overlay and asphalt pavement rehabilitation manual

DESIGN for ROADS and BRIDGES PART 4 – Overlay Design2009

DESIGN for ROADS and BRIDGES

PART 4

MATERIALS & PAVEMENT DESIGN

b) –Overlay Design and Asphalt Pavement Rehabilitation

The Republic of Kenya - Ministry of Roads Draft Document – September 2009

DESIGN for ROADS and BRIDGES PART 4 – Overlay Design20091 Summary...........................................................................................................12 Definitions and Abbreviations............................................................................23 Introduction........................................................................................................54 Network level evaluation....................................................................................7

4.1 Visual Inspection.........................................................................................74.2 Roughness Condition Data..........................................................................8

5 Project Level Evaluation..................................................................................135.1 Detailed Visual Condition Survey..............................................................135.2 Falling Weight Deflectometer (FWD) Survey.............................................155.3 Traffic Estimation.......................................................................................15

5.3.1 Classified Traffic Counts and Axle Loading.......................................................155.3.2 Conversion to design traffic loading..................................................................165.3.3 Effect of Road Geometry...................................................................................165.4 Homogeneous Sections............................................................................17

5.4.1 DCP and Test Pit Investigations........................................................................205.5 Use of DCP data for remedial work...........................................................20

5.5.1 Test Pits............................................................................................................216 Calculation of Structural Number.....................................................................23

6.1 Definitions..................................................................................................236.1.1 variation of bituminous layer coefficient with temperature................................246.2 Use of Structural Number for Overlay Design...........................................256.3 Use of the FWD to estimate SNPExisting.......................................................266.4 Overlay Design Procedure using the FWD................................................27

6.4.1 SNP for Future Traffic (SNPDesign)......................................................................276.4.2 Structural Deficiency.........................................................................................286.4.3 Designing thick overlays....................................................................................306.5 Overlay Design Procedure using the DCP................................................31

7 Remedial Works Prior to Overlay.....................................................................338 References......................................................................................................349 Appendices......................................................................................................35

9.1 Appendix 1 : DCP Test..............................................................................359.1.1 Description........................................................................................................359.1.2 Operation...........................................................................................................359.1.3 Interpretation of results......................................................................................369.1.4 Calculation of Structural Number......................................................................389.2 Test Pit......................................................................................................40

9.2.1 Labour, equipment and materials......................................................................409.2.2 Sampling and testing procedure........................................................................40

9.2.2.1 Field Procedure.....................................................................................41

9.2.2.2 Laboratory procedure...........................................................................42

The Republic of Kenya - Ministry of Roads Draft Document – September 2009

DESIGN for ROADS and BRIDGES PART 4 – Overlay Design

2009

1 SUMMARYThe purpose of this Manual is to update the document of the same title produced for the Ministry of Works, Roads Department in May 1988. The Manual recommends a practical procedure to:

design asphalt overlays; and audit overlay designs submitted by Consultants for major projects.

The procedure is based upon the AASHTO Design Guide (1993) which uses the concept of Structural Number (SN) to establish the thickness of the overlay. The procedure uses a correlation between Falling Weight Deflectometer (FWD) deflection measurements and the Adjusted Structural Number (SNP) of the existing pavement. This correlation must be calibrated for Kenya conditions.

Overlay design thickness is based on the equation:

Where:SNPDesign = Structural Number for future trafficSNPExisting = Structural Number of the existing roada1 = Layer coefficient of asphalt overlay25.4 = conversion mm to inches

SNPDesign values are determined by the AASHTO (1993) design equation. SNPExisting values are based on FWD deflection measurements.

The use of the FWD allows designs to be completed quickly and at relatively low cost. In common with all overlay design procedures the method described in this must be critically reviewed and adjusted according to local experience.

The Manual also provides guidance on a method of designing overlays using the Dynamic Cone Penetrometer (DCP), when FWD results are not available.

Republic of Kenya - Ministry of Roads 1Draft Document – September 2009


2009

2 DEFINITIONS AND ABBREVIATIONSAdjusted structural number A numerical indicator of the overall strength of the pavement

layers including the subgrade. It consists of a summation of the product of the thickness (in inches), layer coefficient and drainage coefficient (if applicable) of each of the pavement layers plus a contribution from the subgrade. It is independent of where the boundary layer of the subgrade is selected.

Asphalt A generic term for any mixture of bitumen, filler and aggregate. This includes asphalt concrete.

Asphalt concrete A mixture of bitumen, filler and crushed stone aggregate proportioned to meet specific strength, deformation and volumetric criteria related to the Marshall test method for asphalt mixes.

Base course A pavement layer lying between the surfacing and the sub-base. This can be constructed from asphalt, granular or stabilised material.

Binder course The lower bituminous course of the pavement, usually asphalt concrete. It is not always present ie the wearing course may rest directly on the base course.

California Bearing Ratio This is the standard test for characterising subgrade material and some granular layers (test method AASHTO T193).

CBR California Bearing Ratio Dynamic Cone Penetrometer This is a portable, hand-operated, percussive penetrometer

for rapidly assessing the strength of subgrade and other granular layers, on site. The results can be converted to CBR values.

DCP Dynamic Cone Penetrometer Empirical A method of engineering design based on observation of the

performance of structures. New designs are extrapolated or interpolated from the observations without necessarily reverting to the calculated stresses and strains in the road structure.

ESAL Equivalent Standard Axle Load

Equivalent Standard Axle This is the standard unit of measurement of the damaging effect of traffic.

Falling Weight Deflectometer A road testing device that generates a pulse load on the road surface and measures the peak vertical deflection at the centre of the loading plate and at several radial positions by a series of sensors.

FWD Falling Weight DeflectometerLayer coefficient A number (a1 value) to indicate the strength of asphalt, base

course or unbound sub-base layers when calculating the structural number of a road pavement.

Maintenance measures undertaken to preserve the pavement, consisting of:

routine: eg grass cutting, ditch & culvert cleaning recurrent: eg patching, pothole-filling, crack-sealing periodic: eg re-sealing road, re-gravelling shoulders



2009 urgent: eg debris removal, erecting warnings

Mechanistic Method of engineering design based on mathematical models of material behaviour and determination of stresses and strains within the structure.

Modified Structural Number A numerical indicator of the overall strength of the pavement layers including the subgrade. It consists of a summation of the product of the thickness (in inches), layer coefficient and drainage coefficient (if applicable) of each of the pavement layers plus a contribution from the subgrade

Overlay A strengthening layer of either granular or asphalt placed on top of an existing road to strengthen the road.

Rehabilitation measures undertaken to increase significantly the functional life of a road pavement

Reliability The reliability of a pavement design is the probability that the pavement section will perform satisfactorily for the traffic and environmental conditions over the design period.

Regulating course A layer of material, usually asphalt concrete, placed on an irregular or unsatisfactory road surface primarily to achieve a substantially smoother surface or a changed surface profile. Its thickness will be variable and is typically used when overlaying existing pavements which have ruts. The maximum particle size may be fairly small as this material is sometimes laid to less than 20mm thickness.

Selected subgrade Imported, good quality soil or rock fill material, which is placed at the top of the subgrade. Its purpose is to increase the strength and stiffness of low strength in-situ material and thus reduce the pavement thickness.

Serviceability The serviceability of a pavement is its ability to serve the type of traffic using the pavement.

SN Structural NumberSNC Modified Structural NumberSNP Adjusted Structural Number Sub-base A medium quality granular layer resting on the subgrade and

supporting the base course.Subgrade All the material below the sub-base. It may consist of in-situ

material, ordinary fill or “selected subgrade “. (Subgrade has the same meaning as the AASHTO term “roadbed”.)

Surfacing The layer(s) of asphalt or surface dressing forming the surface of the pavement. If constructed of asphalt it may include a wearing course and an optional binder course.

Structural Number A numerical indicator of the overall strength of the pavement layers. It consists of a summation of the product of the thickness (in inches), layer coefficient and drainage coefficient (if applicable) of each of the pavement layers above the subgrade.

Structural deficiency The difference between the required strength of a road to be overlaid and its existing strength. Recorded in units of Structural Number

Terminal serviceability index This is the index of the lowest serviceability that will be tolerated by the road users, before rehabilitation, resurfacing



2009or reconstruction becomes necessary. The value will depend on the status of the road and generally lies between 3 and 2.

Wearing course The uppermost bituminous course of the pavement, usually asphalt concrete. The top surface of this layer should provide a smooth surface but with adequate texture to provide adequate friction for safe vehicle braking and turning. See also surfacing

WMAAT Weighted Monthly Average Annual Temperature.



