WAFFLE SLAB - TYPES, ADVANTAGES AND DISADVANTAGES

Waffle Slab

Waffle slab is a reinforced concrete roof or floor containing square grids with deep sides and it is also called as grid slabs.The name waffle comes from the grid pattern created by the reinforcing ribs.Waffle slabs are preferred for spans greater than 40 feet (12 m), as they are much stronger than flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs and one-way joist slabs.A waffle slabs are provide stiffer and lighter slabs than an equivalent flat slab.The speed of construction of waffle slab is faster than conventional slab.It is also light weight slab and uses 30% less concrete and 20% less steel than a raft slab.

This kinds of slab are generally used in the entrance of hotels, malls, restaurants for good illustrative view and to install artificial lighting.This is a type of slab where we find a hollow hole in the slab when the formwork is removed. Firstly PVC trays are placed on shuttering then reinforcement is provided between the pods and steel mesh is provided at top of the pods and then concrete is filled.After concrete sets, the formwork is removed and PVC pods are not removed.This forms a hollow hole in it which hole is closed at one end.The concrete waffle slab is often used for industrial and commercial buildings while wood and metal waffle slabs are used in many other construction sites.

Types Of Waffle Slab

1. Square Pod System
2. Triangular Pod System

Advantages Of Waffle Slab

• Waffle slab are used for large span slabs or floors.
• The load carrying capacity of waffle slab is greater.
• It has excellent vibration control capacity.
• It is lightweight and required less amount of concrete.
• Suitable for spans of 7 m - 16 m longer spans may be possible with post-tensioning.
• Waffle slabs are able to carry heavier loads and span longer distances than flat slabs at these system are light in weight.
• It provides good structural stability along with aesthetic appearance.It is constructed for airports, hospitals, temples, churches etc.

 Disadvantages Of Waffle Slab

• Required skilled labour.
• Required strict supervision.
• It is not used in typical construction projects.
• It is preferred for flat topographical areas not sloped sites.
• Costly construction.

HARDY SLAB - TYPES, ADVANTAGES AND DISADVANTAGES

Hardy Slab

Hardy slab is a type of concrete slab constructed by hardy bricks.Hardy bricks are hollow bricks and it is made up of concrete hollow blocks.These blocks are used to fill portions of the slab.Hardy slabs reduce the amount of concrete in the slab and also its own weight of the slab is also reduced.This types of slab has a more thickness 2.27 m when compared with conventional one.The method of installing hardy slab is different from normal slab.The application of hardy slab are generally seen in Dubai and China.

Types Of Hardy Slab

1. One-way Hardy Slab
2. Two-way Hardy Slab

The Process Of Hardy Blocks Execution is as follows:

1. Formwork is arranged and then shutters are fixed on the formwork.

2. Hardy blocks are placed on the shutter with one brick gap on the entire shutter.

3. The gaps between the bricks are a rib. Reinforcement is provided in a form of the beam within the gap.

4. After placing the rib, the plain steel mesh is placed on the entire slab area  resting on ribs.

5. Now pouring of concrete is done on a slab. 

Advantages Of Hardy Slab

• Economical for span of length upto 5 m.
• Reduced the quantity of concrete below neutral axis.
• Reduced the weight of the slab compared to conventional slab.
• Hardy slab construction is good where temperature is very high.

Disadvantages Of Hardy Slab

• Costly 
• Difficult to repair.
• Difficult to strengthen.
• Required proper handling during transportation, otherwise it get damaged.

HOLLOW CORE SLAB - ADVANTAGES AND DISADVANTAGES

Hollow Core Slab

Hollow core slab is also know as a voided slab, hollow core plank or simply a concrete plank is a precast slab of prestressed concrete concrete typically used in the construction of floor in multi-story apartment buildings.In hollow core slab the main function of core is to maximizing structural efficiency, reduce the self weight of slabs and work as a service ducts.

The precast concrete slab has tubular voids extending the full length of the slab, typically with a diameter equal to the 2/3 - 3/4 the thickness of the slab.This makes the slab much lighter than a massive solid concrete floor slab of equal thickness or strength.The slab are typically 1200 mm wide with standard thickness normally between 150 mm and 500 mm.These type of slabs are precast and it is used where the construction has to be done fast.

The hollow core ribed slabs have between four and six longitudinal core running through them, the primary purpose of the cores being to decrease the weight and material within the floor yet maintain maximal strength.To increase the strength of the slab, it is reinforced with 12 mm diameter steel stand running longitudinally. This is one of the types of concrete slabs.

Advantages Of Hollow Core Slab

• Fast construction.
• High load capacity.
• Reduce self-weight.
• Excellent fire resistance.
• Excellent sound insulation.
• Excellent thermal properties.
• Easy to install and required less labour.
• Long spans without intermediate supports.
• Green product reduce the use of raw materials.
• No additional formwork or any special construction machinery is required.
• It not only reduces the building costs it also reduces the overall weight of the structure. 

Disadvantages Of Hollow Core Slab

• Not economical for small spans.
• Difficult to repair and strengthen.
• Required special equipment for lifting and moving the precast units.
• Required proper handling during transportation other wise it get damaged.
• It is difficult to produce satisfactory connections between the precast members.

FLAT SLAB - TYPES, ADVANTAGES AND DISADVANTAGES

Flat Slab

Flat slab is a reinforced concrete slab supported directly by concrete columns or caps. Flat slab doesn't have beam so it is also called a beam-less slab. Flat slab is defined as one sided or two-sided support system with sheer load of the slab being concentrated on the supporting columns and square slab called drop panels.

