CB-214D, CB-224D and CB-225D Vibratory Compactors Caterpillar

Forces of Compaction

Illustration 1	g00786830

The forces that contribute to compaction are static pressure (1), manipulation (2), impact (3) and vibration (4). The forces of static pressure and manipulation are determined mostly by machine weight, contact pressure and tire pressure. Static pressure and manipulation usually involve lower forces. Impact and vibration are both dynamic forces. Higher compaction is usually achieved by dynamic forces. Combining both static and dynamic forces in one machine makes the most effective compactor.

Static Pressure

Illustration 2	g00735443

Static pressure is determined by dividing the weight of the drum by the contact area of the drum. The area of the drum in contact with the surface is called the contact area (5). The contact area is influenced by the diameter of the drum. Also, the contact area is influenced by the amount of depression on the compact surface. The contact area will increase with a larger diameter and more depression. Static pressure is often quantified as static linear load or pounds per linear inch (PLI). Divide the weight of the drum by the width of the drum in order to calculate pounds per linear inch. PLI is a way to compare machines by specifications, but PLI does not include the contact area. PLI is not an accurate way to describe the interaction between the drum and the ground.

Manipulation

Manipulation rearranges particles into a more dense mass by a kneading process. This force is caused by the rubber tires of a pneumatic compactor or a combi roller. Manipulation is especially effective at the surface of the lift of the material. The longitudinal and transverse kneading action is essential when compacting a soil that is similar to clay type soil. Manipulation is also the desired process for the compaction of the final wear surface of an asphalt pavement. Manipulation helps to close the small, hairline cracks through which moisture could penetrate and cause premature pavement failure.

Vibratory Forces

Illustration 3	g00735448

Vibratory steel drums exert forces which increase compactive effort. Vibratory drums have internal eccentric weights that rotate on a shaft. The rotating eccentric weights cause the drum to move in all directions. The vibratory forces for compaction are the upward movement and the downward movement of the drum. Vibratory forces cause movement in aggregates and in particles of soil. The movement in aggregates and in particles of soil allows easier compaction under the weight of the drum. Each downward movement is an impact that also contributes to compaction. Vibration is an effective tool for an aggregate or a particulate material such as sand or gravel. Vibration is less effective on a cohesive material such as clay.

Amplitude

Illustration 4	g00735625

One part of vibration is amplitude. Amplitude is defined as half the total downward drum movement (6). Compaction occurs when the ground is being vibrated. Compaction also occurs due to the impact that results from the vibration. Amplitude influences the force of impact and the vibration energy. When the amplitude (7) increases, the influence increases.

Frequency

Illustration 5	g00735628

Frequency is a measured rate of ground impact. Frequency is usually rated in impacts, or vibrations per minute (8). For a given speed, a vibratory drum with low frequency strikes the ground with a wide impact spacing. At the same speed, a vibratory drum with high frequency strikes the ground more often and the spacing of impact (9) is closer. The impact spacing that is effective for many applications is 26 - 46 impacts/m (10 - 14 impacts/ft). Desired spacing of impact can be achieved even when the vibratory compactor operates at a relatively high frequency and a relatively high speed.

Rolling Speed

To get the best combination of compaction, productivity, and smoothness, a rolling speed should be chosen on each job. Generally, higher speed reduces compactive effort. The correct rolling speed is crucial for the thicker lifts.

If equipped, automatic speed control allows the operator to set the top speed at full stroke of the propel lever. Automatic speed control helps maintain a consistent speed for all passes.

Note: This feature is not available on the CB-214D, CB-224D, CB-225D, CB-334D and CB-335D.

Machine Vibration

If the mat is very stiff, the machine experiences excess vibration. The excess vibration of the machine occurs when the drums are not fully in contact with the mat during vibration. The excess vibration causes the machine to bounce and the steering to be difficult. The bouncing of the machine and steering difficulty is not good for the machine or for the mat and needs to be minimized. Usually reducing the amplitude setting or changing the rolling pattern will help.

