Concrete vibration – The why and how of consolidating concrete
What factor has a greater effect on concrete compressive strength than any other? Most engineers would say water-cement ratio … as water-cement ratio increases strength decreases. Duff Abrams showed this in 1919, and Abrams’ law is the principle behind most concreting proportioning methods used today. But Abrams ran his tests on fully consolidated concrete.
Unless concrete is properly consolidated, voids reduce strength regardless of the water-cement ratio. And, as shown in Figure1, the effect is significant.
Right after it’s placed, concrete contains as much as 20% entrapped air. The amount varies with mixtype and slump, form size and shape, the amount of reinforcing steel, and the concrete placement method. At a constant water-cement ratio, each percent of air decreases compressive strength by about 3% to 5%. Consolidating the concrete, usually by vibration, increases concrete strength by driving out entrapped air. It also improves bond strength and decreases concrete permeability.
Figure 1. Degree of consolidation can have as much effect on compressive strength as water cement ratio. Low-slump concrete may contain up to 20% entrapped air when placed.
Vibration is a two-part process
How does vibration consolidate concrete? Figure 2 shows it to be a two- part process. A vibrator creates pressure waves that separate aggregate particles, reducing friction between them. Piles of concrete flat-ten as the concrete flows around reinforcing steel and up to the form face. Large voids (honeycomb) disappear. But making the concrete flowable doesn’t finish the compaction proces s. Almost simultaneously, a second stage starts to occur as entrapped air bubbles rise to the surface. This deaeration process continues after the concrete has flattened out. Until both vibration stages are complete, the concrete isn’t fully consolidated. If the vibrator is removed too soon, some of the smaller bubbles won’t have time to rise to the surface. Vibration must continue until most of the air entrapped during placement is removed. It’s usually not practical, though, to remove all the entrapped air with standard vibrating equipment.
Figure 2. A vibrator consolidates concrete in a two-part process. The first waves liquify the concrete so it flows better and the continuing waves knock out air bubbles.
Different vibrators for different jobs
The earliest form of equipment used as a vibrator was a rod stuck into the concrete, pushed down and pulled up. Rodding works for concretes with slumps greater than 3 inches, but it’s rarely used because of the costly labor required. Because rodding doesn’t put extra pressure on forms, howe ve r, it has helped more than one contractor complete a concrete pour when forms were bulging. The most common vibrator used is the electric, flexible shaft type. Other types include electric motor-in- head, pneumatic, and hydraulic. Vibrator output, usually expressed as a frequency, is controlled in a different way for each type of vibrator:
An electric vibrator uses voltage.
A pneumatic vibrator uses air pressure.
A hydraulic vibrator uses pressure and flow rate of hydraulic fluid.
On the jobsite the contractor can check the operating performance of his equipment by measuring frequency. If it’s low he should check for voltage fluctuations, air pressure losses, or hydraulic pressure drops. The type of vibrator must match the requirements of the concrete and the jobsite (Figure 3). Frequency rates determine the amount of vibration time required to complete the two-stage consolidation process. In the 1960s, vibration frequencies were much lower. To compact a 1⁄2-inch-slump concrete took 90 seconds at 4,000 vibrations per minute (vpm), 45 seconds at 5,000 vpm, and 25 seconds at 6,000 vpm. Today’s typical frequency of 15,000 vpm requires only 5 to 15 seconds of vibration time. Internal vibrators chosen for most jobs have a frequency of 12,000 to 17,000 vpm in air. The common flexible shaft-type vibrator reduces its frequency by about 20% when immersed in concrete. Motor-in- head types provide a constant frequency when in air or concrete.
Figure 3. The vibrator head must fit between the rebars and have a high enough frequency to quickly consolidate the concrete.
How to use an internal vibrator
Producing a dense concrete without segregation requires an experienced vibrator operator. Inexperienced operators tend to merely flatten the concrete because they don’t vibrate long enough to deaerate the concrete. Undervibration is more common than over vibration because of a worker’s effort to keep up with the concrete or to increase productivity. The operator can judge whether or not vibration is complete by watching the concrete surface. When no more large air bubbles escape, consolidation is adequate. Skilled operators also listen to the pitch or tone of the vibrator motor. When an immersion vibrator is inserted in concrete, the frequency usually drops off, then increases, becoming constant when the concrete is free of entrapped air. Never use a vibrator to move concrete laterally. Concrete should be carefully deposited in layers as close as possible to its final position in the form. As each layer is placed, insert the vibrator vertically. The distance between insertions should be about 11⁄2 times the radius of action (usually 12 to 24 inches). Radius of action is a distance from the vibrator head within which consolidation occurs. It varies with equipment and concrete mix.
Walls and columns
Special techniques are necessary to blend layers of concrete in walls and columns. Let the vibrator penetrate quickly to the bottom of the layer and at least 6 inches into the preceding layer. Then move the vibrator up and down, generally for 5 to 15 seconds, to blend the layers. Withdraw the vibrator gradually with a series of rapid up and down motions.
Elevated beams and slabs
Beams and joists placed monolithically with slabs should be vibrated separately before slab placement. Place the slab concrete after vibrating the beam, but before the beam concrete is set. Allow the vibrator to penetrate through the slab into the previously placed beam to blend the two structural elements.
Undervibration vs Overvibration
Undervibration is far more common than overvibration. Good quality normal- weight concrete is not readily susceptible to the problems caused by overvibration, so when in doubt, vibrate more. The problems associated with undervibration include:
Honeycombing
Excessive entrapped air
Sand streaks
Cold joints
Subsidence cracking
The problems associated with overvibration include:
Segregation
Sand streaks
Loss of entrained air
Form deflection
Form damage or failure
Overvibrating, because it causes entrained air loss, might be expected to decrease freeze-thaw resistance. Research results don’t bear this out, however. In one study, overvibration of low- slump, air- entrained concrete had no effect on freeze-thaw resistance. Overvibration should not be a concern unless high- slump, improperly proportioned concrete is being placed.
Vibrating around congested reinforcement
To provide good concrete- to- steel bond, vibration is especially important in areas congested with rebar. Vibration alone doesn’t solve the problem. Other actions must be taken to help complete concrete consolidation, such as:
Using admixtures to increase flowability but limit segregation
Changing mix proportions or ingredients to increase flowability
Designing the reinforcing for ease of concrete placing
Figure 4. Vibration alone won’t consolidate concrete adequately when reinforcing is congested. To ensure adequate consolidation it may be necessary to use superplasticizers, reduce aggregate maximum size, or adjust rebar spacing.
To achieve proper consolidation by internal vibration in congested areas, the designer should provide obstruction-free vertical access of 4×6-inch minimum openings to insert the vibrator. Horizontal spacing of these openings should not exceed 24 inches or 11⁄2 times the vibrator’s radius of action. Engineers designing congested reinforcement should also design for proper consolidation, otherwise contractors can’t always guarantee adequate concrete to steel bond (Figure 4).
