Compaction - Asphalt

Compaction of bituminous materials

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

PRE 109006 [Revision 07/02]


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4/93

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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

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


1. Introduction The volume of traffic on our roads has been steadily increasing over the past few years. Increasing traffic density and higher permitted axle loads have placed higher demands on the standard of road construction. Cost effective reconstruction and repair and maintenance techniques are accordingly becoming of increasing importance. In addition to the design properties of materials the quality and durability of the pavement structure is influenced by the choice of construction methods. The use of bituminous materials requires an understanding of the compaction characteristics of the mix for good quality assurance. It is then necessary to employ the correct type of compaction equipment for each construction stage to meet the required compaction standards in the most economical way. Sound compaction techniques must be employed if the desired results are to be achieved.


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2. Objectives of compaction The laid bituminous mix is compacted to increase its density or to reduce the air voids in the material. This results in increased stability in the material and a higher resistance to deformation. Good compaction also has a positive influence on the durability under traffic of the wearing course. Additionally the risk of failure to water penetration (frost damage), fretting and the embrittlement of the binder is reduced. At the same time the roller must produce an even riding surface to meet today’s requirements for driving comfort. Moreover the wearing course must be sealed and uniform. It must also, however, have a surface texture providing resistance to skidding. A high level of density achieved during construction will reduce the potential subsequent compaction under traffic. This is an essential requirement for a durable and even road surface as well as for driving comfort.


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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 of the decisive influences are as follow: Type

Mix

Application condition

Type of roller

Mineral substance

Quality of the subbase

− − − −

− − − −

− stiffness − roughness

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

Table 1:


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

Bituminous binder − type − quantity

Weather conditions − ambient temperature − sunshine − wind Layer thickness

Compactibility

Precompaction by the paver

Compacting temperature

Number of passes

Factors influencing compaction

Rolling technique


4. Compaction characteristics of bituminous materials The design of the mix will vary according to the anticipated traffic volumen and local climatic conditions. Thus the compactibility will also differ from mix to mix and depends upon the nature of the aggregate, on the type and viscosity of the bitumen and on the temperature of the mix. Bituminous mixes for roads with heavy traffic loadings are designed to provide high resistance to deformation. These mixes are characterized by their bulky stone structure, i.e. a high aggregate content, coarse grain, a high proportion of crushed sand and stiff binder (Fig. 1). Due to the high internal friction in the aggregate these mixes are highly stable following laying and are therefore less sensitive to the action of compaction equipment. Rippling, contraction or shoving of material should not occur even with the use of heavy rollers as long as correct operating procedures are observed.

Fig. 1: Compaction characteristics of bituminous materials


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Bituminous mixes used with lightly trafficked roads offer different compaction properties. These mixes contain a lower aggregate content, a relatively high content of natural sand and soft binder. They are compactible and do not therefore require high compactive effort. Due to their decreased stability in hot conditions they may, however, be sensitive to the use of heavy compaction equipment or when vibratory compaction is applied prematurely (Fig. 2).

Fig. 2: Influence of mix stability on rolling procedure

Rippling and shoving of materials may occur very readily. The stability of the mix can only be improved by the cooling which increases the adhesive properties of the bitumen. Filler and bitumen (bituminous mortar) have a strong influence on the stability of most bituminous mixes. The type and quality of filler and bitumen determines whether the mix will react in a stable or unstable manner under the influence of rolling. Relatively small deviations in mix composition can cause difficulties during compaction. It should therefore be noted that an understanding of the compaction characteristics of different mixes and their behaviour during rolling (together with the services of a well qualified and experienced roller operator) are important ingredients for success in bituminous compaction operation.


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The influences of aggregate composition on resistance to deformation, which also serves as an indicator of compactibility, is illustrated in Fig. 3 for readily 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 utmost importance for the given compaction effort. With high mix temperatures the compaction produced by the use of rollers is assisted by the low viscosity of the bitumen. The bitumen acts as a lubricant and reduces the internal friction of the aggregate / filler mix. Due to the increasing stiffness of the bitumen during cooling the required compaction effort increases significantly at low temperatures. In addition to the internal friction of the aggregate/filler mix it is now necessary to overcome 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 most suitable. Compaction should be completed at temperatures between 80 and 100 degrees C.


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Fig. 4: The influence of temperature on compaction

When using harder types of bitumen, compaction should be commended near the upper temperature limit. Difficult to compact mixes require precise observation temperature since compaction is sometimes unobtainable at low temperatures even with a high compaction effort.


