Guidelines for the Use of the
Falling Weight
Deflectometer in Ireland
CC-GSW-04008
July 2000
CC Construction & Commissioning Technical
TRANSPORT INFRASTRUCTURE IRELAND (TII) PUBLICATIONS
About TII
Transport Infrastructure Ireland (TII) is responsible for managing and improving the country’s
national road and light rail networks.
About TII Publications
TII maintains an online suite of technical publications, which is managed through the TII
Publications website. The contents of TII Publications is clearly split into ‘Standards’ and
‘Technical’ documentation. All documentation for implementation on TII schemes is collectively
referred to as TII Publications (Standards), and all other documentation within the system is
collectively referred to as TII Publications (Technical). This system replaces the NRA Design
Manual for Roads and Bridges (NRA DMRB) and the NRA Manual of Contract Documents for
Road Works (NRA MCDRW).
Document Attributes
Each document within TII Publications has a range of attributes associated with it, which allows
for efficient access and retrieval of the document from the website. These attributes are also
contained on the inside cover of each current document, for reference. For migration of
documents from the NRA and RPA to the new system, each current document was assigned with
new outer front and rear covers. Apart from the covers, and inside cover pages, the documents
contain the same information as previously within the NRA or RPA systems, including historical
references such as those contained within NRA DMRB and NRA MCDRW.
Document Attributes
| TII Publication Title | Guidelines for the Use of the Falling Weight Deflectometer in Ireland |
| TII Publication Number | CC-GSW-04008 |
| Activity | Construction & Commissioning (CC) | Document Set | Technical |
| Stream | Guidance on Specification for Works (GSW) | Publication Date | July 2000 |
| Document Number | 04008 | Historical Reference |
NRA DMRB and MCDRW References
For all documents that existed within the NRA DMRB or the NRA MCDRW prior to the launch of
TII Publications, the NRA document reference used previously is listed above under ‘historical
reference’. The TII Publication Number also shown above now supersedes this historical
reference. All historical references within this document are deemed to be replaced by the TII
Publication Number. For the equivalent TII Publication Number for all other historical
references contained within this document, please refer to the TII Publications website.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 1 of 36
National Roads Authority
Pavement and Materials Research Division
Guidelines for the use of the Falling Weight Deflectometer in
Ireland
| NATIONAL ROADS AUTHORITY Date; July 2000 Report Number; RC. |
Deflections Normalised to 40kN Load
| Central Deflection Plot (D1) 0 100 200 300 400 500 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Defl ecti on in micr ons 300mm |
| Surface Curvature Index Plot (D1 – D2) 0 50 100 150 200 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Defl ecti on in micr ons |
| Outer Deflection (D9) Plot 5 0 10 15 20 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Defl ecti on in micr ons FILL CUT |
D1(Roadbase)
D1 (HRA)
100mm
200mm
D1 -D2 (Roadbase)
D1 -D2 (HRA)
D9 (Roadbase)
D9 (HRA)
CUT
TO
FILL
TO
Typical Deflection Bow l Shapes
0
50
100
150
200
250
300
350
400
D1 D2 D3 D4 D5 D6 D7 D8 D9
Deflection S ensors
Deflection in microns
| Light F lexible (1 00mm Blac ktop ) |
| Heavy M oto rw a y Cons tru ction (340m m Blackto p) |
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 2 of 36
Index for FWD Report
1. Introduction………………………………………………………………………………………………………………..3
2. Description of FWD……………………………………………………………………………………………………..4
2.1 General Description of FWD…………………………………………………………………………………..4
Figure 1: Segmented FWD Load Plate …………………………………………………………………………4
Figure 2: Diagramatic Representation of FWD………………………………………………………………5
2.2 Load Pulse …………………………………………………………………………………………………………..6
Figure 3: Typical Longitidunal Strain Profile for Moving Wheel (100 km/h) …………………………6
Figure 4: Deflection Response of FWD Load Pulse ……………………………………………………………7
Figure 5: Design Axle Loads Used in Europe(3)…………………………………………………………………8
2.3 Deflection Sensors…………………………………………………………………………………………………8
2.4 Calibration Procedures …………………………………………………………………………………………..9
2.4.1 Relative Control ………………………………………………………………………………………………..9
Figure 6: Relative Calibration Deflection Bowls………………………………………………………………10
2.4.2 Calibration of Load Cell and Deflection Sensors……………………………………………………10
2.4.3 Correllation Trials for FWD‘s ……………………………………………………………………………10
Figure 7: FWD Correllation Trial in TRL ………………………………………………………………………11
3. Measurement Procedures …………………………………………………………………………………………….12
3.1 Preparation for Measurements……………………………………………………………………………….12
3.2 Choice of Test Lane, Test Load ……………………………………………………………………………..12
Figure 8: Staggered FWD test points on two lane road……………………………………………………..13
3.3 Data Required per Test Length………………………………………………………………………………13
3.4 Pavement Temperature…………………………………………………………………………………………14
3.5 Core/Trial Pit Data………………………………………………………………………………………………15
4. Presentation and Interpretation of FWD Deflection………………………………………………………….16
4.1 Normalisation of deflections to standard load …………………………………………………………..16
4.2 Deflection Parameters ………………………………………………………………………………………….16
Figure 9: Example of FWD Deflection Plot…………………………………………………………………….18
Table 1: Summary of FWD deflection data……………………………………………………………………..19
Table 2: Summary of FWD deflection data (Upper Pavement Layers) …………………………………20
