Introduction While concrete is in its fresh phase it is then being moved

Introduction
While concrete is in its fresh phase it is then being moved, handled and placed and it will be compacted as well. Normally in the time horizon when concrete is mixed and when it starts to set, it will only be in the fresh phase. This phase will only last a couple of hours which is a very small amount of time seeing that the structure has a long lifespan for example 50 years. The qualities of the hard concrete will mainly be influence by the properties of concrete in the fresh state.
In this report the following headings will be discussed:
? Consistency
? Workability
? Settlement & Bleeding
? Plastic shrinkage
? Slump loss
? Water requirements
? Special consistence tests

Picture 11: Concrete test on site.

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Consistency
Consistency means the amount of water needed to prepare a plastic mix. Consistency test is done using the Vicat’s apparatus. The Consistency of the cement should be less than 30%. The consistency test is used to determine the amount of water that should be mixed with cement. It is very important to find the consistency because the amount of water present in the cement paste may affect the setting time of the paste.
The Standard consistency of concrete can be defined as the total percentage water requirement of the cement paste at which viscosity of the paste can become such that the plunger in a specially designed apparatus (known as Vicat’s apparatus) will be able to penetrate to a depth of 5 to 7mm, measured from the bottom of the mould.
Each batch is measured by its consistency normally by measuring the sloppiness and stiffness of the mix. To effectively handle, place and compact the concrete, the consistence must be the same. Thus it is of utmost importance to measure the consistency. Usually in practice consistence measurements are used to act as a means of controlling the amount of mixing water in the concrete.
Table 1: Common mixer types, cycle times & outputs
Mixer Type Cycle time (Seconds) Typical mixer size (Litres)

Free-fall Tilting drum 100-150 100-200
Reversing drum 90-150 200-500
Split drum 90-120 3000-4000
Forced-action Pan 60-110 200-2000
Single & Twin-shaft 60-110 500-2000

The slump test is mainly used to measure consistence of conventional concrete.
The 5 consistency classes with slump ranges are as follow:
Slump class:
S1: Dry (kerb backing, dry fill and sloping slab etc.) Slump in 10mm – 40mm.
S2: Standard (Foundations, slabs and other general structures) Slump in 50mm – 90mm.
S3: Wet (Foundations, slabs, plumbing concrete) Slump in 100mm – 150mm.
S4: Very Wet (Foundations, slabs, plumbing and piling concretes) Slump in 160mm – 210mm.
S5: Self levelling (Foundations, slabs, plumbing and piling concretes) Slump in ? 220mm.

It is known that the wetter the concrete the weaker the strength of the mix. This process can and should be treated by increasing the cement content, however, this might increase the overall cost on some concrete mixes.
Following factors which affects the consistency of cement:-
• Fineness of cement
• Temperature.
• Method of mixing water.
• % of admixtures in cement (flash in case of PPC)
• w/c ratio

The concrete slump test measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete, and therefore the ease with which concrete flows. It can also be used as an indicator of an improperly mixed batch.

Picture 1: Slump test: Test the consistency of concrete.

Workability
• Workability refers to:

– The effectiveness in which the concrete can be transport to a site
– The most easy way in which concrete can be placed into formwork, including the flow of concrete into corners and around obstructions
– The comfort in which concrete can be compacted (concrete must usually be compacted fully on site with the equipment available, in such a way that most air is driven out of the concrete).
– The easiness in which the concrete can be finished, including honey combing (i.e. holes in the concrete, because compaction was not adequate), sand streaking on the surface etc.

• Workability can be affected by the following:

– The cement paste amount: the concrete mix will be more workable, if there is more concrete paste mixed with the aggregate. Higher cement content in the concrete mix while the water content remains the same will make the mix stickier.
– The effective use of the mix will drastically be influence when the proportions in which the components of a concrete mix are combined. The cohesiveness of the mix will increases, if the fines in the mix are decreased. If the fines in the mix are lower the thickness of the mix also gets smaller, while there is a possibility of segregation and bleeding if over-vibrate.
– The aggregates will move more freely around each other which will improve the comfort and use, if rounded aggregate particles are used instead of rough.
– When the stone content is increased in the concrete mix, the mix will then become harsher which will result in more difficult compaction.
– Although the water requirements for any given slump increases, in well-proportioned concrete mixes, a smaller stone size will lead to improved workability.

