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Questions and Answers
What is the primary purpose of a hot mix plant in the production of bituminous mixtures?
What is the primary purpose of a hot mix plant in the production of bituminous mixtures?
Which of the following is NOT a component of a hot mix asphalt?
Which of the following is NOT a component of a hot mix asphalt?
In the context of hot mix production, what is the significance of maintaining a moisture content of less than 0.5%?
In the context of hot mix production, what is the significance of maintaining a moisture content of less than 0.5%?
What is the difference between continuous and batch types of hot mix plants?
What is the difference between continuous and batch types of hot mix plants?
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During the production of bituminous mixtures in the laboratory, what is the first step in preparing specimens?
During the production of bituminous mixtures in the laboratory, what is the first step in preparing specimens?
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What does the term 'output at 2% moisture content' refer to in a hot mix plant?
What does the term 'output at 2% moisture content' refer to in a hot mix plant?
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Which type of hot mix plant involves mixing bitumen with aggregates at desired temperatures?
Which type of hot mix plant involves mixing bitumen with aggregates at desired temperatures?
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What compaction method would you expect to use with a Superpave gyratory compactor?
What compaction method would you expect to use with a Superpave gyratory compactor?
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What is the Bulk Specific Gravity of Mix represented by?
What is the Bulk Specific Gravity of Mix represented by?
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Which factor contributes to an initial increase in the Bulk Specific Gravity of Mix (Gmb) when binder content rises?
Which factor contributes to an initial increase in the Bulk Specific Gravity of Mix (Gmb) when binder content rises?
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What is the purpose of determining Theoretical Maximum Specific Gravity (Gmm)?
What is the purpose of determining Theoretical Maximum Specific Gravity (Gmm)?
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In terms of specific gravity, what does the variable G represent?
In terms of specific gravity, what does the variable G represent?
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What can cause a decrease in the Bulk Specific Gravity (Gmb) after reaching maximum binder content?
What can cause a decrease in the Bulk Specific Gravity (Gmb) after reaching maximum binder content?
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What does the term 'Voids in Mineral Aggregates' (VMA) denote?
What does the term 'Voids in Mineral Aggregates' (VMA) denote?
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How is the percentage of binder absorption into the aggregates calculated?
How is the percentage of binder absorption into the aggregates calculated?
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What is the typical range for the Dust to Binder Ratio (DP) in bituminous mixtures?
What is the typical range for the Dust to Binder Ratio (DP) in bituminous mixtures?
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What does the term 'Voids Filled with Bitumen' (VFB) indicate?
What does the term 'Voids Filled with Bitumen' (VFB) indicate?
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What happens to the Theoretical Maximum Specific Gravity (Gmm) as binder content increases?
What happens to the Theoretical Maximum Specific Gravity (Gmm) as binder content increases?
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What method is commonly used to determine the Effective Specific Gravity of Aggregates (Gse)?
What method is commonly used to determine the Effective Specific Gravity of Aggregates (Gse)?
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Which of these statements is correct regarding the air voids in a compacted mixture?
Which of these statements is correct regarding the air voids in a compacted mixture?
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What impact does improper rolling equipment have on bituminous mixtures?
What impact does improper rolling equipment have on bituminous mixtures?
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What is the main purpose of determining an appropriate blend of aggregate sources in mix design?
What is the main purpose of determining an appropriate blend of aggregate sources in mix design?
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Which of the following best describes the role of air voids in mix design?
Which of the following best describes the role of air voids in mix design?
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What is a primary concern when designing mixtures for heavy traffic?
What is a primary concern when designing mixtures for heavy traffic?
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Which mix design method emphasizes the use of a kneading compactor?
Which mix design method emphasizes the use of a kneading compactor?
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The final goal of mix design is to achieve a balance among which of the following properties?
The final goal of mix design is to achieve a balance among which of the following properties?
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What characteristic must moisture sensitivity testing ensure in bituminous mixtures?
What characteristic must moisture sensitivity testing ensure in bituminous mixtures?
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What is a key performance parameter that mix design seeks to ensure?
What is a key performance parameter that mix design seeks to ensure?
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Which of the following factors does NOT influence the drying process in a hot mix plant?
Which of the following factors does NOT influence the drying process in a hot mix plant?
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What does VFB failing at relatively high air voids indicate in light traffic conditions?
