Corrosion Science 70 (2013) 119–126

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Corrosion Science

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Corrosion behavior of reinforced no-fines concrete

Francesca Tittarelli a,c,⇑, Maddalena Carsana b,c, Tiziano Bellezze a,c

a Department of Scienze e Ingegneria della Materia, dell’Ambiente ed Urbanistica (SIMAU), Università Politecnica delle Marche, Via Brecce Bianche 1, 60131 Ancona, Italyb Department of Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’ (CMIC), Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italyc INSTM, Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via Giusti 9, 50121 Florence, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 June 2012Accepted 20 January 2013Available online 29 January 2013

Keywords:A. ConcreteA. ZincB. PolarizationC. Atmospheric corrosionC. Passivity

0010-938X/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.corsci.2013.01.020

⇑ Corresponding author at: Department of Scienzedell’Ambiente ed Urbanistica (SIMAU), Università PBrecce Bianche 1, 60131 Ancona, Italy. Tel.: +39 02204729.

E-mail address: f.tittarelli@univpm.it (F. Tittarelli)

The CO2 induced corrosion behavior of no-fines concrete manufactured with three different strength clas-ses and reinforcements is compared. The main results showed that black steel corrodes with rates threetimes higher with respect to those monitored in the other reinforcements, with higher corrosion rates inlower strength class concretes. The corrosion rates of steel covered by cement grout and galvanized rein-forcements are not affected by concrete strength class since they protect themselves with the cementgrout coating or zinc passivation, respectively. Among the reinforcements considered in this work, galva-nized steel shows the best corrosion behavior.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction [6–8]. The aim of this work is also to verify the possibility to use gal-

No-fines concrete is porous concrete obtained when fine aggre-gate is omitted [1]. It is currently used in the United States for roadpaving [2,3] under the name of ‘‘pervious concrete’’. However, duethe recent emphasis on sustainability issues in the construction,thanks to the low cement content, the acoustic and/or thermal insu-lation characteristics and water permeability, no-fines concretecould be also considered in a wide range of vertical applications[4,5] where high structural performance is not required, as render-ing mortars, cladding or acoustic panels. The great porosity pro-motes also a great CO2 diffusion inside the conglomerate where itcan be sequestered by carbonation. At present, the only limitationto broaden the potential applications of no-fines concrete is theimpossibility of reinforcing with black steel, when a slight rein-forcement is needed, since in carbonated concrete reinforcing steelloses its passivity and becomes vulnerable to corrosion. In a study itwas shown that no-fines concrete quickly carbonated but corrosionof embedded steel may be low if wetting is prevented [6]. Neverthe-less, the study of the corrosion behavior of steel in moist conditionneeds to be studied in more details evaluating also the use of addi-tional protections. In particular, the role of the application of a thinlayer of cement grout to the steel surface, suggested in the litera-ture [1], has to be verified because the fast rate of carbonation ofthis thin protective layer may soon promote de-passivation of steel

ll rights reserved.

e Ingegneria della Materia,olitecnica delle Marche, Via71 2204732; fax: +39 071

.

vanized steel reinforcements since it has already been experi-mented that their corrosion behavior improves by decreasing thealkalinity of cementitious material as in carbonated cement grout[9]. Moreover, the great porosity of no-fines concrete allows greatdiffusion of atmospheric gases such as CO2 and O2 which react withzinc coating to form zinc carbonate and calcium-hydroxy-zincate,respectively, that are passive films able to preserve zinc from fur-ther corrosion [10,11]. Since in ordinary concretes galvanized steelbehave better in the presence of an hydrophobic admixture [12,13]and, when carbonation reaches steel bars, the corrosion rate is con-trolled by the electrical resistivity of concrete, which depends alsoon its moisture content [6,14], hydrophobic treatment in mass ofthe no-fines concrete was also considered [15,16].

In this work, three different no-fines concrete mixtures weremanufactured with and without a silane based hydrophobic admix-ture and characterized in terms of mechanical performances, phys-ical properties and carbonation rate. Then, the corrosion behavior ofembedded black steel, galvanized steel and black steel covered withcement grout exposed to a CO2 rich atmosphere was compared bymeans of free corrosion potential, polarization resistance measure-ments, optical observations and metallographic cross section eval-uations, carried out on reinforcements at the end of the tests.

