Properties of Concrete

8 Key excerpts on "Properties of Concrete"

• 

  eBook - PDF

  Concrete

  Properties and Manufacture

  • T. N. W. Akroyd(Author)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  The workability will have to be increased, the mix made richer in cement, and the total water content increased, all of which will tend to increase the shrinkage. The main properties o f hardened concrete are strength, permeability, shrinkage, elasticity and creep. T h e y all change with time and depend upon, or are affected by, the moisture content o f the concrete. In building construction, strength, elasticity and creep are important; in water-retaining structures, reduced shrinkage and high impermeability are as important as strength; in a road slab, strength and resistance to deterioration are equally important. Thus it is impossible to say that one property is more important than another. However, as the strength o f concrete increases, the other properties o f concrete improve, so strength is often considered as the most important property o f concrete, and to some extent there is justification for this view. Strength of concrete The strength o f concrete is its resistance to rupture, and may be measured in a number o f ways. Thus we have the strength in compression, in tension, in shear and in flexure. All these define strength by reference to a method o f testing; some methods determine basic properties o f the material whilst others do not. Concrete is a brittle material with a compressive strength about ten times its tensile strength. When it fails under a compressive load the failure is essentially a mixture o f crushing and shear failure. T h e mechanics o f 8 CONCRETE PROPERTIES AND M A N U F A C T U R E failure are not yet fully understood, but an approximation to the failure load can be made by assuming that the concrete, in resisting failure, generates both cohesion and internal friction.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Concrete

  Properties and Manufacture

  • T. N. W. Akroyd(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  Concrete is used for a wide variety of purposes — as a foundation and structural material, as a waUing material, a n d as a Hghtweight material for insulation. It is used for the construction of roads, airfields, buildings, water-retaining ι C O N C R E T E P R O P E R T I E S A N D M A N U F A C T U R E Structures, docks, harbours and sea defences. It follows, there-fore, that all the various Properties of Concrete are of interest to the engineer, their relative importance depending upon the use to which it is put. As a structural material, strength is important; in construc-tion, however, simplicity and control of manufacture are necessary. For docks, harbours and sea defences, resistance to seawater is required as well as strength. In road construction, freedom from cracking and resistance to abrasion and frost are important. T h e various Properties of Concrete can be altered by varying the proportions of the cement, water and aggregate, and by choosing an appropriate aggregate. This proportioning is known as mix design, and is described in Chapter 3. Before the proportions of the various constituents can be decided, it is essential to know something of the general proper-ties of concrete, and it is the purpose of the remainder of this chapter to describe these. T h e first section describes the properties of the plastic concrete after mixing, and the second section deals with the properties of the set concrete. Various other properties are discussed later in the book, e.g. resistance to deterioration is described in Chapter 6. Before considering the general Properties of Concrete, its limitations should be realized. It may have a high compressive strength up to 10,000 p.s.i. but has only a low tensile strength of about one-tenth of the compressive strength, and thus needs to be reinforced with steel to form a structural member. Con-crete also changes with age, its strength gradually increases and it dries out.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Concrete Mix Design, Quality Control and Specification

  • Ken W. Day, James Aldred, Barry Hudson(Authors)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  87 © 2010 Taylor & Francis Group, LLC Chapter 5 Properties of Concrete Before starting to design (or specify) concrete, it is necessary to consider what properties we want the concrete to have and also what properties we do not want it to have. Some properties may come under both headings, such as heat generation, but generally undesirable properties are simply a lack of desirable properties. Important properties include • Durability • Strength • Water/ion transport • Rheology • Dimensional stability • Good appearance • Economy • Sustainability 5.1 DURABILITY Durability must come first on our list because if our concrete does not achieve the required design life, it cannot display any of the other desirable properties (not even economy because the most expensive concrete you can get is that which has to be replaced!). However there is a difference between durability for a few years, a few decades, or a few centuries, between dura-bility at any price and “reasonable” durability of economical concrete, and durability in benign or aggressive environments. More particularly there is a difference between the durability of plain concrete and the durability of reinforced concrete. 5.1.1 Corrosion of reinforcement Generally, reinforcement is the Achilles’ heel of concrete. We are all familiar with cracked, rust-stained concrete caused by the expansion of reinforcing 88 Concrete mix design, quality control and specification, fourth edition © 2010 Taylor & Francis Group, LLC steel. Roman concrete was not reinforced and this is a major reason for its survival for centuries. However, as we shall see, there can be durability problems with unreinforced concrete aside from structural issues. The major factor in the corrosion of reinforcing steel is the thickness and quality of the concrete cover. In theory, without adequate cover, concrete cannot protect the reinforcement.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Concrete Mix Design, Quality Control and Specification

  • Ken W. Day, James Aldred, Barry Hudson(Authors)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  Properties of Concrete

  Before starting to design (or specify) concrete, it is necessary to consider what properties we want the concrete to have and also what properties we do not want it to have. Some properties may come under both headings, such as heat generation, but generally undesirable properties are simply a lack of desirable properties.

