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Welcome to lesson 4.10 design of bituminous mixes part II. This is second part of the
presentation on design of bituminous mixes. And as you recollect this is a part of module
IV which is on pavement design. The main objective of this lesson is to make
the student learn about Marshall Method. In fact in the previous lesson on bituminous
mixed design part I we have covered various aspects such as; what are the important parameters
that are to be considered in designing mixes, what are the requirements of mix design and
we have also identified that air voids which is a volumetric parameter of bituminous mix
is one of the most important parameters to be considered in designing the mixes and mix
design is a process in which we have to identify a proper aggregate skeleton sketcher because
mix after all consists of aggregates of different sizes or any bituminous binder has a binder
and is compacted. So we have to find out the optimum combination of aggregates and binder.
we also have to find out what is an appropriate aggregate skeleton and also the type and then
content of the bitumen that we are going to use and of course we have to also think in
terms of what is an appropriate compaction effort to be used for preparing this specimen
and then for testing and evaluating them.
Therefore we would like to discuss Marshall Method of mix design which is the most commonly
used method for designing bituminous mixes. Also, learn about the selection of optimal
proportions of different components of mixes, what should be the guidelines that we should
adopt in selecting optimum bitumen content, having selected what is the appropriate aggregate
skeleton, aggregate gradation and is also expected that the student would be able to
understand the significance of various test conditions adopted in the mix design procedure
because we are going to test bituminous mixes and then find out various parameters. So these
tests are going to be conducted under various conditions. So, what is the implication or
significance of these different conditions, how they are correlated to actual conditions
and what is the influence of these parameters on the performance of these mixes will be
understood.
As I indicated there are various methods of designing bituminous mixes. A few common methods
are Marshall Method this is the most commonly used method, Hveem method and Superpave mix
design method, as I indicated when we were discussing bituminous binders Superpave refers
to superior performing pavements this is of a more recent development. All these mix designs
mainly involve preparation of laboratory trial mix specimens. That means a number of specimens
would be prepared with various combinations of aggregates and binders and all these specimens
will be tested and then on the basis of the results that we obtain on these trial mixes
optimum combination of binder and aggregates will be selected.
Marshall Method of mix design was developed by a gentleman named as Bruce Marshall in
the late thirties for the Mississippi highway department in the United States. Ministry
of shipping and road transport highways recommends that the Asphalt institute procedure manual
MS2 should be followed for design of bituminous mixes so most of these provisions that we
are going to discuss in terms of Marshall Mix design will be as per Asphalt institute
procedure. The original procedure for designing bituminous mixes that were originally developed
by Marshall especially in terms of the compaction effort used underwent a lot of changes over
all these years so that the mix design corresponds to the actual conditions of traffic, different
climatic conditions that are prevalent now. So the mix design procedure that we are now
adopting is expected to be stimulating different traffic conditions and also various climatic
conditions. And the Asphalt institute MS2 mix design guidelines are mainly meant for
dense graded bituminous mixes.
The main steps involved in Marshall Mix design method are selection of mix type. We have
to first identify what is the type of mix that we are trying to design; is it the bituminous
concrete, is it dense bituminous concrete are any other type of mix. so obviously we
will first try to identify whether I am trying to provide a dense graded mix, or am I trying
to provide a gradation which has got more coarser fraction, am I trying to design a
mix were only the surface characteristics are important, should I get good surface characteristics
or am I designing a mix where rutting is a major problem because of high temperatures,
heavy loads etc.
So, according to the requirement you have to select what is the type of mix that we
are going to select. And having done that we will select what is the maximum aggregate
size and select an appropriate aggregate gradation. Maximum aggregate size as we have indicated
earlier will be selected on the basis of the thickness of the layer that is going to be
provided and usually most agencies have a specified gradation given for each mix. So
normally it is expected that those are to be followed unless it can be shown that deviating
from the specified gradation is for good and if you can convince the agency then you can
go for other gradations other than what has been specified by either a [montage] or other
gradation.
The minimum layer thickness is usually more than two to three times the maximum aggregate
size. This is just to give an indication of what can be the maximum size of aggregate
that you can use. It is a function of thickness of layer that we are going to provide. Similarly,
the aggregate fractions are usually designated as coarse, fine and then filler. Coarse is
the aggregate that is retain on 2.36 mm size, fine aggregate is passing 2.36 mm size and
retained on 75 micron sieve, mineral filler is that which passes 75 micron sieve.
