INTRODUCTION TO REINFORCED CONCRETE

Objectivesdefine important terms related to

reinforced concrete,enumerate the different

materials of reinforced concrete,explain the uses of the different

types of cement,explain why concrete and steel

are compatible structural materials,

Concrete Concrete is a non-homogenous manufactured

stone composed of graded, granular inert materials, which are held together by the action of cement and water.

The inert materials usually consist of gravel or large particles of crushed stone, and sand or pulverized stone. Manufactured lightweight materials are also used.

The inert materials are called aggregates. The large particles are called coarse aggregates and the small particles are called fine aggregates.

 

Concrete behaves very well when subjected to compressive forces but ruptures suddenly when small tension forces are applied. In order to utilize this material effectively, steel reinforcement is placed in the areas subjected to tension.

Reinforced ConcreteReinforced Concrete is a composite

material, which utilizes the concrete in resisting compression forces, and some other material usually steel bars or wires to resist the tension forces.

Steel is also often used to assist the concrete in resisting compression forces. Concrete is always assumed to be incapable of resisting tension, even though it actually can resist a small amount of tension.

The properties of the finished concrete, including its strength, weight, color, and porosity, are subject to considerable variation.

Variables include the type of cement; the ratio of water to cement; the type, size, and proportionate amount of the inert materials; various actions performed while mixing and depositing the concrete mix; and the conditions that occur while the concrete is hardening (called the curing period).

ADVANTAGES OF REINFORCED CONCRETE AS A STRUCTURAL MATERIAL:It has considerable compressive strength as

compared to most other material.Reinforced Concrete has great resistance to

the actions of fire and water.Reinforced Concrete structures are very rigid.It is a low maintenance material.It has very long service life under proper

conditions, Reinforced Concrete can be indefinitely used without reduction of their load carrying abilities.

Usually the only economical material available for footings, basement walls, etc.

ADVANTAGES OF REINFORCED CONCRETE AS A STRUCTURAL MATERIAL:

A special feature of concrete is its ability to be cast into an extraordinary variety of shapes from simple slabs, beams, and columns to the great arches and shells.

In most areas, concrete takes advantage of local materials available (sand, gravel, water).

A lower grade or skilled labor is required for erection as compared to other materials.

DISADVANTAGES OF CONCRETE AS A STRUCTURAL MATERIAL: Concrete has low tensile strength requiring the

use of tensile reinforcement. Forms are required to hold the concrete in place

until it hardens sufficiently, in addition, falsework or shoring may be necessary to keep the forms in place.

The low strength per unit weight of concrete leads to heavy members.

Similarly, the low strength per unit of volume of concrete means members will be relatively large.

The properties of concrete varies widely due to variations in its proportioning and mixing.

MATERIALS FOR CONRETE:CEMENT - the cement used most extensively in building construction is Portland cement and is generally available in a number of different types:

Normal Portland Cement – used for general purposes when specific properties are not required.

Modified Portland Cement – for use when low heat of hydration is desired, such as in mass concrete, huge piers, heavy abutments and heavy retaining walls, particularly when the weather is hot.

High-early-strength Portland cement – for use when very high strength is desired at an early age.

MATERIALS FOR CONRETE: Low-heat-of-hydration Portland cement – for use in

large masses such as dams. Low heat of hydration is desirable to reduce cracking and shrinkage.

Sulfate-resistant Portland cement – for use when the structure will be exposed to soil or water having a high alkali content.

Air-entrained Portland cement – for use when severe frost action is present, or when salt application is sued to remove snow or ice from the structure.

Shrinkage-compressing Portland cement (expansive cement) – which expands as the concrete cures, thus compensating for some shrinkage.

MATERIALS FOR CONRETE:MIXING WATER –

The water used in making concrete should be clean and free from injurious amounts of oil, acid, alkali, organic matter, or other deleterious substances, excessive impurities may affect setting time and concrete strength and cause corrosion of reinforcement.

Although seawater containing as much as 3.5% salt can be employed in making plain concrete, it should not employed for reinforced concrete because of the risk of corrosion of the steel reinforcement.

MATERIALS FOR CONRETE:AGGREGATES – the materials held

together by the paste formed of cement and water. ◦ They form the bulk of the concrete

components. Fine aggregates consist of sand or other fine

grained inert materials usually less than ¼” (6.4 mm) maximum size.

Coarse aggregates consist of gravel or crushed rock usually larger than ¼” (6.4 mm) size and usually less than 3: (76 mm) size.

LIGHTWEIGHT AGGREGATES – the aggregates discussed above are those used in the production of normal-weight concrete, that is concrete weighing about 135 to 160 lb/ft3.

Structural light-weight concretes ranging from about 85 to 115 lb/ft3 are made with expanded shale, clay, slate, and slag as aggregates. ◦ Such materials produce concrete of sufficient

strength for many purposes, and, in comparison with stone concrete reduce the loads appreciably.

MATERIALS FOR CONRETE:ADMIXTURES – substances added

to concrete to improve its workability, accelerate its set, harden its surface, and increase its waterproof qualities are known as admixtures.

◦The term embraces all materials other than the cement, water, and aggregates that are added just before or during mixing.

1. Accelerating Admixtures – reduce time of setting and accelerate early strength development.

2. Air entraining admixtures – form minute bubbles 1 mm in diameter or smaller in concrete or mortar to increase workability of the mix during placing and the frost resistance of the finished product.

3. Water-reducing and set-controlling admixtures – increase strength of concrete, enable reducing the cement content in proportion to the reduction of water content.

