Hello viewers, welcome to lesson 2 of module V. module V is on pavement evaluation and
rehabilitation. In the previous lesson that was lesson 5.1 we discussed about the need
for rehabilitation and various techniques that can be adopted for functional and structural
evaluation of pavements. In this lesson we will be discussing about designing overlays
for existing pavements using Indian Roads Congress method that is IRC: 81 -- 1997 version.
The specific objectives of this instruction will be after completing this lesson it is
expected that the student would be familiar with the background for the IRC method of
overlay design. it is also expected that the student would understand the standard procedure
to be adopted for conducting structural evaluation of pavements using Benkelman beam because
IRC: 81 -- 1997 version is based on the structural evaluation of pavements using Benkelman beam
evaluation technique. it is also expected that the students would be able to carry out
overlay designs for flexible pavements on the basis of Benkelman beam evaluation.
We have briefly discussed about the need for evaluating pavements in the previous lesson
5.1. We know that once constructed pavements gradually deteriorate in terms of its functionality
and also in terms of its structural condition. The structural strength also gets reduced,
there will be cracks, there will be deformation, there will be deterioration of materials in
various forms resulting into reduced structural soundness of the structure.
The functional performance also gets deteriorated because of the surface distresses so the riding
comfort, safety and other aspects that are important to the road users will also be affected
thereby the functional performance gets reduced and simultaneously the structural performance
also gets reduced. We are concerned about these two aspects. So to improve the structural
conditions what is to be done, how to assess the requirement, how to assess the present
condition are the things we are going to discuss. This discussion will be based on the guidelines
that are provided in the Indian Roads Congress guidelines.
So as I have just mentioned pavements deteriorate functionally and structurally with time, this
is because of application of traffic loads and also due to the action of different climatic
factors like action of moisture, action of temperature and other parameters.
It is necessary to evaluate the condition of the existing pavement in terms of its functional
and or structural condition periodically. Some agencies carry out only the function
evaluation, some agencies carried only the structural evaluation but there are agencies
which carry out that carry out both the functional evaluation and structural evaluation and evaluate
the pavement in terms of its capability provide proper functional performance and also proper
The evaluation will enable the timely assessment of the condition of the pavement at any given
point of time like what is the condition of the pavement in terms of its functional performance
and in terms of its structural performance and on that basis one would be able to assess
what is the requirement if any and what is to be done to improve the condition of the
Overlay is that reinforcing layer that we provide over an existing pavement. that is
because pavements that do not have adequate structural strength to carry the projected
future traffic will have to be reinforced by providing additional pavement layer.
Normally there will be situations where we are trying to assess a given facility in terms
of its ability to carry traffic satisfactorily or to serve satisfactorily for the next five
years, ten years, fifteen years period. So if you can take into account all the traffic
that has to be served over the next ten years design life period so we have to make an assessment
in terms of its structural strength it has got at present whether it is capable of serving
the projected traffic that is going to be there over the next ten years satisfactorily.
The pavements that do not have adequate structural strength will have to be reinforced by providing
additional pavement layers. As you can see from this diagram we have an existing pavement
placed over a subgrade of given strength. It consists of two layers, it can be three
layers or four layers also. So if you can assess what is the strength of these two layers
placed over the subgrade then we can assess if we have a criteria which tells us what
is the strength that is required to carry a given number of traffic loads satisfactorily.
An important thing is we need to have a criteria which will tell us whether the existing strength
is adequate or not and if it has to carry more traffic what should be the initial strength
that criteria is very important. So we need to have a performance criteria which correlates
the initial strength in terms of the parameters that we are using to evaluate the existing
strength and its ability to carry further traffic.
So once we are able to assess the condition of the existing pavement and also in terms
of the subgrade strength then using the criteria we can assess whether this pavement is capable
of carrying more loads or not and we can also assess how many more loads it can carry so
this is what we call as remaining life. And on the basis of assessing the remaining life
of the pavement we can calculate the requirement for additional reinforcing layer as depicted
here. So in this case the need for reinforcing was felt so the overlay that is provided is
The guidelines that we are going to discuss here are as per Indian Roads Congress: 81
and its 1997 version. These are guidelines for strengthening of flexible road pavements
using Benkelman beam deflection technique, this is the title of the guidelines.
Guidelines for strengthening of flexible road pavements means we are referring to flexible
roads, flexible pavements and these guidelines are meant for Benkelman beam deflection technique.
