Tóm tắt LA (tiếng anh): 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.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.

UNIVERSITY OF TRANSPORT AND COMMUNICATIONS

THERMAL BEHAVIOR OF EARLY-AGE HIGH-STRENGTH CONCRETE BRIDGE STRUCTURES AND MEASURES TO CONTROL TEMPERATURE

AND MITIGATE THERMAL CRACKING

SUMMARY OF DOCTORAL THESIS

HA NOI- 2022

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

INTRODUCTION

1 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 strength, 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

high-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 do 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 4 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

2 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

3 Research objects and research scope

of 2.0 m x 3.0 m at early ages (0 - 7 days after casting)

- The effect of shrinkage and reinforcement distribution are not considered

5 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

6 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 47,9C, 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 4107; 2.95107; 2.31107 and 1.4107 (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 αu 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) Proposed Equation

- 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 1 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

A typical hydration process includes 5 stages: Figure 1.1

Figure 1 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 1 3 Models for final degree of hydration for Portland cement

Researchers The ultimate degree of hydration αu

Powers and Brownyard (1947) min (1; w /

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

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

Fig 1.7 Thermal stress and tensile strength

development with crack

initiation Figure 1.6 Thermal evolution and formation

cracks in masive concrete

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 7 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 EARLY- AGE 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, strength-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

high-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 box-girder 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

not been quantified

From the above comments, the author selected a study to analyze the temperature

and stress development in high-strength fly ash concrete bridge components to find the

optimal fly ash replacement content while reducing hydrothermal heat while ensuring

the resistance of the structure

CHAPTER 2 THEORETICAL BASIS OF HEAT TRANSFER AND

TEMPERATURE AND STRENGTH CALCULATION MODEL IN

EARLY AGE CONCRETE STRUCTURE 2.1 Theoretical foundations of heat conduction

The differential equation of heat conduction in concrete is expressed in the Cartesian

coordinate system (Oxyz) as follows

Surface convection refers to the heat transfer from the surface of a material to a flow

that is moving over it, as expressed by Eq.(2.9)

q  hA T  T (2.9)

Where:

qc - rate of heat transfer (W)

Ts - temperatures of the surface (°C)

Ta - temperatures of the air (°C)

As - surface area (m2)

h - convection coefficient between the concrete surface and the air

(W/m2.°C)

2.3 The heat rate

A mathematical (three-parameter) degree of hydration model expressed in Eq

(2.11) has been effectively used to estimate temperature evolution in concrete since it

incorporates the temperature effect via the equivalent age The equivalent age (te) of concrete (or maturity) and the ultimate degree of hydration (u) were calculated using

Eq (2.12) and (2.13) In order to use the hydration model in Eq (2.11), the αu, τ, and β parameters are determined by fitting Eq (2.11) with the calculated degree of hydration from the measured ATR

Calculation process:

The computer program “EACTSA” (Early-Age Concrete Thermal Stress Analysis) was used in this study to calculate degree of hydration dependent temperature evolution and the associated thermal stress in concrete The EACTSA program - a combined FD and FE model - was created in MATLAB, verified and presented in the previous research The calculation of early-age stress consists of two stages: temperature and stress analyses The resultant temperature profile at each node of the concrete section is then used as an input loading for the stress analysis, in which EACTSA considers the change of material properties and creep effect at each time increment of 1

h

The flowcharts of the algorithm are shown below:

2.8 Numerical models simulating early-age thermal behavior of concrete

Initial studies mainly focused on using theoretical functions describing heat generation during cement hydration to calculate the adiabatic temperature rise, then applied to the computational model

Recent computational models have taken into account the role of supplementary cementitious materials in the concrete mixture and the complexity of the unequal thermal loads from the heat of hydration as well as the material properties of the concrete The temperature-dependent irregular early age concrete was taken into account

to better predict the thermal behavior of concrete

Such theories led to sophisticated numerical models that, together with the evolution of computer hardware and software, allowed the development of very complex simulation models that successively get closer to reality both in terms of geometry and phenomenological models considered

2.9 Comparison of thermal and thermal stress calculation models

The softwares ABAQUS, TNO DIANA, ANSYS, and Midas Civil have their own advantages and disadvantages, were compared in Table 2.1 base on the input and output parameters

Figure 2.7 Thermal Analysis

Flowchart

Figure 2.8 Stress Analysis Flowchart

Table 2.1 Compare input parameters and outputs of calculation models/software

ABAQUS TNO

DIANA ANSYS

Midas Civil EACTSA

Input material parameters:



Structural inputs and boundary conditions:

Outputs:

Cracking index,  User have

to create

Yes User have

to create