Civil Engineering Materials-Ordinary Concrete

8/10/2019 Civil Engineering Materials-Ordinary Concrete

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Civil Engineering Materials-Ordinary Concrete

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Chapter Four Ordinary Concrete

Catalogue

4.1 Introduction

4.2 Ingredient of Ordinary Concrete

4.3 Workability of Ordinary Concrete

4.4 Strength of Concrete

4.5 Durability of Concrete

4.6 Dimensional Stability

4.7 Mix Proportion Design of Concrete

4.8 Advanced Concrete

4.1 Introduction

4.1.1 What is Concrete?

Concrete is defined as a composite material which consists essentially of a binding medium of

embedded particles or aggregate fragments. In Portland cement concrete, the binder is a mixture of

Portland cement and water. Asphalt and other cements are used to make various types of concrete, but

commonly the term concrete refers to Portland cement concrete.

4.1.2 Origin of Concrete

Concrete can be dated back to Rome palace. In the 1980s, the 100m3 concrete was excavated in

Dadiwan of Qin, an county of Gansu province 5000 years ago in Neolithic age.

4.1.3 Widely usagesConcrete is used exclusively for foundations of buildings, bridges, and other structures. It is used in

dams, canals, aqueducts, and other structures to control and divert water. It is used in highways, streets,

pavements, and sidewalks; thus, it is a major material in the transportation industry.

Concrete is of such importance that almost every civil engineering structure uses concrete in some form.

Materials to make Portland cement and concrete are clean, abundant and non strategic, for they can be

obtained from the earth. Concrete may become more important in the future as the supply of strategic

materials becomes exhausted.

4.1.4 Classification of Concrete

1. In Portland cement concrete, the binder is a mixture of Portland cement and water. Asphalt and other


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cements are used to make various types of concrete, but commonly the term concrete refers to Portland

cement concrete.

2. By apparent density, concrete includes three types:

Heavy concrete: 02600 as shielding materials of atomic energy engineering;

Normal concrete: 0=2000-2500 in several bearing structure;

Lightweight Concrete: 01900 including: Light aggregate concrete and porous concrete.

3. By strength, concrete includes three types: Ordinary concrete, high strength concrete, super strength

concrete.

Ordinary concrete: Compressive strength 60 MPa.

It is applied largely to the high-rise building, large span bridges and high-strength prefabrication

components and so on.

Super-strength concrete: Compressive strength>80 MPa.

4. By forming or construction technology (Fig.4.1.1)

It includes four kinds of concretes: Deposit concrete, Precast concrete, Premixed concrete and Shotcrete

concrete.

4.1.5 Characteristics of Concrete

The concrete is largely applied to the construction for its excellent technical performance and low cost.

It is characterized by rich raw material, the changeability of its performances, excellent plasticity and

durability, and reinforcement by the steels.

Disadvantage of Concrete are large deadweight and brittle.

Fig.4.1.1 Types of concrete

4.2 Ingredient of Ordinary Concrete

Concrete is made up of paste (cement water), aggregate (sand, gravel) and admixture.

4.2.1 Cement


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1. Selection of Types

Select according to the different environment.

The six general cements are commonly used in Portland cement, Ordinary Portland cement,

Furnance-slag cement, Portland Pozzlana cement, Portland Fly-ash cement and Composite Portland

cement.

2. Selection of Grade

Grade must be selected according to the different design strength grades.

For ordinary concrete the cement is 1.5-2 times the strength of the concrete.

For high strength concrete, it is 0.9-1.5 times the strength of the concrete.

4.2.2 Aggregate

1. Classification

Aggregate can be divided into primary aggregate and recycled concrete aggregate. Primary aggregate,

such as sand and gravel are widely used in engineering.

Most rocks found in the earth's crust can be used as aggregate in concrete. Because of the variety of

physical and chemical characteristics among aggregates, their influence on concrete mixtures is also

varied.

2. Features influence concrete

Physical characteristics influence this behavior, such as particle size distribution, particle shape, surface

texture and hardness. Chemical composition and reactivity influence the properties of hardened concrete.

The maximum amount of aggregate should be used in concrete mixtures

Aggregate generally increases both the quality and the economy of concrete. Each cubic meter of

Portland cement water paste will cost approximately 4 times as much as each cubic meter (solid volume)

of aggregate. Aggregates also have greater dimensional stability, particularly less drying shrinkage, than

Portland cement paste.

Workability requirements limit the amount of aggregate that can be used in concrete mixtures.

Aggregate particles, in effect, float in the paste of the mixture. When there is insufficient paste to fill the

voids in the aggregate particles, the mixture becomes granular, crumbly, and unworkable because of

particle interference.

The following describes the factors which influence the amount of aggregate that can be used.

(1)SlumpAs the aggregate content increases, the slump decreases with no changes in the paste content.

(2)GradingProper grading of aggregate particles decreases the volume of voids among the aggregate

particles, which permits a greater aggregate content in a mixture without particle interference.

(3)Entrained airEntrained air increases the volume of paste in effect and permits an increase in aggregate

content at the same slump.

(4)Particle shape and surface textureThere is usually a smaller volume of voids in rounded and smooth

aggregates.

(5)Water-reducing admixturesDispersing agents commonly referred as water-reducing admixtures which

make the paste more fluid by releasing water in the cement. This permits an increase in the aggregate

content at a given slump.

(6)Viscosity of pasteAn increase in viscosity of paste decreases the allowable aggregate content.

(7)TemperatureHigh temperature increases the viscosity of paste and reduces the aggregate content of

the same slump.


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3. Fine Aggregate-Sand

(1) Definition

It refers to the aggregate whose diameter is less than 5mm.It must be in agreement with

GB/T14684-2001(JGJ52-92).

(2)Classification

Sand is the widely used in primary fine aggregate.

By raw materials

Sand can be divided into natural sand and artificial sand by the sand origin. In natural sand, we have

river sand, sea sand and hill sand.

By technical requirement, it includes three kinds of sand

:Those used in the concrete whose strength grade is more than C60;

:Those used in the concrete whose strength grade is between C30~C60;

:Those used in the concrete whose strength grade is less than C30.

(3)Requirements of Aggregate

Impurities

Impurities hinder the hydration or causes harden cement paste corrosion, and reduce the bond between

cement paste and aggregate.

Types:

micaclaysilt and organic substance.

Damage:

Hinder the bond between paste and aggregate, weaken the strength of concrete, increase the requirement

quantity of water, increase the shrinkage of concrete and bring corrosion to harden cement paste.

Processing methods:

Wash impurities out as requested, if too much in the sand.

The amount of impurities must be in agreement with GB/T14684-2001(JGJ52-92) (Tab.4.2.1).

Tab.4.2.1 Amounts of Impurities (GB/T14684-2001)

TypeIndex

Clay content, % < 1.0 2.0 5.0

Clod content, % < 0 1.0 2.0

Mica, % < 1.0 2.0 2.0

Light matter, % < 1.0 1.0 1.0

Organic substance

(colorimetry)Eligible Eligible Eligible

Sulphide and

sulphate, % 4 >4 >4

Insoluble matter

mg/L


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The second ones refer to those used to adjust the setting time and to harden the performance, including

set retarded, hardening accelerating and flash setting admixture.

The third ones are used to improve the durability of the cement, including air entraining admixture,

water-repellent admixture and anti-freezing admixture.

The last ones are used to improve other performances, including air entraining admixtureexpanding

admixtureanti-freezing admixture and coloring admixture and so on.

Application

Using admixtures is one of the most effective ways of improving the concrete strength and performance,

saving on the cement and energy.

Concrete admixtures develop rapidly for their usage in many abroad countries comes up to 60-80%,

even 100%. Nowadays, they have become one of the five components in concrete.

(2) Water-reducing admixture

Definition

Water-reducing admixture permits less water at the same of workability or the mobility increase in the

same amount of water.

Water-reducing admixtures increase workability and quality by more efficient use of the water in a

concrete mixture. By reducing the water, these admixtures densify the concrete and decrease capillary

voids and permeability.

Classification

By effect: It can be classified into ordinary water-reducing admixture and super plasticizer.

The water-reducing rate of the former is less than 10% while that of the latter, also called super

plasticizer or fluid concrete, is more than 10%.

By the effect on setting time: It can also be classified into standard type, set retarder type and coagulant

type.

By the effect on air content: It can also be classified into air entraining type and non air entraining type.

Mechanism

Water-reducing admixture (surfactant) is composed of hydrophilic group-polar group and hydrophobic

group-nonpolar group. When mixed with water, hydrophobic group attaches to cement particle and

hydrophilic group attaches to water.

Mechanism of water-reducing admixture is reflected in the apparent active reaction.

Cement paste flocculation structure is wrapped to reduce the mobility of the admixture. But

water-reducing admixtures can make flocculation structure dissolve so as to improve the mobility and

strength.

The flocculation structure dissolves by the following three ways:

Disassemble cement flocculation by electron repels from the surface of cement particle. Improve the

fluidity of paste by forming a solvate membrane on the particle surface. Extend the hydrating area by

improving the dispersity of the cement particles to improve mobility.

Technical and economic effects


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Technical and economic effects of water-reducing admixture are listed in Tab.4.2.10.

Tab.4.2.10 Technical and economic effects of water-reducing admixture

Group effects

Cement content

Kg/m3W/C

Slump

mmFcu,k(MPa)

Base concrete(not of

water-reducing

admixture)

300 0.62 50 37

Improving mobility 300 0.62 100 38

Improving strength 300 0.56 50 46

Saving cement 270 0.62 50 37.5

Water-reducing admixture in common use

a. Lignin suffocate water-reducing admixture

Definition: Lignin suffocates (M) is the most common one in use. It is made by wood pulp through

sulfonation and dryness.

Application: It applies to large molding, sliding formwork, mass concrete, concrete pumping, and

summer construction and so on.

Performance:

Set retarded activity:

Delay the setting time 1-3hours. Add 0.25% M into lignin suffocate water-reducing admixture. In winter

it must be used with the mixture of hardening accelerator.

Air entraining activity:

Increase the air content from 2% to 3.6% to reduce the strength and improve durability.The amount and its effects: If 0.2-0.3% lignin suffocates is added, the water-reducing rate will be 10%

or the strength increased by 10-20% by increasing the slump of 10cm.

b. Naphthalene suffocate water-reducing admixture

Brief Introduction: At present, there are varieties of naphthalene suffocate water-reducing admixtures in

China. The most common types include NF, NNO, FDN, UNF, MF, AF and so on.

Applications in five fields: high strength concrete, high performance concrete, liquid concrete, concrete

pumping and winter construction concrete.

Performance: It has advantages over lignin suffocate water-reducing admixture for its violent dispersion

in that water-reducing rate is over 15%, reinforcing rate over 20% and saving cement 10-20%.

Minor set retarded activity but doesnt affect the strength. Most of them are non air entraining types. Its

amount is 0.2-1.0%.

c. Resinous water-reducing admixtures

Introduction: Resinous water-reducing admixtures are one of super-plasticizers.

In China, SM, the main type of resinous water-reducing admixtures, has the perfect water-reducing

effect.

Advantages: It is better than naphthalene suffocate in dispersion, water reducing, and reinforcing. 7d

strength of early strength types is the same with 28d strength of ordinary ones. The 28d strength can

increase by 30-60%.

Disadvantages: The cost is high.(3) Hardening accelerator


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Definition

It refers to those that improve the strength in early period without affecting that of the later period.

Application

It applies to the construction that is constructed in winter, emergency engineering and time-limited ones.

The use of hardening accelerator can make C20 reach mould removal strength within 16 hours and the

strength allowing floor slab installment on it within 36 hours so as to speed up the construction.

Common hardening accelerator

a. Chloride hardening accelerator

Typical type:

CaCl2type is most commonly used for its excellent effect, low cost and convenience.

Performance-early strength: Add 0.5-1% chloride and strength of 2-3d can improve 50-100%, and 7d of

strength 20-40%.

Mechanism of early strength:

CaCl2, reacted with C3A and CH, gives new hydrate-calcium oxychloride. Hydrate-calcium oxychloride

is separated in the early period, and formed into the framework. It speeds up the formation of structure of

hardened paste. Meanwhile, the CH concentration reduces. These two factors quicken the C3S hydration.

Thus the strength is improved in the early period.

It can form new hydration, improve speed of hydrate and intensify solubility of cement, so as to improve

early strength.

b. Sulfate hardening accelerator

Typical type:

Sal mirabile are most commonly used.

Performance-early strength:

When it is 1-1.5%, sulfate is added. The concrete strength goes up 70% of design strength in only half of

the time.

Mechanisms (Fig.4.2.7):

c. Organic amine hardening accelerator

Typical type:

TEA is the main type of organic amine water-reducing admixtures. It is pale buttery liquid. Its quantity

is 0.020.05%.

Performance-early strength:

The early strength improves by about 50%. The 28d strength remains the same. It plays the role of slow

setting

Mechanisms:

Compliant in the form of complex ion in aqueous alkali produces complex salt of small solubility

framework with cement hydrate.

Na2SO4Ca(OH)2+H2O NaOH+CaSO42H2O

CH


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speed of hydrate Aft

formed into the framework

speed up the formation of structure of paste

improvement of strength in the early period

Fig.4.2.7 Mechanism of early strength of sulfate hardening accelerator

(4) Air entraining admixture

Definition

It refers to the concrete admixture that is added by large quantities of separate, tiny and even bubbles.

Mechanism

It is hydrophobic apparent active substance. It reduces the surface tension and causes the oriented

adsorption in the bubble surface to change the air mixing into separate, tiny and even bubbles.

Performance

It improves durability and workability as well as decrease concrete strength. Air entraining admixtures

improve durability and plasticity, but air entrainment reduces strength.

Air entraining admixtures can change the pore structure, improve durability, increase lubricate of

aggregates, improve plasticity, increase air content and reduce strength.

Application

This type of admixture should always be used where freezing and thawing deterioration occurs. It

reduces the strength unnecessarily in structural concrete.

Common air entraining admixture

Rosin soap and rosin pyrolytic polymer and so on. The amount is 0.005%-0.012%. Introduce

D=0.05-1.25mm air bladder. Make concrete air content to 2%-6%.

(5) Set retarding admixture / Flash setting admixture

a. Set retarding admixture

Definition: It refers to the admixture that can delay the setting time without apparent effects on the

secondary hardening

Common types: molasses and citric.

Application: It applies to mass concrete, hydraulic construction, large moulding, and high temperature

construction, construction using long time scale of mixing or pouring.

b. Flash setting admixture

Definition: It refers to the materials that make concrete accelerating and improve adhesion and stability

of concrete.

Classification: Inorganic salts can be classified by their main components in sodium aluminates, calcium

aluminates and silicate.

Common types: Red star type 2.5%-4%; 711 type 2.5%-3.5%.


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Application: gunite concrete and caulking engineering

Performance: Initial setting time is 5min and final setting time 10min. Strength can be achieved within

one hour. It will improved 2-3 times one day later and reduces to 80-90% of the concrete without it 28 days

later.

(6) Expanding admixture

Definition: This kind of admixtures can compensate shrinkage of concrete and cause self-stressing in

concrete.

Common types: U type expanding admixture

Performance: amount 10%-15%, impermeability improvement.

(7) Anti-freezing admixture

Definition: It makes concrete gain strength normally in negative temperature and plays the role of

freezing point, anti-freezing and accelerating strength gain in concrete.

Common types: NaNO2and Ca(NO2)2mixing 1%-8%; CaCl2and NaCl mixing 0.5%-1%.

Performance: lowing freezing point; anti-freezing; accelerating strength gain

The admixture used in the construction is composite anti-freezing admixture which is compounded of

anti-freezing component, early strength component, reducing water component, even air entraining

component so as to improve the effect on anti-freezing.

2. Mineral admixture

(1)Definition:

It refers to mineral powdery materials blended during the course of mixing concrete for saving cement

and improving the performance of concrete, such as fly ash, silica fume, grounded slag, sintered, clay,

zeolite, and volcanic tuff and so on.

(2)Types in common use

Fly ash, Silica fume, Grounded slag, Sintered, Clay, Zeolite and Volcanic tuff

(3)Fly ash

Definition

It refers to fine powder that is collected from smoke of burning coal.

Property

=1.77~2.43g/cm3, average 2.08g/cm3(China); 1.9-2.9g/cm3(overseas

Classification

It can be divided into dry ash and wet ash by the form of discharging. It can be divided into ash collected

by static electricity and by machine. It can be divided into ground ash and original ash by the degree of

grinding.

