The reinforced concrete structures ingeneral and reinforced concrete bridges in particular under the seawaterenvironment are gradually being impacted by invading chloride, as a result,th
Trang 1INTRODUCTION Meaning of the topic performed in this thesis
Roadway network system in Vietnam has been much invested and rapidlydeveloped by Vietnamese government since recent decades Demand of thesafety and the durability of old or new bridges is an important task that should
be ensured
Among the bridges of the Vietnamese roadway network, there are manyones running along or near coastal areas The reinforced concrete structures ingeneral and reinforced concrete bridges in particular under the seawaterenvironment are gradually being impacted by invading chloride, as a result,these structures are corroded and their durability as well as service life isreduced
The structures located on the lowest level of tide water and coastalstructures will be strongly affected by the reinforcement corrosion Damageform of these structures is that the reinforcement within concrete iselectrochemically corroded by the diffusion of chloride ions in environmentinto concrete When the chloride concentration at the reinforcement surfacereaches some threshold levels causing reinforcement corrosion, chloride willbreak the passive film on the surface of reinforcing steel, the corrosion willoccur When the corrosion occurs, its product is corrosive rust Rust absorbswater to expand its volume which causes the cover concrete layer cracked andbroken Corrosion of steel reinforcement reduces the bond betweenreinforcement and concrete and the cross-sectional area of steel reinforcement.These lead to the reduction of resistance in bending, compression resistanceand shear strength
The service life of a reinforced concrete structure under chloride impact isdetermined from the time the structure constructed to the time it becomesunsafe for using due to the damages caused by the corrosion The time stageincludes two sub-stages: Initiation corrosion stage and propagation corrosionstage The first stage is the necessary time so that chloride ions penetrate andconcentrate on reinforcement surface with “concentration threshold causingcorrosion”, while the second stage is started from the end of the first stage tothe time the concrete cover layer is entirely cracked due corrosion or the cross-section area of reinforcement is reduced to the disability of bearing capacity ofthe structure
With the purpose to predict the service life of reinforced concrete bridgesimpacted by chloride penetration near coastal areas in Vietnam, the author
selects the thesis title “Prediction of service life due to the exposure of chlorides of reinforced concrete bridges near coastal areas in Vietnam”
Purposes of the thesis
The thesis aims at researching of the penetration and reinforcementcorrosion of chloride in reinforced concrete bridges so as to:
Trang 221) Determine parameters of chloride penetration process into concretesuch as: Chloride diffusion coefficient D; Chloride concentration onconcrete surface Cs; chloride threshold causing reinforcementcorrosion Cth
2) Establish a model to predict the service life of reinforced concrete
bridges impacted by chloride impact near coastal areas in Vietnam.3) Propose the methods to increase the service life To apply the modelfor components of T-shaped bridges
Content of the thesis
The thesis includes 4 chapters which are summarized as follows:
Chapter1 “Literature review of the studies on the service life of concretebridges which are impacted by chloride penetration”
Chapter 2 “Determination of chloride diffusion coefficients in concrete” Chapter 3 “Establishment of a service life prediction model of reinforcedconcrete bridges which are impacted by chloride penetration near coastal areas
in Vienam”
Chapter 4 “Methods to increase the service life and examples”
Part “Conclusion and Recommendation” presents the conclusions,recommendations and future research orientations
CHAPTER 1 LITERATURE REVIEW OF THE STUDIES ON THE SERVICE LIFE OF CONCRETE BRIDGES IMPACTED BY CHLORIDE PENETRATION IN THE WORLD AND IN VIETNAM
The causes of damages in concrete bridges were statistically presented byprofessor Mutsuyoshi (2001) His study showed that the main cause was due
to the chloride penetration which occupied 66% of the damages, while thecarbonation only occupied 5%
The service life of reinforced concrete bridges due to chloride penetration
is often defined as the time from the structures initially exposed to chlorideenvironment until the time the concrete cover layer is entirely cracked due
Trang 33corrosion or the cross-section area of reinforcement is reduced to disability ofbearing capacity of the structures.
