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Concrete Technology

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This book contains fundamentals of the subject concrete technology such as hydration of cement, cement types, concrete making' materials, workability, hardened properties of concrete, durability, mixed design, chemical and mineral admixtures, special concretes, high performance concrete, self-compacting concrete, non-destructive testing, and waste materials in concrete. The book is the first of its kind incorporating high performance concrete, rheology of concrete, some sophisticated and special techniques in concrete technology, use of waste materials in concrete and geopolymer concrete. It will provide up to date information on the subject and ready reference of the relevant, codal provisions. The book will serve as a textbook at undergraduate level in Civil Engineering in Indian Universities, NITs and IITs. It has been written in a simple and lucid manner, incorporating the recent developments of the subject so that the future engineers are well acquainted with the subject during their undergraduate study.

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Prelims

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Foreword

I feel privileged to write the foreword of the book, entitled Concrete Technology by Dr Aminul

Islam Laskar, Associate Professor of Civil Engineering, National Institute of Technology Silchar.

From a pile of books on the subject, what makes this book interesting is the effort taken by the author to make the subject very interesting to the first timers as well as to the professionals. The author presented the concepts in a very lucid and simple manner to be attractive for the target audience. The book covers most of the recent developments in concrete material science. Also, extensive references are made to IS codes and practical design considerations throughout the book for easy clarity and understanding. Thus the book should serve as a textbook for undergraduates and a reference compendium for practicing civil engineers. I hope the prospective readers will greatly benefit from the book.

Dr Bishwajit Bhattacharjee

Professor of Civil Engineering

Indian Institute of Technology,

New Delhi

 

Chp-1

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Chapter

1

Introduc tion

Ever since civilizations first started to build, the human race has sought a material that binds stones into solid formed mass. The Romans mixed lime (i.e., burned limestone) with volcanic ash from Mount Vesuvius that produced structures of remarkable durability. During the Middle

Ages, the art of making hydraulic cement (cement that hardens when it comes in contact with water) became lost and it was not until the year of 1824 that the hydraulic cement (now commonly known as Portland cement) reappeared when it was patented by a Leeds builder named Joseph

Aspdin. The name “Portland cement” was given originally due to the resemblance of the colour and quality of the hardened cement to Portland stone (limestone quarried in Dorset). The most widely used modern construction material is concrete that is made by mixing Portland cement with sand, crushed rock and water. Man consumes no material except water in such tremendous quantities.

Concrete is neither strong nor tough as steel, so why is it the most widely used engineering material? There are number of reasons. Firstly, Concrete possesses excellent resistance to water.

 

Chp-2

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2

Hydraulic Cements

Concrete, made from cement, aggregates, chemical admixtures, mineral admixtures and water, is any product or mass made by the use of a cementing medium. The active constituent of concrete is cement paste and the performance of concrete is largely determined by the cement paste. Admixtures in concrete confer some beneficial effects such as acceleration, retardation, air entrainment, water reduction, plasticity, etc., and they are related to the cement-admixture interaction. Mineral admixtures such as blast furnace slag, fly ash, silica fume, and others, also improve the quality of concrete.

The performance of concrete depends on the quality of the ingredients, their proportions, placement, and exposure conditions. In the production of concrete, amount and the type of cement, fine and coarse aggregate, water, temperature of mixing, admixture, and the environment to which it is exposed will determine its physical, chemical, and durability behavior. Various analytical techniques are applied to study the effect of these parameters and for quality control purposes. In this chapter, physical, chemical, and mechanical characteristics of cement paste are presented because of their relevance to the application of various properties of concrete.

 

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3

Aggregates and Water

Aggregates are those parts of concrete that constitute the bulk of the finished product. They comprise 60%–80% of the total volume of concrete and have to be so chosen that the entire mass of concrete acts as relatively solid, homogenous, dense combination, with the smaller sizes acting as an inert filler of the voids that exist between the larger particles. Aggregates are of two types:

1.

Coarse aggregate such as gravel, crushed stone or blast furnace slag;

2.

Fine aggregate such as natural or manufactured sand.

Since the aggregates constitute the major portion of the mixture, the more the aggregate in concrete, the cheaper is the concrete, provided that the mixture is of reasonable workability for the specified job for which it is used.

COARSE AGGREGATE

Coarse aggregate is classified as such if the smallest size of the particle is greater than 4.75 mm.

