Construction Manual  

 

Storage silos and surge bins work well if certain precautions are followed, but they can cause and/or contribute to segregation of the mix if not operated properly. Hot mix is dumped into the top and falls vertically into the structure. It is good practice to use a baffle plate, batching gob hopper, or similar device at the discharge end of the conveyor used to load the silo. The baffle helps to prevent the mix from coning and segregating as it drops into the silo or bin. It is also recommended to keep the hopper at least 1/3 full when possible to minimize segregation as the hopper empties and to help to keep the mix hot.

 

Mix that is discharged from the silo or bin must meet the requirements of the job mix formula. The system must be designed so that segregation of the mix is held to a minimum. Frequent visual checks of the mix must be made by the Plant Technician to make certain segregation has not occurred during the charging of the silo or bin and/or during loading into trucks. Visual checks for segregation should be made frequently, especially on base and intermediate mixes, since they are more subject to segregation.

 

Samples of the mix for testing purposes will be taken directly from the truck body in accordance with procedures outlined in the HMA/QMS Asphalt Manual.

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610-7 HAULING OF ASPHALT MIXTURE

The truck bodies in which the mix is to be hauled should be inspected to make sure that the bed has been lightly coated with an approved release agent to prevent the mixture from adhering to the bed. Diesel fuels or other petroleum products are not approved releasing agents. After the bed is coated, any excess solution must be adequately drained before any mix is allowed to be loaded. Excess solution can be extremely detrimental to mixture that it contacts.

 

The finished mix should be observed frequently and the temperature should be checked at random intervals and recorded. The Contractor must provide a platform near the truck loading area from which the mix may be observed, temperature of the mix determined and from which samples of the mix can be secured. The Technicians should assure that all trucks are properly covered and covers securely fastened before leaving the plant to protect the mix from chilling due to cool weather or potential rain showers. Covers must be of sufficient length and width to cover the entire load and must be repaired or replaced when damaged or torn.

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610-8 SPREADING AND FINISHING

Spreading, finishing, and compaction are extremely important operations if a high quality, visually pleasing, long lasting pavement is to be constructed. Placing and compacting the asphalt mixture is the operation to which all the other processes are directed.

 

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.

 

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.

 

National surveys of the traveling public (taxpayers) indicate that their perceptions of high quality pavements are those that are smooth and last for a long time. While the public is usually neither aware nor concerned about other properties such as gradation, binder content, voids properties, density, etc., we as Engineers and Contractors know that mix quality, pavement 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, require less maintenance and therefore, last longer. 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 common questions and items of concern, answer questions and to plan the paving operation accordingly. Some common items of interest are listed in Section 9.4 of the HMA/QMS Asphalt Manual.

 

Sections 9 and 10 of the HMA/QMS Asphalt Manual discuss in detail the requirements of the Specifications, good construction practices, typical calculations, and responsibilities and duties of QC and QA Roadway Technicians. Examples of rate of spread and tons of mix required are illustrated in the HMA/QMS Asphalt Manual.

 

There are places on many jobs where spreading with a paver is either impractical or impossible. In these cases, hand spreading may be permitted. Placing and spreading by hand should be done very carefully and the material distributed uniformly so that the segregation of the coarse aggregate and the asphalt mortar will be avoided. When the asphalt mix is dumped in piles, it should be placed far enough ahead of the shovelers and rakers to necessitate moving the entire pile. Also, sufficient space should be provided for the workman to stand on the base and not on the freshly mixed material. If the asphalt mix is broadcast with shovels, almost complete segregation of the coarse and fine portions of the mix will result. The material should be deposited from the shovels into small piles that are spread with lutes or rakes. In the spreading process, all material should be thoroughly loosened and evenly distributed. Any part of the mix that has formed into lumps and does not break down easily should be discarded. After the material has been placed and before rolling is started, the surface should be checked with templates and straightedges and all irregularities corrected.

