This section of the manual addresses hot mix asphalt paving operations, including planning, equipment requirements, spreading and finishing operations, compaction operations, and other related subjects.  Also included are sections on the use and application of prime coats and tack coats. In this section we primarily discuss the actual construction operations.  The following section of the manual will give more detail on the required sampling and testing requirements and procedures.

The Department and the Contractor are required to both have a QMS certified Roadway Technician with each paving operation on the project at all times that mix is being placed. During paving operations, the Department’s certified QMS Roadway Technician has three primary responsibilities.  These are:

(1) to be certain that contract specifications are met,

(2) to provide the Contractor every opportunity to meet the job specifications in the most cost-effective manner, and

(3) Assist the contractor's QMS roadway technician in monitoring traffic control.

(1)  monitoring all roadway paving operations;

(2) monitoring all quality control processes and activities, to include stopping production or implementing corrective measures when warranted; and

(3)  ensuring that the specifications are being met, that a high standard of workmanship is being achieved, that the required sampling and testing is being performed and traffic control devices are being used and maintained properly.

To meet these responsibilities, the Contractor and Department personnel must have a courteous, cordial, cooperative, and professional relationship with each other.  All Supervisors, Technicians, and other involved parties must thoroughly understand the job specifications.  They must be familiar with the equipment necessary to perform all phases of the paving operations and be knowledgeable of  proper use of the equipment required.

Before beginning paving, the certified Asphalt Roadway Technician for both the Department and the Contractor must assure that the subgrade or base course is properly conditioned and true to grade and cross section as shown on the plans before paving operations begin.  Prime coat must be applied when required in accordance with Article 600 of the Standard Specifications.  In addition, tack coat must be applied when required in accordance with Article 605 and procedures detailed in this Manual.  We will first discuss the placement of prime coats, next, the application of tack coats, followed by the placement of the asphalt mix.  The Technicians, for both the Contractor and the Department, must be familiar with all aspects of quality paving practices if long lasting pavements are to be constructed.

9.1.1 The Asphalt Distributor

The asphalt distributor is one of the most important pieces of equipment on a paving project, prime coat, tack coat, or surface treatment operation (See Figure 9-1).  It is made specifically to apply liquid asphalt material uniformly and in proper quantities to a roadway surface.  (See Article 600-5 of the Standard Specifications for distributor requirements). The asphalt distributor includes a truck or trailer-mounted insulated tank containing a heating system that is normally oil-fired to maintain the asphalt at the proper application temperature.  An accurate thermometer must be mounted on the distributor in such a manner that the dial or indicator remains in full view at all times.  The distributor shall also have a hand-held spray attachment for applying asphalt to areas inaccessible to the spray bars.

Asphalt sprayed at an improper temperature may not be applied uniformly and this is sometimes hard to detect until months later when streaking and grooving may develop.  The distributor has a power driven pump capable of handling asphalt products ranging from light, cold application liquid to heavy asphalt cements, heated to spraying viscosity.

At the rear-end of the tank is a system of spray bars and nozzles through which the asphalt is forced under pressure onto the surface of the road.  The spraybar must have a constant uniform pressure along its entire length for equal output from all nozzles.  Although the methods of maintaining pressure may vary, many distributors use gear type pumps to deliver the asphalt to the spray bar.  On some distributors, pressure is regulated by a variable pump speed, while on others by constant pump speed and use of a pressure relief valve.  The correct pressure is that which neither atomizes the asphalt nor distorts the fan of spray.  Too low pressure results in streaking from a non-uniform discharge of material from the individual nozzles, while too high a pressure will atomize the asphalt and distort the spray fan.  The manufacturer normally supplies charts and data which gives the proper pump speed or pressure for determining the discharge in gallons (liters) per minute for each nozzle size.

One of the most important parts of the distributor is the spray bar.  To achieve proper results from the spray bar, the correct size nozzles for the job conditions must be selected.  Nozzles should be checked for damage before use.  The angle of the long axis of the nozzle openings must be adjusted so that the fans of spray will not interfere with each other.  The nozzle angle varies according to the make of distributor, but is usually between 15° and 30°.  It is important that all nozzles be set at the proper angle within close tolerances to provide proper overlap.  In addition to a proper and uniform nozzle angle, the most important adjustment to insure uniformity of asphalt spray is the height of the spray bar above the pavement surface.  It is important that the correct height be maintained during the entire application to provide proper overlap.  An incorrect height of spray bar will result in streaking.  For example, the best result with a 4" (100 mm) nozzle spacing comes from an exact triple lap of the fans, but with the 6" (150 mm) nozzle spacing, the height of the spray bar is too high and subject to wind distortion.  In such a case, a double lap pattern should be used.  For best results, the height of the spray bar should not vary more than 1/2" (13 mm).  Some distributors have mechanical controls to maintain proper height.

Controls for the distributor include a pressure gauge (which registers pump out-put) and odometer (which registers distance traveled in rate of travel).  The odometer has connected to it a rubber tired wheel mounted on a retractable frame with a cable leading to the odometer dial in the cab of the vehicle.  The rate of travel in feet (meters) per minute and the total distance traveled in feet (meters) are indicated on the dial.  The odometer should be checked for accuracy and cleanliness at regular intervals.

Newer distributors usually have float type gauges and measuring sticks for indicating the volume of the contents of the tank.  The stick used should be calibrated so that tank contents can be determined to the nearest 25 gallons (100 liters).  Calibration charts or devices must be kept with the distributor at all times.

9.1.2 Asphalt Delivery Tickets for Emulsified Asphalts
When a shipment of asphalt emulsion that is to be used as either prime or tack coat is received at an asphalt plant or on a project, a copy of the bill of lading will be furnished to the appropriate Resident Engineer and retained in their project records.

9.2 PRIME COAT

Prime Coat is a sprayed application of a low viscosity liquid asphalt to a base course of untreated material.  It is the initial incorporation of asphalt into the surface of a non-asphalt base course preparatory to any superimposed treatment or construction.  The objective of priming a non-asphalt base may be any one or all of the following:

• To waterproof the surface in order to prevent surface water from penetrating the base course or subgrade material
• To seal voids, coat and bond loose mineral particles and stabilize the surface being primed
• To provide temporary covers in cases of delayed pavement construction or planned stage construction
• To promote adherence of overlying asphalt courses or surface treatments to granular bases, including both ABC and soil type bases.

9.2.1 Grades, Application Rates and Temperatures for Prime Coats

All Prime Coat materials must meet the requirements of Article 1020-3 of the Standard Specifications.  This Article does not list any specific brands or grades of materials.  A particular brand or grade must be approved prior to use.  A listing of approved prime coat materials is maintained by the Materials and Test Unit's NCDOT website under "Approved Products List - Prime Coat".  Those parties who may not have access to this listing may contact the Chemical Engineer at the Materials & Tests Unit at phone number 919-329-4090 or may visit their approved products web site located at at https://apps.dot.state.nc.us/constructionunit/mttracking/.

All currently approved prime coat materials are emulsified asphalts that have been pre-thinned with water. Due to the Department's concerns about the environment, the use of cut-back asphalt (ones pre-thinned with another petroleum product) has been phased out and are not included on the approved material listing.  Unless the project special provisions specify that a particular prime coat material be used, the Contractor may select any grade from the approved list.

Prime coat will be uniformly applied at a rate of 0.20 to 0.50 gallons per yd² (0.9 to 2.3 liters per m²) or as noted on the approved list.  The exact rate for each application will be established by the Engineer to 0.01 gallons per yd² (0.1 liters per m²) and will be conveyed to the Contractor  prior to application.  The required rates of asphalt materials will be based on the volume of material measured at the application temperature.  The application temperature of the prime coat material shall be in accordance with the manufacturer's recommendations or as approved by the Engineer.