2009

3 INTRODUCTIONRoads in Kenya vary widely in their geometric standard and the traffic they carry. They have been constructed and maintained over a period of years; indeed, many have ‘evolved’ rather than been designed and constructed by a formalized process. The range in topography and traffic loading results in roads having a wide range of construction thickness and strength. However, the common theme is that they have a granular road base and either a relatively thin asphalt concrete or surface dressing surfacing. The road network, both in flat and hilly terrain, is also criss-crossed with patches and utility trenches. These often contribute to road deterioration through poor reinstatement.

The present overlay practice is either to mill the existing surface and overlay with 40-50mm (periodic maintenance) of asphalt, or to apply a new surface dressing, or to engage consultants to carry out the overlay design for major projects. The overlay designs submitted by Consultants are generally based on the methods described in ‘Design of Pavement Structures’ (AASHTO, 1993).

The proposed empirical overlay design method, described in this , is also based upon the AASHTO recommendations (1993) and uses the concept of Structural Number (SN) to establish the thickness of the overlay. The design process is illustrated in Fig 3.1.The procedure uses a relationship to convert FWD deflection measurements to the Adjusted Structural Number (SNP) of the existing pavement, allowing designs to be completed quickly and at relatively low cost.

In common with all overlay design procedures the method described in this recommends a method to formulate designs which must be reviewed by the Engineer and adjusted based on his/her own local experience.

The design process envisages the following two levels of survey:

Network level surveys, consisting of roughness and visual condition, carried out to demarcate road sections of equivalent condition, followed by:

Project level surveys, more detailed in scope, consisting of visual condition, FWD, DCP and Test Pit investigations, carried out to determine the level of maintenance required.



2009Figure 3.1: Design Process


Results from Network VCS and Roughness Survey identifies sections of road for rehabilitation

Carry out non-destructive tests Project Level VCS FWD deflections

Traffic count (where necessary) Axle load survey (where necessary)

Identify homogeneous sections of road using FWD deflection (do)

Plan and carry out destructive tests

(DCP and Test Pits)

Establish Adjusted Structural Number at each FWD point

Establish required Design Structural Number for future traffic

Design thickness of strengthening overlay for each homogenous section

Calculate Costs

Correct Adjusted Structural Number for temperature

Establish BoQ of remedial works from

VCS.

Calculate Structural Deficiency from Existing Structural Number


2009

4 NETWORK LEVEL EVALUATION

The network level evaluation categorises road sections into the following :

those where only minimal routine or periodic maintenance needed those where major treatment, such as reconstruction needed, and those of intermediate condition where further project-level investigation is needed to

decide what measures to take.

4.1 Visual InspectionThe objectives are to identify the type and severity of the distress in a quantitative manner in order to estimate maintenance interventions and also to enable the function of performance modelling tools (eg HDM) if required.

The inspection is implemented by examining the condition of road cross sections at discreet intervals, or samples. The condition is evaluated at each sample and combined with a qualitative assessment of the interval between each sample.

The inspection should be undertaken by a trained engineer who also has knowledge of the software system that he will use to process the data recorded.

Each sample is subdivided into a number of sub-samples and the distress in each sub-sample is recorded. The severity of the distress is estimated as the proportion of the total number of sub-samples affected.

The total number of sub-samples influences the survey precision. The number of samples influences the reliability of the survey. The level of detail required is governed by the purpose for the data and the resources available to do the work. For network surveys, sample points could be spaced at up to 1km spacings, each with 2 sub-sample points. For project surveys, sample points would be more frequent (from 0.01km to 0.1km spacings), each sample point having 4 sub-sample points.

Table 4.1 gives details of the assessment criteria, Table 4.2 the roughness values to be expected, Table 4.3 the recommended threshold values for all the assessment criteria and Table 4.4 lists the risks associated with these partial surveys and recommended follow-up work. Table 4.5 is a recommended field form.

For 2-lane roads ( 5.5m), the defects will be assessed in 4 transverse strips corresponding to each wheel path, covering the full width of the pavement. For road widths < 5.5 m wide, such that the inner wheel paths overlap leading to 3 rather than 4 wheelpaths, the assessment shall be carried out over three strips corresponding to the wheelpaths. The centre wheel path ratings shall be allocated to both strips 2 and 3.

Table 4.1: Assessment criteria for Visual Condition SurveyFeature Rating

Wide cracks > 2m 0, 1, 2, 3 or 4 depending on number of strips with this defect

Depressions with cracks 0, 1, 2, 3 or 4 depending on



2009

number of strips with this defect

Rutting (Visible, >10mm) 0, 1, 2, 3 or 4 depending on number of strips with this defect

Edge failures 0, 1 or 2 depending on number of edges with this defect

Shallow potholes - No base exposed

(Include shallow local failures on the basis of 1m2 = 1 pothole )

Number per sampling interval

Deep Potholes - Base exposed

(Include shallow local failures on the basis of 1m2 = 1 pothole )

Number per sampling interval

“Shiny” Surface 0, 1 or 2 depending on number of edges with this defect

General Condition

(Surveyor’s estimate of the required maintenance for the sample length)

NB. Not all the indicated defects need to be present to qualify for a particular treatment.

1 Routine: No depressions, only a few cracks and shallow potholes, very minor rutting.

2 Thin Overlays: Slight depressions, some shallow potholes, some cracks, slight rutting.

3 Thick overlays: Major depressions with cracks, some deep potholes, wide cracks and significant rutting.

4 Reconstruction: Broken up pavement areas, deep potholes, depressions with cracks and substantial rutting.

The results of the survey can be evaluated according to Table 4.3 and this will enable the road sections to be categorised.

4.2 Roughness Condition DataRoughness is normally measured using a Bump Integrator and expressed through the International Roughness Index (IRI), in m/km. Typical values of the IRI with reference to the type and condition of the road are indicated in Fig 4.2.

Table 4.2: Roughness criteria



2009

IRI Ranges Road Condition

Lower than 6 very good

6 to 11 good

11 to 15 fair

15 to 19 poor

Larger than 19 very poor



Table 4.3: Proposed Analysis of Network Visual Condition Survey DataClass 1 2 3 4Treatment Routine

(Patching and crack sealing)Thin Overlays 40mm Thick overlays Reconstruction

Purpose Local repairs and sealing Restore surface characteristics

Prevent moisture entry Restore transverse

shape ( + reg. Layer)

Strengthening Reduce roughness

Replace excessively weakened and distorted AC and Base

Typical Pavement Condition

Small number of local failures or cracks(Strength, Ride and Surface OK)

Some local failures, minor rutting, cracked or poor surface.(Strength OK)

More frequent failures and depressions, some weakness in AC and/or Base,

Frequent and severe failures and deformation, general weakness in AC and/or Base.

Road Class Int. P + S Local Int. P + S Local Int. P + S Local Int. P + S LocalRoughness (IRI m/km)

4 5 6 4 5 6 4 to 6 5 to 7 6 to 8 > 6 > 7 > 8

Wide cracks 0 0 1 1 1 2 2 2 3 3 3 4Depressions with cracks

1 1 2 1 1 2 2 2 3 > 3 > 3 4

Rutting 1 1 2 1 1 2 2 2 3 > 3 > 3 > 3Edge Failures 1 2 2 1 2 2 1 2 2 1 2 2Shallow Potholes (No. per km)

20 20 30 30 30 40 > 30 > 30 >40 > 30 > 30 >40

Deep Potholes (No. per km)

10 10 20 15 15 30 20 20 >30 > 20 > 20 >30

“Shiny” Surface 1 1 2 > 1 > 1 > 2 > 1 > 1 > 2> 1 > 1 > 2

General Condition Yes/No Yes/No Yes/No Yes/No

Republic of Kenya - Ministry of Roads 10 Draft Document – September 2009


Table 4.4: Risks involved and action required on completion of Network Surveys

Class 1 2 3 4

Treatment Routine

(Patching and crack sealing)

Thin Overlays 40mm (or Surface Dressing or Microsurfacings)

Thick overlays Reconstruction

Risks of Fast-Track VCS assessment

More serious deterioration between sample lengths may be missed. There is a need for condition checks of some of the non-sample lengths to confirm this classification.

May over or under-estimate deterioration but this will be corrected during follow-up Detailed VCS.

May over or under-estimate deterioration but this will be corrected during follow-up Detailed VCS.

