PII: S0010-938X(99)00102-X

UNIVERSITY OF GAZIANTEP FACULTY OF ENGINEERING CIVIL DEPARTMENT

CE-547Corrosion of Plain &Reinforced concrete

Report #6 About :

(Substance of 11 articles)

Submitted to:Do.Dr. Mehmet GESOLU

Prepared by:Chalak Ahmed Mohammedchalak.mohammed@gmail.com

2014 45056

Date : 12.05. 2015

List of content

1. Permeability properties of self-compacting rubberized concretes2. Permeation Properties of Self-Consolidating Concreteswith Mineral Admixtures3. Effect of initial curing on chloride ingress and corrosion resistancecharacteristics of concretes made with plain and blended cements 4. study on reinforcement corrosion and related properties ofplain and blended cement concretes under different curing conditions

5. Strength development and chloride penetrationin rubberized concretes with and without silica fume

6. Improving strength, drying shrinkage, and pore structureof concrete using metakaolin

7. A study on durability properties of high-performanceconcretes incorporating high replacement levels of slag

8. Comparative study on strength, sorptivity, and chlorideingress characteristics of air-cured and water-curedconcretes modified with metakaolin

9. Transport properties based multi-objective mixproportioning optimization of high performance concretes.

10. Corrosion behavior of reinforcing steel embedded in chloride contaminated concretes with and without metakaolin

11. Durability aspect of concretes composed of cold bonded andsintered flyash lightweight aggregates.

Permeability properties of self-compacting rubberized concretes

The brittleness and low tensile strength of cement-based materials are detrimental to their durability. Researchers are tryingto eliminate brittleness of concrete and they have been working onthe possibility to make the concrete tough by introducing waste rubber phases among the traditional components (cement, water,and aggregates). It has been estimated that around one milliontires are withdrawn from use in the world every year. Disposal of waste tires has been a major issue to the cities all around theworld. Generally, the cheapest and easiest way to decompose the used tires is by burning them. However, the pollution due to enormousamount of smoke makes this method so unacceptable that itis prohibited by law in many countries. The discarded tires arealso buried with the other industrial waste in landfills or stockpiledin huge dumps. The resulting stockpiles, however, may cause majorhealth risks for the public and the environment. Therefore,recycling and utilization of the waste tires seems to be necessaryIn the literate, durability of rubberized concretes has not foundadequate attention. worked on the rapid freezethaw resistance of the concrete containing different amounts ofground rubber aggregates. They reported that the rubberized concretehad lower performance against freezing and thawing damage.The use of rubbersignificantly aggravated the chloride ion penetration through concretesuch that there was a systematic increase in depth of chloridepenetration with the increase in rubber content for concretes withand without silica fume, especially at high w/cm ratio. As the rubbercontent increased from 0% to 25% by total aggregate volume,the chloride permeability of the rubberized concrete with andwithout silica fume was about 640% at 0.60 w/cm ratio and about2759% at 0.40 w/cm ratio greater than that of the controlled concrete.

ConclusionsBased on the result obtained from this study, the following conclusionsmay be drawn:_ A progressive increase was observed in the chloride ion penetrationof the self-compacting rubberized concretes with theincrease in rubber content without fly ash at both of 28 and90 days test results. Addition of fly ash did not considerablyaffect the chloride ion permeability of the self-compacting rubberizedconcretes at the 28 days. However, when the curingperiod was extended to the 90 days, the long-term reaction offly ash refines the pore structure of concrete so that ingress ofchloride ions decreased drastically.

_ Since the presence of the crumb rubber in the concrete, porosityis poorly affected. Therefore, water sorptivity and water absorptionvalues of the self-compacting rubberized concretesincreased. Furthermore, increasing the crumb rubber contentincreased the water sorptivity and water absorption. However,with the addition of fly ash, the negative effect of crumb rubberon these properties eliminated slightly. The amount of reductionincreased with increasing the replacement level of fly ash.

_ Statistical significance of the defined systems (PCCR, PCFA,and PCCRFA) on the measured permeability properties werefound out by the GLMANOVA tests. The results indicated thatthere was a good agreement between the qualitative and statisticalanalysis. The most effective system on the chloride ion permeabilityof the SCRCs was the PCCR system with acontribution of 96.3%.