The next step that we follow in Marshall Mix design method is the selection of binder that
is type of binder also has to be selected. We had in the earlier lessons given guidelines
of what is the type of binder to be selected for different traffic conditions and for different
climatic conditions represented in terms of what is the maximum temperature and also what
is the minimum temperature. So we know how to select an appropriate grade of binder for
different situations.
After selecting the binder we also have to select the aggregates. There are specifications
available for what is the quality of aggregates that we have to use for different types of
layers. If an aggregate is to be used in surface coarse it will have to satisfy different requirements,
if it is used in a binder coarse its requirement will be different so we have to select the
binder and aggregates satisfying the requirements for a specific project and for a specific
layer. These are to be selected on the basis of traffic and climatic conditions and these
metals have to be tested for the source properties. As soon as we get these materials from source
normally these are tested and once they are satisfied subsequently they may be tested
at regular intervals. The next step is to select design aggregate gradation considering
the traffic and climatic conditions.
The next step is preparing test specimens of bituminous mixes using the aggregate gradation
that is selected and the binder that is selected with appropriate compaction effort. We also
have to select what is an appropriate compaction effort. Ministry of shipping and road transport
suggests a specified compaction effort as per as the specified procedure for all highways.
For all highways we are talking about heavy traffic volumes so we are talking about compaction
that is produced by heavy traffic over some period of time.
Therefore we are talking about heavy compaction and rather all these specifications are of
the ministry of shipping and road transport and highways that is based on heavy compaction
effort. But if you are designing mixes for roads having very low traffic we can go for
smaller compaction efforts so the corresponding equipment the corresponding compaction effort
also can be used and the mixes are tested corresponding to that compaction effort. Thus
we also have to select a compaction effort then those specimens that are prepared will
be tested.
We have to make a number of additional trails. if the initial trails are made when these
specimens are tested they do not yield required properties or they do not satisfy the specifications
that are meant for a specific mix. Then the final step would be the selection of optimum
binder content as per the specified criteria; how to select the optimum binder content,
what is the criteria to be adopted for selecting the optimum binder content also is usually
specified in a given design procedure.
The steps include grading of commercially available mineral aggregates because we are
not going to have aggregates crushed and then sieved as per individual sieves, but we are
going to get aggregates like either twenty mm aggregates, twelve mm aggregates or in
whatever manner it is supplied so we would select appropriate sizes of aggregate that
are available, aggregates that are having suitable quality then those aggregates will
have to be blended as we have discussed in the previous lesson. so the proportioning
of mineral aggregates will have to be done by blending then we have to find out this
specific gravities of the binder and the aggregates.
The specific gravity of binder can be determined using a pigma meter method. The specific gravity
of aggregates has to be determined for their bulk specific gravities and also for the apparent
specific gravity though normally apparent specific gravity is not used in mix design.
Then we have to prepare Marshall Specimens because we are talking about Marshall Method
of mix design then using the selected aggregates and after blending the aggregates we mix the
aggregates in the blending proportion that we arrived at. It should then give you a grading
that is within this specification limit.
Therefore using that blended portion and the binder that we have selected both have to
be taken together and then Marshall Specimen has to be prepared. These Marshall Specimens
have to be tested for the bulk specific gravity. Marshall Specimen is nothing but adding aggregates
and bitumen together and compacting them. These specimens have to be tested for their
bulk specific gravity of the compacted specimen, they have to be tested for stability, they
have to be tested for flow and we also have to find out the specific gravity of the loose
mix. We have discussed in the previous lesson terms such as void-less, volume of mix which
would give you Gmm which is the maximum specific gravity of the loose mix, these are the parameters
that we have to measure.
So using this information we can calculate the air void content in the compacted mix,
we can calculate the percentage voids in the mineral aggregate filled by the binder or
bitumen and using all these information we can also calculate other volumetric parameters
and using all these information we will select an appropriate binder content known as optimum
binder content. Then we will check at this optimum binder content what are the various
parameters the mix will have in terms of strength, in terms of flow, in terms of various volumetric
parameters so at optimum binder content what are the properties this mix is going to have.