Table 3: Summary of FWD deflection data (Subgrade Reaction) ………………………………………..20
4.3 Summary of FWD Deflections on Irish Pavements…………………………………………………………21
Figure 10: Plot of central deflections versus Blacktop thickness …………………………………………21
4.4 Subdivision into homogeneous subsections …………………………………………………………………..22
5. Estimation of is situ layer moduli………………………………………………………………………………….23
5.1 Surface Modulus Plot …………………………………………………………………………………………..23
Figure 11a,b. Examples of surface moduli plots. ……………………………………………………………..24
5.2 Backcalculation of Layer Moduli……………………………………………………………………………25
Figure 12: Three Layer Pavement Model ……………………………………………………………………….27
Figure 13: Stiffness versus Temperature relationships………………………………………………………28
Table 4: Expected Stiffness values for Pavement materials………………………………………………..30
Figure 14: Estimated Stiffness Values for Motorway(4) ……………………………………………………..31
6. Overlay design…………………………………………………………………………………………………………..32
Table 5: Overlay design values for Motorway construction site ………………………………………….34
Table 6: Example of summary table for stiffness and overlay values……………………………………34
7. Minimum Information to be Supplied from FWD Survey………………………………………………….35
8. References ………………………………………………………………………………………………………………..36
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 3 of 36
1. Introduction
Non destructive deflection testing of road and airfield pavements are carried
out using a variety of methods including Benklman Beam, Deflectograph,
Dynaflect and Falling Weight Deflectometer (FWD). The FWD is widely used
throughout Europe for evaluating the bearing capacity of pavements. There are
in excess of 100 machines in use in Europe and in excess of 300 worldwide. A
review of the use of FWD’s throughout Europe (COST 336) has recently been
completed and the document is due to be published in late 2000. The
publication will form a guidelines document for the use of the FWD under the
three main headings, Use at Project level, Use at Network level and Calibration
protocols for FWD.
The type of materials which will be referred to in this document are those
which are used in flexible and semi-flexible pavements. Rigid pavements are not
dealt with in this document.
The aim of any non-destructive test device is to provide information on the
bearing capacity of a pavement due to the action of wheel loads. Studies(1) have
shown that the load pulse generated by a FWD is similar to that produced by a
wheel travelling at a speed of 60 to 80 km/h.
An advantage of the FWD system over some other deflection devices is that
deflections are measured at a number of locations remote from the load
application. This provides useful information on the overall bearing capacity of
the pavement being tested.
The FWD machine is static during the testing sequence and so requires safe
traffic management during survey. For this reason, the use of FWD is usually
restricted to project level analysis. A number of prototype devices are currently
being manufactured that are designed to measure deflections at traffic speeds
thus removing the need for traffic management. The advent of such devices
would allow more scope for routine deflection testing on a network level basis.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 4 of 36
2. Description of FWD
2.1 General Description of FWD
During FWD testing, a load pulse is achieved by dropping a constant mass with
rubber buffers through a particular height onto a loading platen. The load is
usually transmitted to the pavement via a 300mm diameter loading plate. The
loading plate has a rubber mat attached to the contact face and should
preferably be segmented to ensure good contact with the road surface. An
example of a segmented loading plate is shown in Figure 1. A load cell placed
between the platen and the loading plate measures the peak load. The resulting
vertical deflection of the pavement is recorded by a number of geophones,
which are located on a radial axis from the loading plate. One of the deflection
sensors is located directly under the load as shown in Figure 1. A typical FWD
test set-up is shown diagrammatically in Figure 2.
Figure 1: Segmented FWD Load Plate
Load Plate
Central Deflection Sensor
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 5 of 36
Figure 2: Diagramatic Representation of FWD
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 6 of 36
2.2 Load Pulse
As stated earlier the load pulse is achieved by dropping a constant mass onto a
loading platen via rubber buffers. Differences in manufacturers design have
resulted in varying pulse shapes for the same peak load. However, most FWD’s
have a load rise time from start of pulse to peak of between 5 and 30
milliseconds and have a load pulse width of between 20 and 60 milliseconds
(1)
.
The shape of the load pulse is intended to be similar to that produced by a
moving wheel load. Figure 3 shows a typical longitudinal strain profile for a
wheel moving at 100 km/h on a rolled asphalt roadbase(2). Figure 4 shows a
typical deflection profile for a FWD load pulse.
Figure 3: Typical Longitidunal Strain Profile for Moving Wheel (100
km/h)
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 7 of 36
Figure 4: Deflection Response of FWD Load Pulse
Most FWD’s have a load pulse range of between 25 and 120kN approximately.
Some machines are capable of achieving larger loads, which may be required
for airfield work. The target load pulse used for analysis is usually either 40 or
50kN (Standard Wheel Load). The range of design axle loads (wheel load x2)
currently used in Europe is shown in Figure 5(3).