Permeation Properties of Self-Consolidating Concreteswith Mineral AdmixturesSelf-consolidating concrete (SCC) emerged in Japan in thelate 1980s as a material that can flow under its own weightso that it can be placed in formwork with dense reinforcementand complicated shapes without the need for additionalmechanical compaction. The critical aspects of this technologyinvolve attaining a highly fluid mixture while preventingsegregation among constituents, especially segregationbetween the aggregate and the cement paste. Theadvantages of SCC include high performance in its fresh andhardened states; economic efficiency (shortened constructiontime, reduced labor, and lower equipment costs); animproved working and living environment (high consumptionof industrial by-products, reduced noise, and reduced healthhazards); and enhancement toward the automation of theconstruction process.In the production of SCC, it is common practice to limitcoarse aggregate content associated with its maximum size andto use a lower water-binder ratio (w/b), along with anappropriate high-range water-reducing admixture (HRWRA).5To achieve an SCC of high fluidity and to prevent segregationand bleeding during transportation and placing, the formulatorshave employed a high binder content and used an HRWRA andviscosity-modifying admixtures.The cost of such concretesassociated with the use of a high volume of portland cement(PC) and chemical admixtures, however, was remarkablyhigher. In some cases, the savings in labor cost might offsetthe increased cost. The use of mineral admixtures, such as flyash (FA), blast-furnace slag, and/or limestone filler,however, reduced the material cost of the SCCs and alsoimproved the fresh and hardened properties of theconcretes.. It has been reported thateconomically competitive SCC can be produced by replacingup to 50% of PC with mineral admixtures.Using mineral admixtures, especially in SCC, necessitatesfurther attention. With the incorporation of such materials,certain properties of the concrete may be enhanced, whereasothers may worsen relative to the plain PC concrete. SF, forexample, substantially increases early concrete strength butimparts a sharp fall in workability to fresh concrete, whereas FA decreases early strength but improvesworkability. These negative effects may be remedied bythe combined use of the mineral admixtures. To date, onlylimited work has been carried out on the binary, ternary, andquaternary blends of mineral admixtures. Some examplesinvolve the combined use of SF-FAPC blends23 and MKFA-PC blends18 in conventional concrete

The SCC mixtures investigated in this study were preparedwith CEM-I 42.5 R PC, a Class F FA, a GGBFS, and MK.The chemical and physical properties of the cement andmineral admixtures used are summarized in Table 1. Thecoarse aggregate used was river gravel with a nominalparticle size of 0.629 in. (16 mm). As fine aggregate, themixture of natural river sand and crushed limestone was usedwith a nominal particle size of 0.196 in. (5 mm). They hadfineness moduli of 2.87 and 2.57, respectively

CONCLUSIONSBased on the findings of this study, the following conclusionsmay be drawn:1. Concretes containing FA had a generally lowercompressive strength, whereas GGBFS and MK concreteshad comparable and higher strength values than those of thecontrol concrete, respectively. Even though the FAdecreased the compressive strength, the ternary use of FAand MK mostly improved the compressive strength of theconcretes. Similarly, the combined use of GGBFS and MKgives the concretes a higher compressive strength than thosecontaining binary blends of GGBFS, especially at 90 days.2. All the concretes produced in this study had UPV valuesgreater than 14,760 ft/s (4500 m/s), indicating excellentratings. Moreover, the concrete with quaternary blends of7.5% FA, 7.5% GGBFS, and 5% MK exhibited the highestUPV values, irrespective of the testing age, whereas the lowestUPV values were measured for 22.5FA + 22.5GGBFS + 15MKand 60FA mixtures at 28 and 90 days, respectively.3. It was observed in the chloride ion permeability test thatconcretes with mineral admixtures showed very low ratings,whereas the control concrete had a low rating. The concretesseemed to be much more resistant to chloride ion permeabilitywhen FA, GGBFS, and MK were used in the ternary orquaternary blends. The use of MK appeared to be the mosteffective in reducing the chloride ion permeability.4. A similar pattern seen in the RCPT was also observedin the water permeability test of the concretes, in that MKmade the concretes highly resistant to the ingress of water.Incorporating MK in the binary blends of 5, 10, and 15%caused a reduction of 65%, 78%, and 82% in the waterpermeability, respectively. Regarding the water permeabilityof the concretes with quaternary blends, it was very interestingto note that the concretes with MK had water permeabilityless than or equal to 0.1968 in. (5 mm), irrespective of MK,FA, and GGBFS content.5. Similar to the water permeability test, incorporating themineral admixtures continuously decreased the sorptivity ofthe SCCs. Apart from the use of MK only, the combinationof FA and/or GGBFS with MK provided a marked decreasein the sorptivity.

