BMC IVConcrete

Water, Admixtures And Reinforcement.

Bansi V. A-0312Achal D. A-0612Dhruva P. B-3712Brinda S. B-5512

Concrete- water, additives admixtures and reinforcements 1. WATER- a) Why is it necessary

b) Water cement ratio per element, on the basis of climatic considerations, advantages and disadvantages c) Types of water-fresh and saline d) Effects when excessive use of water is done-past construction e) Curing necessity-type of water used for curing 2. ADDITIVES AND ADMIXTURES- a) Its needs depending on the place where concreting is to be done b) Different workability with different proportions of different mixtures c) Necessity of water-proofing depending on purpose and situation 3. REINFORECEMENT- a) why Reinforcement is necessary. b) Sizes, location in RCC c) Storage on site- its considerations and logistics

Water

• Concrete materials-Cement-Water-Coarse and fine aggregates.-Admixtures(optional).

• Necessity.-To hydrate cement-To lubricate the mix making concrete workable-The water enters into chemical action with cement releasing heat of hydration that in turn sets and hardens concrete

Quality of waterWATER is mixed with the cement powder to form a paste whichholds the aggregates together like glue.

Water must be clean, fresh and free from any dirt, unwanted chemicals orrubbish that may affect concrete. Many concrete plants now use recycled water.

The amount of water required to hydrate cement is about 25% of the weight of cement.

Fresh water is preferred over sea water as sea water may rust steel reinforcement in the concrete.

Clean and free from oils, acids, alkaline or any organic impurities.

Water Cement Ratio

• Too much water and not enough cement means concrete will be weaker and less durable.

Water Cement Ratio• Ratio of Water to Cement in the making of

concrete is very important for the properties of concrete.

• Increase in water content decreases the strength of concrete producing shrinkage cracks and decreases density.

• Concrete with low water content is unworkable as it is stiff and dry. Hence the right amount of water is very much important in concrete mixture.

• The amount of water required to hydrate cement is about 25% of the weight of cement.

• A mix that has too much water will not be cohesive and may separate and bleed.

Water in Curing of Concrete• The prevention of loss of water from

concrete during its early life is known as curing.

• Hence, water is sprinkled periodically over concrete.Or water is allowed to remain on concrete by ponding on it.

Water in Curing of Concrete.

Moist rugs covered over column to keep it moist

The simplest method of APPLYING WATER is to put acontinuous fine, misty spray of water over the concrete.

Effects of too much mixing water

• Whilst adding water will in some cases facilitate easier placing, the disadvantages of this include the following:-

• Lower compressive strengths• Segregation of the concrete mix under certain conditions resulting in variable quality throughout the

concrete mass.• Cracking - with too much water, there will be lower tensile strength, and a tendency towards high

shrinkage and subsequent cracking.• Dusting and scaling - Bleeding of excess water brings too many fines to the surface of floors• Sand streaks. Excess water bleeding up the sides of forms washes out cement paste and leaves an

unsightly streaked surface.• Contamination. Too much water in concrete placed on grades causes contamination from the subgrade

with the concrete leading to an array of quality problems• Permeability. Voids left as excess water evaporates invite water to seep through walls and floors• Dead losses - costly repairs, or in extreme cases, demolition and re-building at contractor’s expense

Role of water in adverse Climatic ConditionsHot Weather

Cold water should be used in hot regions because merely increasing the amount of water in the mixture will decrease the strength of concrete

As it affects adversely in concrete properties.

Role of water in adverse Climatic ConditionsIn Cold Conditions

There is danger of disintegration of unset concrete due to the disruptive effect set up by expansion of mixing water as it freezes.

Hence Hot Water is used.

Low water cement ratio is less liable to damage by frost.

Post Construction effects of Water on concrete.

Salty water is one of the cause of corrosion of rebars inside and ultimately the concrete.

Post Construction effects of Water on concrete.

