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What is a Retaining Wall ? Types of Retaining Walls

Introduction:

A Retaining Wall is a structure that is designed and constructed to withstand lateral pressure of soil or hold back soil materials.

The lateral pressure could be also due to earth filling, liquid pressure, sand, and other granular materials behind the retaining wall structure.

Retaining walls are vertical or near-vertical structures designed to retain material on one side, preventing it from collapsing or slipping or preventing erosion. They provide support to terrain where the soil’s angle of repose is exceeded and it would otherwise collapse into a more natural form. The principal characteristic of a retaining wall is being able to withstand the pressure exerted by the retained material, which is usually soil.

The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained material to move downslope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes.

Lateral earth pressures are zero at the top of the wall and – in homogenous ground – increase proportionally to a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes hydrostatic pressure on the wall. The total pressure or thrust may be assumed to act at one-third from the lowest depth for lengthwise stretches of uniform height.

It is important to have proper drainage behind the wall in order to limit the pressure to the wall’s design value. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. Drystone retaining walls are normally self-draining. As an example, the International Building Code requires retaining walls to be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift; and that they be designed for a safety factor of 1.5 against lateral sliding and overturning.

There ara various types of retaining wall structures which are used for numerous goals.

Gravity Retaining Wall :

Gravity retaining wall depends on its self weight only to resist earth pressure.

Commonly, gravity retaining wall is massive because it requires significant gravity load to counter act soil pressure.

Slidin, overturning and bearing forces shall be taken into consideration while this type of retaining wall structure is designed.

It can be constructed from different materials such as concrete, stone and masonry units.

Crib retaining wall, gabions and bin retaining wall are types of gravity retaining walls.

Gravity Retaining Wall

Crib Retaining Wall :

Crib Retaining Walls are low cost, of open web construction and can be quickly and inexpensively erected. They can be used almost anywhere a retaining wall is needed – driveways, building sites, garden areas, and when planted out will add beauty and value to your property.

Crib walls are gravity retaining walls constructed from interlocking precast concrete components, filled with free draining material and earth backfill, eliminating the hazards of hydrostatic pressure building up behind the wall.

Crib Retaining Wall

Gabion Retaining Walls :

Gabion retaining walls are multi-celled, rectangular wire mesh boxes which are filled with rocks or other suitable materials.

It is employed for construction of erosion control structures and used to stabilize steep slopes.

Gabion Retaining Walls

Cantiliver Retaining Wall :

Cantiliver retaining wall is composed of stem and base slab and is constructed from reinforced concrete, precast concrete or prestress concrete.

Cantiliver retaining wall is the most common type used as retaining walls.

Sometimes cantilevered walls are buttressed on the front, or include a counterfort on the back, to improve their strength resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.

The portion of the base slab beneath material is termed as heel, and the other part is called toe. It is economical up to height of 10m. Similar to gravity wall, sliding, overturning and bearing pressure shall be taken into consideration during its design.

There are 3 different types of cantiliver retaining walls :

T – shaped cantiliver retaining wall
L – shaped cantiliver retaining wall
T – shaped cantiliver retaining wall with shear key

Cantiliver Retaining Wall

Counter-fort / Buttressed Retaining Wall :

It is a cantiliver retaining wall but strengthened with counter forts monolithic with the back of the wall slab and base slab.

Counter fort spacing is equal or slightly larger than half of the counter-fort height. It’s height ranges from 8 to 12m.

Counter-fort Retaining Wall

A buttress wall is the modified version of the counter-fort retaining wall in which the counter-forts, known as the buttresses, are provided at the other side of the backfill.

A buttress wall is more economical when compared to a counter-fort retaining wall. Buttress walls are not much preferred due to the provision of buttresses in the wall. These buttresses reduc the clearance on the front side of the wall.

Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.

Anchored Retaining Wall :

This type of retaining wall is used when the space is limited or thin retaining wall is required.

Anchored retaining wall is suitable for loos soil over rocks. Considerably high retaining wall can be constructed using this type of retaining wall structure system.

Deep cable rods or wires are driven deep sideways into the earth, then the ends are filled with concrete to provide anchor.

Anchors (tiebacks) actes against overturning and sliding pressure.

Advantages of anchored retaining walls

Mostly used for slope protection and earth retaining works of deep excavations.
Thin walls or very light structures can be designed in combinations with anchored walls.
Anchored walls are one of the most economical system of earth retention.
Combination with sheet piles, cantilever retaining walls, piled retaining walls etc are very much useful for very deep excavations to provide a safe working area

Anchored Retaining Wall

Piled Retaining Wall :

Pile retaining wall is constructed by driving reinforced concrete piles adjacent to each other. Piles are forced into a depth that is sufficient to counter the force which tries to push over the wall.

Sheet pile walls are built using steel sheets into a slope or excavations up to a required depth, but it cannot withstand very high pressure. They are economical till height of 6m.

Piled Retainng wall

Mechanically Stabilized Earth (MSE) Retaining Wall :

It is among the most economical and most commonly constructed retaining walls. Mechanically stabilized earth retaing wall is supported by selected fills (granular) and held together by reinforcements, which can be either metallic strips or plastic meshes.

