ROAD RESEARCH LABORATORY
Ministry of Transport
RRL REPORT LR 405
REVIEW OF LITERATURE ON COMPACTION OF BITUMINOUS MATERIALS
by
W.D. Powell
Construction Methods Section Road Research Laboratory
Department of the Environment Crowthorne, Berkshire
CONTENTS
Abstract
1. Introduction
2. Laboratory investigations
2.1 Methods of compaction
2.2 Investigation of factors that influence compaction
Field studies
3.1 Further densification after construction is complete
3.2 Studies of factors that influence compaction
3.3 Studies of thick lift construction
3.4 Tests for compaction control
Pilot-scale laboratory experiments
Conclusions
Acknowledgements
References
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5.
6.
7.
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C CROWN COPYRIGHT 1971
Extracts from the text may be reproduced
provided the source is acknowledged
Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on ! st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
REVIEW OF LITERATURE ON COMPACTION OF BITUMINOUS MATERIALS
ABSTRACT
The literature reviewed in this report covers laboratory and field work on the compaction of bituminous materials. A large number of factors which influence compaction are considered in an attempt to provide a general indication of the various trends that may be expected to occur in practice. The relevant factors include aggregate type and gradation, mix composition, type of subgrade, rolling equipment and procedures,
lift thickness and rolling temperature.
The difficulties involved in direct application of results of laboratory and field studies to construction work in general
are stressed. Pilot-scale laboratory experiments provide a means of better control of factors and yield information which may be more readily and directly applicable to field construction projects.
I. INTRODUCTION
Compaction is defined as the process of compressing a given volume of material into a smaller volume; the degree of compaction increases as voids are eliminated in the mixture. In bituminous road construction this
is achieved through the pressure applied initially by the screed and tampers of a paving machine and then by
smooth-wheeled or pneumatic-tyred rollers.
The importance attached by many to compaction as an influence on the performance and service life of a road is indicated by the volume of literature published on the subject. The general principles underlying the necessity for adequate compaction have been discussed in 'Bituminous Materials in Road Construction' ! and by Minor 2 and by Lowe 3. Inadequate compaction leads to too great a void content and those voids that
v
are inter-connected permit the intrusion of air and water. This tends to lead to an increase in the rate of hardening of the binder which may produce premature embrittlement of the pavement as well as to the possibility in some types of material of stripping of the binder from the aggregate. Although compaction is related to permeability the relationship is not simple, as permeability is a function of the inter-connection of voids in a mixture and not of voids alone. Excessive void content leads, in addition, to differential compaction under traffic giving ruts and grooves in the wheel track. There must be sufficient cohesion in the mixture to withstand tensile stresses developed under load; this is possible only with proper compaction (except for a material such as mastic asphalt which requires no compaction). In road materials of the coated-macadam type, intimate interlocking of the particles in the mixture is also necessary to generate sufficient interparticle
friction to resist displacement of the mix under load. Finally, the load spreading property of the material is
improved with better compaction. I
In practice, however, the benefits of good compaction must be balanced against the cost of the
compaction process. The attainment of a given standard or degree of compaction is not specified in the
United Kingdom as it is in many other countries, and relatively little guidance is given on compaction in
current British Specifications such as BS 5944 , BS 802 s , BS 16216 and the MOT Specification for Road and
Bridge Works 7 . This has arisen mainly from the use of bituminous materials which have been relatively easy
to compact, especially when laid in thin layers. Minimum roiling temperatures are specified in the appropriate
British Standards and certain restrictions are imposed on the mode of compaction. However, at present the
effectiveness of rolling depends largely on experience and skill of the roller driver and the degree to which
temperature has been controlled. Future developments such as thick-lift construction or new mix designs
may require that more attention be paid in specifications to the compaction stage of road construction.
This literature review is certainly not exhaustive but an attempt is made to present the general findings
and opinions reported in this field of work. Much of the discussion on compaction is subjective in nature.
Also, the reported results of experiments are difficult to analyse collectively because there are so many inter-
related factors in addition to compaction which are effective concurrently. Apart from investigations of these
factors which affect compaction, maoy experiments have included an evaluation of various tests of degree of compaction. These tests are only briefly considered in this Report.
Laboratory studies reported in the literature are considered first, in Section 2, and this is followed in
Section 3 by a discussion of field studies. The results of these two types of investigation are usually difficult
to correlate; pilot-scale studies which narrow the gap between small-scale experiments and field work are
described in Section 4.
2. LABORATORY INVESTIGATIONS
2.1 Methods of compaction
Unless a laboratory method of compaction simulates compaction in the field, results obtained using
laboratory specimens may not be valid for full-scale construction work and many investigations have com-
pared densities of laboratory specimens with cores cut from pavements a,9. However, the state of compaction
is not uniquely characterized by the density of a specimen 1° ; the internal structure is also important and
this involves parameters such as particle orientation and degradation (i.e. breakdown of aggregate particles).
Existing methods of laboratory compaction including static, impact, kneading, rolling, vibrating and
gyratory compaction have been reviewed by salehi I 1. Most methods are either unable to duplicate field
compaction or have not been properly examined in this respect. However, a laboratory roller-compactor
recently developed by Lees and Salehi 12 closely simulated field compaction for the rolled-asphalt wearing
course mixture used in their study. Particle orientation was analysed by statistical techniques after cutting
specimens into thin sections; the method is two-dimensional and does not involve removal of particles from
their original positions. The results indicated a preferred orientation of particles with respect to the direction
of compactive effort. Aggregate particles tend to orientate themselves so that their preferential elongation
direction is nearly in the horizontal plane.
Another method of evaluating particle orientation adopted by Puzinauskas 13 in 1964 involved the
measurement of the ratio of compressive streiagth in the direction of compactive effort to that normal to
the direction of compactive effort. Lees and Salehi 12 have criticized this method in that it gives a measure
of degree of anisotropy rather than a direct measure of particle orientation and fails to differentiate between
particle orientation and packing structure. For example spheres, which have no orientation, may be packed
2
together with different orientation of voids and might be expected to exhibit anisotropic behaviour in
mechanical tests.