References
1. “Guide for Consolidation of Concrete,” ACI 309R-87, ACI Materials Journal, September-October 1987, American Concrete Institute, Box 19150, Detroit, Michigan 48219. 2. Whiting, D., G. W. Seegebrecht, and S. Tayabji, “Effect of Degree of Consolidation on Some Important Properties of Concrete,” SP-96, Consolidation of Concrete, American Concrete Institute. 3. Olsen, Mikael, “Energy Requirements for Consolidation of Concrete During Internal Vibration,” SP-96, Consolidation of Concrete, American Concrete Institute. 4. Troxell, Davis, & Kelly, Composition and Properties of Concrete, McGraw- Hill, New York, New York, 1968.
Simulation of the Morandi bridge collapsed in Genoa, Italy, in 2018, performed with the Bullet Constraints Builder (BCB) structural simulation software for Blender. The model was built from plans true to scale. Reinforcement information was estimated in part from photos of the destroyed bridge. In order to narrow down the likely collapse scenario this video includes simulation results for different initial failure points. The characteristics of the debris heaps in comparison to reality often can provide an indication on what has probably happened.
Types of structural supports – Boundary Conditions
Types of supports
Defining the boundary conditions in a model is one of the most important part of preparing an analysis model, irrespective of the software that you use. Supports are an essential part of building your model to ensure accurate and expected results.
These are not to be ignored nor guessed as it can lead to your structure not behaving in the way you anticipated. To define supports you need to be aware about the support detailing in case of steel structures. For example, a support column in a steel structure can be pinned or fixed, depending upon the detailing adopted.
1. Fixed support:
This is the most rigid type of connection. It restrains the member in all translations and rotations, which means it can’t move or rotate in any direction. The best example of this is a column placed in concrete which can’t twist, rotate or displace. A fixed support in three dimensional model will have 6 degrees of freedom restrained, which are three translations and three rotations in three orthogonal directions, X, Y and Z.
These are beneficial when you can only use a single support. The fixed support provides all constrains necessary to ensure the structure is static. It’s the only support which is used for stable cantilevers.
The greatest advantage provided by this support can also lead to its downfall as sometimes the structure requires a little deflection or some play to protect the surrounding materials. For example, as concrete continues to gain strength, it also expands. Hence it’s crucial that the support is designed correctly else the expansion could lead to reduction in durability.
Fixed support reactions
Beam fixed on the wall as an example
2. Roller support:
This support can’t resist the horizontal support but can resist the vertical support. This connection is free to move in horizontal direction as there is nothing restraining it.
The most common use of this support is in a bridge. Typically a bridge consists of a roller support at one end to account for the vertical displacement and expansion from changes in temperature. It’s required to prevent the expansion causing damage to a pinned support.
The roller support doesn’t resist horizontal force which acts as its limit as the structure will require another support to resist the horizontal force.
For a structure to be stable roller support is used along with pin support.
Roller support reactions
Roller support on one end of a bridge
3. Pinned support:
A pinned support is a common type of support in civil engineering. Like hinge, this support allows rotation to occur but not translation which means that it resists the horizontal and vertical forces but not a moment.
Pinned supports are widely used in trusses. By joining multiple members by pinned connections, the members push against each other which will induce an axial force within the member. The advantage of this support is that the members won’t have internal moment forces, and can be designed only according to their axial force.
The pinned support can’t completely resist a structure on its own as you need at least two supports to resist the moment coming on the structure.
Hinge support reactions
Hinge support in sydney harbor bridge
4. Internal Hinge
Interior hinges are often used to join flexural members at points other than supports. In some cases, it is employed deliberately so that the excess load breaks the weak zone rather than damaging other structural elements.
We are in the middle of a construction boom, fuelled by large civil engineering projects in India and the Middle East. This makes it a great time to work for a civil engineering company. There have rarely been so many opportunities in both developed countries and the developing world. This article is going to give you the lowdown on what the best companies offer to their employees and some criteria you can use to pick out the best of the bunch. It will also list ten companies that are recognised as being industry-leaders in civil engineering and score highly with employees.
Characteristics of a good civil engineering company
The top-ten companies listed below are obviously some of the larger, often internationally-respected, businesses. In reality, there are thousands of smaller companies, consultancies and agencies that you can work for. So, how do you make a decision on which vacancies to apply for? The first thing to consider is a company’s pedigree. In general, companies that are well established will be more accommodating to new staff than start-ups that haven’t yet found their feet. They are also more likely to offer good remuneration packages and benefits. Some of the bigger companies will offer perks like extra holidays, free health insurance and enhanced pension schemes.
Another thing to pay attention to is any staff satisfaction surveys or reviews that are available. Websites like www.glassdoor.co.uk can give you a good insight into what it’s like to work for a particular company. Finally, if you are going to be a site-based civil engineer, make sure you check out each company’s safety record. Look for companies that have robust health and safety processes in place and low accident rates. Let’s take a look at the top ten civil engineering companies to work for, based on a combination of the above criteria.
In a recent survey by the New Civil Engineer publication, 96% of employees agreed that Arup was great, and they had no desire to work anywhere else. That tells you something about the ethos and culture of the company. It offers excellent training and career progression and scores highly on pay and benefits. Arup is a well-established company with a large portfolio of construction and infrastructure projects in Europe and throughout the world, employing over 13,000 people in more than 30 countries. It is well known for its creative approach to structural design and is not afraid to innovate, making it a great company to work for if you relish a challenging position at the cutting-edge of engineering.
Atkins scored a healthy 7.4 out of 10 in a recent job-satisfaction survey, with employees particularly happy with the level of personal support and professional development. As Atkins is the main contractor on large projects such as London’s Crossrail, there will be plenty of opportunities to get stuck into interesting engineering jobs.
French construction company Vinci is one of the largest in the world, employing over 180,000 people globally. Their employees work on large international structural and infrastructure projects, including a multi-million dollar highway system in Atlanta, Georgia and large natural gas projects in Australia. Operating for over 115 years, Vinci definitely ticks the ‘well-established’ box and regularly scores highly on job satisfaction.
Mott McDonald is a fast-growing global construction and engineering company that regularly scores 80% or more on job-satisfaction surveys. It is an employee-owned company, which means that the culture is very people-centred and values professional development and collaboration very highly. It also boasts one of the best graduate training schemes, which consistently ranks highly in comparison tables.
Stantec is a globally renowned engineering firm that has a particularly large presence in North America and the UK. Employees praise the benefits system and the promotion of a work-life balance within the company.
Balfour Beatty specialises in large-scale infrastructure projects and has a solid global reputation for successful delivery. It has a strong focus on helping communities to grow and gets involved with positive initiatives such as local sustainability projects.
If you decide to work for Bechtel, you will probably be working on some of the most challenging engineering projects in the world, possibly in locations such as Africa, where Bechtel has a strong presence. It is a prestigious and world-leading company for structural design, construction and energy provision.
Skanska is a Swedish construction company that is highly regarded worldwide. Employees say that they are happy with the working environment at the company. One of the reasons regularly given is that Skanska is happy to give new recruits and graduate engineers positions of responsibility early on in their careers.
Laing O’Rourke has a large presence in Europe, the Middle East and Asia. Its graduate training programme is highly regarded and it is a company that promotes training and professional development, as well as the opportunity to work on high profile projects.
Arcadis is a large consultancy that focuses on environmental and sustainability projects, including design and build projects such as transit hubs that improve urban living. It’s a popular company to work for, offering a wide variety of projects to work on – ideal for those starting their career in civil engineering or who fancy a new challenge.
