1

CARBONATION-INDUCED REINFORCEMENT CORROSION IN SILICA

FUME CONCRETE

Marlova P. Kulakowskia,∗, Fernanda M. Pereirab, Denise C. C. Dal Molinc

a Programa de Pós-Graduação em Engenharia Civil, Universidade do Vale do Rio dos Sinos (UNISINOS), Brazil.

b Programa de Pós-Graduação em Engenharia: Energia, Ambiente e Materiais, Universidade Luterana do Brasil (ULBRA), Fundação de Ciência e Tecnologia (CIENTEC), Brazil.

c Programa de Pós-Graduação em Engenharia Civil, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.

Abstract This study presents the results of carbonation depth and carbonation-induced

reinforcement corrosion in concrete samples with silica fume additions of up to 20%

and water/binder ratios ranging from 0.30 to 0.80. The behavior of the additions is

determined by the w/b ratios. For w/b ratios lower or equal to 0.45-0.50, carbonation

processes in these materials are controlled by the porosity of the material and the

consumption of Ca(OH)2 has a negligible effect on carbonation. For higher w/b ratios,

the consumption of Ca(OH)2 plays a significant role. At the same time, the results of

reinforcement corrosion indicate that the effect of silica fume additions depends on their

concentration. In concentrations equal to or lower than 10%, silica fume will not reduce

corrosion resistance and it may actually increase it when used in concentrations below

this level. When used in concentrations greater than 10%, silica fume increases the

potential for carbonation-induced reinforcement corrosion.

Keywords: Concrete, Silica Fume, Carbonation, Corrosion

∗ Corresponding author. Programa de Pós-Graduação em Engenharia Civil, Universidade do Vale do Rio dos Sinos (UNISINOS), Av. Unisinos, 950, 93022-000 São Leopoldo, RS, Brazil. a

marlovak@unisinos.br, b

fernanda.pereira@ulbra.br, c

dmolin@ufrgs.br

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1. Introduction

The widespread use of reinforced concrete as a construction material derives

from the excellent compatibility between the materials used in its production. The

synergy between steel and concrete improves physical, mechanical and chemical

properties of both components. The physical-mechanical synergy is evident from the

fact that the use of steel reinforcements makes it possible for engineers to erect

buildings with different shapes and design and build wide spans. At the same time, the

steel is protected from physical and chemical agents by the concrete layer, in this way

solving the major problem that affects metallic structures, namely the corrosion of steel

when exposed to the environment. Concrete also offers chemical protection to the steel

through the alkaline pH of the concrete pore solution, which creates a passivation layer

on the surface of the reinforcements that provides chemical equilibrium and protects the

metal from corrosion. However, the durability of concrete structures is subject to

considerable changes and reinforcements often start showing signs of decay much

earlier than expected for the structure’s service life. There are several factors that can

account for pathological manifestations in shorter time spans, such as: (a) The

development of new improved cement formulations, which achieve the same

mechanical strength as in the best with higher water/cement ratios. This results in higher

porosity and reduces durability, as shown in Table 1 [1]; (b) Modern calculation

methods that allow engineers to design leaner structures with smaller structural

elements. For this reason, the thickness of the concrete layer is sometimes smaller that

the values recommended by applicable standards; (c) Increased pollution levels,

meaning that the concrete is exposed to a more aggressive environment.

Therefore, studies aiming at the development and improvement of reinforced

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concrete structures are of paramount importance. They can help improve the quality of

materials used and their combinations for improved durability and extended service life.

The Federal University of Rio Grande do Sul (UFRGS) has been studying the behavior

of concrete and mortars produced with additions of silica fume for some years in order

to fins ways of improving the quality of materials used in concrete structures and

identify potential uses and limitations in their use.