Asphalt Equipment

Combination rollers, sometimes called combi rollers, use a vibrating drum plus three or four pneumatic tires that are located at the front or rear of the machine. Combination rollers attempt to bring together the compactive techniques of vibratory rollers and pneumatic compactors. The use of combination rollers reduces equipment requirements on method specification jobs that require both vibratory and pneumatic compactors. Combination rollers can be used when tight specification controls do not exist. A combi machine provides the smoothing action of the steel drum and the high ground pressure of the pneumatic tires. The pneumatic tires cause the manipulation of the mat surface.

Pneumatic Tires

Illustration 6	g00786819

Tire inflation pressure is very important to the performance of a pneumatic compactor. A higher tire pressure (10) means a smaller ground contact area and better compaction. Tire inflation pressure is kept high when doing breakdown or intermediate compaction.

Reducing tire inflation pressure (11) causes the bottom of the tire to flatten out and ground contact pressure is reduced. Therefore, the compactive effort is less, but the kneading action is emphasized. Lower tire inflation pressures are used for finish rolling.

The use of the water spray system is another important factor when compacting with a pneumatic compactor. When the roller begins to compact, the rubber tires will be cold and the hot asphalt will stick to the tires unless a nonstick agent is sprayed on the tires. As the roller works, the tires will heat up and asphalt sticking will not occur. The most effective use of the pneumatic or combi compactor requires keeping the tires hot whenever compacting.

Components of Asphalt Mix

Illustration 7	g00735638

The components that make up an asphalt mix depend on the type of structure that is being paved. The components have a major effect on the process of compaction. A cross section of an asphalt core shows the mixture of aggregates and binder. The following parts are in an asphalt mix: coarse aggregates, fine aggregates, asphalt cement or binder, mineral filler and asphalt modifiers. The modifier can be put in the asphalt mix at the refinery or at the plant. Mineral particles constitute 85 to 90 percent of asphalt mix. Another 5 to 8 percent consists of asphalt cement. The balance is air voids.

Angular Aggregate

Illustration 8	g00735660

The angularity of aggregate affects the ability to compact a mat. Round aggregate is easy to compact because of the low internal friction. However, if the aggregate is too round, the aggregate will become unstable. The aggregate will resemble a bag of marbles by continually shifting under a load. Mixtures with angular aggregates will have a higher internal friction. Angular mixtures are more difficult to compact because the sharp edges of the angular aggregate restrict movement.

The relative amounts of gradation also affects compaction. Coarse gradations usually compact with more difficulty than fine gradations.

Asphalt Stiffness

Illustration 9	g00737569

Asphalt cement has a range of stiffness. In hot climates, a cement with a higher stiffness is used. High stiffness is also used on roads with high volume. For cooler climates, a cement with a lower stiffness is used. Low stiffness is also used on roads with low volume. Modifiers are sometimes used to change the stiffness characteristics of binders.

Cement mixes with a higher stiffness make compaction more difficult. Cement with a higher stiffness is often compacted at a higher temperature.

Designation of Mixtures

Many different designations of mixtures exist. The designations show the maximum aggregate size. Also, the designations show the gradation of aggregate. Less compactive effort is needed on the smaller aggregates in surface mixtures and leveling mixtures. More compactive force is needed on the larger aggregates in base mixtures and in binder mixtures. Superpave mixes are near the top of the list in aggregate size.

New Road Construction

Illustration 10	g00786831

New road construction consists of numerous layers. Each individual layer is compacted by using various methods of compaction. The bottom layer of a road is called the sub-grade (17). The sub-grade consists of natural soil. The embankment fill (12) is the fill material that is placed on the sub-grade. A layer of aggregate base (13) is placed on top of the embankment fill. Sometimes, a layer of sub-base is placed on top of the aggregate base. The sub-base is either an embankment fill that is thoroughly compacted or a special material that is directly under the asphalt base course. The asphalt base is then placed on top of the sub-base (16). The base course consists of a hot mix that is made of large aggregate. The large aggregate is able to handle a high load. The base course is usually the thickest layer of hot mix, so high compactive effort is needed to get density. The binder course or the intermediate course is then placed on top of the base course (15). The base course and the binder course make up the asphalt base. Finally, there is a surface course, which is sometimes called a friction course or a wear course (14). The surface course will be made of a mix with the smallest aggregate. The surface course is ordinarily the thinnest layer. A low force of compaction is used on the surface course.

Note: The shoulders of the road often have a different design that requires different techniques of compaction.

Road Rehabilitation

Illustration 11	g00737751

Asphalt paving and asphalt compaction also take place on existing road rehabilitation. This often involves cold planing or milling a portion of asphalt. The road is then repaved. In some cases, the truck lane is milled to a deeper depth than the passing lane. The road may be widened for an additional lane. These techniques will result in a stair step effect and paving with different thicknesses is required. The most effective equipment and the rolling pattern for each step may be different. The equipment set up will vary with the design of the mix and with the thickness of the layer.

Phases of Compaction

Illustration 12	g00786833

The phases of compaction do not change for either new construction or for road rehabilitation. The following three items constitute the phases of compaction: breakdown, intermediate and finish. The process of breakdown (18) should accomplish most of the compaction. During the intermediate phase (19), some additional compaction occurs. There is minor movement of binder and aggregate that creates a strong pavement. The final phase is finish rolling (20). Minimal additional compaction occurs. Finish rolling smooths the mat. Also, marks from previous phases are removed by finish rolling.

Understanding Asphalt Compaction

Asphalt compaction is a complicated process. Compaction is not as simple as just rolling back and forth over the asphalt. Troubleshooting may be necessary if there are problems with meeting the requirements of density or smoothness. Generally, if there are problems, one or two of the following variables needs to be adjusted or compensated:

• mix design

• aggregate type and gradation

• asphalt cement type and properties

• modifiers or additives

• mix temperature

• delivery temperature

• thickness of the lift

• condition of the base

• air temperature

• wind

• weather

• rolling pattern

• rolling speed

• paving speed

Vibration Modes

There are several modes of vibration that help deal with different jobs.

Note: The combi rollers CB-225D and the CB-335D only have front drum vibration and a static mode.

• Use both drums in vibration mode for most applications.

• Use static mode for light compaction work and for finish rolling.

• Use vibration of the lead drum for repair and stiff mixes. Also, use vibration of the lead drum for steering problems and light compaction work.

• If equipped, use the automatic vibratory system shutoff. The system feature starts vibration at a preset speed when the compactor accelerates. The system feature also stops vibration at a preset speed when the compactor decelerates. The automatic vibratory system shutoff prevents the machine from continuing to vibrate once the roller has stopped. The automatic vibratory system shutoff activates the automatic reversing weights (ARW). The system feature (ARW) spins the drum weights in the direction of travel. The ARW reduces the damage to weak mixes.

Note: This feature is not available on the CB-214D, CB-224D, CB-225D, CB-334D and CB-335D.

Amplitude Setting

Note: This feature is not available on the CB-214D, CB-224D, CB-225D, CB-334D and CB-335D.

The chart is useful for selecting a setting for amplitude at the start of a project or for the generation of a test strip. The chart reinforces the need for the operator of the compactor to be familiar with variables that affect compaction. The following parameters determine the amplitude for compaction: mat thickness, base support, asphalt cement viscosity, shape of the aggregate, aggregate gradation and the temperature of the base and the air.

Mat Thickness and Aggregate Size

Mat thickness alone affects compaction, but the combination of thickness and size of aggregate can also be a factor. For example, a mat that is four times thicker than the largest aggregate is usually easier to compact. A mat that is two times thicker than the largest aggregate can be very difficult to compact. Special methods of rolling may need to be used if the mat thickness is less than three times the largest aggregate size.

Test Strip

The test strip should be a straight run that is at least 150 m (500 ft) long. The test strip should be part of the actual job. The following items are revealed from the test strip: measure of the target density, riding quality, proper compactive force, proper amplitude, proper rolling speed, proper rolling pattern and the rate of production for the equipment.