Flat slab construction is less popular in recent years because of the limit on economical spans of about 9.5 m for reinforced slabs about 12 m for pre-stressed slabs. A flat slab is a thick plate or shell where stiffness is high.It resists the loads because of stiffness. The thickness of flat is minimum 8 inches or 0.2 m.The plain dimensions of the drop panels are minimum of 1/3 of the span in the direction under consideration. Flat slab are highly versatile elements widely used in construction, providing minimum depth, fast construction and allowing flexible column grids. Flat slab is easier to construct and required less formwork. 

Types Of Flat Slab

1. Flat slab with drop panel.
2. Flat slab with column head.
3. Flat slab with drop panel and column head.
4. Flat slab without drop panel and column head.

Uses Of Flat Slabs 

• It is mostly used in industrial structures.
• It is used in parking garages, ramps, warehouses, hotels etc.
• They are also used where uses of beams are not required.

Advantages Of Flat Slab

• Fast construction.
• Better quality control.
• Required less formworks.
• It is capable of to carry concentrated loads.
• It reduces the overall height of the structure.
• It gives better fire resistant than other floor systems.
• It gives better appearance and better diffusion of light.
• Reduce the moment in the slab by reducing the clear or effective span.

Disadvantages Of Flat Slab

• Higher slab thickness.
• Not suitable for masonry partitions.
• In flat slab larger span is not possible.

CONVENTIONAL SLAB AND ITS TYPES

Conventional Slab

The slab which is supported on beams and columns is called conventional slab.In conventional slab the thickness of the slab is small whereas the depth of the beam is large and load is transferred to beams and then to columns.It required more formwork when compared with flat slab.The thickness of conventional slab is 4 inches or 10 cm.Normally conventional concrete slabs are in square shape and have a length of 4 m. Reinforcement is provided in conventional slab and the bars which are set in horizontal are called main reinforcement bars and the bars which are set in vertical direction are called distribution bars.These types of slab are generally used in high rise buildings.

There are two types of Conventional slabs based on length and breadth, they are:

1. One-way slab
2. Two-way slab

1. One-way Slab

One way slab is supported by beam on the two opposite sides to carry the load along one direction.In one way slab the ratio of longer span (l) to shorter span (b) is equal or greater than 2.

 i.e Longer span (l)/ Shorter span (b) ≥ 2. 

In this type of slab the main reinforcement bar will be provided in the shorter span where as the distribution bars will be provided in the longer span for the load distribution.

2. Two-way Slab

Two way slab is supported by beams on all the four sides and the loads are carried by the supports along with both directions, it is known as two way slab.In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2.

i.e Longer span (l)/ Shorter span (b)﹤2. 

In this type of slab the length and breadth of the slab are more than 4 m.To resist the formation of stresses distribution bars are provided at both the ends in two way slab.

Difference Between One Way Slab And Two Way Slab

• In one-way slab, the slab are supported by beams on the two opposite sides whereas in two-way slab, the slab are supported on all four sides.

• In one-way slab, the loads are carried along one directions whereas in two-way slab, the load are carried along both directions.

• In one-way slab, the ratio of Longer span to shorter span is equal or greater than 2. (i.e i/b ≥ 2 ) whereas in two-way slab, the ratio of i/b is less than 2. (i.e i/b﹤ 2 ) 

WHAT IS CONCRETE SLAB

Concrete Slab

Slab are constructed to provide flat surfaces, usually horizontal in building floors, roof, bridges and other types of structures.Basically a slab is a structural element, usually made up of reinforced concrete.The slab are usually constructed monolithically with the beams. Concrete slab is a very common and important structural element are constructed to provide flat, useful surfaces.It is a horizontal structural components, with top and bottom surface parallel.

Types Of Concrete Slabs

1. Flat Slab
2. Conventional  Slab
3. Hollow Core  Slab
4. Hardy Slab
5. Waffle Slab
6. Dome  Slab
7. Pitch  Slab
8. Slab with Arches
9. Post Tension  Slab
10. Pre Tension  Slab
11. Cable Suspension Slab
12. Low Roof Slab
13. Projected Slab
14. Grads Slab
15. Sunken Slab
16. Miscellaneous Slab

CONCRETE WATERPROOFING

Concrete Waterproofing

Waterproofing is the process of making an object or structure waterproof or water-resistant so that it remains relatively unaffected by water or resisting the ingress of water under specified conditions.Water-resistant and waterproof often refer to penetration of water in its liquid state and possibly under pressure, whereas damp proof refers to resistance to humidity or dampness.

Types Of Waterproofing

1. Cementitious Waterproofing
2. Bituminous Coating
3. Bituminous Membrane
4. Liquid Waterproofing Membrane
5. Polyurethane Liquid Membrane
6. Crystalline Admixtures

1. Cementitious Waterproofing

Cementitious waterproofing is the easiest method of waterproofing in construction work.The materials which are used for waterproofing of concrete are easily available in the market.This method is used in wet areas such as bathrooms and toilets.

2. Bituminous Coating

Bituminous coating is also called asphalt coating.It is a sticky, black and highly viscous liquid or semi-solid form of petroleum.It is a very good protective coating and waterproof agent, especially on surfaces of concrete foundations.Bituminous coating is made of bitumen based materials and it is not suitable for expose to sunlight.It become very brittle and fragile when long exposure to the sunlight unless it is modified with more flexible material such as polyurethane or acrylic based polymers.