Over the last several thousand years, bridges have served one of the most important roles in the development of our earliest civilizations, spreading of knowledge, local and worldwide trade, and the rise of transportation.
Initially made out of most simple materials and designs, bridges soon evolved and enabled carrying of wide deckings and spanning of large distances over rivers, gorges, inaccessible terrain, strongly elevated surfaces and pre-built city infrastructures.
Starting with 13th century BC Greek Bronze Age, stone arched bridges quickly spread all around the world, eventually leading to the rise of the use of steel, iron and other materials in bridges that can span kilometers.
To be able to serve various roles, carry different types of weight, and span terrains of various sizes and complexities, bridges can strongly vary in their appearance, carrying capacity, type of structural elements, the presence of movable sections, construction materials and more.
Bridges by Structure
The core structure of the bridge determines how it distributes the internal forces of tension, compression, torsion, bending, and sheer . While all bridges need to handle all those forces at all times, various types of bridges will dedicate more of their capacity to better handle specific types of forces. The handling of those forces can be centralized in only a few notable structure members (such as with cable or cable-stayed bridge where forces are distributed in a distinct shape or placement) or be distributed via truss across the almost entire structure of the bridge.
Arch Bridges
Arch bridges – use arch as a main structural component (arch is always located below the bridge, never above it). With the help of mid-span piers, they can be made with one or more arches, depending on what kind of load and stress forces they must endure. The core component of the bridge is its abutments and pillars, which have to be built strong because they will carry the weight of the entire bridge structure and forces they convey.
Galena Creek Bridge, a cathedral arch bridge
Arch bridges can only be fixed, but they can support any decking fiction, including transport of pedestrians, light or heavy rail, vehicles and even be used as water-carrying aqueducts. The most popular materials for the construction of arch bridges are masonry stone, concrete, timber, wrought iron, cast iron and structural steel.
Examples of arch bridge are “Old Bridge” in Mostar, Bosnia, and Herzegovina, and The Hell Gate Bridge in New York. The oldest stone arch bridge ever is Greek Arkadiko Bridge which is over 3 thousand years old. The longest stone arch bridge is Solkan Bridge in Slovenia with an impressive span of 220 meters.
Beam Bridges
Beam bridges – employ the simplest of forms – one or several horizontal beams that can either simply span the area between abutments or relieve some of the pressure on structural piers. The core force that impacts beam bridges is the transformation of vertical force into shear and flexural load that is transferred to the support structures (abutments or mid-bridge piers).
Rio Grande in Las Cruces bridge
Because of their simplicity, they were the oldest bridges known to man. Initially built by simply dropping wooden logs over short rivers or ditches, this type of bridge started being used extensively with the arrival of metalworks, steel boxes, and pre-stressed construction concrete. Beam bridges today are separated into girder bridges, plate girder bridges, box girder bridges and simple beam bridges.
Individual decking of the segmented beam bridge can be of the same length, variable lengths, inclined or V-shaped. The most famous example of beam bridge is Lake Pontchartrain Causeway in southern Louisiana that is 23.83 miles (38.35 km) long.
Truss bridges – is a very popular bridge design that uses a diagonal mesh of most often triangle-shaped posts above the bridge to distribute forces across almost entire bridge structure. Individual elements of this structure (usually straight beams) can endure dynamic forces of tension and compression, but by distributing those loads across entire structure, entire bridge can handle much stronger forces and heavier loads than other types of bridges.
Common types of truss bridges
The two most common truss designs are the king posts (two diagonal posts supported by single vertical post in the center) and queen posts (two diagonal posts, two vertical posts and horizontal post that connect two vertical posts at the top). Many other types of the truss are in use – Allan, Bailey, Baltimore, Bollman, Bowstring, Brown, Howe, Lattice, Lenticular, Pennsylvania, Pratt, and others.
Admiral T.J. Lopez Bridge
Truss bridges were introduced very long ago, immediately becoming one of the most popular bridge types thanks to their incredible resilience and economic builds that require a very small amount of material for construction. The most common build materials used for truss bridge construction are timber, iron, steel, reinforced concrete and prestressed concrete. The truss bridges can be both fixed and moveable.
Cantilever Bridges
Cantilever bridges – are somewhat similar in appearance to arch bridges, but they support their load, not through a vertical bracing but trough diagonal bracing with horizontal beams that are being supported only on one end. The vast majority of cantilever bridges use one pair of continuous spans that are placed between two piers, with beams meeting on the center over the obstacle that bridge spans (river, uneven terrain, or others). Cantilever bridge can also use mid-bridge pears are their foundation from which they span in both directions toward other piers and abutments.
Howrah Bridge, Kolkata
The size and weight capacity of the cantilever bridge impact the number of segments it uses. Simple pedestrian crossings over very short distances can use simple cantilever beam, but larger distances can use either two beams coming out of both abutments or multiple center piers. Cantilever bridges cannot span very large distances. They can be bare or use truss formation both below and above the bridge, and most popular constriction material are structural steel, iron, and prestressed concrete.
Same of the most famous cantilever bridges in the world are Quebec Bridge in Canada, Forth Bridge in Scotland and Tokyo Gate bridge in Japan.
Tokyo Gate bridge in Japan
Tied Arch Bridges
Tied arch bridges – are similar in design to arch bridges, but they transfer the weight of the bridge and traffic load to the top chord that is connected to the bottom cords in bridge foundation. The bottom tying cord can be reinforced decking itself or a separate deck-independent structure that interfaces with tie-rods.
Generic tied-arch bridge with a movable support on the right side
They are often called bowstring arches or bowstring bridges and can be created in several variations, including shouldered tied-arch, multi-span discrete tied-arches, multi-span continuous tied-arches, single tied-arch per span and others. However, there is a precise differentiation between tied arch bridges and bowstring arch bridges – the latter use diagonally shaped members who create a structure that transfer forces similar to in truss bridges.
Tied arch bridges can be visually very stunning, but they bring with them costly maintenance and repair.
The Fort Pitt Bridge is a tied-arch bridge. The arches terminate atop slender raised piers and are tied by the road deck structure
Suspension Bridges
Suspension bridges – utilize spreading ropes or cables from the vertical suspenders to hold the weight of bridge deck and traffic. Able to suspend decking over large spans, this type of bridge is today very popular all around the world.
View of the Chain Bridge invented by James Finley Esq.” (1810) by William Strickland. Finley’s Chain Bridge at Falls of Schuylkill (1808) had two spans, 100 feet and 200 feet
Originally made even in ancient times with materials such as ropes or vines, with decking’s of wood planks or bamboo, the modern variants use a wide array of materials such as steel wire that is either braided into rope or forged or cast into chain links. Because only abutments and piers (one or more) are fixed to the ground, the majority of the bridge structure can be very flexible and can often dramatically respond to the forces of wind, earthquake or even vibration of on-foot or vehicle traffic.
Some of the most famous examples of suspension bridges are Golden Gate Bridge in San Francisco, Akashi Kaikyō Bridge in Japan, and Brooklyn Bridge in New York City.