Currently, studies on the prediction of the service life have been beingattracted and conducted by material and civil engineering associates in overthe world
2 Studies on durability and service life in the world
2.1 Studies on durability theory of concrete
2.2 Studies on chloride penetration mechanisms into concrete
In the study of Nielsen A (1985) [46], the volume of non-hydrated ironoxide Fe2O3 is twice than that of the steel replaced, but the volume of hydratedone is 6.5 times greater than that of the steel replaced This causes the cracksand breaks the cover concrete layer
2.4Tests of chloride penetration resistance in concrete
Three test methods to determine diffusion coefficients D are classifiedbased test time consuming
Table 1.2 Test methods of chloride penetration
Long-term test Salt ponding test
AASHTO T259 90 days after 28 days ofmaintenanceBulk Diffusion Test
6 hours after 28 days ofmaintenance
Electrical Migration Depended on voltage and
concrete Rapid Migration Test 18 hours after 28 days of
maintenance (AASHTO
Trang 42.5Studies on diffusion coefficients
Stanish (2000) [53] performed tests on determining the experimentrelationship between chloride diffusion coefficients in concrete at the age of 28days and at 20oC and the ratio of water/cement
Zhang and Gjorv [67] developed a simple approach to determine thediffusion coefficients of chloride ions as shown in Eq.1.19:
o
s f
f’c compressive strength of concrete MPa; a, b are experiment constants
2.6Studies on initiation corrosion stage and propagation corrosion stage,
service life
Tuutti, K (1980) “The service life of structures relating toreinforcement corrosion” The author proposed a two-stage model of theservice life of reinforced concrete structures, which considered D as aconstant of time
Figure 1.14: Service life of reinforced concrete structures: Two-stage model
Trang 55increase their durability The approaching method is based on semi-probability and full probability of chloride and CO2 penetration to determinethe service life of the operation concrete structures.
A study of ACI Committee in America is named as “Life 365” (2012)[22] – a model to predict the service life of concrete structures contacted tochloride In the study, the serviced life includes two stages: the initiationcorrosion stage and propagation corrosion stage The study assumed that theconcrete is in saturated humidity state The propagation corrosion stage is 6year fixed for all types of structures
3 Studies performed in Vietnam
Prof.Dr Huu et al 2008[7], “High-strength and high-quality concrete”The authors mentioned the durability of concrete and components effecting tothe durability In the study, a method to increase the durability is to use high-quality concrete with low chloride diffusion coefficients
"A study on the chloride deionized-electrochemical technology andalkalinity recovery area around concrete" of the transport institute of scienceand technology in 2007 [20] Their purpose is to reduce the concentration ofchloride ion in concrete and to restore the alkalinity level aroundreinforcement down below the corrosive chlorine thresholds This work is verydifficult and expensive, not basic Moreover, these studies are conducted onexisting structures
Studies of the Institute of construction technology science “Corrosionstatus of reinforced concrete structures in Vietnam sea areas and someexperiences of using the corrosion inhibitors Calcium Nitrite” in 2010 of Dr.Khoan, Pham-Van and Dr Trang, Nguyen-Nam [9]
4 Comment and research orientation of the thesis
The new problems arising from the above studies that are needed to be solved:
- To determine the diffusion coefficients in concrete from fastchloride permeability test according to the Standard ASTM C1202
- The service life due to chloride penetration of reinforced concretebridges with taking the effects of temperatures, humidity in bothstages of corrosion initiation and propagation
- The corrosion propagation stage is needed to have a quantitativeprediction
The thesis are going to deal with the problems mentioned above based oncalculation models The methodology is based on a mathematical model ofdiffusion process to determine the corrosion initiation stage as well as amathematical model of corrosion activation that causes by concrete cracking todetermine the corrosion propagation stage
Trang 6CHAPTER 2 DETERMINATION OF CHLORIDE DIFFUSION
COEFFICIENTS IN CONCRETE 2.1 General
Diffusion coefficient of chloride ion in concrete is an important parameter
to predict the corrosion initiation stage for concrete reinforcement Thecoefficient can be determined based on experiments or predicted based on theconcrete mixing method
The chapter describes the fast permeability tests according to ASTMC1202 based on 16 concrete samples to determine the diffusion coefficient D,and to compare the results obtained with ones predicted