May exaggerate deterioration. There is a need for condition checks of some of the non-sample lengths before proceeding with FWD

Further Surveys

Patching Works Records only Essential:

Detailed VCS (100% in 5m sample lengths)

Traffic survey

Discretionary

FWD + Cores + DCP + Test pits

Essential:

Detailed VCS (100% in 5m sample lengths)

FWD @ 50m, staggered L + R, outer wheel paths.

Traffic survey

Axle weight survey

Discretionary:

Cores + DCP + Test pits at frequencies and locations to suit FWD d1 values.

Essential:

FWD @ 100m, staggered L + R, outer wheel paths.

DCP@ 200m

Traffic survey

Axle weight survey



Region:…………………………………. Road Name:……………………………. Road ID:…………… Sampling Interval:……..

…m Direction:………………………. Single / Dual Width:…………. m Date:……………….. Form Start Time:

………………

Start of Survey: 1 2 3 4 5 6 7 8 9 10

Roughness (IRI m/km)

Ex database

Wide cracks 0 1 2 3 4

Depressions with cracks

0 1 2 3 4

Rutting 0 1 2 3 4

Edge Failures 0 1 2

Shallow Potholes (No. per sample i’val)

Number

Deep Potholes (No. per sample interval)

Number

“Shiny” Surface 0 1 2 3 4

General Condition 1 2 3 4

Remaining interval condition

Worse = 0 Similar = 1 Better = 2

Surveyed by – Name:…………………………….. Signed:…………..………………..



2009

5 PROJECT LEVEL EVALUATION

5.1 Detailed Visual Condition SurveyThe project Visual Condition Survey (VCS) is more detailed than the network survey, covering the whole of the road. It is carried out during on foot, each sample length (5, 10 or 20 metre) of the road being examined to identify defects in the wheelpaths. Notes on the collection of the defects are presented in Table 5.1.

Table 5.1: Explanation of DefectsDefect Unit NotesWide single cracks m Cracks (wider than 3mm) to be sealed, in m Wide connected cracks m Cracks wider than 3mm, separating pavement into

blocks, to be sealed, in mAlligator cracks, no depressions

m2 Cracks separating pavement into small pieces, but no depression or rut

Alligator cracks, depressions

m2 As above, with associated depression

Deep Potholes N Potholes that penetrate through baseStructural rutting m2 >10mm depth, originating in baseEdge failure m Loss of pavement surface >50mmTrench/Patch Failure m2 Rutted (>10mm) or broken-up patchShallow potholes N Potholes that occur just in surfacingAsphalt shoving N Pushing-up of asphalt surfacingSurface rutting m2 >10mm depth but just in surfacingSlippage cracks N Adhesion failure of asphalt surfacing to base

Neither the condition of the road shoulders, nor of the drainage are covered in this type of survey, which refers to bituminous-surfaced roads only.

Any one defect should only be counted once: for example, ‘rutting with cracks should be counted as “Depressions with cracks” only and not also recorded as “Rutting”. The data from the VCS is transferred to a spreadsheet that automatically calculates the Bill of Quantities for the remedial work prior to overlay. Quantities for crack sealing should be adjusted to an area not affected by previous deep or surface patching.

Prior to overlay a number of these defects will need remedial work. The survey enables the quantity of materials required to be estimated.




TABLE: PAVEMENT CONDITION SURVEY (Project Level)

:المنطقةRegion

جاه :االتDirection

Start of Survey:Sta 0+000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Unit Mean

WidthSum Quantity

Wide Single cracks l.m 0 0

Wide Connected cracks

l.m 0 0

Alligator Cracks without Depression

sq.m 0 0

Alligator Cracks with Depression

sq.m 0 0

Base Shoving sq.m 0 0

Deep Potholes sq.m 0 0

Structural Rutting sq.m 0 0

Edge Failure sq.m 0 0

Trench / Patch Failure sq.m 0 0

Shallow Potholes sq.m 0 0

Asphalt Shoving sq.m 0 0

Surface Rutting sq.m 0 0

Slippage Cracks sq.m 0 0

“Shiny” Surface sq.m 0 0

BILL OF QUANTITIES

CR

AC

K S

EA

LIN

GF

ULL D

EP

TH

PA

TC

HIN

GM

ILL A

ND

RE

PLA

CE

ق :إسم الطريRoad Name

ة ني :المسافة العيSample Interval

ق :رقم الطريRoad ID

ق عرض الطريCarriageway Width


2009

5.2 Falling Weight Deflectometer (FWD) SurveyOn 2 lane single carriageway roads FWD tests should be carried in both lanes (ie both directions) in the outer wheel-path (closest to the shoulder of the road). The location of the FWD tests should be ‘staggered’ to allow for maximum coverage. For multi-lane dual carriageways FWD measurements should be carried out, as a minimum, in the outer wheelpath of the heaviest loaded lane. In addition, tests should be carried out in other lanes where the condition of the lane is worse than the heaviest loaded lane.

On Class A, B and C roads, the tests should be carried out at 50 metre intervals. On 2 lane single carriageway roads the location of the tests should be ‘staggered’ by 25 metres so as to result in an FWD test every 25 metres along the road.

On Class D and E roads, the tests should be carried out at 100 metre intervals. On 2 lane single carriageway roads the location of the tests should be ‘staggered’ by 50 metres so as to result in an FWD test every 50 metres along the road.

The FWD tests should be normalised to a standard load of 50 KN.

5.3 Traffic Estimation

5.3.1 CLASSIFIED TRAFFIC COUNTS AND AXLE LOADINGReference is made to the Design for New Bituminous, Gravel and Concrete Roads for procedures to undertake traffic and axle load surveys. Fig 6.1 summarises the steps described.

Figure 5.2: Steps for carrying out Traffic and Axle Load Surveys


Select Design Period

Determine Initial Traffic Volume (Initial AADT) per

Class of Vehicle

Determine Traffic Growth

Determine Cumulative Traffic Volumes over the

Design Period

Estimate Mean Equivalent Axle Load (ESA) per Class

of Vehicle

Estimate Cumulative ESAs over the Design Period (in

one direction)


2009The individual weights of each axle of a particular vehicle class are converted to ESA, which are added to produce a total for the vehicle. It is usual to determine the average ESA for each vehicle class based on the results of an axle load survey, allowing for the proportions of loaded and unloaded vehicles in each class. (These may vary in each direction and between routes.)

It is essential that the individual axle weights are converted to ESA before any aggregation or averaging of data is carried out for either the individual vehicle or for all the weighed vehicles of a single class.

After the total ESA for each vehicle have been calculated, the average value of ESA is calculated for the whole vehicle class. These average values of ESA are sometimes termed “vehicle wear factors” or “vehicle damage factors”.

The total contribution to pavement loading of a vehicle class is the product of the vehicle damage factor of the vehicle class and the number of such vehicles recorded on the road either on a daily or annual basis. The process is repeated for the other classes and the total loading per unit of time is determined by summation.

5.3.2 CONVERSION TO DESIGN TRAFFIC LOADING The pavement loading calculated above must be summed for the whole of the design period (normally 10 to 15 years), adjusted for annual traffic growth. The estimate of annual traffic growth is usually based on historic trends and affected by predictions of future economic activity, but normally increases vary between 2 and 7 per cent per annum. In some instances there is the possibility of a sudden increase (or reduction) of traffic, when for example a new factory, quarry or port comes into operation.

The Design Traffic Loading in the performance period is calculated from Equation 1:

Equation 1: Calculation of design traffic loading

Where:

N = Performance Period in yearsri = Growth rate (%)ESAL1 = Daily number of ESA in the first year in traffic class ‘i’

5.3.3 EFFECT OF ROAD GEOMETRYThe vehicle damage to the road pavement is influenced by the road geometry. On narrow single carriageway roads the wheelpaths can overlap in the centre of the road, causing more damage. On multi lane dual carriageways medium and heavy traffic can use other lanes besides the outer lane (most heavily trafficked lane). The criteria presented in Table 6.1 can be used to calculate the design traffic.

Table 5.6 Effect of road geometry on design traffic loadingSingle/DualCarriageways

No ofLanes

Width of Carriagway(m)

Calculation of ESALs

Single 2 <6.7 80% of the ESALs in both directions is used in order to allow for overlap on the



2009central section of the road.

Single 2 > 6.7 The ESALs in the most heavily trafficked direction is used.

Dual 2 - With less than 2000 commercial vehicles per day in one direction, 90% of the ESALs in one direction is used.