The asphalt distributor is the piece of equipment used to apply prime coat material (See Section 9.1.1).  Most distributors include gauges for measurement of the material within the tank. These may be used to determine the quantity used provided they are accurate. It is also acceptable for the Technician to "strap" the tank with a calibrated measuring stick having increments of 25 gallons (100 liters) or less.  Each distributor must either be measured by the gauge or "strapped" with the stick before and after a "shot" in order to estimate the actual number of gallons (liters) to the nearest 10 gallons (50 liters) applied.  When sticking the tank, the measurements must be made from the same spot on the rim along the centerline of the truck on the top opening with the tank level both times.  The tank should always be level when measured, whether measured with the gauge or the stick.

The Inspector should always check the amount of asphalt material applied against the amount received for each tanker load.  The amount applied obviously cannot be more than the amount received.
 

The directed rate of application and the actual rate of application of the prime coat will be recorded in the Pay Record Book.

*The most current listing of approved Prime Coat Materials is on the Materials & Tests Unit’s web site
under “Approved Products List-Prime Coats” @ https://apps.dot.state.nc.us/constructionunit/mttracking/.

**Actual material to be used in the field must be sampled and approved prior to use.

Application rate of 0.200 to 0.500 gal/sy (0.90 to 2.25 L / SM) as established by the Engineer

9.2.2 Application of Prime Coat

Prime coat shall be applied only when the surface to be treated is sufficiently dry and the atmospheric temperature in the shade away from artificial heat is 40°F (4°C) or above for plant mix and 50°F (10°C) or above for asphalt surface treatment.  Prime shall not be applied on a frozen surface or when the weather is foggy or rainy.  The base shall be cleaned of any objectionable material or other deleterious matter prior to placing prime coat.

Prime coat should not be applied until the base to be treated has been approved by the Engineer.  Requirements include approval by the laboratory of aggregates used in base courses, adequate density, proper grade and proper conditioning of the base material on which the prime is to be applied.

After the base has been conditioned in an acceptable manner and is sufficiently dry, a stringline shall be placed by the Contractor and checked by the Technician to serve as a guide for the distributor.  The use of a stringline may be waived by the Resident Engineer if prime is to be placed adjacent to a curb and gutter or other structure where a true alignment can be obtained.

For best application results, the following points should be observed.

• Maintain uniform pressure on all spray nozzles.  The fan of spray from each nozzle must  be uniform and set at the proper angle with the spray bar (See Figure 9-2).
• The spray bar must be maintained at the proper height above the road surface to provide  complete and uniform overlap of the spray fans.
• The distributor road speed must be uniform.
• Before the start of the work the spread of the distributor spray bars should be checked.   The spraying operation should be checked frequently to be sure that the nozzles are the  proper height from the road surface and are working fully.
• When prime is to be utilized adjacent or parallel to bridge floors, curbs, handrails and all  other appurtenances, care must be taken to prevent spraying, splattering, or tracking the  prime on the structures.

To insure a tight and smooth base, the Engineer may require that the prime be rolled with pneumatic rollers as soon as the prime has penetrated the base enough to prevent it from picking up.  Before beginning placement of any surface treatment or plant mix, all the water must have evaporated and all loose granular material swept from the base.

If in the opinion of the Resident Engineer it is impracticable to keep traffic off the prime, the Contractor may be directed to spread blotting sand over the prime coat to prevent it from being picked up, in accordance with Section 8l8 of the Standard Specifications.

9.2.3 Determination of Prime Coat Rates and Pay Quantities

The procedures which follow serve as a guide by which rate applied and pay quantities for prime coat, curing seal and other similar asphalt items will be computed and recorded.
The quantity of prime coat and curing seal to be paid for will be measured at the application temperature. The pay records must indicate the actual computed rate for each shot.

English Units

Actual Pay Gallons = 945 gals.

Metric Units

9.3 TACK COATS
A tack coat is the spray application of liquid asphalt to an existing asphalt or concrete surface to promote a bond between old pavement surfaces and the new asphalt layer.  The three essential requirements for a satisfactory tack coat application are:

(1) Existing pavement surface must be thoroughly cleaned.
(2) Proper rate of application must be assured.  (See Section 9.3.2 below for application rates)
(3) Uniform coverage over the entire area to be paved must be assured.

The Contractor must remove the grass, dirt and other material from the edge of the existing pavement prior to placement of the tack coat.  This is important since this will prevent the bonding of the asphalt overlay.  However, the Contractor should be cautioned about removing excessive amounts of grass and earth material from the pavement edge.  This operation should not be conducted in a manner which allows the material to be thrown into the roadway ditches or which creates a hazardous traffic condition.

The Contractor should take necessary precautions to limit the tracking and/or accumulation of tack coat material on either existing or newly constructed pavements.  Excessive accumulation of tack may require corrective measures.

9.3.1 Tack Coat Grades

All tack coat materials shall meet the Standard Specification requirements and will be either Asphalt Binder, Grade PG 64-22 or Asphalt Emulsion, Grade RS-1H, CRS-1H, CRS-1, HFMS-1 or CRS-2 unless otherwise approved by the Pavement Construction Engineer.  Asphalt Emulsions shall not be diluted with water.  Different grades must not be intermixed in a tanker or distributor since this can cause the material to break in the tank and become almost impossible to spray.

Unless otherwise specified in the Project Special Provisions, the Contractor may select the grade of tack coat material he anticipates using.  Any approved grades may be used provided the material is accompanied by a certified delivery ticket in accordance with Article 1020 of the Standard Specifications.  The Contractor will advise the Roadway
Technician of the actual brand and grade being used and the QA Technician will indicate same on the Asphalt Roadway Technician's Daily Report (M & T Form 605).  Should there be any concern about the quality of material, samples should be taken in approved containers and submitted to the M&T Unit for testing.

9.3.2 Application Rates and Temperatures

The Standard Specifications state that tack coat shall be uniformly applied at a rate of 0.04 to 0.08 gallons per square yard (0.18 to 0.36 liters per m²).  The exact rate for each application will be established by the Engineer.  A different rate of application for different layers or surfaces may also be established.

Uniformity of application and proper application rate are the keys to success of the tack coat performance.  The exact application rate for the tack coat cannot be given in a blanket statement, which explains the 0.04 to 0.08 gallon per yd² (0.18 to 0.36 liters per m²) range stated in the Specifications.  On an average asphalt material to be overlaid, 0.05 to 0.06 gallons per yd² (0.25 to 0.30 liters per m²) should be adequate, while an asphalt surface that has oxidized for a long period of time and has a gray tint to its color may necessitate 0.08 gallons per yd² (0.35 liters per m²).  An asphalt surface may already be so rich with asphalt that it is bleeding, and may need no tack at all, except a very light fog on the outer edges.  When determining the rate of application, the type of tack material must also be considered.  Generally, when emulsion is being used, the rate must be increased since emulsions may contain up to 45 percent water.  Once the water evaporates, the residual asphalt binder left on the surface has less volume.  A "rule of thumb" when emulsion is being used is to increase the rate 0.02 gallons per yd² (0.10 liters per m²) as compared to the rate if asphalt binder was being applied.

Emulsion and asphalt binder tack is considered equivalent from a service viewpoint when applied at the proper rates.  The primary advantage of emulsion is that it can be applied at a significantly lower temperature than asphalt binder tack and can normally be applied more uniformly.  This lower temperature makes it easier to store and handle and also is much safer to use.  However, regardless of the rate grade used, the tack coat material should be heated to the proper temperature so that it is fluid enough to be sprayed from the nozzles instead of coming out in strings.  The temperature of the tack coat material at the time of application should be within the ranges in Table 605-1 of the Standard Specifications. (Shown Below)

The proper amount of tack coat for any surface is a matter of judgment, and this judgment must be made with the knowledge that too much asphalt could flush into the mix and cause loss of stability or could sometimes cause it to slip, and that too little tack will not properly bond the surfaces.  Regardless of the rate selected, if the tack is not applied uniformly over the surface, the tack will not perform satisfactorily.

The contractor shall provide a distributor for heating and applying the tack coat in accordance with Article 600-5 of the Standard Specifications and Section 9.1.1 of this manual.