Akashi Kaikyō Bridge in Japan
Cable-Stayed Bridges
Cable-stayed bridges – use deck cables that are directly connected to one or more vertical columns (called towers or pylons) that can be erected near abutments or in the middle of the span of the bridge structure. Cables are usually connected to columns in two ways – harp design (each cable is attached to the different point of the column, creating the harp-like “strings” and “fan” designs (all cables connect to one point at the top of the column). This is a very different type of cable-driven suspension than in suspension bridges, where decking is held with vertical suspenders that go up to main support cable.
Suspension bridge
Cable-stayed bridge, fan design
Originally constructed and popularized in the 16th century, today cable-stayed bridges are a popular design that is often used for spanning medium to long distances that are longer than those of cantilever bridges but shorter than the longest suspension bridges. The most common build materials are steel or concrete pylons, post-tensioned concrete box girders and steel rope. These bridges can support almost every type of decking (only not including heavy rail) and are used extensively all around the world in several construction variations.
The famous Brooklyn Bridge is a suspension bridge, but it also has elements of cable-stayed design.
Brooklyn Bridge
Fixed or Moveable Types
The vast majority of all bridges in the world are fixed in place, without any moving parts that forces them to remain in place until they are demolished or fall due to unforeseen stress or disrepair. However, some spaces are in need of multi-purpose bridges which can either have movable parts or can be completely moved from one location to another. Even though these bridges are rare, they serve an important function that makes them highly desirable.
Fixed Bridges
Fixed – Majority of bridges constructed all around the world and throughout our history are fixed, with no moveable parts to provide higher clearance for river/sea transport that is flowing below them. They are designed to stay where they are made to the time they are deemed unusable due to their age, disrepair or are demolished. Use of certain materials or certain construction techniques can instantly force bridge to be forever fixed. This is most obvious with bridges made out of construction masonry, suspension and cable-stayed bridges where a large section of decking surface is suspended in the air by the complicated network of cables and other material.
Small and elevated bridges like Bridge of Sighs, ancient stone aqueducts of Rome such as Pont du Gard, large medieval multi-arched Charles Bridge, and magnificent Golden Gate Bridge are all examples of bridges that are fixed.
Temporary Bridges
Temporary bridges – Temporary bridges are made from basic modular components that can be moved by medium or light machinery. They are usually used in military engineering or in circumstances when fixed bridges are repaired, and can be so modular that they can be extended to span larger distances or even reinforced to support heightened loads. The vast majority of temporary bridges are not intended to be used for prolonged periods of time on single locations, although in some cases they may become a permanent part of the road network due to various factors.
The simples and cheapest temporary bridges are crane-fitted decking made out of construction wood that can facilitate passenger passage across small spans (such as ditches). As the spans go longer and loads are heightened, prefabricated bridges made out of steel and iron have to be used. The most capable temporary bridges can span even distances of 100m using reinforced truss structure that can facilitate even heavy loads.
Moveable Bridges
Moveable bridges – Moveable bridges are a compromise between the strength, carrying capacity and durability of fixed bridges, and the flexibility and modularity of the temporary bridges. Their core functionality is providing safe passage of various types of loads (from passenger to heavy freight), but with the ability to move out of the way of the boats or other kinds of under-deck traffic which would otherwise not be capable of fitting under the main body of the bridge.
Movable Bridge in Chicago, USA
Most commonly, movable bridges are made with simple truss or tied arch design and are spanning rivers with little to medium clearance under their main decks. When the need arises, they can either lift their entire deck sharply in the air or sway the deck structure to the side, opening the waterway for unrestricted passage of ships. While the majority of the moveable bridges are small to medium size, large bridges also exist.
The most famous moveable bridge in the world is London Tower Bridge, whose clearance below the decking rises from 8.6m to 42.5m when opened.
Types by Use
When thinking about bridges, everyone’s first thought are structures that facilitate easy passenger and car traffic across bodies of water or unfriendly terrain. However, bridges can be versatile and can support many different types of use. Additionally, some bridges are designed in such way to support multiple types of use, combining, for example, multiple car traffic lanes and pedestrian or bicycle passageways (such as a present on the famous Brooklyn Bridge in New York City).
Pedestrian Bridges
Pedestrian bridges – The oldest bridges ever made were designed to facilitate passenger travel over small bodies of water or unfriendly terrain. Today, they are usually made in urban environments or in terrain where car transport is inaccessible (such as rough mountainous terrain, forests, swamps, etc.). Since on-the foot or bicycle passenger traffic does not strain the bridges with much weight, designs of those bridges can be made to be more extravagant, elegant, sleek and better integrated with the urban environment or created with cheaper or less durable materials. Many modern pedestrian-only bridges are made out of modern material, while some tourist pedestrian bridges feature more exoteric designs that even include transparent polymers in the decking, enabling users unrestricted view to the area below the bridge.
Charles Bridge as viewed from Petřínská rozhledna
While the majority of modern pedestrian bridges were made from the start to facilitate only on-foot access (such as Venice’s Ponte Vecchio and Rialto bridge), other bridges can be transformed from other purposes to pedestrian-only function (such as Prague’s historic Charles bridge).
Car Traffic
Car Traffic – This is the most common usage of the bridge, with two or more lanes designed to carry car and truck traffic of various intensities. Modern large bridges usually feature multiple lanes that facilitate travel in a single direction, and while the majority of bridges have a single decking dedicated to car traffic, some can even have an additional deck, enabling each deck to be focused on providing travel in a single direction.
Double-decked Bridges
Double-decked bridges – Multi-purpose bridges that provide an enhanced flow of traffic across bodies of water or rough terrain. Most often they have a large number of car lanes, and sometimes have dedicated area for train tracks. For example, in addition to multiple car lanes on the main decking, famous Brooklyn Bridge in NYC features an isolated bicycle path.
Train Bridges
Train bridges – Bridges made specifically to carry one or multiple lanes of train tracks, although in some cases train tracks can also be placed beside different deck type, or on different decking elevation. After car bridges, train bridges are the second-most-common type of bridges.
Cikurutug Bridge, Indonesia
First train bridges started being constructed during the early years of European Industrial Revolution as means of enabling faster shipment of freight between ore mines and ironworks factories. With the appearance of safe passenger locomotives and cars, the rapid expansion of railway networks all around Europe, US and Asia brought the need for building thousands of railway bridges of various sizes and spans.
Pipeline Bridges
Pipeline Bridges – Less common as a standalone bridge type, pipeline bridges are constructed to carry pipelines across water or inaccessible terrains. Pipelines can carry water, air, gas and communication cables. In modern times, pipeline networks are usually incorporated in the structure of existing or newly built bridges that also house regular decking that facilitates pedestrian, car or railway transport.
A pipeline bridge carrying the Trans-Alaska Pipeline
Pipeline bridges are usually very lightweight and can be supported only with the basic suspension bridge construction designs. In many cases, they are also equipped with walkways, but they are almost exclusively dedicated for maintenance purposes and are not intended for public use.
Aqueducts
Aqueducts – are ancient bridge-like structures that are part of the larger viaduct networks intended to carry water from water-rich areas to sometimes very distant dry cities. Because of the need to maintain a low but constant drop of elevation of the main water-carrying passageway, aqueducts are very precisely created structures that sometimes need to reach very high elevations and maintain rigid structure while spanning large distances. The largest aqueducts are made of stone and can have multiple tiers of arched bridges created one on top of each other.
