CONCRETE LIBRARY OF JSCE NO. 34, DECEMBER 1999

A CORROSION NIECHNISM OF CONCRETE IN A SEWAGE TREATMENT FACILITYUSING AN OXYGEN AERATION ACTIVATED SLUDGE PROCESS

(Translation from JOURNAL OF MATERIALS, CONCRETE STRUCTURES AND PAVEMENTSof JSCE, No.599/V-40, August 1998)

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Tatsuo KAWAHIGASHI Hironobu SUZUKI Toyoaki MIYAGAWA Manabu FUJII

In order to study the corrosion and deterioration mechanisms of concrete, concrete prism specimens wereimmersed in the fmal sedimentation tank and troughs of an oxygen-aeration activated sludge facility at asewage treatment plant. Over the years, changes to the roughened surfaces, mass loss and a decreasingrelative dynamic modulus of elasticity were observed. After 10 years, the bending performance ofconventional concrete specimens immersed without a lining was lower in comparison to specimens witha lining. Microstiucture and chemical analyses showed that surface samples underwent more obviouschanges than interior ones, with the degree of change depending on the concrete mixture and whether ornot the surface had been coated with a lining. The production of erosive flee carbon dioxide wasinfluenced by the quality of the sewage water, while the activity of bacteria and protozoa affectedcorrosion and deterioration of concrete more than sulfuric acid in sewage treatment water. Corrosionand deterioration of concrete progressed through chemical reaction and ion transport in concrete andsewage treatment water flows. A method for estimating the corrosion/deterioration of concrete in thissewage facility is proposed from the results and a calculation of free carbon dioxide.

Kev Words: corrosion of concrete, sewage treatment facility, fi'ee carbon dioxide, mass loss,dynamic modulus ofelasticity, pore distribution, chemical analysis

Tatsuo KAWAHIGASHI is an assistant in the Institute for Environmental Science at Kinki University,Higashi-Osaka, Japan. He received his M.Eng. from Kinki University in 1979. His research interestsrelate to the corrosion/deterioration mechanism of concrete and the estimation of durability of concrete.He is a member of the JCI, JSMS, and JSCE. iHironobu SUZUKI is a section chief in the lst Technology Section at Chuken Consultant Corporation,Osaka, Japan. He received his M.Eng. from Ritsumei University in 1986. His research interests relate tothe deterioration of concrete and the examination of concrete structure tmder various environment. He isa member of JCI and JSCE.Toyoaki MIYAGAWA is a Professor in the Department of Civil Engineering at Kyoto University,Kyoto, Japan. He received his D.Eng. from -Kyoto University in 1985. He is the author of a number ofpapers dealing with the durability, maintenance, and repair of reinforced concrete structures. He is amember of the AC1, RILEM, fib, JSMS, JCI, and JSCE.Manabu FUJII was a Professor in the Department of Civil Engineering at Kyoto University, Kyoto,Japan. He received his D.Eng. from Kyoto University in 1972. His research activities were focused onthe optimal design of prestressed concrete cable-stayed bridges, structural safety evaluations, and thedurability of concrete structures. He has written some 50 papers on these subjects. He was a member ofthe ACI, JSMS, JCI, and JSCE. g

1. INTRODUCTION

The corrosion of concrete in sewage facilities varies with sewage quality, the type andquantity of microbes present, the sewage treatment environment, and other factors.The main cause of sewage-pipe corrosion is thought to be acid (sulfuric acid) corrosion,and this is affected by the activity of bacteria such as sulfate reduction bacteria in theliquid phase and sulfur oxidation bacteria in the gas phase. Recent work has graduallyclarified that such corrosion occurs as a result of bacterial activity and chemicalreactions[1],[2],[3]. Thus, to protect sewage facilities from corrosion, the mechanismsat work must be clarified [4],[5]. Examination of corrosion occurring early on concretesurfaces in sewage treatment facilities has been traced to erosive carbon dioxidethrough analysis of the water quality and the corroded concrete [6],[7]. Thus, themechanism was different from the sulfuric acid corrosion of sewage pipes. To furtherexamine such mechanisms, experiments have been done on bacterial metabolism andconcrete deterioration [8],[9]. With the increasing demand for qualitative andquantitative improvements in the water supply after wastewater treatment, there is agreat need to obtain basic information on which to base countermeasures againstcorrosion in concrete sewage facilities.

In this ten-year study, 13 types of concrete test specimen with different mixproportions were placed at three locations in a final sewage treatment facility for anexamination of corrosion mechanisms. Over the 10 years, measurements wereconducted to evaluate changes in the specimens. This report presents our findings withrespect to concrete corrosion and deterioration mechanisms. The main focus of thisstudy was to nondestructively measure the degree of deterioration under the differingconditions of each immersion location. Over the 10 years, the test specimens weresubjected to visual inspections and also to measurements of changes in mass anddynamic modulus of elasticity. After this 10-year period, some of the test specimenswere examined physically for bending performance, water absorption, carbonationdepth, and pore distribution and were also analyzed chemically by electron probemicroanalysis, X-ray fluorescence analysis, and X-ray powder diffraction.

2 .CAUSE OF EROSIVE CARBON DIOXIDE

The material balance in a sewage treatment facility, as in all environments wherenatural reactions take place, depends on various cyclic processes such asdecomposition, linkage, and immobilization, and there are complex interactions amongthem. Various organic and inorganic materials are present in the water, composingelements such as hydrogen, oxygen, carbon, nitrogen, sulfur, and phosphorus. Anotherfactor affecting decomposition and immobilization in the material balance is theenergy metabolism of the bacteria in sewage, and the growth/decline of theirpopulations as kept in balance by the predatory mechanism of protozoa.

The activated sludge process is commonly used for aerobic and anaerobic treatment,and the quantity of aerobic bacteria, anaerobic bacteria, or facultative anaerobes ineach system depends on the concentration of organic substances, the amount ofdissolved oxygen, and other conditions. In aerobic treatment, organic substances are

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decomposed to carbon dioxide, water, nitric acid, phosphoric acid, sulfuric acid, andother inorganic substances using the oxidative characteristic of bacteria,metabolism[10]. In anaerobic treatment, organic substances are decomposed tomethane and carbon dioxide depending on the metabolism mechanism of the bacteria.Both treatment methods were examined, with the oxygen aeration activated sludgeprocess using pure oxygen, termed the "oxygen process", and the conventionalactivated sludge process with air, termed the "air process". In the first sedimentationtank of both systems, the occurrence of hydrogen sulfide and corrosion were minimal inthe process leading up to a final product, since this stage of the process does notinvolve holding the sludge, although anaerobic activity is observed. With aeration, theactivity and proliferation of facultative anaerobes cause nitrification, denitrificationand phosphorus removal, so there is little possibility of the formation of hydrogensulfide and sulfuric acid (Fig.l).

first sedimentation tank aeration systemflo wI I o i i o z b low in g p u r e o x yg en . tr ou g h fi n alvT ^ -s . se dim e ntatio n tan k^C j¥ ,

sh o rt -termw a te r fro m v C Ch C Ch trea ted ¥7se d im e n tati o n -= - i w a ter o u tfl ow

slu d g e resid en c i a i o * treated w ater(little an aero b ica ctiv ity )

C H 4 t C O 2

C O : ga s rele aseda ero b ic-a n a erob ic a ctiv ity by b acteri a

(m tn fi catio n -de nitrific ationand p h osph o ru s re m o v al) m e ta b olism

fa cu lta tiv e an a ero b es ,- ¥ gre ater pro liferationp seu d o m o n as g ro up etc . '- / th an aero -an aerob ic

b acteria

pre d atory Ip red a tory acti v ity b y r- -¥ b ala nc e o fp rotoz oa / p re d atory by

