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CHAPTER 21

Lime, Portland Cement, and Concrete Introduction to Lime

Lime, Portland Cement, and Concrete

Introduction to Lime

Lime is a cement (binder) and has been used for thousands of years in masonry mortars to bind the stone and brick units in walls.

Until the discovery of portland cement, lime-sand mortar was the only masonry mortar available.

Lime is made from limestone, one of the most abundant rocks in the earth’s crust.

Lime is produced by simply heating limestone—a process known as calcining.

Introduction to Lime – cont’d

Calcium oxide is called quicklime.

Quicklime is a caustic substance that corrodes metals and causes severe damage to human skin.

However, it reacts readily with water to form calcium hydroxide.

Calcium hydroxide is called hydrated lime, or slaked lime, because it contains water that is chemically combined with calcium oxide.

It is a relatively benign material and is the one that is commonly used in building construction.


The calcination of limestone results in quicklime; the hydration of quicklime produces hydrated lime; and the carbonation of hydrated lime changes it to limestone.

These three processes constitute a cycle.


Limestone cycle—calcination, hydration and carbonation.

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In addition to its hardening caused by carbonation, lime experiences another type of hardening.

This is caused by lime’s reaction with amorphous (noncrystalline, i.e., glassy) silica, referred to as a pozzolanic reaction.

Romans found that if lime and volcanic ash were mixed, the mixture, when used with sand and water, gave a mortar that set more quickly, was stronger, and was more durable than lime-sand mortar.

The mixture produces a water-resistant, or hydraulic cement—a cement that does not dissolve in water.

Types of Lime Used in Construction



Limestone occurs in two types: High-calcium limestone , consisting of approximately 95%

calcium carbonate

Dolomitic-limestone , consisting of approximately 60 to 80% calcium carbonate


The lime industry produces the following two types of hydrated lime: Type N (normal) hydrated lime

Type S (special) hydrated lime

In construction activities, lime is principally used in Masonry mortar

Plaster and stucco

Soil stabilization

Portland Cement


Portland Cement

Portland cement –a hydraulic cement

Portland cement is no longer a brand name, but a generic one; therefore, a large number of manufacturers produce Portland cement.

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Portland Cement – cont’d (Manufacturing Process)

An outline of the portland cement manufacturing process.

Portland Cement – cont’d (Types of Portland Cement)

Portland Cement (Types of Portland Cement) – cont’d

Type I/II Portland cement. A Portland cement bag (1 ft 3 ) generally weighs 94 lb.

Air-Entrained and White Portland Cement


Air-Entrained and White Portland Cement (Air-Entrainment)

The Portland cement types previously described are the basic types.

These types are also available with an integrally combined air-entraining agent.

Air entrainment in concrete, mortar, or plaster increases the durability of these materials against freezing and thawing.

Air entrainment can be achieved in two ways. The preferred way is to add an air-entraining chemical to the

mix, referred to as an air-entraining admixture.

The alternative is to use an air-entrained Portland cement.

Air-Entrained and White Portland Cement (Air-Entrainment) – cont’d

All five basic types of Portland cement are available as air-entrained Portland cement.

They are identified as Types IA, IIA, IIIA, IVA, and VA.

With an air-entraining admixture, normal Portland cement (Type I, II, III, IV, or V) is used.

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Air-Entrained and White Portland Cement– cont’d (White Portland Cement)

The more commonly used Portland cement is gray in color. For aesthetic reasons, white Portland cement is preferred, particularly for terrazzo flooring, stucco, and architectural concrete.

Like the gray cement, white Portland cement is available in bags, or in bulk.

It is generally produced as Type I or Type III.

These types have the same properties as the gray Type I and Type III Portland cement, respectively.

Basic Ingredients of Concrete


Basic Ingredients of Concrete

Concrete consists of

Aggregate (coarse and fine aggregate) as matrix or filler

Portland cement–water paste as an adhesive

Any other material used in concrete is called a concrete admixture

Unlike other structural materials, concrete can be formed to any desired shape.

Basic Ingredients of Concrete– cont’d (Sculptural Quality of Concrete)

Basic Ingredients of Concrete– cont’d (Sculptural Quality of Concrete)

Basic Ingredients of Concrete– cont’d (Size of Aggregates)

The aggregate in a concrete mix consists of several sizes.

However, the concrete industry divides the aggregate into two size groups: Fine aggregate

Coarse aggregate

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Basic Ingredients of Concrete(Size of Aggregates) – cont’d

Fine aggregate is generally sand, but more precisely it is that material of which 95% passes through a No. 4 sieve.

