Concrete Structures

Concrete Structures

OverviewIntroductionConcrete PropertiesHistory of ManufactureComposition:CementAggregatesAdmixturesSetting and hydrationMaking and Testing of concreteConcrete mix designReinforced steelTypes of concrete

IntroductionRocklike MaterialIngredientsPortland CementCourse AggregateFine AggregateWaterAdmixtures (optional)

Concrete propertiesVersatilePliable when mixedStrong & DurableDoes not Rust or RotDoes Not Need a CoatingResists Fire

History of manufactureEgyptian Pyramids The Egyptians were using early forms of concrete over 5000 years ago to build pyramidsRoman ArchitectureRomans used a material that is remarkably close to modern cement to build many of their architectural marvels1824-Portland Cement Invented Joseph Aspdin of England is credited with the invention of modern Portland cement.

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Egyptian pyramid7Roman Architecture

CementChemical Composition of cement is:Lime63%Silica22%Alumina06%Iron oxide03%Gypsum01 to 04%Types of cementOrdinary Portland Cement (OPC)

Portland Pozzolana Cement (PPC)

Portland Blast Furnace Slag Cement (PBFS)

Oil Well Cement

Rapid Hardening

Sulphate Resisting Portland Cement

White Cement

AggregatesAggregates make up to 59-75% of concrete volume Strength of concrete is dependent on the strength of aggregate particles and the strength of hardened paste

Properties of aggregatesCompressive strength: Should be higher than concrete strength

Voids: Represent the amount of air space between the aggregate particles

Moisture content: The amount of water in aggregates

Gradation: Particle size distribution of a representative sample of an aggregate AdmixturesAdmixture are materials that are added to plastic concrete to change one or more properties of fresh or hardened concrete.

To fresh concrete Added to influence its workability, setting times and heat of hydration.

To hardened concrete Added to influence the concretes durability and strength.

Types of AdmixturesChemical (Accelerators, retarders, water-reducing and air-entraining).Mineral (Strength and durability).

Chemical admixturesAccelerating admixtures Compounds added to cement to decrease its setting time Retarding admixtures Added to concrete to increase its setting timeWater-reducing admixtures and super plasticizers Used to reduce the amount of water used in concrete mixesAir-entraining admixtures Allows dispersal of microscopic air bubbles Foaming agents Vinsol resin; Sulphonated lignin compounds; Petroleum acid compounds; Alkyd benzene compounds.

Mineral AdmixturesUsed in concrete to replace part of cement or sand.When used to replace sand called as supplementary cementing materials.Added in large quantities compared to chemical admixtures.Pozzolans Fly ashSilica fume

Setting and HydrationDue to hydration cement undergoes a series of chemical reactions Constituents slowly crystallizeInterlocking of their crystals gives cement its strengthDuring setting, paste losses its fluids and hardensAt the final stage cement gains its strength

Formation of hydration products over time leads toStiffening (loss of workability) Setting (solidification)

Gypsum reduces the speed of hardeningHot water increases the speed

Portland cement vs rapid set cement

Making and testing of concreteMixingInvolves weighing out all the ingredients for a batch of concrete and mixing them together Pumping and placing: Concrete is conveyed to the construction site or pumped Concrete should be placed as near as possible to its final position Finishing: The concrete must be leveled and surface made smooth/flatin wheel barrows, carts, belt conveyors, cranes21

Transit Mix Truck (Ready-Mix Truck)

Placement Today - Direct From the Transit Mixer, or

Improperly consolidated Concrete

Making and testing of concreteCuring of concrete : Process of maintaining enough moisture in concrete to maintain the rate of hydration

Methods of curing: Ponding or immersionspraying or fogging wet coverings Plastic sheetsSteam curing The most important single step in developing concrete strength

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Top of Slab being protected during cold weather Properties of Fresh ConcreteThe mix should maintain its uniformity and not bleed excessively

Consistency and WorkabilityConsistency is a measure of its wetness and fluidity - Measured by the slump test

StrengthTensile strength: Obtained using split cylinder tests Flexural strength: Determined by third point loading - Modulus of ruptureBleeding is movement and appearance of water at the surface of freshly-placed concrete, due to settlement of heavier particles

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Sample collectedSlump MeasuredCone Removed and Concrete Allowed to SlumpSlump Cone FilledConcrete Mix Design Concept : Determining the suitable proportion of ingredients that would produce a workable concrete mix that is durable, and of required strength, and cost-effective

Principles of Mix Design Workable mixUse as little cement & water as possibleGravel and sand to be proportioned to achieve a dense mixMaximum size of aggregates should be as large as possible, to minimize surface area of aggregates

e Methods of Mix Design Volumetric methodProportioning from field data methodProportioning by trial mixtures methodMass proportioning method

Using the given dataSelect the maximum slump as per the taskSelect the maximum size of aggregatesEstimate the mixing water and air contentSelect the w/c ratioEstimate the weight of dry rodded coarse aggregates Estimate the fine aggregate contentFind the weights of field mix (containing moisture) per unit volume

Concept of reinforcing concrete with steel reinforcementSteel is good at withstanding tensile stressConcrete is good at bearing compressive stress but can crack under tensile stress. The concept of reinforced concrete is to combine these two qualities to produce a material that is stronger than either material alone.

Reinforcing SteelSizesThere are 11 Standard Diameters3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18(Number refers to 1/8ths of an inch)Grades40, 50, 60Steel Yield Strength (in thousands of psi) Details marked on a Reinforcing Steel

Types of ConcreteNo fine concreteLime concretePlum concreteCement concreteShotcrete (sprayed concrete)Air entrained concreteLight weight concrete

No fine concreteConsists of cement and coarse aggregate onlyhas good drainage properties due to its open textureMix proportions of aggregate to cement is in the range 1:5 to 1:10 or as specifiedMaterial should be poured as soon as possible after mixing

Lime concreteMade from a mixture of lime, sand, and gravelGenerally lime & sand are taken in ratio of 1:1 to 1:3 by volumeUsed for foundation bases of load bearing walls, columns, and under layers of floors

Plum concreteConsist of hard rock of approved sizesA layer of concrete not less than 50 mm thick shall be placed Plums placed should be allowed to sink in their own weightPlums should not be completely coveredUsed mostly in mass concrete works like concrete gravity dams