Effect of initial curing on chloride ingress and corrosion resistancecharacteristics of concretes made with plain and blended cements

Durability of reinforced concrete is largely controlled bythe capability of the concrete cover to protect the steelreinforcement from corrosion .Chemical protection isprovided by concretes high alkalinity, and physical protection is afforded by the concrete acting as a barrierto the access of aggressive species. However, despitethese inherent protective qualities, the corrosion of steelreinforcement has become the most common cause offailure in concrete structures. In such structures asbridges and parking garages, failure of the concretestructures may be attributed largely to the use of de-icingsalts. In other structures, salt penetration from seawaterspray may be responsible for the corrosion. In both cases, it is the chloride ion which destroys the protective(passive) environment for the steel reinforcement andresults in the corrosion of the steel reinforcement andeventual concrete distress. Moreover, carbonation inconcrete normally involves a chemical reaction betweencarbon dioxide and the products of cement hydration. Thisreaction results in a significant reduction in the pH of thepore solution due to the removal of the hydroxyl ions,which may lead to steel depassivation and subsequentreinforcement corrosion. As a result, carbonation can beconsidered as a second cause of damaging the passivationlayer over the reinforcement.Blended (or pozzolanic) cements are being used worldwideto produce dense and impermeable concrete. Theycontain a blend of portland cement clinker and a variety ofnatural pozzolans and/or supplementary cementing materialssuch as blast furnace slag, fly ash, and silica fume. Theuse of these materials is also environment friendly becauseit helps to reduce the CO2 emission to the atmosphere.The beneficial effects of incorporating these materials inconcrete are widely discussed in the literature .

However, the addition of a wide range of blendingmaterials of differing chemical composition also introducessignificant diversity into the cementing system. The widevariation in the performance of the blending materials maybe attributed to the variation in their physical, chemical,and mineralogical composition resulting from the industrialprocesses related to their production and the propertiesof the raw materials used. Therefore, it should berecognized that different cements have different propertiesand performance .

ConclusionsFor the concrete mixtures investigated and the initialcuring conditions employed, the following conclusions maybe drawn:

1. Initial curing conditions had a substantial influence onthe rate of chloride penetration for the plain andespecially blended cement concretes. Results indicatedthat the lack of proper initial curing considerablyaggravated the chloride ion penetration through theconcretes but the degree in the rate of increment of thechloride penetration depended mainly on the w/c ratioof the concrete mixture, type of cement used in theproduction of concrete, and the immersion period in thesalt solution.

2. It was observed that the application of controlled initialcuring yielded chloride penetration coefficients within10% of those obtained at wet initial curing condition forthe plain concretes. However, it was scattered for theblended cement concretes and subsequently deviated upto 35%. Moreover, uncontrolled initial curing conditionresulted in great differences in comparison to wet initialcuring in terms of chloride ingress and gave remarkablyhigher chloride penetration coefficients forboth plainand especially blended cement concretes. Therefore, itmay be concluded that 1 week of WC is not enough forblended cement concretes, although it seems enough fornormal portland cement

3. Similar to the results of chloride ingress characteristicsof the concrete mixtures investigated, the half-cellpotential values of the reinforcing steel embedded inplain and blended cement concretes seem to be muchsensitive to the initial curing conditions before exposureto chloride environment. Under the proper initial curin conditions, the potential values at reinforcing steel,particularly for the blended cement concretes with loww/c ratio exhibited lower rate of drop, which implieslower probability of corrosion due to chloride attack.

4. The data developed in this study indicated that theblended cement concrete mixtures showed considerablybetter resistance to chloride ion penetration and lowerchances of reinforcement corrosion than the plainportland cement concrete mixtures, especially when thesuitable initial curing conditions have been applied.

study on reinforcement corrosion and related properties ofplain and blended cement concretes under different curing conditionsPerformance of concrete is generally judged bystrength and durability properties. Probably the mostimportant durability issue with reinforced concrete isdeterioration due to reinforcement corrosion. A detaileddescription of the corrosion process can be foundin the study of Rosenberg et al. In the alkalinecementitious environment, a stable oxide film is formedon the steel surface which protects the interior steel fromcorroding. However, corrosion starts due to the carbonationof concrete leading to a reduction in the alkalinity,or the presence of chloride ions causing pitting damageof the protective film on the steel bar. The corrosionproduct absorbs water and increases in volume. Oncethe expansion becomes excessive, concrete cracking willoccur. Following the approach proposed by Tuutti .the corrosion process can be divided into two parts: aninitiation (depassivation) stage and a propagation (corrosion)stage. During the initiation stage, corrosionagents such as chloride ions and carbon dioxide penetrateinto the concrete cover, but their concentrationaround the steel reinforcement is not high enough tocause corrosion yet. The end of the initiation stage orthe beginning of the propagation stage is the momentwhen corrosion starts at threshold concentration ofaggressive species. Within the propagation stage, steelcorrosion is accompanied by the growth of radial cracksfrom the steel bar, which will eventually lead to spallingof the concrete cover The range of compositions as specifiedby the U.S. and European standards are summarized inTable 1.

