EXTENDED ABSTRACT

Modeling the evolution of carbonation in reinforced concrete elements

Isabel Filipa Garcia Monteiro

Coordenador: Professor Fernando Branco

June 2010

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ABSTRACT

In recent years, the number of reinforced concrete structures with pathologies has grown,

mainly associated with corrosion problems in the reinforcement. One of the main processes

responsible for these anomalies is the concrete carbonation. The concrete natural carbonation

is a process that depends simultaneously on the materials’ characteristics and on the

surrounding environment. It is a process associated with the absorption time of CO2 by

concrete, transforming its ph in an acid environment. There are several models that describe

the relationship between the carbonation depth and concrete age. The most used one is the

equation where k is the carbonation coefficient. The aim of this study was to define,

a range of values for the carbonation coefficient in order to be able to make more accurate

evaluations of the carbonation depth with the concrete age. About 100 measurements of real

data was considered (structure age and carbonation depth), from audits performed by IST.

Correlations were then analyzed. The data was split into groups with similar characteristics that

could influence the carbonation progress, such as age, superficial painting, compression

strength and exposure level. The carbonation coefficients obtained were then compared with

those reported in the latest LNEC specification.

KEYWORDS: Concrete, corrosion, carbonation, service life, carbonation coefficient;

 

1. Introduction 

The service life of reinforced concrete structures is one of the major construction concerns in

security, economical and environmental terms.

The long-term performance of a concrete infrastructure is mainly a function of the deterioration

level. Corrosion is the most common form of reinforcement deterioration and consists of the

chemical or electrochemical disintegration of the metallic material in an aggressive exposition

level. This is a gradual process with a time period for the corrosion initiation that is not always

easy to detect in situ. The corrosion can be induced by chlorides or by carbonation, but in this

work only the corrosion caused by penetration of carbon dioxide in the reinforced concrete

elements will be enhanced.

Several models have been developed in order to predict the service life of reinforced concrete

structures, but this is not a simple phenomenon, to model. It involves several different

processes, from the aggressive environment gas diffusion to the beginning of the corrosion

itself, and also several other parameters whose variability should not be ignored.

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Figure 1  TUUTI Service life model (1982)

The main purposes of the present work were: a) to analyze the existent models for expected

service life evaluation based on data of several studies already carried out; b) to analyze if the

models fit to experimental measurements; c) to compare the obtained values for the

carbonation coefficient with the ones of the latest LNEC specifications;

2. Models for predicting carbonation depth 

The service life of a structural component is the time after construction, during which all their

properties exceed the minimum acceptable level being subjected to periodic actions of

maintenance. The structures must be designed and constructed to maintain their security,

stability and suitability service during their life period, under the expected environmental

conditions and used as recommended.

Over the years, several methods to predict the service life of reinforced concrete elements have

been developed. Nowadays, usually two approaches are used to evaluate the structures’

service life. The first is an experimental method based on deterioration tests which try to

reproduce the conditions and aggressive agents the structure will be facing during life. The

other method is an analytical approach that uses mathematical models that try to simulate the

aggressive agents’ effects over time. They are based on the adjustment equations, according

to inspections data, and most of them are based on Fick's second law, which considers

additional assumptions and can be used as a simple mathematical equation. From the several

approaches used, the most cited in the literature is the TUUTI model, due to its extensive

experimentation. TUUTI (1982) proposed a simplified model for predicting the service life of

reinforced concrete structures, considering the degradation due to the phenomenon of

reinforcement corrosion.

This approach split service life into two distinct periods: the initiation period and propagation

period as presented in Figure 1.

This split occurs because the mechanisms involved in each period are different, in

physicochemical terms. So it is necessary to define two different models that use different

performance properties to consider those mechanisms. The initiation period (t1) is the time

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period from the start of the structure life until a significant part of the steel protection afforded by

the concrete (basic ph) is lost, due to carbonation or the chlorides penetration or a combination

of both effects. In this stage, the aggressive agents are still penetrating through the porosity of

the concrete covering layer without causing damage to the structure. Taking into account the

carbonation, it is considered that corrosion only becomes possible when the carbonation

thickness equals the concrete covering layers of the steel reinforcement.

