untreated wood ash as a structural stabilizing material in forest

Originalscientificpaper

Croat. j. for. eng. 35(2014)1 81

Untreated Wood Ash as a Structural Stabilizing Material in Forest Roads

Gerald Bohrn, Karl Stampfer

Abstract

Due to the euphoric use of »green« energy produced by biomass power plants, up to 350 000 tons of ash are accumulated as a waste product every year in Austria and the estimated costs for landfilling are 1.7 million € per year. For this reason, methods must be found for the utili-zation of wood ash. One solution is to use it as a stabilizing material in forest roads. The pozzolanic characteristic of ash is used to bind the gravel in the road base. Wood ash is ex-pected to reduce the need for gravel on forest roads and at the same time to enhance the load bearing capacity of forest roads.Two different untreated wood ashes were used in two mixture ratios, each on a 50 meter long forest road section, to investigate the load bearing capacity. The ashes were selected by their different properties: high lime and low heavy metal content, production of ashes in Austrian biomass power plants with various furnace technologies and disposal costs. Mixing depth was 0.50 m and the road base was covered by a 0.10 m thick surface layer. Elastic moduli of these sections were measured before the application, and repeated monthly by using a light falling weight deflectometer.After the first vegetation period, the mean elastic modulus of the sections mixed with dry bed ash showed an improvement. The increase of the initial mean load bearing capacity of 32.0 MNm-2 was 65% for 15/85 mixture and 76% for 30/70 mixture. The results for the fluidized bed ash sections fell short of expectations. Only 95% of the initial value could be reached for both mixing values.

Keywords: wood ash, utilization, forest road, load bearing capacity, stabilization

burning facilities, where further use is unknown.125000tonswererecycledandtherestof170000tonswaslandfilled(EnvironmentAgencyAustria2009).Withanaveragefeeof100€pertonofdepositedma-terial, the total costs for theannualashdisposal is1.7million€justforlandfilling.Itisnecessarytode-velopmethodsofwoodashutilizationsoastopreventthisnutrientrichmaterialfrombeinglandfilled.De-pendingon thecharacteristicsofdifferent typesofwoodash,variouspurposescanbeselected.Recyclingnutrientsofwoodashbyusingitasfertilizerhasbeencommonforaverylongtime(Holzner1999).Anothermethodcouldbethereinforcementofunpavedlow-volumeforestroads(Lahtinen2001).InAustria,thestandardofthesecondaryroadnet-

workisveryhigh.Almostallroadsinthisnetworkarepaved.Low-volumeroadsaremostlysituatedonagri-

1. IntroductionTheenergygeneratedbybiomasshasgainedim-

portanceinrecentyears.Itisanessentialcontributiontosustainableregionalenergysupply,especiallyinAustrianwoodedareas.Woodash,producedintheseprocesses,waspreviouslyconsideredtobea»waste-product«(Stupakaetal.2007).Inthelate1990stheAustrianAdvisoryBoardforSoilFertilityhighlightedthepropertiesofthissecondaryrawmaterial.Recy-clingofmineralscorrespondstotheecologicalprin-cipleoftheclosedbiogeochemicalcyclesandhelpstosparenaturalmineralresources(HolznerandObern-berger2011).Intheyear2007,thetotalashproductionofbio-

massuseinAustriawas350000tons;295000tonsinbiomasspowerplantsand55000tonsinsmallscale

G. Bohrn and K. Stampfer Untreated Wood Ash as a Structural Stabilizing Material in Forest roads (81–89)

82 Croat. j. for. eng. 35(2014)1

cultural or forested areas and mainly used by the land owners.InotherEuropeancountriespublicuseoflow-volumeroadsismorefrequentandtheirlegalframe-works allow the use of alternative structural road ma-terials likewoodashes.Due tomismanagementofashes in large road constructions and introduction of stricterregulations,therehadbeensomesetbacksinthe establishment of ash as binding material (Kärrman etal.2004).However,resultsofresearchonecologicalinfluence(Lahtinen2001,Thurdin2004),andtechnicalrequirementsandqualitycriteriaofwoodash(vonBahretal.2006)formedthebasisforsuitableguide-linesofashapplication(Mundeetal.2006).Themainobjectivewastoverifythesuitabilityof

untreated wood ash as a reinforcing structure material in the load bearing layer of low-volume roads based on thehypothesesthatwoodashisself-hardening.UnliketheexperimentsinScandinaviawithpureflyashesorincombinationwithotherindustrialby-products(La-htinen2001,MácsikandSvedberg2006),wheretheinfluence of themixtureproperties ismanagedbychangingtheproportionofdifferentcomponents,inthisstudyuntreateddrybottomashandfluidizedbedashwereused.Inthecaseofsuitability,gravelcouldbereplacedwithwoodashresultinginacost-effectiveuseofwoodashinforestroadconstruction.