The first version was in 1981 and the first revision what we are discussing is the 1997
revision. These guidelines are based extensively on the basis of the findings of MORTH Ministry
of Road Transport and Highways, Government of India, a sponsored research project which
has got the code R6 which was entitled "Development of Methods such as Benkelman Beam Deflection
method for Evaluation of Structural Capacity of Existing Pavements and also for Strengthening
of any Weak Pavement" this was the main objective of the research scheme, it was to develop
Benkelman beam and other similar methods for evaluation of structural capacity of existing
pavements. So the methodology is for evaluation of structural capacity of in service pavements
and also for finding out the strengthening requirements of any weak pavements.
The scope of these guidelines: these are meant for evaluation of the strengthening requirement
of existing flexible road pavements using Benkelman beam technique. So this is meant
for flexible road pavements and is also meant for Benkelman beam pavement evaluation technique.
The pavement performance is closely related to the elastic deflection. It is believed
that pavement performance is closely related to the elastic deflection that the pavement
undergoes when it is subjected to a standard load.
Elastic deformation under standard loading conditions depends on various parameters such
as the condition of the subgrade, its moisture content, degree of compaction and the condition
of various other layers including granule layers, bituminous layers and also it depends
upon various climatic conditions like temperature, position of drainage, thickness of various
layers, quality of different materials and so on. So all the pavement related parameters
affect the elastic deformation of a pavement when it is subjected to wheel loads.
Benkelman beam is a very simple apparatus and it is commonly used for measuring the
surface deflection of a pavement under standard loading conditions. A schematic arrangement
of the Benkelman beam is shown here. This is the Benkelman beam. It is cut at several
places because its full length cannot be shown in sketch. This is the probe point which would
be resting on a pavement surface at a point at which we are trying to measure the deflection.
As you can see here since it is very long its extended portion is connected here by
nuts and bolts so it can be dismantled and then for ease of carrying and other convenience
it can be carried in two different parts but when we actually use it for deflection measurement
we will connect them together so the full length of the beam is going to be attained.
The practical length of the beam that we are going to utilize is 2.44 + 1.22 that is 3.66
m. As you can see here from the probe point as I said that is the point at which we are
going to measure the deflection of the pavement to a hinge so the beam is hinged here and
this is where we are measuring the deflection. We are using a dial gauge here so the movement of the end of this beam is
measured with the help of this dial gauge and the probe of this dial gauge will be resting
over the beam so as the beam moves up and down with reference to the upward and downward
movement of the probe which is placed on the pavement surface as the pavement surface deflects
and rebounds so the probe point also goes down and goes up, the corresponding end point
also will go up and go down and that can be recorded with the help of this dial gauge
which is positioned here. So what we are basically interested at is in the movement of this beam
rather the movement of the end points pivoted and while the beam is pivoted about this hinge.
As you can see the distance from the probe point to the hinge is 2.44 m and from the
hinge to the point at which you are measuring the deflection is about 1.22 m, you can see
the ratio is 2:1. Therefore if the actual deflection of the pavement or actual movement
of the probe is say about 2 mm the deflection that would be measured by the dial gauge will
be 1 mm. So this beam is supported by a support frame with one rear leg, there is one leg
here and there are two front legs here. so with the help of these three legs the support
frame will be supported and the beam will be supported with the help of this arrangement
here and it is hinged about this location.
In its normal condition during transport it will be connected to the main supporting frame
with the help of a connecting pin here. But when we are going to measure the deflections
we will remove this pin and allow this probe to rest on the pavement surface so that it
can be freely rotating about this hinge point. So what we basically have here is three supporting
legs, one supporting frame which supports the beam about this hinge point and the beam
will rotate about this hinge. So as the pavement goes up and down at the probe point the corresponding
end point will also will move accordingly which can be recorded using the dial gauge.
The principle is illustrated here: This is a very slender beam very narrow beam
and the total length as I mentioned is 3.66 m it is hinged at a distance of 2.44 from
the pivot end. So initially what is done is a load is applied here and in its deflected
condition the probe point of the beam rests on the pavement surface this is the corresponding
position of the end of the beam which is rotating about this hinge so this deflection can be
absorbed this portion of the beam can be absorbed using a dial gauge and when this load is removed
this pavement surface rebounds and this would be positioned of the beam corresponding to
the rebounded portion of the pavement surface so the end point will come down so this position
will be recorded using the dial gauge so the absolute difference gives us an indication
of what is the rebound deflection of the pavement.
