Methods of Prestressing

Methods of Prestressing

The prestressing can be performed by two methods:

1. Pretensionong
2. Post-Tensioning

1. Pretensioning

In the pretensioning method, the stress is induced by initially tensioning the steel tendons. These are wires or strands that are tensioned between the end anchorages. After this tensioning process, the concrete casting is performed.

Once the casted concrete has hardened sufficiently, the end anchorages arranged are released. This releasing transfers the prestress force to the concrete. The bond between the concrete and the steel tendons facilitates this stress transfer.

As shown in figure-2, the tendons that are protruding at the ends are cut and a finished look is achieved. In order to induce prestress force in the pre-tensioning method, a large number of tendons and wires are used. This arrangement hence demands a large area of surface contact to make the bond and stress transfer possible.

2. Post Tensioning

The procedure in post-tensioning is depicted in the figure-3 below. Here, the steel is prestressed only after the beam is cast, cured and attain strength to take the prestress. Within the sheathing, the concrete is cast. For the passage of steel cables, ducts are formed in the concrete.

Once the casted concrete hardens completely, the tendons are tensioned. One end of the tendon is anchored and the other end is tensioned. In some cases, the tensioning can be performed from either side and anchored subsequently.

Once the prestressing is complete, there is space between the tendons and the duct. This leads to:

1. Bonded Construction
2. Unbonded Construction

1. Bonded Construction

In bonded construction, the space between the duct and the tendon is filled with cement grout. The grouting process helps the steel to resist corrosion to a large extent. The ultimate strength is increased as this method increases the resistance to live loads acting. The grout mixture is cement and water combined with or without admixture. No sand is used in this grout.

2. Unbonded Construction

If no grout is used to fill the space between the duct and tendon, it is called as unbonded construction. Here, the steel is galvanized to protect from corrosion. A waterproofing material is used for galvanizing.

In Case If Concrete Box Girder Bridges How Is The Number Of Cells Determined?

When the depth of a box girder bridge exceed 1/6th or 1/5th of the bridge width then the design recommended is that of a single cell box girder bridge. But in case the depth of the bridge is lower than 1/6th of the bridge width then a twin-cell or in some cases multiple cell is the preferred choice. One should also note that even in the cases of wider bridges where there depths are comparatively low the number of cells should be minimized. This is so as there is noticeably not much improvement in the transverse load distribution when the number of cells of the box girder is higher than three or more.

In The Design Of Bridge Arguments What Considerations Should Be Made To Select The Orientation Of The Wing Walls?

Some of the most common arrangements of wing walls in cases of bridge arguments are as follows:

• Wing walls parallel to abutments: This method is considered to take least amount of time to build and is simple as well. But on the downside this method is not the most economical. The advantage of this type of design being that they cause the least amount of disturbance to the slope embankment.
• Wing walls at an angle to abutments: This design method is considered to be the most economical in terms of material cost.
• Wing walls perpendicular to abutments: The characteristic of this design is it provides an alignment continuous with the bridge decks lending a support to the parapets.

What Reinforcements Are Used In The Process Of Prestressing?

The major types of reinforcements used in prestressing are:

• Spalling Reinforcement: The spalling stresses leads to stress behind the loaded area of the anchor blocks. This results in the breaking off of the surface concrete. The most likely causes of such types of stresses are Poisson`s effects strain interoperability or by the stress trajectory shapes.
• Equilibrium reinforcements: This type of reinforcements are required where several anchorages exist where the prestressing loads are applied in a sequential manner.
• Bursting Reinforcements: These kinds of stresses occur in cases where the stress trajectories are concave towards the line of action of load. In order to reduce such stresses reinforcements in the form of bursting is required.

Why Are Steel Plates Inserted Inside Bearings In Elastomeric Bearings?

In order to make a elastomeric bearing act/ function as a soft spring it should be made to allow it to bulge laterally and also the stiffness compression can be increased by simply increasing the limiting amount of the lateral bulging. In many cases in order to increase the compression stiffness of the bearing the usage of metal plates is made. Once steel plates are included in the bearings the freedom of the bulge is restricted dramatically, also the deflection of the bearing is reduced as compared to a bearing without the presence of steel plates. The tensile stresses of the bearings are induced into the steel plates. But the presence of the metal plates does not affect the shear stiffness of the bearings.