What does VFB failing at relatively high air voids indicate in light traffic conditions?
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What is the primary role of the cyclonic separators in the primary pollution control device?
What is the primary role of the cyclonic separators in the primary pollution control device?
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Which of the following is NOT one of the objectives of bituminous mix design?
Which of the following is NOT one of the objectives of bituminous mix design?
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What is the initial step in the sequence of aggregate collection in the weigh hopper?
What is the initial step in the sequence of aggregate collection in the weigh hopper?
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Who was responsible for the adoption of the Marshall mix design procedure?
Who was responsible for the adoption of the Marshall mix design procedure?
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Which component of the hot mix plant temporarily stores heated and screened aggregates?
Which component of the hot mix plant temporarily stores heated and screened aggregates?
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What is the significance of maintaining higher aggregate temperatures in a hot mix plant?
What is the significance of maintaining higher aggregate temperatures in a hot mix plant?
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Which of the following best describes the function of the mixing unit (pugmill)?
Which of the following best describes the function of the mixing unit (pugmill)?
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What role does the hot elevator serve in the batch type hot mix plant?
What role does the hot elevator serve in the batch type hot mix plant?
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In the secondary pollution control device, how is the dust handled after it collects on the filter bags?
In the secondary pollution control device, how is the dust handled after it collects on the filter bags?
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What is the main purpose of the control panel in the hot mix plant?
What is the main purpose of the control panel in the hot mix plant?
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What does the bitumen weigh bucket do in the hot mix plant?
What does the bitumen weigh bucket do in the hot mix plant?
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Which type of screening method uses a rotating drum?
Which type of screening method uses a rotating drum?
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Which equipment is essential for protecting mineral filler from moisture?
Which equipment is essential for protecting mineral filler from moisture?
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What is the function of the overflow pipe in the hot bins?
What is the function of the overflow pipe in the hot bins?
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How long can hot mix temperature be maintained in the hot mix surge silo?
How long can hot mix temperature be maintained in the hot mix surge silo?
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What is the target air void content typically established in the Marshall method for optimal bitumen content?
What is the target air void content typically established in the Marshall method for optimal bitumen content?
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Which of the following is NOT a component assessed in the physical properties assessment phase of the Marshall method?
Which of the following is NOT a component assessed in the physical properties assessment phase of the Marshall method?
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Which test evaluates the stability and flow of Marshall specimens?
Which test evaluates the stability and flow of Marshall specimens?
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What aspect of the aggregate blend is NOT addressed in the Marshall method's aggregate blend combinations phase?
What aspect of the aggregate blend is NOT addressed in the Marshall method's aggregate blend combinations phase?
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In the Marshall method, what does VMA stand for?
In the Marshall method, what does VMA stand for?
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Which formula is used to approximate asphalt content in Marshall method mix design?
Which formula is used to approximate asphalt content in Marshall method mix design?
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Which of the following parameters is used to determine the stability of the Marshall mixes?
Which of the following parameters is used to determine the stability of the Marshall mixes?
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What characteristic is analyzed during the density-void analysis in the Marshall method?
What characteristic is analyzed during the density-void analysis in the Marshall method?
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What causes thermal cracking in asphalt pavements?
What causes thermal cracking in asphalt pavements?
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Which test is not used for evaluating tensile properties in asphalt mixtures?
Which test is not used for evaluating tensile properties in asphalt mixtures?
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Which material proportion is recommended for RAP in asphalt mixes?
Which material proportion is recommended for RAP in asphalt mixes?
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What is the main focus of quality control tests in asphalt mixture production?
What is the main focus of quality control tests in asphalt mixture production?
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What does VMA stand for in asphalt mix design?
What does VMA stand for in asphalt mix design?
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Which is a common adjustment made to restore VMA in asphalt mixtures?
Which is a common adjustment made to restore VMA in asphalt mixtures?
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In daily mix verification, what does data dispersion indicate?
In daily mix verification, what does data dispersion indicate?
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What is crucial for the reconciliation of differences between laboratory and field-produced mixes?
What is crucial for the reconciliation of differences between laboratory and field-produced mixes?
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What should the average density of field-produced asphalt be compared to the reference density?
What should the average density of field-produced asphalt be compared to the reference density?
