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Segregation in Asphalt Paving Identification, causes & prevention

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Presentation on theme: "Segregation in Asphalt Paving Identification, causes & prevention"— Presentation transcript:

1 Segregation in Asphalt Paving Identification, causes & prevention
Pennsylvania Asphalt Pavement Association Conference January 30, 2014 Sam Gregory Municipal Transportation Specialist State and Local Road Consultant

2 What Is Segregation? What is segregation and from the picture would you consider this mat segregated?

3 Webster defines Segregation as: “to separate from the main mass and collect together in a new body.”
Applied to Hot Mix Asphalt: “Segregation is the separation of the course and fine aggregate in the mix, collecting and distributing these fractions so that the Hot Mix Asphalt is no longer uniformly textured.

4 What Is The Effect Of Segregation On Pavement?
Premature Distress Raveling Frost Damage Potholes When segregation occurs in a pavement it will lead to premature distress. This distress may be in the form of raveling, frost damage, or may be sever enough to produce potholes.

5 Why Do Premature Failures Occur Due To Segregation?
Weaker Aggregate Structure and Gradation Weaker Mix Higher Voids Segregated mixtures are weak. They have weaker aggregate structures and gradations that tend to produce higher void structures then are recommended for sound long lasting pavements.

6 Sources of Segregation
Truck Hauling Truck Unloading Paver – Hopper, Slat Conveyers, Hopper gates, Auger, Screed Paving Operation Continuity (Balance) Field Responsibility Stockpiling Cold Feed Bins Hot Bins Truck Loading Plant Responsibility Segregation results from mishandling of the HMA at any of several points during the mix production, hauling, and placing operations.

7 Stockpile Segregation – how does it happen?
Large Stockpiles Single Aggregate Blends Although plant segregation is not the responsibility of the Certified Field Technicians, they should be aware of how it could occur at the plant. Large stockpiles are very sensitive to single aggregate blends. Larger particles have a tendency to roll to the outside of the pile thereby segregating the material.

8 Stockpile Segregation – how is it prevented?
Well-trained Operators A good front-end operator can take a segregated pile and by working the pile minimize the problem. By the same token, he can take a good pile and mess it up.

9 Stockpile Segregation – how is it prevented?
Different-sized material Separate piles Generally, different-sized materials are stockpiled separately for feeding to an asphalt plant. To minimize segregation in forming a stockpile use numerous small piles.

10 Stockpile Segregation – how is it prevented?
Build Horizontal Layers Build Sloped Layers Other methods are to build stockpiles in layers, either horizontal layers or sloped layers. Dozer operation should be monitored to assure degradation is not occurring.

11 Cold Feed Bin Segregation – how does it happen?
Bridging Segregation in cold feed bins is usually not a problem unless the aggregate material consists of several sizes. If material bridging takes place non-uniform feeding takes place, resulting in a segregated mix.

12 Cold Feed Bin Segregation – how is it prevented?
Reconfigure Opening – uniform feeding By utilizing a self-relieving bottom, uniform feeding will occur all along the opening of the cold feed bin, eliminating bridging as a source of segregation.

13 Hot Bin Segregation – how does it happen?
Range of Materials Size of Bin Shape of Bin No. 1 Bin Segregation often occurs in the No.1 hot bin due to the size and shape of the large bin and the wide size range of materials in that bin. The ultra-fine dust may lay on the sloping bin wall and then break loose in large slugs, producing an ultra-fine mix that is segregated and uncoated.

14 Hot Bin Segregation – how is it prevented?
Install Baffle – dust slides to center The installation of a baffle causes the dust to slide to the center of the bin where it is mixed with the coarse material

15 Truck Loading Segregation – how does it happen?
Dribbling the material Single drop Segregation can be caused by not loading the truck in mass, but rather loading it by trickling or dribbling the material into the truck. This will cause the larger aggregate to separate and fall into the truck first. Also, a single drop of the materials will cause the larger aggregate to roll to the front and rear of the truck, resulting in coarse material being the first and last material to be discharged from the truck bed.

16 Truck Loading Segregation – how is it prevented?
No dribbling, no topping off Multiple Drops The gates of the bins should be opened and closed as quickly as possible with the mix being dumped in mass into the truck bed, with no dribbling. There should be no topping off, opening the gates slightly to allow a little more mix to bring the truck weight to maximum. In addition, by loading the truck in three different drops, with the first drop being very near the front of the truck, the second drop extremely close to the tail gate and the third drop in the center, segregation caused by incorrect truck loading is eliminated.

17 Truck Hauling Segregation – how does it happen?
Temperature variations causing: Crusting Lumps in mix due to: Long hauls Delays No tarps No Insulation In hauling mix in trucks, temperature variations can occur causing too much crusting and lumps in the mix. Extra long hauls and delays at the plant or on the road due to traffic with no tarps or insulation as required can all cause temperature problems in the mix. Temperature Segregation in truck!

