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.
MINISTRY OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS _ HO XUAN BA DURABILITY ASSESSMENT OF CONCRETE BY WATER PERMEABILITY AND CHLORIDE DIFFUSION WITH CONSIDERATION TO THE STRESS FACTOR, APPLICATION IN BRIDGE STRUCTURE Disciple: Transport Construction Engineering Code : 9580205 Major : Bridge and Tunnel Engineering SUMMARY OF DOCTORAL THESIS HA NOI - 2020 This thesis was completed at University of Transport and Communications Academic supervisors: Prof Dr Pham Duy Huu University of Transport and Communications 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 INTRODUCTION Imperiousness of topic The strength of the structure of construction works in general and bridges and tunnels in particular, reinforced concrete has been a matter of great concern recently in Vietnam A number of requirements on the design of structures based on durability have been included in new design documents, repair designs to maintain the life of the project as expected Once the durability requirements are guaranteed, the work will be sustainable over time, the cost of maintenance and repair of the work will be reduced to a minimum level; project exploitation capacity will be maximized 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 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 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 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 3.2 Scope of Research + Reinforced concrete structure with compressive stress in aggressive environments with chloride ions + Concrete in construction works in general and bridges in particular Research method + 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 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 also will 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 is Micro 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 concrete Stress 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: D28 = 1×10(−12,06+2,4 N / X ) (m2/s) (1.2) Diffusion coefficient, D28 This relationship is shown in Figure 1.3 N/X Diffusion coefficient, CDC (x 10 -8cm2/s) 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) Permeability level, RCP (coulombs) Figure 1.4 - Relationship between permeability level and chloride ion diffusion coefficient (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 (1.6) D = 0,0103 × 10 × (Q ) , Q0 is the electric quantity transmitted over hours according to the ASTM C1202 experiment d Formula for the relationship between diffusion coefficient and compressive strength of concrete C Lim et al (2000) [14] performed an assessment of the effects of microcracks and chloride ion diffusion of concrete when subjected to a pre-compressive load one axle He commented that, when concrete samples are completely unloaded at a pre-compression load level of 0.5f’c, the areas where micro-cracks appear can recover 100% to their original state However, when unloading at a payload level from 0.7 to 0.95f’c, some crack areas are unlikely to recover after unloading This property has a great significance to the permeability of concrete Chloride ion diffusion of concrete (after unloading) is affected by the appearance of pre-compressive stress The chloride ion diffusion in concrete samples does not change significantly when the precompression load levels are small σ/σmax ≤ 0.7 The increasing degree of permeability can be seen clearly when the pre-compressive load levels are large σ/σmax > 0.7 as shown in Figure 1.5 The "ab" line shows that chloride permeability does not change from its original permeability After that, the "bc" line after threshold σ/σmax = 0.7 shows the chloride permeability increased rapidly 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 (1.7) t=t +t ; 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) Conclusion of chapter The evaluation of durability and long-term prediction of transport works by reinforced concrete plays an important role in the management and operation of the system The evidence is that this issue has been of interest and research for a long time in developed countries around the world In particular, the two main factors affecting durability are permeability and diffusion of concrete In addition, carbonation, chemical corrosion due to acid and seawater can also be mentioned 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 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 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 diffusion chloride 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 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 Diffusion of chloride ions of concrete C30 Electric quantity(Coulombs) 4500 4000 3500 3000 2500 