Corrosion resistanceThe accelerated corrosion behavior of steel barsembedded in plain and blended cement concrete specimenssubjected to three different curing conditions werestudied by impressing a constant anodic potential. Thecurrent required to maintain the fixed potential was plottedagainst time and the typical curves of corrosion currentversus time for the concrete specimens made withportland cement (B1) and portland composite cement(B3) are illustrated in Figs. 6 and 7, respectively. Typicalcorrosion specimens after the termination of the test areshown in Fig. 8. As seen from Figs. 6 and 7, currenttimecurve initially descended till a time value after which asteady low rate of increase in current was observed,and after a specific time value a rapid increase in currentwas detected until failure. Almost a similar variation ofthe corrosion current with time has also been observedby other researchers [3034]. The sudden rise of the currentintensity coincided with the cracking of the specimen.Thus, this curve was utilized to determine thecorrosion time of the specimen when the specimencracked due to corrosion and the current started to increasesharply. The first visual evidence of corrosionwas the appearance of brown stains on the surface ofthe specimens. Cracking was observed shortly thereafterand it was associated with a sudden rise in the current.Figs. 911 present the average corrosion times requiredto crack the specimens made with plain and blended cement and subjected to uncontrolled, controlled, andwet curing regimes, respectively. Time to cracking inplain portland cement concrete specimens was in therange of 67170h (37 days) whereas that in blendedcement concrete specimens was in the range of50440h (218 days), depending on the cement type,w/c ratio, curing condition, and age at testing. At similarcuring condition and testing age, the times ofcorrosion cracking for the blended cement concretespecimens were longer than the plain cement concretespecimens, which indicated that the former providedbetter protection to steel reinforcement against corrosion.

ConclusionsBased on the results obtained from this study, thefollowing conclusions may be drawn:

1. Cement type, w/c ratio, age, and curing procedurehad significant effect on both strength and durabilitycharacteristics of concretes. Both plain and blendedportland cement concretes subjected to uncontrolledcuring in air had lower performance in terms ofstrength and corrosion resistance compared to thecontrolled and wet curing procedures.

2. The application of controlled curing gave averagecompressive strengths within 5% of those obtainedat wet curing procedure for both concrete types.However, the strength of the plain and blendedcement concrete specimens under uncontrolled curingcondition deviated within a range of _10% and _20%from those cured under wet curing, respectively. Bothuncontrolled and controlled curing proceduresresulted in great differences with respect to wet curingin terms of electrical resistivity and corrosion timeof the concretes made with plain and blendedcements.

3. The results generally indicated that the strength gainin blended cement concretes was higher than that inplain portland cement concretes, especially undercontrolled and wet curing conditions. The concretesmade with blended cements had mostly lower 28-day compressive strength as compared to the plainportland cement concretes. However, with increasingage, this trend was reversed.

4. For a given curing condition, lowering w/c ratio ofthe mixes increased the concrete resistivity, and fora given w/c ratio, better curing procedure yieldedhigher electrical resistivity for all concretes. Theblended cement concretes had greater electrical resistivitythan the plain portland cement concretes for allw/c ratios and ages.

5. The accelerated corrosion setup used under the presentstudy has been found to be an efficient and simpletool to evaluate the durability performance of concretes,especially in terms of resistance of concreteagainst reinforcement corrosion.

Strength development and chloride penetrationin rubberized concretes with and without silica fume

Disposal of waste tires has been a major issue tocities all around the world. Generally, the cheapestand easiest way to decompose used tires is by burningthem. However, the pollution due to enormousamount of smoke makes this method so unacceptablethat it is prohibited by law in many countries ,The discarded tires are also buried with otherindustrial waste in landfills or stockpiled in hugedumps. The resulting stockpiles, however, may causemajor health risks for the public and the environment. It has been estimated that around one billiontires are withdrawn from use in the world every year. Therefore, recycling of the waste tires seems tobe necessary by means of innovative techniques.Innovative solutions to meet the challenge of the tiredisposal problem involves the use of waste materialsas additives to cement-based materials and theproduction of rubber-powder incorporated asphalt orbituminous materialsRecently, for the recycling purpose, the scientificcommunitys efforts has led to intense research on therubberized concretes in which some part of naturalaggregates have been replaced by rubber aggregates.The use of crumb rubber and tire chips has found a lotof attention as rubber aggregates in the literature The overall results indicated a remarkabledecrease in strength and stiffness properties of theconcrete after the addition of tire rubber particles.The use of coarse rubber particles affected theconcrete properties more negatively than do fineparticles. Interestingly, source type ofwaste tire from which the rubber aggregates havebeen obtained plays an important role in the performanceof the concretes. Rubber aggregates fromtruck tires are much stiffer than those of car tires,leading to stronger and stiffer concrete .The chemical compositions and the physicalproperties of Portland cement and silica fume aregiven in Table 1. The fine aggregate was a mixture ofriver sand and crushed sand whereas the coarseaggregate was a river gravel. Two types of scrap tirerubber came from used truck tires castaway after asecond recapping. Crumb rubber is a fine materialwith gradation close to that of the sand and tire chipsare produced by mechanical shredding and containcoarser particle sizes. The gradation of crumb rubberwas determined based on the ASTM C136 method.However, it was not possible to determine thegradation curve for the tire chips, as for normalaggregates since they were elongated particlesbetween 10 and 40 mm. Specific gravities for thecrumb rubber and tire chips are 0.83 and 1.02,respectively. The particle size distribution for theaggregates and rubber material are shown in Fig. 1. Acommercially available naphthalene formaldehydebasedsuperplasticizer was used to give a consistentworkability. Its specific gravity is 1.18.

