ALKALI AGGREGATE REACTIONS IN CONCRETE:A REVIEW OF THE ETHIOPIAN SITUATION

Abebe Dinku and Birhanu BogaleDepartment of Civil Engineering

Addis Ababa University

ABSTRACT

The reaction of aggregates with alkalis in thecement to produce alkali silica reaction and alkalicarbonate reaction is reviewed. The effects of thetwo reactions on the durability of concretestructures are highlighted. By taking samples ofaggregates and cement test results, the potential ofalkali silica reaction in Ethiopia is preliminarilyassessed. Finally a _ set of' conclusions andrecommendations are forwarded

Keywords:Alkali silica reaction; Alkali carbonate reaction;Cement; Concrete; Durability; Moisture; Sand;

INTRODUCTION

Concrete is one of the most widely usedconstruction materials in the world today. Beingused for over 2000 years, it has now become anindispensable part of the day-to-day life of modemsociety. It is produced by mixing measuredamounts of cementing material, fine aggregate,coarse aggregate, water and chemical admixtures toimprove its various characteristics such asworkability, strength or resistance to chemicalattack. 1be careful selection and proportioning ofthe different ingredients is vital for achieving theapparent engineering properties of concrete. It isnearly always reinforced with steel embeddedwithin it as it has high compressive strength butpoor tensile strength.

Among the reasons .making concrete a popularcons~ction material in the world is its versatility ..With its high compressive strength which continuesto grow up with age, its being water tight, its'durability and its high resistance to heat, chemicalattack and radiation, concrete is used for theconstruction of a broad variety of civil engineeringstructures ranging from the skeletal frameworks ofhigh rise buildings and Jong spanning bridgl;s tohydraulio structures like dams, spillways. andirrigation distribution network. It is also used for

the construction of roads, water tanks, silos forchemical storage, b1JI1kersand nuclear reactors.

Despite its long history of use, our understandingof concrete has only really developed in very recenttimes, particularly with respect to its durability, andas a result""'"of the continuing engineering andscientific developments on the material, practicesare constantly changing and it is essential. to updateoneself with the state-of-the-art methodologies ofmaking it into use to achieve the desired safety andeconomy.

While it has proved to be a very durable material,concrete is susceptible to failure, most. of whicharises from the need to reinforce it with steel. Thebasic role of steel is to take care of tensile stressesdeveloping in the structure, yet because of theexpansion in volume of steel resulting fromcorrosion, it is capable of rupturing the concrete aswell. There are' also less common breakdowns,which occur in concrete due to chemical

inconsistency between the cement paste and somespecific types of aggregates. In cold climates, tlfereis the potential for break down by the freeze orthaw action of water near the concrete' surface as

well. Therefore, more emphasisl which was givenso far for the strength characteristics of concrete,should also be equally focused on its durability toavoid premature failUres.

Because it is not possible to change the nature ofconcrete in existing structures easily~ protective.measures should be taken as deteriorations start tooccur to airest· the damage. But· before this isattempted, it is essential to have a fullunderstanding of the cause and extent of thebreakdown. It may already have reached the pointwhere economical repair is no longer possible.Fortunately, .in most cases the onset of thesedeteriorations does not necessarily guaranteecollaJ1,Se.of 'the structure and' repairs can beachieved su<;:cessfully.

The continued usage of concrete as a constructionmaterial is also to a large extent dependent on its

JournalofEEA, VoL 21,2004

48 Abebe Dinku and Birhanu Bogale

relative economy in comparison with otherstructural materials. ,Especially in developingcountries like Ethiopia, the relative cost ofproducing concrete where there is a .limitedmanufacture of alternative construction materialslike steel is an important factor to be considered inview of the overall economic picture of thecountries.

One of the most important factors in the economyof concrete is that about 70% of its volume is

occupied by what are" known as aggregates.Aggregates are one of the principal constituents ofconcrete consisting of gravel, sand or rockfragments of different mineral composition.Aggregate particles were originally thought to beinert and unaffected by the concrete paste; anedhen~ they were selected only on the basis of theirphysical properties such as shape, density (specificgravity), surface' texture, and amount andinterconnection' of internal surfaces. However, itwas later discovered that there are chemicalreactions that take place between the aggregatesand the cement paste and currently,' an indepthexamination of the microstructure and chemicalconstituents of the reaction products is revealingthe fact that the type of the reactions vary with themineral composition of the aggregate particles.Following is a detailed description of thesedeleterious reactions in concrete.

ALKALI-AGGREGA ~E REACTIONS (AAR)

Alkali':aggregate reactions are .chemical reactionsin concrete involving certain active minerpJconstituents often present in some aggregates andthe sodium and potassium alkali hydroxides rromportland cement paste. The reactions are potentiallyharmful only when they produce significantexpansion and hence cracking of concrete, leadingto loss of strength and elastic modulus.