Although most laboratory methods of compaction do not wholly imitate field compaction, many
laboratory investigations have yielded valuable conclusions which have since been confirmed by full-scale
field studies. The following section deals with laboratory experiments designed to investigate factors that
influence compaction of bituminous materials.
2.2 Investigations of factors that influence compaction
The effect Of compaction temperature on the properties of bituminous materials was examined by
Kiefer 14 in 1960 and his results clearly, indicated the necessity to control temperature during compaction.
The Hveem kneading compactor Is was used to produce test specimens and the void content was found to
decrease with increasing compaction temperature; the void content obtained at a compaction temperature
of 65°C was 40 per cent higher than obtained at 135°C. Two mixtures were used (denoted as A and B);
the aggregate in both cases was crushed limestone with the same gradation and the penetration values of both
binders used were equal at 25°C but the viscosity of the binder in mixture B exceeded that of the binder in
Mixture A at higher temperatures. When mixture A was compacted at 80°C and mixture B at 115°C, each
had the same void content of 4.5 per cent. A previous investigation by Parker 16 using the Marshall method
of compaction ~s and a similar mixture indicated a far greater variation of void content with temperature.
The void content at a compaction temperature of 65°C was four times that at 135°C in this case. This"
temperature dependence is explained by the viscosity-temperature characteristics of the binder; the higher
the viscosity of the binder, the greater the resistance to compaction.
More recent work carried out at the University of Wisconsin 17 ' 18 again showe d the significance of
temperature on the degree of compaction (MarshaU-type compaction Was used). The viscosity-temperature
relationship of the binder was determined and the specific gravity and void content of specimens were then
related to the binder-viscosity at the compaction temperature. Changes in mineral-Filler content were also
found to affect specific gravity and void content, as shown in Table ~I. The reduction in void content with
increase in filler content was believed by the authors to be a result of the increased amount of Idler available
to fill the voids in the aggregate.
T A B L E I
Values of specific gravity and per cent voids for different
Idler contents at the same compaction temperature
Filler Content
(per cent by weight
of total mix)
7.1
3.75
Tar Content
(per cent by weight
of total mix)
5.25
5.25
Specific Gravity
2.42
2.36
Percentage
of Voids
5.5
8.0
3
The mechanical properties of bituminous materials with a high content of filler have been studied in
detail at the Road Research Laboratory 19., 20 . The importance of the filler-tar portion Of a dense tar-
surfacing mixture was demonstrated and it was shown that an optimum binder content could be calculated
from a knowledge of the void content in the dry compacted filler. The effect of compacting load on optimum
tar content was also examiried. A roller-compaction haachine was used to compa,:~ 254 mm. x 51 mm. x
25 mm. specimens which were then mechanically tested to determine the optimum binder content. Increasing
the load on the steel roller from 45.4 kg. tO 227 kg. reduced the optimum binder content from 7.5 per cent
to 6 per cent. The lower the binder content employed the greater was the resistance to deformation of the
mixture.
Lefebvre 21 has examined the variation of density and stability of Marshall test-specimens with a
number of hammer blows of the Marshall mechanical compactor. One mineral aggregate consisting of
crushed dolomite and natural sand was used with a ffLxed gradation and bitumen content. The density was
found to be linearly related to the logarithm of the number of blows. This means that the rate of change
of density becomes progressively less with increasing number of blows. The effect of grade and source of
bitumen was studied; these were found not to affect density when compaction temperatures were chosen
to give the same values of viscosity. Marshall stabilities were also measured and were extremely dependent
on density; if the density at 60 hammer blows is reduced by 5 per cent, the corresponding reduction in
stability at 60°C is as much as 80 per cent. I f these laboratory results are directly applicable to field
construction work then the data show the importance of achieving adequate compaction to obtain roads of
high load-bearing capacity. The binder-viscosity at rolling temperatures is again shown to be a critical factor in compaction.
The ease with which a paving mixture is compacted is extremely important and attempts have been
made to describe this property quantitatively. I f other factors are maintained fixed, the influence of type
of mixtiare and its composition can be investigated in the laboratory.
Ruiz and Dorfman 22 have determined the variation of density with number of hammer blows of a '
Marshall mechanical compactor and their results are shown in figure 1. They described the variation by the equation:
log N = Ie (D N - Dn) ( I ) n
where the variables n and D n refer to the number of hammer blows and corresponding density respectively;
N is a constant number of blows which is greater than n (for example, that number used in the design of the
mix) and D N is the corresponding density. The constant I e is defined as the compactibility index. For
n = N/10 equation (1) may be rewritten:
Ic = D 1. O ........ (2) N N/ IO
Figure 1 indicates the variation of density with number of compactor blows for two very different types of
material, one a mixture containing mainly crushed granite and the other a mixture made entirely from natural
sand and gravel. The mixture containing crushed granite was known to be difficult to compact by roiling
(i.e~ a 'harsh' mixture) and had a low value of I e. The other mixture required little rolling and had a
relatively high value of Ie; however, such a ' tender' or 'slow setting' mix often has low stability and is prone
to displacement during and after rolling.
4
The disadvantage of the compactability index I e, is that its value depends on the specific gravity of the
mineral aggregate as well as the ease of compaction. Equation (2) may be rearranged to give:
i • = 1 o o . . . . . . . . ( 3 )
c Dm ( V N / I O . VN ) where D m is the maximum theoretical' density and V n is the percentage void content. Lefebvre and
Robertson 23 have pointed out that aggregates with different specific gravities may influence I c to such an
extent that any difference in ease of compaction is completely masked. Clearly an index which is independent
of specific gravity is desirable. One could be obtained by converting the density values in Figure 1 into void
contents. An index I c, could then have been defined by I e, = 1/(VN/IO -V N ).
