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MessagePosté le: Sam 21 Mai - 14:18 (2016)    Sujet du message: The Alkali-silica Reaction In Concrete || Répondre en citant




The Alkali-silica Reaction In Concrete || > bit.ly/1YLo7iS





Aggregates...susceptible...to...ACR...tend...to...have...a...characteristic...texture...that...can...be...identified...by...petrographersThe...size...of...the...structure...and...the...environment...to...which...it...will...be...exposed...are...then...considered,...and...the...structure...is...placed...within...specified...risk...levels2.2..ALKALI-SILICA..REACTIONFigure..9At..c)..the..potassium..peak..has..disappeared..entirely..and..the..gel..has..approximately..the..same..composition..as..the..normal..calcium..silicate..hydrate..comprising..the..bulk..of..the..cement..pasteMcGowan.and.Vivian.(1952).disputed.the.osmotic.theory.proposed.by.Hansen,.on.the.basis.that.cracking.of.the.surrounding.cement.paste.membrane.due.to.ASR.would.relieve.hydraulic.pressure.and.prevent.further.expansionProvided...that...sufficient...amounts...of...alkali-hydroxides...are...available,...this...process...continues,...producing...an...alkaline-silicate...solutionOne.reason.for.this.difference.in.behavior.is.that.the.solubility.of.amorphous.silica.increases.significantly.with.pH,.as.illustrated.in.figure.5Alkali-silica..reaction..in..concreteASR.has.been.implicated.in.the.deterioration.of.various.types.of.concrete.structures,.including.dams,.pavements,.bridges,.and.other.structuresPersonal.tools.Not.logged.inTalkContributionsCreate.accountLog.in.However,.good.quality.control.coupled.with.periodic.testing.is.essential.to.ensure.that.the.aggregate.continues.to.be.non-reactiveThis..can..take..up..(imbibe)..water..and..is..mobileTotal...sodium...oxide...equivalent...(or...equivalent...soda)...in...percent...by...mass...is...equal...to...the...sodium...oxide...content...in...percent...plus...0.658...times...the...potassium...oxide...content...in...percent.However,..given..that..these..conditions..often..are..not..met,..and..given..that..some..aggregates..that..were..believed..to..be..nonreactive..(based..on..testing..methodologies..available..at..the..time..of..construction)..have..caused..damaging..ASR..expansion..in..field..structures,..further..precautions..should..be..taken..in..some..situationsLikewise,.local.differences.in.moisture.availability.within.the.same.structure.have.resulted.in.vastly.different.performance.within.that.structure(Note:.we.currently.have.two.Cembytes.subscriber.lists,.and.old.one.and.a.new.one,.and.we.are.gradually.transferring.to.the.new.oneIn..many..structures..this..amount..of..expansion..may..not..cause..problems,..but..in..long..sidewalks..or..median..barriers,..for..example,..the..expansion..may..cause..compression..and..heavingTable.1No...expansion...results.In...the...case...of...ASR,...the...reaction...usually...occurs...much...later,...possibly...years...after...the...concrete...was...placedWhere:..Na2Oe..=..Total..sodium..oxide..equivalent..(or..equivalent..soda),..in..percent..by..mass..Na2O..=..sodium..oxide..content,..in..percent..K2O..=..potassium..oxide..content,..in..percent..This...rule-of-thumb...value...has...been...cited...in...various...specification...limits...and...was...adopted...as...part...of...the...American...Society...for...Testing...and...Materials...(ASTM)...standard...ASTM...C...150Using...Lithium...CompoundsSupplementary.cementing.materials.(e.g.,.fly.ash,.slag,.silica.fume)Gel...may...be...present...in...cracks...and...within...aggregate...particlesLocations...of...Potentially...Reactive...Aggregates...in...Canada...Potentially...reactive...rocks...and...documented...cases...of...alkali-aggregate...reaction...in...concrete...occur...in...virtually...all...parts...of...Canada2.6.CONCLUSIONSThe.basic.mechanisms.of.ASR.are.summarized,.followed.by.discussions.on.ASR.manifestations.and.symptoms.in.field.structures,.relevant.test.methods,.methods.of.preventing.ASR,.and.specificationsWe..have..some..great..training..and..reference..resourcesTest.uses.high-alkali.cement.and.PyrexTM.glassBased.on.Stanton's.early.work.(1940),.it.was.proposed.for.many.years.that.expansion.due.to.ASR.reaction.is.unlikely.to.occur.when.the.alkali.content.of.the.cement.is.below.0.6.percent.Na2OeTherefore,..an..aggregate..should..not..be..rejected..solely..based..on..ASTM..C..1260..results..unless..the..reactivity..is..confirmed..using..ASTM..C..1293It...is...also...important...to...note...that...not...all...ASR...gel...reactions...produce...destructive...swellingThe..minimum..level..of..silica..fume..(as..a..percentage..of..material..content)..is..calculated..on..the..basis..of..the..alkali..content..of..the..concrete..(expressed..as..kg/m3..Na2Oe),..but..in..cases..where..silica..fume..is..the..only..SCM..to..be..used,..the..silica..fume..content..should..be..7.0%..by..mass.This.approach.considers.key.parameters.previously.described.in.this.chapter,.such.as.the.CaO.content.of.fly.ash.and.the.total.alkali.content.of.the.mixture.when.dosing.silica.fumeASTM...C...856:...Practice...for...Petrographic...Analysis...of...Hardened...Concrete...Useful...for...analyzing...concrete...(from...laboratory...or...field)...and...for...identifying...presence...of...reactive...aggregates...or...reaction...productsCanadian...Journal...of...Civil...Engineering,...Vol f5880b0734



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