Alkali-aggregate reactions occur in two forms:alkali-silica reaction (ASR) and alkali-carbonatereaction (ACR). Alkali-silica reaction is thereaction between the alkali hydroxide in portlandcement and certain siliceous rocks and minerals

.present in the aggregates, such as opal,. chert andchalcedony. The products of this reaction oftenresult in significant expansion and cracking of the

, concrete and ultimately, failure of the concretestructure. Alkali-carbonate reaction on the otherhand is the reaction between the cement hydroxidesand certain dolimitic limestone aggregates whichcan also result in deleterious expansion. Alkali­silica reaction is of more concern than alkali-

Journalq[EEA, VoL21,2004.

carbonate reaction because the occurrence ofaggregates containing reactive silica minerals ismore common. Alkali reactive carbonate

aggregates have a specific composition whoseoccurrence is relatively rare [1,2]. Because of thisreason, the phenomenon of alkali-aggregatereaction is these days commonly referred to asalkali-silica reaction.

Because deterioration due to alkali-aggregatereaction is a slow process, the possibility of suddenfailure is, low. Even in some parts of the world,much concrete made with reactive aggregates still

. remains in service. However, concrete with alkali­aggregate reaction can cause serviceabilityproblems and. aggravate other failure mt:chanismssuch as those that can occur due to frost action,deicing salts, or sulphate exposures.

ALKALI-CARBONA TE REACTION (ACR)

The reaction of dolomite in certain carbonate rockswith alkalis from the cement paste has beenassociated with deleterious expansion of concretecontaining such rocks as an aggregate. These rockshave a composition in which the carbonate portionconsists of substantial amounts of both dolomite

anq calcite, and the acid insoluble residue cont!\ins· a significant amount of clay. Under humidconditions, cracks are formed around the activeaggregate particles resulting in a loss of bondbetween the aggregate and the cement paste. Sincethe occurrence of dolomitic limestone is very rarein Ethiopia, more emphasis is given to alkali-silicareaction ..

ALKALI-SILICA REACTION (ASR)

Alkali-silica reaction is a chemical process whichresults from the use of certain aggregates in.concrete in which particular constituents of theaggregates react with alkali hydroxides dissolved inconcrete pore solutions.· These alkali hydroxidesare derived mostly from sodium and pOtassium inportland cement and other cementious materials,but oc.cassionally alkalies' may be introduced intoconcrete from external sources or may be relaeasedslowly from certai~ alkali-bearing rock componentswithin the aggregate.

The first written explanation of alkali-silica· reaction was published in 1940 by Thomas E.· Stanton from his investigation of cracked concrete

structures in California [3,4]. Since then, numerousexamples of concrete deterioration from other parts.of the world have been reported to show that the.

Alkali Aggregate Reactiolls ill COllcrete:A Review of tile Ethiopiall Situatioll 49

MECHANISM OF ALKALI-SILICAREACTION

The second phase is the expansion of this alkali­silica gel when it comes in contact with moisture.

conclusion. is made, detailed case studies andlaboratory investigations need be made with thevariety of locally available aggregates. If forexample there is a need of using high amount ofcement for spciaJ'purposes in the COlllltry,there is apossibility that pronounced deleterious expansioncan still occur. Moreover, although it is rare, alkalismay be contributed from the aggregates themselvesor from the surrounding environment, in addition to

. the amount present in the cement and therefore, endremarks can not be stated without further

inspection into the case.

While many aggregates may react in concrete,damage in structures is observed only whensignificant amounts of expansive reaction products

. are formed, they take up water and expand. Thereaction products concerned are hydrous gelswhose chemical composition always includessilica, alkalies and at least a little calcium. Thereaction between' the alkalis (sodium andpotassium) in concrete and' silica reactiveaggregates is essentially a two step process. Thefirst step is the chemical reaction between thereactive silica in the ag~regate and the alkalipresent in concrete to produce an alkali-silica gel.

Alkali-Silica

gel+ Alkali in

concrete ---.

ReactiveSilica inAggregate

Portland cements contammg more than O.~%equivalent Na20 when used in combination with analkali reactive aggre'gate can cause significantexpansion due to the alkali-aggregate reaction.Therefore, cements with less than 0.6% equivalentNa20 are usually designated as low alkali cementsand with more than 0.6% equivalent Na20 as highalkali cements [1,6].

Although both Sodium and Potassium compoundsare present in Portland cements, alkali content ofcements is usually expressed as acid solublesodium oxide equivalent, which is equivalent toNa20 + 0.658K20. "The constant 0.658 is themolecular ratio of Na20 to K20 (Atomic weights:Na=22.9898, K=39.0983, 0=15.9994; (2(22.9898)"+ 15.9994) -'- (2(39.0983) + 1.5.9994) = 0.65798[5].

As the major source of alkalis in concrete is thecement paste, it is essential to evaluate the'composition of the cement with respect to thisparameter. Normally, raw materials used for theportland cement clinker manufacture are the sourceof alkalies in cement which typically range from0.2-1.5% Na20 equivalent. Therefore, the PH ofthe pore fluid In normal concretes is generally 12.5to 13.5, which represents a strongly alkaline fluidin which certain ,acidic rocks composed of silicaand siliceous minerals do not remain stable on longexposure.

alkali-silica reaction may become one of thepotential causes of distress in structures located inhumid environments such as dams, bridge piers andretaining walls.

International practice suggests that ordinaryconcrete witl1 alkali content of 0.6% or less isusually found insufficient to cause damage due toalkali-aggregate reaction: irrespective of the type ofreactive aggregate [7]. But with concrete mixturescontaining very high cement content, even less than0.6% alkali in cement may prove to be harmful.

