460.7 Mat Problems

From Engineering_Policy_Guide
Jump to navigation Jump to search

The following sections describe some important mat problems, possible causes and solutions, and their effects on the long-term performance of the pavement. However, the solutions may vary with the characteristics of the mix and may not work in all cases. The intent is to give the inspector an idea of what may be causing the problem and the possible solutions that the contractor may employ.

460.7.1 Non-uniform Texture

Non-uniform texture is a difference in the appearance of the mat, both transversely and longitudinally, as it is placed and compacted. Normally, minor differences in surface texture will be apparent because of differences in the alignment of the coarse aggregate particles. In addition, a mix with a high percentage of fine aggregate will have a more uniform surface texture than a mix containing a high percentage of coarse aggregate.

A non-uniform surface texture may be caused at nearly any point in the paving operation. The easiest way to obtain a uniform surface texture is to maintain a constant head of material in front of the screed. Also, the mix should be handled properly at all times. The paver and screed should be well maintained and in good operating condition.

Non-uniform surface texture is usually associated with non-uniform density. As the density decreases, the air void content increases, and the durability of the pavement decreases significantly.

460.7.2 Surface (Auger) Shadows

Surface shadows are dark areas that appear in the surface of the mat. The shadows may extend across the full width of the mat or only partially. In most cases, the shadows cannot be seen until after the pavement has been subjected to traffic and some of the binder has worn off the aggregate near the surface. In severe cases, surface shadows may be visible immediately behind the paver.

Surface shadows are caused primarily by overloading the augers. Whenever the amount of mix in the auger chamber buries the augers, shadows will be formed. The spacing of the shadows normally corresponds to the points where the augers were turned on after being shut off.

The amount of mix carried in the auger chamber should be maintained at a level near the center of the auger shaft. This means the flow gates and the speed of the conveyors and augers should be set so that the conveyors and augers operate as close to 100% of the time as possible and stopping and starting of the conveyors and augers is minimized. In no case should the top of the augers be completely covered with mix.

Surface shadows are not detrimental to the long-term pavement performance of the mix. The main concern with surface shadows is the visual appearance of the pavement.

460.7.3 Bleeding and Fat Spots

Bleeding is the result of asphalt binder flowing to the surface of the pavement. Bleeding usually occurs shortly after traffic is allowed on a new pavement during hot weather and while some moisture is still present in the mix. Traffic essentially continues to compact the pavement, which decreases the air void content and squeezes some of the binder out of the mix. Bleeding may be caused by an excess of binder and/or moisture in the mix, or by a lack of adequate space in the mix for the binder. Another possible cause of bleeding is excessive crack and joint sealant or tack coat on the underlying surface, which may be drawn up through the mat. Bleeding is usually represented by a longitudinal streak in the wheelpaths of the pavement.

Fat spots are isolated areas where asphalt binder has come to the surface of the mat during the laydown and compaction operations. These spots can occur erratically, or may be numerous in a regular pattern. Fat spots are primarily caused by excessive moisture in the mix. This is a common problem with mixes that have a high percentage of fine aggregate and/or aggregates with high porosity. If all of the moisture in the aggregates is not removed at the plant, the moisture vapor will force the binder to the surface as the moisture escapes from the mix and evaporates. Fat spots occur more frequently when aggregate stockpiles are wet or when the moisture content varies within the stockpiles. In addition, fat spots can be associated with segregated areas in the mat.

Variations in the mat temperature behind the paver indicate that the moisture content of the mix may also be variable. Where moisture has evaporated, the temperature is lower. This can contribute to pavement bleeding under traffic and fat spots in the mix during construction. Therefore, the aggregate should be thoroughly dried, especially when dealing with highly absorptive aggregates. Bleeding problems associated with excess binder can be solved by reducing the binder content of the mix. If rutting also occurs, the only solution is to redesign the mix.

Occasional fat spots in the mix are not detrimental to the long-term pavement performance. However, a large number of fat spots in the wheelpaths or bleeding can affect the longterm pavement performance because of the variations of binder and air void content throughout the mat. Shoving, rutting, and reduced skid resistance may occur in a pavement that contains many fat spots or bleeding.

460.7.4 Surface Waves

Surface waves, both short and long, can be caused by an inconsistent head of material in front of the screed. Variations in the amount of mix in the auger chamber causes the screed to “hunt” for an angle of attack and move up and down, resulting in variations in the thickness of the mat and a wavy surface. Variations in the stiffness of the mix as a result of temperature or composition changes can also cause surface waves. As the stiffness varies, the mix pushes on the screed with varying force, causing the screed to rise and fall. Surface waves are also a result of the poor mechanical condition of the screed, improper mix delivery, or improper automatic grade control procedures.