Using a Benkelman beam for measuring the deflections the load that is applied is either static
or creep loading. As we have just discussed Benkelman beam is an apparatus used for measuring
the surface deflection of the pavement subjected to standard truck load. What is important
here is we are using load that is very similar to the load that is being applied on actual
pavement surfaces although we are standardizing the load we are not going to apply any magnitude
of load that is going to be there on the pavement surface but we are going to standardize this
load as to how much load has to be applied that is the standard load which is applied
through a truck. As you can see here what we want to measure
here is either the maximum deflection when the load is applied or the maximum rebound
deflection when the load is removed.
Maximum surface deflection is measured using Benkelman beam in two different modes. As
per a WASHO procedure Western American State Highway Officials method the deflection is
noted as the wheel load approaches the point. so the probe is kept on the pavement surface,
wheel load gradually approaches the point so when the wheel load is directly above the
point we see the maximum deflection that deflection is absorbed, that is what is absorbed in the
case of a WASHO procedure. But if you are adopting Canadian Good Roads Association CGRA
procedure the method of absorbing the deflection is slightly different. What to do here is
we absorb the rebound deflection, it is measured as the load is removed from the point.
Initially the pavement will be subjected to the load. Then we know what is the reading
corresponding to the deflected position of the load. When the load is removed away from
the point the pavement rebounds, we will see the reading again and the difference should
give us the rebound deflection. That is what we measure in the case of CGRA evaluation
The WASHO method of measuring deflection using a Benkelman beam is depicted here. In this
case initially the load is away, the probe point of the beam is kept here so we measure
the corresponding deflection, so as the wheel load gradually moves forward and is directly
above the point that is when we are expected to get the maximum deflection so we closely
monitor the reading in the dial gauge. So when it is directly above the point that is
the deflection we note in the case of a WASHO procedure.
On the other hand in CGRA procedure initially the load is directly above the point at which
we are trying to measure the deflection so the probe point will be here so the dial gauge
reading corresponds to the coefficient of the probe point corresponding to the deflected
shape. When this load is removed away from the point the pavement surface rebounds and
the corresponding absorption of the dial gauge is also made. So what we get here is the rebound
Indian Roads Congress: 81 guidelines suggest that we have to adopt CGRA procedure. One
can also adopt WASHO procedure but the criteria that we adopt has to be corresponding to the
pavement evaluation technique that we are adopting. If you are using CGRA procedure
the corresponding criteria has to be there in terms of the deflection that we measure.
If it is a WASHO procedure the deflection that we measure can be slightly different
so the criteria also has to be different. So the criteria that is available for us is
on the basis of CGRA evaluation of pavements. The salient features of the beam are;
• The length of the beam from hinge to probe point is 2.44 m
• The length of the beam from hinge to dial is 1.22 m
• The distance from the hinge to the front legs is 0.25 m
• The distance from the hinge to the rear legs is 1.66 m
• The lateral spacing of the front legs the spacing between the two front legs is
about 0.33 m
The loading that has to be applied is we have to use a 5 ton truck this is a standard truck
so this is to be used to apply the load. What is important is the rear axle of the truck
has to weigh 8170 kg. If you recollect this is the load that corresponds to standard axle
load eighteen thousand pounds or approximately equal to eighty kilo Newton rear axle.
This load has to be equally distributed over the two wheel sets. The standard truck that
we are going to consider will have two dual wheel sets on either side so each one of these
dual wheel sets will have to 8170 by two wheel load approximately. The clear spacing between
the tires the dual wheel sets that
we are going to use will have to be 30 to 40 mm and standard tires are to be used ten
by twenty twelve ply, the tire pressure is again very important 5.6 kg per centimeter
square or 80 psi tire pressure has to be maintained.
The other accessories we need to have besides the Benkelman beam are the gauge for measuring
the pressure because we said that the tire pressure is very important in the entire exercise
so we have to keep monitoring the tire pressure that the tires have. If they deviate from
5.6 so we have to again adjust that. We also need to have a temperature measuring instrument
especially thermometers which are capable of measuring temperatures in the range of
0 to 100 degree centigrade because we have to measure the pavement temperature because
in the case of bitumen surfaces the deflections get affected by the temperature of the pavement.
