- 1 Asphalt cement
- 2 Consistency Parameters
- 3 Temperature Susceptibility
- 4 Stiffness of Asphalt
- 5 Grading of Asphalt cement
- 6 Asphalt Durability and Age-hardening
- 7 Liquefied Asphalts
- 8 Asphaltic Concrete
- 9 Stiffness of Compacted Asphalt Concrete
- 10 Plant Manufacture of Hot-Mix Asphalt (Asphalt Concrete)
Asphalt cement which is a major part of asphalt paving materials is the residual product of the distillation of heavy crude petroleum crude petroleum.
The material may also occur in a natural form, but very rarely. The distillation is a two-stage process carried out under atmospheric followed by vacuum conditions.
The residual asphalt at the end of the vacuum distillation stage is referred to as straight-run asphalt or sometimes vacuum tower bottoms.
When straight-run asphalts or vacuum tower bottoms are too soft they are given further treatment in order to increase their hardness to a level acceptable for road building purposes.
This is achieved by subjecting the material to a process known as air-blowing. Air-blowing converts the asphalt stock to one of the modified properties (particularly increased hardness) through polymerization by contacting with air (oxygen).
In some cases, vacuum tower bottoms from the distillation process may be treated with propane solvent to extract heavy oils such as lubricants in a process referred to as propane de-asphalting.
The residual asphalt from this process is a hard glassy product known as propane-precipitated asphalt. This type of asphalt cannot be used directly for paving applications but can be blended with any of the types of asphalts already mentioned to obtain a product of intermediate consistency and the resulting product is referred to as a blended asphalt.
Thus depending on the crude type and available process capabilities, several manufacturing options are available for the production of asphalts.
Consistency is a measure of the hardness or the degree of fluidity of an asphalt cement sample. The most commonly used parameters to describe asphalt consistency are penetration and viscosity.
This is the depth in units of 0.1mm that a standard needle loaded to 100g will penetrate an asphalt sample for a standard duration of 5 seconds when the temperature of the sample is maintained at 25oC (see illustration in the image below). The harder an asphalt cement the lower its penetration and vice-versa.
- Viscosity which is a measure of the resistance of a fluid to flow is a fundamental property of fluids. This parameter is considered the most appropriate parameter in describing the consistency of asphalts.
Asphalt viscosity may be evaluated when the sample is in a fluid state at two standardized temperatures; 60oC and 135oC. The viscosity at 60oC commonly called the absolute viscosity and measured in units of poise corresponds to the viscosity of asphalt in asphaltic pavements during hot summer weather and represents the consistency of the asphalt in its most critical state in service. In SI Units, 1 poise=0.1Pa.s.
The viscosity at 135oC is referred to as the kinematic viscosity (measured in centistokes) since at this temperature the asphalt is sufficiently fluid to flow under gravitational forces alone.
This viscosity corresponds approximately to the viscosity of asphalts at mixing and lay-down conditions.
The kinematic viscosity is the ratio between the viscosity and density of the asphalt and has units of m2/s in SI Units and cSt (centistokes) The SI unit of kinematic viscosity is the square meter per second [m2/s].
- The Gaussian unit of kinematic viscosity is the stokes [St = cm2/s].
- Ten thousand stokes equal one square meter per second [10,000 stoke = 1 m2/s].
This is the temperature at which an asphalt cement or binder changes from a solid to liquid state. The harder an asphalt cement, the higher its softening point and vice versa.
The standard method of evaluation of the softening point uses a ring filled with bitumen and then loaded with steel ball. As such, the softening point temperature is also referred to as the Ring and Ball temperature (TR&B).
Extensive research on asphalt cement has indicated that practically all asphalt cement attains the same consistency at the softening point temperature. This value is used to advantage to evaluate the temperature susceptibility of asphalt cement as discussed in the next section.
This measures the effect that changes in temperature will have on the properties of asphalt cement. Asphalt cement are thermoplastic, therefore, their consistency changes with temperature.
An asphalt cement that is highly temperature-susceptible undergoes large changes in properties for small changes in temperature. Such material is not considered suitable for use in road construction because it would become extremely soft at high temperatures and extremely stiff and brittle at low temperatures.
A soft asphalt will have stability problems leading to rut failure; a brittle asphalt is highly prone to cracking when subjected to tension. There are several indices for the quantitative measure of the temperature susceptibility of asphalt cement but the most commonly used is the Penetration Index (PI).
