Mixing Of Concrete

-Ingredients should be properly mixed

-Surface of All Aggregate Particle is coated with cement paste & which is homogeneous and macro scale and therefore possessing uniform properties

Consistency & Slump

-The concrete should flow freely around corners & between the reinforcing bars

-Slump test is used to determine the consistency of concrete



 Recommended Slumps :

Concrete Construction	Maximum inches(mm)	Minimum inches(mm)
RCC Fdn. Walls & Footings	3(76)	1(25)
Plain Footing & Substructure Walls	3(76)	1(25)
Beams & Reinforced walls	4(102)	1(25)
Building Columns	4(102)	1(25)
Pavement & Slabs	3(76)	1(25)
Mass Concrete	2(51)	1(25)

Types Of Slump

Water Cement Ratio

-Compressive &Tensile strength and most of the other desirable properties of concrete is inversely related to w/c ratio

-Low w/c ratio increases freeze thaw resistance & decreases shrinkage

-Minimum amount of water is required to completely hydrate the cement (0.27-0.32)

- 0.65 - 0.7 onward - Less Strength

Compressive Strength, psi(MPa)	w/c Ratio By Weight
6000 (41)	0.41
5000 (34)	0.48
4000 (28)	0.57
3000 (21)	0.68
2000 (14)	0.82

-Strength is based on 6”x12” cylinders, moist cured, 28 days at 23+-2C in accordance with section 9B of ASTM C31

Pumping Admixture

-Used for pumping concrete through pipes

Damp Proofing Admixture

-Used to Avoid Dampness

-Zinc Sulphate, Calcium Chloride

Bonding Admixture

-Used to bond old concrete with new concrete

-Poly Vinyl Chloride, Poly Vinyl Acetate

Self Accelerators

-Used to accelerate the setting process

-Calcium Nitrite

Self Retarders

-Used to slow down the reaction

-Sugar

Water Reducing Admixture

-Used to increase the workability of concrete

-Lignosulphonates

Admixture

-Nearly all concrete contains one or more admixture

-Added in dry or liquid form

-Purpose is to effect certain desirable changes in the concrete

-Types Of Admixtures

-Water Reducing Admixture

-Self Retarders

-Self Accelerators

-Bonding Admixture

-Damp Proofing Admixture

-Pumping Admixture

Water

-An important ingredient in concrete mixture

-Water Must be cleaned and free from salt & other minerals

-Sea water is not recommended

Low Heat Of Hydration Cement

-When desired to reduce cracking and shrinkage

-Mass Construction (Dams, Marine constructions)

-Hydraulic Engineering Concrete

-Retaining wall construction

High Early Strength Cement

-Very high strength is desired at an early stage

-Use when the early remove remove framework is desired

-In cloud weather construction is due to reduce the time require for protection in low temperature

-Use in Road Pavement Works & Precast concrete casting (Beams, Columns etc)

Sulphate Resistant Cement

-Structure exposed to soil or water having alkali content

-It is widely used in chemical industries

-In foundations and basements where soil contains sulphate

Normal Cement

-Used in vast majority of cases

-Good for large structures

-Used in mortar for plastering, masonry work, pointing, etc

-Used for making joints for drains and pipes

-Used for water tightness of structure


-Used in concrete for laying floors, roofs and constructing lintels, beams, stairs, pillars etc

Types Of Cement

-Normal Cement

-Moderate Sulphate Resistant Cement

-High Sulphate Resistant Cement

-High Early Strength Cement

-Low Heat of Hydration Cement

Fine & Coarse Aggregates Pictures

Composition Of Aggregates in Concrete

Aggregates

-Fine Aggregates (¼” or 6.4mm) - Sand

-Coarse Aggregates (Larger than ¼” & less than 3”) - Gravel

-Bulk of solid volume of concrete is composed of Aggregates

-Water Cementitious Material matrix binds the aggregate together & through the hydration of cement the mass becomes a solid

-Concrete Mass can be no stronger than the aggregates

-Aggregates should be stronger

-If aggregates have planes of weakness or are soft then the matrix cannot provide full strength to the hardened concrete

Soundness Of Cement

-Change in Volume after setting

-Once set it should not undergo a large change in volume

-Expansion may happen due to the delayed or slow hydration

Setting Time

-Done By Vicat Apparatus

-Minimum time of 60 mins is prescribed  by ENV 197-1:1992 for cements up to 42.5MPa

-45 mins for cements with higher strengths

-ASTM C 150-94 minimum time for initial set is 45 mins

-final setting time <45mins

Factors Affecting Heat Of Hydration Of Cement

-Components such as tricalcium silicate, dicalcium silicate, and tricalcium aluminate

-Fineness of Cement

-Cement content, water-cement ratio, curing temperature, and chemical admixtures

Heat Of Hydration Of Cement

-Quantity of Heat (J/g) of unhydrated cement evolved upon complete hydration at a given temperature

-Exothermic Reaction (up to 500J/g) between cement & water

-In normal construction heat is dissipated into the soil or air

-For Massive Structures like dams or mat foundations the heat cannot be readily released and thus attains high internal temperatures

Fineness of Cement

-Total Surface Area of Cement that represents the material available for Hydration

-Rate of Hydration depends on the fineness of the cement particles

-High Fineness is necessary for rapid strength and development

Difference Between Hardening & Setting Of Concrete

Hardening Of Concrete

-The gain of strength of a set cement paste

Setting Of Concrete

-Change from fluid to rigid stage

-During setting, the paste acquires some strength

-Setting time decreases with the rise in Temperature

Hydration Of Cement

-Portland Cement Becomes a bonding agent

-Silicates & Aluminates form products of hydration which produce a firm & hard mass

Chemical Composition of Cement

Concreting Picture

Top Cement Producers & Consumers

Portland Cement Consumption

Concrete

-Mixture of ingredients into a paste & triggers a chemical reaction - hydration

-Reaction forms a gel which coats & fills spaces between the stone/sand;

-Hardens into a solid mass that gets stronger & stronger

-The world’s most widely used building material

-Global production is 5 billion cubic yards per year (using approximately 1.25 billion tons of cement)

-Can be shaped in any desired form

-Adaptable to a wide variety of different uses

-Strong, Durable, Economical, Versatile & Economical

Concrete Ingredients

What Is Concrete ?

-Construction Material

-Composite Material

-Cement + Fine Aggregate + Coarse Aggregate +Water +(and in some cases Admixtures)

-Cement & Water form a paste that hardens and bonds the aggregates together