Coarse aggregate is that aggregate of which 95% is retained on a No. 4 sieve. It consists of either crushed stone or gravel.

Although the minimum size of coarse aggregate is limited by the No. 4 sieve, its maximum size is a function of the smallest dimension of the building element and the spaces between steel reinforcement.


A typical sieve used for grading of aggregates in concrete laboratories.


(a) Well-graded aggregate. (b) Poorly graded aggregate. The colored areas in these illustrations represent Portland cement and water paste. Observe that poorly graded aggregate requires a greater amount of portland cement.



Effect of aggregate size on the amount of cement-water paste. The four mixes shown here have the same overall volume. Note that as the size of aggregate increases, theamount of cement-water paste needed for a given volume of concrete decreases. Because Portland cement is the major cost-contributing ingredient in a mix, concrete with larger aggregate is generally more economical.

Normal-weight concrete is obtained using normal-weight aggregate. These aggregates include crushed limestone, granite,

quartz, and so on.

Lightweight structural concrete is obtained using lightweight aggregate. One commonly used lightweight aggregate is

expanded shale

Basic Ingredients of Concrete– cont’d (Weight of Concrete)

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Basic Ingredients of Concrete– cont’d (Weight of Aggregates)

Two commonly used aggregates in concrete—crushed limestone as normal-weightaggregate and expanded shale as lightweight aggregate. The quantities shown here are such that both aggregate piles have the same approximate weight. Expanded shale, being fired clay, is generally brown in color, similar to that of a fired clay brick.

Water is an important component of concrete.

Portland cement derives its cementing property from its reaction with water.

Water used in concrete must be clean.

A rule of thumb in the concrete industry is that if the water is fit for drinking, it is fit for use in concrete.

Basic Ingredients of Concrete– cont’d (Quality of Water)

Important Properties of Concrete


Important Properties of Concrete

Properties of concrete that are of interest to architects, engineers, and builders may be divided into the following two categories: Fresh (plastic-state) concrete properties

Hardened concrete properties

Important Properties of Concrete– cont’d (Concrete Workability)

A good concrete should be easy to pump (when required), place, and compact, and it should set within a reasonable amount of time so that finishing can be done without delay.

A measure commonly used to describe them collectively is called workability.

Concrete that is not workable is referred to as a harsh concrete.

Important Properties of Concrete(Concrete Workability) – cont’d

To obtain a workable concrete: The aggregates should be well graded

There should be adequate fine material in the mix

The aggregates should not be too angular in shape

An adequate amount of water is necessary for workability

A commonly used measure of workability is slump.

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A slump cone is open at the top and bottom.


Measurement of the slump of concrete.

Important Properties of Concrete– cont’d (Fresh Concrete Properties)

If concrete is not carefully placed in the forms or if it is compacted excessively, larger aggregates settle down and smaller ones rise to the top.

The segregation of particles gives a non-homogeneous concrete, reducing its strength.

It is important, therefore, that concrete be placed in position carefully and not thrown from a large distance.

Segregation of aggregates is generally accompanied by bleeding. Bleeding is the rising of water to the top.

Important Properties of Concrete– cont’d (Hardened Concrete Properties)

The two most important properties of hardened concrete are durability and compressive strength.

The first step in determining concrete’s strength is to cast test cylinders from the concrete received at the job site.

The casting of cylinders and the slump test are generally performed at the same time.

Important Properties of Concrete(Hardened Concrete Properties) – cont’d

Casting of concrete test cylinders.

Important Properties of Concrete(Hardened Concrete Properties) – cont’d

Dimensions of a concrete test cylinder

A concrete test cylinder in position in the testing machine, ready to be com-pressed to failure

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Making Concrete


Making Concrete

Concrete is made by mixing coarse and fine aggregates, Portland cement, and water.

A small amount of concrete for a do-it-yourself job may be made by mixing various ingredients with a shovel, adding water, and mixing the ingredients further until all materials have blended.

Alternatively, bags of dry concrete mix (consisting of premixed aggregates and Portland cement) may be obtained from a building material store.

A dry concrete mix needs only the addition of water.

Making Concrete – cont’d

If a slightly larger quantity of concrete is required, an on-site mobile concrete mixer can be used.

However, where even a small degree of control on the quality of concrete is required, the concrete should be obtained from a ready-mix plant.

A ready-mix plant is a concrete-manufacturing facility.

Approximately 95% of all concrete used in contemporary building construction is obtained from such a facility.


A transportable mixer for on-site mixing of small quantities of concrete.