One trial and error method for creating a test strip is reducing the rolling speed of the compactor and performing a nuclear density test after every completed pass of the roller until the target density of the test strip is reached. The next step is increasing the rolling speed while the same number of passes is used. Test the density again. Continue this method until density fails to pass. The result will be the highest rolling speed and the number of passes that will achieve the target density. The results will show the highest production rate that the compactor can operate while the mat still meets the requirements of density and surface finish.

Thin Lift Compaction

Thin lift compaction is defined as work on any layer of asphalt that has a thickness of 50.8 mm (2 inch) or less. The process is commonly used for overlay work or as a finish course. The normal recommendation for a thin lift is selecting low amplitude if vibration is possible. A good choice for thin lift compaction would consist of a lead drum that is vibrating and a static rear drum. Possibly, static mode for both drums may be the best option. The purpose of thin lift compaction is achieving compaction without causing excess vibrations that would mark the mat.

The following tips are for compacting thin mats:

• Do not turn the wheels or turn the drum when the compactor is stopped on a mat. Thin mats are especially susceptible to the turning because thin mats will deform easily.

• Always turn off the vibratory system when you approach a stop or a change of direction. This will prevent the drum from hammering on the mat in one spot. All mat thicknesses can be affected by hammering. Compactors with automatic vibratory system shutoff make easier work of thin mats.

• Roll as close to the laydown machine as possible. Thin mats lose heat rapidly. You will need to quickly reach density. Achieving density on cold, stiff mats can lead to fractures in the aggregate.

Thick Lift Compaction

On lifts that are 50.8 mm (2 inch) thick or greater, select medium amplitude or high amplitude. The following factors affect thick lift compaction: mix design, aggregate type, gradation, mix temperature, base rigidity and the tendency of aggregate to fracture. The mix may tend to bulge in front of the drum on the first pass because the temperature of the mat is usually high and more compactive force is used. If this occurs, consider the following solutions:

• You should change the rolling pattern in order to move back from the laydown machine. This will allow the uncompacted mat to cool.

• The first pass should be made in static mode or in the low amplitude range.

• Use a machine with lower ground pressure, either a lighter machine or a wider drum.

Mix Temperature

Illustration 13	g00786834

The upper temperature limit that permits compaction is around 149 °C (300 °F). The upper temperature limit is affected by the characteristics of a mix at high temperature. If the mix is above 149 °C (300 °F), a bulge (21) might occur on each side of the drum. The mat will constantly shift, and the mat will not compact. Increasing the distance between the paver and the compactor will solve temperature problems (22). The increase in distance will allow the mat to cool. When the mix cools to around 85 °C (185 °F) little gain or no gain in density is possible. One solution to cold mixes involves increasing the force of compaction. A second solution is the use of multiple compactors on the job. A third solution is compacting close to the paver.

Superpave mix designs are normally placed between 140 - 160 °C (290 - 320 °F). Some mixes may be unstable after initial breakdown rolling and may require a delay in rolling. The delay will allow the mat to cool. Also, the delay will allow the mat to gain strength until compaction can continue. Many Superpave mix designs can be compacted below 85 °C (185 °F).

Ambient Temperature and Weather

An increase in air temperature decreases the cooling rate of bituminous mixtures. The increase in air temperature allows more time for compactors to achieve desired density levels in the mix. Higher air temperatures improve the mixture flow during compaction. In some cases, high air temperature can make a mix unstable and difficult to compact.

Wind can also affect compaction. The mix will cool quickly with a strong wind. Velocity of the wind is more of a problem in cold weather.

Rainfall during the production of asphalt can also affect compaction. Rain in aggregate stockpiles can reduce the overall temperature of the mix that is being compacted. Water in the mix can also make the mat unstable during compaction.

Rolling Pattern

A good rolling pattern provides the uniformity and efficiency needed to meet density. A good rolling pattern will meet the requirements for smoothness. A good rolling pattern will allow the compactor to match the production of pavers. The pattern should be determined on the test strip. The pattern will establish the number of passes that are needed to cover the mat. Also, the pattern will establish the number of passes that need to be repeated in order to achieve density.