18 Temperature Variation
Raveling and Moisture Damage 1% increase in air voids over 7% results in a minimum 10% reduction in pavement life Aggregate Segregation It was found that in general: Concentrated areas of cooler hot-mix commonly occur under a variety of paving conditions. Good rolling practices can partially offset temperature differential-related compaction problems However, if the temperature profile across the mat varied by more the 25oF the variability of air voids led to an increase in aggregate segregation, fatigue cracking, raveling and moisture damage. Fatigue Cracking

19 Truck Hauling Segregation – how is it prevented?
Proper planning Proper tarp Proper insulation The solution is proper planning for the right number and size of trucks, proper scheduling with the plant, and making sure trucks are properly tarped and insulated as needed.

20 Truck Hauling Segregation – how is it prevented?
“Strive for Continuous Paving” Proper Planning /Scheduling Proper Coordination with Plant Proper Number of Trucks Coordination with Paver Speed Avoid Bunching of Trucks at Paver or Clean-out Area Use Material Transfer Vehicle Another way to help minimize segregation is to insure a constant flow of material to the paver. This will minimize the number of stops, thus eliminating some of the sources of segregation. Two ways to foster continuous paving are to have sufficient trucks and/ or a materials transfer vehicle. The materials transfer vehicle is so designed to insure continuous movement of the material and is constantly remixing the material as it moves from the truck to the paver. This constant remixing helps minimize not only aggregate segregation but also temperature segregation.

21 How Does It Happen? In Unloading the Truck Dribbling the material
Poor Bed Condition No Truck Release Agent When unloading a truck into a paver hopper, it is important not to dribble the material into the pave. If the bottom of the truck bed is in poor condition the truck release agent will pond and cause material to stick to other areas. With sensitive mixes pockets of coarse aggregates may accumulate in the center of the bed behind the hydraulic cylinder enclosure. As the load moves forward caves in, forcing large stones to accumulate behind the enclosure.

22 Truck Unloading Segregation – how is it prevented?
Discharge in Mass Flood Hopper Truck Release Agent Modify Dump Enclosure Baffles When unloading a truck into a paver hopper, it is important to discharge the material as a mass. To do this the bottom of the truck bed needs to be in good condition and lubricated so that the entire load will slide rearward. Elevate the truck bed just enough to “break” the load before the tailgate is opened, while remaining at a safe angle. A mass discharge prevents an accumulation of coarse material at the outside portion of the paver hopper. The hydraulic cylinder enclosure may have to be modified for sensitive mixes.

23 Paver Segregation – how does it happen?
Hopper Wings Slat Conveyer Hopper Gates Auger Screed Proportional Feed Control Hopper Gates Slat Conveyors Hopper Wings Augers In order to understand how segregation can occur in the paver we have to look at the entire Material Feed System from the hopper to the screed. The importance of fully understanding and controlling the head of material CANNOT BE OVER EMPHASIZED, as most mat problems and paver induced segregation is caused by not maintaining a uniform constant level of material in front of the screed. The feed system should be set up and adjusted so the material is moved from the hopper, to the auger chamber, across the augers and under the screed in a uninterrupted uniform manner. Segregation may occur when ever this flow of material is interrupted or changed.

24 Paver Segregation – how is it prevented?
In the operation of the Auger Material to the midpoint (or just covering) the Shaft Gear Box In the center of the augers is a gear box. Kick back paddles are used to “tuck” mix under the gear box. Make sure the paddles are positioned to push the material toward the gear box. If the paddles were not there, the proper amount of mix would not get under the gear box and can cause segregation. The correct head of material is just covering the auger shaft. The auger should be running 100% of the time while the paver is moving and the speed of the auger approximately 20 to 40 RPM. An off/on or erratic feeder system may cause segregation patches to occur. Excessive high or low auger RPM may cause segregation stripes to occur. High or low levels of material may cause segregation stripes to occur. Auger height should be set so the bottom of the flight is at least 2” above the surface of the mat being placed. If the auger is too close to the surface of the mat being placed segregation can occur. Heavy or large rock designs may require the height be increased. The feed sensors control the level of material at the outboard ends of the augers. They should be positioned and adjusted so the correct auger material level is maintained at the outer ends of the screed. Install Kickback Paddles Feed Sensor

25 Paver Segregation – how is it prevented?
Caution in Cycling Hopper Wings In the Hopper Fillet Corners Cycle Wings onto full slat conveyors Some companies fashion metal plates into the corners of the hopper to prevent coarse aggregate from forming pockets of segregated material during the unloading operation. Caution should be used in cycling hopper wings, because cycling hopper wings when the hopper is less than 25 percent full causes patch type segregation. Make sure that the area of the slat conveyor is filled when the wings are cycled.