Conclusions

Based on the investigation, the following conclusionsmay be drawn.

1. The use of silica fume on the strength developmentof the rubberized concrete was very effective.For example, at 90 days, the rate of strengthincrease due to the inclusion of silica fume was inthe range of 820% and 934% for the plain andrubberized concretes, respectively, dependingmainly on the variation in w/cm ratio andrubbercontent.

2. The UPV of the concrete mixtures increasedwith increasing curing time in a fashion similar tothat observed in the compressive strength. However,it was noticed that the rate of UPV incrementwith curing period was somewhat lower than thatof the compressive strength.

3. For a given w/cm ratio and moist curing period,the use of rubber in the production of concreteconsiderably aggravated the chloride ion penetrationthrough concrete but the degree of the rate ofthe increment of the chloride permeability dependedmainly on the amount of the rubber used.When the curing period was extended in steps of37 days and 728 days, the reduction in themagnitude of chloride penetration depth wassignificantly higher for both plain and rubberizedconcretes, even at a rubber content of as high as25%. This indicated the importance of prolongedmoist curing period, especially for the rubberizedconcrete.

Improving strength, drying shrinkage, and pore structureof concrete using metakaolin

Performance of concrete is determined by itsmechanical and durability properties. There are somany studies in the literature focusing on theimprovement of concrete performance by replacementof Portland cement to some extents of variousmineral admixtures; such as, fly ash, silica fume,blast-furnace slag, etc. Due to pozzolanic and fillingeffects of these certain mineral admixtures, they arecapable of enhancing the durability through the porerefinement and the reduction in the calcium hydroxideof the cement paste matrix. Generally, theeffects of mineral admixtures may be assessed asimprovement in workability, durability to thermalcracking, durability to chemical attacks, and productionof high performance concrete .

ConclusionsThe effectiveness of using MK on the performanceproperties of concretes were investigated in thisstudy. From the above experimental results, thefollowing conclusions are drawn:

Concretes with high strength and low shrinkagecan be made by using Portland cement blendedwith ultrafine MK.

The study showed that the MK provided a significantincrease in both the compressive and splittingtensile strengths when used as a modifier inconcrete with varying amounts. WhenMKreplacescement, its positive effect on the concrete strengthgenerally starts at early ages and also noticeableincrease in the strength was observed at later ages. Itwas observed that the strength of concretes incorporatedwithMKwas up to 30% greater than that ofthe plain concretes, depending mainly on replacementlevel of MK, w/cm ratio, and testing age.

For all replacement levels, the MK modifiedconcretes exhibited remarkably lower shrinkagein comparison to the plain concretes, irrespectiveof w/cm ratio. It is known that the dryingshrinkage is influenced by many factors. Theresults demonstrated that the w/cm ratio was thedominating factor because both the plain andespecially the MK modified concretes with highw/cm exhibited relatively low drying shrinkage.

With regard to the rate of drying shrinkage, it isevident that both plain and MK concretes with loww/cm ratio showed a somewhat faster developmentof shrinkage than those with high w/cm ratio.However, the drying shrinkage rates of the concreteshad a decreasing tendency with increaseddrying time, particularly for the MK concretes.

The inclusion of MK as a partial cementreplacementmaterial provided an excellentimprovement in the pore structure of concrete.Irrespective of w/cm ratio, the pore size distributionwas shifted to the smaller pore size range dueto the incorporation of MK. The total porositydecreased substantially with increasing replacementlevel of MK. The magnitude of thisreduction ranged from 22 to 49%, dependingmainly on w/cm ratio and replacement level ofMK. Moreover, there was a considerable reductionin the mean (or median) pore diameter of thesamples due to the inclusion of MK. The effectwas particularly beneficial at 20% MK content,where the lowest porosity and the pore diameterwere achieved.