Types of MSE retaining wall include panel, concrete block and temporary earth retaining walls.

Mechanically Stabilized Earth (MSE) Walls

Hybrid Systems :

Retaining walls that use both mass and reinforcement for stability are termed as Hybrid or Composite retaining wall systems.

What is LIDAR? How it works?

Introduction:

LIDAR or Light Detection And Ranging uses lasers to measure the elevation of things like the ground forests and even buildings. It is lot like sonar which uses sound waves to map things, or radar which uses radio waves to map things, but a LIDAR system uses light sent out from a laser.

For the record, there are different ways to collect LIDAR data: from the ground, from an airplane or even from space.

Airborne LIDAR data are the most commonly available LIDAR data and airborne LIDAR data will also be freely available through the National Ecological Observatory Network or NEON. Many other sources are becoming free for many countries.

The four parts of LIDAR Sytem:

To understand how lasers are used to calculate height in airborne LIDAR, we need to focus on the four parts in the system.

1. LIDAR Unit – Scans the ground:

First, the airplane contains the LIDAR unit itself which uses a laser to scan the earth from side to side as the plane flies. The laser system uses either green or near infrared light because these wavelengths or types of light reflect strongly off of vegetation.

2. Global Positioning System – Tracks planes x,y,z position:

The next component of a LIDAR system is a GPS receiver that tracks the altitude and X,Y location of the airplane.

The GPS allows us to figure out where LIDAR reflections are on the ground.

3. Inertial Measurement Unit (IMU) – Tracks Plate Position:

The third component of the LIDAR system is what’s called an inertial measurement unit or IMU.

The IMU tracks the tilt of the plane in the sky it flies which is important for accurate elevation calculations.

4. Computer – Records Data:

Finally, the LIDAR system includes a computer which records all that important height information that the LIDAR collects as it scans the earth’s surface.

How these four parts of the system work together to get fantastically useful later dataset?

The laser in the LIDAR system scans the earth actively emitting light energy towards the ground. Now before we go any farther, let us get two key LIDAR terms associated with this emitted light energy out of the way.

First, let’s define the word “pulse”. A pulse simply refers to a burst of light energy that is admitted by the LIDAR system.

And second, lets define the word “return”. Return the first reflected light energy that has been recorded by the LIDAR sensor.

Pulses of light energy travel to the ground and return back to the LIDAR sensor.

To get height the LIDAR system records the time that it takes for the light energy to travel to the ground and back. The system then uses the speed of light to calculate the distance between the top of that object and the plane.

To figure ground elevation, the plane’s altitude is calculated using the GPS receiver and then we subtract the distance that the light travel to the ground.

There are two more things in a LIDAR system to consider when calculating height. First, the plane rocks a bit in the sky as it flies due to turbulence in the air. These movements are recorded by the inertial measurement unit or IMU so that they can be accounted for when height values are calculated for each LIDAR return.

An airborne system scans the earth from side to side to cover a larger area on the ground when flying. So while some light pulses travel vertically from the plane to the ground or directly at nadir, most pulses leave the plane angle or off nadir. The system needs to account for pulse angle when it calculates elevation.

How a LIDAR system works?

The LIDAR system emits pulses of light energy towards the ground using a laser, it then records the time it takes for the pulse to travel to the ground and return back to the sensor. It converts this time to distance using the speed of light.

The system then uses the plan’s altitude, tilt, and the angle of the pulse to calculate elevation. It also uses a GPS receiver to calculate the object’s location on the ground.

All this information is recorded on that handy dandy computer also mounted on the airplane.

Self-compacting concrete (SCC) – Advantages, Disadvantages and Applications

Introduction

Self-compacting concrete (SCC) is a concrete which flows under its own weight and doesn’t require any external vibration for compaction, it has revolutionized concrete placement.

Such concrete should have relatively low yield value to ensure high flow ability, a moderate viscosity to resists segregation and bleeding and must maintain its homogeneity during transportation, placing and curing to ensure adequate structural performance and long termdurability.

Self-compacting concrete (SCC) can be defined as a fresh concrete which possesses superior flow ability under maintained stability (i.e. no segregation) thus allowing self-compaction that is, material consolidation without addition of energy.

It is a fluid mixture suitable for placing in structures with Congested reinforcement without vibration and it helps in achieving higher quality of surface finishes.