2000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 σ/σmax Figure 3.1 - Relationship between electric quantity and pre-compressive stress in concrete C30 11 Diffusion of chloride ions of concrete C40 Electric quantity(Coulombs) 3500 3000 2500 2000 1500 1000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 σ/σmax Figure 3.2 - Relationship between electric quantity and pre-compressive stress in concrete C40 Figure 3.1 and Figure 3.2 show that with two types of concrete considered C30 and C40, the chloride ion diffusion (through electric quantities) increases linearly and fairly evenly Constructing the relationship of increasing chloride diffusion coefficient through concrete (relative value D / D0) and pre-compressive stress as shown in Figure 3.3, Figure 3.4 and Figure 3.5 Concrete C30 1.6 y = 0.4851x + 1.0205 R² = 0.9799 D/D0 1.4 1.2 y = 1.0242e0.4081x R² = 0.9642 1.0 0.8 0.2 0.4 σ/σ 0.6 0.8 max Figure 3.3 – Relationship between the relative ratio of chloride ion diffusion coefficient through concrete and the pre-compressive stress of concrete sample C30 (DO is initial chloride permeability coefficient) D/Do Concrete C40 ,1.6 ,1.5 ,1.4 ,1.3 ,1.2 ,1.1 ,1.0 ,0.9 ,0.8 y = 0.5504x + 1.028 R² = 0.9725 y = 1.0317e0.4537x R² = 0.9521 0.2 0.4 0.6 σ/σmax 0.8 Figure 3.4 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete and pre-compressive stress of concrete sample C40 12 The results from Figures 3.4 and 3.5 show that for C30 concrete, when the pre-compressive stress reaches 0.8σmax, the permeability coefficient increases by 1.4 times higher than the permeability of unloaded concrete Concrete C30, C40 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 D/D0 y = 0.5177x + 1.0242 R² = 0.9603 y = 1.028e0.4309x R² = 0.9455 0.2 0.4 σ/σmax 0.6 0.8 Figure 3.5 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete with pre-compressive stress of both C30 and C40 concrete types The law of increasing chloride ion permeability coefficient according to the precompressive stress of both C30 and C40 concrete is expressed by the following formulas: + Exponential regression: D/Do = 1.028exp(0.4309σ/σmax) (3.1) + Linear regression: D/D0 = 0.5177(σ/σmax) + 1.0242 (3.2) The above regression lines also show that the rule of increasing diffusion of chloride ions of concrete is quite similar to the rule of increasing air permeability through concrete subject to pre-compressive stress ((A Djerbi Tegguer – 2013, Choinska – 2008, Tran - 2009) [15], [17], [10] 3.3 Effect of direct compressive stress on chloride permeability of concrete The relationship of chloride ion diffusion (C) of C30 and C40 concrete according to the rapid permeability test corresponding to stress values when compressing simultaneously concrete samples is shown in Figure 3.6 and Figure 3.7 Experimental results show that chloride diffusion differs drastically in the presence of simultaneous action loads However, before and after diffusion load of chloride ions are in the "average" level according to TCVN 9337-2012 At a stress level of 30% σmax, the average chloride ion diffusion decreased by 11.33% When increasing the stress to 50% and 70% σmax, the permeability of concrete increases by 13.60% and 35.79%, respectively The loss of permeability at the stress level of 30% σmax is explained by the stress causing micro deformation and because the stress is still within the elastic limit, so no crack has been generated, which in turn increases the concentrates and reduces voids of concrete thus reducing permeability In the case of chloride ion diffusion decreases, it will lead to prolong the time of chloride ion diffusion through protective concrete layer to cause corrosion of reinforcement in reinforced concrete constructions From this result it is shown 13 that in a pre-stressed concrete structure, when the compressive stress in the concrete is within the appropriate limits, it can prolong the diffusion time and increase the life due to chloride ion diffusion process Concrete C30 1.40 1.30 1.20 y = 1.3985x2 - 0.5661x + 0.9898 R² = 0.8484 D/Do 1.10 1.00 0.90 Giá trị… 0.80 0.70 0.0 0.2 0.4 σ/σmax 0.6 0.8 Figure 3.6 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete and direct compressive stress of concrete C30 Concrete C40 1.40 1.30 1.20 y = 1.2354x2 - 0.5297x + 0.9929 R² = 0.8453 D/Do 1.10 1.00 0.90 Giá trị đo 0.80 0.70 0.0 0.2 0.4 0.6 0.8 σ/σmax Figure 3.7 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete with direct compressive stress of concrete C40 3.4 Xây dựng mối quan hệ hệ số khuếch tán ion clorua với trạng thái ứng suất nén trực tiếp bê tông Mối quan hệ hệ số khuếch tán ion clorua bê tơng với điện lượng đo được, tính theo cơng thức Berke Hicks Kết tính hệ số khuếch tán ion clorua cho mẫu bê tơng thí nghiệm C30 C40 trình bày hình 