The propagation period (t2) corresponds to the time period between the end of the initiation

period until the reinforcement corrosion assumes a level of unacceptable degradation. After

reaching the reinforcement steel, the factors which contribute to the propagation period are the

moisture and the oxygen concentration surrounding the steel reinforcement. The service life is

considered as the sum of the duration of these two periods t = t1 + t2.

In this work only models that reflect the initiation period, are considered because they have

been tested a lot more than the propagation models. About these latter ones, research is still in

the beginning and it is not yet possible to have confident models.

2.1  Models for the initiation period  

Modeling the advance of carbonation is a difficult task due to the complexity of the mechanisms

complexity that involves this process. The carbonation front is controlled by carbon dioxide

diffusion that occurs through the concrete porosity, which depends on moisture, temperature,

carbon dioxide concentration and cement composition. If all the factors are analyzed

simultaneously, the alkali content of the concrete and its greater or lesser ability to be

penetrated can be estimated.

As the carbonation is estimated by a process of CO2 diffusion, we can model its depth using the

1st Fick law (COSTA, 1999). The model proposed by TUTTI (1982), is based on the diffusion

law and considered that the carbonation rate is proportional to the square root of exposure time

to the aggressive agent, as presented in equation (1).

( 1) 

where,

x- carbonation depth (mm);

K- carbonation coefficient (mm/year-1/2);

t - exposure time to the aggressive agent (year);

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Most of the models for carbonation advance prediction are based on the equation presented

above. Several authors propose some changes to this equation in order to have a better

approximation of the depth carbonation prediction. These include the case of Smolcyk and

Daimond et al (1971). The first takes into account that the exponent n can depend on the type

concrete that is used and also includes a new parameter t0. With this parameter a model can be

developed to estimate the carbonation of existing structures, if their age is known (2). Daimond

takes out an initial period (ti) that is the material drying period while the diffusion of carbon

dioxide is initially blocked by the porosity saturated with water (3)

Smolcyk

(2)

Daimond et al (3) 

Based on the literature, the major difficulty is to estimate the carbonation coefficient (k) present

in the various models. Mostly because of the simplifications, the models do not reflect the

intensity of the real phenomenon. Other methods avoid these simplifications but it is difficult or

expensive to obtain their parameters. Bakker (1988) doubted that it would ever be possible to

make a fully adequate formula to predict the carbonation rate, taking into account all of the

parameters involved.

Basically, the equation TUTTI is the basis for all other approaches, with a very simple formula to

estimate the carbonation depth, where the more complicated task is to find the k value. This

parameter is assumed as a durability concrete coefficient and includes the effects of all

dependent concrete variables and aggressive environment. In the existing literature, a wide

range of values can be found with or without the application of corrective factors to take into

account aspects such as cement type, class or exposure protection systems that may be

applied.

2.2 Modeling the concrete durability according to LNEC E465 specification  

The LNEC specification that established a methodology for estimating the performance

properties of reinforced concrete from the action of carbon dioxide and chlorides, was published

in November 2007 and can satisfy the service life desired (E465-2007). To use this model, in

the determination of concrete performance values, it is necessary to define a number of factors:

service life, environmental level exposure and reliability class.

It is recommended, as an application rule, that the service life of each concrete structure should

be specified within five categories, depending on features and functions of the structure. In the

following analysis of this work a 50 years’ service life was considered.

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It is also mandatory to identify the environmental exposure classes based on the environment

description and informative examples. This simplification reflects, in an incomplete way, the

influence of environment on the corrosion resistance of reinforcement concrete, as well as

moisture, temperature, solar radiation, rain and wind. In the following analysis of this work, it

was taken into account the effects on reinforced concrete elements exposed to aggressive

environments as XC3 and XC4.

Another issue that must be set previously in this model is the structure reliability class: RC1,

RC2 or RC3. Each class is defined by the consequences of failure or malfunction of the

structure. For the analysis of this work, it a reliability class RC2 was chosen to be followed,

because it is a regular reliability situation.

As in the theoretical models discussed above, also in this specification, the model TUTTI is

considered. In LNEC specification two models are used for the initial period. Only the first will be

compared with experimental values. The first model can be presented by equation (4).