4. Finely Divided Admixtures – mineral admixtures used to rectify deficiencies on concrete mix by providing missing fines from the fine aggregates.

5. Admixtures for no-slump concrete – no slump concrete of 1-inch slump immediately after mixing.

6. Polymers – enable producing concretes of very high strength up to a compressive strength of 15,000 psi or higher and a tensile splitting strength of 1500 psi or higher.

7. Superplasticizers – high range, water reducing chemical admixtures.

MATERIALS FOR CONRETE:STEEL REINFORCEMENT – the most

common type of steel reinforcement employed in concrete building construction consists of round bars, usually of the deformed type, with lugs or projections on their surfaces.

MATERIALS FOR CONRETE:◦ The purpose of the surface deformations is to

develop a greater bond between the concrete and the steel. The bars used for reinforcement are made from billet steel, rail steel, or axle steel, conforming to ASTM Specifications A615, A616, and A617, respectively.

◦ The most common grades of reinforcing steel are Grade 40 and Grade 60, with yield strengths (fy) of 40,000 and 60,000 lb/in2 (psi) respectively.

What is PNS 49?

Steel bars are covered by a mandatory standard called the Philippine National Standard 49 or PNS 49, as formulated by the Bureau of Product Standards (BPS) of the Department of Trade and Industry (DTI) with the help of the Steel Industry, the Association of Structural Engineers of the Philippines, and the Philippine Construction Association.

Rebar Grades   ◦ in accordance with Philippine National

Standards for Steel Bars for Concrete Reinforcement, under PNS-49:2002.

Specifications and Grading   ◦The three grades under this specification:

Grades PNS 230, 275, and 415 –trace their roots to ASTM specifications.

the terms ‘Structural’, ‘Intermediate’, and ‘High-tensile’ originated from earlier versions of ASTM-A615 but remained unchanged in deference to custom and familiarity among users.

The table below provides a handy reference as to the common understanding in the market regarding these grades

ASTM vs PNS Codes Popular Nomenclature

Typical Application

Grade 33 / PNS 230 Structural Grade

Buildings and Low Loading Conditions

Grade 40 / PNS 275 Intermediate Grade

Medium-rise Structures / Infrastructure Work

Grade 60 / PNS 415 High-Tensile Grade

Medium & High-rise Structures / Infrastructure

Sample PS Marks of BPS registered companies

Identifying mark on rebarManufacturer's Identifying Mark / Company Logo - registered symbols / logos thru Philippine Patent Office.

The three grades are distinguished by the different color markings painted at the ends of each bar, as shown in the following table:

The Nominal Size of a Rebar For both the plain round bar and the rebar, the word

‘size’ and nominal diameter are used interchangeably. For a rebar, alternatively known as a ‘deformed steel

bar’, its ‘size’ specifically refers to the size of a plain round bar having the same weight per meter as the rebar.

A specific ‘size’ of plain round bar refers to its nominal diameter. This ‘size’ also refers to the diameter of a circular plain round bar having a certain unit mass or weight per meter.

A plain round bar and a rebar of the same ‘size’ will have the same unit mass or weight per meter of bar.

Rebar Sizes   The standard sizes that the Company manufactures and sells are as follows: 10mm, 12mm, 16mm, 20mm, 25mm, 28mm, 32mm, and 36mm. For special orders, it can also make 40mm and 50mm bars

The Proper Way of Measuring a Plain Round Bar  For a plain round bar, determining its 'size'

involves simply taking a direct physical measurement of its diameter since its circular cross section should remain quite uniform throughout its length.

A plain round bar is considered to be dimensionally acceptable if its diameter, as measured, falls within the dimensional tolerances allowed by the Standard.

The Proper Way of Measuring a Rebar

 A rebar however is not measured in the same way as a plain round bar.

Because of its non-uniform cross section, a rebar must comply with the allowable ‘Variation in Mass’ (VIM) in lieu of measurement of its diameter.

Checking for this VIM involves taking and weighing one meter of rebar and comparing the findings against the standard value.

Weight ToleranceBecause variation in cross sectional

area occurs during hot rolling, a corresponding variation in linear weight is to be expected.

For this reason, PNS-49 provides for an allowable ‘variation in mass’ (VIM) of ±6%, measured using one meter of rebar.

Length ToleranceThe cut length of a rebar varies

as well because of shrinkage as it cools down.

The standard however limits this shrinkage, and the minimum lengths of the finished rebar are shown below.

The most important properties of Reinforcing steel:

Elastic modulus, or modulus of elasticity - is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it.

The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region:

stiffer material will have a higher elastic modulus.

The most important properties of Reinforcing steel:

Modulus of Elasticity, (Es ) - the modulus of elasticity of steel is considered to be 29,000,000 psi or 200,000 MPa.

Specifying how stress and strain are to be measured, including directions, allows for many types of elastic moduli to be defined. The three primary ones are:

Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; - it is defined as the ratio of tensile stress to tensile strain. - it is often referred to simply as the elastic modulus.

The most important properties of Reinforcing steel:

shear modulus  or modulus of rigidity (G or ) describes an object's tendency to shear (the deformation of shape at constant volume) when acted upon by opposing forces; - it is defined as shear stress over shear strain. The shear modulus is part of the derivation of viscosity.

bulk modulus (K) describes volumetric elasticity, or the tendency of an object to deform in all directions when uniformly loaded in all directions; - it is defined as volumetric stress over volumetric strain, and is the inverse of compressibility . -The bulk modulus is an extension of Young's modulus to three dimensions.