Dual 2 - With more than 2000 commercial vehicles per day in one direction, a special study shall be carried out to establish the distribution of commercial vehicles.

Dual > 2 - A special study shall be carried out to establish the distribution of commercial vehicles

5.4 Homogeneous Sections

Results from the Network Visual Condition and Roughness Surveys will identify lengths of road in need of rehabilitation. Those lengths of road that require Detailed Design (see Fig 6.1) need further investigation with the FWD or other equipment as described below, as part of the rehabilitation design process.

Figure 5.3: Maintenance Thresholds

All roads vary in pavement thickness and strength along their length. For instance, the strength of the underlying subgrade will vary along the road alignment as the road passes from areas of cut to fill. Rehabilitation measures cannot be tailored to each and every variation in road characteristic, so to produce cost-effective designs the road should be divided into lengths


Surfacing Integrity & Texture

IRI Min

IRI Max

RoughnessIRI

Min< IRI <Max

Cracked & depressed

area %

No

< 10

> 10

< 2

< 5

< 10

< 15

< 20

Shiny area %

Cracked area %

Potholed area %

0

< .5

0 to .3

.5 to 1

<5

5 to 10

< 30

> 30

< 30

> 30

< 1

1 to 2

< 3

> 10

Maintenance Operation

Patch, Seal Cracks

Patch, Seal Cracks, Grinding

Patch, Seal Cracks, Surface Dressing [SD]

Patch, Seal Cracks, Inlay, Double SD

Patch, Seal Cracks, Mill & Replace

Patch, Seal Cracks, Thin Overlay

Patch, Mill & Overlay [Detailed Design]

Patch, Mill & Strengthen [Detailed Design]

Repair, Mill & Strengthen [Detailed Design]

Reclaim Base & Repave [Detailed Design]

Reconstruction & Improvement

Main

tainab

le to R

ehab

ilitate

Surfacing Integrity & Texture

IRI Min

IRI Max

RoughnessIRI

Min< IRI <Max

Cracked & depressed

area %

No

< 10

> 10

< 2

< 5

< 10

< 15

< 20

Shiny area %

Cracked area %

Potholed area %

Shiny area %

Cracked area %

Potholed area %

0

< .5

0 to .3

.5 to 1

<5

5 to 10

< 30

> 30

< 30

> 30

< 1

1 to 2

< 3

> 10

Maintenance Operation

Patch, Seal Cracks

Patch, Seal Cracks, Grinding

Patch, Seal Cracks, Surface Dressing [SD]

Patch, Seal Cracks, Inlay, Double SD

Patch, Seal Cracks, Mill & Replace

Patch, Seal Cracks, Thin Overlay

Patch, Mill & Overlay [Detailed Design]

Patch, Mill & Strengthen [Detailed Design]

Repair, Mill & Strengthen [Detailed Design]

Reclaim Base & Repave [Detailed Design]

Reconstruction & Improvement

Main

tainab

le to R

ehab

ilitate


2009where the strength properties are similar, known as homogeneous lengths. Each homogeneous length is then treated as a separate overlay design exercise. This will result in reduced costs as the overlay thickness changes, reflecting the existing strength of each homogeneous section.

This procedure is best carried out by using the Cumulative Sum Method (CUSUM) on FWD central deflection measurements (do). The method involves plotting the cumulative sum of the differences of the FWD deflection from the mean FWD value calculated from all the results. The calculations are based on Equation 2 and a worked example is shown in Table 6.1:

Equation 2: CUSUM calculation

Where:

FWDmean = Mean FWD deflection of the roadFWDi = FWD deflection at chainage iSi = Cumulative sum of the deviations from the mean deflection

Table 5.7: Cusum Calculations on FWD central deflections (d0)



2009Chainage FWD D0 - Mean Cusum

(m) D0

0 0.381 -0.061 -0.061

50 0.407 -0.035 -0.096

100 0.313 -0.129 -0.225

150 0.404 -0.038 -0.263

200 0.261 -0.181 -0.444

250 0.314 -0.128 -0.572

300 0.305 -0.137 -0.709

350 0.301 -0.141 -0.850

400 0.308 -0.134 -0.984

450 0.435 -0.007 -0.990

500 0.261 -0.181 -1.172

550 0.215 -0.227 -1.398

600 0.261 -0.181 -1.580

650 0.166 -0.276 -1.856

700 0.482 0.041 -1.815

750 0.769 0.327 -1.488

800 0.366 -0.076 -1.564

850 0.247 -0.195 -1.759

900 0.366 -0.076 -1.835

950 0.228 -0.214 -2.049

1000 0.313 -0.129 -2.178

1050 0.273 -0.169 -2.346

1100 0.245 -0.197 -2.543

1150 0.318 -0.124 -2.667

1200 0.304 -0.138 -2.805

1250 0.483 0.041 -2.764

1300 0.559 0.117 -2.647

1350 0.665 0.223 -2.424

1400 1.003 0.561 -1.863

1450 0.559 0.117 -1.747

1500 0.769 0.327 -1.420

1550 0.665 0.223 -1.196

1600 0.559 0.117 -1.080

1650 0.769 0.327 -0.753

1700 0.462 0.020 -0.733

1750 0.467 0.025 -0.708

1800 0.467 0.025 -0.684

1850 0.462 0.020 -0.664

1900 0.479 0.037 -0.627

1950 0.665 0.223 -0.403

2000 0.559 0.117 -0.287

2050 0.404 -0.038 -0.325

2100 0.476 0.034 -0.291

2150 0.559 0.117 -0.174

2200 0.462 0.020 -0.155

2250 0.467 0.025 -0.130

2300 0.435 -0.007 -0.136

2350 0.559 0.117 -0.020

2400 0.462 0.020 0.000

Mean 0.442

The FWD d0 deflection values and CUSUM plot are given in Fig 6.2 and Fig 6.3 respectively. Error: Reference source not foundA change in slope of the graph indicates a change in strength along the road. In Fig 6.3 five distinct homogeneous sections can be identified. These sections should be treated as separate overlay designs.

Figure 5.4: FWD d0 deflection values



2009

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Figure 5.5: CUSUM plot showing homogeneous sections

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5.4.1 DCP AND TEST PIT INVESTIGATIONS

Destructive testing may be needed after the non-destructive testing is completed to establish the thickness and strength of the existing pavement layers and relate these to the road failure. Two methods are available, either the Dynamic Cone Penetrometer (DCP) or Test Pits. Details of these field methods are presented in Appendices respectively

DCP tests are relatively quick and therefore should be used where there is no risk of damaging any utilities in the road pavement. The results from DCP tests are particularly useful in identifying areas of weak base course and sub-base layers which will need deep patching required prior to overlay.

Test pits are best used when the road is to be partially or fully reconstructed. In this case laboratory tests are carried out on the samples collected from the various granular layers in the road to establish whether they can be used in the reconstruction process.

5.5 Use of DCP data for remedial workDCP tests should be carried out at points in the road where the Detailed Visual Condition Survey and FWD deflection profile show the road to be abnormally weak. In Fig 6.4 the FWD test are high at chainages 750 and 1400 metres. DCP would be carried out the outside



2009wheelpath at these chainages to establish the cause of the weakness. Prior to testing a detector should be used to ensure there are no utilities beneath the test location.

Figure 5.6: FWD deflections and points for DCP testing

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DCP Test

DCP Test

The DCP is driven through the road pavement under a standard force to a maximum depth of approximately 800mm. The strength of the layers is related to their resistance to penetration, measured as mm per blow, and there are correlations to convert the DCP values to in-situ values of CBR. The thickness of the road layers are identified by the changes in mm/blow as the apparatus penetrates the pavement layers.

Where the in-situ CBR of the granular base course and sub-base are below 80% and 30% respectively (as measured from the DCP), the base course and sub-base (if necessary) shall be deep patched. Sometimes it is difficult to differentiate between the base and subbase and test pits may be necessary as a last resort to determine the layer interval.

5.5.1 TEST PITSWhere the FWD results indicate that the road should have a thick overlay or be either partially or fully reconstructed then test pits will be needed. If the road is to be overlaid then the pits should be dug in areas where the FWD shows the road to be weak.

If the road is to be partially or fully reconstructed the Test Pits should be dug at regular intervals where the road is weak. Two test pits would normally be dug in every one kilometre of road.