2. Material and Methods

2.1 Road reconstructionTheinitialsituationwasanexistingforestroadin

needofmaintenanceandreconstructioninsomeplac-es.ThegeographicalpositionoftheresearchareawastheAttergau(47°56′N,13°88′E),wheremostoftheparentalmaterialcontainsflysch.Flyschisacombina-tionofclay,argillite,limerockandsandstone.Underdryconditions,thesubgradehasahighloadbearingcapacity,butwithincreasingmoistureitlosesstrength.Sothesuperstructureofaforestroadinflyschareashastoprovideastablelayer.Inmostcasesthisaimisreached by constructing a bulky bearing layer of com-pressiblegravel.For an average subgrade elastic-modulus of

20 MNm-2,itispossibletoreach30–35MNm-2 under dryconditionsandwithcompaction.Basedonthis,aroad surface elastic modulus of 80 MNm-2 with a 0.20–0.30mthickloadbearinglayermadeofgravelwithagrainsizedistributioncurveof0–60mmcanbereached(Dietzetal.1984).Withaflyschsubgrade,theelasticmodulusoftendropsbelow10MNm-2.Forthisreason,0.40–0.50mgravelisusedtoconstructtheloadbearinglayertoreachthenecessaryloadbearingcapacity.

Thisstructuralelementiscoveredbyasurfacelayerofgravelwithagrainsizedistributioncurveof0–32mm.Duringthereconstructionofaforestroadofatotal

lengthof1850m,four50msections(Fig.1)werere-inforcedwithtwodifferenttypesofwoodash,drybottomash(DBA)andfluidizedbedash(FBA)intwomixingratios(15%ashto85%originalforestroadma-terialand30%ashto70%originalforestroadmate-rial).Onafifthsection,theforestroadreceivedthesametreatmentwithoutwoodash,whichservedasacontroltreatment,andasixthsectionremainedun-treatedtoprovidereferencedataofloadbearingca-pacities.Theelasticmoduluswasmeasuredonthesesections,andthedatawerecompared.Inthetextbe-low,thesesectionswillbereferredtoas:DBA–Xfordrybottomash,whereXisthepercent-

ageofdrybottomashmixedwithoriginalmaterial,forexampleDBA-30for30%drybottomashto70%originalmaterial;

FBA–X forfluidizedbedash,whereXistheper-centageoffluidizedbedashmixedwithoriginalmaterial;

TWOA fortreatmentwithoutash(mixing,rollingandgrading);

REF forreferencewithnotreatment.Themixingratiosarerelatedtotheapplicationof

otherbindingmaterialslikelimeorPortlandcement(Dietzetal.1984).Theeffectivenessofthereinforce-mentare,ononeside,dependentontheashcharac-teristics,whichareinfluencedbythetypeofcombus-tion,differingfueltypesstructureofgrainsandfreelimecontent.Ontheotherside,themainfactorsforagoodperformingloadbearinglayerareheterogeneousbasematerialandtheoptimalwatercontent.Earlierresearch showed that a small addition of lime caused asignificantimprovementofstrength(Lahtinen2001).

2.2 Bearing layer constructionDependingonthemixingratio,a0.08m(15/85)

or0.12m(30/70)thicklayerofwoodashwasappliedontotheroadsurface.Afterthatthemixingproce-durewasdonebyaWR2400recycler,aproductoftheWirtgenCompany,withamixingdepthof0.50mandaworkingwidthof2.40m(Fig.2andFig.3).Themachine hydraulic system guaranties a homogenous mixingresultbykeepingtherotationspeedofthemillconstantandvaryingthemachinespeed.Underdrysoilconditions,anadditionalwatertanksuppliestherecyclerwithwatertoreachtherequiredmois-tureandpreventsdustformation.Afterthemixingprocedure,themodifiedloadbearinglayeriscoveredbya0.10mthicksurfacelayerofgravelwithapar-

Untreated Wood Ash as a Structural Stabilizing Material in Forest roads (81–89) G. Bohrn and K. Stampfer

Croat. j. for. eng. 35(2014)1 83

Fig. 1 Layout overview

Fig. 2 WR2400 recycler during mixing process

Fig. 3 Scheme of the reconstruction procedure

ticlesizeof0–32mm.Thefinalizingtreatmentin-cludedconventionalgradingandcompactingwithavibratingroller.

