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

The large amount of used polypropylene baler twine generated from the agricultural community may provide a low-cost, environmentally friendly source of fibre reinforcement that can be used to improve the properties of concrete. However, the performance of such fibres for the application has not yet been explored. The effectiveness of using small amounts of chopped baler twine to control the restrained plastic shrinkage cracking of portland cement mortar was investigated in this study. To determine the influence of baler twine fibre type, length and volume fraction on their performance, two types of baler twine ( one composed of strands with circular cross section, the other composed of flat band shape strands) in two lengths (19 mm and 38 mm) and three volume fractions (0.05%, 0.1%, and 0.3%) were evaluated. To compare the performance of baler twine fibre with that of other commercially available synthetic fibres, fibrillated polypropylene fibres at equal lengths and volume fractions was investigated.<p>The restrained plastic shrinkage tests were carried out by subjecting the fibre-reinforced mortar specimens, cast on rough substrate bases, to a wind speed of 2.6 m/s, and relative humidity less than 3% at 35 °C for 22 hours. To evaluate the effectiveness of the fibres, the crack numbers were recorded, and the maximum crack width and total crack area on the surface of each specimen were measured using an image analysis technique. Unrestrained plastic shrinkage tests were also conducted in which fibre-reinforced mortar specimens without the substrate bases were tested under the same environmental conditions.<p>Test results indicate that both types of baler twine are capable of controlling restrained plastic shrinkage cracking to some extent, but are not as effective as fibrillated polypropylene. The baler twine composed of band shape strands performed better than the one composed of strands with circular cross section. Compared with plain specimens, the total crack area was reduced by 95.3, 77.5 and 38.7% when 0.3% volume fraction of 38 mm fibrillated polypropylene, band shape baler twine and circular baler twine fibres, respectively, were added. Similar reductions in maximum crack width were observed. Fibre length did not significantly influence cracking behaviour. Free plastic shrinkage was significantly reduced only when long fibre lengths (38 mm) and high volume fractions (0.3%) were used.

The large amount of used polypropylene baler twine generated from the agricultural community may provide a low-cost, environmentally friendly source of fibre reinforcement that can be used to improve the properties of concrete. However, the performance of such fibres for the application has not yet been explored. The effectiveness of using small amounts of chopped baler twine to control the restrained plastic shrinkage cracking of portland cement mortar was investigated in this study. To determine the influence of baler twine fibre type, length and volume fraction on their performance, two types of baler twine ( one composed of strands with circular cross section, the other composed of flat band shape strands) in two lengths (19 mm and 38 mm) and three volume fractions (0.05%, 0.1%, and 0.3%) were evaluated. To compare the performance of baler twine fibre with that of other commercially available synthetic fibres, fibrillated polypropylene fibres at equal lengths and volume fractions was investigated.<p>The restrained plastic shrinkage tests were carried out by subjecting the fibre-reinforced mortar specimens, cast on rough substrate bases, to a wind speed of 2.6 m/s, and relative humidity less than 3% at 35 °C for 22 hours. To evaluate the effectiveness of the fibres, the crack numbers were recorded, and the maximum crack width and total crack area on the surface of each specimen were measured using an image analysis technique. Unrestrained plastic shrinkage tests were also conducted in which fibre-reinforced mortar specimens without the substrate bases were tested under the same environmental conditions.<p>Test results indicate that both types of baler twine are capable of controlling restrained plastic shrinkage cracking to some extent, but are not as effective as fibrillated polypropylene. The baler twine composed of band shape strands performed better than the one composed of strands with circular cross section. Compared with plain specimens, the total crack area was reduced by 95.3, 77.5 and 38.7% when 0.3% volume fraction of 38 mm fibrillated polypropylene, band shape baler twine and circular baler twine fibres, respectively, were added. Similar reductions in maximum crack width were observed. Fibre length did not significantly influence cracking behaviour. Free plastic shrinkage was significantly reduced only when long fibre lengths (38 mm) and high volume fractions (0.3%) were used.

La presente investigación buscara la dosificación optima de microfibra para controlar la retracción plástica comparando microfibras sintéticas de polipropileno FIBRA ULTRAFINA de la marca CHEMA y microfibra sintética acrílica DRYMIX RC4020 de la marca SUDAMERICANA DE FIBRAS, considerando las dosificaciones que recomiendan los proveedores por cada metro cubico de concreto. Se realizarán ensayos de laboratorio en 17 mezclas para luego medir la retracción plástica en cada una de ellas utilizando los paneles normados por el ASTM C1579-13, midiendo también las demás propiedades que serán comparadas con el desempeño de un concreto convencional. El resultado de esta investigación será el valor de dosificación óptima con la que se logre disminuir la retracción plástica sin afectar otras propiedades del concreto, tales como resistencia a la compresión, tracción, flexión y trabajabilidad. De igual manera se realizará un análisis económico de acuerdo con las dosificaciones realizadas de las fibras anteriormente mencionadas. / The present investigation will search the optimal dosage from microfiber to control the plastic shrinkage comparing the polypropylene synthetic microfiber Fibra Ultrafina by the brand CHEMA and the acrylic synthetic microfiber Drymix RC4020 by the brand SUDAMERICANA DE FIBRAS, considering the dosages per cubic meter that the providers recommend. It will perform laboratory tests in 17 mixes by then measure the plastic shrinkage in each of them using the standardized panels by the ASTM C1579-13, also will measure the other properties that will be compared with the performance from a conventional concrete. The investigation result will be the optimal dosage valor that can reduce the plastic shrinkage without affecting the other concrete properties like the compressive strength, tensile strength, flexural strength and slump. Likewise, it will perform an economic analysis according the fiber dosages aforementioned. / Tesis

El concreto es uno de los materiales más utilizados en el mundo de la construcción, de las cuales cada material en la mezcla depende de la resistencia que se requiera de acuerdo al análisis estructural. Dentro del proceso de producción de concreto debemos garantizar que el cemento reaccione químicamente y desarrolle la resistencia para la cual fue diseñada, para esto es importante mantenerlo hidratado en ese tiempo mediante el proceso de curado. Una técnica aún no tan conocida es el auto-curado del concreto, por lo cual es una necesidad saber cuál es su influencia en el desarrollo de la resistencia y en la disminución del porcentaje de agrietamiento del concreto en estado plástico. El objetivo de esta tesis es determinar la influencia que tiene el reemplazar un cierto porcentaje de ladrillo triturado como reemplazo del agregado grueso; evaluando la resistencia a la compresión, resistencia a la flexión, y el agrietamiento por contracción plástica del concreto. Se desarrolló con 3 diferentes porcentajes de reemplazo de ladrillo triturado que son: 15%, 21%, 27% del peso del agregado grueso para la resistencia a la compresión (f’c) de 280 kg/cm2. Se concluyó que reemplazo del agregado grueso por ladrillo triturado es efectivo cuando es usado hasta un máximo de 21%. Los resultados obtenidos son óptimos y viables en el tiempo, mostrándonos un aumento en la resistencia a la compresión, resistencia a la flexión y la disminución del porcentaje de fisuras en estado plástico. / Concrete is one of the most used materials in the world of construction, of which each material in the mixture depends on the strength required according to the structural analysis. Within the concrete production process, we must ensure that the cement reacts chemically and develops the resistance for which it was designed, for this it is important to keep it hydrated at that time through the curing process. A technique not yet so well known is the self-curing of concrete, so it is a necessity to know what its influence is in the development of resistance and in the reduction of the percentage of cracking of concrete in the plastic state. The objective of this thesis is to determine the influence of replacing a certain percentage of crushed brick as a replacement for coarse aggregate; evaluating the compressive strength, flexural strength, and cracking by plastic shrinkage of concrete. It was developed with 3 different percentages of crushed brick replacement that are: 15%, 21%, 27% of the weight of the coarse aggregate for the compressive strength (f’c) of 280 kg / cm2. It was concluded that replacement of coarse aggregate with crushed brick is effective when used up to a maximum of 21%. The results obtained are optimal and viable over time, showing an increase in compressive strength, flexural strength and a decrease in the percentage of cracks in the plastic state. / Tesis