Describe Briefly The Various Methods Of Concrete Curing?

Curing is the process of maintaining the moisture and temperature conditions for freshly deployed concrete. This is done for small duration of time to allow the hardening of concrete.

The methods that are involved in saving the shrinkage of the concrete includes:

• Spraying of water: on walls, and columns can be cured by sprinkling water. 
• Wet covering of surface: can be cured by using the surface with wet gunny bags or straw
• Ponding: the horizontal surfaces including the slab and floors can be cured by stagnating the water.
• Steam curing: of pre-fabricated concrete units steam can be cured by passing it over the units that are under closed chambers. It allows faster curing process and results in faster recovery. 
• Application of curing compounds: compounds having calcium chloride can be applied on curing surface. This keeps the surface wet for a very long time.

What Are The Steps Involved In The Concreting Process, Explain?

1. Batching: The process of measurement of the different materials for the making of concrete is known as batching. batching is usually done in two ways: volume batching and weight batching. In case of volume batching the measurement is done in the form of volume whereas in the case of weight batching it is done by the weight.
2. Mixing: In order to create good concrete the mixing of the materials should be first done in dry condition and after it wet condition. The two general methods of mixing are: hand mixing and machine mixing.
3. Transportation and placing of concrete: Once the concrete mixture is created it must be transported to its final location. The concrete is placed on form works and should always be dropped on its final location as closely as possible.
4. Compaction of concrete: When concrete is placed it can have air bubbles entrapped in it which can lead to the reduction of the strength by 30%. In order to reduce the air bubbles the process of compaction is performed. Compaction is generally performed in two ways: by hand or by the use of vibrators.

Cement and History of cement

Cement

• A cement is a binder, a substance used in construction that sets and hardens and can bind other materials together.

• The most important types of cement are used as a component in the production of mortar in masonry, and of concrete, which is a combination of cement and an aggregate to form a strong building material.

Cement History

• Joseph Aspedin of Yorkshire (U.K.) was the first to introduce Portland cement in 1824 formed by heating a mixture of limestone and finely divided clay in a furnace to a temperature high enough to drive off the carbonic acid gas.
• In 1845, Issac C. Johnson invented the cement by increasing the temperature at which the mixture of limestone and clay were burned to form clinker. This cement was the prototype of the modern Portland cement.
• From then onward, a gradual improvement in the properties and qualities of cement has been made possible by researchers in U.S.A., U.K., France and Germany

Cement Clinker

In the manufacture of Portland cement, clinker occurs as lumps or nodules, usually 3 millimeters (0.12 in) to 25 millimeters (0.98 in) in diameter, produced by sintering (fused together without melting to the point of liquefaction) limestone and alumino silicate materials such as clay during the cement kiln stage.

Today cement finds extensive use in all types of construction works; in structures where high strength is required e.g. bridge piers, light houses, lofty towers, and large structures such as bridges, silos, chimneys. And also in structures exposed to the action of water, e.g. reservoirs, dams, dock yards etc. Cement mortar, concrete, reinforced brick work, artificial stones, plastering, pointing and partition walls are routinely used in buildings.

Uses of Lime

Uses of Lime

• Lime has been used in building techniques for over 5,000 years. Archaeological evidence shows it to have been in existence for this time frame due to its resilience, durability, and water resistant qualities.
• The Romans used lime extensively in their building program in Britain, and refined its application into mortars and plasters, which remained the principal surface finish for buildings until the nineteenth century, when cements took over this function. For this reason, many historic buildings in the UK contain large amounts of lime within their fabric
• Also used for pointing and plastering.

Lime vs Cement

Lime and types of lime

Lime : 

• The word “lime” refers to products derived from heating limestone.
• It originates with its earliest use as building mortar and has the sense of “sticking or adhering“
• The rocks and minerals from which these materials are derived, typically limestone or chalk, are composed primarily of calcium carbonate (CaCO3).

