CONCRETECONCRETE

TopicsTopics

ConcreteConcrete Water-cement Water-cement

ratioratio Workability testsWorkability tests BleedingBleeding SegregationSegregation

ConcreteConcrete

Is one of Is one of the most commonthe most common construction materialconstruction material

Is used in a Is used in a wide varietywide variety applications applications ranging from piles, multistory ranging from piles, multistory buildings, dams, foundations, buildings, dams, foundations, pavements, storage tanks, bridges, pavements, storage tanks, bridges, and many other structures.and many other structures.

Is one of Is one of the most economicalthe most economical materials of construction, very materials of construction, very versatile in nature and applicationversatile in nature and application

Concrete ConstituentsConcrete Constituents

CementCement Fine aggregateFine aggregate Coarse aggregateCoarse aggregate WaterWater Admixtures – Admixtures – chemical and chemical and

pozzolanicpozzolanic

Important Properties of ConcreteImportant Properties of Concrete

Compressive Compressive strengthstrength

Durability (weather Durability (weather resistance)resistance)

ImpermeabilityImpermeability Resistance to Resistance to

environmental environmental attacksattacks

Abrasion resistanceAbrasion resistance

Types of ConcreteTypes of Concrete

Based on Based on aggregate:aggregate:

1)1) Lightweight Lightweight concreteconcrete

2)2) Normal weight Normal weight concreteconcrete

3)3) Heavyweight Heavyweight concreteconcrete

Based on strengthBased on strength

Normal strength Normal strength concrete concrete ( 0 to 50 ( 0 to 50 MPa)MPa)

High-strength High-strength concreteconcrete (51 to 80 (51 to 80 MPa)MPa)

Very High-Very High-strength concrete strength concrete ( greater than 80 ( greater than 80 MPa)MPa)

Based on constructionBased on construction

In-situ In-situ concreteconcrete

Precast Precast concreteconcrete

Prestressed Prestressed concreteconcrete

WATER-CEMENT RATIOWATER-CEMENT RATIO

W/C RATIOW/C RATIO

The The ratio ratio of weight of water to of weight of water to weight of cement used in the weight of cement used in the mixmix

Weight of water = Weight of water = water addedwater added + + free waterfree water

Water-cement ratio will have an Water-cement ratio will have an effect on effect on strength and durabilitystrength and durability of concreteof concrete

Abram’s LawAbram’s Law

Assuming Assuming full compactionfull compaction, , and at a given age and and at a given age and normal temperature, normal temperature, strengthstrength of concrete can be of concrete can be taken to be taken to be inversely inversely proportionalproportional to the water- to the water-cement ratiocement ratio

W/C vs Compressive strengthW/C vs Compressive strength

W/C RatioW/C Ratio At a given degree of At a given degree of

hydration, the water cement hydration, the water cement ratio ratio determines the porositydetermines the porosity of the cement paste (gel of the cement paste (gel pores, capillary pores and pores, capillary pores and entrapped air)entrapped air)

Lower w/c ratio will Lower w/c ratio will increase increase concrete strengthconcrete strength due to due to lower porositylower porosity

With an increase in age, the With an increase in age, the degree of degree of hydration generally hydration generally increasesincreases so that strength so that strength increasesincreases

W/C RatioW/C Ratio

One of the One of the most importantmost important factors factors affecting the strength and durability affecting the strength and durability of concreteof concrete W/C ratio for normal strength W/C ratio for normal strength

concrete concrete – 0.45-0.6– 0.45-0.6 W/C ratio for high strength W/C ratio for high strength

concrete – concrete – less than 0.45less than 0.45 W/C ratio will also W/C ratio will also affect the affect the

workabilityworkability of concrete of concrete

PROPERTIES OF FRESH PROPERTIES OF FRESH CONCRETECONCRETE

WORKABILITYWORKABILITY

The amount of The amount of work required in work required in mixing, placing, mixing, placing, and compacting and compacting the fresh the fresh concreteconcrete without without segregationsegregation

1)1) ConsistencyConsistency

2)2) MobilityMobility

3)3) CompactibilityCompactibility

Factors Affecting Factors Affecting WorkabilityWorkability

1)1) Water contentWater content

2)2) Cement content and finenessCement content and fineness

3)3) Aggregate type and gradingAggregate type and grading

4)4) Size of aggregatesSize of aggregates

5)5) Aggregate-cement ratioAggregate-cement ratio

6)6) AdmixturesAdmixtures

7)7) Weather/temperatureWeather/temperature

WorkabilityWorkability Higher water content will increase the Higher water content will increase the

inter-particle lubrication. Hence inter-particle lubrication. Hence increasing the workability of fresh increasing the workability of fresh concreteconcrete