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

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concrete within the first few hours after the concrete has been cast. It results in unsightly surface cracks that serve as pathways whereby corroding agents can penetrate the concrete which shortens the expected service life of a structure. PSC is primarily a problem at large exposed concrete surfaces for example bridge decks and slabs placed in environmental conditions with high evaporation rates. Most precautionary measures for PSC are externally applied and aimed to reduce the water loss through evaporation. The addition of a low dosage of polymeric fibres to conventional concrete is an internal preventative measure which has been shown to reduce PSC. The mechanisms involved with PSC in conventional and low volume fibre reinforced concrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidance leads to neglect and ineffective use of preventative measures. The objective of this study is to provide the fundamental understanding of the phenomena of PSC. To achieve the objective, an in depth background study and experiments were conducted on fresh conventional concrete and LV-FRC. The three essential mechanisms required for PSC are: 1→ Capillary pressure build-up between the particles of the concrete is the source of shrinkage. 2→ Air entry into a concrete initiates cracking. 3→ Restraint of the concrete is required for crack forming. The experiments showed the following significant findings for conventional and LV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated and once air entry has occurred. The critical period where the majority of the PSC occurs is between the initial and final set of concrete. Any preventative measure for PSC is most effective during this period. The bleeding characteristics of a mix have a significant influence on PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the added resistance that fibres give to crack widening, which increases significantly from the start of the critical period. The fundamental knowledge gained from this study can be utilized to develop a practical model for the design and prevention of PSC in conventional concrete and LV-FRC. / AFRIKAANSE OPSOMMING: Plastiese krimp krake (PSK) is die krake wat gevorm word a.g.v. die vroeë krimping van beton binne die eerste paar ure nadat die beton gegiet is. Dit veroorsaak onooglike oppervlak krake wat dien as kanale waardeur korrosie agente die beton kan binnedring om so die dienstydperk van die struktuur te verkort. Dit is hoofsaaklik ʼn probleem by groot blootgestelde beton oppervlaktes soos brug dekke en blaaie wat gegiet is in klimaat kondisies met hoë verdamping tempo’s. Meeste voorsorgmaatreëls vir PSK word ekstern aangewend en beperk die water verlies as gevolg van verdamping. Die byvoeging van ʼn lae volume polimeriese vesels is ʼn interne voorsorgmaatreël wat bekend is om PSK te verminder. Die meganismes betrokke ten opsigte van PSK in gewone beton en lae volume vesel versterkte beton (LV-VVB) is vaag. Die vaagheid en tekort aan riglyne lei tot nalatigheid en oneffektiewe aanwending van voorsorgmaatreëls. Die doel van die studie is om die fundamentele kennis oor die fenomeen van PSK te gee. Om die doel te bereik is ʼn indiepte agtergrond studie en eksperimente uitgevoer op gewone beton en LV-VVB. Die drie meganismes benodig vir PSK is: 1→ Kapillêre druk tussen die deeltjies van die beton is die hoof bron van krimping. 2→ Lugindringing in die beton wat krake inisieer. 3→ Inklemming van die beton is noodsaaklik vir kraakvorming. Die eksperimente het die volgende noemenswaardige bevindinge opgelewer: PSK is slegs moontlik indien al die bloeiwater van die beton oppervlakte verdamp het en indien lug die beton ingedring het. Die kritiese periode waar die meerderheid van die PSK plaasvind is tussen die aanvanklike en finale set van die beton. Enige voorsorgmaatreël vir PSK is mees effektief gedurende die periode. Die bloei eienskappe van ʼn meng het ʼn noemenswaardige effek op die PSK. Die byvoeging van ʼn lae volume polimeriese vesels tot beton verminder die PSK deur die addisionele weerstand wat die vesels bied teen die toename in kraakwydte. Die weerstand vergroot noemenswaardig vanaf die begin van die kritiese periode. Die fundamentele kennis wat in die studie opgedoen is, kan gebruik word vir die ontwikkeling van ʼn praktiese model vir die ontwerp en verhoed van PSK in gewone beton en LV-VVB.

