Concrete - Mert Yücel YARDIMCI · Fresh concrete must have the desired degree of workability for a...

Materials of Construction-Concrete 1

Concrete

Chapter 10

Properties of Fresh Concrete

Wikipedia.org


What is Concrete?

Concrete is a composite material that consists of a

cement paste within which various sizes of fine and

coarse aggregates are embedded.

Concrete is a versatile material that can easily be

mixed to meet a variety of special needs and formed

to desired shape.

Concrete contains entrapped air and may contain

purposely-entrained air by the use of air-entraining

admixtures.


Concrete Constituents

Cement

Water

Fine Agg.

Coarse Agg.

Admixtures


Advantages

Ability to be cast

Economical

Durable

Fire resistant

Energy efficient

On-site fabrication


Disadvantages

Low tensile strength

Low ductility

Volume instability

Low strength to weight ratio


Specific operations must be performed in a certain order final quality is influenced by every step

I. mix design (proportioning) II. trial mixes & testing III. batching ------------------------------------------------------------start the clock IV.mixing V. transporting VI. pouring (placing) VII. vibrating (consolidating) -------------------------------------------------------------initial set here VIII. finishing -------------------------------------------------------------final set here IX. curing X. maintenance

Order of Operations for Concrete

In lab.

In plant

In site

Sampling is here


Fresh? Hardened?

The plastic state of the concrete starting from the

time that the concrete making materials are

mixed until the concrete gains rigidity is called

fresh concrete

The state of the concrete starting from the time

that it gains rigidity is called hardened concrete

Basic Properties of Fresh Concrete

Workability

Consistency

Segregation

Bleeding

Setting time

Unit weight

Uniformity



FRESH CONCRETE

Fresh concrete must have the desired degree of workability for a given job. It must;

be easily mixable and transportable,

be sufficiently flowable so that it can fill the forms completely for which it was designed,

have the ability to be compacted fully without an excess amount of energy being applied,

be uniform, and the materials in it must not show segregation during mixing, placing and consolidation and,

its surface must be easily finishable to the required amount of smoothness.


FRESH CONCRETE

Other fresh concrete properties;

It must show very little or no bleeding.

It must have a proper setting time.

It must have the required amount of unit

weight.


WORKABILITY

Workability is the ease with which fresh concrete can be mixed, placed, compacted and finished without harmful segregation.

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

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


Factors Affecting the Workability

- Quantity of the mixing water,

- Quantity and properties of the cement,

- Gradation and Dmax of the aggregate,

- Shape and surface texture of aggregate particles,

- Quantity of mineral admixtures,

- Quantity and characteristics of chemical admixtures,

- Quantity of the entrained air,

- Temperature of concrete.


Consistency is the fluidity or degree of

wetness of concrete.

It is generally dependent on the shear

resistance of the mass.

It is a major factor in indicating the

workability of freshly mixed concrete.

Consistency


Flow test → measures the amount of flow

Kelly-Ball test → measures the amount of penetration

Slump test (Most widely used test!)

Test Methods for Measuring

Consistency


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.

Concrete

Concrete Concrete

Slump cone

(Abram’s cone)


Slump Test is related with the ease with

which concrete flows during placement (TS

EN 12350-2, ASTM C 143)


Class Slump , mm

S 1 10 – 40

S 2 50 – 90

S 3 100 – 150

S 4 160 - 210

S 5 > 220

Slump Classes acc. to TS EN 206-1


SLUMP TEST VIDEO RECORD


In a properly proportioned concrete, the water

content required to produce a given slump

depends on several factors:

Shape and surface texture of aggregate,

Dmax of well graded aggregate,

Entrained air.

Segregation

Seperation of the coarse aggregates from the

mortar in the fresh concrete mixture.

The segregation may take place either by the

settling of heavy particles to the bottom of the

fresh concrete or by the seperation of the

coarse aggregates from the body of the

concrete due to improper placing or vibration

of the concrete.


Factors leading to segregation

Large maximum particle size (over 25 mm) and

proportions of the large particles.

Decreased amount of sand and cement (decreased

amount of paste volume in concrete mix).

