compaction - asphalt

Compaction ofbituminous materials

Dipl. Ing. Hans-Josef KloubertBOMAG GmbH, Hellerwald, D-56154 Boppard

PRE 109006[Revision 07/02]

VMA/Kl-wi [Rev. 07/02]

Compaction of bituminous materials Date : 4/93Page : 1 of 31

CONTENT

1. Introduction ................................................................................................2

2. Objectives of compaction ...........................................................................3

3. Factors influencing successful compaction................................................4

4. Compaction characteristics of bituminous materials ...............................5

5. Cooling and compaction time.....................................................................9

6. Pre-compaction by paver ......................................................................... 10

7. Primary compaction with rollers ............................................................. 12

8. Compaction output ................................................................................... 18

9. Rolling techniques..................................................................................... 21

10. Literature ................................................................................................. 30



1. Introduction

The volume of traffic on our roads has been steadily increasing over the pastfew years. Increasing traffic density and higher permitted axle loads haveplaced higher demands on the standard of road construction. Cost effectivereconstruction and repair and maintenance techniques are accordinglybecoming of increasing importance.

In addition to the design properties of materials the quality and durability ofthe pavement structure is influenced by the choice of construction methods.The use of bituminous materials requires an understanding of thecompaction characteristics of the mix for good quality assurance. It is thennecessary to employ the correct type of compaction equipment for eachconstruction stage to meet the required compaction standards in the mosteconomical way. Sound compaction techniques must be employed if thedesired results are to be achieved.



2. Objectives of compaction

The laid bituminous mix is compacted to increase its density or to reducethe air voids in the material.

This results in increased stability in the material and a higher resistance todeformation. Good compaction also has a positive influence on thedurability under traffic of the wearing course. Additionally the risk offailure to water penetration (frost damage), fretting and the embrittlement ofthe binder is reduced.

At the same time the roller must produce an even riding surface to meettodays requirements for driving comfort. Moreover the wearing coursemust be sealed and uniform. It must also, however, have a surface textureproviding resistance to skidding.

A high level of density achieved during construction will reduce thepotential subsequent compaction under traffic. This is an essentialrequirement for a durable and even road surface as well as for drivingcomfort.



3. Factors influencing successful compaction

Successful compaction is determined by the type of compaction equipment,the compactibility of the mix and specific site conditions (table 1). Some ofthe decisive influences are as follow:

Type Mix Application condition

Type of roller Mineral substance Quality of the subbase

static roller pneumatic roller vibratory roller combination roller Design characteristics weight weight distribution geometry and quantity

of drums or tyres resp.

Machine parameters frequency amplitude tyre pressure rolling speed

max. grain size chip content crushed/natural sand type of filler and

content

Bituminous binder type quantity

Compactibility

Compacting temperature

stiffness roughness Weather conditions ambient temperature sunshine wind

Layer thickness

Precompaction by the paver

Number of passes

Rolling technique

Table 1: Factors influencing compaction



4. Compaction characteristics of bituminous materials

The design of the mix will vary according to the anticipated traffic volumenand local climatic conditions. Thus the compactibility will also differ frommix to mix and depends upon the nature of the aggregate, on the type andviscosity of the bitumen and on the temperature of the mix.

Bituminous mixes for roads with heavy traffic loadings are designed toprovide high resistance to deformation. These mixes are characterized bytheir bulky stone structure, i.e. a high aggregate content, coarse grain, a highproportion of crushed sand and stiff binder (Fig. 1). Due to the high internalfriction in the aggregate these mixes are highly stable following laying andare therefore less sensitive to the action of compaction equipment. Rippling,contraction or shoving of material should not occur even with the use ofheavy rollers as long as correct operating procedures are observed.

Fig. 1: Compaction characteristics of bituminous materials



Bituminous mixes used with lightly trafficked roads offer differentcompaction properties. These mixes contain a lower aggregate content, arelatively high content of natural sand and soft binder. They are compactibleand do not therefore require high compactive effort. Due to their decreasedstability in hot conditions they may, however, be sensitive to the use ofheavy compaction equipment or when vibratory compaction is appliedprematurely (Fig. 2).

Fig. 2: Influence of mix stability on rolling procedure

Rippling and shoving of materials may occur very readily. The stability ofthe mix can only be improved by the cooling which increases the adhesiveproperties of the bitumen. Filler and bitumen (bituminous mortar) have astrong influence on the stability of most bituminous mixes. The type andquality of filler and bitumen determines whether the mix will react in astable or unstable manner under the influence of rolling. Relatively smalldeviations in mix composition can cause difficulties during compaction.

