Summary of Doctoral thesis Transport construction engineering: Durability assessment of concrete by water permeability and chloride diffusion with consideration to the stress factor,

Determine the coefficient of water permeability, waterproofing marks, chloride ion diffusion coefficients of some types of concrete commonly used in bridge construction subjected to pre-compressive and direct-compression loads. Develop a predictive model for life expectancy using reinforced concrete bridges taking into consideration the effect of the load according to the criteria of initial corrosion of reinforcement in concrete. Application of life expectancy for the structure of reinforced concrete bridges.

HO XUAN BA

DURABILITY ASSESSMENT OF CONCRETE BY WATER PERMEABILITY AND CHLORIDE DIFFUSION WITH CONSIDERATION TO THE STRESS FACTOR,

APPLICATION IN BRIDGE STRUCTURE

MINISTRY OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS

_

HA NOI - 2020

Disciple: Transport Construction Engineering Code : 9580205

Major : Bridge and Tunnel Engineering

SUMMARY OF DOCTORAL THESIS

This thesis was completed at University of Transport and Communications

Academic supervisors:

1 Prof Dr Pham Duy Huu

University of Transport and Communications

2 Assoc Prof Dr Tran The Truyen

University of Transport and Communications

Referee 1: Prof Dr Nguyen Thuc Tuyen

Referee 2: Prof.Dr Nguyen Nhu Khai

Referee 3: Prof.Dr Nguyen Dong Anh

The thesis will be defended in front of the university-level

doctoral thesis judgement panel at ………

At…… , dated ……… 2020

The thesis can be found at

- The Vietnam National Library

- The Library of the University of Transport and Communications

One of the criteria of the durability of structures of reinforced concrete is the durability

of concrete materials against water repellency and chloride ion permeability In addition to the study of the application of concrete with good waterproofing ability to apply in practice, the evaluation of the water permeability, chloride ion permeability through concrete, especially the studies considering the influence of the effect of load on water permeability and chloride ion permeability have also been studied in recent times These include the latest studies of Banthia

&al (2008), A Antoni & al (2008), Tran & al (2009), H Wang & al (2011), W Zhang &al (2011), G.P Li & al (2011), A Djerbi &al (2013), Junjie Wang & al (2015)

The studies of the above authors point out the effects of stress in concrete due to mechanical impact (load) on water permeability and chloride ion permeability through concrete through empirical studies or building theoretical model based on Fick or Darcy's law and hollow structure model of concrete considering the movement of liquid through concrete

Some points not mentioned in the above studies are empirical studies related to stress state outside the elastic limit of concrete as well as the relationship between chloride ion diffusion and water permeability of concrete If results of these issues are available, it will allow easy diffusion of chloride ion diffusion (which is difficult to experiment) from water permeability (easier measurement) of concrete; At the same time, forecasting the effect of the existence of micro-crack (not yet large cracks) on the service life of reinforced concrete bridges

In Vietnam, the study of the effect of the load on water permeability and chloride ion permeability of concrete is still a new issue, not many studies have mentioned it Partly due to the complexity of the experiments, the evaluation of water permeability and the diffusion of chloride ions through concrete, especially those that take into account the effect of direct compression loading

The study to evaluate the relationship between water permeability and chloride ion diffusion with consideration of stress factors in concrete is necessary, with many meanings for the assessment and forecast of the life of the building; suitable with the exploitation characteristics of constructions in general and bridges in particular, especially bridges in Vietnam

From the urgent requirements and the important implications in proposing a model to assess the effect of load on the permeability of concrete and its application in predicting the life

of a building with reinforced concrete in general and the bridge works in particular, the research topic " Assessment durability of water permeability and chloride ion of concrete with consideration to the stress factor, application in bridge structures " was selected as the thesis topic

The thesis content consists of 4 chapters, introduction, conclusion and recommendation

+ Introduction

+ Chapter 1: Overview of concrete durability and studies related to water permeability, chloride ion diffusion of concrete

+ Chapter 2: Experimental analysis of water permeability of concrete considering to the state of compressive stress

+ Chapter 3: The chloride diffusion analysis test of concrete considering to the state of compressive stress

+ Chapter 4: Calculation the durability of reinforced concrete bridge construction with consideration to the simultaneous effect of load effect and environmental impact

+ Conclusion and recommendation

2 Objective of Thesis

The objectives of the thesis are:

+ Determine the coefficient of water permeability, waterproofing marks, chloride ion diffusion coefficients of some types of concrete commonly used in bridge construction subjected to pre-compressive and direct-compression loads