Too wet and too dry mixtures.

Improper mixing, improper placing, and improper

consolidation (prolonged vibration) of the concrete.


Bleeding

Bleeding is a form of segregation in which some of the

water in the mix tends to rise to the surface of the

freshly placed concrete.


It is caused by the inability

of the solid particles in the

fresh concrete mix to hold

all of the mixing water as the

solid particles settle

downward with the effect of

gravitional forces.

Harmful effects of bleeding

With the movement of water towards the top, the top

portion of the concrete becomes weak and porous

(relative w/c ratio increment in bleeded region!)

Since the top part of the concrete section is porous

(and permeable), the resistance of the concrete to

freezing and thawing or to sulfates is decreased.

Relatively weak and non durable surface, which is not

resistant to abrasion, may be obtained.


Bleeding can be reduced!

By increasing the cement fineness and/or by using

finely divided mineral admixtures.

By entraining ait into the concrete mix (use of air

entraining chemicals).

By reducing the water content of the concrete

mixture, if this can be done while maintaining an

acceptable workability.

By placing the concrete in relatively shallow layers.



Setting Time

Setting time is the length of time from the moment of mixing concrete making material until the moment that concrete gains rigidity. During this time concrete is plastic and shapeable.

The setting time for ordinary concrete is usually 3-5 hours.

The setting time of concrete is affected by:

- mix proportions,

- type and amount of cement,

- type and amount of admixtures,

- the ambient temperature.


Unit Weight

Unit weight of fresh concrete is the weight of it per unit volume (including the volume of the solid particles and the spaces between them). It is expressed in kg/m3 or ton/m3.

Unit weight of concrete is affected by:

- the gradation of the aggregates,

- the specific gravity of the aggregates,

- the degree of compaction applied to the concrete

- the air content of concrete and,

- obviously mix proportions of concrete.


Unit Weight

The unit weight of concrete made with normal weight aggregates is around 2400 kg/m3. A concrete that has a relatively higher unit weight is a more compact concrete, which leads to higher strength and durability.

The unit weight of concrete is generally used to check the uniformity of concrete batches (produced using same types and proportions of materials) and in mix design calculations.


Uniformity

Uniformity: Concrete should be uniform

and similar in appearance and quality

within-batch and batch-to-batch.

Within-batch variations: Differences in the

quality of concretes obtained from one

section of a concrete batch to another

section of the same batch.


Determination of uniformity

Concrete uniformity is

checked by conducting

tests on fresh and

hardened concretes.

Slump, unit weight, air

content tests

Strength tests


Specific operations must be performed in a certain order final quality is influenced by every step

I. mix design (proportioning) II. trial mixes & testing III. batching ------------------------------------------------------------start the clock IV.mixing V. transporting VI. pouring (placing) VII. vibrating (consolidating) -------------------------------------------------------------initial set here VIII. finishing -------------------------------------------------------------final set here IX. curing X. maintenance

Order of Operations for Concrete

In lab.

In plant

In site

Sampling is here

Proportioning of Concrete Mixes

(Mix Design)

Determine proportions of mix ingredients

• be economical

• be practical

• use available materials

• satisfy requirements & specs

• acceptable workability of fresh mix

• quality (durability, strength, appearance) of hardened

concrete

• economy

31

Mix Design: Volumetric Method

1. Strength requirements

2. Determine W/C

3. Estimate coarse aggregate mass

4. Air entrainment requirements

5. Workability needs

6. Estimate water content

7. Determine cement content requirements

8. Evaluate admixture needs

9. Estimate fine aggregate mass

10. Determine moisture corrections

11. Trial Mix


Trial Mixes

check proportions with trial batches

• air content

• slump

• 28 day compressive strength:

•3 cylinders – 15cm in dia x 30 cm

adjust for optimum workability & economy


Batching Concrete

Batching is the process of measuring

concrete mix ingredients by either mass or

volume and introducing them into the

mixer. To produce concrete of uniform

quality, the ingredients must be measured

accurately for each batch.

Most specifications require that batching

be done by mass rather than by volume


Mixing Concrete

All concrete should be mixed thoroughly until it is uniform in appearance, with all ingredients evenly distributed.