It should therefore be noted that an understanding of the compactioncharacteristics of different mixes and their behaviour during rolling(together with the services of a well qualified and experienced rolleroperator) are important ingredients for success in bituminous compactionoperation.



The influences of aggregate composition on resistance to deformation,which also serves as an indicator of compactibility, is illustrated in Fig. 3 forreadily compactible and difficult to compact mix designs.

Fig. 3: The influence of air voids on resistance to deformation

The temperature of the mix during the compaction process is of the utmostimportance for the given compaction effort. With high mix temperatures thecompaction produced by the use of rollers is assisted by the low viscosity ofthe bitumen. The bitumen acts as a lubricant and reduces the internal frictionof the aggregate / filler mix.

Due to the increasing stiffness of the bitumen during cooling the requiredcompaction effort increases significantly at low temperatures. In addition tothe internal friction of the aggregate/filler mix it is now necessary toovercome the adhesion of the bitumen.

Compaction should therefore generally be commenced as early as possible(Fig. 4).

Compaction temperatures of 100-140 degrees C have been found to be mostsuitable. Compaction should be completed at temperatures between 80 and100 degrees C.



Fig. 4: The influence of temperature on compaction

When using harder types of bitumen, compaction should be commendednear the upper temperature limit. Difficult to compact mixes require preciseobservation temperature since compaction is sometimes unobtainable at lowtemperatures even with a high compaction effort.



5. Cooling and compaction time

The time available for compaction depends upon speed at which the asphaltlayer cools after laying. It is determined by the layer thickness, weatherconditions, application temperature and the minimum specified rollingtemperature.

Further loss of temperature in the layer is caused by heat exchange with thesubbase. Additional temperature loss on the surface is caused by theevaporation of rain water or water from the sprinkler system of the roller.

The thickness of the layer is also very important. The thinner the layer, thefaster the material will cool due to its low heat retention. The compaction ofthin asphalt layers at ambient temperatures below 10 degrees C will veryoften need to be completed after just a few minutes.

With a thick layer at summer ambient temperatures, it may be necessary todelay compaction to avoid difficulties such as rippling or displacement ofthe material.

Strong wind reduces the compaction time considerably whereas highambient temperatures and sunny weather extends the available time (Fig. 5).



Cooling curve of a 4 cm bituminous layer at different conditions:

1. Summer afternoon, air temperature 22 C,road surface temperature 35 C

2. Fall morning, air temperature 2 C,road surface temperature 2 C

Fig. 5: Cooling curves for different weather conditions

Available compaction time for a bituminous material containing 200 penbinder at an ambient temperature of 0 degrees C in relation to the layingtemperature LT and the layer thickness.

Bituminous material containing 200 pen binderAmbient temperature 0 CLaying temperature T1 = 160 C, T2 = 130 C

Fig. 6: Compaction time as a function of the layer thickness at variouslaying temperatures



6. Pre-compaction by paver

The level of pre-compaction is an important factor in the selection of theroller to be used immediately behind the paver. Low paver pre-compactionrequires the use of a lightweight roller to pre-compact the material. Heavyrollers tend to have a negative effect on the eveness of the layer anddepending upon the stability of the hot mix material, could causedisplacement of the material. In such cases, tandem vibratory rollers shouldmake the first two passes without vibration.

High pre-compaction by the paver plays an important part in establishinguniform thickness of layer and provides the possibility of early compactionat high mix temperatures. This assists the compaction effect of the rollersand final compaction can be achieved in fewer passes. High pre-compactionby the paver very often makes the use of a lightweight roller for pre-compaction purposes unnecessary.

High pre-compaction is therefore desirable and the development of the highcompaction screed is to be welcomed. However, the sole use of this screedcan assure full compaction only in rare instances such as on straight runs ofeasy to compact material. In most instances final compaction can only beachieved by using rollers with high compacting capabilities following onfrom the use of high compaction screeds.



7. Primary compaction with rollers

The specified level of compaction can seldom be achieved only withprecompaction by the paver. In most cases additional compaction withrollers is necessary whereby primary compaction is obtained with the use ofeither static rollers, pneumatic tyred rollers, tandem vibratory rollers orcombination rollers.

