A model of concrete carbonation depth under the coupling effects of load and environment.pdf

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A model of concrete carbonation depth under the coupling effects of load and environment

Y. Ren*, Q. Huang, X. L. Liu and Z. J. Tong

Based on the durability test data of the existing bridges in Hainan province, this study explored how

the following factors, namely, the strength grade of concrete, stress condition, chloride ion content

and environment influenced the depth of the concrete carbonation. A prediction model for the

concrete carbonation depth of the in-service bridges in Hainan was established. The actual

bridge carbonation data collected in recent years were employed to calculate the concrete

quality impact coefficients. The impact on the carbonation depth of concrete from stress

condition is large, the results show that the greater the concrete compressive stress, the slower

the concrete carbonation rate; the larger the tensile stress, the faster the concrete carbonation

rate. After inspection, it was found that the calculation results of the model agreed well with the

bridge field test results, which suggested that the model would have good applicability.

Keywords: Concrete, Carbonation, Coupling effects, Load, Prediction

Introduction

Besides being eroded by carbon dioxide and chloride,concrete bridges in coastal or marine environment arealso subjected to the dead load and vehicle load. Someconcrete bridges, only in operation for approximately10 years, start to have reinforcement corrosion and pro-

tection layer spalling, which will seriously affect the struc-ture’s safety, serviceability and durability. To ensure theservice life of bridges, the durability of the concretematerial itself plays a key role.

In marine-atmosphere environment with sufficientoxygen supply, the coupling effect of concrete carbona-tion and chloride ion erosion can cause the accelerationof concrete damage and intensification of concrete cor-rosion. According to the mechanism of concrete cor-rosion, micro-cracks inside the concrete, under theheavy load of a bridge, will be expanded to a certainextent, then the damage and deterioration of a concretestructure will be further accelerated and intensified, thusthe service life of the concrete structure will be shortened

correspondingly.1

Under the coupling effects of environment and heavytraffic, concrete bridges in Hainan have serious steel cor-rosion. In particular, the concrete carbonation depth of these bridges is significantly higher than those in theinland areas. Based on the durability test data of thebridges in Hainan, this research explored how the follow-ing factors, namely, the strength grade of concrete, stresscondition, chloride ion content and environment, influ-enced the depth of the concrete carbonation. In the mean-time, a prediction model for the concrete carbonation

depth applicable to the in-service bridges in Hainan iscarried out. It provides a theoretical basis for furtherservice life evaluation.

Durability tests on the in-service

concrete bridges in HainanOn the basis of thorough consideration of the impact of the marine environment and the structural characteristicsof the in-service bridges in Hainan province, this researchtook typical bridges in the east and west expressways inHainan to conduct the durability tests. The tested itemsincluded bridge condition survey, environmental con-dition survey, traffic volume survey, investigation of theconcrete carbonation depth, detection of the chlorideion content, detection of the thickness of the concreteprotective layer, detection of the reinforcement corrosionand concrete crack detection.

The results showed that the concrete bridges in Hainanexperienced a relatively serious durability damage.Common deficiencies include cracks induced by concretecorrosion, concrete protective layer spalling and exposedsteel bars. Some bridges, only in operation for 10-oddyears, had serious concrete corrosions. In addition, forthe bridges in this region, concrete carbonation depth isdeep; chlorine ion content is high; the thickness of protec-tive layer is small; especially, the detection results of thecarbonation depth of concrete are significantly higherthan those in the inland areas, and they are also higherthan the results derived from classic carbonation depthprediction models. For example, a bridge had been inservice for only 13 years, but the carbonation depthreached 33.11 mm. The following reasons might explain

this phenomenon:(i) chloride content is high in Hainan;

School of Transportation, Southeast University, Nanjing 210096, China*Corresponding author, email [email protected]

© W. S. Maney & Son Ltd 2015Received 3 November 2014; accepted 12 March 2015DOI 10.1179/1432891715Z.0000000001970 Materials Research Innovations 2015 VOL 19 SUPPL 9S9-224

mailto:[email protected]





(ii) annual temperature and relative humidity are rela-tively higher in this region;

(iii) concrete grade of the bridges is high;(iv) since there is no road toll station in Hainan, roads

are overloaded by vehicles.Prediction models of the concrete carbonation depth of the in-service bridges in Hainan were studied from theabove four aspects and discussed as follows.

