1. CVL 2407 Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
Dr. Eng. Mustafa Maher Al-tayeb
2nd Semester 2013/2014
CVL 2407

2. Low durability against weigh
Properties of Fresh Concrete
Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structure
Operations: batching, mixing, transporting, placing, compacting, surface finishing
Treatment (curing) of in-placed concrete 6-10 hours after casting (placing) and during first few days of hardening is important.
Concrete properties
Negative properties
Positive properties
Low tensile
Able to be cast
Volume unstable
Caste on the site
Low durability against weigh
Economically
Low elasticity
Heat resistance
Durability
Aesthetic properties

3. Properties of Fresh Concrete
Fresh state properties enormously affect hardened state properties.
The potential strength and durability of concrete of a given mix proportion is very dependent on the degree of its compaction.
The first 48 hours are very important for the performance of the concrete structure.
It controls the long-term behaviour, influence f'c (ultimate strength), E (elastic modulus), creep, and durability.

4. Properties of Fresh Concrete
Main properties of fresh concrete during mixing, transporting, placing and compacting.
• Fluidity or consistency: capability of being handled and of flowing into formwork and around any reinforcement, with assistance of compacting equipment.
• Compact ability: air entrapped during mixing and handling should be easily removed by compaction equipment, such as vibrators.
• Stability or cohesiveness: fresh concrete should remain homogenous and uniform. No segregation of cement paste from aggregates (especially coarse ones)
Fluidity & compactability known as workability
Higher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability

5. Properties of Fresh Concrete
Main properties of fresh concrete during mixing, transporting, placing and compacting.
• Fluidity or consistency:
• Compactability
• Stability or cohesiveness
Fluidity & compactability known as workability
Compaction of Concrete
Finishing of Concrete

6. Workability
Definition: Effort required to manipulate a concrete mixture with a minimum of segregation.
Factors affecting concrete workability:
w/c ratio: increase in w/c ratio increase workability (decrease strength)
grading of agg.: increase agg. to cement ratio decrease workability
agg. properties: rounded agg. have high workability (occur segregation)
time and temperature: high temperature decrease workability
cement: increase of cement fineness decrease of workability
use new cement decrease workability
admixture: increasing concrete workability

7. Workability measurement methods
1. Slump test
2. Compacting factor test
3. Vebe test
4. Flow table test
1. Slump test - simplest and crudest test
Fill concrete into frustum of a steel cone in three layers
Hand tap concrete
In each layer
Lift cone up. Define slump as downward Movement of the concrete

8. Workability

9. Workability

10. Workability Concrete consistency Compacting factor (AC)%
Uses & recommended of compaction
Stiff
Dames-retaining walls (Vibration)
Plastic
All mass structures (hand compaction)
Flowing
Slabs-reinf. Structures (vibration)
Self compacting
> 95
Thick steel renif structures(no compaction)

11. Workability Concrete consistency Compacting factor (AC)%
Uses & recommended of compaction
Stiff
Dames-retaining walls (Vibration)
Plastic
All mass structures (hand compaction)
Flowing
Slabs-reinf. Structures (vibration)
Self compacting
> 95
Thick steel renif structures(no compaction)

12. Workability

13. Workability
Vebe test – like slump test measures only the consistency but more meaningful for mixtures of concrete with low consistency.
The equipment for the test, as shown in the above figure, was developed by Swedish engineer V. Bahrner .
Vebe test equipment consists of a vibrating table, a cylindrical pan, a slump cone, and a glass or plastic disk attached to a free-moving rod, which serves as a reference end point
The cone is placed in the pan, filled with concrete, and removed like slump test
The disk is brought into position on top of the concrete cone, and the vibrating table is set in motion
The time required to remold the concrete, from the conical to the cylindrical shape until the disk is completely covered with concrete, is reported as the Vebe time in seconds

14. Workability / Vebo test

15. Workability

16. Segregation and Bleeding
From placing to final set, concrete is in a plastic, semi-fluid state. Heavier particles (aggregates) have tendency to move down (SEGREGATION). Mix water has a tendency to move up (BLEEDING)
Segregation is when the coarse and fine aggregate, and cement paste, become separated.
Segregation may happen when the concrete is mixed, transported, placed or compacted.
Segregation makes the concrete:
WEAKER,
LESS DURABLE,
and will leave A POOR SURFACE FINISH.