Static rollers

Static rollers can be designed as three-wheel rollers or tandem rollers.Three-wheel rollers are provided with two bigger driven rear wheels andone smaller non-driven front wheel. The operating weight of three-wheelrollers is approximately 4-16 tonnes.

Tandem rollers are fitted with two drums of almost similar size. On most ofthese machines only one drum is driven. The operating weight of theserollers is between 1 and 12 tonnes.

The compaction effect of static rollers is based on the influence of theirweight thereby inducing a predominantly vertical pressure onto the layer tobe compacted. This pressure overcomes the internal friction in the mix andproduces a higher density. The vertical pressure of a steel drum changeswith increasing compaction and cooling of the layer. At the commencementof the compaction process the drum sinks slightly into the material so that abigger section of the drum circumference is in contact with the surface. Thisresults initially in a contact area of greater size and accordingly produces alower surface pressure. In the center of the contact area the pressure ishigher than at the periphery. The surface pressure increases compaction orcooling of the layer as the penetration of the drum into the layer reduces.The vertical pressure of the roller drum increases continuously throughoutthe rolling process. This is a benefit in compaction terms but it is also thereason that the surface pressure cannot be specified for a smooth drum. Asthe vertical pressure depends upon the static linear load, the latter is acommonly used indicator of the compaction potential of a static roller.

The effectiveness at depth of a dead-weight roller is relatively low and isnormally efficient to approximately 10 cm. The surface smoothnessproduced by these rollers is generally good. The working speed is up to5 km/h.



Three-wheel rollers are suitable for pre-compaction and primarycompaction, for compaction of joints, for coated chipping work and forsurfaces rolling and finishing off. Heavy dead-weight tandem rollers areused predominantly for primary compaction duties whereas light tandemrollers are mainly used for pre-compaction and finishing work.

All tandem vibratory rollers can obviously be used without vibration asstatic rollers.

Pneumatic tyres rollers

The operating weight of pneumatic tyred rollers lies between 5 and 25tonnes (Fig. 7). Depending on their weight they are fitted with 5-11 smoothtyres which are capable of oscillation.

The compaction effect is a function of the deadweight of the machine anddepends upon the wheel load, tyre pressure and the rolling speed. Therubber tyres flatten on contact with the surface thereby producing lowsurface pressure. Crushing or damage to aggregates near the surface isvirtually eliminated even with the use of heavy pneumatic tyred rollers.Despite this low surface pressure effective compaction is achieved usingPTRs due to the combination of vertical pressure and horizontal forcesbelow the tyre which are directed multilaterally. The combination of forcesproduced by the PTR produce on a number of material types a denseraggregate structure than the predominantly vertical forces generated by astatic smooth drum roller.

The resulting compaction is confined mainly the upper part of thebituminous layer. The penetration effect can be increased by ballasting thePTR and increasing the tyre pressure. The tyre pressure must be adjusted sothat the wheel produces even surface contact over the entire width.



Fig. 7: 24 t Pneumatic tyred roller

The combined action of vertical and horizontal forces causes a flexing andkneading effect resulting in excellent sealing of the surface. Stress fracturescaused by other rollers can be closed by the use of pneumatic tyred rollers.Pneumatic tyred rollers have advantages over rollers with smooth drumswhen used on steep inclines due to the good adhesion between rubber tyresand the mix material.

For the compaction of thick layers the rolling speed should lie between 3and 4 km/h. On thin layers and for surfacing sealing the rolling speed maybe increased to 6 to 10 km/h.

Pneumatic tyred rollers are primarily used for pre-compaction and only veryrarely for primary compaction. When sealing high bitumen content roadsurfaces at summer temperatures, too many passes can cause a concentrationof bitumen on the surface which leads to an initial lack of skid resistance.

Tandem vibratory rollers

The introduction of vibratory rollers has led to a reduction in the use ofstatic rollers in the compaction of bituminous materials due to the greaterinfluence of vibratory compactors. The majority of modern tandemvibratory rollers with an operating weight between 1 and 12 t are equippedwith hydrostatic travel and vibration drive to both drums (Fig. 9).Theserollers are provided with either articulated or pivot steering.



Fig. 8: Medium weight tandem vibratory roller

The compaction effect of vibratory rollers is a function of the influence ofvibration on the material to be compacted. The vibration reduces the internalfriction in the material mix so that the combined action of basic weight anddynamic load increases the density. Static linear load, vibrating mass andfrequency and amplitude are the main determinants of the compactionresult.