Properties of the coarse aggregate affect the final strength of hardened concrete and its resistance to disintegration, weathering, and other destructive effects. The mineral coarse aggregate must be clean of organic impurities and must bond well with the cement gel. The common types of coarse aggregate are:

 

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4

Workability

Concrete has become the most widely used structural material today. Concrete has two distinct stages: fresh and hardened stage. Hardened concrete should possess definite shape, good appearance, adequate strength and durability. The performance requirements of hardened concrete are more or less well defined with respect to shape, finish, strength, durability, shrinkage and creep. To achieve these objectives economically, the fresh concrete, in addition to having a suitable composition in terms of quality and quantity of cement, aggregates and admixtures, should satisfy a number of requirements from mixing stage till it is transported, placed in forms and compacted. The requirements may be summarized as follows:

• It should be able to produce a homogeneous fresh concrete from the constituent materials of the batch under the action of mixing forces. A less mixable concrete mix requires more time to produce a homogeneous and uniform mix. This property is termed as mixability.

 

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5

Concre te in Plastic and Early Stage

The contents of this chapter, cover the problems of concrete that may take place in plastic stage and early stage of concrete. The occurrences such as settlement, plastic shrinkage and thermal cracks may lead to future durability problems if these are not properly appreciated and treated.

SETTLEMENT CRACKS

Settlement cracks arise due to the differential settlement of the concrete. Settlement of concrete happens during the plastic state when the concrete had been already placed but still does not reach its time of setting. Settlement is usually accompanied by bleeding of the concrete. Settlement cracks may occur by various mechanisms as stated below.

(i)

There are obstructing subjects to the setting concrete such as reinforcing steels

(Figs. 5.1 and 5.2). In slabs or beams, cracks can happen on the top surface of the concrete in the direction along the reinforcing bars. Cracks may be happen along the main reinforcements (Fig. 5.2(a)) or along the stirrups (Fig. 5.2(b)). Voids under the reinforcing bars are frequently formed due

 

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6

Chemical Admixtures

Admixtures for concrete are defined as materials other than hydraulic cement, aggregates or water that are added immediately before or during mixing. The most important admixtures are ones added to accelerate or retard setting, to decrease the quantity of water needed to obtain a given degree of workability, or to entrain air in order to increase freeze-thaw resistance of concrete.

RETARDING ADMIXTURES

Admixtures that retard setting are of value for concreting in hot weather, oil well cementing and other purposes. Many organic materials have this property; sucrose, glucose, calcium citrate and calcium lignosufonate are examples of retarders. The retarders most widely used in practice appear to be hydroxyl-carboxylic acid and their salts. Because of low concentration, these are commonly added as solutions. They act by adsorption and hence are used in low concentration. The time of initial set increases with the content of retarder and generally decreases with temperature and cement content. The increased retardation occurs especially with cements having high C3A content because once some C3A has hydrated, it does not absorb retarder and the retarder is available for action with calcium silicates. In a typical case, addition of 0.1% sucrose by weight of cement might increase the initial setting time by 10 hours, while a 0.25% addition might delay it by 6 days. High dose of retarders can permanently “kill” setting of cement.

 

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7

Mineral Admixtures

Mineral admixtures are finely divided siliceous materials that are added to concrete in relatively large amounts. By-products from thermal power plants, ferrosilicon alloys, blast furnace slag being produced at the rate of millions of tonnes every year and these waste products pose problems of disposal, and are a source of air and land pollution. These waste products exhibit excellent pozzolanic property and therefore can be effectively utilized in concrete industry. Such pozzolans are also sometimes called artificial pozzolana. ASTM defines pozzolana as a siliceous or siliceous and aluminous material which in itself possesses little or no cementing property but will, in a finely divided form and in the presence of water react with calcium hydroxide at ordinary temperature and pressure to give calcium silicate hydrate. All pozzolanic materials contain mainly amorphous silica (reactive silica) which is responsible for pozzolanic reaction in concrete.

Mineral admixtures also called cement replacement materials or supplementary cementitious materials in Concrete Technology. As discussed in Chapter 2, calcium silicate hydrate (C-S-H) gel is responsible for imparting mechanical properties of concrete such as strength. The pozzolanic reaction is as follows:

 

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Chapter

8

Rheology of Concrete

In this chapter, a description of fluid rheology and the different measurement techniques of rheological parameters of concrete are presented. Some important definitions related to concrete rheology has also been included so that the reader may easily understand the principles of fluid rheology. Constitutive equations of fluid flow, thixotropy, dilatancy of concrete, wall effect, plug flow, particle migration, particle sedimentation and end effect have made the chapter more comprehensive to the readers. Finally, complete description of the available concrete rheometers including the conceptual design, actual design and derivation of torque-speed relationship is provided.

RHEOLOGY

Rheology is the scientific study of the deformation and flow of materials under load. A material is said to flow, if under the action of constant shear stress, its deformation increases with time.

