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Early plastic shrinkage cracking of concrete is an important factor affecting the durability of modern concrete structures. Early cracking (within 24 hours after pouring) may become a problem for any concrete structure. It will promote the entry of harmful materials, destroy the beauty of concrete members, and reduce their durability and performance. In addition, due to long-term shrinkage and/or load, these cracks may gradually expand in the service life of components. Scientific research and engineering technicians often have to face the difficulties caused by early plastic shrinkage cracking of concrete. From the aspects of shrinkage mechanism, measurement method, prediction model and strength development, this paper reviews the scientific and technological status of plastic shrinkage and strength development of early-age concrete, and based on this, summarizes the important conclusions in existing research and establishes the relevant concrete strength prediction model.:1 Introduction 2. Shrinkage in concrete 2.1 Classification and mechanism of concrete shrinkage 2.2 Main factors causing concrete shrinkage 2.3 Concluding remarks 3. Plastic shrinkage in early age concrete 3.1 Method for determining the time of initial and final setting 3.2 Mechanism of plastic shrinkage 3.3 Evaporation 3.4 Capillary pressure 3.5 Main factors affecting plastic shrinkage cracking 3.6 Concluding remarks 4. Different methods for determining the resistance to plastic shrinkage cracking 4.1 Rectangular mould test setup 4.2 ASTM C 1579 4.3 Ring test method (NT BUILD 433) 4.4 Capillary pressure test 5. Development of early age strength of concrete 5.1 Mechanical properties 5.1.1 Compressive strength 5.1.2 Tensile strength 5.1.3 Early-age shrinkage of concrete 5.2 Test and prediction model evaluation 6. Test and quantitative model 6.1 pullout tests on early-age concrete 6.1.1 Tests principle 6.1.2 test result 6.2 Compilation of existing pullout capacity prediction models 6.2.1 Strength and pullout force model based on 𝒉𝒆𝒇 6.2.2 Strength and pullout force model based on 𝒉𝒆𝒇 and ∅𝒉 6.2.3 Tensile strength and pullout force model 6.3 Application of existing prediction model in early age concrete 7. Conclusions 8. Literature

Over the past fifty years, many states have recognized the benefits of making precast, prestressed multi-girder bridges continuous by connecting the girders with a continuity diaphragm. Although there is widespread agreement on the benefits of continuous construction, there has not been as much agreement on either the methods used for design of these systems or the details used for the continuity connections. To aid designers in choosing the most appropriate method, an analytical and experimental study was undertaken at Virginia Tech. Analyses were done to compare the differences in the predicted continuity moments for different design methods and assumptions over a range of commonly used systems of Precast Concrete Bulb Tee (PCBT) girders and cast-in-place slabs. The results of the analyses were used to develop three continuity connection details for testing during the experimental study. Three different continuity connections were tested using full depth PCBT 45 in. deep girders made continuous with a 6 ft wide slab. The bottom of the ends of the girders were made continuous with the continuity connection by extending prestressing strands for one test and extending 180 degree bent bars for the other test. Both connections adequately resisted service, cyclic, and ultimate loads. But, the test with the extended bars remained stiffer during cyclic loading and is recommended for use. A third test was performed on a system using only a slab cast across the top of the girders. Two primary cracks formed above the ends of the girders at the joint during service testing, after which no significant increase in damage took place. Results from the analytical study indicate that the predicted positive thermal restraint moments may be significant, similar in magnitude to the actual positive cracking moment capacities. Results from the experimental study indicate that restraint moments develop early due to thermal expansion of the deck during curing and subsequent differential shrinkage; however, the magnitudes of the early age restraint moments are much less than conventional analyses predict. / Ph. D.