Although many studies [2, 3, 4, 5], show improvements in the performance of

concrete with silica fume in terms of chloride resistance and mechanical properties,

some questions requiring further research remain. These questions refer mainly to the

properties of concrete formulations in terms of carbonation and carbonation-induced

reinforcement corrosion. The aim of the present study is to evaluate carbonation

processes in concrete samples with the addition of silica fume and their behavior

regarding carbonation induced corrosion in order to answer these questions.

2. Methods and materials

The tests of carbonation and reinforcement corrosion were conducted

simultaneously, using the same materials. The test methods used for carbonation

measurements over time and the tests of carbonation-induced reinforcement corrosion

were carried out on concrete.

2.1. Variables

The experiments were planned using a fractionated crossed factorial design for

the carbonation tests and a fully crossed factorial design for the corrosion tests, with two

repetitions for each combination of variables. The combinations of independent

variables for the carbonation tests are shown in Table 2. As the corrosion tests extend

over a longer period, for this case the matrix of variables was reduced and was kept

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within the limits of the matrix of carbonation tests, as shown in Table 3.

2.2. Materials

High initial strength Portland cement (Brazilian grade CP V-ARI) from a single

lot was used in the production of concrete samples. This type of cement was chosen

because it does not have any pozzolanic additions, which eliminated the possibility of

influences from other reactive additions in the results. The chemical and

physical/mechanical properties of this cement are shown in Table 4.

This study used uncompacted silica fume powder supplied as a single lot by a

Brazilian manufacturer. The chemical composition and the physical characteristics of

the silica fume are shown in Table 5.

The morphologic characterization of the silica fume was made through X-ray

diffraction analysis (XRD) at the XRD Lab of the Geosciences Institute of the Federal

University of Rio Grande do Sul (UFRGS) in a SIEMENS D5000 Kristaloflex

Diffractometer with CuKα radiation operated at 30 mA and 40kV. Fig. 1 shows the

resulting diffractogram, with the visible amorphous halo that characterizes the spectrum

of amorphous silica fume. The characterization of fine and coarse aggregates is

presented in Table 6.

Some compositions required the use of a superplasticizer addition to yield the

necessary workability (70±10 mm by slump test). A sulphonated naphthalene

superplasticizer with density = 1.21 kg/dm3 and 40% solids content was used.

2.2.Concrete compositions

The compositions were prepared according to the dosage method developed by

IPT/EPUSP and described by Helene and Terzian [6] and a concentration of 50% of

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mortar was selected. A consistency of 70±10 mm was selected as a control parameter

using the slump test according to Brazilian Standard NBRNM67 [7]. The dosage

diagram was prepared from the results of compressive strength at 1, 3, 7 and 28 days, as

shown in Fig. 2. Silica fume was added to the concrete in concentrations ranging from

5% to 20% by weight of cement mass and the superplasticizer was added in

concentrations ranging from 0.2 to 1.3% by weight of cement.

2.4. Carbonation

Test specimens measuring 100×100×300 mm were cast in two blocks and one

sample was removed from each block so that two samples for each composition

resulted. The blocks were cast on two different days. Accelerated carbonation tests were

carried out in a chamber with controlled CO2 concentration (5%) in an open system and

continuous feeding. The concentration of 5% was selected because some of the w/b

ratios used in this study would yield extremely high carbonation rates if higher

concentrations of CO2 were used. The tests were carried out in a environment with

controlled temperature (25 ± 1ºC) and relative humidity (70 ± 2%). The test specimens

were cured for fourteen days in a moisture chamber (RH ≥ 95% and T = 23 ± 2º C) to

reach a state of equilibrium with the RH of the test environment [8]. After 28 days, they

were exposed to the carbonation environment. The carbonation depth of the specimens

was determined by the phenolphthalein method recommended by RILEM CPC-18 on a

vertical section of the specimen. Concretes were exposed to the carbonation

environment for 7, 28, 56 and 98 days and a slice measuring 100 x 100 x 50 mm was

removed from each of the test specimens after each of these periods.