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 8 of 36
Figure 5: Design Axle Loads Used in Europe(3)
2.3 Deflection Sensors
The deflection sensors must be capable of reading deflections to resolutions of
1um (0.001 mm). At the same time they must be sufficiently robust to
withstand site conditions. There must also be sufficient number of sensors to
ensure that the full influence of the load pulse on the pavement is recorded.
The position of the sensors is usually chosen from the following list
(1)
;
0, 200, 300, 450, 600, 900, 1200, 1500, 1800, 2100, 2400mm
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 9 of 36
2.4 Calibration Procedures
2.4.1 Relative Control
Calibration of FWD devices is extremely important and is described in
FEHERL(1) document. A substantial section of the COST 336 document is also
given over to calibration protocols. Relative calibration of the machine should
be carried out approximately every month depending on usage. This can be
done by testing in a location where deflections under the load plate of the order
of 300 to 600um can be obtained for a load of 50kN. Both the load and sensor
repeatability checks can be carried out at the same time.
a) A series of 12 drops at 50kN should be carried out. The first two drops
are then discarded. In the case of each deflection sensor the standard
deviation of the remaining ten drops, normalised to 50kN is then
calculated. The standard deviation for each sensor must be less than or
equal to 2um or 1.25% of the mean value of the reading + 1.5um
(whichever is greater).
b) The first two drops are also discarded in the case of the load
calibration. The standard deviation of the load should be less than 2%
of the mean of the remaining ten readings.
An example of a series of relative calibration tests is shown in Figure 6 for a
period of four years and a temperature range of approximately 5oC to 25oC.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 10 of 36
Figure 6: Relative Calibration Deflection Bowls
2.4.2 Calibration of Load Cell and Deflection Sensors
The load cell and deflection sensors should be calibrated every year. This can
be done by removing the load cell and deflection sensors from the machine and
having them calibrated. There is also the possibility of calibration of the load
cell and deflection sensors while still mounted on the FWD machine. An
example of a calibration certificate is shown in Appendix A.
2.4.3 Correllation Trials for FWD‘s
Correlation trials have been set-up in Europe and more recently in the UK as a
quality control process for FWD machines. It is now mandatory in Ireland
that any FWD to be used on the national road network must have a current
certificate of acceptance from a correlation trial. The first full correlation trial
took place on the small roads system in the TRL on 7 March 2000.
The trial consisted of a series of twelve test points of varying bearing capacity.
Each FWD carried out a series of tests on each test point. The FWD operators
travelled in convoy as shown in Figure 7. The results were then checked for
consistency using predefined statistical limits. Certificates were issued by the
FWD Relative Calibration Summary
0
50
100
150
200
250
300
350
400
D1
D2
D3
D4
D5
D6
D7
D8
D9
Deflection Densor
Deflection in microns (40 kN)
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 11 of 36
Highways Agency following the trials. An example of such a certificate is
shown in Appendix B.
Figure 7: FWD Correllation Trial in TRL
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 12 of 36
3. Measurement Procedures
3.1 Preparation for Measurements
The appropriate traffic management must be arranged well in advance of the
FWD survey being carried out. This is usually done in consultation with the
relevant local Authority Engineers. The type of management required will
depend on the particular site.
The FWD survey can commence as soon as the traffic management has been
set up. FWD tests are generally carried out at 25 or 50m intervals on flexible
roads. The load and deflection data is recorded using a laptop computer inside
the towing vehicle. The testing sequence including number of drops and drop
heights is set-up using software supplied with the FWD device.
3.2 Choice of Test Lane, Test Load
The location of the FWD tests will usually be governed by the information,
which is required from the FWD survey. In many cases the tests will be carried
out in the inner wheel track of the slow lane (if applicable). The reason for this
choice is that this is often the first location to show distress signs on a road
pavement. Tests can also be carried out between the wheel tracks for
comparison purposes and to ascertain the residual life of the relatively
untracked pavement.
FWD surveys on two way single carriageway roads can be carried out in one
direction or alternatively in both directions using “staggered” locations as
shown in Figure 8.
It is generally recommended
(1)
that at least three loading cycles, excluding a
small drop for settling the load plate should be made at each location. The first
drop is usually omitted from calculations. A drop sequence of four drops
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 13 of 36
ranging from 27kN to 50kN approximately allows data analysis to be carried
out at either the 40 or 50kN load level as required. Each drop sequence takes
approximately one minute or less.
Figure 8: Staggered FWD test points on two lane road
3.3 Data Required per Test Length
The following data should be recorded for each test length: This data should be
recorded and saved in a file format similar to the example shown in Appendix C
(.F20).
· Deflection sensor offsets
· Base plate diameter
· Deflection sensor numbers and gain factors
· Test program filename and drops stored on file
· Name and number of test length, carriageway
· Name of operator
· Date of survey
· State of filtering/ smoothing option and cut off frequency
The following data should be recorded for each test point:
· Location (chainage, lane, transverse position in the lane)
· Time and date
50 m
FWD
FWD
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 14 of 36
· Air temperature
· Pavement temperature (if measured)
· Peak Load and Peak Deflections for each drop recorded
· Drop number
· Relevant comment e.g. Marker Plate number
3.4 Pavement Temperature
In general FWD measurements can be carried out over a wide range of surface
temperatures. The range for testing flexible pavements should be 10 to 250 C.