Concrete undergoes volumetric changes throughout its service life. These changes are a result of applied loads and shrinkage. Applied loads result in an instantaneous recoverable elastic deformation and a slow, time dependent, inelastic deformation called creep. Creep without moisture loss is referred to as basic creep and with moisture loss is referred to as drying creep. Shrinkage is the combination of autogeneous, drying, and carbonation shrinkage. The combination of creep, shrinkage, and elastic deformation is referred to as total strain. The prestressed concrete beams in the Chickahominy River Bridge have been fabricated with a lightweight, high strength concrete mixture (LTHSC). Laboratory test specimens have been cast using the concrete materials and mixture proportions used in the fabrication of the bridge beams. Two standard cure and two match cure batches have been loaded for 329 and 251 days, respectively. Prestress losses are generally calculated with the total strain predicted by the American Concrete Institute Committee 209 recommendations, ACI 209, or the European design code, CEB Model Code 90. Two additional models that have been proposed are the B3 model by Bazant and Baweja, and the GL2000 model proposed by Gardner and Lockman. The four models are analyzed to determine the most precise model for the LTHSC mixture. Only ACI 209 considered lightweight aggregates during model development. GL2000 considers aggregate stiffness in the model. ACI 209 was the best predictor of total strain and individual time dependent deformations for the accelerated cure specimens. CEB Mode Code 90 was the best predictor of total strain for the standard cure specimens. The best overall predictor of time dependent deformations was the GL2000 model for the standard cure specimens. / Master of Science

In the precast construction market, the competitive situation is significantly affected by price, cost, productivity, and quality factors. Since self-consolidating concrete (SCC) was first introduced to the concrete industry in the late 1980s, it has been used worldwide in variety of applications. Despite the documented technical and economic advantages of SCC in precast, prestressed applications, the use of SCC has been limited in some countries due to some technical uncertainties of such innovative material. To explore some unsolved issues related to SCC and to contribute to a wider acceptance of SCC in precast, prestressed applications, this study was undertaken to assess the effect of mixture proportioning and material characteristics on the performance of SCC and recommend performance-based specifications for use of SCC in the precast, prestressed applications. The thesis presents an experimental program that contains four parts: (1) a parametric study to evaluate the influence of binder type, w/cm, coarse aggregate type, and coarse aggregate nominal size on the modulus of elasticity and compressive strength developments; (2) a parametric study to evaluate the effect of mixture proportioning and material characteristics on fresh and hardened properties of SCC; (3) a fractional factorial design to identify the relative significance of primary mixture parameters and their coupled effects on SCC properties; and (4) a field validation using full-scale AASHTO Type II girders cast to investigate constructability, material properties, and structural performance (the latter part was carried out by the research team of Professor Denis Mitchell at McGill University). Based on the experimental test results, SCC exhibits similar compressive strength and modulus of elasticity to that of conventional high-performance concrete (HPC) of normal slump consistency. SCC and HPC mixtures made of a given binder type exhibit similar autogenous shrinkage. However, SCC exhibits up to 30% and 20% higher drying shrinkage and creep, respectively, at 300 days compared to HPC made with similar w/cm but different paste volume. The results of the experiment program show that among the investigated material constituents and mix design parameters, the w/cm has the most significant effect on mechanical and visco-elastic properties. The binder content, binder type, and sand-to-total aggregate ratio (S/A) also have considerable effect on those properties. The thickening-type viscosity modifying admixture (VMA) content (0 to 150 ml/100 kg CM) does not significantly affect mechanical and visco-elastic properties. Based on the findings, some mixture parameters regarding the overall performance of SCC designated for precast and prestressed applications can be recommended: SCC made with relatively low w/cm (such as 0.34 vs. 0.40) should be selected to ensure desirable compressive strength, modulus of elasticity (MOE), flexural strength, as well as less drying shrinkage and creep; the use of crushed aggregate with 12.5 mm MSA is suggested since it provides better mechanical properties of SCC compared to gravel; the use of low S/A (such as 0.46 vs. 0.54) to secure adequate mechanical and visco-elastic properties is recommended; the use of thickening-type VMA can help to secure robustness and stability of the concrete in the case of SCC proportioned with moderate and relatively high w/cm; and the use of Type MS cement can lead to lower creep and shrinkage than Type HE cement and 20% Class F fly ash. However, SCC mixtures made with Type HE cement and 20% Class F fly ash can result in better workability and mechanical properties. Therefore, it is recommended to use Type HE cement and 20% Class F fly ash and reduce binder content (such as 440 kg/m[exposant 3] vs. 500 kg/m[exposant 3]) to assure better overall performance of SCC. Validation on full-scale AASHTO-Type II girders using two HPC and two SCC mixtures show that girders casting with SCC can be successfully carried out without segregation and blocking for the selected optimized mixtures. The surface quality of the girders cast with SCC is quite satisfactory and of greater uniformity than girders cast with HPC. Both HPC and SCC mixtures develop similar autogenous shrinkage for mixtures made with similar w/cm. Again, the two evaluated SCC mixtures develop about 20% greater drying shrinkage than comparable HPC mixtures. Modifications of existing models to assess mechanical and visco-elastic properties of SCC used in the precast, prestressed applications are proposed. Based on the comparisons of various code provisions, the ACI 209 and CEB-FIP codes with suggested material coefficients can be recommended to estimate compressive strength. The modified AASHTO 2007 model can be used for predicting the elastic modulus and flexural strength. The AASHTO 2004 and 2007 models with suggested material coefficients can be used to estimate drying shrinkage and creep, respectively. The CEB-FIP 90 code model can be used to predict both drying shrinkage and creep. Finally, the modified Tazawa and Miyazawa 1997 model with material modifications can be used to estimate autogenous shrinkage of SCC.