Mixers should not be loaded above their rated capacities and should be operated at the mixing speed recommended by the manufacturer.

If the blades of a mixer become worn or coated with hardened concrete, mixing action will be less efficient. These conditions should be corrected.


Stationary Mixing

Concrete is sometimes mixed at the jobsite in a stationary mixer or a paving mixer.

Stationary mixers include both onsite mixers and central mixers in ready mix plants. They are available in sizes up to 9.0 m3 and can be of the tilting or nontilting type or the open-top revolving blade or paddle type.

All types may be equipped with loading containers and some are equipped with a swinging discharge chute.




Stationary Mixing

Careful attention should be paid to the required

mixing time. Many specifications require a

minimum mixing time of one minute plus 15

seconds for every cubic meter.

Short mixing times can result in

nonhomogenous mixtures, poor distribution of

air voids (resulting in poor frost resistance),

poor strength gain, and early stiffening

problems.


Stationary Mixing

Under usual conditions, up to about 10%

of the mixing water should be placed in

the drum before the solid materials are

added.

Water then should be added uniformly

with the solid materials, leaving about

10% to be added after all other materials

are in the drum.


Ready-Mixed Concrete

Ready mixed concrete is proportioned and mixed off the project site and is delivered to the construction area in a freshly mixed and unhardened state. It can be manufactured by any of the following methods:

1. Central-mixed concrete is mixed completely in a stationary mixer and is delivered either in a truck agitator, a truck mixer operating at agitating speed or a truck

2. Shrink-mixed concrete is mixed partially in a stationary mixer and completed in a truck mixer.

3. Truck-mixed concrete is mixed completely in a truck mixer



Pouring (Placing)

Chute Conveyor


Pouring (Placing)

During (or just before) the placing, slump and air content

of the fresh concrete should be determined and samples

for further strength tests should be prepared.

Test on site

• Slump

• Air content

Prepare samples for determining

concrete strength class

• Standard cylinders (or cubes)

• Beams (if required)


Pouring (Placing)


Directly into form

Wheel barrow/ buggy Bucket

Pump


Placing Issues

Drop height

• < 90 cm

Horizontal movement

• Limit to prevent segregation

Pumping

• Adjust mix design

tremie chute to limit drop height


Vibration of Concrete

Consolidation (compaction)

should be completed before initial set

Manually by

– ramming

– tamping

Mechanically using vibrators

– Internal – poker

• 5 sec to 2 min in one spot

• <10 sec. typical

• avoid segregation

• through entire depth

• penetrate layer below if still plastic

– External – by vibrating the mould itself


Finishing Concrete

smoothing and imprinting the surface of the

concrete with the desired texture.

must be completed before final set


Screeding – strike concrete off to desired level

Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. Power float


Curing Concrete

Maintain moisture and

temperature in the

concrete to promote

continued hydration and

strength gain

Hydration will resume if

curing is stopped and

resumed

Curing affects:

• durability

• strength

• water-tightness

• abrasion resistance

• volumetric stability

• resistance to freezing

and thawing

• resistance to de-icing

chemicals


Curing Approaches

1. Maintaining presence of water in the

concrete

2. Seal the surface so mix water can’t

escape.

3. Providing heat & additional moisture to

accelerate the strength development


Approach 1. Maintaining Presence

of Water

Must water periodically

Also provides cooling

Methods

• ponding: smaller jobs flat-work (floors and

pavement) and laboratory

• spraying or fogging: expensive and a lot of water

• wet coverings: burlap, cotton, rugs, etc.


Wet Covering

Fogging

Spraying


Approach 2. Seal the Surface

impervious paper or

plastic sheets

membrane forming

compounds

leave forms in place

Plastic Sheets


Approach 3. Heat

Insulate

Hot steam application (around 60C0)

good for early strength gain and in freezing weather

Electrically heated forms

Usually in precast plants only

Can be applied in very cold weather conditions


Compressive strength of concrete at different

ages & curing levels

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Concrete - Mert Yücel YARDIMCI · Fresh concrete must have the desired degree of workability for a...

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