Types of Lime:

• Quick Lime (CaO)
• Fat Lime
• Hydraulic Lime 
• Hydrated Lime
• Lump Lime
• Milk Lime

Quick Lime (CaO) :

Pure lime, generally called quick lime, is a white oxide of calcium. Much of commercial quick lime, however, contains more or less magnesium oxide, which gives the product a brownish or grayish tinge. Quick lime is obtained after the calcination of limestone. It is also called caustic lime. It is capable of slaking with water and has no affinity for carbonic acid. The specific gravity of pure lime is about 3.40.
(Calcination  is used to mean a thermal treatment process in the absence or limited supply of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition. A calciner is a steel cylinder that rotates inside a heated furnace and performs indirect high-temperature processing (550-1150 °C, or 1000-2100 °F) within a controlled atmosphere.)

Fat Lime

has high calcium oxide component and, sets and hardens by the absorption of CO2 from atmosphere. These are manufactured by burning marble, white chalk, calcareous tufa, pure limestone, sea shell and coral.

Hydraulic Lime 

contains small quantities of silica, alumina, iron oxide in chemical combination with calcium oxide component. These are produced from carboniferous limestones and magnesian limestone. It has the property to set and harden under water.

Hydrated Lime

When quick lime is finely crushed, slaked with a minimum amount of water, and screened or ground to form a fine homogeneous powder the product is called hydrated lime.

Lump Lime

    is the quick-lime coming out of the kilns.

Milk Lime

    is a thin pourable solution of slaked lime in water.

Materials for 12 mm or half inch thick plastering in wall for 100 square meter

Materials for 12 mm or half inch thick plastering in wall for 100 square meter

• First of all, we have to multiply 100 square meter surface with 12 mm.
• 100 square meter x 0.012 m = 1.2 cubic meter. (12 mm can also be written as 0.012)
• 1.2 cubic meter is wet mixed mortar for uniform thickness.
• Add 30% in this value to fill up joints, uneven surfaces, etc., the quantity of mortar comes out 1.2 + 0.36 = 1.56 cubic meter.
• Increasing by 25% the total dry volume will be 1.2+0.36+0.39 = 1.95 cubic meter or 2 cubic meter.
• For cement sand mortar, cement = dry volume / ( sum of ratios) x numeral of cement.
• For 1:4 cement sand mortar, cement will be 2/5 x 1 = 0.4 cubic meter.
• For 1:4 cement sand mortar, sand will be 2/5 x 4 = 1.6 cubic meter.
• In this way you can calculate the dry volume of any ratio of mortar ingredients.

process involved in the concreting

following Process are involved in concreting.

1. Batching
2. Mixing
3. Transporting and placing of concrete
4. Compacting

1- Batching: 

The process of measurement of the different materials for the making of concrete is known as batching. batching is usually done in two ways: volume batching and weight batching. In case of volume batching the measurement is done in the form of volume whereas in the case of weight batching it is done by the weight.

2-Mixing: 

In order to create good concrete the mixing of the materials should be first done in dry condition and after it wet condition. The two general methods of mixing are: hand mixing and machine mixing.

3-Transportation and placing of concrete: 

Once the concrete mixture is created it must be transported to its final location. The concrete is placed on form works and should always be dropped on its final location as closely as possible.

4-Compaction of concrete: 

When concrete is placed it can have air bubbles entrapped in it which can lead to the reduction of the strength by 30%. In order to reduce the air bubbles the process of compaction is performed. Compaction is generally performed in two ways: by hand or by the use of vibrators.

Concrete Mixing Grades

GRADE = CEMENT : SAND : AGGREGATE
M5 = 1 :5 :10
M7.5 = 1 :4 :8
M10 =1 :3 :6
M15 =1 :2 :4
M20 =1 :1.5 :3
M25 =1 :1 :2

Describe briefly the various methods of concrete curing.

Describe briefly the various methods of concrete curing.