Cracked pavement slabs lead to uncomfortable and eventual unsafe driving conditions for motorists. Replacement of cracked pavement slabs can interrupt traffic flow in the form of lane closures. In Florida, the traffic demands are high and pavement repairs need to be carried out swiftly typically using concrete with high cement contents and accelerators to create rapid setting and strength gain. The concrete used in these pavement replacements is usually accompanied by a high temperature rise, making the replaced slabs susceptible to cracking. Cracking is a result of developed tensile stresses in the concrete, which exceed the concrete's tensile strength capacity. This research is being conducted to determine the risk of cracking for pavement slabs with varying dosages of chloride based accelerator used to promote high early strength. To analyze the effect of the accelerator, five different concrete mixtures including a control were assessed in a series of tests with varying accelerator dosages. Experiments included: mortar cube testing, concrete cylinder testing, autogenous deformation measured with a free-shrinkage frame, and restrained stress analysis using a rigid cracking frame. The findings indicate that accelerators are necessary to meet the strength requirements, and that the higher the accelerator dose, the higher the early shrinkage in the first 24 hours determined from the free shrinkage frame. Accidental overdose of the chloride-based accelerator results in the highest cracking potential and the highest shrinkage when tested under field generated temperature profiles.

The industrial concrete floor is a very important part of an industrial building, distribution center, storage or shopping mall, and it must have high quality surfaces for operation. To achieve the high quality we must know the problems and how to treat them. The most important problems on the concrete floors are: (i) cracks which are caused by shrinkage and creep, (ii) curling resulting in a loss of contact between concrete slab and sub-base, and (iii) unevenness In this thesis, it is aimed to investigate the effect of optimizing the concrete mix with and without additional shrinkage reducing agents (SRA) to reduce the crack risk in industrial concrete floors. Four types of concrete recipes are used (A-D) which include a recipe with optimized mix design for minimum shrinkage, a reference recipe (standard mix), an optimized mix with SRA and a fourth recipe with the reference plus SRA. The testing program extended to 224 days of age and comprised e.g. free-shrinkage, restrained shrinkage, weight change, modulus of elasticity, compressive strength, splitting tensile strength and creep of concrete. At early ages, a 28 days, there are large differences in shrinkage-time relations for different mixes. Later than 28 days, the relations are closer. A comparison among shrinkage and creep test results of four recipes shows that recipes A and C have greater crack risk than recipes B and D. The recipe D has also the best result in restrained shrinkage test. These results are because of the aggrega-te graduation, type of cement and shrinkage reducing agents which all have a direct influence on the concrete properties. These tests were done by CBI (The Swedish Cement and Concrete Research Institute) during 2009.

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Der 3D-Druck mit Beton zählt zu den vielversprechendsten Methoden der automatisierten Bauweise. Er bietet zahlreiche Vorteile gegenüber konventionellen Bauverfahren, wie beispielsweise Kostenersparnis, erhöhte Produktivität und architektonische Gestaltungsfreiheit. In den letzten Jahren hat sich der 3D-Druck mit Beton von einer gewagten Vision zu einer zukunftsweisenden Baumethode entwickelt. In mehreren Ländern konnte die praktische Anwendbarkeit der neuen Technologie durch zahlreiche Demonstratorobjekte bewiesen werden. Um eine breite Anwendung in der Baupraxis zu ermöglichen, müssen jedoch noch einige material- und technologiespezifische Fragestellungen gelöst werden. Eine davon ist die Rissbildung der gedruckten Betonelemente aufgrund von Schwindverformungen. Das Ausmaß der Schwindverformungen ist vor der Verfestigung der gedruckten Schichten am größten. Diese Verformungen werden als plastisches Schwinden bezeichnet. Das plastische Schwinden wird maßgeblich durch die hohe Wasserverdunstung im jungen Alter des Betons und dem dadurch folgenden inneren Spannungsaufbau in den Kapillaren hervorgerufen. Im Fall, dass die Verformungen eines Elements z. B. durch Schichtverbund oder Bewehrungselemente gehindert werden und daraus resultierende Spannungen höher als die Zugfestigkeit des Betons sind, kann es zur Rissbildung kommen. 