Concrete is mixture of Cement, Fine Aggregate and Coarse Aggregate with water and Admixtures which upon setting forms the building block of construction.

Introduction to Concreting

This section contains process of formation of concrete, placing of concrete and process of curing. It also covers concepts like workability, segregation and bleeding of concrete.

Process of Concrete Manufacturing :

• Batching
• Mixing
• Transporting
• Placing
• Compacting
• Curing
• Finishing

Vibration of Concrete :

It is done to expel entrapped air from concrete mix to make it more rigid, dense and stable.

It is carried out using special equipments, called vibrators.

Types of Vibrators :

• Internal Vibrator or Needle Vibrator : Power, unit, shaft and needle are parts; used for congested reinforcement
• Formwork Vibrator or External Vibrator :
  • For Column
  • thin wall
  • precast unit.
  • It uses more power.
• Table Vibrator :
  • Concrete cubes
  • Prefabricated RCC member.
• Platform Vibrator : Large prefabricated parts, electric poles and sleepers
• Surface Vibrators : (Screen board), Floor slab, roof slab, and roads

Over Vibration :

For poorly graded mix, it cause segregation.

It produce concrete with poor resistance to abrasion.

Re Vibration :

• It is delayed vibration of Concrete that has already been compacted.
• It occur while placing successive layers.
• Not harmful and may be beneficial
• Rearrange aggregate particle
• Improve compression and bond strength, reduce honey comb, remove air-water pocket.

Curing :

Process of applying layer of water after concrete is placed and compacted.

It replenishes loss of water and keeps them moist and warm.

Cement requires water content of 0.23 for hydration and 0.15 for filling voids in gel pores, which forms total of 38 % requirement.

Methods of Curing of Concrete :

• Precast elements : Dipped in curing tank, steam curing
• Slab : Ponding
• Vertical Wall and column: Spraying water, wet covering
• Membrane curing : seal off evaporation by covering surface with polythene, bitumen rubber etc.
• Application of heat : It accelerate hydration in presence of moisture, steam curing
• Electrical curing : for very cold regions
• Surface coating : CaCl₂ as coating
• Steam curing : it increases compressive strength also.
• Radiation curing : It also cause very accelerated gain in strength.

Workability :

Ease of mixing, placing, compaction and finishing of Concrete.

Factors affecting Concrete workability :

• Water Content : more water, more fluidity
• Mix Proportions : more aggregate/cement ratio, less paste, low mobility, low workability.
• Size of Aggregate : big size of aggregate, less paste required for wetting more workability.
• Shape of Aggregate : round aggregate, more workability; flaky angular or elongated, more void, harsh
• Surface Texture : smooth texture, more workability
• Grading of Aggregate : well graded, least voids, more paste, higher workability
• Use of Admixture : Plasticizer increase workability, air entrailing agents increase workability.

Measurement of Workability :

Slump Test :

• For very wet or very dry mix
• Apparatus : Metallic mould in frustum shape, steel tamping rod
• Mould is filled in 4 layers, tamped 25 times. Mould is removed immediately; subsidence is noted which is called slump (mm).

K- Slumper Test : directly measure slump in one minute, after tester is removed.

Compacting Factor Test :

• Mix of low workability
• Principle : degree of compaction achieved by standard amount of work done by allowing mixture to fall through standard height.
• Compacting factor : Ratio of density actually achieved in test to density of same concrete fully compacted.

\color{blue}\large{Compacting \:factor = \frac{W_{2}-W_{0}}{W_{1}-W_{0}}\leq 1}

W₀ : weight of pot , W₁ : weight after standard compaction

W₂ : weight after partial compaction (test)

\color{blue}{Maturity = \sum (Time \times Temperature) }

Vee – bee consistometer Test :

• For low workability mix (very dry)
• Concrete is placed in steel truncated cone and then removed.
• Vibrator is started and time taken to loss in conical shape and becoming horizontal is noted down.
• Higher time : low workability

Flow Test :

• For very high workability mix, self compacting concrete
• Spread of concrete after jolting is measured in flow table

Segregation of Concrete :

Separation of constituent material of concrete.

It is caused by lack of homogeneity.

Bleeding :

• Segregation of water from concrete
• Observed in highly wet mix, badly proportional and insufficiently mixed concrete.
• Water takes out some cement, forming paste, called Laitance.
• If w/c > 0.7, several channels form
• Reduction :
  • Revibration for good bond
  • Delayed finishing
  • Finely divided pozzolanic material
  • Air entrailing agent
  • Cement of low alkali, finer texture
  • Rich mixes

Admixtures and Chemical Action on Concrete

In this section we will learn about various admixtures added to enhance the properties. We will also learn about chemical reactions that takes place in concrete exposed to chemicals.

Admixtures

Plasticizers :

• Water reducers
• Helps in obtaining higher workability
• Anionic Surfactant :
  • Lignosulphonate
  • Salts of Sulphonate
  • Hydrocarbon

• Non ionic Surfactant :
  • Polyglycol ester
  • Hydroxylated carboxylic acid

• Amount : 0.1 % – 0.4 %
• Action : Dispersion which is deflocculation of cement particles with trapped water ; retardation of hydration.