Fineness of cement is of minor Fineness of cement is of minor influence on workabilityinfluence on workability

Higher cement content increases the Higher cement content increases the workability (W/C)workability (W/C)

Finer particles require more water to Finer particles require more water to wet their larger specific surfacewet their larger specific surface

Cont.Cont. Irregular shape and rougher Irregular shape and rougher

texture of an angular aggregate texture of an angular aggregate demand more waterdemand more water than than rounded aggregaterounded aggregate

For a For a constant w/c ratioconstant w/c ratio, , the the workability increases as the workability increases as the aggregate-cement ratio is aggregate-cement ratio is reduced because the amount of reduced because the amount of water relative to the total water relative to the total surface of solids is increased surface of solids is increased

Cont.Cont. The use of chemical The use of chemical

admixture increases the admixture increases the workability of concreteworkability of concrete

Fly Ash also known to Fly Ash also known to increase the workability due increase the workability due to its spherical shapeto its spherical shape

Higher temperature reduces Higher temperature reduces workability and increases the workability and increases the slump lossslump loss

WORKABILITY TESTSWORKABILITY TESTS

Slump testSlump test Compacting Factor Compacting Factor testtest

Vebe testVebe test

Slump TestSlump Test

The mould for the slump test is The mould for the slump test is aa frustumfrustum of a cone, 300 mm of a cone, 300 mm high. The base of 200 mm high. The base of 200 mm diameter is placed on a smooth diameter is placed on a smooth surface with the smaller surface with the smaller opening of 100 mm diameter at opening of 100 mm diameter at the top.the top.

The container is filled with The container is filled with concrete in concrete in three layersthree layers

Each layer isEach layer is tapped 25 timestapped 25 times with a standard 16 mm with a standard 16 mm diameter steel rod, rounded at diameter steel rod, rounded at the endthe end

Cont.Cont. The top surface is leveled using The top surface is leveled using

troweltrowel The mould must be firmly held The mould must be firmly held

against its base during the entire against its base during the entire operationoperation

The cone isThe cone is slowly lifted upwardslowly lifted upward, , and the unsupported concrete will and the unsupported concrete will slumpslump

The decrease in the height of the The decrease in the height of the center of the slump concrete is center of the slump concrete is called called SLUMP (measured in mm) SLUMP (measured in mm)

SLUMP TESTSLUMP TEST

Types of SlumpTypes of Slump

True slump - True slump - acceptaccept Shear slump – Shear slump – need to need to

redo the test, indication redo the test, indication of lack of cohesion of the of lack of cohesion of the mixmix

Collapse - Collapse - rejectreject

Types of SlumpTypes of Slump

Without superplasticizer

With superplasticizer

ExampleExample

Concrete A – Concrete A – slump 150 slump 150 mmmm

Concrete B – Concrete B – slump 30 mmslump 30 mm

Compacting Factor TestCompacting Factor Test

The The degree of compactiondegree of compaction, , called the compacting factor, called the compacting factor, is measured by the density is measured by the density ratioratio

Density ratio – The ratio of the Density ratio – The ratio of the density density actually achievedactually achieved in in the test to the density of the the test to the density of the same concrete same concrete fully compactedfully compacted

Compacting Factor TestCompacting Factor Test

Cont.Cont. Compacting factor = Compacting factor = Uncompacted Uncompacted

concreteconcrete Compacted Compacted

concreteconcrete

Value – maximum is one (1)Value – maximum is one (1) Example: Example: Concrete A – Concrete A – 0.80.8 Concrete B – Concrete B – 0.40.4

Vebe testVebe test

Slump Slump test is performed inside the test is performed inside the containercontainer

The disc rider is placed on top of the The disc rider is placed on top of the unsupported concreteunsupported concrete

The vibrating machine is The vibrating machine is switch onswitch on and the concrete will be compacted and the concrete will be compacted inside the containerinside the container

Compaction is complete when the Compaction is complete when the transparent rider is totally covered transparent rider is totally covered with concretewith concrete

The time taken for the whole process The time taken for the whole process is known asis known as VEBE TIMEVEBE TIME

Vebe TestVebe Test

ExampleExample

Concrete AConcrete A – – Vebe time is Vebe time is 10 10 secondsseconds

Concrete BConcrete B – – Vebe time is Vebe time is 55 secondsseconds

Factors Affecting Consistency Factors Affecting Consistency and Workability of Fresh and Workability of Fresh

