WATER

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

                                                                                                                   

                                                                                                                   

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

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 )

Mixing

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

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

P1500                                                                                                

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.

Mixing is faster in natural aggregates MIXING (contd -vi) MIXING TIME 25 to 30 revolution @ 15-20 rpm is minimum required for proper mixing. Therefore 2 minute mixing is desirable. Mixing is faster in natural aggregates

WORKABILITY

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

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 0.97 125-200 0-2

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.’

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.

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.

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

Effect of inadequate workability Honey combing Less strength Less durability

PLACEMENT

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’

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

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

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

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

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

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

curing

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.

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)

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.

As per IRS : CBC for OPC – 14 days Curing mainly depends upon PERIOD OF CURING As per IS : 456 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

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.

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.

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.

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