Notes:**
This table compares two of the viscosity graded Table 1 specifications. The viscosity grading system provides immediate information as to the mean anticipated viscosity at 60 oC.  For example, an AC 25 will have a mean viscosity of 250 Poise (2.5 times 100).  Because of the limited allowable range (coefficient of variance of 20 percent), there is no overlap between the AC grades. 

## Viscosity, 60C (140F)

**100**

**50**

**10**

**5**

**Viscosity, 60C (140F)**

**Notes:**
This figure provides a general comparison of the various traditional specifications.  While there is no direct relationship between the specifications, there is a general relationship between stiffness and viscosity.  Higher penetration numbers correspond with lower viscosities.

## Aggregates

**Usually refers to a soil that has in some way been processed or sorted.**

**Notes:**
Soils are materials that are used as-is.  An example would be a finished subgrade surface.  Aggregates are materials that have been specifically sorted or processed to achieve given properties.  This block will present general background information about how aggregates are obtained and processed.

## Excavation

***Excavation***

**Notes:**
Some aggregate sources are obtained by removing materials from the bottom of lakes and rivers.

## Crushing

**Crushing**

**Notes:**
The first step in preparing stockpiles for specific uses is the crushing the larger boulders and aggregates  into usable sizes.

## Transportation

**Notes:**
Once materials are obtained,they are usually transported by land, rail, or barge to a centrally located plant for separation into specific sizes.

## Sizing

***Sizing***

***Stockpiling***

**Notes:**
Once the rock has been sufficiently reduced in size, it is separated into individual stockpiles with specific ranges of particle sizes.  Large screening operations have a number of wire mesh screen decks with each deck having progressively smaller openings between the wires.  These screens are slanted and empty the material retained on each screen out onto conveyor belts.  These belts then move the material into individual stockpiles of a particular size.

## Desired Aggregate Properties

**Toughness**
**Soundness**
**Deleterious Materials**
**Gradation**

**Notes:**
Source aggregate properties are those properties which are measured for the aggregate as-stockpiled and are commonly used for aggregate source acceptance control.  These properties are toughness, soundness, and deleterious materials.  In addition, the gradations of individual stockpiles may be evaluated.

## Soundness

- Before

-    After

**Notes:**
Damage to the aggregate after a number of wet-dry cycles can be seen by visual examination as well as in the change in gradation.

## Gradations

**The distribution of particle sizes expressed as a percent of total weight.**
**Determined by sieve analysis**

***Gradations***

## Mechanical Sieve

- Individual Sieve

- Stack of Sieves

**Notes:**
Each sieve has wire mesh in the bottom.  The sieve size (4.75 mm, 2.36, mm etc.) denote the distance between each wire.  Sieves are stacked from largest openings  on the top to smallest openings on the bottom.  There is always a pan (no openings) at the bottom of the stack. 

## Mechanical Sieve

**Stack in Mechanical Shaker**

**Notes:**
Once the stack is assembled and the oven dry aggregate added to the top, it is placed in a mechanical shaker.  This is run from 5 to 15 minutes.  Care needs to be taken to ensure that any given sieve is not overloaded.   That is, that there is so much material on the screen that material cannot be efficiently separated.

## Types of Gradations

**-  Few points of contact**

**-  Poor interlock (shape dependent)**

**-  High permeability**

***  Well graded**

**-  Good interlock**

**-  Low permeability**

***  Gap graded**

**-  Only limited sizes**

**-  Good interlock**

**-  Low permeability**

**Types of Gradations**

**Notes:**
There are several general types of aggregate gradations.   Uniform gradations have large percentages of one size.  Well graded aggregates have approximately equal amounts on each sieve in the stack.  Gap graded aggregates have large and small but few intermediate sizes.

The properties of the aggregate gradation depends strongly on the distribution of aggregates sizes.

## Aggregate Size Definitions

***Nominal Maximum *****Aggregate Size**
**one size larger than the first sieve to retain more than 10%**
***Maximum***** Aggregate Size**
**one size larger than nominal maximum size**

**Notes:**
For HMA pavements these are the definitions for gradations.

## Percent Passing

**100**

**0**

** ****.075****	****.3****	****   2.36****	****    4.75****	****9.5****	****12.5         19.0**

**Percent Passing**

***control point***

***restricted zone***

**max density line**

**max**
**size**

**nom**
**max**
**size**

**Sieve Size (mm) Raised to 0.45 Power**

**Notes:**
To specify aggregate gradation, two additional features are added to the 0.45 chart:  control points and a restricted zone.  Control points function as master ranges through which gradations must pass.  They are placed on the nominal maximum size, an intermediate size and the dust size.
The restricted zone resides along the maximum density gradation between the intermediate size (either  4.75 or 2.36 mm) and the 0.3 mm size.  It forms a band through which gradations should not pass.  Gradations that pass through the restricted zone have often been called “humped gradations” because of the characteristic hump in the grading curve that passes through the restricted zone.  In most cases, a humped gradation indicates a mixture that possesses too much fine sand in relation to total sand.  This gradation practically always
 results in tender mix behavior, which is manifested by a mixture that is difficult to compact during construction and offers reduced resistance to permanent deformation during its performance life.  Gradations that violate the restricted zone possess weak  aggregate skeletons that depend too much on asphalt binder stiffness to achieve mixture shear strength.  These mixtures are also very sensitive to asphalt content and can easily become plastic.  

## Questions - ?