V ort icella , E p isty lis gro up e tc. flo ck o f b acteria

activ ity o f bacterian d p ro to zo a(sim ilar to ae ratiosy stem )

Fig.1 Outline of oxygen activity sludge treatment system

Identifying the main corrosion factor among nitric acid, carbonic acid, and others,including hydrogen sulfide and sulfuric acid, is difficult because many factors are atwork in a complicated system such as found in sewage treatment facility. For instance,the corrosion mechanism taking place in sewage-pipe is influenced primarily by theprocess of sulfuric acid formation. However, other contributing factors includehydrogen sulfide and sulfuric acid. Carbon dioxide occurs in gas and liquid phases ataeration points, according to the results of existing measurements [7]. As for theoccurrence of carbon dioxide in the oxygen process, corrosion is more obvious than inthe air process. However, there are no significant differences with respect to othercorrosion factors between the oxygen and air processes. Therefore, at the immersionlocations in the facility used for these experiments, erosive free carbonic acid fromcarbonic acid in the liquid phase, is likely to be the primary cause of corrosion. Thecarbon dioxide is formed by the bacterial metabolism and the conditions of its growthor decline are affected by the oxygen demand. If the concentration of organicsubstances in the sewage is high, the sewage load becomes high, and much carbondioxide is formed in the liquid phase. The sewage load in the oxygen process is higherthan in the air process, and the concentration of carbon dioxide rises to a high level inthe liquid phase. Also, the carbon dioxide concentration in the liquid phase increasesbetween the aeration system and the final sedimentation tank, as carbon dioxidediffuses to the gas phase from the liquid phase and dissolves into the liquid phase.

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All the dissolved carbonic acid in the liquid phase is called the "free carbonic acid",referring to the carbon dioxide remaining after linkage of carbonic acid with calcium,magnesium, and other cations. Of the free carbonic acid, the equilibrium level ofdissolved carbonic acid is called the "dependent free carbonic acid", and the remainderis called the "erosive free carbonic acid" [11] (Fig.2). Although the carbon dioxidecontent of water is affected by the partial pressure and temperature of the water, afterdissolution, the carbonic acid assumes various forms such as C02, H2C03, HC03" andC032", which vary in proportion with changes in pH,Free carbonic acid, and especially the erosive free carbonic acid, is affected most bychanges in the inflow load, bacteria metabolic activity, alkalinity, and pH of the liquidphase or the system environment. In the oxygen process in large treatment facilities,concrete corrosion may be largely due to the formation of carbon dioxide rather than ofhydrogen sulfide and sulfuric acid, because of greater bacterial metabolic activity thanin a sewage-pipe.

carbonic acid Table 1 Materials

Ca tLCOj HCCV CO)2' IlinkageLWitlr~ substancesfree form in liquid phase

link ageC aCO) , MgCCh ect.

C a(HCO,). , Mg(HCO3): etc

freeCOi (equilibrium)

COi (errosive)

action with cement substance "^corroding concrete

Fig.2 Formation and action ofcarbonic acid

M a t e ria l N o t e

C e m e n t O .P .C

A g g re g a t e C o a rs e C ru s h e d st o n e

F in e B e a c h s a n d

A d m i x t u r e W a te r- r e d u c in g a g e n t L ig m n s u lf o n ic a c id t y p e

H ig h w at e r- re d u c in g a g e n H ig h c o n d e n s a tio n t n a z m e ty p e

A ii- e n t ra m in g a g e n t N a tu ra l re s in a c id t y p e

P o ly m e r S B R a te x

L in in g E p o x y r e s in

T a i- e p o x y re s in

C o n d e n s at io n c o m p o u n d

o f b is p h e n o l A a n d e p ic h lo ro h y d rinT a r d e n a tu r e d c o m p o n e n t

o f e p o x y re s in

T able 2 Mix proportion factorsN o n - lin in g c o n c re t e L ru n g c o n c r e t e N o n - lin in g c o n c re t e

C o n v e n t io n a l c o n c re t e E p o x y (r e s n T a r- e p o x y M o r t a r F lo w P o ly m e r

lin in g

c o n c r e t e

(r e s in ) lin in g

c o n c re t e

lin in g

c o n c r e t e

c o n c re t e c e m e n t

c o n c re t eS p e c im e n N o . D 2 3 4 5 6 7 (1 r 7 0 )*2 8 0 r 8 (3 f 9 1 0 E D

W a t e r- c e m e n t r a t io I 5 0 5 5 6 0 5 0 5 5 6 0 5 5 5 1

S a n d p e rc e n t a g e (s / a , 4 4 4 2 4 8 4 2

U n it c e m e n t w e ig h t (k g / m ) 3 0 0 3 3 0

(1)* : one coating (lining thickness of 400-600 ju m)(3)* : three coatings (lining thickness of 600-1000 n m)

3. EXPERIMENTAL OUTLINE

3 1 TmTTifirfiinn Conditions

The main location selected for immersion of the test specimens was the trough of thefinal sedimentation tank in the oxygen treatment process. The water depth in thetroughwas 13~32 cm. The flow speed was about 96 cm/sec at a depth of4.5 cm. Forcomparison, other immersion locations, such as the final sedimentation tank of theoxygen process and the trough in the air treatment process, were also selected. Thewater level in the trough of the air treatment process system was 14~21 cm and theflow speed at a depth of6 cm in the center of the trough was about 80 cm/sec [12].

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3 .2 Test Specimen Factors

The materials and mix proportion factors are shown in Table 1 and Table 2. OrdinaryPortland cement was used with sea sand (specific gravity: 2.56; F.M.: 2.79) and crushedstone (specific gravity: 2.71; F.M.: 6.92) as aggregates. The water-cement ratio was setat the largest amount of water acceptable for AE concrete based on durability as givenin the sections on "water-tightness concrete" and "mix proportion" of the Japan Societyof Civil Engineers Standards at the time of experiment planning. As a result, threewater-cement ratios of 50, 55,.and 60 %, and two unit cement weights of 300 and 330kg/m8 were chosen ("conventional concrete" ; specimen number in Table 2 : 1~6). Theslump and air volume of all concrete specimens were, respectively, 8±1 cm and 4±0.5 %. In addition to these six types of concrete, specimens with different coating("lining") specifications ("lining concrete"; specimen number: 7(1) and 7(3) ("epoxylining concrete"), 8(1) and 8(3) ("tar-epoxy lining concrete"), and 9 ("mortar liningconcrete") ), flow (high slump; "flow concrete"; specimen number: 10) and polymer-cement concrete ("polymer-cement concrete"; specimen number: ll) were also prepared.The basic mix proportions of the base concrete for the lining specification, unit cementweight and water-cement ratio, were respectively 330 kg/m3 and 55 %. The slump offlow concrete was 18± 1 cm. The polymer cement ratio of the polymer-cement concretewas set at 10 % and the water-cementratio at51 %. For the lining, two kinds of resinand one kind of mortar were used. Each epoxy resin and tar epoxy resin were used asresin for lining. The epoxy resin was combined with paint or other materials to formcondensation compounds of bisphenol A and epichlorohydrin. The denatured aliphaticpolyamine was used as a hardener component. Also, the tar-epoxy resin was made witha denatured component of tar, which was mixed with bitumen material to form epoxyresin. The thickness of the resin lining applied to the concrete was 400-600 Mm or600-1000 Mm.The thickness of 400-600 Mmrepresents one coating (specimen number :7(1) and 8(1)) and 600-1000 Mmrepresents three coatings (specimen number: 7(3) and8(3)). The mortar for the mortar lining concrete was made with a 34 % water-cementratio ; sand : cement =2 : 1 ; and a flow value of 183. The lining quantitywas measuredfrom the difference in test specimen mass before and after the lining was applied. Thelining thickness was calculated as the mean value by using the unit mass of the liningmaterial and the test specimen surface area. Furthermore, there was a possibility thata lining primer would impregnate the concrete and change its physical properties.Therefore, no primer was used.The concrete materials were mixed using a 50-L tilting mixer. The concrete was castin accordance with JIS A 1132. All test specimens were cured in water for two weeksand then under atmospheric conditions for two weeks. Test specimens were subjectedto an immersion test after.which the,concrete surface was coated with the lining orsubjected to other forms of processing.

