Chapter V Fresh Concrete...If one-half of the cone slides down along an inclined plane, it is regarded as shear slump. Shear slump is caused by insufficient cohesiveness and the concrete

Chapter VFresh Concrete

Assist.Prof.Dr. Mert Yücel YARDIMCI

Advanced Concrete Technology - Zongjun Li 1

Fresh Concrete


Fresh concrete is defined as a fully mixed concrete in a rheological state that has not lost its plasticity.

The fresh concrete stage covers the cement hydration stages I and II.

The plastic state of fresh concrete provides a time period for transportation, placing, compaction, and surface finishing.

The properties of fresh concrete have a large influence on construction speed and decision making.

Workability of fresh concrete


The properties of fresh concrete are short-term requirements in nature, and should satisfy the following requirements:

1. It must be easily mixed and transported.2. It must be uniform throughout a given batch, and between batches.3. It must keep its fluidity during the transportation period.4. It should have flow properties such that it is capable of completely filling the forms.5. It must have the ability to be fully compacted without segregation.6. It must set in a reasonable period of time.7. It must be capable of being finished properly, either against the forms or by means of troweling or other surface treatment.


Compaction plays an important role in ensuring the long-term properties of thehardened concrete, as proper compaction is vital in removing air from concreteand in achieving a dense concrete structure.

The compressive strength of concrete can increase with an increase in the density. Traditionally, compaction is carried out using a vibrator.

Nowadays, the newly developed self-compacting concrete can reach a dense structure by its self-weight without any vibration.

Workability of fresh concrete

Definition of workability


Workability of concrete is defined in ASTM C125 as theproperty determining the effort required to manipulate afreshly mixed quantity of concrete with minimum loss ofhomogeneity (uniform).

The workability of fresh concrete can be defined as “theamount of mechanical work, or energy, required toproduce full compaction of the concrete withoutsegregation (Mindess et al. 2003).”

Measurement of workability


There is no universally accepted test method that candirectly measure the workability. Main tests for assessingthe consistency of normal-weight and vibrating freshconcrete:

• The most widely used test, which mainly measures theconsistency of concrete, is called the slump test.

• The second test in order of importance is the Vebe test, which ismore meaningful for mixtures with low consistency.

• The third test is the compacting factor test, which attempts toevaluate the compactability characteristic of a concrete mixture.

Testing fresh concrete


Part 8: Self Compacting Concrete – Slump Flow Test


Slump test

a: Fill the fresh concrete into Abram’s cone in three layers by rodding each layer 25 times b: Making smooth surfacec: Lift-up the cone upward in a constant speed.d: Measure the slump value

Slump test


If slumping occurs evenly all around, it is regarded as a true slump.If one-half of the cone slides down along an inclined plane, it isregarded as shear slump.

Shear slump is caused by insufficient cohesiveness and theconcrete proportions should be adjusted.

The slump test is unreliable for very dry and lean mixes.

Slump test


https://www.youtube.com/watch?v=lwZf217v5XA

https://www.youtube.com/watch?v=lwZf217v5XA

Slump classes (EN 206-1)


VeBe test


The test equipment, which was developed by Swedish engineer V. Bahrner, is shown inFigure.

The Vebe test is a good test,particularly for very dry mixes.

The cohesiveness of concretecan be easily distinguished byVebe test through theobservation of distribution of thecoarse aggregate after vibration.

VeBe test


https://www.youtube.com/watch?v=XWJviHtlTsc

https://www.youtube.com/watch?v=XWJviHtlTsc

VeBe Classes (EN 206-1)


Degree of compactability


Compacting Factor (EN Norms do not cover this test)

16

It consists of two hoppers and one cylindrical mold stacked in three levels.

Usually, the range of compaction factor is from 0.78 to 0.95 and concrete with high fluidity has a higher compaction factor.