The PI of asphalt cement is evaluated as follows;
and T1 and T2 are two different temperatures.
In practice, PI can be evaluated from a knowledge of the standard penetration value of the asphalt cement and the softening point temperature.
The parameter A then becomes
Asphalts that are considered suitable for road building purposes have PI values ranging between –2 and +2.
Stiffness of Asphalt
Asphalt Stiffness (also known as Elastic Modulus) measures the relationship between stress and strain as a function of time of loading and temperature.
This parameter must be known in order to assess the behavior and performance of engineering structures such as asphalt concrete pavements of which the asphalt forms a part.
Because asphalt stiffness is dependent on temperature and time or rate of loading, its characterization is rather difficult.
At very short times of loading, such as occurs on highway pavements, the behavior of the material is almost elastic and the stiffness is analogous to the Elastic Modulus. At longer times of loading and higher temperatures, the stiffness is simply a relationship between stress and strain.
Asphalt stiffness may be measured directly in the laboratory by creep and relaxation tests and indirectly by use of monographs.
Grading of Asphalt cement
Asphalt cements are placed under different standard grades by way of their hardness or degree of fluidity in order to make it possible for users to differentiate between them and/or to select the type appropriate for an intended application.
Asphalt cements may be graded on the basis of either penetration at 25oC or viscosity at 60oC.
Based on penetration, five standard asphalt grades exist. These are;
- 40-50 or 40/50
- 60-70 or 60/70
- 85-100 or 85/100
- 120-150 or 120/150
- 200-300 or 200/300 penetration grades.
The numerical values for each grade represent the range within which the penetration for that particular grade must lie.
For example, any asphalt sample whose penetration lies between 40 and 50 (including the lower and upper limits) is classified as a 45-50 pen. grade.
Because asphalt manufacturers would be required to produce a specific grade for application, there is no question of having an asphalt binder not belonging to any of the above grades, i.e., manufacturers control their production methods in order to obtain the required grade.
Grading of asphalts may also be based on the viscosity of the fresh sample at 60oC (the absolute viscosity). The standard viscosity grades are;
- AC- 20
where AC designates “asphalt cement” and the numerical value of each grade represents one hundredth of the absolute viscosity with a tolerance of ±20%. Thus an AC-20 grade, for example, has an absolute viscosity value that lies in the range 1600-2400 poise.
It is must be noted that there is no direct correspondence between viscosity grades and penetration grades. Whereas, for example, most 60-70 pen grades will qualify as AC-20 grades and vice versa, AC-20 grades are not equivalent to 60-70 pen. grades.
As a result, specification for asphalt for road construction is based either on penetration or viscosity but not both.
However, in some cases, an additional requirement in terms of viscosity for penetration grades or in terms of penetration for viscosity grades may be imposed on a particular grade just to restrict the range of possible materials.
Asphalt Durability and Age-hardening
To serve as a good binder, an asphalt cement must have the following characteristics:
- good adhesion to aggregates
- good viscosities at processing and application temperatures
- ability to deform easily without rupture in order to resist stresses whilst existing as a component of an asphaltic pavement (visco-elastic behaviour)
The durability of an asphalt cement in relation to its performance as a binder is its resistance to changes (for the worse) in these original properties during in-service use.
An asphalt pavement with a durable binder should be able to support traffic-induced stresses and strains and detrimental weather conditions for a long time.
Characteristic features of a pavement experiencing durability problems are pavement disintegration, raveling, and all kinds of cracking. The asphalt binder in such pavement is considered to have age-hardened or become brittle.
Age-hardening is a process by which asphalt become brittle as a result of chemical changes in the asphalt during service conditions.
The major cause of aging is the oxidation of the material. Oxidation reactions take place when asphalt comes into contact with atmospheric oxygen. The process is aided by high temperatures. It is believed that significant hardening of asphalt takes place in the pugmill during plant production of asphaltic concrete due to the high mixing temperatures.
The high temperatures cause dehydrogenation of the asphalt with a resultant increase in material hardness. There is also loss of volatiles from the asphalt and polymerization reactions which allow the asphalt molecules to combine to form larger molecules and contribute to asphalt age-hardening.
As a result, it is part of quality control in hot-mix asphalt production to check that production temperatures do not go beyond a certain range so as to prevent premature aging.
Age-hardening continues in service although at a much lower rate for the first 2-3 years until the pavement approaches its maximum density under traffic.
In that condition, the rate of hardening is significantly reduced due to the low pavement permeability and the reduced potential for oxygen to diffuse into the pavement and react with the asphalt.
