Concrete and Masonry Movements

Jeffrey Brooks

• Publisher: Butterworth-Heinemann
• Year: 2014
• Print ISBN: 9780128015254
• eBook ISBN: 9780128017678
• Topic: Mathematics
• Subtopic: Applied Mathematics
• Index: Mathematics

Widely used in the construction of bridges, dams and pavements, concrete and masonry are two of the world's most utilized construction materials. However, many engineers lack a proper understanding of the methods for predicting and mitigating their movements within a structure. Concrete and Masonry Movements provides practical methods for predicting and preventing movement in concrete and masonry, saving time and money in retrofitting and repair cost. With this book in hand, engineers will discover new prediction models for masonry such as: irreversible moisture expansion of clay bricks, elasticity, creep and shrinkage. In addition, the book provides up-to-date information on the codes of practice.- Provides mathematical modelling tools for predicting movement in masonry- Up-to-date knowledge of codes of practice methods- Clearly explains the factors influencing all types of concrete and masonry movement- Fully worked out examples and set problems are included at the end of each chapter

1

Introduction

Abstract

“Concrete and masonry movements” is a compilation of existing and up-to-date knowledge of movements of two traditional construction materials based upon the author's research and teaching and over a period of 30 years. It is a reference book that brings together theory and engineering practice with worked examples and is suitable for the practising engineer, research student and undergraduate student studying civil engineering. Each type of movement of concrete and masonry is considered in separate chapters except where behaviour and features are so common that separate treatment is not warranted. The book emphasizes the property of clay brick units exhibiting irreversible moisture expansion which, under some circumstances when combined with mortar to build free-standing masonry, can manifest itself as an enlarged moisture expansion due to the occurrence of cryptoflorescence at the brick/bond interface.

Keywords

Civil Engineers; Concrete movements; Masonry movements; Postgraduate students; Research students; Undergraduate students

“Concrete and masonry movements” is a compilation of existing and up-to-date knowledge of movements of two traditional construction materials, based upon the author's research and teachingover a period of 30 years. It is a reference book that brings together theory and engineering practice with worked examples and, consequently, is suitable for the practising engineer, research student, and undergraduate student studying civil engineering.

The presentation is somewhat different because it considers deformation properties of plain concrete and plain masonry together. Structural concrete and masonry containing steel reinforcing bars or prestressing tendons are not included. Conventionally, properties of concrete and masonry have been treated as separate composite materials by their respective professional institutions in spite of having common constituents: cement, sand, and coarse aggregate (brick or block). The theme of the book is to consider each type of movement of concrete and masonry in separate chapters, but to emphasize common features, except where behaviour and features are so common that treatment in different chapters is not warranted. It is the author's belief that the mutual exchange of knowledge in this manner will lead to a greater understanding of the movement properties of both materials.

What is essentially different about the two materials is when the clay brick or block is used as the “coarse aggregate” constituent, because of its behaviour under normal ambient conditions and how it can react with mortar to influence the movement of masonry. The book emphasizes the property of clay brick units exhibiting irreversible moisture expansion, which, under some circumstances, when combined with mortar to build free-standing masonry, can manifest itself as an enlarged moisture expansion due to the occurrence of cryptoflorescence at the brick/bond interface. When occurring in a control wall, this feature appears to increase creep of masonry because of the way in which creep is defined but, in practice, the enlarged moisture expansion is suppressed in masonry provided there is sufficient dead load or external load.

Summaries of all topics discussed are now presented chapter by chapter.

After defining terms and types of movement in Chapter 2, composite models for concrete and masonry are presented for: elasticity, creep, shrinkage or moisture expansion, and thermal movement. A new composite model is developed for masonry. Composite models are useful in understanding how individual components having different properties and quantities interact when combined. The models are applied and verified in other chapters, particularly for masonry, which has the advantage that movement properties of units can be physically measured in the laboratory. With concrete, this approach is not practicable because of the much smaller size of the coarse aggregate, a feature that makes it difficult to measure representative movement characteristics.