A study on durability properties of high-performanceconcretes incorporating high replacement levels of slag

The worldwide demand for high-performance cement-based materials has increased and predictionsare that it will be widely used in construction industryduring the early 21st century. Economical andenvironmental considerations had a crucial role inthe supplementary cementing material usage as wellas better engineering and performance properties. From the viewpoints of the development of highperformanceconcrete and the reuse of industrialwaste products, the use of blast-furnace slag (BFS) asa cementitious ingredient in either cement or concretecomposites has been increasing.On the other hand, blast furnace slag is a quitevariable material due to the variation in its chemicalcomposition together with both content and compositionof the glass fraction of the slag. The crystallinepart of the slag does not hydrate interfering only asfine aggregate and crystallization seed. The hydrationmechanism of the slag is also different from that ofcement. When the slag is mixed with water, initial hydration is much slower than Portland cement mixedwith water. Hydration of the slag in the presence ofPortland cement depends upon the breakdown anddissolution of the glassy slag structure by hydroxylions released during the hydration of Portland cementand also the alkali content in cement. Furtherinformation on its characteristic can be found in the literature

Conclusions

The following conclusions are drawn from the testresults and analysis presented in this paper:

1. For the mixtures with high replacement levels ofslag, curing played a critical role in realizing thefull potential of concrete in terms of strength andespecially durability characteristics.

2. Generally, there was a systematic decrease inboth compressive and splitting tensile strengthswith the increase in slag content, especiallyunder air curing condition. However, the incorporationof up to 60% slag to partially replacedPortland cement in concrete caused an increasein long-term compressive and splitting tensilestrengths.

3.Slag concrete exhibited marginally lower absorptioncharacteristics than control concrete. Anincrease in slag content (from 50 to 80%)reduced the water penetration by total immersionand capillary action, particularly under wetcuring condition.

4. It was observed that the concretes containing50% and above-replacement levels of slagshowed sharply reduced values of the charge,irrespective of curing condition and testing age.Results indicated that the chloride permeabilityof the air cured control concrete was about 1.1times higher than that of the wet cured controlconcrete, whereas for slag blended cement concrete,the equivalent increase in the dry/wet ratioranged from 1.3 to 1.7 times, depending onreplacement level of slag and testing age. Thisimplies that concretes containing high replacementlevels of slag are very sensitive to thecuring method adopted.

Comparative study on strength, sorptivity, and chlorideingress characteristics of air-cured and water-curedconcretes modified with metakaolin

Concrete is the most important element of theinfrastructure and well-designed concrete can be adurable construction material. However, the environmentalaspects of Portland cement are a growingconcern, as cement manufacturing is responsible forabout 2.5% of total worldwide emissions fromindustrial sources. One effective way to diminishthe environmental impact is to use natural pozzolansand/or supplementary cementing materials, as apartial cement replacement. This strategy will havethe potential to reduce costs, conserve energy, andreduced waste volumes [1]. The cementing materialsthat are widely used, concrete constituents, are flyash, granulated blast furnace slag, and silica fume [2].Metakaolin (MK), produced by controlled thermaltreatment of kaolin, is the most recent mineraladmixture to be commercially introduced to theconcrete construction industry. The utilization ofcalcined clay in the form of high-reactivity MK aspozzolans for concrete has received considerableinterest in recent years. This interest has been focusedon the consumption of calcium hydroxide (CH)produced by cement hydration which is associatedwith poor durability. Thus, the use of MK improveslong-term strength and durability. In addition, it isalso possible to obtain early strength enhancementthrough the filling effect

Conclusions

From the results presented in this paper, the followingconclusions can be drawn:

1. This study indicated that the inclusion of MKinto concrete significantly enhanced the strengthand especially permeability-related durabilitycharacteristics of the concrete in varying magnitudes.Concrete containing MK shows higherstrength than that of the plain concrete butmarginally lower chloride penetration depths andsorptivities. The order of the magnitude isdepended mainly on replacement level of MK,w/b ratio, concrete age, and curing condition.

2. Curing played a critical role in realizing the fullpotential of concrete. It is necessary to paycareful attention when using MK in concrete dueto the fact that the performance properties of the MK-modified concretes are more sensitive tocuring method adopted.

3. Irrespective of w/b ratio and replacement levelsof MK, air-cured concretes exhibit lower strengthand considerably higher permeability relative toequivalent concrete that is water cured. It wasobserved that the strength of the plain and MKmodifieconcretes subjected to air curing deviatedup to _24% and _34% from those curedunder water, respectively. The application of aircuring also resulted in marked differences withrespect to the water curing in terms of sorptivityand chloride ingress characteristics of the concretes,particularly for those made with MK.

4. The comparison of the order of the variation inthe concrete properties (strength, sorptivity coefficient,depth of chloride penetration at 90 days,and chloride penetration coefficient) pointed outthat there is a strong correlation between them.The four measured concrete properties hadsimilar tendency for both plain and MK-modifiedconcretes in that the latter had better performancein comparison to the former.

