19 Chapters
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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.

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Chp-15

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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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Chp-3

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Chapter

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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Chp-11

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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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Chp-5

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Chapter

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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