Long-term pavement performance (LTPP) study – an overview

Introduction

The Austroads-funded long-term pavement performance (LTPP) study ran from 1994 to 2018. Its four primary objectives were to:

1. Enhance road asset management strategies by using improved pavement performance models.
2. Compare the performance of a range of Australian pavement sections with United States (US) sections established under the Strategic Highway Research Program Long Term Pavement Performance (SHRP-LTPP) Program.
3. Compare the performance of accelerated loading facility (ALF) test pavements with actual road pavement performance.
4. Investigate the quantitative influence of various maintenance surface treatments on long-term pavement performance, as determined by the long-term pavement performance maintenance (LTPPM) portion of the study.

These objectives were achieved using regular functional (roughness, rutting, cracking) and structural (deflection) observations on specific LTPP sites observations in conjunction with the long-term pavement performance maintenance (LTPPM) observational data. This observational data was collected from a range of in-service pavement LTPP and LTPPM sites set up for the study.

These objectives were also achieved in combination with the data obtained from the ALF experiments, under controlled environmental conditions and aimed at quantifying the effect of: (i) surface maintenance treatments and; (ii) increased axle loads (Martin 2010, 2011).

Findings – observational data

Estimation of deterioration rates

The deterioration rate of a pavement is a good indicator of pavement performance. Pavements are expected to deteriorate over time under traffic loading and climatic conditions. However, the deterioration process is often slowed due to the influence of surface maintenance and can be reversed due to the restorative impact of rehabilitation works.

The estimation of the deterioration rate, excluding the impact of either minor or major restorative works, was addressed by the ARRB in-house software tool that calculates the linear rate of progression (LRP) over part, or the whole, of the monitored pavement segment that is undergoing deterioration. The tool adopts a set of decision rules to validate each time-series data point for the calculation of the progression rate. The LRP software tool was written in an MS Excel macro and run on a spreadsheet with input data and output results worksheets attached to the file (Martin 2008).

The tool uses the maximum limit (ML), which is defined as the upper limit of consecutive deterioration change that can be acceptable, and the tolerance limit (TL), which is the maximum improved change condition allowed between two consecutive measurements, to tighten or relax the selection of input data. The ML and TL values set in the LRP interface for the calculation of progression rates for structural and functional distresses are given in Table 1.

Results of LRP processing for individual LTPP sites, the estimated rates of deterioration, are summarised in Table 2 for the various road types. The deterioration rate calculation requires a minimum number of three sequential valid data points. In the case where this condition is not satisfied, no valid result is produced by the tool. Table 2 summarises the range of observed rates of annual deterioration of deflection, roughness, rutting and cracking on the LTPP sites.

Findings – comparisons of deterioration rates

Comparative analysis studies of the LTPP pavement data with ALF experimental data were undertaken at an early stage of the project. The studies concluded that the pavement performance predictions made from ALF were generally comparable with that of the LTPP pavements (Clayton 2002, Tepper et al. 2002).

A comparison of the LTPP asphalt pavement sites’ performance with that of US SHRP-LTPP sites of similar loading and climate revealed that the sites from both groups experienced similar structural and functional deterioration, in terms of roughness and rutting (Austroads 2009). Cracking data was not available for the comparative analysis.

Findings – performance models

A number of road deterioration (RD) and works effects (WE) models were developed during the LTPP study. These included:

• The interim network-level functional and structural RD models for flexible pavements (Austroads 2010a, 2010b).
• The interim WE models for a wide range of surface treatments (Austroads 2007, 2017).
• The probabilistic RD model development, using the decision tool @Risk and a data condensation technique known as stochastic information packets (SIP) (Austroads 2016).
• The asphalt and seal life prediction models based on bitumen hardening Austroads 2010c).

Findings – impact of maintenance on performance

An investigation of the influence of surface maintenance treatments on pavement deterioration was based on the data collected from eight LTPPM sites from 2000 to 2018. This influence was assessed by treating each treatment as a variable while keeping all other factors – including pavement strength, traffic, climatic and topographic conditions – constant. Five different surface treatments were accommodated in any one LTPPM site.

