Shrinkage and plastic of concrete samples.ppt

Shrinkage study of Concrete

1. Relationship between slump value and compaction factor

2. Concrete Tests (Hardened Concrete)
1. Rebound Hammer Test

3. Concrete Tests (Hardened Concrete)
1. Schmidt Rebound Hammer Test
Basically higher the rebound number higher is the strength

4. Concrete Tests (Hardened Concrete)
2. Ultrasonic pulse velocity test
Transmitting transducer-receiving transducer –time is recorded

5. Concrete Tests (Hardened Concrete)

6. Concrete Tests (Hardened Concrete) –Destructive tests
Concrete Compression test in UTM (Universal testing machine)
Cube 150mmx150mmx150mm for maximum aggregate size >20mm
When maximum aggregate size < 20mm 100mmx100mmx100mm
When cylinder is used dimensions are diameter 15 cm and height 30 cm (2:1)
were cured and tested after 7 and 28 days
In Great Britain, Germany and Europe cubes are used. In the United States of America, France,
Australia, and Canada cylinders are used as standard specimens
IMPORTANT-1. Results obtained from cylinder are found to be more precise than results from cubical moulds
2. Strength of concrete measured using cylindrical mouldsis approximately 0.8 times than that obtained
from cubical moulds
Question for Home: Why Cube has more compressive strength than cylinder ?

7. Tensile strength of concrete
Unreinforced concrete specimen: Size of specimen 70cmx15cmx15cm
Done by casting a small beam of above dimensions
For aggregate size < 20 mm; 50cmx15cmx15cm

8. Tensile strength of concrete
Split test or Brazilian test
In this test, cylinder of 15 cm diameter and 30 cm height is loaded along
its length in compression.
Tension is created along the diameter and cylinder splits into two halves
Tensile strength = 2P/pixDxL

9. READING RESOURCE
PLEASE GO THROUGH CHAPTER 11 -CEMENT CONCRETE -BT P.C
VARGHESE PAGE 89 -99

10. Not a Liquid -an Unstablemixture
Will segregate if handled improperly
Deposit in Formwork (methods)
Direct From the Truck
Bucket
Pump
Consolidate Mix
Apply Finish (if Required)
Concrete Placement

11. Concrete Placement (1929)

12. Placement Today -Directly From the Transit Mixer

13. Concrete Bucket being Filled

14. Placement of a Wall with a Crane & Concrete Bucket

15. Placing of Concrete through bucket

16. Placement with a Concrete Pump

17. Placement of Concrete with a Boom Pump

18. Placing of Concrete

19. Placement with a Conveyor

20. Vibration of concrete
•Thepurposeofcompactionofconcreteistoachieve
highestpossibledensityofconcretebyremovingthe
entrappedair
Typesofvibrators
Internalvibrators
Externalvibrators
Vibratingtables

21. Compaction of Concrete

22. Compaction of Concrete

23. Compaction of Concrete

24. Improperly Consolidated -“Honeycomb”

25. Improperly Consolidated Concrete

26. Improperly Consolidated Concrete

27. Cohesive Non
Cohesive
Segregation
Workability & Consistency
Cohesiveness of concrete
Is a measure of its ability to resist the segregation of its
constituents during handling, placing and compaction

28. Cohesiveness

29. Cohesiveness of concrete
Factors affecting the cohesiveness of concrete:
Water
-The higher the water content the greater is the risk of
segregation and bleeding
-High water content retards the stiffening of concrete allowing
sedimentation of heavier particles
Cold weather can also retard setting and promote bleeding
Dry mix can be friable and also prone to segregation
Grading
-Mixes that are deficient in fine particles tend to segregate more
readily during handling and bleed after finishing (entrained air
can help)
-Excessive fines make concrete “sticky” and very cohesive

30. Setting time
Can be determined by the test method described in ASTM C
403 by means of penetration resistance measurements
Made on mortar sieved from the concrete mixture
Initial and final time of setting corresponds to a penetration
resistance equals 500 psi (3.4 MPa) and 4000 psi (27.6 MPa)
Typical values range
Initial set: 2 to 6 hours
Final set: 4 to 12 hours
Factors affecting setting time of concrete:
W/C ratio
Cement type and content
Temperature
admixtures

31. Measurement Setting time0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
180210240270300330360390420450
Elapsed time (min)
Penetration resistance (psi)
Final set
Initial set

32. Segregation and Bleeding
Segregation
(concentration of heavy particles)
Bleeding
(concentration of water)

33. Bleeding and settlement
Bleeding is the development of a layer of water at the top or surface
of freshly placed concrete. Water concentration at bottom of
aggregate and the reinforcing steel
Bleeding is normal and should not diminish the quality of properly
placed, finished and cured concrete. It occurs on the surface of
specimen due to the compacting process.
Some bleeding is helpful to control plastic cracking

34. Excessive bleeding?
Cracks

35. Fresh Concrete : Segregation (Separation)

36. Fresh Concrete –Slump Loss
It is defined as the loss of fluidity in fresh concrete with
time.
Due to: hydration of cement, loosing of water
Can be delayed or retarded by admixture

37. Fresh Concrete –Curing of Concrete
Curing is a process to control the loss of moisture from
concrete after placed and finished
Provides adequate time for the hydration of cement
particles therefore assures adequate strength
development and durability of the concrete
Curing period depends on:
The properties required of the concrete
The purpose for which it is to be used
And the ambient conditions (T°, RH..)

