Moisture Influence on Structural Behaviour of Pavements : Field and Laboratory Investigations

The structural behaviour of pavements in cold regions can considerably be affected by seasonal variation in environmental factors such as temperature and moisture content. Along with the destructive effect of heavy traffic loads, climatic and environmental factors can considerably contribute to pavement deterioration. These factors can influence the structural and functional capacity of the pavement structures which, as a result, can trigger and accelerate pavement deterioration mechanisms. Studies on the influence of variation of the environmental factors on the response and behaviour of pavement materials have shown that proper consideration to these factors must be given in realistic pavement design and analysis. In flexible pavement structures, particularly with a thin hot mix asphalt (HMA) layer, unbound materials and subgrade soil largely contribute to the overall structural behaviour of the pavement system. In unbound materials, moisture content and its variation can significantly affect pavement layer stiffness and permanent deformation characteristics. Therefore, the moisture condition of pavements and its influence on the mechanical behaviour of pavement materials has been of interest among the pavement research community. A proper understanding of moisture transformation in pavement systems and its effects on pavement performance are important for mechanistic pavement design. The present summary of this doctoral thesis is based on four main parts. The first part of the thesis covers field measurements and findings from a test section along county road 126 in southern Sweden and consists of two journal papers (paper I and II) tackling different aspects of the research topic. This test section is located in a relatively wet ground condition and consists of a thin flexible pavement structure with a deep drainage system. It is instrumented with subsurface temperature, volumetric moisture content and groundwater probes. The mechanical response of the pavement structure was investigated using Falling Weight Deflectometer (FWD) measurements. The second part of the thesis (paper III and IV) are based on laboratory experiments and investigates different recent approaches that have been proposed to apply principles of unsaturated soil mechanics for incorporating seasonal variation of moisture content into the resilient modulus models using matric suction. The third part of the thesis (paper V) builds a bridge that spans between the laboratory and field investigations with an attempt to evaluate one of the predictive models presented in Paper III. The fourth part of the thesis (paper VI) mainly focuses on the laboratory-based investigation of the permanent deformation characteristic of subgrade soils. In this part, the permanent deformation characteristics of two different silty sand subgrade soils were investigated and modelled using the data obtained from repeated load traxial tests. Paper I mainly focuses on the spring-thaw weakening of the pavement structure. The environmental data collected using different sensors and the FWD tests were used to investigate variations in moisture content with thaw penetration and its influence on the stiffness of unbound layers and the pavement’s overall bearing capacity. Using the backcalculated layer stiffness and corresponding in situ moisture measurements in the unbound layers, a degree of saturation-based moisture-stiffness model was developed for the granular material and the subgrade. In Paper II, the drainage system of the structure was manually clogged during a three month period in summer to raise the groundwater level and increase the moisture content of the layers. Along with the subsurface groundwater level and moisture content monitoring, the structural response of the pavement was studied. In this research work, the FWD tests were conducted at three different load levels. The stress dependent behaviour of the unbound granular layer and the subgrade soil were further studied using the multilevel loads FWD test data. Additionally, parameters of a nonlinear stress-dependent stiffness model were backcalculated and their sensitivity to in situ moisture content was studied. In Paper III and IV, series of suction-controlled repeated load triaxial (RLT) tests were conducted on two silty sand (SM) subgrade materials. Several resilient modulus prediction models that account for seasonal moisture content variation through matric suction were summarized and after optimizing the model parameters, the capability of the prediction models in capturing the material response were evaluated. In Paper V, an attempt was made to evaluate the proficiency of one of the suction-resilient modulus models using the field moisture content and FWD measurements from the Torpsbruk test site. The backcalculated subgrade stiffness dataset at different moisture contents were compared with resilient modulus models obtained from the suction-resilient modulus predictive model. Paper VI presents an evaluation of several permanent deformation models for unbound pavement materials that incorporate the time-hardening concept using a series of multistage repeated load triaxial (RLT) tests conducted on silty sand subgrade materials. The permanent deformation tests were conducted at four different moisture contents with pore suctions measurement throughout the test. The effect of moisture content (matric suction) on the permanent deformation characteristics of the materials and the predictive model parameters were further investigated.QC 20150324</p

Pavement structural behaviour during spring thaw : interpretation of FWD measurements by monitoring environmental data from county road 126 at Torpsbruk

Vägnätet i stora delar av Norden är utsatt för relativt stor omgivningspåverkan under året. Under vinterhalvåret exponeras vägar för temperaturer under nollstrecket, vilket ger upphov till tjälbildning som penetrerar vägkonstruktionen och undergrunden. Vatten ackumuleras då i islinser i konstruktionen genom lateral transport av fukt, vertikal infiltration av nederbörd samt kapillär stigning av fukt från grundvattenytan. Under tjällossningsperioden omvandlas sedan isen till vatten igen, vilket leder till att konstruktionen håller relativt hög fuktkvot tills vattnet gradvis dräneras bort och vägen återhämtar det tillstånd den hade innan frysningen påbörjade. Ökat fuktinnehåll i de granulära delarna i konstruktionen under tjällossningsperioden resulterar i nedsatt skjuvhållfasthet i materialet samt minskade friktionskrafter mellan enstaka stenpartiklar. De obundna lagren förlorar därför delar av sin styvhet och bärighet, vilket kan ge upphov till en relativ rörelse mellan partiklarna med accelererad ackumulering av permanenta deformationer som följd. Ökning i utmattning av den bundna delen kan också observeras eftersom det underliggande stödet från det granulära materialet har minskat. Det hävdas ofta att den mest betydelsefulla faktorn för nedbrytning av vägar i regioner med kallt klimat är tjälperioden. Ökad förståelse av vägkonstruktionens strukturella beteende och uppförande under denna period är därför viktigt och kan leda till förbättringar i dimensioneringen (alternativa lösningar) med förhöjd förmåga att motstå nedbrytningsprocesser associerade med tjälperioden. I denna rapport har det strukturella tillståndet av länsväg (Lv)126 i södra Sverige följts upp under tjällossningsperioden 2010. Vägen är traditionellt uppbyggd, tvåfilig landsväg med flexibel beläggning och den öppnades för trafik 1985. Vägsträckan som observerades ligger nära Torpsbruk, 5 km norr om Moheda i Alvesta kommun i Småland. Den aktuella undersökningsplatsen var instrumenterad med temperatur- och fuktkvotsmätare som registrerade mätvärdena regelbundet under tjäl- och tjällossningsperioden. Det strukturella tillståndet undersöktes sedan med KUAB 50 fallviktsmätare. Totalt elva fallviktsmätningar utfördes våren 2010. Genom bakåtberäkning av fallviktsdatan bestämdes sedan styvhetsegenskaperna hos de individuella lagren i vägkonstruktionen. Analyserna gav därför styvhetsändringen under såväl tjällossningsperioden som den efterföljande återhämtningsperioden. Fallviktsdata användes för att klassificera vägens respons och för att undersöka energiåtgången under tjällossningsperioden. Slutligen jämfördes de bakåtberäknade modulerna med den uppmätta fuktkvoten för de obundna lagren, vilket gav styvhetsmodulens fuktkvotsberoende.The present research project has been carried out based on the environmental data and Falling Weight Deflectometer (FWD) measurements from the county road 126 in southern Sweden during the year 2010. The county road 126 has a relatively thin flexible pavement structure with unbound aggregate base and subbase layers. The major intention of this study was to investigate the behaviour of the pavement structure during spring thaw. Temperature and moisture content of the pavement structure profile were continuously monitored throughout the year 2010. Layer moduli backcalculation as well as deflection basin analyses were performed using the FWD measurement data. A comprehensive study on the effect of environmental factor variations and pavement structural capacity were carried out during the spring thaw and the recovery period. The result showed a considerable decrease in the bearing capacity of the pavement structure during the spring thaw period when the highest annual moisture content was also registered. Both deflection basin indices and backcalculated layer modulus indicated that the pavement was weakest during the subgrade thawing phase. Backcalculation on the FWD measurements showed a 67% loss in stiffness of the subgrade soil and 43% in the granular base and subbase course during the spring thaw compared to the summer values. In addition, the compatibility of the analysis with a predictive moduli-moisture content model for unbound materials was studied. The measured field data from the test road pavement in Torpsbruk showed promising agreement with the resilient modulus predictive model, both for the granular layer and subgrade material. Similar models could be developed or calibrated for other soils and granular materials if sufficient data become available in the future

Påverkan av grundvattennivån på den mekanistiska responsen av en vägkonstruktion : en studie på provväg Torpsbruk på länsväg 126 med fallviktsmätningar

Excess moisture presence in pavement unbound layers can result in lower structural stiffness and reduce the service life of road systems. A field study was conducted along the county road 126 near Torpsbruk to investigate the effect of the moisture content on the bearing capacity of a flexible pavement structure and its unbound layers stiffness. The subsurface drainage system of the test road was clogged during a three-month period in summer 2010, raising the groundwater level under pavement structure and therefore increasing the moisture content of the pavement unbound layers. During this period, the groundwater level and the volumetric moisture content of the pavement was continuously monitored using the instrumentations at test site and the structural response of the pavement was evaluated by conducting frequent Falling Weight Deflectometer (FWD) tests with multilevel loads. Using the FWD data, the unbound layers stiffness were back-calculated and their sensitivity to the moisture content and load level were studied. The field measurements showed that the groundwater level can significantly affect the moisture condition of the unbound layers and therefore their stiffness. Both the unbound granular layer and the subgrade showed stress dependent behavior to multilevel FWD loads.Ökad fuktkvot i vägars obundna lager kan inverka på vägkonstruktionens strukturella styvhet och minska dess livslängd. En fältstudie gjordes längs riksväg 126 nära Torpsbruk för att undersöka de obundna lagrens fuktkvoter på vägkonstruktionens strukturella respons och styvhet. Vägkonstruktionens dräneringssystem täpptes till under tre månader under sommaren 2010. Tilltäppningen av dräneringssystemet resulterade i en snabb höjning av grundvattennivån, samtidigt ökade de obundna lagrens fuktkvot. Under studieförloppet har grundvattennivåns läge och fuktkvoten i vägkonstruktionens obundna lager mätts kontinuerligt med hjälp av fältinstrumenteringar. Dessutom gjordes frekventa fallviktsmätningar med tre lastnivåer för att mäta konstruktionens strukturella respons. På så sätt kunde lastnivåns och fuktkvotens inverkan på de obundna lagrens beteende studeras med målsättningen att bestämma egenskaperna hos de obundna materialen genom bakberäkning av fallviktmätningsdata. Enligt bakberäkningarna resulterade konstruktionens fuktökning till en betydande reducering av de obundna lagrens styvhet. Det observerades dessutom att de obundna lagren visade ett spänningsberoende beteende under de olika lastnivåerna vid fallviktsmättningarna

Characterisation of Permanent Deformation of Silty Sand Subgrades from Multistage RLT Tests

AbstractIn thin flexible pavements, unbound aggregate layers and subgrade, contribute largely to the rut formation of the pavement system. Therefore, realistic prediction of surface rutting requires models that can reliably capture the cumulative plastic deformation of pavement unbound materials under repeated loads. Here four models were evaluated to capture the accumulation of permanent deformation of two fine grained subgrade materials. The models were modified based on the time hardening concept allowing them to be used in a multistage repeated load triaxial (RLT) test procedure, in which a single specimen is exposed to a series of consecutive stress paths of varying magnitudes. The RLT tests were conducted on two non-plastic silty sand subgrade soils with different fines content and at four different moisture contents. The used triaxial testing system was capable of controlling the pore-air and pore-water pressures of the specimen and therefore the permanent deformation tests were carried out with controlled matric suction of the soil samples throughout the test. An effective stress approach was therefore used in the modelling. Using the test data, the model parameters were determined using a parameter optimisation algorithm and the qualities of fits achieved was analysed and compared through goodness-of-fit statistics. In general, the four permanent deformation models were found to work satisfactorily in capturing the accumulation of the permanent deformation of the tested subgrade materials. It was concluded that the multistage RLT test procedure has the potentials to be used for characterizing the permanent deformation behaviour of subgrade soils. This can considerably reduce the effort and time required for permanent deformation characterization of subgrade materials