3. Bituminous Membrane

Bituminous membrane waterproofing is a popular method used for low-sloped roofs due to their proven performance.Bituminous waterproofing membrane have touch on membrane and self-adhesive membrane.This type of waterproofing is good for commercial and residential buildings.

4. Liquid Waterproofing Membrane

Liquid waterproofing membrane is a thin coating which consist of usually a primer coat and two coat of top coats which are applied by spray, roller or trowel.It offers more flexibility than the cementitious type of waterproofing. The liquid cures into a rubbery coating on the wall.It is suited to all types of roof including flat, pitched and domed.Such coatingare usually reinforced with secondary materials such as glass reinforced plastic to provide additional tensile strength.

The process of liquid roofing provides a cost efficient method of making a new or existing roof waterproof.It can deliver upto 25 years performance depending on the coating system employed.It is estimated that liquid roofing is 70% less expensive than overall roof replacement in refurbishment situations.

5. Polyurethane Liquid Membrane

Polyurethane liquid membrane method of waterproofing is used for the flat roof area and exposed to weathering.This waterproofing method is expansive.Polyurethane liquid membrane can offer higher flexibility.Polyurethane is very sensitive to moisture content present, therefore before application, one has to be very careful evaluating the moisture content of the concrete slab, otherwise peeling or de-bonding of membranes may happen after some time.

ULTRA HIGH PERFORMANCE CONCRETE

Ultra High Performance Concrete

Ultra High Performance Concrete is defined as concrete that has a minimum specified compressive strength of 150 MPa with specified durability tensile ductility and toughness requirements; fiber are generally included to achieve specified requirements as per ACI 239R-18.

Ultra High Performance Concrete is known as reactive powder concrete.The material is typically formulated by combining Portland cement, supplementary cementitious materials, reactive powders, limestone and quartz flour, fine sand, high-range water reducers and water ranging from 0.15 to 0.25.The material can be formulated to provide compressive strength in excess of 29000 pounds per square inch (psi) (200 MPa). The use of fine materials for the matrix also provides a dense, smooth surface valued for its aesthetics and ability to closely transfer form details to the hardened surface.

Materials and Mixture Proportions

• Water
• Fine Sand
• Steel Fibers
• Silica Fumes
• Portland Cement
• Limestone and Quartz Flour
• High-Range Water Reducers

Applications Of UHPC

• Piles
• Facades
• Rehabilitation
• Seismic Retrofit
• Highway Bridges
• Impact Resistance
• Aggressive Environments
• Spend Nuclear Fuel Storage
• Security and blast mitigation applications
• Thin-bound overlays on deteriorated bridge deck

Advantages of UHPC

• Extended life span
• 
  
• Improve resiliency
• Improved durability
• Reduce maintenance
• Reduce out of service 
• Speed of construction
• Minimum interruption
• Reduced element size and complexity

SLUMP TEST OF CONCRETE

Slump Test Of Concrete

Slump test is the most common and simple test adopted to measure the workability of fresh concrete.This test is adopted to check the workability or consistency of freshly mixed concrete in a specific batch.The slump test is used to ensure uniformity for different loads of concrete under field conditions.

Apparatus Used In Slump Test

1. Slump cone with Height of 300 mm, Bottom diameter 200 mm and Top diameter 100 mm.
2. Standard Tamping Rod.
3. Non-Porous Base Plate.
4. Measuring Scale.

Procedure For Slump Test

1. First clean the inner surface of the empty mould and then apply oil to it.
2. Set the mould on a horizontal non-porous and non-absorbent base plate.
3. Fill the mould fully by pouring freshly mixed concrete in the three equal layers.
4. Each layer is tempted 25 times with a standard 16 mm diameter steel rod.
5. When the mould is completely filled with concrete, the top surface is stuck off by means of screening and rolling motion of the temping rod.
6. After filling is completed and the concrete is leveled, the cone is slowly and carefully lifted vertically, an unsupported concrete will now slump.
7. The decrease in the height of the centre of the slumped concrete is called slump.
8. The slump is measured by placing the cone just besides the slump concrete and the temping rod is placed over the cone so that it should also come over the area of slumped concrete.
9. The decrease in height of concrete to that of mold is noted with scale.

Types Of Slump

1. True Slump
2. Shear Slump
3. Collapse Slump

1. True Slump

In true slump concrete just subside shortly and more or less maintain the mould shape.This type of slump is most desirable.

2. Shear Slump

If one-half of the cone slides down in an inclined plane, it is called a shear slump.It indicates lack of cohesion in the concrete mix.

3. Collapse Slump

When the sample is collapse due to adding excessive water, it is known as collapse slump.

SILICA FUMES CONCRETE

Silica Fume Concrete

Silica fume concrete is composed of cement, silica fume, fine aggregate, coarse aggregate and water.

Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys.Because of its chemical and physical properties, it is a very reactive pozzolan. Silica fume concrete can have very high strength and can be very durable. The durability of this type of concrete is superior to conventional concrete.In silica fumes concrete segregation and bleeding is low and the mixture is adhesive compared to traditional concrete.

Because of its extreme fineness and high silica content, silica fume is very effective pozzolanic material.The applications of silica fume concrete in construction are seen in high-rise building, parking structure, dams, nuclear waste storage facility and shotcrete rehabilitation.