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5. Cooling and compaction time The time available for compaction depends upon speed at which the asphalt layer cools after laying. It is determined by the layer thickness, weather conditions, application temperature and the minimum specified rolling temperature. Further loss of temperature in the layer is caused by heat exchange with the subbase. Additional temperature loss on the surface is caused by the evaporation 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, the faster the material will cool due to its low heat retention. The compaction of thin asphalt layers at ambient temperatures below 10 degrees C will very often need to be completed after just a few minutes. With a thick layer at summer ambient temperatures, it may be necessary to delay compaction to avoid difficulties such as rippling or displacement of the material. Strong wind reduces the compaction time considerably whereas high ambient temperatures and sunny weather extends the available time (Fig. 5).


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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 pen binder at an ambient temperature of 0 degrees C in relation to the laying temperature LT and the layer thickness. Bituminous material containing 200 pen binder Ambient temperature 0 °C Laying temperature T1 = 160 °C, T2 = 130 °C

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


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6. Pre-compaction by paver The level of pre-compaction is an important factor in the selection of the roller to be used immediately behind the paver. Low paver pre-compaction requires the use of a lightweight roller to pre-compact the material. Heavy rollers tend to have a negative effect on the eveness of the layer and depending upon the stability of the hot mix material, could cause displacement of the material. In such cases, tandem vibratory rollers should make the first two passes without vibration. High pre-compaction by the paver plays an important part in establishing uniform thickness of layer and provides the possibility of early compaction at high mix temperatures. This assists the compaction effect of the rollers and final compaction can be achieved in fewer passes. High pre-compaction by the paver very often makes the use of a lightweight roller for precompaction purposes unnecessary. High pre-compaction is therefore desirable and the development of the high compaction screed is to be welcomed. However, the sole use of this screed can assure full compaction only in rare instances such as on straight runs of easy to compact material. In most instances final compaction can only be achieved by using rollers with high compacting capabilities following on from the use of high compaction screeds.


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7. Primary compaction with rollers The specified level of compaction can seldom be achieved only with precompaction by the paver. In most cases additional compaction with rollers is necessary whereby primary compaction is obtained with the use of either static rollers, pneumatic tyred rollers, tandem vibratory rollers or combination 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 and one smaller non-driven front wheel. The operating weight of three-wheel rollers is approximately 4-16 tonnes. Tandem rollers are fitted with two drums of almost similar size. On most of these machines only one drum is driven. The operating weight of these rollers is between 1 and 12 tonnes. The compaction effect of static rollers is based on the influence of their weight thereby inducing a predominantly vertical pressure onto the layer to be compacted. This pressure overcomes the internal friction in the mix and produces a higher density. The vertical pressure of a steel drum changes with increasing compaction and cooling of the layer. At the commencement of the compaction process the drum sinks slightly into the material so that a bigger section of the drum circumference is in contact with the surface. This results initially in a contact area of greater size and accordingly produces a lower surface pressure. In the center of the contact area the pressure is higher than at the periphery. The surface pressure increases compaction or cooling of the layer as the penetration of the drum into the layer reduces. The vertical pressure of the roller drum increases continuously throughout the rolling process. This is a benefit in compaction terms but it is also the reason that the surface pressure cannot be specified for a smooth drum. As the vertical pressure depends upon the static linear load, the latter is a commonly used indicator of the compaction potential of a static roller. The effectiveness at depth of a dead-weight roller is relatively low and is normally efficient to approximately 10 cm. The surface smoothness produced by these rollers is generally good. The working speed is up to 5 km/h.


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Three-wheel rollers are suitable for pre-compaction and primary compaction, for compaction of joints, for coated chipping work and for surfaces rolling and finishing off. Heavy dead-weight tandem rollers are used predominantly for primary compaction duties whereas light tandem rollers are mainly used for pre-compaction and finishing work. All tandem vibratory rollers can obviously be used without vibration as static rollers.