The aqueduct at Querétaro city
The modern equivalent of the ancient aqueduct bridges are pipeline bridges, but while the viaduct network used natural force of gravity to push water toward the desired destination, modern pipeline networks use electric pumps to propel water and other material.
Commercial Bridges
Commercial bridges – These are bridges that host commercial buildings such as restaurants and shops. Most commonly used in medieval bridges created in urban environments where they took advantage of the constant flow of pedestrian traffic, today these kinds of bridges are rarely constructed with a notable amount of them being found in modern India. Slovakia’s city of Bratislava is a home of a car passageway bridge with a large tower that hosts a restaurant on top of it.
Medieval bridges are much more commonly known for their commercial applications. Italy is home to two of the best known commercial bridges in the world – the famous multi-tiered Ponte Vecchio in the city center of Florence, and brilliant white Rialto Bridge that spans the scenic Grand Canal in Venice. Both feature numerous shops that offer tourist memorabilia and jewelry.
Types by Materials
The core function of the bridge is to span a stable decking intended for the transport of pedestrians, cars or trains while enduring weight of its core structure, the weight of the traffic, and the natural forces that slowly but surely erode its durability. Various materials can help bridge designers to achieve their goal, and provide stable and long-lasting bridges that require varying levels of maintenance (and in cases of historic bridges, restorations). Here is the breakdown of all the common types of materials that are used in historical and modern bridge building:
Natural Materials
Bridges of natural materials – The first bridges ever made were constructed from unprocessed natural materials, starting from simple wooden logs that were placed across small rivers or ditches, to the large rope-tied bridges that are constructed over large canyons and mountain ranges in inhospitable areas of Asia.
Wood
Wood (Wooden bridges) – Wood is an excellent material that can be used for the creation of small to medium-sized bridges that are best suited for pedestrian or low-weight car transport. In modern times, wooden bridges are most commonly found for spanning short distances or being used to transport people, cars, and livestock over rough terrain or small rivers in Covered Bridges.
Stone
Stone (Stone bridges) – Stone is an excellent long-lasting natural material that can be used for the construction of bridges that can last for centuries. Stone pieces can even be used to construct very large bridge structures that don’t even use concrete – such as in Pont du Gard aqueduct in southern France that uses the weight of individual stones to make an entire 48.8 m high and 275 m structure stable for two thousand years.
Concrete and Steel
Concrete and Steel bridges – Durable, long-lasting and highly versatile modern materials that are today used for the creation of countless types of bridge designs. Coupled with the presence of cables and other modern materials, these types of bridges represent the majority of all the bridges that are currently in public pedestrian, car, and train transport use today.
Advanced Materials
Bridges of advanced materials – As decades go on, modern industry enables bridge builders to gain access to wide array of advanced materials that offer noticeable advantages over traditional construction processes.
A Two-Way Slab “TWS” is simply a slab carrying loads in both directions (length and width) to its supports (Beams or Columns). This is because it’s length to width ratio is (Long/Short) not big enough for loads to travel in the short direction only (width).
Theoretically speaking:
Loads are lazy, they travel in the shortest path possible, they like to spend the least effort.
Slabs have 4 supports to transfer its loads to (2 in each direction).
Slab: A structural element that has large width and length compared to its thickness. Designed to carry whatever loads the building is meant to carry, and transfer it to its the supporting elements.
Loads: Slab’s own weight, Slab covering and finish, live loads (people, furniture, machines…etc) etc.
Supports: Beams or Columns that the Slab is resting on at its 4 edges (could be on . The slab transfers its loads to the supports > Supports transfer its loads to the foundations on the ground > Foundations transfer all these loads to the grounds below the building.
That being said, let’s assume a (10 x 2) Rectangular Slab (width=2m and length=10m) the ratio is large enough (Long/Short = 10/2 = 5) in this case all the loads will travel much faster through the Short Direction which is the 2m width to the nearest supports.
This makes a One Way Slab “OWS”. On this slab loads will not travel in the Long Direction (the 10m length). The slab will carry 100% of the loads to half of its supports through one direction only. (supports on the other direction will not receive any loads from the slab, therefore these slabs can have 2 supports only at the short direction).
The Length is larger than Double the Width (Long/Short > 2)
100% of the Loads Travel in the Short Direction only.
Only the 2 Supports on this Short Direction will carry the slab and 100% of its the loads.
We need Reinforcement (steel bars or other) in this direction only to resist the straining actions caused by loads traveling through this direction to the supports.
We will study and design based on one direction only.
Slab thickness is design based on short direction only.
Now assume a (5 x 5) Square Slab with ratio (Long/Short = 5/5 =1), in this case loads will travel in both directions equally, since Long=Short they will spend almost equal effort. Each direction will carry 50% of the loads to its supports.
This makes a Two Way Slab “TWS”. In this case all 4 supports are working and they receive almost equal loads from the slab. (The Slab must be supported from all directions).
The Length is equal to the Width (Long/Short = 1)
Loads Travel in Both Directions. 50% of the total loads through each direction.
All supports (2 pairs in each direction) will carry half of the slab and its loads.
We need equal reinforcement in both directions.
We will study two directions, but since they are equal, we can design one direction and apply the same to the other.
Slab thickness is design based on either direction, since they are equal.
If we have something in between the above two cases, assume a (4×6) Rectangular slab (width=4m, length=6m) Which direction will the loads travel through?
The ratio is (Long/Short = 6/4 = 1.5) which is smaller than double the width (1.5 < 2) which means this a “TWS” as well.
The number we compare to the length to width ratio (2) might be different depending on the Application and the Code we are following (which is based on studies and experiments), but here we assume that the breakpoint is the length being over or under double the width.
So this is also a “TWS” where we predict loads travel in both directions, but will each direction carry 50% of the loads? We know that one direction is shorter than the other, therefore more of our loads will prefer to travel through the easier-shorter direction, and less will travel through the longer direction. Supports will receive a percentage of the Slab loads based on the distance it has to travel.
The Length is smaller than Double the Width (Long/Short <2)
Loads Travel in Both Directions. The short direction transfers a larger percentage of the loads, the long direction transfers a smaller percentage of the loads.
All support (2 pairs in each direction) will carry the slab, but each pair will receive different percentage of the loads.
We need more reinforcement in the short direction (larger percentage of loads > larger bending moment) than we need in the long direction (smaller percentage of loads > smaller bending moment)
We will study and design each direction separately (different loads, different bending moments, different reinforcement)
Slab thickness is designed based on the worst case, the larger straining actions we calculate from the Short or Long direction (most probably the short direction)
The exceptional durability of portland cement concrete is a major reason why it is the world’s most widely used construction material. But material limitations, design and construction practices, and severe exposure conditions can cause concrete to deteriorate, which may result in aesthetic, functional, or structural problems.
Concrete can deteriorate for a variety of reasons, and concrete damage is often the result of a combination of factors. The following summary discusses potential causes of concrete deterioration and the factors that influence them.
Corrosion of reinforcing steel and other embedded metals is the leading cause of deterioration in concrete. When steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which can eventually cause cracking, delamination, and spalling (Figs. 1 and 2).
Fig. 1. Corrosion of reinforcing steel is the most common cause of concrete deterioration.