Chemical Composition Of  Silica Fume Material

• It contains more than 90% of silicon dioxide.
• Other constituents are carbon, sulfur, iron, calcium, magnesium, sodium, potassium and oxides of aluminum.

Physical Properties Of  Silica Fume Material

• The diameter of the silica fume particle range from 0.1 micron to 0.2 micron.
• The surface area is about 30000 m2/kg.
• Density varies from 150 to 700 kg/m3.
• The shape of particle is spherical.

Properties Of Fresh Silica Fume Concrete

• Low workability.
• Low slump value.
• High plastic shrinkage.
• The mixture is cohesive.
• Bleeding and segregation is low.
• It requires higher water content.

Properties Of Hardened Silica Fume Concrete

• The modulus of elasticity is higher than that of ordinary concrete.
• Creep of silica fume concrete is lesser than conventional concrete.
• The compressive strength of silica fume concrete is higher than ordinary concrete (6000 - 8000 psi).

Advantages Of Silica Fume Concrete

• High durability.
• High bond strength.
• High flexural strength.
• High modulus of elasticity.
• High compressive strength.
• It reduces segregation and bleeding.
• High electrical resistivity and low permeability.
• It is suitable for mass concreting since it prevents thermally induced cracking.

Disadvantages Of  Silica Fume Concrete

• Expensive.
• Availability issue.

Applications Of  Silica Fume Concrete

• Dam structure.
• Parking structure.
• High rise building.
• Shotcrete rehabilitation .
• Nuclear waste storage facility.

DAM  STRUCTURE

PARKING  STRUCTURE

HIGH  RISE  BUILDING

TYPES OF CONCRETE FINISHES

Concrete Finishes

The concrete is a flexible construction material used globally for residential, commercial and industrial applications.The basic types of concrete finishes are as fallows:

1. Troweled Finish
2. Broom Finish
3. Salt Finish
4. Stamped Finish
5. Exposed Aggregate Finish
6. Swirl Finish
7. Polished Finish
8. Colored Finish
9. Other Finish

1. Troweled Finish

Trowel finish is the most common and popular type of concrete finish used for major types of applications.After concrete is laid in the formwork and leveled, the trowel is used to smooth and fine-level the surface of concrete.

Trowels are available in both manual and mechanical types.A manual towel consists of  a flat steel blade with an attached handle that is pushed and pulled across the concrete surface.A mechanical towel is used for large commercial projects which resemble large fans with the blade sitting directly against the concrete.

2. Broom Finish

The broom finish of the concrete is rough textured finished.In order to make concrete surface slip resistant, a broom finish can be applied.This is done after placement, leveling and troweling of concrete.Once a smooth surface has been created a broom is dragged across the surface of the concrete to create small ridges that provide for traction control, particularly when the concrete is wet. 

3. Salt Finish

Salt finish is one of the older and more conventional decorative concrete finishes.It is created by applying rock salt to the top of the wet concrete and then washing it away, which leaves small pits in the finished surface.This finish is mainly used for swimming pool decks.

4. Stamped Finish

Stamped concrete finish is obtained by pressing the required patterns on to the freshly levelled concrete.A stamped finish is made by placing panels with designs onto the uncured cement.This transfers the design of the panel, giving the concrete a textured look.Concrete stamps come in many different designs.These include stone, bricks, tiles or other patterns.The main objectives of stamping technique is to give concrete a textured look.It is mainly applied for floor in patios, car porch, driveways and sidewalks.

5. Exposed Aggregate Finish

The construction of exposed aggregate finish, concrete is poured, levelled and troweled.It is then treated overnight with a chemical that slows the hardening of the surface cement.The surface is then washed with water to remove the top layer, exposing the aggregate that was below the surface.

In exposed aggregate finish with normal concrete materials other materials may be added into the mix to provide exposed finishes with unique looks.Example are rose quartz, limestone, dark grey or black basalt, red or blue granite and even colored glass or seashells.This provides an attractive and slip resistant finish.Exposed aggregate finished concrete provides a safe, non-slip surface.This makes it both functional and attractive.It's great for sidewalks, porches and breezeways between buildings.

6. Swirl Finish

Swirl finish is one of the most exceptional types of concrete finish that is generated by smearing the concrete surface in a uniformly overlapping circular pattern.Swirl finish is less common these days but is still a great option for creating a beautiful and slip resistant surface.

The swirl pattern provides an attractive appearance to the concrete surface which is favored for outdoor applications. The swirl pattern, or fan pattern is created by moving a trowel in swirls or circle that overlap each other.The swirls can be made in varying sizes and depth depending on the aesthetic the customer prefers.This pattern especially looks nice when you add a standard smooth border around it similar what you see on sidewalks with a brushed finish.

7. Polished Finish

Polished  concrete is a multi-step process where a concrete floor is mechanically ground, honed and polished with bounded abrasives in order to cut a concrete floor's surface.It is then refined with each cut in order to achieve a specified level of appearance.

The polishing process is typically accomplished using concrete floor grinders that are outfitted with diamond abrasives.First, concrete is stripped of any existing sealer or coating and any visible cracks are repaired.This is followed by the polishing process using the floor grinders mentioned above.During polishing process, chemical hardeners are often added to the concrete to provide future protection against water infiltration.Finer and finer abrasive are used until the desired surface finish is achieved.