Pneumatic tyres rollers The operating weight of pneumatic tyred rollers lies between 5 and 25 tonnes (Fig. 7). Depending on their weight they are fitted with 5-11 smooth tyres which are capable of oscillation. The compaction effect is a function of the deadweight of the machine and depends upon the wheel load, tyre pressure and the rolling speed. The rubber tyres flatten on contact with the surface thereby producing low surface pressure. Crushing or damage to aggregates near the surface is virtually eliminated even with the use of heavy pneumatic tyred rollers. Despite this low surface pressure effective compaction is achieved using PTRs due to the combination of vertical pressure and horizontal forces below the tyre which are directed multilaterally. The combination of forces produced by the PTR produce on a number of material types a denser aggregate structure than the predominantly vertical forces generated by a static smooth drum roller. The resulting compaction is confined mainly the upper part of the bituminous layer. The penetration effect can be increased by ballasting the PTR and increasing the tyre pressure. The tyre pressure must be adjusted so that the wheel produces even surface contact over the entire width.


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Fig. 7: 24 t Pneumatic tyred roller

The combined action of vertical and horizontal forces causes a flexing and kneading effect resulting in excellent sealing of the surface. Stress fractures caused by other rollers can be closed by the use of pneumatic tyred rollers. Pneumatic tyred rollers have advantages over rollers with smooth drums when used on steep inclines due to the good adhesion between rubber tyres and the mix material. For the compaction of thick layers the rolling speed should lie between 3 and 4 km/h. On thin layers and for surfacing sealing the rolling speed may be increased to 6 to 10 km/h. Pneumatic tyred rollers are primarily used for pre-compaction and only very rarely for primary compaction. When sealing high bitumen content road surfaces at summer temperatures, too many passes can cause a concentration of 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 of static rollers in the compaction of bituminous materials due to the greater influence of vibratory compactors. The majority of modern tandem vibratory rollers with an operating weight between 1 and 12 t are equipped with hydrostatic travel and vibration drive to both drums (Fig. 9).These rollers are provided with either articulated or pivot steering.


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Fig. 8: Medium weight tandem vibratory roller

The compaction effect of vibratory rollers is a function of the influence of vibration on the material to be compacted. The vibration reduces the internal friction in the material mix so that the combined action of basic weight and dynamic load increases the density. Static linear load, vibrating mass and frequency and amplitude are the main determinants of the compaction result. For the varying compaction requirements of different layer thicknesses the larger size tandem vibratory rollers are usually equipped with two amplitudes and two frequencies. For high compaction output on thin layers or easy to compact mixes a low amplitude and high frequency (higher than 45 Hz) should be selected. A high amplitude and low frequency setting is recommended 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 on thick layers. When stopping or reversing automatic vibration control switches off the vibration in time to avoid the formation of ruts or the shoving of material. Comparison with other types of roller show that tandem vibratory rollers can achieve the required degree of compaction after fewer passes especially on difficult to compact mixes. Better surface sealing is achieved by making the last two passes without vibration.


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Too many passes, however, can have an adverse effect on density. This can decrease stability and will probably cause loosening of aggregates and of the material structure. Failures of this nature can also be caused by the use of vibratory compaction on cool or already cold layers. These risks should be taken into account when working on materials such as high stone content hot rolled asphalt layers or previous macadam. Due to the frequent presence of stabilizing additives, difficult to compact high stability asphalts should be compacted with heavy vibratory rollers at high temperatures. To avoid loosening of material and fatting up of binder at the surface, the layer should only be compacted at low amplitude and a limited number of vibrating passes applied. Where previous macadam is to be compacted excessive vibratory compaction can lead to a reduction in design air voids. The number of vibrating passes should also therefore be moderated or lighter rollers employed (Table 2). Tandem vibratory rollers are powerful compactors which are used for the primary 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 vibratory rollers consisting of one smooth drum and, in most cases, 4 smooth rubber tyres (Fig. 10). They are manufactured in the operating weight range of 2-18 tonnes. Compaction is achieved by the vibration of the roller drum. Its characteristics are similar to those of drums on vibratory rollers. The compaction effect of the rubber tyres is relatively low. As with pneumatic tyred rollers, the tyre pressure is adjusted so that the tyre contacts the bituminous layer evenly over the entire width. Combination rollers combine the high compaction performance of vibratory rollers and the kneading and flexing effect of pneumatic tyred rollers. Combination rollers can be used on the same applications as tandem vibratory rollers but with the advantages of increased gradeability, better bonding of layers and improved surface sealing.