Bituminous bound material behaves in a visco-elastic manner under load and
therefore stiffness is temperature dependent. The temperature of the
bituminous material must therefore be measured at the time of test and
corrected if necessary to a reference temperature. Ideally, FWD testing should
be carried out at a temperature, which is as close as possible to the reference
temperature. It is not necessary to carry out temperature measurements on thin
bituminous pavements such as surfaced dressed granular roads as the thickness
of bituminous material is such that it would not have any significant effect on
the overall pavement structure.
The temperature of the bituminous material is measured by first drilling a hole
in the bituminous layer and inserting a temperature probe into this hole. Holes
for temperature measurement should be pre drilled at least ten minutes before
recording the temperature in order that the heat generated by drilling has time
to dissipate. A drop of glycerol or similar fluid can be used to ensure good
thermal contact between the temperature probe and the bituminous material.
This procedure takes approximately 15 minutes and should be carried out at
least every 4 hours during testing.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 15 of 36
3.5 Core/Trial Pit Data
An attempt should be made to establish as full a picture as possible of the
pavement construction and maintenance history. On recently constructed
pavements this information is usually readily available. However, in the case of
older less formal pavements this type of information may be more difficult to
ascertain. Information relating to layer thickness, materials used, ground
conditions, date of opening etc. are of particular interest. Trial pits can be
excavated in many cases to determine the type and thickness of the various
pavement layers. In cases where trial pits are not possible coring can be used as
a means of determining the thickness of the bituminous bound layers,
particularly in Urban areas. Laboratory test can also be carried out on the
pavement materials and subgrade soil in order to assess existing ground
conditions. Information obtained on the pavement constituents is useful in the
later analysis of the FWD deflection data.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 16 of 36
4. Presentation and Interpretation of FWD Deflection
4.1 Normalisation of deflections to standard load
The actual peak load achieved during a FWD test will depend on the reaction
of the pavement to the load application. The normalising of deflections to
standard load makes the comparison of deflections possible. The deflections are
normalised to 40 kN target load by linear extrapolation. This means that the
deflections are multiplied by the factor (ptarget/pmeasured). The contact pressure
equivalent of the target load (40 kN) on a 300-mm diameter plate is 566 kPa.
For example, if the deflections of a specific drop are due to a 570 kPa load,
then the measured deflections are multiplied by 566/570 = 0.993 to give
normalised deflections.
4.2 Deflection Parameters
There are a number of different ways of presenting FWD deflection data. One
useful method of deflection analysis is to plot more than one deflection
parameter against distance on the same graph. These plots may also contain
marker information, which can be used to identify features along the test
sections (e.g. changes in construction, bridges etc.). An example of such a plot
is shown in Figure 9.
The first plot shows the central deflection (D1). This plot gives an indication of
the overall structural condition of the pavement. The second plot is the Surface
Curvature Index(D1-D2), which indicates the condition of the upper pavement
layers. Low values of (D1-D2) suggest good load spreading ability of these
layers. In cases where this plot takes the same shape as the D1 plot then the
upper layers have a large influence on the pavement structural condition. This
is usually the case with flexible pavements.
The third plot (D9) relates to the subgrade strength. Low values here indicate a
stiff subgrade. In cases where this plot takes the same shape as the D1 plot then
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 17 of 36
the subgrade layer has a large influence on the pavement structural condition.
The deflection parameters for test lengths can be summarised in tabular form as
shown in Table 1.