8. Colored Finish

The colored finish on a concrete surface can be carried out by either pigment or stains.The pigmented colored concrete can be accomplished by adding the pigments directly to the concrete mix prior to pouring. The strained colored concrete can be obtained by applying stains to concrete of any age to obtain a more vibrant colored finish.The application of stain is typically followed up with the installation of a seal over the concrete to protect the surface.

9. Other Finish

The other finish of concrete are listed below:

• Dyed Finish
• Etched Finish
• Flashed Finish
• Over lay Finish
• Engraved Finish
• Color Chip Finish
• Marbleized Finish
• Sand Blasted Finish

HONEYCOMB IN CONCRETE

Honeycomb In Concrete

Honeycomb in concrete refers to cavities in the concrete formed due to presence of air bubbles present in the concrete.Honeycombs are hollow spaces and cavities left in concrete mass on surface or inside the concrete mass where concrete could not reach.These looks like honey bees nest.Honeycombs which are on sides are visible to naked eyes and can be detected easily as soon shuttering is removed.

Honeycomb in concrete is normally seen at the column and beam junction due to overcrowding of reinforcement bars which leads to poor concrete fill.Concrete honeycomb not only reduces the strength also provides passage for water which in turn corrodes reinforcement bars.

Types Of Honeycomb In Concrete

1. Small Size Honeycomb
2. Medium  Size Honeycomb
3. Large  Size Honeycomb

1. Small Size Honeycomb

Small size honeycomb has a depth of less than 25 mm.

2. Medium Size Honeycomb

Medium size honeycomb is deeper than 25 mm but in which steel bar is not exposed.

3. Large Size Honeycomb

Large size honeycomb is deeper than 25 mm and in which steel bar has come out.

Causes Of Honeycomb In Concrete

• Improper workability in concrete.
• Insufficient compaction to concrete.
• Improper clear cover to reinforcement bars.
• Improper vibration of concrete in formwork.
• Formwork is not rigid and watertight.
• Use of larger size aggregates in excessive amount.
• Concrete is poured from more than allowable hight.
• Improper placement of bars at column and beam junction.
• Over reinforcement.
• Bad workmanship.

Effects Of Honeycomb In Concrete

• Honeycombing makes the concrete weak.
• Water and air penetrate inside the structure.
• Due to honeycombing moisture easily entered in RCC.
• It reduces the structural strength of concrete components. 
• It reduces the load-bearing capacity eventually affecting the strength of the structure.

How To Repair Honeycomb In Concrete

1. Mark the affected area of honeycombing and remove all loose aggregate.
2. With the help of wire brush clean the affected area.
3. Remove the finer particles and wash the concrete surface with water.
4. After 2-3 hours apply Chemi-fix glue on the affected area.
5. Prepare a grout mix with white cement and add the required amount of water.
6. Fill the affected area of the concrete surface.In case of large-area affected by honeycombing make patch hole before applying grout for proper bonding.
7. Remove formwork if any applied and cure surface properly for at least 24 hours.

BLEEDING OF CONCRETE

Bleeding Of Concrete

Bleeding can be defined as the tendency of water to rise to the surface of freshly placed concrete.It is another from of segregation where some amount of water comes to the concrete surface after placing and compacting, before setting.

Effects Of Bleeding In Concrete

• Concrete loses its homogeneity due to bleeding.
• It makes concrete permeable.
• Bleeding of concrete causes high water-cement ratio.
• It delays the surface finishing in pavement construction.
• The bond between two concrete layers become weaker.

How To Reduce Bleeding In Concrete

• Reduce the water content. b
• Increase the amount of cement content.
• Increase the amount of finer particles.
• Use air-entrained admixtures.
• Design the concrete mix properly.
• Use cementitious materials with finer particles.
• Use rounded natural sand rather than an angular.

SEGREGATION OF CONCRETE

Segregation Of Concrete

Segregation of concrete is the separation of cement paste and aggregates of concrete from each other during handling and placement.Segregation also occurs due to over-vibration or compaction of concrete, in which cement paste comes to the top and aggregates settles at the bottom.

Generally, it is observed that segregation occurs in plastic stage of concrete, mostly occurs in lean concrete and wet concrete.

Causes Of Segregation In Concrete

• It may cause due to excess water content in the mix.
• It may cause due to the separation of coarser particles.
• It may cause due to the use of poor graded aggregates.
• It occurs due to the use of over vibration of concrete.
• It may cause due to the transporting concrete mixes for long distance.
• It occurs due to poor handling of concrete, i.e drooping concrete more than 1 m.

How To Reduce Segregation In Concrete

• The water cement ratio in the concrete mix should be properly designed.
• The placement of concrete must be from normal height, i.e less than 1.5 m.
• Formwork should be water tight to prevent leakage.
• The combination of cement and sand in the concrete must be according to the design.
• Transport the concrete mix effectively.Select shortest route for transportation of concrete mix.
• Use vibrator correctly and never use the vibrator to spread a heap of concrete over a large area.
• Use admixtures in concrete such as air entraining agents in the mix. Entrained air reduces the danger of segregation.

METHODS OF CONCRETE CURING

Concrete Curing

Concrete curing is the process of maintaining adequate moisture in concrete within a proper temperature range in order to aid cement hydration at early age.Curing will increase the strength and decrease the permeability of hardened concrete.Curing is also helps in reducing thermal and plastic cracks, which can severely impact durability of structure.

