Aggregate characterisation in relation to
bitumen-aggregate adhesion
D.Q. van Lent, A.A.A. Molenaar, M.F.C. van de Ven
TU Delft
Summary
Adhesion between bitumen and aggregate is one of the most important factors affecting the
service life and durability of asphalt mixtures, especially for Porous Asphalt Concrete
mixtures. The Road and Hydraulic Engineering Institute of the Dutch Ministry of Transport,
has started a research into ravelling of Porous Asphalt Concrete. As part of this research, a
quantification of the adhesion between bitumen and aggregate is made in the laboratory of
Road and Railway engineering at the Delft University of Technology. For this quantification
of the adhesion between bitumen and aggregates several experiments are conducted on
bitumen-aggregate specimens. The bitumen-aggregate specimens are assembled from two
stone columns with a small bitumen film in between. To obtain the stone columns some stone
treatments were performed. First large boulders are sawed in slices. These slices are
sandblasted to improve the reproducibility. From the slices small stone columns are drilled
using a column drill. Afterwards the stone columns are cleaned by boiling them in
de-mineralised water. Because of these treatments the surface characteristics of the stone
samples might have changed. In this study the effect on the aggregate surface characteristics
of the treatments is investigated and how these surface characteristics of the stone samples
relate to the same characteristics of 4/8 aggregates used by contractors in Porous Asphalt
mixtures.
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1. Introduction
General
For reasons of traffic noise reduction much of the Dutch primary road-network is surfaced
with Porous Asphalt Concrete. Additional advantage of Porous Asphalt Concrete is the
significant reduction of splash and spray in wet weather conditions. However, the service life
of porous asphalt concrete is much lower than the service life of dense asphalt concrete. The
surface life of Porous Asphalt Concrete is limited by the development of ravelling of the
surface. Ravelling may cause a reduction of driving comfort to the road users, loss of
aggregate from the top layer, a decrease of traffic safety and a strong increase of traffic noise.
The DWW, the Road and Hydraulic Engineering Institute of the Dutch Ministry of Transport,
has started research into ravelling of Porous Asphalt Concrete. As part of this research the
Delft University of Technology developed a mechanistic design tool for Porous Asphalt
Concrete, called Lifetime Optimisation Tool (LOT).
In the LOT research program [1,2,3,4] a characterisation is made of the mechanical behaviour
of the adhesive zone between the aggregate particles and the bituminous mortar. Also the
bituminous mortar in Porous Asphalt Concrete mixtures was tested. Several mechanical tests
were conducted on mortar and stone-bitumen specimen. To avoid scale effects it was decided
to perform the tests at a similar scale as in practise, i.e. the scale of individual aggregates. It
was considered of great importance to get the size of the aggregate specimens as used in the
test program as constant as possible. To get similar sized samples with similar surface
properties, the aggregates received all kinds of treatments.
Stone column specimens are used in DMA en DSR fatigue tests to give insight into adhesive
zone fatigue behaviour. In principle circular column specimens with a 6.7 mm diameter are
used. Between two stone columns a 15 μm interface binder layer is assembled (figure 1).
Figure 1: Stone column specimen with a 15 μm interface binder layer for use in DSR. [4]
To obtain the stone columns some stone treatments were performed. First slices of about 10
mm thickness were sawed from boulders of Bestone and Greywacke of approximately 5 kg.
The sawed surfaces of the slices are sandblasted to get a more reproducible surface (figure 2,
left). The small columns were drilled from the sandblasted slices using a column drill (figure
2, right). To remove grease from the sawing and drilling process the stone columns were
boiled in de-mineralised water for 15 minutes.
2
Figure 2: Sandblasting of slice (left) and drilling of the columns from a sandblasted slice. [4]
Research question
The effect of these treatments on the adhesive zone characteristics for the individual tests is
unknown. The question arises how the surface characteristics of the stone samples were
affected by the different treatments and how the surface characteristics of the stone samples in
the laboratory relate to the same characteristics of stones used by contractors in asphalt
concrete mixtures.
The stone characteristics affecting bitumen-aggregate adhesion are investigated. Especially
the effects of the surface treatments, used in the LOT research program on these stone
characteristics are investigated. The results of this exploring study will give an insight in the
aggregate characteristics affecting the bitumen-aggregate adhesion as well as an insight into
the effects of the different treatments on the surface characteristics.
2. Experimental programme
A literature review [5] showed that the chemical and mineralogical composition of the
aggregates has a strong influence on adhesion. The minerals and chemical elements at the
surface do not only affect the Arrhenius acidity of the aggregate, but the surface free energy is
affected as well. Geometry has, especially according to the mechanical adhesion theory, an
important influence on the adhesive bond between bitumen and aggregate. Shape, angularity,
size and roughness all determine this geometry and have their effect on the specific surface
area of the stones, which is an important parameter on adhesion according to all four adhesion
theories. According to the mechanical adhesion theory also the geometry of the aggregates,
the porosity and the pore size distribution are affecting the interlock with the bitumen. The
chemicals interacting with the bitumen can be from the aggregate surface, but also from dust
and other surface impurities. The adhesive bond is affected by it.