3D-gedruckte Betonelemente sind stärker als konventionell gefertigte vom plastischen Schwinden bedroht. Dies hängt vor allem mit der schalungsfreien Bauweise und den spezifischen Zusammensetzungen der druckbaren Betonrezepturen zusammen. Risse, die durch das plastische Schwinden entstehen, können sich über den gesamten Querschnitt eines gedruckten Elements ausbreiten. Die dadurch verursachten Schäden gefährden die Dauerhaftigkeit, die Gebrauchstauglichkeit, beeinträchtigen die Ästhetik und können sogar zum Stabilitätsverlust führen. Trotz der Signifikanz dieser Problematik und der möglichen Schäden durch später auftretende Schwindarten wie z.B. Trocknungsschwinden und autogenes Schwinden, wurden bis jetzt nur wenige Studien diesem Thema gewidmet. Auch wurden die Quantifizierungs- und Vorbeugungsmethoden bisher ungenügend erforscht. Die vorliegende Dissertation befasst sich eingehend mit den Mechanismen des plastischen Schwindens und der damit verbundenen Rissbildung bei 3D-gedruckten Betonelementen. Da es keine standardisierte oder allgemein anerkannte Methode zur Quantifizierung des plastischen Schwindens und der damit verbundenen Rissbildung von 3D-druckbaren Betonen gibt, wurde in dieser Arbeit eine zuverlässige und einfach anwendbare Messmethode entwickelt. Diese Methode ermöglicht gleichzeitig die Quantifizierung des ungehinderten und gehinderten plastischen Schwindens sowie die Ermittlung relevanter Materialeigenschaften. Die durchgeführten statistischen Analysen bestätigten die Reproduzierbarkeit der erzielten Ergebnisse. Die Ergebnisse dieser Arbeit tragen zur Etablierung einer einheitlichen Methodologie für die Untersuchung des plastischen Schwindens und der damit verbundenen Rissbildung bei 3D-gedruckten Betonen bei. Auf Grundlage der entwickelten Versuchsaufbauten wurden spezifische Mechanismen des plastischen Schwindens und der damit verbundenen Rissbildung von 3D-gedruckten Elementen erforscht. Die experimentellen Untersuchungen wurden durch eine numerische Simulation von der Entwicklung des Kapillarporendrucks in gedruckten Elementen ergänzt. Ein besonderes Augenmerk lag auf dem Einfluss der Schichtdicke und dem Ausmaß der der Austrocknung ausgesetzten Fläche. Es wurde ein spezifisches Verformungsverhalten bei 3D-gedruckten Betonelementen festgestellt. Der Zeitpunkt, die Richtungen und das Ausmaß der schwindbedingten Verformungen wurden umfassend analysiert. Überdies wurde an einem analytischen und numerischen Modell zur Vorhersage der Schwindverformungen in 3D-gedruckten Betonelementen gearbeitet. Praktische Empfehlungen auf Grundlage der Analyse verschiedener Maßnahmen zur Vorbeugung und Reduzierung des plastischen Schwindens und der damit verbundenen Rissbildung bilden den Abschluss dieser Arbeit.:Abstract I Kurzfassung II Vorwort des Herausgebers IV Acknowledgement V Contents VI Notations and abbreviations XI 1 Introduction 1 1.1 Motivation 1 1.2 Relevance of the research 2 1.3 Objectives and research questions 2 1.4 Dissertation structure 3 2 Theoretical background 5 2.1 Plastic shrinkage of cementitious materials 5 2.1.1 Mechanisms of plastic shrinkage 5 2.1.2 Mechanisms of plastic shrinkage cracking 6 2.1.3 Experimental methods 7 2.1.4 Numerical methods 8 2.1.5 Mitigation techniques 9 2.1.5.1 Active mitigation approaches 9 2.1.5.2 Passive mitigation approaches 9 2.2 3D concrete printing 11 2.2.1 Flashback to history 11 2.2.2 Current state 14 2.2.3 Significance of the PS and PSC for 3D-printed concrete elements 14 2.2.3.1 Specifics of material compositions 14 2.2.3.2 Production related issues 14 2.2.3.3 Case studies 15 2.2.4 Previous studies on PS and PSC of 3D-printed concrete elements 19 2.3 Chapter summary 19 3 Materials and methods 21 3.1 Reference composition 21 3.2 Experimental methods 22 3.2.1 3D concrete printing test device 22 3.2.2 Wind tunnel and climate control chamber 23 3.2.3 Determination of the specific material properties 23 3.2.3.1 Air content and spread flow 23 3.2.3.2 Capillary pressure 24 3.2.3.3 Ultrasonic pulse velocity 24 3.2.3.4 Tempe cell and self-desiccation tests 24 3.2.3.5 Falling-head method 25 3.2.3.6 Tea bag test 25 3.2.3.7 Confined uniaxial compression test 26 3.2.3.8 Penetration test 26 3.2.3.9 Microscopy 27 3.2.4 Digital image correlation 27 3.3 Numerical method 28 3.4 Chapter summary 28 4 Quantification