A curing practice involves keeping the concrete damp or moist until the hydration of concrete is complete and strength is attained.Hydration is the chemical reaction between cement and water that results in the formation of various chemicals contributing to setting and hardening.Curing of concrete should being soon after initial setting time of concrete or formwork/shuttering is removed and must continue for a reasonable period of time as per specified standards, for concrete to achive its desired strength and durability.

Method Of Concrete Curing

1. Pounding Method
2. Sprinkling of Water
3. Steam Curing
4. Membrane Curing
5. Covering with Wet Sheets
6. Shading Concrete 

1. Pounding Method

Pounding is the best method of curing.It is suitable for curing horizontal surface such as floors, roof slabs, lintels, road and air field pavements.A temporary pond is made by mortar made up of less amount cement and sand or clay.The area is thus divided into a number of rectangles and this is filled with water for curing.This is the best and efficient method curing however this is very water extensive.

2. Sprinkling Of Water

Sprinkling of water continuously on the surface provides an efficient curing.It is mostly used for slabs curing.In this method water is sprayed over the concrete surface when ambient temperature is more than freezing temperature and humidity is low for keeping surface moist, as low humidity and high temperature causes quick evaporation.

3. Steam Curing

Steam curing method is adopted during cold weather condition that facilitates rapid hardening of concrete.This is the fastest method of curing compared to other curing methods.This method is used in Pre-cast concrete work.In steam curing the temperature of steam should be restricted to a maximum of 75℃ as in the absence of proper humidity (about 90%) the concrete may dry soon.In case of hot water curing, temperature may be raised to any limit at 100℃.At this temperature the development of strength is about 70% of 28 days strength after 4-5 hours.

4. Membrane Curing

Membrane curing comes under the category of moist curing.In this method of curing is to cover the wetted concrete surface by a layer of water proof material, which is kept in contact with the concrete surface of 7 days. A membrane will prevent the evaporation of water from the concrete.This membrane can be either in solid or liquid form.They are also known as sealing compounds. Bituminised water proof paper, wax emulsions, bitumen emulsions and plastic films are common types of membrane used.

Whenever bitumen is applied over the surface for curing, it should be done only after 24 hours curing with gunny bags.The surface is allowed to dry out so that loose water is not visible and then liquid asphalt sprayed throughout.The moisture in the concrete is thus preserved.It is quite enough for curing.

5. Covering with Wet Sheets

This is a widely used method of curing, especially for structural concrete.In this method exposed surface of concrete is prevented from drying out by covering it with jute bags or empty cement bags.The covering over vertical and sloping surface should be secured properly.These are periodically wetted.The interval of wetting will depend upon the rate of evaporation of water.It should be ensured that the surface of concrete is not allowed to dry even for a short time during the curing period.

6. Shading Concrete

The object of shading concrete work is to prevent the evaporation of water from the surface even before setting.This is adopted mainly in case of large concrete surface such as road slabs.This is essential in dry weather to protect the concrete from heat, direct sun rays and wind.It also protect the surface from rain.In cold weather shading helps in preserving the heat of hydration of cement thereby preventing freezing of concrete under mild frost conditions.Shading may be achived by using canvas stretched on frames.This method has a limited application only.

WORKABILITY OF CONCRTE

 Workability Of Concrete

Workability of concrete is the property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished as defined by ACI Standard 116R-90 (ACI 1990b).

Workability is a property that directly impacts strength, quality, appearance and even the cost of labor for placement and finishing operations.Concrete is said to be workable when it is easily placed and compacted homogeneously i.e without bleeding or segregation.

Types Of  Workability Of Concrete

1. Unworkable Concrete
2. Medium Concrete
3. Highly Workable Concrete

1. Unworkable Concrete

An unworkable concrete also called as harsh concrete, is a concrete with a very little amount of water in it.The hand mixing of such concrete is very difficult.This type of concrete has a high segregation of aggregates and it is very difficult to maintain the homogeneity of concrete mix and compaction of concrete requires much effort.Water cement ratio of such concrete is below 0.4.

2. Medium Concrete

Medium workable concrete is preferred in most construction works.This type of concrete is relatively easy to mix, transport, place and compact without much segregation and loss of homogeneity.Water cement ratio for medium workable concrete is 0.4 to 0.55.

3. Highly Workable Concrete

A highly workable concrete is very easy to mix, transport, place and compact because of high water content.It is used for concreting where effective compaction of concrete is not possible.The problem in using this type of concrete is that there are high chances of segregation and loss of homogeneity.Such concrete is used in case of heavy reinforcement is used where vibration of concrete is not possible.Water cement ratio in such type of concrete is more than 0.55.

Factors Affecting Workability Of Concrete

• Water cement ratio.
• Type of construction work.
• Method of mixing of concrete.
• Method of placement of concrete.
• Method of compaction of concrete.
• Method of transmission of concrete.
• Temperature of the concrete mix.
• Thickness of concrete sections.
• Environmental condition.

Method Of Improving The Workability Of Concrete

• Increase water cement ratio.
• Increase the size of aggregate.
• Increase the mixing time of concrete.
• Increase the mixing temperature of concrete.
• Use non porous and saturated aggregates.
• Adding appropriate admixtures in concrete.
• Use well rounded and smooth aggregate instead of irregular shape.

Methods Of Determining Workability Of Concrete

• Slump test.
• Flow test.
• Kelly ball test.
• Compacting factor test.
• Vee Bee consistometer test.