The definition of workability of concrete refers to the mix of consistency and cohesiveness of concrete
? Consistency can be classified to the stiffness, sloppiness or fluidity of a concrete mix.
? The water content can also be one of the major influences of consistence of a concrete mix.

• Cohesiveness:

– The cohesiveness of the paste describes how well a concrete mix keeps together when it still in the plastic, i.e. not yet hardened.
– This means it describes the ability of the concrete mix that should remain well mixed.
– When cohesiveness is being discussed, the degree of segregation of the aggregation in the mix should also be taken into consideration.
– If separation of the aggregates in the concrete mix takes place it is very likely that the mix will not compact well enough and that honeycombing can take place.

• How to improve the cohesiveness of a concrete:

– By increasing the slump, when the concrete mix is already dry, i.e. add more water.
– Reduce the slump, when concrete mix is too wet, i.e. use less water. To reach full compaction, care must be taken that the concrete stays sufficiently workable.
– Use less stone or smaller stone.
– By adding finer blending sand to the mix, it will increase the fines content. The cementitious content can also be increased.
– Add a mineral admixture like Condensed Silica Fume (CSF).
– Add an air-entraining agent, by which air bubbles are incorporated into the concrete mix.

• Factors which affect workability of concrete are:
– Cement content of concrete
– Water content of concrete
– Mix proportions of concrete
– Size of aggregates
– Shape of aggregates
– Grading of aggregates
– Surface texture of aggregates
– Use of admixtures in concrete
– Use of supplementary cementitious materials
The primary materials used for concrete are cement, fine aggregates (sand), coarse aggregates and water. Most of the time admixtures are added in the concrete to improve its properties. Therefore, the properties of these materials and their content will substantially affect the workability of the concrete.

Type of measurements to use for workability:
• Slump test

Picture 2: Steps of Slump test.

• Compacting factor test

Picture 3: Compacting factor test.

• Flow test

Picture 4: Flow test.

• Kelly ball test

Picture 5: Kelly ball test apparatus.

• Vee Bee consistometer test

Picture 6: Vee Bee Consistometer.

Settlement & Bleeding
The average Cement and aggregate particles has a densities of about three times that of water. In fresh concrete they consequently tend to settle and displace mixing water which migrates upwards and may be collected on the top surface of the concrete. This upwards movement of mixing water can be defined or seen as bleeding; this is the action where the water separates from the rest of the concrete and is called bleed water. Settlement and bleeding will continue until the concrete sets.

Beneficial effects of settlement and bleeding on concrete:
– Bleeding, by removing water from the concrete, reduce water-cement ratio (W/C) which could lead to improve potential strength and impermeability. However, the negative effects are usually far greater than any benefit from a reduction in W/C ratio.
– A film of bleed water on the surface of concrete being placed out of doors, e.g. paving, reduces the evaporation of water from the concrete and so reduces the harmful effects of plastic shrinkage.

Harmful effects:
– The Bleeding water then gets trapped under the reinforced bars and aggregate particles, especially stone, thus creating voids and weak internal surfaces and increasing permeability of the hardened concrete.
– Where settlement is restrained, e.g. by horizontal reinforcement or by steps in the formwork surface, the concrete may crack. This occurrence often causes cracking at links in reinforced concrete columns.
– Bleeding increases W/C of the mortar layer at the top of the concrete, leading to the weakening the surface.

