Concrete

• Artificially built-up Stone with Cement, Water, Aggregates with or without Admixtures

Constituents of Concrete:

Aggregates = (60-70) %

Water = (7- 15) %

Cement = (14-21) %

Air = (0 – 3) %

Properties of Concrete:

• Air Content
• Fluidity
• Strength
• Setting Time
• Durability

Types of Concrete:

• Based on Binding Materials
• Cement Concrete
• Lime Concrete

• Based on Design
• Plain Concrete (no reinf.)
• Reinforced Concrete
• Prestressed Concrete (High Compressive Stress are artificially induced before use)

• Based on Density
• Light Weight Concrete (18 KN/M³) (i.e., by using pumice from volcanoes or bloated aggregates)
• Normal Weight Concrete (24 KN/M³)
• Heavy Weight Concrete (32 KN/M³)

Boated Aggregates?

• Slightly Salted and lightly smoked

Cement

Composition of Cement Clinker:

Tricalcium Silicate (C₃S) – 40%

• Cement hydrates more rapidly
• Generates more heat of hydration
• Develops high early strength
• Possesses less resistance to Sulphur Attack

Dicalcium Silicate (C₂S) – 32 %

• Cement hydrates slowly
• Generates less heat of hydration
• Provides good ultimate strength
• More resistance to sulphate attack

Tricalcium Aluminate (C₃A) – 10.5 %

• Reacts fast with water
• Generates a large amount of heat of hydration when reacts with water.
• Causes initial setting of cement

• First compound which reacts with water

Tetra Calcium Alumino Ferrite (C₄AF) – 9 %

• Poor cementing value
• Reacts slowly with water
• Generates small heat of hydration

Types of Cement:

Ordinary Portland Cement (OPC)

• Common type of cement
•  Resistance to dry shrinkage and cracking
• Less resistance to chemical attack
• Shouldn’t be used for construction work exposed to sulphate

Rapid hardening Portland Cement (RHPC)

• High early strength cement
• Lighter than OPC
• Shouldn’t be used for massive concrete structures

Low Heat Portland Cement (LHPC)

• Low percentage of tri-calcium silicate
• High percentage of di-calcium silicate
• Hydrates slowly
• Less lime than OPC
• Possesses less compressive strength
• Widely used in retaining walls
• Shouldn’t be used for thin concrete structures

Sulphate Resisting Portland Cement (SRPC)

• Percentage of C₃A kept below 5%
•  Resisting power against sulphates
• Used for structures subjected to severe alkaline conditions such as canal linings, culverts, etc.

High Alumina Cement (HAC)

• 35% of Alumina
• Sets slowly
• Obtained higher ultimate strength in a short period of time.
• Used for structure subjected to the action of Sea water, chemical plants and furnaces.

Blast Furnace Slag Cement (BFSC)

• Made by intergrading of OPC clinker and granulated blast furnace slag.
• Cheaper than OPC
• Develops low heat of hydration
• Has less early strength
• Frequently used in dams, abutments, and retaining walls.

Colored Cement (CC)

•  prepared by adding (5-15) % of a suitable coloring pigment before the Cement is finally ground.
• Commercial term used is colocrete.
• Used for the finishing of floors, external surfaces, decorative concrete, aesthetics, etc.

Portland Pozzolana Cement (PPC)

Made by intergrinding of OPC clinker and pozzolana

• Pozzolana Materials are a silicious material containing clay up to 80%.
• 30% of Pozzolana added to OPC
• Used for hydraulic structures as dams, weirs

Aggregates

Aggregates = Boulder (> 200 mm)

                        Cobble (80 – 200 )mm

Properties of Aggregate:

• Strong
• Chemically inert
• Hard
• Durable

Types of Aggregates:

• Natural Aggregates = sand, gravel, crushed stone, crushed rock etc.
• Artificial aggregates = furnace clinker, coke – breeze, saw dust, foamed slag

(A) Based on size

                              (0.002 – 0.06) mm – silt

                              < 0.002 mm – clay

(ii)Coarse Aggregates: (4.75 – 75) mm

Max. size of Coarse < (1/4) * min. dia. of PCC

                                    < (1/5) * min. dia. of RCC

(iii) Cyclopean Aggregates: (> 75 mm)

(iv) All- in Aggregates:

• Consists of different fractions of fine & coarse aggregates

• Not used as an aggregate for making high-quality concrete.

Based on type of Rock

• Aggregates from Igneous Rock
• Aggregates from Sedimentary Rock
• Aggregates from Metamorphic Rock

Based on Strength

• Hard Aggregates
• Soft Aggregates

Based on Shape

• Rounded (33 % void, good workability, low strength due to poor interlocking)
• Flaky (least diameter < 0.6 mean diameter)
• Elongated (Greater diameter > 1.8 mean diameter)
• Angular ((38 – 45) % void, less workability, high strength due to best interlocking)
• Irregular ((35 – 37) % void, low workability, average strength due to average interlocking)

Based on Surface Texture

• Glassy
• Smooth
• Granular
• Rough
• Crystalline
• Honey combed
• Porous

Based on surface moisture

• Very Very Dry
• Dry
• Saturated Surface Dry
• Wet or Moist

Properties of Aggregates:

Crushing Value:

• It is the value which shows resistance of an aggregate to  the crushing under any gradually applied compressive load.

Impact value:

• Resistance to sudden shock/impact
• Used as alternative to crushing value
• Know quality of aggregates

Abrasion Value:

• Resistance to wear
• Rotated in cylinder with some abrasive charges
• By Los-Angeles machine

Soundness :

• Resistance to effect of hydration of cement & weather

Water

• Insufficient water – harsh & unworkable concrete
• Excess water – bleeding & segregation
• Presence of CO3, HCO3^—of Na– adversely affect on the setting time of cement
• Presence of CaCl2 -accelerates setting & hardening of Cement (restricted to 1.5% by wt. of Cement)

W/C Ratio: (Experiment by Duffs Abrahms known as w/c ratio law)

• Law is valid only when concrete is of workable plasticity
• Strength depends on w/c and not on C/A ratio
• Strength  1/(w/c)
• w/c < 0.45: – concrete is not workable/honey combed

Workability:

• ease and homogeneity in mixing, placing, compacting, finishing

Factors affecting Workability:

Water Content

• Increases with water content

Size of Aggregates

• Large size aggregates more workable than small size

Shape of Aggregates

• Round shape more workable than flaky, angular & elongated

Surface texture of Aggregates

• Smooth texture more workable than rough

Grading of Aggregates

• Continuous lean concrete mix more workable

Air entraining Agents

• Increases the workability (bubbles produced)

Temperature

• Reduces at high temperature

Admixtures

Admixtures are used for accelerating or retarding initial set and strength, can be positive or negative, either chemical or mineral.

• Air- entraining Admixtures, Retarding and Water reducing Admixtures, Accelerating Admixtures, Water Proofing Admixtures, Pozzolanic Admixtures, Coloring/Pigment Admixtures

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