Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.Nghiên cứu ứng xử nhiệt và một số giải pháp kiểm soát nhiệt, hạn chế vết nứt trong bê tông cường độ cao tuổi sớm kết cấu cầu.
MINISTRY OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS THERMAL BEHAVIOR OF EARLY-AGE HIGH-STRENGTH CONCRETE BRIDGE STRUCTURES AND MEASURES TO CONTROL TEMPERATURE AND MITIGATE THERMAL CRACKING Major : Transport Construction Engineering Code No : 9580205 SUMMARY OF DOCTORAL THESIS HA NOI- 2022 MINISTRY OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS The thesis was completed at the University of Transport and Communications Academic Supervisor 1: Assoc Prof Dr Do Anh Tu Academic Supervisor 2: Assoc Prof Dr Nguyen Huu Thuan Reviewer 1: Reviewer 2: Reviewer 3: This thesis will be defended before Doctoral-Level Evaluation Council at the University of Transport and Communications at … The thesis can be found at: - National Library - University Transport and Communications Library INTRODUCTION Research background Portland cement concrete is widely used in construction of transport infrastructure Heat released during cement hydration causes an uneven temperature distribution in a concrete structure This problem may be concerned when the concrete is in the hardening stage: heat is still generated from the cement hydration while the surface of the concrete is cooling down to ambient temperature The temperature difference between the concrete core and its outer surface can cause significant tensile stresses that can increase the risk of cracking in early age concrete Cracking in large concrete structures due to thermal stress is a problem that has existed for a long time, most obviously when it was first discovered in dams in the early 20th century The concept of "massive concrete" is also often understood to mean large-sized concrete structures such as dams and foundation Recently, however, this term is also used for large-sized bridge components such as foundations, piers, beams, box girders, etc Standards for concrete engineering are always required to control the temperature difference between the core and surface of concrete, thereby minimizing or limiting thermal cracking in the construction phase Currently, the bridge construction industry has applied many types of highstrength, high-performance, ultra-high-strength concrete materials The concept of mass concrete is no longer simply a large-sized structure, as it can be a slender structure that uses high strength concrete (HSC) (with a high cement content) Thus, the problem of thermal cracking needs to be carefully examined The current trend of manufacturing high-strength concrete is to use a high Portland cement content and reduce the water/cement ratio In addition, supplementary cementitious materiels such as silica fume, blast furnace slag, and fly ash are also used to decrease the amount of cement thus consequently reducing heat, but still ensure the desired strength of concrete Concrete mixes using fly ash and blast furnace slag also contribute to the reduction of CO2 emission to the environment Many studies both in the world and Vietnam suggested that the introduction of fly ash into concrete would significantly reduce the heat of hydration However, such studies not quantitatively consider how much heat will be reduced by replacing cement with fly ash and how much concrete strength will be reduced, while the desired strength is guaranteed Therefore, the thesis entitled "Thermal behavior of early-age high-strength concrete bridge structures and measures to control temperature and mitigate thermal cracking" aims to answer the above question The thesis will conduct experiments on heat of hydration and strength for high strength concrete mixes using fly ash as the supplementary material Based on the experimental results, the thesis quantitatively evaluates the influence of the percentage of fly ash replacement on the heat of hydration, strength and thermal cracking risk of early-age high-strength concrete in bridge piers Research objectives - Experimentally determine the heat of hydration characteristics including: adiabatic temperature rise, cumulative heat, heat generation rate, and heat of hydration parameters for HSC incorporating fly ash - Evaluate the influence of the replacement percentage of fly ash on the thermal effect, strength development and thermal cracking risk of bridge piers, therefore providing a material solution for HSC incorporating fly ash Research objects and research scope a) Research objects Heat of hydration parameters and mechanical properties of HSC incorporating fly ash, thermal stress, and thermal cracking risk of early-age concrete b) Research scope - High-strength concrete (with a target compressive strength of 55 MPa) with the fly ash replacement of 0% to 30%; bridge piers selected for analysis with a cross section of 2.0 m x 3.0 m at early ages (0 - days after casting) - The effect of shrinkage and reinforcement distribution are not considered Research methods - Experimental research: measuring the adiabatic temperature rise and testing the strengths of concrete - Theoretical research: establishing mathematical relationships on experimental data, setting up numerical models using the finite difference and finite element methods for investigation and evaluation The scientific and practical significance This research has experimentally studied the strength at early age and the heat of hydration parameters using an adiabatic calorimetry of fours HSC mixtures incorporating fly ash Through comparisons, a reasonable amount of fly ash replacement from 