The three properties that characterise a concrete as self-compacting Concrete are :

Flowing ability: the ability to completely fill all areas and corners of the formwork into which itis placed
Passing ability: the ability to pass through congested reinforcement withoutseparation of the constituents or blocking
Resistance to segregation: the ability to retain the coarse components of the mixin suspension in order to maintain a homogeneous

Applications of Self-Compacting Concrete

The main applications of this type of concrete are the following:

Construction of raft and pile foundations
Retrofitting and repairing constructions
Structures with complex reinforcement distributions
Construction of earth retaining systems
Drilled shafts
Columns

Advantages of (SCC) Self Compacting Concrete

Self-compacting concrete comes with several advantages compared with regular concrete. Some of these benefits include:

Reduces labor costs.
Improved constructability.
High durability, strength, and reliability.
Minimizes voids in highly-reinforced areas.
Reduces permeability in concrete structures.
Fast placement without mechanical consolidation.
The SCC construction is faster than normal concrete.
SCC enables freedom in designing concrete structures
Creates smoother and more aesthetic surface finishes.
Eliminates problems associated with concrete vibration.
Creates high-quality structures with improved structural integrity.
Allows for easier pumping, and there are many placement techniques available.
Allows for innovative architectural features, since it can be used in complex forms.
Reduced noise during placement as no vibration is required
SCC requires a lower pumping pressure. Hence, concrete can be easily pumpedover longer distances and heights compared to traditional concrete

Disadvantages of (SCC) Self Compacting Concrete

As with any construction material, self-compacting concrete faces the following limitations:

Material selection is more strict.
Construction costs are much more higher, compared with regular concrete.
Higher precision is required when measuring and monitoring.
There is no globally accepted test standard to undergo an SCC mix design.
The cost of construction is costlier than conventional concrete construction.
Many trial batches and laboratory tests are required to use a designed mixture.
There is no internationally accepted test standard for self-compacting concrete mix.
The higher flow rate of SCC compared to traditional concrete can cause adynamic pressure, in addition to the hydrostatic pressure of placed concrete, and thismust be taken into consideration for formwork design

Material Use In (SCC) Self Compacting Concrete

Cement :

Ordinary Portland Cement, 43 or 53 grade can be used

Aggregates :

The maximum size of aggregate is generally limited to 20 mm. An aggregate of size 10 to 12mm is desirable for structures having congested reinforcement.

Wherever possible size of aggregate higher than 20mm could also be used. Well graded cubical or rounded aggregates are desirable.

Aggregates should be of uniform quality with respect to shape and grading. Fine aggregates can be natural or manufactured.

The grading must be uniform throughout the work. The moisture content or absorption characteristics must be closely monitored as the quality of SCC will be sensitive to such changes.

Particles smaller than 0.125 mm i.e. 125-micron size are considered as FINES which contribute to the powder content

Mixing Water :

Water quality must be established on the same line as that for using reinforced concrete or prestressed concrete.

Chemical Admixtures :

Super plaseizers are an essential component of SCC providing necessary workability. The new generation super plasticizers termed poly-carboxylated ethers (PCE) is particularly useful for SCC.

Other types may be considered as necessary, such as Viscosity Modifying Agents (VMA) for stability, air-entraining agents (AEA) to improve freeze-thaw resistance, and retarders for Control of Setting.

Mineral Admixtures:

This may vary according to the mix design and the properties required. Below is a list of the different mineral admixtures used, and the properties they provide to the concrete mixture:

Fly ash: Used to improve the filling of the internal concrete matrix, resulting in fewer pores. This reduces permeability and improves the quality of structures.
Ground granulated blast furnace slag (GGBS): GGBS helps improve the rheological properties of concrete.
Stone Powder: Incorporated to improve the powder content of the mixture.
Silica Fumes: Used to improve the mechanical properties of the structure.

Transport, pumping, placing and finishing

Transport

Self-compacting concrete has higher fluidity compared to traditional concrete. Hence,there is a higher chance of spillage during transport. Additional caution is advised by reducing the batch size on the back of a truck, as well as ensuring the water-tightnessof the drum.

Extreme weather conditions (very high or low temperature) can affect the self-compacting properties of concrete. Under such conditions, the transport duration must be minimized by choosing non-peak hours in congested areas and also by choo-ing the closest concrete production plant to the placement site.

Overall, the averagetime that an SCC mixture can spend on the back of a truck before placement needs to be considered and the mixture design needs to be optimized accordingly. Otherwise,flowability properties might not be achieved.

Pumping, placing and formwork

An advantage of SCC is its excellent flow properties, which results in easier pumping and placing of SCC compared to traditional concrete. After discharging, self-compacting concrete can flow up to 10m in horizontal directions.

The excellent flowability of SCC also results in a much higher filling capability compared to traditional concrete, i.e. it can quickly fill inaccessible voids between reinforcements and formwork.

The placement rate of SCC can occur over ashort amount of time. Therefore, it is essential that all formwork, linings, reinforcing steel and any other embedded objects are secured and tightened before placement.

SCC can be placed with chutes, buckets and pumps. Pumping is the most common method of SCC placement because of excellent flowability without segregation.

Pumping of SCC from a truck using a crane pump at a building site

Surface finish of SCC

SCC is generally used for architectural concrete because the surface finish of SCC is of high quality, often more appealing with sharp edges compared to traditional concrete. The improved surface finish is attributed to the self-levelling and filling capabilities of SCC, which allows concrete to flow smoothly, and thereby fill holes.

The surface finish of traditional concrete often has discolouration because of hydrationby-products and segregation.

