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Solution of the sulfates of various bases including sodium, potassium, magnesium and calcium react with hydrated cement paste forming gypsum or a compound called ettringite (sulphoaluminate) which leads to the expansion and disruption of the concrete and mortar this process is referred as sulphate attack.




Sulphate attack is a chemical break down mechanism where suphate ion attacks on components of cement paste. Sodium Sulphate attacks calcium hydroxide and forms gypsum that becomes more than doubles the volume. Sulphate attack might show itself in different forms depending upon the chemical form of the sulfate and the atmospheric environment which the concrete is exposed to. 

Groundwater or soil contains soluble sulfates naturally or sometimes it comes from the industrial effluents or fertilizers which sometimes also contains amounium sulfate which attacks hydrated cement paste by producing gypsum. 

Calcium sulfate attacks only calcium aluminate hydrate, forming calcium sulfoaluminate which is known as ettringite. Magnesium sulfate attacks calcium silicate hydrate as well as calcium hydroxide and calcium aluminate hydrate. 

The consequences of sulfate attack include not only disruptive expansion and cracking, but also loss of strength of concrete due to the loss of cohesion in the hydrated cement paste and of adhesion between it and the aggregate particles. 

Sulfates combines with the C-S-H, or concrete paste, and begins destroying the paste that holds the concrete together. As sulphate dries, new compounds are formed, often called ettringite.

These new crystals occupy empty space, and as they continue to form, they cause the paste to crack further damaging the concrete.

Physically sulfate attack, often evidenced by bloom (the presence of sodium sulphates Na2SO4 and/or Na2SO4.10H2O) at exposed concrete surfaces.

Concrete attacked by sulfates has a characteristic whitish appearance. The damage usually starts at edges and corners and is followed by progressive cracking and spalling which reduce the concrete to a firable or even soft state.

Sources of Sulfates 

Internal sources


It is more rare but, originates from such concrete-making materials as hydraulic cements, fly ash, aggregate, and admixtures.
portland cement might be over-sulphated.
presence of natural gypsum in the aggregate.
Admixtures also can contain small amounts of sulphates.

External Sources


External sources of sulphate are more common and usually are a result of high-sulphate soils and ground waters, or can be the result of atmospheric or industrial water pollution.
Soil may contain excessive amounts of gypsum or other sulfate.
Ground water be transported to the concrete foundations, retaining walls, and other underground structures.
Industrial waste waters.

Main factors affecting sulphate attack


1: Cement type and content:
2:Fly  ash addition
3: sulphate type and concentration
4: Chloride ions

1.Cement type and content:


The most important mineralogical phases of cement that affect the intensity of sulphate attack are: C3A, C3S/C2S ratio and C4AF.

2. Fly ash addition


The addition of a pozzolanic admixture such as fly ash reduces the C3A content of cement.

3. Sulphate concentration:


The sulphate attack tends to increase with an increase in the concentration of the sulphate solution up to a certain level.

4. Chloride ions



 It is the physical resistance to penetrate into the concrete of all liquids, by reducing the water/binder ratio, and adequately consolidating and curing the concrete

Control of sulphate attack


The quality of concrete, specifically a low permeability, is the best protection against sulphate attack.
Adequate concrete thickness
High cement content
Low w/c ratio
Proper compaction and curing

Quality of concrete depends on many factors among which the most vulnerable and defenseless is workmanship. During pouring the concrete and execution of work cautious supervision is imperative. The supervisor has to look for many things including the finishing of concrete, pouring quantity, at-site testing, unremitting and continual supply of material from the batching plant. Beside all those points a very important one is vibration of concrete by any suitable means or as specified by the contract documents of the project. 



In the past the placing of concrete is always followed by some densification by means of ramming or punning but this old technique is superseded. The purpose of that ramming was to compact concrete to achieve maximum possible density of the concrete. This process is usually termed as consolidation of concrete. 

Nowadays the concrete is consolidated by means of vibration. The freshly mixed concrete has always air bubbles which usually occupy 5 to 20 percent of the total volume of the concrete. These air bubbles are more in low-slump concrete and less in loose slump or high workable concrete. During pouring when vibrator (a device used to produce vibration effect for consolidation of concrete) is poked into the concrete the internal friction between coarse aggregates reduces to a suitable amount which results in fludifying the mortar and thus packing of the same is done. The purpose of vibration is to achieve a maximum possible close configuration of the coarse aggregate with respect to their shape. 

As we all know that excess of everything is bad same is the case with vibration. The term over-vibration is used to define the amount of vibration that will result segregation in the concrete mass which is very destructive. Segregation is the separation of the fine and coarse materials like cement and sand get separated and the aggregates loose the coating of fines around them. 
Therefore it is always advised to uniformly apply vibration within the concrete mass so that it is consolidated equally throughout. However, with a sufficiently stiff and well-graded mix, the ill effects of over-vibration can be largely eliminated. 

Different vibrators require different consistency of concrete for most efficient compaction so that the consistency of the concrete and the characteristics of the available vibrator have to be matched. It is worth noting the flowing concrete although it may be self-levelling, does not achieve full compaction by gravity alone. However, the necessary duration of application of vibration can be reduced by about one-half compared with ordinary concrete. 