2.5. Carbonation-induced corrosion of reinforcing steel

The accelerated tests of carbonation-induced corrosion consisted of exposing the

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test specimens to an atmosphere with a concentration of CO2 ≥ 50%. The test specimens

for the accelerated carbonation test were cured for 28 days in a moisture chamber (RH ≥

95% and T = 23 ± 2ºC) and were stabilized in an oven at 50ºC for seven days in a room

with controlled temperature (25 ± 2ºC) and relative humidity (70 ± 5%) for a period of

14 days. After that, the test specimens were placed in the accelerated carbonation

chamber inside a room with controlled temperature. In this way, an optimal level of

humidity for the development of carbonation reactions was ensured.

The assessment of corrosion on the steel bars was made through an

electrochemical technique of polarization resistance using an Gill AC potentiostat

(ACM Instruments) with automatic IR drop compensation. The potentiostat was

connected to a computer and electrochemical measurements were made using data

acquisition (Sequencer) and data analysis software (V4 Analysis).

From the polarization resistance data, the intensity of corrosion over time was

obtained and this yielded the intensity of total corrosion [9]. Electrochemical

measurements to determine ohmic resistance (IR drop), corrosion potential and

polarization resistance in the test specimens were made after 24 hours and 96 hours in

the carbonation chamber and after that were made once a week. Before making the

electrochemical measurements, the test specimens were weighed to monitor their mass

along the experiment.

3 Results and analysis

3.1. Carbonation

As the four periods of exposure to CO2 resulted in a large number of graphs and

analyses produced by, a choice was made to present only the results and analyses of the

samples after 126 days, which means 28 days of set time and 98 days of exposure to

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CO2.

The statistical analysis tool used non-linear multiple regression with results

presented in tables of analysis of variance (ANOVA), mathematical models and curves

representing resulting models. A graph shows a comparison between forecast values and

actual values. Table 7 presents the analysis of variance for the model obtained and

Table 8 presents the analysis of variance for the factors studied. A value of ‘p’ lower

than 0.01 indicates that the relationship between the variables is statistically significant

for a confidence level of 99%. The determination coefficient r2 obtained (0.9481)

indicates that the model explains 94.81% of the variability of the values observed for

the carbonation depth in concrete.

Considering the values analyzed in the model and their interactions, the results

in Table 8 indicate that only the variable water/binder ratio (wb) and test age (t) are

statistically significant for a confidence level of 99% because their value of ‘p’ is lower

than 0.01. Therefore, the contribution of silica fume to carbonation depth is not

significant because the value of ‘p’ for factors ‘sf’ (silica fume content) and ‘wb x sf’ is

greater than 0.10 (the lowest value usually accepted for a factor to be considered

significant at a confidence level of 90%). However, factors ‘sf’ and ‘wb x sf’ with a

significance of 53% were considered in the model in order to forecast correlations with

other response variables in the tests performed. Eq. 1 shows the model for carbonation

depth.

( ) ( ) ( )[ ] 6424110417589916

,,,,,exp tsfwbsfwbdc ×××+×+×+−= Eq. 1

Where:

dc = carbonation depth (mm); wb = water/binder ratio (with values between 0,30 and 0,80); sf = silica

fume content (with values between 0 and 0,20); t = test age (126 days).

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Fig. 3 presents graphs with the behavior curves forecast by the adjusted model

and the actual values observed (experimental data) for carbonation depth. The

carbonation curves presented, which show forecast carbonation values, can be used as

reference for carbonation forecasts in concrete compositions with silica fume in the

experimental conditions of the program because r² = 0.9481. This is reinforced by Fig.