Fig. 2. The expansion of corroding steel creates tensile stresses in the concrete, which can cause cracking,
delamination, and spalling.
Steel corrodes because it is not a naturally occurring material.Rather, iron ore is smelted and refined to produce steel. The production steps that transform iron ore into steel add energy to the metal. Steel, like most metals except gold and platinum, is thermodynamically unstable under normal atmospheric conditions and will release energy and revert back to its natural state — iron oxide, or rust. This process is called corrosion.
For corrosion to occur, four elements must be present: There must be at least two metals (or two locations on a single metal) at different energy levels, an electrolyte, and a metallic connection. In reinforced concrete, the rebar may have many separate areas at different energy levels. Concrete acts as the electrolyte, and the metallic connection is provided by wire ties, chair supports, or the rebar itself.
a – Concrete and the Passivating Layer
Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1 μm per year.
Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI 222 2001). Because of concrete’s inherent protection, reinforcing steel does not corrode in the majority of concrete elements and structures.
However, corrosion can occur when the passivating layer is destroyed. The destruction of the passivating layer occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level.
b – The Role of Chloride Ions
Exposure of reinforced concrete to chloride ions is the primary cause of premature corrosion of steel reinforcement. The intrusion of chloride ions, present in deicing salts and seawater, into reinforced concrete can cause steel corrosion if oxygen and moisture are also available to sustain the reaction (Fig. 3). Chlorides dissolved in water can permeate
through sound concrete or reach the steel through cracks.
Fig. 3. Deicing salts are a major cause of corrosion of reinforcing steel in concrete.
Chloride-containing admixtures can also cause corrosion. No other contaminant is documented as extensively in the literature as a cause of corrosion of metals in concrete than chloride ions. The mechanism by which chlorides promote corrosion is not entirely understood, but the most popular theory is that chloride ions penetrate the protective oxide film easier than do other ions, leaving the steel vulnerable to corrosion.
c – Carbonation
Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. In the reaction with calcium hydroxide, calcium carbonate is formed:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction reduces the pH of the pore solution to as low as 8.5, at which level the passive film on the steel is not stable. Carbonation is generally a slow process. In high-quality concrete, it has been estimated that carbonation will proceed at a rate up to 1.0 mm (0.04 in.) per year. The amount of carbonation is significantly increased in concrete with a high water-to-cement ratio, low cement content, short curing period, low strength, and highly permeable or porous paste.
Carbonation is highly dependent on the relative humidity of the concrete. The highest rates of carbonation occur when the relative humidity is maintained between 50% and 75%. Below 25% relative humidity, the degree of carbonation that takes place is considered insignificant. Above 75% relative humidity, moisture in the pores restricts CO2 penetration (ACI 201 1992).
Carbonation-induced corrosion often occurs on areas of building facades that are exposed to rainfall, shaded from sunlight, and have low concrete cover over the reinforcing steel (Fig. 5).
Fig. 4. Carbonation-induced corrosion often occurs on building facades with shallow concrete cover.
d – Dissimilar Metal Corrosion
When two different metals, such as aluminum and steel, are in contact within concrete, corrosion can occur because each metal has a unique electrochemical potential. A familiar type of dissimilar metal corrosion occurs in an ordinary flashlight battery. The zinc case and carbon rod are the two metals, and the moist paste acts as the electrolyte. When the carbon and zinc are connected by a wire, current flows. In reinforced concrete, dissimilar metal corrosion can occur in balconies where embedded aluminum railings are in contact with the reinforcing steel.
Below is a list of metals in order of electrochemical activity:
Zinc / Aluminum / Steel / Iron / Nickel / Tin / Lead / Brass / Copper / Bronze / Stainless Steel / Gold
When the metals are in contact in an active electrolyte, the less active metal (lower number) in the series corrodes.
2. FREEZE-THAW DETERIORATION
When water freezes, it expands about 9%. As the water in moist concrete freezes, it produces pressure in the capillaries and pores of the concrete. If the pressure exceeds the tensile strength of the concrete, the cavity will dilate and rupture. The accumulative effect of successive freeze-thaw cycles and disruption of paste and aggregate can eventually cause significant expansion and cracking, scaling, and crumbling of the concrete (Fig. 5).
The resistance of concrete to freezing and thawing in a moist condition is significantly improved by the use of intentionally entrained air. Entrained air voids act as empty chambers in the paste for the freezing and migrating water to enter, thus relieving the pressure in the capillaries and pores and preventing damage to the concrete.
Concrete with low permeability is also better able resist the penetration of water and, as a result, performs better when
exposed to freeze-thaw cycles. The permeability of concrete is directly related to its water-to-cement ratio—the lower the water-to-cement ratio, the lower the permeability of the concrete.
Fig 5. Freeze-thaw cycles can cause scaling of concrete surfaces
a – Deicer Scaling
Deicing chemicals used for snow and ice removal, such as sodium chloride, can aggravate freeze-thaw deterioration. The additional problem caused by deicers is believed to be a buildup of osmotic and hydraulic pressures in excess of the normal hydraulic pressures produced when water in concrete freezes. In addition, because salt absorbs moisture, it keeps the concrete more saturated, increasing the potential for freeze-thaw deterioration. However, properly designed and placed air-entrained concrete can withstand deicers for many years.
In the absence of freezing, sodium chloride has little to no chemical effect on concrete. Weak solutions of calcium chloride generally have little chemical effect on concrete, but studies have shown that concentrated calcium chloride solutions can chemically attack concrete. Magnesium chloride deicers have come under recent criticism for aggravating scaling. One study found that magnesium chloride, magnesium acetate, magnesium nitrate, and calcium chloride are more damaging to concrete than sodium chloride (Cody, Cody, Spry, and Gan 1996). Deicers containing ammonium nitrate and ammonium sulfate should be prohibited because they rapidly attack and disintegrate concrete.
b – Aggregate Expansion
Some aggregates may absorb so much water (to critical saturation) that they cannot accommodate the expansion and
hydraulic pressure that occurs during the freezing of water. The result is expansion of the aggregate and possible disintegration of the concrete if enough of the offending particles are present. If a problem particle is near the surface of the concrete, it can cause a popout.
D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more years of service. Due to the natural accumulation of water in the base and subbase of pavements, the aggregate may eventually become saturated. Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate at the bottom of the slab and progresses upward until it reaches the wearing surface. D-cracking usually starts near pavement joints.
Aggregate freeze-thaw problems can often be reduced by either selecting aggregates that perform better in freeze-thaw cycles or, where marginal aggregates must be used, reducing the maximum particle size.
Fig 6. D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more
years of service.
3. CHEMICAL ATTACK
Concrete performs well when exposed to various atmospheric conditions, water, soil, and many other chemical exposures. However, some chemical environments can deteriorate even high-quality concrete. Concrete is rarely, if ever, attacked by solid, dry chemicals. To produce significant attack on concrete, aggressive chemicals must be in solution and above some minimum concentration.
a – Acids
In general, portland cement concrete does not have good resistance to acids. In fact, no hydraulic cement concrete, regardless of its composition, will hold up for long if exposed to a solution with a pH of 3 or lower. However, some weak acids can be tolerated, particularly if the exposure is occasional.