2. Experimental

2.1. Materials and mixtures

A commercial Portland-limestone blended cement type CEM II/A-L 42.5 R according to EN-197/1 and crushed limestone gravel

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Diameter (mm)

Cum

ulat

ive

Volu

me

%

Fig. 1. Particle size distribution of aggregate.

Table 1Main properties of no-fines concrete mixtures.

Mixtures Rl0% Rl1% Rm0% Rm1% Rh0% Rh1%

w/c 0.39 0.39 0.37 0.37 0.34 0.34a/c 7 7 5 5 4 4Water (kg/m3) 82 88 113 113 142 136Cement (kg/m3) 210 231 307 318 419 417Gravel (kg/m3) 1478 1628 1571 1625 1678 1671Silane admixture (kg/m3) – 5.13 – 7.06 – 9.27Specific weight (g/cm3) 1.77 1.95 1.99 2.06 2.24 2.23Porosity (%) 29 22 19 16 9 9Rc28days (MPa) 7.8 12.2 17.6 15.6 31.3 25.5

120 F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126

with 15 mm maximum size were used as binder and aggregate,respectively. The particle size distribution curve of the adoptedaggregate is reported in Fig. 1. A 45% aqueous emulsion of an al-kyl-triethoxy-silane was used as hydrophobic silane-basedadmixture.

No-fines concrete properties depend on the used materials,water/cement and aggregate/cement ratios adopted, and themodality of compaction [3]. Three mixtures were selected andmanufactured with water/cement ratio (w/c) and aggregate/ce-ment ratio (a/c) ranging respectively from 0.39 to 0.34 and from7 to 4 and labeled, in term of strength class, as low (Rl), medium(Rm) and high (Rh) concrete (Table 1). The mixtures were manu-factured with the addition of hydrophobic admixture correspond-ing to a dosage of 0% (reference) and 1% of active ingredient bycement weight (six mixes, in total).

Each mixture was cast in cubic moulds in two layers. Each layerwas compacted for ten seconds with a drill equipped with a flatsquare with side equal to that of the formwork.

The cubic specimens were de-molded after 24 h from the castand cured at relative humidity (RH) equal to 100 ± 5% and temper-ature (T) equal to 22 ± 2 �C until test.

Density and porosity of no-fines concrete were measured on notreinforced cubes after de-moulding.

Every mixture was characterized by compressive test of the cu-bic specimens after 28 days of curing.

2.2. Carbonation resistance

The resistance to carbonation of no-fines concrete is of a pri-mary importance in relation to the protection of embedded steel.After 28 days of curing (RH = 100 ± 5% and T = 22 ± 2 �C) six cubicspecimens for each mixture were exposed to a carbonation cham-ber at CO2 = 3 ± 0.2%, T = 21 ± 2 �C and RH = 60 ± 10%. After 10 and30 days of exposure the progress of carbonation was evaluated bycolorimetric test performed with an alcoholic solution of phenol-phthalein applied on the cross-section surfaces of the specimens

submitted to indirect tensile test. Differently from conventionalconcrete, carbonation of no-fines concrete does not only occurfrom the external surface inward. High porosity of no-fines con-crete promotes the carbonation penetration also in depth andcauses also the partial carbonation of cement grout in contact withinner macro-pores. Thus each single thin layer of cement grout thatcovers the coarse aggregates is subjected to carbonation. For thisreason, the resistance to carbonation could not be evaluated bycarbonation depth measurements, as is the case for traditional con-crete, but an image processing software was used to estimate thecarbonation degree, defined as the percentage of carbonated sur-face with respect to the total cross-section surface. The percentageof carbonated surface (carbonation degree) was obtained by count-ing the number of pink pixels (x), corresponding to not carbonatedconcrete, and subsequently, by calculating the ratio of complemen-tary pixels (y), corresponding to carbonated concrete, with respectto the total ones (x + y) of the specimen cross-section surface, andfinally by converting the obtained result in %:

Carbonation degree ¼ yxþ y

100 ð1Þ

2.3. Corrosion tests

2.3.1. SpecimensFor each mixture reported in Table 1, reinforced cubic speci-

mens were manufactured and reinforced with bars (U = 1 cm,l = 10 cm).