So these deflections have to be standardized to correspond to a standard temperature so
we should know what is the temperature at the time of measuring the deflections and
for that we need to have thermometer. We also need to have a small mandrel using
which we can drill a hole into the pavement and into the hole we can put some glycerol
and then we can put the thermometer to measure the temperature. So we have to have all this
arrangement to put a small hole in the pavement temperature which is 4.5cm deep and put glycerol
and then put the thermometer there and we should be able to measure the temperature
of the pavement periodically.
The procedure that has to be followed for measuring deflections using Benkelman beam
is like this. The first thing that we were to do is mark the point on the pavement surface
this has to be selected at a distance of 60cm from the pavement edge as per the guidelines
given in IRC: 81 this is for single lane roads. So if you have single lane roads we assume
that the wheel path is approximately at a distance of 60cm from the pavement edge so
that is the path along which we are going to measure the deflections also. So for single
lane road the points will be selected at a distance of 60cm from the pavement edge.
On the other hand if you are using wider lanes then the wheel path can be considered to be
at a distance of about 90cm from the pavement edge.
For divided 4-lane highways these are divided highways 4-lanes that means in each direction
you have 2-lanes. In such cases the wheel path can be considered to be at a distance
of 1.5 m from the pavement edge and that is the path along which we are going to measure
the deflections. We have to place the outer dual wheel of the truck at the location so
we have to center the outer wheel load. If this is the point at which you want to measure
the deflection so the outer dual wheel load will have to directly positioned about this
point. The probe of the beam has to be inserted between
the dual wheels. The probe will be directly resting on the selected point.
The locking pin as shown in the previous diagram will be removed so that the beam is free to
rotate about the hinge. The support frame has to be leveled.
This sketch here shows how to select the location of the points for measuring the deflections.
From the pavement edge it is 0.6 m, 0.9 m, 1.5 m for different types of facilities.
The beam plunger has to be brought in contact with the stem of the dial gauge. Beam plunger
is the arrangement that we have at the end of the beam above which the plunger of the
dial gauge will be resting. So as the beam rotates in this direction the plunger of the
beam will be supporting the plunger of the dial gauge.
Initial reading in the dial gauge will be noted. as you understand load is already placed
on the point, the probe of the beam is placed between the wheel loads so it is the initial
deflected portion and the corresponding position of the end of the beam we are noting using
a dial gauge because the pavement is already loaded it has already taken a deflected shape
so the probe point is on the deflected shape, the end point goes up so that is what we are
observing using the dial gauge. Then we move the truck forward to a distance
of 2.7 m from the initial point. Then we observe what is known as initial intermediate reading
in the dial gauge. So as the wheel load moves away the pavement rebounds, the end of the
beam comes down and that position will be noted by the dial gauge reading.
The truck wheel will then be asked to move away by a further distance of 9 m so starting
from initial point of 0 the next position of the rear wheel road will be 2.7 m and the
next position will be after the wheel load is moved by a further distance of nine meters.
For this position if there is any change in the dial gauge reading we will note that as
final dial gauge reading. So there are three portions of the wheel loads
such as wheel load directly over the point initial reading, wheel load placed at a distance
of two point seven intermediate reading, and wheel load moved by a further distance of
9 m final reading in the dial gauge. These are the three readings that we are going to
note down. We have to note down these dial gauge readings
when either the rate of deformation or the rate of recovery is less than .025 mm. So
in its deformed condition the rate of recovery has to be less than this so either the rate
of deformation or rate of recovery has to be less than this.
A photograph here shows the dual wheel set of the outer wheel with the probe point of
the beam is inserted between the two dual wheels. You may not able to see it very clearly
because it is on the shadow. These are the two front legs other leg you cannot see and
this is the extension arrangement that we have and this is the support frame of the
While carrying out the deflection survey we also have to measure the pavement temperature
at least once every one hour by inserting the thermometer in the hole made in a bituminous
surface and after filling the hole with glycerol. Hence we have to measure the pavement temperature
at one hour interval. Tire pressure is also checked at two to three hours intervals and
if it is not 5.6 kg per centimeter square we will have to adjust them.
With three positions of the wheel load and the corresponding measurements that we take
are represented here. In the topmost sketch we have here the wheel
load is directly placed over the point and the probe point is at the bottom of the deflected
wall, it is rotating about this hinge and this is the position of the end of the beam
for this loading position so we get the initial reading.