Some paving specifications require that the pavement be compacted to a certain percentage of the
design density. Lefebvre and Robertson 23 converted the data of Ruiz and Dorfman 22 on density and
compactive effort into percentages of the design values as shown in Figure 2. They defined the slope of this
relationship as the compaction resistance index, C a' which is given by:
Ca = A [ 1 0 0 D n / D N ] ........ (4)
a log(100 n/N)
C R is a measure of the resistance of.the mix to compaction and is independent'of the specific gravity of the
aggregate. The index C a is also found in practice to be approximately independent of the number of hammer
blows N chosen for the design density. Mixes which are easily compacted have low values of C a and 'harsh'
mixes that require a large amount of rolling have high C R values.
Lefebvre and Robertson carried out an extensive investigation of the effect of mineral-aggregate
characteristics on the compactibility of paving mixtures. Values of C R for various aggregates indicated that
the shape and surface texture of the fine aggregate played an important part during compaction and had
more influence in general than characteristics of the coarse aggregate. Mixes with rounded natural sand:
had low C n values regardless of type of coarse aggregate. Mixes containing fine aggregates with angular'and
irregular Shaped particles naa medium-to-high resistance to compaction depending on the characteristics of
the coarse aggregates, in particular the surface texture of the coarse aggregate. There was also some evidence
to confirm the findings of Ruiz and Dorfman that the compactibility of a mixture increases with increasing
binder content.
Aggregate gradation was also shown to be important in compaction; for all types of aggregate examined,
it was found that gradings near the maximum-density gradation gave mixes with high resistance to compaction
and a low percentage of voids in the mineral aggregate. These results indicated that such gradings should be
avoided, not only to prevent early failure due to excessively low void content but also to facilitate compaction
during construction. Too low a resistance to compaction was also undesirable because such mixtures had a
low stability and were easily 'overstressed' by rollers, resulting in lateral flow of material rather than
compaction. Mixes giving a medium value of C a therefore had the best overall combination of properties.
The values of C a obtained by Lefebvre and Robertson have not been correlated with actual field
experience but the laboratory results for the particular mixes used suggest that mineral-aggregate combinations
consisting entirely of crushed materials or of rounded natural sand and gravel should be avoided if possible.
If, by correlation with field studies, a range of acceptable values of C a were to be established, this would aid
the design of paving mixtures with optimum compaction characteristics and good performance in service.
5
3. FIELD STUDIES
A large number of field experiments have been carried out to investigate the various factors which may
influence compaction. Inevitably, the results obtained only hold for the particular conditions under which
the trials were performed but a consideration of the data as a whole enables a general picture to be obtained.
Apart from compaction during construction of a road, further densification occurs under traffic. This
aspect of compaction is considered first because it indicates the possible need for adequate compaction during construction.
The construction variables involved during comPaction may be considered as arising from either the
properties of the mix or the roiling procedures. Most investigations have been concerned with both aspects
in order to attempt to make a comprehensive study of the various factors. In recent years considerable
attention has been paid to compaction of bituminous materials in thick lifts and this topic is considered separately.
Finally, a number of tests are briefly considered which were designed to evaluate the use of different instruments to measure the degree of compaction.
3.1 Further densification after construction is complete
The importance of adequate initial compaction was established by Goode and Owings 24 when a number
of flexible roads were constructed near Washington. Samples of the wearing course were cut from the road,
densities were measured and the penetration of the extracted bitumen was determined. The results of tests
carried out over a period of years indicated that the materials of a given type with the lowest initial density
suffered the greatest densification due to traffic. In addition, a pavement with 8 per cent air-voids
immediately after construction showed a change in penetration 6f the bitumen over four years of service
which was twice the change for a pavement with only 5 per cent air-voids. This demonstrated that a high
void-content is conducive to an increased rate of hardening of the binder and it was found that this led to
premature deterioration of the road. Goode and Owings found that for asphaltic concrete wearing course
mixtures approximately 6 per cent air-voids was the optimum for good pavement performance. This value
prevented a high rate of hardening of the binder and early deterioration of the pavement and in addition is large enough to prevent excessive loss in stability.
Adam 2s has shown that the minirnum further densification under traffic occurs when optimum rolling
temperatures are used; that is, when greatest compaction is achieved. These experiments are described in more detail in the next section.
3.2 Studies of factors that influence compaction
A recurring theme throughout these investigations is the importance of binder-viscosity on compaction.
Adam 2s has reported on a project to examine the influence of binder-viscosity where comparative field tests
were made using three-wheel steel rollers and pneumatic-tyred rollers. Measurements of the viscosity of the
bitumen were made in the laboratory at different temperatures, for several grades of bitumen. Samples were
cut from sections of the road that had been rolled at different temperatures. The density was found first to
increase with increasing temperature and then to decrease after passing through a maximum. The variation
of density with the binder-viscosity measured at the rolling temperature for two entirely different mixtures
6
is such that the maximum density of the mixture occurs at approximately the same viscosity even though the
corresponding rolling temperatures for the two mixtures are different. Similar results were obtained when
steel-wheeled or pneumatic-tyred rollers were used. Adam has pointed out the danger of specifying the
penetration of a binder at a single temperature; the temperature-susceptibility of the binder-viscosity is
critical and determines the viscosity of the binder at mixing, rolling and service temperatures.
In 1962, the New York State Department of Public Works 26 initiated a project to examine the
influence of variables such as mix composition, pavement thickness, mix temperature and number of roller
passes on compaction. Marshall specimens were compacted from samples of wearing-course mixtures and
temperature measurements were made of the mixtures during compaction at twelve construction projects.
Full details of rolling procedures were also recorded. Bulk-density, binder-extraction, aggregate-gradation
and specific-gravity tests were conducted on core-samples taken from the road. Finally pavement-rebound
deflection measurements were performed near each core location to take into account the different stiffnesses
or supporting strengths of the underlying layers.