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


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8. Compaction output For bituminous road bases the logistics of mixing - transport - laying compaction - must be carefully co-ordinated to ensure a continuous supply of material which is necessary to achieve a high quality pavement. Planning of compaction procedures requires knowledge of the compaction output required, selection of machine type number of rollers necessary and rolling patterns to be employed etc. Compaction output For the compaction of bituminous materials output is expressed in area terms as m²/h or in volume terms as t/h and calculated by means of the following formulae: Area output:

QA = f x

Volume output:

Qv = f

x

b x v x 1000 z b x v x h x 1000 z

[m²/h]

x

ζA [t/h]

Where: f = Efficiency factor The efficiency factor is the ratio of actual average output and theoretic basic output. The efficiency factor considers all significant influencing factors arising from the condition and operation of the machine, the organization and condition of the site as well as the weather conditions. In asphalt construction an efficiency factor of f = 60 is usually 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 passes The number of passes can vary considerably. It depends primarily on the compactibility of the mix, the precompaction by the paver, the temperature of the mix during the compaction, the thickness and type of layer and the characteristics of the roller (Table 2.)


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Experience shows that static rollers and pneumatic tyred rollers need 8 to 12 passes, whereas tandem rollers only need 4 to 8 passes to achieve the required compaction. Combination rollers provide a similar compaction output as tandem vibratory rollers. h = Layer thickness of the material to be compacted in m ζA = Density of the compacted asphalt mix in t/m³

Thickness of asphalt layer d (cm) 3t

Number of vibratory passes of various tandem vibratory rollers 6t 9t

2

2-4

1 - 2 (K)

1 - 2 (L)

4

4-6

2 - 4 (K)

2 - 4 (L)

6

4-8

4 - 6 (K)

2 - 4 (L)

10

6-8

4 - 8 (K, G)

4 - 6 (L, H)

14

--

6 - 8 (G)

4 - 6 (H)

18

--

6 - 8 (G)

4 - 8 (H)

d=2 d=4

---

1 - 2 (L) + stat. passes 4 - 6 (L) + stat. passes

1 - 2 (L) + stat. passes 4 - 6 (L) + stat. passes

Previous coated macadam d=4

--

1 - 2 (L) + stat. passes

1 - 2 (L) + stat. passes

Chip mastic

L = low amplitude; H = high amplitude; 3 t = Machine with only the amplitude

Table 2:

Assumption: Compaction temperature > 100 °C

Reference values for the number of passes with tandem vibratory rollers

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


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Roller type / operating weight CECE

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Area output Productivity [m²/h] at specified layer thickness

(including ROPS + cabin) t

Wearing course 2 - 4 cm

Binder course 6 - 8 cm

Base course 10 - 14 cm

BW 80 AD-2 BW 80 ADH-2 BW 90 AD-2 BW 100 ADM-2 BW 100 AD-3 BW 120 AD-3 BW 125 ADH BW 135 AD BW 138 AD BW 141 AD-2 BW 144* AD-AM BW 151 AD-2 BW 154* AD**-AM BW 161 AD-2 BW 170 AD** BW 174* AD** BW 170 AD**-AM BW 174* AD-AM BW 202 AD-2 BW 180 AD** BW 184* AD** BW 184* AD**-AM

1.5 1.6 1.5 1.6 2.4 2.7 3.4 3.6 4.2 6.9 8.1 7.3 8.5 9.7 8.5 9.0 9.0 9.6 10.7 11.9 12.9 12.9

250 – 350 250 – 350 250 – 400 300 – 500 300 – 500 350 – 600 350 – 600 500 – 800 500 – 800 800 – 1200 800 – 1400 1000 – 1500 1000 – 1600 1200 – 1700 1100 – 1700 1100 – 1700 1100 – 1800 1100 – 1800 1400 – 2200 1300 – 1800 1300 – 1800 1300 – 2000

200 – 250 200 – 250 210 – 280 220 – 300 250 – 300 250 – 350 270 – 350 320 – 450 320 – 500 500 – 700 500 – 800 600 – 800 600 – 900 700 – 900 600 – 850 600 – 850 600 – 900 600 – 900 800 – 1000 800 – 1000 800 – 1000 800 – 1100

170 – 200 170 – 200 200 – 250 220 – 280 250 – 300 250 – 350 270 – 350 300 – 380 300 – 400 400 – 500 400 – 600 500 – 600 500 – 700 600 – 700 500 – 650 500 – 650 500 – 700 500 – 700 700 – 900 600 – 850 600 – 850 700 – 1000