Three different types of reinforcement were compared:

a. black steel obtained by corrugated steel bars FeB44K(labeled as N);

b. hot dip galvanized hypo-Sandelin steel obtained by immer-sion in a molten zinc bath [17] (labeled as Z);

c. steel covered with cement grout (w/c = 0.4) before casting, topromote passivation of reinforcements, (labeled as B);

with a total of 18 different types of specimens (Fig. 2).For each typology, three specimens were manufactured to aver-

age results on three specimens of each type (54 specimens, totally).The ends of each bar were masked by epoxy resin to hinder the di-rect contact with water during the corrosion tests and the exposedsurface was 22 cm2.

The reinforcements were placed in the centre of the cubic spec-imens with the aid of a plastic septum (0.6 cm deep, 5 cm high and10 cm wide) placed on the opposite sides of the mould and prop-erly shaped to place the reinforcing bar (Fig. 3). Each mixturewas compacted in two different layers with the aid of a drill as de-scribed in Section 2.1.

After casting, all specimens were cured for 28 days (RH = 100 ± 5%and T = 22 ± 2 �C) and then exposed in a carbonation chamber atCO2 = 3 ± 0.2%, T = 21 ± 2 �C and RH = 60 ± 10% for 3 months.

2.3.2. Electrochemical measurementsThe corrosion risk of reinforced concrete specimens exposed to

a CO2 rich atmosphere was evaluated by free corrosion potentialmeasurements using a Ag/AgCl electrode as reference. The mea-surements were carried out periodically on the specimens ex-tracted from the chamber and previously immersed in water for1 h, to minimize the ohmic drop, with the reinforcing bar placedin vertical position, the water level 1 cm below the top surface ofthe specimen and the reference electrode and the counter elec-trode (an external graphite bar) immersed both in water.

The kinetics of the corrosion process was followed by polariza-tion measurements performed by means of a three-electrodeconfiguration cell using an Amel workstation managed by a

Z-Rl0% Z-Rm0% Z-Rh0% 0% N-Rl0% 0% N-Rm0% 0% N-Rh0%

B-Rl0% B-Rm0% B-Rh0% Rl Rm Rh

Z-Rl1% Z-Rm1% Z-Rh1% 1% N-Rl1% 1% N-Rm1% 1% N-Rh1%

B-Rl1% B-Rm1% B-Rh1%

Fig. 2. Schematization of all types of samples used for corrosion test. (Rl, Rm, Rh mean low, medium and high strength class, respectively. 1% and 0% mean the dosage of thehydrophobic admixture by cement weight. Z, N, B mean galvanized steel, black steel, and steel covered with cement grout, respectively).

Fig. 3. Cast of no-fine concrete specimens reinforced with black steel (a), galvanized steel (b) and steel covered with cement grout (c).

F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126 121

National Instrument acquisition board and by a software suitablydeveloped. The polarization resistance was obtained by calculatingthe slopes of two potential/current curves obtained after two sub-sequent galvanodynamic scans (0.5 lA/s): the first in the cathodicdirection and the second in the anodic direction, each of themstarting from the open circuit potential. The polarization reachedafter these two scans was always equal or less than 5 mV to thecorrosion potential, depending on the particular corrosion stateof the steel sheet analyzed. An average value for the polarizationresistance was taken considering the obtained cathodic and theanodic branch. From the polarization resistance value, the corro-sion current density icorr was calculated using the Stern–Gearyequation as reported in [18]:

icorr ¼BRp¼ babc

2:3RpðbabcÞð2Þ

where ba and bc are the absolute values of anodic and cathodic Tafelconstants, respectively, and B is the Stern–Geary constant that is afunction of ba and bc. The B value was taken equal to 26 mV/decadeas reported in [17,19,20]. Considering this B value and the exposedsurface of the reinforcement (S = 22 cm2), the corrosion rate vcorr, aspenetration depth versus time (lm/year) can be calculated applyingthe Faraday’s law.

The polarization resistance was measured periodically, as thecorrosion potential, during wetting cycles after 1 h of immersion;for polarization measurement a counter-electrode (an externalgraphite bar) was used.