No more tack coat material may be applied than can be covered with base, intermediate, or surface course material during the next day's operation, except where public traffic is being maintained.  Where public traffic is being maintained, no more tack coat may be applied than can be covered during the same day’s operation.  However, the Resident Engineer may limit the application of tack coat in advance of any paving operation depending on traffic conditions, project location, proximity to business or residential areas, or other reasons.  In the event that tack coat material is not covered in the same day's operation, the
Resident Engineer may require the application of suitable granular material or other means to provide a safe traffic condition at no additional cost to the Department.

Tack coat must be applied only in the presence of and as directed by the Engineer.  No base, intermediate, or surface mixture may be placed until the tack coat has been placed and sufficiently cured.  Tack coat shall be uniformly applied with the spray bar on a pressure distributor in the presence of, and as directed by, the Resident Engineer or his Technician.  In places where the distributor bars cannot reach, it will be necessary to apply the tack coat with a hand spray attached to the distributor by a hose.  When hand spray methods are used, care should be taken to give the surface a adequate and uniform application of tack coat.  All pavement contact surfaces of headers, curbs, gutters, manholes, core sample holes, vertical faces of old pavement and all exposed transverse and longitudinal edges of each course must be painted or sprayed with tack before any mixture is placed adjacent to such surfaces.

Sometimes, a tack coat application on newly placed asphalt may not be needed.  This is a judgement call that will be made only by the Resident Engineer or his representative; otherwise, all pavement surfaces to be overlaid will be tacked.  The contractor may request not to tack whenever he thinks it’s not needed but it is the Engineer’s responsibility to make this determination.  For example, when an asphalt mixture is used to overlay a lane that has just been placed (within the same day), tack coat may not need to be applied, since the surface is still very tacky and the heat in the freshly placed mat will absorb the tack and leave no visible trace of it being applied.  This same heat will keep the asphalt in the mixture soft enough to bond to the newly placed mix.  Still there are other factors to be considered other than the fact the mix being overlaid is new.  Traffic, pavement cleanliness, and uniformity of surface texture are some things that could dictate the need for a tack coat and/or  the rate of application, even though the mix being overlaid is a new mix.

After the tack coat has been applied, it shall be protected from all traffic until it has cured sufficiently.  It can be considered sufficiently cured when it is tacky to the touch.  If an emulsified asphalt is used, adequate time should be allowed for the water to evaporate leaving only the asphalt binder residue.  Normally emulsified asphalt is brownish in color when first sprayed but will be black and tacky once the water has evaporated out of it.  If a PG 64-22 binder is used for tack, plant mix can normally be placed on it almost immediately.  After the tack has cured, it should still be protected as much as possible from all nonconstruction traffic.  In the event that a rain or shower falls on the freshly placed tack coat, the Contractor shall at the direction of the Resident Engineer or his Inspector place whatever signs, lights, and pilot cars that are necessary to protect the traveling public from the slippery tack coat and shall maintain this protection as long as the hazardous condition prevails.

9.3.4 Determination of Tack Coat Rate

NCDOT specifications require that tack coat be applied within a range of 0.04 - 0.08 gallons per yd² (0.18 - 0.36 liters per m²) as directed by the Engineer. It is the Engineer’s responsibility to determine the exact rate within this range  to be applied and to convey the directed rate to the Contractor.  The tack coat rate must be regularly checked by the DOT Technician to determine that the specified amount is being placed.  The rate of application may be obtained at intervals by using the total gallons (liters) applied divided by the square yards (square meters) upon which the tack coat was placed.  At the end of each day's operation, a Technician must compute the actual rate of tack coat applied and record this on his daily report (M&T 605 form).   The rate of application should be calculated separately for each individual application or “shot”.   An example of a tack coat rate calculation is shown below.

275 gallons (1040 liters) of  CRS-1 was applied 12 feet (3.6 meters) wide from Sta. 12+00 to 45+00 (3+66 to 13+72) at a directed rate of 0.06 gals / yd² (0.30 L / m²).  What is the actual tack coat rate of application in gals. per square yard (liters per square meter)?

English Units

Metric Units

NOTE:  Significant Decimal for Tack Coat is 0.01

9.4 HOT MIX PLACING AND COMPACTION OPERATIONS - GENERAL

9.4.1 Introduction

Placing and compacting the asphalt mixture is the operation to which all the other processes are directed.  Aggregates have been selected and combined; the mix designed; the plant and its auxiliary equipment set up, calibrated and inspected; and the materials mixed together and delivered to the paver.

Asphalt mix is delivered to the paving site in trucks and may be deposited directly into the paver, or in windrows in front of the paver, or transferred to the paver by specially designed materials transfer  equipment.  The paver then spreads the mix to the required grades, cross-section thickness, and widths shown on the plans and typical sections as it moves forward.  In doing so, the paver partially compacts the material and  provides a smooth, uniform texture.  Immediately thereafter and while the mix is still hot, steel-wheeled, vibratory or rubber-tired rollers or some combination of these are driven over the freshly paved mat, further compacting the mix to the required density and texture.  Rolling is usually continued until the pavement is compacted to the required density, or the temperature has dropped to a point where further compaction may produce detrimental results.

After the pavement course has been compacted and allowed to cool, it is ready for additional paving courses or ready to support traffic loads.

9.4.2 Planning Paving Operations

Paving operations require careful planning, preparation, co-ordination, and communication between all parties.  The surface to be paved must be properly prepared.  Enough vehicles and equipment must be available and in good operation to provide a steady flow of materials and progress without delays.  Plant production must be closely coordinated with the paving operation, and the compaction of freshly placed mixture must be prompt and adequate.

Nowhere in the construction of hot-mix asphalt pavements are the efforts and skills of workers, operators, and  technicians more apparent than in the placing and compacting of the hot-mix in the roadway.  Having the necessary knowledge and skills of the paving operation and having pride in the final product can mean the difference between a durable, smooth-riding pavement and a rough, unsound, unsightly  pavement that will not perform as was intended, but  also,  is a nuisance to drive on.

Recent national surveys of the traveling public (taxpayers) indicate that their perception of high quality pavements are those which are smooth and last for a long time. While the public is usually neither aware or concerned about other properties such as gradation, binder content, voids properties, density, etc., we as Engineers and Contractors know that mix quality, smoothness, and density are significantly related. Smoothness is an indicator of a pavement that has uniform and consistent mix properties without segregation during placement. Achieving uniform density at the proper level during placement and compaction means a pavement which will have more rut resistance, less permeability, less oxidation, less fatigue cracking, be more durable,  last longer and therefore, require less maintenance. The key is communication and consistency. To meet these objectives requires substantial planning on the part of all parties involved.

Because planning and communication are so essential for successful paving operations, a pre-paving construction conference should be held before work begins.  Such a conference allows the Department’s Project Engineer, the Contractor’s Paving Superintendent, Traffic Control personnel, Trucking personnel, Roadway and Density Technicians, and others directly involved with the operation the opportunity to discuss topics such as the following and to plan the paving operation accordingly:

The pre-paving construction conference is the time for questions to be answered, problems to be solved, and channels of communication and command to be established.  It is a time to establish relationships with everyone involved in the project so that confusion and friction can be avoided once paving operations begin. As many questions and issues as possible should be resolved prior to beginning paving operations. In most circumstances, this will be reflected in a higher quality finished product.

9.4.3 Weather, Temperature and Seasonal Limitations

Article 610-4 of the Standard Specifications addresses air temperatures, road surface temperatures, seasonal limitations, weather requirements, the layer being placed, and layer thickness that apply when producing and/or placing the various mixture  types.

Asphalt mixtures must not be produced or placed during rainy weather,  when the subgrade or base course is frozen, nor when the moisture on the surface to be paved would prevent proper bond.  Asphalt material must not be placed when the air temperature, measured in the shade away from artificial heat at the location of the paving operation and the road surface temperature in the shade at the paving site is less than the following temperatures.

In addition, surface course material which is to be the final layer of pavement shall not be placed between December 15 and March 16, except that OGAFC will not be placed between October 31 and   April 1 of the next year, unless otherwise approved by the Engineer.

As an exception to the above, when in any day's operations, the placement of a layer of asphalt base course material or intermediate course material 2" or greater in thickness has started, it may continue until the temperature drops to 32°F (0°C).