Superplasticizers :

• Reduction of water content upto 30 %
• For self compacting concrete, and high strength concrete
• Sulphonated melanic-formaldehyde, modified Lignosulphonate
• Higher molecular weight : more efficiency
• Amount : upto 3 % after 3 minutes of mixing
• No change in strength

Retarder :

• Slows down chemical process of hydration
• To make them remain plastic and workable for longer time
• Overcome acceleration effect of temperature
• Calcium sulphate (gypsum), starches, cellulose, sugar, acids, tartaric acid
• Sugar (0.2 %) is very effective

Accelerator :

• Increase the rate of early strength development of concrete
• Permit earlier removal of formwork, reduce period of curing, used for emergency repair work.
• Calcium chloride, triethenolamine, fluosilicate, lithium salt, sodium silicate

Air Entrailing Agent :

• Incorporate millions of non-coalescing air bubbles, which act as flexible ball bearing.
• Modify workability, reduce segregation, bleeding and improve finishing
• Improve frost resistance and impermeability
• Wood resin, animal and vegetable fat (olive oil, oleic acid), soaps, Al powder
• Water content of 0.4 – 0.6 % gives abundant action.
• Action decreases with temperature, compaction, and addition of fly ash and calcium chloride.

Pozzolanic Mineral Admixture :

• Lower heat of hydration and thermal shrinkage
• Reduce alkali-aggregate reaction.
• Improve resistance to sulphate soil, sea water
• Improve workability
• They are siliceous, and in finely divided form
• Chemically react with calcium hydroxide to form cementitious product.
• Pozzolana + calcium hydroxide + water makes C-S-H gel
• Make paste dense and impervious
• Clay, Fly ash, silica fume, Rice husk, Surkhi, Metakaolin, blast furnace slag

Fly Ash :

• It is residue of combustion of powdered coal
• Collected by electrostatic precipitator
• It develop strength at later stage

Silica Fume :

• Microsilica
• Amorphous glassy sphere
• Formed when SiO gas in furnace mixes with oxygen to give silicon dioxide.

Metakaolin :

• Unpurified
• Thermally activated
• Ordinary clay and Kaolinite Clay

Gas Forming Agent :

• Al powder
• React with hydroxide to form hydrogen bubble

Air Detrailing Agent :

• Tributyl phosphate
• Silicones
• Removes entrapped air

Chemical Action :

Sulphur Attack :

Attack calcium hydroxide in form of CaSO₄ , Na₂SO₄ , K₂SO₄ etc.

Calcium sulphate attack CAH gel producing Calcium Sulpho Aluminate (Ettringite).

Control :

• Use cement with low C₃A
• Air entailment
• Pozzolana
• Steam Curing
• High Aluminate Cement

Alkali Aggregate Reaction :

• Reactive silica react with alkalis present in cement.
• Alkali : Na₂O and K₂O
• Temperature : (10 – 38)° C
• Silica gel expands and exert osmotic pressure, cause cracking.

Control :

• Alkali < 0.4 %
• Non reactive aggregate
• Pozzolanas
• Controlling moisture

Acid Attack :

If p^H < 6.5, but it is severe if p^H < 5.5

Cement compounds are broken down and leached away.

Carbonation :

CO₂ penetrates into concrete and reacts with Calcium hydroxide to form CaCO₃

CO₂ change into carbonic acid in moisture, and then attack reinforcement.

Chloride Attack :

Corrosion of reinforcement reduce protective layers of reinforcement.

Chlorine content < 0.5 %

Binding with C₃A to form Calcium Chloroaluminate.

Mineral Oil :

Negligible harm

Organic Acid :

Harmful

Suger :

Retarder, Cause corrosion

Sewage :

H₂S evolved from sulphuric acid and sulphate salt attack concrete.

Special Concrete and Strength of Concrete

In this section we will learn about some special concretes with their properties and uses. We will also learn numerically about strength parameters of concrete and their durability.

Special Concrete :

High Strength Concrete :

• Strength greater than 35 MPa
• Unconventional method of making HSC
  • Seeding : ground fine hydrated Portland cement added to mix
  • Revibration
  • High speed slurry mixing
  • Use of Admixture
  • Inhibitor of crack
  • Sulphur Impregnation : immersing in molten Sulphur
  • Cementitious aggregate

• Ultra High Strength concrete
  • Pressure compaction
  • Helical binding
  • Polymerisation
  • Reactive powder

High Performance Concrete :

• Greater workability
• High strength, modulus of elasticity, density, dimensional stability, low permeability
• W/C ratio : 0.25 – 0.3
• Use of silica fume
• Use of super plasticizers for high slump
• Maximum size of aggregate : 20 mm

Light Weight Concrete :

• Use of porous or light weight aggregate.
• Introducing air bubble in mortar.
• Omitting sand fraction from aggregate (no fines)
• Maximum slump : 100 mm
• Unit weight : 1850 Kg/m³

Aerated Concrete :

Introduce air or gas into slurry of Portland cement or lime and crushed filler.