2.3.3. Visual evaluation of reinforcements and metallographic analysisIn order to validate the results obtained with the electrochem-

ical study, after 3 months of exposure in carbonation chamber, thereinforcements were extracted by splitting the concrete specimensand the corrosion extent was assessed by visual observation. Animage processing software was used to estimate the corrosiondegree, defined as the percentage of corroded surface, of steel

reinforcements (not galvanized ones). The percentage of corrodedsurface was obtained by calculating the ratio between the numberof red pixels (r), corresponding to corroded surface, with respect tothe surface bar total ones (z) and by converting the obtained resultin %:

Corrosion degree ¼ rz

100 ð3Þ

For this work, different images of the cylindrical surface of eachbar were used in order to consider the whole exposed metalsurface to concrete matrix. For each mixture, the results wereaveraged among three bars of the same type.

In the case of galvanized reinforcements, metallographic analy-sis were carried out on the cross section of bars to evaluate, afterthe corrosive attack, the thickness (D) of pure zinc layer (g phase)on the top of the coating. This phase is particularly present when ahypo-Sandelin steel is hot-dip galvanized, as that used in this work[21].

In particular, 24 measurements were taken around each bar fora total of 72 measurements for each specimen typology (threespecimens for each type) and the average final thickness for eachspecimen typology was calculated (Df). Other 24 measurementswere taken around each type of bar, but under the resin layer,and the average initial thickness for each specimen typology wascalculated (Di). The decrease of the g phase due to the corrosive at-tack (DD) was calculated for each specimen typology:

DD ¼ Df � Di

Di100 ð4Þ

3. Discussion of test results

3.1. Mixtures characterization

Table 1 reports the different mixture proportions and therelative properties (averaged among three specimens for each

Fig. 4. Phenolphthalein test in no-fines concrete without (up) and with (down) hydrophobic admixture after 30 days of exposure to carbonation chamber. (Rl, Rm, Rh meanlow, medium and high strength class, respectively. 1% and 0% mean the dosage of the hydrophobic admixture by cement weight).

0102030405060708090

100

Mixtures

10 dd30 dd

Car

bona

tion

Deg

ree

(%)

Fig. 5. Quantitative evaluation of the carbonation degree by photo image process-ing. (Rl, Rm, Rh mean low, medium and high strength class, respectively. 1% and 0%mean the dosage of the hydrophobic admixture by cement weight. dd means daysof exposure to carbonation chamber).

122 F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126

mixture). It can be observed that by varying the adopted w/c (0.34,0.37 and 0.39) and a/c (4, 5 and 7), it is possible to obtain no-finesconcrete with relatively low (Rl ffi 5–10 MPa), medium (Rm ffi 15–20 MPa), and high (Rh ffi 25–30 MPa) compressive strength. Thedensity of the mixtures studied ranges from 1770 kg/m3 for mixRl to 2240 kg/m3 for mix Rh. The porosity, calculated from densityvalues, ranges from 29% to 9%. As expected, the compressivestrength and density increase, as well as porosity decreases, withthe decrease of w/c and a/c. It is evident that the hydrophobicadmixture decreases the mechanical performance of about 11%and 20% in Rm and Rh no-fines concrete, respectively. This behav-ior is similar to that observed in ordinary concretes where thehydrophobic admixture causes a reduction in compressivestrength up to 20% [12,13,15,22]. Indeed, in Rl no-fines concrete,the hydrophobic admixture slightly increases the strength, densityand, consequently, decreases porosity of concretes probably due toa certain plasticizing effect of silane that improves the low com-pactibility of the mixture due to the lack of fine particles suppliedonly by cement.

3.2. Carbonation resistance

The carbonation degrees of the different mixtures obtainedfrom the phenolphthalein tests (Fig. 4) after 10 and 30 days ofexposure in the accelerated carbonation chamber are comparedin Fig. 5. After 10 days in accelerated conditions, 60% of thecross-section surface of Rl no-fines concrete is carbonated. Byincreasing the time of exposure, the carbonation degree increased.A lower carbonation degree is observed on mixtures Rm and Rh,which have lower w/c and a/c (i.e. more cement grout with lowercapillary porosity). To evaluate the possible effects of the hydro-phobic admixture on the carbonation degree, tests were carriedout also on no-fines concrete with 1% of hydrophobic admixture.The phenolphthalein test shows that the hydrophobic admixturefavors the carbonation of no-fines concrete, especially in moreporous concrete (Rl) with the highest w/c ratio. These results arein agreement with the expected effect of hydrophobic admixturewhich hinders penetration of liquid water, keeping the cementgrout dry and favoring the diffusion of gases as CO2 [16,23].