2.3 Falling weight deflectometerTheloadbearingcapacitywasmeasuredwitha

fallingweightdeflectometer(FWD).Itwasoriginallyusedtoevaluatethephysicalpropertiesofpavementslike the in situ base and subgrade moduli during con-struction.FWDdataallowstoestimatethepavementstructuralcapacityforoverlaydesignandtodeter-mineifapavementisbeingoverloaded.Inthistest,aLightFallingWeightDeflectometer

TerraTest3000GPSfromtheTerraTestCompanywasused(Fig.4andFig.5).Thebigadvantage,incom-parisontoothermeasuringmethods,isthefastandeasyrepetitionofmeasurementsondifferentsampleplots(Tholenetal.1985).Themeasurementisbasedontheimpulseofthe

light fallingweight,whichdropsthe loadingplatewithadiameterof0.30m.Thisimpulsegeneratesamaximumforce(Fmax)of7.07MN.Thisforceisgauged


84 Croat. j. for. eng. 35(2014)1

Table 1 Elastic modulus of all sectio

DBA–30 DBA–15 FBA–15 FBA–30 TWOA REF

Date mean N mean N mean N mean N mean N mean N

06.05.2010 19.996 39 29.934 12 18.220 27 16.653 36 21.256 6 36.554 24

12.05.2010 18.481 36 30.161 18 22.076 30 21.371 33 19.506 6 44.569 24

27.05.2010 22.392 33 31.701 27 22.167 30 17.897 33 13.027 6 44.619 24

23.06.2010 31.740 42 39.931 27 25.758 33 19.696 36 21.466 6 42.924 24

22.07.2010 51.373 36 42.607 24 41.996 33 36.162 33 41.190 6 62.766 24

23.08.2010 47.853 27 53.294 15 28.276 24 19.696 36 39.327 6 50.216 24

22.09.2010 56.360 48 52.937 27 30.300 39 30.241 48 32.049 6 49.666 24

04.11.2010 55.817 36 52.821 24 31.858 33 28.500 36 28.817 6 50.133 24

Fig. 4 Falling weight deflectometer TerraTest 3000

during the calibration to ensure a normal tension of 0.1MNm-2undertheplatewhileperformingthetests.Deflectionsensorsmountedontheloadplatemeasurethedeformationofthepavementinresponsetotheload.ThedeformabilityparameterofthesoilcausedbythisimpulseistheelasticmoduluscalledEvd and iscalculatedbytheLightFallingWeightDeflectome-terTerraTest3000(TERRATEST2009).

E r svdmax

= 1 5. maxs (1)

Where:smax meanvalueofthedisplacementsσ4max,σ5max,

σ6maxof3measurementsafter3preconsolida-tionmeasurements,m;

r radiusoftheplate,m;σmax normaltensionundertheplate,MNm-2.

Fig. 5 TerraTest 3000 in use


Croat. j. for. eng. 35(2014)1 85

Themeasurementswere takenon the trackwayandonthemedialstripoftheroad(Fig.6).Thereasonforthislayoutwastofinddifferentmeasurementpa-rameters for the influenceof theupcoming traffic.Whilethewholeloadoftheupcomingtrafficwasanadditionalcompactingforceonthetrackways,theme-dialstripstayeduntreatedaftertheroadconstructionwascompleted.

