Guideline for Pavement Strength Testing

2012 Road Asset & Information Forum

The Project•The project is commissioned by the RIMS Group•Researcher - Graham Salt et al of Tonkin and Taylor Ltd•Project Manager – Theuns Henning•RIMS Champion – Jim McQueen

Project ObjectivesTo provide a good practise guideline to assist Road Controlling Authorities in understanding the collection and interpretation of pavement strength data using the falling weight deflectometer.

The Falling Weight Deflectometer (FWD)FWD is currently the most practical system for accurate measurement of deflection responseGeophones determine the deflection bowl produced by the impulse of the falling weight.

Background•RCA’s are required to manage assets in cost effective and sustainable

manner•Understanding failure modes and strength of pavements at project level

promotes design of best long term treatments•Knowledge of pavement strength at network level aids sustainable long

term renewals planning•Pavement strength data stored in AMIS is utilised in optimised decision

making and deterioration modelling (eg dTIMS)•FWD testing has been widely used in NZ for may years for design and

network level management•As testing becomes more affordable authorities are increasingly

surveying extended parts of their networks

Background (cont)•Technology behind analysis of FWD data is well advanced•Various reports cover many aspects of FWD testing and

analysis for project and network level•There is no commonly adopted sampling approach adopted

by authorities in NZ•Combining information available into one guideline would

assist asset owners•In recent research, Salt et al (2010) have developed

structural indices for rutting, roughness, flexure and shear•Testing of these indices indicate their potential value

Aspects covered by Guideline•Use and basic principles of FWD testing•Explanation of empirical bowl parameters and structural

indices (SI)•Guidance for planning surveys and sampling•Data requirements•FWD data that should be stored in AMIS•SI concepts for alternative pavement types (unbound

granular, thin AC, stabilised etc)•SI application at network and project level•Reporting for Project/network

Sampling for network analysisProject level testing carried out for pavement rehabilitation requires sufficient data to provide reliability in terms of the 5 or 10 percentile parameters applied to design.

Network testing for asset management is often constrained by budget and to get coverage spacing of tests is extended.

The following table is used for network level testing(The remaining life can be determined as survey progresses)

dTIMS network level FWD spacings

Centreline Length  

FWD Test Spacing based on Field Calculation of Residual Life 

Life > 15 Years  Life < 15 Years 

0 m ‐ 200 m  5 Tests (3 in IRP lane, 2 in DRP lane) 

200 m ‐ 500 m  100 m intervals in each lane 

500 m ‐ 2 km  10 tests in IRP lane only  10 tests in each lane 

2 km ‐ 5 km  200 m intervals in IRP lane only  200 m intervals in each lane 

> 5 km  200 m intervals in each lane, or 400 m intervals if geologically uniform terrain 

 

Structural indices•The SNP while being easy to calculate, has a limitation that it

does not distinguish between the various modes that can lead to the failure of a pavement

•The SNP could suggest a strong pavement with a long life ahead but failure occurs sooner due to tensile failure in surface

•A set of “Structural Indices” has been introduced by Salt et al(2010), representing a structural number for each of the common distress modes

Structural indices and Distress Modes•SI Rutting – Vertical surface deformation, initially some

bedding in then a rapid increase at end of pavement life•SI Roughness – loss of shape longitudinally from variations

in rut depth and shear failures•SI Flexure – Surface cracking caused by surface tensile

strain in the deflection bowl as the wheel travels along the road and also in structural AC on the bottom of the layer

•SI Shear – lateral deformations or shoving due to shear failure in the pavement layers.

Obtaining the structural indices•Supplied by FWD provider after mechanistic analysis•Calculated by dTIMS analyst after mechanistic analysis

outlined in guideline•Generated by regressions currently being considered as an

application within RAMM•Analyst might generate SIs using a more rigorous approach

(layered elastic theory) using data in RAMM - may need to get additional data from the FWD provider

Mechanistic approachEach SI is mechanistically derived and has the same range and general distribution as the traditional SNP

The following chart shows a process of determining Structural indices (Stevens et al 2009)

Mechanistic approach (cont) 

No 

Yes 

Establish general form of relationships for determining ESA to  a  term inal condition for  each distress mode  

Using known distress and traffic data (from LTPP sites or specific case  histories), determine  coefficients for each distress general form relationship 

Create a transfer function that maps the  refined relationship onto a structural index with the  same  range  

Use  structural index until more  data comes to hand that  can fu rther  refine the  above  models 

Determine layer moduli, stresses  and strains  at  LTPP sites under  1 ESA 

More  pavement performance  data  now available? 

Mechanistic approach (cont)Data from LTTP sites is used to determine coefficients in transfer functions that derive SIsChart shows distribution of SNP for all national LTTP sites.

Mechanistic approach (cont)The pavement structural life (Rutting) is converted to the corresponding structural index using a transfer function

An example of SIs and SNP from one LTTP site BM01

Chainage  SNP  SIRUTTING  SIROUGHNESS  SIFLEXURE  SISHEAR 

0.10  3.9  4.5  4.9  3.1  3.6 

0.15  3.9  4.6  4.7  3.5  3.8 

0.20  3.5  4.0  3.9  3.3  3.6 

0.25  3.6  4.1  3.8  4.0  3.9 

0.30  1.8  2.6  2.7  4.2  4.2 

0.35  2.2  3.1  3.1  3.8  3.8 

 

Application of Structural Indices•The application of Sis has been tested on some networks (eg

Hastings)•Correlations between SIs and Radius of curvature etc have

been tested (Ashwin Sashi and Sine Foulgar) at Auckland university

•The box and whisker plot below is sample from wellington

Application of Structural indices (cont)

This is a section of Motorway that is AC on a relatively stiff base. The chart supports the expected behaviour with isolated areas indicated as cracking risk with other indices not an issue.

Some Conclusions•Structural indices give an indication of the bearing capacity

of road lengths in terms of specific failure modes•Capable of highlighting performance issues which may not

yet be visible – risk of future failure•Provides strong supporting evidence for maintenance

•Indices cannot be used in isolation as deterioration is also a function of traffic loading

•May also be a function of material types

Progress with Guideline ProjectThe document has been written and will go to the review stage next

Peer review and final editing to be completed – approx 2 months

Publishing will be later in this calendar year

Questions?More Information?www.rims.org.nz

Presenter NameJim McQueenjmcqueen@dcc.govt.nz