The soluble alkali content of the cement producedin Ethiopia in the Mugher Cement Factory, whichhas been used for several years and 'is still in use inthe country is 0.6% which perhaps indicates that itmay not cause severe alkali-aggregate expansion.The Messebo cement which is receptly introducedto the construction industry by the newlyestablished Messebo Cement Factory also has analkali content of 0.6% which is again on the saferside and hence may not be a cause for alkaliaggregate reaction by itself. But before an~

ExpandedAlkali-Silica Moisture ---. Alkali-Silica gel+

gel

Depending on the location and on the amount ofwater available, this gel expands more and moreand the increasing expansion and swefling of thegel generates an excessive pressure within theconcrete. ASR swelling results from the relativevolume increase between the produst and reactantphases involved in the reaction., 'The productsexpand in pores and microcracks of the cementiousmatrix. Once this free expansion space is filled, theswelling is restrained and the product phases exert10c.allY'a' pressure on the surrounding concrete

. skeleton: It .is this pressure that triggers themacroscopic expansion which gradually developsto an extent that is capable of disrupting the entireconcrete matrix [8,9].

JourllalofEEA, VoL 21,2004

50 Abelie Dinku and Birhanu Bogale.

In conclusion, the' following three factors areessential for ASR to occur in concrete.1. There must be reactive minerals present in the

aggregates.2. There must be sufficient alkali in the concrete

to react with the aggregates.f There must be sufficient moisture available toenable the chemical reaction.

Alkali-silica reaction and movement mechanisms

of the substances responsible for it in the above tworeactions are affected· by temperature (ASRmechanisms are enhanced by heat, that is, thehigher the temperature, the faster"they occur) andrelative humidity [9]. This effect of temperature onASR results from the thermoactivation of bothprocesses involved: the dissolution of reactivesilica at the aggregnte-cement interface and thereaction product formation.

It is an established fact that the relative humidity inconcrete is essential for ASR, affecting bothkinetics and magnitude. Thus the higher therelative humidity, the faster and more vigorous thereaction would be. Water plays the role of thesolvent for the silica dissolution; intervenes astransport media for the diffusion of ions throughthe pore solution and is a necessary compound forthe formation of the various reaction products.

ASR usually takes place at 20°C temperature and85% relative humidity. The ASR induced crackingresults in a decrease in strength and an increase inpermeability that increases the susceptibility of theconc~ete to subsequent damage from processessuch as freezing and thawing, corrosion of thereinforcing steel and sulphate attack. The overealleffect is reduced structural integrity and shortenedservice life of the concrete structure.

In Ethiopian conditions, both the temperature andrelative humidity suitable for ASR to take place areprevalent all the year round and throughout thecountry. Hence, provided other conditions aresatisfied, damage due to ASR could be aggravatedas a result of the surrounding environmentalconditions.

POTENTIALLY REACTIVE SILICAMINERALS AND ROCKS

,As presefited ;earlier, siliceous aggreglltes 'arepotentially reactive with alkalis from portlandcement in the presence of adequate moisture in theconcrete paste. Silicate minerals are estimated toconstitute the largest part of the earth's crust, as

Journal of EEA, Vol. 21, 2004

high as 80-90%, and the alkali reactive componentshave been identified in most silicate rocks. Some ofthe potentially reactive minerals present in thesesilicate rocks include the following [1,10]:

1. Opal (Hydrated Silica Glos.'i) - Opal ishydrated silicon dioxide with no ,appreciablecrystallirie structure. It contains 5-10% or evengreater percentage of water. It is a white 'or·colourless mineral which usually occurs in avariety of volcanic rocks, cherts, and sandstones. It tends to appear in cracks andfractures in rocks. It is actually an ornamentalmineral if it appears in massive quantities, butits presence even in small quantities in rocks tobe used as aggregates for concrete productioncould result in a severe'damage as it is themost reac5ive of all silica minerals. The tinyglassy particles present in some sands androcks in Ethiopia may contain small amount ofthis mineral and laboratory investigationsshould be conducted to arrive at a conclusion.

2. Chalcedony - Chalcedony is ,mother form of~lica mineral which is harder and more

crystalline than opal. It is a microcrystallineform of quartz. It is basically of hexagonalq'ystal structure with various colors and whitestreak. It is less lustrous than opal. If it occursabundantly, Chalcedony is also a preciousstone and it is in fact pre~nt in smallquantities in almost all' types of silica rocks.Observing just the physical properties andcommon occurrences of Chalcedony, thesmooth rounded· pebbles usually present insome sands in Ethiopia may contain'· thisreactive mineral.

3. VolcanicGloss (Obsidian)- Volcanic glass is

another amorphous fo~ of silica miner2-l,usually darker than the above two types mainlydue to the presence of iron or magnesium in it.It is also a semi-ornamental mineral if it occurs

in large quantities. Aggregates containing thismineral may be reactive depending on its silicacomposition. Volcanic glass of silica contentabove 55% is commonly regarded as reactive-while that with silica content below 55% isless so. As its name suggests, volcanic glaSs is

. formed as a result of volcanic activity. Hence,in Ethiopia, it is necessary to pay piuticularattention to aggregates 'quarried from rockdeposits of volcanic origin (igneous rocks).