+ Establish a relationship between the coefficient of water permeability and the chloride ion diffusion coefficient, taking into account the effect of direct compression load

+ Develop a predictive model for life expectancy using reinforced concrete bridges

taking into consideration the effect of the load according to the criteria of initial

corrosion of reinforcement in concrete Application of life expectancy for the structure

of reinforced concrete bridges

3 Object and Scope of Research

3.1 Object of Research

The object of research is some types of concrete commonly used in bridge construction The coefficient of water permeability, chloride ion permeability and the correlation between them considering the effect of load Service life of reinforced concrete structure bridges

+ Methods of synthesis, analysis and comparison

+ Research methods are mainly theoretical and experimental methods Use advanced theories of durability of concrete to identify experimental correlations (formulas) and conduct empirical research to verify

+ Modeling to predict the service life of reinforced concrete bridges affected by chloride ion diffusion taking into account the effect of the load

5 New Contributions of Thesis

+ The values of water permeability coefficient, waterproofing marks, chloride ion

diffusion coefficient of some types of concrete commonly used in bridge construction are subject to pre-compressive load and direct compression load

+ The relationship between water permeability and chloride ion diffusion coefficient, taking into account the effect of direct compression load

+ Predictive models for longevity using reinforced concrete bridges take into account the effect of the load according to the criteria of initial corrosion of reinforcement in

concrete Application of forecast life expectancy structure of reinforced concrete

bridges

CHAPTER 1: OVERVIEW OF CONCRETE DURABILITY AND STUDIES RELATED TO WATER PERMEABILITY, CHLORIDE ION DIFFUSION OF

CONCRETE

Overview of durability of concrete

The durability of cement concrete according to ACI 201.2R-08 [1] defines "Resistance to weathering, chemical corrosion, abrasion, or any other degradation process", meaning Is durable concrete that retains its original appearance, quality, and serviceability when exposed to the environment

The deterioration of reinforced concrete structure when exposed to the environment has many mechanisms In the scope of the research in this thesis, along with the conditions and characteristics of Vietnam's climate, especially in coastal areas, the mechanism leading to the decline is mainly assessed through the criteria of durability such as the waterproofing ability of liquids and the resistance to diffuse chloride ions through concrete In addition, carbonation, sulfate corrosion, acidic and seawater corrosion can be included

The waterproofing of concrete, a hollow material, is highly dependent on parameters of the concrete environment such as porosity, zigzagness of voids and interconnection between voids Scrivener [2] said that, when the porosity and the gap between pores in concrete increases, the waterproofing strength of concrete is reduced; and the more straight the pores are, the faster the flow of permeation flows Under mechanical actions such as creep, shrinkage, or temperature, , will lead to destruction in concrete accompanied by cracks that increase the above parameters, permeability of concrete so alsowill increase rapidly The permeability diagram is illustrated in Figure 1.1

Figure 1.1 - Effect of porosity, form, size of hollow lines and interconnection of porosity on

permeability of concrete (Scrivener (2001) [2])

Corrosion of reinforcement in concrete is a common problem affecting the durability of reinforced concrete structures In particular, the corrosion caused by chloride ions is one of the main causes leading to the decline, affecting the long-term durability of structural structures [3]

As long as the reinforcement is still surrounded by dense protective layers of concrete, the alkaline environment in the concrete is high enough to create a protective film on the reinforcement If this protective film is destroyed, reinforcement corrosion will occur, resulting

in a reduction of cross-sectional area and bearing capacity of reinforced concrete structural parts

The process of carbonation of concrete under the effect of CO2 in the environment by reaction:

Ca(OH)2 + CO2 = CaCO3 + H2O (1.1) This process takes place from outside to inside of concrete, over time However, in order for the carbonation process to take place completely (then the pH of concrete is only about 9) even if the thickness of the protective layer is thin, it needs a long time (20 ÷ 30 years or more)

On the other hand, the results of many practical studies prove that even concrete structures have very high pH (greater than 11.5) but corrosion still occurs

The cause of chloride ion concentration, which exceeds the limit, destabilizes the passive membrane layer:

Chloride ions are present in concrete for the following reasons: Concrete structures working

in marine environment or other chloride-containing environments, concrete made from salty materials, when treating concrete thawing agents or when using rapid solid additives containing chloride ions,

The study of the water permeability and diffusion of chloride ions of concrete in the world

Water permeability of concrete is always an important issue for concrete structures in contact with water because water permeability affects the durability of reinforced concrete structures Water permeability through concrete is a cause of reinforced corrosion in concrete when the structure of the project is exposed to corrosive water environment such as groundwater, mineral water, seawater, domestic wastewater and industrial contains corrosive agents In addition, much infiltrated concrete will lose water in reservoirs, canals, water tanks, causing roof leakages, standing water,