No plant mix base course or intermediate course shall be placed that will not be covered with surface course during the same calendar year or within 15 days of placement if the plant mix is placed in January or February.  Failure of the Contractor to cover the plant mix as required above will result in the Engineer notifying the Contractor in writing to cover the plant mix with a sand seal.  The sand seal shall be applied in accordance with the requirements of Section 660 of the Standard Specifications, except that Articles 660-3 and 660-11 will not apply.  This work shall be performed by the Contractor at no cost to the Department.  In the event the Contractor fails to apply the sand seal within 72 hours of receipt of such notice, the Engineer may proceed to have such work performed with Department forces and equipment.  The cost of such work performed by Department forces will be deducted from payments due or to become due to the Contractor.

 Meeting the requirements of the weather and temperature limitations does not preclude the enforcement of compaction and surface requirements of the Specifications.  If the required density, surface tolerances and/or an acceptable surface finish cannot be achieved, the Contractor must be so advised and paving operations should cease until these requirements can be met.  See Section 10 of this Manual for limited production procedures for these problems.

 Asphalt mixtures shall not be produced or placed during rainy weather.  In no event should mixture be placed in standing water or when the moisture on the surface to be paved would prevent proper bond.  In the event unpredictable rain begins after paving operations have started, the plant production should immediately cease.  If the Contractor requests and the Engineer grants approval, he may be allowed to place any mixture which is in transit at his own risk.  This material will be subject to removal if problems are encountered, including but not limited to poor bond, low density or unsatisfactory laydown.

9.5 SPREADING AND FINISHING OF ASPHALT PAVEMENTS

9.5.1 Spreading and Finishing Equipment

Most asphalt plant mixtures are placed by asphalt pavers and compacted by either steel-wheeled static or vibratory rollers, pneumatic-tired rollers, or some combination of these.  This is the basic paving equipment.  Other equipment used in connection with the paving operation may include: milling equipment, the asphalt distributor, haul trucks, materials transfer devices, motor grader, wind-rowing equipment, hand tools, and other machinery and implements. Specification requirements for hauling, placing, and compaction equipment are included in Division 6 of the 2006 Standard Specifications and in Sections 8 and 9 of this Manual.

The Contractor must furnish and utilize equipment, which meets the requirements of the Specifications, unless otherwise approved by the Engineer.  Prior to beginning paving operations, the Resident Engineer or the Roadway Technician must inspect the Contractor's spreading and finishing equipment to see that it meets all requirements of the Specifications and is in good working order.  If the equipment meets Specifications and is in satisfactory operating condition a statement shall be entered in the Technician's Daily Diary.  If not, the Contractor should be advised accordingly and corrective actions taken before paving begins.  See the Technicians Checklist for QC/QA Roadway Operations in Section 10.1 and the following information.

(A) INCIDENTAL TOOLS
 Adequate hand tools and proper equipment for cleaning and heating them should be available for the paving operation.  Incidental tools to be furnished by the Contractor include:

1. Rakes;
2. Shovels;
3. Lutes;
4. Tool heating torch;
5. Cleaning equipment;
6. Hand tampers;
7. Small mechanical vibrating compactors;
8. Blocks and shims for supporting the screed of the paver when beginning operations;
9. Heavy paper, or timbers for construction of joints at ends of runs;
10. Joint cutting and tacking tools
11. 10 foot (3 meter) straightedge (See Section 610-12 of Standard Specifications)
12. 6" (150mm) Core Drill
13. 4 Foot (1.2 meter) Level
14. Depth Checking Device
15. Infrared Thermometer
16.  Stringline for paver alignment
17.  North Carolina Hearne Straightedge (when required by contract)

 An asphalt distributor is required to apply tack coat material before paving operations begin. Details on requirements and use of the asphalt distributor are covered in Article 600-5 of the Standard Specifications and Section 9.1.1 of this Manual.

(C) HAUL TRUCKS

Hot-mix is delivered to the jobsite by trucks.  Article 610-7 of the specifications state the requirements for hauling vehicles.  The technician must be certain that the mixture being delivered is within specifications and that it is being delivered in a manner that is safe.

Article 610-7 requires trucks to have tight, clean, smooth metal beds and free of holes.  All trucks must meet minimum safety criteria.  Each truck must be clearly numbered for easy identification and must be equipped with a tarpaulin.  A 3/8 inch (10 mm) hole must  be located on each side for the purpose of inserting a thermometer to check the mix temperature.

Before being loaded, the truck bed must be cleaned of foreign material and hardened asphalt and then lightly coated with an approved truck release agent that aids in preventing fresh hot-mix asphalt from sticking to the surfaces of the bed. (For a current list of approved release agents, contact the Materials and Tests Unit at phone number 919-329-4060 or visit their approved products web site located @ https://apps.dot.state.nc.us/constructionunit/mttracking/. Fuel Oil is not allowed for this purpose.  After the bed is coated, any excess release agent must be drained from the bed.

Before loading, the truck must also be weighed to establish a tare weight (unloaded weight).  The tare weight is later subtracted from the loaded weight of the truck to determine the weight of hot-mix the truck is hauling.  (See weigh ticket requirements in section 6.8.1)

The number of trucks required on the project is determined by many factors: the mix production rate at the plant, the length of the haul, the type of traffic encountered, and the expected time needed for unloading.

The truck must be inspected to be certain the rear of the bed overhangs the rear wheels enough to discharge mix into the paver hopper.  If it does not, an apron with side plates must be added to the truck body to increase the overhang and prevent spillage of mix in front of the paver.

The bed must also be of a size that will fit into the hopper without pressing down on the paver.  The hydraulic system for the truck bed hoist should be frequently inspected to guard against hydraulic fluid leakage.  Such leakage on the roadway surface will prevent good bonding between the roadway and the new mat.  If enough oil or fuel is spilled that the mix can absorb it, the mix can become unstable at the spot.  As a result, leaking trucks must not be used.

Tarpaulins are pulled over the mixture during hauling to protect the mixture from excessive cooling and/or the intrusion of water.  Care must be taken to be sure it is securely fastened to the top of the truck bed so that cold air cannot funnel under it and that moisture cannot enter the mix.  A cool mix forms lumps and a crust over its surface.  A mix with excessive moisture in it will probably blister and not lay smoothly, as it will pull and tear.

During the haul operation, the Contractor should take necessary precautions to limit the tracking and/or accumulation of tack coat material on either existing or newly constructed pavements.  Trucks should minimize their distance traveled over freshly tacked pavement and avoid tack that has not broken.

During delivery, the driver must direct the truck squarely against the paver, and should stop the truck a few inches from the paver, before the truck tires make contact with the paver push roller bar.  Backing the truck against the paver can force the screed back into the mat leaving a bump in the pavement even after the mat is rolled.

The truck bed should be partially raised and the load allowed to “break” before the tail gate is opened to prevent the mix from dribbling from the load into the paver hopper.  This technique will help to minimize segregation that occurs between loads.

(D) THE ASPHALT PAVER

Article 610-8 of the Standard Specifications requires the Contractor to utilize a self-contained, power propelled paver capable of spreading and finishing the asphalt mixture to the required grades, cross sections, thickness, and widths shown on the plans and typical sections and to uniform density and texture..  The asphalt paver spreads the mixture in a uniform layer of desired thickness, shape, elevation and cross section, ready for compaction.  Modern pavers are supported on crawler tracks or wheels.  These machines can place a layer of less than 1 in. (25mm) to around 8 in. (200mm) in thickness over a width of 6 to 32 ft. (2 to 10 meters).  Working speeds generally range from 10 to 70 ft. (3 to 21 meters) per minute. The basic asphalt paver consists essentially of a tractor unit and a screed unit. The paver used in highway construction is a relatively large machine with many intricate parts and adjustments.  Most pavers in use today may differ in detail, but they are all similar in principle and operate based on the principle of the self-leveling, floating screed.

The plan and side views shown in Fig. 9-3 trace the flow of asphalt mix from the receiving hopper at the front of the paver to the finished pavement behind the screed unit at the rear of the machine.  The mix is dumped into the receiving hopper at the front of the machine from a truck that is pushed ahead by the paver.  Rollers mounted on the front of the paver contact the rear tires of the truck and allow the paver to push the truck while it is dumping into the hopper.