Factors that tend to increase settlement and bleeding:
– High aggregate particle relative density (RD)
– Lack of very fine material in the mix
– High water content
– Delayed setting

Controlling settlement and bleeding
In some cases it may be necessary to reduce the amount of bleeding of a concrete mix. In practice this is normally done by adding more fine materials in the mix, reducing the water content, or by entraining air in to the mix. Increasing the cement content or using a finer cement is a relatively ineffective and expensive means of reducing bleeding.
Also alternative Crusher sand with a high- 75-µm content may be used; otherwise incorporating a proportion of fine pit sand with either crusher or river sand is beneficial. Another method is the use of condensed silica fume is a very efficient but expensive way of minimising bleeding. The water content of the mix can also be reduced by improved mix design or by the use of suitable and alternative admixtures or materials with a lower water requirement. Air-entraining agents may be used but the modifications to the concrete mix design will be needed.
Overcoming harmful effects
Voids and weak internal surfaces under reinforcement and stone particles, and settlement cracking, and any other sort off failure of this nature can be eliminated by vibrating the concrete just before it sets.

Picture 7: Bleeding of concrete.

Plastic Shrinkage
When water is removed from the compacted concrete mix before it sets properly, the volume of the concrete is reduced by the amount of the water removed. We call this volume reduction plastic shrinkage. Water may be removed from the plastic concrete by evaporation or by being absorbed by dry surfaces such as soil or old concrete.
Plastic shrinkage is not that harmful in itself, but where shrinkage is restrained, e.g. by formwork or by other concrete, the concrete may crack or be affected. Sometimes in certain cases plastic shrinkage cracks can easily penetrate the full depth of concrete slabs. Plastic-shrinkage cracking is especially harmful and destructive in floor slabs and paving placed in conditions where it is exposed to direct sun, high ambient temperatures and wind. Such conditions cause rapid evaporation from the surface of the concrete. The resulting shrinkage of the upper part of the slab is restrained by the underlying concrete cracks, from the surface downwards.
Avoid the harmful; effects of plastic-shrinkage cracking by preventing the water getting lost from the fresh concrete, e.g. by making the soil wet before placing concrete on it and by covering the compacted concrete with plastic sheeting or use a water spray on the concrete paving. Any surface Cracks that have been caused by restrained plastic shrinkage can be closed by vibrating the concrete before it sets.
Plastic shrinkage cracks are most likely to occur on horizontal surfaces that has been exposed to the outdoor elements. These types of cracks are different from other early cracks because they tend to be much deeper and wider. Plastic shrinkage cracks are normally small about two to four inches deep and exactly one-eighth inch wide. They may also become longer with several feet in length adopting a crow’s-foot pattern or even a tree branch pattern. As a nature of these cracks most of the time they form before any bond has developed between the aggregate particles and mortar. Therefore, the cracks tend to follow the edges of large aggregate particles or reinforcing bars and never break through the aggregate particles. Although plastic shrinkage cracks usually do not really affect the structural performance of the slab, cracks might be to blame in case where leaks occur.
There are many corrective and effective procedures that can be followed listed below to reduce the risk of experiencing plastic shrinkage cracks.
1. Moisten subgrades and forms to prevent absorption.
2. Dampen dry aggregates that are absorptive.
3. Reduce the temperature of the concrete by
a. Precooling aggregate with water.
b. Cooling the cement.
c. Using chipped ice to cool mixing water.
d. Shading aggregates, water tanks, and lines.
1. Avoid overmixing.
2. Place concrete early in the morning or late afternoon.
3. Construct temporary walls to reduce wind velocity.
4. Provide sunshades for concrete.
5. Reduce time between placing and start of curing by working efficiently during construction.
6. Use evaporation retardant (usually polymers).
7. Use fog sprays to keep the humidity high and the air temperature low.