of plastic shrinkage and plastic shrinkage cracking of the 3D-printable concretes using 2D digital image correlation 29 4.1 Novel setups for quantification of the PS and PSC 29 4.2 Materials and methods of investigation 30 4.2.1 3D printing and preparation of samples 30 4.2.2 Evaluation of the deformations 32 4.2.3 Experimental setup and procedure 33 4.3 Experimental results 33 4.3.1 Penetration force 33 4.3.2 Free shrinkage behaviour 34 4.3.2.1 Vertical settlement 34 4.3.2.2 Horizontal shrinkage 35 4.3.3 Shrinkage behaviour in partially and fully restrained tests 35 4.3.3.1 Vertical shrinkage 35 4.3.3.2 Horizontal shrinkage 36 4.3.3.3 Shrinkage-induced cracking 38 4.3.4 Influence of the paint on the surface 41 4.4 Discussion of the test setups and measuring techniques 42 4.5 Chapter summary 43 5 Repeatability of the experimental results 45 5.1 Followed statistical approach for assessment of the repeatability 45 5.2 Experimental program 46 5.3 Preparation of the samples and the experimental procedure 46 5.4 Results and discussion 48 5.4.1 Repeatability of the experimental results in previous studies 48 5.4.2 Free shrinkage 49 5.4.2.1 Spread flow, density and air content 49 5.4.2.2 Ambient conditions 49 5.4.2.3 Water loss 50 5.4.2.4 Evolution of the capillary pressure 50 5.4.2.5 Temperature 51 5.4.2.6 Shrinkage 52 5.4.2.7 Evaluation of the repeatability 55 5.4.3 Restrained shrinkage 56 5.4.3.1 Basic fresh-state properties 56 5.4.3.2 Ambient conditions 56 5.4.3.3 Water loss 57 5.4.3.4 Evolution of the capillary pressure 58 5.4.3.5 Temperature 58 5.4.3.6 Shrinkage 59 5.4.3.7 Cracking 60 5.4.3.8 Evaluation of repeatability 62 5.5 Chapter summary 63 6 Specifics of plastic shrinkage and related cracking in 3D-printed concrete elements 65 6.1 Materials and methods 65 6.1.1 Impact of layer width 66 6.1.2 Reduction of the area exposed to desiccation 67 6.2 Results and discussion 68 6.2.1 The influence of the width of the layer 68 6.2.1.1 Evolution of the capillary pressure 68 6.2.1.2 Waterloss and temperature 69 6.2.1.3 Plastic shrinkage 71 6.2.1.4 Plastic shrinkage cracking 72 6.2.1.5 Discussion 74 6.2.2 The impact of formwork-free production technique 75 6.2.2.1 Plastic shrinkage 75 6.2.2.2 Evaporative behaviour 77 6.2.2.3 Evolution of the capillary pressure 78 6.2.2.4 Discussion 80 6.3 Chapter summary 83 7 Deformation behaviour of the 3D-printed concrete elements due to plastic shrinkage 85 7.1 Materials and methods 85 7.2 Experimental results 86 7.2.1 Shrinkage-induced deformations 86 7.2.2 Allocation of the deformations to the reference coordinate system 88 7.2.3 Deformations dependent on the considered surface plane and position 88 7.2.3.1 Surface A 88 7.2.3.2 Surface B 90 7.2.3.3 Surface C 93 7.3 Proposed deformation model of the 3D-printed concrete elements due to PS 94 7.4 Formulation of the deformation functions 95 7.5 Verification of the proposed model 97 7.5.1 Experimentally obtained deformations 97 7.5.2 Modelled deformations 99 7.6 Discussion 99 7.6.1 Differences between cast and formwork-free produced elements 99 7.6.2 Applicability and limitations of proposed deformation models 102 7.7 Chapter summary 102 8 Evolution of capillary pressure in 3D-printed concrete elements: numerical modelling and experimental validation 105 8.1 Introduction to the modelling approach 105 8.1.1 Flow in the saturated medium 106 8.1.2 Flow in the unsaturated medium 107 8.1.3 Shrinkage 108 8.2 Boundary conditions and mesh 109 8.3 Experimental investigations 110 8.3.1 Preparation of the specimens 110 8.3.2 Experimental setup and procedure of the experiment 111 8.3.3 Determination of the input parameters for numerical simulation 112 8.4 Results and discussion 112 8.4.1 Model input parameters 112 8.4.1.1 Temperature and evaporation of the water 112 8.4.1.2 Bulk modulus 114 8.4.1.3 Water retantion curve 117 8.4.1.4 Air entry curve 117 8.4.1.5 Summary of the input parameters 118 8.4.2 Experimental results 118 8.4.2.1 Capillary pressure 118 8.4.2.2 Shrinkage test 119 8.4.3 Verification of the model output 121 8.4.3.1 Effect of the bulk modulus 121 8.4.3.2 Effect of the Poisson's ratio 122 8.4.3.3 Influence of the defined boundary conditions 122 8.4.4 The final model output result 124 8.4.4.1 Capillary pressure 124 8.4.4.2 Plastic shrinkage 125 8.5 Chapter summary 126 9 Advancement of the experimental technique for quantification of the plastic shrinkage cracking 127 9.1 Experimental program 127 9.2 Preparation of the samples and the experimental procedure 129 9.3 Results and