Types and classification of Vibrators 


Internal Vibrators

The most widely used and efficient among the vibrators is the internal vibrator. It consists of a poker which is connected through a flexible drive with motor. The poker is immersed into the concrete which through harmonic forces induces the vibration within the concrete mass. 
The frequency of vibration varies up to 12,000 cycles of vibration per minute minimum of which is 3500 to 5000 cycles per minutes but now a days 4000 to 7000 is favorable. 
The duration of immersion as well as the radius of action of immersion depends on the consistency of the mix but in most of the cases the poker needs to be immersed after every 0.5 to 1 m for duration of 5 to 30 seconds. 

Completion of compaction through vibration is judged by the appearance of the surface of the concrete. It should be as so that it does not have honeycombed surface and also should not be having excess of mortar. The poker should be with-drawl slowly at rate of about 80 mm / sec so that the gap or space being taken by the poker gets level without any entrapped air within the concrete volume. 

External vibrators


External vibrators are usually used for precast or in situ sections of such shape or thickness that an internal vibrator cannot be conveniently used. 
During usage of external vibrators it is ensured that the thickness of layer of concrete is not large so as to avoid the air to get entrapped. The position of the vibrator may need to change in such cases during pouring of concrete. 

External vibrators are rigidly clamped to the formwork resting on elastic support, so that both the form and the concrete are vibrated. As a result, a considerable proportion of the work done is used in vibrating the formwork. 

Vibrating Tables 


In such cases formwork is clamped to the vibrator unlike external vibrators where vibrator is fixed to the formwork. Usually a rapidly rotating eccentric mass is fixed to the bottom of the table that makes it to vibrate in a circular motion. With two shafts rotating in opposite directions, the horizontal component of vibration can be neutralized so that the table is subjected to a simple harmonic motion in the vertical direction only. 

A vibrating table provides a reliable means of compaction of precise concrete and has the advantage of offering uniform treatment. 

Surface vibrators


A surface vibrator applies vibration through a flat plate direct to the top surface of the concrete. In this manner, the concrete is restrained in all directions so that the tendency to segregate is limited; for this reason, a more intense vibration can be used. 

An electric hammer can be used as a surface vibrator when fitted with a bit having a large flat area, say 100 mm by 100 mm (4 in. by 4 in.)

One of the main applications is in compacting test cubes. A vibrating roller is used for consolidating thin slabs. For road construction various vibrating screeds and finishers are available.  

Materials added to concrete during or before mixing are referred to as admixtures. They are used to improve the performance of concrete in certain situations as well as to lower its cost. There is a rather well-known saying regarding admixtures, to the effect that 
Different Types of Admixtures

“they are to concrete as beauty aids are to the populace.”

An admixture can be defined as :- 

A chemical product which, except in special cases, is added to the concrete mix in quantities no larger than 5 percent by mass of cement during mixing or during an additional mixing operation prior to the placing of concrete, for the purpose of achieving a specific modification, or modifications, to the normal properties of concrete. 



Admixtures, unlike cement, aggregate and water, are not an essential component of the concrete mix, they are an important and increasingly widespread component; in many countries a mix which contains no admixtures is nowadays an exception. 

Benefits / Advantages of Admixtures


They are capable of imparting considerable physical and economic benefits with respect to concrete. These benefits include use of concrete under circumstances where previously there were considerable difficulties. They also make possible the use of a wide range of ingredients in the mix. 
Admixtures although are not always cheap but they do not necessarily represent additional expenditure because their use can result in economical savings. 

Classification and Types of Admixtures


Admixtures are classified according to ASTM C 494-10 as follows :- 


  • Type A – Water reducing 
  • Type B Retarding
  • Type C – Accelerating
  • Type D – Water-reducing and retarding 
  • Type E – Water reducing and accelerating
  • Type F – High range water-reducing or superplasticizing 
  • Type G – High range water reducing and retarding or superplasticizing and retarding 




Air-Entertaining Admixtures


Conforming to the requirements of ASTM C260 and C618, are used primarily to increase concrete’s resistance to freezing and thawing and provide better resistance to the deteriorating action of de-icing salts. The air-entraining agents cause the mixing water to foam, with the result that billion of closely spaced air bubbles are incorporated into the concrete. When concrete freezes, water moves into air bubbles, relieving the pressure in the concrete. When the concrete thaws, the water can move out of the bubbles, with the result that there is less cracking than if air-entrainment had not been used. 

Accelerating Admixtures


The addition of accelerating admixtures such as calcium chloride to concrete will accelerate its early-strength development. The results of such additions (particularly useful in cold climates) are reduced times required for curing and protection of the concrete and earlier removal of forms. Other accelerating admixtures that may be used include various soluble salts as well as some other organic compounds. 

Retarding admixtures


Are used to slow the setting of the concrete and to retard temperature increases. They consists of various acids or sugars or sugar derivatives. Some concrete truck drivers keep sacks of sugar on hand to throw into the concrete in case they get caught in traffic jams or otherwise delayed. Retarding admixtures are particularly useful for large pours where significant temperature increases may occur. they also prolong the plasticity of the concrete, enabling better blending or bonding together of successive pours. 

Superplasticizers 


They are admixtures made from organic sulfonates. Their use enables engineers to reduce the water content in concretes substantially while at the same time increasing their slumps. Although superplasticizers can also be used to keep constant water-cement ratios while using less cement, they are more commonly use to produce workable concretes with considerably higher strengths while using the same amount of cement. 