10÷20% (in the range from 0% to 30%) has determined to ensure a lower risk of thermal cracking This can be considered as a material solution to controlling temperature and limiting thermal cracking in concrete bridge piers The research methodology of this thesis can be applied in analyzing and evaluating different types of concrete and different components of a concrete bridge thus ensuring the expected durability and service life of the structure The new contribution of this research - The adiabatic temprature rises (ATRs) of four high-strength concrete (HSC) mixtures were obtained using an adiabatic calorimeter The highest temperature rises for the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 during the test were 58,1; 55,5; 52,9 and 47,9C, respectively The ATR of CĐC-TB00 was highest due to the largest cement content In contrast, the ATR of CĐC-TB30 was lowest due to its low cement content among the four experimental mixtures - The rates of hydration heat and peak values were determined At approximately 9.5 hours after mixing, the peak heat rates of the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 occurred at 4107; 2.95107; 2.31107 and 1.4107 (J/h/m3), respectively - The hydration parameters of HSC mixtures (u, τ and β) were also determined using the measured ATR and the curve fitting method The ultimate degree of hydration (u) increases with the increasing amount of fly ash replacement The α u values of the four HSC mixtures CĐC-TB00; CĐC-TB10; CĐC-TB20 and CĐC-TB30 were 0.6100; 0.6515; 0.7027 and 0.7136, respectively - The ultimate degree of hydration (u) of the four HSC mixtures are smaller than those calculated using the equation of Schindler and Folliard (2005) Thus, the coefficient accounting for the effect of fly ash replacement on αu needs to be adjusted for HSC as follows: the coefficient is 0.4 compared to 0.5 determined using the equation of Shindler and Folliard In these equations, w/cm is the water-to-cementitious materials ratio and pFA is the content of fly ash in the blended cement Shindler and Folliard (2005) u 1,031.w / cm 0,5 pFA 0,194 w / cm Proposed Equation u u ,0 0, pFA - Quantitative analysis based on four HSC mixtures was performed to evaluate the cracking risk in rectangular bridge piers at early ages The result shows that the thermal cracking risk of the pier using CĐC-TB30 was highest among the four mixtures Furthermore, with economic, technical and environmental advantages of using fly ash, use of an HSC containing 10% to 20% fly ash replacement is recommended to minimize cracking risk in early-age concrete CHAPTER OVERVIEW OF THE THERMAL BEHAVIOUR OF EARLY- AGE CONCRETE BRIDGE STRUCTURES AND MEASURES TO CONTROL TEMPERATURE AND MITIGATE THERMAL CRACKING 1.1 Hydration process of Portland cement 1.1.1 Hydration reactions Heat of hydration is a direct result of the chemical reactions between cement and water Heat of hydration is a property of Portland cement, the amount of heat released is dependent upon the cement composition, curing temperature, water to cement ratio, and cement fineness 1.1.2 Heat of hydration A typical hydration process includes stages: Figure 1.1 Figure 1 Stages of hydration 1.1.3 Factors affecting total heat of hydration and development of heat of hydration 1.1.4 Degree of hydration Several authors proposed empirical equations to determine the final degree of hydration that could be reached for Portland cements (Table 1.3) In these models, w/c is the water-to-cement ratio and Slag and FA are the content of slag and fly ash in the blended cement Table Models for final degree of hydration for Portland cement Researchers The ultimate degree of hydration αu w/c ) 0, 42 Powers and Brownyard (1947) (1; Mills (1966) 1,031 w / c 0,194 w / c Schindler and Folliard (2005) u 1,031.w / cm 0,5 pFA 0,3 pSlag 0,194 w / cm 1.2 Early-age properties The early age is the first few hours or days after casting concrete that are characterised by two main processes: setting (progressive loss of fluidity) and hardening (gaining strength) During these processes, the fluid multiphase structure of the fresh concrete transforms into a hardened structure due to the progress of hydration reactions, leading to the development of mechanical properties, heat liberation and deformations (Pane and Hansen, 2002) Hence, the coupling between thermal and mechanical characteristics of early-age concrete is more critical compared to that in mature concrete Furthermore, proper curing after placement is crucial to maintain a satisfactory moisture content and adequate temperature in concrete during this early stage so that the desired properties can develop later (Huo and Wong, 2006) 1.3 Thermal effect of early age concrete During early age, the nonuniform temperature profile distribution causes disproportionate thermal expansions within the concrete body The surface of concrete in lower temperatures can be under high tensile stresses due to relative thermal expansions from internal concrete Therefore, the surface of concrete is under tension once concrete is set until the hydration heat is fully dissipated to the environment Whether the high surface tensile stresses can cause cracking is depending on the stress to-strength ratio at the critical locations During the hydration process of the early-age concrete, both the thermally induced stresses and the concrete strength are being developed but at different rates Cracks are most likely to occur at the critical locations where tensile stress exceeds the tensile strength This phenomenon is known as thermal cracking problem Figure 1.7 originally