Other imperfections such as sand textured areas, honeycombing (aggregate bridging), and some problems caused by mortar loss canal so occur. Using SCC can increase the chance of eliminating these surface imperfections. However, a well-balanced concrete mixture with optimized rheological properties is required to achieve a high-quality surface finish for SCC, i.e. aesthetic appeal for exposed architectural use.

Mixtures with a lower viscosity, i.e. higher slump flow allow for entrained air to escape more efficiently and thereby provide a better surface finish.

The quality of formwork surfaces, type and amount of releasing agent, as well as production and placement methods also affect the surface finish of SCC.

What is Polymer Concrete? Advantages and disadvantages – Applications

Introduction

Polymer concrete is the composite material made by fully replacing the cement hydrate binders of conventional cement concrete with polymer binders or liquid resins, and is a kind of concrete-polymer composite.

For hardening of polymer concrete, most liquid resins such as thermosetting resins, methacrylic resins and tar-modified resins are polymerized at ambient or room temperature. The binder phase for polymer concrete consists only of polymers, and does not contain any cement hydrates. The aggregates are strongly bound to each other by polymeric binders.

The different ways in which the polymer is introduced into the concrete (hardened concrete) will vary widely based on the commercial objective. The polymers can be employed in concrete in different ways.

They are:

Polymer Impregnated Concrete (PIC)
Polymer-Modified Concrete (PMC)
Polymer Concrete (PC)
Polymer as Protective Coating
Polymer as Bonding Agent

Advantages and disadvantages:

The advantages and disadvantages of polymeric binders are directly given to the polymer concrete. Accordingly, in comparison with ordinary cement concrete, its properties such as strength, adhesion, watertightness, chemical resistance, freeze-thaw durability and abrasion resistance are generally improved to a great extent by polymer replacement. Since the bond between polymeric binders and aggregates is very strong, its strength properties depend on those of the aggregates.

On the other hand, its poor thermal and fi re resistance and its large temperature dependence of mechanical properties are disadvantages due to the undesirable properties of the polymer matrix phases. Therefore, the glass transition point (or temperature) of the polymer matrix phases should be noted from the viewpoint of such thermal properties.

Thermoplastic resins generally retain their practical properties at temperatures below the glass transition point and lose them at temperatures exceeding the point, beginning to thermally decompose at somewhat higher temperatures.

The practical temperature range of the thermoplastic resins may be improved by the addition of suitable cross-linking monomers or comonomers having higher glass transition points.

Thermosetting resins do not commonly show a glass transition point, and retain their mechanical properties up to the thermal decomposition temperature. Such essential disadvantages of the polymer concrete can be considerably improved by controlling the necessary polymeric binder content by volume to a minimum.

Practical applications

Structural precast products:

Manholes and handholes for telecommunication cable lines,
electric power cable lines and gas pipelines,
prefabricated cellars or stockrooms,
tunnel liner segments for telecommunication cable lines and sewerage,
pipes for sewage,
hot spring water and seawater,
piles for port or hot spring construction,
FRP-reinforced frames or panels for buildings,
machine tool structures, e.g.
beds and saddles, etc.

Non structural precast products :

Gutter covers,
U-shaped gutters,
footpath panels,
permanent forms for checkdams with acidic water and offshore or marine structures,
terrazzo tiles and panels, and large-sized or curved decorative panels for buildings,

partition wall panels, sinks, counters, washstands, bathtubs, septic tanks, electrolytic tanks, works of art, e.g. carved statues and objets d’art, tombs for Buddhists, etc.

Cast-in-place applications :

Spillway coverings in dams,
protective linings of stilling basins in hydroelectric power stations,
coverings of checkdams,
foundations of buildings in hot spring areas,
acid-proof linings for erosion control dams with acidic water,
patch materials for damaged concrete structures,

overlays for pavement repairs,
overlay strengthening for bridge decks,
drainage pavements using porous polymer concrete, etc.

Precast applications :

Transportation applications such as railroad crossings,
railroad ties, median barriers, etc.

Structural and building panels

Sewer pipes, equipment vaults, drainage channels, etc.

Corrosion-resistant tiles, bricks and linings

Small water-flow control structures

Stair treads and nosings

Non conductive, non magnetic support structures for electrical equipment

Manhole structures and shims

Components for the animal-feeding industry

Large-scale pre-insulated wall panels for segmental building construction

Electrical insulators

Machine tool bases

Cast-in-place applications :

Patching materials for reinforced concrete structures
Overlays for reinforced concrete structures in the transportation industry

What Is a Culvert? – Types of Culverts

Introduction

Culverts are one of those things that seem so obvious that you never take the time to even consider them. They are also so common that the practically blend into the background. But without them, life in this world would be a bit more complicated.

Imagine you are designing a brand new roadway to connect point A to point B, it would be nice if the landscape between these points was perfectly flat, with no obstructions or topographic relief. But, that’s rarely true. More likely, on the way, you will encounter hills and valleys, structures and streams, and you will have to decide how to deal with each one.

Your road can go around some obstacles, but for the most part you will have to work with what you have got.

A roadway has to have gentle curves both horizontally and vertically, so you might have to take some soil or rock from the high spots and build up the low spots along the way, also called cut and fill.