For the varying compaction requirements of different layer thicknesses thelarger size tandem vibratory rollers are usually equipped with twoamplitudes and two frequencies. For high compaction output on thin layersor easy to compact mixes a low amplitude and high frequency (higher than45 Hz) should be selected. A high amplitude and low frequency setting isrecommended for thick layers and difficult to compact mixes.

The rolling speed should be 3 to 6 km/h on thin layers and 2 to 4 km/h onthick layers. When stopping or reversing automatic vibration controlswitches off the vibration in time to avoid the formation of ruts or theshoving of material.

Comparison with other types of roller show that tandem vibratory rollerscan achieve the required degree of compaction after fewer passes especiallyon difficult to compact mixes. Better surface sealing is achieved by makingthe last two passes without vibration.



Too many passes, however, can have an adverse effect on density. This candecrease stability and will probably cause loosening of aggregates and of thematerial structure. Failures of this nature can also be caused by the use ofvibratory compaction on cool or already cold layers.

These risks should be taken into account when working on materials such ashigh stone content hot rolled asphalt layers or previous macadam. Due to thefrequent presence of stabilizing additives, difficult to compact high stabilityasphalts should be compacted with heavy vibratory rollers at hightemperatures. To avoid loosening of material and fatting up of binder at thesurface, the layer should only be compacted at low amplitude and a limitednumber of vibrating passes applied. Where previous macadam is to becompacted excessive vibratory compaction can lead to a reduction in designair voids. The number of vibrating passes should also therefore bemoderated or lighter rollers employed (Table 2).

Tandem vibratory rollers are powerful compactors which are used for theprimary compaction of road bases, base courses and wearing courses.Where all drum drive is available operation on gradients is practical.

Combination rollers

These rollers are a combination of pneumatic tyred rollers and vibratoryrollers consisting of one smooth drum and, in most cases, 4 smooth rubbertyres (Fig. 10). They are manufactured in the operating weight range of 2-18tonnes.

Compaction is achieved by the vibration of the roller drum. Itscharacteristics are similar to those of drums on vibratory rollers. Thecompaction effect of the rubber tyres is relatively low. As with pneumatictyred rollers, the tyre pressure is adjusted so that the tyre contacts thebituminous layer evenly over the entire width.

Combination rollers combine the high compaction performance of vibratoryrollers and the kneading and flexing effect of pneumatic tyred rollers.Combination rollers can be used on the same applications as tandemvibratory rollers but with the advantages of increased gradeability, betterbonding of layers and improved surface sealing.



Fig. 9: Combination roller with an operating weight of 9.2 t



8. Compaction output

For bituminous road bases the logistics of mixing - transport - laying -compaction - must be carefully co-ordinated to ensure a continuous supplyof material which is necessary to achieve a high quality pavement. Planningof compaction procedures requires knowledge of the compaction outputrequired, selection of machine type number of rollers necessary and rollingpatterns to be employed etc.

Compaction output

For the compaction of bituminous materials output is expressed in areaterms as m/h or in volume terms as t/h and calculated by means of thefollowing formulae:

b x v x 1000Area output: QA = f x z [m/h]

b x v x h x 1000Volume output: Qv = f x z x A [t/h]

Where:

f = Efficiency factor

The efficiency factor is the ratio of actual average output and theoreticbasic output. The efficiency factor considers all significant influencingfactors arising from the condition and operation of the machine, theorganization and condition of the site as well as the weatherconditions. In asphalt construction an efficiency factor of f = 60 isusually employed for calculation purposes.

b = Working width of the compaction machine in m

v = Working speed of the compaction machine in km/h

z = Number of passesThe number of passes can vary considerably. It depends primarily onthe compactibility of the mix, the precompaction by the paver, thetemperature of the mix during the compaction, the thickness and typeof layer and the characteristics of the roller (Table 2.)