Study and establishment of the concretecarbonation depth model affected by multi-factors

So far, scholars from home and abroad have had a rela-tively mature cognition on the concrete carbonationmechanism and influencing factors. The law that carbo-nation depth is proportional to the square root of the car-bonised time has been widely accepted.2 There are threecategories of concrete carbonation depth models: theor-etical model,3 empirical model and practical modelbased on diffusion theory and experiments.4,5

According to the existing data and taking into accountthat the depth of the concrete carbonation of thebridges in Hainan is related to the factors such as thestress condition of concrete, bridge environment and con-crete quality, a modified random carbonation depthmodel with multiple coefficients was derived on thebasis of Refs. 5 – 7, as shown in equation (1).

X t( ) = K mck j k Clk CO2k pk sK eK f

t

√ (1)

where X (t) is the carbonation depth of concrete (mm),K mc is the undetermined random variable of calculationmodel, k j is the corner correction coefficient, k Cl is theinfluence coefficient of the concentration of chloride,

k CO2 is the CO2 concentration impact coefficient, k p isthe casting surface correction coefficient, k s is the stressimpact coefficient, K e is the environment impact coeffi-cient, K f is the concrete quality impact coefficient and t

is the carbonised time. Considering the fact that thecorner concrete carbonation is actually a double diffu-sion, it is suggested that corner k j = 1.4 and non-cornerk j = 1.0. In addition, k CO2= 1.1 – 1.4 according to thelocal environmental condition, and k p= 1.2 accordingto Ref. 8.

Equation (1) indicates that the key to establish the pre-diction model is to reasonably determine the coefficients

in the formula. On the basis of the durability test of theconcrete bridges in Hainan, the present study mainlyfocused on the methods of determining the four key par-ameters mentioned above: K f , k s, K e and k Cl.

Determination of the concrete quality impactcoefficientAt present, most concrete carbonation models are estab-

lished on the basis of the results of low-grade concretetest or survey of actual engineering project. However, ina real bridge structure, the grade of concrete strength ishigher. The concrete strength grade of the in-servicebridges in Hainan generally ranges from C30 to C50.There is a large deviation between the carbonationdepth calculated by the existing model and the measuredresults.

In order to determine the suitable concrete qualityimpact coefficient for Hainan province, a standardenvironment was set to analyse the data of the concretecarbonation of 63 bridges in recent years. A diagramwas obtained to demonstrate the relation between thespeed of concrete carbonation and the standard value of

the concrete compressive strength (Fig. 1).The results showed that the concrete carbonation depth

was proportional to the reciprocal of the compressivestrength. According to the trend of the scatterdiagram,3,5 equation (2) was selected for a regressionanalysis:

K f = 68.83 f −1cu,k − 0.7307 (2)

where K f is the concrete quality impact coefficient, and f cu,k is the standard value of the concrete compressivestrength.

After calculation by equation (2), comparing the

detected value and the calculated value, the goodness of fit R2 was 0.8352, whereas R2 was 0.2747 by using themethod in Ref. 5. Thus, the concrete quality influencingcoefficient model established in this study was more suit-able for Hainan.

Determination of the stress impact coefficientSince 1 January 1994, Hainan province started collectingmotor vehicle fuel surcharge and removed all highway tollstations, including those on the expressways. Then a fol-lowing problem emerged: the number of overweightvehicles was increasing year by year in Hainan province.

1 Relation schema of the carbonation speed and the standard value of the concrete compressive strength

Ren et al. A model of concrete carbonation depth

Materials Research Innovations 2015 VOL 1 9 SUPPL 9 S9-225



Some vehicles overweigh 400 times than their verifiedweight, which makes the bridges overload. Manybridges are in operation with deficiencies. Increasingload will inevitably affect the stress condition of the con-crete structure, and the stress change will narrow or widen

the cracks, and thus affect the compactness of the struc-ture and the rate of concrete carbonation. An experimentwas conducted to prove that the compressive stress couldslow down the rate of concrete carbonation. When stressis less than 0.7 f c, the greater the compressive stress, theslower the carbonation rate; on the contrary, the tensilestress can accelerate the rate of concrete carbonation,and the larger the tensile stress, the faster the carbonationrate.6

– 8 Therefore, the tensile stress has an obvious impacton the concrete carbonation depth.

Since the bridges in Hainan bear large traffic loads andthe overload phenomenon is serious, the designed stan-dard load cannot sustain the current situation.