Some guidance on the relevance of recorded deflection values id given in
Tables 2 and 3 for a 40 kN test load.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 18 of 36
| FWD Deflection Parameter Plot Motorway Construction Site Deflections Normalised to 40kN Load |
| Central Deflection Plot (D1) 0 100 200 300 400 500 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Deflection in microns d |
| Surface Curvature Index Plot (D1 – D2) 0 50 100 150 200 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Deflection in microns |
| Outer Deflection (D9) Plot 5 0 10 15 20 14.5 14.7 14.9 15.1 15.3 15.5 15.7 15.9 16.1 16.3 16.5 Chainage in km Deflection in microns |
D1(Roabase)
100mm
200mm
D1 -D2 (Roadbase)
D9 (Roadbase)
CUT
TO
FILL
FILL
TO
CUT
Figure 9: Example of FWD Deflection Plot
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 19 of 36
Table 1: Summary of FWD deflection data
| Area Dublin Corporation Location Whitworth road, Clonliff road | ||||||||
| Test 1 Test Site | Informati 2 Length in km | on 3 # Tests | Average 4 D1 (Under Load) | Deflection 5 D1 – D2 (SCI) | Values 6 D9 ( @2.1 m) | Constr 7 Blacktop Layers (mm) | uction 8 Granular Layers (mm) | Comments 9 Comments |
| Whitworth road (EB) | 0.8 | 30 | 292 | 92 | 21 | 75 – 250 | 300 – 400* | Some concrete slabs |
| Whitworth road (WB) | 0.8 | 31 | 367 | 118 | 17 | ” | “ | |
| Clonliff road (EB) | 0.8 | 33 | 309 | 117 | 17 | 100 – 140 | 300 – 400* | Recent trench work |
| Clonliff road (WB) | 0.8 | 32 | 291 | 96 | 19 | ” | “ |
* Assumed thickness of granular layers
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 20 of 36
| National Roads Note All Deflections are normalised to 40kN Load | ||
| D1 Criteria | SCI Criteria (D1 – D2) | Comment |
| <100 | <40 | Very Strong Pavement |
| 100 – 200 | 40 – 80 Microns | Strong Pavement |
| 200 – 350 | 80 – 140 Microns | Reasonably Strong – May requir overlay depending on traffic volume |
| 350 – 500 | 140 – 200 Microns | Moderate Pavement Probably requires overlay depending on traffic volume |
| 500 – 700 | 200 – 300 Microns | Moderate to weak pavement requiring overlay(possibly granular layer required) |
| >700 | > 300 Microns | Poor Pavement(Granular layer or reconstruction required) |
Table 2: Summary of FWD deflection data (Upper Pavement Layers)
| Use of FWD on National and Regional Roads Note All Deflections are normalised to 40kN Load | |
| D9(2,100mm) Criteria | Comment |
| <10 | Very Stiff Subgrade |
| 10 – 20 | Stiff Subgrade |
| 20 – 30 | Stiff to Moderate Subgrade |
| 30 – 40 | Moderate to Weak Subgrade |
| 40 – 50 | Weak Subgrade |
| >50 | Very Weak Subgrade |
Table 3: Summary of FWD deflection data (Subgrade Reaction)
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 21 of 36
4.3 Summary of FWD Deflections on Irish Pavements
In order to assess the bearing capacity of any pavement, the deflections and other
relevant data must be analysed as discussed throughout this document. In order to give
an overall comparison of a series of pavement constructions the average central
deflection (under the load) has been plotted against thickness of bituminous bound
material for a range of road projects. The results are shown in Figure 10. Both new
and existing pavements have been included for comparison.
In this graph, a band of one standard deviation either side of the average deflection
value for the new pavements tested has been included. This is done so as to give an
expected range of deflections for new pavements. It is clear from this graph that
pavements with relatively thin bituminous bound thickness (approximately 100 mm)
have the most variation after time in service. It appears that pavements that are well
constructed initially with thick bituminous bound thickness have tended to maintain
low deflection values.
Figure 10: Plot of central deflections versus Blacktop thickness
Central FWD Deflections vs Thickness
0
100
200
300
400
0 50 100 150 200 250 300 350
Pavement Thickness in mm (blacktop)
Central Deflection in microns
(40 kN)
| Average D1 (New ) Average D1 (Trafficked) Log. (Average D1 plus std.) |
Log. (Average D1 minus Std.)
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 22 of 36
4.4 Subdivision into homogeneous subsections
Subdivision of a road section may be carried out for a number of reasons and using a
variety of techniques. Within a given road section, the measured deflections on one
part of the section are often significantly higher or lower than those measured on
another part. In this case, it is desirable to divide the main section into subsections,
each with a significantly different load bearing behaviour.
A homogeneous subsection is a part of the road in which the measured deflection
bowls have approximately the same magnitude and where it is not possible to
subdivide it into subsections with significantly different behaviour.
Along with visual assessment of deflection plots, there are several statistical techniques
available to divide a series of data into homogeneous parts. One of these techniques is
the cumulative sum method. With plots of the cumulative sums of the deviations from
the mean of the deflections against test point it is possible to discern these subsections.
The cumulative sum is calculated in the following way:
S1 = x1 – xm
S2 = x2 – xm + S1
Si = xi – xm + Si-1
| where xi is | deflection measured at test point i |
| xm SI | mean deflection of each main section cumulative sum of the deviations from the mean deflection at test point i |
Using the cumulative sums, the extent to which the measured deflections on a certain
part of a road section are different from the mean deflection of the whole section can
easily be determined. Changes in slope of the line connecting all cumulative sum values
will indicate inhomogeneity.
When continuous information on layer thickness is available, this information can be
used for the subdivision of the project into homogeneous subsections. This data can be
obtained using Ground Penetrating Radar (GPR), with pointwise information of layer
thickness from core samples for calibration purposes.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 23 of 36
5. Estimation of is situ layer moduli
5.1 Surface Modulus Plot
In order to obtain an impression of the stiffness of the pavement layers a face modulus
plot can be constructed. Such a plot gives an indication of the stiffness at different
equivalent depths.
The equivalent thickness of the layers above layer No. n can be calculated from(1) :
h f h E
e i i E
i m
i
= å * *3 ,
| where he is fi | equivalent depth, mm factor, f = 0.8 – 1.0, depending on the modular ratio, thickness and number of layers in the structure thickness of layer i, mm modulus of layer i, MN/m2 |
| hi Ei | |
| Em | modulus of , MN/m2 |
This calculation is based on a modification of the original theory of Odemark.