So those properties have to satisfy these specifications that are given for the mix.
For testing the samples we have to determine the bulk specific gravity of the specimens.
Once we compact the specimen prepare a specimen by adding aggregates and binder together and
compact them and then we have a specimen prepared. That compacted specimen's bulk specific gravity
has to be determined and all these specimens will have to be conditioned by keeping them
in a water bath and maintained at 60 degree centigrade for period of thirty to forty minutes
duration.
Basically the idea is to test these specimens at a temperature of 60 degrees. For this we
have to condition these specimens by putting them a water bath. This is the way how it
has to be conditioned which is maintained at 60 degrees and the conditioning has to
be done for about thirty to forty minutes. These condition specimens will have to be
tested in a Marshall testing apparatus for determining the stability and flow of each
one of these specimens. These stability values have to be corrected for non standard height
or volume.
We will discuss about what are the standard dimension of this specimen that we are expected
to maintain in terms of its diameter and also in terms of the height that is expected to
be attained. So, we are talking about a standard size specimen but it is not always possible
to get the same height because of the compaction effort that we put and also because of the
mass of the total aggregate and binder that we take so we may get different heights. So
the volume of the specimen is going to be different and as the volume differs this stability
value that is attained in the Marshall testing machine will have to be corrected to correspond
to a standard volume. There are correction factors available that can be done. Then the
test should be completed within thirty seconds after removing from the water bath. This is
to ensure that the temperature does not fall below 60 degree centigrade, there should not
be any significant difference so we should not wait for two minutes or three minutes
before the test process is completed. As soon as this specimen is removed from the water
bath which is at 60 degree centigrade the test should be completed quickly.
The standard Marshall Method involves preparing a 4 inch diameter specimen, 102 mm dia and
2.5 inch height or thick specimen which is about approximately 64 mm height of bituminous
mix with a selected gradation of aggregates and binder content. As we have already discussed
we are going to select aggregate gradation and also binder type and then certain binder
content. So with this compaction effort that we adopt we should be able to produce 4 inch
dia specimen having a height of about 2.5 inches. The standard compaction effort used
is by dropping a hammer of 4.5 kg 10 pounds mass through a free fall of 457 mm that is
18 inches. So the compaction is done by using a compaction Marshall hammer weighing 4.5
kg and falling from a height of 457 mm. As we are using a 4 inch dia specimen normally
the maximum size of aggregate that we can use is one inch or 25.4 mm. But if you want
to test the mix having a gradation having a larger size more than 25.4 mm where some
of these mixes have got more than 30, 40 mm size so if you want to test those mixes you
have to prepare larger size specimen. Normally a 6 inch specimen also can be tested.
We have to prepare a series of test specimens with the selected aggregate blend and with
different binder contents. So we will keep the aggregate gradation fixed the blend fix
but we will go on varying different binder contents. Thus for each binder content we
will prepare number of samples. therefore will have specimens prepared at varying binder
contents and then we can find out what is effect of varying binder contents on various
mix parameters and then select one of these binder contents or any optimum binder content
which gives us optimum performance.
if you do not have an idea of what could be the range within which optimum binder content
is going to lie the selection of initial trial binder content can be made as P = 0.035 multiplied
by a + 0.045 multiplied by b + K into c + f. P is the approximate binder content this
is expressed as the percentage by weight of total mix whereas 'a' is the percentage of
aggregate retained on 2.36 mm sieve, 'b' is the percentage of aggregates passing 2.36
mm sieve and retained on 0.075 mm sieve, 'c' is the percentage of aggregate passing 0.075
mm sieve, 'k' has got different values 0.15, 0.18, 0.20 for various percentage of filler
which is the material passing 75 micron sieve.
For example, if the filler content is 11 to 15% the value of k is taken as 0.15, if the
filler content is less than 5% the value of K is taken as 0.2, F is a value that is to
be selected on the basis of the assessment of absorption by aggregates of bitumen. So
what is the expected quantity of bitumen that is going to be there in the aggregate pores?
If you know the absorption of aggregates the F can vary from 0 to 2%. For a completely
non absorbable aggregate we can take a value of 0 and for highly pores aggregates the value
of two can be taken.