Long waves may also be caused by continual adjustment of the thickness control cranks, the condition of the underlying surface, and if the direction of the rollers is suddenly changed. If the distance between the waves corresponds to the distance between truckloads, the waves may be a result of emptying the hopper and conveyors between loads.

The only way to eliminate surface waves is to prevent their formation. One way this can be accomplished is by keeping the head of material as consistent as possible by the proper setting of the flow gates and continuous operation of the conveyors and augers. In addition, the stiffness of the mix should be maintained as consistent as possible. Mix stiffness is controlled at the plant by keeping the mix temperature, aggregate gradation, and the binder and moisture contents as constant as possible. Finally, proper mix delivery, paving, automatic grade control, and rolling procedures should be used.

Long-term pavement performance is affected by surface waves in two ways. First, the waves reduce the smoothness of the pavement. If the waves are severe enough to increase the dynamic or impact loading of the pavement under heavy truck traffic, the structural integrity of the pavement will decrease. Secondly, the factors that cause surface waves can affect the pavement density.

Washboarding is a roughness built into the pavement during compaction. A vibratory roller operated at a high speed or at an improper frequency and/or amplitude setting usually causes washboarding. Proper operation of the vibratory roller is the only way to eliminate washboarding. Because washboarding affects the density obtained during compaction, the long-term durability of the pavement can be significantly reduced.

460.7.5 Tearing

Tearing is defined by an open texture in the mat behind the screed. Tearing may appear as longitudinal streaks. These streaks are usually caused by improper crown in the screed. A streak in the center of the mat may be caused by a lack of lead crown. On the other hand, an excess of lead crown may result in streaks that occur along both edges of the mat. The proper relationship between the lead and tail crowns will result in a uniform mat texture across its full width.

A longitudinal streak may also occur in center of the mat as a result of a lack of mix being pushed under the gearbox because of missing, worn, or improperly set reverse augers or paddles. Gearbox streaks are more prevalent in mixes with a large nominal maximum size aggregate or a low binder content. A gearbox streak can be eliminated by increasing the amount of mix forced under the gearbox by adjusting or installing reverse augers or paddles on each side of the earbox.

Tearing that occurs at the edge of the mat can be caused by improper flow gate settings or incorrect installation of the screed extensions. Partial-width tearing of the mat can result if the screed has not been uniformly preheated before paving begins. Full-width tearing of the mat can be caused by warped or worn screed plates, high paver speed, a mat thickness that is too thin for the size of aggregate in the mix, and low mix temperatures.

The long-term pavement performance is affected because of the decreased density and increased air voids that occur as a result of tearing. In addition, the torn areas will be more susceptible to raveling and to the effects of moisture (stripping).

460.7.6 Shoving and Rutting

Shoving is the displacement of mix in a longitudinal direction and may occur during compaction or under traffic. In most cases, shoving during construction is accompanied by a large bow wave in front of the breakdown roller, particularly at the reversal points near the paver. However, shoving can occur under any roller that is operated improperly. Generally, shoving and rutting occur in an unstable mix because of excess binder and/or moisture content, an improper aggregate gradation, or the binder and aggregate properties. Excess crack and joint sealant or tack coat from the underlying surface may be pulled into the mix, increasing its tenderness and fluidity. Shoving may also occur if the underlying surface was not properly cleaned and tacked.

Rutting is distortion of the mat in the vertical and transverse directions. Consolidation (purely vertical) rutting occurs when heavy traffic passes over an unstable mat because adequate compaction was not achieved during construction. The traffic is essentially finishing the compaction process. The most common form of rutting is transverse distortion. In this case, the mat shoves as a result of the lateral flow of the mix under traffic.

The solution to a mix that shoves under the compaction equipment is to increase its internal stability. This can be accomplished by reducing the binder and moisture content of the mix, lowering the production temperature, or adjusting the aggregate gradation. The compaction process may need to be changed in order to obtain the required density during construction and reduce the amount of shoving and/or rutting that may occur under traffic. Also, the underlying surface should be cleaned and a tack coat properly applied.

Generally, shoving and rutting are caused by an unstable mix. These mixes will continue to distort under traffic, both longitudinally and transversely. Shoving during construction is a strong indication that the pavement will rut and not perform adequately under traffic.

460.7.7 Checking

Checking is defined as short transverse cracks that occur in the surface of the mat during the compaction of a tender mix. Severe checking results in longer cracks that are spaced closer together. However, checking does not extend through the full depth of the mat. When checking occurs, the cracks are formed in the bow wave as a result of the mix shoving in front of the steel wheel rollers.

