1. WATER

2. Water requirement in concrete
Minimum water required for hydration reaction - 23% ( by wt of cement)
Further requirement for continued hydration- 15% (by wt. Of cement)
Total requirement thus = 38%
Any excess causes ‘CAPILLARY FORMATION’ and results into
Less strength
Less durability

5. The pH value not less than 6.
Water
It should be clean &free from oils,acids,alkalis,salts,sugar,organic materials etc.
The pH value not less than 6.
If water is satisfactory for mixing, it is safe for curing.
No strain on curing

6. Limits Chlorides shouldn’t be more than 10,000 ppm
Sulphates shouldn’t be more than 3,000 ppm
Turbidity shouldn’t be more than 2,000 ppm
Carbonate & bi Carbonate if more than 2,000 ppm tests for setting time & strength be conducted
(causes quick setting )

7. Mixing

8. Mixing should ensure that mass becomes
Homogeneous
Uniform in colour
Uniform in consistency
Types of mixing
Hand mixing (generally not recommended. If used then 10% extra cement to be used but not to the advantage of contractor)
Machine mixing – efficient and economical

9. MIXING (contd - ii) MACHINE MIXING Batch Mixers Pan type Drum type
Continuous
Batch Mixers
Pan type
Drum type
Tilting
Non tilting
Reversing or forced action

10. P                                                                                                

11. Non Tilting – 200 NT, 280 NT, 340 NT, 400 NT, 800 NT
MIXING (contd - iii)
Designation of mixers (As per IS : 1791 – 1968)
Tilting – 85T, 100T, 140T, 200T
Non Tilting – 200 NT, 280 NT, 340 NT, 400 NT, 800 NT
Reversing – 200R, 280 R, 340R, 400R.
Figures – capacity in litres.

12. Mixing is faster in natural aggregates
MIXING (contd -vi)
MIXING TIME
25 to rpm is minimum required for proper mixing. Therefore 2 minute mixing is desirable.
Mixing is faster in natural aggregates

13. WORKABILITY

14. Workability
Compactibility- or ease with which concrete can be compacted
Mobility- or ease with which concrete can flow into formwork & around reinforcement.
Stability- or ability of concrete to remain stable, cohesive and homogeneous mass while handling, vibrating without segregation

15. Categories of Workability
Category CF
Slump mm
Vee Bee sec
Extremely low
0.68
> 20
V. Low
0.78
0-10
7-20
Low
0.85
10-25
5-10
Medium
0.92
25-50
3-5
High
0.95
50-125
2-3
V. High
0.97
0-2

16. SLUMP TEST
Vessel shape – frustum of cone of 30 cm ht. 10 cm. dia at top & 20 cm at bottom
Concrete filled in four layers each compacted with 25 strokes of bullet points 16 mm dia, 60 cm long, steel rod.
Cone is removed & concrete allowed to settle vertical settlement is ‘SLUMP.’

17. COMPACTION FACTOR TEST
Here workability
is the amount of work required
to place concrete and to compact it thoroughly
Two hoppers are one above other and cylindrical mould 30 cm. high, 15 cm dia at bottom to collect concrete. Wt. Of concrete collected in cylinder divided by theoretical wt. of concrete gives CF.

18. VEE BEE TEST
Vibrating table 36 x 26 cm, with fixed cylindrical container & a transparent disc.
Ordinary slump test done by putting cone into the cylinder. Slump measured with plastic disc.
Then vibration started with disc resting on concrete,
Time required for the concrete to take horizontal surface in sec. is Vee Bee degree.

19. FACTORS AFFECTING WORKABILITY
Water
Size of particles
CA/FA ratio
Particle interference – gap grading helps
Shape of aggregates
Admixtures – Air entraining agents, pozzolanas.

20. Effect of inadequate workability
Honey combing
Less strength
Less durability

21. PLACEMENT

22. Methods of transportation
Mortar pan
Wheel barrow/hand cart
Bucket & rope way
Truck mixer & dumper
Belt conveyor
Chute
Skip & hoist
Pump & pipe line ( 8-70 cum concrete/ hr)
8-20 cm pipe, mix should be truly plastic
Spherical ball to clean the pipe called ‘GO DEVIL’

23. COMPACTION Expulsion of entrapped air
Each 1% entrapped air reduces 6% strength

24. Methods of compaction
Rodding
Ramming
Tamping
Internal vibration
External vibration
Surface vibration
Platform or table vibration

25. Details of equipment Frequency Range - 100 Hz for 40 mm agg.
Needle Diameter
-20 mm to 75 mm (length- 25 to 90 cm.)
Thickness of layer
- Not less than 100 mm, not more than 600mm
-preferably bet. 2/3 L to L of needle

26. Proper Internal Vibration
Increase compressive strength & bond
Decrease permeability
Decrease cold joints
Decrease honeycombing
Decrease excessive entrapped air
Decrease segregation

27. Spacing tips Overlapping field of action Watch the concrete
High powered vibrators
High slump concrete
Field of action 8 times vibrators’ head diameter

32. Stop vibration - when
Concrete surface- shining appearance
Large air bubbles no longer escape
Vibration change pitch or tone
Over vibration is preferable to under vibration

33. curing

34. CURING OF CONCRETE Curing is a procedure used for
Promoting the hydration of cement
Control of temp and humidity
As a result of curing
Strength is improved
Durability is improved and permeability of concrete is reduced.

35. WHY CURING IS IMPORTANT ?
Hydration progresses only when pores are saturated (80% relative humidity is reqd.)
Initially the concrete has sufficient amount of water to begin with hydration. Water is lost with time due to
Evaporation
Self desiccation (Relative humidity fall
below 80% due to hydration reaction)

36. Replenishing lost water
METHOD OF CURING
Replenishing lost water
Immersion
Ponding
Sprinkling
Saturated covering i.e. Jute bags
Preventing moisture loss
Curing compounds
Impermeable membrane covering.

37. As per IRS : CBC for OPC – 14 days Curing mainly depends upon
PERIOD OF CURING
As per IS : for OPC – 7 days
As per IRS : CBC for OPC – 14 days Curing mainly depends upon
Type of cement
Ambient atmospheric condition
Mass to surface area ratio

38. The process of curing shouldn’t be interrupted
CURING (cont..)
The process of curing shouldn’t be interrupted
Because Partial hydration makes capillaries discontinuous & on curing again water may not enter the concrete
High strength concrete should be cured at an early stage.

39. PRACTICAL ASPECTS IN RELATION TO CURING
Widespread belief that humid climate is sufficient for curing
Generally the person responsible for curing is most unskilled. He doesn’t appreciate the importance & therefore doesn’t care much
Curing is not a measurable item in the agreement.

40. CURING (cont..) It is better to Use curing compounds
Curing be kept as a separately payable item in bill of payment
Keep a set of cube in vicinity of structure & let it be cured in the same way as the structure. Strength & Permeability test on these cubes will reveal the history
Educate the person responsible for curing.

41. EFFECT OF CURING ON PERMEABILITY AND POROSITY
Permeability (m/sec unit x 10-17)
Porosity (%)
Curin (days)/
W/C 1 3 7 28 90
0.32
5.60
0.30
0.12
Very low
20.80
19.7
14.4
9.80
5.90
0.40
18.70
0.59
0.07 ”
33.30
28.6
20.9
16.80
11.10
0.50
214.0
14.7
2.35
0.19
43.50
37.8
32.2
14.50