WHAT IS SHOTCRETE

Shotcrete

Shotcrete or gunite or sprayed concrete is a method of applying concrete projected at high velocity primarily on to a vertical or overhead surface.The impact created by the application consolidates the concrete.Although the hardened properties of shotcrete are similar to those of conventional cast-in-place concrete, the nature of the placement process results in an excellent bond with most substrates, and rapid or instant capabilities, particularly on complex form or shape.The shotcrete required less formwork and can be more economical than conventionally placed concrete.

Shotcrete is mortar or concrete conveyed through a hose and pneumatically projected by a nozzle with high velocity on the prepared surface.The whole system is also known as spraycrete.Shotcrete is mainly used in underground construction projects as preliminary or permanent structural supports.About 90% of the shotcrete applied into underground construction projects like road tunnel,rail tunnel, waterways etc.Annually more than 12 million cubic meters shotcrete by volume used globally. Shotcrete was invented by Mr.Carl Ethan Akeley (1864-1926) in 1910.

Types of Shotcrete

1. Dry-mix process
2. Wet-mix process

Advantages Of Shotcrete

• Shotcrete technique is flexible and easily modified.
• Shotcrete method has less impact on nearby properties.
• Excellent bounding nature makes the concrete very strong.
• The concrete can be applied by a nozzle from a safe distance.
• Shotcrete required less formworks.
• Economical.

Disadvantages Of Shotcrete

• Too much dust.
• Too much wastage of concrete.
• The production cost is very high.
• It requires skilled and experienced labours.

Applications Of Shotcrete

• Waterproofing walls.
• Stabilized rock slopes.
• Thin overhead vertical or horizontal surfaces.
• Excavation stabilization in tunnelling and underground construction.
• Restoration and repairing of old building and fire-damaged structure.
• Rehabilitation of reinforced concrete structures such as bridges, chemical processing and handling plants.

WHAT IS GREEN CONCRETE - APPLICATION, ADVANTAGES AND DISADVANTAGES

 Green Concrete

Concrete which is made from concrete wastes that are eco-friendly, uses less energy in the production and produces less carbon dioxide than ordinary concrete is called green concrete.Most people associate it with the concrete that is colored with pigment, but green concrete does not refer to the color of the concrete.It is considers every aspect starting from raw materials to its manufacture, concrete mix design and ultimately the structural design, construction and service life.

Green concrete is very cheap to produce, because waste products are used as a partial substitute for cement, charges for the disposal of waste are avoided, energy consumption in production is lower and

durability is greater.Green concrete is a type of concrete which resembles the conventional concrete but the production or usage of such concrete requires minimum amount of energy and causes least harm to the environment.Green concrete was first invented in Denmark in the year 1998 by Dr.WG.

Why Green Concrete?

The main constituents in concrete is cement which consists of limestone.During the manufactured of cement, its constituents are heated about 800-1000 ℃.During this process, carbon dioxide is driven off.Approximately 1 kg of cement produces about 900 gm of Co2. When it comes to green concrete , it reduces the Co2 emmision by at least 30%.It is also energy-saving and eco-friendly than normal concrete.

Materials Used

1. Fly Ash
2. BFS (Blast Furnace Slag).
3. GGBS (Ground Granulated Blast Furnace Slag).
4. Recycled demolition waste aggregates.
5. Recycled concrete aggregates.
6. Glass aggregates.
7. Manufactured sand.
8. Rice husk ash.
9. Silica fume.
10. Metakaolin.

Environmental Goal of Green Concrete

• Reduction of CO2 by 30%.This is in accordance with the Kyoto Protocol of 1997.
• Reduce the use of fossil fuels by increasing the use of waste derived fuels in the cement industry.
• The recycling capacity of the green concrete must be less compared to existing concrete types.
• The structures do not impose much harm to the environment during their service life.
• The production and the use of green concrete must not deteriorate the working environment.

Advantage of Green Concrete

• Eco-friendly.
• Better workability.
• Reduced CO2 emissions by 30%.
• No environmental pollution.
• Helps in recycling industrial waste.
• Reduce the consumption of cement.
• Better damping resistance in building.
• It requires less maintenance and repairs.
• It is as economical as conventional concrete.
• Greater strength and durability than normal concrete.
• It has good thermal resistant and fire resistant properties.
• Sustainable development without environmental pollution.
• Low production costs as wastes directly substitute the cement.
• Flexural strength of green concrete is almost equal to that of conventional concrete.

Disadvantage of Green Concrete

• Water absorption is more than conventional concrete.
• It has less split tensile strength than conventional concrete.
• Its compressive strength is less than conventional concrete.
• The cost of reinforcement increases as it uses stainless steel.
• The effect of creep and shrinkage is more than conventional concrete.

Application of Green Concrete

• It is used in building construction.
• It is used in bridge construction.
• It is used in dam construction.
• It is used in road construction.
  
  

CREEP OF CONCRETE

Creep of Concrete

Creep can be defined as the elastic and long term deformation of concrete under a continuous load.Generally, a long term pressure changes the shape of concrete structure and the deformation occurs along the direction of the applied load.When the continuous load is removed  the strain is decreased immediately.The amount of decreased stain is equal to the elastic stain at the given age.The quick recovery is then followed by a continuous decrease in stain known as creep recovery that is a part of total creep stain suffered by the concrete.

Creep Coefficient

The ratio of the ultimate creep stain to the elastic stain at the age of loading is termed as creep coefficient.