Requirement of quality

It includes three grades according to CaO content, fineness, loss on ignition, water requirement ratio,

SO3content and water content.

First class: reinforced concrete, prestressed concrete (span


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There are three effects of fly ash: Pozzolana effect, rolled ball effect and filling effect.

Change porosity structure and improve properties of concrete including workability, strength and

durability.

Blending methods

There are three methods: equal replacement, excess replacement and adscititious.

(4)Silica fume

Definition

It refers to the smoke and dust of absolute fine particle collected through the smoke during the

production of ferrosilicon or silicon steel, whose density 2.1-2.2 g/cm3.

Effect

Improve viscidity and water retention of concrete admixture. Take mobility into consideration.

Improve concrete strength, pore structure and durability.

4.2.5 Influence of Each Ingredient on Concrete Properties

The proportions of ingredients used in concrete must be carefully balanced to produce an optimum

mixture for the purposes intended. Too much or too little of any one ingredient may have an adverse effect

on one of the important properties of the resulting concrete (Tab.4.2.11).

Tab.4.2.11 Influence of each principal ingredient on the properties of concrete

Ingredient Quality Workability Economy

Aggregate Increases Decreases Increases

Portland cement Increases Increases DecreasesWater Decreases Increases Increases

1. Aggregates

Most of the rocks found in the earth's crust can be used as aggregate in concrete. Because of the variety

of physical and chemical characteristics among aggregates, their influence on concrete mixtures is also

varied. Physical characteristics such as particle size distribution, particle shape, surface texture, and

hardness influence this behavior. Chemical composition and reactivity influence the properties of hardened

concrete.

The maximum amount of aggregate should be used in concrete mixtures. Aggregate generally increases

both the quality and the economy of concrete. Each cubic meter of Portland cement water paste will cost

approximately 4 times as much as each cubic meter (solid volume) of aggregate. Aggregates also have

greater dimensional stability, particularly less drying shrinkage, than Portland cement paste.

Workability requirements limit the amount of aggregate that can be used in concrete mixtures.

Aggregate particles, in effect, float in the paste of the mixture; and when there is insufficient paste to fill

the voids in the aggregate particles, the mixture becomes granular, crumbly, and unworkable due to particle

interference. The following describes the factors which influence the amount of aggregate.

a. SlumpAs the aggregate content increases, the slump decreases with no changes in the paste content.

b. GradingProper grading of aggregate particles decreases the volume of voids among the aggregate

particles, which permits a greater aggregate content in a mixture without particle interference.


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c. Entrained airEntrained air, in effect, increases the volume of paste and permits an increase in

aggregate content at the same slump.

d. Particle shape and surface textureThere is usually a smaller volume of voids in rounded, smooth

aggregates.

e. Water-reducing admixturesDispersing agents commonly referred as water-reducing admixtures make

the paste more fluid by releasing water in the cement. This permits an increase in the aggregate content at a

given slump.

f. Viscosity of pasteAn increase in viscosity of paste decreases the allowable aggregate content.

TemperatureHigh temperature increases the viscosity of paste and reduces the aggregate content for the

same slump.

2. Portland Cement

Portland cement and water react chemically and produce a calcium silicate hydrate. It takes a

comparatively small amount of water (estimated to be about 30 percent of the cement) to complete the

hydration reaction. A paste with this much water, however, is stiff and unworkable, and an additional

amount of water is required for sufficient fluidity of the paste and workability of the concrete. The ratio of

water to cement (w/c ratio) determines the quality of the paste and, to a large extent, controls the quality of

concrete. It follows that additional cement added to concrete improves the quality by reducing the w/c

ratio.

In lean concrete, additional amount of Portland cement improves the workability of the concrete. This

trend is limited, however, and an excessive amount of cement tends to make concrete mixtures sticky and

lacking of mobility. Hardened concrete containing excessive cement and water will increase drying

shrinkage.

3. Water

Water is essential to hydrate Portland cement and provides workability to concrete mixtures. Too much

water added to the mixture is detrimental to the quality of concrete. More water than required added to

hydrate Portland cement dilutes the paste, separates the calcium silicate gel crystals, and weakens the gel

structure. Uncombined water leaves capillary voids in the paste, which are involved in most freezing and

thawing mechanisms. The proportion of water to cement is critical in producing quality, and the ratio of

aggregate to paste is critical in obtaining workability and volume stability.

4. Admixtures

An admixture is any material added to concrete besides the three principal ingredientsaggregates,

Portland cement, and water. Admixtures are used in concrete to improve the principal requirements for

concrete mixtures. For example, air-entraining agents improve durability and plasticity, but air entrainment

reduces strength. This type of admixture should always be used where freezing and thawing deterioration

occurs, but it reduces the strength unnecessarily in structural concrete not exposed to freezing and thawing

in a saturated condition. Water-reducing admixtures increase workability and quality by more efficient use

of the water in a concrete mixture. By reducing the water, these admixtures densify the concrete and

decrease capillary voids and permeability.


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Admixtures which retard or accelerate the time of set or strength gain do not generally influence the

properties of plastic concrete. The following are the use of some main admixtures in common use in

concrete.

Air-entraining agentsImprove durability and workability; Allow the additional aggregate

Water-reducing agentsIncrease slumpReduce w/c ratioAllow the additional aggregate at the same

slump and same w/c ratio

Accelerating agentsAccelerate hydration and setting of cement; Produce early strength

Retarding agentsRetard hydration and setting of cement

It should be noticed that Pozzolans also can be added to concrete composite as admixtures to improve

concrete properties. For example, Pozzolans can be used to reduce alkali-aggregate reaction, temperature,

permeability and increase sulfate resistance.

4.3 Workability of Ordinary Concrete

4.3.1 The principle requirements for concrete

The principle requirements for concrete listed in the order of importance are workability, strength,

durability and economy.

4.3.2 Workability of Fresh Concrete

1. Definition workability

(1)Fresh Concrete

Fresh Concrete is the concrete before hardened, e.g. concrete composite.

(2)Workability (Fig.4.3.1)

The properties of Fresh Concrete are sufficiently workable to be mixed, transported, and placed

properly.

Fig.4.3.1 concrete in construction

2. Importance of workability

Concrete which cannot be placed properly will not produce quality structures regardless of the qua1ity

of the mixture. Concrete may be considered workable for one structure, but may not be suitable for

another.

3. Mobility

The capacity for a concrete mixture to readily respond to vibration and completely fill all parts and


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corners of a form is defined as mobility. It overcomes interior friction, paste viscidity and fiction between

aggregates in composite.

4. Viscidity

Viscidity is the property of a concrete mixture which allows molding without segregation.

Segregation: Composition of concrete mixture segregates by their density and grain size.

5. Water retention

It refers to the capacity to prevent water bleeding from concrete composite.

Bleeding:

It refers to the phenomenon that water flows from composite and up to the concrete surface.

Harm:

(1) Bleeding path or water package can be caused deteriorate durability;

(2) Settlement can be caused crazing in concrete;

(3) Floating paste can be caused wreck the bond to new concrete.

4.3.3 Evaluation of Workability1. Slump Test

(1)Index

Slump is a common field control test for concrete mobility. It is the subsidence of fresh concrete upon

removal of a 12-in truncated cone form. Plasticity and water retention also can be evaluated by slump test.

(2)Application range:

Dmax40mmSlump10mm

(3)Testing method:

Put the concrete composite in a slump cone (Fig.4.3.2) in standard methods, and smooth the top surface,

and then remove the cone, in the end measure the composite subsidence due to gravity.

Slump=height before subsidence-height after subsidence

Fig.4.3.2 Slump Cone

(4)Evaluation

Mobility

The larger the slump is, the greater the mobility is.

Viscidity

There are possibilities after kicking the side of composite. (good, poor)

Water retention


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More paste bleeding means poor capacity of water retention and less paste bleeding means good

capacity of water retention.

2. Vebe Consistometer Test

(1) Application range

Dry composite with D40mm, Vebe Consistometer is 5~30sslump10mm

(2) Testing Method

Put the composite of truncated cone shape in the container of Vebe Consistometer instrument and make

the transparent disc touch to the top of cone, then put on vibrating table and record the time when the disc

is covered with paste.

Fig4.3.3 Vebe Consistometer Instrument

4.3.4 Selection and classification of composite slump

1. Principle

We should abide by the principle to select low mobility with the guarantee of construction operation and

dense placement.

2. Selection

Select mobility by member cross section, condensing methods and reinforcement spacing.