Although core densities did not vary significantly in the longitudinal direction, initial values varied
significantly across lanes as shown in Table 2; the average density was greatest at the centre and least near
the edges. This variation was reduced over a period of two years whenfurther densification occurred under
traffic. One of the factors which may have resulted in this initial difference was the significant variation (in
the transverse direction) of the amount of rolling.
TABLE 2
Density of pavement cores immediately after construction
in and out of wheel path
Outer Wheel Path
Percent of Marshall density 94.9
(50 blows)
Between Wheel Paths Inner Wheel Path
96.5 95.4 '~,
Regression analyses were applied to determine the effect of various factors on pavement density when
similar materials were used. The factors in order of greatest influence were:- (1) binder content, (2) support
of underlying layers, (3) aggregate gradation, (4) binder-viscosity, (.5)' number of roller passes. The influence
of binder-viscosity on density may not have been truly examined in this study as the viscosity values were
related only to temperature when rolling began and no further record was made at later stages of rolling.
A number of investigations have been carried out by the Ontario Department of Highways to study
compaction processes in the field and to determine the best procedure to achieve adequate compaction
Results reported by Fromm 27 ! 28 showed the effect of various positions of rollers within the rolling sequence
and the differences in compaction achieved when a 30 Mg pneumatic-tyred roller was substituted for a 9 Mg
pneumatic-tyred roller. The most effective rolling sequence was where a steel-wheeled roller used for
'breakdown' roiling was followed by a pneumatic-tyred roller and, finally, by a steel roller used for fmishing.
The lighter pneumatic-tyred roller gave adequate compaction for 'easy to compact' carbonate aggregates, but
granitic aggregates that were more difficult to compact required the use of a 30 Mg pneumatic-tyred roller.
The optimum tyre pressure was found to depend on the size of pneumatic-tyred roller used.
7
In these investigations in Ontario, it was also found that the value of the function:
Marshall Flow
Void Content x Marshall Stability
gave a good indication of the degree to which a bituminous mixture could be compacted.
The need for factual data relating to methods of compacting bituminous materials led the U.S. Bureau
of Public Roads to conduct an extensive series of field-research investigations. The work was reported by
Kilpatrick and McQuate 29 in 1967. Eleven different construction projects in a number of states were
selected for study. A large variety of mix designs and construction equipment was used over a range of "lift'
thickness from 25 to 100 mm. Density measurements were made on pavement cores and also on the pave-
ment in place by nuclear density and air-flow tests. The results of these 'in situ' tests are considered in a later section.
A number of different makes of pavers were used at different speeds in the range 4 to 20 m./min.
Over this range, paver speed was not a significant factor in determining final lane density. Large lateral
variations in pavement density were again found in this work but the variations were greatly reduced after
completion of rolling. Normal procedures used by the roller operators resulted in a lateral variation in
compactive effort; densities measured in the wheeltrack were often lower than in the centre of the lane
after construction but this transverse variation of density disappeared after several years of trafficking.
Higher densities were obtained when a pneumatic-tyred roller was used between steel-wheel 'breakdown'
and finishing rolling. This procedure also reduced the variation of density with depth and sealed the top
surface. Attempts to establish an optimum number of passes for different rollers did not enable a simple
universal formula applicable to all conditions to be established. A number of other studies 26 , a0 have shown
that no simple function exists which accurately describes the relationship between rolling and density. The
highest densities obtained in the work reported by Kilpatrick and McQuate occurred when 'breakdown'
rolling (either steel wheel or pneumatic) was accomplished at temperatures exceeding 105°C. Measurements
of temperature losses under different conditions on the various projects indicated that this would require
'breakdown' rolling to be completed within six minutes after laydown under the worst conditions that
existed. Most of the research findings presented by Kilpatrick and McQuate are from projects which were considered to be representative for several states.
The actions of steel-wheeled and pneumatic-tyred rollers on hot bituminous materials have been
described by McLeod al . He stressed the importance of pneumatic-tyred rollers equipped so that the tyre
inflation-pressure is rapidly adjustable to enable pressures at all times to be at the maximum that the paving
mixture can tolerate without detrimental lateral deformation. Figure 3 is an idealised representation of the
performance of such a roller. The tyre pressure may be adjusted in step with the increasing resistance to
compaction whereas the pressure applied by the steel-wheel roller increases at a rate which is not controllable;
the steel-wheel roller settles into the mix until the area of contact is sufficient to support the roller. This
contact area depends on the stability of the mix and changes accordingly during compaction of the mix. It
may be necessary to delay steel-wheel rolling to allow the mix to cool and thereby develop sufficient stability
to give adequate support to the roller. McLeod suggested that with steel-wheel rollers it is frequently not
possible to take advantage of the low resistance to compaction of 'softer mixes' at relatively high temperatures
when the stability is low. Although it may be possible to overcome these stability problems with pneumatic-
tyred rollers equipped with adjustable tyre inflation pressure, there are practical difficulties which have been
encountered with such rollers; for example, there is a tendency for hot-mix to stick to tyres when they are
cold and grooves are sometimes left on the surface of the pavement (see Section 3.3).
8
3.3 Studies of thick-lift construction
Considerable attention has been paid in recent years to the laying of bituminous materials in thick lifts.
McLeod 31 has pointed out that fully flexible pavements remain relatively cool in the deeper layers even during
the summer season and, as these layers have a reduced pressure transmitted to them, little further compaction
by traffic can be expected. The degree of compaction during construction is therefore critical to ensure good
load-bearing capacity.
Beagle32,33 has shown that a paving mixture compacted in a single 128 mm. thick layer resulted in an
average pavement density approximately one per cent greater than that obtained by rolling the same mix in
two layers each 64 mm. thick. Beagle explained this in terms of the slower rate of cooling of thicker layers
which could therefore be rolled effectively for a longer period of time. This was confLrmed by later work in
which thermocouples were placed at regular intervals in the laid materials. Cores taken from the pavement
indicated that the middle section of the single-lift construction, which was at the highest temperature during
rolling, was also the most dense. Further confirmation that the higher temperature assisted compaction effort
was obtained when 25 ram. thick foamed polystyrene sheets were stapled to the subgrade before applying the
base material; this ensured that the loss of heat downwards was much reduced and the major loss of
temperature was then to the atmosphere.' Later, single lifts up to 450 mm. thickness were laid. Beagle also
examined various rolling procedures; again the introduction of a pneumatic-tyred roller between 'breakdown'
and finishing rolling increased the pavement density.