Therefore, the actual vehicle load distribution has to betaken into consideration so as to evaluate the real stresscondition of the bridge. Through a survey on the trafficvolume of the expressway in Hainan, a vehicle loadmodel based on the measured data was established bystudying the statistical distribution of the componentsand proportion of the traffic volume, as well as theweight and speed of vehicles, axle load, time headwayand other parameters. At the same time, finite elementmodels were established for the bridges under the dura-bility test in Hainan. By loading the actual vehicle loadon the model and analysing the results of the finite

element calculation, the actual stress in the concrete car-bonation detected region was obtained.

Through normalisation of the concrete carbonationdepth data of the in-service bridges in Hainan area tothe uniform environment standards, a schema about the

relationship between the concrete carbonation rate andthe concrete stress level was obtained (Figs. 2 and 3).

It indicated that the greater the concrete compressivestress, the slower the concrete carbonation; the largerthe tensile stress, the faster the concrete carbonation(Figs. 2 and 3). This was in consistent with the resultsof Ref. 9, where the concrete carbonation velocity andthe stress level was a quadratic function relation.According to the trend of the scatter diagram, quadraticpolynomial can be selected as the most reasonablemethod for the regression analysis. The followingequations can be obtained for a standard environmentin Hainan:

For the compressive stress:

k s = 0.6973 + 4.363sc − 18.18s2c (3)

for the tensile stress:

k s = −0.1291 + 11.74st − 5.991s2t (4)

where k s is the concrete stress impact coefficient, sc is thecompressive stress level and st is the tensile stress level.

2 Schema of the relation between the carbonation speed and the concrete compressive stress level

3 Schema of the relation between the carbonation speed and the concrete tensile stress level


Materials Research Innovations 2015 VOL 1 9 SUPPL 9S9-226



It is known from the figure that, by applying equations(3) and (4), comparing the detected value and calculatedvalue, R2 is 0.853 and 0.861, respectively. But in Ref. 10,the stress condition influencing coefficient k s was 1.0 forthe compression stress and was 1.1 for the tensile stress.Thus, the stress influencing coefficient model establishedby this study is more suitable for Hainan.

Determination of the environment impactcoefficientAs for the environmental impact coefficient K e, the influ-ence of temperature and humidity on the concrete carbo-nation is considered. The calculation is conductedaccording to equation (5):11

K e = α

T

4√

1 − RH( )RH (5)

where α is the environment impact factor, RH is the rela-tive humidity of environment (%) and T is the meanannual temperature of environment (°C).

The value of α reflects the environment impact on thecarbonation depth in the area. According to the data inRefs. 5 and 12, combining with the measured data of Hainan province, for the region of Hainan, a standardenvironment is determined as RH= 80% and T = 25°C.Through calculation, the value of α is 2.816.

Determination of the impact coefficient of chloride ion erosion on carbonationAt present, a lot of researches focus on the effect of car-bonation on chloride ion erosion, but few studies on theimpact of chloride ion erosion on the carbonation of con-crete are reported. A relevant research13 showed thatchloride ion erosion could refine the pore structure of concrete, prevent carbon dioxide gas from entering the

structure, thus greatly improve the carbonation resistanceof concrete. The study also believed that, in a marine-atmosphere environment, the concrete carbonationdepth was generally small, and the chloride ion erosionwas dominant.6 This conclusion was inconsistent withthe detection result that concrete carbonation depth isrelatively large in Hainan. Therefore, for the region of Hainan, the influence of chloride ion on the carbonationof erosion was not taken into consideration, and the valueof k Cl was set 1.0.

Establishment of the concrete carbonation depthmodel under the impact of multi-factors

In conclusion, the concrete carbonation depth predictionmodel for the bridge components in Hainan can beexpressed in equation (6).

X (t) = 2.816k jk Clk CO2k pk s

T

4√

RH 1 − RH( )× 68.83 f −1

cu,k − 0.7307

s

t

√ (6)

The physical meanings and specific expressions of eachparameter in equation (6) can be referred from theabove equations.

Model verificationIn order to verify the applicability and practicability of this model and predict the concrete carbonation depthof the in-service bridges in Hainan, the study conducted T

a b l e

1

C o m p a r i s o n b e t w e e n t h e c a l c u l a t e d

v a l u e s a n d t h e m e a s u r e d v a l u e s o f t h e

c o n c r e t e c a r b o n a t i o n d e p t h o f t h e b r i d

g e s i n H a i n a n ( a p a r t o f t h e r e s u l t s )

N o .

D e t e c t e d

s i t e

S e r v i c e

t i m e / y e a r

M e a s u r e d v a l u e

s / m m

C a l c u l a t e d v a l u e s i n

t h i s s t u d y / m m

C a l c u l a t e d

v a l u e s

i n R e f . 5 / m

m

N o .