In this paper the aggregate properties affecting the bitumen-aggregate adhesion are
investigated. Based on the literature study [5] the aggregate properties influencing the
bitumen-aggregate adhesion are given in figure 3. Figure 3 also shows the order in which the
aggregate properties are discussed in chapter 3 and the evaluation methods of the aggregate
properties.
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Figure 3: Aggregate properties affecting adhesion and their evaluation methods.
Materials
In this paragraph the sample preparations of the materials before starting the experiments are
discussed.
In the LOT research program two different aggregate types are analysed, being Greywacke
and Bestone. On these stones a number of treatments is carried out. Some stone samples are
sandblasted, some are only drilled and some are both drilled and sandblasted. An example of
some samples is given in figure 4.
Figure 4: From left to right: untreated sample, sandblasted sample, drilled column samples,
drilled and sandblasted column samples, 4/8 aggregate.
In order to allow comparisons to be made, also real aggregate particles of Greywacke and
Bestone as used in asphalt mixtures are obtained. These Greywacke and Bestone aggregate
particles have a sieve size of 4/8 mm.
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3. Test results
It was not possible to give all the test results in this paper. For complete and quantified test
results reference is made to [5]. In this chapter the analyses and discussion of the results are
given.
Roughness
Macro roughness
It is found that the 4/8 aggregates of both the Greywacke and the Bestone have a higher
measured roughness than the sawed slices at 7 times magnification. This is an indication that
the sawing results in a less rough surface on macro level. In the same way it is found that the
slices of both the Greywacke and the Bestone have less roughness than the same slices after
sandblasting. So this sandblasting results in an increase in roughness. This increased
roughness is still less than the macro surface roughness of the 4/8 aggregates.
It should be mentioned that not too much weight should be given to the values and the
differences between the treatments at this stage, because only one measurement per sample
has been conducted. The results show however that the macro roughness of the 4/8 aggregates
is higher than the roughness of the slices. The sandblasted slices have a higher roughness than
the sawed slices.
Meso roughness
The meso level roughness is determined at 20 and 50 times magnification. The measured
roughness of the 4/8 aggregates is larger than the measured roughness of the column samples.
Here as well, the results indicate that sawing reduces the roughness index in comparison to
the 4/8 aggregates. Sandblasting the sawed columns increases the roughness, but the
roughness is still less than the measured roughness of the 4/8 aggregates. At meso level this
holds for Greywacke as well as for Bestone.
Micro roughness
At 100 times magnification the electron microscope pictures of the surface of the Greywacke
sandblasted columns and the sawed columns show no clear visual difference in roughness.
Over the entire surface of the samples areas with lower and higher roughness are found.
However the number of areas with a relative higher roughness dominates. It can be concluded
that sandblasting of the Greywacke columns doesn’t result in a visual difference in micro
roughness.
Also for the Bestone column samples the micro roughness is difficult to differentiate visually.
All the Bestone columns show areas with a low roughness and areas with a high roughness on
their surfaces. So also here, no clear indication is found that sandblasting of the columns
results in a difference in the micro roughness. The visual difference of the micro roughness
between the Bestone 4/8 aggregates and the columns is larger than the difference between
Greywacke 4/8 aggregates and columns. The Bestone 4/8 aggregates show on their surface
more areas with a relative low roughness compared to the sandblasted and sawed columns.
An explanation for this can be that more dust is present on the surface of the 4/8 aggregates.
Another possibility is that the 4/8 aggregates are more scraped and crushed onto each other,
resulting in flat areas that are visible at 100 times magnification.
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To quantify the micro roughness on the electron microscope pictures the wavelet method [6]
is used. The quantitative results of the Greywacke pictures give a more clear distinction
between the micro roughness of the differently treated samples. On the sandblasted
Greywacke columns more areas with fine texture are found than on the Greywacke sawed
columns. More coarse texture areas are found on the sawed columns than on the sandblasted
columns. This indicates that the sandblasting of the Greywacke columns results in more areas
of fine texture and thus micro roughness is added. Comparing the quantitative results of the
Greywacke columns to the Greywacke 4/8 aggregates results in the observation that the 4/8
aggregates have less areas with fine texture and more areas with coarse texture than both
sawed and sandblasted column samples. So in contrast to the macro and meso roughness, the
Greywacke sawed columns have more fine texture at micro level than the Greywacke 4/8
aggregates. With sandblasting additional fine texture is added to the Greywacke samples.