Both viscous and plastic materials flow and, as they exhibit little or no deformational recovery on unloading, the strain energy of distortion is usually stored as potential energy.

 

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Chapter

9

Strength

The usual primary requirement of good concrete is a satisfactory compressive strength in its hardened state. The strength of a material can be defined as the ability of the material to resist the applied load without failure. Since hardened concrete already contains some micro-cracks before it is subjected to any load, failure in this case is therefore, not directly identified with the appearance of cracks. Concrete strength is taken as the maximum stress concrete sample can withstand before failure. In concrete design, strength is usually specified because it is the property which can be easily determined in the laboratory. Compressive strength is closely related to same concrete micro-structural features that govern other properties such as elastic modulus and durability. Universally, 28 day uniaxial compressive strength of concrete is taken as the index of the concrete strength.

CONCRETE IN COMPRESSION

When the failure surface of usual concrete tested in uniaxial compression is examined with naked eye, it can be seen that the rupture developed either within the matrix or along the interface mortar and coarse aggregate. The interfacial zone is called the transition zone which is considered as the third phase of concrete in addition to matrix phase and aggregate phase. However, if concrete contains weak aggregates, some of the failure planes propagate through the aggregate particles.

 

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10

Durability

The principle for good design practice is to design a structure that is able to sustain design load up to its required service life without excessive cost of repair and maintenance. Concrete can deteriorate when subjected to severe environment. Various physical, mechanical and chemical processes, poor detailing of reinforcement, poor workmanship can cause deterioration of concrete which may cause reduction in the mechanical properties and performance of structures. Deterioration can occur in various forms, such as alkali-aggregate expansion reaction, freeze-thaw expansion, salt scaling by deicing salts, shrinkage and enhanced attack on the reinforcement of steel due to carbonation, sulfate attack on exposure to ground waters containing sulfate ions, sea water attack, and corrosion caused by salts.

SULFATE ATTACK

The process of sulfate attack is one of the chemical processes that can lead to sever damages in concrete structures. Unlike carbonation and chloride attack, sulfate damages the concrete directly but does nothing to the steel reinforcement. The deterioration by sulfate can be in the form of expansion leading to crack, strength loss and mass loss. Sulfate that attack cement paste in concrete are in the form of solution. Examples of sulfate that are often found in the environment are sodium sulfate (Na2SO4), magnesium sulfate (MgSO4) and calcium sulfate (CaSO4). Since the solubility of calcium sulfate is negligible in normal ambient condition, it does not attack the concrete directly. Sulfates can be found mostly in sea water, brackish water or soils, waste water.

 

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Chapter

11

Concrete Mix Design

Proper design of concrete mixtures is intended to obtain such proportioning of ingredients that will produce concrete with high durability performance during the desired designed life of a structure. It is always difficult to develop a theoretical mix design method that can be used universally with any combination of cement, any aggregates because the criteria of all the components are too broad. Moreover, the same properties of fresh and hardened properties of concrete can be achieved in different ways from the same materials. A mix design method only provides a starting mix design that will have to be modified to meet the desired concrete characteristics. In spite of the fact that mix proportioning is an art, it is unquestionable that some scientific principles can be used as a basis for mix calculation.

STRENGTH REQUIREMENTS

Most proportioning methods are based on concrete compressive strength. Therefore, it is important to define the exact value of concrete compressive strength that has to be achieved before using any mix design method. Many other engineering properties of concrete appear to be generally related to its compressive strength. Indian Standards such as IS 456: 2000 stipulates concrete compressive strength in terms of grades of concrete. For example, M 20 refers to a concrete mix having characteristics strength 20 MPa. Here M stands for mix and 20 refer to the characteristic strength. Characteristics strength (fck) of concrete is defined as the value of the strength of concrete below which not more than 5% of the test results are expected to fall. The following basic assumptions are made for the design of conventional concrete:

 

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Chapter

12

High-Performance Concrete

High strength concrete seems to have become the key word in today’s concrete technology. In the early 1940s, 30 N/mm2 (at 28 day) was considered to be the representative of high strength concrete. This level jumped to 50 N/mm2 in the late 1950s and early 1960s. Concrete strengths of 100–130 N/mm2 is now being viewed as the criteria for high strength. Just how far we can go to reach an ultimate in strength in the future is nobody’s guess. High strength concrete is among the most significant ideal materials available in the market to rehabilitate and enhance the performance of the nation’s crumbling infrastructure such as assisting the widespread problems of deteriorated bridge structures and tall buildings. In the precast and prestressed concrete industries, the use of high strength concrete has resulted in a rapid turnover of moulds, higher productivity and less loss of products during handling and transportation. The well-known ‘Laws’ and ‘rules of thumb’ that apply to normal strength concrete may well not apply to high strength concrete. ACI 363R and other ACI guidelines address some recommendations on placing, compacting and curing of high strength concrete. However design of high strength concrete entails detailed knowledge of properties of local materials, i.e., aggregates, cement and pozzolanic admixtures.