According to Banthia N et al [4] the permeability of concrete is influenced by two main factors: One is the porosity characteristics such as size, zigzagness, and the connection between voids, Two isMicro crack in concrete, especially at the bonding surface between aggregate and binder The porosity factor is controlled mainly by the N / X ratio, the degree of hydration and the degree of compaction Meanwhile, the density and location of micro-cracks at the bonding surface are determined by the effective stress level, appearing inside or outside the structure of concreteStress occurs inside the concrete due to shrinkage, temperature difference, sudden change of heat-moisture

in the environment and factors that cause volumetric instability

The effect of stresses due to external influences on concrete permeability is still poorly understood Questions such as the degree of stress, at what age of concrete are acceptable when assessing permeability need to be clarified

Figure 1.2 - Effect of capillary voids on permeability

(Powers (1958))

Water permeability of concrete under load

Researchs on the effect of load on the water permeability of concrete have been published

by a number of authors in the world such as Kermani (1991) [5], Gerard (1996) [6], Lion & al (2005) [7], Banthia & al (2008) [8], Tran & al (2009) [9] However, these results are quite dispersed, partly because the water permeability of the concrete is affected by the preparation

of test samples (sample form, concrete composition, aggregate aggregate, conditions of curing, etc.) and testing process (loading process, pressure of water used ); On the other hand, the

water permeability measurements of concrete obtained are lower than the actual permeability values due to the chemical and physical reactions of water with the micro structure of concrete

It is these physico-chemical reactions that make it harder to assess the water penetration mechanism of concrete and also make the water permeability measurement results lower than the air permeability measurement results

In 2009, in his doctoral thesis at Liege University, Tran [10] showed that the water permeability of concrete depends significantly on the residual effect of pre-compressive load and water pressure during the experiment Water pressure is too small or too large, causing the effect of preventing water permeation through concrete The occurrence of mechanical damage due to the applied load has an effect on the increase in the water permeability of concrete similar

to that of air permeability Water permeability of concrete is approximately 100 times smaller than air permeability When concrete is not under the initial water permeability K0 can be approximated to be 10-19 m2 or 10-11 cm / s An increase in water permeability has also begun

to be noted when σ/σmax > 0.4 – 0.6

Stanish, K (2000) [11] developed a relationship between the diffusion coefficient D28 and the N / X ratio for standardized concrete at 20 ° C Based on the large database of diffusion experiments, he proposed empirical relations as follows:

This relationship is shown in Figure 1.3

Figure 1.3 - Relationship between N / X ratio and chloride ion diffusion

coefficient (Stanish, K (2000)) Ahmad S., (2003) [12] on the equation expressing the relationship between chloride ion diffusion coefficient and electric quantity of concrete This correlation is used to determine chloride ion diffusion coefficients when chloride ion diffusion levels are known (Figure 1.4)

Figure 1.4 - Relationship between permeability level and chloride ion

(Ahmad S.( 2003)) The formula for calculating the chloride ion diffusion coefficient is as follows:

Normal concrete: D = 0,0056Q0 - 8,45 ; (1.3) Concrete uses silicon soot: D = 0,0005Q0 + 0,99 ; (1.4)

Concrete uses fly ash: D = 0,0019Q0 - 0,86 ; (1.5) With:

D: Chloride ion diffusion coefficient of concrete (x10-8 cm2/s);

Q0: Chloride ion diffusion level (Coulombs)

Berke et al (1992) [13] proposed a correlation between diffusion coefficients and the amount of coulombs transferred in the experiment

Figure 1.5 - Fast chloride ion permeability at various pre-compression load

levels (C Lim (2000))

In 2013, A Djerbi Tegguer et al [15] conducted experiments, evaluated the effect of uniaxial compressive load on air permeability and chloride ion diffusion coefficient of concrete and showed the relationship of them The correlation between air permeability and chloride ion diffusion coefficients is established by introducing a destructive variable due to the deterioration

of the damaged concrete under the effect of uniaxial compressive load Common concrete (OC) and high strength concrete (HPC) samples were used in the experiment to examine the effect of

the mechanism of the appearance and spread of cracks in concrete to the air permeability and chloride ion permeability of the concrete

Figure 1.6 - The relationship between the relative chloride diffusion coefficient and the

damage value d of concrete (A Djerbi Tegguer (2013))

Researches on corrosion initiation time and corrosion propagation time, service life

