Methods Of Curing

-1.Water Curing

-a.Immersion

-b.Ponding

-c.Spraying/Fogging

-d.Wet Covering

-2.Membrane Curing

-3.Application Of Heat

-a.Steam Curing

-b.Infra Red Radiation Curing

-c.Electrical Curing

Immersion Curing

Ponding Curing

Spraying Curing

Wet Covering Curing

Membrane Curing

Steam Curing

Reasons For Curing

-Laboratory tests show that concrete can lose 50% strength if it is in dry environment

-Higher Temperature = Early Strength Gain

-Better surface hardness

-Makes concrete more water tight

-Better appearances

Curing Of Concrete

-Maintaining of an adequate moisture content & temperature

-Develop such properties it was designed to achieve

-Begins immediately after placement & finishing

-Concrete may get its desire strength

-Cementitious material in concrete cannot react to form a quality product without adequate supply of m/c

-Drying may remove the water needed for hydration

-Rate of hydration & strength development is faster at higher temperatures

-Temperatures should be higher than 10C

-Uniform temperature should be provided

Compacting Factor Apparatus

-Determine the workability

-Compacting factor apparatus is used

-1.Weigh the empty cylinder

-2.Place fresh concrete in upper hopper

-3.Open the gate & allow concrete to fall into lower hopper

-4.Open gate of lower hopper so that it falls into cylinder

-5.Cut off excess concrete from top of cylinder & weigh the cylinder containing concrete

-6.Deduct the weight of empty cylinder & new weight is called as the weight of partially compacted concrete

-7.Cylinder is filled again with fresh concrete & vibrated or tamped to obtain full compaction

-8.Concrete in cylinder is weight again & this is called as weight of fully compacted concrete

• Compacting Factor = Wt. Of partially compacted conc./Wt. Of fully compacted conc.

                                                                   

Measurement Of Workability

-Many different methods to measure workability

-Most common are Slump Cone Test & Compacting Factor Test

-Ball Penetration

-Flow Table Test

-Vebe Test

7.Use Of Admixture

-Most Important Factor which affects the workability

-Plasticizers, Super-Plasticizers

6.Grading Of Aggregates

-Maximum influence on workability

-Well graded aggregate has least amount of voids

-Better grading>Less Void Content>Higher Workability

5.Surface Texture Of Aggregate

-Surface area of rough texture is more

-Smooth aggregates will have higher workability

4.Shape Of Aggregate

-Angular, elongated or flaky aggregates makes the concrete very harsh

-Rounded or cubical shape aggregates contributes to better workability

3.Size Of Aggregate

-Bigger the size of aggregate the less is the surface area

-Less amount of water is required

-Bigger size of aggregate will give higher workability

2.Mix Proportion

-Higher the aggregate cement ratio the lower will be workability

-If rich concrete with lower aggregate cement ratio then workability of concrete will be more

1.Water Content

-Significant influence on the workability

-Higher the water content higher will be workable concrete

-Usually at site supervisors add more water

-Higher quantity of cement should also be added if water is being added

Factors Affecting Workability

-Water Content

-Mix Proportion

-Size Of Aggregate

-Shape Of Aggregate

-Surface Texture Of Aggregate

-Grading Of Aggregate

-Use Of Admixture

Workability of Concrete

-Flow of concrete

-Its placement without segregation, without loss of homogeneity

-Can be compacted & finished easily

Bleeding

-Some water from the concrete comes out to surface being of the lowest Specific gravity among all the ingredients

-Usually observed in highly wet mixes, badly proportioned mixes, and improperly mixed concrete

-Water Comes up & accumulates at surface

-Sometimes cement also comes to the surface

-The aggregates goes down

-Formation of cement paste at the surface is Laitance

-Bleeding won't be harmful if rate of evaporation of water from surface is equal to the rate of bleeding

-Removal of water after it had played its role in providing workability from body of concrete by way of bleeding will do good to the concrete

Segregation

-Separation of constituents of the heterogeneous mixture

-Difference in size of particles are causes of segregation

-Suitable grading & by care in handling minimizes segregation

-Two forms of segregation

-1.Coarser particles tends to separate out

-2.Separation of Grout

Hand Mixing Of Concrete

-Cement is then spread over the aggregates

-Dry Materials are mixed by turning over from one end of the try to the other (turning 3 times is usually required)

-Water is slowly added so that neither water by itself nor with cement can escape

-Mix is turned over again usually 3 times until it appears uniform in color & consistency

Mixing Time

-Often a tendency to mix concrete as rapidly as possible

-Mixing time should be enough to get concrete uniform in composition and satisfactory strength

-It is not the mixing time but the number of revolutions of mixer that is the criteria of adequate mixing

-Generally about 20 revolutions are sufficient

Pan Mixers

-Consists of circular pan rotating about its axis with 1or2 stars of pedal rotating about a vertical axis

-Relative movement between the pedal & the concrete is same

-Concrete in every part of the pan is thoroughly mixed

-Scraper blades prevent concreting sticking to the sides of the pan & height of the pedals can be adjusted to prevent the coating of concrete at the bottom of the pan

Non-Tilting Mixers

-Axis of the mixer is always horizontal

-Discharge is obtained by inserting a chute or by reversing the direction of the drum or rarely by splitting of drum

-Slow rate of discharge can cause segregation of concrete

-Larger size of aggregate tend to stay in the mixer & the discharge sometimes starts as a mortar and finishes as a collection of coated coarse aggregate particles

Tilting Mixers

-Mixing chamber known as the drum is tilted for discharging

-Usually bowl shaped drum with veins inside

-Discharge is good because concrete can be tipped out rapidly as an unsegregated mass as soon as the drum is tilted

-Preferable for low workability and for large size aggregates

General Types Of Mixers

-Tilting Mixers

-Non-Tilting Mixers

-Pan Mixers

Concrete Mixers

-Achieve uniformity of the mix

-Should discharge the mix without disturbing that uniformity

-Method of discharging is one of the basis of classification of concrete mixers