Concrete Technology: Cement, Aggregate, Water (W/C, Workability), Admixtures

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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

(i) Fine Aggregates (0.075 – 4.75) mm

                              (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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