The surface modulus at the top (equivalent thickness= 0 mm) is calculated as:
| ( ) – * n * * s | E | a | 0 |
| d r |
2
2 1 0
=
é êêë
ùúúû
The surface modulus at the equivalent depth her (> 2a) can be calculated from:
| E ( ) 1 = é – n * * s | a 2 |
r d
r
0
2
0
êêë
ùúúû
*
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 24 of 36
where Eo is the surface modulus at the centre of loading plate
Eo(r) the surface modulus at a distance r (MPa)
| n s0 a r dr | Poisson’s ratio contact pressure under the loading plate (kPa) radius of the loading plate (mm) distance from sensor to loading centre (mm) deflection at distance r (mm) |
Figure 11 shows two examples of a ‘surface modulus’ plot. Figure 11a shows an
increasing surface modulus with decreasing equivalent depth. This means that the
stiffness modulus of the lower layers is less than that of the upper layers. The stiffness
of the subgrade will be around 100 MPa. Figure 11b shows a pavement that has a
‘soft’ interlayer between the upper layers and the subgrade. The stiffness of the
subgrade is about 300 MPa in this example and the stiffness of the ‘soft’ interlayer will
be approximately 150 MPa.
0
200
400
600
800
1000
1200
1400
1600
0 200 400 600
Surface modulus, E0, MN/m2
Equivalent depth, mm
0
200
400
600
800
1000
1200
1400
1600
0 200 400 600
Surface modulus, E0, MN/m2
Equivalent depth, mm
Figure 11a,b. Examples of surface moduli plots.
Figure 11a Figure 11b
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 25 of 36
5.2 Backcalculation of Layer Moduli
The insitu stiffness moduli of the constituent layers can be used to assess the
bearing capacity of a pavement. Pavement layer moduli values can be estimated
from FWD deflections using a number of methods. Most methods use an
iterative process or a database method to reduce the error between the
measured and calculated deflections. This process is referred to as “BackAnalysis”. There is a wide range of programs available to carry out this
analysis. A straightforward linear elastic approach is generally favoured in
routine FWD analysis.
5.2.3.1 Bowl matching by manual iteration
In this method, the stiffness is changed using engineering judgement. The backcalculation is begun by making a surface modulus plot, then calculating the
subgrade modulus, then the granular modulus and finally the bituminous bound
layer modulus. These values can then be manually altered in an iterative manner
until predicted and measured deflections match acceptably.
5.2.3.2 Bowl matching by automatic iteration
This method involves forward calculation using an iterative approach. In this
system, theoretical deflections are calculated for a set of layer moduli which
may or may not be user defined (seed moduli). These layer moduli are then
adjusted in an iterative manner until the error between the measured and
calculated deflections is sufficiently small. The acceptable error can usually be
set by the user. A maximum number of iterations can usually be set also to
account for situations where solutions are impossible.
| 5.2.3.3 database | Bowl matching by interpretation of bowl |
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 26 of 36
This approach involves the generation of a database containing a large number
of deflection bowls. A set of seed moduli or upper and lower bounds are used
as input for the initial database. The measured deflection bowls are then
compared to those in the database in order to reduce the error between the
measured and calculated deflections. This is usually done either by regression
or interpolation techniques. The acceptable accuracy can usually be user
defined.
Different programs can handle various numbers of layers usually up to four or
five. Most programs tend to work best however when the number of layers is
restricted to three. Therefore the modelling of pavements will often require that
layers of similar stiffness behaviour be grouped together in order to reduce the
overall number of layers. A three-layer structure is shown in Figure 12. Some
programs recommend that modular ratios be set in the case of more than three
layers. This method can be used in cases where there are two distinct granular
layers with different stiffness values. Generally, it is recommended that the
model should contain only one stiff layer(bituminous bound) and that moduli
decrease significantly with depth( an Ei/Ei+1 ratio of greater than two is
sometimes recommended).
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 27 of 36
Figure 12: Three Layer Pavement Model
5.1 Normalisation of Pavement Temperatures
The method used for measuring pavement temperatures is described in 3.3.
The stiffness of the bituminous bound layers depends on both the test
temperature and the loading time. The loading time will be constant for a given
FWD device. However, in order to compare deflections/layer moduli they
should be normalised to a standard temperature. This will usually be the design
temperature for the country or region.
The stiffness moduli of the various layers can be calculated from the measured
deflections and the bituminous bound layer stiffness then normalised. There are
a number of normalisation methods available(ELMOD etc), some of which are
contained within backcalculation packages. An example of three such
| Layer 1 (Blacktop) Layer 1 (Blacktop ) |
| Layer 3 (Subgrade) Layer 3 (Subgrade ) |
| Layer 2 (Granular ) |
T1
T2
FWD Load
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 28 of 36
temperature stiffness relationships(4,5,6) is shown in Figure 13 for a reference
temperature of 20 0C.
Figure 13: Stiffness versus Temperature relationships
5.4.1.2 Pavement Modelling
For calculation of stiffness moduli it is usually recommended that the thickness
of the bituminous bound layer be at least half the radius of the FWD loading
plate. In cases where this criterion is not met, a realistic stiffness value based
on temperature and degree of cracking is usually assumed for thin layers. It is
generally recommended also that the thickness of layers increase significantly
with depth.