3, which shows minimal dispersion between measured and forecast values. The adjusted

mathematical model for carbonation does not show a significant effect of the addition of

silica fume. However, for w/b ratios in excess of 0.70 an increase of 50% in carbonation

depth is seen when samples with 20% silica fume are compared with control samples

(no silica fume), showing that silica fume has an influence on carbonation. It must be

pointed out that these concrete formulations have an fck below 20 to 25 MPa and are

not suitable for use in the construction industry. Carbonation data highlight the

importance of educating construction workers about the correct use and control of lower

w/c and w/b ratios because even with no additions, concrete compositions with w/b

ratios of 0.80 (corresponding to an fck of approximately 15 MPa) are highly susceptible

to carbonation.

For the values observed, the carbonation depths were null up to 126 days in the

samples with w/b ratios of 0.30, 0.35 and 0.45 for all addition concentrations. For the

forecast values, the highest value reached for single combination of variables was 1.05

mm. The interval between w/b 0.30 and w/b 0.51 in Fig. 3 shows that for all

concentrations of addition used, carbonation depths do not exceed 2 mm, a value much

lower than the usual thickness of the concrete layer over the reinforcements. This

analysis agrees with Khan and Lynsdale [10].

These data are in partial agreement with the results observed by Venquiaruto et

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al [11] in a study using 5% CO2, RH 75% and 24ºC with a range of pozzolans at

different concentrations (including 8% silica fume) combined with w/b ratios of 0.35,

0.50 and 0.60. For a w/b ratio of 0.35, the concrete with 8% silica fume show no

carbonation depth and for the other two w/b ratios, carbonation depths were lower than

that observed in the reference concrete. At 84 days of exposure, carbonation depth

values of approximately 11 mm were observed for w/b = 0.50 and of 18mm for w/b =

0.65. The test conditions were similar to the ones used in the current study. The results

also agree with the observed by Khunthongkeaw et al [12] shows that for a given fly ash

content, a lower w/b leads to a lower carbonation coefficient, due to the pore structure

densification.

The data collected in this study indicate that carbonation is null in the range of

w/b ratios between 0.45 and 0.50, which could be termed a critical threshold between

different regions of carbonation behavior. For w/b ratios above this threshold,

carbonation depths become more evident. The mathematical model shows distinctive

behaviors in terms of carbonation depth and porosity depending on the w/b ratio used.

For w/b ratios below the critical threshold, the carbonation depths observed are far

lower than those produced by the model. However, carbonation values observed for w/b

ratios above the critical threshold are very high. As these values have a great weight in

adjustments of the model, the calculated values tend to be far higher than those

observed in the interval below the critical threshold. These observations are in

agreement with the very low or null carbonation depth data found by Cabrera et al. [13]

when studying several parameters linked with the corrosion of reinforcing steel, using

concrete with 0 and 20% substitutions of silica fume for cement and w/b ratios of 0.30

and 0.46.

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However, the adjusted mathematical models picture very clearly the “critical

point” found at the threshold w/b ratio. Data for w/b = 0.5 (Fig. 4) indicate that the

carbonation depth in concrete at 126 days increases by 0.48 mm when the concentration

of addition is increased from 0 to 20% and by 0.22 mm when the concentration of

addition is increased from 0 to 10%. For a w/b ratio = 0.45, the carbonation depth

increases by 0.30 when the concentration of addition is increased from 0 to 20% and by

0.14 mm when the concentration of addition is increased from 0 to 10%. If these

findings are transferred to actual building site conditions, it could be said that increases

in carbonation depth in this range would not be harmful to the structure of concrete.

3.2 Carbonation x Compressive strength

The relationship between carbonation and compressive strength in the concrete

formulations used in the carbonation tests of this study is shown in the graph of Fig. 4.

By analyzing the graphs in Fig. 4 and using the curves of fc and dc without the addition

of silica fume as reference, the behavior often described in the literature is seen, as

pointed out by Roy et al. [14, 15] and others. In other words, the compressive strength is

lowered and the carbonation depth increases as the w/b ratio increases in samples with

the same concentration of addition. However, the same behavior cannot be extended to

the curves of silica fume additions. While the compressive strength of concrete shows

improvements when additions are used, carbonation depth values increase. However,

the carbonation depth showed no technically significant changes with increased

concentrations of additions with w/b ratios below the 0.45-0.50 range (“carbonation

threshold”).