When the load is moved to a distance of 2.7 m it rebounds. It may or may not fully rebound
so it rebounds like this so the corresponding intermediate reading is noted. In the final
position the load is moved by a further distance of 9 m. In this position it can be expected
that the probe point fully rebounds the pavement fully rebounds and the corresponding dial
gauge reading is noted as the final reading.
For a given stretch of may be 1 km, 2 km, 10 km we will be making number of deflection
measurements. It is not just that we measure this deflection at one point and then design
the overlay on the basis of that. We have to select a sample of points at which we are
going to measure the deflections and on the basis of all the deflections that we measure
at all these points we will work out a characteristic deflection which is representative of the
entire pavement stretch under consideration and we will use that characteristic deflection
for design and assessing the condition of the pavement.
At a given point how we calculate the rebound deflection is we subtract the final dial reading
from the intermediate dial reading we also subtract the intermediate dial reading from
the initial reading. If the difference between the final and the intermediate dial readings
is less than 0.025 mm we should normally expect that the difference should be negligible because
when the load is moved to a distance of 2.7 we should expect the pavement should have
fully recovered, rebounded and when the load is moved to a further 9 m distance there should
not be any further rebound that's the normal expectation. But it is quite possible that
when the load is at 2.7 the pavement has not fully recovered it is still influenced where
the load placed at 2.7 m distance and when the load is further moved to a 9 m distance
that's the distance which is significantly large so that position should not have any
influence on this point so that's the position we consider that the pavement has fully rebounded.
If the difference between the intermediate reading and the final reading is more than
.025 mm we have to make some corrections to the deflection that we get. If the difference
is more than 0.025 mm compute the rebound deflections as follows.
Pavement rebound deflection is twice this is when the intermediate and final readings
differ by more than .025 mm. Pavement rebound deflection is given as twice
final minus intermediate readings plus 2.91 times twice the difference between final and
intermediate readings. This is when the intermediate and final readings differ from each other
by more than .025 mm. If the difference is less the pavement rebound deflection is equal
to twice final minus initial reading.
As I said we may have to be conducting Benkelman beam evaluation survey for let's say hundred
kilometers and we cannot give one single value representing all the hundred kilometer stretch
so what we have to do is this complete stretch will have to be divided into smaller stretches
having uniform characteristics it can be 1 km, 2 km stretches each one two or three kilometer
stretch will have to have uniform characteristics either in terms of the materials used, in
terms of traffic pattern, in terms of the subgrade condition or especially in terms
of the distress condition that we absorb on the pavement surface. So we have to identify,
we have to initially conduct some visual examination and also carry out a few physical measurements
to find out what is the present distress condition and on the basis of that classify that into
different types of pavement stretches so each pavement stretch will be handled differently.
So, for selecting homogenous sections uniform sections the pavements will have to be classified
in terms of good, fair and poor sections represented by pavement condition of good being no cracking,
rutting less than 10 mm average, fair being no cracking or cracking confined to only single
crack in the wheel track rutting in the range between 10 to 20 mm. It can also be classified
as very poor if extensive cracking is absorbed and rutting is more than 20 mm. And sections
with cracking exceeding 20% shall be treated as failed.
So we can classify the road in terms of cracking, rutting and select homogenous sections on
this basis. On the basis of surface conditions survey
the total stretch and can be divided into uniform sections.
Length of each section is to be kept to a minimum of at least 1 km because would not
want to design overlays for every 200 m.
For each homogenous or uniform section of road a minimum of ten points should be selected
at equal distance in each lane of traffic. The points are to be selected along outer
wheel path. Normally we have to select these points along the outer wheel path.
The interval between points should not be normally less than 50 m.
On roads with more than one lane the points on the adjacent lanes can be staggered.
In case of extreme deflection values the additional deflection measurements are to be made.
For example, if the highest or lowest deflection differs from the mean by more than one third
of the mean then extra deflection measurements should be made at 25 m on the either side
of the pavement. We have to be careful about extreme points or extreme deflections then
we have to make additional deflections if this criteria is satisfied.
The measured deflections have to be corrected to correspond to standard pavement temperatures.
As I said if you have a bituminous surfacing the pavement deflections are going to be affected
by the temperature of the pavement, at higher temperature the bituminous surface is going
to be weak then you get higher deflections. On the other hand if you do the deflection
survey in very cold temperature the deflections will be low so this has to be normalized to
a standard temperature. The measured deflections also have to be corrected
to correspond to the worst conditions because we are going to consider the structural condition
of the pavement when it is in worst condition. This worst condition will normally be attained
soon after monsoon.