[pt] A aplicação de compósitos cimentícios usando fibras vegetais, em substituição a fibras de asbestos, é uma realidade em indústrias de fibrocimento em vários países do mundo, pois, apesar das boas propriedades mecânicas e durabilidade, a utilização de asbestos acarreta problemas de insalubridade. Fibras vegetais, pela disponibilidade e adequação à preservação ambiental, apresentam vantagens sobre fibras sintéticas. O bambu é excelente fornecedor de fibras, pelo rápido crescimento, baixo custo e qualidade das fibras. Usando-se a polpa do vegetal, pode-se inserir maiores teores de fibras que, distribuídas aleatoriamente, conferem características isotrópicas ao compósito. Estudos são realizados, visando melhorar o desempenho dos compósitos com fibras vegetais. Retração e fluência se constituem em formas de deformação ao longo do tempo que podem comprometer o desempenho e reduzir a durabilidade do material. Tratando-se de materiais heterogêneos e sujeitos à presença de falhas, em diversos níveis, a aplicação da mecânica da fratura pode tornar-se valiosa ferramenta para projeto e controle da integridade desses compósitos, sendo a inibição da iniciação e propagação de trincas uma das principais funções do reforço de fibras curtas. Esse trabalho buscou analisar o comportamento de compósitos cimentícios reforçados com polpa de bambu, quanto à retração e à fluência, e obter parâmetros que descrevessem seu modo de fratura. Enquanto a capacidade de sofrer retração plástica foi reduzida, a retração livre na secagem cresceu com o aumento do teor de polpa de bambu no compósito, chegando a 40% de incremento para 14% de polpa, após um ano. Sob retração restringida, resultados mostraram melhor desempenho dos compósitos com fibras, pela ausência de fissuras detectáveis por fissurômetro, em relação à matriz sem reforço, que apresentou fissura em torno de 4 horas de exposição à secagem. Estudo da reversibilidade da retração mostrou que para os compósitos predominam as deformações de contração. Houve aumento da fluência sob compressão simples, com a inserção do reforço fibroso na mistura. Na fluência sob flexão, houve aumento da fluência específica na face comprimida com o aumento do teor de polpa na mistura. A fluência específica sob tração na flexão resultou maior para a matriz sem reforço do que para os compósitos com polpa de bambu. No estudo sobre mecânica da fratura, os corposde- prova entalhados de compósito com polpa apresentaram melhoria considerável no comportamento à flexão em relação à matriz sem reforço. Os compósitos com polpa mostraram-se menos sensíveis ao entalhe, com o incremento do teor de reforço fibroso. Observou-se considerável amolecimento (softening) precedendo a ruptura devido à propagação da trinca, nos compósitos. As curvas de resistência (curvas-R) permitiram identificar os valores de KIR que, nos compósitos, mostrou manter certa constância, com o aumento do comprimento da trinca. Nesse platô da curva, os valores médios para KIR foram de 1,88 MPa.m1/2 e 1,84 MPa.m1/2, respectivamente, para compósitos com 8% e 14% de polpa de bambu. Nos compósitos, os perfis dos caminhos trilhados pelas trincas no crescimento foram tortuosos, sendo o mecanismo de fratura mais intensamente dominado pela presença do entalhe inicial na matriz sem reforço que nos compósitos. / [en] The application of cimentitious composites using vegetal fibers in substitution of asbestos is a worldwide fact in the fiber cement industry. Despite their good mechanical properties and durability, the use of asbestos fibers causes well-known health hazards. Although vegetal fibers have relatively poor mechanical properties compared with synthetic fibers, they have other advantages such as low cost and low energy demand during manufacture. Bamboo is an excellent fiber supplier, due to its fast growth and the quality of its fibers. Using vegetal pulp it is possible to insert considerable amounts of fiber in a cement matrix, which randomly distributed confer isotropic characteristics to the composite. Studies are carried out aiming to improve the performance of composites with vegetal fibers. Shrinkage and creep are sorts of time depending deformation that may significantly reduce the durability and performance of the cement based composite. Cementitious composites are essentially heterogeneous materials subject to the presence of flaws at different levels due to the presence of many internal microcraks in the material prior to loading. Therefore, the application of fracture mechanics could become a suitable tool for the design and control of the integrity of these composites, since the inhibition of crack initiation and propagation is one of the main functions of the short fiber reinforcement. This work sought to analyze the behavior of cimentitious composites reinforced with bamboo pulp under shrinkage and creep and to provide sufficient fracture parameters to describe the failure mode of the material. The results show that, whereas