Experience shows that static rollers and pneumatic tyred rollers need 8to 12 passes, whereas tandem rollers only need 4 to 8 passes toachieve the required compaction. Combination rollers provide asimilar compaction output as tandem vibratory rollers.

h = Layer thickness of the material to be compacted in m

A = Density of the compacted asphalt mix in t/m

Thickness of asphaltlayer d (cm)

Number of vibratory passes of varioustandem vibratory rollers

3 t 6 t 9 t

2

4

6

10

14

18

2 - 4

4 - 6

4 - 8

6 - 8

--

--

1 - 2 (K)

2 - 4 (K)

4 - 6 (K)

4 - 8 (K, G)

6 - 8 (G)

6 - 8 (G)

1 - 2 (L)

2 - 4 (L)

2 - 4 (L)

4 - 6 (L, H)

4 - 6 (H)

4 - 8 (H)

Chip mastic d = 2d = 4

Previous coatedmacadam d = 4

----

--

1 - 2 (L) + stat. passes4 - 6 (L) + stat. passes

1 - 2 (L) + stat. passes

1 - 2 (L) + stat. passes4 - 6 (L) + stat. passes

1 - 2 (L) + stat. passes

L = low amplitude; H = high amplitude; Assumption: Compaction temperature > 100 C3 t = Machine with only the amplitude

Table 2: Reference values for the number of passes with tandem vibratoryrollers

The indications about the average compaction output of tandem vibratoryrollers and combination rollers in the tables 3 and 4 can be used asguidelines for choosing the right machines.



Roller type / Area outputoperating weight CECE Productivity [m/h] at specified layer thickness(including ROPS + cabin)

t Wearing course2 - 4 cm

Binder course6 - 8 cm

Base course10 - 14 cm

BW 80 AD-2 1.5 250 350 200 250 170 200BW 80 ADH-2 1.6 250 350 200 250 170 200BW 90 AD-2 1.5 250 400 210 280 200 250BW 100 ADM-2 1.6 300 500 220 300 220 280BW 100 AD-3 2.4 300 500 250 300 250 300BW 120 AD-3 2.7 350 600 250 350 250 350BW 125 ADH 3.4 350 600 270 350 270 350BW 135 AD 3.6 500 800 320 450 300 380BW 138 AD 4.2 500 800 320 500 300 400BW 141 AD-2 6.9 800 1200 500 700 400 500BW 144* AD-AM 8.1 800 1400 500 800 400 600BW 151 AD-2 7.3 1000 1500 600 800 500 600BW 154* AD**-AM 8.5 1000 1600 600 900 500 700BW 161 AD-2 9.7 1200 1700 700 900 600 700BW 170 AD** 8.5 1100 1700 600 850 500 650BW 174* AD** 9.0 1100 1700 600 850 500 650BW 170 AD**-AM 9.0 1100 1800 600 900 500 700BW 174* AD-AM 9.6 1100 1800 600 900 500 700BW 202 AD-2 10.7 1400 2200 800 1000 700 900BW 180 AD** 11.9 1300 1800 800 1000 600 850BW 184* AD** 12.9 1300 1800 800 1000 600 850BW 184* AD**-AM 12.9 1300 2000 800 1100 700 1000

BW 90 AC-2 1.7 250 350 200 250 170 200BW 100 AC-3 2.3 250 400 220 300 200 250BW 120 AC-3 2.5 300 500 250 350 220 280BW 138 AC 4.0 500 800 320 500 250 370BW 151 AC-2 7.0 950 1400 550 700 450 550BW 161 AC-2 9.4 1100 1500 600 800 550 650BW 174* AC** 8.6 1000 1500 600 800 500 700BW 174* AC**-AM 9.0 1000 1600 600 900 500 700

AD = Tandem vibratory roller * = split drumsAC = Combination roller ** = pivot steeredAM = Asphalt Manager

Table 8: Average output (m/h) of tandem vibratory and combination rollers



Roller type / Volume outputoperating weight CECE Productivity [t/h] at specified layer thickness(including ROPS + cabin)

t Wearing course2 - 4 cm

Binder course6 - 8 cm

Base course10 - 14 cm

BW 80 AD-2 1.5 10 30 25 45 35 70BW 80 ADH-2 1.6 10 30 25 45 35 70BW 90 AD-2 1.5 15 30 30 50 40 80BW 100 ADM-2 1.6 15 40 35 60 50 90BW 100 AD-3 2.4 15 40 40 60 60 100BW 120 AD-3 2.7 20 45 40 65 70 110BW 125 ADH 3.4 20 45 40 65 70 110BW 135 AD 3.6 30 55 50 85 75 130BW 138 AD 4.2 30 55 50 90 75 135BW 141 AD-2 6.9 35 65 70 130 100 170BW 144* AD-AM 8.1 35 70 70 150 100 180BW 151 AD-2 7.3 40 75 80 150 120 190BW 154* AD**-AM 8.5 40 80 80 170 120 200BW 161 AD-2 9.7 50 90 100 180 150 210BW 170 AD** 8.5 40 90 90 165 130 190BW 174* AD** 9.0 40 90 90 165 130 190BW 170 AD**-AM 9.0 50 100 90 180 140 210BW 174* AD-AM 9.6 50 110 90 180 140 210BW 202 AD-2 10.7 70 145 120 250 190 320BW 180 AD** 11.9 65 120 110 210 190 300BW 184* AD** 12.9 65 120 110 210 190 300BW 184* AD**-AM 12.9 65 130 110 230 190 320

