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HOT MIX ASPHALT (HMA) MIXTURE DESIGN HMA = aggregate + asphalt binder…
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
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
2. Batch Plant
better production quality at rate of 100 ton/hour
most preferred plant in industry
screening unit / screen deck
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
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)
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
Gradation Limits for Asphaltic Concrete
Design Bitumen Contents
Test and Analysis Parameters
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
Tolerance for Asphalt Concrete Mix
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