Acids react with the calcium hydroxide of the hydrated portland cement. In most cases, the chemical reaction forms water-soluble calcium compounds, which are then leached away by aqueous solutions (ACI 201 1992).
The products of combustion of many fuels contain sulfurous gases which combine with moisture to form sulfuric acid. Also, certain bacteria convert sewage into sulfuric acid. Sulfuric acid is particularly aggressive to concrete because the calcium sulfate formed from the acid reaction will also deteriorate concrete via sulfate attack (Fig. 7).
Fig. 7. Bacteria in sewage systems can produce sulfuric acid, which aggressively attacks concrete
In addition to individual organic and mineral acids which may attack concrete, acid-containing or acid-producing substances, such as acidic industrial wastes, silage, fruit juices, and sour milk, will also cause damage.
Animal wastes contain substances which may oxidize in air to form acids which attack concrete. The saponification reaction between animal fats and the hydration products of portland cement consumes these hydration products, producing salts and alcohols, in a reaction analogous to that of acids. Acid rain, which often has a pH of 4 to 4.5, can slightly etch concrete, usually without affecting the performance of the exposed surface.
Any water that contains bicarbonate ion also contains free carbon dioxide, a part of which can dissolve calcium carbonate unless saturation already exists. This part is called the “aggressive carbon dioxide.” Water with aggressive carbon dioxide acts by acid reaction and can attack concrete and other portland cement products whether or not they are carbonated.
Calcium-absorptive acidic soil can attack concrete, especially porous concrete. Even slightly acidic solutions that are lime-deficient can attack concrete by dissolving calcium from the paste, leaving behind a deteriorated paste consisting primarily of silica gel.
To prevent deterioration from acid attack, portland cement concrete generally must be protected from acidic environments with surface protective treatments. Unlike limestone and dolomitic aggregates, siliceous aggregates are acid-resistant and are sometimes specified to improve the chemical resistance of concrete, especially with the use of chemical-resistant cement. Properly cured concrete with reduced permeability experience a slightly lower rate of attack from acids.
b – Salts and Alkalis
The chlorides and nitrates of ammonium, magnesium, aluminum, and iron all cause concrete deterioration, with those of ammonium producing the most damage. Most ammonium salts are destructive because, in the alkaline environment of concrete, they release ammonia gas and hydrogen ions. These are replaced by dissolving calcium hydroxide from the concrete. The result is a leaching action, much like acid attack. Strong alkalies (over 20 percent) can also cause concrete disintegration (ACI 515 1979).
c – Sulfate Attack
Naturally occurring sulfates of sodium, potassium, calcium, or magnesium are sometimes found in soil or dissolved in ground-water. Sulfates can attack concrete by reacting with hydrated compounds in the hardened cement. These reactions can induce sufficient pressure to disrupt the cement paste, resulting in loss of cohesion and strength.
Calcium sulfate attacks calcium aluminate hydrate and forms ettringite. Sodium sulfate reacts with calcium hydroxide and calcium aluminate hydrate forming ettringite and gypsum. Magnesium sulfate attacks in a manner similar to sodium sulfate and forms ettringite, gypsum, and brucite (magnesium hydroxide). Brucite forms primarily on the concrete surface, consumes calcium hydroxide, lowers the pH of the pore solution, and then decomposes the calcium silicate hydrates.
Environmental conditions have a great influence on sulfate attack. The attack is greater in concrete exposed to wet/dry
cycling (Fig. 8). When water evaporates, sulfates can accumulate at the concrete surface, increasing in concentration
and their potential for causing deterioration.
Fig 8. The bases of these concrete posts have suffered from sulfate attack
Porous concrete is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of field concrete include sodium carbonate and sodium sulfate (laboratory studies have also related saturated solutions of calcium chloride and other salts to concrete deterioration). Under drying conditions, salt solutions can rise to the surface by capillary action and, as a result of surface evaporation, the solution phase
becomes supersaturated and salt crystallization occurs, sometimes generating pressures large enough to cause cracking and scaling (Mehta 2000).
Sulfate attack is a particular problem in arid areas, such as the Northern Great Plains and parts of the Western United States. Seawater also contains sulfates but is not as severe an exposure as sulfates in groundwater.
4. ALKALI-AGGREGATE REACTIVITY
In most concrete, aggregates are more or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of years. This alkali-aggregate reactivity has two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). ASR is of more concern than ACR because aggregates containing reactive silica materials are more common.
a – Alkali-Silica Reactivity
Aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it draws water from the surrounding cement paste or the environment. In absorbing water, these gels can swell and induce enough expansive pressure to damage concrete:
1. Alkalies + Reactive Silica → Gel Reaction Product
2. Gel Reaction Product + Moisture → Expansion
Typical indicators of alkali-silica reactivity are map (random pattern) cracking and, in advanced cases, closed joints and spalled concrete surfaces (Fig. 9). Cracking usually appears in areas with a frequent supply of moisture, such as close to the waterline in piers, from the ground behind retaining walls, near joints and free edges in pavements, or in piers or columns subject to wick action.
Because sufficient moisture is needed to promote destructive expansion, alkali-silica reactivity can be significantly reduced by keeping the concrete as dry as possible. The reactivity can be virtually stopped if the internal relative humidity of the concrete is kept below 80%. In most cases, however, this condition is difficult to achieve and maintain. Warm seawater, due to the presence of dissolved alkalies, can particularly aggravate alkali-silica reactivity.
Fig. 9. Typical indicators of alkali-silica reactivity are map cracking and, in advanced cases, closed joints and spalled
concrete surfaces
b – Alkali-Carbonate Reactivity
Reactions observed with certain dolomitic rocks are associated with alkali-carbonate reaction (ACR). Dedolomitization, or the breaking down of dolomite, is normally associated with expansive alkali-carbonate reactivity. This reaction and subsequent crystallization of brucite may cause considerable expansion.
The deterioration caused by alkali-carbonate reaction is similar to that caused by alkali-silica reaction (Fig. 10); however, alkali-carbonate reaction is relatively rare because aggregates susceptible to this reaction are less common and are usually unsuitable for use in concrete for other reasons, such as strength potential.
Fig. 10. Map cracking pattern caused by alkali-carbonate reactivity.
5. ABRASION/EROSION
Abrasion damage occurs when the surface of concrete is unable to resist wear caused by rubbing and friction. As the outer paste of concrete wears, the fine and coarse aggregate are exposed and abrasion and impact will cause additional degradation that is related to aggregate-to-paste bond strength and hardness of the aggregate.
Although wind-borne particles can cause abrasion of concrete, the two most damaging forms of abrasion occur on vehicular traffic surfaces and in hydraulic structures, such as dams, spillways, and tunnels.
a – Traffic Surfaces
Abrasion of floors and pavements may result from production operations or vehicular traffic. Many industrial floors are subjected to abrasion by steel or hard rubber wheeled traffic, which can cause significant rutting.
Tire chains and studded snow tires cause considerable wear to concrete surfaces (Fig. 11). In the case of tire chains, wear is caused by flailing and scuffing as the rotating tire brings the metal in contact with the concrete surface.