Compacting factor


https://www.youtube.com/watch?v=0d2dUXbdVzs

https://www.youtube.com/watch?v=0d2dUXbdVzs

Flow Table Test


Flow table test


https://www.youtube.com/watch?v=RO5GHlTvGnA

https://www.youtube.com/watch?v=RO5GHlTvGnA

Flow classes


Self Compacting Concrete-Slump-Flow Test-


https://www.youtube.com/watch?v=dGd8zU0XyNU

https://www.youtube.com/watch?v=dGd8zU0XyNU

Slump flow classes (EN 12350-8)


Air Content of Fresh ConcreteEN 12350-7


https://www.youtube.com/watch?v=qaUamXhjqKA

https://www.youtube.com/watch?v=qaUamXhjqKA

Factors affecting workability


• Water content• Cement content• Aggregate characteristics• Admixtures• Temperature and time

Through the influences on consistency and/or cohesiveness

Water content

25

• The most important factor influencing the workability ofconcrete.

• Some of the added water is absorbed on the surface of thethe cement and aggregates. Additional water fills the spacesamong the particles and “lubricates” the particles by a waterfilm.

• Decreasing the water content will result in a low fluidity.

• If the water content is too small, the concrete will becometoo dry to mix and place.

• Too much water will reduce cohesiveness and leads tosegregation and bleeding, and reduces the concretestrength.

• The water content in a concrete is determined by w/c or w/band cement or binder content.


Cement content influences the workability of concrete in two ways.

• For given constant w/c ratio, the larger the cement content, thehigher the total water amount in the concrete; hence, theconsistency of concrete will be enhanced due to the increasedcement paste content.

• Cement paste itself plays the roles of coating, filling, andlubrication for aggregate particles.

In normal concrete, a considerably low cement content tends toproduce a harsh mixture, with poor consistency and, subsequently,poor finishability.

High cement content implies that more lubricant is available forconsistency improvement.

Cement content

Cement content


With an increase of the cement content at a low w/c ratio, bothconsistency and cohesiveness can be improved. Under the same w/cratio, the higher the cement content, the better the workability.

Increasing the fineness of the cement particles will decrease thefluidity of the concrete at a given w/c ratio, but will increase thecohesiveness.

Concretes containing a very high proportion of cement or very finecement show excellent cohesiveness but tend to be sticky.


Aggregate CharacteristicsAggregates can influence the workability of concrete through their need for surface coating and their friction and mobility during mixing, placing, and compaction.

Aggregate Properties Influencing Workability• Maximum aggregate size, • Aggregate/cement ratio, • Fine aggregate/coarse aggregate ratio, • Aggregate shape and texture


For a given w/c ratio, as the maximum size of aggregate increases, the fluidity increases.

Aggregate Characteristics

Very fine sands or angular sands will require more paste for agiven consistency.They will produce harsh and unworkable mixtures at watercontents that might have been adequate with coarser or well-rounded particles.

In general, to get a similar consistency of concrete, morewater is needed when crushed sand is used instead ofnatural sands.



The aggregate/cement ratio influences the paste requirement.

A higher aggregate/cement ratio implies more aggregates and lesscement paste.

The concrete consistency decreases with aggregate/cement ratioincrease due to less cement paste being available for lubrication.


Fine aggregate/coarse aggregate ratio also affects the cement paste requirement.


With an increase of the fine aggregate/coarse aggregateratio, concrete contains more fine aggregates and lesscoarse aggregates. The total surface area of the aggregatesincreases, which leads to a higher demand on the cementpaste for surface coating.

As a result, by an increase in fine/coarse aggregate ratio, the consistency of concrete decreases and the cohesiveness improves. Increasing the fine aggregate/coarse aggregate ratio is the most effective measure to increase the cohesiveness of concrete.


The shape and texture of aggregate particles can affect theworkability of concrete through the influence on pasterequirement, particle moving friction, and moving ability.

Cubical, irregular, granular, and rough aggregates requiremore coating cement paste and have higher friction thanspherical, glassy, and smooth aggregates.

As a general rule, the more spherical the particles, the moreworkable is the concrete.


Temperature and time


Freshly mixed concrete stiffens with time due to loss of water by • evaporation of the mixing water.• absorption by the aggregate.• consumption in the formation of hydration products.