38. Fresh Concrete –Curing of Concrete
Curing may be done in three ways:
1.By preventing an excessive loss of moisture from the
concrete for some period of time
Leaving formwork in place
Covering concrete with an impermeable membrane
after formwork have been removed
Or by a combination of such methods
2.By continuously wetting the surface
Ponding or spraying the surface with water
3.By raising the temperature on concrete (steam curing) to
accelerate the rate of strength gain at early age

39. Curing methods and materials
Methods of curing concrete fall broadly into three
categories:
1.Those which prevent moisture loss by continuously
wetting the surface of the concrete (spraying water,
saturated wet coverings…)
2.Those which minimize moisture loss from the
concrete by covering it with a relative impermeable
membrane (plastic sheets, curing compound..)
3.Those which keep the surface moist and, at the
same time, raise the temperature of the concrete,
thereby increasing the rate of strength gain

40. Influence of Curing on strength development

41. Curing methods and materials
Ponding and Immersion
Flat surfaces, pavements, floors
Earth, sand or mortar dikes around the perimeter of the
concrete to retain a pond of water
Keep uniform temperature of the concrete
DT between water and concrete < 11°C (prevent thermal
cracks)
Water free of substances that will stain or discolor the
surface
Used only for small jobs

42. Curing methods and materials
Ponding and Immersion

43. Curing methods and materials
Fogging minimizes moisture
loss during and after placing
and finishing

44. Curing methods and materials
Wet coverings
Fabric coverings saturated with cotton mats, rugs or other
moisture-retaining fabrics are commonly used for curing
Treated burlaps that reflect light and resistant to rot and fire are
available (ASTM C 171, AASHTO M182)
Must be free of any substance harmful to concrete or causes
discoloration
Should be placed immediately concrete has sufficiently hardened
/ including the edges of slab
Avoid wetting and drying / may cause crazing (cracks)
Avoid wet coverings of earth, sand or sawdust to prevent
possible discoloration of the concrete surface

45. Curing methods and materials
Impervious paper
Consists of two sheets of kraft paper cemented together by
bituminous adhesive with fiber reinforcement (ASTM C 71)
Efficient means of curing horizontal surfaces and other elements
with simple shapes
Advantage: no need for periodic addition of water
Applied immediately after concrete has hardened and wetted

46. Curing methods and materials
Plastic sheets
Consist of polyethylene film –ASTM C 171
Effective moisture retarder
Easy to apply to complex as well as simple shapes
May be clear or colored
Polyethylene film may also be placed over wet burlap to retain
the water in the wet covering material
Disadvantage: can cause patchy discoloration

47. Curing methods and materials
Curing compounds
Liquids formulated from wax emulsions, chlorinated rubbers,
synthetic and natural resins, and PVA emulsions
Sprayed directly onto concrete surfaces to form a relatively
impermeable membrane
Retards the loss of moisture from the surface
Should be able to maintain the
RH of the concrete surface above
80% for seven days to sustain
cement hydration

48. Curing methods and materials
Steam curing
Advantageous where early strength gain is needed
Or where additional heat is required to accomplish
hydration as in cold weather
Two methods of steam curing are used:
Live steam at atmospheric pressure (cast-in-place
structures and large pre-cast concrete units
High-pressure steam in autoclaves (for small
manufactured units)

49. Curing methods and materials
Live steam at atmospheric pressure
Widely used in the pre-cast concrete industry
Cost justified by higher turnover of formwork & productivity
Generally done in an enclosure to minimize moisture and
heat losses
A typical steam-curing cycle consists of
1.Initial delay prior to steaming3 to 5 hours
2.Temperature increase period2.5 hours (20°C /h)
3.Constant temperature period6 to 12 hours (to achieve f’
c)
4.Temperature decrease period2 hours

50. Selecting a Method of Curing
Factors which affect the selection of a curing method include:
Type of member to be cured (slab, column, wall…)
Specified finish for the concrete element (will the final
“bond” be affected by the curing compound)
Whether the curing process will influence the appearance
of the concrete
The construction schedule of the project (will work need to
continue in the area during curing ?)
The cost and availability of materials
Weather conditions, exposure and location

51. Fresh Concrete –Plastic Shrinkage
Plastic shrinkage
Result of improper curing
Surface layer shrinks; inside concrete is plastic

52. Interaction Between Bleeding & Evaporation
Evaporation
Bleed water
surface water
Bleed water = evaporation

53. Plastic Shrinkage Cracks
Usually associated with hot weather concreting or any time
when ambient conditions produce rapid evaporation
Occurs when water is lost from concrete during plastic state
(water evaporation > bleeding water)
by evaporation (bleeding, humidity, wind, T°C)
by suction of underlying dry concrete or soil

54. Plastic Settlement Cracks

55. Time of Occurrence

56. Plastic Shrinkage cracks
Special precautions in hot weather concreting
1. Moisten concrete aggregate that are dry and absorptive
2. Keep concrete temperature low by cooling aggregate and
mixing water
3. Dampen the subgrade and fog forms prior to placing
concrete
4. Erect temporary windbreaks to reduce wind velocity over the
concrete surface
5. Erect temporary sunshades to reduce concrete surface
temperatures
6. Protect the concrete with temporary coverings such as
polyethylene during delay between placing and finishing
7. Fog the slag immediately after placing and before finishing
8. Add plastic fibres