3.3 Test Specimens and Tests

Test specimens measuring 10X 10x40 cm were immersed in the sedimentation tankwhile those in the trough measured 5X 10X40 cm. Thinner test specimens were usedfor the trough tests to allow easy installation, to minimize disruption to the flow, andto prevent the loss of the test specimen due to flooding or other reasons. The testspecimens were placed in a stainless steel net and immersed in the sedimentation tank.

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There was a possibility that test specimens immersed in the trough could be washedaway by the water flow. Therefore, they were attached by metal fittings to a stainlesssteel chain stretched in the flow direction of the trough. Test specimens were measuredafter being raised from their location and washed. Three test specimens were preparedfor each factor. Each test specimen was also inspected visually in its immersionlocation (roughness of surface, swelling, peelings and cracks in the lining layer),change in mass and measurement of the resonance number of vibrations (JIS A 1127)by the deflection-vibration method. One test specimen for each factor was subjected toa bending performance test (JIS A 1106).

After the test specimen had been ruptured by the bending performance test, it wassubjected to visual section inspection and measurement of carbonation depth(phenolphthalein method), water absorption rate, pore distribution (mercury intrusionmethod) as well as electron probe microanalysis, X-ray powder diffraction analysis,and X-ray fluorescence analysis. Multiple test specimens were examined in caseswhere the resin lining showed significant swelling and peeling. An overall outline ofthis research is shown in Fig.3.

thirteen kinds of specim enD

0sedim entation; difference¥ tank I 1 ofprocess!_in I oxygen;_____ ｣__ _

i ^^BInunHHndaHdunHnH

visual inspectionroughness of surfaceswelling, peeling of liningexamining re sin with SEM

m ech an ical p erform an cebending performance

ch ange in com position¥

I trough *mm,I in oxygen^

n

ifference of environm en

durabilitydynam ic m odulusof elasticity

carbonation depth

physical changesesti m ati oncorrosion water absorption

D m ass loss pore distribution corrosion m echanismp redi ction

1 troughIin air i

ch em ical changeselectron probe m icroanalysisX-ray pow der diffraction

deterioration

D tt

/

y

nnng

;�ｫ N4o+DDN¥raBaaIS

fluorescence X -ray analysis

start of exp

starting test exposure period

osure test corrosion factors

calculation of free carton dioxide

Fig.3 Outline of study

4. RESULTS AND CONSIDERATION

4.1 Change in Specimens with Exposure Period

a) Visual inspectionAll test specimens without surface lining treatment (conventional concrete) showed achange in color (oxygen process) at an early stage (3 months after immersion), whilethose with a 60 % water-cement ratio immersed in the trough showed a loss of corners.Erosion proceeded gradually. After more than 10 years, the surface was very rough,with exposure of coarse aggregate as shown in Photo 1. For specimens with a cementratio of 50 %, some mortar remained among the coarse aggregate on the surface.However, for specimens with 55 % and 60 % water-cement ratios, the surface mortar

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had almost all eroded away and only coarse aggregates were seen. Even this wasoften lost. Therefore, we concluded that immersion has a stronger erosive effect asthe water-cement ratio increases, with more mortar falling away from the surface anderosion occurring at the interface between coarse aggregate and mortar.

For 3 months after immersion had begun, specimens with lining showed no colorchange or other changes except in one case where swelling of the surface occurred. Asthe exposure period increased, the lining of some test specimens began to swell andpeel. Almost all tar epoxy resin linings peeled from the concrete surface leaving arough concrete finish. With the epoxy resin linings, swelling occurred over almost theentire surface with cracking and peeling of the single-application resin layer (400-600Mm). In cases where the lining had been applied three times, swelling was observedin some specimens. In the case of the mortar lining, long-term immersion caused lossof corners and exposure of the fine aggregate. However, the concrete did not becomeexposed, because the mortar was I cm thick and had a low water-cement ratio. Withspecimens, the flow concrete and the polymer-cement concrete showed the same degreeof roughness on the surface as the conventional concrete. No advantage of the flowconcrete and the polymer-cement concrete in comparison to the conventional concretewas observed visually.

Environmental differences had a greater effect on erosion in the oxygen processingsystem than in the air processing system. The degree of erosion differed with both theprocessing system and the specimen location. The most severe erosion occurred withspecimens placed in the trough of the oxygen process. This is because these testspecimens were influenced by the difference in the content of the immersion solutionas well as the mechanical action of the processing water flow. In all environments,the damage with the mortar lining and the three-layer epoxy lining was less than witha one-layer epoxy lining and a one- or three-layer tar-epoxy lining.

No.3 "" No.6 No.ll

Photo 1 Results of visual inspection (trough in oxygen process)

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b) Corrosion of resinTo examine the resin lining, it was peeled off from test specimens and observed with anelectron microscope. Photo 2 shows representative results. These samples were thethree-layer epoxy and tar-epoxy linings from specimens which had been immersed inthe troughs of the oxygen process for about 13 years. Low-magnification images of thecross section show many bubbles of about 100 fJ>min the epoxy, but few in the tar-epoxylining. However, high-magnification images show the epoxy lining to be almostuniform while the tar-epoxy lining is not. Both types of lining were coated with matterwhich had been floating in the processing water. The high-magnification images of thesurface also revealed that there were almost no faults in the epoxy lining but pinholesof 1~10 Mmover the whole of the tar-epoxy lining. The difference between the twolinings was evident. Examination of the back surface of the lining showed clear tracesof cement hydrates in the case of the epoxy. This indicated that the epoxy resin hadadhered well to the concrete. However, in the case of the tar-epoxy, swelling/peelingwas observed in many places, and the hydrates of cement or the resin itself had eroded.

(1) Low-magnification (2) High-magnification (1) Low-magnification (2) High-magnification

(a) Epoxy resin lining (b) Tar-epoxy resin liningPhoto 2 SEM micrographs of resin lining (trough in oxygen process,

I : section, II: surface, HI: reverse)

c) Nondestructive testingChanges in the relative dynamic modulus of elasticity during the exposure period areshown in Fig.4. In the case of the sedimentation tank in the oxygen process, therelative dynamic modulus of elasticity was high until 1 year in all cases. However,the value decreased for some resin-lining test specimens and for most non-liningspecimens immersed in the trough of oxygen process. Most non-lining specimensaside from the polymer concrete one immersed in the troughs of the air process showedvalues above 100 % with a peak at 3 months. On the other hand, for some resin-

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140

120

3100

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Specimen No-0-7(1) -0- 7(3) -X-8(1) ---X--- 8(3)

-*-9 -I-10 -t-ll

1 60

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ensto msedimentation tankOxygen process

4 6 8 10 12

Exposure period Cyoar)

4 6 8 10 12

Exposureperiod (year)

Lining, flow and polymai-ooretain trough

-0-7(1) -0-- 7(3) -X-8(1) -X- 8(3)

gao.1 60

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Oxygon process^>.