Water proofing materials 


Are applied to hardened concrete surfaces, but they may be added to concrete mixes. These admixtures generally consist of some type of soap or petroleum products, as perhaps asphalt emulsions. They may help retard the penetration of water into porous concretes but probably don’t help dense, well-cured concrete very much. 


Structural lightweight aggregate concrete is an important and versatile material in modern construction. I has many and varied applications: multistory building frames and floors, curtain walls, shell roofs, folded plates, bridges, prestressed or precast elements of all types, and others. In many cases the architectural expression of form combined with functional design can be achieved more readily in structural lightweight concrete than in any other medium.

Structural light weight concrete is defined as :-

Concrete made with lightweight aggregate; the aid-dried unit weight at 28 days is usually in the range of 90 to 115 lb/ft3 (1440 to 1850 kg/m3) and compressive strength is more than 2500 psi. 


During the 1950s many multistory structures were designed from the foundations up, taking advantage of reduced dead weight using lightweight concrete. Examples are the 42 story Prudential Life Building in Chicago, which incorporated lightweight concrete floors, and the 18 story Statler Hilton Hotel in Dallas, which was designed with a light-weight concrete frame and flat plate floors.

Economy


Use of lightweight aggregate concrete in a structure is usually predicted on lower overall costs. While lightweight concrete may cost more per cubic yard than normal weight concrete, the structure may cost less as a result of reduced dead weight and lower foundation costs.

This is the basic reason, in most cases, for using structural lightweight concrete. Economy then depends on attaining a proper balance among cost of concrete per volume, unit weight and structural properties.

In all cases the light weight aggregates used in structural concrete are light in weight due to the cellular structure of individual aggregate particles. This cellular structure within the particles is formed at high temperatures, generally 2000 F or higher

Raw materials used in commercial production of structural lightweight aggregates are generally
a. Suitable natural deposits of shales, clays or slates
b. By products of other industries such as iron blast furnace slags.

Reparation of raw materials can range from negligible to extensive prior to treatment to produce expansion. In many cases crushing to suitable sizes is the only prerequisite. In the cases of finely divided materials such as silty and laminar clays, and fly ash, the raw material may need to be agglomerated with water, or possibly require addition of supplementary binder, fuel, gas-forming or fluxing agents, prior to heating.

Production 


Several different methods are used to produce structural lightweight aggregates, and the aggregates produced may vary widely in their characteristics. Process such as rotary kiln process is widely used throughout the world for the production of light weight aggregates.

Basically, the rotary kiln is a long, nearly horizontal cylinder lined with refractory materials. Raw material is introduced in a continuous stream at the upper end, and due to slow rotation and slope of the kiln, it progress to the lower or burner end. The heat causes simultaneous formation of gases and onset of a pyroplastic condition in the material. The viscosity of the softened mass is sufficient to entrap the gases and to form an internal cellular surface. This structure is retained on cooling as a vitrified hard material.

Sometimes the bloated material is discharged, cooled, and then crushed and screened to required aggregate gradation. The resultant particles tend to be cubical or angular in the shop and to have a varying percentage or particles with a smooth shell.

Properties of light weight aggregate


Particle Shape and Surface Texture : shape may be cubical, essentially rounded or angular and irregular. Surface texture may vary from smooth to small exposed pores

Bulk specific gravity – lower than normal weight aggregate – generally around 1/3 to 2/3 of that of normal weight aggregate

Unit weight – is significantly lower, due to cellular structure.

Strength – may be strong and hard and may be weak and friable – no correlation between aggregate strength and concrete strength

Moisture content and absorption – light weight aggregate tend to absorb more water due to cellular structure – light weight aggregate generally absorb from 5 to 20 percent by weight of dry aggregate.


Properties of light weight concrete


Compressive Strength – Normally vary between 3000 to 4000 psi and less frequently between 6000 psi and above

Unit weight – depends on unit weight of light weight aggregate and also proportion of the material while mixing – properties are mostly depending on unit weight thus very important

Slump – should be the lowest value consistent with the ability to satisfactorily place, consolidate and finish.

Entrained Air content – more air entrained and thus more durable against freeze and thaw etc.

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Mostly concrete structures are failed due to poor workmanship which is a very tricky factor to control. Structures build with concrete has a life both from serviceability point of view and from structural point of view and this life depends solely on the quality of the material while placing.
The quality of concrete is tested by using following experiments and tests :-



1. Workability or consistency 


Possible tests for workability or consistency include :-

a. Slump test by means of the standard ASTM code. The slump in inches recorded in the mixture indicates its workability
b. Remolding tests using Powers’ flow table.
c. Kelly’s ball apparatus

2. Air Content


 Measurement of the air content in fresh concrete is always necessary, especially when air entraining agents are used.

3. Compressive Strength of Concrete


This is done by loading cylinders 6 in. in diameter and 12 in. high in compression perpendicular to the axis of the cylinder. For high-strength concrete, cylinders 4 in. dia. X 8 in. – height can be used applying proper dimensional correction.

4. Flexural strength of Plain Concrete Beams :-


This test is performed by three-point  loading of plain concrete beams of size 6 in. x 6 in. x 18 in. that have spans three times their depth.