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What does the term 'Job Mix Formula' (JMF) relate to?
What does the term 'Job Mix Formula' (JMF) relate to?
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What can inadequate VMA and air voids in asphalt mixes lead to?
What can inadequate VMA and air voids in asphalt mixes lead to?
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What is the typical range for in-place density of asphalt mixes compared to laboratory compacted bulk density?
What is the typical range for in-place density of asphalt mixes compared to laboratory compacted bulk density?
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What is the purpose of temperature measurements in density specifications?
What is the purpose of temperature measurements in density specifications?
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Which of the following is true regarding unconventional mixes?
Which of the following is true regarding unconventional mixes?
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Study Notes
Bituminous Mixtures
- Bituminous mixtures are a combination of aggregate and bitumen, heated and uniformly mixed to obtain aggregate coated with bitumen.
- Hot-mix asphalt is a type of bituminous mixture.
Contents
- Production of Bituminous Mixtures
- Behavior of Bituminous Mixtures
- Desirable Properties of Bituminous Mixtures
- Volumetric Properties
- Mix Design
- Performance Testing
- Field Verification of Bituminous Mixtures
Production in Laboratory
- Procedure for preparing specimens:
- Heat and mix the aggregate and bitumen.
- Place the material into a heated mold.
- Apply compaction force.
- Allow the specimen to cool and extrude from the mold.
- Compaction process uses tools like a Marshall hammer (impact force) and a Superpave gyratory compactor (shearing action).
Production in Field - Hot Mix Plant
- Purpose:
- Blend different sizes of aggregates.
- Dry and heat aggregates (moisture content < 0.5%).
- Maintain mixing temperature.
- Feed bitumen and filler.
- Create a homogeneous mix of aggregates, bitumen, and filler.
- Ensure safe and environmentally friendly operation.
- Maintain accurate and consistent production.
- Process:
- Drying (heating) and screening of cold aggregate bins.
- Measuring and mixing of mineral filler and asphalt cement.
Classification of Hot Mix Plant
- Capacity of Hot Mix Plant: Output at 6% and 2% moisture content by aggregates.
- Hot mix plant methodology: continuous type and batch type.
- Direction of flow of aggregates and hot gases: counter flow type and parallel flow type.
- Mobility of hot mix plant: stationary type and mobile type.
- Examples of hot mix plant capacity: 40-60 ton/hr output at 6% moisture content, 60 ton/hr output at 2% moisture content.
Classification of Hot Mix Plant (continued)
- Hot mix preparation methodology:
- Continuous type: Continuous induction of aggregates, fines, and bitumen into pugmill/drum mix in desired proportion, with continuous discharge.
- Batch type: Adding hot bitumen with a batch of hot aggregates and filler (if necessary) at desired temperature and proportion in the mixing unit. The prepared mix is then transferred to a silo for storage or directly to transportation tippers.
Classification of Hot Mix Plant (continued)
- Direction of flow of aggregates and hot gases:
- Counter flow type: Virgin aggregates enter opposite direction to exhaust gases inside dryer drum to improve heat transfer efficiency, reduce exit gas temperature, and lower plant emissions.
- Parallel flow type: Aggregates and hot gases flow in the same direction inside the dryer drum, which might reduce thermal efficiency, increase fuel consumption due to low heat transfer near the drum mixer, and increase environmental hazards.
Classification of Hot Mix Plant (continued)
- Mobility of hot mix plant:
- Stationary type: Fixed location with rigid construction, less mobility, but more work quantity and frequent shifting.
- Portable type: Fitted with pneumatic tires, for more frequent shifting, lower work quantity of short duration.
- Integrated foundations.
Batch Mix Plant & Drum Mix Plant
- Diagrams illustrating the components of a batch mix and the drum mix plant.
Components of Batch Type Hot Mix Plant
- Descriptions of each component, such as the cold bin feeder, cold elevator, dryer drum, primary dust collection device, screening unit, hot bins, mixing unit, and control panel.
- Details of functions and processes.
Components of Batch Mix Plant (continued)
- Cold bin feeder: Minimum four bins with separators, first bin for fines, conveyors fitted, single deck vibrating screen at discharge, to remove oversized aggregates, cold elevator or cold feed conveyor, conveyor to dryer drum.