3. Slump selection for concrete structure (vibrated by machine) (Tab4.3.1).

Tab4.3.1 Slump selection for concrete structure (vibrated by machine)

Slump

(mm)Type Application

>160 Large viscidityPumping concrete, structures that have narrow transact and reinforcing

steel bar distribute especially densely

100-150 ViscidityPumping concrete, structures that have narrow transact and reinforcing

steel bar distribute densely

50-90 Plasticity Normal concrete structure

10-40 Low plasticity Vibrate intensely, prefabricate member, base and steel less thick structure

4.3.5 Main influencing factors of workability

1. Influence from ingredients

(1) W/C (water to cement ratio, cement consistency)When cement quantity is fixed, W/C is critical in producing quality.


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Low W/C will produce dry composite and small slump.

If excessively low W/C, Composite will breakdown and low viscidity will be condensed, which results

in poor strength and durability.

High W/C will produce thin composite and large slump.

If excessively high W/C, it tends to segregate and bleed, which results in poor strength and durability.

With proper W/C, composite can be condensed favorably and stability.

W/C should be selected according to the requirements of concrete strength and durability.

(2) Quantity of paste (quantity of mixing water, ratio of paste to aggregate)

When W/C is fixed, quantity of paste is crucial in obtaining workability and volume stability.

More paste will produce greater mobility.

Excessive paste will produce excessive mobility and low viscidity, which results in poor strength and

durability.

Proper quantity of paste can fulfill the requirements of mobility, viscidity and capacity of water

retention.

It should be determined according to the requirements of construction.

How to adjust mobility:

Function of water:

Water is essential to hydrate Portland cement and provides Workability to concrete mixtures.

The mixture with too much water is detrimental to concrete quality.

Reasons:

Dont add water singly when the mobility of composite is to be adjusted; otherwise it will cause

segregation and deteriorate the strength and durability of concrete.Harmfulness of too much water

Too much water added to the mixture is detrimental to the quality of concrete. More water in-required to

hydrate Portland cement dilutes the paste, separates the calcium silicate gel crystals, and weakens the gel

structure. Uncombined water leaves capillary voids in the paste, which are involved in most freezing and

thawing mechanisms.

The proper method is to alter the quantity of cement paste to improve the mobility with the stability of

W/C.

(3) Sp

Definition

Sp is the ratio of sand to gravel. It determines the void and surface area of aggregates (w and c are

fixed).


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Influence

With low Sp, it lacks of mortar so it results in poor lubrication of the surface of aggregates and poor

mobility, which tends to segregate. With high Sp in large surface area, cement mortar wraps up sand and

fills paste, which results in low mobility.

Optimized Sp (Fig.4.3.4): When W and C are fixed; the Optimized Sp makes the composite gain the

highest mobility with favorable viscidity and water retention. Or, the Optimized Sp makes the composite

gain the required mobility with the least cement used.

The principle of Sp selection

Method: selecting Sp according to test and experience.

General principle: select small Sp with the guarantee that segregation will not happen and the composite

can be placed densely

Fig.4.3.4 Sketch map of optimized Sp

Select small Sp when gravel has great maximum dimension, proper size distribution and smooth surface.

Select small Sp when fine sand is used. Select small Sp when composite is thick with low w/c. Select large

Sp when composite has a high mobility avoiding segregation. Select small Sp when using admixture.

Select large Sp when there is a requirement for permeability.

How to adjust workability

Low slump increases the cement paste with fixed W/C; High slump increases the use of aggregates with

slumpAmount of cement


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fixed Sp.

(4) Aggregates

Shape and surface texture

The relationship between shape/surface texture and mobility is that: Gravel or pit sand in coarse texture

and multi-angle results in low mobility; smooth and round scree has high mobility.

Size distribution

The relationship between size distribution and mobility: Proper size distribution has small voidage (W is

certain) and high mobility; poor size distribution has large voidage (W is certain) and poor mobility.

Large particle size

The relationship between larger particle size and mobility: Large particle size has small surface area

(cement paste is certain) and high mobility.

(5) Admixture

Admixture also has effects on the mobility of the concrete composite. This problem has been discussed

in previous section.

2. Influence from environment conditions

(1) Time (Fig.4.3.5)

By hydration and vaporization, water is absorbed by aggregates and mobility decreases accordingly.

The relationship between slump and time is shown in Fig.4.3.5. Slump decreases while time increases.

Slump test should be preceded in 15 minutes after preparation of the composite in construction.

time

Fig.4.3.5 Relationship between Slump and Time

(2) Temperature (Fig.4.3.6)

Mobility decreases with temperature rising. Temperature must be noticed to get a required workability.

More mixed water in summer should be used than in winter.

slump

slump


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Fig.4.3.6 Relationship between Slump and Temperature

(3) Test condition

Test conditions also have effects on the mobility of the concrete composite. Device condition-parameter.

Concrete condition-humidity, smoothness.

4.3.6 Approach to improve workability

1. When slump is too small, improve the quantity of cement paste for the mobility with the stability of

W/C.

2. When slump is too large, improve the quantity of aggregates to decrease the mobility with the stability

of Sp.

3. Choose the optimized Sp.

4. Improve the gradation of aggregates.

5. Choose coarse aggregates if possible.

6. Use admixture.

4.4 Strength of Concrete

4.4.1 Quality of Concrete

The quality of concrete is measured by its strength, durability, and dimensional stability.

1. Strength

Hardened concrete must have sufficient strength to resist stresses from 1oads which may be imposed on

a structure. The strength must also be sufficiently high to allow for variations in concrete mixtures.

2. Durability

Concrete must be able to withstand forces of deterioration such as freezing and thawing, wetting and

drying, erosion and chemical attack.

3. Dimensional stability

Quality concrete should have a minimum of shrinkage or expansion because of either outside forces or

chemical reactions within the concrete itself.

4.4.2 Deformation under load and damage process

1. Deformation under load and damage process

(1) Damage type under compressive load;

(2) Original crack;

(3) Damage process-one-axis static compression.

2. Damage type under compressive load

There are three damage types of concrete under compressive load. Damage types, the reason and

possibility of these kinds are illustrated in Tab.4.4.1 and Fig.4.4.1.

Tab.4.4.1 Damage type under load, reason and possibility

damage type reason to cause possibility

temperature


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hardened cement damage cause by low grade cement appear often

Interface damage caused by interface cracks; appear often

coarse aggregates damage normally, frock>fcu appear seldom

Fig.4.4.1 Damage types under compressive load

3. Original crack

The interface damage is likely to happen because there are original cracks in interface.

Original cracks refer to the cracks formed in the interface of coarse aggregates and mortar before the

concrete is loaded. Sketch map of original crack is illustrated in Fig.4.4.2.

Fig.4.4.2 Sketch map of original crack

Types of original crack: dry shrinkage, cold contract, volume decrease, settlement, plastic shrinkage and

bleeding path forming.

4. Damage process under one-axis static compression

(1) Method

The relation between deformation and interior crack development can be found by mechanic test and

microscope observation.

(2) Mechanism

With load increasing, concrete deformation increases with micro-cracks appearing and developing in the

concrete until the concrete is damaged. Concrete compressive instrument is shown in Fig.4.4.3.

water package

coarse aggregates

crack

mortar


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Fig.4.4.3 Picture of concrete compressive instrument

Compression-deformation curve of concrete (deformation, ultimate load)

interface cracks develop slowly;

Interface crack increase;

cracks appear in mortar and continuous cracks appear;

continuous cracks develop quickly;

cracks develop slowly;

cracks develop quickly

5. Relation between damage process and interior crack development

The following Tab.4.4.2 illustrated the relation between damage process and interior crack development.

Tab.4.4.2 Relation between damage process and interior crack development

Period Load Interior cracks

-OAelastic limit

(30% of ultimate load)= not obvious development

-ABcritic load (70%-90% of

ultimate load)


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Tep=203, RH>90% cured in standard condition.

(3) Instruments

Instruments of strength test are shown in Fig.4.4.4.

Fig. 4.4.4 Instruments of strength test

Fig.4.4.5 Sketch map of assurance factor of concrete -P%

2. Assurance factor of concrete (Fig.4.4.5)

Assurance factor of concrete-P% refers to the probability of the strength beyond the designed strength in

strength ensemble.

3. Standard cubic compressive strength of concrete

Standard cubic compressive strength of concrete with the assurance factor of 95%.