5. Tensile Splitting Tests

These tests are performed by loading the standard cylinder by a line load perpendicular to its longitudinal axis with the cylinder placed horizontally on the testing machine pattern.

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Forces that are acting perpendicular to the longitudinal axis of the beam cause bending stresses which are termed as flexural stresses, beside flexural stresses beams also undergo shear stresses and normal stresses.




We know from the basic concepts of internal forces in the beams that whenever some load either dead load, live load, imposed load, superimposed load or whatever is applied, it produces some internal forces and resultant stresses within the fibers of the beam. These internal forces within the fiber includes, Bending Moment M, Shear Forces V and normal forces P.

Bending Moment as the name suggests is a bending force that is caused as a result of the moment of the force given by the magnitude of the force multiplied by the distance to the point of consideration along the length of the beam. Bending moment varies throughout the length of the beam and is thus given by a diagram called Bending Moment Diagram.

Therefore it is concluded that the Stresses that are caused as a result of bending is called flexural stresses.

In order to calculate flexural stresses there is a very well-known formula called flexural formula. Flexural formula is derived while considering some assumptions which are as follows :-

        1.    plane section of the beam normal to its longitudinal axis prior to loading remains plane after the forces and couples have been applied,
2.       the beam is initially straight and of uniform cross section
3.       the moduli of elasticity in tension and compression are equal.

α = My/I

Where α = Flexural Stresses
M is the bending moment at any point along the longitudinal axis of the beam
Y is the internal fiber distance from neutral axis
I is the moment of Inertia of the Beam

.

Concrete is one of the widest used construction material whether it is a dam or a house or a bridge or flyover. Concrete is basically of three components water, aggregate and cement. Aggregate acts as a filling agent while cement mostly Portland cement act as a binding agent.



Concrete is basically the most popular artificial material on earth. It has many advantages beside easy availability of the constituents it is powerful in compression however weak in tension which can be compensated by using reinforcing bars of steel as in Reinforced Cement concrete.

Concrete is easily and readily prepared and fabricated in all sorts with advantage being turned into any shape desired at site. Concrete is leveled by screeding and smoothed out with trowel or a float. Normally it would be poured into a wood formwork and then finished.

There are different types of concrete out there depending upon type of constituents used in there 
keeping in view of the demand at site.

Some common types are as follows :-

1.       Normal concrete
2.       High performance concrete
3.       High strength concrete
4.       Air entrained Concrete
5.       Light weight concrete
6.       Self compacting Concrete
7.       Shortcrete
8.       Pervious concrete
9.       Roller compacted concrete


Normal Concrete


Normally concrete also known as normal weight concrete is relatively strong in compression and weak in tension. It has low coefficient of thermal expansion and it shrinks while setting. Density of concrete is around 2240 - 2400 Kg/meter cube. (150 lb/cu.ft.). Compressive strength is usually around 3000 to 6000 psi.  Normal concrete has 1-2 % air content but is not durable for severe conditions like freezing and thawing. A freshly poured concrete usually sets within 30 – 90 minutes depending upon local temperature and moisture conditions. Just after coming in contact with water the normal hydration reaction starts and concrete sets. It mostly develops its strength after 7 days and usually attain more than 80% strength after 28 days.

High Strength Concrete


Whenever we are talking about strength of non-reinforced concrete we are talking about compression strength or compressive strength denoted by fc’. As the name suggests this type of concrete has extraordinary strength than normal strength or normal concrete. By one definition HSC is mostly the one having cube strength between 60 to 100 N/mm2, although higher strengths can also be achieved. Mix design of high strength concrete is influenced by properties of cement, sand aggregates and water cement ratio. The strength according to one standard is said to be greater than 40 MPa. Mostly high strength is achieved by lowering water cement ratio upto 0.35 or even lower. Often Silica fume is also added to prevent the formation of free calcium hydroxide crystal in the cement matrix. It is usually less workable and thus if needed super-plasticizers can be used.

High Performance concrete


As the name suggests high performance concrete refers to all those sorts of concrete which are optimum in all the standards usually adopted in common application of concrete, thus not only strength it has ease of placement, optimum heat of hydration, compaction without segregation, have early age strength, permeability is sufficient, density is more, life is more even in severe environments, toughness is more.

High workability is attained by super plasticizers, they lower the water cement ratio to 0.25 which is the amount required only for hydration process.

High durability is attributed to fly ash and silica fume which modify the e mineralogy of the cement; it enhances the compatibility of ingredients in concrete mass and reduces the CH amount. Fly ash also causes ball bearing effect increasing workability.

The admixtures are 20-25% fly ash of partial replacement of cement and rest 70% is Ordinary Portland Cement.

As it is not usually durable against freezing and thawing so air entrained agents can also be utilized.
Properties of high performance concrete mix

Strength of high performance concrete ranges from 10000 psi - 15000 psi

Water cement ratio can be reduced to 0.25

Air entrained Concrete

In this type of concrete tiny air bubbles are intentionally created. Mostly an air entraining agent called surfactant (a type of chemical) is used. Air bubbles are created during mixing of the plastic concrete which mostly survive upto end as hardened concrete. The basic purpose of air entrainment is to make it durable for concrete in extreme climate where freeze-thaw effect is there while second purpose is to increase workability of concrete while in plastic state.