- Dryer drum: Revolving, inclined cylindrical drum, burner and blower fan, longitudinal troughs (channels), flights to lift the aggregates, and drop them through hot gases to remove moisture, 4-6% moisture content for maximum efficiency.
- Dryer drum (Dwell time): Dwell time of aggregates within the dryer drum to account for the material's removal of moisture, slope of the dryer drum. Diameter, and length, number and arrangement of flights, efficiency of burner.
- Primary pollution control device: Primary dust collection system, cyclone separators to remove dust from the exhaust.
- Exhaust stack, to eliminate exhaust gases.
- Hot elevator: Carries aggregates to gradation units after heating and drying, location at edge of hot elevator discharge chute, to smoothens the flow.
- Screening unit: Multi-deck vibratory screening, series of vibrating screens, hot elevator to vibrating screens to hot bins, order of coarse, intermediate, and fines. Rotary screening (rotating screening drum - screening + heating), cold aggregates to screening drum to hot bins, order of fines to intermediate to coarse.
- Hot bins: Temporarily store heated and screened aggregates, individual compartments with min. four partitions, overflow pipe, discharge gates, and leveling and temperature measurement devices.
- Weigh hopper: Discharge gate of an aggregate bin is opened and aggregates pour into weigh hopper when scale reaches a preset weight, the discharge gate closes.
- Bitumen unit: Equipment (bitumen tank, heating system, bitumen pump, delivery pipe, bitumen weighing bucket), weigh and pump bitumen into pugmill through spray bar, for measuring and delivering desired bitumen quantities into the pugmill.
- Mixing unit (pugmill): Chamber for mixing aggregates and bitumen, feed material quantity, types of mixing units (underfilled pugmill, overfilled pugmill). Batch mixing time, time between opening weigh hopper gate and closing of pugmill discharge gate, mixing cycle and optimum.
- Mineral filler/dust control system: Storing mineral filler (rock dust, hydrated lime or cement) to prevent moisture, choking, and hardening from moisture, equipment (1st and 2nd screw feeders) to carry filler/dust and a common chute for both.
Components of Batch Mix Plant (continued)
- Control panel: Controls operations of complete plant, displays information such as plant load, composition of materials, running weights of materials, total quantity flow during a specified period, and temperature.
- Secondary pollution control device (bag house filter): Two chambers (dirty gas chamber, clean gas chamber) to filter dust laden flue gases. Filter bags used to capture exhaust dust, and shaker arrangement for dust from the bags to hopper are necessary.
- Hot mix surge silo: To store heated and screened aggregates temporarily, hot oil circulation for maintaining temperature for 16 hours approximately.
Components of Drum Mix Plant
- Rotary shell function: Remove moisture from aggregates, blending aggregates and bitumen, fines pipes, J-flights, W-flights, aggregate retention rings, sweep discharge, bitumen pipe, cup flight, notched flight, and kicker flight, Burner.
Components of Drum Mix Plant (continued)
- Dryer cum mixing drum (zones): Drum with two zones (combustion and mixing); Heating and drying of aggregates, mixing of aggregates, filler, and bitumen; Rate of drying of aggregates - Dependent on moisture content, dwell time; Factors - Length to diameter ratio (4-6), capacity of drum mixer.
- Burner fuel: Light diesel oil/furnace oil; Forced and induced draft principle; Exhaust fan = 55% air; Air blower = 45%. Bitumen line: Liquid bitumen discharge by gravity into drum; Bitumen fines receiver; Dust from mineral filler system mixing zone.
Advantages of Drum Mix Plant
- Less cost, less space requirement, easy to transport, availability of different capacity, less operation and maintenance cost.
Advantages of Batch Mix Plant
- Well-graded aggregates, secondary gradation control unit, homogenous mix, measurement of aggregates and bitumen in desired quantity, and mixing in pugmill.
Limitations of Drum Mix Plant
- No secondary gradation control unit, no system to measure aggregate temperature, aggregate temperature = exhaust gas temperature ≈ 12°C, and quantity of hot mix not as homogeneous as batch mix plant.
Factors Affecting Hot Mix Plant
- Presence of moisture in the aggregates, adequate aggregate heating, dust content in the mix; adequate surface area to be coated with bitumen, moisture content versus output, dust content versus output, and altitude versus output.
Bitumen Behavior
- Temperature susceptibility, viscoelasticity, and aging characteristics.