the plastic shrinkage reduces, the free drying shrinkage increases proportionally to bamboo pulp content in the composite, reaching a 40% increment for a 14% pulp content, after one year. Under restrained shrinkage, the composite with bamboo pulp presents better performance than unreinforced matrix. Namely, under same boundary conditions, while the unreinforced matrix presents cracks after about four hours, the composites present no cracks visible through a 10x magnifying glass, even after forty five days of drying. Study of the shrinkage reversibility of the composite showed that there is contraction deformation prevalence. Under simple compression, the creep capacity of the bamboo pulp composites increases proportionally with the fiber content. Under bending stress, there was an increase of the specific creep in the compressed face of the specimen, as the pulp content of the mixture increases. The specific creep under bending tension for the tensile face was greater for the unreinforced matrix than in the bamboo pulp composites. As revealed through the assessment of fracture behavior of composites with bamboo pulp, notched specimens presented a considerable improvement in bending behavior when compared to the unreinforced matrix. The composites with pulp became less sensible to the notch with the increment of pulp content. In the bamboo pulp composites, considerable softening was observed in the load-displacement curve, as load gradually decreases after the peak load and before the rupture due to crack propagation. Using resistance curves (R-curves) it was possible to identify the KIR values that, for the composites, kept certain constancy as the crack length increased. At this plateau of the curve, the average values for KIR reached 1,88 MPa.m1/2 and 1,84 MPa.m1/2 for composites with bamboo pulp content of 8% and 14% respectively. In the composites, crack profiles and crack surfaces were tortuous, while in the unreinforced matrix the fracture mechanisms were more intensely dominated by the presence of the initial notch.

Dans une démarche de valorisation des déchets, les granulats recyclés sont introduits dans la formulation des bétons pour donner naissance à de nouveaux bétons recyclés qui représentent l’objet du projet national RECYBETON et du projet ANR ECOREB. Cette thèse se concentre sur l’étude du béton frais et en particulier la maîtrise du retrait plastique et l’effet du degré de saturation initial des gravillons recyclés sur le comportement des bétons recyclés. Deux familles de bétons avec des rapports eau/ciment respectifs de 0,6 et 0,45 ont été testés en conditions endogènes ainsi qu’en dessiccation (Vvent = 8 m/s). Chaque famille de bétons est constituée d’un béton naturel de référence et de deux bétons recyclés différenciés pas le degré initial de saturation des gravillons recyclés (50 et 120 % de l’absorption nominale). Les résultats expérimentaux soulignent la capacité des gravillons recyclés initialement partiellement saturés à capter rapidement l’eau contenue dans la pâte de ciment, modifiant ainsi le rapport E/C, les propriétés rhéologiques du béton frais et les résistances mécaniques du béton durci. Après saturation en eau, si les conditions de séchage conduisent à un manque d’eau dans le béton, les gravillons recyclés peuvent fournir de l’eau à la pâte. Ils constituent donc un potentiel de cure interne. Le retrait plastique sous vent est explicitement lié au ressuage, au développement de la pression capillaire et à la fissuration. Le temps d’initiation de la fissuration dépend de la quantité d’eau totale dans le béton et de sa capacité à ressuer tandis que l’ouverture de fissure varie avec la valeur de retrait plastique mesurée. Dans cette étude, le développement de la pression capillaire est la cause de la fissuration qui apparaît dès l’entrée d’air dans le matériau poreux. Les différences de comportements les plus importantes sont observées entre bétons ayant une quantité d’eau totale différente plutôt qu’entre bétons naturel et recyclé. La compilation des résultats expérimentaux a permis de mettre sur pied des modélisations qui illustrent les comportements observés. Les pores concernés par l’entrée d’air dans les bétons recyclés et naturels au moment de la fissuration sont les plus gros pores de la pâte. Enfin, un couplage hygrothermique séchage-température du béton peut influer sur le démarrage de l’hydratation. / In the context of sustainable development, the reuse of construction and demolition waste is necessary to conserve nonrenewable natural aggregate resources, so recycled aggregates are introduced in concrete mix design. This is the aim of the national projet RECYBETON and the research project ECOREB. This study deals with the fresh concrete and more specifically with shrinkage control and the effects of the initial saturation degrees of recycled coarse aggregates on concrete behavior.Two concrete families, with two different water/cement ratios 0,60 and 