presented by Tia et al depicts an example of thermal stress and concrete tensile strength development The cracking zone in the figure refers to the time when tensile stress exceeds tensile strength In practice, this cracking time zone is most likely to occur within 1–2 days after concrete placement, depending on the member geometry, size, boundary restraint and the ambient temperature variations Figure 1.6 Thermal evolution and formation Fig 1.7 Thermal stress and tensile strength development with crack cracks in masive concrete initiation 1.4 Material solutions to control temperature and minimize thermal cracking risk in early-age concrete Concrete materials can be optimized to control the temperature and thus control the thermal stresses Most measures focus on indirectly reducing the possibility of cracking by reducing temperature differences and thermal gradients in concrete Design of the optimal concrete mix is considered as the easiest way to minimize negative effects in early-age concrete The selection of adequate mixes to mitigate heat development is generally based on the control of one or several material variables mentioned as below: type, amount and fineness of cement, including type and amount of supplementary cementitious materials, water content and water-to-binder ratio, type and composition of aggregate Mineral additions (silica fume, fly ash, slag) are more and more used to partially replace Portland cement to limit the temperature increase in massive concrete structures * Effect of fly ash when replacing cement on strength development and heat of hydration: Initially, fly ash was used as a partial mass or volume replacement of portland cement for economical reasons As fly ash usage increased, researchers recognized the potential for improved properties of concrete containing fly ash When fly ash is used, the reactions are due to silica and alumina content in the mineral additions As this kind of mineral addition does not contain calcium, the additions are going to react with the calcium hydroxide produced by the reaction of clinker Silica reacts with the portlandite to create C–S–H Alumina reacts with the portlandite to create aluminates (C–A–H) which are similar to the one created by clinker additions Because fly ash reacts with the alkali hydroxides in portland cement paste, it reduces alkali-aggregate reactions In addition, fly ash may increase resistance to deterioration when exposed to sulfates, improve workability, reduce permeability, and reduce peak temperatures in mass concrete Fly ash lowers the rate of hydration reaction and thus also the rate of self-heating, but also the strength development (Flower and Sanjayan) The lubricating effect of the glassy spherical fly ash particles, generally finer than cement, and the increased ratio of solids to liquid make the concrete less prone to segregation (and bleeding) and increase concrete workability On an equal mass replacement basis of portland cement with fly ash, early compressive strengths (less than days) may be lower After the rate of strength contribution of portland cement slows, the continued pozzolanic reactivity of fly ash contributes to increased strength gain at later ages if the concrete is kept moist The ability of fly ash to aid in achieving high ultimate strengths has made it a very useful ingredient in the production of high-strength concrete 1.5 STRUCTURAL AND CONSTRUCTION MEASURES TO CONTROL TEMPERATURE AND MITIGATE THERMAL CRACKING IN EARLYAGE CONCRETE Measures to reduce the temperature of the concrete mix: - Using materials with low heat of hydration, - Cooling aggregates, - Using lowering temperature mixing water, - Casting at night, - Covering tank trucks Measures to limit temperature difference in concrete: - Post-cooling by embedding cooling pipes - Use of insulation materials and formwork insulation - Increase the temperature for the concrete area which cool down quickly, - Divide into smaller blocks to pour the concrete 1.6 Overview of the thermal behaviour of early- age concrete 1.6.1 Research on thermal control in early age concrete in the world In the past, the use of temperature control measures was limited to dams and very large structures Temperature control and thermal stress development in smaller concrete members, such as bridge structures, are often not considered With the advent of high-performance concretes, cracking at the early ages is no longer a peculiarity of massive structures The term ‘massive concrete’ is used in a broad sense, comprising all types of concrete elements for which the effects of cement hydration can lead to thermal cracking risks In JSCE 2007 and JCI 2008, a relatively large slab having a thickness of 80 to 100 cm or more and a wall with a restrained bottom having a thickness of 50 cm or more may be thought of as mass concrete structures According to Neville when the temperature difference between the surface and the core of the concrete block exceeds 20oC, cracking will occur, either at the surface or inside the concrete block The specifications of the Florida, Iowa, Virginia, and West Virginia departments of Transportation currently include a requirement that the temperature differential in elements designated as mass concrete be controlled to a maximum 20oC(or 35oF) (FDOT 2007) ACI 207.2R 1997 also recommends a maximum temperature differential of 20oC and a maximum temperature (usually 71oC) to control thermal cracking and prevent delayed ettringite formation (DEF) in concrete In 2001, Committee 363 adopted the following definition of HSC: concrete, highstrength-concrete that has a