Reaction of cement in concrete

Scientific Principles

What is in This Stuff?
The importance of concrete in modern society cannot be overestimated. Look around you and you will find concrete structures everywhere such as buildings, roads, bridges, and dams. There is no escaping the impact concrete makes on your everyday life. So what is it?
Concrete is a composite material which is made up of a filler and a binder. The binder (cement paste) "glues" the filler together to form a synthetic conglomerate. The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate. The role of these constituents will be discussed in this section.
Cement, as it is commonly known, is a mixture of compounds made by burning limestone and clay together at very high temperatures ranging from 1400 to 1600 [[ring]]C.
Although there are other cements for special purposes, this module will focus solely on portland cement and its properties. The production of portland cement begins with the quarrying of limestone, CaCO₃. Huge crushers break the blasted limestone into small pieces. The crushed limestone is then mixed with clay (or shale), sand, and iron ore and ground together to form a homogeneous powder. However, this powder is microscopically heterogeneous. (See flowchart.)

Figure 1: A flow diagram of Portland Cement production. The mixture is heated in kilns that are long rotating steel cylinders on an incline. The kilns may be up to 6 meters in diameter and 180 meters in length. The mixture of raw materials enters at the high end of the cylinder and slowly moves along the length of the kiln due to the constant rotation and inclination. At the low end of the kiln, a fuel is injected and burned, thus providing the heat necessary to make the materials react. It can take up to 2 hours for the mixture to pass through the kiln, depending upon the length of the cylinder.

Figure 2: Schematic diagram of rotary kiln. As the mixture moves down the cylinder, it progresses through four stages of transformation. Initially, any free water in the powder is lost by evaporation. Next, decomposition occurs from the loss of bound water and carbon dioxide. This is called calcination. The third stage is called clinkering. During this stage, the calcium silicates are formed. The final stage is the cooling stage.
The marble-sized pieces produced by the kiln are referred to as clinker. Clinker is actually a mixture of four compounds which will be discussed later. The clinker is cooled, ground, and mixed with a small amount of gypsum (which regulates setting) to produce the general-purpose portland cement.
Water is the key ingredient, which when mixed with cement, forms a paste that binds the aggregate together. The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process. The role of water is important because the water to cement ratio is the most critical factor in the production of "perfect" concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.. walls, domes, etc.). Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.
Aggregates are chemically inert, solid bodies held together by the cement. Aggregates come in various shapes, sizes, and materials ranging from fine particles of sand to large, coarse rocks. Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used. 70 to 80% of the volume of concrete is aggregate keeping the cost of the concrete low. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete. For example, the density of concrete is determined by the density of the aggregate. Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion.
Aggregates should be clean, hard, and strong. The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste. It is then separated into various sizes by passing the material through a series of screens with different size openings.
Refer to Demonstration 1

Table 1: Classes of Aggregates

class	examples of aggregates used	uses
ultra-lightweight	vermiculite ceramic spheres perlite	lightweight concrete which can be sawed or nailed, also for its insulating properties
lightweight	expanded clay shale or slate crushed brick	used primarily for making lightweight concrete for structures, also used for its insulating properties.
normal weight	crushed limestone sand river gravel crushed recycled concrete	used for normal concrete projects
heavyweight	steel or iron shot steel or iron pellets	used for making high density concrete for shielding against nuclear radiation

Refer to Demonstration 2
The choice of aggregate is determined by the proposed use of the concrete. Normally sand, gravel, and crushed stone are used as aggregates to make concrete. The aggregate should be well-graded to improve packing efficiency and minimize the amount of cement paste needed. Also, this makes the concrete more workable.
Refer to Demonstration 3
Properties of Concrete
Concrete has many properties that make it a popular construction material. The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal.
Good-quality concrete has many advantages that add to its popularity. First, it is economical when ingredients are readily available. Concrete's long life and relatively low maintenance requirements increase its economic benefits. Concrete is not as likely to rot, corrode, or decay as other building materials. Concrete has the ability to be molded or cast into almost any desired shape. Building of the molds and casting can occur on the work-site which reduces costs.
Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures. It is resistant to wind, water, rodents, and insects. