TECHNICAL PAPER Towards a mechanistic structural …

1 Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese, 46(3) 20042 TECHNICAL PAPERJ ournal of the south AfricanInstitution of Civil engineering , 46(3)2004, Pages 2 8, PAPER 554 JACO LIEBENBERG holdsthe degrees BEng(Civil),(Hons)(Transportation)and MEng(Transportation)from the University ofPretoria, and in 2003received the SAICE awardfor the best postgraduatetransportation engineer-ing student in SouthAfrica (Northern Region) for his dissertation onthe structural design of emulsion-treated materi-als. Between 1996 and 1999 he held the posi-tion of regional roads engineer for the Winburgregion in the Free State Department of PublicWorks, Roads and Transport, before joining thePavement and Materials unit of Stewart ScottInternational in 2000.

2 Jaco was appointed anassociate in the company in 2003 and is cur-rently involved in various projects throughoutsouthern VISSER is the SARoads Board Professor inTransportationEngineering in theDepartment of Civil andBiosystems engineering atthe University of holds the degreesBSc(Eng) (Cape Town),MSc(Eng) (Wits), PhD(University of Texas at Austin) and BComm (SA).His fields of research interest are primarily low-volume road design and maintenance, roads forultra-heavy applications, and road managementsystems. He is a fellow and past president of theSouth african Institution of Civil engineering (SAICE). In 1998 he received the SAICE Awardfor Meritorious Research for his contributions tolow-volume road technologies.

3 In 2004 hereceived the Chairman s Award from theTransportation Division for his contributions totransportation a mechanistic structural designprocedure for emulsion-treated base layersJ J E Liebenberg and A T VisserExisting guidelines on the use of emulsion-treated materialsdo not provide the necessary information for mechanisticanalysis and design, and the structural design of these materi-als is mainly based on the experience of practitioners. Recentlaboratory and heavy vehicle simulator testing provide someinsight into the behaviour of emulsion-treated materials whichhas led to the development of interim transfer functions thatmay be used in a mechanistic analysis.

4 Emulsion-treatedmaterials behave in two phases, a pre-cracked and post-cracked phase. In the pre-cracked phase, the material has astiffness that is similar to that of lightly cemented material,while in the post-cracked phase it has a stiffness that is com-parable to that of the untreated material. During the pre-cracked phase the material will behave similarly to a lightlycemented material and will fail in fatigue. In the second phasethe material will behave similarly to a granular material with areduced resistance to permanent deformation. This paperdescribes a mechanistic design procedure for the structuraldesign and analysis of emulsion-treated materials, based onthe principles of the south african mechanistic PavementDesign Procedure.

5 An interim design catalogue based on theseprinciples was developed and is also the past 30 years, success has beenachieved by south african road engineerswith the technique of adding small quanti-ties of bitumen emulsion to gravels of fair togood quality. The introduction of in-siturecycling makes the use of bitumen emul-sion in the rehabilitation of existing pave-ments attractive and this technology isbecoming increasingly popular. Althoughemulsion-treated materials have been usedwith great success for a number of years,their structural performance has not beeninvestigated in detail. The objective of this PAPER is to definethe life cycle behaviour and failure criteria ofpavement layers treated with bitumen emul-sion and to present interim transfer func-tions for the mode of failure that can beused in mechanistic pavement design.

6 Thiswork is based on an extensive laboratorystudy and heavy vehicle simulator testing onseveral test sections where a ferricrete materi-al was stabilised, as reported in the firstauthor s MEng thesis (Liebenberg 2003).HISTORY OF ANDBACKGROUND TO THESTRUCTURAL DESIGNOF EMULSION-TREAT-ED MATERIALSE mulsion was added to pavement layers ini-tially to enhance the water-resisting proper-ties of the layer and to improve cohesion onthe surface to allow the road to be opened totraffic soon after construction in order toprevent or limit ravelling of the base the first experiments in south Africa (Otte& Marais 1979) no cement was added to theemulsion-treated layer and it was treated as agranular layer during the structural (1977) did research on emul-sion treated material with high bitumencontents (11 % by volume or 5 5,5 % bymass).

7 He developed transfer functions foremulsion-treated as well as emulsion- andcement-treated materials. The maximumhorizontal tensile strain at the bottom of thetreated layer and the maximum verticalcompressive strain at the top of the subgradewere used to determine the thickness of thepavement. The method assumed that theproperties of the emulsion-treated layer weresimilar to that of asphalt. Marais and Tait (1989) made someadjustments to the method of Santucci(1977) to allow for south african conditions. In 1993 SABITA published manual 14(SABITA 1993) and provided differentapproaches for modification and stabilisa-tion. The structural design for the stabilisa-tion approach was based on the work ofSantucci (1977) and Marais and Tait (1989).

8 In the modification approach, the emulsion-treated material was treated similarly to agranular material. De Beer and Grobler (1994) developedtransfer functions based on research done onthe Heilbron heavy vehicle simulator (HVS)test sections. The method was regarded astoo conservative because it proposed thickstructures, contrary to the experience (1998) provided guidelines and aJournal of the south african Institution of Civil engineering , 46(3) 20043proposed design catalogue for pavementscontaining emulsion-treated materials withlow bitumen content (less than 1,8 % netbitumen) based on the DCP designapproach. SABITA manual 21 (SABITA, 1999) pro-vides guidelines on the mix design and con-struction of emulsion-treated layers.

9 It onlyproposed the use of a catalogue included inthe document for structural design laboratory testing and heavyvehicle simulator testing on Road P243/1between Vereeniging and Balfour in Gautengwas carried out by CSIR Transportek in testing includes static and dynamic tri-axial tests, flexural beam tests, UCS, ITS andCBR tests, and heavy vehicle simulator testsat 40, 80 and 100 kN wheel loads. Theresults of the testing and research are report-ed by Liebenberg (2001, 2003) andLiebenberg and Visser (2003) and are notrepeated STABILISATION OFMATERIALS WITHBITUMEN ANDCEMENTThe addition of chemical agents to soil andgravel to improve the engineering propertiesand thus performance of materials are com-monly used in pavement engineering .

10 Theinteraction of bitumen and the cement onthe properties are not always 1 provides an outline of the range ofmaterials that is found by including differentpercentages of bitumen and materials lie somewherebetween asphalt concrete materials with itshigh bitumen contents and cement-treatedmaterials. Cement-treated materials are usu-ally very stiff, but with low flexibility whichmake them prone to cracking once the bend-ing strength has been exceeded under load-ing. Asphalt materials have high flexibilityand stiffness, but are usually expensive solu-tions that are restricted to BEHAVIOUR OFEMULSION-TREATEDMATERIALSThe general behaviour of emulsion-treatedmaterials is similar to that of lightly cement-ed materials in the sense that emulsion-treat-ed materials also exhibit a phased first phase is a fatigue life phase similarto cemented materials, while the secondphase is an equivalent granular phase simi-lar to granular unbound materials.