After receiving the material in the hopper, two independently controlled slat conveyors, sometimes called flights, carry the mix back through the control gates to the spreading screws (augers).  Each auger and its respective conveyor are automatically controlled to allow the mix to be uniformly distributed and maintained in front of the screed unit.

The screed unit is attached to the tractor unit by two long pull arms that pivot well forward on the paver.  The arms provide no vertical support for the screed when it is in operating position.  As the tractor pulls the screed into the material, the screed will seek the level where the path of its bottom surface is parallel to the direction of operating position.

An often overlooked but important item is the proper cleanup of the paving machine at the end of the working day.  While the machine is still warm, the hopper, feeders, spreading screws, tamper bars, and screed plates should be given a light spray of release agent to ensure a smooth start the next day.

(1) The Tractor Unit

It is impractical to describe in detail all tractor units in current use.  There are several features, however, which are generally common to all and should be checked at the beginning of the paving operation and examined periodically thereafter.  Most points that should be checked involve moving or working parts.  For more detailed information, service manuals provided by the manufacturer should be carefully studied.

The governor on the engine should be checked for proper operation.  It is important that the tractor unit provide a smooth steady pull on the screed arms.  If the paver is equipped with pneumatic tires, air pressure should be as recommended.  On crawler machines, crawlers should be snug but not tight.  Any unnecessary movement caused by low tire pressure or loose crawlers when the machine starts or stops will be reflected in the surface of the mat as the screed is pulled forward.

The specifications require that a the paver with a receiving hopper and an automatically controlled distribution system capable of uniformly maintaining a proper head of material in front of the full length of the screed, including screed extensions, be utilized.  In the bottom of the paver hopper are two slat conveyers.  These conveyors are used to carry the asphalt mix from the hopper through the tunnel on the paver and back to the augers. The slat conveyor and auger on one side of the paver operates independently from the movement of the slat conveyor and auger on the other side of the machine. However, the auger and slat conveyor on  each side of the paver are interlocked such that they operate simultaneously. Thus, the amount of mix that can be carried back through the paver on one side can be varied from the amount of material that is being delivered on the other side.  This capability allows the automatic controls or the paver operator to manually feed more or less material to one side of the paver or the other for various reasons, including paving ramps, mail box turnouts, tapers, variable widths, and variable depth areas.

Flow control gates at the back end of the hopper over each of the slat conveyors can be individually adjusted to control the material flow rate to the augers.  These gates regulate the amount of material that can be delivered by the conveyors to the augers.

Sensors mounted on the paver near the outer end of the augers detect the amount of material being carried in front of the screed and activate the automatic controls when material is needed. The automatic controls actuate the augers and the slat conveyors to keep a constant depth of material in front of the screed, including screed extensions.  The feed system should be adjusted so as to cause the conveyors and augers to operate as close to 100% of the time as possible (at least 85% of the time) with a uniform head of material at a level at or just above the center of the auger shaft in front of the screed at all times. Some newer pavers are equipped with systems which allow the augers to run 100 percent of the time.

On some newer pavers, the slat conveyor system has been replaced by a screw conveyor system. The purpose of this new system is to provide remixing of the mixture in the paver hopper to reduce both temperature and aggregate segregation in the mat behind the screed.

The mix deposited in the auger chamber from the slat conveyors is distributed across the width of the paver screed by the movement of the augers.  At the junction of the two augers in the center of the paver, adjacent to the auger gear box, there typically is a different shaped auger (reverse auger or paddle) to tuck mix under the gear box and assure that mix placement at this location is the same as that across the rest of the width of the mat being laid.  If the reverse auger paddles are not operating properly or are in poor condition, a coarse segregated streak of material will most likely show up in the middle of the mat.  It is important that the augers carry a consistent amount of mix across the front of the screed so that the pressure (head of material) on the screed is kept as constant as possible.  When the screed width is extended, consideration should be given to extending the augers.  The Specifications require that auger extensions be used when needed to provide a proper head of mix in front of the full screed length.  A general rule of thumb is that when the screed is extended by more than one foot, the auger on that side of the paver should be extended by an equivalent amount.

(2) The Screed Unit

The specifications (Article 610-8) require that screeds be "activated".  Normally this means that the screed must have vibrators.  The vibrators cause the mixture to feed more uniformly under the screed and also impart some initial compaction to the mat.  This results in a more uniform mat thickness, increased density, improved smoothness and better surface texture. This initial compaction also helps to minimize the amount the screed settles when the paver is stopped for various reasons. Vibration may be accomplished with electrically operated mechanical vibrators or eccentrically loaded turning shafts which produce vibration.  The frequency of vibration may be controlled, thus helping to obtain a maximum compacting and smoothing effect. The optimum frequency and amplitude for best results in surface texture, smoothness and density is a trial and error process; however, vibrators must be used at all times on all layers of mixture.

Both the leading and trailing edges of the screed have a crown adjustment.  The leading edge (front) should always have slightly more crown (normally about 1/8 inch or 3 mm) than the trailing edge (rear) to provide a smooth flow of material under the screed.  Too much lead crown, however, creates an open texture along the edges of the mat while too little lead crown may create an open texture along the center of the mat.  Optimum crown adjustment also is a matter of trial and error. Crown adjustments to the leading edge or trailing edge of the screed may be made independently or simultaneously during the paving operation. The objective is to achieve a smooth, uniform texture across the entire width of the mat.

The screed must be equipped with heaters to prevent the mix from sticking to the screed plate.  They must always be used to pre-heat the screed up to the mix temperature at the start of paving operations. Sometimes they are needed when paving during cool, windy days.  However, they should never be used to attempt to heat cool mixture being delivered to the paver. Pre-heating helps to minimize the pulling and tearing that usually occurs on start-up.

Before paving begins, the screed should be raised and the bottom surface carefully checked for smoothness and excessive wear.  Screed plates first wear out about 4 to 6 inches in (100 to 150 mm) from the trailing edge.  Extensions should be flush with and in the same true plane as the bottom surface.  Tampers should be checked for excessive wear, adjustment, and proper operation.  Excessive wear causes a pitted surface in the mat, and improper adjustment gives the mat a scuffed appearance.  The limit of the bottom stroke of the tamper bar should extend 1/64 in (0.4 mm) below the bottom of the screed plate (the thickness of a fingernail).  If the screed is of the vibrating type, the vibrators should be started with the screed in a raised position to see that adequate vibration is being achieved.  Their performance must also be checked during the paving operation.  Either the tamper bars or the vibrators, depending on the screed type, must be utilized at all times during the laydown operation.

While the screed is raised, the strike-off device in front of the screed should be checked for condition and adjustment in accordance with manufacturer's recommendations.  An uneven, damaged, worn or improperly adjusted strike-off can greatly affect the smoothness, texture and uniformity of the mat.

Pavers must be equipped with a screed control system which will automatically control the longitudinal profile and cross slope of the pavement through the use of either a mobile grade reference(s), including mechanical, sonic and laser grade sensing and averaging  devices, an erected string line(s) when specified, joint matching shoe(s), slope control devices or other methods or combination of methods approved by the Engineer.  An erected fixed stringline must be used when required by the contract; otherwise, a mobile grade reference system capable of averaging the existing grade or pavement profile over a minimum 30 foot (9 meter) distance or by a non-contacting laser or sonar type ski with at least four referencing stations mounted on the paver at a minimum length of 24 feet shall be used.  Position the system such that the grade sensor is at the approximate midpoint of the grade reference system. The transverse cross-slope shall be controlled as directed by the Engineer.

A spirit level mounted on the screed or on a wedge board should be available so that a check on the roadway crown can be made at any time.  The heating unit should also be checked for proper operation by lighting the burner and allowing it to burn a few minutes prior to beginning paving operations.

