Introduction
The subject of this term project was an asphalt paving
process utilizing a paving machine and 20 tons capacity tri-axle trucks. The location of
the process was at the corner of Main and Madison in Greenwood ( South of Indianapolis ).
The project is being run by the Reith-Riley Construction Company. - Indianapolis. The
overall process involved :
- Hot-mix batch plant cycle
- Tri-axle truck cycle
- Roller cycle
- Spreader cycle
- Crew cycle
Because of the complexity of the overall construction
process, we chose to observe, report on, analyze and model the paving process on the base
layer of the 15' lane road. At that time, the other lane of the road was not paved yet.
The road has slightly increasing grade and curve along the process. The preliminary
process of gathering the data used in this project, the efficiency of the operation, a
model and MicroCYCLONE simulation of the process, and illustrations will be discussed and
presented.
Asphalt has been used by man for its adhesive and
waterproofing properties. Asphalt was used in 3800 B.C. in the Euphrates and 2500 B.C. in
Egypt. The Sumerians used asphalt in 6000 B.C. for its shipbuilding industry. Today,
asphalt is applied to roofing, sealants, caulking, brake linings, paints, enamels, and
most widely used in the paving industry (Asphalt - Science and Technology, 1968).
Process Description
Batch Plant Production
First, aggregate travels through the cold feed bins, where
initial proportioning of the aggregate takes place. The quantity of material leaving each
bin is regulated by the size of the gate opening, or the speed of a belt, or a combination
of the two. The aggregate is sent to a drier. Here the moisture is removed and is heated
to provide the proper mixing temperature in the pugmill. The aggregate continues to the
hot elevator by screens to the hot bins. The screens provide the final separation of the
aggregate.
The different sizes of aggregate are released into the
weight hopper one bin at a time. The aggregate is dropped into the pugmill for mixing with
the asphalt. The mixture is then dropped into a waiting truck or moved to a storage silo.
Samples are taken from each hot bin for testing. A sieve analysis is conducted as well as
gradation test. From the gradation information, the weight of the aggregate must be equal
to the design gradation. A trial run should be performed and the weights adjusted until
the desired mix is produced. (U.S. Department of Transportation, December 1984)
Placing Asphalt Pavement
Placing the Coat
Before the paving operation starts, an asphalt distributor
is used to spray asphalt on the unpaved surface. This film of asphalt serves as the prime
and tact coats. The coats are then allowed to cure before the actual paving resume. The
purpose of having these coats is to prevent any slippage between the surface and overlay
during or after the compaction. (The Asphalt Institute)
Placing the Asphalt Mix
To start the paving operation, the paver is positioned
properly onto the road. The screed of the paver is lowered onto block of the same depth of
the loose asphalt mat that is going to be laid on the road. (The screed is responsible for
the setting the depth of the asphalt mix.) After that, the block can be removed and paving
can start. As soon as the haul truck arrives at the job site, the paving inspector must
check that the asphalt delivered must be in a satisfactory condition. The paving inspector
usually check for these criteria listed below:
- blue smoke - blue smoke indicate that the mix is too hot.
- stiff appearance
- mix slumped in truck.
- lean, dull appearance - this indicates that the mix has
insufficient asphalt.
- rising steam - too much moisture.
- segregation.
- contamination.
If there is any of the signs above is observed, the mix will
be sent back to the batch plant to be reprocessed. After all conditions are satisfied, the
haul truck can load the mix into the receiving hopper of the paver.
When loading the mix into the receiving hopper, the haul
truck is placed carefully in front of the paver. The rear wheels of the truck should be in
contact with the truck roller of the paver to avoid any misalignment with the paver. The
paver will push the truck forwards as it paves the road. If skewness happens, the whole
process will be delayed because they have to reposition the truck in front of the paver.
Most paver used are self-propelled paver. Each of them
consists of two main units:
- tractor unit. -it includes the receiving hopper, slot
conveyor, flow control gates, spreading crew, power plant, transmission, operator control
for use on either side, and operator's seat. This unit will move the whole system forward.
- screed unit. -it is attached to the tractor unit by long
screed arms on both sides of the machine. It consists of screed plate, vibrators or tamper
bars, thickness control, crown control, and screed heater.
As soon as the the first load of asphalt mix has been
spread, the uniformity of the asphalt texture should be checked. Operators will adjust the
the appropriate adjustment points to correct any nonuniformity. Any segregation of
materials also should not be allowed. Operation should be stopped immediately if any
segregation is detected. The operators should also be aware of is the crown control.
Pavement with crown has to be redone all over again. In addition to that, operators should
continuously loosen the mix that clings to the sides of the hopper and push it back into
the active mix. If the asphalt mix grow cold, it cannot be properly compacted and thus,
looses its strength.