4. Tridymite and Cristobalite - These are als0crystalline forms of silicate minerals that are

Alkali Aggregate Reactions in Concrete:'AReview of the Ethiopian Situation 51

usually white or colorless. They are glassy andof tetragonal or hexagonxal crystal structure.They are commonly found in cavities involcanic rocks. So, like volcanic glass,aggregates of igneous rock origin in Epliopiashould be investigated if they containsubstantial amounts of these reactive minerals.

5.. Quartz - Quartz is the most abundant mineralwhich is found in almost every type of rork.Pertaining to .metamorphism, tridymite andcristobalite could gradually convert to a wellcrystallized quartz that may also be reactive. InEthiopia therefore, aggregates. obtained fromrocks that are formed as' a result of

meta:morphism of igneous rocks should besuspected of containigthis reactive mineral.

6. Silicates - Silicates are minerals of either

sedimentary or metamorphic rock origincontaining clays or micas. They have beenobserved to be slowly reactive with alkalis andsometimes, their reactivity may not be detected

. by the ordinary tests. They principally occur insuch rock types as graywackes, argillites,phyllites, siltsjones, etc. It is stronglyrecommended that if the aggregate to be usedcontains significant contents of such rocktypes, low-alkali cement should be used whereavailable.

In general, aggregates contammg most types ofsilica minerals are believed to be alkali reactivealthough to differ~t degrees. Some aggregates arepotentially reactive while others may not show anysign of reactivity for as .long as 20 years. Opal,obsid~an, cristobalite, tridymite, chalcedony, chert,andesite, rhyolites, and strained or metamorphicquartz have been found to be' alkali reactive in tl).edecreasing order of reactivity.

METHODS OF DETECTING ALKALI­SILICA REACTION .POTENTIAL·

Several test methods have been used for detection

of ASR potential. The following are some of thestandard and most common ones ordained byASTM [11].

1. Mortar-Bar Method (ASTM C 227)2. Chemical Method (ASTM C 289)3. Petrographic Examination (ASTM C 295)4. Accelerated Mortar-Bar Method

(ASTM C 1260)5. Concrete Prism Test (ASTM C 1293)

DIAGNOSIS OF ALKALI-SILICAREACTION IN CONCRETE STRUCTURES

Once the necessary conditions for alkali-silicareaction are sustained, the reaction will soonproceed and alkali-silica gel will be formed. Then,in the case of aggregetes of sedimentary rockorigin, the expansion proceeds by shedding of thesheet structure and further water absorptionwhereas in aggregates containing dense particles,reaction rims will be formed around the particleswhich result in progressive. peeling of theperipheral layer continuing inwards. In both cases,the increasing expansion obviously initiates thebuilding up of excessive pressure in the concretematrix and development of microcracks in both theaggregate and the concrete.

If there is a continued supply of water, themicrocracks will extend and enlarge eventuallyreaching the outer surface of the concrete.

In the case of ordinary concrete which is free toexpand equally in all directions, the cracks so,formed will be irregular and unifoqnly distributedover the entire concrete surface, but in the case of acontinuously reinforced concrete section or inconcrete units that are much longer than their

. width, the cracks will be parallel to thereinforcement or to the longer dimensionrespectively. The resulting crack pattern IS

commonly referred to as map cracking [5,6].

Figure 1 below indicates typical map crack patternsinduced by ASR in airfield concrete pavement androad pavement.

(a)

Journal of EEA, VoL 21, 2004

52 Abebe Dinku and Birltanll Bogale

....... ' ....... ~'.'.'.:.'."

.;.;.;.;.; .....;.;.;.;.;.;.;.;.;.;.;.; ,...... ",." ...... ' ..................................... ' ,

.. ·::::::::::m··li~II·;I~llillll!

(b)

Figure I ASR Induced Cracks (a) High severitymap cracks in airfield concrete pavement [12].(b) Map cracking in pavement [13]..

In most of the cases, ASR induced cracks vary insize from O. 1mm for less severe cases, to as muchas 10mm in extreme cases. They may be as deep as50mm but, in most cases, their· depth is limited to25mm. With these dimensions, ASR cracks may bea§sumed to cause serviceabilitY'problems only, butas described earlier, they allow entrance of harmfulagents making the. structure more susceptible. tofailure.

Another indication of the increased expansioncaused by ASR is closing of expansion joints.Because of the excessive expansion induced byASR, these expansion joints may be permanel1etlyclosed and in worst cases, the concrete sectionsmay be seen bumping and crushing one other.There is also a.possibility that the joints m~y bemisaligned from their original position [6,14].

The third probable indication of potential ASRtaking place in concrete' structures is emission ofthe expansive silica gel through the cracks formedon to the surface of the concrete. Initially, the gelis sticky and moist but it will progressivelyeffloresce to' a white coating as a result ofdehydration and carbonation [6,13]. Occasionally,small pieces of concrete may also pop-out from theconcrete surface [2]. In both cases, the exudationscan be tested in the laboratory to c~nfirm the cause.

MITIGATION OF ALKALI-SILICAREACTION' ...

Va,rious alkali-silica reaction mitigation techniquesare continuously being developed to prevent earlyage deterioration of concrete structures. In recent·

JournalofEEA, VoL21,2004

constructions, methods of avoiding ..this harmfulreaction should be implemented; however, oncethis reaction is identified in existing structures,selected treatmeo.ts should be applied which willhelp to prolong the useful service life of theconcrete structure.