^^5=^Lining, flow ind polymar-aoonorote in aadimantation Unk

Specimen No.-0-7(1) -à"-<>à"à"7(3) -*-8(1) -X-8(3)

-*-9 -I-10 -H-ll

8 10 12

t period (year)

4 6 8 10 12

Exposure period (yaar)E xposure period (ymr)

* Rolative D.M.E. : Rolativs dynamic modulus of elasticity!*)

Fig.4 Change in relative dynamic modulus of elasticity

lining test specimens, the value fell below 100 %. The results for the threeenvironments differed as a result of the variations in processing water quality,mechanical action of water, and the period of hydration of the immersed concrete. Forexample, a comparison of specimens from the sedimentation tank of the oxygen processand the trough of the air process shows that the former had a more detrimental effecton the concrete. However, in the sedimentation tank of the oxygen process almost noerosion due to mechanical action occurs, in contrast with the trough of the air process.Also, the differences in the degree of hydration among the test specimens themselvesled to differences.

Based on these findings, the differences were considered to arise from the environmentand depend on the moisture condition, caused by immersion (non-lining testspecimens), the environment during the drying before lining, and the lining (theresin-lining test specimens). Therefore, test specimens immersed for 1 year wereconsidered to have undergone a complex process including improvement of the physicalproperties due to the progression of hydration, which was then offset by the chemicalaction of the processing water or the physical action of the trough water flow. Therelative dynamic modulus of elasticity of conventional concrete in the trough of theoxygen process decreasedby around 5 % between 1 year and 3 years. However, exceptfor the tar-epoxy lining specimens, most others showed values above 100 %.Differences in relative dynamic modulus of elasticity were also found to arise due tovariations in the unit weight of cement and the water-cement ratio. The rate ofdecrease in relative- dynamic elasticity modulus of the conventional, flow andpolymer-cement concrete in each environment were comparatively large as theexposure period increased, but very small for the resin-lining test specimens.

The conventional, flow and polymer-cement concrete showed decreases in relativedynamic modulus of elasticity of 10-30 % at the end of the exposure period in the airprocess. Some of the lining specimens showed about a 10 % decrease, but mostshowed almost no decrease even after long exposure. Many specimens immersed in

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the oxygen process could not be measured due to the roughness of the specimensurfaces. Thus, it is difficult to compare the effects of the sedimentation tank and thetrough. However, most of the lining specimens in the sedimentation tank indicatedno decrease, except for the 5 % decrease found for the mortar lining. For the troughspecimens, the decrease was about 5~20 %. The value of non-lining specimens in thesedimentation tank decreased about 20 % (flow and polymer-cement concrete)~25%(conventional concrete), and the value of non-lining specimens in the trough decreasedabout 35 % (flow and polymer-cement concrete) and also about 25~40 % (conventionalconcrete). Therefore, the influence of the trough in the oxygen process was greatestamong the three environments. In general, decreases in the relative dynamicmodulus of elasticity of the conventional, flow and polymer-cement concrete specimenswere more obvious than for the lining concrete. Comparison of the environmentsshows that the decrease in the air process was of the same degree or larger than in thesedimentation tank in the oxygen process, and was largest in the trough in the oxygenprocess. The decrease began within a year for the conventional, flow and polymer-cement concrete. With the lining concrete, almost no decrease was noted under theseexposure environments for 10 years or more. This led us to conclude that liningconcrete can offer protection for a long period.

d) Mass changeThe change in mass over the exposure period is shown in Fig.5. Almost all specimensshowed loss of corners and other changes after 3 months of immersion. Nevertheless,their mass increased due to absorption because the initial mass value was the dryvalue before immersion. The specimens were affected by erosion in the immersionliquid, mechanical erosion from the water flow and other causes of mass loss.However, the mass loss was small until 1 year because absorption and hydrationexceeded the influence of erosion. Mass loss of specimens in the air process and in thesedimentation tank of the oxygen process was not observed until 1 year. On the otherhand, all specimens of lining concrete in the trough of the oxygen process haddecreased in mass after 1 year. This result shows that the trough environment in the

C onvolitional oonorato in troughAir process

E xposureperiod (y«r)

-7(1) à"à"-<>" 7(3) -

E xposureperiod (yaar)

C onventional oonorata in troughOxygen process

Speoknan No.-2 -6-3à"

à"5 --A--6

E xposureperiod (yo«r)

Lining, flow ists in trough

-0-7(1) à"--<>à"à"7(3) -X-8(0 à"à"-X---8(3)

-*-9 -I-10 ---t---1l

4 6 8 10 12Exposure period (yo.r)

14

E xposureperiod (year)

E xposureperiod (yair)

Fig.5 Changes in mass in each system

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oxygen process system is severer than the other environments. Until the third year,the non-lining specimen immersed in the air process decreased in mass about 3 %, andthere was a 3~7 % mass loss after that period. Ahigher unit weight of cement gavesome protection against mass loss. The mass loss of specimens with a water-cementratio of 50 % was less than that of 55 % or 60 % specimens. Therefore, in non-liningconcrete, low water-cement ratio and high unit-cement weight was useful for corrosionresistance. Although mass loss was not observed for most lining specimens, themortar lining specimen showed a few percent mass loss. The flow and polymer-cementconcrete was in the loss range of conventional concrete and decreased by about thesame degree.

The ultimate mass loss at the end of the exposure period was as follows. In the airprocess, non-lining concrete specimens decreased by 6~10 % and the lining ones 0~~2 % (resin) and 4 % (mortar). Also, in the sedimentation tank of the oxygen process,the mass losses of non-lining ones were 6~8 % and those of lining ones were 0 %(resin)~3.5 % (mortar). Furthermore, in the trough of the oxygen process, the massloss of the non-lining specimens was 9~16 % and that of the lining ones was 0~5 %(resin) and 8 % (mortar). The relative surface area of the test specimens in the troughof the oxygen process was large, about 1.4 times that in the sedimentation tank of theoxygen process. Even if this is considered, the mass loss of the non-lining specimensin the tough of the oxygen process was the largest. There was no obvious difference inmass loss with unit weight of cement, though there was influence with the water-cement ratio in mass loss. These results show the need for a low water-cement ratioand a high unit weight of cement to ensure concrete durability. The mass loss ofnon-lining specimens in the trough of the oxygen process was 15 % or more, and themass loss of the lining specimens was near that of the non-lining specimens in the airprocess and in the sedimentation tank of the oxygen process. From these masschange results, again the trough of the oxygen process is shown to have the severestenvironment.

4.2 Physical and Chemical Properties

a) Bending strengthBending strength tests were done in conformity with JIS A 1106. The bending sectionof the test specimens in the trough had a width of 10 cm and a bending height of 5 cm.The bending test specimens were divided into three equal 30-cm spans at loading.Measurements were done for one conventional, flow and polymer-cement concretespecimen without the resin lining and mortar lining for each factor. For the liningconcrete specimens in the trough of the air process and the oxygen process, allspecimens with a single epoxy coating and a single or triple tar-epoxy coating showedobvious swelling/peeling on the surface, and all the lining specimens except those witha mortar lining in the sedimentation tank of the oxygen process were measuredindividually.

Loading in the bending test, was done in the casting direction. Because the erosion ofthe test specimen surface was severe, accurate comparison of cross sectionmeasurements could not be done. Instead of bending strength, the maximum loadresults were compared. These results are shown in Fig.6. As noted earlier, the section

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K g liM Jm Environm entx 800 D Trough (air process)

S 600o? 400T35 200_QI oI sp

H Trough (oxygen process)Q Sedim entation tank (oxygen process)

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-Trough (air process) -Trough (oxygen process) - A - Sedim entation tank (oxygen process)

Specimen No.1 2 3 4 5 6 7(1) 7(3) 8(1) 8(3) 9 10 ll '

Fig.7 Effective section height by estimating maximumbending load

height of the test specimens in the sedimentation tank of the oxygen process was twicethat of the test specimens in other environments. Considering this, the measuredvalues of bending strength for test specimens in the sedimentation tank of the oxygenprocess would be 4 times greater. Therefore, these maximumloads are multiplied bya factor of 1/4 in Fig.6. Since most of the measured values are from a single testspecimen only, a considerable variance is conceivable. However, as a whole, themaximumload determined from bending tests on test specimens in the trough of theoxygen process is lower than that in the trough of the air process. Therefore, theresults clearly show the differences between the influence of the environment in thetrough of the air and oxygen process. Thus, these test results suggest that a testspecimen in a particular environment is influenced by the unit cement weight, water-cement ratio, and lining on the surface. Therefore, durability increases with a highunit cement weight, low water-cement ratio, and a lining. All the test specimens weresubjected to destruction within the bending span in the bending test. The maximumbending load depend on mix proportion factors and the system environment.