 

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Chapter

13

Self-compacting Concrete

There remains always a doubt regarding the full compaction of concrete during its placing to achieve the required strength and durability. Due to the lack of skilled manpower and other social constraints, it is very difficult to compact concrete with vibrators at construction sites. SelfCompacting Concrete (SCC) is that type of concrete which requires no inner or outer vibration for the compaction. SCC compacts itself alone due to its self-weight and de-aerated almost completely while flowing in the formwork. SCC flows like ‘honey’ and has nearly a horizontal concrete level after placing. Originally developed in Japan by Professor Hajime Okamura in the late 1980’s, the SCC has now been taken up with enthusiasm throughout the world for both site and precast work. SCC is a type of high-performance concrete where a compressive strength of

M150 has been achieved.

DEVELOPMENT OF SCC

The concept of SCC was first proposed by Professor Hajime Okamura of Kochi University of

 

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14

Fiber Reinforced Concrete

Fiber reinforced concrete (FRC) is defined as concrete made with hydraulic cement, fine or fine and coarse aggregate and discontinuous discrete fibers. For structural applications, steel fibers are used as complementary reinforcement to increase the cracking resistance, flexural and shear strength, impact resistance and ductility of RCC elements. Fibers are used in cementitious materials in order to improve the characteristics in the hardening or the hardened state.

Historically, fibers have been used to reinforce brittle materials since ancient times; straws were used to reinforce sun baked bricks, horse hair was to reinforce plaster and more recently, asbestos fibers are being used to reinforce Portland cement. Patents have been granted since the turn of the century for various methods of incorporating wire segments or metal chips into concrete. The low tensile strength and brittle charter of concrete have been bypassed by the use of reinforcing rods in the tensile zone of the concrete since the middle of the nineteenth century.

 

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Chapter

15

Non-destruc tive

Testing of Concrete

The need for non-destructive testing (NDT) of concrete may arise due to variety of reasons.

These are: assessment of structural integrity or safety following material deterioration, structural damage caused by fire, fatigue or overload, adequacy of members suspected to contain unspecified material, fault in design, and monitoring long term changes in material properties and structural performance. NDT is generally defined as not impairing the intended performance of element or member under test, and when applied to concrete is taken to include methods which cause localized surface zone damage. All NDT methods can be performed directly on the in-situ concrete without removal of a sample.

Broadly speaking, there are two classes of NDT methods. The first class consists of those methods that are used to estimate strength. The surface hardness, penetration resistance, pullout, pull-off, break-off, and maturity techniques belong to this category. Some of these are not truly

 

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Chapter

16

Additional Topics in

Concrete Technology

Several physical, chemical and mechanical techniques are being applied in research of concrete over recent years. They provide important information including characterization of raw materials, cured concrete, quantitative estimation of products of hydration. Information on the specialized techniques are scattered in literature and hence latest knowledge on the various methods are compiled and presented in this chapter.

MERCURY INTRUSION POROSIMETRY

Mercury intrusion porosimetry (MIP) is used for determining pore size distribution of cement paste, cement mortar and concrete. The method is based on the relationship between pressure and corresponding volume of pores filled with mercury. MIP is relatively straight forward and generally yields reproducible pore size distribution. Parameters such as total porosity, threshold diameter, mean pore diameter, permeability, diffusion coefficient and retention factor can be deduced from the distribution.

 

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Chapter

17

Waste Materials in Concrete

WASTE MATERIALS

Waste is defined as any material which is by-product of human and industrial activity and has no residual value. Many of the non-decaying waste materials will remain in the environment for hundreds, perhaps thousands of years. The non-decaying waste materials cause a waste disposal crisis, thereby contributing to the environmental problems. The problem of waste accumulation exists worldwide, specifically in the densely populated areas. Most of these materials are left as stockpiles, landfill material or illegally dumped in selected areas. Table 17.1 shows the solid waste disposed at landfill in some countries. Large quantities of this waste cannot be eliminated.

However, the environmental impact can be reduced by making more sustainable use of this waste. This is known as the “Waste Hierarchy’’. Its aim is to reduce, reuse, or recycle waste, the latter being the preferred option of waste disposal. Figure 17.1 shows a sketch of the waste hierarchy.

 

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