Checking rarely occurs during the first or second pass of the rollers. If checking is going to occur, it will normally happen after the temperature of the mix has cooled to a certain point and additional passes are made with the steel wheel rollers. Checking usually does not occur with a pneumatic tire roller. Checking may be caused by excessive deflection of the mat under the compaction equipment because of a weak or yielding underlying surface. A mix that checks does not have enough internal stability at elevated temperatures to support the weight of the rollers causing excessive movement of the mat. The checks are then formed when the surface of the mat is pulled apart as the underlying surface deflects during compaction. The checks should only appear where the underlying surface yields.

The most common cause of checking is a poor mix design. Excessive binder and/or moisture content, particularly in addition to low VMA, results in a tender mix that is easily displaced under the rollers. If the mix is tender because of excessive binder, checking should occur on a regular basis. If the mix is tender because of excessive moisture, checking should only occur when the aggregate has not been sufficiently dried. Mixes that contain a high percentage of fine aggregate or lack adequate VMA will be tender and check continuously. If the gradation varies, however, checking may occur only when the gradation is improper. Finally, the aggregate properties can significantly affect the amount of checking. Checking is greatly increased if the mix temperature is too high for the binder grade. Secondary causes of checking are an overheated mix, improper rolling procedures, and a poor bond between the mat and the underlying surface. Occasionally, checking may be caused by temperature differences within the depth of the lift (heat checking).

If checking is the result of a poor underlying surface, it should be repaired and properly prepared. Soft spots should be removed and replaced. Areas of excessive deflection should be stabilized or removed and replaced. If the mix contains an excess of binder or moisture content, the binder content should be decreased or the aggregate properly dried. In order to reduce the moisture content of the mix, the production rate of the plant may need to be slowed down or the plant operating conditions may need to be changed. The VMA may also need to be increased. If checking is caused by excessive fine aggregates in the mix, the gradation should be adjusted. The best approach to rolling a mix that has tendency to check is to compact the mat as much as possible while the mix is hot and before checking starts, stay off the mat in the middle temperature zone when checking is most likely to occur, and then finish the mat once it has cooled enough to support the weight of the finish roller. Also, if the mix is at a high temperature when it reaches the paver, it should be allowed to cool before compaction. Improper rolling procedures should be corrected and the underlying surface should be properly prepared.

Although checking only extends a short distance into the mat, they are highly detrimental to the long-term performance of the pavement because of the lack of density associated with the checks. Inadequate density leads to increased air voids. Therefore, the pavement life will be significantly reduced.

460.7.8 Poor Compaction

The mat should be compacted so that the density and air void content are within specifications. If the air voids are too high, the mat will be permeable and will not have the desired rability. If the air voids are too low, the mix may become unstable under traffic resulting in shoving and/or rutting.

When the mix is too stiff or too tender, compaction is difficult. Some possible causes of poor compaction are inadequate support from the underlying surface, improper type and weight of rollers, improper tire pressure in rubber tire rollers, improper rolling procedure, improper mix design, segregation, high moisture content, and a low mix temperature or variations in the mix temperature.

Solutions to compaction problems include taking the necessary steps to ensure adequate support, producing a satisfactory mix, and using proper paving and rolling procedures. If the binder content is too high, the mix may compact too easily. If the binder content is too low, the mix may be stiff and difficult to compact. A satisfactory mix design will produce a mix with an optimum binder content that can be compacted to the required density with reasonable effort. Reducing the speed of the rollers and/or increasing the weight and number of rollers can increase density.

Inadequate compaction is detrimental to the long-term pavement performance because the mix will be permeable to air and water. Water can flow through the mix and reduce the strength of the underlying base. The high air voids also result in excessive oxidation of the mix, which leads to raveling, cracking, and general deterioration of the mix over time. When the air voids are excessively low after compaction, because of an unsatisfactory mix, the mix is likely to rut and shove under traffic.

460.7.9 Joint Problems

Joint problems are caused by improper construction of the joint, improper start-up procedures, or inadequate compaction. Poor transverse joints are associated either with a bump at the joint and/or a dip in the pavement near the joint. Poor longitudinal joints are usually characterized by a difference in elevation between the two lanes and/or raveling of the joint. See Compaction and Transverse Joints in Paving Operations for the proper joint construction procedures.