After recognising the temperature-compaction relationship, Beagle attempted to relate the void content
achieved after compaction with the 'lay-down' temperature and the thickness of the lift. He devised a chart
based on the data collected from the projects to indicate the correct 'lay-down' temperatures whicl~ would
have given the specified laboratory-design v~ue of void content. This chart however cannot be applied in
general because it only refers to the specific conditions existing in Beagle's experiments.
Field trials carried out with bituminous base materials at the Road Research Laboratory 34 have
indicated that the average density of cores removed from a single lift 140 mm. thick is slightly higher than
that for a combination of two lifts, 64 mm. and 76 mm. thick. This led to an increase in the maximum
permitted lift thickness of bituminous materials from 80 mm. tO 100 mm. A further trial at the Sevenoaks
By-Pass as with gravel-asphalt material laid from 62 mm. to 240 mm. thick indicated that the average density
of the thicker lifts was 1 per cent higher than that for the thinner lifts. It was also shown that an appreciable
drop (up to 15 per cent) in density occurred at the interface between layers.
Recent unpublished work at the Road Research Laboratory on measurement of temperature in
bituminous mixtures during compaction has again indicated an appreciable variation of temperature with
depth in the material as found by Beagle. Although a minimum rolling temperature is specified for bituminous
road base materials in the United Kingdom, results indicate that the position of the temperature measurement
is critical. A typical variation of temperature with depth is shown in Figure 4 for a dense bitumen macadam
roadbase 125 mm. thick laid at the Road Research Laboratory. Further work is required to specify more
accurately the rolling temperature of bituminous materials.
Extensive work into the compaction of bituminous materials (chiefly with 'difficult to compact'
aggregates) has been carried out in the State of Washington, 36,37 including a study of the effect of thickness
of a lift. A 106 mm. thick base was laid in one, two and three lifts and various rolling procedures were
examined. Equal or higher densities were obtained with the single lift with no detrimental reduction in
9
surface regularity of the surface. It was reported that pneumatic-tyred rollers gave better compaction when
used in the 'breakdown' position rather than the intermediate position, and in the 'breakdown' position these
rollers gave higher densities, regardless of thickness of lifts. Heavier pneumatic-tyred rollers (in the region of
20 Mg. gross weight) with relatively high tyre pressures (approximately 600 kN/m 2) gave a more uniform
compacted density. It was also found that much more efficient compaction was achieved for stable dense-
graded mixtures when rolling temperatures exceeded 95°C. The advantage of laying in thick lifts was again
shown to be the better heat retention of thicker layers and consequent improvement in compaction.
The problem of material pick-up with pneumatic-tyred rollers was alleviated by maintaining the tyres
hot and dry or by the use of additives or detergents. Rollers equipped with rapidly adjustable tyre inflation
pressures were reported to take longer to warm up but were still found to give the best performance.
3.4 Tests for compaction control
As each mix has its optimum rolling conditions and there are many other variables involved on different
construction projects, there is clearly a need to control compaction. Considerable effort has therefore been
directed towards development of tests which can be used for this control. Any acceptable method for site
work should be rapid and non-destructive; the most common techniques used to date are the nuclear density gauge and the air-permeability test.
Briefly, the general conclusions of studies evaluating these tests 29,38"43 are that there are many
potential sources of error involved in the use of these instruments including pavement characteristics such as
surface texture, operational procedures, and instrumentation errors. It is therefore unlikely that they can be
used to measure compaction on an absolute basis. However, they have been successfully used on trial sections
early in paving operations to correlate pavement densities and non-destructive test measurenaents with roller
passes; cores taken from the trial sections determine the number of passes necessary to obtain the optimum
density for the particular mix used and then non-destructive test measurements, related to core densities in
the trial sections, provide a continuing check on the state of compaction.
4. PILOT-SCALE LABORATORY EXPERIMENTS
Difficulties involved in correlating laboratory test data with those obtained in field work have already been
referred to. It is sometimes impossible to obtain a correlation because field conditions contain unknown
factors not included in the laboratory investigation. The disadvantages of an investigation involving ~ series
of field experiments are that the cost may be prohibitive and the rate of progress in the long term is slow.
An alternative method of study is to simulate field conditions as closely as possible under controlled conditions in a laboratory.
Swanson, Nemec and Tons 44 attempted to simulate full-scale rolling conditions as far as possible.
They examined the important variables affecting compaction for a Massachusetts Type 1 surface mix
containing 6.5 per cent by weight of binder. The influence of viscosity was of principal interest but the
effects of rolling procedures, hardness of supporting medium and environmental temperature were also
studied. The roller compactor machine employed consisted of a rubber-tyred roller (750 x 16 mm.) with a
load of 48.4N and a tyre pressure of 690kN/m 2. The action of a steel roller was simulated by placing a
curved steel plate under the rubber-tyred roller. The curved plate, 1.52 m. diameter, gave a load per unit
width of 4.4N/m. The machine was capable of compacting 300 x 300 mm. slabs, 50 mm. thick, and
different supports were used under the specimens, including foam rubber, urethane elastomer, hard rubber
and concrete. The surface area of specimen was selected to be the smallest size that still simulated a
10
'continuous' bituminous layer.