But you have got to be careful about filling in low spots, because that’s where water flows. Sometimes it’s obvious like rivers or perennial streams, but lots of watercourses are ephemeral, meaning they only flow when it trains.

If you fill across any low area in the natural landscape, you run the risk of creating an inpoundment. If the water get through your embankment, it’s going to flow over the top. Not only this lead to damage of the roadway, it can be extremly dangerous to motorists and other vehiclets.

One obvious solution to this problem is a bridge : the classic way to drive a vehicle over a body of water. But, bridges are expensive. You have to hire a structural engineer, install supports, girders and road decks. It’s just not feasible for most small creeks and ditches.

So instead we do fill the low spots in, but we include a pipe so the water can get through. That pipe is called a culvert, and there’s actually quite a bit of engineering behind this innocuous bit of infrastructure.

A culvert really only has two jobs : it has to be able to hold up the weight of the traffic passing over without collapsing, and it has to be able to let enough water pass through overtopping the roadway. Both jobs are pretty complicated, but it’s the second one which is the most important.

Factors influencing the hydraulics of a culvert :

In fact, there are eight factors that can influence the hydraulics of a culvert :

Headwater, or the depth of flow upstream of the culvert

The cross-sectional area of the culvert barrel
The cross-sectional shape of the culvert barrel
The configuration of the culvert inlet
The roughness of the culvert barrel
The length of the culvert
The slope of the culvert
The tailwater or depth of flow downstream

Types of culverts:

Following are the types of culverts generally used in construction:

1- Pipe Arc Culvert

The pipe-arch culvert is a simple structure that looks like a half-circle-shaped culvert.

It is suitable for larger waterway opening, but the flow should be stable where fishes can be provided with greater hydraulic advantage and they are artistic and it provides low clearance.

2. Box Culvert

The culverts are constructed in the form of one or more rectangular or square openings, in their top slabs.

The box culverts made up of concrete specially R.C.C. materials. They used to dispose of rainwater so, these are not useful in dry periods.

This culvert’s construction is preferable, especially in loose soil conditions, and for a larger span and also it requires a good foundation, and not be used for larger velocity.

For a single span of 3 m or for a double span of 6 m width, such type of culverts can be used. The thickness range of the R.C.C slab should be kept within 1.25 to 2.5 m.

There is a sudden change that occurs in the section of bending moment and shearing force due to the sinking of the culvert. Box culvert is a rigid frame and simple construction.

Pressure on the soil is reduced due to the bottom slab of a culvert.

Box culvert is economical due to there is no need to provide a separate foundation and also rigidity.

3. Arc Culvert

The arch culvert involves the construction of a superstructure its superstructure consisting of one or two segmental arches consisting of brick masonry, concrete, stone masonry is commonly used.

The arch culverts are not provided with the piers to the sides of the abutment.

Advantages Of Arc Culvert

Following are the advantages of Arc Culvert,

The arch culvert and artificial floors both are made up of concrete.
The pipe arch culvert and arch culvert are similar but in the case of an arch culvert, an artificial floor is provided below the arch.
It is normally used for narrow passages.
The arch culvert is similar to the Masonry bridges.
The arch culvert is very easy to install.
The arch culvert is also made of steel but it is very extortionate.

4. Pipe Culvert (Single or Multiple)

Pipe culverts are widely used culverts and rounded in shape. The culverts may be of single in number or multiple. If single pipe culvert is used then larger diameter culvert is installed. If the width of channel is greater than we will go for multiple pipe culverts. They are suitable for larger flows very well. The diameter of pipe culverts ranges from 1 meter to 6m. These are made of concrete or steel etc..

5. Bridge Culvert

Bridge culverts are provided on canals or rivers and also used as road bridges for vehicles. For this culverts a foundation is laid under the ground surface. A series of culverts are laid and pavement surface is laid on top this series of culverts. Generally these are rectangular shaped culverts these can replace the box culverts if artificial floor is not necessary.

What is the difference between Pre-tension and Post-tension in concrete?

Pre-tension Concrete

Pretension in concrete is the method when the concrete is prestressed with tendons before the placing of the concrete. and It is a suitable method for small structural elements. the pre-tensioning members are produced in the mold.

To pretension concrete the steel is first tensioned in a frame or between anchorages external to the member. The concrete is then cast around it. After the concrete has developed sufficient strength the tension is slowly released from the frame or anchorage to transfer the stress to the concrete to which the tendons have by that time become bonded. The force is transmitted to the concrete over a certain distance from each end of a member known as the transfer length.

Post-tension Concrete

Post tension in concrete is the method when the prestressing process is done after the concrete attains its strength. It is suitable when the structure is heavy and this method is developed due to bearing.

Post-tensioned concrete is made by casting concrete that contains ducts through which tendons can be threaded. An alternative is to cast the concrete around tendons that are greased or encased in a plastic sleeve.

When the concrete has sufficient strength the tendons are tensioned by means of portable jacks. The load is transmitted to the concrete through permanent anchorages embedded in the concrete at the ends of the tendons.