  Important properties include

  • Durability
  • Strength
  • Water/ion transport
  • Rheology
  • Dimensional stability
  • Good appearance
  • Economy
  • Sustainability

  5.1 durability

  Durability must come first on our list because if our concrete does not achieve the required design life, it cannot display any of the other desirable properties (not even economy because the most expensive concrete you can get is that which has to be replaced!). However there is a difference between durability for a few years, a few decades, or a few centuries, between durability at any price and “reasonable” durability of economical concrete, and durability in benign or aggressive environments. More particularly there is a difference between the durability of plain concrete and the durability of reinforced concrete.

  5.1.1 Corrosion of reinforcement

  Generally, reinforcement is the Achilles’ heel of concrete. We are all familiar with cracked, rust-stained concrete caused by the expansion of reinforcing steel. Roman concrete was not reinforced and this is a major reason for its survival for centuries. However, as we shall see, there can be durability problems with unreinforced concrete aside from structural issues.

  The major factor in the corrosion of reinforcing steel is the thickness and quality of the concrete cover. In theory, without adequate cover, concrete cannot protect the reinforcement. However excessive cover means that the surface concrete is essentially unreinforced and can crack due to thermal stresses or shrinkage, sometimes with the reinforcing cage acting as a crack inducer. Therefore, specifications that call for 100 mm cover or more may result in no effective cover whatsoever! The good durability of spun pipes, ferrocement, and steel fibre reinforced concrete with limited or negligible cover to the reinforcement highlights that corrosion is a complicated subject and there can be exceptions to the general rules.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Concrete Materials and Technology

  A Practical Guide

  • Kambiz Janamian, José Aguiar, José B. Aguiar(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  1 Introduction to Concrete Technology

  DOI: 10.1201/9781003384243-1

  Concrete is the most important construction material with a high amount of usage in the structures, because:

  • It is very simple to find its constituent materials everywhere.
  • Fresh concrete is a flexible material. So, you can shape it with all types of forms.
  • The compressive strength of hardened concrete is very good. We can make a concrete with more than 300 MPa compressive strength nowadays.
  • When we use concrete combined with steel bars or steel fibers, the tensile and flexural strength of the mix will be very good.

  So, it is very important for a civil engineer to know about this magic material. The technique of making and controlling the quality of this material defined as the concrete technology.

  Concrete technology is the technique for the preparation of high-quality constituent materials and mixing them. It seems that it is a simple work. But really it is not. The importance of choosing suitable constituent materials and good proportions is one of the most advanced techniques in civil engineering.

  This chapter is the start for a concrete technologist. You should start with some of the most important expressions and definitions of concrete technology. This fundamental knowledge is necessary to continue the other subjects of the book.

  Every page of this chapter contains many important definitions and concepts that you should learn about the concrete technology. We will use these concepts in the following chapters too many times. So, for beginners, this chapter is the base of other chapters.

  Let’s start our journey with the concrete constituent materials.

  1.1 Concrete Constituent Materials

  Concrete is a mixture of below materials:

  • Portland cement (Figure 1.1 ): It is a kind of powder, which contains calcium silicates and calcium aluminate chemicals. This is the main binder in concrete, which reacts with water to harden. This is also the main material for the smoothness of concrete. We can use about 300–600 kg of Portland cement in 1 m3 of concrete which is about 10%–25% by weight of concrete and about 9%–18% by volume of concrete.
    FIGURE 1.1

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Practical Civil Engineering

  • P.K. Jayasree, K Balan, V Rani(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  9      Concrete Technology

  9.1     Fresh Concrete

  Concrete remains in its fresh state from the time it is mixed until it sets. It is during this time that the concrete is handled, transported, placed, and compacted. Properties of Concrete in its fresh state are very important because it influences the quality of the hardened concrete. Fresh concrete is that stage of concrete in which concrete can be moulded and it is in plastic state. This is also called “Green Concrete.”

  9.1.1     Properties of Fresh Concrete

  9.1.1.1     Consistency

  Consistency of a concrete mix is a measure of the stiffness or fluidity of the mix. It indicates the easiness of flow of concrete. Slump test is commonly used to measure consistency of concrete.

  9.1.1.2     Setting of Concrete

  The hardening of concrete before its hydration is known as setting of concrete. It is the transition process of changing of concrete from plastic state to hardened state. Following are the factors that affect the setting of concrete:

  1. Water cement (w/c) ratio
  2. Suitable temperature
  3. Cement content
  4. Type of cement
  5. Fineness of cement
  6. Relative humidity
  7. Admixtures
  8. Type and amount of aggregate

  9.1.1.3     Workability

  The workability of a concrete mix is the relative ease with which concrete can be placed, compacted, and finished without separation or segregation of the individual materials. As the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. Presence of voids in concrete reduces the density and greatly reduces the strength, 5% of voids can lower the strength by as much as 30%. Slump test can be used to find out the workability of concrete.