- Shorter loading time, stiffer bitumen, higher temperatures; mixing temperature, compaction temperature.
Mineral Aggregate Behavior
- Types of aggregates: Natural (bank-run or pit-run materials), processed (mined, quarried, crushed), and synthetic (man-made material – Slag).
- Permanent deformation - shear strength, overloading of mix, shear plane develops, aggregate particles slide past each other, shear stress, which is greater than shear strength; dependent on resistance to movement or inter-particle friction; preferred aggregates are angular, rough-textured aggregates over rounded, smooth-textured aggregates.
Bituminous Mixture Behavior
- Primary stresses: vertical compressive stress, shear stress, horizontal tensile stress, tensile strain at the edge of high pressure radial tires, and top-down cracking.
Desirable Properties of Bituminous Mixtures
- Resistance to permanent deformation, stability, fatigue resistance, resistance to low-temperature cracking, moisture resistance, durability, skid resistance, and workability.
Desirable Properties (continued)
- Resistance to permanent deformation - stability: accumulation of small amounts of unrecoverable strain from repeated loads, primary causes: inadequate mix stability and subgrade failure, methods to improve rutting resistance - internal friction (aggregates) - angular and rough aggregates, aggregate gradation, cohesion (bitumen), the lesser extent, stiffer/modified binder.
Desirable Properties (continued)
- Fatigue resistance: pavement's resistance to repeated bending under wheel loads; primary causes: insufficient pavement thickness, air voids, and asphalt binder properties; Methods to overcome fatigue cracking: adequate traffic load and composition during mix design, use thicker pavements, provide adequate subgrade drainage, use modified binder pavement materials, use HMA resilient to withstand normal deflections.
Desirable Properties (continued)
- Low-temperature cracking: occurrence of transverse cracks when the temperature drops sufficiently to induce stress in the HMA layer that exceeds the tensile strength of the mixture; primary causes: magnitude and rate of cooling, frequency of low-temperature occurrences, stiffness of asphalt; Methods to overcome cracking: Proper choice of bitumen, avoid highly absorptive aggregates, and aggregates with high dust content.
Desirable Properties (continued)
- Moisture resistance - impermeability: moisture damage results in water penetrating and pulling from voids in the pavement as traffic loads occur; stripping - water vapor gets between the bitumen film and the aggregates, breaks the adhesive bond; Methods improve moisture resistance - sufficient binder in mix, provide sufficient compaction - impermeable mat, use anti-stripping agents, and controlling dust and clay content.
Desirable Properties (continued)
- Durability: ability of the mix to resist factors like aging of asphalt, disintegration of aggregates, and stripping of asphalt film from aggregates; primary causes: traffic and weather; methods to improve durability: Dense gradation; sound, tough, moisture-resistant aggregate, maximizing asphalt film thickness, compacting mixture to be impervious.
Desirable Properties (continued)
- Skid resistance: ability of an asphalt surface to minimize skidding or slipping of vehicle tires; good skid resistance: tire treads maintain contact, rough pavement surface; methods to overcome skid resistance: aggregates - rough textured and resist polishing, minimize hydroplaning.
Desirable Properties (continued)
- Workability: ease with which a paving mixture can be placed and compacted tender mix - internally unstable mix that tends to displace laterally and shove rather than compact under roller loads; causes of tender mix - aggregate issues, bitumen issues, and construction issues.
Volumetric Properties in Mix Design
- Aggregates: specific gravity (bulk, effective, apparent).
- Mix: bulk specific gravity of mix (Gmb), theoretical maximum specific gravity of mix (Gmm), air voids, voids in mineral aggregates (VMA), voids filled with bitumen (VFB), binder absorption (Pba), and effective binder content (Pbe).
- Dust to binder ratio (DP).
Volume Calculations
- Bulk Specific Gravity of Mix (Gmb): Methods (SSD method, Automatic Vacuum Sealing) formula.
- Theoretical maximum specific gravity of mix (Gmm): steps, apply vacuum, fill with water and weigh.
- Effect of binder content on Gmb and Gmm: binder increases initially, binder fills voids => more mass, after maximum value, Gmb reduction; voids filled with binder => more binder => Volume increases, Increase in binder content => Gmm reduces; % aggregates reduces; volume of binder increases.