The last process of paving is compaction. This process is
highly influenced by major mix proportion; (1) asphalt content: aggregate size, shape
texture and distribution gradation; (2) filler content, and; (3) mix temperature.
Appropriate rollers and rolling methods should be used in accordance with these
proportion. There are several roller combinations used for maximum results:
- steel-tired static and pneumatic-tired rollers,
- vibratory and steel-tired static rollers, or
- vibratory rollers used in vibrating and static modes.
These combinations are highly recommended by the asphalt
institute.
Rollers should be moved in a slow but uniform speed to
achieve the best result. (See table) These rollers should also be in good conditions. Any
irregularities in the rollers' performances will result in poor compaction of the asphalt;
thus, the pavement will not last long. The rollers should not reverse suddenly while
compacting because this action can displace the mix. If displacement happens, the whole
mat should be loosened with lutes or rakes and restored to the original grade before
rolling can restart. A pattern that is economical and provides the maximum compaction
result should be established. (The Asphalt Institute)
Testing Method
Why Evaluate Density of Hot Asphalt Concrete
As we know, lacking of density during construction of
asphalt concrete causes many problems. It is necessary to obtain high density to insure
that the asphalt concrete will provide the necessary stability and durability for
performance. For instance, low density generally causes long-term deterioration when the
asphalt begins cracking. Therefore various methods have been used to measure the density
in the asphalt concrete.
Procedures Used to Obtain Density
Proper aggregate gradation and asphalt content are important
parameters to ensure that the density of asphalt concrete meets the requirement.
Generally, poor gradation results in a reduction of voids in the mixture; thus, reduces
the asphalt content which serves as the lubricant for aggregates in the mix. The stiff mix
is more difficult to compact. Both the aggregate gradation and the asphalt content are
interrelated and equally important.
Paving asphalt is really difficult in cold climate. The hot
mix cools down faster and harder to compact. To overcome this, contractors usually
increase the temperature of the mix. Unfortunately excessively increasing the temperature
of asphalt mixture may cause problems during compaction and increase oxidation of the
asphalt cement. This may result in a hard and brittle pavement. The mix temperature should
be selected so that the mixture would be able to support the roller immediately behind the
paver. Since there is less time to roll the mixture before it cools, more rollers or
larger rollers are required for the compaction process.
After the mixture is transported to the site, the next step
is to ensure proper density while laying down the asphalt with a spreader. A continuos
availability of the asphalt mix for the paver is crucial. The spreader cannot afford to
start and stop while waiting for the materials. It important to have material that has the
same texture and appearance.
Evaluation of In-Place Density
An evaluation of the in-place material is necessary to
ensure that a satisfactory density is obtained. Most of the time, a nuclear gage is used
to estimate the density. However, the results obtained using this equipment are not
accurate. It has to be calibrated by taking a number of measurements from different
location as soon as the project starts. After calibrations, a series of readings are taken
and then, the readings are compared to the density obtained from the laboratory. The
laboratory results are the density of core samples taken from the same location.
(Placement and Compaction of Asphalt Mixtures, 1982)
Laboratory and Field Pavement Stability Tests
The American Association of State Highway Officials (AASHO)
and the American Society for Testing and Materials (ASTM) are two agencies which set forth
methods and test procedures the paving industry must follow. Five test methods readily
used in the field and laboratory are:
- The Hubbard Field Stability Test (ASTM-D-1138-52 or AASHO
Test 169) - tests the resistance to plastic flow of fine aggregate mixtures.
- The Unconfined Compression Test (ASTM-D-1074-52-T) - measures
the cohesion of paving binder and performance of the internal friction of the aggregate.
- The Marshall Test (ASTM-Method-D-1559) - measures the flow
value or flow index by distorting the specimen until fracture.
- The Hveem Test (ASTM-Method-1560) - uses stabilometer and
cohesiometer apparatus. The stabilometer determines the maximum amount of asphalt which
will obtain the greatest stability by measuring the internal fiction of the mineral
aggregate. The cohesiometer determines the cohesion properties and the strength of asphalt
films by bending and breaking a specimen.
- The Triaxial Compression Test - is useful in determining the
cohesion of the mix and asphalt contents and the angle of internal friction of the entire
mixture by applying lateral pressures. Theses are the most widely used although a variety
of other methods exist. It must be stated that different local, state, and federal
organizations will require certain tests to be performed and may accept a different range
of values.