Post Construction effects of Water on concrete.

Post Construction effects of Water on concrete

Additives & Admixtures• Concrete materials

-Cement-Water-Coarse and fine aggregates.-Admixtures(optional).

• Added immediately before or during mixing• Are used to improve or give special properties to concrete

• Why are add mixtures used?1. To accelerate or retard hydration or setting of cement2. To improve workability3. As water proofers4. As air-entraining agents5. To produce colored concrete

1.Air entraining agents

• Objectives of entraining air in concrete are:

i. To increase workability :a) More cohesive concreteb) Less liable to segregationc) Or bleeding-formation of water on surface after compaction.d) Hence final finishing can be started immediately after compaction.

ii. To increase resistance to weathering esp. to action of frosta) Reduces the danger of surface scaling in frosty weather.

• Drawback of entrained air:

i. Reduction in strength of concrete.

• Hence used where a good surface finish and resistance to frost are more important.

• Air entraining agents are 0.005 to 0.05 of 1% of the weight of cement

• Methods of incorporating air:

i. By use of gas forming agents like aluminium or zinc powder, hydrogen peroxide.

ii. By use of surface active agent which reduces surface tension.

iii. By use of cement dispersing agents.

• Effects of air entrainment:

i. the entrainment of air increases compacting factor.

ii. Maximum aggregate size is decreased with increase in amount of air entrainment

iii. Considerable reduction in water requirement

iv. Water cement ratio can be reduced even then concrete remains more workable

v. Increases resistance to freezing and thawing

vi. Minute dispersed air bubbles acting like expansion chambers, of stress and pressure-caused by expansion of moisture on freezing.

vii. Increases the resistance of concrete to chemical attack

viii. Reduces risk of segregation during transport

ix. Enables percentage of sand to be kept low while making light weight concrete

2.Retarders

• Rate of chemical reaction decreases and setting time increases.

• Calcium sulphate in form of gypsum is generally added during manufacture of cement

• 0.2% adding of sugar can extend the final setting time to 72 hours or more.

• 0.1% of sodiumhexametaphosphate can allow initial and final setting time of 12 to 13 hours

3.Accelerators

• Agents added to cement to make it set and acquire strength rapidly

• Use of accelerators is desirable as they:

i. Reduce form time

ii. Shorten curing time

• Most commonly used accelerator is calcium chloride(CaCl2)

• 2% of CaCl2 by weight of cement reduces initial setting time from 3 to 1 hours and final setting time from 6 to 2 hours

• At 21⁰C it approximately doubles the 1 day strength.

• Disadvantages of calcium chloride:

i. If over 3% of it is added then there is instantaneous setting of cement.

ii. It impairs volume stability

iii. It increases the rate of heat liberation

iv. Drying shrinkage may be increased by 50%

• Aluminium chloride is used when concrete is required to set in a few minutes in order to stop water seepage under pressure.

4. Water proofers

• A waterproof concrete has to fulfill two functions:

i. To be impervious to water under pressure

ii. To resist absorption of water.

• Water proofers are obtained in powder paste or liquid forms

• Can consist pore filling or water repelling materials

• Pore filling materials:

i. Alkaline silicates are chief pore filling materials. They are chemically active hence accelerate the setting time Thus making it more impervious at early stage.

ii. Talc in a very finely ground form is chemically inactive pore filler.

• Water repelling materials:

i. Soda and potash soaps are chemically active.

ii. Calcium soaps, resins, vegetable oils/fats, waxes are chemically inactive and pore blocking agents.

5. Pozzolonas• As mineral admixture we use FLY ASH(A waste material) directly, or indirectly as an additive in

PORTLAND POZZOLANA cement.• Siliceous Material and itself has no Cementitous Properties.• Natural.

-Clay and Shales (needs to be calcined to be active)-Volcanic tuffs and pumicites.

• Artificial-Ground blast Furnace slag.-Fly Ash.