discussion 130 9.4 Chapter summary 131 10 Mitigation of plastic shrinkage and plastic shrinkage cracking 133 10.1 Experimental program 133 10.2 Methods of investigation and materials 134 10.2.1 Passive mitigation approaches 134 10.2.1.1 Reduction of the paste content 134 10.2.1.2 Substitution of the cement content 134 10.2.1.3 Addition of the SAP 134 10.2.1.4 Addition of the SRA 138 10.2.1.5 Addition of fibres 138 10.2.2 Active mitigation approaches 138 10.2.3 Production of the specimens 138 10.2.3.1 General investigations 138 10.2.3.2 3D-printing of the demonstrator structure 140 10.3 Results and discussion 141 10.3.1 Modification of the reference composition 141 10.3.1.1 Reduction of the paste content 141 10.3.1.2 Substitution of the cement content 142 10.3.1.3 Addition of the SAP 142 10.3.1.4 Addition of the SRA 144 10.3.1.5 Addition of the fibres 144 10.3.2 Efficacy of mitigation strategies 145 10.3.2.1 Evolution of the capillary pressure 145 10.3.2.2 Plastic shrinkage 146 10.3.2.3 Cracking 148 10.3.3 Demonstrator structures 149 10.3.3.1 Evolution of the temperature and capillary pressure 149 10.3.3.2 Horizontal shrinkage 150 10.3.3.3 The effect of thermal expansion 151 10.3.3.4 Alteration of the surface qualities 152 10.3.4 Discussion 153 10.4 Chapter summary 154 11 Final conclusions and outlook 155 11.1 Summary and conclusions 155 11.2 Application of the findings 158 11.3 Future research topics 158 References 160 Appendices 170 A.1 Mixture compositions 170 A.2 Implementation of the deformation model 172 A.3 Implementation of the numerical model 173 A.4 Complementary results 175 A.4.1 Repeatability of the experimental results 175 A.4.2 Specifics of plastic shrinkage 180 A.4.3 Deformation behaviour 181 A.4.4 Numerical modelling and experimental validation 183 A.4.5 Mitigation methods 186 Curriculum vitae 190 List of publications 191 / Among various techniques for automated construction, 3D concrete printing (3DCP) counts as the most promising. 3D printing with concrete offers multiple advantages in cost savings, increased productivity and design freedom. 3DCP has rapidly transformed from a bold vision to a promising construction method in recent years. Manufacturing numerous demonstrators in several countries has proven the applicability of the new technology in various construction fields. Despite this, some issues still need to be resolved before 3DCP can be widely applied in construction practice. One among them is the early-age cracking of printed concrete elements due to shrinkage-induced deformations. Volumetric contractions related to shrinkage are at their highest before the solidification of 3D-printed layers. This type of shrinkage is attributed to the plastic shrinkage. Plastic shrinkage occurs due to the extensive water loss followed by the rise of the negative capillary pressure in the system. The negative pressure in the capillaries of concrete forces the system to contract. If the volumetric contractions are hindered by, e.g., layer bonding or rebar, and the occurred stresses are higher than the tensile strength of concrete, cracks begin to form. 3D-printed concrete elements are suspended to a much higher propensity to plastic shrinkage and related cracking than conventionally cast concrete. Cracks initiated due to plastic shrinkage can propagate through the entire cross-section of the printed wall. The damages caused by plastic shrinkage can severely affect durability, serviceability, and aesthetics and even jeopardise structural stability. Despite the importance of controlling and mitigating plastic shrinkage and later appearing shrinkage types, such as drying and autogenous shrinkage, until now, only a few studies have been dedicated to these topics. This dissertation focuses on the mechanisms of plastic shrinkage and related cracking of 3D-printed concrete elements. Since there is no standardized or commonly recognized method for quantification of the plastic shrinkage and related cracking of the printable concretes, in this study, affordable and easy-to-apply experimental setups for measuring unrestrained and restrained shrinkage-induced deformations along with relevant material properties of 3D-printed concretes were developed. The statistical analysis verifies the reliability of the experimental results obtained with developed setups. The findings of this study contribute to establishing a unified testing framework for studying the shrinkage and related cracking of 3D-printable concretes. On the basis of the developed experimental