Chloride diffusion coefficient in concrete
The coefficient is D which is used in Fick’s law The unit of D in SI is
m2/s
x
C D J
RILEM-TC-178 proposes two types of diffusion coefficients as follows:
a) Effective diffusion coefficient Deff
A coefficient in a stable state
b) Apparent diffusion coefficient
A unstable diffusion coefficient The coefficient is obtained based onimmerging test or structure observation with the applying of the second Fick’slaw of diffusion
Chloride diffusion coefficient D is a function of material, environment,time and humidity
2.2 Test and determination of chloride diffusion coefficient in concrete
2.2.1 Fast chloride permeability test ASTM C1202
Table 2.2 Chloride ion permeability level
Charge Passed (coulombs ) Chloride Ion Penetrability
2.2.2 Results of fast chloride permeability test ASTM C1202
Table 2.3 Test results of C1202
No Sample name W/C
Date of sample casted
Date
of test
eratur e
Q 0 - 6 hours (C)
I=Q 0 /t (mA)
1 C30-1 0.4 22/3/12 20/4/12 30 16 3264 2536 117.4
2 C30-2 0.4 22/3/12 20/4/12 30 16 3317 2577 119.3
Trang 7E C D RT
Trang 8It
J zF (2.6)
Equation 2.6 is substituted into Eq.2.5 with taking x=L; E=E, Ci=Co ,
we obtain the formulation of the diffusion coefficient in steady state asfollows:
Q 0
(C)
I mA
Migration speed (mol/l.s)
D C1202 ( m 2 /s)
16 C50+15%FA-1 2475 1981 91.7 3.8E-06 5.47E-12
17 C50+15%FA-2 2500 2001 92.7 3.84E-06 5.52E-12
18 C50+15%FA-3 2357 1887 87.4 3.62E-06 5.21E-12
2.3 Prediction of the apparent diffusion coefficient
2.3.1 Apparent diffusion coefficient
The thesis summarizes the studies of the apparent diffusion coefficient inthe world The coefficient is predicted as follows:
28
w exp( 0,165 ) ( ) ( ) ( ) cr 2.18
Trang 9SF is percentage of silica fume which used to substitute cement amount
in concrete; f(t),f(T),f(H) are effective coefficients of concrete,temperature, relative humidity to chloride diffusion coefficient inconcrete
( 12,06 2,4 / ) 2
2.4 Comparison of results and discussion
2.4.1 Comparison of obtained diffusion coefficient D
Table 2.7 DC1202, D based on prediction and experience formulas
No Name of sample W/C DC1202
( m 2 /s)
D based
on Stanish ( m 2 /s)
D based on Zhang – Gjor( m 2 /s)
D based
on Berke ( m 2 /s)
D based on Omar S Baghabra ( m 2 /s)
1 C30-1 0.4 6.83E-12 7.94E-12 1.15E-11 7.45E-12 2.36E-11
2 C30-2 0.4 6.94E-12 7.94E-12 1.17E-11 7.55E-12 2.36E-11
3 C30-3 0.4 6.90E-12 7.94E-12 1.16E-11 7.52E-12 2.36E-11
4 C35-1 0.375 6.36E-12 6.92E-12 1.06E-11 6.97E-12 1.37E-11
5 C35-2 0.375 6.11E-12 6.92E-12 1.02E-11 6.73E-12 1.37E-11
6 C35-3 0.375 6.22E-12 6.92E-12 1.03E-11 6.84E-12 1.37E-11
7 C40-1 0.35 5.39E-12 6.03E-12 8.91E-12 6.06E-12 8.53E-12
8 C40-2 0.35 5.49E-12 6.03E-12 9.08E-12 6.16E-12 8.53E-12
9 C40-3 0.35 5.36E-12 6.03E-12 8.86E-12 6.03E-12 8.53E-12
10 C50-1 0.33 4.78E-12 5.40E-12 7.80E-12 5.45E-12 3.87E-12
11 C50-2 0.33 4.95E-12 5.40E-12 8.09E-12 5.62E-12 3.87E-12
12 C50-3 0.33 4.92E-12 5.40E-12 8.03E-12 5.59E-12 3.87E-12
13 C50+8%SF-1 0.3 8.57E-13 1.22E-12 1.37E-12 1.29E-12 1.06E-12
14 C50+8%SF-2 0.3 8.19E-13 1.22E-12 1.31E-12 1.25E-12 1.06E-12
15 C50+8%SF-3 0.3 9.29E-13 1.22E-12 1.49E-12 1.38E-12 1.06E-12
16 C50+15%FA-1 0.35 5.47E-12 6.03E-12 8.90E-12 6.06E-12 1.06E-12
17 C50+15%FA-2 0.35 5.52E-12 6.03E-12 8.99E-12 6.11E-12 1.06E-12
18 C50+15%FA-3 0.35 5.21E-12 6.03E-12 8.48E-12 5.81E-12 1.06E-12
2.4.2 Discussion of diffusion coefficients D obtained
Based on results obtained of D, a following comment raises: chloridediffusion coefficient determined from the test based on C1202 as in Eq 2.9 isbasically suitable to results of Stanish used in Life 365
2.5 Conclusion of chapter 2
1 Experiment study of fast chloride permeability level according toASTM C1202 for typical concrete samples was conducted The test
Trang 1010results have shown that concrete grades C30-C40-C50 with electricityquantity transmitted from 2577 to 1799 (coulomb) belong to low andaverage chloride ion permeability level
2 A formulation to determine the chloride diffusion coefficient based oncharge transfer according to ASTM C1202 is proposed as follows:
2 16
on C1202 as in Eq 2.9 is basically suitable to results of Stanish used
in Life 365 as well as those of Berke
4 The thesis has synthesized studies of foreign authors and to propose apredictive formulation of apparent chloride diffusion coefficient asshown in Eq 2.18
CHAPTER 3 ESTABLISHMENT OF SERVICE LIFE PREDICTION MODEL USED IN REINFORCED CONCRETE BRIDGES DUE TO CHLORIDE PENETRATION AT COASTAL AREAS IN VIETNAM
5 General introduction
3.1.1 Definition of the service life
The service life of reinforced concrete bridges under chloride impact
is determined from the time the structure exposed to chloride ion environment
to the time the concrete cover layer is entirely cracked due corrosion or the cross-section area of reinforcement is reduced to disability of bearing capacity of the structure.