Obviously, to prevent ASR, mineral constituents of. the aggregates to be used in the concrete should beinvestigated and their potential alkali reactivityshould .be tested. If they are found to bedeleteriously reactive, they should not be used inthe concrete. But if there is no any otheralternative, the alkali contribution from the cementand all other possible sources like some admixturesand the aggregates themselves, should' besubstantially reduced (to less than 0.6% Na20equivalent; and to less than 0.4% Na20 equivalentfor potentially reactive aggregates [15]) to preventalkali attack. In other words, low alkali cementshould be used with such reactive aggregates orcement replacement materials should be used toreplace some part (25-40%) of the high alkalicement.

ALKALI-SILICA REACTION IN ETHIOPIA

As has been mentioned in the previous discussions,alkali-silica reaction can occur in concretestructures if the three preconditions are satisfied atthe same tLl11e,i.e. if reactive constituents, alkali~and moisture are aU present. In the absence of oneof these preconditions, deleterious ASR will nottake place and hence the structure will not besubjected to ASR induced failure. Therefore,before concluding whether there is a potential forASR .in Ethiopia or not, the above threepreC<?nditions should be assessed carefully anddeeply. Following is a preliminary assessment ofthe potential occurence of each precondition inEthiopia.

OCCURRENCE AND COMPOSITION OFSOME LOCALLY AVAILABLE

AGG~EGATES

As can be seen from the simplified geological mapof Ethiopia shown in Figure 2, majority of thecountry's rocks are predominantly of volcanicorigin .e~ending from the north eastern and northwestern parts ,of the country to the central, south

.central and south western parts. This includes thewestern, sQuth western and eastern parts of Tigrayregion, Afar, majority of the Amhara regioncovering Gojam, Gondar and parts of Wollo,central Ethiopia including Addis Ababa, Western

Alkali Aggregate Reactions in, Concrete:A Review 0/ the Ethiopian Situation 53

and south western parts of Oromia an~ northernpart of SNNP. The most common of such ~olcanicrocks are basalt, ignimbrite and tuffs. The later twoare primarily found in areas near the Rift Valley.

Th~ other considerable part of the country isdominated by sand and lime stone depositsincluding parts of Tigray, southern Amhara aroundAbay basin and predominantly eastern and southeastern parts of the country covering Harrar,western parts of Ogaden and eastern SNNP.

The third type of major rock deposit found inEthiopia is predominantly granite 'located at thefour comers of the country, i.e. in the southern part

around Sidamo, in the eastern part around Harrar,in the northern part around central and northwestern Tigray and in the western part mainly inWellega, Gojam and Benishangul Gumuz. Thisrock deposit mainly constitutes the minerals quartz,feldspar and mica. '

From these major rock ocurences in the country,the ones which could be suspected of undergoingexpansive alkali-siliCa reaction are the graniterepresentative samples from the different localities,to determine their potential reactivity. Table 1shows sample data of which, chemical andmineralogical composition have, been carried out[17,18,19].

Table 1: Chemical and mineralogical composition of selected rocks.--

SamplesBurll and , DehanNegashAmbo

Akaki-

FeldsparAngerguten GraniteGraniteSandstoneOlivine

Basalt,

Granites Basalt

Source

GojamandBenishangulWestShooShoaShoa

WellegaGuinuzTigray

Location

Bekotabo andMetekelNegash

Ambo-AddisAddis

AngergutenSenkeleAbabaAbaba

Si02

68,1270.9470.08· 4761.62

c-;;-

Ah0314.8514.1515.78 15.715.1-15.2 '

Q <l.l

F~03 3.512.141.18 14.88.42;-0 MnO 0.050.070.02 '0.2-0.3";; .~ ~0.2

8.~ CaO 1.421.871.14::c8.03.2-3.6S Q'"

Q-MgO 1.010.410.31:a5.61.4-1.7U~ Na20

3.754.284.70:>3.44.9

-; 'Qj'"

,~ ~.K2O' 5.024.583.64'00.92.6-2.7 -'.

S >.H2O 0.47 0.50

Z0.7,

<l.l~

- -e~ LO! 0.440.92._. 0.74 1.1.0.5-0.9'-' Ti02 0.68 0.17,3.31.2-1.3-

P2050.380.010.21 --

Quartz

25303595--

K-Feldspar5527 -.- -,-.

~ Plagioclase 920.27 -60-6570-800 '-' , Biotitec 54----,~ Muscovite 5- ,5----';j Microc1ine 338.

-- ---S

Sericite -6-,-~.

- --Q Epidote 6U ------; Chlorite -3.0" -

---u

Clinopyroxene.''6il ----

-10-20-,Q Pyroxene -'".