The correction for standard temperature has to be done for a temperature of 35 degree
centigrade. The guidelines are the correction has to be
0.01 mm for each degree variation from 35 degree centigrade.
For example, the deflections are measured at 38 degree centigrade pavement temperature.
Because the deflection is measured at higher temperatures more than 35 degree centigrade
so we have measured higher deflections so they have to be reduced to correspond to 35
degree centigrade so the corrected deflection will be 0.8 -- 3 into 0.01 that's the correction
for each degree of difference in temperature so the corrected deflection will be 0.8 -- 3
into 0.01 that is 0.77 mm.. Similarly correction for seasonal variation
also can be done. This has to be done for the weakest condition which will be soon after
monsoon. Deflection will vary with variation in subgrade strength which is affected by
the variation moisture content with season and also it is a function of the type of soil
that is there.
For correction of seasonal variation field moisture content of the subgrade soil sample
has to be determined during the deflection survey.
While we are conducting deflection survey we have to collect soil samples from the subgrade
and determine the moisture condition. We also have to find out the type of soil. So, on
the basis of the field moisture content moisture correction factors or seasonal correction
factors are given in the guidelines. Hence correction factors are available for different
types of subgrade soils, different rainfall condition and for different field moisture
contents. Three categories of soils are considered namely
clayey with low plasticity Pl less than 15, clayey with high plasticity Pl greater than
15 and sandy gravelly soils.
The correction for seasonal variation can be obtained from the charts that are given
in the guidelines. For a given moisture content this is the field
moisture content that is measured on the basis of the soil sample that is collected and for
a given soil type and for a given rainfall category.
Two categories are considered for rainfall, less than 1300 mm annual rainfall and greater
than thirteen hundred millimeter annual rainfall. So for these two categories and for different
types of soils a series of charts are available. So for a given soil type or for a given rainfall
category and for the measured field moisture content the moisture correction factor can
On the basis of number of deflections that we measure in a given stretch representative
of a rebound deflection value for the length of the uniform stretch can be selected. This
is known as characteristic deflection which represents the overall condition of the pavement.
This is obtained Dc is obtained as mean of all the measured deflections plus some factor
multiplied by the standard deviation of all the measured deflections.
For major arterial roads like national highways and state highways the characteristic deflection
can be obtained as mean plus two times standard deviation. But for all other roads it can
be mean plus one standard deviation.
For designing overlays the following steps have to be followed:
First is you have to select the design period. Next we have to project the commercial traffic
that is going to be there during the design period. If you select design period to be
ten years so after the pavement is constructed for a further period of ten years how many
standard axle load repetitions this pavement is expected to have so we have to have that
number We have to estimate the cumulative standard
axle load repetitions for the design period. We have to select characteristic rebound deflections
for the existing pavement on the basis of rebound deflections survey conducted using
Benkelman beam. That is what we have discussed so far.
Actually what is representative of the given stretch is the following:
• How to select the characteristic deflection • How to conduct the survey
• How to get the sample of deflections and on the basis of that
• how to calculate the characteristic deflection •
Traffic projection can be made in the following manner:
Traffic A which is commercial vehicles per day in the year of completion of construction
then can be obtained from the traffic count that we have made sometime back let us say.
So P is the commercial vehicles per day at last count, A is the annual rate of increase
of commercial vehicles, if you have more authentic information about this we will use that value
otherwise IRC suggests that you can use the value of 7.5%.
n is the number of years between the last count and the year of completion of construction
of overlay because we might have obtained the value of P two years back and may be the
overlay is going to be constructed after another three years so the time that has elapsed between
the last count and the completion of construction of the overlay could be five years so the
traffic intensity would vary by that time so we are going to estimate A value on the
basis of the P that we have obtained about less than five years back using this expression.
From the value of A that we have estimated N which is the cumulative number of standard
axles to be catered by the pavement during the design period N is in terms of cumulative
standard axles is given as; N = 365 A into 1 + r to the power x -- 1 into
F divided by r where r is the annual rate of growth of commercial vehicles authentic
projections have to be made on the basis of various data one has to collect for projecting
this. If it is not available a value of 7.5% can be taken.
x is the design life period in years normally taken as ten for major roads and five for
less important roads and F is the vehicle damage factor.