(1) fcu,kis the strength according which to design structures

(2) fcu,kis used in quality control

(3) fcu,kis used in engineering acceptance check 1.15 fcu,k, fcu, min0.95 fcu,k

(4) mf is average value of strength.

4.4.4 Strength grade of common Concrete

Normal concrete strength is classified according to fcu,k. Method to explain strength grade and the 12

t

t 0

l


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grades of normal concrete are C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60.

4.4.5 Other strength

1. Prism compressive strength fcp

The compressive strength of 150150300mm prism is cured in standard condition in age of 28 days.

The relations of Prism compressive strength and compressive strength are:

fcp=0.70.8fcu

2. Tensile strength

Tensile strength is tested through split tensile test.

(1) Principle

Fig.4.4.6 is the ketch map of split tensile test. Apply linear load

distributed evenly on two opposite surfaces, thus the tension

stress will be produced on the vertical surface affected by the

external force. This force will be calculated by the elastic theory.

(2) Formula

Split tensile test strength-fts

Pdamage load, N;

Asplit area of specimen, mm.

3. Bending strength (Fig.4.4.7)

Middle third point loading experiment test;

Specimen: 150150600(550) beam specimen;

Formula:

4.4.6 Influencing factors

1. Strength grade of cement

Strength grade of cement are important to the strength of concrete.

When the mix proportion of concrete is fixed, the higher the cement grade, the higher the strength ofhardened cement paste, binding with aggregate and the aggregate strength. The prerequisite of the above

rules is the density of the concrete.

2. W/C

The concrete strength is determined by W/C, when the types and strength of cement are the same.

(1) If W/C in a certain range(concrete density)drops, the strength will climb up.

(2) If excessively small W/C (concrete non-density) W/C drops, voidage will increases and the strength

drops.

A

P

A

pfts 637.0

2

2bh

PLfm

Fig.4.4.6 Sketch map of split tensile test

Fig.4.4.7 Sketch map of bending test


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The water needed in normal hydration is 23% of the cement (W/C=0.23). However, in order to have the

excellent mobility for the concrete composite, the water should be 40%~70% of the cement (W/C=0.4 ~

0.7).

The extra water leaves lots of pores in the concrete, thus reduces the actual loaded area, concentrates the

stress, and decreases the strength of the concrete.

Velocity mixing, ultrasonic vibration, high frequency vibration, multi frequency vibration used in the

current project will lessen the mixing water, exclude air, improve the density and strength of the concrete.

3. Strength formula of the concrete

W/C0.30.8

fcu-compressive strength of concrete in the age of 28 days (MPa);

fce-actual compressive strength of cement in the age of 28 days (MPa);

b

cef -compressive strength of cement in the age of 28d;

Kcextra-coefficient of cement (1.13);A, Bempirical coefficient, related to the types of aggregate;

gravel A =0.46 B=0.07

scree A =0.48 B=0.33

Application range: plastic concrete and low plastic concrete4. Factors of aggregate

The strength of coarse aggregate, particle size and gradation are important to the concrete strength.

(1) When the aggregate strength is high, the crack expands and passes over the interface between aggregate

and cement, thus the concrete strength is increased.

(2) surface texture: W/C0.65, no effect; W/C0.4, fcu=1.38fcu,k

(3) DMAXhas smaller effect on ordinary concrete. When HSC and DMAXgoes up, strength will drop (effect

of dimension)

Obviously aggregates play an important role in producing strength in concrete. The shearing strength of

an aggregate particle itself may control the strength across a shear plan when the bond between the

aggregate and the paste is strong enough to force the shear plane through the aggregate particles rather thanaround them. This is usually true in lightweight aggregate concretes.

a. surface texture

Angular particles and rough surface textures will generally produce higher-strength concrete with the

same quality of paste (equal water-cement ratio) but require a higher paste content. The gel structure of

hydrated Portland cement tends to permeate a rough, porous surface of an aggregate particle and creates

better bond. Angular aggregate particles will also create greater aggregate interlock and force shearing

stresses through the aggregate particles.

b. particle shape

Since the hardened cement paste provides stability, there is no special requirement for crushedaggregates like in asphaltic concrete pavements. Rounded aggregates have a lower void content requiring

BW

CAff cecu

b

cecc fKf


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less paste and less cement for a given water-cement ratio. Rounded aggregates also produce more

workable concrete.

The advantages and disadvantages of crushed and rounded aggregates tend to balance out. It has been

found that in concrete mixes containing an equal amount of cement, the strength is also nearly equal,

regardless of whether crushed or rounded aggregates are used. The lower water-cement ratio obtained

when rounded aggregate is used balances the better strength-producing properties achieved when crushed

aggregate is used.

c. maximum size of aggregate

There has long been consensus that the largest size of aggregate practicable should be used to produce

quality concrete. The larger the maximum size of aggregate, the fewer voids are left to fill with paste. This

is important in the construction of concrete dams and other massive structures where the heat of hydration

cannot readily escape and the thermal coefficient of expansion becomes a problem in dimensional stability.

With large-size aggregates, a minimum amount of cement is required for high-quality, economical

concrete.

d. aggregate-paste bond

It is possible to produce concrete with higher strength when smal1er maximum-size aggregates are used

because of the increase in total bonding surface of the aggregate particles. For example, the surface area of

an equal amount of 3/4 aggregate is about double that of 11/2 aggregate, if we assume that the aggregate

particles are spheres.

In a given plane of shear, therefore, the 3/4 aggregate will have about 2 times as much bonding surface

as the 11/2 aggregate.

The maximum size of aggregate has the influence on the strength of concrete. In this case, the maximum

strength that a given concrete can reach is limited by the aggregate size. Larger-size aggregates have the

highest strength when the quality of paste controls. As the bond strength between the paste and theaggregate particles begins to control, the concretes having smaller maximum-size aggregate produce

higher strengths.

5. Curing condition

The concrete strength is influenced by the degree and speed of cement hydration, which is affected by

the humidity and temperature. Higher the temperature is, faster the speed of cement hydration is, higher the

concrete strength is. Larger the humidity is, higher degree of cement hydration is. Relationship among

strength, temperature and damping age of concrete are shown in flash, Fig.4.4.8 and Fig.4.4.9.

Fig.4.4.8 Influence of temperature to strength

10

0

age

20

30

40

non-frozen

frozen after 7 days

frozen after 1 day


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Fig.4.4.9 Relationship between strength and damping age of concrete

The fixed temperature and relative humility in a certain time will guarantee the normal development of

strength and other performances of concrete after the formation of concrete. There are three types of curing:

Natural cure, steam cure and autoclaved cure.

(1) Natural cure

Cured at the natural temperature and in the natural condition.

P.P.P.OP.S should be cured in more than 7 days;

P.P and P.F should be cured in more than 14 days;

The aluminous cement should be cured in more than 3 days.

(2) Steam cure

Steam cure:pressure is1 standard atmospheric pressure; temperature>100.The steam cure can improve Portland cement with mineral admixture for 1040% in the age of 28

days;

P.P.and P.O can reduce 1015%.

(3) Autoclaved cure

Pressure8 standard

atmospheric pressure;

Temperature > 174.5 ;

Place: autoclave.

(4) ageFormula:f28=fn (lg28/lgn)f28=fn (log28/logn)fnn days strength of concrete

f2828 days strength of concreteApplication range:

Cured in standard condition; 32.542.5 grade P.O (n>3)

6. Testing conditions

140

100

9 365Age (d)

20

0

40

60

80

120

Damped for a long time

Damped for 14 days

Damped for 7 days

Damped for 1 day

f

Mpa

7d 28dage

Fig.4.4.10 The relations of compressive strength and age

f28(%)


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The shapes, dimensions and surface texture of specimen and experimental instrument and operation are

important to the concrete strength.

(1) Hoop effect

When the concrete specimen is suffered by the axis compression, the traverse deformation of

compression panel is smaller than that of the concrete. Thus the contacting surface between specimen and

compression panel is influenced by the centric constraint, which is effective in the range of a2/3 and

improve the concrete strength. After the damage of the specimen, a complete pyramid is formed upper and

below.

(2) Size effect

Effects of size to concrete strength are listed in Tab.4.4.3.