(E) MATERIAL TRANSFER VEHICLE

A Material Transfer Vehicle (MTV) is basically a surge bin on wheels which  transfers the mixture from the haul vehicle to the paver hopper at a uniform and continuous rate so as to allow continuous movement of the paver between truck exchanges, provided a continuous supply of material is received from the plant.  This allows the paver to operate almost continuously, without stopping between truck exchanges. In addition, the MTV also remixes the mixture prior to discharge into the paver conveyor system to minimize aggregate segregation and temperature variation that may have occurred during mix production, loading from silos, and cooling during hauling.

The proper use of an MTV significantly improves the uniformity and ride quality of any pavement and is highly encouraged by the Department.  Article 610-8 specifically requires the use of a MTV when placing all asphalt plant mix pavements, including open-graded asphalt friction course, which require the use of asphalt binder grade PG 76-22, unless otherwise approved by the Engineer. The MTV must be used when placing all full width travel lanes, including shoulders, collector lanes, ramps, and loops which require mixes with PG 76-22.

One of the key factors in achieving smooth pavements is to maintain a uniform head of material in front of the screed without stopping the paver.  Keeping a constant stream of trucks supplying mix to the MTV is necessary if a continous paving operation is to be achieved.  However, if a gap does occur, the MTV should be stopped with out being completely emptied when waiting fir trucks, so that a consistent minimum amount of mix is retained on the augers to mix with the new, possibly segregated, material delivered from the next haul truck.  In addition, the paver should be stopped with the hopper half full so that the amount of mix in front of the paver screed remains constant and the proper smoothness of the mat can be achieved.

The specifications require that an MTV that  provides to the paver a uniform, non-segregated mixture that is of uniform temperature be used such that there is no more than 20?F difference between the highest and lowest temperatures when measured transversely across the width of the mat in a straight line at a distance of one foot to three feet from the screed while the paver is operating.  The temperature measurements are taken approximately one foot from each edge and at least once in the middle of the mat.

Some MTVs are very heavy.  Empty the MTV when crossing a bridge and move across without any other Contractor vehicles or equipment being on the bridge.  Move the MTV across a bridge in a travel lane and not on the shoulder.  While crossing a bridge move the MTV at a speed no greater than five miles per hour without any abrupt acceleration or deceleration.

In the event the MTV malfunctions during paving operations, immediately discontinue plant operations and do not resume operations until the MTV malfunctions have been remedied, unless otherwise directed by the Engineer due to safety concerns.  The Contractor may continue placement of the mix until any additional mix in transit has been placed, provided satisfactory results are achieved.  This procedure in no way alleviates the Contractor from meeting contract requirements.

9.5.2 Coordinating Plant Production and Paver Speed

Article 610-8 requires that the paver be operated at a forward speed consistent with plant production, material delivery, and satisfactory laying of the mixture so as to ensure a uniform and continuous laydown operation. Coordinate and adjust the paving operation and loading operation so as to maintain an adequate amount of asphalt mixture in the paver hopper between truck exchanges.  Do not allow the paver hopper to become empty between loads.  Should unevenness of texture, tearing, segregation, or shoving occur during the paving operation due to unsatisfactory methods or equipment, immediately take such action as may be necessary to correct  such unsatisfactory work. Excessively throwing back material will not be permitted.

Uniformity of operations is essential in asphalt hot-mix paving.  Uniform, continuous operation and forward  speed of the paver produces the highest quality pavement.  A smooth pavement with uniform density and surface texture is the ultimate goal.

There is no advantage in the paver traveling at a speed that allows the mix to be placed faster than the plant can produce mix  and/or the mix can be delivered to the roadway. Paving too fast can result in the paver having to stop frequently to wait for trucks to bring more mix.  If the wait is too long (more than a few minutes on a cool day) the smoothness and surface texture of the pavement will suffer. In manual operation, when the paver starts up again, the mix in the paver that has cooled causes the screed to rise and then fall as warmer mix feeds through the paver and the screed seeks the equilibrium level.  If automatic screed controls are being used, the automatic system overcorrects the screed, causing it to dip before finally leveling off, and thereby causing a rough riding surface in the pavement. In addition, these areas are usually different in surface texture (open), many times the mix is segregated, and density is almost always lower, if not failing.  A POTHOLE WAITING TO HAPPEN. Obviously, then, it is essential that plant production and paving operations be coordinated.  The paver must be continuously supplied with enough mix, and at the same time, the trucks should not have to wait a long time to discharge their loads into the paver hopper.

The paving machine should never be operated at a speed in excess of that which will result in a properly placed mixture.  If the paver cannot properly place the mixture at a rate equal to plant capacity, the plant production rate will need to be slowed, or other corrective measures taken.  A chart similar to the one shown in Figure 9-5 may be helpful in balancing paving machine speeds with plant production. However, it doesn’t take a rocket scientist to realize that if the paver is waiting for extended periods between every load, then the paver needs to be slowed down.

9.6 THE PLACING OPERATION

Construct pavements using quality paving practices as detailed herein. Construct the pavement surface smooth and true to the plan grade and cross slope.  Immediately correct any defective areas with satisfactory material compacted to conform with the surrounding area.  Pavement imperfections resulting from unsatisfactory workmanship such as segregation, improper longitudinal joint placement or alignment, non-uniform edge alignment and excessive pavement repairs will be considered unsatisfactory and if allowed to remain in place will be accepted in accordance with Article 105-3.  When directed due to unsatisfactory laydown or workmanship, operate under the limited production procedures.  (See Section 10.2.4)

Prior to beginning operations a string line must be placed by the Contractor along the edge of the proposed pavement to provide horizontal alignment control for the paver operator.  The objective is to ensure a true and uniform line for the pavement edge(s).  A string line will not be required when the first course is placed adjacent to a curb section.  The Contractor and the Inspector must frequently check the stringline to assure that it is correctly located and of uniform alignment and is being followed.

Apply tack coat in accordance with the provisions of Section 605 of the Specifications and Section 9.3 of this Manual.

The Specifications state that mixtures produced simultaneously from different plant sources can not be intermingled by hauling to the same paver on the roadway unless the mixtures are being produced from the same material sources and same mix design.

The Specifications require that the paver be operated as continuously as possible.  Pave intersections, auxiliary lanes, and other irregular areas after the main line roadway has been paved, unless otherwise approved.

Some contracts require the use of an erected fixed stringline for both and longitudinal profile and cross slope control. When an erected fixed string line is required, the Contractor must furnish and erect the necessary guide line for the equipment.  Support the stringline with grade stakes placed at maximum intervals of 25 feet (7.6 meter) for the finished pavement grade.

9.6.1  The Spreading Operation

After the paver has been checked and has been positioned on the road, the screed should be lowered onto “starting blocks” (shims) of the same thickness as the loose mat to be laid and the thickness control screws adjusted for this depth.  Or, if starting from a previously laid mat, “starting blocks” of the same thickness as the difference between the loose and compacted mats should be used.  A general rule of thumb is to increase the loose thickness by ¼ inch per inch of compacted thickness. The thickness control screws on the screed are then adjusted for this depth. When this is done, the paver will then begin spreading the loose material at a depth such that after compaction, the desired depth of mat will be achieved. It is desirable that the starting blocks (shims) be as long, or longer than the distance from the front edge to the rear edge of the screed plate.  This gives the screed plate full bearing at each end on a surface, which is close to parallel with the grade, upon which the screed can be nulled out.

As soon as the first load of asphalt mix has been spread, the texture of the unrolled surface should be checked to determine its uniformity.  Adjustment of the screed, tamping bars or vibrators, spreading screws, hopper feed, and other adjustment points should be checked frequently to assure uniform spreading of the mix to proper line and grade.  A straightedge should be used to determine whether or not a smooth surface is being obtained.

When the truck is dumping its load into the hopper, the  wheels should firmly contact the truck push rollers of the paver.  This is done automatically on many pavers, with oscillating push rollers that permit some misalignment of the truck.  When the truck is skewed, the oscillating push rollers automatically adapt to that condition and the truck load is concentrated at the center of the paver.  If the paver is not equipped with oscillating push rollers and a truck is skewed so that the rollers are pushing against one set of dual wheels only, the spreader tends to skew also.  In this case, continual correction is required by the operator, resulting in a ragged line with consequent irregular and poorly compacted joints. In addition, the rollers must be clean and free to rotate to allow smooth forward travel of the paver.