The value of A has to be adjusted for lane distribution. For single lane having a width
of about three point seven five meter the total 2-way commercial traffic multiplied
by two has to be taken. Though this does not appear to be very logical but this is the
guideline that IRC: 81 has. If you recollect the guidelines given in IRC thirty-seven which
is for design of flexible pavement which has been revised subsequent to 1997 there the
lateral dispersion factor taken for single lane roads is 1 that is we are going to take
hundred percent of the total 2-way traffic. However, in the case of IRC eighty-one this
provision of multiplying the total 2-way traffic by 2 still remains.
But for 2-lane single carriageway having 2-way traffic we are going to take 75% of the total
2-way traffic. For 4-lane single carriageway forty percentage of total 2-way commercial
traffic we have 4-lanes total but at a single carriageway there is no division between the
2-lanes and the two directions it's a single carriageway it is not a divided facility and
in that case we are going to take forty percent of the total two way traffic. We are interested
only in commercial traffic. Similarly, if you have dual carriageway 75%
of the commercial traffic in each direction for dual 2-lane carriageway is going to be
considered. For each additional lane reduce the distribution factor by 20%.
And for estimating the traffic we also need vehicle damage factors. We know that this
has to be obtained from axle load survey. But if axle load survey data is not available
indicative VDF values are; for different commercial traffic intensities 0 to 150 initial traffic,
we can select a value of 1.5 for rolling and plain terrain, 0.5 for hilly terrain, for
150 to 1500 commercial vehicles per day 3.5 for rolling and plain terrain and 1.5 for
hilly terrain. Similarly, if the commercial intensity traffic
is more than 1500 it is 4.5 for rolling and plain terrain and 2.5 for hilly terrain. This
is how we can select the vehicle damage factors. Thus we can estimate 'n' which is the cumulative
standard axle load repetitions that the pavement has to cater to.
IRC: 81 has typical design charts using which we can select the overlay thickness.
Characteristic deflection depending upon the importance of the facility mean plus two standard
deviation or mean plus one standard deviation that value has to be selected.
On the basis of characteristic deflection and on the basis of design traffic it is twenty
million standard axles 10 million standard axles. We estimated that also and we get the
thickness of the overlay to be provided in terms of bituminous macadam material in millimeters.
This is quite simple. What we have to obtain is characteristic deflection from the Benkelman
beam survey conducted corrected for temperature and corrected for season. So we select either
mean plus two sigma or mean plus one sigma depending on the importance of the facility.
Obviously if we select mean plus two sigma we are using the same chart so we get higher
bituminous macadam thickness. If you use mean plus sigma for lesser important
roads we get lesser characteristic deflection so for this naturally we will get lesser overlay
thickness. We have the overlay thickness in terms of bituminous macadam.
We may not be using bituminous macadam. obviously bituminous macadam cannot be used for surface
so this has to be replaced by the total beam requirement that has to provided in terms
of DBM, also it can be BM, BM, BBM, BC or other types of bituminous layers. So using
the conversion factors that are given here we can convert the total requirement of BM
into different component layers. For example, BM can be converted into other
materials using equivalence factors that is 1 BM = 1.5 WBM or bituminous macadam or built
up spray grout layer if you are going to replace some requirement of BM in terms of WBM, WMM
or built up spray grout. Alternatively if you are trying to replace BM in terms of DBM
AC or BC bituminous concrete or semi dense concrete equivalence is 1 BM = 0.7 DBM, AC
or SDC. Minimum thickness of overlay that has to be
provided irrespective of all the calculations that we get is 50 mm BM with an additional
surfacing course of 50 DBM or 40 BC.
Let us take an example; The data that is available is:
• Subgrade is of sandy soil • Moisture content that was measured during
the deflection survey was 8% • Pavement temperature was observed to be
35 degree centigrade. You remember 35 is the standard temperature to which we have to adjust
all the measured deflections. And incidentally if the temperature itself is 35 degree centigrade
then we will not be making any correction for pavement temperature.
• The area has an annual rainfall less than 1300 mm
• The existing traffic is five thousand commercial vehicles per day
• Design period has to be ten years • Traffic growth rate is given as 7%
• Vehicle damage factor on the basis of axle load survey is found to be 4.5.
We have to design an overlay for these pavements which is a national highway.
