29
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
CONDITION ASSESSMENT OF REINFORCED CONCRETE
ELEMENTS EXPOSED TO CARBONATION
Samindi Samarakoon
(1)
, Jan Sælensminde
(2)
, Cecilie Myklebust Helle
(1)
(1) University of Stavanger, Stavanger, Norway
(2)
Betec AS, Bergen, Norway
Abstract
Onshore reinforced concrete structures are vulnerable to deterioration due to carbonation.
Condition assessment is a vital task, which helps to determine the reliability of structural
elements over the remaining service life. This study assesses the condition of reinforced
concrete structural elements (i.e. consoles) which are 53 years into their service life. The
reinforced concrete consoles, exhibiting no visible signs of corrosion, were chosen to measure
carbonation depths. In addition, non-destructive testing method: half-cell potential
measurements were taken over the surfaces of the consoles. A full-probabilistic service life
prediction model was used to calculate the expected service life, and a comparison was made
based on the actual carbonation depth measurements.
1.
Introduction
Condition assessment of the existing reinforced concrete structures is an important task in the
planning of maintenance and modification activities. Reinforcement corrosion can be
considered as one of the potential mechanisms which affect the durability of reinforced
concrete structures. There are many reasons for the corrosion of steel reinforcement, with
carbonation and chloride-induced corrosion being dominant mechanisms among them.
Therefore, it is important to choose the dominant phenomenon based on the exposure
condition of the structure. Hence, in this study, the diffusion of CO
2
is considered as the
dominant transport mechanism for the corrosion in steel reinforcement of residential
buildings.
Deterministic and probabilistic models have been developed for service life design, to predict
the time to initiate corrosion and the time to propagate corrosion. However, the probability
based models can help to make more realistic decisions than deterministic models. Therefore,
in this study, the full probability based model given in fib_bulletin_34 [1] is used to calculate
30
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
the probability of corrosion initiation. In addition, there are various proactive and reactive
approaches developed for controlling the condition of reinforced concrete structures, such as
measuring carbonation depths, using non-destructive testing methods (i.e. Half-Cell Potential
(HCP) measurements), visual inspection, etc. In this, study, potential mapping has been
carried out over the surface of three consoles, and carbonation depths were measured at
selected locations. Moreover, this manuscript discusses a case study of three reinforced
concrete consoles in a residential building. In addition, it determines the probability of
corrosion initiation using the DuraCrete [2, 3] model and compares it with HCP data.
Moreover, the measured carbonation depths are compared with calculated average
carbonation depths.
2.
Modelling of reinforcement corrosion due to carbonation
Carbonation is a chemical process, in which the carbon dioxide in air diffuses into the
concrete, dissolves in the pore solution, and
reacts with calcium hydroxide, forming insoluble
calcium carbonate and water. This results in a reduced pH-value of the concrete, which is one
reason for initiating the corrosion of steel reinforcement. The level of damage due to the
corrosion of embedded steel in a concrete structure over time can be described using Tuutti’s
model [4]. Essentially, the model categorizes the service life of a structure into the corrosion
initiation period and the corrosion propagation period. This manuscript focuses on the
corrosion initiation phase.
Many researchers have used Fick’s second law to model the carbonation to the un-cracked
concrete, considering diffusion as the dominant transport mechanism. In
this paper, the
DuraCrete [2, 3] model, derived using Fick’s second law, is adopted to include environmental
and material parameters, as given in Eq. (1).
*w(t)
(1)
X
c
(t): carbonation depth (mm)
k
e
:
environmental function (-)
k
c
: execution parameter (-)
k
t
: regression parameter (-)
R
ACC, 0
-1
: inverse effective carbonation resistance ((mm
2
/year)/(kg/m
3
))
t
: error term ((mm
2
/year)/(kg/m
3
))
C
s
: CO
2
concentration of the ambient environment (kg/m
3
)
w(t): weather function (-), where w(t)=(t
0
/t)
w
, where t
0
: time of reference (years), w: weather
exponent (-)
t: time (years)
Time to initiate corrosion (T
i
), when X
c
(t)= X
cover
(concrete cover) is given in Eq. (2).