Asphalts from the distillation process usually exist in a solid form at room temperatures and lower but may be produced in a liquefied form to enable use without or with a minimum application of heat.
Liquefied asphalts may exist in the form of asphalt emulsions (bituminous emulsions) or cutback asphalts (cutback bitumen).
An asphalt emulsion is a liquefied asphalt obtained by the dispersion of asphalt globules in water containing an emulsifying agent (a soap-like substance which acts as a stabiliser).
The process of manufacture involves passing hot asphalt cement and water containing an emulsifying agent under pressure through a colloid mill to produce extremely small (less than 5-10 microns) globules or droplets of asphalt cement which become suspended in the water.
Without the emulsifying agent the asphalt globules will coalesce and stay out of suspension since asphalt is an organic material and will therefore not normally mix with water.
The emulsifying agent imparts an electric charge to the surface of the asphalt globules which causes the globules to repel one another.
On the basis of the charge carried on the globules, the emulsified asphalt may be termed cationic (electro-positively charged) or anionic (electro-negatively charged).
When it comes into contact with the surface of an aggregate an emulsified asphalt breaks or sets (i.e. the asphalt globules react with the surface of the aggregates, lose their charge and coalesce to form a continuous film on the aggregate as the water evaporates).
Depending on the rate of set or break, an emulsion may be described as rapid setting (RS), medium setting (MS) or slow setting (SS).
In general the rate at which an emulsion-aggregate mixture will set depends on the following:
- composition of the emulsion
- the porosity of the surface to which the emulsion is applied
- the rate of evaporation of the water which is dictated by wind conditions, relative humidity and environmental temperature and
- the surface chemistry of the aggregates with which the emulsion comes into contact.
The following grades of anionic and cationic emulsions are available.
The “h” designation means a harder base asphalt cement is used in the emulsion. The “HF” designation refers to a high float residue which is an indication of chemical gelling of the emulsion residue.
Selection and uses of emulsified asphalts are generally as follows (see ASTM D3628)
Rapid-setting grades: Surface treatments and penetration macadam
Medium-setting grades: Open-graded cold asphalt-aggregate mixtures
Slow-setting grades: Tack coat, dense-graded cold asphalt–aggregate mixtures, slurry seals.
Cutback Asphalts (or Cutbacks)
A cutback is a liquefied asphalt obtained when an asphalt cement is liquefied by dissolution in an organic solvent (cutter).
The type of organic solvent or cutter used will determine the rate at which the cutback will lose its liquid component and become solid, and hence the type of cutback.
If the cutter used is of high volatility (such as naphtha or gasoline) the resulting cutback is described as “rapid curing” (RC); if a kerosene-type solvent which is of medium volatility is used, the cutback is described as “medium curing” (MC).
A cutback containing low-volatile oils (such as diesel or gasoil) is described as “slow curing” (SC). Curing of cutback-aggregate mixtures occurs simply by the evaporation of the cutter from the cutback and does not involve any chemical reaction between the aggregates and the asphalt material.
Cutbacks are commercially available in different grades: the thinnest and most fluid grade is designated by the suffix number 30 which is available in “medium curing” type only (MC-30).
The Table below provides the grades of cutbacks commercially available in the paving industry.
The numbers shown against the various grades represent the minimum kinematic viscosity (in centistokes) at 60oC for the particular grade.
Each grade has an upper viscosity limit which is double the minimum viscosity (or grade number)).
The largest use of asphalt cement or binder is for the production of hot-mix asphalt (HMA) which is primarily used for the construction of flexible pavements.
HMA variously referred to as asphalt concrete, asphalt paving mix or bituminous paving mixture etc. is a mixture of asphalt cement and graded mineral aggregates.
Asphalt concrete is used to provide structural strength to pavements that come under very heavy loads from traffic on roads and airfields. Depending on the intended application, asphaltic concrete mixtures may be designed as “open-graded” or “dense-graded”.
Compared to the dense-graded mixtures which are the traditional and predominant mixtures used in flexible pavement construction, open-graded mixtures tend to have relatively large stone or higher coarse aggregate content and are more permeable.
Large-stone open-graded mixtures are more suitable for supporting heavy truck traffic.
Aggregate for Asphalt Concrete Design
Aggregates for asphalt concrete mix must be sound and pass the strength and shape characteristics determined by the following tests:
- Los Angeles Abrasion test,
- Soundness test,
- Flakiness Index test
- Elongation Index test
- Aggregate Impact test
- Aggregate Crushing test
- 10% Fines test.