(4)

where,

k0 - factor that takes the value 3 when the test conditions are the same as LNEC specification

E391;

k1 - factor that takes in account the relative humidity influence, depending on the exposure

level, as is presented in Table 1;

k2 - factor that takes in account the cure influence, and takes the value 1 on a standardized

cure and 0.25 when the formwork has controlled permeability and the cure is 3 days;

n - factor that takes in account the influence of wetting/drying over time, whose values are also

presented in Table 1.

As seen, this model depends on the concrete exposure level and thereby the k's and n, but also

the carbonation resistance (RC65). This value is not easy to estimate and induces some

uncertainty when used to calculate the carbonation depth. This value can either be found

through a table provided in the LNEC specification, but one has to assume certain parameters.

Another way is by using equations developed according to the cement specifications. The

formulas used in this specification model to calculate the carbonation resistance (RC65) can be

found in Table 2.

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Table 1 Parameters values, k1 and n 

XC1 XC2 XC3 XC4

k1 1,0 0,20 0,77 0,41

N 0 0,183 0,02 0,085

 

 

 

  Table 2 (RC65) calculation 

Rc65 Cement specifications

Rc65 = 0,0016.fcm3,106 CEM I; CEM II/A

 (5) 

Rc65 = 0,0018.fcm2,862

CEM II/B; CEM III; CEM IV; CEM V

 

(6)  

As stated above, the data processing in this work was based on achieving a carbonation

coefficient (k) that can relate the carbonation depth with the concrete age. So, there was the

requirement to transform the formula proposed in the LNEC specification to a formula that leads

to a k single value, (7).

 

 

By observing the LNEC model equation, it can be seen that, the concrete age is not a power of

0.5 but a power of 0.5-n. Dealing only with exposure environments XC3 and XC4, these values

will be 0,5-0,02 (0.48) and 0,5-0,085 (0.415), respectively. So, it was necessary to estimate the

RC65 value. For this, the equations described above can be used or one table provided by LNEC

where the value can be taken directly. It is assumed the environment XC4 in the dry region, 50

years of service life, reliability class RC2 and a normal 35 mm covering. So it was assigned a

value of RC65 of 41 kg.year/m5.

At first sight, this seemed completely unbalanced. It was necessary to find another value of k

but this time using the equation (5). This required a compression strength concrete value (Fcm).

The mean value found in the available data was 41.8 MPa, of the experimental data. To make

the time being associated to the reference 28 days, it was used the normal REBAP, and 28.8

MPa was obtained for the reference strength. For the latter value, and using equation (5), the

RC65 value obtained was 54.7 kg.year/m5, which corresponds to a carbonation coefficient of:

 

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3. Measurement Campaign

This work was based on data collected about carbonation depth from inspections carried out by

ICIST during the time period between 1998 and 2008. This is a wide range of reports that cover

different types of structures and ages between 4 and 65 years. Assuming that the carbonation

depth is linear dependent to the square root of exposure time, x = K × √ t, and using the data

collected it is possible to obtain carbonation coefficient values. These carbonation coefficients

were achieved in various circumstances and then they were analyzed.

For the initiation period 113 values were achieved. It is possible to distinguish between several

structure groups that were subject to inspection by the ICIST and were used in the

measurements, like: Viaducts, Parking Decks of Hypermarkets, Bullring Arenas and High

Schools. Each structure has a different functionality, with very specific characteristics and all

this will influence the values of their measurements. Therefore each case must be processed

and analyzed in a separate attempt to interpret all the factors that may lead to the carbonation

depths and carbonation corresponding coefficient.

Despite all the work developed mainly around the carbonation depth of the various components

of concrete, other values have been compiled, with equal interest and have proved useful for

the interpretation of some tests performed. Examples of that are the initial covering concrete of

the elements, as well as the concrete compression strength at the time of their audits. In almost

all reports, the experimental trials that led to the data used were all prepared following the same

procedure.

Carbonation depth measurements, in most examinations, were performed by direct

measurement of the cores taken from concrete elements and through holes opened in other

locations, using a spray-based ph indicator (fenofetaléina ver em inglês). In regard to concrete

compression strength measurements, they were determined by sclerometer.