BW 90 AC-2 1.7 10 30 25 40 40 - 60BW 100 AC-3 2.3 15 35 35 40 45 90BW 120 AC-3 2.5 20 40 40 60 55 105BW 138 AC 4.0 30 55 50 90 65 115BW 151 AC-2 7.0 30 60 60 120 90 155BW 161 AC-2 9.4 40 80 90 160 135 190BW 174* AC** 8.6 40 80 90 160 130 185BW 174* AC**-AM 9.0 50 90 100 170 140 200

AD = Tandem vibratory roller * = split drumsAC = Combination roller ** = pivot steeredAM = Asphalt Manager

Table 9: Average output (t/h) of tandem vibratory and combination rollers



9. Rolling techniques

At site level the compaction result is determined by the experience and skillof the roller operator and the rolling pattern he chooses. Here are some basicrules for compaction work and rolling patterns which have been shown to beuseful.

Basic rules for compaction

1. Start compaction as early as possible. This also applies when usingheavy rollers immediately behind the paver. The eveness of the layerproduced by the paver must, however, not be jeopardized.

2. Drive with the driven drum towards the paver to avoid the formation ofriples and cracks. Combination rollers should be used with the rubbertyres towards the paver.

Exception: When working on steep gradients, the driven drum shouldface downhill so that the high shear forces induced by the drum will beabsorbed by the pre-compacted layer without disturbance to thematerial. When working downhill, the driven drum should trail uphill.This problem does not arise when using modern rollers with all drumdrive.

3. To avoid mix material sticking, drums and tyres must be lightly sprayedwith water. They should be moist but not wet.

The water is evaporated by the hot mix which loses heat. This reducesthe time available for compaction. Sprinkler systems with intervalcontrol switches reduce the amount of water and additives in the water.

4. Drive smoothly and do not change direction in jerky movements. Theuse of an automatic speed control system can improve the rollingquality.

5. Do not use vibration when the machine is at standstill. This will avoidthe formation of waves.

6. Switch the vibration on only when the machine is moving. Whenreversing, stop the vibration in good time before the machine comes toa halt (or use an automatic vibration control).



7. If the road is cambered start compaction from the lower edge andoverlap each pass towards the higher edge. In this way the compactedmix works as a support for the machine.

8. Steer and offset the machine only on compacted material to avoidshoving of material.

9. Never stop the roller on hot mix material since the machine willprobably cause deformation of the layer.

10. Always park the machine at an oblique angle to the direction of work sothat any marks can be smoothed out later.

ROLLING PATTERN

As a general rule all rolling lanes should overlap by at least 15 cm intransverse direction so that no uncompacted strips will remain. The numberof passes should be identical for all lanes to achieve a uniform compactionover the entire width of the mat.

The roller must stay in lane until it reaches the cool and stable area where itcan manoeuvre (Fig. 10). If the road has a kerb, compaction should start atthe outer edge.

1 2

3

5

7 8

6

4

Fig. 10 Rolling pattern



If the road is unsupported with no kerb and the layers (base and wearingcourses) are thick, there is a risk that the roller will push the materialoutwards when starting to compact at the edge. To avoid this, a strip of 30-40 cm should be left uncompacted on each side to allow the material to coolin order to provide a stable support for the roller (Fig. 11).

1

2

3

4

5

Fig. 11 Rolling pattern where there is a risk of lateral displacement ofmaterial



When paving in echelon, rollers should compact from the outer edgestowards the middle leaving a 30 - 40 cm strip uncompacted at the middlejoint. This joint is compacted last with a roller to produce a tight bondbetween both mats as shown in fig. 12.

1

1

3

2

34

2

Fig. 12 Rolling pattern for echelon paving

Joints must be compacted to be closed and level. This requires specialattention by an experienced roller operator.

