CONCRETE

What is Concrete?• Concrete is one of the most commonly used

building materials.• Concrete is a composite material made from

several readily available constituents (aggregates, sand, cement, water).

• Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape.

The word concrete comes from the Latin word "concretus"

(meaning compact or condensed).

The first major concrete users were the Egyptians in around

2,500 BC and the Romans from 300 BC

• OPUS CAEMENTICIUM• LAYING BARE ON TOMB

• ROMAN PANTHENON

Opus caementicium laying bare on a tomb near Rome. In contrast to modern concrete structures, the concrete walls of Roman buildings were covered, usually with brick or stone.`

Outer view of the Roman Pantheon, still the largest unreinforced solid concrete dome to this day.

• There are various types of concrete for different applications that are created by changing the proportions of the main ingredients.

• The mix design depends on the type of structure being built, how the concrete will be mixed and delivered, and how it will be placed to form the structure.

Examples include:

• Regular concrete• Pre-Mixed concrete• High-strength concrete• Stamped concrete• High-Performance concrete• UHPC (Ultra-High Performance Concrete)

TYPES OF CONCRETE

• Self-consolidating concretes• Vacuum concretes• Shotcrete• Cellular concrete• Roller-compacted concrete• Glass concrete• Asphalt concrete• Rapid strength concrete• Rubberized concrete• Polymer concrete• Geopolymer or Green concrete• Limecrete• Gypsum concrete• Light-Transmitting Concrete

EXAMPLES OF CONCRETE TYPE

Basic Composition for Main Concretes• Regular Concrete

– Cement, Aggregate, and water

• Geopolymer (Green concrete)– Fly Ash and Regular– Concrete

• High Strength Concrete ~<0.35%– Silica Fume– Strong Aggregates

• Ultra High Performance Concrete (UHPC) – Cement– Coarse/Fine Aggregate– Air– Silica Fume– Polypropylene Fibers

Admixtures and Properties

Chemical Admixtures• Dispersing Agents

– Water Reducers• Accelerators• Retarders

Water Reducers• DEFINITION: Water Reducers are used for the

purpose of reducing the quantity of mixing water required to produce a concrete of given consistency.

Accelerators• DEFINITION: Accelerating admixtures are added to

concrete for the purpose of shortening set time and accelerating early strength development.

Retarders

• DEFINITION: Retarding, and Water-reducing and retarding admixtures are used to offset acceleration and unwanted effects of high temperature and keep concrete workable during placement and consolidation.

Shrinkage Reducing Admixtures• DEFINITION: Shrinkage Reducing Admixtures are used to

minimize drying shrinkage cracking in concrete .

Corrosion Inhibitors

• DEFINITION: Corrosion Inhibitors are used to mitigate corrosion of reinforcing steel in concrete.

ASR Inhibitors

• DEFINITION: ASR Inhibitors (primarily Lithium) are used to mitigate alkali-silica reactivity in concrete.

Concrete Materials

• Aggregate is the second most influential ingredient in concrete.

• Aggregate–Occupies 60-75 % of volume–Fine Aggregate is typically 35-45 % of

total aggregate–Mortar (Air, water, cement, fly ash,

sand) is typically 50 - 65 % of total volume of a mixture

Aggregates in Concrete

• Fine: Sand or Crushed Stone (< 5mm)• Coarse: Gravel or Crushed Stone (5-50 mm) • Aggregate must be washed in many areas

– Granite & other crushed stone– Recycled concrete

• All must satisfy ASTM C33

Concrete Construction

• Significance of aggregate grading– smooth grading curve

• (sieve size vs. % passing)– more voids will lead to more cement.– undersanded mixes tend to be harsh– large sizes have less surface area

Desired Properties of Fresh Concrete

WorkabilityConsistencySegregationBleedingSetting TimeUnit WeightUniformity

Concrete - Workability

• cement: too fine of material – stickiness – increased water demand

• water: too much water– segregation– bleeding

• water: too little water– harshness– compaction problems

• fly ash: increases flow– ball bearing effect– ionic effect– reduced water demand

• aggregate– rounded particles flow more

easily– Too much sand “stickiness”– Poor gradation - harsh

Compressive Strength

Compressive Strength Uses of Concrete

41 MPa Highways and Structures

17-24 MPa Sidewalks and Driveways

> 55 MPa Prestressed Concrete

> 90 MPa High Strength Columnsand Beams

Concrete Mixture Pre-Design

• Engineer

• Architect

• Contractor

• Concrete Supplier

• Define strength, congestion and durability properties

• Defines color, texture,• Defines workability,

setting time, ..• Defines aggregates,

cement, fly ash, admixtures....

MIXING OF CONCRETE

Ready-Mix concrete: In this type ingredients are introduced into a mixer truck and mixed during transportation to the site.

• Wet – Water added before transportation• Dry – Water added at site

Mixing at the site• Hand mixed• Mixer mixed

Ready Mix Concrete

Hot Weather Concrete• Rapid hydration early setting rapid loss of

workability• Extra problems due to

– Low humidity– Wind, excessive evaporation– Direct sunlight

Solutions– Windbreaks– Cooled Concrete Ingredients– Water ponding (cooling due to evaporation)– Reflective coatings/coverings

Cold Weather Concrete• Keep concrete temperature above 5 °C to

minimize danger of freezing

Solutions– Heated enclosures, insulation– Rely on heat of hydration for larger sections– Heated ingredients --- concrete hot when placed– High early strength cement

Structure of “un-damaged” Concrete

Macrostructurem Microstructure

VIBRATION OF CONCRETE The process of compacting concrete consists

essentially of the elimination of entrapped air. This can be achieved by:

– Tamping or rodding the concrete– Use of vibrators

Internal Vibration

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1½ R

Vibrator

Internal vibrator: The poker is immersed into concrete to compact it. The poker is easily removed from point to point.

EXTERNAL VIBRATORSExternal vibrators: External vibrators clamp direct to the formwork requiring strong, rigid forms.

Advantages

• Ability to be cast• Economical• Durable• Fire resistant• Energy efficient• On-site fabrication

Disadvantages• Low tensile strength• Low ductility• Volume instability• Low strength to weight ratio

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Submitted ByAdesh GuptaB.Arch