Normal concrete will have small capillaries produced by the evaporation of water in concrete. These capillaries are invaded by water from the environment and the freezing of this water can cause a lot of stress in concrete because of expansion in volume that accompanies the freezing. An Air entrained concrete allows the expansion to take place without causing any further stresses as air bubbles are capable of being compressed.

The amount of entrained air is maintained between 4 and 7 percent for best freezing thawing resistance though variations can be made depending on specific conditions.
Normally air entrained concrete has a drawback it has low strength as compared to normal concrete.

Light weight concrete


Structural light weight concrete has an in-place density or unit weight on the order of 90 to 115 lb/ft3 or 1440 to 1840 kg/m3. Lightweight concrete is defined as having an air dry density not exceeding 2000 Kg/m3, but can be as low as 400 kg/m3.
One of the most common uses for light weight concrete is with floor, roof or bridge decks; other include pavement system, masonry blocks etc.
Lightweight aggregates used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other products such as air-cooled blast furnace slag are also used. There are other classes of non-structural LWC with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete.

The required properties of the lightweight concrete will have a bearing on the best type of lightweight aggregate to use. If little structural requirement, but high thermal insulation properties, are needed then a light, weak aggregate can be used. This will result in relatively low strength concrete.

Self Compacting Concrete


Self compacting concrete or SCC is a flowing concrete mixture that is able to consolidate under its own weight. The high fluid nature of SCC makes it suitable for placing in difficult conditions and in sections with congested reinforcement. Use of SCC can also help minimize the bearing related damages on the formwork that are induced by vibration of concrete. Another advantage of SCC is that the time required to place large sections is considerably reduced.
Self compacting concrete has more finer content and less coarser aggregate. Self compacting concrete also uses superplasticisers in large amount and a viscosity modifying agent (VMA) in small doses.
As a high-performance concrete, SCC delivers these attractive benefits while maintaining all of concrete's customary mechanical and durability characteristics. Adjustments to traditional mix designs and the use of superplasticizers creates flowing concrete that meets tough performance requirements. If needed, low dosages of viscosity modifier can eliminate unwanted bleeding and segregation.

Shortcrete


Shortcrete is basically not a product or something you can pick or touch, it is basically a process of placing concrete to achieve high strengths and low permeability.
Shortcrete is a concrete conveyed through a hose and pneumatically projected at high velocity onto a receiving surface.

They are strong, durable, resistant to disasters, fires, mold, insects and vermin, and have low permeability, good thermal mass, and create tight envelopes. Although the hardened properties of Shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as very fast erection, particularly on complex forms or shapes, including curved walls and arches.

Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork. It is sometimes used for rock support, especially in tunneling. Shotcrete is also used for applications where seepage is an issue to limit the amount of water entering a construction site due to a high water table or other subterranean sources. This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.

Pervious Concrete

Pervious concrete (also called porous concrete, permeable concrete, no fines concrete and porous pavement) is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass directly through, thereby reducing the runoff from a site and allowing groundwater recharge. When set, typically between 15% and 25% of the concrete volumes are voids, allowing water to drain.
The majority of pervious concrete pavements function well with little or no maintenance. Maintenance of pervious concrete pavement consists primarily of prevention of clogging of the void structure.

In preparing the site prior to construction, drainage of surrounding landscaping should be designed to prevent flow of materials onto pavement surfaces. Soil, rock, leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasing the utility of the pervious concrete pavement.  

Roller Compacted Concrete


Mostly abbreviated as R.C.C but it must not be confused with Reinforced Cement Concrete. The word is enough to define itself; roller compacted concrete is a relatively stiff concrete mix that is spread with a paving machine and is than compacted with the help of a roller which is usually steel drum vibratory roller.

Method of construction used for roller compacted concrete is simple and conventional.
Roller-compacted concrete has the same basic ingredient as conventional concrete: cement, water, and aggregates, such as gravel or crushed stone.

But unlike conventional concrete, it's a drier mix—stiff enough to be compacted by vibratory rollers. Typically, RCC is constructed without joints. It needs neither forms nor finishing, nor does it contain dowels or steel reinforcing.

These characteristics make roller-compacted concrete simple, fast, and economical.
These qualities have taken roller-compacted concrete from specialized applications to mainstream pavement. Today, RCC is used for any type of industrial or heavy-duty pavement. The reason is simple. RCC has the strength and performance of conventional concrete with the economy and simplicity of asphalt. Coupled with long service life and minimal maintenance, RCC's low initial cost adds up to economy and value.

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Intersection is an area shared by two or more roads. This area is designated for the vehicles to turn to different directions to reach their desired destinations. Its main function is to guide vehicles to their respective directions. Traffic intersections are complex locations on any highway. This is because vehicles moving in different direction wan to occupy same space at the same time.



In addition, the pedestrians also seek same space for crossing. Drivers have to make split second decision at an intersection by considering his route, intersection geometry, speed and direction of other vehicles etc. A small error in judgment can cause severe accidents. It also causes delay and it depends on type, geometry, and type of control.

Overall traffic flow depends on the performance of the intersections. It also affects the capacity of the road. Therefore, both from the accident perspective and the capacity perspective, the study of intersections very important for the traffic engineers especially in the case of
urban scenario.