Water is lost from the concrete mass in two main ways:
When drying from the top Moisture rises to the top surface of a concrete element during placement this is a process that is known as bleeding. Bleed water dries out through evaporation; when the rate of evaporation exceeds the rate of bleeding, this is when the problems start to show and the surface dries and tends to crack.
While drying from the base Water in a concrete slab may be absorbed into the subgrade or ground below. In addition to affecting bleeding this could rapidly increase the settlement of the concrete and the risk of surface cracking.
The following preventive measures will help to reduce the occurrence of concrete cracking:
o Pouring the concrete during the part of the day that is the coolest
o Protecting the surface of the concrete from wind and sun with shade cloth and wind breaks
o Gently applying a good quality curing compound to the mix immediately after the surface is finished
o Protecting the surface with damp sand, hessian or plastic sheet immediately after the surface is finished
o Using water/fog sprays to keep the surface of the concrete constantly and continuously damp
o Delaying the finishing of the surface in order to close up any cracks that may have occurred
Main Causes of Plastic Shrinkage Cracking
In general Plastic shrinkage cracks occurs on concrete surfaces exposed to high surface temperatures, strong wind or low humidity. Any environmental factor which increases the rate of evaporation of bleed water from the surface of the concrete increases the potential for concrete cracking. When the evaporation rate exceeds the bleeding rate it is likely that cracks might occur In essence, cracking is likely if the evaporation rate exceeds the bleeding rate.
Concrete mix design also influences the probability of cracking. Mixes designed to be cohesive with low bleeding capacity are highly likely to crack if the environmental conditions are continues. For example, cracking of pump mixes is particularly prevalent in the Western Cape in summer, as is cracking of some mixes containing very fine grounded cements or extenders. Retardation of set, from whatever cause, also increases the probability of concrete cracking.
Settlement cracks occur when the settlement of the concrete is restrained; this is mainly caused by the formwork, the reinforcement or, sometimes, void formers.
When cracks are caused by restraint by reinforcement, the crack pattern will always follow the steel underneath and the cracks will proceed straight to the steel. In effect this will also mean that the concrete settles away from the steel which negatively affects the cohesive bond. This is a very bad and unfavourable and undesirable occurrence, especially in conditions of unmoral environmental exposure conditions.

Picture 8: Plastic shrinkage.

Slump Loss
From the starting time of mixing, the fresh concrete slowly loses consistence, this is also known as slump loss. This opens up possibilities for problems but only if the concrete becomes too hard or stiff to use, place and compact properly.
Slump loss is caused by:
• Cement Hydration
• Evaporation causing Loss of water
• The aggregates Absorbing the water
• Absorption of water by surfaces in contact with the concrete
The rate as the slump is lost increases with increasing concrete temperature and increase as cement content.
The following steps can be taken to minimise loss slump or overcome its adverse effects:
• The concrete can be batched at a higher slump than initially needed for placing and compacting.
• Reduce concrete temperature, e.g. by shading aggregate stockpiles, keeping stone stockpiles wet and using flaked ice as some of the mixing water.
• Minimise delays in handling the concrete.
• Under certain circumstances superplasticisers may be added to the concrete to regain slump. This can be done only if concrete is delivered in mixer trucks, and prior trails have been done.
The act of Adding water to the concrete mix in order to restore the lost slump must not be approved because this will increase the W/C ratio, with a high loss of strength and durability.
All concrete consist of a slump loss or it would never harden. In the normal slow concealing we call ‘setting’; the concrete firstly slowly loses its entire slump and then gradually begins to harden.
What can be a cause for concern to the concrete user is an abnormally high rate of slump loss.

There are several causes of troublesome slump loss such as:

1) TIME
the Time span is commonly ignored when listing and discussing the causes of slump loss. Actually it is by far one of the most important factors of all. Elapsed time, from mixing to placement, is always there and working every minute to reduce the slump.
Normal slump loss is merely a function of time:

Prevention
• Eliminate any possible delay
• Workable concrete must be established by trial and adjustment made as required under actual job condition
• One important adjustment that should be made after a suitable trial on the job, is to ensure that there is enough extra slump in the concrete as mixed to offset normal slump loss and allow enough time for transportation and placing

2) TEMPERATURE
The rate in which slump is lost is increased when concrete is mixed, handled or place of elevated temperature.
Whatever other causes of slump loss may be at work in a given situation, these seem to be quickly magnified at higher temperature.