The test pit data are used to determine the reasons for the weaknesses identified from the FWD investigation, which could include:

whether the existing granular base course and sub-base meet normally acceptable material standards for partial or full reconstruction.

whether the existing granular base course and sub-base meet normally acceptable standards for thickness for the appropriate road class.

confirmation of the pavement layers identified during DCP analysis. to enable mechanistic analysis of FWD measurements

Test Pits will be dug at points in the road where the Detailed VCS and FWD deflection profile show the road to abnormally weak. The measurements and tests required are listed in Table 6.2.



2009The results of these tests should be compared to standard material specifications, listed in the Design for New Bituminous, Gravel and Concrete Roads. Where the road base and sub-base material do not meet these specifications the length of road affected should be deep patched.

Table 5.8Field/Lab Test

Pavement material Test Description Test

Field Asphalt surfacingRoad baseSub base/Selected SubgradeSubgrade

ThicknessDescriptionMoisture Content KS 999 Part 2 2001

Road baseSub base/Selected SubgradeSubgrade

Layer density KS 999 Part 9 2001

Laboratory Road baseSub base/Selected SubgradeSubgrade

Atterberg Limits

Grading

KS 999 Part 2 2001

KS 999 Part 2 2001Sub base/Selected SubgradeSubgrade

CompactionCBR

KS 999 Part 4 2001KS 999 Part 2 2001



2009

6 CALCULATION OF STRUCTURAL NUMBER

6.1 Definitions

The Structural Number approach is probably the most reliable method of evaluating the ‘strength’ of pavements of similar type in terms of their likely traffic carrying capacity. It is calculated from the following:

Equation 3: Definition of Structural Number

Where:ai = Layer coefficient of layer ihi= Thickness of layer i (mm)

The calculation of layer coefficients for existing pavement layers is based on the stiffness of bituminous materials and the CBR of granular materials. They are indicated in Tables 6.1 and 6.2.

Table 6.9: Layer Coefficients for Existing Asphaltic Concrete and Granular MaterialsMATERIAL SURFACE CONDITION COEFFICIENT, ai

AC Surface Little or no alligator cracking and/or only low-severity transverse cracking

0.35 to 0.40

<10 percent low-severity alligator cracking and/or<5 percent medium- and high-severity transverse cracking

0.25 to 0.35

>10 percent low-severity alligator cracking and/or<10 percent medium-severity alligator cracking and/or>5-10 percent medium- and high-severity transverse cracking

0.20 to 0.30

>10 percent medium-severity alligator cracking and/or<10 percent medium-severity alligator cracking and/or>10 percent medium- and high-severity transverse cracking

0.14 to 0.20

>10 percent high-severity alligator cracking and/or>10 percent high-severity transverse cracking

0.08 to 0.15

Granular Roadbase or Subbase

No pumping, degradation, or contamination by fines.Some pumping, degradation, or contamination by fines.

0.10 to 0.14

0.00 to 0.10

Table 6.10: Layer Coefficients for Existing Stabilised Road BasesMATERIAL SURFACE CONDITION COEFFICIENT, ai

Stabilized Roadbase

Little or no alligator cracking and/or only low-severity transverse cracking

0.20 to 0.35

<10 percent low-severity alligator cracking and/or<5 percent medium- and high-severity transverse cracking

0.15 to 0.25

>10 percent low-severity alligator cracking and/or<10 percent medium-severity alligator cracking and/or>5-10 percent medium- and high-severity transverse cracking

0.15 to 0.20



2009>10 percent medium-severity alligator cracking and/or<10 percent high-severity alligator cracking and/or>10 percent medium- and high-severity transverse cracking

0.10 to 0.20

>10 percent high-severity alligator cracking and/or>10 percent high-severity transverse cracking

0.08 to 0.15

The Structural Number was developed during the AASHO Road Test, which considered the performance of trial sections constructed over a uniform subgrade having a particular strength. A further parameter, the Modified Structural Number (SNC) (Hodges et al, 1975), was later developed to take into account different subgrade strengths. This relationship is defined in Equation 4:

Equation 4: Definition of Modified Structural Number

Where:SNSG = Structural Number contribution from the subgrade= 3.51 Log10 (CBR) – 0.85 (Log10 (CBR))2 – 1.43SNC = Modified Structural NumberCBR = In situ CBR of the subgrade.

6.1.1 VARIATION OF BITUMINOUS LAYER COEFFICIENT WITH TEMPERATURE

The AASHO Road Test was carried out in Illinois, USA. The layer coefficient taken for a new asphalt concrete surfacing during the Road Test was 0.44. This was for asphalt concrete having an elastic modulus of 3100 MPa at a temperature of 20oC. It is therefore necessary to derive a strength coefficient suitable for Kenya, where the ambient temperatures, and hence road temperatures, are different to those in Illinois. This is obtained by calculating the effective elastic modulus of asphalt concrete using the Shell Method of Weighted Monthly Average Annual Temperature (WMAAT) (Shell, 1978), shown below. The analysis shows that the layer coefficient of asphalt concrete used in Kenya should be:

Altitude 0 – 600 metres = 0.38 (WMAAT=22.5 oC)Altitude 600 – 1200 metres = 0.40 (WMAAT=19.6C)Altitude > 1200 metres = 0.44 (WMAAT) = 11.3C

The analysis is shown below:

Step 1Calculate equivalent modulus at the AASHO Road Test site (WMAAT = 15oC) of asphalt concrete having an elastic modulus of 3100MPa at 20oC tested in laboratory, using the equation 5:

Equation 5: Variation of Elastic Modulus with temperature

where b = 0.024 and T1, T2 are two asphalt temperatures.

ET=15 = 3100*10-0.024(15-20) = 4086 MPa



2009Step 2Calculate the elastic modulus of similar material, for instance, in the Coastal Region (WMAAT = 22.5oC) to that in Illinois (WMAAT = 15oC):

ET=22 = 4086*10-0.024(22.5-15) = 2700 MPa

Step 3Calculate layer coefficient of asphalt concrete in the Coastal Region having an elastic modulus of 2700MPa:

aT1/aT2 = (ET1/ET2)0.333

Where:aT1 and ET1 are the layer coefficient and elastic modulus respectively at temperature T1.

aT=22/aT=15 = (2700/4086)0.333 = 0.87 aT=22 = 0.44*0.87 = 0.38

6.2 Use of Structural Number for Overlay Design

The overlay thickness is derived from:

Equation 6: Derivation of overlay thickness from Structural Number

Where:SNPDesign = Structural Number for future trafficSNPExisting = Structural Number of existing roada1 = Layer coefficient of asphalt overlay

Therefore to calculate the thickness of required overlay, the Structural Number of the existing road (SNExisting) has to be measured. There are a number of ways of doing this, all of which have various advantages and disadvantages, as enumerated in Table 6.3.

Table 6.11: Advantages and Disadvantages of Investigative methodsMethod Procedure to calculate

SNExisting

Requirements Operational restrictions

Test Pits Direct calculation from thickness and strength (laboratory) of the different pavement layers

Field and Laboratory testing

Poor coverage

DCP tests

Direct calculation from estimated thickness and in situ strength of the different pavement layers

Test Pits needed to gain information on actual pavement layer thickness and material

Fair Coverage

FWDBack calculation DCP or Test Pits

needed to establish pavement layer thickness

Good coverage

Estimate of SNC from FWD deflection bowl (SNP)

- Good coverage



2009The Structural Number and Modified Structural Number concept, whilst simple in principle, gives rise to a number of practical difficulties, especially on roads that have been in existence for many years. When DCP tests and Test Pits are carried out, the boundaries between the different materials are sometimes indistinct and differentiating base courses from sub-bases, and sub-bases from the subgrade can be difficult. Changes of strength are expected to occur when passing from one layer to another but significant changes of strength also occur within reasonably well-defined layers. When the same pavement is tested with a DCP a more complex, many-layered structure is often revealed.

This can cause a problem in defining the layers in Test Pits for calculating the Modified Structural Number. The same difficulty also applies when trying to define the appropriate layer thickness for back-analysis of FWD data and often makes this form of analysis somewhat unreliable.

A procedure is therefore required which takes account of the contribution to Structural Number of a pavement from all the pavement layers and the contribution of the subgrade, which is independent of where the subgrade boundary is defined. This value is called the Adjusted Structural Number (SNP) (Rolt and Parkman, 2000).

6.3 Use of the FWD to estimate SNPExisting

The most suitable tool to measure the Adjusted Structural Number of an existing road (SNPExisting) is the DCP; its use to design overlays in Kenya is, however, often not ideal. This is because:

it may not be practicable to take sufficient DCP measurements along each road to cope with the possible high variability found in Kenya, and

the coarse granular road base in the Kenya roads prevent the instrument’s penetration.