Conflicts at an intersection are different for different types of intersection. Consider a typical four-legged intersection as shown in figure. The number of conflicts for competing through movements are 4, while competing  right turn and through movements are 8. The conflicts between right turn traffics are 4, and between left turn and merging traffic is 4. The conflicts created by pedestrians will be 8 taking into account all the four approaches. Diverging traffic also produces about 4 conflicts. Therefore, a typical four legged intersection has about 32 different types of conflicts.



The essence of the intersection control is to resolve these conflicts at the intersection for the safe and efficient movement of both vehicular traffic and pedestrians. Two methods of intersection controls are there: time sharing and space sharing. The type of intersection control that has to be adopted depends on the traffic volume, road geometry, cost involved, importance of the road etc.



Levels of intersection control


The control of an intersection can be exercised at different levels. They can be either passive control, semi control, or active control. In passive control, there is no explicit control on the driver . In semi control, some amount of control on the driver is there from the traffic agency. Active control means the movement of the traffic is fully controlled by the traffic agency and the drivers cannot simply maneuver the intersection according to his choice.


Passive control


When the volume of traffic is less, no explicit control is required. Here the road users are required to obey the basic rules of the road. Passive control like traffic signs, road markings etc. are used to complement the intersection control. Some of the intersection control that are classified under passive control are as follows:


1. No control If the traffic coming to an intersection is low, then by applying the basic rules of the road like driver on the left side of the road must yield and that through movements will have priority than turning movements. The driver is expected to obey these basic rules of the road.


Traffic signs: With the help of warning signs, guide signs etc. it is able to provide some level of control at an intersection. Give way control, two-way stop control, and all-way stop control are some examples. The GIVE WAY control requires the driver in the minor road to slow down to a minimum speed and allow the vehicle on the major road to proceed. Two way stop control requires the vehicle drivers on the minor streets should see that the conflicts are avoided.

Finally an all-way stop control is usually used when it is difficult to differentiate between the major and minor roads in an intersection. In such a case, STOP sign is placed on all the approaches to the intersection and the driver on all the approaches are required to stop the vehicle. The vehicle at the right side will get priority over the left approach. Thetraffic control at ’at-grade’ intersection may be uncontrolled in cases of low traffic. Here the road users are required to obey the basic rules of the road. Passive control like traffic signs, road markings etc. are used to complement the intersection control.

Traffic signs plus marking: In addition to the traffic signs, road markings also complement the traffic control at intersections. Some of the examples include stop line marking, yield lines, arrow marking etc.


Semi control


In semi control or partial control, the drivers are gently guided to avoid conflicts. Channelization and traffic rotaries are two examples of this.
Channelization:
The traffic is separated to flow through definite paths by raising a portion of the road in the middle usually called as islands distinguished by road markings. The conflicts in traffic movements are reduced to a great extent in such a case. In channelized intersections, as the name suggests, the traffic is directed to flow through different channels and this physical separation is made possible with the help of some barriers in the road like traffic islands, road markings etc.


Traffic rotaries:


It is a form of intersection control in which the traffic is made to flow along one direction around a traffic island. The essential principle of this control is to convert all the severe conflicts like through and right turn conflicts into milder conflicts like merging, weaving and diverging. It is a form of ‘at-grade’ intersection laid out for the movement of traffic such that no through conflicts are there. Free-left turn is permitted where as through traffic and right-turn traffic is forced to move around the central island in a clock-wise direction in an orderly manner. Merging, weaving and diverging operations reduces the conflicting movements at the rotary.

Active control


Active control implies that the road user will be forced to follow the path suggested by the traffic control agencies. He cannot maneuver according to his wish. Traffic signals and grade separated intersections come under this classification

Traffic signals: Control using traffic signal is based on time sharing approach. At a given time, with the help of appropriate signals, certain traffic movements are restricted where as certain other movements are permitted to pass through the intersection. Two or more phases may be provided depending upon the traffic conditions of the intersection. When the vehicles traversing the intersection is very large, then the control is done with the help of signals. The phases provided for the signal may be two or more. If more than two phases are provided, then it is called multiphase signal.

The signals can operate in several modes. Most common are fixed time signals and vehicle actuated signals. In fixed time signals, the cycle time, phases and interval of each signal is fixed. Each cycle of the signal will be exactly like another. But they cannot cater to the needs of the fluctuating traffic. On the other hand, vehicle actuated signals can respond to dynamic traffic situations. Vehicle detectors will be placed on the streets approaching the intersection and the detector will sense the presence of the vehicleand pass the information to a controller. The controller then sets the cycle time and adjusts the phase lengths according to the prevailing traffic conditions.


Grade separated intersections: 


The intersections are of two types. They are at-grade intersections and grade-separated intersections. In at-grade intersections, all roadways join or cross at the same vertical level. Grade separated intersections allows the traffic to cross at different vertical levels. Sometimes the topography itself may be helpful in constructing such intersections. Otherwise, the initial construction cost required will be very high. Therefore, they are usually constructed on high speed facilities like expressways, freeways etc. These type of intersection increases the road capacity because vehicles can flow with high speed and accident potential is also reduced due to vertical separation of traffic.