3.3. Corrosion tests

3.3.1. Free corrosion potential measurements: effect of hydrophobicadmixture, concrete strength class and the type of reinforcement

Fig. 6 compares the free corrosion potential values of steel rein-forcements, with and without the hydrophobic admixture in low(Fig. 6a), medium (Fig. 6b) and high (Fig. 6c) resistance no-finesconcrete, respectively. Fig. 7 reports the same measurements car-ried out on galvanized steel reinforcements. It is evident that thefree corrosion potential values are not significantly influenced bythe presence of the hydrophobic admixture both for steel (Fig. 6)and galvanized steel (Fig. 7) bars. Also the surface treatment ofsteel bars with cement grout does not give a significant decreaseof corrosion probability of steel (Fig. 6). Generally, less negativecorrosion potentials are observed in specimens with the highestmechanical performances meaning that the porosity of the matrixis the major factor affecting the free corrosion potentials of rein-forcements. However, all the potentials remain in the passive rangefor steel (>�450 mV vs SCE) and for galvanized steel (>�950 mV vsSCE) showing an apparent low corrosion probability for all thereinforcements.

-400

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-50

0

0 10 20 30 40 50 60 70 80 90 100

Time (days)

0 10 20 30 40 50 60 70 80 90 100

Time (days)

0 10 20 30 40 50 60 70 80 90 100

Time (days)

N-Rl0%B-Rl0%N-Rl1%B-Rl1%

-400

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-50

0N-Rm0%B-Rm0%N-Rm1%B-Rm1%

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-50

0N-Rh0% B-Rh0%

N-Rh1% B-Rh1%

Pote

ntia

l (m

V vs

Ag/

AgC

l)Po

tent

ial (

mV

vs A

g/Ag

Cl)

Pote

ntia

l (m

V vs

Ag/

AgC

l)

(a)

(b)

(c)

Fig. 6. Free corrosion potentials of steel reinforcements in low (Rl) (a), medium(Rm) (b) and high (Rh) (c) strength class no-fines concrete. (1% and 0% mean thedosage of the hydrophobic admixture by cement weight. N, B mean black steel andsteel covered with cement grout, respectively).

-950

-850

-750

-650

-550

-450

-350

0 10 20 30 40 50 60 70 80 90 100

Pote

ntia

l (m

V vs

Ag/

AgC

l)

Time (days)

Z-Rl0% Z-Rl1%Z-Rm0% Z-Rm1%Z-Rh0% Z-Rh1%

Fig. 7. Free corrosion potentials of galvanized steel reinforcements in different no-fines concrete. (Rl, Rm, Rh mean low, medium and high strength class, respectively.1% and 0% mean the dosage of the hydrophobic admixture by cement weight. Zmeans galvanized steel).

F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126 123

3.3.2. Corrosion rate of reinforced no-fines concrete: effect ofhydrophobic admixture and the type of reinforcement

Figs. 8–10 compare the corrosion rates of different types ofreinforcements in no-fines concrete with the same strength classwith and without the hydrophobic admixture (in these figuresthe x-axis is considered a discrete variable, assuming the values

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 43 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

N-Rl0% B-Rl0%

N-Rl1% B-Rl1%

Z-Rl0% Z-Rl1%

Fig. 8. Corrosion rate of reinforced low strength class no-fines concrete (Rl). (1%and 0% mean the dosage of the hydrophobic admixture by cement weight. Z, N, Bmean galvanized steel, black steel and steel covered with cement grout,respectively).

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 43 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

N-Rm0% B-Rm0% N-Rm1%

B-Rm1% Z-Rm0% Z-Rm1%

Fig. 9. Corrosion rate of reinforced medium class strength no-fines concrete (Rm).(1% and 0% mean the dosage of the hydrophobic admixture by cement weight. Z, N,B mean galvanized steel, black steel and steel covered with cement grout,respectively).

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 43 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

N-Rh0% B-Rh0% N-Rh1%

B-Rh1% Z-Rh0% Z-Rh1%

Fig. 10. Corrosion rate of reinforced high strength class no-fines concrete (Rh). (1%and 0% mean the dosage of the hydrophobic admixture by cement weight. Z, N, Bmean galvanized steel, black steel and steel covered with cement grout,respectively).