In addition, the aggregates must also be rough-textured with angular cleavage and perhaps cubical in shape.
The gradation must also represent the most economical blend that satisfies all specification requirements for the intended use of the asphaltic concrete.
Marshall Mix Design Method.
The Marshall method of mix design is the most widely used method for designing asphalt concrete mixes. This method is standardised under the designation ASTM D 1559. The procedure for the method is as follows.
a. Mixture preparation
About 1150-1200g of the graded aggregates meeting specifications are placed in a mixing pan. The actual quantity of material required is that which will result in a compacted specimen height of 63±1.27mm.
Usually, it is recommended to prepare a trial specimen prior to the complete test schedule so that if this height is no met the quantity of aggregates taken could be adjusted per the following formula:
where Q=the quantity of aggregates required
Qo=quantity used for trial specimen
h=trial specimen height
To provide adequate data, three test specimens must be prepared for each asphalt content and aggregate combination. The asphalt content is by definition given as
A separate pan is required for each specimen. Specimens must be prepared to asphalt contents that vary in 0.5 % increments with at least two of the test specimens having asphalt content above the optimum and two below.
Usually, six different asphalt contents are selected requiring a total of 18 Marshall specimens to be scheduled.
The weighed aggregates are dried in an oven and maintained at a temperature about 30oC above the mixing temperature. The mixing temperature is the temperature which corresponds to asphalt viscosity of 170±20cSt. This temperature is obtained from the viscosity-temperature data on the asphalt cement being used.
A quantity of the required asphalt cement corresponding to the desired asphalt content of the mix is added to the heated aggregates and then quickly and thoroughly mixed to obtain a uniform coating of asphalt on the aggregates.
b. Compaction of mixture
The whole of the mixture is transferred into the compaction mould for compaction to proceed.
Compaction takes place when the mixture has attained the compaction temperature which is the temperature corresponding to a viscosity of 280±30cSt.
This temperature which is slightly lower than the mixing temperature is obtained from the viscosity–temperature curve for the asphalt cement sample being used and is usually in the region of 130-140oC.
Samples that cool below the compaction temperature are not recommended for use and must be discarded. The hammer and mould to be used for the compaction must be heated to temperatures around 100oC or a bit more.
It is recommended that the inside of the mould be given a light application of oil to aid later extrusion of compacted samples.
In compacting, one face of the specimen receives the required number of blows of the hammer: the faces are reversed and the same number of blows is administered to the reversed face.
The number of blows per face is dictated by the level of traffic as follows:
After the compaction, samples are identified and allowed to cool preferably overnight before being extruded from the compaction moulds.
c. Tests on compacted specimens
i. Bulk density test
This test is to be carried out on the extruded compacted specimen before any destructive test.
Volumes for density evaluation are determined by the water displacement method instead of the physical dimensions of the sample because of the unevenness of the sample surface due to the presence of pores and crevices.
There are two approaches to the determination of the compacted volume; one approach uses the saturated surface dry sample and the other uses the compacted sample with a coating of wax.
Approach 1: Saturated surface dry sample method
In this approach the compacted sample is weighed in air (Wa), then weighed fully submerged in water (Wsub).
After the sample has been removed from water the surface is wiped dry of any free-flowing water. In this condition, the sample is said to be saturated surface dry (i.e. it is saturated but the surface is dry).
The saturated surface dry sample is then weighed in air (WSSD). At the end of the weighings, the bulk (total) volume of the compacted specimen is given as
Hence the bulk density of the compacted sample becomes
Approach 2: Wax-coated sample method
In this approach, the sample is first weighed in air (Wa), then coated with paraffin wax (candle) and weighed in air again (Wc). It is then fully submerged in water and weighed submerged (Wcsub).
At the end of the weighings the following volumes obtain if the weightings are recorded in grams:
ii. Stability and flow test
This is a destructive test and must be carried out only when bulk density determinations have been completed on the compacted samples.
Marshall specimens for the stability and flow test are conditioned for 30 to 40 minutes in a water bath maintained at 60oC. At the end of the conditioning period, a specimen is removed from the bath and carefully wiped dry. It is then positioned in the testing head and the flow meter set in position.
A compressive load is applied at a constant rate of 51 mm/min until failure. The failure load is the stability value and the corresponding compression measured in one-hundredth of an inch is the flow.