ConcreteConcrete

Water contentWater content – – higher water content higher water content will increase the workabilitywill increase the workability

Fineness of cementFineness of cement – – workability workability decreases as the fineness increasesdecreases as the fineness increases

Chemical admixtureChemical admixture – – increase increase workabilityworkability

Pozzolanic admixturePozzolanic admixture – – PFA increase PFA increase workabilityworkability

AggregatesAggregates – – depends on the shapes depends on the shapes and sizes of aggregatesand sizes of aggregates

TemperatureTemperature – – higher temperature higher temperature reduces workabilityreduces workability

Test 1Test 1

Wednesday 19Wednesday 19thth Feb Feb 20082008

8.30 – 9.30 pm8.30 – 9.30 pm C09-407, 408C09-407, 408 Two Questions – topic Two Questions – topic

up to concrete on siteup to concrete on site

SEGREGATIONSEGREGATION

SEGREGATIONSEGREGATION Is defined as the Is defined as the tendency for tendency for

separationseparation of large and fine of large and fine particles in a fresh concrete mixparticles in a fresh concrete mix

Results in a Results in a non-homogenous mixnon-homogenous mix that that affects the strength and affects the strength and durabilitydurability of the hardened of the hardened concreteconcrete

Segregation one of the Segregation one of the causes of causes of pores and honeycombedpores and honeycombed surface surface

Segregation occurs due Segregation occurs due to:to:

Dry mixDry mix Very wet mixVery wet mix Coarser mixCoarser mix

Types of segregationTypes of segregation

During placing and During placing and compaction – compaction – concrete mix concrete mix contains too many coarse contains too many coarse aggregateaggregate

Cement paste segregate Cement paste segregate from concrete mix – from concrete mix – the mix the mix is too wetis too wet

Effect of SegregationEffect of Segregation

Lower compressive Lower compressive strength – strength – strength not strength not uniformuniform

Lack on durability – Lack on durability – higher higher porosityporosity

Methods to minimize Methods to minimize segregationsegregation

Concrete hasConcrete has good good workabilityworkability

Concrete Concrete not over not over compactedcompacted

Correct placingCorrect placing of of concreteconcrete

Nearby construction Nearby construction (piling)(piling) should be should be avoidedavoided

BLEEDINGBLEEDING

water

concrete

BLEEDINGBLEEDING Is defined as the Is defined as the

process of process of separation of water separation of water from the fresh from the fresh concreteconcrete

Happens when the Happens when the concrete mix does concrete mix does not possess the not possess the proper consistencyproper consistency that makes it that makes it unable to hold the unable to hold the mixing watermixing water

Bleeding results in Bleeding results in the the movement of movement of water and the finer water and the finer particles to the topparticles to the top of the form and of the form and produces a non-produces a non-homogenous mixhomogenous mix

Over vibration, over Over vibration, over trowelingtroweling and and lean lean mixesmixes increases the increases the potential for potential for bleedingbleeding

Effect of BleedingEffect of Bleeding Weaker Weaker

concreteconcrete Causing fine Causing fine

cracks below cracks below large large aggregate aggregate particlesparticles

Minimize BleedingMinimize Bleeding

1)1) Reduce water contentReduce water content

2)2) Increase finer particles Increase finer particles in the mixin the mix

3)3) Use air-entrainment Use air-entrainment admixtureadmixture

4)4) Proper compactionProper compaction

FACTORS AFFECTING FACTORS AFFECTING CONCRETE STRENGTH and CONCRETE STRENGTH and

DURABILITYDURABILITY

Water-cement ratioWater-cement ratio – – higher higher w/c ratio will decrease concrete strengthw/c ratio will decrease concrete strength

Good mix designGood mix design – – proportions proportions of concrete constituentsof concrete constituents

Cement contentCement content – – higher cement higher cement content increases compressive strength content increases compressive strength by the production of more C-S-H gelby the production of more C-S-H gel

Cont.Cont. Types of aggregatesTypes of aggregates – – size, grading, size, grading,

strength of aggregates will affect concrete strength of aggregates will affect concrete strengthstrength

Use of chemical – Use of chemical – air-entraining admixture air-entraining admixture reduces compressive strengthreduces compressive strength

Pozzolanic admixturePozzolanic admixture – – increase C-S-H increase C-S-H gel, make concrete densergel, make concrete denser

Proper Compaction – Proper Compaction – good compaction good compaction reducing the amount of voidsreducing the amount of voids

CuringCuring – – improve concrete strength by improve concrete strength by enhancing the hydration process of cementenhancing the hydration process of cement

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DeSiRe!!!!!!!

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