As mentioned above, a difference in micro roughness between the Bestone columns and the
Bestone 4/8 aggregates is detected by means of the electron microscope pictures. This finding
is contradicted by the quantitative analyses. According to the quantitative analyses, the
Bestone sawed columns and the Bestone 4/8 aggregates have an equal percentage of areas
with fine texture. It is found, similar to the Greywacke samples, that by sandblasting the
sawed Bestone columns, micro texture is added to the columns. The quantitative results show
a shift from medium and coarse texture at the Bestone sawed columns to the fine texture at
the Bestone sandblasted columns.
The differences found between the visual pictures and the quantitative results can however be
explained. For instance clear visible flat areas are found on the pictures of the Bestone 4/8
aggregates. Such areas are not found on the pictures of both the sawed and sandblasted
columns. However these areas can be included in very coarse and very fine texture in the
quantitative analyses. For instance when these flat areas are viewed with a higher
magnification than 100 times, they might still be flat. In this case the flat areas should be
included in the coarse areas. However when the flat areas show roughness at a higher
magnification the areas should be included in the fine roughness areas.
Figure 5: Indicated areas on Bestone 4/8 aggregate which are fine or coarse textured in the
quantitative analyses.
Specific surface area
At all magnifications the measured specific surface area of the 4/8 aggregates of both the
Greywacke and Bestone is larger than that of the sandblasted and sawed columns. At all
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magnifications the sandblasted columns have a larger specific surface area than the sawed
columns. This indicates that, for both the Greywacke and the Bestone columns, sandblasting
results in an increase of the specific surface area. This increased specific surface area of the
sandblasted columns is however found to be less than the specific surface area of the 4/8
aggregates of both Greywacke and Bestone, at 7, at 20 and at 50 times magnification.
It is important to mention that the measured specific surface area of all samples increases with
increasing magnification. An explanation for this could be that with increasing magnifications
one obtains a better representation of the real surface. The problem however is that at higher
magnifications the measurements become less representative, because a smaller area is
measured. It is not expected that the real values could be measured using whatever
microscope magnification, because of the unknown errors and the increasing values of the
measured specific surface area. Therefore other methods should be searched to measure the
specific surface area of stone samples.
Porosity
No significant differences were found between the aggregate samples. Also between Bestone
and Greywacke no significant differences were found.
Chemical and mineralogical composition
The ESEM chemical analysis of the surface of the samples doesn’t show a clear distinction
between the treated Greywacke samples and between the treated samples on one hand and the
Greywacke 4/8 aggregates on the other. However for Bestone, a difference in concentrated
individual elements on the surface was found between the 4/8 aggregates and the Bestone
columns. The Bestone 4/8 aggregates didn’t show any concentrated elements on the surface,
but the Bestone columns did. No clear distinction was visible between the Bestone sawed
columns and the Bestone sandblasted columns. This indicates that sandblasting has no effect
on the presence of major chemical elements on the surface of the Greywacke and the Bestone
samples. However a difference in chemical elements between the Bestone columns and the
Bestone 4/8 aggregates is found.
The result from this ESEM chemical analysis is just qualitative. By performing the analyses
over an area of at least 0.5 cm², a more reliable and predictable result was tried to achieve for
stones with the same treatment and origin. It should be noticed that stones in general have a
very heterogeneous character and that the elements found on the surface are influenced by a
large number of uncontrollable external factors. In this particular case the ESEM chemical
analyses gave no evidence that sandblasting and sawing has influenced the concentrated
individual elements visible on the surface of the Greywacke samples. The difference in result
between the Bestone 4/8 aggregates and the Bestone treated columns doesn’t directly have to
do with the treatments. A possibility is that dust of crushing at the quarry is present on the 4/8
aggregates. The possible presence of dust on the 4/8 aggregates however would affect the
bitumen-aggregate adhesion, which is not taking into account by using the columns.
The quantitative XRF chemical analyses of the Greywacke samples show the largest
difference between the Greywacke 4/8 aggregates samples and the Greywacke laboratory
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samples which are prepared from the 5 kg boulder. This difference is found in the CaO
content. The CaO content of the 4/8 aggregates is 11.48 % and 14.57 %, but for the
Greywacke laboratory samples, which are made from the 5 kg boulder this content, is 5.09 %,
5.11 % and 5.10 % respectively. However the silica content is almost constant over the
Greywacke samples. The explanation that due to sandblasting more silica is present on the
sandblasted columns is not likely. The contents of all oxides like, aluminium oxide (Al2O3),
potassium oxide (K2O), magnesium oxide (MgO), sodium oxide (NaO) and iron oxide
(Fe3O2), show larger differences between Greywacke 4/8 aggregates samples and the
Greywacke laboratory stones than between the 4/8 aggregates mutually and the laboratory
stones mutually. Therefore it is expected that not the treatments of the samples have caused a
distinguishable effect on the chemical content, but more the origin of the samples. It is known
that the content per stone type can differ, not only per quarry, but per location in a quarry as
well.