The effective section was estimated using the calculation formula fb = PL/bd2 for thebending strength based on the compression test results at the start of exposure, whereis P is the maximumload on the test specimen, L is the span, b is the width offailedsection, and d is the height of the failed section. Estimation of the effective sectionwas based on the following relationship. The specimen exposed in the sedimentationtank of the oxygen process showed the relation ofd ^ b and also the relation ofd =b/2 in the trough of the oxygen and air process. The fb used was 1/7 of the value of the28-day compressive strength at the time the test specimen was cast. The value of destimated using this assumption is shown in Fig.7. Comparison of the estimatedvalues showed the effective section of the non-lining test specimens to decrease morethan the epoxy resin lining ones. Also, looking at the difference in environments, theeffective section of the oxygen process specimens decreased more than that of the airprocess specimens. If the effective section of the resin lining specimen estimated tobe larger than the standard value before immersion was taken as equal to the standardvalue, the ratio of the effective section of each test specimen would be as shown inFig.8. However, the estimated effective section was calculated from the standardvalue of the bending strength of the concrete based on the compressive strength for the

64-

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Specimen No. 6 7(1) 7(3) 8(1) 8(3) 9 10 ll

Fig.8 Relative height ratio (effective height ofresin lining concrete: 1)

Fig.

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P T ro u gh ( a ir p r o c e s s )

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Carbonation depth (mm)

10 Relationship betweencarbonation depth andnon-effective depth

SpecimenNo.1 2 3 4 5 6 7(1) 7(3) 8(1) 8(3)

Fig.9 Mean value ofcarbonation depth

10 ll Fig. ll Geometric section ofspecimen at cutting

compressive test specimen cured for 28 days. Therefore, the estimated effectivesection may have been too large. Also, the d value of the lining test specimens showsa larger value in Fig.7 than the standard value, may have been possible the influenceby the lining. Further consideration of this influence is needed.

b) CarbonationA 1% of phenolphthalein solution was sprayed on the section tested for bendingperformance and the depth of color change was observed. The specimen was observedto have three regions from the outside to inward. The outside periphery ofa few to 10mmwasa faint brown, the next about 1 mmwas white, and the innermost section wasgray. Spraying with phenolphthalein, the innermost gray portion changed to redwhile the outer brown and white sections did not change. Therefore, the outer brownand white regions had undergone carbonation while the inner section had not. Theinner-gray region may not have been chemically affected by erosion. It was difficultto judge whether the white region between the outermost and innermost regions wasdue to direct chemical action of the immersion liquid or the secondary action ofequilibrium reactions maintaining the chemical balance between outside and inside.The carbonation depth can be compared if the specimen is not damaged by chipping.However, the degree of erosion of the cross section differs with test specimen and it isdifficult to compare carbonation depths. Carbonation depth was estimated to saveextent from the initial face of specimen before immersion by assuming that even thepart eroded and washed away passes through the process of carbonation ->à"dissolution ->à"outflow. All conventional, flow and polymer-cement concrete specimensshowed carbonation depths of about 4 mmto 10 mm,while most the lining specimensshowed no carbonation, as can be seen from Fig.9. Also, differences were apparentaccording to the type of concrete and the environment. Therefore, the non-effectivedepth was taken to be half the height of the section (the standard section) beforecorrosion minus the height of the estimated effective section in Fig.7. Comparing thenon-effective depth and carbonation depth showed the latter to be larger, as can beseen in Fig.10. Thus, the part of carbonation region contributes to the mechanicalperformance. However, as mentioned above, if the effective section of the resin liningspecimen is assumed to be the basic effective section, the section of each non-liningtest specimen decreases, and non-effective depth approaches the carbonation depth.

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c) Water absorption and pore distributionThe test specimen used for the bending performance test were examined for absorptionand pore distribution characteristics. The test specimens were those from the troughin the oxygen process and which were thought to have been exposed to the most severecorrosion. To eliminate influence from the edge face, the specimen was cut about 100mmalong the long axis, and about 50 mmalong the short axis and at about 10 mmsections from the surface to the depth shown in Fig.ll. The specimens were thendivided into samples for pore distribution, chemical analysis, and absorption studies.Before measurement, all samples were dried in a vacuum and then immediatelysubjected to measurements.

Water absorption depends on the physical properties of the hardening substance andthe absorption time. However, it was assumed that water absorption does not dependon time and that the moisture distribution gradient within the sample was zero in thiscase because of its thinness. Fig.12 shows the relation between water absorption rateand time. This is compared with absorption over 24 hours as the standard value (thefinal value). Water absorption in the early period approaches the final value. Thistendency is especially evident at the surface region of sample. In most samples, thewater absorption rate near the surface at 10 minutes was about 7/10~8/10 of thestandard value. On the other hand, when there was a resin lining (except for thesingle coating with tar-epoxy), the interior samples had a the 10-minute absorption ofabout 3/10~5/10 of the standard value. Therefore, the physical properties of thesurface without a lining differ from those of the interior.The water that comes into contact with concrete reaches close to saturation in thepores near the surface. Water penetration can be thought to advance inside viasmaller pores. The pore distribution near the surface can be considered to change asthe pore size increases through contact with the eroding liquid. Therefore, the poredistribution structure near the surface changes from the original pore distribution,which is depend on the water- cement ratio and the unit weight of cement.

0 .2

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Absorption period (min)500 1 000 1500 2000

Absorption period (min)

N o.7(1) - O- No.7(3)N o.8(1) - A- No.8(3)

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Absorption period (min)2000

( a) Surface region (b) InsideFig.12 Changes in water absorption

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Pore size of 6 nm to 200 Mm was measured by the mercury intrusion method. Thetotal pore volume of the surface region and the interior is shown in Fig.13 (a) and theratio of each to the total pore volume in Fig.13 (b) (surface) and (c) (interior). Thetotal pore volume of the surface region is larger than that of the interior. With theexception of samples with a resin lining, the difference in total pore volume betweenthe surface region and interior is large. These total pore volumes differ with mixproportion factors, unit weight of cement and water-cement ratio. The total surfacepore volume of the mortar lining sample is larger than that of other samples becausethe surface mortar region contains many fine aggregate particles with a large relativesurface area. Therefore, the inter facial zone attacked by the eroding liquid is greaterin the mortar than in the concrete [13]. In the resin lining samples, the difference intotal pore volume between the surface region and the interior may be caused by the drycuring carried out prior to lining.

0 .2

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(a) T otal po re volum e

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Specimen No. 1 6 7(1)7(3)8(1)8(3) 9 10 ll

IV U5ｫI s 8 0o-+-> 6 0

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Specimon No. 1 10 ll

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W m6n-50nm n50n-100nm n100n-2um 02u-10um D10urn-

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Fig.13 Total pore volume and pore volume ratio

0.05 0.1 0.1 5 0.2

Absorption (24 hr)

Relationship betweenwater absorption andpore volume

C omparing the various pore volumes in the surface region shows that pores of 2 Mm~10 Mmare present in conventional concrete, although they were thought not to exist.The surface region is thought to be more influenced by corrosion because pores of2 Mm~10 Mmare rare in interior. The pore distribution was divided into small (below 20nm~30 nm) and large (over 20 nm~30 nm). Fig.14 (a) shows a correlation betweenabsorption over 10 minutes and the absorption over 24 hours as well as between theratio of pore volume over 30 nm to total pore volume. There was also a correlationbetween the ratio of absorption over 24 hours and the total pore volume (Fig.14 (b)).In the early period, absorption is thought to be influenced largely by the capillarypores, including those in the inter facial zone between aggregate and cement paste.Also, although the final absorption ratio exceeded the total pore volume, thisdifference was large for interior of the specimens. This result is mainly influenced bygel pores less than 6 nm and shows that the gel pores of interior were not attacked bythe eroding liquid.