Transport properties based multi-objective mixproportioning optimization of high performanceconcretes

The use of concrete possessing both high strength anddurability, hereinafter called high performance concrete(HPC), has been increasing all over the world.The factors which justify its popularity are highworkability, high strength, and high durability, forvarious structural purposes. Although the definitionsof HPC are varied, the essence of HPC emphasizesthree main characteristics. Apart from the three basicingredients (cement, aggregates and water) in conventionalconcrete, mineral additives like fly ash,silica fume, and admixtures such as high range waterreducers (superplasticizers), have been incorporatedto make highly workable, high-strength and durableconcrete .Designers of concrete structures havebeen mostly interested in the strength characteristicsof the materials; however, for a variety of reasons,they should now consider durability. According tothe ACI committee , the durability of Portlandcement concrete is defined as its ability to resistweathering action, chemical attack, abrasion, or any other process of deterioration. According to Mehtaand Monteiro ,a durable concrete will retain itsoriginal form, quality, and serviceability, whenexposed to its intended service environment.Permeation is one of the most important parametersof measuring the durability of concrete .Permeation properties of the near surface concreteand the various transport mechanisms which governthe ingress of chloride into concrete, are themajorfactors that influence concrete durability .The permeability of concretedepends on the pore structure of concrete. Manyresearchers have found that the microstructure ofconcrete can be improved and permeability ofconcrete can be decreased, by adding mineral additivessuch as fly ash, silica fume, and blast furnaceslag. To achieve high-strength, workable and moredurable concretes, researchers suggested usinghigh range water reducers and mineral additives inhigh-performance concrete.

Conclusions

An experimental program was set up in order to examinethe slump, compressive strength, split tensile strength,static elastic modulus, ultrasonic pulse velocity, waterabsorption, water penetration, and chloride ion penetrationvalues of the HPCs. The effects of mix designparameters on the permeation properties of HPCs wereinvestigated. Furthermore, transportation propertiesbased multi-objective mix proportioning optimizationofHPCwas performed.Based on the findings of the studythe following conclusions can be drawn:

The use of silica fume in concrete productionconsiderably improved the transport properties ofHPCs.

Increasing the amount of fine to total aggregateratio and superplasticizer contents did notsignificantly influence the permeation propertiesof HPCs.

As expected, increasing the w/b ratio remarkablyincreased the permeation properties, and increasingthe total binder content (cement ? silicafume) decreased the permeation properties ofHPCs, noticeably.

Forty-two different optimum mix proportionswere obtained at the end of the multi objectiveoptimization study. Mixture that has the highestdesirability function value was experimentallyproduced, and it was seen that theoreticallyobtained optimum mix proportions can be usedto minimize permeation properties of HPCs.

Corrosion behavior of reinforcing steel embedded in chloride contaminatedconcretes with and without metakaolin

Reinforced concrete (R/C) is the most commonly used compositematerial in structural practices due to ease in applications andlower cost of construction. Besides, reinforced concrete structuresoffer good service under certain environmental conditions. Theworldwide demand for high performance concrete with improvedcorrosion resistance has increased and it is expected that it will bewidely used in construction industry during next decades. The corrosionresistance of concrete has an important effect on the durabilityand hence its performance. Therefore, it can be said thatconcrete performance depends mainly on the environmental conditionsand the quality of the concrete.

The presence of chloride ions R/C plays a major role in reinforcementcorrosion and hence for the durability and service lifeof R/C structures .The existence of chlorides within reinforcedconcrete accelerates the initiation of reinforcement corrosion andresults in severe deterioration of concrete structures. Once thechloride content at the reinforcement reaches a threshold valueand enough oxygen and moisture are present, the reinforcementcorrosion will be initiated . When corrosion is initiated, activecorrosion results in a volumetric expansion of the rust aroundthe reinforcing bars against the surrounding concrete . It isknown that, in well designed and high quality concrete, the riskof corrosion is expected to be minimal since it provides chemicaland physical conservation to the embedded steel reinforcementbars. The corrosion of rebar in concrete is generally considered asanelectrochemical process. Therefore, the use of electrochemicaltechniques for the appraisal of corrosion behavior ofR/C in this regard, becomes a prominent field of durability study.