specified compressive strength for design of 8000 psi (55 MPa) or greater Demand for and use of HSC for tall buildings began in the 1970s, primarily in the U.S.A, mainly in the construction of columns and walls of high buildings Since then, HSC continued to be widely used around the world The use of HSC in bridges began in the U.S in the mid 1990s through a series of demonstration projects High-strength concrete has also been used in long-span boxgirder bridges and cable stayed bridge 1.6.2 Research on thermal control in early age concrete in Viet Nam In 2012, the Ministry of Construction published standard TCXD VN 9341: 2012 which stipulates that concrete or reinforced concrete structures are considered as massive concrete when the dimensions are sufficient to cause tensile stress due to heat of hydration of the cement which exceed the tensile strength of the concrete, causing cracking the concrete, therefore measures should be taken to prevent cracks Large concrete structures such as foundations and piers with cement-rich mixtures have higher temperature peaks, and an increased temperature difference between the surface and the core can increase the risk of thermal cracking Figure 1.10 shows the crack in Vinh Tuy bridge pier, which is concluded to be a temperature difference cracking in the pier concrete at an early age in the construction phase Figure1.10 Thermal cracking of Vinh Tuy Bridge pier Some case studies must be mentioned such as: Do Van Luong (2005), Nguyen Thong (2010), Ho Ngoc Khoa and Nguyen Chi Cong (2012), Le Quoc Toan (2015), Nguyen Van Lien (2018) and Nguyen Van Huong (2019) Most of these studies refer to structures using ordinary concrete, and offer some solutions to limit heat and thermal cracking in masive concrete 1.7 Conclusions - The evolution of knowledge on the subject has led to the development of theories that consider the hydration reaction as exothermic and thermally activated Also, the properties of the material and phenomena related to hydration evolution, such as strength, Young’s modulus, autogenous shrinkage and creep, will vary according to the extension of the reaction - Up to now, research works in Vietnam have mainly studied the distribution of temperature and stress in normally concrete blocks, and tested in the hydrothermal phase These studies only stop at using the theoretical curve for the thermal characteristics of concrete as input for the computational model The regulations in the world and in Vietnam on heat control of concrete all take the temperature difference between the surface and the core to be 20oC In Vietnam, there are currently no regulations or criteria to evaluate and control the heat of high cement content concrete used for bridge structures - The use of fly ash to replace cement brings many other benefits such as increasing the workability of concrete, reducing the amount of water required, increasing the durability of concrete Besides, using fly ash is also a way to make use of dust and gas emitted from coal-fired power plants, contributing to environmental protection - Studies in Vietnam and around the world have acknowledged that the use of fly ash to partially replace cement in traditional concrete mixes can reduce the heat released in the hydration process of cement How much fly ash can be used to replace cement in order to be both thermally beneficial and maintain strength has not been studied and has 12 Table 2.1 Compare input parameters and outputs of calculation models/software ABAQUS TNO DIANA ANSYS Midas Civil EACTSA Yes Yes Yes Yes Yes User have to create User have to create User have to create Yes Yes Poisson’s ratio, Yes Yes Yes Yes Yes CTE Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes k Yes Yes Yes Yes Varies by cp Yes Yes Yes Yes Yes Initial temperature Yes Yes Yes Yes Yes ATR Yes Yes Yes Yes Yes R, Ea, Qc, u, , User have to create User have to create User have to create No Yes Calculating degree of hydration dependent temperature evolution in concrete User have to create User have to create User have to create No Yes Input material parameters: Elastic modulus, E Effective modulus of elasticity, Eeff Structural inputs and boundary conditions: Size Yes Yes Yes Yes Yes Air temperature Yes Yes Yes Yes Yes convection coefficient, h Yes Yes Yes Yes Yes Boundary Conditions Yes Yes Yes Yes Yes Temperature Yes Yes Yes Yes Yes Stress Yes Yes Yes Yes Yes User have to create Yes User have to create Yes Yes Outputs: Cracking index, 13 2.10 Conclusions Chapter presented the theoretical basis of heat conduction in concrete and heat exchange with the open surface, including the heat generation process due to the cement hydration reaction The FD and FE methods are applied to calculate temperature and stress in early-age concrete structures, through a description of the calculation process of the EACTSA program Models built on ABAQUS, TNO DIANA, ANSYS, Midas Civil and EACTSA software were compared in terms of analysis capabilities, inputs and outputs Since then, the program EACTSA was selected to calculate because it considers the influence of temperature on the rate of hydration, the heat generation, and can receive material and thermal parameters from the experiments performed conveniently This is a proven tool and will be used for calculations, surveys and analysis in Chapter CHAPTER EXPERIENCE TO DETERMINATE THERMAL CHARACTERISTICS AND STRENGTH OF HIGH-STRENGTH FLYASH CONCRETE USED IN BRIDGE CONSTRUCTION 3.1 Experiment purpose The aim of this study is to find quantitative heat evolution and strength for HSC mixtures with different fly ash content, thereby