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SERVICE LIFE PREDICTION OF CONCRETE STRUCTURES
Mohamad Nagi and Robert Kilgour
GHD Global Pty Ltd, Dubai, U.A.E
Synopsis: Methodologies and computer models for predicting the
service life of reinforced concrete structure were developed over
the years. Methods were mainly used to define the remaining
service life of existing structures based on the durability
characteristics of concrete. Currently, these technologies were
adopted to define the service life at the design stage. In the
Arabian Peninsula and the Gulf region, authorities are requesting
extended service life (75 to 100 years) of their key infrastructures
such as bridges and towers with minimum maintenance and life
cycle cost. Since the Gulf is considered the most corrosive place
in the world, corrosion of reinforcement is the main durability
element controlling the service life of structures in this region.
Prediction models were used to assess the service life of reinforced
concrete towers and bridges and provide guidance to achieve such
targeted life. High performance concrete, corrosion resistant steel
and inhibiting admixtures are currently used in the region to
enhance concrete durability and extend the service life of
structures.
Keywords: Service life, diffusion coefficient, corrosion, concrete
durability
INTRODUCTION
During the first construction booming in the Gulf and Arabian
Peninsula in the early 1970’s, many concrete structures marine
structures were built based on foreign codes without paying
attention to the unique environment in the region. As a result, the
high temperature and severe environment have lead to a major
durability-related deterioration, in some of these structures within
10 to 15 years (1).
Currently, the Gulf and Arabian Peninsula region is on the top of
the world’s list in concrete construction and the daily consumption
of concrete is probably the highest in the world. From super tall
towers to marine, industrials and highways structures, reinforced
concrete stands as the main material used in construction.
In the last decade, Supplementary Cementing Materials (SCM)
such as fly ash, silica fume and ground granulated blast furnace
slag (GGBS) made their way to the Gulf and are commonly used to
produce high strength and high performance concretes.
Considering the high initial construction cost, Developers and
authorities are demanding much longer service life of their
structures (75 to 100 years or more) with minimum maintenance
and life cycle cost. Due to the harsh and severe environments in
the Gulf region, durability characteristics of concrete control its
service life. Production of durable and good quality concrete is the
key to extend the service life of the structures. Designers are now
looking into durability modeling to assess the service life of the
designed facilities.
SERVICE LIFE PREDICTION
In general, service life is the period of time during which a
structure meets or exceeds the minimum requirements set for it.
The requirements limiting the service life can be technical,
functional or economical. (2). The technical requirements are
related to the structural functions of the structure.
As mentioned above, the main deteriorating factors affecting the
service life of concrete structures are durability related ones.
Durability by definition is the ability of concrete to resist
weathering action, chemical attack, and abrasion while maintaining
its desired engineering properties. Concrete ingredients and their
proportions, and interaction among ingredients, curing and placing
of concrete control its ultimate durability (3). Alkali-aggregate
reaction, sulfate attack, and corrosion are the main factors affecting
the performance of concrete structures. In the Gulf region, while
other factors exist, the corrosion of reinforcing steel is the main
factor controlling the service life of reinforced concrete structures.
Mechanism of corrosion is well covered and understood. Steel
reinforcement is usually protected in concrete as far as the
passivated layer (protective iron oxide film) formed in the concrete
high-alkali environment is existed (4, 5). Whenever this layer is
damaged either due to carbonation or the presence of chloride ions,
and in the presence of oxygen, corrosion will start. The chloride-
induced corrosion is the common form of corrosion in the region.
The Gulf area is predominantly ex-seabed sand. There is a very
high chloride content in the sand and ground water. Salt content
can be several times the seawater combining with the high ambient
temperature and high humidity, making the Gulf one of the most
corrosive location in the world.
Service life prediction models
Service life models can be divided into two groups: deterministic
and probabilistic. Deterministic models are based on empirical
relationship, while probabilistic models are based on stochastic
behavior of structures (2,6). It is based on the idea that the service
life cannot be accurately predicted.
