HOT MIX ASPHALT (HMA) MIXTURE DESIGN
HMA = aggregate + asphalt binder

Asphalt Mixing Plant


where aggregates are blended, heated, dried, and mixed with bitumen

1. Drum Mix Plant


  • simplest plant, high production rate of 300 ton/hour
  • heating and mixing of the bitumen is done in the drum mix

2. Batch Plant

  • better production quality at rate of 100 ton/hour
  • most preferred plant in industry
  • screening unit / screen deck
  1. Continuous Plant
  • process similar up to hot bins but differs from the batch plant during mixing
  • uses a continuous flow of aggregates into the pugmill with the amount of each size fraction controlled by adjustable gate openings.
  • bitumen is sprayed into pugmill continuously
  • thus mixing is a continuous process
  • continuous mixing process and also unloading process (truck is loaded with continuously discharge from the pugmill)

Process :
(a) aggregate with controlled gradation are stored in cold bins from which they are continuously fed in exact proportions on to a conveyor
(b) before flowing into the drum mixer, aggregate is weighed
(c) delivered into drum mixer in which 4 phases of aggregate coatings are undergoing
(d) as they are thoroughly mixed in the drum, they move slowly towards the discharge end of the drum
(e) by the time they reach the end, they are thoroughly mixed.
(f) from the drum, the hot mix is transported by a conveyor to a surge silo
(g) from surge silo, the mix is loaded into tip-truck.

Phase 1: (IN DRUM)

  • early heating phase
  • surface and free moisture begin to leave the aggregates as temperature rises

Phase 2: (IN DRUM)

  • most of the heat rise occur
  • 77-82 'c
  • drive off majority of the moisture
  • rate of increase in mixing temperature levels off

Phase 3: (IN DRUM)

  • 82-93'c
  • asphalt is introduced to the mix
  • moisture driven off now causes asphalt to foam
  • surface area of the asphalt become greatly enlarged
  • thus entrapping dust as well as larger particles and coating the aggregate rapidly
  • aggregate coating in a drum is the aggregate particles being engulfed by the foaming, rapidly spreading asphalt, instead of asphalt being forcibly mixed

Process:
(a) aggregates are fed from cold bins into a dryer in a controlled amounts.
(b) after aggregates are dried and heated, it the pass over a screening unit / screening desk separates material into different sizes and keep in hot bins
(c) aggregates and mineral filler then fed into pugmill in a fixed proportions and mixed with a known quantity of hot bitumen as a batch
(d) mix is then loaded into tip-trucks and been delivered to the paving site.

Precautions:

1. Bitumen heating


  • bitumen should be between 140-160'c when delivered to pugmill
  • bitumen should not be exposed during storage and mix production to temperature of more than 177'c

2. Mix production temperature


  • temperature of mix immediately after discharged from the pugmill may be as low as can be demonstrated to get good coating and compaction to reduce binder hardening
  • temperature immediately before unloading from the tip-truck should be less than 125'c
  • at the commencment of initial rolling should be more than 110'c

3. Dry and wet mixing times


  • dry mixing time of aggregates and filler in the pugmill should be minimal (not more than 10s)
  • wet mixing time of bitumen, aggregates and filler should be no longer than needed to uniformly mix the aggregate and coat the aggregate with bitumen (30-35s)
  • unnecessary long wet mixing time can reduce plant output and promote additional age hardening of the mix

4. Pugmill live zone


  • aggregates, mineral filler and bitumen are mixed in the pugmill
  • should be operated at or just below nominal capacity
  • avoid overloading or underloading that can decrease efficiency of the mixing process

Before road construction works start, these steps must be carried out!


Step 1: Laboratory Mix Design

  • propose a job mix formula for each type of mix required in the road construction
  • Marshall mix design procedure is one of the methods
    end of process, final optimum mix is produced / defined as in terms of the following parameters:
    (a) bitumen content
    (b) VTM (voids of total mix)
    (c) VMA (voids in mineral aggregate)
    (d) VFB (voids filled with bitumen)
    (e) flow
    (f) stability

Refer tables for standards

Step 2: Plant Trial Mix


  • optimum mix is manufactured in asphalt plant (at least 20 tons)
  • verify laboratory mix design
  • test on
    (a) aggregate grading
    (b) BC- bitumen content
    (c) VTM- voids of total mix
    (d) VMA- voids in mineral aggregates
    (e) VFB- voids fill with bitumen
    (f) flow
    (g) stability

Refer table for standards

Step 3: Trial Paving


  • test performance of the mix
  • test workability / constructability of the mix
  • take properties of field sample after paving
  • establish required compaction efforts in terms of number of rolling in order to achieve targeted compaction density
  • establish production mix parameters
  • set tolerances for acceptance control

Step 4: Field / site quality control


  • project mix is finalised after successful evaluation of trial plant mix & trial paving
  • parameters monitored for compliances:
    (a) aggregate grading
    (b) bitumen content
    (c) VTM- voids in total mix
    (d) density
    (e) layer thhickness
    (f) levels

Gradation Limits for Asphaltic Concrete

Design Bitumen Contents

Test and Analysis Parameters

Tolerance for Asphalt Concrete Mix