Viscosity data were presented in terms of an average compaction-viscosity which related to an average
temperature of the mix during rolling. It was found that the void content of the compacted mix was
proportional to the logarithm of the average bitumen viscosity over the range studied. The rubber-tyred
roller used was less effective than the steel roller even though from surface appearance the reverse seemed
true. The differences in densities achieved by the two rollers diminished with increasing number of
coverages. As expected the effect of coverage depended on the time interval between coverages because
this influenced the mix temperature during compaction. It was found that the efficiency in densification
decreased at each successive coverage and density was approximately a semi-logarithmic function of number
of coverages. A similar variation has been observed with the Marshall method of compaction. A change in
• environmental temperature from 27°C to 5°C did not give a large increase in void content and this suggested
that material could be laid and roiled in cold weather provided that the required rolling is done within a short
time. Also any differences in densities attributable to different base support stiffness were very small from
a practical point of view.
Research work has been carried out on a much larger scale in the laboratory under controlled conditions
by Schmidt et al at the California Research Corporation 4s" 49. An attempt was made to reproduce the
processes of commercial hot-mix plants as closely as practical prior to spreading of materials. Full-size
rollers were used for compaction and temperatures were measured by thermocouples buried in the mix. The
equipment was used in a general study of the rolling of hot-mixes; it was shown that optimum steel-roller
weights and diameters, and number of passes for maximum compaction, were dependent largely on mix
characteristics.
The important factors involved during compaction were studied for mixes classified as 'understressed'
or 'overstressed'. A mix is defined as understressed when an increase in compactive effort results in higher
densities, and this mix becomes over-stressed when additional rolling causes a reduction in density. The
optimum compactive effort lies between these two conditions. Mixes at both extremes of bearing capacity
were examined; low-bearing-capacity mixes made from smooth-textured rounded gravel, and low in filler
content, were easily over-stressed. On the other hand, high-stability mixes made from crushed aggregates
were able to tolerate heavy rollers and a large number of passes.
The maximum density at optimum roiling depended on roller pressure and diameter, mix properties
and temperature, and the thickness of the layer. Further rolling beyond the optimum reduced density, due
to the formation of fissures. A study of the variation of density with depth in this case confirmed the
decompaction at the surface. The layer thickness was important because, for thin layers, the close proximity
of the stable base inhibiteddecompaction at the surface and thus thin layers could tolerate higher pressures.
For under-stressed mixes the density increased with increasing temperature during compaction and the
consequent decreasing viscosity of the binder. The source of the binder had no appreciable effect even
though the binder viscosity was important. These results confirm the laboratory studies of Parker 16 and
Kiefer 14. The binder-viscosity and the temperature during compaction had the opposite effect for over-
stressed mixes; for a low-stability rounded-gravel mix that was easily over-stressed, the density increased
with decreasing rolling temperatures. An increase in binder-viscosity in this case increased the resistance of
the mix to the decompactive action of the roller.
11
Santucci and Schmidt 4s showed that when the sum of filler and bitumen volume was maintained
constant, an optimum filler/binder ratio (on a volume basis) existed for maximum compaction. For a rounded-
gravel mixture with a filler-plus-binder volume fixed at 15 per cent, the optimum filler/binder ratio was found
to be 0.2. The 'toughness' of pavement was also evaluated by penetration tests. The 'toughness' depended on
the filler/binder ratio as well as density, binder-viscosity and aggregate characteristics. As the filler/binder ratio increased, the 'toughness' also increased.
From their general conclusions of this work using steel-wheel rollers, Schmidt and his associates were
able to make suggestions to attain optimum compaction for different situations encountered in field
construction work. Briefly, these are:-
A. For mixes stressed below optimum:
(1) (2) (3) (4) (5)
Increase roller weight
Decrease roller diameter (usually not recommended)
Increase number of passes of roller
Reduce binder viscosity
Finally reduce mix stability provided there is no danger of instability under traffic.
B. For mixes overstressed:
(1) (2) (3) (4) (5) (6)
Increase roller diameter
Decrease number of roller passes
Decrease roller weight
Decrease thickness of each layer rolled
Increase binder viscosity
Finally, increase mix stability.
5. CONCLUSIONS
Published results of research work on the compaction of bituminous materials provide an overall picture of
the factors influencing compaction and the various trends that may be expected to occur in practice. Direct
application of results to construction work in general is, however, often not possible due to the number of
inter-related factors which influence compaction and which are effective concurrently. Published data
therefore regularly refer only to a particular set of conditions which are often not specified or controlled.
The various factors which influence compaction are indicated in Table 3 together with the appropriate
references dealing with these factors.
12
TABLE 3
Factors involved in compaction of bituminous materials together
with references to publications dealing with these factors
Factors
Aggregate type 13 22 23
Aggregate gradation 23 26
Binder proportion 23 26 ~A5[49 19/20
f Filler proportion 17[ 18
Temperature/ Viscosity 14 16 17/18 21
Subgrade 24 26 27/28 44
Thickness 29 31 32/33 36/37
Roiling equipment 16 26 27/28 29
Rolling procedures 26 27/28 29 30
Densification after
construction 24 25
References
25 26 31 32/33 44-49
46 34 35
30 31 32/33 36/37
31 44 45-49
44-49
The overall impression gained from the literature is that the mode of compaction must be tailored to the particular conditions existing for a construction project in order to give optimum results. This is difficult
with our existing knowledge of the subject and further research work is clearly required.
The limitations of laboratory work have been discussed. However this approach is often useful because
it is generally less time-consuming and cheaper. Field work is costly and conditions exist which are difficult
to identify and control. The best approach must be with pilot-scale laboratory experiments which provide
a means of better control of factors and the information thus gained is more readily applicable to field
construction projects. In this respect, the studies of the compaction of concrete carried out at the Road
Research Laboratory s° ' s l provide an interesting example of the necessary methodology, particularly in
relation to the investigation of the effects of vibratory compaction. However for thick-lift studies of
bituminous materials there are problems associated with handling the large quantities of material involved in
addition to the high temperatures of materials during laying. In this case pilot-scale laboratory experiments
require completely new equipment to be designed and constructed.
qhere is a lack of quantitative information on the compactibility of mixes used in the United Kingdom
and laboratory work similar to that of Lefebvre and Robertson 23 could give useful results in this respect.