3.3 3.4 Từ kết tính tốn tiến hành xây dựng mối quan hệ hệ số khuếch tán ion clorua qua bê tông ứng suất nén trực tiếp mẫu bê tông C30, B40 14 Diffusion coefficient Dx10-12 (m2/s) D/Do Bê tông C30, C40 1.40 1.30 y = 1.317x2 - 0.5479x + 0.9914 1.20 R² = 0.8367 1.10 1.00 0.90 Giá trị… 0.80 0.70 0.0 0.2 0.4 0.6 0.8 σ/σmax Figure 3.8 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete with direct compressive stress of both C30 and C40 concrete types From Figure 3.8 shows, the law of changing chloride permeability through directly compressive concrete of two types of concrete is quite similar When the compressive stress is less than 0.5, the permeability change is negligible, but when the pre-compressive stress reaches 0.7σmax, the permeability coefficient increases by about 1.3 times compared to the permeability of unloaded concrete The law of increasing chloride ion permeability coefficient according to the precompressive stress of two types of concrete C30 and C40 is expressed by the following formula: Exponential regression: D/Do = 1.317(σ/σmax)2 – 0.5479(σ/σmax) + 0.9914 (3.3) The regression function above also shows that the law of increasing chloride diffusion of concrete is quite similar to the rule of increasing air permeability through compressive stress concrete (Banthia & al (2006)) 3.5 Relationship between coefficient of water permeability and chloride diffusion coefficient of concrete Plot thechloride diffusion coefficient relationship based on Banthia theoretical formula and experimental results, as shown in Figure 3.9 and Figure 3.10 14 12 10 Lý thuyết 0 0.2 0.4of concrete 0.6 0.8 Grade Hình 3.9 - Diagram of coefficient of chloride ion diffusion relationship based on Banthia theory and experimental results of concrete C30 15 Diffusion coefficient Dx1012(m2/s) 10 Lý thuyết Thí nghiệm 0.1 0.2 0.3 0.4 0.5 0.6 Grade of load 0.7 0.8 0.9 Figure 3.10 - Diagram of chloride diffusion coefficient relationship based on Banthia theory and experimental results of C40 concrete As shown in Figures 3.9 and 3.10, the results of the calculation of Chloride ion diffusion coefficients are theoretical, and the results of chloride ion diffusion experiments are quite close Experimental results show that, when the stress level in concrete σ/σmax ≤0.3, the chloride ion diffusion coefficient decreases, when this stress level increases, the diffusion coefficient increases gradually Increasing sharply when stress levels in concrete exceed σ/σmax ≥ 0.6 3.5.1 Propose a formula to determine chloride ion diffusion coefficient from water permeability factor when considering stresses in concrete The calculation results in section 3.5.1 allow to propose the formula for calculating the chloride ion diffusion coefficient from water permeability coefficient as follows: - With concrete C30: Kw = 144.93 S0.5 D (3.4) - With concrete C40: Kw = 176.72 S0.5 D (3.5) With these two formulas, it is easy to calculate chloride ion diffusion coefficient from water permeability coefficient of some commonly used concrete 3.6 Conclusion of chapter The author conducted experiments analyzing chloride ion permeability through concrete affected by the load with concrete samples with f’c = 30MPa and f’c = 40MPa There are components as designed in chapter The results of chloride ion permeability test with concrete samples subjected to precompressive stress show that, when the pre-compressive stresses in concrete σ/σ ≤ 0,8, the chloride ion diffusion increases linearly and fairly evenly; after this threshold chloride ion diffusion increased sharply The relationship between diffusion of chloride ions with the state of pre-compressive stress of two types of concrete C30, C40 that the author of the construction thesis has proposed is: D/D = 1.028 × exp(0.4309 × (σ/σ )); (3.6) In which : D0 chloride diffusion coefficient of unloaded concrete The relationship between diffusion of chloride ions with the state of direct compressive stress of two types of concrete C30, C40 that the author of the thesis has proposed is: D/D = (1.317 × (σ/σ ) − 0.5479(σ/σ ) + 0.9914 (3.7) In which : D0 chloride diffusion coefficient of unloaded concrete The results of chloride ion permeability test with concrete samples under direct load show that, Chloride ion diffusion drastically changes in the presence of concurrent load However, before and after the incremental load, chloride ion diffusion is in the "average" level according to TCVN 9337-2012 The decrease in permeability at 30% σmax stress is explained by the stress 16 which causes micro deformation and because the stress is still within the elastic limit, no crack has been generated, which increases the density and decreases pores of concrete, thereby reducing permeability The chloride