3.1.2 Service life based on the penetration of chloride in sea environment
To predict the service life of concrete structures, the end of the servicelife needs to be determined
In this thesis, the author is going to determine the service life for two cases:
1 The end of the service life is considered as the time the concrete cover layer is cracked due to corrosion
2 The end of the service life is the time that corrosion caused the damage to the structure in strength limit state
There are many opinions of damage process of concretereinforcement According to Tuuti (1980) 59, the process has two stages:initiation stage and propagation stage as shown in Eq.3.1
t t t
Trang 116.2 Parameters of the model
3.2.2.1 Chloride diffusion coefficient (D)
3.2.2.2 Chloride concentration accumulation on concrete surface
, ax
, ax , ax
3.3
s m s
Table 3.1 Build-up Rates and maximum concentration of surface chloride
(%/ year) C(%/)s,max
The thesis selects the rule of surface chloride concentration accumulation
of Michael Thomas as shown in Eq.3.3, but the maximum surface chloride concentration (Cs,max) is different for different sea regions of Vietnam
3.2.2.3 Chloride concentration threshold causing the corrosion forconcrete reinforcement
- For normal reinforced concrete Cth=0.05%
- For pre-stressed concrete Cth=0.012%
6.3 Construction of corrosion initiation stage prediction model
3.2.3.1 One-dimension problem: (1D diffusion)
When the chloride concentration C(x,t) at the cover concrete layerdepth reaches the corrosion concentration threshold Cth, the reinforcement isinitially corroded
1
( , ) c
th
x d
C x t t t C t1 is corrosion initiation stage
3.2.3.2 Two-dimension problem: (2D diffusion)
Trang 12123.2.3.3 The problem is solved based on finite difference method
Crank-Nicolson because D and Cs are changeable
6.4 Establishment of programming algorithm diagram in MATLAB
to determine the corrosion initiation stage
Two examples to determine the corrosion initiation stage in MATLAB:1D diffusion problem is used for a wall plate and inner reinforcements ofbridge piers 2D diffusion problem is used for angular reinforcements ofbridge piers and beams
The programming algorithm of both examples are presented inAppendix 1
7 Construction of corrosion propagation stage prediction model 7.1 General
7.2 Available applying models
2) Stresses and strains in concrete appear due to the volume dilation
of corrosive products Concrete cover is considered as a linearlyelastic material
3) There are porous areas around concrete-reinforcement interface,corrosive products will diffuse into the gaps
4) During the development of cracking, a part of corrosive productswill be filled into the cracks
Trang 13Figure 3.6: Idealization of concrete cover layer as a thick-walled cylindrer:
(a) initial concrete sample; (b) concrete deformation, (c) deformation of
corrosive products (d) Rust filled into open cracks
Figure 3.7 : Time stage from the iniatiation of reinforcement corrosion to concrete cover completely cracked and the load capacity risk
In the crack propagation time, a small part of corrosive products will bepenetrated into through-center cracks After that, required amount of corrosivereinforcement to cause the completely cracked concrete cover can bedetermined based on two components (Figure 3.7):
7.3.2 The relationship between reinforcement weight loss of
reinforcement and through-center pressure
Where ρ is the symbol of percentage of the reinforcement weight loss
Mloss per initial reinforcement weight Ms over a unit length:
