PROPERTIES OF FRESH PROPERTIES OF FRESH CONCRETECONCRETE

WorkabilityWorkability ConsistencyConsistency SegregationSegregation BleedingBleeding Setting TimeSetting Time Unit WeightUnit Weight UniformityUniformity

WORKABILITYWORKABILITY

It is desirable that freshly mixed It is desirable that freshly mixed concrete be relatively easy to concrete be relatively easy to transport, place, compact and finish transport, place, compact and finish without harmful segregation. without harmful segregation.

A concrete mix satisfying these A concrete mix satisfying these conditions is said to be conditions is said to be workableworkable..

Factors Affecting Factors Affecting WorkabilityWorkability

Water-Cement ratioWater-Cement ratio Amount and type of Aggregate Amount and type of Aggregate Amount and type of Cement Amount and type of Cement Weather conditions Weather conditions

– TemperatureTemperature– WindWind

Chemical Admixtures Chemical Admixtures Sand to Aggregate ratioSand to Aggregate ratio

WORKABILITYWORKABILITY

Workability is the most Workability is the most important property of important property of freshly mixed concrete. freshly mixed concrete.

There is no single test There is no single test method that can method that can simultaneously measure simultaneously measure all the properties involved all the properties involved in workability. in workability.

It is determined to a large It is determined to a large extent by measuring the extent by measuring the “consistency”“consistency” of the of the mix.mix.

Consistency is the fluidity or degree Consistency is the fluidity or degree of wetness of concrete. of wetness of concrete.

It is generally dependent on the shear It is generally dependent on the shear resistance of the mass. resistance of the mass.

It is a major factor in indicating the It is a major factor in indicating the workability of freshly mixed concrete.workability of freshly mixed concrete.

CONSISTENCYCONSISTENCY

Test methods for measuring Test methods for measuring consistency are:consistency are:

Flow test Flow test → measures the amount of flow→ measures the amount of flow Kelly-Ball test → measures the amount of Kelly-Ball test → measures the amount of

penetrationpenetration Slump test (Most widely used test!)Slump test (Most widely used test!)

CONSISTENCYCONSISTENCY

10 cm

20 cm

30 cm

The slump cone is filled in 3 layers. Every layer is evenly rodded 25 times.

Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.

Segregation Segregation refers to a separation of the refers to a separation of the components of fresh concrete, resulting in a components of fresh concrete, resulting in a non-uniform mix non-uniform mix

SEGREGATIONSEGREGATION

The primary causes of The primary causes of segregation are segregation are differences in specific differences in specific gravity and size of gravity and size of constituents of concrete. constituents of concrete. Moreover, improper Moreover, improper mixing, improper placing mixing, improper placing and improper and improper consolidation also lead consolidation also lead to segregation.to segregation.

Some of the factors affecting Some of the factors affecting segregation:segregation:– Larger maximum particle size (25mm) and Larger maximum particle size (25mm) and

proportion of the larger particles. proportion of the larger particles. – High specific gravity of coarse aggregate. High specific gravity of coarse aggregate. – Decrease in the amount of fine particles. Decrease in the amount of fine particles. – Particle shape and texture. Particle shape and texture. – Water/cement ratio.Water/cement ratio.

SEGREGATIONSEGREGATION

Bleeding is the tendency of water to Bleeding is the tendency of water to rise to the surface of freshly placed rise to the surface of freshly placed concrete. concrete.

BLEEDINGBLEEDING

It is caused by It is caused by the inability of the inability of solid constituents solid constituents of the mix to hold of the mix to hold all of the mixing all of the mixing water as they water as they settle down.settle down.

A special case of A special case of segregation.segregation.

Undesirable effects of bleeding are:Undesirable effects of bleeding are:

• With the movement of water towards the top, the With the movement of water towards the top, the top portion becomes weak & porous (high w/c). top portion becomes weak & porous (high w/c). Thus the resistance of concrete to freezing-Thus the resistance of concrete to freezing-thawing decreases.thawing decreases.

• Water rising to the surface carry fine particles of Water rising to the surface carry fine particles of cement which weaken the top portion and form cement which weaken the top portion and form laitance. This portion is not resistant to abrasion.laitance. This portion is not resistant to abrasion.

• Water may accumulate under the coarse agg. and Water may accumulate under the coarse agg. and reinforcement. These large voids under the reinforcement. These large voids under the particles may lead to weak zones and reduce the particles may lead to weak zones and reduce the bond between paste and agg. or paste and bond between paste and agg. or paste and reinforcement. reinforcement.

BLEEDINGBLEEDING

The tendency of concrete to bleeding The tendency of concrete to bleeding depends largely on properties of depends largely on properties of cement. It is decreased by:cement. It is decreased by:

Increasing the fineness of cementIncreasing the fineness of cement Increasing the rate of hydrationIncreasing the rate of hydration Adding pozzolansAdding pozzolans Reducing water contentReducing water content

BLEEDINGBLEEDING

MIXING OF CONCRETEMIXING OF CONCRETE

The aim of mixing is to blend all of The aim of mixing is to blend all of the ingredients of the concrete to the ingredients of the concrete to form a uniform mass and to coat the form a uniform mass and to coat the surface of aggregates with cement surface of aggregates with cement paste.paste.