ion diffusion rate through concrete decreases when the stress is at 0.3σmax and increases at 0.5σmax and 0.7σmax Finally, the author of the thesis proposes the relationship between water permeability coefficient and chloride ion diffusion coefficient of concrete as follows: - With concrete C30: Kw = 144.93 S0.5 D - With concrete C40: Kw = 176.72 S0.5 D CHAPTER 4: CALCULATE THE LIFE PREDICTION OF REINFORCED CONCRETE BRIDGE CONSTRUCTION REGARDS THE SIMULTANEOUS EFFECT OF LOAD EFFECTS AND ENVIRONMENTAL IMPACT 4.1 Problem The purpose of this chapter is to build a model to predict the impact of load and environment on the service life of reinforced concrete bridge structures according to the criteria of initiation of corrosion in reinforced concrete The experimental results in chapter will be used as the basis for setting up life forecasting models These models will be applied in predicting the life of a specific bridge This chapter is structured into main parts The first part is the construction of a forecasting model that takes into consideration the effects of load and environmental conditions simultaneously The second part is the estimation of life expectancy for a specific bridge structure taking into consideration the change of protective concrete thickness, surface chloride ion concentration, pre-compressive stress and direct compression in concrete 4.2 Building a predictive life model of reinforced concrete bridge construction The input parameters in the problem are important This thesis will be based on input parameters from experiments in chapter and chapter with results of domestic and foreign authors Those parameters will be recommended for the model to be built 4.2.1 Develop a predictive model of life expectancy for reinforced concrete bridges according to the criteria of initial corrosion of reinforcement In 1975, Crank [18] proposed a mathematical model for the diffusion process based on Fick II's law In the case of the diffusion coefficient is constant, the chloride ion concentration on the reinforcement surface in Equation 4.1 with boundary conditions C0 = C (0, t) (i.e the content of chloride ion ions is constant) and the initial condition C=0, x>0 and t=0, is determined by: x (4.1) C = C − erf ; 2√Dt In which: - Cx is the chloride concentration in depth x; - erf is the error function; - Cs chloride concentration at the concrete surface of a structure; - t is review time; - x is the depth from the concrete surface of the structure; - D is the chloride ion diffusion coefficient The process of reinforcing corrosion starts when Cx = Ccr; then x = h (thickness of protective concrete layer) we have: h (4.2) C = C − erf 2√Dt 17 In fact, the life of buildings in general and traffic works in particular according to the corrosion criteria is significantly higher than the results calculated according to the formula above because of chloride diffusion and surface chloride concentration are time-dependent factors To consider the time factor in the representation of chloride diffusion value of usually intact concrete, Mangat & Molloy (1994) [19] suggest that the law of changing Kc over time has the following form: t (4.3) D=D ; t In which: - D28: is the chloride diffusion coefficient at the age of 28 days; - t0 : concrete age (t0 = 28 days) ; - m : is the experimental coefficient taken as follows: (according to A.Costa and J.Appleton (1998)) + The area affected by ocean waves: m = 0.245 ; + The tide goes up and down: m = 0.2 ; + Coastal climates: m = 0.29 To consider the time factor in representing value of surface chloride concentration of Cs in this thesis, the author took or exchanged at the proposal of A Costa & J.App skeleton (1998) as follows: (4.4) C = C t ; In which: Cso is the surface chloride concentration after year; n is the empirical coefficient According to different environmental conditions, values Cso (% concrete) and n for typical concrete are taken as follows (A Costa & J.Appeleton (1999)): - The area affected by ocean waves Cso = 0.24; n = 0.47; - The tide goes up and down:: Cso = 0.38; n = 0.37; - Coastal climates: Cso = 0.12; n = 0.54 Therefore, considering the change over time of chloride diffusion coefficient and surface chloride concentration, (4.2) is rewritten as follows: x C = C t (1 − erf (4.5) D t The minimum thickness of the protective concrete layer h required to prevent reinforcement corrosion in concrete is calculated as follows: C (4.6) h = 3D t × erf C t 4.2.2 Building a predictive model of life expectancy for reinforced concrete bridges according to the criteria of reinforced corrosion taking into consideration the stress state of concrete Different from the state when no load, the concrete structure is intact, when subjected to a load large