In 1980, at the international conference on concrete in the marine environment organized

by the American Concrete Institute (ACI), Tuuti [16] suggested that the reinforced concrete structures working in the marine environment will be ionized: Chloride diffuses into concrete and accumulates on the reinforced surface When the chloride ion concentration at the reinforcement surface reaches the critical concentration threshold, it will begin to cause reinforcement corrosion Corrosion of the reinforced steel will have two consequences Firstly,

it reduces the cross-sectional area of rebar leading to reduced resistance to loads Secondly, corroded reinforcement will produce corrosive products, volumetric corrosive products cause tensile stress in the protective concrete layer and cause concrete cracking, splitting and rupture Modeling the service life forecast of reinforced concrete structures due to chloride ion diffusion needs to show the processes leading to steel corrosion in concrete caused by chloride ions These processes are basically described as follows:

- Chloride ions in the environment accumulate on concrete surfaces

- Chloride ions are diffused into concrete through a number of mechanisms, mainly diffusion

- The concentration of chloride ions accumulates over time at the surface of the reinforcement

- When the chloride ion concentration at the reinforcement surface reaches the critical threshold, the passive membrane on the reinforcement surface is broken and corrosion begins

- The product of corrosion has a larger volume than the reinforced steel has been corroded, causing tensile stress in the protective concrete layer

- Concrete has poor tensile strength, so cracks will appear either perpendicular or horizontally forming a layer between the reinforcement

- Cracks forming cracks or breaks making the structure degraded as the use function is no longer guaranteed or unsafe This may be the time that repairs are required

- Corrosion causes loss of steel cross-sectional area, resulting in a state of load-bearing limit which is no longer satisfied

Tuutti, K proposed a two-stage model of service life of reinforced concrete structures as shown in Figure 1.7 Accordingly, the service life consists of two successive stages: the initial corrosion stage and the corrosion propagation stage according to Equation 1.7

With:

- t is the lifespan used;

- t1 is the beginning of corrosion;

- t2 is the stage of corrosion propagation

Figure 1.7 - Service life of reinforced concrete structures: Tuuti's two-stage model (1980)

Through many researches on water permeability of concrete, it has been shown that the permeability of concrete is influenced by two main factors: One is the porosity characteristic; such as size, zigzagness, and the connection between pores, the two are micro cracks in concrete, especially at the bonding surface between aggregate and binder In particular, the effect of stresses due to external influences on concrete permeability remains unclear Experiments to measure water permeability of concrete are classified as follows: steady water flow test, unstable water flow test, water immersion test

Meanwhile, for reinforced concrete construction works in the marine environment, the important damage phenomenon that needs to be considered is the corrosion of steel reinforcement in concrete due to chloride ions Many studies have proposed the relationship between the chloride ion diffusion coefficient of concrete, the water / cement ratio, the time, the number of Coulombs In addition, researches to evaluate the effect of pre-stressed state in concrete have been conducted Ion diffusion experiments through concrete include stable state diffusion experiments, unstable state diffusion experiments, electric field migration test In general, the implementation of chloride ion permeability tests is complex (especially considering stress states in concrete) Therefore, indirect determination of chloride ion diffusion coefficient through simpler experiments such as water permeability test is important in the evaluation of durability and durability of reinforced concrete structure

CHAPTER 2 : EXPERIMENTAL ANALYSIS OF WATER PERMEABILITY OF CONCRETE CONSIDERING TO THE STATE OF COMPRESSIVE STRESS 2.1 Introduction

The purpose of the experiments in this chapter is to assess the water permeability of some typical concrete types commonly used in bridge constructions in Vietnam Two types of concrete with 30 MPa (symbol C30) and 40 MPa (symbol C40) respectively were used in these

experiments Experimental program includes the following experiments:

- Experiment to determine compressive strength of concrete

- Experiment to determine of water permeability of concrete under stress

- Experiment to determine water permeability of concrete under direct compression stress This chapter is structured into 3 main parts The first part of the chapter deals with the preparation of test samples, including the preparation of materials, casting and maintenance of test samples The second part presents the process of carrying out the test to determine compressive strength and the test to determine the water permeability of concrete subject to pre-compression stress and direct compression stress The third part is the analysis and evaluation

of the experimental results obtained

In order to design graded concrete with compressive strength fc '= 30 MPa (C30) and fc' = 40 MPa (C40), the post-graduate used Bim Son cement - PC 40 (meeting the requirements of TCVN 2682: 2009)

v Fine aggregate (sand)