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610-9 COMPACTION

Compaction is accomplished by arranging the aggregate particles closer together in a position in which the asphalt binder can hold them in place. Compaction accomplishes 2 important goals:

 

The need for a pavement to be compacted to the required density is better understood when the effect of air, water, and traffic on an under-compacted pavement is realized. The voids in an under-compacted mix tend to be interconnected and therefore, permit the intrusion of air and water throughout the pavement. Air and water carry oxygen, which in turn, accelerates the oxidation of the asphalt binder in the mix, causing it to become brittle. Consequently, the pavement itself will ultimately fail as it can no longer withstand the repeated deflections due to traffic loading. The internal presence of water at freezing temperatures can also cause an early failure in the pavement due to expansion of the freezing water.

 

A pavement that has not been adequately compacted during construction has not developed its potential design strength and therefore, may push, shove, and rut from traffic utilizing the pavement. However, unless the mix is properly designed and adequate voids remain in the compacted mix, the pavement will likely flush and tend to become unstable due to further reduction of void content under traffic and/or thermal expansion of the asphalt. The desired as-constructed void content is approximately 8 percent or less for dense-graded mixes. At this level, the voids are usually not interconnected. When the air void content is too high, the pavement will tend to ravel and disintegrate. When the air-void content is too low, there is a danger of the pavement flushing leading to poor skid resistance and becoming unstable.

 

Compaction is the final stage of hot-mix asphalt paving operations. It is the stage at which the full strength of the mixture is developed and the smoothness and texture of the mat is established. Therefore, the Technicians must be particularly observant of the compaction process.

 

There may be occasions during resurfacing operations where the mixture is totally within the job mix formula and the Contractor is applying every effort to achieve density and is unable to do so due to the poor condition of the existing pavement or underlying base or subgrade. If it is determined that this is the cause, the Engineer should advise the Division Engineer and the State Construction Engineer, and a determination will be made if the lower density results will be accepted, and/or other appropriate action(s) be taken. In some cases, revisions in the job mix formula such as adding additional asphalt cement may be made in order to obtain a pavement that will not have a high void content and, therefore, be subject to intrusion of air and water. However, we must assure ourselves that failure to obtain density is in fact due to yielding base or the condition of the existing pavement and not due to lack of proper equipment or lack of compactive effort being applied.

 

Rolling and compaction of asphalt pavements becomes more difficult due to cooler ambient temperature and cooler temperature of the base on which the mix is being placed. This is particularly true when placing the thinner surface course mixtures.

 

The temperature of a mixture during the compaction process is probably the single most important factor in achieving the required density on an asphalt pavement. This temperature must be high enough as well as uniform throughout the thickness of the mat if compaction is to be achieved. The mix temperature affects the viscosity of the asphalt binder, which in turn determines the ability of the mixture to be compacted. The hotter the mix during compaction the more fluid the asphalt binder and the less resistant the mix is to compaction. At the same time, we must be careful not to overheat the mixture to temperatures, which may result in damage to the asphalt binder. Also, if the mix is excessively hot, the rollers may need to wait for the mix to cool in order to prevent “picking up” on the roller wheels and to support the weight of the roller. This delay results in differential cooling between the surface and interior of the mat and may result in “heat checking” when rolled. Heat checking is the formation of small hairline cracks in the mat perpendicular to the direction of roller travel. Many times this “heat checking” is misconstrued as overrolling when, in fact, it is caused by not rolling the mat while the temperature is uniform from top to bottom. The relatively cool non-plastic surface deflects because the interior of the mat is still hot and in a plastic state, cracks appear, and density becomes very difficult if not impossible to achieve.

 

Naturally, cooler air temperatures, high humidity, stronger winds, and cooler base temperatures on which the mix is being placed drastically increases this rate of cooling and shortens the time in which compaction must take place. It becomes even more critical that the rolling operation follows closely behind the paver, so that there is no delay between placing and compacting the mat. We have all heard the expression “bump the spreader” with the roller. When thin lift pavements are being placed from late fall to spring or during any cool weather, this expression literally describes what must be done if specification density and tight smooth surface textures are to be obtained.

 

Many times, roller operators and inspection personnel tend to let the rolling operation lag behind the paver as they did back during the hot summer months when the rate of cooling of the mix was not as critical. As the weather cools, we sometimes fail to get out of our “bad habits” and to adjust rolling procedures to compensate for the faster rate of cooling with the result being inadequate density and poor surface texture. It should be noted, however, that increasing the roller speed is not the solution to this problem. Steel wheel roller speeds in excess of 3 miles per hour (5 km per hour) only tends to reduce the amount of compactive effort to a given area of pavement and very possibly will result in a rough surface. Some possible solutions to this problem are (1) slow the paver down in order that rolling operation can keep up, (2) add another roller, or (3) increase roller weight by adding ballast.

 

In addition to keeping accurate detailed records and observing that the operation is performed safely, the Technician must also be sure that compaction is done properly and that the finished pavement meets all specifications. To achieve this, the Technician must understand the compaction procedure and the equipment involved. The Technician must acquire samples of the compacted mat or take readings with special instruments to determine mix density and smoothness.

 

Section 9 of the HMA/QMS Asphalt Manual covers in detail compaction specifications, roller types, rolling and compaction procedures, factors affecting compaction, rolling phases, and procedures for rolling and compacting transverse and longitudinal joints. Pay close special attention to page 9-17 noting that some contracts require the use of an erected fixed stringline and/or machine guidance for line, grade and cross slope control. When required, the Contractor must furnish and/ or erect the necessary guide for the equipment. 

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610-10 DENSITY REQUIREMENTS

This Article of the Specifications specifies the minimum density requirements for the various Superpave mix types and which categories of pavements must be tested for density. Also defined are areas that do not require density testing provided appropriate compaction equipment and procedures are used.

 

For QMS projects, the Contractor selects the density acceptance method to be used on a project.  Testing to verify field compaction can be done by either obtaining cores from the pavement, testing with a nuclear density gauge, or testing with a non-nuclear density gauge on surface mixes.  However, nuclear density readings and non-nuclear density readings still must be correlated with core sample densities.  The Engineer is normally advised at the pre-construction conference which method will be used.​

 

When the Contractor elects to utilize nuclear methods as the means of density control, the requirements and procedures shall be as specified in  Subarticle 609-5(D),  and Articles 609-6, and 609-7. Reference should also be made to the QMS Nuclear Gauge Operator's Manual published by the Materials & Tests Unit and the HMA/QMS Asphalt Manual . These manuals describe in detail the procedures for nuclear density control for both QC and QA testing.

 

When the Contractor elects to utilize core sample methods as the means of density control, the requirements and procedures shall be as specified in Articles 609-5(D), 609-6, and 609-7. Reference should also be made to the appropriate sections in the HMA/QMS Asphalt Manual pertaining to core sample methods of density control.

 

Regardless of the method used, density tests must be made on a random location basis. The Nuclear Gauge Operator’s Manual outlines the use of a random numbers table to locate test sites when using nuclear gauge control. A similar random concept must be utilized when determining the core sample location within a lot as specified in Section 10 of the HMA/QMS Asphalt Manual.

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610-11 JOINTS

(A) TRANSVERSE JOINTS

A transverse joint is constructed at any point where the paving operation is interrupted for a period of time (an average time of 15 minutes or more) and the paving operation is to be resumed later. The type of transverse joint to be constructed depends primarily on whether traffic will be traveling over the mat before paving is resumed.

 

The proper construction of transverse joints is essential if a smooth riding surface is to be obtained and a durable, ravel-free joint is to be achieved. A poorly constructed transverse joint is noticeable as a pronounced bump in the pavement, many times with a very open surface texture and low density. The formation of transverse joints must be done while the mix is still hot enough to be easily workable and the joint must be thoroughly rolled and compacted before the mix becomes chilled. Consequently, the Technicians, both QC and QA, must be on hand whenever a transverse joint is made in order to ensure it is done properly. Discovering hours after construction that a transverse joint is unsatisfactory does no good, because joint construction can only be corrected while the mix is still hot and workable. Once the mix cools, corrections can be made only by cutting out and replacing the joint. Unfortunately, removing and replacing asphalt due to poor transverse joint construction creates two transverse joints where only one previously existed. Paving should not progress until the joint has been completed.