comparing and evaluating according to the criteria of thermal cracking The study selected the characteristic compressive strength of high-strength concrete is 55 MPa All mixtures have the same amount of cementitious materials and the same water/cementitious materials (w/cm) ratio to investigate the variation in heat of hydration and strength developement when changing fly ash content The fly ash content partially repelaced the cement increases gradually from 0%, 10%, 20% to 30% in four mixtures 3.2 Strength testing 3.2.1 Concrete mix design The proportions of the four high-strength concrete mixtures are designed follow ACI 211.4R-08 and summarized in Table 3.10 All mixtures have the same amount of cementitious materials and the same water/cementitious materials (w/cm) ratio The fly ash content partially repelaced the cement increases gradually from 0%, 10%, 20% to 30% in mixtures Table 3.10 Concrete mix design Mixture Percentage replacement w/c w/cm CĐC-TB00 0% 0,32 CĐC-TB10 10% 0,36 Water Fly Coarse ash aggregate (kg) (kg) (l) Cement (kg) Sand (kg) HRWR (kg) 0,32 170 530 1050 723 5,5 0,32 170 477 53 1050 709 5,5 14 Mixture Percentage replacement w/c w/cm CĐC-TB20 20% 0,40 CĐC-TB30 30% 0,46 Water Fly Coarse ash aggregate (kg) (kg) (l) Cement (kg) Sand (kg) HRWR (kg) 0,32 170 424 106 1050 695 5,5 0,32 170 371 159 1050 680 5,5 3.2.2 Compressive strength testing The compressive strengths of the four concrete mixes at the ages of day, days, days, days, and 28 days were tested on 0,15m0,3m cylindrical samples Table 3.11 Concrete compressive strength (MPa) Age(day) CĐC-TB00 CĐC-TB10 CĐC-TB20 CĐC-TB30 29,3 25,07 22,93 18,21 49,54 41,75 40,50 30,29 55,58 50,11 47,30 38,80 62,47 60,30 56,00 46,91 28 76,19 70,6 68,00 63,50 3.2.3 Spliting tensile strengths Testing The spliting tensile strengths of the four concrete mixes at the ages of day, days, days, days, and 28 days were tested on 0,15m0,3m cylindrical samples (ASTM 496 -04 Standard) Table 3.13 Concrete spliting tensile strength (MPa) Age(day) CĐC-TB00 CĐC-TB10 CĐC-TB20 CĐC-TB30 2,65 2,44 2,21 1,88 3,37 3,19 2,67 2,56 3,72 3,69 3,48 3,16 4,48 4,40 4,10 3,83 28 5,98 5,01 4,62 4,30 3.3 Adiabatic Temperature Rise Testing The concrete mixes were tested to obtain the adiabatic temperature rise (ATR) to be used as the input for the temperature prediction model The ATR was measured using an adiabatic calorimeter developed in this study based on the concept described by Gibbon et al (1997) and improved by Lin and Chen (2015) The adiabatic calorimeter is shown in Fig 3.8 15 Figure3.8 Placing concrete sample in adiabatic calorimeter The ATR of four HSC mixtures were obtained using the above adiabatic calorimeter (Fig.3.8) The measured ATRs versus time are plotted in Fig 3.10 The initial temperature of concrete mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 were 31.1; 32; 31.7 31.3C, respectively The peak temperature of these mixtures were 89.2, 87.5, 84.6 and 79.1C at 50h, 77h, 50h, 40h and 36h after casting, respectively Figure 3.10 Measures ATR for HSC mixtures Figure 3.12 Nhiệt lượng tích lũy mẫu hỗn hợp BTCĐC tro bay 3.3.4 Determining the cumulative heat and the heat rate of HSC mixture The cumulative heat was determined and shown in Figure 3.12 The heat of hydration of four HSC mixtures were plotted in Figure 3.13 a) and the peak values were represented in Figure 3.13 b) a) Time from – 80 h b) Time from từ – 30 h Figure 3.13 The heat rate of four HSC mixtures 3.3.5 Determining the degree of hydration and hydration parameters 16 Table 3.16 Thermal properties of HSC Ea (J/mol) (kg/m3) cp (J/kg.°C) Mixture Hu (J/g) Qc (J/m3) CĐC-TB00 457,95 242714348 36011 2479 1042 CĐC-TB10 414,41 219635413 35226 2465 1042 CĐC-TB20 370,86 196556478 34528 2450 1043 CĐC-TB30 327,32 173477544 33916 2436 1044 The hydration parameters (u, , ) were determined using the least-squares method and are also shown in Table 3.17 The experimental degree of hydration curves and the fitted curves for the concrete mixtures versus equivalent age are plotted in Figure 3.14 Bảng 3.17 Heat of hydration parameters Mixture (h) u CĐC-TB00 19,73 1,387 0,6100 CĐC-TB10 20,92 1,836 0,6515 CĐC-TB20 21,54 1,685 0,7027 CĐC-TB30 23,26 1,567 0,7136 a) CĐC-TB00 c) CĐC-TB20 b) CĐC-TB10 d) CĐC-TB30 Figure 3.14 Fitted curves for experimental degree of hydration for HSC 17 In this study, the ultimate degree of hydration (u) for HSC mixtures are 5% to 9% smaller than those calculated using the fomular proposed by Shindler and Folliard (2005), as shown in Table 3.18 Table 3.18 Compare the experimental ultimate degree of hydration and the result calculated using E.q proposed by Shindler and Folliard (2005) Mixture Experimental u u calculated using E.q (3.5) Difference (%) CĐC-TB00 0,61 0,6424 5,0% CĐC-TB10 0,6515 0,6924 5,9% CĐC-TB20 0,7027 0,7424 5,3% CĐC-TB30 0,7136 0,7924 9,9% To represent the influence of fly ash content replaced cement on the ultimate degree of hydration for HSC mixtures, the fitted linear was calculated using the least-square method The fitted equation was proposed as follow: (R2 = 0.98): u u ,0 0, pFA (3.16) Where αu,0 is the experimental ultimate degree of hydration for CĐC-TB00 mixture containing 100% cement content, and pFA is the content of fly ash in the blended cement Fitted linear for ultimate degree of hydration is plotted in Figure 3.16 Figure 3.16 Fitted linear for ultimate degree of hydration versus the percentage fly ash content replacement for HSC 3.4 Conclusions (1) The compressive strength and spliting tensile strengths of the four concrete mixes at the ages of day, days, days, days, and 