Further work on thermal aspects of laying bituminous materials may enable the heat losses to the atmosphere
and underlying layer to be reduced and thus improve compaction. Finally a comparison of the different
13
modes of compaction including compaction plant and operation procedures, under the various conditions
existing in construction projects, would help to improve the efficiency of achieving adequate compaction.
The relationship between compaction of bituminous materials and pavement performance has not been
accurately established and it is, therefore, not possible to indicate with confidence a degree of compaction
which is adequate for good pavement performance. Further research is necessary to relate compaction of base,
base course and wearing course mixtures currently used in the U.K. to load spreading properties of pavements in addition to stability and durability.
6. ACKNOWLEDGEMENTS
This literature review has been prepared as part of the programme of co-operation research work between the Asphalt and Coated Macadam Association and the Road Research Laboratory. The assistance of staff of the Road Research Laboratory, in particular Mr. D.H. Mathews, and members of the Asphalt and Coated Macadam Association in the preparation of this Report is gratefully acknowledged.
7. REFERENCES
1. DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, ROAD RESEARCH
LABORATORY. Bituminous materials in road construction. London, 1962 (H.M. Stationery Office).
2. MINOR, C.E. Asphalt pavement - placed and compacted in thick lifts.The Asphalt Institute,
Information Series No. 138 (1S- 138). Maryland, 1966 (The Asphalt Institute).
3. LOWEI G. Compaction of bituminous materials. FCMIFederation Lectures 9th Series. 1963 (Federation of Coated Macadam Industries).
4. BRITISH STANDARDS INSTITUTION. British Standard 594 : 1961. Rolled Asphalt (Hot Process). London 1961 (British Standard's Institution).
5. BRITISH STANDARDS INSTITUTION. British Standard 802 : 1967. Tarmacadam with crushed rock or slag aggregate. London, 1967 (British Standards Institution).
6. BRITISH STANDARD'S INSTITUTION. British Standard 1621 : 1961. Bitumen macadam with
crushed rock or slag aggregate. London, 1961 (British Standards Institution).
7. MINISTRY OF TRANSPORT. Specification for road and bridge works. London, 1969 (H.M. Stationery Office).
DILLARD, J.H. Comparison of density of Marshall specimens and pavement cores. Proc. tech. sess. Ass. Asph. Pay. Technol., 1955, 24, 178-209.
RUTH, B.E. and SCHAUB, J.H. Gyratory testing machine simulation of field compaction of asphaltic concrete. Proc. tech. sess. Ass. Asph. Pay. Technol., 1966, 35, 451-80.
.
.
14
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
GOETZ, W.H. Comparison of Triaxial and Marshall test results. Proc. tech. sess. Ass. Asph. Pay.
Technol. 1951, 20, 200-245.
SALEHI, M. A study into the internal structure and flexural strength properties of bituminous paving materials. Ph.D. Thesis, University of Birmingham, Department o f Transportation and Environmental
Planning, 1968 (University of Birmingham).
LEES, G., and SALEHI, M. Orientation of particles with special reference to bituminous paving
materials, Highw. Res. Rec. No. 253, 1969, 63-75.
PUZINAUSKAS, V.P. Influence of mineral aggregate structure on properties of asphalt paving
mixtures, Highw. Res. Rec. No. 51, 1964, 1-15.
KIEFER, R.W. Effect of compaction temperature on properties of bituminous concrete, ASTM spec.
tech. pub. No. 294, 19-25, Philadelphia, 1960 (American Society for Testing Materials).
AMERCIAN SOCIETY FOR TESTING MATERIALS. Book of standards. Part 4. Philadelphia, 1958
(American Society for Testing Materials).
PARKER, C.F. Steel tired rollers, Highw. Res. Bd. Bull. 246, Washington, 1959.
BAHRI, G.R., and RADER, L.F. Effects of asphalt viscosity on physical properties of asphaltic
concrete, Highw. Res. Rec. No. 67, 1965, 59-83.
KHANNA; S.K., and RADER, L.F. Effects of mixing viscosity and compacting viscosity on physical
properties of tar concrete,Highw. Res. Rec. No. 158 1967, 32-48.
RIGDEN, P.J. The rheology of non-aqueous suspensions. Department o f Scientific and Industrial
Research, Road Research TechnicalPaper No. 28. London, 1954 (HaVI~ Stationery Office).
LEE, A.R. and RIGDEN, P.J. The use of mechanical tests in the design of bituminous road-surfacing
mixtures. Part 1. Dense tar surfacings. J. Soc. Chem. lnd., London, 1954, 64(6), 153-61.
LEFEBVRE, J.A. Effect of compaction on the density and stability of asphalt paving mixtures. Proc.
lOth Annual Conf. Can tech Asph. Assoc. 1965, 23-109.
RUIZ, C.L and DORFMAN, B. Sobre la medida de la compactacion y de la compactibilidad de las mezclas asfalticas del tipo superior. Comision Parmanente del Asphalto, Buenos Aires Argentina
Decimoquinta Reunion del Asfalto, Mar del Plata, 1968, 189-208.
LEFEBVRE, J.A. and ROBERTSON, W.D. Effect of mineral aggregate characteristics on the compactibility of asphalt paving mixtures, Proc. 14th Annual Conf. Can tech Asph. Assoc., 1969.
GOODE, J.F., and OWlNGS, E.F. A laboratory - field study of hot asphaltic concrete wearing course mixtures. ASTM spec. Techn. Publ. No. 309, 1-21. Philadelphia, 1961 (American Society for Testing
Materials).
15
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
ADAM, V., Effects of viscosity in bituminous construction. ASTM spec. tech. Publ. No. 309, 121-132. Philadelphia, 1961 (American Society for Testing Materials).
GRAHAM, M.D., BURNETT, W.C., THOMAS, J.J., and DIXON, W.C. Pavement density - what influences it,Proc, tech. sess. Ass. Asph. Pay. Technol., 1965, 34, 286-304.
FROMM, H.J., The compaction of asphaltic concrete on the road. Proc. tech. sess. Ass. Asph. Pay. Technol, 1964, 33, 241-284.
