Durability assessment of concrete by water permeability and chloride diffusion with consideration to the stress factor, application in bridge structure

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 9 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. v Water Use domestic water to produce and maintain concrete. Water must be clean according to TCVN 4056: 2012 Water for concrete and mortar - Technical requirements. 2.2.Results of water permeability test with concrete samples subjected to pre- compressive stress Based on the results of the above experiments, we construct a chart of waterproofing of concrete C30 and C40 when considering the following compressive stresses (Figure 2.1): 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 s/smax ≤ 0.3, the increase in water permeability is quite slow. When the relative stress is greater s/smax > 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 s/smax < 0.4; After this threshold, the permeability coefficient begins to increase rapidly. When the stress is relative s/smax ³ 0.6, the water permeability increases very quickly; this can be explained by the micro-structure of 0 5 10 15 0 0.2 0.4 0.6 0.8 w at er pr oo f m ar ks W s/smax C 10 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 pre- compression showed that, when the relatively pre-compressive stress is small s/smax ≤ 0.3, the water permeability is quite slow. When the relative stress is greater s/smax > 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 0 5E-10 1E-09 1.5E-09 2E-09 2.5E-09 3E-09 0 0.2 0.4 0.6 0.8 w at er p er m ea bi lit y co ef fic ie nt (m /s ) Relative stress s/smax K5at m 0.00E+00 5.00E-10 1.00E-09 1.50E-09 2.00E-09 2.50E-09 3.00E-09 0 0.2 0.4 0.6 0.8 W at er p er m ea bi lit y co ef fic ie nt (m /s ) Relative stress s/smax K5atm K4atm K3atm 11 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 s/smax < 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 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 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 2000 2500 3000 3500 4000 4500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 El ec tri c qu an tit y( C ou lo m bs ) s/smax Diffusion of chloride ions of concrete C30 12 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. 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). Figure 3.4 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete and pre-compressive stress of concrete sample C40 1000 1500 2000 2500 3000 3500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 El ec tri c qu an tit y( C ou lo m bs ) s/smax Diffusion of chloride ions of concrete C40 y = 0.4851x + 1.0205 R² = 0.9799 y = 1.0242e0.4081x R² = 0.9642 0.8 1.0 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 1 D /D 0 s/smax Concrete C30 y = 0.5504x + 1.028 R² = 0.9725 y = 1.0317e0.4537x R² = 0.9521 ,0.8 ,0.9 ,1.0 ,1.1 ,1.2 ,1.3 ,1.4 ,1.5 ,1.6 0 0.2 0.4 0.6 0.8 1 D /D o s/smax Concrete C40 13 The results from Figures 3.4 and 3.5 show that for C30 concrete, when the pre-compressive stress reaches 0.8smax, the permeability coefficient increases by 1.4 times higher than the permeability of unloaded concrete. 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 pre- compressive stress of both C30 and C40 concrete is expressed by the following formulas: + Exponential regression: D/Do = 1.028exp(0.4309s/smax) (3.1) + Linear regression: D/D0 = 0.5177(s/smax) + 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% smax, the average chloride ion diffusion decreased by 11.33%. When increasing the stress to 50% and 70% smax, the permeability of concrete increases by 13.60% and 35.79%, respectively. The loss of permeability at the stress level of 30% smax 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 y = 0.5177x + 1.0242 R² = 0.9603 y = 1.028e0.4309x R² = 0.9455 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0 0.2 0.4 0.6 0.8 1 D /D 0 s/smax Concrete C30, C40 14 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. Figure 3.6 – Relationship between relative ratio of chloride ion diffusion coefficient through concrete and direct compressive stress of concrete C30 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 giữa hệ số khuếch tán ion clorua với trạng thái ứng suất nén trực tiếp của bê tông Mối quan hệ giữa hệ số khuếch tán ion clorua của bê tông với điện lượng đo được, được tính theo công thức của Berke và Hicks Kết quả tính hệ số khuếch tán ion clorua cho mẫu bê tông thí nghiệm C30 và C40 được trình bày ở hình 3.3 và 3.4. Từ kết quả đã tính toán tiến hành xây dựng mối quan hệ giữa hệ số khuếch tán ion clorua qua bê tông và ứng suất nén trực tiếp của cả 2 mẫu bê tông C30, B40 y = 1.2354x2 - 0.5297x + 0.9929 R² = 0.8453 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 0.0 0.2 0.4 0.6 0.8 D /D o s/smax Concrete C40 Giá trị đo y = 1.3985x2 - 0.5661x + 0.9898 R² = 0.8484 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 0.0 0.2 0.4 0.6 0.8 D /D o s/smax Concrete C30 Giá trị 15 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 0.7smax, 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 pre- compressive stress of two types of concrete C30 and C40 is expressed by the following formula: Exponential regression: D/Do = 1.317(s/smax)2 – 0.5479(s/smax) + 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. Hình 3.9 - Diagram of coefficient of chloride ion diffusion relationship based on Banthia theory and experimental results of concrete C30 y = 1.317x2 - 0.5479x + 0.9914 R² = 0.8367 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 0.0 0.2 0.4 0.6 0.8 D /D o s/smax Bê tông C30, C40 Giá trị 0 2 4 6 8 10 12 14 0 0.2 0.4 0.6 0.8 Di ffu sio n co ef fic ie nt Dx 10 -1 2 (m 2/ s) Grade of concrete Lý thuyết 16 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 s/smax ≤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 s/smax ≥ 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 3 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 2. The results of chloride ion permeability test with concrete samples subjected to pre- compressive 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% smax stress is explained by the stress 0 1 2 3 4 5 6 7 8 9 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9D iff us io n co ef fic ie nt D x1 0- 12 (m 2/ s) Grade of load Lý thuyết Thí nghiệm 17 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 0.3smax and increases at 0.5smax and 0.7smax. 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 3 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 2 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 2 and chapter 3 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: C = C 1 − erf x2√Dt ; (4.1) 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: C = C 1 − erf h2√Dt (4.2) 18 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: D = D t t ; (4.3) 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: C = C . t ; (4.4) In which: Cso is the surface chloride concentration after 1 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: C = C t (1 − erf x2 D t (4.5) The minimum thickness of the protective concrete layer h required to prevent reinforcement corrosion in concrete is calculated as follows: h = 2 3D t × erf C C t (4.6) 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