Factors Affecting the Creep of Concrete

Following are the factors affecting the creep of concrete:

1. Water-Cement Ratio

When the water-cement ratio is increased, the higher water-cement ratio will result in more porous and weaker cement paste which will deform more under a given load.

2. Humidity

The rate and ultimate magnitude of creep increased as the humidity of atmosphere decreases.The relation between relative humidity and creep is not linear concrete under sustained load in air at 70% relative humidity will have an ultimate about twice as large as concrete in air at 100% relative humidity.The ultimate creep in air at 50% relative humidity will be about three times as large.

3. Aggregate

When aggregate volume is increased, creep will be less as the aggregate is more rigid than the cement paste.Aggregate with moisture movement and low modulus cause a large amount of creep.

4. Admixtures

Some admixtures (mainly accelerators) are also responsible for causing creep in concrete.

5. Age of Concrete

The age at which a concrete member is loaded will have a predominant effect on the magnitude of creep.This can be easily understood from the fact the quality of gel improves with time.Such gel creeps less, whereas volume gel under load being not so stronger creeps more.The rate of creep rapidly decreases with time.The time taken by a concrete structure to attained creep is 5 years.

WHAT IS LIGHT WEIGHT CONCRETE - TYPES, USES AND ADVANTAGES

Light Weight Concrete

Light weight concrete is a mixture of lightweight coarse aggregate and sometimes a portion or entire fine aggregates.A structure light weight concrete mix has a density of about 105 pound per cubic foot.The light weight concrete density is less because of light weight coarse aggregates and sometimes light weight fine aggregates are used, which make the concrete weight less.

The light weight aggregate used are usually expanded shale, clay or slate materials that have been fired in a rotary kiln to give them a porous structure, similar to volcanic rock.In some mixes air cooled blast furnace slag is also used.

Structure light weight concrete mixes can be designed to achieve similar compressive strength as normal weight concrete.They also have the same durability and mechanical characteristics as regular weight concrete.There are other classes of non-structural light weight concrete with lower density made with other aggregate materials and higher air voids in the cement paste matrix such as in cellular concrete.

Types of Light Weight Concrete

1. Lightweight Aggregate Concrete
2. Aerated Aggregate Concrete
3. No Fines Concrete

1. Lightweight Aggregate Concrete

Light weight aggregate concrete can be produced using a variety of lightweight aggregates.In early 1950s the use of lightweight concrete blocks was accepted in the UK for load bearing inner leaf of cavity walls.

 The following are the types of light weight aggregate concrete:

• Formed Slag: It was the first light weight aggregate suitable for reinforced concrete that was produced in large quantity in the UK.

• Pumice: It is used for reinforced concrete roof slab, mainly for industrial roofs in Germany.

• Expanded Clays and Shales: It is capable of achieving sufficiently high strength for prestressed concrete.Well established under the trade name of Aglite and Leca (UK), Haydite, Rocklite, Gravelite and Aglite (USA).

2. Aerated Concrete

Aerated concrete has the lowest density, thermal conductivity and strength.Like timber it can be sawn, screwed and nailed but there are non-combustible.Aerated concrete is a lightweight, cellular material consisting of cement and sand or lime or other silicious material.It is made by either a physical or a chemical process during which either air or gas is introduced into a slurry which generally contains no course materials.

3. No Fines Concrete

No fines concrete may be defined as concrete mixtures containing little or no fine aggregates.The fine aggregate is not added in this concrete so that there are voids left in the coarse aggregate.The coarse aggregate may be any of the usual type or the light weight aggregate.The coarse aggregate used should be finer than 20 mm size and not more than 10% should pass the 10 mm sieve.Generally no fine concrete is produced with a cement aggregate ratio of 1:6 to 1:10, but practical mixes vary rather widely.The variation of cement aggregate ratio for lean mixes is from 1:10 to 1:20.The corresponding cement contents are 130 kg/m3 to 70 kg/m3.In case of fines concrete there is a narrow optimum value of w/c ratio for any given aggregate.

Although the strength of no-fine concrete is considerably lower than that of normal-weight concrete, this strength ,coupled with lower dead load of the structure, sufficient in building up to about 20 storeys high and in many other applications.

Properties of Light Weight Concrete

• It is light in weight because of fly ash as aggregate.
• Compressive strength varies from 0.3Mpa to 40Mpa.
• It has high thermal and acoustical insulating qualities.

Uses of Light Weight Concrete

• It is used in general insulation of walls.
• It is used in insulating water pipes.
• It is used in heat insulation on roofs.
• It is used for reinforced concrete.
• It is used for fixing bricks to receive nails from joinery.
• It is used in casting structure steel to protect its against fire and corrosion or as a covering for architectural purposes.

Advantages of  Light Weight Concrete

• Self compaction.
• Easy to transport.
• Reduced seismic forces.
• Improved constructability.
• Improved structural efficiency.
• It has low drying shrinkage.
• Pumping concrete to large distance.
• It has good sound insulation properties.
• Formwork will withstand low pressures.
• Reduced the dead load of a structure due to less density.
• It is eco-friendly as it used clinker, fly ash, slag etc.

Disadvantages of  Light Weight Concrete

• It is porous and shows poor resistance.
• Very sensitive with water content in the mixture.
• Cement mixtures have difficult in placing and finishing.
• Foam concrete mixtures is susceptible to water content in the mixture.
• Mixing time is longer than conventional concrete to assure proper mixing.