10'i -,',:" .."-- -,

It., Olivine'"

10-20Trace<l.l ----c~

Hematite ----5-10.-Glass

-----UP to 25Others

1 '4-5--

JournallJ/ EEAJ VoL 21, 2004"

A.bebe Dinku and Birhanu Boga.le

'1.:::,D<::t':~~~(~rfjrd;.;:~:;, l'.fKJ0~9iJ;~nq:f.1n:1)f:'

'14, ~:.I;~~W··v·:i:-?':

15. :'-;'~::l!.l:~:":G"J~~:"~16 ':)I:>J.:"p CtHt·sl iiIHl,I~,,';;:f7. ;·iQ:c.:(n,;:':}:'Q lir·1~r-;t~_~:~;.::. -.': .~;••• k:;P~::1(~(,~l<.'(;(~nj;

~:'~:~~~~~ & ~i:~~::¥.·!;:n€'S-:{:'~:~J

.~.:::. :-V~';:-:(••~")("~::n·~~::t(:::w,7.1. /";nlx~· :(10·::1:;, ;)f\<.'1

8. A~;:':':C:"~r1v..~rt.~k::9. t'.;:;.~l~i:1(l(r>:.·'·'''.1rt?1:~

10, HJ.~n;«)l>:'~Q,.:;";::lr~~~:

:; ;,\IJ~);*' gran:~,

: . ~\"k~;:'.')~\~:..-;:1):·:~·

2. g~;l~~""5: {;:.onzl ;~':~J:~>lf'~~.T~;lu·~k.;f;)·t? :y:orble<1.~",l(Hw:t~~n:"ql':Jd:~:;:

~~" f)(;i~:.:·:i {:·}\.-J:t.ll{~

C. :::'f.:io iik\:r,r ry,:~'t;k.;!.~"J"?\A/: rnc1rt';B

=~;:~::::;::~::~:;~::::.;::::.::.~~i~~;i:I;;::::Xhjjjj;t~tmm;\·h:~L~f):~~${:.(~im(~:.::t~fY r~)~k~i(s~\~:;.l~:·(·HW~M:~~ !i:":1~~:st<;r}~)

~~~f:1~~~~~~~i~r:~·.jJ~c{'rt.ujf;~'{.V.)"~

~. PH:~~.m~:.i.:rh:i.;;jgJJ·:: .•..!'\lS f()(::':" (pr':dor~1111.:.(;.:.d.:.;gl::U;jt<::)

!:effff ~:~;~;,':~~~:;~;~;'~:I:~~~~I<;IY:li(;:,xh l!~""":.~,'iI,h:"I. ,,::whi·~ ron·:l

;g:·:':/';1:'::

~~-:':)({:k.3;'::~:~~~~!t

:(::~";:·~l:.':r:;',··,,::,,

{;~'::;:i~:::~f:¥~'

.f{'f.t:,: J'.t;i;;~~'~.~::;:;~{:!''t:l.'':.:!~;l~ ;~'~':;I1~:':"

Figure 2 Geological Map of Ethiopia, GSE 2000 [16].

Journalo/EEA, Vol. 21,2004

Alkali Aggregate Reactions in Concrete:A Review of the Ethiopian Situation 55

Refering to Table 1, the first. three granites havealmost the same amount 'of Si02 content, about70% which is high of course, but only 25-35% is inthe form of quartz which itself is least reactive ofall the silica minerals. On the other hand, theycontain appreciable quantities of the alkali oxideswhich may gradually contribute to the total solublealkali content in the concrete besides the cement.Hence, althoughix>tential reactivity might not· beexpected of these. rocks because of their limitedcontent of quartz, the high silica content and thepresence of the two strong alkali. hydroxidesobviously initiate. further investigation.

In the case of the sand stone deposit at Ambo, it ispredominantly co~posed of the mineral quart~

.which under high alkali exposures is reactive to anappreciable degree. Therefore; one of the. quick andless expensive tests could possibly be conducted onit to assess its degree of reactivity .

The alkali-olivine-basalt, being a fine grainedextrusive rock, its glass content is generally low.Hence, it is not, deleteriously' reactive with thealkalis in hydraulic cement paste. However, the'relatively higher concentrations of Si02, alkalis andglass in the feldspar basalt are apparently clear andtherefore its alkali reactivity should be assessed, ina more detail. '

The other assessment could possibly be that of thelocally available sand which is used as a fmeaggregate in constructions. Table 2 gives the oxidesconcentration of silica sand obtained from ~eelocalit!es [17].

Table 2: Oxide concentration of silica sandobtained from different localities'. Sample

Silica SandSourceShoaTigrayTigray

LocationMugherAdigratSinkata

Si0296.199.598.4-99.6

c-::'

Al'P31.910.270.1-0.6Q 41

Fe2030.320.040.1-0.33:="'0 MnO

'0; .~ -<0.1-Q Q CaO0.1<0.1c. ••• -E Q MgO0.1<0.1Q- -

u~ Na200.1<-D.1--;G;

,~ ~K200.1<0.1-

ES H2O -0.13.41..c~ LOI0.490.24-u· '-'Ti02 <0.02- -

P20S---

In this case, ,although alkali contribution from thesand is minimal, the concentration of "SiOl is So

high that substantial ASR can take place if theother favourable conditions are satisfied. The majorsand supply to Addis Ababa is derived from theAwash basin which is located about 120km southeast of the city and a similar investigation IS

required to assess the potential of reactivity.

ALKALI CONTENT OF CEMENTSPRODUCED IN ETHIOPIA

For so many years, the Mugher Cement Factory inAddis Ababa was the major producer and supplierof cement in Ethiopia. But, because of theproliferation of concrete qonstructions in' .thecountry. in recent times, this factory alone wasunable to meet the entire cement demand in thecountry !\Ild therefore, Messebo Cement Factory is.recently established in Tigray region where there isabundant supply offue raw material.

The soluble alkali content of the cement producedin the Mugher Cement Factory is 0.6% which isright at the margin of low and high alkali cements.According to the very recent (20.10.04) test resultof the quality control laboratory of MesseboCement Factory, the Na20 and K20 concentrationsof the OPC produced are 0.35% and 0.38%respectively which again yield Na20 equivalent of0.6% [i.e. 0.35+0.658*0.38=0.600 / Na20 eq =

%Na2O,+ 0.658%K20].

Accordi-Rg to the foregoing discussion, forpotentially reactive aggregates, the total solublealkali content in the concrete should be sufficientlybelow .0.4% to avoid deleterious ASR. From the

above 'observation however, there is no clue as tosay 'that such potentially reactive aggregates occurin Ethiopia. And therefore, the soluble alkalicontent of both cements i's in the acceptable rangein our conditions.

Practical experience in some European countriessuggests that deleterious ASR will not take place asfar as the total alkali contribution from all sources

is kept below 3kglm3 [6]. This me#ns in our case,ass~ing that all the alkalis are supplied only fromthe cement paste, as high as 500kglm3 cement isrequirec;i to reach this limiting value using ouriqcaily'. produced. cements. Comparlng this value

'With the quantity of cement used in eVerydayconstruction reveals that there is a good safetymargin which may account for contribution of

.. alkalis from other sources like from the aggregatesthemselves, from some chemical admixtures or

JournalofEEA, VoL 21,2004

56 Abebe Dinku and Birhanu Bogale

from surrounding alkaline environments. -HQwever,keeping this in mind, more investigation is quitenecessary to arrive at tangible conclusions.

NATURAL CONDITIONS OF MOISTUREAND TEMPERATURE

The surface layer of a concrete section is the onlydriest portion which does not react 'much and hencedoes not expand appreciably. The inside part of anyconcrete section however is ,seldom dry and hencesubjected to ASR. Moreover, as a result ofpermeability of or~inary concrete, water enters tothe inside easily and keeps the relative hUmidity inthe concrete high enough for ASR to take place.

In addition to the high relative humidity (>50%) in.the air present in rpany places in Ethiopia,highlands of the country (the -temperate zone)receive a high amount of rainfall ranging from1300 to 1800mm annually, with a correspondingaverage temperature of about 16°c. Even thesubtropical zone which covers most of the highlanoplateau of the country receives an annual rainfall of500 to 1500mm with an average tetnperature of22°c [20]. Hence, as far as ASR is not precluded,there is a potential replenishment of moisture to theconcrete in Ethiopian conditions.

Moreover, in recent studies, it is found out thatnearly all concrete will have an internal relativehumidity of more than 80% provided that one of itssides is on the grourid and the relative humidity ismeasured at .a depth of 2 in. (5.08cm) from theexposed' surface-even in the desert [14]. Thismeans, important structures like concrete bridges,pavements, dams and foundations are always liableto contain high moisture inside them irrespective oftheir location. Therefore, abundance of moisturefor ASR to take place is not questionable and henceit.- is imperative to limit its amount as far aspracticable to avoid deleterious expansion.