All models developed over the years are based on the idea that the
service life is the total of the initiation time and the propagation
time of corrosion. Figure 1 illustrates the principle of service life
analysis. It is assumed that corrosion is initiated when chloride
content at the level of reinforcing steel reaches the defined
corrosion threshold. The estimated time for corrosion initiation
can be calculated using Fick’s second law of diffusion, assuming
diffusion is the main mechanism of chloride ingress into concrete
DC/dt = D. d2C/dx
2
C Chloride content
T time
X depth (form exposed surface)
D apparent diffusion coefficient
The general solution of the above-mentioned equation is as
follows:
Dt
xERFCCCCC isstx
2,
with Cx,t = the chloride concentration at concrete depth x and
at time t,
Cs = the projected chloride concentration at the surface,
Ci = the initial chloride concentration,
D = the apparent chloride diffusion coefficient (m2/sec,
in.2/year), and
ERFC is an error function.
One of the earliest and simplest model for predicting service life of
structures was the one developed by Tutti in the early 1980’s (2,
8). The propagation time is considered to be a constant period of
time. Tutti’s model analysis was supported by experimental data.
A more complicated model combining both deterministic and
probabilistic models was developed by Gannon and others (9)
using Monte Carlo statistical simulation. The Monte Carlo
simulation is used to generate values for an equation whose
variables have a specified distribution. Variables such as
reinforcing cover and diffusion coefficient are used to solve the
diffusion equation.
Determination of Diffusion Coefficient.
As mentioned above, diffusion coefficient is a key factor in
predicting the time to corrosion. For existing structures, concrete
cores are taken and tested to establish chloride profile. Following
the procedures of ASTM C 1556 “Standard Test Method for
Determining the Apparent Chloride Diffusion Coefficient of
Cementitious Mixtures by Bulk Diffusion” the diffusion
coefficient can be determined. Ligozio and Nagi (10), as part of
their study to determine the remaining service life of Chamberlain
Bridge over the Mississippi river in the U.S. measured the
diffusion coefficient of concrete in different parts of the bridge.
The diffusion coefficient for the 50-year old concrete elements
ranged from 0.8 to 2 x10-12
m2/s.
For new constructions, concrete samples can be prepared and
tested in accordance to ASTM C 15556 or NT Build 443. These
tests require an average of two months to be completed. Recently,
a rapid test (NT Build 492) has been introduced in the region to
measure the chloride migration coefficient. The test is based on
applying an external electrical potential to force chloride ions into
the samples. It was reported by Tang (11) that this test has a good
correlation with the diffusion coefficient measured using the NT
Build 443.
Determination of Surface Chloride Concentration
The surface chloride concentration to which an element may be
exposed is not quantified. Codes often provide broad qualitative
exposure classifications such as submerged, spray or splash zones but
these do not provide adequate information to determine the surface
chloride level. For sulfate-bearing ground, a more quantitative
approach has been adopted. For example, AS 2159 refers to five
exposure classifications based on the actual level of sulfate present in
the ground. For coastal structures, Bamforth (12) suggested the
following number of exposure classes and associated surface chloride
concentrations .
PRE-CONSTRUCTION SERVICE LIFE ASSESSMENT
(CASE STUDY)
A large-scale reinforced concrete structure with nominal design
life of a 100 year is under construction in the Gulf region.
Assessment of the durability of the concrete elements of the
structures subjected to defined deterioration scenarios was required
prior to finalizing the design and commencing construction.
Recommendations were made for the mix designs, protective
measures and construction quality assurance.
The main concrete elements of the structure considered in the
durability design were:
Bored piles and piles caps
Retaining walls
Raft slab
Ground floor slabs
Exposed superstructures elements
The durability and serviceability of concrete in aggressive
environment is addressed at the design stage by the selection of
appropriate mix designs and the specification of additional
protective measures and construction quality assurance measures.