In addition to this data that was explicitly demonstrated in the reports, other important analysis

was observed about certain characteristics that could influence, positively or negatively the

depth of carbonation. Like whether or not they finish were protected by painting, if they were in

an area of pollution and high humidity and the fact that they are located indoor or outdoor.

Finally, using the photographs that were in the photographic survey of each report it was

possible to fill a table with deterioration development in concrete elements. Three main states

of transition from deterioration were considered: cracking, peeling and corrosion. Each state

was split into three sub-states: an area with only one boar, an area with many bars or many

areas. As references taken from photographs, it was always necessary to take into account

their subjectivity, because different observers may draw different conclusions from the same

concrete element.

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There are 113 different elements, from over 37 structures. For a better analysis of the different

groups of elements, the amount of data that was accessed is presented in Table 3 and Table 4.

Analysis on the carbonation depth evolution was only made in groups that had more than 15

items.

Table 3 Amount of values          Table 4 Amount of values for each element 

 

4. Measurements Analysis 

To estimate the carbonation coefficient (k), the initial general analysis made for this purpose,

uses all available data without making any distinction of different structures. All of the data of

carbonation depth vs. age was put in function like y = x √ K. The obtained value from this

analysis was k = 3.50 mm/year0,5, as is presented in Figure 2.

 

Figure 2 Relationship between carbonation depth and age with all values

Elements Pillars 30 Beams 29 Slab 15

Walls 15

Bridge deck 7

Walls forehead 3

Borders 3

Others 11

Compression strength ≥ 35

69

Compression strength 35

26

painted 40 No painted 70

Elements With pollution 34

Painted Without pollution

7

With pollution 58 Outside

No painted Without pollution

1

With pollution 0 Painted Without

pollution 1

With pollution 4 Inside

No painted Without pollution

8

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This adjustment was done using an Excel function, the function Linear Projection (PROJL). This

approach was used for all of the data in Figure 2, as well as for the other tests, described in the

following section. By observing the chart, it was noted that some points were clearly far from the

progress of the function. Then there was the need to recheck these points in order to detect

some outliers. A little clean-up was made to eliminate these outliers but the carbonation

coefficient has only changed in the 3rd decimal places, which is not relevant.

   

Later, the same procedure was made for obtaining the carbonation coefficient, but this time the

elements were split into various groups depending on certain characteristics (painting,

compression strength, environment level, age and function). The aim was to evaluate whether

some of them interfered, and how, in advance of carbonation, comparing the values of k

obtained in the overall analysis. The results are presented in Table 5.

Table 5 Carbonation coefficient summary 

Elements k R Comments

Painted 4,27 0,870 In this case the k value was higher than the average

value of 3,50. This was not expected because the paint

protects the concrete, and should reduce carbonation

process;

Not painted 3,89 0,766 The value of k was slightly superior to that found in the

overall analysis. This fact can be explained by the

absence of paint, which does not give protection to the

elements in question, thus allowing a further advance

of carbonation;

Res.≤35 MPa 4,07 0,711 In less resistance, the carbonation moves much mores

easily , hence the value of k is greater than the general

analysis;

Res.≥35 MPa 3,38 0,847 In concrete with more compression strength it is

harder to make progress in carbonations processes;

Outside 3,32 0,906 Coefficient is within the value found in the overall

analysis;

Interior Polluted 4,09 0,705 All the points analyzed in this section, were in Parking’s

Deck. These elements are constantly in contact with

heavily polluted environments, a factor that makes up

the whole process of carbonation, increasing the value

of k; 

 

 

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Elements k R Comments

Age ≤ 20 anos 3,94 0,668 The value is greater than 3.50. In the youngest

structures the carbonation process tends to develop

faster and eventually develop slower past a few years;

Age ≥ 20 anos 3,37 0,839 Value lower than that found in the overall analysis. For

older structures the carbonation process tends to

develop slowly.

In many situations, it is important to have a range of values associated with a confidence

probability. For this, it was considered a definition of the confidence interval (CI) assuming a

standard normal distribution.

Using all data from the carbonation depths and the structure age of the points discussed, it was

possible to calculate, for the carbonation coefficient, mean, standard deviation and variance

coefficient, as presented in Table 7. As can be noticed, the standard deviation has a quite high

value. This occurred because there is a large discrepancy between the various values of k

considered. They were determined for the confidence intervals 95%, 80% and 50%, as

presented in Table 6.