The maintenance treatments were broadly classified into six groups as per Table 3. Routine maintenance, minimum maintenance, polymer modified binder (PMB) reseal and normal resealing occurred on every one of the eight LTPPM sites, whereas the geotextile reseal and a reseal with shape correction were only used on five and two sites respectively.

From Table 3 the following is observed:

• All six groups have a similar rate of structural deterioration as indicated by mean rate of deflection. These treatments, therefore and as expected, either had no influence or caused similar effects on the pavement structural performance.
• In terms of roughness progression, the impact of the geotextile reseal, PMB reseal and a reseal with shape correction on roughness was superior to the rest of the treatments (Austroads 2018).
• In terms of rutting progression, however, the results were slightly different to that of roughness progression, with the normal reseal and geotextile reseal giving the best performance while the remaining treatments had a similar effect on rutting performance.
• As expected, typical periodic maintenance treatments such as the geotextile reseal, PMB reseal, normal reseal and a reseal with shape correction, are the best treatments for long-term pavement protection. Routine and minimum maintenance treatments can initially be a quick and low-cost method of treatment and are only a short-term maintenance solution.

Monitoring – lessons  

After nearly 24 years of continuous monitoring experience, lessons were learnt about certain aspects of fieldwork, from the site establishment to the management and execution of the fieldwork program, which are summarised as follows:

• The majority of the original SHRP and ALF-LTPP test sections were set up on long-life pavements with thick asphalt or a bound base overlying a bound subbase in Victoria, NSW and Queensland. These sections represent major highways on national road networks but do not represent most of Australia’s roads, which are typically built on an unbound granular base with sprayed seal surfacing. These long-life pavement sites showed extremely slow functional and structural deterioration rates and required a longer period of monitoring to obtain useful data.
• All sites were annually monitored in the early stages of the study, which was later reduced for those sites with low rates of deterioration. Some long-life pavement LTPP sites were structurally monitored every five or six years, while those with conventional or weak pavements on a low volume traffic were tested for deflection on a yearly base. The annual monitoring frequency for functional performance remained for all sites.
• Both the survey method and equipment used for the study changed over the years in response to changes in road safety policy (which require lane closure) by participating road agencies in combination with the benefits of technological advancement – for example, the switch from the walking profilometer (WP) to the multi-laser profilometer (MLP) and the change from field-based visual crack assessment to manual visual rating from digital images. These necessary changes initially required further data processing efforts to ensure a smooth transition from one system to another.

Concluding remarks

The in-service pavement LTPP/LTPPM observational data was obtained through a rigorous monitoring approach that consumed a significant portion of the annual budget. The quality of this data is currently superior to that collected from a network survey.

The LTPP database and webpage will continue to exist as a valuable data source for research in pavement related areas. The data has been used by many researchers and is frequently used by academia inside and outside Australia. The LTPP database can be used to store additional data from other studies, such as the past local road deterioration study and any other future performance studies.

From the outset, the Australian LTPP study provided valuable pavement performance input to the US SHRP program. While the Australian sites were relatively limited in number, the initial intention was to use these as reference sites to ‘calibrate’ the performance of Australian pavements against the vastly numerically superior US SHRP study of approximately 1200 sites.

The LTPP study has fulfilled its original objectives. The study created an environment, or working platform, to facilitate the activities required to achieve the primary objectives of the study, such as tools for estimating the influence of climate on pavement distress and progression rates of pavement distress. In addition, technology and knowledge transfer activities involved establishing the LTPP/LTPPM database and webpage and the annual newsletter to promote further investigation and research. International activities involved the comparison of the performance of Australian LTPP sites with those of US-LTPP and a review of the 2002 AASHTO pavement design guide performance relationships to see if these were applicable to Australian LTPP sites.