Tab.4.4.3 Effects of size to concrete strength

aggregate Dmaxmm size of specimenmmhoop effect

strengthconversion coefficient

60

40

31.5

200

150

100

feebleness low

strong high

1.05

1.00

0.95

Size of specimen 3Dmax

7. Influence of Admixtures on Strength

Admixtures influence the strength of concrete in different ways. Air-entraining agents reduce strength;

water-reducing admixtures increase strength by permitting a reduction in the water-cement ratio.

Accelerating agents produce higher early strength, and retarding agents delay the initial strength gain.

(1)Air-entraining Agents

As the gel structure of Portland cement paste becomes more porous, its resistance to stress decreases.

Air voids between the surface of aggregate and the gel structure decrease the bond. The influence of

entrained air on concrete strength is shown in Fig.4.4.11. This figure indicates that where the water-cement

ratio is the same, entrained air will reduce the strength of concrete about 5 percent for each percent of

entrained air.

Fig.4.4.11 Influence of entrained air on the strength of concrete having the same w/e ratio.

Entrained air increases the workability of concrete, which permit a reduction in water requirement. This


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increases the strength for a given amount of cement.

For lower cement contents, entrained air actually increases the strength of concrete. For high-strength

concrete, however, entrained air causes a significant loss in strength.

(2)Water-reducing Admixtures

Water-reducing admixtures are widely accepted as a means of increasing slump and workability without

sacrificing strength and qua1ity. A second option is to reduce the water content and water-cement ratio

without decreasing the slump or workability.

For example, assume the water content is reduced by 10percent. In a typical mix, a 10 percent reduction

in water will lower the water-cement ratio from 55 percent by weight of cement to 50 percent. From

Fig.4.4.12, this indicates an increase in strength of 4MPa.

Fig.3.2 Typical W/C ratio-strength curves.

(3)Pozzolans

When pozzolans are added to a concrete mix, they add another cementing material which combines with

lime and soluble alkalies. When a suitable pozzolan is used as a partial replacement for Portland cement,

the early strength of the concrete is reduced, but the strength of concrete at later ages, when the pozzolanic

action is, complete, is not significantly changed.

When pozzolans are added, in addition to the regular amount of Portland cement, they increase the

strength of concrete; in some cases, very high strength concrete has been produced.

Accelerating and Retarding Admixture

As their name implies, accelerating and retarding admixtures either accelerate or retard the hydration

reaction of Portland cement.

(4) Accelerating agent

Calcium chloride is the most common accelerating agent. The major advantage of accelerating agents is

to speed up the hydration and setting of cement in cold weather, which produces reasonable finishing times

and reduces the time of protection from freezing. Calcium chloride cannot be used in prestressed concrete

and should not be used in any concrete where corrosion of reinforcing steel is critical.

(5)Retarding admixtures

Retarding admixtures are useful in hot weather to keep concrete plastic and prevent setting and cold

jointsbefore successive layers of concrete can be placed. Retarding agents do not decrease slump loss of

fresh concrete, nor do they reduce the strength gain of concrete after set has taken place. Many water-

reducing admixtures are also set-retarding admixtures.

4.4.7 Approaches to improve strength


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1. High-grade cement and quick hardening and high early strength cement

2. Using dry and hard concrete makes porosity decrease and density improved, fierce vibration, ordinary

concrete strength.

3. Hydrothermal treatment can improve efficiency, save space and increase strength.

4. Employing the mechanic mixing, vibration, strong mixing and high frequency vibration

5. Blending agent and mineral admixture; predicting the concrete development; producing HSC, HPC and

so on.

4.5 Durability of Concrete

4.5.1 Definition

It refers to that concrete can resist the influence of exterior corrosive substance and maintain good

usability and complete appearance so that it can maintain the safety and usability of the structure. That is

to say that concrete can maintain stable quality after being used for a long time.

It includes:

(1) Anti-permeability;

(2) Anti-freezing;

(3) Anti-corrosion;

(4) Anti-carbonate;

(5) Anti-abrasion;

(6) Resistance to alkali-aggregate reaction and so on.

4.5.2 Importance of Durability

Concrete as a suitable structural material must be able to withstand the forces of deterioration. The most

important of these include freezing and thawing action; chemical attack; abrasion; alkali-aggregate

reaction.

In cold climates the most serious and persistent deterioration force in concrete is freezing and thawing.

When water freezes into ice, it expands about 9 percent of its original volume and exerts tremendous force

on any confining vessel. In concrete, water-filled voids are subject to these pressures. Hydrated Portland

cement gel contains voids of different sizes.

The loss of life and properties caused by sudden early destruction of material should be paid much

attention to.

Examples of deteriorate of concrete in engineering are shown in Fig.4.5.1.

Fig.4.5.1 Deteriorate of concrete in engineering


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The costs of structure repair and the maintenance caused by material destruction account much

proportion in the whole construction budget. 40% of the construction investment in developed countries is

spent on repair and maintenance of the existing structure while the rest goes to the new structure

(Fig.4.5.2).

Fig.4.5.2 Percent of the constructure investment

4.5.3 Anti-permeability

1. Definition

It refers to the ability to resist the permeability from compressive water. It is one of the most important

symbols of durability. It influences the anti-freezing and anti-corrosion of the concrete directly. Instrument

in anti-permeability test is shown in Fig.4.5.3.

Fig.4.5.3 Instrument in anti-permeability test

2. Void system in hardened concrete

Several types of voids occur in concrete and are classified according to size and origins:

(1) Gel pores. Gel pores are the interstitial cavities among the hydration products of Portland cement. Gel

pores are estimated to be about 1.5 to 2.0 nm in diameter.

(2) Capillary cavities. Capillary cavities are estimated to average about 500 nm in diameter and are usually

formed by excess uncombined water not required for hydration.

(3) Entrained air. When air-entraining agents are used, billions of tiny spheres of air are introduced into the

gel structure. These are many times larger than capillary voids and will vary from lm to l mm or more in


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2. The following mechanisms have been suggested as being contributing factors in freezing and thawing

deterioration.

(1) Critical saturation

Unless a void is filled, freezing water can expand and fill the void. The critical saturation point is above

91 percent since water will expand about 9 percent upon freezing.

(2) Osmosis

The gel structure may be considered a porous membrane, and any void containing a solution of water

which has a greater concentration of alkalies and sa1ts will draw water from the surrounding gel structure.

Moisture may also be drawn from the soil or water in contact with the concrete, which increases the

likelihood of critical saturation. Forces resulting from osmosis are in an opposite direction to the expansion

forces of freezing water and therefore add to the pressures exerted in the gel structure.

(3) Hydraulic pressure

Water does not freeze instantaneously, but ice crystals start forming within the solution. As freezing

progresses, the ice crystals grow larger until they completely fill a void. During the freezing process in

capillaries of hardened cement, the frozen water expands and forces the unfrozen water into the

surrounding gel structure. Water in the gel pores will not freeze because the gel pores are so small that ice

crystals cannot form and the gel-pore water exists as super-cooled water. The pressure will build, unless

there is flow away from the void, and the gel structure wil1be destroyed.

The comparatively large entrained-air voids provide an escape for the water pressure. The pressure

developed in the gel structure will depend on:

The coefficient of permeability of the gel structure;

The distance from the capillary void to the air void boundary;

The rate of freezing.

When the entrained-air voids in concrete are spaced sufficiently close, the hydraulic pressure

mechanism does not operate.

(4) Diffusion

When water freezes in a capillary, its energy level is reduced. The surrounding gel water has a higher

energy level and will migrate to the frozen ice crystal where it, too, will freeze. In soils this is referred to as

frost heaveand will continue as long as unfrozen water is available. In concrete, ice lenses will form and

fracture the concrete.

Any water drawn from the gel structure by diffusion will cause shrinkage and, if near the surface, may

result in fine, hair1ine cracks.

Diffusion is particularly damaging when it occurs near a pavement surface. If the surface of a pavement

is compacted by poor, premature finishing methods or by rapid drying, any subsequent excess mixing

water that bleeds to the surface wi1l be trapped below the compacted surface and create an abundance of

capillary voids. When these voids are filled with frozen water, diffusion of the gel water is often sufficient

to force the compacted surface off the pavement. This is known as scaling. Hairline cracks which form

open channels for surface moisture to now to the ice crystals may also contribute to scaling.

(5) Deicing salts

The use of salts to melt the snow and ice on concrete pavements develops other mechanisms of concrete

deterioration.


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Lowers temperature of concrete. Snow and ice melting in a salt solution draws heat from a concrete

pavement and may lower the temperature of the concrete significantly.