BW 90 AC-2 BW 100 AC-3 BW 120 AC-3 BW 138 AC BW 151 AC-2 BW 161 AC-2 BW 174* AC** BW 174* AC**-AM

1.7 2.3 2.5 4.0 7.0 9.4 8.6 9.0

250 – 350 250 – 400 300 – 500 500 – 800 950 – 1400 1100 – 1500 1000 – 1500 1000 – 1600

200 – 250 220 – 300 250 – 350 320 – 500 550 – 700 600 – 800 600 – 800 600 – 900

170 – 200 200 – 250 220 – 280 250 – 370 450 – 550 550 – 650 500 – 700 500 – 700

AD = AC = AM =

Table 8:

Tandem vibratory roller Combination roller Asphalt Manager

* = split drums ** = pivot steered

Average output (m²/h) of tandem vibratory and combination rollers



Roller type / operating weight CECE

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Volume output Productivity [t/h] at specified layer thickness

(including ROPS + cabin) t

Wearing course 2 - 4 cm

Binder course 6 - 8 cm

Base course 10 - 14 cm

BW 80 AD-2 BW 80 ADH-2 BW 90 AD-2 BW 100 ADM-2 BW 100 AD-3 BW 120 AD-3 BW 125 ADH BW 135 AD BW 138 AD BW 141 AD-2 BW 144* AD-AM BW 151 AD-2 BW 154* AD**-AM BW 161 AD-2 BW 170 AD** BW 174* AD** BW 170 AD**-AM BW 174* AD-AM BW 202 AD-2 BW 180 AD** BW 184* AD** BW 184* AD**-AM

1.5 1.6 1.5 1.6 2.4 2.7 3.4 3.6 4.2 6.9 8.1 7.3 8.5 9.7 8.5 9.0 9.0 9.6 10.7 11.9 12.9 12.9

10 – 30 10 – 30 15 – 30 15 – 40 15 – 40 20 – 45 20 – 45 30 – 55 30 – 55 35 – 65 35 – 70 40 – 75 40 – 80 50 – 90 40 – 90 40 – 90 50 – 100 50 – 110 70 – 145 65 – 120 65 – 120 65 – 130

25 – 45 25 – 45 30 – 50 35 – 60 40 – 60 40 – 65 40 – 65 50 – 85 50 – 90 70 – 130 70 – 150 80 – 150 80 – 170 100 – 180 90 – 165 90 – 165 90 – 180 90 – 180 120 – 250 110 – 210 110 – 210 110 – 230

35 – 70 35 – 70 40 – 80 50 – 90 60 – 100 70 – 110 70 – 110 75 – 130 75 – 135 100 – 170 100 – 180 120 – 190 120 – 200 150 – 210 130 – 190 130 – 190 140 – 210 140 – 210 190 – 320 190 – 300 190 – 300 190 – 320

BW 90 AC-2 BW 100 AC-3 BW 120 AC-3 BW 138 AC BW 151 AC-2 BW 161 AC-2 BW 174* AC** BW 174* AC**-AM

1.7 2.3 2.5 4.0 7.0 9.4 8.6 9.0

10 – 30 15 – 35 20 – 40 30 – 55 30 – 60 40 – 80 40 – 80 50 – 90

25 – 40 35 – 40 40 – 60 50 – 90 60 – 120 90 – 160 90 – 160 100 – 170

40 - 60 45 – 90 55 – 105 65 – 115 90 – 155 135 – 190 130 – 185 140 – 200

AD = AC = AM =

Table 9:

Tandem vibratory roller Combination roller Asphalt Manager

* = split drums ** = pivot steered

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 skill of the roller operator and the rolling pattern he chooses. Here are some basic rules for compaction work and rolling patterns which have been shown to be useful. Basic rules for compaction 1.

Start compaction as early as possible. This also applies when using heavy rollers immediately behind the paver. The eveness of the layer produced by the paver must, however, not be jeopardized.

2.

Drive with the driven drum towards the paver to avoid the formation of riples and cracks. Combination rollers should be used with the rubber tyres towards the paver. Exception: When working on steep gradients, the driven drum should face downhill so that the high shear forces induced by the drum will be absorbed by the pre-compacted layer without disturbance to the material. When working downhill, the driven drum should trail uphill. This problem does not arise when using modern rollers with all drum drive.

3.

To avoid mix material sticking, drums and tyres must be lightly sprayed with water. They should be moist but not wet. The water is evaporated by the hot mix which loses heat. This reduces the time available for compaction. Sprinkler systems with interval control switches reduce the amount of water and additives in the water.

4.