The initial binder content is normally assessed in terms of the film thickness that is required
depending on the gradation. so depending on the size of particles that we have, percentage
of different fractions we can approximately calculate the surface area and we can find
what is the minimum thickness of binder that has to be there coating these aggregates from
the point of view of durability of these mixes. Thus normally the initial trial binder is
on the basis of the film thickness that is required to code these aggregates and also
taking into consideration if these aggregates are absorptive then some amount of bitumen
is going go into these pores. Therefore taking into consideration this is an empirical formula.
Obviously this will serve only as an initial trail thickness so we can take this initial
trail binder content. You can start with this binder content and select other binder contents
on either side of this.
Normally specimens have to be tested are prepared at six different binder contents. And at each
binder content normally three specimens have to be prepared. So, if you take about 25 kg
of blended aggregate and about four liters of binder that would be normally be sufficient
to cover these six binder contents and three specimens at each binder content.
Before we go about preparing these specimens we have to determine the viscosity of binder
at different temperatures using rotational viscometer. We have discussed about Brookfield
viscometer in the previous lessons and then measuring absolute viscosity using rotational
viscometers. So we have to determine the viscosity of bituminous binder at various temperatures.
This exercise is necessary to select the mixing and compaction temperatures.
There are guidelines that are available about what is the consistency of the binder that
we should adopt for compaction process, what is the consistency of the binder that is to
be adopted for mixing the aggregates with binder. Therefore as you seen in this sketch
the specifications are; for mixing typically these the viscosity should be ranging from
0.17 +/-- 0.02 Pascal seconds.
Similarly the viscosity range for compaction is 0.28 +/-- 0.03 Pascal seconds. For example,
if you typically have a plot between temperature and then viscosity on y axis given by this
line you can select the range within which mixing can be done, the range within which
this requirement is satisfied so this temperature range can be selected and similarly you can
find the range within which compaction can be done given by these two temperatures. So
this is normally how we select the range of temperatures for mixing and also the range
of temperature for compaction operation.
For preparing the specimens the compaction of loose hot mix obviously we have to add
aggregates and then binder and then heat it to the temperature that we have just indicated
and then that loose mix will have to be compacted so that we get standard test specimen dimensions.
The mixing temperature is corresponding to a viscosity of 0.17 Pascal seconds similarly
the dry aggregates are heated to a temperature not exceeding mixing temperature plus about
28 degree centigrade, the binder should be heated to mixing temperature which will be
different for different types of binders because those viscosity ranges are attained at different
temperature ranges for different types of binders. The binder plus aggregate mix is
placed in the compaction mould which is to be pre-heated and the loose mix heated mix
is compacted.
As we indicated in the previous slide we have the compaction of loose hot mix to obtain
a test specimen of standard dimensions. Standard compaction effort used is by a 4.5 kg hammer
having a free fall of 457 mm, the maximum size of aggregate that we can use is 25.4
mm and for largest size of aggregates we can also use 6 inch dia moulds.
On the left hand side you see typically a sketch of specimen that we are trying to prepare
and this is the mould that we are using and this is the loose mix that we have placed
here and this is the pedestal of the hammer and this is the mass that is going to fall
from a specified height. The hammer is not seen here so we have the hammer here and this
is the height of fall that we are going to get and below this there is a base plate.
You see the hammer here now, so we also see the height of fall so the hammer will be lifted
up and allowed to drop freely and this exercise is repeated a number of times.
You see two different photographs of an automated Marshall compaction equipment. On the left
hand side you see the hammer. This is the hammer and this is an arrangement in which
you can set the number of blows you want to apply and then the hammer gets lifted up and
then repeatedly dropped and this is where the mould is fixed.
This is a manual Marshall hammer and these are moulds, base plate. Typically for preparing
this specimen MORTH specifies for heavy compaction 75 blows are to be applied using Marshall
compaction hammer on both faces. first you have to compact it on one side then the specimen
has to be reversed then again 75 blows of Marshall hammer have to be applied on the
other face also. So this is the standard compaction that is recommended for all the mixes that
we use for highways. So the specimens that are compacted will have to be extracted from
the mould.
Of course before that we will how to determine this specific gravity of aggregates and also
binders and the specimen that is extracted from the mould we have to find out its bulk
specific gravity of the compacted mix that is Gmb. This is obtained by measuring the
dry mass of the mix, you take the dry mass of this specimen and find out the volume of
water replaced by the saturated surface dry specimen.