An overlay procedure based on DCP results is described in Section 6.6 for Secondary and Local roads where FWD results are not available.

As FWD deflection data can be measured very quickly and accurately, the proposed overlay procedure uses the data to estimate the SNPExisting of the existing road, rather than the DCP. Previous work (Rolt, 2000) showed that the most effective form of the correlation between FWD measurements and SNP takes the form below:

Equation 7: Correlation between SNP and FWD

Where:

d0 = Central deflection (mm)d900 = Deflection at 900mm from the load (mm)d1200 = Deflection at 1200mm from the load (mm)(FWD deflection is measured in mm at a load of 50KN)

Figure 6.7 : Correlation between SNP and Deflection



2009

R2 = 0.96

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P(E

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tin

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The equation above has been used to convert FWD measurements taken from hypothetical Trial Sites. The predicted values of SNP are shown plotted against the central deflection d0 in Fig 6.1. The limited scatter around the ‘line of best fit’ (R2 = 0.96) shows the suitability of this form of general relationship for the analysis of FWD results. However, to enable the equation above to be used in for Kenya a series of comparative tests between the FWD and the DCP must be carried out on a selection of Category A and B roads.

6.4 Overlay Design Procedure using the FWD

The required overlay thickness is calculated based on a comparison of the strength of the road required for the future traffic and the existing strength of the road, as assessed by FWD measurements.

6.4.1 SNP FOR FUTURE TRAFFIC (SNPDESIGN)The first step in the process is to establish the value of Structural Number (SNPDesign) that is required for each homogeneous section of road for future traffic loading. This is achieved by using the AASHTO (1993) equation for flexible pavements, shown below:

Equation 8: Computation of SNP Design

Where:

W8.16 = predicted number of 8.16 tonne ESALs,ZR = Standard normal deviate for required reliability, S0 = Combined standard error of the traffic and performance predictions - see below, PSI = drop in serviceability over the performance period,MR = subgrade resilient modulus in psi,SN = structural number to carry W8.16 ESALs.



2009The recommended Reliability factors and decrease in Pavement Serviceability Index (PSI) used in the equation are shown in Table 6.4. The Standard Deviation is set at 0.49 as recommended by AASHTO (1993). The calculated values of SNPdesign for various values of ESA are presented in Table 6.5.

Table 6.12: AASHTO Design Criteria: Reliability factors and Servicability IndicesRoad Class Reliability Standard

DeviationTerminalPSI

Decrease in PSI

International 90 0.49 2.7 1.5Primary 90 0.49 2.2 2.0Secondary 85 0.49 2.0 2.2Local 50 0.49 1.7 2.5

Table 6.13: Design SNPFuture Traffic (Million ESA)

Road Class <0.5 0.5–1 1-2 2-5 5-10 10-20 20-50A - - - 5.68 6.25 6.84 7.67B - - - 5.22 5.76 6.28 -C 3.54 3.93 4.32 4.90 5.40 -Local 2.93 3.25 3.57 4.05 4.45

To use the AASHTO design equation when the Adjusted Structural Number (SNP) is used rather than SN and subgrade strength separately, the subgrade resilient modulus value must be assumed at 4325 psi in the equation. This was the subgrade resilient modulus used in the Road Test and therefore at this value the subgrade contribution is zero. Thus SN is then the same as SNP. In using either method, the difference between SN and SNP needs to be understood. In the normal AASHTO design method SN is used rather than SNP. The overlay procedure described in this Manual uses SNP.

This results in the principle that if any two pavements have the same value of Adjusted Structural Number (SNP) then they should carry the same level of traffic.

In the following paragraphs that describe the overlay procedure it has been assumed that the road under investigation is a Category A road with a design traffic loading of between 5-10 million ESA. Therefore, from Table 6.5, the SNPDesign is 5.76.

6.4.2 STRUCTURAL DEFICIENCY

It is necessary to plot the ‘Structural Deficiency’, that is the difference between the required design Structural Number of the road (SNPDesign) and the existing Structural Number at each FWD test (SNPExisting), for each FWD test. This is simply :

Equation 9: Definition of Structural Deficiency

After calculation the Structural Deficiency is plotted as a bar chart, which allows the engineer to identify the following actions for the homogeneous sections based on the criteria given in Table 6.6:



2009Table 6.14: Structural Deficiency Criteria

Mean Structural Deficiency Action Notes

Zero or negative Maintain A thin overlay may be required to

correct other defects

0 to 0.6 Thin overlay Remedial works possible

0.6 to 1.5 Thick overlay

(40/50mm)

Remedial works probable

> 1.5 Reconstruction probable

Fig 6.2, the results of an actual FWD survey, illustrates these principles:

Figure 6.8: Structural Deficiency



2009

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Str

uc

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efi

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Mean Structural Deficiency = -0.92

Mean Structural Deficiency = -0.95

Mean Structural Deficiency = +0.55

Mean Structural Deficiency = +1.45

No strengthening overlayNo strengthening

overlayThick strengthening overlay

plus patchingThin strengthening overlay

plus patchingPatching

No strengthening is required if the Structural Number Deficiency is either zero or predominantly negative. Any occasional positive values should be investigated and deep patched where necessary. If the road has been identified as having a poor profile (ie high IRI value) a thin overlay can be constructed as periodic maintenance. The minimum thickness of these thin overlays is governed by the aggregate grading of the overlay material. Where the mix has a Maximum Stone Size of 25mm the overlay will need to be 50mm thick. Where the Maximum Stone Size is 19mm the material can be laid with a minimum thickness of 40mm.

If the mean structural deficiency lies in the range ranges from 0 to 0.6 a thin overlay should be constructed. Points with high structural deficiency should be investigated and deep patched where necessary.

If the mean structural deficiency ranges from 0.6 to 1.5 then a thick overlay is necessary. The need for some deep patching is also very likely to be required. The thickness design procedure is described in Section 6.4.3.

The need for partial or full reconstruction is less easy to define, but becomes probable if the structural deficiency is greater than 1.5. Under such circumstances the visual condition data, DCP and test pit data needs to be re-assessed.

The design of roads that require reconstruction should be done in accordance with design recommendations set out in the Design of New Bituminous, Gravel and Concrete Roads. In general, roads with good foundations can be partially reconstructed by making use of much of the existing material in the form of enhanced sub-base or even lower base course layers. Roads which have a very weak or non uniform pavement structure and or sub-grade require more elaborate remedial works and full reconstruction is possibly required.

6.4.3 DESIGNING THICK OVERLAYSThe final step in the process is to calculate the thickness of overlay for those homogeneous section where a thick strengthening overlay (>50mm) is required.



2009The overlay at each FWD test is calculated using the equation below. Where no overlay is required at an FWD test, a value of zero is assigned.

Equation 10: Calculation of Overlay Thickness

Where a1 = layer coefficient for the asphalt overlay.

The overlay thickness for each homogeneous section is then calculated as follows:

Equation 11: Calculation of Overlay Thickness for Homogeneous Section

Where:SD = Standard deviation of the overlay thickness in the homogeneous sectionCF = Probability of achieving design life.

Values of CF that should be used for different levels of probability are given below:

Table 6.15: Values of 'CF'Probability of Achieving Design Life CF Factor

90% 1.282

85% 1.037

80% 0.841

75% 0.674

50% 0.0

A value of 85% is usually recommended. The use of a higher level of probability can result in overlays being too thick if the road construction is highly variable. In Fig 6.3 a value of 1.037 has been used:

Figure 6.9: Design of Overlay Thickness from FWD data

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Thick Overlay 112mmplus patching



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6.5 Overlay Design Procedure using the DCP

This procedure should only be used on Secondary and Local roads where FWD data is unavailable and where the road structure allows the DCP to penetrate the road structure to a depth of 800mm. The required overlay thickness is calculated based on a comparison of the strength of the road required for the future traffic and the existing strength of the road, as assessed by DCP measurements. The following steps should be followed:

1. Establish the value of Structural Number (SNPDesign) that is required for each homogeneous section of road for future traffic loading. This is done using the AASHTO (1993) equation in the same way as described in Sections 6.4.2 and 6.4.3. The resultant values of SNPDesign are given in Table 6.5 for different levels of traffic for Category B roads and Local roads.