28 days were obtained The average compressive strength at the ages of 28 days of four HSC mixtures were over 60 MPa, the average tensile strength at the age of 28 days above MPa (2) The adiabatic temprature rises (ATRs) of four high-strength concrete (HSC) mixtures were obtained using an adiabatic calorimeter The highest temperature rises for the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 during the test were 58,1; 55,5; 52,9 and 47,9C, respectively 18 (3) The rates of hydration heat and peak values were determined At approximately 9.5 hours after mixing, the peak heat rates of the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 occurred at 4107; 2.95107; 2.31107 and 1.4107 (J/h/m3), respectively (4) The hydration parameters of HSC mixtures (u, τ and β) were also determined using the measured ATR and the curve fitting method The ultimate degree of hydration (u) increases with the increasing amount of fly ash replacement (5) The ultimate degree of hydration (u) of the four HSC mixtures are smaller than those calculated using the equation of Schindler and Folliard (2005) Thus, the coefficient accounting for the effect of fly ash replacement on αu needs to be adjusted for HSC as follows: the coefficient is 0.4 compared to 0.5 determined using the equation of Shindler and Folliard In these equations, w/cm is the water-to-cementitious materials ratio and pFA is the content of fly ash in the blended cement Shindler and Folliard (2005) u Proposed Equation u u ,0 0, pFA 1,031.w / cm 0,5 pFA 0,194 w / cm CHAPTER MEASURES TO CONTROL TEMPERATURE AND MITIGATE THERMAL CRACKING OF EARLY-AGE HIGHSTRENGTH CONCRETE BRIDGE STRUCTURES 4.1 Material measure 4.1.1 Srength parameters of concrete The relationship between the compressive strength (fc) and splitting tensile strength (fct) of concrete at early age with the degree of hydration were adjusted as the following equations: f c (t ) pc (t ) (4.9) f ct (t ) pct (t ) (4.10) a b where: pc, pct are coefficients determined from the regression curve of the experimental data fc(t), fct(t) – the compressive strength and splitting tensile strength versu time Fitted curves for tensile strength versus time of HSC mixtures are shown in Figure 4.2 19 Table 4.4 Parameters for strength development curves Mixture pc a pct b CĐC-TB00 111,70 1,055 6,3258 0,7335 CĐC-TB10 105,31 1,291 5,7552 0,7656 CĐC-TB20 84,33 1,139 5,2152 0,7715 CĐC-TB30 69,33 1,122 4,9292 0,8246 a) CĐC-TB00 b) CĐC-TB10 c) CĐC-TB20 d) CĐC-TB30 Figure 4.2 Fitted curves for tensile strength versus time of HSC mixtures The development of modulus of elasticity of concrete can be determined using model B3 (ACI 209.2R-08) 4.1.2 Creep parameters Creep parameters of the Bazant-Baweja B3 Model were determined according to ACI 209.2R-08 and shown in Table 4.5 Table 4.5 Creep parameters are determined according to ACI 209.2R-08 Mixture q1 (10-5/MPa) q2 (10-5/MPa) q3 (10-7/MPa) q4 (10-6/MPa) CĐC-TB00 1,452 8,641 2,627 8,718 CĐC-TB10 1,509 8,785 4,279 8,143 CĐC-TB20 1,557 8,759 6,502 7,550 CĐC-TB30 1,591 8,517 11,06 6,909 20 In this study, the coefficient of thermal expansion of concrete CTE = 8,510-6/C was taken from the work of Lin and Chen (2016), this is also a reasonable value according to ACI 363R-10 and Neville 4.1.3 Comparison and evaluation of HSC fly ash mixes in terms of thermal temperature and thermal cracking risk The experimental results in Chapter show that: increasing fly ash content can reduces the strength and the heat of hydration (adiabatic temperature rise): both quantities are inverse This relationship can be represented in the Figure 4.3 Both the strength at early age and heat of hydration decreased with increasing fly ash content (total binder remained constant) It is necessary to have a quantitative assessment, in which the relationship between strength and temperature must be considered, thereby making a comparison of the cracking risk under the same conditions, answering the question "How much fly ash is used is reasonable: both for thermal benefits, while ensuring strength and mitigating cracking risk of the structure" a) b) Figure 4.3 Scenarios between thermal stress and tensile strength versus content fly ash replacement This thesis will compare the development of temperature, strength, thermal stress, and cracking ability of bridge pier structures with the same cross-section using different types of HSC mixtures mentioned above The piers selected for analysis have the usual dimensions used in bridge construction with a cross section of 2.0 m x 3.0 m EACTSA program was used to analyze the heat distribution and thermal stress development in the pier Thermal conductivity k versus time (day – 7) and degree of hydration were calculated and shown in Table 4.8: Table 4.8 Degree of hydration and thermal conductivity of HSC mixtures versus time Age (day) CĐC-TB00 k /u (W/m.°C) 2,4871 CĐC-TB10 k /u (W/m.°C) 2,4871 CĐC-TB20 k /u (W/m.°C) 2,4871 CĐC-TB30 k /u (W/m.°C) 2,4871 21 0,7568 0,9215 0,9521 0,966 0,9738 0,9787 0,982 2,0201 1,9184 1,8995 1,891 1,8862 1,8831 1,8811 0,795 0,9585 0,9786 0,9865 0,9904 0,9928 0,9942 1,9965 1,8956 1,8832 1,8784 1,8759 1,8745 1,8736 0,7282 0,9364 0,9659 0,9778 0,984 0,9877 0,99 2,0377 1,9093 1,891 1,8837 1,8799 1,8776 1,8761 0,6238 0,8981 0,9442 0,9631 0,9731 0,9791 0,9829 2,1022 1,9329 1,9045 1,8928 1,8866 1,8829 1,8805 The other parameters were used in calculation process: the initial concrete temperature was assumed to be 27°C, the ambient temperature is taken according to the actual measured value, so there is an increase and decrease in the day and night; the convection coefficient is hc = 13,9 W/(m2.