Grade separated intersections

As we discussed earlier, grade-separated intersections are provided to separate the traffic in the vertical grade. But the traffic need not be those pertaining to road only. When a railway line crosses a road, then also grade separators are used. Different types of grade-separators are flyovers and interchange. Flyovers itself are subdivided into overpass and underpass. When two roads cross at a point, if the road having major traffic is elevated to a higher grade for further movement of traffic, then such structures are called overpass. Otherwise, if the major road is depressed to a lower level to cross another by means of an under bridge or tunnel, it is called under-pass.

Interchange is a system where traffic between two or more roadways flows at different levels in the grade separated junctions. Common types of interchange include trumpet interchange, diamond interchange , and cloverleaf interchange.


1. Trumpet interchange: 

Trumpet interchange is a popular form of three leg interchange. If one of the legs of the interchange meets a highway at some angle but does not cross it, then the interchange is called trumpet interchange.



2. Diamond interchange:

 Diamond interchange is a popular form of four-leg interchange found in the urban locations where major and minor roads crosses. The important feature of this interchange is that it can be designed even if the major road is relatively narrow. 



3. Clover leaf interchange: 

It is also a four leg interchange and is used when two highways of high volume and speed intersect each other with considerable turning movements. The main advantage of cloverleaf intersection is that it provides complete separation of traffic. In addition, high speed at intersections can be achieved. However, the disadvantage is that large area of land is required. Therefore, cloverleaf interchanges are provided mainly in rural areas. 



Channelized intersection



Vehicles approaching an intersection are directed to definite paths by islands, marking etc. and this method of control is called channelization. Channelized intersection provides more safety and efficiency. It reduces the number of possible conflicts by reducing the area of conflicts available in the carriageway. If no channelizing is provided the driver will have less tendency to reduce the speed while entering the intersection from the carriageway. The presence of traffic islands, markings etc. forces the driver to reduce the speed and becomesmore cautious while maneuvering the intersection. A channelizing island also serves as a refuge for pedestrians and makes pedestrian crossing safer.


Mass transportation is any kind of transportation system in which large numbers of people are carried within a single vehicle or combination of vehicles. Airplanes, railways, buses, trolleys, light rail systems, and subways are examples of mass transportation systems. The term mass transit is commonly used as a synonym for mass transportation.



Mass transit system refers to public shared transportation, such as trains, buses, ferries etc that can commute a larger number of passengers from origin to destination on a no-reserved basis and in lesser time. It can also be termed as Public Transport.

Impacts - Advantages of Mass Transit Train:


The following will be the impacts or advantages of Mass Transit Train in Pakistan.

Environmental Impacts:

Mass transit train is believed to be more environmental friendly or eco-friendly than other public transport facilities. Private vehicles emit about twice as much carbon monoxide and other volatile organic compounds than public vehicles. Mass transit reduces the number of cars on the road which in turn reduces the pollution caused by individual cars.

Social Impacts of Mass Transit:

All members of the society irrespective of their financial status, religion or cast are able to travel which enhances the social integrity of the country. The necessity of a driving license is also eliminated. It is a blessing for those individuals who are unable to drive.

Economic Impacts of Mass Transit:          

Mass transit train in Pakistan can improve the both usefulness and efficiency of the public transit system as well as result in increased business for commercial developments and thus serves to improve the economy of the country. Transit systems also have an indirect positive effect on other businesses. Mass transit systems offer considerable savings in labor, materials, and energy over private transit systems.

 Reasonable Capacity:

 Also mass transit train will allows a higher amount of load to be transported to far away destinations in lesser time because of its reasonable capacity than private vehicles. Because of their larger capacity offering them to carry high efficient engines they also help in saving fuels.

Employment opportunities:

The Mass transit train project will provide employment opportunities in Pakistan because the whole labour required for that project will be from Pakistan.

Other Positive Impacts:

Reduces congestion:

The main idea behind mass transit train in Pakistan is to reduce the number of vehicles on the road by providing a larger facility which carries higher number of passengers thus eliminating congestion.

Saves Time:

Mass transit train will reduces the travel time to a great extent as it moves at high speeds and stops only at specific spots.

Cost Effective:

Mass transit is comparably cheaper than other modes of public transport.

Disadvantages of Mass Transit Train:

Pakistan has to face the following consequences in order to establish the whole Mass Transit Train project. The disadvantages of that project are explained as under:

High Fares:

Rs. 140 aside. Will anyone pay such a fare? It’s too much for worker class who will be the main consumers of this service.

Expensive to import the technology:

 Too expensive to import the technology and maintain the service.  Pakistan has to pay the huge amount for the import of the technology as well as has to pay the engineers of china because it have been decided that the engineering staff will be from China.

Large investment:

The Mass Transit Train project requires a large investment of capital. The cost of construction, maintenance and overhead expenses are very high as compared to other modes of transport. Moreover, the investments are specific and immobile. In case the traffic is not sufficient, the investments may mean wastage of huge resources

Noise Pollution:

Another disadvantage of the Mass Transit Train is that it will create noise pollution in Pakistan.

Require Huge Capital:


The Mass Transit Train project require huge capital outlay, they may give rise to monopolies and work against public interest at large. Even if controlled and managed by the government, lack of competition may breed in inefficiency and high costs.