An example of the above-mentioned problem is in Chapter 4, which deals with elasticity of concrete. Modulus of elasticity is related to strength empirically because of the difficulty in measurement of aggregate modulus in order to apply theoretical composite models. Short-term stress–strain behaviour in compression leading to different definitions of modulus of elasticity is described together with Poisson's ratio. Main influencing factors are identified and effects of chemical and mineral admixtures are discussed in detail. Relations prescribed by U.S. and European standards are given for estimating modulus of elasticity from strength in tension as well as corresponding relations in compression, but there is a large scatter mainly because of the failure to quantify the influence of aggregate precisely.

Chapter 5 deals with elasticity of masonry and, besides presenting current empirical relations between modulus of elasticity and strength, composite models are tested and developed for practical application. In the first instance, it is demonstrated that modulus of elasticity of units and mortar may be expressed as functions of their respective strengths so that the composite model for modulus of elasticity of masonry can be expressed in terms of unit and mortar strengths. However, a limitation of the theoretical approach is demonstrated in the case of units laid dry, which causes moisture transfer at the unit/mortar bond during construction. This mainly affects the elastic properties of the bed joint mortar phase. However, this effect can be quantified in terms of the water absorption of the unit, which is thus an additional factor taken into account by the composite prediction model.

The different types of deformation arising from moisture movement that occur in concrete are described in Chapter 6. These range from plastic, autogenous, carbonation, swelling, and drying shrinkage, but emphasis is given to autogenous shrinkage and drying shrinkage especially, in view of the recent developments in the use of high strength concrete made with low water/binder ratios, very fine cementitious material, and chemical admixtures. Influencing factors are identified and quantified, such as the effects of mineral admixtures: fly ash, slag, microsilica, and metakaolin, and the effects of chemical admixtures: plasticizers, superplasticizers, and shrinkage-reducing agents. Methods of determining autogenous shrinkage are described and the latest methods of prediction are presented with worked examples.

The drying shrinkage behaviour of calcium silicate and concrete masonry, and their component units and mortar joints, are the subjects of Chapter 7. After considering influencing factors, the importance of the moisture state of the units at the time of laying is emphasized because of its effect on shrinkage of the bonded unit, mortar, and masonry. A mortar shrinkage-reducing factor is quantified in terms of water absorption and strength of the unit. The geometry of the cross section of masonry, quantified in terms of the ratio of its volume to the drying, exposed surface area, is also shown to be an important factor. The main influencing factors are accommodated in the composite models, which are developed for practical use to estimate shrinkage of calcium silicate and concrete masonry. Methods prescribed by Codes of Practice are also presented and their application is demonstrated with worked examples.

Moisture movement of masonry built from most types of clay units behaves in a different manner to other types of masonry and to concrete due to the property of irreversible expansion of clay units, which begins as soon as newly-made units have cooled after leaving the kiln. The effect is partially restrained when units are bonded with mortar since the mortar joints shrink, but the net effect in masonry depends on the type of clay used to manufacture the unit and the firing temperature. In fact, masonry shrinks in the long-term when constructed from a low, expanding clay brick. In Chapter 8, a detailed review of irreversible moisture expansion of clay units is undertaken before proposing a model to estimate ultimate values from knowing the type of clay and the firing temperature. Laboratory methods of measuring irreversible moisture expansion of clay units are given. It is then demonstrated that prediction of moisture movement of clay brick masonry can be achieved successfully by composite modeling.

The phenomenon of enlarged moisture expansion of clay brickwork is the subject of Chapter 9, which occurs in special circumstances when certain types of clay unit are bonded with mortar to create conditions for the development of cryptoflorescence at the interface of the brick/mortar bond. In many instances, the clay units responsible for the phenomena are of low strength, have high suction rate, and are laid dry. The degree of enlarged expansion also depends on in-plane restraint of the masonry and, hence, can be suppressed by wetting or docking units before laying, and ensuring there is sufficient dead load acting on the masonry. Enlarged moisture expansion is of particular relevance in measuring creep of clay brickwork by using laboratory-sized specimens, and recommended test procedures are suggested. The chapter examines the nature of efflorescence, the influencing factors, and the mechanisms involved.