• Advantages.-Workability with less amount of water.-Reduction in heat of hydration.-Protection form Salts And Sulphates-Preventionn of Calcium Hydroxide leaching.

• Disadvantages-Strength Development is slowed.-Drying, Shrinkage may be increased.

Construction of massive structures, has use of pollozonas because of saving of cement cost and reduction in heat of hydration.

• Fly Ash-It reduces Segregation and Bleeding.-When used in replacement of cement gives less compressive strength at 7 and 28 days, becomes equal after 3 months.

6. Pigments• Pigments are substances used to produce coloured cement.• Pigments must be permanent and should not get affected by free lime in concrete.

Chief Pigments Used in Concrete.

Brown- Ferrous Oxide and hydroxides and manganese oxide.

Black- Carbon Black, magnetic ferrous oxide(gives purple tint)

Red- naturally occurring red oxides, gives variety of shades.

Green- Chromium Oxide and hydroxide are suitable and also produced artificially.

Blue- Barium Manganete.

Yellow- hydroxide of iron gives colour. Naturally/chemically produced yellow ochres.

7. Workability Agents• They increase workability by increasing the amount of paste on concrete hence aloows more

cohesiveness.• Larger amount requires mixing of water and leads to loss of strength.

• Classified into 3 chemical types.-Lignosulphonates-Organic acid-Carbohydrates.

Reinforcement

WHY USE REINFORCEMENT?As a force is applied to concrete there will be compressive, tensile and shear forces acting on the concrete. Concrete naturally resists compression (squashing), very well, but is relatively weak in tension (stretching). Horizontal and/or vertical reinforcement is used in all types of concrete structures where tensile or shear forces may crack or break the concrete. HORIZONTAL reinforcement helps resist tension forces. VERTICALreinforcement helps resist shear forces.

Reinforcements

• Steel roads are used as Reinforcements which are called ReBars. (Reinforcement Bars)

Images shown above are known as Deformed steel bars.

Lugs

Ribs

Sizes Of Rebar.

The common type of reinforcement known as flexible reinforcement consists of 6mm to 40 mm steel bars which are plain round or intermittently deformed.

The cross-sectional area of effective reinforcement is computed in accordance with resulting forces and the installation bars tie the effective reinforcement into mats or blocks for the prescribed concrete member.

Placement in Concrete.Bars are arranged in the manner shown in the picture before concreting is done.They are binded with one another with horizontal rings at regular interval.

Placement in Concrete.

• Ends are tied, not continuous or welded.

• Interior and exterior rebar should exchange at corners

• Intersections should extend to the exterior

Placement in Concrete.

Reinforcement in different elements of construction.

Placement in Concrete.

Binding of Bars

Types

Fibre Reinforcement.

• Polypropylene and Nylon fibres can:• Improve mix cohesion, improving pump-ability over long

distances• Improve freeze-thaw resistance• Improve impact resistance• Increase resistance to plastic shrinkage during curing• Steel fibres can:• Improve structural strength• Reduce steel reinforcement requirements• Improve ductility• Reduce crack widths and control the crack widths tightly,

thus improving durability• Improve impact– and abrasion–resistance• Improve freeze-thaw resistance

Logistics And Storage• The reinforcement delivered to site should be stored neatly in a location specially prepared for

this purpose. Reinforcement should be stored on a platform off the ground to prevent corrosion and contamination due to deleterious matter (mud, grease, oil, paint, loose rust, etc). If the reinforcement is to be stored for a long period of time or where stored in a marine environment (within 10km of sea) the reinforcement piles should be covered

• They are ordered in package of 75kgs BHARI(local term).

Examples of Poor Storage.

Sources

• Concrete technology. –Krishnaswamy KT• Reinforced Concrete.- Sigolov S. Strongin• CONCRETE BASICS A Guide to Concrete Practice• Site Images by Achal D. and Savan G.