Designing, Specifying & Constructing with Modern Concrete

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Understanding Concrete

• PORTLAND CEMENT + H2O + ROCK = HARD CONCRETE + ENERGY (HEAT)

• Cement only a component of concrete and concrete is structural material


Objectives • What is concrete and how it work? • Basic chemistry of concrete • Analyze the strengths and weaknesses of

concrete• Basic concrete design • Extreme weather concreting • Analyze development and prevention of

cracks in concrete• Precautions while working with concrete


Concrete as a Structural Material

Advantages of concrete over other construction materials

Economical

Long life

Low maintenance

Unlikely to rot, corrode, or decay

Fire-safe and able withstand high temperatures

High fire resistance

Useful for storm shelters

Resistant to wind, water, rodents, and insects

Ability to be molded or cast into almost any desired shape

Molds and casting can occur on the work-site which reduces costs


Aggregates • Plays an important role in determining properties of concrete

• Occupies 75% of the volume of concrete

• Chemically inert, solid bodies held together by the cement

• Come in various shapes, sizes

• Materials ranging from fine particles of sand to large, coarse rocks

• Keep the cost of the concrete low

• Two basic types:–Corse: crushed rock, gravel or screenings–Fine: fine and coarse sands and crusher fines

• Selection depends upon required characteristics of concrete


Special Aggregates• Classified by specific gravities : • Lightweight - High internal porosity. Most of these materials are synthetic. Clays, shale,

or slates will bloat at high temperatures resulting in an expansion in volume. Lightweight aggregates have high absorption capacity associated with their high porosity. Some materials have a coating resulting from the fusion process. May get damaged during handling resulting in an abrupt increase in absorption

• Normal-weight• Heavy-weight - A material with a high specific gravity. Mostly used for radiation

shielding and application where a high mass-to-volume ratio required

Source: http://home.howstuffworks.com/home-improvement/remodeling/concrete-countertop4.htm


Aggregates With Specific Properties:

• Abrasion and Skid-Resistant Aggregates - Hard, dense aggregates used in heavy-industry applications where high resistance to abrasion is required. The strength of the cement paste and the cement-aggregate bond are more important than the aggregate hardness

• Marginal Aggregates - Require more care and thought in design, and generally more costly. Four areas of interest:

Concrete properties

Weaknesses of aggregate

Beneficiation

Use of protective measures


EN 197: Classification of Cement

Type Name Properties

I Portland cementComprising Portland cement and up to 5% of minor additional constituents

IIPortland-composite cement

Portland cement and up to 35% of other single constituents

III Blastfurnace cementPortland cement and higher percentages of blastfurnace slag

IV Pozzolanic cementComprising Portland cement and higher percentages of pozzolana

V Composite cementComprising Portland cement and higher percentages of blastfurnace slag and pozzolana or fly ash


Cement Manufacturing Process

Common raw material used : • common sources of calcium in cement:

– Limestone, marl, and chalk

• Common sources of silicon: – Clay, sand, and shale, certain waste products, such as fly ash

• Iron, aluminum recycled metals can also be used • About 5% of cement by weight is gypsum, a common

calcium- and sulfur-based mineral


Cement Manufacturing Process Continued…

Cement Compound

Weight Percentage

Chemical Formula

Tricalcium silicate 50 % Ca3SiO5 or 3CaO.SiO2

Dicalcium silicate 25 % Ca2SiO4 or 2CaO.SiO2

Tricalcium aluminate

10 % Ca3Al2O6 or 3CaO .Al2O3

Tetracalcium aluminoferrite

10 % Ca4Al2Fe10 or 4CaO.Al2O3

.Fe2O3

Gypsum 5 % CaSO4.2H2O

• Composition of Portland cement :


Cement Manufacturing Clinker


Cement Manufacturing Process Continued…

PRODUCTION OF PORTLAND CEMENT

1. LIMESTONE 2. CLAY/SHALE

CRUSHED LIMESTONE + CLAY/SHALE

MIXTURE IS HEATED IN A KILN

GYPSUM IS ADDED AND THE MIXTURE IS

PORTLAND CEMENT

Raw materials

QUARRYING PROCESS

CRUSHING PROCESS

CLINKER

ARE MIXED AND GROUND TOGETHER

GROUND TO A POWDER RESULTING IN


Heat of Hydration


Concrete Proportioning

• For strong, durable concrete rests in careful proportioning and mixing of ingredients

Ingredient Proportion

Cement 10-15%

Sand & Aggregates 60-75%

Water 15-20%

Air 5-7%

Admixtures <1 -3% (varies with requirement)


Permeability

• Should be controlled in order to prevent corrosion of reinforced steel bars

•Porosity

Expressed in terms of percentage by volume of concrete

Matrix pore structure that ultimately affects its permeability, sorptivity, and diffusivity

Size, distribution, interconnectivity, shape, and tortuosity of pores are all determining factors in overall permeability of a concrete matrix


Reinforced Cement Concrete

Low tensile strength

High compressive strength

Normal concrete has:• HIGH compressive strength• VERY LOW tensile strength• VERY LOW shear strength

Reinforced Concrete has:• VERY HIGH compressive strength• VERY HIGH tensile strength• VERY HIGH shear strength


Cracking in Concrete • Types of Cracks

Pre-setting cracks that happen before concrete hardens, while it is still workable

Hardened Cracking Cracks which happen after concrete hardens

• Types of pre-setting cracks :

• Formwork Movement

Form during placing, compaction and finishing caused by movement of concrete before it is dry

Develop cracks from sudden contraction

• Thermal Shock

Presetting Cracks


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Engineering

Designing, Specifying & Constructing with Modern Concrete