The deterioration scenarios assessed in the project were sulfate
attack, carbonation of concrete and chloride induced corrosion of
PC Blended Cement
Extreme exposure Csn = >0.75% >0.9%
Severe exposure Csn = 0.5% to 0.75% 0.6% to 0.9%
Moderate exposure Csn = 0.25% to 0.5% 0.3% to 0.6%
Mild exposure Csn = < 0.25% <0.30%
the reinforcement. A durability plan that outlines the requirements
for durability and the assessment of compliance with final
requirements was prepared. The assessment of service life based
on chloride-induced corrosion is presented in this paper.
Ground Conditions
Chemical testing undertaken on soil and ground water indicated
high level of chlorides, up to 21 g/L. Groundwater pH was
reported to be between 7.1 and 7.6. The structure foundations are
located below groundwater table.
Basis of Analysis
The study was based on the assumption that the diffusion process
governs chloride ingress into the concrete over the longer term.
The diffusion coefficient for concrete is generally influenced by
the permeability and porosity of the concrete, which in itself is
influenced by the cement content, the aggregate grading, the use of
cement replacements, the water cement ratio compaction and
curing. Data of measured diffusion coefficients for various
concrete mixes using blends of OPC with GGBS, and OPC with
PFA as well as ternary blends that include silica fume was used to
show the possible diffusion coefficient variability, prior to
measurement of diffusion coefficient of the trial mixes conducted
prior to construction.
Minimum Requirements for Atmospherically Exposed Concrete
The following requirements are based on the guidance from AS
5100.5 (2004) and assume the concrete will have a minimum
strength of 45 MPa. The requirements apply to formed slabs,
beams, walls and columns.
Exposure classification B2
Cover 55 mm
Probabilistic Corrosion Model
The acceptable degree of deterioration considered in the study of
the buried and atmospherically exposed elements is to avoid
spalling of concrete. In this case, corrosion initiation is allowed,
with progression of corrosion sufficient to just cause cracking of
the concrete after the required service life
The in-house probabilistic model determines the probability that
sufficient corrosion will occur to cause cracking of the concrete
over a range of diffusion coefficients, surface chloride levels,
concrete covers, activation thresholds and corrosion rates. The
reliability index is the number of standard deviations from the
mean of the failure equation:
P(t) = (- ) = {CCx(t) – Xc 0}
Where CCx(t) is the cumulative corrosion (in microns) at
reinforcement depth at time t
Xc is the amount of corrosion necessary to induce
cracking at the reinforcement depth
is the standard normal distribution function
is the reliability index (no. standard deviations from the
mean)
In the elements of many structures, loss of cover and minor section
loss do not in themselves constitute any significant loss of
structural capacity or serviceability. Significant loss of section
would be required for structural safety to be compromised.
Accordingly the limiting value of Beta for design does not need to
be the same as in structural considerations, where 3.8 is often used
(e.g. Eurocode).
The study considered a value of 1.65 (5% probability of cracking)
was an appropriate minimum value to adopt for a long service life
(50 years plus) for civil engineering structures. A reliability index
of 2.3 was used where cracking itself carried a safety risk (I.e., for
atmospherically exposed concrete n above pedestrian and vehicular
access ways.
Data Analyses
The probabilistic model assesses the risk of deterioration due to the
ingress of chlorides. The following scenarios (Table 1 and 2) are
considered for the various possible configurations of concrete mix
design, concrete cover to reinforcement as well as bar size (based
on the design development documentation). The diffusion
coefficients (determined by bulk diffusion tests such as those
described by NT Build 443 or ASTM C 1556) are assumed based
on what could be achieved with a carefully designed mix and good
batching and construction quality control. It is possible that these
mixes could achieve much lower diffusion coefficients.
Cover to the reinforcement is taken as the cover to the external
bars of the reinforcing cage –typically identified as
ligatures/stirrups/hoops depending on the element in question.