Provides a supply of water for diffusion to capillary ice.

Increases the concentration of salt in capillary water which increases saturation and stress through

osmosis.

Makes eutectic expansion possible.

(6) Eutectic expansion

A second expansion in sa1t solutions may cause damage such as scaling and deterioration of concrete

and explains some conflicting test results.

With relative dilute solutions of salt, the phase diagram of water-salt solutions shows the relative

freezing point of solutions with various percentages of salt. In weak salt solutions, the water freezes first

and gradually concentrates the salt solution until it reaches the eutectic concentration of about 23 percent,

at which time it freezes at a temperature of about -20 C. If the eutectic concentration of salt solution

freezes in a capillary, the hydraulic pressure mechanism operates and can be overcome with entrained air.

In more dilute solutions, however, the water freezes solid, leaving pockets of unfrozen salt solution. When

the temperature reaches -20 C, the remaining salt solution freezes. The resulting expansion occurs in solid

ice, so that the capi1lary ice expands as a unit. This secondary expansion may cause deterioration in

concrete otherwise protected, including air-entrained concrete, and will possibly explain unanswered

problems in concrete scaling.

Recommendations to avoid freezing and thawing deterioration

Keep concrete from becoming saturated. Unless the capillary voids are filled, freezing and thawing

mechanisms will not operate.

(a) Good drainage.

(b) Protective coatings.

Reduce the number of capillary voids by using a low water-cement ratio.

Entrain about 5 percent air in the concrete.

Avoid compaction of pavement surface by finishing before all bleed water has come to the surface.

Prevent compaction of surface by rapid drying of fresh concrete.

Prevent exposure to freezing and thawing action while concrete is immature and contains uncombined

water.

Avoid the use of deicing salts.

(7) Chemical attack

Acids


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The cement paste of concrete is composed of several hydration products containing calcium. These

compounds will react with any acid; as a result, the concrete will disintegrate. Common types of acid

attack come from food, processing plants, and certain types of sewage. Lactic and acidic acids found in the

dairy and food, processing industries may show a mild attack on concrete and, over a long period of time,

cause deterioration (Tab.4.5.1).

Tab.4.5.1 Effect of commonly used chemicals on concrete

Rate of attack

at ambient

temperature

Inorganic

acids

Organic

acids

Alkaline

solutionsSalt solutions Miscellaneous

Rapid

Hydrochloric

Hydrofluoric

Nitric Sulfuric

Acetic

Formic

Lactic

Aluminum chloride

Moderate Phosphoric Tannic

Sodiumhydroxide

20

percent

Ammonium nitrate

Ammonium sulfate

Sodium sulfate

Magnesium sulfate

Calcium sulfate

Bromine (gas)

Sulfite Liquor

Slow Carbonic

Sodiumhydroxide

10 to 20

percentSodium

hypochlorite

Ammonium chloride

Magnesium chloride

Sodium cyanide

Chloride (gas)

Seawater Soft

Water

Negligible Oxalic

Tartaric

Sodiumhydroxide

10 percent

Sodium

hypochlorite

Ammonium

hydroxide

Calcium chloride

Sodium chloride Zinc

nitrate

Sodium chromate

Ammonia

(liquid)

Sewage

Concrete pipe is used extensively in the conveyance of domestic sewage without appreciable

deterioration. Hydrogen sulfide gas (H2S) may be formed under certain conditions by decomposition of

sulfur compounds by bacteria. If this gas combines with oxygen and condenses on the surface of exposed

concrete, it will change to sulfuric add, which will attack the concrete. Certain types of industrial sewage

containing acids or sulfates will cause deterioration of concrete.

Sulfate attack

Alkali soils and drainage water in certain regions and, to a lesser extent, seawater contains magnesium

and sodium sulfate and other salts. These chemicals, particularly magnesium salts, will react with hydrated


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calcium aluminate in hardened concrete and produce ca1cium sulfoaluminates. This cause expansion and

disintegration of the hardened cement paste.

The severity of sulfate attack depends on the percentage of tricalcium aluminate (C3A) in the cement

and the concentration of the sulfate salts. Specifications for sulfate-resisting cements limit the percentage

of tricalcium aluminate.

3. Freezing resistance grade

It can be expressed by circle times of the ultimate freezing and thawing which specimen can stand after

saturation in 28 days. The specimen should stand continuous freezing and thawing. The decreasing of

strength should not be more than 25% and the mass loss should not be more than 5%.

For example: F25, F50, F100, F300

Selection: Selection should be made with considerations of climate, environment and building types.

4. Important factors determining the anti-freezing

(1) Density;

(2) Pore quantity and structure;

(3) Pores water filling.

4.5.5 Carbonation /Neutralization

When calcium hydroxide, Ca(OH)2, is exposed to the air, it will react with CO2 and form CaCO3.

Calcium hydroxide is one of the hydration products of Portland cement and is available to react with CO2

in the atmosphere. This process occurs slowly and is not usually important in hardened concrete with the

possible exception of dimensional instability of lightweight masonry units.

Carbonation can be a serious problem when freshly placed concrete floors are exposed to excessive

concentrations of CO2. This problem occurs frequently when open-flame space heaters, used to prevent

freezing, exhaust the oxygen in the air and leave excessive concentrations of CO2. This produces a soft

inferior layer on the surface of the concrete.

1. Major factors influencing carbonation:

(1) Types of cement (quicker carbonation with mineral admixture);

(2) W/C;

(3) Time of carbonation;

(4) Humidity;

(5) Concentration of CO2;

(6) Admixture.

2. Influences on concrete performance

(1) Reducing concretes alkalinity;

(2) Lessening the protection to reinforcement;

(3) Improving concrete shrinkage;

(4) Leading to minor crack;

(5) Improving compressive strength (taking resilience method into consideration);

(6) Fill the pore with CaCO3 to produce water after carbonation, favorably cement hydrate.

3. Instrument in carbonation test (Fig.4.5.5)

OHCaCOOHCOOHCa 23222 2)(


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Fig.4.5.5 Instrument in carbonation test

In short, the disadvantage of carbonation overweighs its advantage, so carbonation speed should be

slowed down.

4.5.6 Alkali-aggregate Reaction

OnHSiOONaOnHSiOONa 222222

The volume of sodium silicate hydrate gel is over 3 times than that of the admixture, leading to

expending crack.

Reaction conditions:

(1) Cement with high alkali content;

(2) Active aggregate;

(3) Water.

4.5.7 Steps to Improve Durability

1. Selecting cement appropriately;

2. Controlling the maximum W/C and the minimum quantity of cement;

3. Selecting appropriate aggregate and admixture etc.;4. Guaranteeing construction quality.

4.6 Dimensional StabilityThe volume and dimensions of hardened concrete will change slightly for the following reasons:

1. Elastic deformation (strain) resulting from stress

2. Creep or permanent deformation because of stress

3. Shrinkage of plastic and hardened concrete

4. Thermal volume change

5. Expansion because of chemical reactions

1. Elastic and Plastic Deformation.

In the initial stages of applying a compression load to a concrete cylinder, it exhibits elastic properties;

that is, it will deform under load and return to its original length when the load is released. As the stress

approaches the ultimate strength of the concrete, strain increases more for a given stress and the

stress-strain relationship is no longer a straight line, but shows increasing curvature. When the load is

released, part of the deformation is recovered. The permanent deformation is called creep.

Modulus of Elasticity. The modulus of elasticity is defined as the ratio of stress over strain, and in

elastic materials it is a straight-line relationship,

E=stress/strainIn determining E for concrete, strain is usually measured at zero load and at some load well below the


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ultimate strength of the concrete. This gives the secant modulus of elasticity, which is generally used in

design. The chord will indicate a lower modulus, and the initial tangent will indicate a higher modulus.

The modulus of elasticity is important in reinforced-concrete design, since deflection of beams and

floors is inversely proportional to the modulus of elasticity:

D=PL3/48EI

E = modulus of elasticity

P= applied center load

D=deflection

L=distance between simple supports

I=moment of inertia

Since deflection controls many design considerations, the modulus of elasticity is of first importance.

The modulus of elasticity of normal structural concrete varies with the strength and can be estimated from

the following ACI formula

'5.1

33 cc fWE

Ec= modulus of elasticity of concrete

W= density of concrete (unit weight, lb/ft3)

'

cf = strength of concrete

2. Creep.

When concrete is loaded, elastic deformations and some nonelastic deformations take place. If the load

is maintained, the nonelastic deformation or creep continues for long periods (Fig.4.6.1).