The sides, or wings, of the hopper are movable.  Mix, if left to stand for a long period of time in the corners of the hopper, will cool and may appear as chunks of mix back of the screed when it passes through the paver.  Thus, the mix is periodically moved from the sides of the hopper into the middle of the hopper by folding the wings (sides) and allowing the mix to be deposited into the area of the slat conveyors.

Many paver operators dump (fold) the wings of the paver after each truckload of mix has been emptied into the hopper.  Further, to prevent spillage of the mix out the front of the hopper, the operator often pulls the amount of mix left in the hopper down during discharge and after the truck has left the hopper by continuing to run the slat conveyors to feed mix back to the augers.  This may result in the slat conveyor running completely empty.  This practice can lead to increased mat problems if segregated mix is deposited on the conveyor slats, either from the paver wings or from the haul truck, and carried back to the augers and screed.  It is not good practice to dump the paver wings after each truckload of mix has been delivered or to deposit the mix held in the wings into an empty paver hopper, because either procedure can cause segregation and decrease the quality of the finished mat.

Take necessary precautions during production, loading of trucks, transportation, truck exchanges with paver, folding of the paver hopper wings, and conveying material in front of the screed to prevent segregation of the asphalt mixtures.

To minimize segregation, the paver operator should fold the wings as seldom as possible.  The frequency at which the wings are dumped depends on the rate of delivery of the mix to the paver, the temperature of the mix, and the environmental conditions.  The wings should be emptied before the mix that collects in the corners of the hopper cools so much that chunks are formed that cannot be broken up as that mix moves through the paver to the augers and under the screed.  On colder days, the hopper wings will need to be dumped more frequently than on warmer days.  In some cases, it may be better to allow the mix to remain on the wings until the end of the day and then remove and discard the cool, hardened mix.

When the hopper wings are folded, the paver hopper should be at least partially full.  The amount of mix in the hopper should be approximately at the level of the bottom of the flow gates at the back of the hopper.  This will provide enough mix to heat the cooler material in the wings before it goes through the screed.  The slat conveyors should not be visible at the time that the wings are raised.  As discussed later, keeping the hopper relatively full between truckloads of mix keeps the head of asphalt mix in front of the paver screed constant and also reduces any segregation that might be present in the mix.  In addition, the wings should not be “banged” repeatedly as they are emptied.

Segregation of the mix  must not occur.  If segregation should occur, the spreading operation should be stopped immediately and not resumed until the cause is determined and corrected.  (See “SEGREGATION OF MIXES ON THE ROADWAY” in Section 10 of the printed version of the 2008 QMS Manual).

The amount of material carried ahead of the screed should be kept uniform in height.  Variation may result in surface roughness.  If the mix pulls and tears under the screed, the condition should be investigated immediately.  Common causes of pulling are moisture in the mix, too cold a mix and too high a percentage of screenings in the fine aggregate.  For example, when the fine aggregate portion of the mix is composed of stone screenings and sand, an excessively high proportion of screenings may toughen the mix unduly.  A tough mix is very desirable for stability but sometimes pulls badly.  Even a slight change in the fine aggregate sometimes makes an appreciable difference in placing and rolling characteristics.

Some pavements may develop extreme crowns (either steep or flat) after many overlays have been placed.  When not restricted by existing curbs, leveling wedges may be placed on both sides of the crown.  The mix should contain small-size aggregates and be feathered near the center or edge of the existing crown.

Many paving operations consist of variable widths not common to multiples of the normal paver width.  Cutoff shoes or screed extensions may be used to vary the paver width.  In multiple lane paving, the cutoff shoe should always be opposite the joint matching side of the paver.  The final lane should be at least the width of the paver.  When adding extensions, it is important that they match the screed on the paver, i.e., tamping, vibrating, or oscillating.

Urban paving poses additional problems.  Intersections require changes in the crown to facilitate rainfall runoff.  This is achieved by reducing crown in the leading and trailing edges of the screed equally.

Manholes and drop inlets require handwork on base courses.  The screed is lifted over the structure and relocated on the opposite side.  Then the area around the structure is filled in by hand.  When the surface course is placed, the screed should ride across the structure and the excess material removed.

When placing base courses next to the curb it is advisable to operate the paver 3 to 4 in (75 to 100 mm) away from the curb.  This prevents the screed from becoming jammed or bound by the curb.  Material is allowed to  feed out of the spreading screw chamber by raising the end plate and spreading by hand.  The loose hand finished material must be slightly higher than that placed by the screed to compensate for the lack of compaction by the screed.

After one or two truckloads have been spread, both the Department’s Technician and the Contractor’s Technician should check the unrolled depth and rate of spread of the mix. The unrolled depth is determined with a ruler. To determine the rate of spread, they must determine the area covered and the weight of  material spread over that area. The specifications usually require that the mix be placed at a specified rate in pounds per square yard (kilograms per square meter) at an approximate depth. By dividing the mass (weight) in pounds (kilograms) of mix placed by the number of square yards (square meters) covered will give the average rate in pounds per square yard (kilograms per square meter) being placed.

Once the proper adjustments are made on the screed control system such that the required rate of mix is being placed, an occasional check of the pounds per square yard (kilograms per square meter) being placed and an occasional check on the depth of the unrolled mix will be sufficient to determine if the correct spread is being maintained.

If the specifications require that the mix be placed at a specified depth in inches (millimeters), the compacted depth of the mix should also be checked so that a correlation ratio can be established between the loose depth and compacted depth.

Note:  See “Determination of Rate of Spread and Tonnage Required” in Section 10 of the printed version of the 2008 QMS Manual.

9.6.2 Fundamentals of Screed Operation

The screed unit, exclusive of any type of automatic control, is attached to the tractor unit by two tow arms that pivot about a hinge point (tow point) just beyond the midpoint of the paver. In manual operation, these tow arms are locked in a fixed position at the tow point.  The basic principle of  screed operation is that when pulled into the material deposited in front of it by the spreading screws (augers), it floats on the mix, moving up or down seeking the level where the forces acting on the screed are in equilibrium and the path of its flat bottom surface is approximately parallel to the direction of pull.

Forces acting on the screed unit during paving operations are shown in Fig. 9-6.  While the paver is moving, the pull, (P), at the pivot point always exceeds the horizontal resistance, (H), on the screed plate.  When the thickness of the mat is to be increased, the screed is tilted upward to allow more material to crowd under it.  The result is that the vertical uplift, (V), exceeds the weight, (W), and causes the screed plate to rise.  As it rises, V becomes less until it again equals W, at which time the vertical motion stops and the screed plate once again moves only in the horizontal direction in a path parallel to the direction of pull.  The thickness of the mat is changed by either by tilting the screed plate using the screw or jack or by moving the pivot point of the pull arm vertically, assuming all other factors remain uniform.

"Nulling out the screed" is an expression used to describe adjusting the screed angle on both ends so that the screed plate rests flatly on starting blocks or other surface.  When both the front edge and the rear edge of screed plate rest firmly on a surface, the adjusting screws will have a limited amount of free rotary movement. This means that the angle of attack of the screed is in the neutral or flat position. This will indicate that the screed is nulled out.  CAUTION:  Before attempting to null out the screed, be sure the screed lift is not restricting the full weight of the screed from resting on the surface or surfaces upon which it is being nulled out. Once the screed is nulled out, it is good practice to increase the angle of attack by turning each depth screw handle approximately one full turn (depending on the make of paver) from the nulled out position of the screed and start paving, checking and adjusting until proper mat depth is obtained. This practice usually results in an attack angle very close to that need to maintain the desired depth.

The weight of the screed exerts a compacting and compressing force as it passes over the asphalt mix. To compensate for this,  ANGLE OF ATTACK is an adjustment which raises only the front edge of the screed by an amount which causes the screed to climb enough to equal the amount of compaction by the screed. Exact angle of attack is always an unknown factor.  Variation in mixes, temperature of mix, and paving speed are all variables which effect the amount of angle of attack which is required.  These variations must be kept to a minimum if a smooth mat is to be accomplished.