It is very important that the layer thickness information be as accurate as
possible. There are a variety of methods available to measure layer thickness
including road construction information, coring, trial pits, Ground Penetrating
Stiffness v Temperature for Bituminous Mixes
0
0.5
1
1.5
2
2.5
0 10 20 30 40
Temperature (C)
E / E @20C
| ELMOD 4.0 [4] Portugal [5] TRL [6] |
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 29 of 36
Radar(GPR) etc. The type of method used to obtain layer thickness will often
be governed by the particular site conditions.
The existence of a stiff layer close to the pavement surface will have a large
influence on the calculated layer moduli. Some programs attempt to take this
into account when calculating layer moduli. The estimated depth to a stiff layer
can be calculated from the shape of the deflection bowl or be user defined.
Once the depth to the stiff layer has been calculated the pavement can be
modelled as having a fixed bottom boundary.
The upper layer in flexible pavements will usually be a bituminous bound layer.
Bituminous bound materials are visco-elastic and so stiffness is a function of a
number of factors including loading time and test temperature. The stiffness of
a bituminous material can be measured in the laboratory by a variety of
methods. One method is to carry out indirect tensile tests on cores cut from the
road surface. Great care must be taken when comparing these results with
those from FWD deflections due to the visco-elastic nature of the material.
Granular mixes are sometimes stress-dependent and therefore the measured
stiffness is dependent on the applied stress. The temperature of granular
materials does not generally affect stiffness except in the case of freezing
temperatures. Moisture content usually has a large affect on the measured
stiffness of these materials. Some granular materials such as limestone can
undergo cementing actions, which also have a large effect on material stiffness.
Many subgrade soils are also stress dependent. As in the case of granular
materials the measured stiffness will be greatly influenced by the moisture
content present at the time of test. Some work has been carried out to relate
soil stiffness to other well-known parameters such as CBR(7).
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 30 of 36
Table 4 contains expected stiffness values and Poisson’s ratios for a range of
road making materials. The estimated stiffness values may also be plotted
against distance as shown in Figure 14. The data in this figure was produced
using the ELMOD(4) backcalculation package using bowl matching.
Table 4: Expected Stiffness values for Pavement materials
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 31 of 36
Estimated Stiffness Moduli for Motorway
10
100
1000
10000
15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 20
Chainage in km
Stiffness Moduli in MPa
E1,MPa
E2,MPa
E3,MPa
Low E1
Low E2
Low E3
Figure 14: Estimated Stiffness Values for Motorway(4)
5.4.1.7 Limitations of modelling
Great care must always be used when modelling pavement structures. All
estimated stiffness values are based on inputted parameters such as layer
thickness and type. Therefore errors in the accuracy of inputted information
will lead to errors in the output data.
Some pavement conditions can also be difficult to model effectively. One
example of this is leanmix concrete or cemented subbase material that has been
overlaid with thin bituminous layers. Very often these type of materials crack at
irregular intervals due to variations in material properties. Modelling of these
type of pavements is therefore difficult due to the inhomogenity of the
pavement structure.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 32 of 36
6. Overlay design
The addition of a new structural layer to an old or distressed pavement is a
widely used method of prolonging the service life of a pavement. This is often
done by overlay with new bituminous bound material. A new overlay will
reduce the stresses in the existing pavement and will also seal small cracks in
the surface, thus reducing water ingress into the pavement layers.
The two most important overlay design parameters are the design traffic
volume and the design overlay material. Traffic can be estimated in a number of
ways. The LR1132(7) method uses a formula based on the initial daily traffic
flow, expected growth rate over the design life and the proportion of
commercial vehicles using the slow lane. The method outlined in COST 333(3)
also takes into account such factors as width of driving lane, slope etc.
The design modulus and fatigue characteristics of the overlay material are also
used as input into the overlay design procedure. It is important that the design
characteristics of the overlay material are similar to that which will ultimately
be used on the road.
The average and 85th Percentile overlay values are usually calculated. The 85th
Percentile value is usually used for overlay on National Roads. The calculated
overlay values can be plotted to give a visual indication of the range of overlay
requirement. In many cases remedial action will be required prior to the use of
an overlay carpet. An example of an overlay design plot is shown in Figure 15.
A series of overlay design calculations were carried out on a motorway
construction site using the overlay design method in ELMOD(4). Overlay design
calculations were carried out for a range of design traffic loading and on a
range of existing pavement thickness. The design characteristics of the overlay
material are similar to those of dense bitumen macadam basecourse material(8).
The results of this exercise are shown in Table 5. These overlay values should
be taken as examples only as the actual overlay design values will always
depend on the construction and subgrade soil conditions present along a
particular test length. In this case, there were substantial depths of granular
material (capping and CL. 804) present and the construction was supervised by
a full Resident Engineering staff.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 33 of 36
The average estimated stiffness values and overlay design calculations can be
summarised for a particular project as shown in Table 6.