0,45, are tested under endogenous and drying (wind speed equal to 8 m/s) conditions. Each concrete family contains a reference natural concrete and two recycled concretes. The initial saturation degree is the difference between them (recycled coarse aggregates saturated or semi saturated).Experimental results underline the capacity of non-saturated aggregates to quickly absorb water from cement paste, modifying the W/C ratio, rheological properties of the fresh concrete and the mechanical strength (at 28 days) of recycled concretes. After saturation in water, recycled aggregates can release water into the cement paste if the undergone drying conditions lead to a lack of water in the cement matrix. The recycled coarse aggregates can be seen as an internal curing potential.Experimental plastic shrinkage studies carried out under drying conditions highlight a link between bleeding, capillary pressure, plastic shrinkage and cracking. It should be pointed out that the initial cracking is dependent on the total quantity of water in the concrete and on its bleeding capacity. The opening cracks vary with the plastic shrinkage values measured during the test. The analysis of the results emphasize that the capillary pressure is the determining parameter and that the air entry value matches the cracks. The major behavior differences are found between concretes with different volumes of water rather than between natural and recycled concretes.Finally, the analysis of all the experimental results have allowed concrete modelling and understanding why concretes do not behave in the same way. When it cracks, the air come in the biggest pores of the concrete paste. Moreover, a hygrothermal coupling exists between the drying and the temperature in concrete. It can affect hydration start up.

La presente tesis contempla el diseño de una gama de concretos de baja contracción, empleando cementos con adición de puzolanas, fibras de polipropileno y fibras metálicas para mitigar la fisuración, mejorando la durabilidad de las edificaciones. Para proponer los diseños se investigó cuáles de las contracciones son la que tienen mayor incidencia en la fisuración del concreto, siendo la contracción plástica y la contracción por secado las más importantes. Asimismo, se estudiaron qué variables son las que provocan la contracción y posterior fisuramiento, afirmando que son producidos por factores ambientales, los componentes del concreto y malas prácticas constructivas. En la primera etapa, se realizó la caracterización de los agregados (fino y grueso), realizándose ensayos como granulometría, absorción, peso específico, contenido de humedad y %pasante de la malla #200. En la segunda etapa se realizaron los ensayos en concreto fresco, siendo el de mayor importancia el ensayo de simulación de contracción plástica, para el cual empleamos la ASTM C1579. Para realizar este ensayo se fabricaron los paneles que simulan restricciones y se construyó una cámara en la que se controla la velocidad del aire, temperatura y humedad relativa. En la tercera etapa se realizaron los ensayos en concreto endurecido, siendo el más importante el ensayo de contracción por secado, para lo cual empleamos la ASTM C490. Para ello, se realizaron probetas rectangulares para la medición de la variación del cambio de longitud durante 31 días. Finalmente, se realizará el análisis costo – beneficio para demostrar la viabilidad de la propuesta. / This thesis contemplates the design of a range of low-shrinkage concretes, using cements with the addition of pozzolans, polypropylene fibers and metallic fibers to mitigate cracking, improving the durability of buildings. In order to propose the designs, it was investigated which of the contractions have the greatest incidence in the cracking of the concrete, being the plastic contraction and the drying contraction the most important. Likewise, the variables that cause contraction and subsequent cracking were studied, stating that they are produced by environmental factors, concrete components and poor construction practices. In the first stage, the characterization of the aggregates (fine and coarse) was carried out, performing tests such as granulometry, absorption, specific weight, moisture content and% passing through of the # 200 mesh. In the second stage, tests were carried out on fresh concrete, the most important being the plastic shrinkage simulation test, for which we used ASTM C1579. To carry out this test, the panels that simulate restrictions were manufactured and a chamber was built in which the air speed, temperature and relative humidity were controlled. In the third stage, tests were carried out on hardened concrete, the most important being the drying shrinkage test, for which we used ASTM C490. For this, rectangular test tubes were made to measure the variation of the change in length during 31 days. Finally, a cost-benefit analysis will be carried out to demonstrate the viability of the proposal. / Tesis