°C) Case 1: The bridge pier uses CĐC-TB00 mixture The predicted temperature histories at the center, mid-side, bottom, and corner of the section are portrayed in Fig 4.4 The predicted temperature contour in the cross section 40 h after placement is plotted in Fig 4.5 Figure 4.4 Temperature histories in the pier using CĐC-TB00 Figure 4.5 Temperature contours °C at 40h in the pier using CĐCTB00 The stress analysis was then performed using the thermal loading obtained previously The predicted tensile stress profiles yy at mid-side and xx at corner of the section are plotted in Fig 4.7 The 40h principal stress contours in the section is shown in Fig 4.6 whereas the principal tensile stress profiles at mid-side, bottom, and corner of the section are portrayed in Fig 4.8 As can be seen in the figure, the highest principal tensile stress experiences at mid-side of the section In order to evaluate earlyage thermal cracking risk in the concrete, the maximum principal stress at mid-side is compared with the tensile strength at this point as shown in Fig 4.9 22 Figure 4.6 Principal stress contours (MPa) at 40h in the pier using CĐCTB00 Figure 4.7 Stress histories yy at midside and xx at the corner in the pier using CĐC-TB00 Figure 4.8 Principal stress histories in the pier using CĐC-TB00 Figure 4.9 Max principal tensile stress and tensile strength at mid-side in the pier using CĐC-TB00 As can be seen in Figure 4.9, the thermal tensile stress development curve is always below the tensile strength development curve in the first 168 h after casting, showed that concrete does not have a high cracking risk at an early age It is noted that after 70 h, the thermal stress tends to decrease rapidly, related to a decrease in the temperature difference between the core and the surface of concrete Similar analysis for the concrete mixes CĐC-TB10, CĐC-TB20, CĐC-TB30, the results are as follow: Figure 4.11 Max principal tensile stress and tensile strength at mid-side in the pier using CĐC-TB10 mixture Figure 4.13 Max principal tensile stress and tensile strength at mid-side in the pier using CĐC-TB20 mixture 23 Figure 4.15 Max principal tensile stress and tensile strength at mid-side in the pier using CĐC-TB30 mixture Figure 4.16 Max cracking index for four HSC mixtures As can be seen in the figures, when using the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20, the tensile strength is greater than the tensile stress within days of concrete placement revealing low thermal cracking risk during this construction stage; and in the case of the mixture CĐC-TB30, the thermal stress exceeds the tensile strength in the period from 39 to 68 h This shows that the risk of thermal cracking of the CĐCTB30 mixture is the highest among the four experimental concrete mixes, so it is not recommended to use too much fly ash replacement (> 20%) To quantify the risk of thermal cracking risk of each HSC mixture, the ratio between the thermal stress and the tensile strength of the concrete was used, is defined as the cracking index The maximum cracking index (at the point of the section with the highest principal stress) of different HSC mixtures were calculated and shown in Figure 4.16 It can be seen that the cracking index of CĐC-TB00, CĐC-TB10, and CĐC-TB20 mixtures are always less than during days, while the cracking index of CĐC-TB30 mixture is greater than during the period of time from 39 h to 68 h after casting (within the first days) Thus, when comparing quantitatively, the CĐC-TB30 mixture has a higher cracking risk than the other three mixtures under the same conditions of structural size and ambient temperature 4.2 Other Measures 4.2.1 Reasonable time to remove formwork As can be seen in the figures 4.9, 4.11, 4.13 and 4.15, the cracking risk is fairly high during the 72h period (within days) of concrete placement For larger sized members, the thermal cracking risk can last for days or more Therefore, it can be recommended that: for the reinforced concrete mixes, although the compressive strength can be achieved at early age as 2-3 days, to mitigate thermal cracking risk, the formwork should not be removed before 3-4 days after concrete placement 4.2.2 Using insulation to cover the concrete at early age The thickness of the insulation layer must be sufficient to ensure the allowable temperature difference When removing the formwork, the insulation material should still be left in place and must be removed slowly to avoid thermal shock 24 4.3 Conclusions In chapter 4, the thesis has analyzed quantitatively about material solutions and some other measures to mitigate thermal cracking of early-age high-strength concrete bridge structures The thesis has compared the temperature development, strength, thermal stress and cracking risk of bridge pier structures with the same cross-section using types of high-strength concrete mixtures studied To evaluate the risk of thermal cracking at an early age in concrete, the cracking coefficient (which is the ratio between thermal stress and tensile strength) is used The results show that CĐC-TB30 mixture has a slightly higher risk of thermal cracking than the other blends Therefore, it is reasonable to use a high strength fly ash concrete mix with an alternative fly ash content of less than 30% to minimize the risk of thermal cracking As analyzed in Chapter 1, the use of fly ash in concrete offers many economic, technical and environmental benefits Therefore, within the scope of the study, the author proposes to use a high strength fly ash concrete mix with a reasonable replacement fly ash