 

Curing is the process of maintaining the moisture and temperature conditions for freshly deployed concrete. This is done for small duration of time to allow the hardening of concrete. The methods that are involved in saving the shrinkage of the concrete includes:
(a) Spraying of water: on walls, and columns can be cured by sprinkling water.
(b) Wet covering of surface: can be cured by using the surface with wet gunny bags or straw
(c) Ponding: the horizontal surfaces including the slab and floors can be cured by stagnating the water.
(d) Steam curing: of pre-fabricated concrete units steam can be cured by passing it over the units that are under closed chambers. It allows faster curing process and results in faster recovery.
(e) Application of curing compounds: compounds having calcium chloride can be applied on curing surface. This keeps the surface wet for a very long time

How Portland cement is made??

How Portland cement is made??

Bricklayer Joseph Asp din of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove.
Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Lime and silica make up about 85% of the mass. Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.

Describe briefly the various methods of concrete curing.

Curing is the process of maintaining the moisture and temperature conditions for freshly deployed concrete. This is done for small duration of time to allow the hardening of concrete. The methods that are involved in saving the shrinkage of the concrete includes:
(a) Spraying of water: on walls, and columns can be cured by sprinkling water.
(b) Wet covering of surface: can be cured by using the surface with wet gunny bags or straw
(c) Ponding: the horizontal surfaces including the slab and floors can be cured by stagnating the water.
(d) Steam curing: of pre-fabricated concrete units steam can be cured by passing it over the units that are under closed chambers. It allows faster curing process and results in faster recovery.
(e) Application of curing compounds: compounds having calcium chloride can be applied on curing surface. This keeps the surface wet for a very long time.

Setting of Concrete and Work ability of Concrete

Setting of Concrete

The hardening of concrete before its hydration is known as setting of concrete. OR

The hardening of concrete before it gains strength. OR

The transition process of changing of concrete from plastic state to hardened state. Setting of concrete is based or related to the setting of cement paste. Thus cement properties greatly affect the setting time. 

Factors affecting setting:

Following are the factors that affect the setting of concrete.

1. Water Cement ratio
2. Suitable Temperature
3. Cement content
4. Type of Cement
5. Fineness of Cement
6. Relative Humidity
7. Admixtures
8. Type and amount of Aggregate 

Work ability of Concrete

  Work abilityis often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation.

OR

The internal work done required to overcome the frictional forces between concrete ingredients for full compaction. It is obvious that no single test can evaluate all these factors. In fact, most of these cannot be easily assessed even though some standard tests have been established to evaluate them under specific conditions.

In the case of concrete, consistence is sometimes taken to mean the degree of wetness; within limits, wet concretes are more workable than dry concrete, but concrete of same consistence may vary in workability.

Because the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. This requires sufficient workability for virtually full compaction to be possible using a reasonable amount of work under the given conditions. Presence of voids in concrete reduces the density and greatly reduces the strength: 5% of voids can lower the strength by as much as 30%.

Slump Test can be used to find out the workability of concrete. View Procedure of Slump Test

Factors affecting concrete workability:

1. Water-Cement ratio
2. Amount and type of Aggregate
3. Amount and type of Cement
4. Weather conditions
   1. Temperature
   2. Wind
5. Chemical Admixtures
6. Sand to Aggregate ratio

i. Water content or Water Cement Ratio

More the water cement ratio more will be workability of concrete. Since by simply adding water the inter particle lubrication is increased. High water content results in a higher fluidity and greater workability but reduces the strength of concrete. Because with increasing w/c ratio the strength decreases as more water will result in higher concrete porosity. So, the lower the w/c, the lower is the void volume/solid volume, and the stronger the hardened cement paste. Also See: Rate of Strength Gain of Concrete

Increased water content also results in bleeding, hence, increased water content can also mean that cement slurry will escape through the joints of the formwork (Shuttering).

ii. Amount and type of Aggregate

Since larger Aggregate sizes have relatively smaller surface areas (for the cement paste to coat) and since less water means less cement, it is often said that one should use the largest practicable Aggregate size and the stiffest practical mix. Most building elements are constructed with a maximum Aggregate size of 3/4" to 1", larger sizes beingprohibited by the closeness of the reinforcing bars. Also See: Effects of Different Aggregates on Properties of Concrete

Because concrete is continuously shrinking for years after it is initially placed, it is generally accepted that under thermal loading it will never expand to it's originally-placed volume. More the amount of aggregate less will be workability.