2.4 Traffic observation and load calculationForthecalculationofthebearingload[t],thetraffic

was observed during the whole growing season from 6th May to 11thNovember2010(Fig.7)withscoutingcameras,whichhaveanintegratedmotiondetectorcombinedwithanautomaticrelease.Picturesweretakenofeverytypeofmovingvehicle,truckwithorwithouttrailer,agriculturaltractororcarontheforest

Fig. 6 Layout of elastic-modulus measurement depending on the surface location

Fig.7 Loads of the upcoming traffic

road.For the loaddetection,a secondcamerawasmountedfacingtheoppositedirection.Thepicturesfrom the backside of the trucks were used to estimate theirload.Theseestimationswerebasedonsampleloads recorded on delivery notes of the saw mill and summedupdaily.Atotalof3570tonswastransport-edontheroadduringtheobservation.

2.5 Basic meteorological dataTheclimateintheresearchareaaroundWeyregg

amAtterseeiswarmandtemperate.Thereissignifi-cantrainfallthroughouttheyear,eveninthedriestmonth(81mm).TheregionisdescribedasCfbintheKöppen-Geigerclimateclassificationsystem.Theav-erageannualtemperatureis8.2°Candtheprecipita-tionapproximately1318mmperyear(Fig.8).

3. Results and discussionIntotal1251elasticmodulusmeasurementswere

takenoneightdifferentdays.Thetotalelasticmodu-lusrangedfrom5.70–114.20MNm-2(Table1).Themean elastic modulus of the whole forest road was measured before any treatment and had a mean value of32.0MNm-2.Theelasticmodulusofthetreatedsec-tions was reduced by the disaggregation of the mixing process.Ittookapproximatelythreemonthsforeachsection to reach the initial value of the forest road again.Themeanelastic-modulusofthereferencesec-tionvariedintheobservedtimebetween36.55and62.78MNm-2andattheenditwas50.13MNm-2.


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Fig. 8 Climograph

Fig.9 Development of elastic moduli of all sections over growing season

Overthe6-monthperiod,thesectionstreatedwithdrybottomashincreasedtheirelasticmodulisignifi-cantly.An improvementof theelasticmodulusof5.4%(52.8MNm-2)fortheDBA–15sectionand11.3%(55.8MNm-2)fortheDBA–30sectionwasdetected.Theresultsforthefluidizedbedashsectionsfell

shortofexpectations.Only31.9MNm-2fortheFBA–15sectionand28.5MNm-2fortheFBA–30sectioncouldbereachedforbothmixingvalues.Thiswasnearly95%oftheinitialvalueand63.5%(FBA–15)and56.8%(FBA–30)ofthereferencevalue(Fig.9).

Inthecaseofatotalreconstruction,comparisonwiththeinitialsituationisnotpossible,becausethewholeroadneedsstabilizingorconditioningtomakesure that theroadcanbeusedfor traffic.With theDBA,itispossibletoincreasetheelasticmodulusbymore than 20 MNm-2,comparedtomechanicalstabi-lization.FBAwasnotsuitableforreinforcingtheroadconstruction,neither15%nor30%(Fig.10).Thetworeasonsforthisbehaviourarethedifferentcontentoffreelimeinbothashtypesandthegrainstructureandsize(vonBahretal.2004).Thedifferencebetweentheinfluenceofthemixing

valuesandashtypeswascheckedwithANOVAandtheDuncanTest.Theresultsof5thMayconfirmednosignificantdifferenceoftheelasticmodulusbetweenFBA-30,FBA-15,DBA-30andTWOA.DBA-15andREFexhibitedasignificantlyhigherelastic-modulus(Table2).Theobservationon4th November 2010 shows

nosignificantdifferencebetweenbothFBAsectionsandtheTWOAsection.TheDBAsectionsshowednosignificantdifferencefromtheREFsection(Table3).Thehypothesisthatforestroadscanbereinforced

bytheapplicationofwoodashcouldbeconfirmedforDBAwiththemixingvaluesof15/85and30/70,butdisprovedforFBAwithasimilarmixingvalue.Afteronegrowingseasontheelastic-modulusoftheDBAsectionshadsurpassedthevaluesoftheREFsection.Theinfluenceofthetrafficdidnotshowtheex-

pectedresults.Thehypothesisthattheadditionalcom-


Croat. j. for. eng. 35(2014)1 87

Fig. 10 Comparison of elastic moduli of sections treated with and without ash

Table 2 ANOVA (Duncan Test) for results of 6 May 201

Section NSubcategory for Alpha = 0.05.