However, the average increase in compressive strength with additions of 10%

silica fume amounts to 10% and the mean reduction in the consumption of cement by

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m² is 23%. The comparison of compressive strength and carbonation depth curves in

concrete also highlights the fact that compressive strength is not the only parameter that

should be used to estimate the service life of concrete structures. The w/b ratio used

must be assessed in combination with the type of binder used. To confirm this assertion,

Table 1 shows the improvements in compressive strength properties of cements over

time, which indicates that using higher w/c ratios present-day cements can reach the

same strength that in the past was only possible with lower w/c ratios. To reach an fck

of 25 MPa in the past, a w/c ratio of 0.40 was used (below the “carbonation threshold”)

and nowadays the same strength is achieved with a w/c ratio of 0.55, a value above the

carbonation threshold.

3.3. Carbonation-induced corrosion of reinforcing steel

The statistical analysis of the results of total corrosion intensity for accelerated

carbonation-induced corrosion was performed in order to obtain a mathematical model

that represented the effect of the w/b ratio and the concentration of silica fume on the

final variation in the intensity of corrosion. The mathematical model proposed, shown in

Eq. 2, was obtained through multiple non-linear regression analysis. Like in the case of

carbonation data, the corrosion results observed are presented in the same graphs where

the curves of the mathematical models obtained from the statistical analysis are plotted.

Table 9 presents the analysis of variance (ANOVA) of the proposed model,

which indicated a determination coefficient (r2) of 0.888 while Table 10 shows the

parameters of the factors considered in this model, with estimations for the data of this

experimental program, as well as the statistical parameters calculated for the factors

being investigated. The determination coefficient r2 = 0.888 shows that the model

explains 88.8% of the variability of the values observed for the final variation in

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corrosion intensity. The results presented in Table 10 indicate that both the w/b ratio

(wb) and the interaction between the w/b ratio and the concentration of silica fume have

a significant influence on the intensity of corrosion to a confidence level of 95% (“p” <

0.05). The results in Tables 9 and 10 show that Eq. 2 presents the regression model that

explains the relationship between the factors investigated and the final variation of the

intensity of corrosion.

( )[ ] ( ) ( )[ ]24253001033300221 −×−××+−×=∆ − sfwbwbicorr ,,,, Eq. 2

Where:

∆icorr = final variation in the intensity of corrosion (µA/cm2); wb = water/binder ratio (with values

between 0,40 and 0,70); f = silica fume content (with values between 0 and 0,20).

Fig. 5 presents the behavior curves obtained from the model proposed, as well as

the values observed for the final variation of the intensity of corrosion. The influence of

the w/b ratio on the intensity of corrosion is clearly visible in the curves of behavior in

Fig. 5. These results indicate that the mean final variation of the intensity of corrosion

for w/b = 0.70 is twenty times greater than the value for w/b = 0.40. This fact is related

to the increase in porosity of the concrete with higher w/b. In addition to facilitating

CO2 diffusion, this accelerates the onset of corrosion reactions. This behavior is similar

to the results published by other researchers [16, 17, 18] who found greater corrosion

intensities for higher w/b ratios when investigating carbonation-induced corrosion.

The effect of silica fume in the variation of the intensity of corrosion is more

significant for higher w/b ratios, as Fig. 5 shows. The performance of concrete with an

10% addition of silica fume is equivalent to the performance of the reference concrete

and the addition of higher concentrations of this admixture results in greater variations

in corrosion intensity, particularly for higher w/b ratios. A small reduction in the final

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variation of corrosion intensity in the concrete with 5% silica fume was also observed.

Silica fume can react with calcium hydroxide and lower the pH of the pore

solution, in this way facilitating the carbonation process. However, BRANCA et al [19]

point out that while the lowered alkalinity caused by carbonation or the pozzolanic

reaction is a necessary condition to promote the corrosion of reinforcements, it is not

enough to trigger this phenomenon. These researchers claim that permeability and

porosity are key factors that can limit or accelerate corrosion processes. This claim is in

agreement with the results obtained for the variation of corrosion intensity, where the

reduction in porosity caused by the formation of a finer pore structure as a result of the

addition of up to 10% of silica fume compensates for the consumption of calcium

hydroxide, and the protection of the steel structure is not compromised. The finer pore

structure results in increased resistivity and decreased oxygen diffusion in concrete,

which hinders the onset of corrosion reactions. On the other hand, for higher

concentrations of additions, the reduction in porosity provided by silica fume does not

seem to be a determinant factor in corrosion and the effect of reduced alkalinity seems

to prevail, which results in an increase in corrosion intensity.

4. Conclusions

Regarding the carbonation study, this study showed:

• The existence of a ‘critical threshold’ in the carbonation behavior of

concrete, delimited by an interval of w/b ratios, which this study showed to be 0.45 and

0.50;

• Below the lower w/b ratio limit, carbonation is determined mainly by the

porosity of the cementitious matrix while the concentration of Ca(OH)2 and pH have

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little influence on carbonation depths at this ratio;

• For values above the upper w/b ratio limit, chemical characteristics start to

play a more significant role in carbonation depth and the consumption of Ca(OH)2 in the

pozzolanic reactions caused by silica fume starts to have a detrimental effect on

carbonation;

• Therefore, carbonation processes are determined by the porosity provided

to the materials as a result of the w/b ratio. The effect of silica fume, in practice, is only

detrimental for w/b ratios above the ‘critical carbonation threshold’;

However, in the case of carbonation-induced reinforcement corrosion, it was

seen that the effect of silica fume on the performance of concrete is affected by the

concentration of addition used. In this way, it was possible to conclude that:

• For concentrations of additions below 10%, silica fume increases the

resistance to carbonation-induced corrosion even though it increases carbonation depth.

• In concrete preparations with 10% silica fume there is an increase in

carbonation depth, but silica fume will not affect carbonation-induced corrosion. At the

same time, it improves other properties considerably.

• When the concentration of silica fume exceeds 10%, it not only increases

carbonation depth but it also increases the variation in the intensity of carbonation-

induced corrosion, particularly for higher w/b ratios. It must be highlighted, however,

that in the current stage of concrete technology, addition concentrations exceeding 10%

are rarely used and common sense dictates that ideal concentrations fall in the 5 to 10%

range.

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References

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[7] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBRNM67: Concreto - Determinação da consistência pelo abatimento do tronco de cone. Rio de Janeiro; 1998. p. 8.

[8] Kulakowski MP. Contribuição ao estudo da carbonatação em concretos e argamassas compostos com adição de sílica ativa. Porto Alegre, 2002. 200p. Tese (Doutorado), Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais, Universidade Federal do Rio Grande do Sul.

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[11] Venquiaruto SD, Isaia GC, Gastaldini ALG. A Influência do teor e da quantidade de adições minerais na carbonatação do concreto. In: Proceedings of the 43th Congresso Brasileiro do Conreto, Foz-do-Iguaçu, Brasil; 2001. São Paulo: IBRACON, 2001. 1v. (cd), p.14

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[13] Cabrera J.G, Claisse P.A, Hunt D.N. A statistical analysis of the factors which contribute to the corrosion of steel in Portland cement and silica fume concrete. Construction Building Mat 1995; 9: 105-13.

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[14] Roy SK, Beng PK, Northwood DO. The carbonation of concrete structures in the tropical environment of Singapore and a comparision with published data for temperate climates. Mag Concrete Res 1993; 48: 293-300.

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[16] Bauer E. Avaliação comparativa da influência da adição de escória de alto-forno na corrosão das armaduras através de técnicas eletroquímicas. São Paulo, 1995. 236 p. Tese (Doutorado). Escola Politécnica, Universidade de São Paulo.

[17] Monteiro ECB. Estudo da capacidade de proteção de alguns tipos de cimentos nacionais, em relação à corrosão de armaduras sob a ação conjunta de CO2 e íons cloretos. Brasília, 1996. 138p. Dissertação (Mestrado), Faculdade de Tecnologia, Universidade de Brasília.

[18] Cascudo O. Influência das características do aço carbono destinado ao uso como armaduras para concreto armado no comportamento frente à corrosão. São Paulo, 2000. 310p. Tese (Doutorado), Escola Politécnica, Universidade de são Paulo.

[19] Branca C, Fratesi R, Moriconi G, Simoncini S. Influence of fly ash on concrete carbonation and rebar corrosion. In: Proceedings of the 4th International conference on fly ash, silica fume, slag, and natural pozzolans in concrete, 1992, Istanbul, Turkey. Detroit: American Concrete Institute, 1993. v.1, p.245-255 (ACI Special Publication, 132).

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4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72

0

400

800

1200

1600

2000

XRD - Silica fume

Inte

nsity

(cps

)

2θθθθ Fig. 1. X-rays diffractograms of silica fume.

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Time Setting

0.40 0.50 0.60 0.70 0.80

10

20

30

40

50

60

70fc (MPa)

3

4

5

6

7

8

9m(kg/kg)

Workability = 70 ± 10 mm

w/c

600C(kg/m3)

550 500 450 400 350 300 250

1 day

3 days

7 days

28 days

m = 11,7684*w/c – 0,7814r2=0,9885

Time Setting

0.40 0.50 0.60 0.70 0.80

10

20

30

40

50

60

70fc (MPa)

3

4

5

6

7

8

9m(kg/kg)

Workability = 70 ± 10 mm

w/c

600C(kg/m3)

550 500 450 400 350 300 250

1 day

3 days

7 days

28 days

1 day

3 days

7 days

28 days

m = 11,7684*w/c – 0,7814r2=0,9885

Fig. 2. Proportioning diagram.

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0,3 0,4 0,5 0,6 0,7 0,8

0

5

10

15

20

25

30

obs 0% obs 5% obs 10% obs 15% obs 20%

0% 5% 10% 15% 20%

Car

bona

tion

dept

h -

d c (m

m)

water/binder ratio

0 5 10 15 20

obs 0,30 obs 0,35 obs 0,45 obs 0,60 obs 0,80

silica fume content (%)

0,30 0,35 0,45 0,60 0,80

Fig. 3. Concrete carbonation – 126 days concrete age (98 days accelerated carbonation test).

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0,3 0,4 0,5 0,6 0,7 0,8

15

20

25

30

35

40

45

50

55

60

65

70

75

80

fc

0% 5%

10% 15% 20%

Obs0%Obs5%Obs10%Obs15%Obs20%

Com

pres

sive

str

engt

h -

f c (M

Pa)

Water/binder ratio

0,3 0,4 0,5 0,6 0,7 0,8

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30 %0% %5% %10% %15% %20% obs0% obs5% obs10% obs15% obs20%

Water/binder ratio

Car

bona

tion

dept

h -

d c - 1

26 d

ays

(mm

)

Fig. 4. Relationship between carbonation and compressive strength in the concrete formulations used in the carbonation tests of this study.

21

0 5 10 15 200,00

0,03

0,06

0,09

0,12

0,15w/b

0,40 obs.0,55 obs.0,70 obs.

Water/binder ratio

0,40 0,45 0,50 0,55 0,60 0,65 0,70

∆ico

rr (

µA/c

m2 )

Silica fume content (%)

0,40 0,45 0,50 0,55 0,60 0,65 0,70

0% obs.10% obs.20% obs.