The specimen has to be saturated then the surface water has to removed and its weight
has to be taken and then its weight in air has to be taken. Therefore the difference
in weights will give us the volume of water replaced by saturated surface dry specimen
so the dry mass of the specimen divided by volume of water replaced by the saturated
surface dry specimen which is nothing but the bulk volume gives us bulk specific gravity
of the compacted mix.
Then the specimen is put in a Marshall testing machine and Marshall Test is conducted. Marshall
Test is nothing but these are the breaking head put on both sides of the specimen so
a compressive load is applied along the diameter of the specimen at a rate of 51 mm per minute,
we know that the temperature of the specimen is going to be maintained at 60 degree centigrade
and the inside radius of the breaking head is going to be approximately equal to that
of the specimen which is 51 mm so load at this rate is applied.
What we observe is the load at which the specimen breaks. so either in a proving ring or in
a dial case or in any automatic measurement we see the load increasing then after a certain
stage once the specimen fails the load starts decreasing. So we have to observe what the
failure load is and we also have to observe what deformation this specimen undergoes when
this specimen fails. Starting from an initial deformation of zero the deformation at failure
condition has to be observed. So the breaking load is known as stability and the deformation
at failure is known as flow. So these are known as Marshall Stability and Marshall Flow.
This is an automated Marshall testing machine which uses LVDTs and load cells to automatically
measure the load and the corresponding deformation and it can automatically be recorded onto
a computer. Of course you can use other simple equipment also.
This is a water bath which is used to maintain constant temperature. The stability that is
obtained from Marshall testing machine as I said ought to be corrected for nonstandard
volume. If the dimensions attained are 4 inch dia and 2 ½ inch height there would not be
any correction that is required but if mostly the height varies there is certain correction
that is to be applied. For example, if the volume is within 509 to 522 cc there would
not be any correction but if it is more the stability will be reduced and if the volume
is less the stability will be increased.
After the Marshall test is done we will have to carry out volumetric analysis. This is
to estimate important volumetric parameters such as air void content, voids and mineral
aggregate, mineral aggregate voids filled with bitumen and so on for each of these specimens.
And for each binder content we have to determine the maximum specific gravity. This is the
specific gravity of the void-less loose mix this has to be determined and using all this
information we calculate the effective specific gravity of aggregates and carry out the volumetric
analysis to compute air voids VMA and VFB.
A typical example of volumetric analysis is given here. For example, if you have the initial
data where in we have taken three different sources A B C of aggregates and they have
been blended in let us say this proportion 25%, 45% and 30% to obtain the desired gradation
and the bulk specific gravity of source A B C are 2.954, 2.896 and 2.835 respectively
these have been measured. So the bulk specific gravity of the combined aggregate will be
100 divided by 25 is a proportion of aggregate A in the blend divided by the corresponding
bulk specific gravity 2.954 + 45 by 2.896 + 30 by 2.835 so the bulk specific gravity
of the blended combined aggregate is 2.8915.
Let's say we have added five percent bitumen by weight of the total mix and the specific
gravity of the bitumen was measured as 1.03 Gb. This is one specimen that we are talking
about, we have measured the bulk specific gravity of the specimen that is 2.552 and
maximum specific gravity of the loose mix for 5% binder content is 2.729 so the effective
specific gravity of the aggregate Gse which is calculated taking into consideration all
the voids except those that absorb bitumen is given as Pmm -- Pb divided by Pmm by Gmm
-- Pb by Gb where Pb is the proportion of binder, Gb is the specific gravity of binder,
Gmm is the maximum specific gravity of loose mix, Pmm is the percentage of total loose
mix this will of course be 100 so 100 -- 5 divided by 2.729 is the maximum specific gravity
of loose mix for 5% binder -- 5 by 1.03 which gives us 2.9884.
The maximum specific gravity of loose mix for other binder contents can be determined
by preparing specimen set of other binder contents. but that has to be calculated on
the basis of what has been determined for one binder content so this expression can
be used to calculate the maximum specific gravity of loose mixes for other binder contents
given by Gmm = Pmm by Ps divided by Gsc + Pb by Gb. For example, for 6% binder content
100 by 94 divided by 2.9884 + 6 by 6 is the binder that we are referring to divided by
1.03 so that's about 2.6823.