- Specific gravity - ratio of density of material to density of water (G = m/v, or G= ρ).
- Aggregate blends: calculation of Gsb from various aggregate materials, and their proportions.
- Apparent specific gravity of aggregates: calculation of Gsa; water absorption for aggregate blend (WA).
- Calculations of Gmm at trial binder contents: steps (determine Gmm at one trial binder content, determination of Gse - effective specific gravity of aggregates, check: Gsa ≥ Gse ≥ Gsb (acceptable)).
% Air Voids in Compacted Mixture
Calculations involving volume of air voids in a compacted mixture, expressed as (%) of total mix volume, in terms of specific gravity; using volumes and masses of components of the mix, air voids calculated.
% VMA in Compacted Mixture
Calculation of VMA (Voids in Mineral Aggregate), defined as the intergranular void space of compacted paving mixture including air voids and effective asphalt content In terms of specific gravity) and by calculating fractions of volumes and masses of components of the mix.
% VFB in Compacted Mixture
Calculation of VFB (Voids Filled with Bitumen), defined as the percentage of VMA that is filled with the effective binder using volumes and masses of components, and relating to the specific gravity of each material in the mix.
Binder Absorption
Calculate percentage by mass of binder that is absorbed into the aggregates from the mixing process.
Dust to Binder Ratio
The ratio of percentage of aggregates passing the 0.075 mm sieve to the effective binder (P0.075/ Pbe).
Exercise
- Solving for Pa, VMA, VFB, and Pbe with given conditions, and using formulas, for bitumen content as 5.5 %.
Discussion on Volumetric properties
- Voids in mineral aggregates (VMA), discussing increase in bitumen content vs VMA decrease, workable mix→ easily compact, and aggregate displaced and pushed.
- Discussing increase in bitumen content and VMA discussion, identifying cases of dry side, wet side (prone to segregation, high air voids, and rutting), modifying aggregate gradation.
- Discussing VMA curve completely below specification, redesign/change in aggregate source, avoid bitumen contents on the wet side and minimum, bleeding and/or exhibitsplastic flow, and susceptibility to rutting.
- Discussing factors like nominal maximum aggregate size (NMAS), decreasing NMAS => total surface area of aggregates increases, % binder requirement increases, higher Vbe and same target Va, higher VMA.
- Type and amount of laboratory compactive effort, higher compactive effort leads to higher VMA, VMA for gyratory > Marshall, aggregate gradation, aggregate shape, strength and texture, bitumen type and quantity, and sample temperature.
- Compaction level, increase the number of blows = increase in VMA, Reduction in asphalt content = minimum VMA; Increase in air voids => increase in number of blows= reduction in air voids.
Mix Design
- Methodology: Aggregate Selection, Binder Selection, Material Selection & Batching, Sample Preparation & Specific Gravity Testing, Volumetric Analysis, Optimum Binder Selection, Moisture Sensitivity, Performance Testing (Issue Job Mix Formula), Field Verification testing.
Objectives of Mix Design
- To determine the combination of bitumen and aggregate that has long-lasting performance as a part of the pavement structure.
- Procedures in mix design:
- Determining an appropriate blend of aggregate sources to produce proper gradation.
- Selecting the type and amount of bitumen.
- Pavement performance parameters:
- Impermeability, durability, strength, flexibility, stability, stiffness, workability, and fatigue resistance.
- Overall objective: sufficient bitumen ⇒ Ensure durability, sufficient mix stability ⇒ Meet traffic demands, sufficient air voids, additional compaction under traffic loading, thermal binder expansion, limit maximum void content ⇒ Restrict permeability of air and moisture, sufficient workability ⇒ Placement of mix without segregation, aggregate texture and hardness, skid resistance.
Marshall Method of Mix Design
- Bruce Marshall, Modified and improved by the US Corps of Engineer, Major features: Density-void analysis, stability-flow test, optimum bitumen content, 4% air voids, and check for stability, flow, VMA, VFB, and dust to binder ratio.
Marshall Method of Mix Design (continued)
- Physical properties assessment
- Aggregate blend combinations - Gradation requirements
- Marshall specimen preparation
- Density-void analysis
- Stability-flow testing
- Job mix formula
- Satisfying all design criteria
Preparation of Test Specimens
- Expected design asphalt content, computational formula, specimens (binder contents=5, expected design asphalt content), total=15 specimens, weight = 1.2 kg.