Sand used to make concrete is natural sand with a grain size of 0.14 to 5mm - according to TCVN 7570-2008; from 0.075 to 4.75 mm - according to American standards and from 0.08 to 5mm according to French standards

The sand used in this research is Da river sand

v Coarse aggregate (Crushed stone)

Use Hoa Binh Crushed stone

Stone materials for making concrete must have adequate intensity and wear Macadam has good roughness, closely associated with cement mortar, so the flexural strength of macadam concrete is higher than gravel concrete

Figure 2.1 – Relationship between waterproof marks of concrete C30 and

C40 according to the pre-compressive stress

When the relative pre-compressive stress is small σ/σmax ≤ 0.3, the increase in water permeability is quite slow When the relative stress is greater σ/σmax > 0.5, the water permeability increases very quickly The appearance of cracks destroying concrete has made the process of water penetration increase faster

In Figure 2.2 and Figure 2.3 we first see that the water permeability of the concrete is almost unchanged or changes slowly when the relative stress value σ/σmax < 0.4; After this threshold, the permeability coefficient begins to increase rapidly When the stress is relative σ/σmax≥ 0.6, the water permeability increases very quickly; this can be explained by the micro-structure of

051015

concrete being destroyed after this stress threshold - which is the threshold for the occurrence

of dispersed destruction zones (according to the approach of concrete destruction mechanics) – making increase water permeability of concrete The rule of increasing water permeability of concrete after 28 days of age in this experiment is similar to the rule of increasing water permeability of premature concrete published by Banthia & al (2005) when mechanical damage has not been appears in concrete

Figure 2.2 – Relationship between water permeability coefficient of concrete K (m / s) and

direct compressive stress in concrete (Concrete C30 according to different water pressure

levels)

Figure 2.3 – Relationship between water permeability coefficient of concrete K (m / s) and

direct compressive stress in concrete (Concrete C40 according to different water pressure

levels)

2.3 Conclusion of chapter 2

In chapter 2, the author conducts experiments, analyzes water permeability through concrete taking into account the compressive stress factor Two grades of concrete were chosen, namely f’c = 30MPa and f’c = 40MPa

Experimental results to determine the waterproofing of concrete under stress compression showed that, when the relatively pre-compressive stress is small σ/σmax ≤ 0.3, the water permeability is quite slow.When the relative stress is greater σ/σmax > 0.5, the water permeability increases very quickly The appearance of cracks destroying concrete has increased the water permeability faster For C40 concrete samples, the rate of deterioration of waterproofing marks when pre-compressive stresses in concrete increases, is lower than that of

pre-05E-101E-091.5E-092E-092.5E-093E-09

0.00E+005.00E-101.00E-091.50E-092.00E-092.50E-093.00E-09

C30 concrete samples

Results of the water permeability test of directly stressed concrete show that the water permeability of the concrete is almost unchanged or changes slowly when the relative stress value σ/σmax < 0.4; After this threshold, the permeability coefficient begins to increase rapidly, which can be explained by the micro structure of concrete being destroyed after this stress threshold - the threshold that causes the occurrence of dispersed destruction zones (according

to approach of mechanical destruction of concrete) - increases the water permeability of concrete

CHAPTER 3: THE CHLORIDE DIFFUSION ANALYSIS TEST OF CONCRETE

CONSIDERING TO THE STATE OF COMPRESSIVE STRESS

3.1 Introductions

The purpose of the experiments in this chapter is to evaluate the chloride diffusion of some typical concrete commonly used in bridge constructions in Vietnam Two types of concrete with expected strength of 30 MPa (symbol C30) and 40 MPa (symbol C40), respectively, are considered in these experiments as in the case of water penetration Experimental program includes the following experiments:

- Experiment to determine the chloride diffusion of concrete subjected to pre-stressed stress

- Experiment to determine of chloride diffusion in concrete subjected to direct

compression

This chapter is structured into 3 main parts The first part of the chapter is chloride ion permeability test with pre-stressed concrete samples including testing principles, material preparation, molding and curing samples, conducting experiments, building the relationship between diffusionchloride ions with prestressed state of concrete The second part presents the procedure for performing chloride ion permeability testing with concrete samples subjected to direct compressive stress including the same content as in part 1 The final part is to propose the relationship between water permeability coefficient and chloride ion diffusion coefficient of concrete

3.2 Effect of pre-compressive stress on chloride permeability of concrete

Based on the above experimental results, draw a graph of the relationship between diffusion of chloride ions of concrete C30 and C40 when reaching the pre-compressive stress as shown in Figures 3.1 and 3.2

Figure 3.1 - Relationship between electric quantity and pre-compressive stress in concrete

C30

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