17. Segregation and Bleeding
A layer of water ( 2 % or more of total depth of concrete) accumulates on surface, later this water evaporates or re-absorbed into concrete.

18. Segregation and Bleeding
Types of segregation:
There are two kinds of segregation
Separation of the mortar (paste + fine aggregate) from the body of concrete (for example, due to over vibration or mishandling of concrete)
Bleeding
It is defined as phenomenon by virtue of which water appears on the surface after a concrete has been placed and compacted but before it has set.
Bleeding takes place due to sedimentation of the solids in voids of concrete
Laitance is a form of bleeding in which water rising in the internal channels within concrete, carry with it very fine particles of cement, sand, and clay and deposit them in the form of a scum at the concrete surface.

19. Segregation and Bleeding
Measurement:
There are no tests for the measurement of segregation.
Visual observation and inspection of cores of hardened concrete are generally adequate to examine the segregation.
There is, however, an ASTM standard test for the measurement of rate of bleeding and the total bleeding capacity
According to ASTM C 232, a sample of concrete is placed and consolidated in a container 250 mm diameter and 280 mm height. The bleed water accumulated on the surface is withdrawn at 10 min. intervals during the first 40 min., and thereafter at 30 min. intervals.
Bleeding is expressed in terms of the amount of accumulated water as the percentage of net mixing water in the sample

20. Segregation and Bleeding
Causes and Control:
The following are the general causes segregation and bleeding problems in concrete:
Improper consistency
Excessive amount of large particles of coarse aggregate with either too high or too low density
Presence of less fines (due to low cement and sand contents or the use of a poorly graded sand)
Inappropriate placing and compacting methods
The problems of segregation and bleeding can be reduced or eliminated by
paying attention to mix proportioning and to
handling and placement methods

21. Segregation and Bleeding
Methods of reducing segregation and bleed and their effects
Workability:
The amount of mechanical work or energy required to produce full compaction of the concrete without segregation or bleeding.
It means (how easy it is to: PLACE, HANDLE, COMPACT and FINISH a concrete mix)

22. Properties of Fresh Concrete (Cont.)
Water content of the mix
- The most important factor
- Increasing water Increase the ease of flows and compaction.
Reduce strength and durability.
May lead to segregation and bleeding.
- mixing water is divided into three parts
1- adsorbed on the particle surfaces
2- filled the spaces between the particles.
3- lubricates the particles by separating them with a film of
water finer particles require more water.

23. Properties of Fresh Concrete (Cont.)
To make a more workable mix:
Add more CEMENT PASTE.
Use WELL GRADED aggregates.
Use an ADMIXTURE.
Never try to make a mixture more workable by just adding more water because this lowers the strength and durability of concrete.

24. Properties of Fresh Concrete (Cont.)
Aggregate properties
There are two important factors here
1- amount of aggregates.
2- the relative proportions of fine to coarse aggregates.
increase of aggregate/cement ratio decreases workability
more cement is needed when finer aggregate grading are used.
Harsh concrete: deficiency in fine aggregate resulting in lack of the desired consistency resulting in segregation.
Shape and texture of aggregate particles.
Nearly spherical particles give more workable concrete. Spherical particles give lower surface –to –volume ratio, less mortar to coat the particles, leaving more water to enhance workability.
The porosity of the aggregates can absorb a great deal of water and less will be available to provide workability.

25. Properties of Fresh Concrete (Cont.)
Time and temperature.
Considerable evidence that temperature increase will decrease workability as higher temperatures will increase both the evaporation rate and hydration rate.
Very warm weather will require more water to maintain the same workability.
Cement characteristics.
Less important factor in determining workability than the aggregate properties.
However, increased fineness of type III (rapid –hardening 0 cements will reduce workability at a given w/c ratio.