The quantitative XRF chemical analyses of the Bestone samples showed the same trend as the
Greywacke samples. The difference in mass percentage of all oxides mentioned before is
smallest between the 4/8 aggregates mutually and the laboratory stones mutually. For instance
the iron oxide contents of the laboratory stones are found to be 2.29 %, 2.67 % and 2.60 %.
The 4/8 aggregates showed to have an iron oxide content of 3.50 % and 3.48 %. Also in this
case it is believed that not the treatments of the samples have caused a distinguishable effect
on the chemical content, but more the origin of the samples and therefore the heterogeneity of
the stone type. Important is the content of silica of the sandblasted Bestone column and the
sawed Bestone column. The explanation that due to sandblasting more silica is present on the
sandblasted columns is not likely, because the silica content of the sandblasted columns
(53.19 %) is lower than the silica content of the sawed columns (60.88%).
The XRD mineralogical analyses showed that all Greywacke samples contain the same
minerals. The most significant differences are found in the Greywacke results. A difference in
the presence of the mineral Clinochlore (Mg,Al,Fe)6(Si,Al)4O10(OH)8 between the two 4/8
aggregate samples and the other samples was found. Also did the 4/8 aggregates show a
difference in the presence of the mineral Muscovite (K,Na)(Al,Mg,Fe)2(Si3.1Al0.9)O10(OH)2
compared to the laboratory samples. The most significant difference was found between the
4/8 aggregates samples and the other samples in the presence of Calcite (CaCO3). The results
indicate that the 4/8 aggregates have higher Calcite content. This is substantiated with the
XRF analyses, which also shows a higher content of CaO and CO2 of the 4/8 aggregate
samples in comparison to the laboratory stone samples.
The minerals Quartz, disordered Albite, Calcite, Clinochlore, Muscovite and Orthoclase are
found in all investigated Bestone samples. Only one major difference between the Bestone
samples was found. Between the laboratory samples and the 4/8 aggregates a difference in the
presence of the mineral Clinochlore (Mg,Al,Fe)6(Si,Al)4O10(OH)8 was found.
The results of the mineralogical analyses indicate that the difference between the 4/8
aggregates samples and the laboratory stones is larger than between the 4/8 aggregates
mutually and the laboratory stones mutually. Therefore it is expected that the treatments of
the samples have caused minor effect to the exposed mineralogical content in comparison to
the effect due to the origin of the samples and therefore the heterogeneity of the stone types.
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Acidity
The pH acidities of the differently treated samples are found to be almost the same. One can
argue that the setup of the acidity test was not sensitive enough, because 8 of the 10 stone
groups have almost the same measured acidity. These groups all have a deviation in pH
values of less than 0.1. It is possible that, if the stones were immersed in a little less water, the
measured pH values would deviate much more and the acidity test would be more distinctive.
Surface free energy
The surface free energy of the stones is measured by means of the Sessile Drop Method. The
contact angles were calculated to surface free energy by using the van Oss, Good and
Chaudhury theory [7].
The results showed that the sawed samples have a lower surface free energy than the
sandblasted stone samples. The other stone samples are difficult to compare, because for these
samples only a minimum value was measured. This means that those samples have at least
one non-stable droplet of an used reference liquid (distilled water, diiodomethane and
glycerol). A non-stable droplet is taken into account as 0°. This means that the surface free
energy of the surface might be larger than calculated. Because for all 4/8 aggregates only a
minimum value for the surface free energy was found, the Sessile Drop Method is not suitable
for these surface free energy measurements of stone surfaces.
Conclusions
Surface characteristics of the prepared stone samples and of untreated 4/8 aggregates that are
determined, are:
• Roughness (measured at 7 X, 20 X, 50 X and 100 X magnification)
• Specific surface area (measured at 7 X, 20 X and 50 X magnification)
• Porosity
• Chemical and mineralogical composition
• pH Acidity
• Surface free energy
It was showed that sandblasting influences the roughness of the stone column samples.
Sandblasting increases the surface free energy of the sawed stone samples.
The specific surface area is increased if the sawed stone samples are sandblasted. However
the specific surface area of the 4/8 aggregates is larger than the specific surface area of the
sawed and sandblasted stone columns.
No indication is found that the porosity and the chemical and mineralogical compositions of
the stone samples are affected by any of the treatments.
From the results of this research it is concluded that the treatments used for preparations of
the stone samples for the laboratory experiments have influenced the surface characteristics of
the stone samples.
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