Longitudinal joints (hot on cold) can be compacted in two different ways.Compaction can be started along the joint whereby the drum covers only10 - 20 cm of the new layer whilst rest of the drum remains compacted andcool asphalt (Fig. 13). The subsequent rolling lanes must then be offset fromthe outer edge towards the longitudinal joint.



2

4

3

1

Fig. 13 Rolling pattern for longitudinal joint hot on cold

The longitudinal joint can also be compacted by rolling with 10-20 cm onthe compacted layer with the rest of the drum on the uncompacted layer(Fig. 14).

1

2

3

Fig. 14 Alternative rolling pattern for longitudinal joint hot on cold



Lateral joints should be rolled at right angles to the paving direction ifpossible. The roller must therefore drive with 10 - 20 cm of the drum on thehot and uncompacted material. The overlap should be gradually increaseduntil the roller is working completely on the new material (Fig. 15).

If the room to manoeuvre the machine is very limited, it is advantageous touse a small and manoeuvrable roller. If the site situation does not permit anymanoeuvring the transverse joint may be rolled at an oblique angle. Thejoint itself may even be cut at an oblique angle to the paving direction.

10 20 cm

hei kalt

Fig. 15 Compacting a transverse joint

Around bends, compaction should be started at the lower inner side of thecurve by driving straight ahead as far as possible before starting to cut thecorner. The roller must be offset at a tangent to previously compactedmaterial. The steering speed must be adapted to the rolling speed, i.e. whendriving slowly the steering movement must also be slow. This reduces theshear forces which are created when rolling around a bend.

The use of rollers with split drums and crab steering to both sides isrecommended.

coldhot



23

4

5

1

Fig. 16 Compacting on bends

The split drum halves the shear forces which are created between drum andmaterial when rolling on bends and reduces the risk of cracks near the outeredge of the rolling lane. When compacting the edge of the road surface thecrab steering offers the facility to offset one drum away from the kerb of theroad surface so that the operator can monitor one drum with ease. This is aparticular benefit when working on bends or moving away from kerbs(Fig. 16).



ROLLING CRACKS AND THEIR CAUSES

Compaction with rollers can lead to the development of longitudinal andtransverse cracks as well as to displacement of material. These cracks can bethe result of several factors and are therefore very difficult to diagnose.

Transverse cracksTransverse cracks are usually shallow. Possible causes are:

The roller pushes a bow wave ahead (pre-compaction by the paver toolow, application too early of rollers which are too heavy, the drivendrum of the roller does not face towards the paver).

Delayed rolling after laying thick layers (the surface has cooled down,the core is still hot and the roller sinks into the surface).

Layer slippage: the roller displaces subbase material (subbasecontaminated or inadequately tack coated).

Surface saturation (rain or excessive water sprinkling).

Compaction of thick layers on gradients (the shear forces of the rollercannot be absorbed).

Poor mix composition (e.g. high proportion of poorly graded naturalsand and a low bitumen content).

Over-compaction.

Segregation of aggregates in mix.

Longitudinal cracks

Longitudinal cracks tend to run through the entire depth of the layer.They are generally caused by:

Defects in the subbase.

Shearing of the mix under heavy rolling (on thick layers, a heavy rollermust delay operations until surface cools: temperature sandwich occursand mix then shears on hot core during rolling).

Inadequate pre-compaction.

Over-compaction.

Unstable mix composition (especially with high sand content).

Temperature too high.

Poor layer bonding.

Binder content too high.

Segregation of mix.



Displacement of materialThe most common reasons for displacement of material are:

Excessive delay before commencing compaction. Temperaturesandwich develops (cool surface skin, hot core). Roller penetratessurface displacing hot material in direction of travel.

Roller too heavy.



LITERATURE

[1] Dbner, R.:Asphalt road construction - application and compaction ofasphalt mix, Arbit-Schriftenreihe Bitumen, Heft 31, 1982

[2] Henrich, H.:Handbook of asphalt technology for road construction,technical publication of Bomag-Menck GmbH, 1982

[3] Kirschner, R. and Kloubert, H.-J.:Vibratory compaction in earth and asphalt construction,technical publication of Bomag-Menck GmbH, 1988

[4] Kirschner, R. and Kloubert, H.-J.:Compaction with combination rollers, BOMAG-test reports(not published), 1987

compaction - asphalt

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