D e t e c t e d

s i t e

S e r v i c e

t i m e / y e a r M

e a s u r e d v a l u e s / m m

C a l c u l a t e d v a l u e s

i n

t h i s s t u d y / m m

C a l c u l a t e d v a l u e s i n

R e f . 5 / m m

1

H o l l o w

s l a b

1 3

2 4 . 7

5

2 2 . 3

4

5 . 3 5

7

P i e r

1 3

1 1 . 7

5

1 1 . 4

2

9 . 7

3

2

T b e a m

1 2

1 7 . 0

2

1 6 . 5

0

2 . 5 5

8

P i e r

1 3

1 2 . 1

0

1 2 . 0

6

1 0 . 4

5

3

B o x g i r d e r

1 3

3 3 . 1

1

3 3 . 3

7

7 . 9 6

9

P i e r

1 2

1 4 . 6

0

1 3 . 7

6

1 2 . 9

3

4

T b e a m

1 1

1 5 . 2

0

1 6 . 7

2

2 . 4 7

1 0

P i e r

1 1

1 1 . 9

0

1 2 . 8

4

1 1 . 4

9

5

T b e a m

1 3

2 5 . 6

9

2 8 . 9

9

4 . 5 4

1 1

P i e r

1 1

3 . 2

0

3 . 5

8

4 . 1

9

6

H o l l o w

s l a b

1 3

2 1 . 3

6

2 0 . 8

0

2 . 6 5

1 2

P i e r

1 3

1 2 . 7

1

1 3 . 0

8

1 1 . 2

3


Materials Research Innovations 2015 VOL 1 9 SUPPL 9 S9-227



a comparison between the prediction results and thedetection results of the bridge carbonation depth. Partof the results is shown in Table 1. All the values of environmental impact parameters (such as CO2 concen-tration, temperature and humidity) and the concrete com-pressive strength were obtained from field tests. Theconcrete stress level was determined by a survey on thetraffic volume and finite element analysis.

The R2 for the comparison between the measuredvalues and the calculated values in the model of thisstudy was 0.9854, whereas the R2 obtained from the com-parison with Ref. 5 was 0.2230. It could be concluded thatthe prediction model of the concrete carbonation depth of the in-service bridges in Hainan provided by this studyhad higher accuracy and stronger applicability comparedwith other models.

Conclusions

1. Through the durability test on the in-service concretebridges in Hainan, it can be concluded that under the

combined effects of environment and vehicle loads,there are serious reinforcement corrosions in con-crete, and the concrete carbonation depth is signifi-cantly higher than that in the inland areas, and alsohigher than the calculated results from the classicalprediction model for carbonation depth.

2. Most concrete carbonation models are based on theresult of low-grade concrete test or survey of actualengineering project, but the actual strength grade of concrete bridge structure is higher. The carbonationdepth calculated by the existing model greatlyvaried from the measured results. In this study, theactual bridge carbonation data collected in recentyears were employed to calculate the concrete

quality impact coefficients. The results fit betterwith the measured results.

3. The phenomenon of vehicle overloading is serious inHainan. The impact on the carbonation depth of con-crete from stress condition is big. Through the statisti-cal analysis of traffic volume and finite elementcalculation, the relationship between the real stresslevel of concrete and the carbonation rate is obtained:the greater the concrete compressive stress, the slowerthe concrete carbonation rate; the larger the tensilestress, the faster the concrete carbonation rate.

4. Taking into consideration of the impact on the concretecarbonation depth from factors such as the strengthgrade of concrete, stress condition and environment,a prediction model for the concrete carbonationdepth of the in-service bridges in Hainan has beenestablished. After inspection, the calculated results of the model fit well with the field test results, suggestingthis model is of good applicability.

Acknowledgement

This study was supported by the National NaturalScience Foundation of China (51208096).

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C o p y r i g h t o f M a t e r i a l s R e s e a r c h I n n o v a t i o n s i s t h e p r o p e r t y o f M a n e y P u b l i s h i n g a n d i t s

c o n t e n t m a y n o t b e c o p i e d o r e m a i l e d t o m u l t i p l e s i t e s o r p o s t e d t o a l i s t s e r v w i t h o u t t h e

c o p y r i g h t h o l d e r ' s e x p r e s s w r i t t e n p e r m i s s i o n . H o w e v e r , u s e r s m a y p r i n t , d o w n l o a d , o r e m a i l

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A model of concrete carbonation depth under the coupling effects of load and environment.pdf