enough, the concrete structure is destroyed, leading to the increase in permeability of concrete, which will create conditions for chloride diffusion into concrete increases faster, increases chloride ion concentration in the surface of reinforcement and consequently causes corrosion of reinforcement earlier To explain this, when the stress in concrete exceeds the crack limit, it will cause the concrete to crack and facilitate the increase in water permeability and chloride diffusion To consider the effect of stress state on the diffusion of chloride ions into concrete, the formula determines the relationship between the increase of chloride ion diffusion coefficient over time and the state of pre-compressive or direct compressive states in the chapter will be used in calculations 18 Therefore, from formulas 4.5 and 4.6 we formulate the formula to determine the life of reinforced concrete works according to the criteria of initiation of corrosion in reinforced concrete a) The case considering the state of pre-compressive stress C (4.7) h=2 D t t 1.028 exp 0.4309(σ/σ ) × erf C t b) The case considering the state of direct compressive stress h=2 D t t 1.317 − 0.5479 + 0.9914 × erf (4.8) h (cm) Applying two formulas (4.7) and (4.8) that change the life of the building according to the thickness of protective concrete layer corresponding to the pre-compressive and direct compression stress levels as shown in Figures 4.1 and 4.2 15 0.0 13 0.3 0.5 11 10 40 50 60 70 80 t (year) Figure 4.1 - Relationship between thickness of protective concrete layer and construction life according to the pre-compression stress In Figure 4.1, we see in the case of pre-compressive load; the rules of changing the life of a building by the thickness of the concrete layer are quite similar; An increase in pre-stressed stress will require a greater layer of protective concrete thickness 13 30 0.0 0.3 0.5 0.7 11 h (cm) 20 10 20 30 40 50 60 70 80 t (year) Figure 4.2 - Relationship between thickness of protective concrete layer and construction life according to direct compression stress In Figure 4.2 we see, in the case of direct compression load, the rule of changing the life of a building according to the thickness of protective concrete layer depends on the state of precompressive stresses in different stages When σ/σmax = 0.3, the thickness of the protective 19 concrete layer decreases, but when σ/σmax = 0.5, the thickness of the protective concrete layer increases and increases significantly at σ/σmax = 0.7 4.3 Apply the forecast for bridge life In the calculation examples applied below, the life of reinforced concrete structures of bridge constructions is calculated according to the formulas (4.7) and (4.8) Concrete-related parameters that have generally been carried out in experiments in the previous sections of section will be used in this section Other parameters are taken as recommended by some typical research results and standards in the world which have been analyzed in the above sections The initial chloride diffusion coefficient D, determined by experiments in chapter 3, has a value of D28 = 7.9 x10-12 (m2/s) (concrete C30) D28 = 6.243 x 10-12 (concrete C40) When considering the time factor, in this thesis, the author uses the model of Mangat & Molloy (1994) [19] The concentration of chloride ion in the surface of reinforced concrete structure Cs in the following calculations is taken from the research results of A Costa & J.Appeleton (1999) [20] and taking into account the time factors These values are as presented in section 4.2 According to EN206 concrete, the limit value of chloride ion concentration at the reinforced surface is taken as : Ccr = 0.4% by weight of cement or approximately 0.05 to 0.07% by weight of concrete (KLBT) In this calculation, Ccr is taken as 0.05% by weight of concrete The proposed model and the experimental values presented in 4.2 are applied in this calculation Three regions considering the piers reinforcement corrosion rate considered in this calculation are: - + Marine air zone - + Wave zone - + The tide goes up and down Marine air zone Wave zone Tide zone Highest tidal water level +1.0m Lowest tidal water level Frequently Flooded zone Figure 4.3 – Divide the impact areas of marine environment conditions to the rate of reinforcement corrosion in piers concrete 4.3.1 Calculates the life of pier when only considering permanent load Use the formula (4.8) with the calculated parameters as above The reason for this is that the pier only has the own weight of the abutment and the span structure on the bridge Regardless of the presence of live load The model considering reinforcement corrosion rate in concrete is a direct compression model with stress in concrete