A poor transverse joint will not affect the long-term pavement performance if density is achieved. However, the joints will be felt by traffic. An improperly constructed longitudinal joint, on the other hand, can seriously decrease the serviceability of the pavement. A poorly placed and compacted longitudinal joint will ravel and, eventually, wear away under traffic. Also, the joint may be porous, allowing the environmental elements to damage the underlying subgrade or base material, which may lead to structural failures.

460.7.10 Segregation

Segregation is the separation of the aggregate in the mix resulting in areas with an undesirable gradation. Segregation results from the improper handling of the mix at any point during the production, hauling, and paving operations. It can occur as the mix is delivered from the plant to a surge silo, as the mix is discharged into the haul truck from the silo, and as the mix is deposited into the paver hopper by the truck. Some mixes are more prone to segregation than others. Mixes that have a large nominal maximum size aggregate, low binder content, or are gap-graded readily segregate when handled.

Rock pockets are areas of coarse aggregate that occur randomly across the length and width of the mat. They are generally caused by improper handling of the aggregate in the stockpiles and cold-feed bins or by improper storage of the mix at the plant. Rock pockets are more prevalent when the mix is produced by a drum-mix plant. If the loader places a bucketful of segregated aggregate in the cold-feed bin, the aggregate can pass through the drum, silo, haul truck, and paver without ever being completely mixed in with the other aggregate. This is possible because a drum-mix plant operates on a continuous-flow instead of a batch basis. Rock pockets generally do not occur in a batch plant because the screens and hot bins recombine any segregated material before it is fed into the pugmill. Further, the pugmill blends all the aggregates together, eliminating any segregation that might have occurred previously. However, if a surge or storage silo is used with a batch plant, segregation may occur because of improper loading of the silo. The solution to this type of segregation is proper handling of the mix at all times.

When a batch plant produces the mix, segregation may occur longitudinally on one side of the mat (side-to-side segregation) because of improper loading of the haul truck. If the mix is not delivered from the pugmill into the center of the width of the truck bed, the coarse aggregate particles in the mix may roll to one side of the truck and accumulate. When the mix is delivered to the paver, the segregated mix will be placed on the mat along the same side, and the segregation will appear as a longitudinal streak on that side of the mat. On the other hand, if a batch plant with a silo or a drum-mix plant produces the mix, segregation may occur on one side of the mat because of improper loading of the surge silo. As the mix is deposited into the silo, the mix is thrown to one side of the silo, and the coarse aggregate particles are separated from the finer materials. When the silo is emptied, the coarse aggregate is deposited into one side of the truck. This segregated material then passes through the paver and is seen on one side of the mat. Also, if the truck is not loaded in the center of its width, the coarse aggregate particles may roll to one side of the bed, and longitudinal segregation will appear on the corresponding side of the mat. One solution to this type of segregation is to load the mix directly in the center of the truck. If a silo is used for temporary storage of the mix, the necessary steps should be taken to ensure that the mix is deposited into the center of the silo.

Truckload-to-truckload segregation occurs at transverse locations across the width of the mat. The most common cause of truckload-to-truckload segregation is the improper loading of the haul truck from the silo. If mix is placed in the truck bed in one drop from the silo, the coarse aggregate particles in the mix have a tendency to roll to the front and back of the bed. Also, if the load is “topped off” by dribbling mix into the truck, segregation will occur. If the hopper or conveyors on the paver are emptied, or if the wings of the hopper are folded, after each truckload, any coarse aggregate particles that have accumulated near the tailgate of the truck will be deposited into the empty hopper and carried back to the auger chamber with the next load of mix. By loading the haul trucks with multiple drops of mix, truckload-to-truckload segregation can be significantly reduced by decreasing the distance that the coarse aggregate particles can roll. Also, the proper truck unloading procedures should be used to minimize any segregation that may have occurred in the truck loading procedure before it is introduced to the paver. Keeping the hopper full of mix between truckloads can also help reduce segregation. Using an MTV that reblends the mix can almost eliminate segregation.

Temperature segregation is also a concern. During the haul, the mix in the truck cools more quickly near the edge, bottom, and top of the load. If the cooler material is not remixed with the hotter material, variations in the temperature of the mat will occur. The results may be variations in mat density and surface texture. An MTV that reblends the mix can be used to significantly reduce temperature differences in the mat.

Segregation is extremely detrimental to the long-term pavement performance because the increased air void content of the mix in the segregated areas increases the potential for moisture damage. Further, the segregated areas are susceptible to raveling, and possibly, total disintegration under traffic. When segregation occurs, it is likely to lead to forms of long-term pavement distress such as wavy surface and poor compaction. Superpave - Lessons Learned provides additional information on causes and remedies for segregation.