In all, the test should not last more than one minute from the beginning to the end in order to keep the specimen temperature practically unchanged at 60oC.
d. Voids analysis
The important properties of the compacted specimens of the paving mixture are the bulk density, and voids in the mixture. Three different kinds of voids are identified. These are;
a. air voids or voids in total mix (VTM)
This is the total volume of the small pockets of air between the coated aggregate particles throughout the compacted paving mixture, expressed as a percentage of the total specimen volume.
b. volume in mineral aggregates (VMA)
This is the volume of inter-granular voids space between the aggregate particles of a compacted paving mixture that includes air voids and volume of the asphalt not absorbed into the aggregate and expressed as a percent of the total volume
c. Voids filled with asphalt (VFA)
This is the proportion of the volume of inter-granular voids space between the aggregate particles of a compacted paving mixture that has been filled with asphalt.
The image below represents the equivalent weights and volumes of a compacted Marshall specimen.
Vmb= bulk volume of compacted specimen
Va=volume of air voids
Vb=volume of asphalt or bitumen
Vba=volume of asphalt absorbed into aggregates
Vma=volume of voids in mineral aggregates
Vmm=voidless volume of compacted mix
Vsb=bulk volume of aggregates
Vse=effective volume of mineral aggregates
Wb=weight of asphalt
Ws=weight of aggregates
By substituting weights for volumes, it can be shown that
Gmb=bulk specific gravity of the compacted specimen
Gsb=bulk specific gravity of the aggregates
Pb=asphalt (bitumen) content in %
Again, by substituting weights for volumes, it can be shown that
Gmm=theoretical maximum specific gravity of the compacted specimen.
The voids filled with asphalt is given by the expression
In the above equations, Gsb is a constant of the aggregates used but Gmm and Gmb are dependent on the asphalt content of the compacted specimen and must be evaluated before the voids calculation for each asphalt content can be made. Gmb is derived directly from the bulk density values of the specimen.
Gmm may be evaluated for each asphalt content by the standard test ASTM D2041. A more practical way of determining Gmm for each asphalt content is to use the standard test (ASTM D2041) to determine the value at an asphalt content close to the optimum.
The value so determined is used in the following equation to determine the parameter Gse which is a theoretical constant of the aggregates.
Gse = the effective specific gravity of the aggregates
Gb=specific gravity of asphalt (values range between 1.01-1.03)
Once Gsehas been evaluated, subsequent values of Gmm for any other asphalt contents can then be calculated using the equation above.
e. Interpretation of test data
Once the density-voids analysis has been completed, the relevant data obtained must be interpreted. This is accomplished by using the data to prepare the following plots:
- asphalt content vs density or unit weight
- asphalt content vs. Marshall Stability
- asphalt content vs flow
- asphalt content vs air voids (VTM)
- asphalt content vs VMA
- asphalt content vs VFA
As a precaution, it is necessary to check that the plots exhibit characteristics similar to the corresponding plots in the image shown below.
The following characteristics must be exhibited by the plots
- Stability increases with asphalt content, reaches a peak and then decreases. With many recycled mixtures, however, stability may decrease with increasing asphalt content and not show a peak.
- Flow increases with increasing asphalt content
- Density increases with increasing asphalt content up to a peak and then decreases. Note that peak density and peak stability do not necessarily occur at the same asphalt content.
- VTM decreases with increasing asphalt content
- VMA decreases with increasing asphalt content reaches a minimum and then increases.
- VFA increases with increasing asphalt content.
It is to be noted that peak density and peak stability do not necessarily occur at the same asphalt content. Maximum stability tends to occur at an asphalt content slightly lower than for maximum density.
Determination of the optimum asphalt content
The optimum asphalt content may be determined by any of the following two methods:
1. Asphalt Institute’s Method
In this method, the following steps are followed.
a. Determine the asphalt content at
- maximum stability
- maximum density
- mid-point of specified air voids range
and find the average of the three values obtained.
b. Enter the appropriate curves in the previous plots and determine the
- air voids (VTM)
- VMA and
corresponding to the average asphalt content determined previously.
c. Compare the values of the above parameters obtained from the plot with the specification values or limits. If any of the values fails to meet the specifications, the mixture should be redesigned, otherwise the mix formulation is accepted.