MicroCYCLONE Model
Resources Description
- Material:
- Aggregates
- Hot mix asphalt material
- Equipment:
- Trucks
- Spreader
- Roller
- Batch plant (hot mix)
- Laborer :
- 4 laborer
- 1 roller operator
- 1 paver operator
- 1 superintendent
- 1 truck operators
- 1 foreman
Discussion
Discussion Preliminary Procedure to Obtain Data
Initially contacting the company and project engineers
involved was necessary before we could obtain access to the site and accumulate data and
information about the operation. Dan Patrick, the superintendent of Reith-Riley
Construction, provided general information regarding this operation. This included cost (
by providing us with the Company's Job Calculation Sheet ), specific details concerning
the operation described and modeled in this paper and details of the crew sizes,
equipment, materials, efficiency and variables of the operation. The site was visited
often to observe and obtain the details of the operation. Pictures were taken, and
individual questioning of the crews and inspector involved in the plant and job site were
employed to get an accurate idea and necessary data. Mr. Patrick couldn't provide a pool
data regarding production times. We were given an estimate of schedule of the job
calculation sheet which were used to compare with the observed/actual production.
Data Collection
The general description of the project, the process
involved, and the equipment used was obtained from Dan Patrick, the superintendent of
Reith-Riley Construction.
The actual project site and time duration for each
activities were obtained from field observation on October 9th, 1991. A digital watch was
used to time every activities involved in the process. The data collected were averaged
for the ease of calculation. The data include the average for :
- loading at batch plant
- travelling to the job site
- dumping the asphalt to spreader
- back-cycle of the truck
- spreading the asphalt
- compacting the asphalt
- checking the level
All the data obtained were approved by the superintendent as
standard time for this particular paving operation. This information is listed in table 1.
The high and low data in table 2.1 and 2.2. were given by the superintendent. Therefore,
the average values of the deterministic input were used as the mode values for the
Triangular and Beta distribution. The Beta 'a' and Beta 'b' values were calculated by the
Vibes program.
Material Handling and Processing System
- Loading hot mix asphalt to the truck
- Hauling asphalt to job site
- Dumping the asphalt to the spreader
- Spreader paving the asphalt
- Roller breaking down the asphalt
- Roller finishing the surface
Productivity Comparison
The productivity level can be measured in three different
ways
- Deterministic time
- Triangle distribution
- Beta Distribution
The values for all three methods were closely related (table
3.). Generally the deterministic values were higher while Beta values were lower. Triangle
values were located somewhere in between the aforementioned. The range of differences
between the distribution is 0.0289 to 0.4278. These difference were not significant;
therefore, any distribution could be used in an actual situation. The production values
from Beta distribution were used for determining the theoretical productivity because they
were more conservative.
From the company's Job Calculation Sheet, they estimated the
capacity of the truck to be 20 tons and working 30 cycles per day. The company's estimated
productivity is 600 tons per day which was equal to 3.75 truck-loads per hour. Refer to
table 3 for comparison of productivity. The productivities of the simulated result of the
MicroCYCLONE model were found to be a little bit lower. The percentage differences as
given in table 3 were as followed :
- compared with deterministic time : 4.67 %
- compared with Triangle distribution time : 10.78 %
- compared with Beta distribution time : 12.87 %
Improvement of Productivity
Using sensitivity analysis, the productivity of the whole
asphalt paving operation could be increased by adding one more roller. With only one
roller, the deterministic productivity at cycle 30 was 3.575 truck-loads per hour. By
adding the second roller, the productivity was increased to 6.4632 truck-loads per hour.
The productivity was increased by about 81 %. ( look at appendix for the output files ).
Also the sensitivity analysis showed that increasing truck
did not increase the productivity at all.
Conclusion
The rate of the operation was determined by the rate of the
roller. This was because the roller took 15 minutes to compact the surface of the asphalt.
Thus, to improve the productivity, more roller should be added into the operation.
Nevertheless, the company chose to use only one roller. Perhaps, the decision to only have
one roller was determined by cost factor. The productivity obtained from the MicroCYCLONE
model was within the expected productivity by the company.
References
- Barth, Edwin J., "Asphalt-Science and Technology,"
Gordon and Breach, New York, New York, 1986.
- Wagner, F.T., "Placement and Compaction of Asphalt
Mixture," ASTM Publication, Philadelphia, PA 19103.
- The Asphalt Institute, "Asphalt Paving Manual,"
Manual Series No. 8, Third Edition, April 1978.
- U.S. Department of Transportation, Federal Highway
Administration, "Hot-Mix Bituminous Paving Manual," December 1984.
- Personal interview with Reith Riley Construction Company
Indianapolis, Job Superintendent: Mr. Dan Patrick, October 2nd, 1991.
- Job Calculation Sheet of Greenwood Man and Madison, Bid date:
September 30, 1991, Courtesy of Reith Riley Construction Company.
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