Overlay Design for 1.5 km
0
50
100
150
200
3.804
3.849
3.895
3.944
3.999
4.049
4.099
4.148
4.222
4.272
4.321
4.371
4.421
4.474
4.525
4.573
4.62
4.673
4.724
4.774
4.823
4.876
4.919
4.94
4.961
4.98
4.999
5.021
5.041
5.073
5.124
5.174
5.3
Chainage in km
Overlay in mm
| Overlay Thickness (mm) 85 Percentile |
Figure 15: Example of Overlay Design Plot
| Overlay Design Calculations for Motorway Construction site | ||||||||
| Overlay design Thickness for Traffic Load | ||||||||
| Existing Thickness of Blacktop | Max Deflection D1 (40kN Load) | SCI(D1 – D2) | D9 | 1 MSA | 3.5 MSA | 12 MSA | 24 MSA | 40 MSA |
| 75 | 450 | 227 | 9 | 40 | 70 | 120 | 145 | 165 |
| 75 | 400 | 188 | 6 | 20 | 60 | 110 | 135 | 155 |
| 100 | 350 | 160 | 10 | 15 | 40 | 90 | 120 | 140 |
| 100 | 300 | 126 | 9 | 15 | 45 | 95 | 120 | 145 |
| 100 | 250 | 102 | 9 | 0 | 20 | 70 | 90 | 120 |
| 200 | 200 | 68 | 8 | 0 | 0 | 30 | 60 | 80 |
| 200 | 150 | 68 | 7 | 0 | 0 | 0 | 0 | 15 |
| 300 | 100 | 34 | 5 | 0 | 0 | 0 | 0 | 0 |
| 300 | 50 | 25 | 2 | 0 | 0 | 0 | 0 | 0 |
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 34 of 36
Table 5: Overlay design values for Motorway construction site
Table 6: Example of summary table for stiffness and overlay values
| Area Dublin Corporation Location Whitworth road, Clonliff road | ||||||||
| Test 1 Test Site | Informati 2 Length in km | on 3 # Tests | Average Es 4 E1 (Bituminous @ 20C) | timated 5 E2 (Granular ) | oduli 6 E3 (Soil) | Overla 7 Design MSA | y Design 8 85 Percentile Overlay | Comments 9 Comments |
| Whitworth road (EB) | 0.8 | 30 | 8,700 | 610 | 70 | 30 | 40 | Some concrete slabs |
| Whitworth road (WB) | 0.8 | 31 | 2,900 | 340 | 40 | ” | 40 | |
| Clonliff road (EB) | 0.8 | 33 | 3,700 | 400 | 100 | 20 | 50 | |
| Clonliff road (WB) | 0.8 | 32 | 6,000 | 400 | 90 | ” | 30 |
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 35 of 36
7. Minimum Information to be Supplied from FWD Survey
The following information should be supplied as a minimum requirement for an
FWD survey report;
1. Copies of the Calibration and Correlation certificates for the FWD device
should be available on request (Appendices A, B).
2. The relevant data outlined in section 3.3 of this report should be supplied
per test length.
3. Recorded deflection parameters (normalised to standard load) plotted
against distance for each test length.
4. Summary of deflection parameters and construction details for each test
length or homogeneous sub section.
5. Summary of estimated layer moduli values and overlay design values (if
any) for each test length or homogenous sub section. The methods and
assumptions used to produce the stiffness and overlay design values should
be outlined.
6. All deflection (.F20), layer moduli, Overlay data should be made available
to the client in digital format and on paper if requested.
7. The FWD survey report should contain advice on the relevance of the
deflection results and the most appropriate remedial measures to be
undertaken.
K:\Fwd\FWDGuidelines.DOC
16/03/10 Page 36 of 36
8. References
1. FEHRL (1996). Harmonisation of the Use of the Falling Weight
Deflectometer on Pavements (Part 1), FEHRL Report 1996/1, Crowthorne:
Transport Research Laboratory.
2. Raithby, R.D., Sterling, A.B., Some Effects of Loading History on the
Fatigue Performance of Rolled Asphalt, TRRL Report LR469, Transport
and Road Research Laboratory, Crowthorne, 1972.
3. COST 333, Development of New Bituminous Pavement Design Methods,
Final Report of Action, European Commission Directorate General
Transport, 1999
4. ELMOD Backcalculation and Overlay Design Computer Package,
Dynatest.
5. Research Document (Unpublished) from Portugal.
6. Research Document (Unpublished) from TRL, Crowthorne, UK.
7. Powel, W.D, Potter, J.F., Mayhew, H.C, Nunn, M.E, LR 1132 The
Structural Design of Bituminous Roads, 1984, TRL, Crowthorne,
Berkshire, UK.
8. British Standards Institution, BS 4987: Part 1: 1993, Coated Macadams for
Roads and Other Paved Areas, Part 1: Specification for Constituent
Materials and for Mixtures
Ionad Ghnó Gheata na
Páirce,
Stráid Gheata na Páirce,
Baile Átha Cliath 8, Éire
www.tii.ie +353 (01) 646 3600
Parkgate Business Centre,
Parkgate Street,
Dublin 8, Ireland
[email protected] +353 (01) 646 3601