 

Transverse joints are constructed in three basic steps: (1) ending the lane or width of pavement (with proper compaction and thickness) at the point of work stoppage, (2) resumption of paving operations at a subsequent time, and (3) rolling the transverse joint. These steps are described in detail in Section 9 of the HMA/QMS Asphalt Manual.

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(B) LONGITUDINAL JOINTS

The proper location and construction of longitudinal joints is essential if a smooth riding surface is to be obtained and a durable ravel-free joint is to be achieved. Care must be exercised in the formation of longitudinal joints between adjacent lanes of pavement and at the edges of concrete gutter or concrete pavement. It is most important that these joints be tightly sealed to prevent the intrusion of water into and through the joint. Maximum compaction must be obtained at joints to prevent further compaction when exposed to traffic thereby resulting in low uneven joints and also to prevent the raveling of the pavement after exposure to harsh weather conditions and traffic.

 

When placing the final surface course adjacent to concrete gutter or concrete pavement, adequate mixture should be placed such that the surface of the fully compacted mat will be 1/8 to 1/4 of an inch higher than the adjacent concrete after rolling. Rolling of the joint between hot mix and concrete should be done as soon as possible. With possibly a few exceptions, a joint matching shoe should not be used.

 

There are several key factors in achieving tight, smooth, and durable longitudinal joints. These include: (1) providing adequate mix when placing the adjoining lane to allow adequate compaction without bridging (2) keeping the joint straight, (3) proper tacking of the previously placed mat exposed edge, (4) offsetting the location of joints in multi-lane multi-layer pavements, (5) locating the joints outside of the wheel path, (6) properly overlapping the previously placed mat at the joint, and (7) location of the rollers when compacting the unconfined edge of the first lane and the confined edge of the adjoining lane. These factors are described in detail in Section 9 of the HMA/QMS Asphalt Manual.

 

It should be noted that currently recommended procedures for constructing and compacting longitudinal joints is somewhat different than procedures used several years ago. The current recommended procedures are covered in detail in Section 9.10 of the HMA/QMS Asphalt Manual.

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610-12 SURFACE REQUIREMENTS AND ACCEPTANCE

This Article of the Specifications requires the Contractor to construct quality pavements using quality paving practices and procedures as required by the specifications and as recognized by the asphalt paving industry. The finished pavement surface must be uniform, smooth, and true to plan grade and cross slope. Many of these practices and procedures are addressed in Sections 9 and 10 of the HMA/QMS Asphalt Manual.

 

Pavement imperfections resulting from unsatisfactory workmanship, including but not limited to segregation, improper longitudinal joint construction, placement or alignment, non-uniform edge alignment, non-uniform texture or excessive pavement repairs will be considered unsatisfactory and if allowed to remain in place will be accepted in accordance with Article 105-3.

 

Continuity and uniformity of operations are two key elements essential to securing a good quality pavement surface; therefore, the speed of the paver should be regulated by the plant's production capacity and delivery of material to the paver, within limits. By observing the surface texture behind the machine and behind the roller and checking the surface with a straightedge, a malfunction in the paver, rolling operation, or non-uniformity of mixture may be detected. Both QC and QA Roadway Technicians must insist on prompt action to locate any deficiencies or defects that occur. Some of the most common problems encountered with possible causes are listed in the Mat Problem Trouble Shooting Guide and in the Segregation Diagnostic Guide in the Appendix of the HMA/QMS Asphalt Manual. Mat deficiencies, segregation, slope and thickness, surface texture, and smoothness are also discussed in detail in the HMA/QMS Asphalt Manual.