Silica fume

0% 5% 10% 15% 20%

Fig. 5. Final variation of the intensity of carbonation-induced corrosion

22

Table 1

Mechanical performance evolution of Brazilian cements in concrete[1].

fck (MPa)

1975 2002

w/c ratio

Cement content (kg/m3)

w/c ratio

Cement content (kg/m3)

25 0,40 378 0,55 305 20 0,47 321 0,61 272 16 0,54 281 0,67 252 12 0,62 246 0,73 223

23

Table 2 Carbonation tests design - independent variables.

Silica fume (%)

Water/binder ratio 0,30 0,35 0,45 0,60 0,80

0 2x -- 2x -- 2x 5 -- 2x -- 2x -- 10 2x -- 2x -- 2x 15 -- 2x -- 2x -- 20 2x -- 2x -- 2x

24

Table 3 Corrosion tests design - independent variables.

Water/binder ratio Silica fume (%) 0,40 0,55 0,70 0 2x 2x 2x 10 2x 2x 2x 20 2x 2x 2x

25

Table 4 Characteristics of cement

Characteristics Properties Chemical SiO2 18,20% Fe2O3 2,10% Al2O3 3,90% CaO 61,30% MgO 1,46% SO3 3,47% Na2O 0,20% K2O 0,89% Ignition loss 2,36% Physical Density 3,11 (kg/dm3) Finess - specific surface 4650 m2/kg Initial setting time 232 min Final setting time 328 min Mechanical Compressive strength (1 day) 23,7 MPa Compressive strength (3 days) 37,6 MPa Compressive strength (7 days) 42,7 MPa Compressive strength (28 days) 49,2 MPa

26

Table 5 Characteristics of silica fume.

Characteristics Properties Chemical SiO2 94,00% Fe2O3 0,07% Al2O3 0,05% CaO 0,33% MgO 0,55% Na2O 0,20% K2O 1,28% Ignition loss 3,01% Physical Density 2,20 (kg/dm3) Finess - specific surface 20780 m2/kg

27

Table 6 Characteristics of coarse and fine aggregates.

Characteristic Coarse aggregate Fine aggregates Fineness modulus 6,86 2,75 Maximum size (mm) 19 4,80 Specific gravity (g/cm3) 2,80 2,62

28

Table 7 ANOVA for the model of carbonation depth.

Source DF a SQ b MQ c Fcal Significance - p Regression Model 5 1591,0 318,2 203,4 0,0000 Residual 47 73,5 1,6 - - Uncorrected Total 52 1664,5 - - - Corrected Total 51 1417,38 - - -

a Degrees of freedom; b Sum of squared; c Squared mean.

29

Table 8 ANOVA for the factors studied for carbonation depth.

Factor Parameter Estimate Std Error t test p Constant b0 -16,99 1,99 -8,54 0,0000 wb a b1 8,75 2,14 4,09 0,0017 sf b b2 1,04 13,40 0,08 0,4700 wb x sf b12 1,41 16,79 0,08 0,4676 t c b3 2,64 0,21 12,57 0,0000

a water/binder ratio; b silica fume content; c test age

30

Table 9 ANOVA for the model of the final variation in the intensity of corrosion.

Source DF a SQ b MQ c Fcal Significance - p Regression Model

2 0,09224 0,04612 279,51 0,000

Residual 32 5,29 x 10-3 1,65 x 10-4 - - Uncorrected Total

34 0,09753 - - -

Corrected Total 33 0,04713 - - a Degrees of freedom; b Sum of squared; c Squared mean.

31

Table 10 ANOVA for the factors studied for the final variation in the intensity of corrosion.

Factor Parameter Estimate Std Error t test p wb a b1 1,122 0,0755 14,86 0,000 wb x sf b b12 0,00033 0,00008 4,13 0,000

a water/binder ratio; b silica fume content.