We can compare the specific gravity that we obtained for 5% which is 2.729. The bitumen
absorption for 5% binder content case can also be calculated using this expression that
is 1.155% and the effective bitumen content after deducting the bitumen that has gone
into the surface pores of aggregates can be calculated using this expression Pb -- Pba
into Ps where Ps is the proportion of aggregates divided by 100 so 5 -- 1.155 is the percentage
of bitumen that is observed into 95 is the proportion of aggregates divided by 100 that's
about 3.9%. So we have put 5% bitumen but 3.97% is what is available to effectively
coat the aggregates.
Then we calculate the voids and mineral aggregates using the expression 100 -- Gmb into Ps by
Gsb so it works out to 16.15% and air void content using specific gravity of loose mix
and bulk specific gravity of the compacted mix so -- Gmb by Gmm is expressed as percentage
so air void content in this case for this specimen is worked out to be 6.49%. Voids
filled with bitumen is nothing but total voids in mineral aggregate minus air void and the
rest is bitumen so VMA -- Va divided by VMA is expressed as percentage so this is working
out to 59.81%.
If for example, we have approximately about six binder contents, for each binder content
we have about three specimens so we can take the average of those three specimens. So basically
we have all results for six different binder contents. Thus we have to select an optimum
binder content that is giving us satisfactory properties.
This can be selected by observing various mix parameters with binder content. What we
normally examine is how stability varies with binder content, how flow varies with binder
content, how the unit weight of the total mix varies with binder content, what is the
variation of percentage of air voids, variation of percentage voids and mineral aggregate,
and variation of percentage of voids filled with bitumen.
Typically this is the trend that we expect to get in terms of stability. As binder content
varies it is normally expected to initially increase and reach a peak and then start decreasing
afterwards. The unit weight also is expected to display a similar trend, it will start
increasing initially because of the increased density that is made possible by re-orientation
of the particles which are lubricated by the binder but subsequently once it attains its
densest position any addition of binder having low specific gravity is only going to decrease
its unit weight so we can expect that unit weight is going to decrease after some binder
content. Therefore we are normally interested in binder content that give us maximum unit
weight, we are also interested in binder content that give us maximum stability but they may
not exactly coincide.
This is how the flow is going to increase. As you go on increasing the binder content
typically the flow is going to increase because of increasing binder contents. So we normally
have specifications in terms of range of flow and this is the corresponding range of binder
content within which the specified range of flow is satisfied.
This is how the air void content is going to varying with binder content. As the binder
content is increased air void content is decreased so in the case of air void also this specification
will normally be in terms of range and we can identify what is the corresponding binder
content within which the given range of air void contents can be obtained.
Voids and mineral aggregate usually start decreasing and then start increasing. It is
not necessary that in all cases you exactly get a similar shape. So once you get the shape
depending upon where the minimum of VMA specification is, whether it is here or here or there so
accordingly we can identify what is the binder content that can be selected.
Similarly these are voids and mineral aggregates that are filled with bitumen. So, as the binder
content increases the voids filled with bitumen will go on increasing. So the specification
for this will also be available and the corresponding binder content can be identified.
Thus for selecting the optimum binder content we normally have to select a binder content
that satisfies all the mix requirements that is specifications given by a given agency,
these are to be selected. The specifications should normally be developed on the basis
of performance of mixes under specified conditions. So we believe that whatever specifications
are given by MORTH or Asphalt institute or other agencies are on the basis of observation
of the mixes and about their performance under varying conditions.
The Asphalt institute's main criterion for selection of optimum binder content is a median
value of 4% air void content. So the binder content that gives us 4.4% air void content
is the optimum binder content provided it's satisfies all the other requirements that
are given. If it does not satisfy any particular requirement we can make slight adjustment
to the binder content that we obtain for 4% air void content.
These are the specifications for heavy traffic from the Ministry of Shipping Road Transport
and Highways. The number of hammer blows that are to be applied in preparing a specimen
on each face of the specimen are 75 blows, the minimum Marshall Stability that has to
be attained is 900 kg, the Marshall Flow should be ranging between 2 to 4 mm, the air voids
in the compacted mix should be ranging from 3 to 6% the voids in the mineral aggregates
VMA percentage which is calculated or rather based on maximum size of aggregate we have
different specifications for different maximum aggregate size, I will put this information
in the next slide,
Voids in the mineral aggregates filled by bitumen range from 65 to 75 and the retained
stability on immersion in water at 60 degree centigrade should be a minimum of 80 degree
centigrade. This is the test that has to be conducted the retained stability test on immersion.