26 Paver Segregation – how is it prevented?
In the Slat Conveyer Never Run the Hopper empty Running the hopper dry before recharging causes random segregation, which is difficult to distinguish from spotty segregation caused by loading improperly from the silos. May appear as end-of load segregation, inverted ‘V”

27 Preventing Segregation at Slats
Truck dumper must communicate with paver operator MTV operator is responsible to not let hopper insert run “dry” Smooth and effective truck exchange when working without an MTV

28 Paver Segregation – how is it prevented?
In setting the Hopper Gates Gates too HIGH - augers overloaded Gates too LOW - insufficient material supply Correct adjustment - uniform material volume/flow The hopper gates setting and the slat feeder speed should be adjusted so that the level of material across the main screed just covers the auger shafts and the speed of the auger maintained at a constant speed. High or low levels of material in this area may cause segregation to occur. When paving with extension on one side, gate should be raised to keep auger speeds equal

29 Paver Segregation – how is it prevented?
In the operation of the Screed Manual Extension Hydraulic Auger & Tunnel Extension Hydraulic Extension Most paver screeds can be extended either hydraulically or with fixed extensions. When paving a level surface, it is important that the extendable portion of the screed is set at the same angle of attack as the main screed. If the extensions are not properly aligned with the main screed, a longitudinal line or ridge will appear in the mat. Also, a difference in surface texture between the main screed and extension may occur. Excessive amounts of material that is not contained properly may cause segregation stripes to occur. Auger & Tunnel Extensions missing

30 In the Operation of the Paver
Paver Segregation – how is it prevented? In the Operation of the Paver Proper Starting & Stopping Continual & Constant Paver Speed Maintaining a consistent paver speed is important in the operation of the paver, as the material feed system is set and affected by paver speed. Paver should be moving at a consistent paving speed, with a minimum goal of 85 percent of the time; spending 15 percent or less of the time stopped waiting on trucks. Paving speed is determined by the delivery rate of material from the hot plant to the paving site. When stopping is necessary, you should stop the paver as quickly as possible, without being erratic in nature; and, when starting again, accelerate as quickly as possible to the previous paving speed, again with out being erratic in nature. The goal is to have “Non-Stop Paving”. The purpose of non-stop paving is to eliminate the problems that occur at each stop and to establish a constant paving speed. The feeder system can be set up and adjusted to where the materials are moved from the hopper, to the screed in a uniform constant manner that prevents segregation.

31 Goal: Non-Stop Paving Industry’s Answer Material Elevator
Mobile Conveyors Material Transfer Vehicle When using the material elevator type equipment, the materials are dumped on the roadway in a windrow. When using a material transfer vehicle the materials are dumped into a receiving hopper. Then conveyed to a storage bin. Then moved to the discharge conveyor. There are other machines that can be used to develop the non stop paving procedure. The use of these pieces of equipment will not guarantee the elimination of segregation.

32 Segregation can still occur!
Problem: Dropping Materials Solution: Limit drop Keep hopper insert full In the case of Mobile Conveyors and the Materials Transfer Vehicles extreme care has to be used to prevent severe segregation. To decrease drop-caused segregation, limit the drop and keep the hopper insert full.

33 How to Prevent Segregation

34 Key Points in Prevention of Segregation
Proper Stockpiling Proper Plant Operation Proper Truck Loading Proper Truck Unloading Continuity of Paving Operation

35 Is this Pavement Segregated?
Is this pavement segregated? It is very difficult to teach an individual how to identify segregated areas. However there are tools available to assist in the development of this talent. The sequence of events could be as follows. Does Every One Agree? Segregation can be difficult to identify Available tools to assist in this determination

36 If Pattern Segregation Is Suspected
Notify Department and contractor Contractor continues work at his own risk The Department will initiate an investigation Surface texture depth (PTM 751) can be used to determine if segregation is present A field technician, representing the contractor or the agency, observes an area that looks to be pattern segregation. The Contractor may elect to continue work at his own risk and immediately and continually adjust the operation until the pattern segregation is eliminated from the finished mat. The Department technician will immediately begin an evaluation to determine the severity of the pattern segregation. PennDOT Specs Section 409.3(h)3.a

37 P.T.M. 751 Carefully spread the material into a circular patch with the spreader tool (large rubber stopper or a hockey puck), filling the surface voids flush with the aggregate particle tips. Spread the material as wide as possible, try to create a circle

38 P.T.M. 751 PROCEDURES Perform the test at 3 locations in the suspected segregated area Calculate the average radius of these 3 locations Obtain the texture depth from Table 1 on page 7 of PTM 751 For areas suspected of Pattern Segregation. Perform a minimum of three tests in the suspected segregated area. Calculate the average radius of the area and determine the average texture depth. Also………

39 P.T.M. 751 PROCEDURES Perform the test at 3 locations in a non- segregated area Calculate the average radius of these 3 locations Obtain the texture depth from Table 1 on page 7 of PTM 751 Perform an equal number of tests in an acceptable area. Calculate the average radius for each area and determine the average texture depth.