2. Calculate the adjusted Structural Number of the existing road (SNPExisting) from DCP tests. On Secondary and Local roads, the DCP tests should be carried out at 100 metre intervals. The location of the tests should be ‘staggered’ by 50 metres to result in a DCP test every 50 metres along the road. The DCP data shall be analysed in purpose designed software called UKDCP. This software enables the user to analyse each DCP test and then calculate the SNPExisting for each test.

3. Identify homogeneous sections of road for strengthening. Each length is treated as a separate overlay design exercise. This procedure is best carried out by using the Cumulative Sum Method (CUSUM) on the value SNPExisting calculated from each DCP test. The homogenous sections are identified in the same way as is shown in Section 5.4. The UKDCP software allows the designer to identify the homogeneous sections automatically and this process is described in the User for the software.

4. Calculate the ‘Structural Deficiency’ for each DCP test. The value of Structural Deficiency is simply the difference between the required design Structural Number of the road (SNPDesign) and the existing Adjusted Structural Number at each DCP test (SNPExisting). After calculation the Structural Deficiency should be plotted as a bar chart and the required actions are described above.

5. Calculate the thickness of overlay for those homogeneous section where a thick strengthening overlay (>40/50mm) is required. The overlay at each DCP test is calculated using the equation below. Where no overlay is required at a DCP test, a value of zero is assigned.

Equation 12: Calculation of Overlay Thickness from DCP data

Where a1 = layer coefficient for the asphalt overlay (See Section 6.1.1).

The overlay thickness for each homogeneous section is then calculated using the following.

Equation 13: Overlay Thickness for Homogeneous Section



2009

Where:

SD = Standard deviation of the overlay thickness in the homogeneous section

CF = Probability of achieving design life

Values of CF that should be used for different levels of probability are given in Table 6.7.

Under most circumstances a value of 80% is recommended for Secondary roads and 75% for Local roads. The use of a higher level of probability can result in overlays being too thick if the road construction is highly variable.



2009

7 REMEDIAL WORKS PRIOR TO OVERLAY

A bituminous overlay will only perform as designed if the correct remedial works are carried out before overlay. Otherwise defects in the existing road will cause the new overlay to deteriorate and premature failure will occur.

The type of remedial work will depend on the type of road defect and these are recorded during the Detailed Visual Condition Survey. The remedial works are summarised in Table 7.1.

Table 7.16: Remedial Works prior to OverlayDefect Remedial Works

Wide single cracksWide connected cracks

Crack Sealing2

Alligator cracks with depressionsDeep potholesBase course shovingTrench/Patch failure

Deep patch affected area

Alligator cracking without depressionsShallow potholes

Shallow patch affected area

Asphalt shovingSlippage cracksPolished surface1

Mill and patch affected areas prior to overlay

Note 1. Where the existing surface has a poor texture and polished stone, the top surface should be lightly milled to ensure the new overlay does not ‘slip’ on the old surface and fail prematurely.

2. Wide cracks should be sealed prior to overlay to prevent water entering the granular base course if reflection cracking occurs.

When deep patching is needed the required minimum thickness of base course and sub-base materials are given in Table 11. For low levels of traffic, granular base course materials should be used. However, for higher levels of traffic on Category A and Category B roads a bituminous base course material can also be used.

Table 7.17: Minimum layer thickness for patching prior to overlay

Road ClassFuture TrafficMillion ESAL

Base course (mm) Sub-base (mm)

Granular Bituminous Granular

International> 20 250 200 200

< 20 225 175 200

Primary> 5 200 150 200

< 5 175 - 200

Secondary> 1 175 - 200

< 1 150 150

Local> 2 175 200

< 2 150 - 150

Where the high deflections are related to lengths of road with poor or poorly maintained drainage then these shortcomings should be rectified prior to overlay construction. Details on the construction and maintenance of road drainage are described in Design for New Bituminous, Gravel and Concrete Roads.



2009

8 REFERENCES

AASHTO (1993). Guide for the design of pavement structures. AASHTO, Washington DC, USA

HODGES J W, J ROLT and T E JONES (1975). The Kenya road transport cost study: research on road deterioration. TRL Report 673, TRL, UK.

ROLT J and C PARKMAN (2000). The characterisation of pavement strength in HDM-III and improvements adopted for HDM-4. 10th REAAA Conference, Tokyo, 2000.

ROLT J (2000). Pavement structural number from FWD measurements for network analysis. TRL Unpublished Report PR\INT\664\00

SHELL INTERNATIONAL PETROLEUM CO. (1978). Shell Pavement Design , London.


9 APPENDICES

9.1 Appendix 1 : DCP Test

The DCP is an instrument which can be used for the rapid measurement of the in situ strength of existing pavements constructed with unbound materials. Measurements can be made down to a depth of approximately 800mm and where the pavement layers have different strengths, the boundaries between them can be identified and the thickness of each layer estimated.

9.1.1 DESCRIPTIONThe DCP uses an 8 Kg hammer dropping through a height of 575mm and a 60° cone having a maximum diameter of 20mm. The instrument is assembled as shown in Figure 3.1. The instrument is usually split at the joint between the standard shaft and the coupling for carriage and storage and it is important that when in operation the joints do not become loose. Operating the DCP with any loose joints will significantly reduce the life of the instrument.

· 60° INC

Ø 20mm

Key:-1 Handle2 Hammer (8kg)3 Hammer shaft4 Coupling5 Handguard6 Clamp ring7 Standard shaft8 1 metre rule9 60° cone

7

8

9

5

4

9

1

2

3

6

9.1.2 OPERATION After assembly, the first task is to record the zero reading of the instrument. This is done by standing the DCP on a hard flat surface, such as concrete, checking that it is vertical and then entering the zero reading in the appropriate place on DCP Test Data Sheet shown in Figure 3.2.

The DCP needs three operators, one to hold the instrument, one to raise and drop the weight and a technician to record the readings. The instrument is held vertical and the weight raised to the handle. Care should be taken to ensure that the weight is touching the handle, but not lifting the instrument, before it is allowed to drop. The operator must let it fall freely and not partially lower it with his hands.


It is recommended that a reading should be taken at increments of penetration of about 10mm. However it is usually easier to take a reading after a set number of blows. It is therefore necessary to change the number of blows between readings, according to the strength of the layer being penetrated. For good quality granular bases readings every 5 or 10 blows are usually satisfactory but for weaker sub-base layers and subgrades readings every 1 or 2 blows may be appropriate. There is no disadvantage in taking too many readings, but if readings are taken too infrequently, weak spots may be missed and it will be more difficult to identify layer boundaries accurately, hence important information will be lost.

After completing the test the DCP is removed by tapping the weight upwards against the handle. Care should be taken when doing this; if it is done too vigorously the life of the instrument will be reduced.

The DCP can be driven through surface dressings but it is recommended that thick bituminous surfacings are cored prior to testing the lower layers. Little difficulty is normally experienced with the penetration of most types of granular or lightly stabilised materials. It is more difficult to penetrate strongly stabilised layers, granular materials with large particles and very dense, high quality crushed stone. Penetration rates as low as 0.5mm/blow are acceptable but if there is no measurable penetration after 20 consecutive blows it can be assumed that the DCP will not penetrate the material. Under these circumstances a hole can be drilled through the layer using an electric or pneumatic drill, or by coring. The lower pavement layers can then be tested in the normal way. If only occasional difficulties are experienced in penetrating granular materials, it is worthwhile repeating any failed tests a short distance away from the original test point.

If, during the test, the DCP leans away from the vertical no attempt should be made to correct it because contact between the shaft and the sides of the hole can give rise to erroneous results. If the lean becomes too severe and the weight slides down the hammer shaft, rather than dropping freely, the test should be abandoned and the tests repeated approximately one metre away from the first test. DCP is used extensively for hard materials, wear on the cone itself will be accelerated. The cone is a replaceable part and it is recommended that it should be replaced when its diameter is reduced by 10 per cent. However, other causes of wear can also occur hence the cone should be inspected before every test.

9.1.3 INTERPRETATION OF RESULTS

The correlation between DCP readings and CBR value has been determined by a number of authorities and a selection of these are given in Figure 3.3. Agreement is generally good over most of the range but differences are apparent at low values of CBR in fine grained materials. It is expected that for such materials the relationship between DCP and CBR will depend on material state and therefore, if more precise values are needed it is advisable to calibrate the DCP for the material being evaluated.


Until local calibration is carried the following relationship given should be used

Log10 (CBR) = 2.48 - 1.057 Log10 (mm/blow) The results can be either be plotted by hand, as shown in Figure 3.3, or processed in a spreadsheet.