Ducts are usually grouted later or filled with grease to protect the tendons against corrosion. In some applications the post-tensioning tendons are run alongside the concrete member.

Pre-tension Concrete vs Post-tension Concrete

Sr.No	Pretensioning	Post-tensioning
1	Pretension is the technique in which we are imparting tension in strands before placing the concrete.	Post tensioning is done by forming a duct in which strands are pulled (tensioned) after the concrete gains it’s full strength.
2	In this type of concrete, the pre-stressing cables called the strands are tensioned before casting the concrete and then concrete is casted enclosing the tensioned cables.	In this type of concrete, the strands are enclosed within a duct in the form and then concrete is casted. The process of tensioning the strands is carried out after the concrete attains its sufficient strength.
3	Post-tension Concrete	In this method, products are changed according to a structure.
4	Pre-tensioning members are produced in a mould.	Cables are used in place of wires and jacks are used for stretching.
5	It is cheaper because cost of sheathing is not involved in pretensioning	It is costlier because cost of sheathing is required.
6	Pre-tensioning is preferred when the structural element is small and easy to transport	Post-tensioning is preferred when the structural element is heavy.
7	Loss of prestressing isn’t less (about 18 %)	Loss of prestressing isn’t more (about 15 %)
8	Small sections are to be constructed	Size of a member is not restricted, long-span bridges are constructed by post-tensioning
9	It is more reliable and durable	The durability depends upon the two anchorage mechanism

Segregation and Bleeding of concrete – Causes and Mitigation

1. Segregation of Concrete:

Segregation is the separation of constituent materials in concrete. There are three common types of segregation:

Separation of coarse aggregate from the concrete mixture
Separation of cement paste from the concrete during its plastic stage
Separation of water from the concrete mix (Bleeding in concrete)

Segregation of concrete affects strength and durability in structures. When this is detected, elements affected should be demolished.

The segregation includes undesirable properties in the hardened concrete; therefore, it can cause honeycombing and may leads to the development of cavities in the concrete surface.

a – Causes of concrete segregation:

The main causes are:

Use of high water-cement ratio in concrete. This general happens in case of concrete mixed at site by unskilled workers
Excessive vibration of concrete with mechanical needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top.
When concreting is done from height in case of underground foundations and rafts, which causes concrete to segregate.

b – Mitigation:

Segregation can be controlled by maintaining proper proportioning the mix.
By peculiar handling, placing, transporting, compacting and finishing of concrete.
Adding air entraining agents, admixtures and pozzolanic materials in the mix segregation controlled to some extent.
Wherever depth of concreting is more than 1.5 meters, it should be placed through temporary inclined chutes.
The delivery end of chute should be as close as possible to the point of deposit.
Handling, placing and compaction of freshly mixed concrete should be done carefully. A proper vibration also reduces the chances of segregation.

2. Concrete Bleeding:

Bleeding is a form of segregation in which water present in the concrete mix is pushed upwards due to the settlement of cement and aggregate. The specific gravity of water is low, due to this water tends to move upwards.

Some bleeding is normal but excessive bleeding can be problematic.

Not all bleed water will reach the surface of the concrete but some water may rise and remain trapped under aggregates and reinforcing. This results in the weakening of the bond between the paste and those elements.

a – Bleeding Causes:

The principal causes of bleeding are:

High water-cement ratio to highly wet mix
Badly proportioned and insufficiently mixed concrete

b – Bleeding Effects:

Due to the formation of Laitance, structures may lose its wearing capacity and decreases its life.

Water while moving from bottom to the top, forms continuous channels. Due to these channels, concrete becomes permeable and allow water to move, which forms water voids in the matrix and reduces the bond between aggregate and the cement paste.

Forming of water at the top surface of concrete results in delaying the surface finishing and so concrete becomes permeable and loses its homogeneity.

Excessive bleeding breaks the bond between the reinforcement and concrete.

c – Mitigation:

To avoid concrete bleeding, it is recommended to:

Reduce Water content
Use finer cements
Increase amount of fines in the sand
Use supplementary cementitious materials
Use air entraining admixtures

What is Plum Concrete?

The word plum means large stones which are termed as boulders or coarse aggregates if technically speaking.

The use of plum concrete is preferred if the required thickness of PCC is excessive or large. This is mainly done below the foundations where due to sleep slope of the strata, the quantity of leveling course could be excessive.

The plum concrete is actually an economical variation of mass concrete.

Plumbs above 160 mm and up to any reasonable size may be used in plain concrete work up to a maximum limit of 20 percent by volume of concrete when specifically permitted by the engineer-in-charge. The plums shall be distributed evenly and shall be not closer than 150 mm from the surface.

Uses of plum concrete:

Plum concrete is used at the water channel beds. It is used mostly in mass concrete works like concrete gravity dams or bridge piers in such cases of rocks about 150 mm in size are used as coarse aggregates to mix a plum concrete.

It is used at side slopes of the embankment to provide a protective laver to earthen foundations and bases.