Prevention

Cooling the Concrete truck with liquid nitrogen
• Cooling concrete materials such as aggregate and water ( ice, liquid nitrogen)
• Slowing the rate of setting time by retarder or supplementary cementing material
• By Follow the guidelines step for step on Hot-Weather Concreting

3) AGGREGATE
Aggregates are often blamed for causing slump loss because they are dry or because they have latent absorption.

Prevention
• Sprinkled to make the aggregates wet and pre-saturate so that they will not take the water from the concrete after mixing or during pumping
• Use shading for aggregate
• Highly absorption poor aggregate preferably should not be used

The following methods are helpful to manage slump loss in fresh concrete:
? Initial high slump
? Using retarders
? Using plasticizers or super plasticizers
? By repetitive dose
? By dosing at final point
? By keeping temperature low
? By using compatible super plasticizers with cement
Procedure of Concrete Slump test:
I. The mould for the slump testis a frustum of a cone, 300 mm of height. The base is 200 mm in diameter and it has a smaller opening at the top of 100 mm.
II. The base is placed on a smooth surface and the container is filled with concrete in three layers, whose workability is to be tested.
III. Each layer is temped 25 times with a standard 16 mm diameter steel rod, rounded at the end.
IV. When the mould is completely filled with concrete, the top surface is struck off (leveled with mould top opening) by means of screening and rolling motion of the temping rod.
V. The mould must be firmly held against its base during the entire operation so that it could not move due to the pouring of concrete and this can be done by means of handles or foot – rests brazed to the mold.
VI. Immediately after filling is completed and the concrete is leveled, the cone is slowly and carefully lifted vertically, an unsupported concrete will now slump.
VII. The decrease in the height of the center of the slumped concrete is called slump.
VIII. The slump is measured by placing the cone just besides the slump concrete and the temping rod is placed over the cone so that it should also come over the area of slumped concrete.
IX. The decrease in height of concrete to that of mold is noted with scale. (Usually measured to the nearest 5 mm.

Picture 9: Slump lost.

Water Requirements

The water requirement mentioned of fresh concrete is the water content of the concrete, measured in litres per cubic meter, this mix of water and concrete is required to bring the concrete mix to the specified consistence. Because the strength of the stiff or hardened concrete depends on the W/C ratio, water requirement has major implications and impact for the material cost of concrete. Water requirement also affects the dimensional stability of the hardened concrete.
Water requirement is determined by:
– The properties and contents of materials in the mix
– The consistence of the concrete.
The fineness and smallest modulus of sand does not influence water requirement at all but only if the concrete is correctly proportioned.
Table 2: Material factors influencing the water requirement of concrete:
Material Factors that influence water requirement Water requirement decreases with:
Stone Average particle size Increasing size
Packing capacity: – Shape
-Grading Improving packing capacity
Surface texture Increasing smoothness
Sand Particle shape Improving roundness
Grading Improving particle size distribution
Surface texture Increasing smoothness
Ultra-fines: – Type, e.g. clay content
– content Decreasing content, especially clay
Fineness Coarser cement
CEM I cement Source Some sources have lower water requirements than others
Cement blends containing extenders Type Use of fly ash
Zero CSF content
Proportion of extender in blend Increasing FA content
Decreasing CSF content
Admixture Type Use of plasticiser
Or superplasticiser
Dosage Increasing dosage

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.
Hydration: can be classified as the result of a chemical reaction that happens between the cement and water. Initially the cement grains are dispersed throughout the system and are separated by water. During this stage of hydration, which mostly occurs in the first 15 minutes of the mix, a rapid chemical reaction takes place, which produces a considerable amount of heat. Following this initial reaction, the hydration process starts to enter a dormant period of as much as two to four hours. This dormant period allows for the moving, removing and placing of the concrete.

Picture 10: Water in concrete.