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d) Electron probe microanalysis (EPMA)The surface of a vertical section taken toward the axis of the sample were subjected toelemental analysis. The main measurement conditions were as follows: accelerationvoltage, 15 kV; absorption current, 2.5X10"7 A; and probe diameter, 10~200 Mm.Point analysis of neighboring corroded parts was also done. Fig.15 shows about a20-mmwidth section from the middle of a cross section subjected to the bending test(Photo 3) with a probe diameter of200 Mm(Fig.15 (a)). The elements in the corrodedpart included carbon, calcium, silicon, and even sulfur. There were many cases inwhich the carbon was concentrated mainly at the corroded surface and decreasedtoward the inside, while there was little calcium at the surface but much further inside.These results show that carbon (main corrosion factor) and calcium (main componentof cement) show concentration gradients which change with specimen depth.No calcium concentration gradient was observed in the sample with the resin lining,though gradients including one for calcium were confirmed in conventional, flow andpolymer-cement concrete. Thus, the coating with resin lining appears to protectagainst or reduce the effect of the erosion factors at the surface. Therefore, thepossibility is great that erosive carbon dioxide originates from carbon, although sulfurcan also be suspected.

tec. vol. 13.0 BcVl

<) W-rf TAP3 CH-1Oft) CH-2 PET1J OM »BTE

0 ' i' ] i » i 5 "

Photo 3 EPMAarea

?tu^?S?4>gt2£y£?1««£ i&»ii"& fjf

à"*«?*'*TOW w*?*;-\.**ie£

i -wt- r+!smp--sv - *f**t

serr. |-| S.MO à"'

M 85.000 E3.WO

Ca Si (b) Point analysis (the arrow(a) Surface analysis point in Ca, result in (a) )

Fig.15 Typical results ofEPMA (No.5 sample in trough of oxygen process)

e) Fluorescence X-ray and powder X-ray diffractionEach sample described in 4.2 c) was analyzed by X-ray fluorescence and diffraction atthe three points: the softening part, the surface, and the inside. Each sample wasprepared by removing the coarse and fine aggregates with a sieve. The measurementconditions for X-ray fluorescence analysis were as follows: anticathode, Rh; appliedvoltage-current, 50 kV-50 mA; analyzer crystal, RX-60, TAP, GE, LiF; and detector, PC,SC. Also, the powder samples of constant mass were molded into thin disks of less

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than 1 mm to decrease measurement errors by Compton scattering. On the otherhand, the measurement conditions for X-ray powder diffraction were: target, Cu-Ka ;applied voltage-current, 40 kV-80 mA; and slit system, 1 degree-1 degree -0.15 mm.Standard samples were several types of Portland cement of NBS (National Bureau ofStandards) and Standard Reference Material.

The principal component elements of cement and carbon were analyzed by X-rayfluorescence. However, the concrete-attaching carbon was judged to be difficult toobtain with good precision in comparison with other elements because it is a lightelement and gave 10 or fewer measuring counts even in the softening part and surfaceportion. Also, every effort was made to prevent contamination of the test specimeneven by fine aggregate, but the major elements were not considered and the focus wasdirected mainly at calcium and magnesium. Fig.16 shows the count ratios of calciumor magnesium in the softening part, surface, and inside of each sample. The countratio of calcium is about 0.8 to 1.0 in the inside, although the ratio fluctuates. Theratio in the softening part was shown to be about 0.2 irrespective of mix proportionfactors, although this value is very low in comparison with the inside. The ratio forconventional, flow and polymer-cement concrete is lower than that for lining concrete,although the ratio of the lining series is almost 1.0, and this result shows acharacteristic different from the softening part and the inside. This resultdemonstrates that calcium decreased due to dissolution and ionization in the surfaceregion in comparison with the inside, where there is much calcium hydrate from thecement, and shows that little calcium is present in the softening part. The lack ofcalcium may be due to its dissolving out of the sample. This agrees with theconcentration gradient of calcium determined from surface analysis. On the otherhand, for magnesium, the relative concentration is large inside each sample comparedwith the surface and the softening part. The difference between the amount ofcalcium and magnesium may arise due to the calcium concentration before corrosionbeing greater than that of magnesium or due to the magnesium not dissolving becauseof its higher solubility product. The sulfuric acid, nitric acid, carbonic acid, and othercomponents suggested as corrosion factors are present in sewage treatment water.

3 '"6 0.8

( a) Ca.-A

A - "A- -A .A. "A "A

7(1 ) 7(3) 8(1 )

Specimen No.

teaimentation Trou^tank(Oxygen),.. %., ,-

«�ð (Air)No.5

*14212

(b) Mg

- - -A - 一�" S o ft e n i n g p a r t - O - - S u r f a c e - O - n n e r

7(1 ) 7(3} 8(1 )

Specimen No.

8(3)tank (Oxygen)(Air) No.5

Fig.16 Count ratio (No.7(3) specimen count: 1) by fluorescence X-ray analysis

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Acid-attacking hydration products of cement, such as calcium hydroxide, ettringite,and calcium silicate hydrate, react with acid and decompose, although the extent of thereaction differs with the acid. Calcium hydroxide was not found in the softened partand the surface of samples without lining, and even in the lining samples the amountof calcium hydroxide was small. On the other hand, calcium hydroxide clearly existedin the interior, which remained in better condition as it was not affected as much bycorrosion in comparison with the surface. Although sulfur was detected at the surfaceby surface analysis, no gypsum was found in the softening part and at the surface byX-ray diffraction [14]. Therefore, it is likely that almost no corrosion due to sulfuricacid occurred. Identification of the components was done by referring to a report on asimilar deterioration case [6]. The main peak of calcium hydroxide and calciumcarbonate and the peaks of the quartz and feldspar groups included in the aggregateand other materials were identified. Magnesium compounds were also identified butat a very low level in comparison with the other components. .However, they could notbe ignored in forming a hypothesis. The results are presented in Fig.17. Theheadings used in the figure are those given by JCPDS (Joint Committee on PowderDiffraction Standards) and thus some may not have been present in the actualsamples.

Aggregate componentM: mica , Q: quartz, FG: feldspar group

g Cement hydrate or reaction substances30CSH: CaO-SiCh- HaO type hydrateP: calsium hydroxide, Ett: ettringiteC aASoH: monosulfateL I I

30

&20

TankTrough A

0

i) o u r ia utj r tjg iuu ｣ O C SH :P: calC aA SC aCC

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ok n. o

nn

H HH M fis ^^ ^^ S

n~ Sd _-A J _- 3_

m s m mH -¥ir_ 5 I J .