An overview of a ready-mix concrete plant. Liquid air may be needed in concretemix in warm weather to control its temperature. Ice may be used as an alternative, depending on availability and cost.


Stockpiles of coarse and fine aggregates in a concrete ready-mix plant. Observe the separation between different aggregates to ensure correct proportioning of the mix.

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A truck mixer being charged with the mix. The control room is located adjacent to the charging station to allow the technician to observe and control the process. The control room has glazed openings toward the charging station.

A typical control room in a ready-mix plant.

Placing and Finishing Concrete


Placing and Finishing Concrete

Because concrete begins to set within a few hours after the addition of water to the dry mix, it is a perishable material.

Therefore, it must be placed in the desired position soon after being received at the construction site.

Placing and Finishing Concrete– cont’d

Ready-mix concrete: Chute

Bucket

Pump


Concrete being brought into the formwork of a grade beam through an open chute from the mixer.


Concrete being filled in the bucket, which is hoisted into position using a crane. The bucket method of transporting concrete is generally used for small quantities of concrete, where pumping of concrete is uneconomical. Note that a large quantity of concrete is required to fill a pump line, which is wasteful if the quantity of concrete to be placed is small.

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Pumping of concrete from mixer truck into a pump truck and then to its final destination,which, in this case, is a slab-on-ground.


(a) Concrete being transferred from a mixer truck to the hopper of a pump truck. (b) Details of the pump hopper, which has a wire screen to arrest any undesirable element in concrete that may clog the pump or the pipeline.

Placing and Finishing Concrete– cont’d (Concrete Consolidation)

Once the concrete has been placed in the form, it must be consolidated.

Consolidation is the process of compacting concrete to ensure that it has no voids and air pockets.

Placing and Finishing Concrete(Concrete Consolidation) – cont’d

Consolidation of concrete using a power-driven vibrator.

Placing and Finishing Concrete– cont’d (Finishing the Concrete Surface)

After the concrete has been compacted, its exposed surfaces are finished while the concrete is still plastic.

The exposed surfaces are those that are not covered by the formwork.

The finishes include: Strikeoff (screeding)

Floating (darbying)

Troweling

Placing and Finishing Concrete(Finishing the Concrete Surface) – cont’d

(a) Striking concrete in a beam with a wood straightedge. (b) Striking concrete in a slab with a wood straightedge.

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Floating of a small concrete surface with a hand float and a large surface with a long-handle bull float.


Troweling of concrete with a power-driven troweling machine, which is available in two different types, as shown. Each machine has metal blades that rotate over the concrete surface.

Portland Cement and Water Reaction


Portland Cement and Water Reaction

Concrete and other mixes made from Portland cement gain their strength due to the reaction of Portland cement with water, referred to as the hydration of Portland cement.

The amount of water required for complete hydration is about 40% of the weight of Portland cement.

In other words, for complete hydration, the water-cement ratio (referred to as the w-c ratio) should be 0.40.

Portland Cement and Water Reaction– cont’d

Often, however, a larger quantity of water is needed to provide the requisite workability of concrete.

Experiments have indicated that the strength of concrete is inversely proportional to the w-c ratio.

Therefore, a concrete should contain the minimumamount of water that gives it the required workability.

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Portland Cement and Water Reaction– cont’d (W-C Ratio)

Strength of concrete as a function of the w-c ratio. For a given concrete (type ofaggregate and amount of Portland cement), concrete’s strength increases as the amount of water used is reduced.

Portland Cement and Water Reaction– cont’d (Curing)

The hydration reaction begins as soon as water and Portland cement come into contact, but the rate at which this reaction proceeds is extremely slow.

It takes up to 6 months or longer for concrete to gain its full strength.

However, approximately 80% of concrete strength develops in 28 days.

Portland Cement and Water Reaction(Curing) – cont’d

The following methods of field curing are common: Keeping concrete wet with water

Covering concrete with a plastic sheet

Using curing compounds


Compressive strength of concrete as a function of its age. Observe that concretekeeps gaining strength well beyond 28 days. Since we generally use concrete’s 28-day strength as the design strength, the additional strength adds to the safety of a concrete structure.


Curing of a recently placed concrete slab-on-ground. Notice the blankets, water pipe and sprinkler.

Hydration of Portland cement is temperature sensitive.

The rate of hydration increases as the ambient air temperature increases, and vice versa.

Below 55°F, the rate of hydration decreases significantly. The use of Type III Portland cement is helpful under

these circumstances.