Illustration 14	g00786835

The figure shows a typical rolling pattern. The compactor begins the pattern on material that has been previously compacted. The compactor begins at the right edge or the left edge of the mat (23). The operator turns on the vibratory system when rolling speed has been reached. The compactor should be operated in a straight line at a constant speed until the vibratory system is shut off and the machine slows to a stop. The machine should stop at a predetermined distance from the paver (24). The application and the type of mix affects the distance between the machines. The distance between the machines is usually 6 m (20 ft). The operator completes the first cycle by repeating the first pass in reverse. When the machine approaches the starting point, the operator should gradually turn the machine to a new starting point (25). The new starting point should provide a 152.4 mm (6 inch) overlap from the uncompacted mat to the compacted mat. The third pass has an identical form to the first pass. The third pass will be longer than the first pass. The pass will be longer because the paver has moved forward by this time. The sequence continues until the full width of the lane is compacted. The reversal point of the first set of passes is the starting point for the next set of passes.

If marks occur, the reversal should be completed differently. Marks in the mat occur most often because of a thick mat, a soft mix, or round aggregate in the mix.

In order to prevent a mark, the operator should slightly turn the machine as the machine comes to a stop. Turning the machine will allow the reversal line to be easily rolled.

Rolling Methods for a Longitudinal Joint

Illustration 15	g00786836

The figure refers to a rolling method for a longitudinal joint. The rolling method that is chosen is affected by the project design.

The procedure for compacting a longitudinal joint involves the vibration of both drums while the drum surface rolls along the joint with all the drum surface on the hot side (26). The drums force the hot asphalt into the joint. During the second pass, overlap the cold mat (27) about 152.4 mm (6 inch). Roll in the static mode during the second pass in order to avoid crushing the aggregate on the cold side.

Unconfined Edge and Confined Edge

Illustration 16	g00736768

Compacting unconfined edges is done differently than a longitudinal joint. Make the first pass about 152.4 mm (6 inch) away from the edge. The first pass adds strength to the mat. During the second pass, the asphalt that was compacted supports the weight of the drum. The edge is compacted but the edge is not pushed out.

Edges that are confined require a similar rolling pattern to the method for the longitudinal joint. Make the first pass along the joint. On thick lifts, some of the material that is hot can be forced onto the cold side. This material cannot be adequately compacted and much handwork must be done in order to clean up the process. An alternate procedure involves rolling the first pass in the static mode with most of the drum on the cold side and a 152.4 mm (6 inch) overlap on the hot side. The position of the drum will close the joint before compacting the hot side.

Transverse Joints

Illustration 17	g00786828

Transverse joints should be compacted in a manner that is transverse rather than in the direction of paving. Position the compactor mostly on the cold side of the joint. Use the static mode and then overlap the hot side by 152.4 mm (6 inch) in order to pinch the joint (28) together. You might need to build a ramp on each side of the road in order to get the compactor into position. Finish rolling should be done in the direction of paving in order to eliminate any marks left by the drums.

Reversing

Illustration 18	g00786839

Compactors are constantly changing direction during asphalt compaction. Improper reversing can cause mat defects. There are different ways to reverse the compactor so that marks are not left on the asphalt. Turn across the hot mat when you prepare to stop and reverse. Any created marks will be in the direction of the roller. The created marks will be easier to roll out on successive passes. Some mats are very stiff and the mats can tolerate a straight stop without deforming. In order to determine the proper technique for reversing, you should use various techniques. When possible, you should stay on the hot side of the mat (29). If you roll off the mat, the compactor will probably cause a bump when you pass over the joint onto the cold side. Stay on the hot mat and turn to the area that will be compacted on the next pass (30). Finally, you should avoid stopping on the hot mat. You should roll off the hot mat onto the cold side or the shoulder (31). You should find a level spot on the joint when you roll off a mat in order to prevent a bump in the mat.

Rolling of Crowns

Illustration 19	g00786844

When you compact a road with a crown or a superelevation, you should compact upward from the outside edge toward the center crown. The pattern creates strength in the mat at the outside in order to support the compactor as the compactor moves toward the center of the road. Overlap the crown by no more than 152.4 mm (6 inch) in order to avoid distorting the crown (32) .