20

26C)60

Fig.17 Results of powder X-ray diffraction

5 . EXPLAINING THE CORROSION MECHANISMANDPREDICTING DETERIORATION

5.1 Explaining the Corrosion Mechanism

The main factor causing corrosion from the outside toward the inside of the testspecimen is, according to the results of this study, erosive carbon. The erosive carbonpresent in sewage process water includes free carbonic acid which is released onreaction with the calcium ions in concrete. Quantitative analysis of the interactionbetween this carbonic acid and cement compounds or hydration products is difficultdue to the effects of changing water quality, water flow, and the other factors relatedsewage process water. However, in general, the erosion mechanism due to freecarbonic acid is thought to be as follows. Sewage process water quickly penetrates intothe concrete, with the degree of penetration and the resulting corrosion depending on

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the composition of the concrete. The erosive free carbonic acid links with calcium ionsat the surface or penetrates into the concrete through the inter facial zone betweenaggregate particles and cement. As it penetrates, it reacts with hydration products,such as calcium hydroxide, deposited at the and forms calcium carbonate.By penetrating into capillary pores, it reacts with hydration products or unhydratedcement compounds. In this process, carbonation occurs and causes corrosion of theconcrete, which is then affected by water flow and other conditions. The degree ofcorrosion differs with water-cement ratio and unit cement weight, because the reactionrate of the penetrating process water is influenced by pore volume and reaction areaand is controlled by the pore structure, including interface area. For instance, even ifthe total pore volume is equal, differences can occur in the area of the wall that thepore forms due to the pore diameter. The total pore surface area becomes large ifthere are many micropores. Therefore, with a constant process water quality, thereaction is greater for concrete with small water-cement ratio. The reaction ratedecreases rapidly toward the inside of the sample. This decrease also becomes moresignificant as the total reaction area increases.

When there is much free erosive carbonic acid in the sewage process water, calciumcarbonate is the main product at the reaction site. Although the pores becometemporarily less porous, calcium carbonate dissolves on attack by free erosive carbonicacid. The calcium ionized in the process water again becomes linked with free erosivecarbonic acid. As a result, the calcium dissolves andis consumed. In such areactionprocess, there is a boundary phase of alkali type compounds such as calcium ormagnesium compounds between the corroded portion and the non-corroded portion,and this can be observed from the discoloration described in section 4.1 b). This typeof reaction is repeated near the surface of the eroding concrete. The concentrationgradient of calcium ions between the surface and the inside was confirmed even bysurface analysis in section 4.2 d). This mechanism shows that calcium ions move tothe surface due to the concentration gradient and may cause gradual changes in thestructure of the intact portion. Thus, the following may occur : secondary corrosion-»erosion-^deterioration of structure further inside.

5.2 Predicting Deterioration

Deterioration of the system under consideration is mainly controlled by the corrosionrate of concrete. However, a decline in the performance of reinforced concrete mayoccur if corrosion occurs in the concrete cover and the reinforcing rods. Therefore,corrosion may invade more deeply than the index indicates. Thus, the progress ofcarbonation and decrease of cover concrete become the most important indices ofdeterioration. We have reported that the existence of free erosive carbonic aciddiffers over the years with fluctuations in alkalinity and other factors of the processwater [7],[15]. As described in Section 2, the free carbonic acid in the process watercan be divided into subordinative and erosive carbon dioxide, with both consideredrelated to alkalinity [16].

Calculating the free carbon dioxide using a previously reported method [17] from thepH and alkalinity values, which have been regular test items in water quality testsover the past 20 years, the change in free carbon dioxide over time at the facility is

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shown in Fig.18. For the calculation, the mean value for one year was adopted, withmeans found for each month. Also, the relation between the free carbon dioxide anderosive free carbon dioxide was calculated and is shown in Fig.19, which presents thecorrelation between erosive free carbon dioxide and free carbon dioxide. The increasein free carbon dioxide suggests the possibility of an increase in the amounts of erosivefree carbon dioxide.

50

40

"x 30

20

3 10

it 0

^50D.

^40CD

à"D

'S 30

o 20-e

10

- -A- -Inflow

S edimentationProcessing

60

50

40

à"= 30

10 12 14

Period (year)16 18 20 22 -g

g 20>fc

« 10

0 20 40 60 80

Free carbonic acid (C02 ppm)

16 18

Fig.

10 .12 14

Period (year)

18 Changes in free C02 in each system

20Trier IP RplfltinnQViin hptwppn

22 «= à" Jf

free CO2 anderosive free C02

Comparison of the free carbon dioxide in each system shown in Fig.18 indicates littlefluctuation among locations, such as the inflow, processing, and sedimentationprocesses, in the air process. However, with the oxygen treatment process, with someexceptions, there was a large amount offree carbon dioxide in the process water. Thisshows that a large amount of carbon dioxide is discharged when organic substances aredecomposed by bacterial activity with aeration. Also, the mean values for the first10 years slightly exceed those for the latter 10 years. In water supply facility design,there is a general possibility of a large amount of erosive free carbon dioxide beingpresent when the free carbon dioxide level exceeds 20 mg/1. Although aerationprocessing or alkali processing has been recommended previously [11], the calculatedfree carbon dioxide value exceeds 20 mg/1. According to previous work, severe erosionmayoccur when the level of erosive free carbon dioxide reaches 60 mg/1 or more [18].This value does not exceed the calculated value of the erosive free carbon dioxide.However, even if each value is below 60 mg/1, the degree of the erosion may be high ifthe hardness of the water is low. Fig.20 shows the relation between the calculatedfree carbonic acid in the process water and the mean value of the mass loss ofconventional concrete in the trough of the oxygen process. The value of free carbondioxide was calculated from values accumulated until the year prior to massmeasurement. The mass loss shows close correlation to the accumulated value offreecarbon dioxide. Therefore, the mass loss of conventional concrete is considered to beinfluenced by the free carbon dioxide. The relation between the mass loss of the testspecimen in the trough environment and the accumulated value of free carbonic acidover one year, shows the state on the same regression curve in all values of the air and

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20

s

° 10

O Trough (air)

à" Trough (oxygen)

O Tank(oxygen)

20

15

10

0 100 200 300 ' 400'

Accumulated value of carbon dioxide (ppm-year)

Fig.20 Relationship betweenfree C02 and erosive C02

Regression curve-Y=0.80l V~X-3.433

Y=0.946V X-2.906à" Y=1.131V~X-4.386à"Y=0.695V X-3.207à" Y=0.864V X-2.343

Y=1.031 V~X-4.963

FT0.9730.971

0.9710.9650.9810.980

Conventional concrete in trough(Oxygen process)

0 1 00 200 300 400

X: Accumulated value of carbon dioxide (pprrryear)

Fig.21 Prediction (regression curve accumulatedC02 against mass loss)

oxygen process. In the test specimen in the sedimentation tank of the oxygen process,a different tendency was observed in the regression curve than for the result in thetrough of the oxygen process in the same free carbon dioxide environment. Theseresults show that the erosion rate is affected not only by the water flow but also by thewater quality in the processing system environment, and that the action of the flow inthe trough promotes physical erosion in addition to chemical erosion. Fig.21 showsthe regression curve applied to the relation between the accumulated value of freecarbon dioxide and the mass loss of conventional concrete in the trough of the oxygenprocess. Mass loss is related to the square root of the accumulated value of freecarbon dioxide as follows:

Y =aV~ X+jS (1)

where, Y: mass loss (%); X: mean value of accumulated free carbon dioxide until theyear prior to mass measurement (ppmà"year); a , j8 : regression coefficient andconstant of regression curve.Each regression curve based on expression (1) showed extremely good correlation.As described above in predicting deterioration, the most important factors are massloss, which is an estimate of the decrease in the amount of cover concrete, and theprogression of the carbonation depth, or erosion depth, which are closely related tomass loss. The relations between mass loss, carbonation depth, and erosion depth inthe trough are shown in Fig.22. Variations were observed in the linear regressionresults but the mass loss correlated with the depth ofcarbonation and erosion. Thus,carbonation depth and erosion depth can be expressed in terms of the annualaccumulated value of free carbon dioxide in a similar way to expression (1).