Portland Cement and Water Reaction– cont’d (Effect of Temperature)

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Water-Reducing Concrete Admixtures


Water-Reducing Concrete Admixtures

Ever since it became known that reducing the water content in concrete increases its strength, the concrete industry began to find ways of reducing the amount of water without decreasing the workability of concrete.

This finally became possible with the discovery of chemicals known as plasticizers or, more commonly, as water-reducing admixtures (WRAs).

High-Strength Concrete


High-Strength Concrete

The realization of high concrete strengths means that concrete competes with steel for the structural frame of tall buildings.

Concrete advantages: Inherent fire resistance

Modulus of elasticity of concrete increases with increasing strength

High-Strength Concrete – cont’d

Progressive increase in concrete’s strength in the structural frame of buildings.

High-Strength Concrete – cont’d (Classifications)

The concrete industry has, therefore, divided concrete into two classifications based on its strength: Conventional concrete—compressive strength <66,000

psi

High-strength concrete—compressive strength ≥6,000 psi

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High-Strength Concrete – cont’d (Disadvantages)

The cost of concrete increases with increasing strength.

Ultra-high-strength concrete requires a great deal of quality control at the site, which increases the cost further. Quality of aggregates and admixtures

Grading of aggregates

Quantity of portland cement.

Placing

Compacting

Curing

Steel Reinforcement


Steel Reinforcement

Concrete is much weaker in tension than in compression.

Its tensile strength is approximately 10% of its compressive strength.

Therefore, concrete is generally used in conjunction with steel reinforcement, which provides the tensile strength in a concrete member.

The use of plain concrete—concrete without steel reinforcement—is limited to pavements and some slabs-on-ground.

Steel Reinforcement– cont’d (Material Bonding)

The bond between steel and concrete is due to the chemistry of the two materials, which produces a chemical bond between them.

Additionally, as water from concrete evaporates, it shrinks and grips the steel bars, making a mechanical bond.

Steel Reinforcement– cont’d (Deformations in Rebar)

Steel Reinforcement– cont’d (Steel Grade)

Rebar are hot rolled from steels of the following yield strengths: 40,000 psi—referred to as grade 40 steel

60,000 psi—referred to as grade 60 steel

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Steel Reinforcement– cont’d (Diameter and Length of Bars)

Eleven different diameters of rebar are available, from 3/8 in. to 2 ¼ in.

The diameter of a bar is generally stated in terms of a number.

In structural calculations, the cross-sectional area of a bar is more important than the bar diameter.

The diameter of a deformed bar is its nominal diameter. The nominal diameter is the diameter that gives the same cross-

sectional area as a plain bar.

Steel Reinforcement(Diameter and Length of Bars) – cont’d

Steel Reinforcement(Diameter and Length of Bars) – cont’d

Two commonly used identification markings on reinforcing bars.

Steel Reinforcement– cont’d (Bending of Bars)

Bars generally arrive at a construction site bent to shape and cut to required lengths. Tags attached to the bundles identify the building component (beam, column, or slab) to which the bars belong.

Steel Reinforcement– cont’d (Tying of Bars)

(a) Bars are assembled into cages at construction site, which requires tying the bars together with a steel wire. Assembled cages are seen in the background. (b) Close-up of tied bars showing the wire.

Steel Reinforcement– cont’d (Epoxy-Coated Bars)

Concrete members that are exposed to corrosive atmospheres require coated bars.

Fusion-bonded, epoxy-coated bars are commonly used in concrete bridges and parking garages exposed to deicing salts, which gradually penetrate through concrete to corrode the bars.

Epoxy-coated bars may also be used in other corrosive atmospheres.

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Welded Wire Reinforcement (WWR)


Welded Wire Reinforcement (WWR)

Welded wire reinforcement (WWR) is a prefabricated reinforcing steel available in rolls or mats. Rolls come in widths of 5 to 7 ft.

Mats come in various dimensions.

They are commonly used in ground-supported slabs (or pavement) and steel deck–supported slabs, where reinforcement requirements are marginal, that is, much smaller than those needed for reinforced concrete suspended slabs.

Welded Wire Reinforcement (WWR)– cont’d

(a) A stack of welded wire reinforcement (WWR) matts. (b) Close-up of WWR.

Welded Wire Reinforcement (WWR) –cont’d (Designation of a WWR Roll or Mat)

Both plain and deformed bars are used in a WWR in steel grades of 40 and 60.

A WWR roll or mat is designated by either W or D, implying the use of plain wires or deformed wires, respectively.

Additional designation includes the cross-sectional areas of wires and their spacing.

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