No Fines Concrete :

• Omit fine aggregate fraction, only coarse aggregate used
• Lower drying shrinkage
• Use : external wall

High Density Concrete :

• Unit weight : (3360 – 3840) Kg/m³ (50 % higher than normal)
• Use in radiation shield
• Use of iron as aggregate

Sulphur Infiltrated Concrete :

• Improved physical strength, more water impermeability and resistance to corrosion.
• Made by sulphur impregnation
• Used in pre cast

Fiber Reinforced Concrete :

• High Tensile strength, more ductility and more resistance to cracking
• Less microcracks
• Fiber used are polypropylene, nylon, asbestos, coir, glass and carbon

Special Concreting Methods

Prepacked Concrete

First aggregates are placed, then grouting with mortar.

For complicated reinforcement.

Vacuum Concrete :

High workable concrete and excess water is removed by pump.

The Gunite or Shotcrete :

• Mortar conveyed through hose and pneumatically projected at high velocity on surface
• Small sized coarse aggregate
• Reduce cement content
• shotcrete is recent and achieve greater thickness.
• Dry mix as well as wet mix

Ferrocement :

• Closely spaced wire meshes which are impregnated with rich cement mortar mix.
• Wire diameter : 0.5 – 1 mm
• Mortar : 1:2 or 1:3
• w/c : 0.4 – 0.45
• For shells, roofs, silos, tanks and complicated shapes.

Self Compacting Concrete :

• Superplasticizers provide workability
• Admixture : fly ash, GGBFS, Silica fume, fiber
• Cement : ordinary portland cement 43 and 53 grade.
• Aggregate : 10 – 12 mm

Cyclopean Concrete :

• Aggregate size > 150 mm
• Low slump value
• High heat of hydration cause high temperature rise.

Strength of Concrete :

Strength depends upon following factors :

• Water Cement Ratio
• Cement Aggregate Ratio
• Grading, Surface Texture, Shape, Strength and Stiffness of aggregate particles.
• Maximum Size of Aggregate

Water/Cement ratio :

Strength of Paste ∝ CementContent

\color{blue}{Strength\:of\:Paste \propto  \frac{1}{Air/Water\:Content}}

\color{blue}\large{S= \frac{A}{B^{x}}}

x : water/cement ratio by volume  
A,B : Constant
A = 14,000 lbs/sq.in
B = 7

Gel/Space ratio : Powel’s Expression

\color{blue}\large{S= 240x^{3}}

x : Gel/Space ratio

Relation between 28 days strength and 7 days strength :

\color{blue}\large{\sigma _{28}= 1.4\sigma _{7}+150}

\color{blue}\large{\sigma _{28}= 1.7\sigma _{7}+850}

28 days strength is taken as standard strength.

\color{blue}\large{\frac{\sigma _{7}}{\sigma _{28}}\approx0.65}

Maturity Concept of Concrete :

Datum line for Maturity : -11° C

Measurement : degree centigrade hours

Fully matured concrete : Cured at 18° C for 28 days

Maximum size of aggregate : reduce strength

Relation between compressive and tensile strength :

Strength in tension increases with increase in compressive strength, but rate of increase keeps on decreasing.

Third point loading :

Tensile strength = K [Compressive strength]ⁿ

K = 6.2 – 10.4

n = (1/2 – 3/4)

\color{blue}\large{Flexural \: Strength = 0.7\sqrt{f_{ck}}    }

f_ck = characteristic compressive strength (N/mm²)

Relation between modulus of Elasticity and strength :

\color{blue}\large{E_{c} = 5000\sqrt{f_{ck}}}

Poisson’s Ratio of concrete : 015 – 0.2

For M20 : f_cr/f_ck = 0.14

Creep :

Gradual increase in strain with increase in stress, with time

• Rheological phenomena (gel structure of paste)
• creep takes place only under stress
• Readjustment of colloidal concrete particle due to yield of gel pore water.

Rheology :

Study of gel pore structure, strength, deformation and rate of shear.

Effect of Creep :

\color{blue}\large{E_{ce} = \frac{E_{c} }{1 + \theta}}

θ : Creep

𝜏 : Shear stress

μ : At point plastic viscosity

γ : At point rate of shear

\color{blue}\large{\tau = \tau_{o} + \mu \gamma}

Durability :

Ability to resist weathering action, chemical attack, abrasion and other deterioration.

Use of pozzolanic material reduce permeability due to conversion of calcium hydroxide into cementitious product.

Fire Resistance :

• Concrete is not a refractory material, but has good fire resistance.
• Loss of strength take place at 250 – 300 ° C
• Quartz in aggregate reduce fire resistance.
• Dense limestone is good resistant, and blast furnace slag aggregate is best.

They exhibit linear relationship between stress and strain at low stress.

E depends on elasticity of aggregate and curing.