  Factors affecting workability of concrete are as follows:

  1. Water content or w/c ratio

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Advanced Concrete Technology

  • Zongjin Li, Xiangming Zhou, Hongyan Ma, Dongshuai Hou(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  CHAPTER 5 PROPERTIES OF HARDENED CONCRETE With the development of hydration, concrete will change from a fluid to a plastic state, and eventu- ally to a solid hardened state. In the hardened state, concrete is ready to support external loads as a structural material. The most important properties of hardened concrete include various strengths, dimension stability, complete stress–strain relationship, various moduli and Poisson’s ratio, and durability. 5.1 STRENGTHS OF HARDENED CONCRETE 5.1.1 Introduction 5.1.1.1 Definitions To understand the concept of strength, it is necessary to understand what stress and strain mean. Nominal stress is defined as the load divided by the original cross-section area. This stress defini- tion can be expressed as [𝜎] = F A (5-1) where F is the load (in N) and A is the cross-section area (in m 2 ). The dimension used for stress in the SI system is Pa, with the following definition: Pa = N m 2 , MPa = 10 6 N m 2 = N mm 2 (5-2) Conventional strain is defined as the change in length per unit original length, and can be expressed as 𝜀 = ΔL L 0 (5-3) where ΔL is the change in length; and L 0 is the original length. Strength is defined as the ability of a material to resist the stress generated by an external force without failure. For concrete, failure is frequently identified with the appearance of cracks. Since the development of a crack is closely related to the development of deformation, in fact, the real criterion of failure for concrete is the limiting strain rather than the limiting stress. The limiting strain for a concrete is different for different loading conditions and different strength levels. For instance, the limiting strain for concrete under uniaxial tension is 100 × 10 −6 to 200 × 10 −6 , while for uniaxial compression it is 4 × 10 −3 for a concrete with a strength of 14 MPa, and 2 × 10 −3 for a concrete with a strength of 70 MPa.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Structural Aspects of Building Conservation

  • Poul Beckmann, Robert Bowles(Authors)
  • 2012(Publication Date)
  • Routledge

    (Publisher)

  Concrete as a construction material did over the last decades of the twentieth century acquire a bad reputation in the popular mind. This was not entirely unfounded; the 1960s and 1970s saw a great amount of ill-considered and/or badly executed use of the material as structure and, particularly, as cladding to buildings. Various, initially promising, concrete-like materials were also used in construction before all their characteristics were properly understood. All of this resulted in premature needs of drastic repairs or, in some cases, demolition after a useful life of only 15–20 years. Most of the demolitions were prompted by consideration of factors, which were not inherently structural, and reinforced concrete, when properly executed, is an excellent form of construction, which has the added advantage that it can be moulded to almost any desired shape and size. This was a property that was (and is) much appreciated by architects and there are a great number of buildings with reinforced concrete structure, which have considerable architectural merit. Some of them date from the first decade of the twentieth century, but many of them of are much more recent. These are all worthy of conservation and in Britain a few of them have in fact been listed, although some very good early examples were allowed to be demolished for reasons of financial gain.

  Successful conservation of such buildings requires assessment of the load-carrying capacity of the structure and of the significance of any apparent defects for their continued serviceability.

  7.1 PROPERTIES AND BEHAVIOUR OF MATERIALS

  7.1.1 Composition of Ordinary Concrete

  Concrete is a man-made ‘conglomerate’, generally consisting of stones and sand with a binder. This can be of lime, sometimes with a pozzolan as in Roman concrete, or it can be of Portland cement as in modern concrete.

  For the purposes of the following, ‘ordinary concrete’ is taken to mean the Portland cement-based concrete, as used nowadays for the majority of construction work.

  The rubble-and-mortar filling, found in cores of mediaeval walls in northern Europe, could be described as a primitive concrete. The lime, used as binder, was usually only capable of hardening by exposure to the carbon dioxide of the atmosphere. In walls several metres thick, the air could not penetrate to the core if the mortar was dense, so the ‘concrete’ inside these walls is sometimes not hardened. In others, the poor quality is due to very sparing use of lime in the mortar.

  The limes used in Roman concrete and in engineering structures up to the mid-1800s, are commonly described as ‘hydraulic’, due to their ability to harden under water. Their essential property is that, due to their content of substances other than calcium hydroxide, they harden in the absence of air, as opposed to the pure limes. These substances, which are sometimes natural impurities and sometimes deliberate additives such as crushed brick, are known collectively as pozzolans.

  Sign up to read

  Learn more about book