This has to be conducted to assess the damage that could be caused to the mixes when it
is subjected to moisture especially in locations where you have heavy rainfall and also when
you are using aggregates that are likely to strip. So we are concerned about the loss
in stability because of moisture. so what we do in this test is we test normal specimens
which are conditioned to normal testing procedure say thirty to forty minutes and we also prepare
a separate set of specimens and put them in water bath for longer specified periods at
60 degree centigrade and test them also and find out the Marshall Stability of those specimens
and compare the condition specimens and see what is the loss in Marshall strength. So
they should have a minimum of eighty percent of retained Marshall Stability.
This is the criteria for minimum voids in mineral aggregates depending on nominal maximum
aggregate size, if we are targeting an air void content of 3% then for 9.5 mm aggregate
which is the maximum size then 14% is the minimum voids in mineral aggregate that has
to be provided. As you see for smaller aggregate size the voids in mineral aggregates are specified
to be larger because we have to put more bitumen into the mix so we have to create more voids
in the mineral aggregates so that we can put more bitumen. More bitumen is required for
smaller size of aggregates because the surface area is going to be more. So the binder that
is required to coat the smaller size fractions will be larger that's why we have to create
more voids in the mineral aggregates so that we can put more bitumen there. On the other
hand when you consider larger size aggregates for example when you see 25 mm size and we
are trying to create let's say 3% air void content then the minimum VMA is 11% for smaller
size.
Those are considered as for MORTH for normal mixes. But as per IRC special publication
53 - 2002 which deals with specifications of bituminous mixes with modified binders,
polymer modified, rubber modified and various other types of modified binders the number
of hammer blows is of course 75 only and these are the parameters that we consider. The requirements
for various climatic conditions such as hot climate, cold climate, high rainfall area
are; the minimum stability for hot climate conditions is 1200, for cold condition it
is 1000 and again if it is high rainfall area we have to have a minimum of 1200 kg minimum
stability. Flow range is 2.58 to 43.5 to 5324.5.
Similarly there is another parameter that is considered which is called as Marshall
Quotient which is nothing but Marshall Stability divided by Marshall Flow which should range
between 250 to 500. Retained stability requirement is also there which is of a minimum of 90%,
95%, 100% and air void requirement is 3 to 5%.
As I indicated normally the binder content will be selected either corresponding to 4%
air void content and that is optimum binder content if it satisfies all the other requirements.
Or on the other hand we can also examine the binder content range that satisfies all the
criteria. For example, if this is the acceptable range for minimum stability on this side of
the binder content you will have low stability and on this side of the binder content you
will have low stability. So this is the range within which we get acceptable stability and
this is the range of binder content within which the flow is going to be satisfied, this
is the binder content range within which air void content specification is going to satisfied
and let us assume in a given case for all binder contents VMA is satisfied and more
than this binder content is required we have the VFB consideration satisfied. So from this
we can identify this is the range of binder content which satisfies all the requirements.
So possibly then you can select the mid point of this as your optimum binder content.
The main advantage of adopting Marshall Mix design method is it is relatively inexpensive,
inexpensive especially when we are comparing this with more recently developed superpave
mix design procedures. This is convenient for design and also for quality control. we
can have a Marshall mix equipment kept in field laboratory also, even the compactor
can be taken to the field and then mixes can directly be collected from field and then
compacted there itself then the specimen can be brought to the laboratory and then tested.
So it can be considered to be a convenient method for field quality control and for laboratory
testing also. Lot of importance is given to air void in this mix design method. As we
already established earlier air void content is a key parameter towards the performance
of the pavements. It also accounts for the strength and durability requirements of the
mix, it can be used on site also.
But the limitations of this method are, this is an impact method of compaction so it does
not really simulate what is the happening in field. In the field there is some needing
action that is taking place so that is not exactly simulated in this impact method of
compaction. It does not consider the shear strength in the method of testing the specimen
which is diametrical loading, it does not take into account the shear strength of the
specimen because the load is perpendicular to the compaction axis. Marshall Test data
cannot normally predict fatigue and permanent deformation behavior of in service pavements.