• Using smooth and round aggregate increases the workability. Workability reduces if angular and rough aggregate is used.
  
• Greater size of Aggregate- less water is required to lubricate it, the extra water is available for work ability
  
• Angular aggregates increases flakiness or elongation thus reduces work ability. Round smooth aggregates require less water and less lubrication and greater work ability in a given w/c ratio
  
• Porous aggregates require more water compared to non absorbent aggregates for achieving sam degree of work ability.

iii. Aggregate Cement ratio

More ratio, less work ability. Since less cement mean less water, so the paste is stiff.

iv. Weather Conditions

1. Temperature

If temperature is high, evaporation increases, thus work ability decreases.

2. Wind:

If wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of water and ultimately reducing work ability.

Grade Of Concrete

Concrete Grade
M5	1:4:8
M10	1:3:6
M15	1:2:4
M20	1:1.5:3
M25	1:1:2

Rcc concrete

RCC (Reinforced Cement Concrete)

RCC (Reinforced Cement Concrete) is the combination of using steel and concrete instead of using only concrete to offset some limitations. Concrete is weak in tensile stress with compared to its compressive stress. To offset this limitation, steel reinforcement is used in the concrete at the place where the section is subjected to tensile stress. Steel is very strong in tensile stress. The reinforcement is usually round in shape with approximate surface deformation is placed in the form in advance of the concrete. When the reinforcement is surrounded by the hardened concrete mass, it form an integral part of the member. The resultant combination of two materials are known as reinforced concrete. In this case the cross-sectional area of the beam or other flexural member is greatly reduced. You can also read the article onreinforced concrete beam behavior.

TYPES OF PILES BASED ON CONSTRUCTION METHOD

There are three types of pile foundations according to their construction methods:

1. Driven piles,

2. Cast-in-situ piles, and

3. Driven and cast-in-situ piles.

Driven Pile Foundations:

Driven pile foundations can be made from concrete, steel or timber. These piles are prefabricated before placing at the construction site. When driven piles are made of concrete, they are precast. These piles are driven using a pile hammer.

When these piles are driven into the granular soils, they displace the equal volume of soil. This helps in compaction of soil around the sides of piles and results in the densification of soil. The piles which compact the soil adjacent to it is also called as compaction pile. This compaction of soil increases its bearing capacity.

Saturated silty soils and cohesive soils have poor drainage capability. Thus these soils are not compacted when driven piles are drilled through it. The water have to be drained for the soil to be compacted. Thus stresses are developed adjacent to the piles have to be borne by pore water only. This results in increase in pore water pressure and decrease in bearing capacity of the soil.

Cast-in-situ Pile Foundations:

Cast-in-situ piles are concrete pile. These piles are constructed by drilling holes in the ground to the required depth and then filling the hole with concrete. Reinforcements are also used in the concrete as per the requirements. These piles are of small diameter compared to drilled piers.

Cast-in-situ piles are straight bored piles or with one or more bulbs at intervals are casted. The piles with one or more bulbs are called as under-reamed piles.

Driven and Cast-in-situ Piles

Driven and cast-in-situ piles have the advantages of both driven and cast-in-situ piles. The procedure of installing a driven and cast-in-situ pile is as follows:

A steel shell of diameter of pile is driven into the ground with the aid of a mandrel inserted into the shell. After driving the shell, the mandrel is removed and concrete is poured in the shell.

The shell is made of corrugated and reinforced thin sheet steel (mono-tube piles) or pipes (Armco welded pipes or common seamless pipes). The piles of this type are called a shell type piles.

The shell-less type is formed by withdrawing the shell while the concrete is being placed. In both the types of piles the bottom of the shell is closed with a conical tip which can be separated from the shell. By driving the concrete out of the shell an enlarged bulb may be formed in both the types of piles. Franki piles are of this type. In some cases the shell will be left in place and the tube is concreted. This type of pile is very much used in piling over water.