Special Consistence Tests
Although the slump test is mostly used for construction work, the consistence can also be measured in a variety of other ways, depending on the nature and what the concrete will be used for.

There are 2 special consistence tests, namely:
1) Vebe test

This test works the best for stiff mixes.

The test is carried out as follow:
? Place a standard slump mould (but without the foot pieces) in the cylinder.
? Fill the mould and tamp as for the slump test but using the funnel to prevent spillage.
? Remove the funnel and level the concrete flush with the rim of the mould.
? Remove the slump from the mould and then measure the slump.
? Place the transparent rider plate on the concrete. Ensure that the rider is free to move downwards.
? Switch on the vibrating table and measure the time it takes to completely compact the concrete, which is when the rider plate is in full contact with the concrete.
? Record the time as perfectly as you can up to the second.

2) Flow test

This test is only suitable for highly workable concrete, for example concrete that is pumped and concrete with a slump better/higher than 150mm.

The test is carried out as follow:
? Clean and sweep the inside of the mould and the top side of the table with a damp cloth.
? Fill the mould with concrete in two different layers of more or less equal depth, tamping each layer ten times. Slightly overfill the mould when putting in the second layer.
? Strike off the concrete flush with the rim of the mould and remove any superfluous concrete.
? Remove the mould.
? Lift the table top to the stops without jolting and allow it to drop. Do this 15 times, each cycle taking three to five seconds. The concrete spreads with each drop.
? Measure the diameter of the concrete in two directions parallel to the edges of the table top.
? Calculate flow as the average of the two diameters.

Conclusion
In conclusion we can see that concrete is very versatile and it is also much more complicated than a person would think concrete has a lot of aspects to consider such as the slump, consistence, hardness/stiffness the workability of the concrete, settlement and the bleeding, special water requirements and the different types of tests that can be conducted on the concrete as well as the handling and transportation of the concrete mixes. There are a lot of factors that can negatively affect the durability, workability and cohesiveness of the concrete which will leave it unusable.
Consistency means the amount of water needed to prepare a plastic mix. Consistency test is done using the Vicat’s apparatus. The Consistency of the cement should be less than 30%. The consistency test is used to determine the amount of water that should be mixed with cement. It is very important to find the consistency because the amount of water present in the cement paste may affect the setting time of the paste.
Workability means the effectiveness in which the concrete can be transport to a site. The most easy way in which concrete can be placed into formwork, including the flow of concrete into corners and around obstructions, the comfort in which concrete can be compacted (concrete must usually be compacted fully on site with the equipment available, in such a way that most air is driven out of the concrete). The easiness in which the concrete can be finished, including honey combing (i.e. holes in the concrete, because compaction was not adequate), sand streaking on the surface etc.
The average Cement and aggregate particles has a densities of about three times that of water. In fresh concrete they consequently tend to settle and displace mixing water which migrates upwards and may be collected on the top surface of the concrete. This upwards movement of mixing water can be defined or seen as bleeding; this is the action where the water separates from the rest of the concrete and is called bleed water. Settlement and bleeding will continue until the concrete sets.
When water is removed from the compacted concrete mix before it sets properly, the volume of the concrete is reduced by the amount of the water removed. We call this volume reduction plastic shrinkage. Water may be removed from the plastic concrete by evaporation or by being absorbed by dry surfaces such as soil or old concrete.
From the starting time of mixing, the fresh concrete slowly loses consistence, this is also known as slump loss. This opens up possibilities for problems but only if the concrete becomes too hard or stiff to use, place and compact properly.
The water requirement mentioned of fresh concrete is the water content of the concrete, measured in litres per cubic meter, this mix of water and concrete is required to bring the concrete mix to the specified consistence. Because the strength of the stiff or hardened concrete depends on the W/C ratio, water requirement has major implications and impact for the material cost of concrete. Water requirement also affects the dimensional stability of the hardened concrete.
Although the slump test is mostly used for construction work, the consistence can also be measured in a variety of other ways, depending on the nature and what the concrete will be used for.