Main bars will have additional cover equivalent to at least the
diameter of the ligature/stirrup/hoop reinforcement.
For piles it is assumed that the coefficient of variability (CoV) of
the cover will be significant (the model assumes CoV is 25% as
evidenced on various foundation projects in the region.
The variability of cover for elements such as slabs, columns and
beams should be less (the model assumes CoV is 10%.
The model assumes a target cover of 100 mm for piles, 75 mm for
slabs (all faces exposed to soil).
In either case, normal code tolerances for cover to the
reinforcement should not be exceeded.
Table 3 and 4 summarise the outputs of the model. The acceptable
reliability index will be influenced by the structural impact of
cracking (if any) – typically a minimum reliability index of 1.65 is
considered acceptable, however higher values (> 2.3) may be more
appropriate where cracking and spalling presents a risk to safety.
Typical time to corrosion and time to cracking charts for the
selected rebar sizes and type of steel are shown in Figures 2 and 3
for GGBS and PFA concretes.
Actual cover required for these elements would be subject to the
type of formwork used and the application of coatings and
cladding systems. For inaccessible elements, a minimum of 65 mm
cover will be required to surfaces where CPF is not used or where
coatings are not applied. Where coatings are applied, they should
be able to provide the equivalent protection of at least 10 mm of
concrete and be maintained in accordance with manufacturer’s
recommendations.
The model is based on a service-ability limit state of sufficient
corrosion to just cause cracking, i.e. approximately 100 microns.
This amount of corrosion will not affect the structural performance
in any way. If the analysis had assumed a greater degree of
corrosion to cause say loss of 20% bar section, or more, which may
affect structural integrity, then a much higher reliability index
would be appropriate, however the model considers a conservative
service-ability limit state therefore we consider that the minimum
RI=1.65 is adequate (5% of the element affected), as achieving
such an amount of corrosion will not cause failure.
SUMMARY AND CONCLUSIONS
Service life prediction of concrete structures based on concrete
durability characteristics is well-established methodology.
Computers models were developed to predict remaining service
life of existing structures taking into consideration exposure
conditions, design issues (e.g., concrete cover) and concrete
properties. Diffusion coefficient of concrete is the essential
property used in the service life assessment.
In the Arabian Peninsula and Gulf region, due to harsh and severe
environments, chloride-induced corrosion of reinforcing steel
dominates the durability factors leading to deterioration of concrete
structures. Foundations, raft slabs and piles are the critical
elements of the structures since they are buried with contaminated
soil and ground water and cannot be repaired.
Due to the high initial cost of massive structures being built in the
Gulf region, developers and authorities are demanding extended
service life of the structures with minimum life cycle cost.
The authors used in-house probabilistic model to assess the service
life of large-scale tower to be built in the region at the design stage.
Using the recommended concrete mix design with the design-
related inputs, the structure could be expected reach the 100-year
design life with minimum repair or maintenance. The use of other
corrosion protection system such as ASTM A 1035 steel will add
assurance to the owner that structure will reach its service life even
if there were changes in the input items (e.g., covers) occur during
construction. Measurement of diffusion coefficient of concrete
trial mixes was added to the specification of the project. Authors
fond that rapid migration coefficient test can be used as indication
for assessing the finding of the model.
REFERENCES
1. “Guide to the Maintenance and Repair of Reinforced Concrete
Structures in the Arabian Peninsula,” Concrete Society,
presented at the Bahrain 6th
International Conference,
November 2000.
2. Vesikari, E, “Service Life of Concrete Structures with regard
to Corrosion of Reinforcement, ”Technical Research Centre of
Finland, ESPOO 1988.
3. Kosmatka and et.al, “Design and Control of Concrete
Mixtures,” Fourteenth Edition, Portland Cement Association,
Skokie, IL, U.S.A
4. Nagi, M and Whiting, D, “Corrosion of Prestressed
Reinforcing Steel in Concrete Bridges, State-of-the-Art,”
Concrete Bridges Aggressive Environments Symposium, SP
151, 1994 American Concrete Institute, Detroit, Michigan,
U.S.A.