This method also does not give proper guidelines for selecting the quality of bitumen.
The laboratory mix design that is aggregate gradation, optimum binder content and the
corresponding mix parameters like stability, density, air voids etc is normally considered
as the target to be attained in the field within permissible tolerances. Whatever is
established in the laboratory and you say this is the binder content and these are the
corresponding properties that we obtained in the laboratory these are to serve as target
values to be checked for in the field.
MORTH specification refers to mix parameters to be attained after several years of traffic.
Normally if we start with six to eight percent initial air void content, this is soon after
initial compaction. That should be considered to be okay assuming that a minimum of 98%
laboratory density is attained. Normally the specification is that whatever is laboratory
density that you obtain corresponding to optimum binder content at least ninety eight percent
of that should be attained in the field.
So, if you can maintain about 6 to 8% assuming that we are getting only 98% in the field
that would correspond to 100% compaction of about 4% which is expected to be attained
after several years of traffic. So we are starting with initial of 6 to 8 we expect
that there is going to be some secondary compaction about another 2%, 3% then the air void content
after several years can get reduced to 3 to 4% or 2% which should be considerably an acceptable
thing. But if it gets reduced further that is a problematic mix.
To summarize; in this lesson we have learnt about various steps involved in the Marshall
Method of mix design. We also discussed about the preparation and testing of specimens of
bituminous mixes. We also discussed how various mix parameters vary with binder content and
also discussed how to select optimum binder content on the basis of Marshall Test results.
Let us take a few questions from this lesson. What does the compaction effort used in preparing
Marshall Specimens correspond to? Why are the Marshall specimens tested at 60
degree centigrade? Estimate the air void content in a specimen.
If its bulk specific gravity is 2.50 and the maximum specific gravity of the loose mix
is 2.60. What are the advantages and limitations of
using Marshall Method for designing bituminous mixes?
Now we will take up the answers for the questions that we asked in lesson 4.9, this was part
one of bituminous mixes or design of bituminous mixes.
What are the main modes of failures of bituminous mixes?
Bituminous mixes normally fail in various modes one of them being cracking of different
types. cracking that can start from bottom, bottom up cracks, cracking that can start
from top, top down cracks caused by various reasons; reputed application of loads, climatic
conditions, cyclic variation of thermal stresses, various parameters can cause cracking of these
mixes they can be starting from bottom or they can start from top also so cracking either
fatigue or other type of cracking is a major problem.
Also, rutting is one of the major failures of many bituminous pavements in India because
of the high temperature conditions. So rutting or permanent deformation which occurs mostly
in bituminous mixes if they are thick at high temperatures and it can of course also occur
in other layers starting from sub grade, sub-base, base and which then gets reflected onto the
surface. But of course in this lesson we were concerned about the failures that were occurring
in bituminous layer so we were concerned about the rutting that is occurring in bituminous
layer.
Other types of failures are bleeding that is seen on the surface because of the presence
of excess of bitumen on surface, because of very little air void that was present, secondary
compaction so bitumen coming to the top which was another failure we saw. Similarly these
three or four types of failures can lead to secondary types of failures also.
How to draw FHWA 0.45 chart for 19 mm nominal maximum aggregate size?
FHWA chart makes use of 0.45 rule for finding out the percentage part to be passing through
a particular sieve size if you know what is the maximum size of aggregate that we are
referring to.
For example, in this case if 13.2 is the maximum nominal size then we take a convenient length
of x axis and also convenient length for y axis. The y axis will be percentage passing
0 to 100 then let us join this line, on this if you want to identify a 2.36 mm size so
we calculate the percentage to be passing through 2.36 so we will calculate 2.36 divided
by 13.2 to the power 0.45 into 100 so we will identify that percentage here and then locate
2.36 here. So on this any given gradation can be plotted and this is the densest gradation
and then any gradation can be compared to the densest gradation.
The last question was how to check aggregate gradations for possibility of tender mix formation.
This can be identified by checking whether a given gradation has a deviation of more
than 3% from a line which connects origin to 4.75 mm sieve size. So, if a gradation
has got deviation by more than 3% that is consider to be leading to a tender mix, thank
you.