26. Properties of Fresh Concrete (Cont.)

27. Properties of Fresh Concrete (Cont.)

28. Properties of Fresh Concrete (Cont.)
Sample collected
Slump Cone Filled
Cone Removed and Concrete
Allowed to ‘Slump’
Slump Measured

29. Properties of Fresh Concrete (Cont.)

30. Compacting Concrete
WHAT IS COMPACTION:
Compaction is done by shaking, or vibrating, the concrete which liquefies it, allowing the trapped air to rise out. The concrete settles, filling all the space in the forms
WHEN TO COMPACT:
Compaction must be done as concrete is placed, while it is still plastic. Never let concrete dry-out and stiffen because it will be too hard to compact
WHY COMPACT:
Properly compacted concrete is more dense, strong and durable. Off-form finishes will also be better.

31. The Mechanical Screed:
Compacting Concrete
EXTERNAL VIBRATION
Screeding:
The Mechanical Screed:
Concrete is screeded TWICE.
The first screed levels the concrete roughly and compacts it.
The second screed levels and compacts the concrete more.
The screed is pulled along the top of the forms by two workers.

32. Compacting Concrete
INTERNAL VIBRATION
is done with a mechanical vibrator or poker vibrator.
The POKER is put into concrete and vibrates it from the inside.
Method of internal vibrations:
Make sure there are enough workers so some can compact while others continue to place.
Put the poker into the concrete QUICKLY.
Take the poker out very SLOWLY otherwise a hole, or weak spot, may be left in the concrete

33. Compacting Concrete INTERNAL VIBRATION
The SIZE of the poker determines how much concrete is vibrated at one time.
The area vibrated at one time is called the RADIUS OF ACTION.
This can be seen by over what radius air bubbles rise to the surface.
The radius of action will be greater with a LARGER poker and more-workable concrete.
Always compact in a definite pattern so the radius of action overlaps and covers the whole area of the concrete.

34. Compacting Concrete
PRECAUTIONS:
Taking the poker out TOO QUICKLY will leave a hole in the concrete.
To close the hole, vibrate near the hole and take the poker out VERY SLOWLY.
NEVER touch the form face with the poker as it can damage the formwork and the concrete.
NEVER touch the reinforcement with the poker.
NEVER spread or move concrete sideways with the poker, always use a shovel.

35. Compacting Concrete
HOW LONG TO COMPACT:
For concrete of average workability ( slump of 60 mm) with a poker size between 25–75 mm, concrete should usually be vibrated for between 5 and 15 seconds.
It is worse to UNDER-VIBRATE than to OVER-VIBRATE concrete.

36. Properties of Fresh Concrete (Cont.)
Curing
Curing; protection of concrete from moisture loss from as soon after placing as possible, and for the first few days of hardening
Curing means to cover the concrete so it stays MOIST
By keeping concrete moist the bond between the paste and the aggregates gets stronger. Concrete doesn’t harden properly if it is left to dry out.
WHEN TO CURE Curing is done just after finishing the concrete surface, as soon as it will not be damaged.
WHY CURE Concrete that is cured is:
LESS LIKELY TO CRACK.
More DURABLE.
HOW TO CURE Concrete is cured by:
APPLYING EXTRA WATER to the surface of the concrete, or
STOPPING water loss from the concrete.

37. Properties of Fresh Concrete (Cont.)
Effect of curing temperature
Hydration reactions between cement and water are temperature dependent and rate of reaction increases with curing temperature
At early ages, rate of strength gain increases with curing temperature (higher temperatures increases rate of reaction, thus more C-S-H and gel is produced at earlier times, achieving a higher gel/space ratio and thus higher strength)
At later ages, higher strength are obtained from concrete cured at lower temperatures.
(C-S- H gel is more rapidly produced at higher temperature and is less uniform and hence weaker than produced at lower temperatures)
Standard curing temperature is 22 ± 1 º C
Hydration proceeds below 0 º C, stop completely at -10 º C

38. Properties of Fresh Concrete (Cont.)
Curing methods
Spraying surface of concrete with water
Protecting exposed surfaces from wind and sun by windbreaks and sunshades
Covering surfaces with wet hessian and/or polythene sheets
Applying a curing membrane, a spray-applied resin seal, to the exposed surface to prevent moisture loss