Note: Never straddle the crown in an attempt to save a pass from a roller.

Bends or Junctions

Illustration 20	g00786883

When you are compacting a tight turn, you should roll the inner side or the lower side of the bend (33) first. You should drive in a straight line as far as possible. You should reverse the compactor until the compactor reaches the pavement that was previously compacted. Move the compactor to the next path. The process is repeated until the bend is complete. Each path is rolled in a straight line. Do not turn the compactor in order to follow the radius of the bend. This will damage the hot, uncompacted mat. Also, maintain the same rolling speed, frequency and amplitude that was established on the test strip.

Compaction of Harsh Mix

Illustration 21	g00786889

When you compact harsh mixes, you need to develop a rolling pattern that allows you to work very close to the paver in order to work on the mix while the mix has the highest temperature. Adding compaction units to the project is often done in order to increase production while the mat is still hot.

The recommended process for the compaction of harsh mixes depends on the phase of paving that is being done. A heavy pneumatic roller in the breakdown position will often produce the best results for base courses (34) or binder courses. A vibratory double drum is also very effective. A double drum vibratory compactor in the intermediate position will be able to meet the required density. The mat should be finished with a double drum compactor that is in the static mode.

The compactor sequence is different for the wear course (35). You should use a double drum vibratory compactor for breakdown. A heavy pneumatic compactor in the breakdown position may leave marks in the mat that cannot be removed. Use the pneumatic compactor in the intermediate position in order to complete compaction without acquiring tire marks. A double drum vibratory compactor with the setting of the vibratory mode on low amplitude can also be used. Again, finish the mat with a double drum in the static mode.

Compaction of Tender Mix

Illustration 22	g00786891

Tender mixes can be divided into two categories:

• tender mixes throughout the compaction process

• tender mixes in a specific zone

Superpave mixes often show a tender zone.

The recommended rolling pattern for a tender mix (36) should be in the following order: a double drum vibratory compactor in the breakdown position, a pneumatic compactor in the intermediate position and static mode for a double drum vibratory compactor in the finish position. A light pneumatic compactor could also be used for breakdown if tire marks are removed by the finish roller. A double drum vibratory compactor on low amplitude could also be used in the intermediate position.

For tender mixes in a specific zone (37), a similar pattern is recommended with slightly different machines. For breakdown, two double drums are often required in order to quickly achieve compaction. On some Superpave mixes, using a heavy pneumatic roller for breakdown has been successful. In the intermediate position, a pneumatic compactor with wide tires is recommended or not using a roller at all until the temperature changes and the softness disappears. A double drum vibratory compactor on low amplitude could also be used in the intermediate position. Finishing with one or two double drums at the end position is recommended.

Compaction of Fragile Mix

Illustration 23	g00786893

In order to avoid damaging fragile mixes (38), use a double drum vibratory compactor in the breakdown position. The vibratory setting of the compactor should be set on low amplitude. The next compactor in the series should be a heavy pneumatic compactor. Finish rolling with a double drum vibratory compactor with the vibration at half setting. Also, you could leave the compactor in static mode.

Vibratory compaction may not be possible for a mix with soft aggregate (39). Use pneumatic compactors for breakdown and intermediate passes. The pneumatic compactor should be very heavy in order to get the majority of the density during breakdown. Use a static double drum to finish the mat. Pneumatic rollers are used in the first position because pneumatic compactors will not damage the aggregate. Double drum vibratory compactors can be used for breakdown on soft aggregate if no damage occurs to the aggregate.

Compaction of Soil and Aggregate

This machine has been specifically designed for asphalt compaction projects. These operating techniques will also apply, in general, for soil compaction. The following are the main differences between soil and asphalt compaction:

• There usually is no asphalt component in the soil mix

• The soil mix temperature is generally the same as ambient conditions

• Moisture content of the soil mix significantly affects compaction

• The soil mix is generally spread using devices other than a paver so the conditions are less level and uniform

• The vibratory amplitude may not be sufficiently high for optimal compaction