5. Broomfield, J., “Corrosion of Steel in Concrete,” 1997 E &
FN Spon, U.K.
6. “Chloride Penetration into Concrete, State-of-the-Art,”
HETEK, Report No. 53, 1996, Road Diroctorate, Denmark.
7. Weyers, et al, “Concrete Bridge Protection and Rehabilitation:
Service Life Estimates,” SHRP-S-668, Transportation
Research Board, Wahsingtion, D.C., 1994
8. Tutti, K., “Corrosion of Steel in Concrete,” Swedish Cement
and Concrete Research Institute, Report No. 4-82, 1982.
9. Gannon, et.al, “Deterioration Model for Corrosion in Concrete
Using Monte Carlo Simulation,” Structural Engineering in the
21st Century. ASCE, 1999.
10. Ligozio, C. and Nagi, M., “Remaining Service Life
Evaluation, Chamberlain Bridge Substructures,” South
Dakota Department of Transportation, U.S.A. 2004
11. Tang, L and Sorensen, H.E., “Precision of the Nordic Test
Methods for Measuring the Chloride Diffusion/Migration
Coefficients of Concrete,” Materials and Structures, Vol. 34,
October 2001.
12. Bamforth, P.B and Pocock, D.C. (2000) “Design for durability
of reinforced concrete exposed to chlorides” Workshop on
Structures with Service life of 100 years- or more, Bahrain
TABLES
Table 1. Scenarios – Buried concrete
Scenario BG 1 BG 2
Mix 40% PFA + SF 70% GGBS + SF
Diffusion coefficient
(×10-12
m2/s)
1.5 2.0
CoV 15%
Cover (mm) 75, 100
Bar size (mm) 12, 16,
16 (ASTM A 1035 steel)a, 32
b
aAssumes ASTM A1035 steel – corrosion threshold is increased
bassumes main bars with additional cover due to tie bars (typically
T12)
Table 2. Scenarios – Atmospheric concrete
Scenario AC 3
(Exterior columns)
Mix 25% PFA + SF
Diffusion coefficient
(×10-12
m2/s)
2.0
CoV 15%
Cover (mm) 55, 65, 75
Bar size (mm) 16, 32
Table 3 – Calculated Reliability Index for Time to Cracking –
Buried elements
Scenario Mix basis Bar
diameter
(mm)
RI 100 y
(Piles –
100 mm
cover)
(wet)
RI 100 y
(Slabs,
Rafts – 75
mm cover)
(wet)
BG 1 40% PFA +
SF
D=1.5x10-
12m
2/s
12 3.1 2.6
16 2.7 2.1
16 (ASTM
A 1035
steel)
5.7 8.5
32 3.8 7.5 (100
mm cover)
BG-2 70% GGBS
+ SF
D=2.0x10-
12m
2/s
12 4.3 4.2
16 3.7 3.6
16 (ASTM
A 1035
steel)*
8.5 8.5
32 5.5 2.7
Table 4 - Calculated Reliability Index for Time to Cracking –
Atmospherically exposed elements
Scenario Bar
diameter
(mm)
RI 100 y
(55 mm
cover)
(dry)
D=2.0×
10-12
m2/s
RI 100 y
(65 mm
cover)
(dry)
D=2.0×
10-12
m2/s
RI 100 y
(75 mm
cover)
(dry)
D=2.0×
10-12
m2/s
AC 3 16 0.9 2.3 4.2
32 0.3 1.6 3.2
FIGURES
Fig. 1 - Principles of Concrete Service Life (Chloride-Induced
Corrosion)
Fig. 2 - Time to Corrosion and Time to Cracking (GGBS Mix)
Fig. 3 - Time to Corrosion and Time to Cracking (PFA Mix)