MIXING OF CONCRETEMIXING OF CONCRETE

Ready-Mix concrete: In this type Ready-Mix concrete: In this type ingredients are introduced into a ingredients are introduced into a mixer truck and mixed during mixer truck and mixed during transportation to the site. transportation to the site.

• Wet – Water added before transportationWet – Water added before transportation• Dry – Water added at siteDry – Water added at site

Mixing at the siteMixing at the site• Hand mixedHand mixed• Mixer mixedMixer mixed

Ready Mix Ready Mix ConcreteConcrete

MixMixing at Siteing at Site

Mixing time should be sufficient to Mixing time should be sufficient to produce a uniform concrete. The time produce a uniform concrete. The time of mixing depends on the type of of mixing depends on the type of mixer and also to some properties of mixer and also to some properties of fresh concrete.fresh concrete.

Undermixing Undermixing → non-homogeneity→ non-homogeneity Overmixing → danger of water loss, Overmixing → danger of water loss,

brekage of aggregate particlesbrekage of aggregate particles

MIXING OF CONCRETEMIXING OF CONCRETE

CONSOLIDATING CONSOLIDATING CONCRETECONCRETE

Inadequate consolidation can Inadequate consolidation can result in:result in:– Honeycomb (Honeycomb (defect in the defect in the

hardened cement hardened cement concreteconcrete consisting of many air  consisting of many air pockets or voids)pockets or voids)

– Excessive amount of Excessive amount of entrapped air voids (bugholes)entrapped air voids (bugholes)

– Sand streaksSand streaks– Placement linesPlacement lines ( (Cold jointsCold joints))

Consolidation is the process of removing entrapped air from freshly placedconcrete.

VIBRATION OF CONCRETEVIBRATION OF CONCRETE

The process of compacting concrete The process of compacting concrete consists essentially of the elimination consists essentially of the elimination of entrapped air. This can be of entrapped air. This can be achieved by:achieved by:

– Tamping or rodding the concreteTamping or rodding the concrete– Use of vibrators Use of vibrators

VIBRATORSVIBRATORS

Internal vibratorInternal vibrator:: The poker is The poker is immersed into concrete to compact immersed into concrete to compact it. The poker is easily removed from it. The poker is easily removed from point to point.point to point.

External vibrators:External vibrators: External vibrators External vibrators clamp direct to the formwork clamp direct to the formwork requiring strong, rigid formsrequiring strong, rigid forms..

Systematic VibrationSystematic Vibration

CORRECTCORRECT

Vertical penetration a few inches into previous lift (which should not yet be rigid) of systematic regular intervals will give adequate consolidation

INCORRECTINCORRECT

Haphazard random penetration of the vibrator at all angles and spacings without sufficient depth will not assure intimate combination of the two layers

To aid in the removal of trapped air the To aid in the removal of trapped air the vibrator head should be rapidly plunged vibrator head should be rapidly plunged into the mix and slowly moved up and into the mix and slowly moved up and downdown..

Internal VibratorsInternal Vibrators

The actual The actual completion of completion of vibration is judged by vibration is judged by the appearance of the appearance of the concrete surface the concrete surface which must be which must be neither rough nor neither rough nor contain excess contain excess cement paste.cement paste.

External vibrators are rigidly clamped to the External vibrators are rigidly clamped to the formwork so that both the form & concrete formwork so that both the form & concrete are subjected to vibration. are subjected to vibration.

A considerable amount of work is needed to A considerable amount of work is needed to vibrate forms. vibrate forms.

Forms must be strong and tied enough to Forms must be strong and tied enough to prevent distortion and leakage of the grout.prevent distortion and leakage of the grout.

External VibratoExternal Vibratorsrs

Vibrating Table: Vibrating Table: used for small used for small amounts of amounts of concrete concrete (laboratory and (laboratory and some precast some precast elements)elements)

External VibratoExternal Vibratorsrs

CURING OF CONCRETECURING OF CONCRETE

Curing is the procedures used for Curing is the procedures used for promoting the hydration of cement and promoting the hydration of cement and consists of a control of temperature and consists of a control of temperature and the moisture movement from and into the the moisture movement from and into the concrete.concrete.

Hydration reactions Hydration reactions can take place in can take place in only saturated water only saturated water filled capillaries. filled capillaries.

CURING OF CONCRETECURING OF CONCRETE

The primary objective of curing is to keep The primary objective of curing is to keep concrete saturated or as nearly saturated concrete saturated or as nearly saturated as possible.as possible.

Curing MethodsCuring Methods

1.1. Methods which supply additional Methods which supply additional water to the surface of concrete water to the surface of concrete during early hardening stages.during early hardening stages.

– Using wet coversUsing wet covers– Sprinkling Sprinkling – PondingPonding

Curing MethodsCuring Methods

2.2. Methods that prevent loss of moisture Methods that prevent loss of moisture from concrete by sealing the surface.from concrete by sealing the surface.

– Water proof plasticsWater proof plastics– Use liquid membrane-forming Use liquid membrane-forming

compoundscompounds– Forms left in placeForms left in place