methodology, specifics of the mechanisms involved in the plastic shrinkage and related cracking of the 3D-printed elements were studied. The numerical simulation of the evolution of capillary pressure in 3D-printed elements supplemented experimental investigations. Special attention was paid to the analysis of the effect of the layer width and the influence of the surface area exposed to desiccation on the extent of the plastic shrinkage and cracking in 3D-printed concrete elements. It was found that the deformative behaviour due to shrinkage-induced stresses greatly differs from those of the cast concrete elements. The onset, directions and extent of the shrinkage-induced deformations in 3D-printed elements were thoroughly analysed, and as a result, analytical and numerical models for the prediction of shrinkage-induced deformations in the 3D-printed concrete elements were developed. Finally, various approaches for mitigating plastic shrinkage and cracking are analysed, and practical solutions for reducing the damages caused by shrinkage-induced deformations are suggested.:Abstract I Kurzfassung II Vorwort des Herausgebers IV Acknowledgement V Contents VI Notations and abbreviations XI 1 Introduction 1 1.1 Motivation 1 1.2 Relevance of the research 2 1.3 Objectives and research questions 2 1.4 Dissertation structure 3 2 Theoretical background 5 2.1 Plastic shrinkage of cementitious materials 5 2.1.1 Mechanisms of plastic shrinkage 5 2.1.2 Mechanisms of plastic shrinkage cracking 6 2.1.3 Experimental methods 7 2.1.4 Numerical methods 8 2.1.5 Mitigation techniques 9 2.1.5.1 Active mitigation approaches 9 2.1.5.2 Passive mitigation approaches 9 2.2 3D concrete printing 11 2.2.1 Flashback to history 11 2.2.2 Current state 14 2.2.3 Significance of the PS and PSC for 3D-printed concrete elements 14 2.2.3.1 Specifics of material compositions 14 2.2.3.2 Production related issues 14 2.2.3.3 Case studies 15 2.2.4 Previous studies on PS and PSC of 3D-printed concrete elements 19 2.3 Chapter summary 19 3 Materials and methods 21 3.1 Reference composition 21 3.2 Experimental methods 22 3.2.1 3D concrete printing test device 22 3.2.2 Wind tunnel and climate control chamber 23 3.2.3 Determination of the specific material properties 23 3.2.3.1 Air content and spread flow 23 3.2.3.2 Capillary pressure 24 3.2.3.3 Ultrasonic pulse velocity 24 3.2.3.4 Tempe cell and self-desiccation tests 24 3.2.3.5 Falling-head method 25 3.2.3.6 Tea bag test 25 3.2.3.7 Confined uniaxial compression test 26 3.2.3.8 Penetration test 26 3.2.3.9 Microscopy 27 3.2.4 Digital image correlation 27 3.3 Numerical method 28 3.4 Chapter summary 28 4 Quantification of plastic shrinkage and plastic shrinkage cracking of the 3D-printable concretes using 2D digital image correlation 29 4.1 Novel setups for quantification of the PS and PSC 29 4.2 Materials and methods of investigation 30 4.2.1 3D printing and preparation of samples 30 4.2.2 Evaluation of the deformations 32 4.2.3 Experimental setup and procedure 33 4.3 Experimental results 33 4.3.1 Penetration force 33 4.3.2 Free shrinkage behaviour 34 4.3.2.1 Vertical settlement 34 4.3.2.2 Horizontal shrinkage 35 4.3.3 Shrinkage behaviour in partially and fully restrained tests 35 4.3.3.1 Vertical shrinkage 35 4.3.3.2 Horizontal shrinkage 36 4.3.3.3 Shrinkage-induced cracking 38 4.3.4 Influence of the paint on the surface 41 4.4 Discussion of the test setups and measuring techniques 42 4.5 Chapter summary 43 5 Repeatability of the experimental results 45 5.1 Followed statistical approach for assessment of the repeatability 45 5.2 Experimental program 46 5.3 Preparation of the samples and the experimental procedure 46 5.4 Results and discussion 48 5.4.1 Repeatability of the experimental results in previous studies 48 5.4.2 Free shrinkage 49 5.4.2.1 Spread flow, density and air content 49 5.4.2.2 Ambient conditions 49 5.4.2.3 Water loss 50 5.4.2.4 Evolution of the capillary pressure 50 5.4.2.5 Temperature 51 5.4.2.6 Shrinkage 52 5.4.2.7 Evaluation of the repeatability 55 5.4.3 Restrained shrinkage 56 5.4.3.1 Basic fresh-state properties 56 5.4.3.2 Ambient conditions 56 5.4.3.3 Water loss 57 5.4.3.4 Evolution of the capillary pressure 58 5.4.3.5 Temperature 58 5.4.3.6 Shrinkage 59 5.4.3.7 Cracking 60 5.4.3.8 Evaluation of repeatability 62 5.5 Chapter summary 63 6 Specifics of plastic shrinkage and related cracking in 3D-printed concrete elements 65 6.1 Materials and methods 65 6.1.1 Impact of layer width 66 6.1.2 Reduction of the area exposed to desiccation 