9.1.4 CALCULATION OF STRUCTURAL NUMBER

If required the Structural Number of the pavement can then be calculated from the DCP results using the following general equation.

SN = 0.0394 I aI dI

where aI = Layer coefficient of layer IdI = Thickness of layer I (mm)


DCP TEST DATA FORM

Date: Wheelpath

Road No: Started test at:

Test No: (Surfacing / Base/ Sub-base / Subgrade)

Chainage: Operator:

Direction: Zero reading of the DCP (mm):

No of

Blows

Blows

mm No of

Blows

Blows

mm No of

Blows

Blows

mm


9.2 Test Pit

Test pits should only be necessary on roads requiring rehabilitation. Roads requiring maintenance with thin overlays (periodic maintenance) will only have test pits dug where FWD or DCP measurements indicate short lengths of weak pavement.

The purpose of carrying out a test pit investigation is to confirm the information obtained from surface condition survey, and FWD and DCP surveys. Pit digging is a time consuming and expensive operation and for this reason the location of each test pit should be carefully selected to maximise the benefit of any data collected.

The responsible engineer will select the number and position of the test pits to establish:

the thickness and material properties of the road pavement in each homogeneous section the thickness and material properties of any lengths of road pavement, within any

homogeneous length, which have been shown to be significantly weaker by either FWD or DCP testing.

The minimum number of test pits dug in any one homogeneous length of road should not be less than one every 2 kms. In general the test pits will be dug in the near-side wheelpath. ie the wheelpath adjacent to the shoulder of the road.

9.2.1 LABOUR, EQUIPMENT AND MATERIALS

Test pits can be excavated by hand or by machine, depending on the availability of plant and the test pit programme required. Machine operations are usually more productive but more costly than methods.

The following personnel are required:

• traffic controllers - a minimum of one at each end of the site (but see above);• 2 (if machine excavation) or 3 (if excavation) labourers;• 1 machine operator if applicable;• 1 driver for vehicle; and• 1 supervising technician.

The following equipment and materials are required:

• 1 backhoe (for machine excavation);• 1 jack hammer with generator (to assist with excavation);• 1 pick;• 1 or 2 spades (a fence post hole digger can also be useful);• 1 tamper or plate compactor for backfilling test pit;• material to backfill and seal test pit : gravel, cement for stabilising gravel, water and cold

mix for resurfacing;• 1 broom to tidy area on completion;• 1 chisel is often useful to assist with inspecting the wall of the test pit;• equipment necessary to complete any required on-site testing;• 1 tape measure and thin steel bar to span pit (to assist with depth measurements);• sample bags and containers, with some means of labelling each;• test pit log forms and clipboard; and• sample log book.

9.2.2 SAMPLING AND TESTING PROCEDURE


9.2.2.1 Field ProcedureBefore commencing the survey in the field, the responsible engineer should be clear as to the information required from each test pit. This will depend on the results of previous surveys, the materials specifications in use and an understanding of the pavement behaviour. Some field testing might be necessary as well as subsequent laboratory testing of samples extracted from the pit. Table 9. summarises the various tests that may be required and references the relevant standards. Not all these tests may be necessary , depending on the situation found.

A safe working environment should be maintained at all times. Reference should be made to the appropriate regulations in this regard.

Once it has been decided what testing is to be carried out and the location of the trial pits has been confirmed, the following procedure should be adopted:

1. Set up traffic control.

2. Accurately locate position of test pit and record this on the Pavement Test Pit Log (see Figure G1). Usually, the position of a pit will be apparent after completion due to the patched surface. However, if long term monitoring is required, a permanent location marker should be placed at the roadside. Record any relevant details such as surrounding drainage features, road condition and weather.

3. Define the edge of the test pit and remove surfacing. The required size of pit will depend on the sample sizes necessary for the selected tests, but it can be increased later if found to be too small. Usually an area of about 0.8m by 0.8m will be sufficient for excavation, and the minimum working area required for a backhoe operation will be sufficient for machine excavations. The edge of the pit can be cut with a jack hammer or pick and the surfacing ‘peeled’ off, taking care not to disturb the surface of the aggregate roadbase. The average thickness of surfacing should be recorded.

4. If density tests are to be performed, a smooth, clean and even surface is required. It is important for the accuracy of the test that the layer is homogeneous. For the sand replacement method, no prior knowledge is required of the layer thickness since this becomes obvious as the hole is excavated. If a nuclear density meter is used, the thickness of the layer can either be estimated from previous DCP results or construction details to determine the depth of testing.

5. On completion of any required density testing, the layer can be removed over the extent of the trial pit, a visual assessment made of the material and samples taken for laboratory testing. Care should be taken not to disturb the adjacent lower layer. The thickness of the layer and the depth at which samples are taken should be measured. All information should be recorded on the Pavement Test Pit Log.

6. Continue to sample, test and excavate each pavement layer following the procedure above. Once it has been decided that there is no need to excavate further, the total depth of pit should be recorded along with any other information such as appearance of water in any of the layers.

7. All samples should be clearly labelled and proposed tests for the pit materials should be logged in a sample log book to avoid later confusion in the laboratory.

8. The pit should be backfilled in layers with suitable material which should be properly compacted. It is often good practice to stabilise the upper layer with cement accepting that


full compaction will not be achieved. A bituminous cold mix can be used to patch the backfilled pit.

9. The site should be cleared and left in a tidy and safe condition for traffic.

9.2.2.2 Laboratory procedure

Table 18: Tests to be carried out

Property Possible Tests Field or Lab

Procedure Remarks

Particle size distribution

Sieve analysis Lab KS 999:Part 2:2001 Initial visual assessment on site.

Plasticity Plastic and Liquid Limits, Plasticity Index

Lab KS 999:Part 2:2001 Initial visual assessment on site.

Linear Shrinkage Lab KS 999:Part 2:2001 Correlated to PI

Particle Shape4 Elongation Index LabKS 1238 Part 6 2003

Flakiness Index LabKS 1238 Part 6 2003

Particle Strength4 Aggregate Crushing Value

LabKS1238 Part 11 2003 Los Angeles

Abrasion Value given in ASTMC 131-96 and C 535-96

10% Fines Value LabKS1238 Part 12 2003

Aggregate Impact Value

LabKS1238 Part 13

Particle Durability4

Aggregate Abrasion LabKS 1238

Los Angeles Abrasion Value given in ASTMC 131-96 and C 535-96

Accelerated Polishing Lab ASTM D 3319-90

Particle Soundness

Sulphate test LabKS 1238

Particle Density Particle density LabKS 1238

For soils

Particle density LabKS 1238

For aggregates

Moisture Content Oven dry7 LabKS 1238

Recommended method

‘Speedy’ Field Suppliers instructions

Nuclear Density Meter Field Suppliers instructions

Hazardous radioactive material

Moisture Density Relationship

Tests at various levels of compaction

LabKS 999

Layer5 Density Sand Replacement FieldKS 999


Method

Core Cutter Method LabKS 999

Nuclear Density Meter Field Suppliers instructions

Hazardous radioactive material

Bearing Capacity DCP Field See Appendix 9.1

California Bearing Ratio

Lab or Field

KS 999

Shear Strength6 Vane test FieldKS 999

Various load tests LabKS 999

In some cases, the possible tests listed for a given property are alternatives. In other cases all the tests listed for a given property might be required. The engineer must decide for which properties information is required and then design a suitable testing programme.

Field tests require testing at the site and possibly further analysis in the laboratory. Laboratory tests require only sampling in the field. All sampling should be carried out in accordance with the general guidance of KS 999 or KS 1238, whichever is applicable, as well as any specific requirements for each test.

Kenya Standards (KS) are quoted where available. Where no Kenya Standard is available, an alternative is quoted.

These tests will only be required for surfacing or base materials.

The layer must consist of homogeneous material for these tests.

These tests will only be required where a slope stability or settlement problem is being evaluated and will only apply to subgrade materials.

For moisture content determination, the oven-drying method is recommended since it provides a fundamental measure of the moisture content. Both the `Speedy' and the Nuclear Density Meter methods require accurate calibration and validation, since they derive the moisture content by indirect analysis, but they have the advantage of providing instant results. Validation should always be made with reference to the oven-dry method.


overlay and asphalt pavement rehabilitation manual

Documents

overlay design procedure

roads department

practical procedure

testing procedure

field procedure

laboratory procedure

use of structural number

use of dcp data