Chapters 10 and 11, respectively, deal with creep of concrete and standard methods of prediction of creep. Two chapters are allocated because of the number of factors influencing creep, and the numerous methods available to the designer for estimating elasticity, shrinkage, and creep of concrete, especially with the advent of high performance concrete containing mineral and chemical admixtures. Besides creep in compression, Chapter 10 highlights creep under tensile loading and creep under cyclic compression; prediction of creep under both those types of loading is included. Standard methods of estimating creep of concrete from strength, mix composition, and physical conditions are presented in Chapter 11 and their application demonstrated by worked examples. For greater accuracy, estimates by short-term testing are recommended and, finally, a case study is given to illustrate the recommended approach when new or unknown ingredients are used to make concrete.

Creep of masonry is the topic of Chapter 12. Compared with concrete, there has been only a small amount of research, and therefore there are fewer publications dealing with the subject. A brief historical review is given and a data bank of published results is complied. The chapter draws on the experience of knowledge of creep of concrete to develop a practical prediction model for masonry by quantifying creep of mortar and creep of different types of unit in terms of their respective strengths, water absorption of unit, and geometry of masonry. Current European and American Code of Practice guidelines are presented with worked examples. The association of creep with the presence of cryptoflorescence in certain types of clay brick masonry is also investigated.

Thermal movement of both concrete and masonry is considered together in Chapter 13 in terms of practical guidance prescribed in design documents and by composite modeling using thermal expansion coefficients of constituents: aggregate or unit and mortar, and their volumetric proportions. In practical situations, thermal movement and all the other various deformations of concrete or of masonry occur together and are often partially restrained in a complicated manner. The resulting effects, which may result in loss of serviceability due to cracking, are discussed in Chapter 14, together with remedies adopted in structural design to accommodate movements and to avoid cracking. Types and design of movement joints are described in detail and their application is demonstrated with worked examples.

Existing theories of creep and shrinkage of cement-based materials are based on those proposed for concrete. However, since none explain all the experimentally observed behaviour, a different theory is proposed and developed in Chapter 15, which is based on the movement of absorbed and interlayer water within and through the C-S-H pore structure. A key assumption is that the adsorbed water is load-bearing in having a structure and modulus of elasticity greater than that of “free” or normal water. If adsorbed water is removed, stress is transferred from adsorbed water in the pores to the solid gel of the cement paste, thus increasing its deformation. Drying shrinkage may be regarded as an elastic-plus-creep strain due to capillary stress generated by the removal of water. The theory is applied to several test cases of creep previously unexplained by existing theories.

The final Chapter 16 deals with the important subject of testing and measurement of elasticity, creep, and shrinkage of concrete and masonry. Measurement of the other types of movement are discussed in relevant chapters, and Chapter 16 concentrates on uniaxial-compressive and tensile-loading techniques and types of strain measurement with practical guidance for good, experimental practice in the laboratory. Prescribed American and European methods of test for determining creep of concrete are included, there being no equivalent standards for determining creep of masonry. Other prescribed, standard test methods are included in this chapter, which use length comparators for determining, independently of creep, shrinkage of concrete, mortar, and masonry units.

2

Classification of Movements

Abstract

The types of movements discussed are shrinkage and swelling (including plastic shrinkage, autogenous shrinkage, carbonation shrinkage, and drying shrinkage); thermal expansion and contraction; irreversible moisture expansion; elastic strain and creep. For normal strength concrete exposed to the environment, drying shrinkage predominates but in the case of exposed high performance (high strength and low permeability) concrete, autogenous shrinkage contribution occurs along with drying shrinkage. When shrinkage and creep occur simultaneously, the common practice is to consider the tw...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Acknowledgments
• 1. Introduction
• 2. Classification of Movements
• 3. Composite Models
• 4. Elasticity of Concrete
• 5. Elasticity of Masonry
• 6. Shrinkage of Concrete
• 7. Shrinkage of Calcium Silicate and Concrete Masonry
• 8. Moisture Movement of Clay Brick Masonry
• 9. Enlarged Moisture Expansion due to Cryptoflorescence
• 10. Creep of Concrete
• 11. Methods of Predicting Elasticity, Shrinkage, and Creep of Concrete
• 12. Creep of Masonry
• 13. Thermal Movement
• 14. Effects of Movements, Restraint and Movement Joints
• 15. Theoretical Aspects of Creep and Shrinkage of Mortar and Concrete
• 16. Testing and Measurement
• Author Index
• Subject Index