-CONCLUSIONS AND RECOMMENDATIONS

I. From the above discussion, the apparentreason for not expecting potential alkali-silicareactivity in Ethiopia is the low alkali contentof the cements produced and distributed allover the country. As stated previously, theNa20 equival~nt of both the MUghei and,Messebo cements is 0.6% which can causedeleterious ASR rnly with potentially reactiveaggregates. From the data obtained above,there is no indication of the occUrrence of such

potentially reactive aggregates in the cOlmtry.

JournalofEEA, Vol. 21,2004

However, only this assessment is not deemedto be sufficient to make a fmal conclusion'and

therefore, detailed laboratory analysis of anumber elf representative aggregate samplesfrom allover the country should be conductedwith particular focus towards suscep\ibility ofthe aggregat~s to ASR.

2. Although the pot!illtial contributor of alkalis inconcrete. is the cement paste, there is also apossibility that alkalis maybe supplied fromother sources like the surroundingenVironment, some chemical admixtures and

'the aggregates themselves.' Again from theforegoing assessment, the two strong alkalioxides, namely Na20 and K20, are present insome of the rocks available in the country to ashigh as 9% combined: As aggregates constitute70-80% of the total volume of concrete, thecumulative contribution of alkalis from these

aggregates shouldn't also be regarded asinsignificant and hence it initiates furtherinvestigation too.

3. Quantitative assessment of possible alkalicontributions from other sources like thegroupd water and other marine environmentsin the country should also be conducted beforedecisions are arrived at hastily. Detailedanalysis and specification for. chemicaladmixtures to be used in the constructionindustry is also helpful to limit their solublealkali content. '

4. Apart from these, as water is essential for ASRto take place, gross moisture' entry to cOncretestructures should also' be prevent&! as much aspossible in case there is unseen poteIiti8I. foralkali-silica reactivity, and hence the followingmeasures will be helpful to address this lispect.

a) Proper drainage of concrete structures toavoid water retention on them.

b) Avoiding the use of high water-cementratio for merely enhancing workabilitywhich ffi!lkesthe concrete more permeableand hence more susceptible to grossmoisture entry.

,,} Taking adequate precautions whil~ placmgand compacting concrete and sttictlypracticing proper curing right afterplacement,

d) Introducing the use of cemen\ replacementmaterials like GGBS to obtain, morecompact and hence less permeableconcrete.

Alkali Aggregate Reactions in Concrete:A Revlew1Jfthe Ethiopi4n Situation 57

It . is the responsibili~of all concernedprofes..~otlJlls wor.king ine COWltry.to provethat thett is in fact no pro lem of ASR in theooupby. :petalled laboratory and . fieldinvestisati~ ,as well as selected case studie~,

. sbouldbe ~td out in areas suspected:ff this,reaction .. Jri. case any deleteri,ous ASR is'di~, it should be d~y addressed before. it get$ out of persPective.

6. Modem construction techniques)ike the use ofcement rePlacement materials· (for instance,GOBS and PFA). should be increasinglyexercised in the >. COWltry as they havemultidirectional advantages with respect toreducing ASR.The use of other beneficialconcrete· admixtures should also be widelypromoted whichparallely improve the oyeraU··quality of construction in the COWltry. '

[2]

[3]

[4]

[5]

[6]

Cement Association of Canada - AlkaliSilica Reactivity.http://www.cement.ca/cement. nsf

Moskvin V., Concrete and Reinforced·Concrete Deterioration and Protection, MirPublishers, Moscow, 1983.

Lea F. M., The Chemistry of Cement andConcrete, Third Edition, Edward ArnoldPublishers Ltd., Glasgow, 1970.

Bensted J. and Barnes P., Structure andPerformance of Cements, Second Edition,Edmundsbury Press, Suffolk, Great Britain,2002.

Concrete Durability.http://www. ceoberkeley .edul-paulmonUCE6

· ONew/chapter5.txlf

[12] Evaluation of ASR Distress in Airfield'Concrete Pavement.

http://www.airoorttech.tc.faa. gov/att04/20020/020TRACK%20P.txlfIP-34.pdf

[13] Master Builders, Concrete TechnologyUpdate; Inhibiting ASR with LithiumAdmixture.

b!1P.JL~S!~!m.u!t~r..~&9JJ.!!.lU'Ii~.~m~iDI..~J:LCon~!ID.t!£.Qf.~QTIJ.AS.R..P.9J

Buildmg Research Establishment Digest,Garston, Watford WD2 7JR, England,February 1982.

Neville A. M. and Brooks l l,'ConcreteTechnology, Third Indian Reprint, ReplikaPress Pvt. Ltd., India, 2003.

Thermo-Chemo-Mechanics of ASR

.Expansion in Concrete Structures.http://eist. mit. edulresearchandproi ects/expan~!~~~_~~jQn~'p'!I~I~~_~I~~p.!m_~_tm)d~gJ

Standard Practice for Concrete for CivilWorks Structures, Appendix D, Alkali-Silica

·'Aggregate Reactions.http://www.usace.army.mil/ineUusace­docs/eng-manuals/emlll0-2-2000/a-d.txlf

[II] .Annual Wook of ASTM Standards, Section4, Volume 04.02, Oct. 2002.

[10]

[9]

I8]

8. Finally, further detail study on the subject oralkali-silica reaction by taking morerepresentative samples is recommended.

We would like to extend our deepest gratitude andprofoUnd thanks to the administrations of theGeoscience Inforniation Center of the GeologicalSUrveyo(Ethiopia, Ministry of Mines, for keepingtheir library open for visitors', making importantdata a"ailable to graduate .students and researchers.Without ..their exemplary collaboration;' it· wouldhave.been n(}teasy to obtain relevant test results.

REFERENCES

[I} Sidney Mindess, Francis YOWlg J. aridDavid Darwin, Concrete, SecopdEdition,:Prentice Hall, USA, 2003.

7. The current Ethiopian. practice ,in aggregateproduction and' usage' is nonsystematic andlacks consistency creatmg problem.s bOth forproducers and consumers. This calls forStandardiZation of aggregates interms of theirchemical composition, specific gra~ty, sizes'and .oU~r relevant parameters. Attempt shouldthus be' made by' the concerned relevantauthority to ,standardize aggregates, both fmeand course, so that the required quality couldbe achieved consistently and at a reasonablecost.

ACKNOWLEDGEMENT •

Journal of EEA, VoL 21, 2004

58 Abebe Dinku and Birhanu Bogale

[14] Petrographic Methods of ExaminingHardened Concrete:. A Petrographic Manual,Alkali-Aggregate Reactions.http'//www.timc.gov/pavement!pccp/petro/

[15] ACl Materials Journal, Vol. 99, NQ 5, Sept.­Oct. 2002.http://www.nfesc.navy.mil/shore/files/ AClas~

[16] Geological Map of Ethiopia, GSE 2000.

[17] Tibebu Mengistu and HaileMichel Fentaw,Industrial Minerals and Rocks ResourcePotential of Ethiopia, Norway, 2003.

[18] NGU Report 2001.078, Natural Stone inEthiopia, Report from the ETHlONORProgram 1996-2001.

[19] Karstaedt H. and Mamo W., SummarizedReport on Building Raw Materials East of,Addis Ababa (Bole Area), Nov. 1986.

[20] Microsoft Encarta Encyclopedia Deluxe2004, Microsoft Corporation 1993-2003.

Journal of EEA, VoL21, 2004'