2. NAPA (National Asphalt Paving Association) Method
In this method emphasis is placed on the air voids in the total mix. The following are the steps to follow in determining the optimum asphalt content.
i. Determine the asphalt content corresponding to the median of the specifications air voids content (usually 4%) and take this value as the tentative optimum asphalt content.
ii. Determine the values of the following parameters at the tentative optimum asphalt content:
- Marshall stability
- VMA and
iii. Compare the values of each of the above parameters against the specification values or limits. If all are within specification requirements, then the tentative optimum asphalt content is indeed the optimum asphalt content otherwise the mixture should be redesigned.
f. Selection of Job Mix Formula
The Job Mix Formula refers to the gradation and the asphalt content which satisfy all specification requirements and based on which plant mixtures are to be produced for the construction.
This is chosen at the final stage of the laboratory design as the mix that was most economical and gave the most satisfactory results.
Stiffness of Compacted Asphalt Concrete
For engineering calculations relating to asphaltic concrete mixture behavior under loads, the stiffness of the mix is used as a term analogous to Young’s Modulus.
It is sometimes referred to as the Resilient Modulus (MR). The stiffness of mixtures just like the stiffness of asphalt cement is dependent on temperature and duration of loading.
The relationship between stiffness (Smix or MR), time (t) and temperature (T) is expressed as
where s, e= stress and strain respectively. The dependence of asphalt concrete stiffness on temperature is illustrated below.
The following times are considered representative of the duration of the given traffic load application conditions.
Mixture stiffness may be determined by conducting indirect tensile tests on the compacted specimen or estimated from the stiffness nomograph attached using the following data input:
% volume of mineral aggregate (Vg)=100-VMA
% volume of bitumen (Vb)=VMA-VTM
stiffness of bitumen within the mix (Sb)
For the evaluated mix stiffness to be representative of service conditions, the input stiffness (Sb) of the asphalt binder should be that of the asphalt within the mix and not that of the original asphalt selected for the mix.
Hence the stiffness must be that of the asphalt recovered from the mixture or that of the Thin Film Oven Test residue.
Plant Manufacture of Hot-Mix Asphalt (Asphalt Concrete)
Plant productions of HMA involve combining aggregates of different sizes to meet specifications and mixing them with the selected asphalt at elevated temperatures in the proportion as per the Job Mix Formula in a hot-mix asphalt plant.
Aggregates are obtained as a blend of 2 to 4 separate stockpiles of materials contained in cold bin silos.
Materials from the silos are combined in the required proportions by means of an adjustable gate and a variable speed feeder located at the bottom of each bin.
The asphaltic concrete mixture may be produced in a drum mix facility or in a batch mix plant.
Drum Mix Operations
In a drum mix facility, aggregates proportioned from cold bin silos are brought to a cold feed elevator by means of a conveyor and thence to a dryer.
An automatic weighing system on this elevator continuously monitors the amount of aggregates plus moisture going into the drum mixer so that the dry weight of the aggregates can be established in a control room and the right quantity of asphalt pumped into the drum mixer by the asphalt proportioning system.
A burner at the entry end of the drum heats and dries the aggregates as they travel down the length of the drum to the discharge end.
At a point about a third way to the discharge end, asphalt cement is added from a storage tank, and just about at the same point mineral filler is also added and the whole mass mixed together to produce a uniformly coated mixture.
The HMA exits the drum through a discharge chute onto a conveyor system that transports the mix into a surge silo for storage or discharge into trucks.
Batch Mix operations
In a batch mix operation, materials proportioned from the cold bin silos are fed by a cold feed conveyor into a dryer exactly in the case of the drum mix operations.
A burner in the dryer provides the heat energy required for evaporating the moisture in the aggregates. Unlike in the drum mix, however, where aggregates being dried move in the same direction as the exhaust gases, aggregate in the batch mixing facility move counter-flow to the exhaust gases.
The hot dust-laden gases from the dryer are passed through a dust collection system so that dust particles can be removed to cut down on dust emission. Dust so collected is returned to the filler silo for re-use.
The heated aggregates from the dryer are conveyed via an enclosed elevator to a tower plant where they are discharged onto vibrating screens that separate the materials into a number of sizes.
The screened aggregates are then stored in hot bins from where a control system proportions them into a weigh box. The weighed mixture is then discharged into a pugmill where the required amount of asphalt cement already weighed and stored in a weigh bucket is introduced after a few seconds of dry mixing.
The pugmill is equipped with a counter-rotating twin shaft that helps coat the aggregates with asphalt quickly. The HMA produced is transferred into a storage silo or discharged directly into trucks for transfer to the lay-down site.
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