1 2 3

FBA–30 36 16.65 – –

FBA–15 27 18.22 – –

DBA–30 39 20.00 – –

TWOA 6 21.26 – –

DBA–15 12 – 29.93 –

REF 24 – – 36.55

Significance 0.19 1.00 1.00

Table 3 ANOVA (Duncan Test) for results of 4 November 2010

Section NSubcategory for Alpha = 0.05.

1 2

FBA–30 36 28.50 –

TWOA 6 28.82 –

FBA–15 33 31.86 –

REF 24 – 50.13

DBA–15 24 – 52.82

DBA–30 36 – 55.82

Significance – 0.46 0.20

pactionduetotrafficleadstohigherelasticmodulionthetrackwayscouldnotbeverified.Themeanelasticmodulus (Mean), elasticmodulus of the trackway(Track)andmedialstrip(Middle)ofthreedifferentsectionsweremeasuredandcompared.

On 23thofAugust2010themeasurementsonthebottomashsectionshowedhigherelasticmodulusatthemedialstripthanatthetrackways.Thesameeffectwas measured on 26thofJuneand22thofSeptemberforthesectiontreatedwithoutash(Fig.11).Thereferencemeasurementsshow(Fig.9)avaria-

tionoverthewholegrowingseason,whichcouldbecausedbytheinfluenceofweatherconditions.Forsub-sequentmeasurements,meteorological stationswillprovideexactweatherdataforcloserconsideration.

Toreachthetargetelasticmodulusof45MNm-2,theself-hardeningprocessofwoodash,initiatedbywatercontact,isimportant.Thedrynessoftheusedashesmustbeguaranteedvialogisticalorganization.Theashistransporteddirectlyfromthepowerplanttotheconstructionsite,wheretheashismixedwiththerawmaterial.Itstartshardeningbywatercontactanditissealedwiththesurfacelayer.Outdoorstorageandthecontactwithairhumiditywilldefinitelyinflu-encetheself-hardeningprocessofashesandshouldbeprevented.Ifthereisnopossibilityofsubsequenttreatment,theasheshavetobecoveredwithwaterproofmaterial(vonBahretal.2004).Thelimitingfac-tor in forest road construction will be the cost in mon-ey.Longtermobservationswillshowthepotentialofthecosteffectivenessofthisreinforcingmethod.


88 Croat. j. for. eng. 35(2014)1

Fig. 11 Mean elastic moduli depending on the surface location

4. SummaryFor the evaluation of untreatedwood ash as a

structuralstabilizingmaterial,twodifferentashtypeswereappliedintwomixtureratios,eachona50meterlongforestroadsection,toinvestigatetheloadbearingcapacity.Theasheswereselectedbytheirdifferentproperties:highlimeandlowheavymetalcontent,theirproductioninAustrianbiomasspowerplantswithvariousfurnacetechnologiesanddisposalcosts.Themixingdepthwas0.50mandtheroadbasewascoveredbya0.10mthicksurfacelayer.Theelasticmoduliofthesesectionsweremeasuredbeforetheap-plication,andrepeatedmonthlybyusingalightfall-ingweightdeflectometer.Afterthefirstvegetationperiod,themeanelastic

modulus of the sections mixed with dry bed ash showedanimprovement.Theincreaseoftheinitialmeanloadbearingcapacityof32.0MNm-2was65%(52.821MNm-2) for 15/85mixtureand76% (55.817MNm-2)for30/70mixture.Theresultsforthefluidizedbedashsectionsfellshortofexpectations.Only99%(31.858MNm-2)forthe15/85mixtureand89%(28.500MNm-2) for the 30/70 mixture of the initial value could bereached.Theresultsofthefieldtrialshowthatdrybedashis

suitableasastructuralstabilizingmaterial.Thiscouldnotbeconfirmedforthefluidizedbedash.Inthiscaseotherutilizationmethodsshouldbeexploited.

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Received:October16,2013Accepted:February10,2014

Authors’address:

GeraldBohrn,MSc.*e-mail:[email protected],PhD.e-mail:karl.stampfer@boku.ac.atUniversityofNaturalResourcesandAppliedLifeSciencesViennaDepartmentofForestandSoilSciencesInstituteofForestEngineeringPeterJordanStraße82/31190 ViennaAUSTRIA

*Correspondingauthor

untreated wood ash as a structural stabilizing material in forest

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