Table 6  Confidence Interval      Table 7 Statistics values for carbonation coefficient 

.

In addition to the statistical analysis deduced by the experimental data, there was also the need

for comparison with the LNEC values. As mentioned above, the direct comparison with the

value of k would be approximate because of the variable concrete age in the LNEC specification

is a power of 0,48 and not a power of 0,5.. In addition, to obtain the carbonation coefficient by

LNEC, it would be necessary to have access to the value of the resistance of the concrete,

which in our case is not necessary because a general approach was used.

   

A direct relationship between the carbonation depths was obtained for both tests. It began by

the observation of the progress of three curves, drawn for the same compression strength (fcm

= 28.8 MPa). The blue curve followed the average value of carbonation coefficient used in this

work (k = 3.50) while the red curve returned values of carbonation depth determined by the

LNEC specification formula. In the middle, is the green curve corresponding to the confidence

interval of 95%.

Intervals Lower Upper 95% 3,18 4,06

80% 3,34 3,91

50% 3,47 3,77

Average (µ) 3,6

Standard deviation 2,26

Amount of values (n) 103

Variance coefficient (cv) 0,677

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Figure 3 Comparison of progress curves by LNEC and dates analysis, with  constant resistance fcm = 28.8 MPa 

As we have seen from observing Figure 3, the progress of LNEC curve is significantly higher

than in the blue curve, as it was calculated.

Another graph was made, but this time the carbonation depth is a function of the resistance in

the LNEC equation. As presented in Figure 4, the curve that uses the carbonation coefficient

found in this work is fully fit in the middle of the curves illustrating the progress of concrete with

compression strengths of 40 MPa and 35 MPa. The only major difference is between our curve

and the curve of resistance LNEC 25 MPa.

Figure 4 Comparison of progress curves by LNEC and dates analysis, with several resistances 

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There was also a need to analyze the bar cover values according to the recommended

approach for a 60 years’ service life. The k values obtained previously and the equation (1)

were used to determine the following bar covers as it can be confirmed in Table 8.

Table 8 Bar cover values (mm) 

Average 95 % LNEC

General 25 29 40

concrete ≥ 35 MPa 24 26 22

concrete ≤ 35 MPa 29 42 50

By observing Figure 5, it appears that most of the bar covers analyzed (39 data) are still well

below the average recommended in general. This factor is a major contributor to the

advancement of corrosion in reinforced concrete elements. Furthermore, it was found that the

approach of LNEC, returns a bar cover with a value somewhat higher, which is only fulfilled by 4

points in this analysis.

 

Figure 5 Comparison of bar covers for a 60 years’  service life 

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5. Main Conclusions

- The correlation values that were estimated concluded that the carbonation depth has a

variation with the square root of time.

- Variations of the carbonation coefficient led to conclude that factors such as age, strength, and

painting exposition will influence the carbonation process.

- The values accepted by LNEC, are rather under-sized. These values can be explained

because the tests were only based on a single structure, Funchal airport.

- Awareness of the use of recommended bar covers is a concern that must be taken into

account because they will influence the overall carbonation progress.

- It would be useful to follow the studies of this work, using tests that could show the importance

of each factor in the carbonation progress. Consequently, allowing a better evaluation of the

carbonation coefficient.

REFERENCES

ANDRADE, C.; ALONSO, C., Vida útil y vida residual das estructuras de hormigon, Seminário,

Laboratório Nacional de Engenharia Civil, Lisboa, Outubro; Portugal, 1996.

BAKKER, R.F.M., Initiation period. In: Corrosion of Steel in Concrete, P Schiessl,

RILEM,1988.

COSTA, António, Anomalias e Mecanismos de deterioração, Módulo 2, Apontamentos da

Cadeira de Reabilitação e Reforço Estrutural, Instituto Superior Técnico, 1999.

SCHRODER, F. & SMOLCZYK, H. G. Carbonation and protection against steel corrosion. In:

Fifth Int. Symp. Chemistry of Cement, vol. 4, Tokyo, 1968.

TUUTTI, K. Corrosion of steel in concrete. Stockholm, Swedish Cement and Concrete

Research Institute, 1982.