All about Shrinkage Cracks in Concrete – Types and Causes of Shrinkage Cracks

The shrinkage in concrete can be defined as the volume changes observed in concrete due to the loss of moisture at different stages due to different reasons.

Types of Shrinkage in Concrete:

The shrinkages can be classified into the following:

Plastic Shrinkage
Drying Shrinkage
Antogenous Shrinkage
Carbonation Shrinkage

Plastic Shrinkage in Concrete

The water required for concrete strength gain is escaped into the atmosphere due to the process of evaporation, from the surface of the structure, creating cracks on the surface of the structure.

Another reason for shrinkage cracks under plastic shrinkage type is due to the water absorption from the concrete by the aggregate.

Plastic Shrinkage in Concrete

In the case of floors and the pavements, where the surface area is exposed to drying in a large extent compared to the depth when are subjected to the sun and the drying wind, the surface dries very quickly causing plastic shrinkage.

In the case of a mix design where the water cement ratio is high, there is the possibility of excess water pathways causing bleeding. This excess water due to bleeding will accumulate at the surface of the slabs. When these are exposed to dry weather conditions, the surface dries up and collapse creating cracks.

Prevention of Plastic Shrinkage:

The escape of water from the surface can be prevented by covering the surface with the help of polyethylene sheeting. Prevention of water evaporation will prevent plastic shrinkage.
Proper vibration of the concrete can prevent plastic shrinkage.
Plastic shrinkage in concrete structures can be reduced by use of aluminium powder.
The use of expansive cement can also help in the control of plastic shrinkage.

Drying Shrinkage

Drying shrinkage is caused by the loss of surface-absorbed water from the calcium silicate hydrate (C-S-H) gel and also due to the loss of hydrostatic tension in the small pores.

Swelling is the opposite phenomenon of shrinkage.

Drying Shrinkage in concrete

This shrinkage is mainly due to the deformation of the paste, though the aggregate stiffness also influences it. It takes place once the concrete has set is called as the drying shrinkage.

Most of the kinds of drying shrinkage take place in the first few months of the concrete structure life.

Autogenous Shrinkage

The water is necessary for the hydration of hydrated cement. This process of water withdrawal from the capillary pores to carry out the hydration of hydrated cement is called as self-desiccation.

The shrinkage dealt with such conservative system can be named as autogenous shrinkage or autogenous volume change.

Autogenous Shrinkage in concrete

This can be largely avoided by keeping the surface of the concrete continuously wet; conventional curing by sealing the surface to prevent evaporation is not enough and water curing is essential. With wet curing, water is drawn into the capillaries and the shrinkage does not occur. Note that autogenous shrinkage is separate from and additional to conventional drying shrinkage, which will start when water curing ceases.

Carbonation Shrinkage:

Carbonation Shrinkage is a decrease in either length or volume of a material(concrete) resulting when carbon dioxide present in the atmosphere reacts in the presence of water with the hydrated cement. Calcium hydroxide gets converted to calcium carbonate and also some other cement compounds are decomposed. Such a complete decomposition of calcium compound in hydrated cement is chemically possible even at the low pressure if carbon dioxide in normal atmosphere. Carbonation penetrates beyond the exposed surface of concrete only very slowly.

Carbonation Shrinkage in concrete

The rate is penetration of carbon dioxide depends also on the moisture content of the concrete and the relative humidity of the ambient medium. Carbonation is accompanied by an increase in weight of the concrete and by shrinkage. Carbonation Shrinkage is probably caused by dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place.

Factors affecting Shrinkage:

The main factors affecting shrinkage are listed below:

Material Selection
Water cement ratio: Shrinkage is mostly influenced by the water-cement ratio of concrete. It increases with the increases in the water-cement ratio
Environment conditions : It is one of the major factors that affect the total volume of shrinkage. Shrinkage is mostly occurred due to the drying condition of the atmosphere. It increases with the decrease in the humidity.
Chemical composition of Cement :The chemical composition of cement used for concrete & mortar also has some effect on shrinkage. Rapid hardening cement has greater shrinkage than Ordinary Portland Cement.
Aggregates : Aggregates with moisture movement and low elastic modulus cause large shrinkage. The rate of shrinkage generally decreases with the increase of the size of aggregates. It is found that concrete made from sandstone shrinks twice than that of concrete of limestone.
Type of cement used
Admixture in concrete : The shrinkage increases with the addition of accelerating admixtures due to the presence of calcium chloride (Cacl2) in it and it can be reduced by lime replacement.
Size and shape of concrete specimen
Temperature

Highway Design – Introduction to Horizontal and Vertical Alignment

The layout of a highway is comprised of two components: A horizontal component which is viewed from above and the vertical component which is viewed from the side.

The horizontal alignment dictates the left or right turning required to remain on the roadway, while the vertical alignment exerts forces on the vehicle as the grade along the roadway changes.

Horizontal Alignment:

In the horizontal perspective, a roadway is primarily comprised of tangent, or straight, sections which are smoothly connected by curves.

The horizontal curves that are used to provide drivers with the transition from one tangent to the next tangent are typically simple curves which are an arc of circle.