 

Article 610-12 of the specifications requires that in the event the laydown of the mix is unsatisfactory due to ride quality, excessive tearing, corrugation, rough surface, segregation, or other mat deficiencies, the roadway Technician will advise the Contractor that the work is unsatisfactory and that the work will continue under limited procedures. The definition of limited and the subsequent operating procedures are described in detail in the HMA/QMS Asphalt Manual. Mix placed under the limited production procedures for unsatisfactory laydown or workmanship will be evaluated for acceptance in accordance with Article 105-3.

 

This Article also includes the pavement smoothness requirements. Pavement smoothness is an extremely important factor in pavement construction for several reasons. The traveling public (taxpayers) demand smooth pavements and it is a proven fact that smooth pavements last longer, require less maintenance, and are cheaper to drive on. Smoothness is usually a good indicator of a uniform pavement and therefore a good quality pavement.

 

All pavements on which there will be traffic include smoothness requirements. There are 3 basic methods of measuring smoothness: (1) a 10 foot stationary straightedge, (2) the Hearne Profilograph and the Inertial Profiler as required in the Project Special Provision FINAL SURFACING TESTING – ASPHALT PAVEMENTS and Article 610-13 – Final Surface Testing and Acceptance. The requirements and procedures for all methods are described in detail in the Specifications and HMA/QMS Asphalt Manual.

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610-13 FINAL SURFACE TESTING AND ACCEPTANCE

International Roughness Index

 

The International Roughness Index (IRI) is the roughness index most commonly obtained from measured longitudinal road profiles. It is calculated using a quarter-car vehicle math model, whose response is accumulated to yield a roughness index with units of slope (in/mi, m/km, etc.). IRI has become the road roughness index most commonly used worldwide for evaluating and managing road systems.

 

Inertial profilers measure the pavement surface profile using a distance measurement transducer, noncontact vertical displacement transducer (line laser technology), an accelerometer, and a computer. The distance measurement transducer operates similarly to a car’s odometer, but more precisely. The noncontact vertical transducer measures the distance the device and the pavement surface at selected intervals. The accelerometer determines the inclination of the profiler as it ascends and descends hills and corrects for any movement of the vertical displacement transducer relative to the ground caused by the suspension. The computer collects and records all the data from the devices and uses it to calculate the IRI index to describe the ride quality of the pavement.

 

The Department gives the option of using either the Hearne straightedge or an inertial profiler to quantify ride quality of newly placed HMA pavements which required final surface testing and acceptance.

 

The measurement of IRI is covered in several standards from AASHTO and ASTM: AASHTO M 328, ASTM E1926, AASHTO R 56, AASHTO R 57, and others.

 

The equipment should be configured to record actual elevation of the pavement surface. The profiler’s internal IRI calculation mode should not be used. The software is required to produce electronic inertial road profiles in a format compatible with the latest version of FHWA’s ProVAL (Profile Viewing and Analysis software). The profile data shall be filtered with a cutoff wavelength of 300 feet. The interval at which relative profile elevations are reported shall be two inches.

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610-14 DENSITY ACCEPTANCE

The Department will evaluate the asphalt pavement for density acceptance on a lot by lot basis after the asphalt mix has been placed and compacted. This acceptance process will be conducted using the Contractor's quality control test results, the Department's quality assurance test results, including verification samples, and by observation of the Contractor's density quality control process conducted in accordance with Section 609. This Article defines how lots will be established, what constitutes a failing lot and how applicable pay factors are determined.

 

When the pavement is considered “other” construction, the pay factor for density deficiency is typically reduced in half of the new construction formula.

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610-15 MAINTENANCE

The Contractor is required to maintain the pavement in an acceptable condition and to repair any defects or damage that may occur until final acceptance of the project.

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610-16 MEASUREMENT AND PAYMENT

This article outlines the various contract descriptions under which the work will be paid, including the various mix types. Asphalt binder will be paid for as provided in Article 620-4.

 

There is no direct payment for the use of a materials transfer vehicle when required by the specifications as it is considered incidental to the cost of the mix.

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ADDITIONAL RESOURCES