FROMM, H.J.~ and PHANG, W.A. The compaction of asphaltic concrete on the road. Part II. Proc.
tech. sess. Ass. Asph. Pay. Technol. 1966 351 529-47.
KILPATRICK, M.J. and MCQUATE, R.G. Bituminous pavement construction, U.S. Department of
Transportation, Research and Development Report. Washington D.C., June 1967. (U.S. Bureau of Public Roads).
SERAFIN, P.J., and KOLE, L.L. Comparative studies of pneumatic tyre rolling, Proc. tech. sess. Ass. Asph. Pay. Technol. 1962, 31, 418-39.
MCLEOD, N.W. Influence of viscosity of asphalt-cements on compaction of paving mixtures in the field. Highw. Res. Rec. No. 158, 1967, 76-111.
BEAGLE, C.W. Compaction of deep lift bituminous stabilized base. Proc. tech. sess. Ass. Asph. Pay. Technol, 1966, 35, 549-65.
BEAGLE, C.W. Single lift construction with hot plant mix base. Highw. Res. Board, Highw. Res. Circ. No. 46 Washington, D.C., 1966 (National Research Council).
MINISTRY OF TRANSPORT, ROAD RESEARCH LABORATORY, Road Research 1967. Annual Report of the Road Research Laboratory, London 1968 (H.M. Stationery Office).
MINISTRY OF TRANSP.ORT, ROAD RESEARCH LABORATORY, Road Research 1968. Annual Report of the Road Research Laboratory, London 1969 (H.M. Stationery Office).
36. MINOR, C.E. Asphalt paving techniques. Pacif. Bldg. Engr., 1967 (February), 65-7.
37.
38.
39.
40.
MINOR, C.E. Construction of asphalt pavement in thick lifts. 52nd Annual Meeting of the Am. Ass. State Highw. Officials, Kansas, 1966.
HUGHES, C.S., and RALSTON, H.H. Field testing of a nuclear density device on bituminous concrete. Proc. tech. sess. Ass. Asph. Pay. Technol, 1963, 32, 106-42.
BROWN, W.R. Development of nuclear density tests for hot asphalt pavement. High Res. Rec. No. 107, 1966.
BEHR, H. Uber die tiefenwirkung von oberflachensonden bei dichtemessungen im strassenbau mit gammastrahlung. Str. Autobahn, 1966, 17, (6), 207-13.
16
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
KARl, W.J. and SANTUCCI, L.E. Control of asphalt concrete construction by air permeability test. Proc. tech. sess. Ass. Asph. Pay. Technol. 1963, 32, 148-62.
O'FLYNN, O.T. Compaction control of bituminous concrete by air permeability measurement. Aust. Road Res. Board Proc. 3rd Conf. 1966 Volume 3 Part 2, 1451-71.
HEIN, T.C., and SCHMIDT, R.J. Air permeability of asphalt concrete. ASTM spec. tech. publ.
No. 309, 49-62. Philadelphia, 1961 (American Society for Testing Materials).
SWANSON, R.C., NEMEC, J. and E. TONS. Effect of asphalt viscosity on compaction of bituminous concrete. Highw. Res. Record No. 117, 1966, 23-52.
SCHMIDT, R.J. Full-scale asphaltic construction in the research laboratory. Highw. Res. Bd. Bull. 251, 1960, 1-1 I.
SCHMIDT, R.J., KARl, W.J., BOWER, H.C., and HEIN, T.C. Behaviour of hot asphaltic concrete under steel wheel rollers, Highw. Res. Bd. Bull. 251, 1960, 18-37.
HEIN, T.C., and SCHMIDT, R.J. Density changes in asphalt pavement core samples. ASTM Annual Meeting, Atlantic City, N.J., June 1959. (American Society for Testing Materials).
SANTUCCI, L.E., and SCHMIDT, R.J. Setting rate of asphalt concrete. Highw. Res. Bd. Bull. 333, 1962, 1-8.
SCHMIDT, R.J., and SANTUCCI, L.E. Influence of asphalt type on pavement setting rate. Highw.
Res. Bd. Bull. 333. 1962, I0-19.
KIRKHAM, R.H.H., and WHIFFIN, A.C. Experiments on the vibration of freshly placed concrete
The Engineer, 1952, February 15, 1-8;
KIRKHAM, R.H.H. Recent research into the construction of concrete pavements. Proc. Instn. cir.
Engrs, 1964, 27, 241-62.
17
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P R I N T E D I N E N G L A N D
ABSTRACT
Review of literature on compaction ofbituminous materials: W D POWELL: Department of the Environment, RRL Report LR 405: Crowthorne, 1971 (Road Research Laboratory). The literature reviewed in this report covers laboratory and field work on the compaction of bituminous materials. A large number of factors which influence compaction are con- sidered in an attempt to provide a general indication of the various trends that may be expected to occur in practice. The relevant factors include aggregate type and gradation, mix composition, type of subgrade, rolling equipment and procedures, lift thickness and rolling temperature.
The difficulties involved in direct application of results of laboratory and field studies to construction work in general are stressed. Pilot-scale laboratory experiments provide a means Of better control of factors and yield information which may be more readily and directly applicable to field construction projects.
ABSTRACT
Review of literature on compaction of bituminous materials: W D POWELL: Department of the Environment, RRL Report LR 405: Crowthorne, 1971 (Road Research Laboratory). The literature reviewed in this report covers laboratory and field work on the compaction of bituminous materials. A large number of factors which influence compaction are con- sidered in an attempt to provide a general indication o f the various trends that may be expected to occur in practice. The relevant factors include aggregate type and gradation, mix composition, type of subgrade, rolling equipment and procedures, lift thickness and rolling temperature.
The difficulties involved in direct application of results of laboratory and field studies to construction work in general are stressed. Pilot-scale laboratory experiments provide a means of better control of factors and yield information which may be more readily and directly applicable to field construction projects .