- Mixing and compaction temperature (Viscosity range - Unmodified binder, mixing = 170 ± 20 cSt, Compaction = 280 ± 30 cSt, Viscosity (log-log cSt) v/s temperature (log °R) and °R = °F + 459.7).
- Preparation of mold and hammer (modified binder-DSR, Heat mold assembly and face of compaction hammer, 95°C to 150°C).
- Preparation of mixtures, adjust batch weights, adjusted mass, weigh aggregates and bitumen, mix at mixing temperature, condition for 2 hours at compaction temperature, packing the mold (pour mixture and spade 15 times around perimeter, 10 times over interior)
Preparation of Test Specimens (continued)
- Compaction of specimens (apply blows), based on traffic category (axis perpendicular to base plate, height of free fall =457 mm, and cool at room temperature and extrude).
Test Procedure
- Volumetric data (dimensions - height and diameter, Gmb), conditioning (water bath - 60°C for 30-40 min), testing (within 30 sec, loading rate = 50 mm/min, plot load and deformation), max load = stability, corresponding vertical deformation =flow .
Interpretation of Test Data
- Measuring stability and flow values, applying corrections to stability values, preparing graphs, fitting second-order polynomials, optimum binder content, 4% air void content, and satisfying other performance requirements.
Superpave Method of Mix Design
- Superpave (Superior Performing Asphalt Pavements) system, 1987-5 year FHWA study to improve the performance of HMA pavements, consists of two interrelated elements (bitumen specification as per Asphalt Institute MS-26, mix design system that specified aggregate criteria and volumetric properties).
- Superpave mix design procedure: material selection, aggregate blending, mixing and short-term aging, compaction, volumetric analysis, selection of best aggregate and asphalt blend, and performance testing.
Material Selection - Aggregates
- Consensus aggregate properties (20-year design ESALs, coarse aggregate, Uncompacted void content of fine aggregates, sand, nominal maximum aggregate size, exception (4.75 mm mixes); coarse-graded mixes).
- Source aggregate properties (20-year design).
Mixture Requirements - Volumetric
- Calculations for air voids, VMA, and VFB for specific aggregate and binder types.
Density Specifications
- Method Specification (no reference density, comparison, number and size of rollers, number of passes per roller, use of temperature measurements, and applicability), and control-strip, and specified density (control strip - minimum length, average density - reference density, compare-Gmm of field produced HMA, field density > 102% of reference density=new control strip)
- Bulk specified density - In-place density × 100/Lab Bulk Density.
- Theoretical maximum specified density - In-place density × 100/Theoretical Max. Density.
Quality Control Tests and Calculations
- Bitumen content, aggregate gradation, maximum specific gravity of mix, bulk specific gravity of mix, recommendation, air voids, stability, and flow.
Job Mix Formula Verification
- Compare field-produced mixture properties with Job Mix Formula (JMF).
- Bitumen content, gradation, void analysis, other specified tests, adjustments, significant changes, new mix design, adjusted JMF, average results of plant-produced mix.
Daily Mix Verification
- Random sampling, control charts, data dispersion, indication of problems, values, gradual or erratic shifts, systematic cycling of data.
Volumetric Adjustments
- Most common problem: Inadequate VMA and air voids; Reason: Differences in curing time and temperature, breakdown of aggregates.
- Suggestions to restore VMA: Standardizing curing parameters, sampling and testing procedures, gradation change, reduce natural sand components, sand components and increase usage, introduce highly fractured, durable, intermediate-sized "chips" into aggregate structure, reduce dust, increasing fine aggregates, and wash aggregates.
Density Specifications
- Method specification: No reference density, comparison (number, type, and size of rollers, number of passes each roller, use of temperature measurements), applicability (smaller projects, light traffic).
Density Specifications (continued)
- Control strip specified density (control strip - minimum length, average density - reference density, compare - Gmm of field produced HMA, field density > 102% of reference density), and new control strip. Least effective in ensuring pavement performance, high variability.
- Bulk specified density (% of bulk density = In-place density × 100/Laboratory Bulk Density), and theoretical maximum specified density (% of theoretical maximum density = In-place density × 100/Theoretical Max. Density).
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