In  order to keep the forces acting on the screed constant, the amount of material carried ahead of the screed should be kept uniform in height.  Variation will likely result in surface roughness.  This will require maintaining sufficient material in the hopper to supply the spreading screws (augers) with just enough mix to cover the midpoint of the screws out to their ends. The material feed system, including the flow  gates, augers, automatic feed control sensors and paver speed should be set so the slat conveyors and augers feeders operate as close to 100% of the time as possible.

The screed is continually attempting to bring or keep all of the forces in balance.  This is why  it is important to set the flow gates properly, keep the slat feeders operating uniformly and as continuously as possible, keep a uniform height of material in front of the screed, not over-control the screed, and maintain a uniform forward speed of the paver.  The temperature of the mix must be kept uniform so that the viscosity of the mix does not change and influence the balance of forces acting on the screed.  Uniformity of the mix, uniform and continuous operation of the paver and proper adjustment are the keys to smooth, dense pavements.

The asphalt mixture must be spread and finished to the required grades, cross sections, thickness and widths shown on the plans and typical sections and to uniform density and texture by the paver.  Many working conditions and adjustments can be checked only by the end results or more specifically, by the quality of the mat that is placed.  However, before work begins, certain checks on items that would obviously affect the work should be made.

The paver must be equipped with a fully activated and heated screed plate which is of adequate length to spread and finish the full uniform width travel lane being placed.  The use of strike off devices, either mechanically or manually operated shall not be permitted in spreading and finishing the mixture within the uniform width travel lane(s) since poor texture, low densities in these areas and an uneven mat are likely to occur.  Strike off devices may be permitted where curve widening, tapers, varying pavement widths and aprons are occasionally encountered.

Where the required uniform width of mat placement is different from the basic paver screed width, the use of cut-off shoes may be used to reduce the width of the mat being placed.  Likewise, factory manufactured, bolt on extensions capable of being heated and vibrated may be used to extend the width beyond the basic width of the screed.  Extensions which will produce a finished mat of the same texture and density as that of the basic screed must be used.  When screed extensions are used, the specifications require augers to be extended if necessary in order to maintain a uniform head of mix in front of the full length of the extended screed.

9.6.3   Use of Automatic Screed Controls

Article 610-8 of the Specifications require that the 30 foot (9.1 meter) minimum length mobile grade reference system or a non-contacting laser or sonar type ski with at least four referencing stations mounted on the paver at a minimum length of 24 feet be used to control the longitudinal profile when placing the initial lanes and all adjacent lanes of all layers, including resurfacing and asphalt in-lays, unless otherwise specified or approved by the Engineer.  A joint matching device short (6 inch shoe) may be used only when approved by the Engineer.

Utilize the automatic slope control system unless otherwise approved by the Engineer. The Engineer may waive the use of automatic slope controls in areas where the existing surface (subgrade, base, asphalt layer, etc.) exhibits the desired cross slope of the final surface.  The Engineer may also waive the use of automatic slope controls in areas where the use of such equipment is impractical due to irregular shape or cross section (such as resurfacing).  When the use of the automatic slope controls is waived, the Engineer may require the use of mobile grade references on either or both sides of the paver.  Manual screed operation will be permitted in the construction of irregularly shaped and minor areas, subject to approval.  Waiver of the use of automatic screed control does not relieve the Contractor of achieving plan grades and cross slopes.

In the case of malfunction of the automatic screed control equipment, the paver may be manually operated for the remainder of the work day provided this method of operation produces acceptable results.  Do not resume work thereafter until the automatic system is functional.

The primary purpose of automatic screed controls is to produce a smoother pavement layer than the paver can accomplish by its self  or the screed operator can  accomplish by continually changing the setting of the thickness-control cranks.  The automatic screed control functions by maintaining the elevation of the screed tow points in relation to a reference other than the wheel base of the paver itself.  The elevation of the tow point is kept at a constant elevation in relation to a given grade reference.  Automatic screed controls have five main components: (1) Grade sensor, (2) Pendulum, (3) Control box, (4) Command panel, and (5) Motors or cylinders to change the screed tilt.  A diagram showing the components of one make of automatic screed control is shown in Figure 9-7.  The automatic screed controls operate on the principle that if the screed tow point(s) are made to follow a smooth line and all other forces acting on the screed are kept constant, a smoother pavement profile grade will result, regardless of irregularities in the surface being paved.  Slope, or transverse profile, is controlled by a pendulum adjusted for a particular slope, or may be controlled by a second profile control system on the opposite side of the paver.

Once the screed is set for the desired depth of spread, the automatic system takes over to produce a smooth mat.  The command panel, the grade sensor, and the pendulum feed electronic impulses to a control box which activates the motors or hydraulic cylinders to change the relative elevation of the screed arm pull points, thereby changing the screed tilt (angle of attack) and automatically compensating for road surface irregularities.

The automatic screed controls get information from a sensing device riding on either an erected stringline that has been set as the grade reference, from a mobile reference device capable of averaging the grade over a minimum 30 foot (9 meters) distance or a non-contacting laser or sonar type ski with at least four referencing stations mounted on the paver at a minimum length of 24 feet, a joint matching device or some combination of these.  The devices references from either the subgrade, base, adjacent lane, curb, or gutter or an erected stringline. These devices can be used for overlaying old pavements or for new construction.  Sometimes a carefully installed stringline is recommended for new construction.  The stringline can be placed on either or both sides of the paver.  When placing the initial lanes and adjacent lanes of all courses, the paver must be equipped with a mobile reference device and should always be used where possible.  This will automatically improve the pavement smoothness as adjacent lanes and courses are placed. When done properly provide depth control such that when completed it will match the depth of the existing lane.  The joint matching device (short ski) is used only when permitted by the Engineer.  The joint matcher should not be permitted to ride on the gutter when placing the final layer in a curb and gutter section.  On all lanes of all layers, a minimum 30 foot mobile reference (ski pole) or non-contacting laser or sonar type ski with at least four referencing stations mounted on the paver at a minimum length of 24 feet should  be used when placing the initial lane and all adjacent lanes.  Paver manufacturers are now recommending using the long referencing devices even when matching joints of adjacent lanes on the final layer.  Different types of grade followers are shown in Figs. 9-8, 9-9, and 9-10.

It should be noted that new automatic screed control systems are now available that utilize sonic and/or laser technology in lieu of or in conjunction with the 30-40 foot (9-12 meter) mobile string line.  These systems have been used very successfully and are permissible under current NCDOT specifications.

Sensors on pavers equipped with electronic controls can be checked by varying the positions of the sensors and observing if the power on the screed controls respond and make the correct compensating adjustment in the screed pull points.

The sensor riding on either an erected stringline or mobile stringline transmits signals to the screed control to produce a paved mat behind the screed at a predetermined grade.  The cross slope of the screed is normally set on the control panel.

When a sensor or grade follower is linked in with a short ski, long ski, or other traveling reference device, the average thickness or average rate of spread may be adjusted by raising or lowering the sensor. Do not use the screw jacks to change the angle of attack of the screed.  The average rate of spread or thickness may then be rechecked as explained earlier.

If the mat being placed is uniform and satisfactory in texture, and the thickness is correct, no screed adjustments are required.  But when adjustments are required, they should be made in small increments and time should be allowed between the adjustments to permit the paver screed to complete reaction to the adjustments sequentially.  The paver must travel the equivalent of approximately 5 lengths of the leveling or tow arm before the adjustment is fully accomplished.

It is equally important that the thickness controls on the screed not be adjusted excessively either in amount or frequency.  Every adjustment of the thickness controls results in a change in elevation of the mat surface.  Excessive changes in the surface elevation at the edge of the first mat are extremely difficult to match in the companion lane when constructing the longitudinal joint.

The automatic screed control should be used all times possible because it can, in most cases, do a much better job than the manual control. The NCDOT Specifications, Article 610-8, state that if the automatic control equipment malfunctions, the manual controls may be used for remainder of that work day, provided satisfactory laydown is being achieved.

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Figure 9-8
Short Mobile Grade Reference (Ski) (Joint Matching Device)