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 44 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

N-Rl0% N-Rl1% N-Rm0%

N-Rm1% N-Rh0% N-Rh1%

Fig. 11. Corrosion rate of black steel reinforcements (N). (Rl, Rm, Rh mean low,medium and high strength class, respectively. 1% and 0% mean the dosage of thehydrophobic admixture by cement weight).

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 43 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

B-Rl0% B-Rl1% B-Rm0%B-Rm1% B-Rh0% B-Rh1%

124 F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126

corresponding to the time of each observation to better highlightthe behavior in the early days of exposure). It is evident that inlow resistance specimens (Rl) (Fig. 8) black steel corrodes withcorrosion rate up to 25 lm/year. The application of a cement grouton the surface of black steel rebar improves slightly the corrosionbehavior; however the corrosion rate values remain stationary at10 lm/year for all the test period. Concerning the galvanized steel,the effect of zinc passivation is well evident: at the beginning zinccorrodes with high values of about 30 lm/year, but subsequently,thanks to zinc passivation, these values decrease slowly up to belower than 5 lm/year.

Concerning the effect of the hydrophobic admixture on corro-sion rates, at the end of exposure, corrosion rates are not muchinfluenced by the hydrophobic admixture. However, in the firstdays of exposure, higher and lower corrosion rates are detectedin black and galvanized reinforcements, respectively in thepresence of silane. The hydrophobic admixture, which hinderspenetration of liquid water, keeping the cement grout dry, favorsthe diffusion of CO2 and O2 [11–13,15,16,23], which promotes onzinc a faster passivation (in the initial period of exposure, galva-nized steel in hydrophobic concrete corrodes less than galvanizedsteel in concrete without hydrophobization) and in black steel afaster stabilization to the high final corrosion rates. These differ-ences are not evident in steel reinforcements previously coveredby cement grout which show relatively low corrosion rates forthe whole test period.

In medium (Rm, Fig. 9) and high (Rh, Fig. 10) resistance no-finesconcrete the relative corrosion behaviors are analogous to thosediscussed in Rl no-fines concrete, even if the lower concrete poros-ity, which limits CO2 and O2 diffusion, progressively levels thedifferences.

Fig. 12. Corrosion rate of steel reinforcements covered with cement grout (B). (Rl,Rm, Rh mean low, medium and high strength class, respectively. 1% and 0% meanthe dosage of the hydrophobic admixture by cement weight).

0

5

10

15

20

25

30

35

0 1 2 3 6 10 16 25 32 38 43 49 56 63 69 83 91 97

Cor

rosi

on ra

te (u

m/y

ear)

Time (days)

Z-Rl0% Z-Rl1% Z-Rm0%

Z-Rm1% Z-Rh0% Z-Rh1%

Fig. 13. Corrosion rate of galvanized steel reinforcements (Z). (Rl, Rm, Rh mean low,medium and high strength class, respectively. 1% and 0% mean the dosage of thehydrophobic admixture by cement weight).

3.3.3. Corrosion rates of reinforced no fines concrete: effect of strengthclass

Figs. 11–13 compare the corrosion rates of the same type ofreinforcements embedded in concretes with different strengthclass (in these figures the x-axis is considered a discrete variable,assuming the values corresponding to the time of each observa-tion, to better highlight the behavior in the early days of exposure).A certain influence of the strength class of no-fines concrete, re-lated to concrete porosity, can be observed on the corrosion rateof black steel reinforcements: higher corrosion rates are detectedin specimens with lower compressive strength and higher porosity(Fig. 11); however, the differences in term of corrosion rate remainin the order of 10 lm/year. On the contrary, the strength class doesnot significantly affect the corrosion behavior of steel reinforce-ments covered by cement grout (Fig. 12) and galvanized reinforce-ments (Fig. 13). In these cases reinforcements show the samecorrosion rate regardless the strength class and concrete porosity,meaning that they ‘‘protect themselves’’ thanks to the coating ofcement grout or zinc passivation, respectively.

Fig. 14. Steel reinforcements extracted from low strength class specimens with silane: black steel (N-Rl1%) (a); black steel covered with cement grout (B-Rl1%) (b); andgalvanized steel (Z-Rl1%) (c).

F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126 125

3.3.4. Visual evaluation of reinforcements and metallographic analysisFor the sake of brevity, only pictures (Fig. 14) of reinforcements

embedded in no-fines concrete with the lowest strength class andwith silane are showed. Fig. 15 summarizes the quantitative eval-uation of the corrosion degree of steel reinforcements (not galva-nized ones) carried out with the aid of the image processingsoftware.

The black steel reinforcements show a general corrosive attackwith red rust appearing on all surface (Fig. 14a), especially in more

0

10

20

30

40

50

60

70

Cor

rosi

on D

egre

e (%

)

Fig. 15. Quantitative evaluation of corrosion degree of steel reinforcements after90 days of exposure in the carbonation chamber. (Rl, Rm, Rh mean low, mediumand high strength class, respectively. 1% and 0% mean the dosage of thehydrophobic admixture by cement weight. Z, N, B mean galvanized steel, blacksteel, and steel covered with cement grout, respectively).

0

10

20

30

40

50

60

70

80

90

100

Rh0% Rh1% Rm0% Rm1% Rl0% Rl1%

ΔD (%

)

Fig. 16. Thickness decrease (DD) of g phase as evaluated by metallographicobservations. (Rl, Rm, Rh mean low, medium and high strength class, respectively.1% and 0% mean the dosage of the hydrophobic admixture by cement weight.)

Fig. 17. Metallographic cross section of a galvanized steel bars extracted from Rl1% specrespectively. 1% and 0% mean the dosage of the hydrophobic admixture by cement weig

porous matrix and in the presence of the hydrophobic admixture(Fig. 15). A slight decrease in corrosion process was observed inreinforcements previously covered by cement grout (Figs. 14band 15).

When galvanized reinforcements are used, no red rust appearson the surface even in more porous cement matrix, meaning thatthe zinc layer is not consumed by the corrosion process, and awell-adherent and compact, surface compound, covered the rein-forcements (Fig. 14c).

The metallographic observations carried out on galvanized steelreinforcements confirmed the electrochemical results. The de-crease of g phase due to the corrosive attack (DD) is lowest in gal-vanized steel reinforcements extracted from low strengthconcretes, especially in the presence of silane, while it is highestin high strength concretes without the hydrophobic admixture(Figs. 16 and 17).

However after the corrosive attack, regardless the concretestrength class, the whole zinc layer remains always about100 lm thick (Fig. 17) and able to protect steel and guaranteelow corrosion rates in any case (Fig. 13). The differences in the gphase thickness decrease (Fig. 16) are due to O2 and CO2 diffusionaffected by the hydrophobic admixture and, especially, by concreteporosity, as already discussed in Sections 3.3.2 and 3.3.3.

4. Conclusions

By adopting different mix-designs, no-fines concrete withmechanical strength ranging from 7 to 30 MPa and relative poros-ity ranging from 9% to 29% can be manufactured.

The main conclusions obtained by comparing the CO2 inducedcorrosion behavior of black steel, steel covered by cement groutand galvanized steel in no-fines concrete with different strengthclass are:

1. The carbonation process is not hindered by the hydrophobicadmixture.

2. The hydrophobic admixture affects only marginally and in thefirst period of exposure the corrosion behavior ofreinforcements.

3. The corrosion behavior of black steel is affected by the concretestrength class, related to concrete porosity, with higher corro-sion rates in lower mechanical strength class concretes.

4. The corrosion rates of steel reinforcements covered by cementgrout and galvanized reinforcements are not significantlyaffected by concrete strength class and porosity since they pro-tect themselves with the cement grout coating or zinc passiv-ation, respectively.

imen (a) and from Rh0% specimen (b). (Rl and Rh mean low and high strength class,ht.)

126 F. Tittarelli et al. / Corrosion Science 70 (2013) 119–126

5. Black steel corrodes with rates three times higher with respectto those monitored in galvanized steel and in steel covered bycement grout.

6. Among the reinforcements considered in this work, no-finesconcrete reinforced with galvanized steel bars show the bestcorrosion behavior.

Acknowledgment

This work was financed by PRISMA projects of INSTM (ItalianInteruniversity Consortium on Materials Science and Technology).

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