These curves have a single radius value which represents the sharpness or flatness of the curve.

Highway Geometric Design – Horizontal Curve Equations

A tangent roadway section has an infinite radius, since it is a straight line and a horizontal curve has a single, finite radius. Therefore, a spiral transition is used in some instances to help make the shift from a tangent to a curve a little smoother.

Geometric relationships and equations can be used to find important information for reach curve. This information includes the radius, the length of the curve, the change in direction of the two tangents, and other factors depending on our needs.

Several points of interest along the curve include the location where the tangents intersect, which is known as the Point of Intersection. The location where the vehicle leaves the tangent section and begins to drive along the curve, is known as the point of curvature.

Types Of Horizontal Curves:

Horizontal curves are of different types as follows:

1.Simple circular curve

2.Compound curve

3.Reverse curve

4.Transition curve

1. Simple Circular Curve

Simple circular curve is normal horizontal curve which connect two straight lines with constant radius.

2. Compound Curve

Compound curve is a combination of two or more simple circular curves with different radii. In this case both or all the curves lie on the same side of the common tangent.

3. Reverse Curve

Reverse curve is generated when two simple circular curves bending in opposite directions are meet at a signle point and that points is called as point of reverse curvature. The center of both the curves lie on the opposite sides of the common tangent such that the radii of both the curves may be same or different.

Types of horizontal curves

4. Transition Curve

A curve of variying radius is termed as transition curve. It is generally provided on the sides of circular curve or between the tangent and circular curve and between two curves of compound curve or reverse curve etc. Its radius varies from infinity to the radius of provided for the circular curve.

Transition curve helps gradual introduction of centrifugal force by gradual super elevation which provides comfort for the passengers in the vehicle without sudden jerking.

Spiral Curve

Spiral is a type of transition curve which is recommended by Indian Road Congress as ideal transition curve because of its smooth introduction of centrifugal acceleration. It is also called as clothoid.

TS = Tangent to spiral
SC = Spiral to curve
CS = Curve to spiral
ST = Spiral to tangent
LT = Long tangent
ST = Short tangent
R = Radius of simple curve
T_s = Spiral tangent distance
T_c = Circular curve tangent
L = Length of spiral from TS to any point along the spiral
L_s = Length of spiral
PI = Point of intersection
I = Angle of intersection
I_c = Angle of intersection of the simple curve
p = Length of throw or the distance from tangent that the circular curve has been offset
X = Offset distance (right angle distance) from tangent to any point on the spiral
X_c = Offset distance (right angle distance) from tangent to SC
Y = Distance along tangent to any point on the spiral
Y_c = Distance along tangent from TS to point at right angle to SC
E_s = External distance of the simple curve
θ = Spiral angle from tangent to any point on the spiral
θ_s = Spiral angle from tangent to SC
i = Deflection angle from TS to any point on the spiral, it is proportional to the square of its distance
i_s = Deflection angle from TS to SC
D = Degree of spiral curve at any point
D_c = Degree of simple curve

Bernoulli’s lemniscate

In this curve, the radius decreases as the length increases and this causes the radial acceleration to keep on falling. The fall is, however, not uniform beyond a 30 o deflection angle. We never use this type of curve on railways.

Vertical Alignment:

In the vertical perspective, a roadway is also comprised of tangents which are smoothly connected by curves. For vertical alignment, the tangents represent grades which can either be flat, uphill or downhill.

The typical vertical curve is a symmetric, parabolic curve whose shape is defined by the parabolic equation.

The information required to fully define a vertical curve is the elevation of the beginning of the curve, the grades of the two tangents that are connected ad the length of the curve.

The naming convention of vertical alignment is similar to horizontal alignment. Several points of interest along the curve include that the location where the tangents intersect, which is known as the Point of Vertical Intersection.

The location where the vehicle leaves the tangent grade and begin to drive along the curve, is known as the point of vertical curvature.

The point where the curve ends and the vehicle returns back to the tangent grade is known as the Point of Vertical Tangency.

Highway Geometric Design – Vertical Curve Equations

Types Of Vertical Curves:

In general parabolic curve is preferred as vertical curve in the vertical alignment of roadway for the ease of movement of vehicles. But based on the convexity of curve vertical curves are divided into two types

1.Valley curve

2.Summit curve

1. Valley Curve (sag curve)

Valley curve connects falling gradient with rising gradient so, in this case convexity of curve is generally downwards. A second name for valley curve is sag curve.

They are formed when two gradients meet as any of the following four ways:

When a negative gradient meets another mild negative gradient
When a negative gradient meets a level zero gradient
When a negative gradient meets with a positive gradient
When a positive gradient meets another steeper positive gradient

2. Summit Curve (crest curve)

Summit curve connects rising gradient with falling gradient hence, the curve has its convexity upwards. A second name for summit curve is crest curve.

They are formed when two gradients meet as in any of the following four ways:

When a positive gradient meets another positive gradient.
When positive gradient meets a flat gradient.
When an ascending gradient meets a descending gradient.
When a descending gradient meets another descending gradient.