Drive smoothly and do not change direction in jerky movements. The use of an automatic speed control system can improve the rolling quality.

5.

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

6.

Switch the vibration on only when the machine is moving. When reversing, stop the vibration in good time before the machine comes to a halt (or use an automatic vibration control).


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7.

If the road is cambered start compaction from the lower edge and overlap each pass towards the higher edge. In this way the compacted mix works as a support for the machine.

8.

Steer and offset the machine only on compacted material to avoid shoving of material.

9.

Never stop the roller on hot mix material since the machine will probably cause deformation of the layer.

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

ROLLING PATTERN As a general rule all rolling lanes should overlap by at least 15 cm in transverse direction so that no uncompacted strips will remain. The number of passes should be identical for all lanes to achieve a uniform compaction over the entire width of the mat. The roller must stay in lane until it reaches the cool and stable area where it can manoeuvre (Fig. 10). If the road has a kerb, compaction should start at the outer edge.

7

8

5

6

3 1

Fig. 10 Rolling pattern


4 2

;;; ;;; ;;; ;;; ;;;


If the road is unsupported with no kerb and the layers (base and wearing courses) are thick, there is a risk that the roller will push the material outwards when starting to compact at the edge. To avoid this, a strip of 3040 cm should be left uncompacted on each side to allow the material to cool in order to provide a stable support for the roller (Fig. 11).

4 3 2 1 5

Fig. 11

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;;; ;;; ;;; ;;; ;;; ;;;

Rolling pattern where there is a risk of lateral displacement of material


Date :


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

1 2 3 4

2

1

Rolling pattern for echelon paving

Joints must be compacted to be closed and level. This requires special attention 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 only 10 - 20 cm of the new layer whilst rest of the drum remains compacted and cool asphalt (Fig. 13). The subsequent rolling lanes must then be offset from the outer edge towards the longitudinal joint.


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;;;;;; ; ;;;;;; ; ;;;;;; ;;;; ; ;;;; ; ;;;; ; ;;;;; 3

Fig. 12

Date :


1 4 3 2

Fig. 13

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;;; ;;; ;;; ;;;

Rolling pattern for longitudinal joint hot on cold

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

3 2 1

Fig. 14

;;; ;;; ;;; ;;;

Alternative rolling pattern for longitudinal joint hot on cold



Lateral joints should be rolled at right angles to the paving direction if possible. The roller must therefore drive with 10 - 20 cm of the drum on the hot and uncompacted material. The overlap should be gradually increased until the roller is working completely on the new material (Fig. 15). If the room to manoeuvre the machine is very limited, it is advantageous to use a small and manoeuvrable roller. If the site situation does not permit any manoeuvring the transverse joint may be rolled at an oblique angle. The joint itself may even be cut at an oblique angle to the paving direction.

10 – 20 cm

heiß hot

Fig. 15

kalt cold

Compacting a transverse joint

Around bends, compaction should be started at the lower inner side of the curve by driving straight ahead as far as possible before starting to cut the corner. The roller must be offset at a tangent to previously compacted material. The steering speed must be adapted to the rolling speed, i.e. when driving slowly the steering movement must also be slow. This reduces the shear forces which are created when rolling around a bend. The use of rollers with split drums and crab steering to both sides is recommended.


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;;;; ;;;; ;;;; 1 2 3 4 5

Fig. 16

Compacting on bends

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


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ROLLING CRACKS AND THEIR CAUSES Compaction with rollers can lead to the development of longitudinal and transverse cracks as well as to displacement of material. These cracks can be the result of several factors and are therefore very difficult to diagnose. Transverse cracks Transverse cracks are usually shallow. Possible causes are: >

The roller pushes a bow wave ahead (pre-compaction by the paver too low, application too early of rollers which are too heavy, the driven drum 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 (subbase contaminated or inadequately tack coated).

>

Surface saturation (rain or excessive water sprinkling).

>

Compaction of thick layers on gradients (the shear forces of the roller cannot be absorbed).

>

Poor mix composition (e.g. high proportion of poorly graded natural sand 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 roller must delay operations until surface cools: temperature sandwich occurs and 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.


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4/93

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Displacement of material The most common reasons for displacement of material are: >

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

>

Roller too heavy.


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4/93

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30 of 31


LITERATURE [1]

Dübner, R.: Asphalt road construction - application and compaction of asphalt 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


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4/93

Page :

31 of 31

Compaction - Asphalt

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