Y '=a'/~X+j3' (2)

where, Y': carbonation depth or erosion depth (mm); X: annual accumulated freecarbon dioxide (ppm-year); a', j8 ': coefficient and constant.The annual variation was predicted from the variation in free carbon dioxide over thepast 20 years in Fig.18. Little fluctuation in the level of free carbon dioxide isexpected over the long term. About 30 ppm deterioration can be expected annually asthe mean value from the movement average over time. For example, the largest mass

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14

.2 2

cbo "�"-O

o CDo o

o o

cPo o Y = 0 .25

(R 7: 0.83 3X33 4 )

10 15

X: Mass loss (X)

(a) Relationship between massloss and carbonation depth

20 5 10 15

X: Mass loss (X)

20

(b) Relationship between massloss and erosion depth

Fig.22 Relationship between mass loss due to carbonation depth or erosion

loss of 20-25 % over 20 years is predicted for concrete in the trough of the oxygenprocess, which is the severest environment. Calculating the test specimen area basedon a mass decrease of20% gives an erosion depth of about 3 mm. On the other hand,an erosion depth of about 5 mm is obtained from the relation with mass loss based onexpression (2). The difference in erosion depth is nearly twofold. Changes in erosiondepth or carbonation depth might be estimated from measured mass loss data andcalculated free carbon dioxide. This method can be used to predict deterioration andsuitable repair times. However, some structures may require repair earlier thanpredicted due to invasion by corrosion greater than the deterioration index and forother reasons. Further detailed studies are needed, because the corrosion anddeterioration of concrete is influenced by the water quality, flow, and other changes inthe environment. Continuous control of water quality is necessary based onconsideration of bacterial and protozoa activity, the water flow, and other factors.

6 . CONCLUSION

The following conclusions were reached from the results of exposure tests conductedover 10 years in the end processing system of a sewage treatment facility.

(1) Changes in appearance, decreases in mass, and changes in relative dynamicmodulus of elasticity were observed with exposure time for various test specimens.Differences in environment led to variations in the degree of corrosion, with the mostsevere suffered by specimens in the trough of the oxygen system. Looking at mixproportion factors in the case of non-lining concrete, the mass loss at a water-cementratio of55 % and 60 % was larger than thatat 50 %. Therefore, a low water-cementratio is one effective way to control corrosion. Also effective is to increase the unitcement weight. Still another is treating the concrete surface with a lining. Almostno corrosion was observed for test specimens with epoxy-resin lining even after morethan 10 years.

(2) The decrease in maximumbending load was large after exposure for more than 10

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years, with particularly serious decreases in specimens with high water-cement ratiosand without lining in the trough of the oxygen treatment process. Therefore, even thebending strength decreased in the case of high water-cement ratio or without lining.In contrast, corrosion can be prevented by adapting a high unit cement weight and lowwater-cement ratio, and also by lining the concrete.

(3) The decline of the physical properties of the concrete as seen in the pore structurewas obvious in the surface region. Therefore, sewage process water was judged tohave a great influence on the pore structure. Change in the pore structure clearlydiffered with or without lining. Therefore, the use of a lining gives effectiveprotection against long-term attack by process water, as in this facility.

(4) In this processing system, corrosion is considered to have been mainly caused byerosive free carbonic acid. The carbonates decompose cement hydration products ofconcrete and finally reduce them to ions. The structure is gradually destroyedthrough the mechanism of ion dissolution.

(5) The mass loss, carbonation depth, and erosion depth were judged to be related tochanges in the amount of free carbon dioxide, based on calculations offree and erosivefree carbon dioxide and various experimental results. Applying a regression curve tothe above relation showed a good correlation, especially between the free carbondioxide and mass loss. This allows the deterioration to be estimated and the repairtime predicted. Control of the water quality can reduce the progress of deterioration,which is influenced by changes in bacterial and protozoa activity or water flow andother factors.

Acknowledgements

Wewould like to thank the manager of the sewage treatment facility, who allowed us toconduct this study at that facility and offered us long-term support. Also, weappreciate the support of the our colleagues who helped us in carrying out thisresearch.

References

[l]Nakamoto, I. "Study on the Chemical Corrosion of Concrete Structure in SewerageFacilities, " Journal of Materials, Concrete Structures and Pavemen ts, JSCE, No. 472/V-20,pp.1-ll,1993 (in Japanese)[2]Mishina, F. "Study on the Formation and Control of Hydrogen Sulfide in SewerageFacilities," Tohoku University Doctoral Thesis, 1990 (in Japanese)[3]Morinaga,T., Kawai.K., Teranishi,S., and Douzono,A. "Effect of Microorganisms on theDeterioration of Concrete," ExtendedAbstracts, 46th Annual Meeting of CAJ, pp.568-573,1992 (in Japanese)[4]Tanaka, S., Kitagawa, M., Fukatani, W., and Ochi, T. "Rational Preventive Measures forCorrosion of Sewerage Facilities," Civil Engineering Journal, Vol.37, No.10, pp. 42-47,1995 (in Japanese)[5]Kinoshita, I. "Improvement for the durability of facilities," Gekkan Gesuido (MonthlySewerage), vol.19, No. 5, pp. 7-ll,1996 (in Japanese).

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[6]Tazawa, E., Kaneko, S., Sakamoto, M., and Yasu, S." Concrete Surface Corrosion inOxygen Aeration Activated Sludge Process Facility," Semento Gijyutsu Nenpo (CementTechnical Annual Reports of the CAJ), 37, pp. 374-377, 1983 (in Japanese)[7]0kada, K., Miyagawa, T., and Yoshimura, K. "Concrete Corrosion in Oxygen ActivatedSludge System," Proceedings of the JCI, Vol.6, pp. 229-232, 1984 (in Japanese)[8]Kawai, K., Morinaga, T., and Tazawa, E. "Effect of Metabolite Content of AerobicMicroorganism on Concrete Deterioration," CAJProceedings of Cement and Concrete, No.49, pp.704-709, 1995 (in Japanese)[9]Sudo, R. "Control of Biological Treatment II," Mizu (Water),Vol. 37-2 (No. 517), pp. 10-40,pp. 61-108, 1995 (in Japanese)[10]Araki, S., et al., Environment Science Dictionary, Kagaku Dojin (Chemistry Member),1991 (in Japanese)[11]Japan Society of Water Supply, Guide and Note for Water Supply Facility Design, pp.311-313, 1990 (in Japanese)[12]0kada, K. "Examination of Concrete Wall Corrosion in Oxygen Aeration ActivatedSludge Process Facility (Part 1)," The Society of Materials Science, Japan, 1981 (inJapanese)[13]Kawahigashi,T., Shingu,Y., Miyagawa,T., Hattori, A., Inoue,S., and Fujii, M. "Influence

of Inter facial Zone between Aggregate and Cement on Mass Transport Properties ofConcrete," CAJ Proceedings of Cement and Concrete, No. 48, pp.198-203, 1994 (inJapanese).[14]Kawahigashi,T., Tamano,!., Inoue,S., Miyagawa,T, and Fujii, M. "Deterioration ofConcrete Caused by a Combination of Salt/Acid," Abstracts of the 47th JSCEAnnualMeeting, Section V, pp. 334-335, 1992 (in Japanese).[15]Kawahigashi,T., Inoue,S., Miyagawa,T, Fujii, M., Nagaoka, S., and Kobayashi, S."Study on Concrete Corrosion in a Sewage Treatment Facility," CAJProc. of Cement andConcrete, No. 47, pp.492-497, 1993 (in Japanese)[16]Bureau of Water Supply Environment, Department of Life Sanitation, Ministry ofHealth and Welfare, Waterworks Test Method, pp. 222-223, 1985 (in Japanese).[17]Bureau of Water Supply Environment, Department of Life Sanitation, Ministry ofHealth and Welfare, Waterworks Test Method, pp. 149-153, 1970 (in Japanese).[18]Calleja, J. "Durability," 7th International Congress on the Chemistry of Cement, Vol. 1,VH-2/l~Vtt-2/48I 1980

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