Fig 11. Tire chains and studded snow tires can cause considerable wear to concrete surfaces
b – Hydraulic Structures
Abrasion damage in hydraulic structures is caused by the abrasive effects of waterborne silt, sand, gravel, rocks, ice, and other debris impinging on the concrete surface. Although high-quality concrete can resist high water velocities for many years with little or no damage, the concrete may not withstand the abrasive action of debris grinding or repeatedly impacting on its surface.
In such cases, abrasion erosion ranging from a few millimeters (inches) to several meters (feet) can result, depending on flow conditions. Spillway aprons, stilling basins, sluiceways, drainage conduits or culverts, and tunnel linings are particularly susceptible to abrasion erosion. Abrasion erosion is readily recognized by its smooth, worn appearance, which is distinguished from the small holes and pits formed by cavitation erosion.
As is the case with traffic wear, abrasion damage in hydraulic structures can be reduced by using strong concrete with hard aggregates. Cavitation is the formation of bubbles or cavities in a liquid. In hydraulic structures, the liquid is water and the cavities are filled with water vapor and air. The cavities form where the local pressure drops to a value that will cause the water to vaporize at the prevailing fluid temperature. Cavitation damage is produced when the vapor cavities collapse, causing very high instantaneous pressures that impact on the concrete surfaces, causing pitting, noise, and vibration.
6. FIRE/HEAT
Concrete performs exceptionally well at the temperatures encountered in almost all applications. But when exposed to fire or unusually high temperatures, concrete can lose strength and stiffness (Fig. 12).
Fig. 12. When exposed to fire or unusually high temperatures, concrete can lose strength and stiffness
Numerous studies have found the following general trends:
• Concrete that undergoes thermal cycling suffers greater loss of strength than concrete that is held at a constant temperature, although much of the strength loss occurs in the first few cycles. This is attributed to incompatible dimensional changes between the cement paste and the aggregate.
• Concrete that is under design load while heated loses less strength than unloaded concrete, the theory being that
imposed compressive stresses inhibit development of cracks that would be free to develop in unrestrained concrete.
• Concrete that is allowed to cool before testing loses more compressive strength than concrete that is tested hot. Concrete loses more strength when quickly cooled (quenched) from high temperatures than when it is allowed to cool
gradually.
• Concrete containing limestone and calcareous aggregates performs better at high temperatures than concrete containing siliceous aggregates (Abrams 1956). One study showed no difference in the performance of dolostone and limestone (Carette 1982). Another study showed the following relative aggregate performance, from best to worst: firebrick, expanded shale, limestone, gravel, sandstone and expanded slag.
• Proportional strength loss is independent of compressive strength of concrete.
• Concrete with a higher aggregate-cement ratio suffers less reduction in compressive strength; however, the opposite is true for modulus of elasticity. The lower the water-cement ratio, the less loss of elastic modulus.
• If residual water in the concrete is not allowed to evaporate, compressive strength is greatly reduced. If heated too quickly, concrete can spall as the moisture tries to escape.
7. RESTRAINT TO VOLUME CHANGES
Concrete changes slightly in volume for various reasons, the most common causes being fluctuations in moisture content and temperature. Restraint to volume changes, especially contraction, can cause cracking if the tensile stresses that develop exceed the tensile strength of the concrete.
a – Plastic Shrinkage Cracking
When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth (Fig. 12). These cracks are often fairly wide at the surface.
Fig. 13. Plastic shrinkage cracks can occur when water evaporates from the surface faster than it is replaced by bleedwater
Plastic shrinkage cracks can be prevented by taking measures to prevent rapid water loss from the concrete surface. Fog nozzles, plastic sheeting, windbreaks, and sunshades can all be used to prevent excessive evaporation.
Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. Restraint to shrinkage, provided by the subgrade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete. Restraint to drying shrinkage is the most common cause of concrete cracking.
In many applications, drying shrinkage cracking is inevitable. Therefore, control joints are placed in concrete to predetermine the location of drying shrinkage cracks. Drying shrinkage can be limited by keeping the water content of concrete as low as possible and maximizing the coarse aggregate content.
c – Thermal Cracking
Concrete expands when heated and contracts when cooled. An average value for the thermal expansion of concrete is about 10 millionths per degree Celcius (5.5 millionths per degree Fahrenheit). This amounts to a length change of 5 mm for 10 m of concrete ( 2 ⁄ 3 in. for 100 ft of concrete) subjected to a rise or fall of 50°C (90°F).
Thermal expansion and contraction of concrete varies with factors such as aggregate type, cement content, water-cement ratio, temperature range, concrete age, and relative humidity. Of these, aggregate type has the greatest influence.
Designers should give special consideration to structures in which some portions of the structure are exposed to temperature changes, while other portions are partially or completely protected. Allowing for movement by using properly designed expansion or isolation joints and correct detailing will help minimize the effects of temperature variations.
Construction Equipment Earthwork & Soil Compaction
1.Cable Excavator
Cable excavators are large earthmoving machines used for heavy excavation of materials with the use of cables or wire ropes. Almost obsolete, cable excavators were once used in mining and some construction applications and usually consisted of a variety of attachments that could transform it into a backhoe, skimmer, dragline, and more. Aside from a few companies that still manufacture them, cable excavators have been replaced by hydraulic excavators due to their cheaper costs, easier operation and faster mobility.
2. Hydraulic excavators (slewing excavators)
A hydraulic excavator (digger) is a large vehicle that is designed for excavation and demolition purposes. Hydraulic excavators consist of a chassis, boom, and bucket, and move via tracks or wheels. They range in size and function, an example of which is the similar but smaller “mini excavator.” All versions are generally designed for the same purposes. Hydraulic excavators weigh between 3,000 and 2 million pounds and their speed ranges between 19 HP and 4,500 HP.
3.Backhoe excavators
Backhoe is another widely used equipment which is suitable for multiple purposes. The name itself telling that the hoe arrangement is provided on the back side of vehicle while loading bucket is provided in the front.
This is well useful for excavating trenches below the machine level and using front bucket loading, unloading and lifting of materials can be done.
4. Bulldozers (dozers)
A bulldozer is a crawler (continuous tracked tractor) equipped with a substantial metal plate (known as a blade) used to push large quantities of soil, sand, rubble, or other such material during construction or conversion work and typically equipped at the rear with a claw-like device (known as a ripper) to loosen densely compacted materials.
Bulldozers can be found on a wide range of sites, mines and quarries, military bases, heavy industry factories, engineering projects and farms.
5. Scrapers
Scraper, in engineering, machine for moving earth over short distances (up to about two miles) over relatively smooth areas. Either self-propelled or towed, it consists of a wagon with a gate having a bladed bottom. The blade scrapes up earth as the wagon pushes forward and forces the excavated material into the wagon. When the wagon is filled, the gate is closed, and the material is carried to the place of disposal. The scraper is the dominant tool in highway construction.
6. Graders
Grader, in excavation, precision finishing vehicle for final shaping of surfaces on which pavement will be placed. Between its front and rear wheels a grader carries a broad mechanically or hydraulically controlled blade that can be extended from either side. Either end of the blade can be raised or lowered. Graders may be used for shallow ditching, but most models are used to assist other earth-moving equipment and to smooth roads, fills, and cuts.
7. Compactors
A compactor is a machine or mechanism used to reduce the size of material such as waste material or bio mass through compaction. A trash compactor is often used by a home or business to reduce the volume of trash it produces.