67 6.2 Results and discussion 68 6.2.1 The influence of the width of the layer 68 6.2.1.1 Evolution of the capillary pressure 68 6.2.1.2 Waterloss and temperature 69 6.2.1.3 Plastic shrinkage 71 6.2.1.4 Plastic shrinkage cracking 72 6.2.1.5 Discussion 74 6.2.2 The impact of formwork-free production technique 75 6.2.2.1 Plastic shrinkage 75 6.2.2.2 Evaporative behaviour 77 6.2.2.3 Evolution of the capillary pressure 78 6.2.2.4 Discussion 80 6.3 Chapter summary 83 7 Deformation behaviour of the 3D-printed concrete elements due to plastic shrinkage 85 7.1 Materials and methods 85 7.2 Experimental results 86 7.2.1 Shrinkage-induced deformations 86 7.2.2 Allocation of the deformations to the reference coordinate system 88 7.2.3 Deformations dependent on the considered surface plane and position 88 7.2.3.1 Surface A 88 7.2.3.2 Surface B 90 7.2.3.3 Surface C 93 7.3 Proposed deformation model of the 3D-printed concrete elements due to PS 94 7.4 Formulation of the deformation functions 95 7.5 Verification of the proposed model 97 7.5.1 Experimentally obtained deformations 97 7.5.2 Modelled deformations 99 7.6 Discussion 99 7.6.1 Differences between cast and formwork-free produced elements 99 7.6.2 Applicability and limitations of proposed deformation models 102 7.7 Chapter summary 102 8 Evolution of capillary pressure in 3D-printed concrete elements: numerical modelling and experimental validation 105 8.1 Introduction to the modelling approach 105 8.1.1 Flow in the saturated medium 106 8.1.2 Flow in the unsaturated medium 107 8.1.3 Shrinkage 108 8.2 Boundary conditions and mesh 109 8.3 Experimental investigations 110 8.3.1 Preparation of the specimens 110 8.3.2 Experimental setup and procedure of the experiment 111 8.3.3 Determination of the input parameters for numerical simulation 112 8.4 Results and discussion 112 8.4.1 Model input parameters 112 8.4.1.1 Temperature and evaporation of the water 112 8.4.1.2 Bulk modulus 114 8.4.1.3 Water retantion curve 117 8.4.1.4 Air entry curve 117 8.4.1.5 Summary of the input parameters 118 8.4.2 Experimental results 118 8.4.2.1 Capillary pressure 118 8.4.2.2 Shrinkage test 119 8.4.3 Verification of the model output 121 8.4.3.1 Effect of the bulk modulus 121 8.4.3.2 Effect of the Poisson's ratio 122 8.4.3.3 Influence of the defined boundary conditions 122 8.4.4 The final model output result 124 8.4.4.1 Capillary pressure 124 8.4.4.2 Plastic shrinkage 125 8.5 Chapter summary 126 9 Advancement of the experimental technique for quantification of the plastic shrinkage cracking 127 9.1 Experimental program 127 9.2 Preparation of the samples and the experimental procedure 129 9.3 Results and discussion 130 9.4 Chapter summary 131 10 Mitigation of plastic shrinkage and plastic shrinkage cracking 133 10.1 Experimental program 133 10.2 Methods of investigation and materials 134 10.2.1 Passive mitigation approaches 134 10.2.1.1 Reduction of the paste content 134 10.2.1.2 Substitution of the cement content 134 10.2.1.3 Addition of the SAP 134 10.2.1.4 Addition of the SRA 138 10.2.1.5 Addition of fibres 138 10.2.2 Active mitigation approaches 138 10.2.3 Production of the specimens 138 10.2.3.1 General investigations 138 10.2.3.2 3D-printing of the demonstrator structure 140 10.3 Results and discussion 141 10.3.1 Modification of the reference composition 141 10.3.1.1 Reduction of the paste content 141 10.3.1.2 Substitution of the cement content 142 10.3.1.3 Addition of the SAP 142 10.3.1.4 Addition of the SRA 144 10.3.1.5 Addition of the fibres 144 10.3.2 Efficacy of mitigation strategies 145 10.3.2.1 Evolution of the capillary pressure 145 10.3.2.2 Plastic shrinkage 146 10.3.2.3 Cracking 148 10.3.3 Demonstrator structures 149 10.3.3.1 Evolution of the temperature and capillary pressure 149 10.3.3.2 Horizontal shrinkage 150 10.3.3.3 The effect of thermal expansion 151 10.3.3.4 Alteration of the surface qualities 152 10.3.4 Discussion 153 10.4 Chapter summary 154 11 Final conclusions and outlook 155 11.1 Summary and conclusions 155 11.2 Application of the findings 158 11.3 Future research topics 158 References 160 Appendices 170 A.1 Mixture compositions 170 A.2 Implementation of the deformation model 172 A.3 Implementation of the numerical model 173 A.4 Complementary results 175 A.4.1 Repeatability of the experimental results 175 A.4.2 Specifics of plastic shrinkage 180 A.4.3 Deformation behaviour 181 A.4.4 Numerical modelling and experimental validation 183 A.4.5 Mitigation methods 186 Curriculum vitae 190 List of publications 191