content of 10÷20% CONCLUSIONS AND RECOMMENDATIONS CONCLUSIONS: (1) This research presented an overview of the hydration process of cement and emphasized that the hydration reaction as exothermic and thermally activated The rate of heat generation at a point in concrete depends on the time and temperature at that point The thesis also presented an overview of heat transfer, heat exchange in concrete and numerical methods (FD and FE) to calculate temperature and stress in concrete (2) The compressive strength and spliting tensile strengths of the four HSC mixes at the ages of day, days, days, days, and 28 days were obtained The average compressive strength at the ages of 28 days of four HSC mixtures were over 60 MPa, the average tensile strength at the age of 28 days above MPa (3) The adiabatic temprature rises (ATRs) of four high-strength concrete (HSC) mixtures were obtained using an adiabatic calorimeter The highest temperature rises for the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 during the test were 58,1; 55,5; 52,9 and 47,9C, respectively (4) The rates of hydration heat and peak values were determined At approximately 9.5 hours after mixing, the peak heat rates of the mixtures CĐC-TB00, CĐC-TB10, CĐC-TB20 and CĐC-TB30 occurred at 4107; 2.95107; 2.31107 and 1.4107 (J/h/m3), respectively (5) The hydration parameters of HSC mixtures (u, τ and β) were also determined using the measured ATR and the curve fitting method The ultimate degree of hydration (u) increases with the increasing amount of fly ash replacement (6) The ultimate degree of hydration (u) of the four HSC mixtures are smaller 25 than those calculated using the equation of Schindler and Folliard (2005) Thus, the coefficient accounting for the effect of fly ash replacement on αu needs to be adjusted for HSC as follows: the coefficient is 0.4 compared to 0.5 determined using the equation of Shindler and Folliard In these equations, w/cm is the water-to-cementitious materials ratio and pFA is the content of fly ash in the blended cement Shindler and Folliard (2005) u 1,031.w / cm 0,5 pFA 0,194 w / cm Proposed Equation u u ,0 0, pFA (7) Quantitative analysis based on four HSC mixtures was performed to evaluate the cracking risk in rectangular bridge piers at early ages The result shows that the thermal cracking risk of the pier using CĐC-TB30 was highest among the four mixtures Furthermore, with economic, technical and environmental advantages of using fly ash, use of an HSC containing 10% to 20% fly ash replacement is recommended to minimize cracking risk in early-age concrete RECOMMENDATIONS: (1) Based on the research methodology from this thesis, it is possible to conduct thermal research for various types of HSC, HPC and UHPC with different cementitiuos supplementary materials such as blast furnace slag, silica fume, (2) In bridge structures using high-strength concrete mixture, fly ash was recommended to be used with a content of 10÷20% replacement of cement by weight to ensure strength and mitigate early-age thermal cracking risk 26 PUBLICATIONS Đỗ Anh Tú, Hoàng Việt Hải, Vũ Xuân Thành, Hoàng Thị Tuyết, Nguyễn Hoài Nam(2019), “Mức độ thủy hóa phát triển cường độ bê tơng cường độ cao”, Tạp chí Khoa học GTVT, Tập 70 số (tháng 8/2019) Tu Anh Do , Thuan Huu Nguyen, Thanh Xuan Vu, Tuyet Thi Hoang, Tam Duc Tran, Thanh Tien Bui (2019), “Adiabatic temperature rise and thermal analysis of high-performance concrete bridge elements”, The International Conference on Sustainable Civil Engineering and Architecture (ICSCEA) 2019, 24-26 October 2019, Ho Chi Minh City, Vietnam (indexed in Scopus) Đỗ Anh Tú, Nguyễn Xuân Lam, Hoàng Thị Tuyết, Thẩm Quốc Thắng, Nguyễn Văn Trường (2020), “Thực nghiệm xác định nhiệt độ đoạn nhiệt từ q trình thủy hóa xi măng cho bê tông thông thường dùng công trình cầu”, Tạp chí Cầu đường Việt Nam, số 1+2/2020 Tuyet Thi Hoang, Tu Anh Do, Linh Ha Le , and Thang Quoc Tham (2020), “Experimental determination of adiabatic temperature rise and hydration parameters for concrete”, Science Journal of Transportation, Special Issue No.10 Tu Anh Do, Tuyet Thi Hoang*, Thanh Bui-Tien, Hai Viet Hoang, Tuan Duc Do, Phan Anh Nguyen (2020),“Evaluation of heat of hydration, temperature evolution and thermal cracking risk in high-strength concrete at early ages” Case Studies in Thermal Engineering, Vol.21, October 2020 https://doi.org/10.1016/j.csite.2020.100658 (SCIE) Chuc Trong Nguyen, Tu Anh Do, Tuyet Thi Hoang, Tam Duc Tran (2021) “Evaluation of early-age cracking risk in mass concrete footings under different placement conditions”, Revista Ingenieria de Construccion, Vol 36 No1, December 2021 https://dx.doi.org/10.4067/S0718-50732021000100005 (ESCI) Tu Anh Do, Mang Tia, Thuan Huu Nguyen, Tuyet Thi Hoang , Tam Duc Tran (2022),“Assessment of Temperature Evolution and Early-Age Thermal Cracking Risk in Segmental High-Strength Concrete Box Girder Diaphragms”, KSCE Journal of Civil Engineering, Vol 26, September 2021 http://dx.doi.org/10.1007/s12205-021-2148-5 (SCIE) ... Xuân Thành, Hoàng Thị Tuyết, Nguyễn Hồi Nam(2019), “Mức độ thủy hóa phát triển cường độ bê tông cường độ cao? ??, Tạp chí Khoa học GTVT, Tập 70 số (tháng 8/2019) Tu Anh Do , Thuan Huu Nguyen, Thanh... Scopus) Đỗ Anh Tú, Nguyễn Xuân Lam, Hoàng Thị Tuyết, Thẩm Quốc Thắng, Nguyễn Văn Trường (2020), “Thực nghiệm xác định nhiệt độ đoạn nhiệt từ q trình thủy hóa xi măng cho bê tơng thơng thường dùng... fume, fly ash, slag) are more and more used to partially replace Portland cement to limit the temperature increase in massive concrete structures * Effect of fly ash when replacing cement on