Conclusions

Based on the findings presented in this study, the following conclusionscan be drawn:

_ Times to failure in chloride contaminated concretes were shortenedas the chloride concentration increased. The shortest failuretime was observed at control concrete with 3.03% chloridecontent (5 h). However, the longest time was observed at15MK concrete (132 h). It was observed that there are large differencesbetween time to failure values of the plain and MKconcretes. This situation implies that the utilization of MK iseffective for enhancing the corrosion resistance to concrete The minimum corrosion current density values were measured15MK concretes irrespective of the chloride contaminationlevel. The values obtained for 5MK concretes were fall betweenthose of plain and 15MK concretes. However, 5MK concretesdemonstrated a close trend to that of 15MK. For example, at0.91% chloride contamination level, corrosion current densitiesof control and 5MK concretes were 1.80 and 1.11 times that of15MK concrete, respectively. When the chloride concentrationincreased to 1.82%, these ratios become 1.97 and 1.02 for controland 5MK concretes, respectively.

_ Corrosion rates of the concretes seemed to have similar trendswith the aforementioned findings. The highest corrosion ratewas measured as 0.0058 mm/yr in control concrete at 3.03%chloride contamination. However, use of MK provided approximately50% reduction in corrosion rate.

_ Increased level of chloride contamination resulted in significantreduction in electrical resistivity of concretes. The lower theelectrical resistivity the higher the corrosion risk occurs in reinforcedconcrete. However, the utilization of MK notablyimproved electrical resistivity of the concretes, especially at15% level of replacement.

Durability aspect of concretes composed of cold bonded and sintered flyash lightweight aggregates

Management of industrial waste materials is one of the mostimportant environmental issues. Concrete technology can proposesome solutions for recycling some industrial wastes such as fly ash(FA), silica fume (SF), and ground granulated blast furnace slag(GGBFS). For couple of decades usage of such minerals as a cementreplacement substance has been practiced by many investigators.However, utilization of industrial waste powder materials suchas FA and GGBFS in production of artificial aggregate has attractedthe attentions of investigators and practitioners as an alternativeway for larger consumption.Since aggregate is the main occupants of concrete (about 6575% of total concrete volume), it may be considered as an effectivesolution to use such waste materials as artificial aggregate in concrete.Artificial aggregates can be manufactured through processingof different materials and production methods like coldbonding pelletization and sintering . Cold bondingis a type of bonding method which accounts for the ability of pozzolanicpowder material to react with calcium hydroxide at ordinarytemperatures to form a water resistant bonding material.

Pelletized aggregates are left to cure for several days to producean aggregate with proper strength to be used in concrete production. On the other hand, sintering method which is mainlybased on atomic diffusion is a common application for mass productionof lightweight aggregates. Because, aggregate particles,immediately after pelletization process are treated with high temperaturesup to 1200 _C, and become ready for use without keepingfor long term curing periods.

ConclusionsBased on the findings presented above, the following conclusionsmay be drawn.

_ Cold bonded (CB) and sintered (S) aggregates were producedwith water absorption values of 16.3% and 11.7%, respectively.The aggregate crushing strength of S aggregates was about34 times greater than that of CB aggregates, depending ongrain size. Higher strength aggregate provides opportunity forproduction of concrete with improved mechanical property.

_ Inclusion of silica fume (SF) resulted in significant enhancementof compressive strength of the LWCs. The combined use of Saggregate and SF provided the compressive strength values of54 MPa and 44 MPa for w/b ratios of 0.35 and 0.55, respectively.

_ The improvement in water sorptivity due to pozzolanic andmicrofilling effect of SF was observed at both groups of LWCs.The aggregate type was also appeared to be influential on theimprovement of capillary water penetration behavior ofconcretes.

_ Reflecting the chloride penetrability into concrete, total chargevalues were significantly reduced in LWCs due to incorporationof SF. However, the utilization of S aggregates seemed to bemore effective than inclusion of SF in diminishing the chargevalues. For example, addition of SF provided 14% decrease forLWC-CB concrete with w/b ratio of 0.35. However, for the samew/b ratio, LWC-S concrete had 34% less value than LWC-CBconcrete.

_ Gas permeability coefficients of the LWCs were ranged between3.0414.02 (_10_16) m2 and 4.7818.54 (_10_16) m2 for LWC-Sand LWC-CB groups, respectively. Due to enhancement incement paste matrix, SF modified LWCs revealed less permeability.The significance of aggregate type on the gas permeabilitybehavior of concretes were also observed in this test.

_ The resistances of the LWCs against corrosion cracking wereproved to be enhanced through incorporating SF and especiallyS aggregate. The crack time in the accelerated corrosion test wasextended up to five times when S aggregate was used instead ofCB aggregate. The highest failure time was measured as138 days for SF modified LWC-S with 0.35 w/b ratio while theminimum crack initiation time was observed as 18 days forplain LWC-CB with w/b ratio of 0.55.

_ The correlations between compressive strength and other propertiesof LWCs revealed that concretes with S aggregate hadvery high R2 values between 0.91 and 0.99 while these valuesfor CB aggregate incorporated ones ranged from 0.39 to 0.85.

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