This is a typical deflection data that was measured. Initial deflection and intermediate
deflection and then this is how we calculate the mean and standard deflection. This is
the mean value, this is the standard deviation value, and from this, this is how we are getting
character deflection as mean + 2 sigma. We are not making correction for temperature
but we are making correction for moisture, this is the correction factor that is obtained.
Traffic is estimated in this fashion; A is given as 5000, lane distribution factor
of 7.5 this is a 2-lane two way highway, vehicle damage factor is 4.5, growth rate is 0.07
so the traffic that the pavement has to cater to is eight point 85 million standard axles.
The corrective deflection works out to be 1.06, this is corrected for temperature there
is no correction and this is corrected for moisture or season which is 1.03 which is
the correction factor so the corrected deflection is 1.09.
So for 95 million standard axles and 1.09 characteristic deflection using the thickness
charts given in IRC the total overlay thickness works out to be 160 mm of bituminous macadam.
So I split this into 50 mm of BC and 70 mm of BM. So you can verify whether this is correct
or not. BM is bituminous macadam and BC is bituminous concrete.
To summarize; in this lesson we have discussed about the basis for IRC: 81 - 1997guidelines
for design of overlays. We also discussed detailed procedure for conducting Benkelman
beam deflection survey. We also discussed about the corrections to be made to the deflections
that are measured in terms of temperature correction, in terms of seasonal correction.
We also discussed how to select overlay thickness on the basis of Benkelman beam deflection
study and also on the basis of other information that we collect during the survey.
Let us take the answers to the questions that we asked from lesson 5.1. Lesson 5.1 was on
evaluation of pavements. 1) What are RTRMMs?
RTRMMs are Response Type Roughness Measuring Machines. these are equipment that give an
index of roughness of the pavement and this is the index that is sensitive to the road
2) Compare MERLIN with fifth wheel bump integrator: MERLIN and fifth wheel bump integrator are
entirely two different equipments. Fifth wheel bump integrator is a response type equipment
which has got a suspension system and what we measure there is the cumulative stroke
of the suspension system. In the case of MERLIN what we measure is how there is variation
of the midpoint of a straight line drawn between two points on the pavement surface, how it
is distributed so that distribution gives us the index of the roughness of the pavement.
The roughness parameter that we obtain using a fifth wheel bump integrator can vary over
time and can be different for different equipments. MERLIN is more or less standard so that's
why it is normally used to calculate response type equipments.
3) Why is it necessary to calibrate RTRMMs? Response type equipment as I just mentioned
their response will be varying with time and similarly for a given road if we use different
types of RTRMMs we get different roughness values. That's why these are to be standardized
to correspond to a standard index. That's why we have to calibrate this RTRMMs.
4) What is golden car? Golden car is hypothetical one fourth of an
equipment which is used in the stimulation of its response to a given profile to calculate
international roughness index. so the parameters of this; the body of the mass, body of the
axle, suspension system and the stiffness of this tire are so selected fine tuned so
that the IRI value that is computed correlates well with the response type measurements.
So the finely tuned tool parameters and the corresponding system is called as golden car.
In fact this is a quarter car.
5) Why is FWD evaluation better than Benkelman beam evaluation?
In FWD evaluation we get information about the deflection bowl of the pavement whereas
in the case of Benkelman beam we get only one deflection which is the maximum deflection
whereas in FWD we get a number of deflections. So as a result we get more information about
the pavement compared to the information that we get from Benkelman beam evaluation. So
FWD is always better than BB evaluation.
We will have a few questions from this lesson; 1) Why do we multiply the deflection obtained
from the dial gauge readings by TWO? This we have to do because the beam is hinged
about a location which has got two is to one ratio in terms of length so that's the reason
why we measure the deflections obtained from the dial gauge by 2.
2) What are the corrections to be applied to the surface deflections measured using
Benkelman beam? We have to apply temperature correction so
that it corresponds to a standard temperature of 35 degree centigrade. We also have to apply
seasonal correction on the basis of the moisture content that we obtain so that the deflection
corresponds to the worst condition.
3) What is the load applied to the pavement for measuring surface deflections in the Benkelman
beam procedure? The load applied is through a truck. The rear
axle has to be 8170 equally distributed on both the dual wheel sets, the tire pressure
has to be 0.56 MPa. 4) What are the parameters on which the overlay
thickness as per IRC: 81 -- 1997 is based? We can select the overlay thickness using
characteristic deflection and also on the basis of the cumulative standard axle load
repetitions, thank you.