The stiffening of concrete is effectively measured by a loss ofworkability with time, known as slump loss, which varies with• richness of the mix,• type of cement,• temperature of the concrete,• initial workability.

A high ambient temperature reduces the workability and increases the slump loss because the hydration rate is higher and the loss of water is faster at a higher temperature. In practice, when the ambient conditions are unusual, it is best to perform actual site tests to determine the workability of the mix.

Segregation and bleeding


SEGREGATION

Cohesiveness is an important characteristic of the workability.

A proper cohesiveness can ensure concrete to hold all the ingredients in a homogeneous way without any concentration of a single component, and even after the full compaction is achieved.

An obvious separation of different constituents in concrete is called segregation





In the case of concrete, it is the differences in the size and weight ofparticles (and sometimes in the specific gravity of the mixconstituents) that are the primary causes of segregation, but theextent can be controlled by the concrete proportion, choice ofsuitable grading, and care in handling.

Segregation can be defined as concentration of individual constituents of a heterogeneous (nonuniform) mixture so that their distribution is no longer uniform.



Bleeding is a form of local

concentration of water in somespecial positions in concrete,usually the bottom of the coarseaggregates, the bottom of thereinforcement, and the topsurface of the concrete member.

Bleeding can be expressed quantitatively as the total settlement(reduction in height) per unit height of concrete, and bleedingcapacity as the amount (in volume or weight) of water that rises tothe surface of freshly placed concrete.



As a result of bleeding, an interface between aggregates and bulk cement pasteis formed, and the top of every lift (layer of concrete placed) may become toowet. If the water is trapped by the superimposed concrete, a porous and weaklayer of nondurable concrete may result.

If the bleeding water is remixed during the finishing process of the surface, aweak wearing surface can be formed. This can be avoided by delaying thefinishing operations until the bleeding water has evaporated, and also by the useof wood floats and avoidance of overworking the surface.

ATTENTION, if evaporation of water from the surface of the concrete is fasterthan the bleeding rate, plastic shrinkage cracking may be generated. In such acase the fast evaporation should be prevented (by keeping the concrete surfacewet…)

Slump loss


Slump loss can be defined as the loss ofconsistency in fresh concrete with elapsed time.

Slump loss is a normal phenomenon in all concretesbecause it results from gradual stiffening and setting ofhydrated cement paste, which is associated with theformation of hydration products such as ettringite andcalcium silicate hydrate.

Slump loss


Slump loss occurs when the free water from a concrete mixture is removed by hydration reactions, by absorption on the surface of hydration products, and by evaporation.

Slump loss should be controlled to an acceptable value, especially for concrete transported with a long delivery time, to ensure that it is still placeable and compactable when shipped to the construction site.

Slump loss can be minimized by using a setting retarder.

Setting of Concrete


Setting of concrete is distinguished asthe initial and final setting of cementpaste.

The initial setting is defined as theloss of plasticity or the onset ofrigidity (stiffening or consolidating) infresh concrete.

The final setting is defined as the onset point of strength.

It is different from hardening, which describes the development of useful andmeasurable strength.

Setting time


The measurement of the setting time for concrete is very differentfrom that of cement paste.For cement paste, it uses the samples made of the water amountneeded for consistency. For concrete, it uses the sieved mortarfrom a concrete with different water/cement or water/binderratios.For cement paste, it measures the penetration depth of the Vicatneedle, 1mm in diameter, under a constant weight.For concrete, it measures the resistance of the mortar to a rodunder an action of the load.

Setting time


ASTM C 403 (1995) defines two points as the initialsetting time and the final setting time, correspondingapproximately to the point at which the concrete will nolonger be plastic during vibration, and a concretestrength gain of about 0.7 MPa.

Initial setting and final setting of concrete are determinedby the times at which the penetration resistance reaches3.5 and 27.6 MPa, when a designated rod penetrates25.4mm into the mortar, sieved from the fresh concrete.

Documents

Chapter V Fresh Concrete...If one-half of the cone slides down along an inclined plane, it is regarded as shear slump. Shear slump is caused by insufficient cohesiveness and the concrete