40 “Unacceptable Pattern Segregation”
P.T.M. 751 PROCEDURES If average texture depth between areas exceeds inches, suspected area is considered: “Unacceptable Pattern Segregation” If the average difference in pavement texture depth exceeds inches between the measured areas, the area is considered Unacceptable Pattern Segregation.

41 If Unacceptable Pattern Segregation Is Evident
[PennDOT Specs Section 409.3(h)3.b]: Stop Paving Department will evaluate segregation to determine corrective work DO NOT resume paving until Department reviews corrective actions & authorizes paving to continue Test section then placed, <200 tons Resume normal paving after successful test section Immediately stop all paving. The Department will evaluate the pattern segregation to determine the extend of the corrective work. This work will be in accordance with the Quality Control Plan or as directed by the Engineer. Do Not Restart paving until the corrective action is accepted by the Department. When the Engineer permits work to resume, place a test section limited to 200 tons. If the corrective action corrects the problem, work will be allowed to continue.

42 If Unacceptable Pattern Segregation Is Evident
Evaluating Segregation [PennDOT Specs Section 409.3(h)3.c]: Take 6” Cores Remove & Replace Segregated Areas if sum of any 2 sieves vary 20% or more from the JMF or core Density is less then 90% Remove full lane width of segregated area plus 5 feet minimum beyond each end Evaluate Segregation- Take 6-inch cores from the unacceptable areas. The Inspector will deliver them to the plant and observe the testing for density, extraction and gradation analysis. Remove and replace any area if the summation of absolute deviations from any two sieves is 20% or more from the job mix formula, or the core density indicates less than 90% of Maximum Theoretical Density. Remove not less then full width of the affected lane and 5 feet minimum beyond each end of the affected area. New replacement area should meet surface tolerance requirements. When the test results exceed the above limitations, the pavement is accepted. (This does not mean that surface Segregation does not exist. In fact in most cases the surface is segregated and during the life of the pavement will ravel.)

43 Segregation Trouble Shooting
Begin/end of load segregation This pictures illustrates beginning and end of load segregation.

44 Segregation Source & cause
Begin/end of load segregation Material segregated in truck Running conveyor deck dry. Cycling hopper wings too soon . End of load segregation can be caused by the material being segregated in the truck, running the slat conveyor deck dry or cycling the hopper wings too soon or onto empty slats.

45 Segregation Trouble Shooting
Center line segregation (single) Centerline segregation is identified by a segregated single line down the center of the lane.

46 Segregation Source & cause
Center line segregation (single) Missing or damaged baffle plates Overloaded material feed system Worn reversing augers Low auger RPM Warped or improperly set screed The source could be attributed to missing or damaged baffle plate in the plant, overloaded material feed system in the paver, worn or missing reverse auger paddles, low auger RPM, or a warped or improperly set screed. NOTE: Raising (or lowering) pre- strike off plate may hide this, but it WILL show up again as the pavement ages. Extension height setting may also cause this centerline stripe.

47 Segregation Trouble Shooting
Stripe in line with outer auger bearing support Segregation in the form of a single stripe in line with the outer auger bearing support.

48 Segregation Source & cause
Stripe in line with outer auger bearing support Excessively slow auger RPM In most instances the segregated stripe of material is caused by slow auger RPM.

49 Segregation Trouble Shooting
Stripe in line with conveyors Segregation in the form of two stripes in line with the slat conveyor.

50 Segregation Source & cause
Stripe in line with conveyors Overloaded feeder system Low auger RPM Maintain proper head of material Once again the form of segregation can be caused by low auger RPM or an overloaded feeder system. The head of material should be at the center line of the augers or just covering the center shaft.

51 Segregation Trouble Shooting
Stripe at outboard edge of mat A segregated stripe of material at the outer edge of the mat/ paving lane. May be caused by improperly set extensions, or rolldown due to insufficient head of material at outer edge of pave related to augers speed/flowgate setting.

52 Segregation Source & cause
Stripe at outboard edge of mat Insufficient material Not enough auger extensions Cold material dragging at extension This form of edge segregation is usually caused by insufficient material or failure to use auger extensions and tunnels. Failure to add auger extensions when you extend the screed will lead to the material not being carried to the endplate. This material will spill out into the void at the end of the auger causing the material to segregate.

53 Segregation Trouble Shooting
What is Wrong?

54 Segregation Source & cause
Nothing! This is one beautiful mat. Not all pavements are segregated or have segregated areas.

55 Questions? Did I go too fast?

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