Guideline for Pavement Strength Testing2012 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 assistRoad Controlling Authorities in understandingthe collection and interpretation of pavementstrength data using the falling weightdeflectometer.
The Falling Weight Deflectometer (FWD)FWD is currently the most practical system for accuratemeasurement of deflection responseGeophones determine the deflection bowl produced by theimpulse 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 rehabilitationrequires sufficient data to provide reliability in terms of the 5or 10 percentile parameters applied to design.Network testing for asset management is often constrained bybudget 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 200 m intervals in each lane, or 400 m intervals if geologically > 5 km 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 rangeand general distribution as the traditional SNPThe following chart shows a process of determining Structuralindices (Stevens et al 2009)
Mechanistic approach (cont) Determine layer modu li, stresses an d strains at LTPP sites under 1 ESA Establish gen eral form o f relationships fo r determining ESA to a term inal conditio n for each distress m ode Usin g kn own distress and traffic data (from LTPP sites o r specific case histo ries), determine coefficients fo r each distress general form relationship Create a transfer fun ction that m aps the refined relationship onto a structural index with the same range More pavem ent performan ce Yes d ata now available? No Use structural index until mo re data co mes to hand that can fu rther refine the above models
Mechanistic approach (cont)Data from LTTP sites is used to determine coefficients intransfer functions that derive SIs Chart shows distribution of SNP for all national LTTP sites.
Mechanistic approach (cont)The pavement structural life (Rutting) is converted to thecorresponding structural index using a transfer functionAn 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 chartsupports the expected behaviour with isolated areas indicated as crackingrisk 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 stagenextPeer review and final editing to be completed – approx 2monthsPublishing will be later in this calendar year