Construction on the palm islands began in 2001. Divers surveyed the seabed and worke­rs constructed a crescent-shaped breakwater from blasted mountain rock. The Crescent of Palm Jumeirah stands a little more than 13 feet above low tide sea level and sits in 34 feet of water at its deepest point.



Palm Islands are two artificial islands, Palm Jumeirah and Palm Jebel Ali, on the coast of Dubai, United Arab Emirates. As of November 2014, only Palm Jumeirah has been completed. This island takes the form of a palm tree, topped by a crescent. After completion, Palm Jebel Ali will take a similar shape; both islands will be host to a large number of residential, leisure and entertainment centers and will add a total of 520 kilometers of non-public beaches to the city of Dubai.



In 2001, there was nothing off the coast of Dubai but warm, shallow gulf water. Then Nakheel, a local real estate conglomerate, dredged 3 billion cubic feet of sand from the seafloor and used GPS precision to shape it a 17-fronded palm tree. Seven million tons of mountain rock was piled around the island to form a crescent-shaped breakwater seven miles long, designed to protect the newborn island from waves and storms.

To facilitate tourism and make life easier for residents, the six-lane Sub-Sea Tunnel connects Palm Jumeirah to the mainland. Workers used a dam to drain the area and excavate the seabed before rereleasing the water. Developers have plans for a four-stop monorail that will race the length of the palm.


Environmentalists have criticized many Dubai megaprojects, but perhaps none moreso than the Palm Islands. The massive dredging required to build the island has drastically changed the wave, temperature, and erosion patterns in the Persian Gulf, and a whole square mile of coral was killed.

A bridge is a structure that is used to cross some form of barrier, making it easier get to one place from another.  Other barriers, such as rivers, have always confronted travelers and traders who wanted to take the shortest, quickest and safest route to complete their journeys.


Arch bridges 

These bridges uses arch as a main structural component (arch is always located below the bridge, never above it). They are made with one or more hinges, depending of what kind of load and stress forces they must endure. Examples of arch bridge are “Old Bridge” in Mostar, Bosnia and Herzegovina and The Hell Gate Bridge in New York.




An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

Arch bridges are one of the oldest types of bridges and have been around for thousands of years. Arch bridges have great natural strength. They were originally built of stone or brick but these days are built of reinforced concrete or steel. The introduction of these new materials allow arch bridges to be longer with lower spans. Instead of pushing straight down, the load of an arch bridge is carried outward along the curve of the arch to the supports at each end. The weight is transferred to the supports at either end. These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out.

Grider or Beam Bridges

Girder bridges are the simplest bridge type in structure and consist of steel beams shaped to an I-section or box section, called a plate girder bridge or a box girder bridge, respectively. Girder bridges are comprised of deck slabs, on which vehicles and people pass, and of main girders supporting the deck slabs. Deck slabs include RC deck slabs, steel deck slabs, composite deck slabs, and PC deck slabs. Bridges where the deck slabs and the main girders work together to resist loads are called composite girder bridges, and bridges designed to resist loads with the main girders only are called non-composite girder bridges. In general, effective spans of about 25 to 150


A girder bridge, in general, is a bridge that uses girders as the means of supporting the deck.[1] A bridge consists of three parts: the foundation (abutments and piers), the superstructure (girder, truss, or arch), and the deck. A girder bridge is very likely the most commonly built and utilized bridge in the world. Its basic design, in the most simplified form, can be compared to a log ranging from one side to the other across a river or creek. In modern girder steel bridges, the two most common shapes are plate girders and box-girders.

Cable Stayed Bridges

At first glance, the cable-stayed bridge may look like just a variant of the suspension bridge, but don't let their similar towers and hanging roadways fool you. Cable-stayed bridges differ from their suspension predecessors in that they don't require anchorages, nor do they need two towers. Instead, the cables run from the roadway up to a single tower that alone bears the weight. The tower of a cable-stayed bridge is responsible for absorbing and dealing with compressional forces. The cables attach to the roadway in various ways.


A cable-stayed bridge has one or more towers (or pylons), from which cables support the bridge deck. A distinctive feature is the cables which run directly from the tower to the deck, normally forming a fan-like pattern or a series of parallel lines. This is in contrast to the modern suspension bridge, where the cables supporting the deck are suspended vertically from the main cable, anchored at both ends of the bridge and running between the towers. The cable-stayed bridge is optimal for spans longer than cantilever bridges, and shorter than suspension bridges.

Suspension Bridges

A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 19th century, Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.



As the name implies, suspension bridges, like the Golden Gate Bridge or Brooklyn Bridge, suspend the roadway by cables, ropes or chains from two tall towers. These towers support the majority of the weight as compression pushes down on the suspension bridge's deck and then travels up the cables, ropes or chains to transfer compression to the towers. The towers then dissipate the compression directly into the earth.

Cantilever Bridge

A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.



First cantilever bridges appeared in 19th century when a need for longer bridges presented itself. To solve the problem of length, engineers of that time found out that many supports would distribute the loads among them and help to achieve length.

Truss bridge

A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges.



Truss bridge is a type of bridge whose main element is a truss which is a structure of connected elements that form triangular units. Truss is used because it is a very rigid structure and it transfers the load from a single point to a much wider area. Truss bridges appeared very early in the history of modern bridges and are economic to construct because they use materials efficiently.


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