Fig.4.6.1 Elastic and creep deformation of mass concrete under constant load

followed by load removal

Creep occurs because of the shifting of the internal structure of the cement gel, movement of moisture,

and possibly the adjustments in the aggregate-cement bond. Creep is desirable in some cases since it tends

to relieve stress concentrations. Weaker concrete will deform, shifting stresses to stronger concrete or to

reinforcing steel.

A portion of inelastic deformation is permanent and must be considered in the design of concrete


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structural members since all structures have a permanent dead load.

In prestressed concrete a camber, with the center of the member slightly higher than ends, is designed

into the member so that when creep and elastic deformation of concrete and steel cables take place, the

member will be level.

3. Shrinkage.

(1)Plastic shrinkage.

Freshly placed concrete will decrease in volume as the solid particles settle and water bleeds to the

surface. When there is excessive evaporation the surface will shrink because of loss of moisture and

compaction. Plastic shrinkage cracks will develop. This usually occurs before final finishing. These cracks

can be avoided by preventing rapid evaporation by protection from sun and wind or by covering with

plastic sheets or a monomolecular film.

(2)Drying shrinkage.

After the concrete has hardened, drying shrinkage will take place principally by the contraction of the

gel structure as moisture leaves. Concrete shrinkage takes place in the hardened paste fraction and has been

found to be a function of the water content and water-cement ratio of a concrete mix.

Since drying shrinkage is an inherent property of concrete, it must be restrained with reinforcing steel or

controlled by contraction joints. A contraction joint or deep groove every 3 m will prevent intermediate

cracking in concrete slabs, which are relatively free to move on a subgrade.

4. Thermal Volume Change.

The coefficient of thermal expansion averages about 1410-6 cm/ C. This mount of expansion or

contraction with changes in temperature is not usually significant, since it is compatible with reinforcing

steel and readily dissipates in a structural member.

Severe problems develop in massive structures, however; where heat cannot be dissipated, thermal

contraction on the surface without a corresponding change in the interior will cause cracking. In this type

of structure, the interior temperature rise; resulting from heat of hydration must be within acceptable limits

from e the mean ambient temperature, or the interior of the concrete block must be cooled with cooling

coils to eliminate incompatible temperature differentials.

5. Chemical Reactions.

Alkali-aggregate reaction causes expansion in concrete because of the formation of unstable silica gel.

This expansion is localized around individual aggregate particles. When the expansion stresses exceed the

tensile strength of the gel structure, cracks occur and the structure is damaged. The volume of the concrete

increases; but more importantly, internal stresses develop.

Certain carbonate rocks will also react with alkalies, causing damage. This type of reaction has not been

as extensive or severe but can cause problems.

4.7 Mix Proportion Design of Concrete

4.7.1 Mix proportion

It is expressed by relative ingredients quantities of concrete.

Representation method:

(1)Expressed by ingredients quantities of 1m3concrete.

C S G W (Kg/m)

300 720 1200 180


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(2)Expressed by relative ingredients quantities of concrete (the cement quantity is 1)

C:S:G=1:2.4:4.0 W/C=0.6

Mix proportion can be expressed by the percentage of cement weight if admixture is blended.

4.7.2 Tasks of mix proportion design

1. Picking out the appropriate raw material according to technical properties, structure and construction.

2. Ascertaining the required technical economic index.

3. Ascertaining the quantity of each material.

4.7.3 Basic requirements of mix proportion design

1. Strengthconformed with structure design

2. Feasibilityconformed with construction condition

3. Durabilityconformed with engineering environment

4. Economical

4.7.4 Principle of mix proportion design of concrete

The basic theory of mix proportion design is based on the change rule of concrete performance. Normal

concrete mix proportion has four basic variables: C, W, S, G.

Three kinds of proportion:

1. The ratio of water to cement;

2. The ratio of sand to gravel;

3. The quality of water in 1m3concrete- ration of cement paste to aggregates.

4.7.5 Steps and methods

Four steps:

Design of preliminary mix;

Ascertaining the basic mix proportion;

Laboratory mix proportion;

Working mix proportion.

1. Design of preliminary mix

(1) Determining the produce strength of concrete

According to the specification for mix proportion design of ordinary concrete (JGJ55-2000), the produce

strength of concrete should be determined by the following formula:

fcu0- the produce strength of concrete, MPa;

fcuk- designed cubic standard compression strength of concrete;

645.1,, kcuocu ff


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- standard deviation of concrete strength.

If statistical data is not available, 0can be calculated according to the following formula:

If statistical data is not available, select0according to the table below (Tab.4.7.1):

Tab.4.7.1 Selection of

Concrete strength grade Less than C20 C20C35 Higher than C35

4.0 5.0 6.0

(2) Selection of water-cement ratio

according to strength ratio:

13.1c

according to durability:

To meet the demands of durability, the calculated W/C should not excess the value in following table

(Tab.4.7.2).

Tab.4.7.2 Maximum of W/C and minimum of cement

nvironment condition Structure type

Maximum of w/c Minimum of cement, kg

ConcreteReinforced

concrete

Pre-

stressed

concrete

Con

crete

Reinforced

concrete

Pre-

stressed

concrete

Dry environment Normal residences and offices - 0.65 0.60 200 260 300

Damp

vironment

Without

thawing

High humidity room; Outside

the house; in the earth and water

(non-corrosive)

0.70 0.60 0.60 225 280 300

1

1

22

,

0n

fnf

n

icuicu

)/(0, BWCAff cecu

0,cuce

ce

fABfAf

CW

b

cecce frf


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Thawing

Exterior members under

thawing; member under thawing

in the earth and water

(non-corrosive)

Interior members under thawingin a High humidity

0.55 0.55 0.55 250 280 300

amp environment with

thawing and deice

Exterior and interior members

with thawing and deice0.50 0.50 0.50 300 300 300

(3) Estimating the quantity of mixing water W0.

According to slump, coarse aggregate, maximum particle diameter and mixing water can be

estimated from the following table (Tab.4.7.3)

Tab.4.7.3 Mixed water in stiff concrete and plastic concrete

thickness of concrete

compositemaximum size of gravel (mm) maximum size of crushed stone (mm)

items indexes 10 20 40 16 20 40

thickness

S

1520

1015

510

175

180

185

160

165

170

145

150

155

180

185

190

170

175

180

155

160

165

slump

mm

1030

3050

5070

7090

190

200

210

215

170

180

190

195

150

160

170

175

200

210

220

230

185

195

205

215

165

175

185

195

Mixed water quantity is a constant to meet the requirement of mobility in the certain range of W/C

quantities, when the materials are fixed.

The principle means that different W/C will produce different strength concrete with common mobility

when the water quantity is the same.

(4) Calculating cement quality in 1m3concrete

Cement quality can be calculated from the following equation:

In addition, the calculated C0should be no less than the value in the table before for durability demands.

(5) Selecting proper ratio of sand to gravel, Sp.

Calculating method

WCWC /00

ogos

osp

pS

'''

''


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Table

Sp can be selected from the following Tab.4.6.4

Testing Method

Select Sp by slump test results

(6) Calculating volume of sand (S0) and gravel (G0).

There are two methods to calculate S0and G0:

Absolute volume

Assume an apparent density

The first method is that the volume of concrete composite equals to the whole volume of ingredients.

Tab.4.7.4 Selection of Sp%

W/CMaximum size of crushed stone,mm Maximum size of gravel, mmmm

16 20 40 10 20 40

0.40

0.50

0.60

0.70

3035

3338

3641

3944

2934

3237

3540

3843

2732

3035

3338

3641

2632

3035

3338

3641

2531

2934

3237

3540

2430

2833

3136

3439

C0, S

0, G

0, W

0-quantities of each gradient in 1concrete composite;

0s, 0g- apparent density of sand and gravel;

- quantity of entrained air; and if there is no entrained air agent, =1.

The other method is to assume an apparent densityof concrete composite at first. Then,

When C0, S0, G0, W0are determined through the steps above (We call it primary proportion), there are

still some work to make the last proportion of concrete.

At first, use the primary proportion to make the concrete composite, and evaluate feasibility of it. Adjust

proportion according the following table.

pSGS

S

00

0

ohWSGC 0000

%%10000

0pS

GS

S

LaWSGC

wasagC

1000100000

8/10/2019

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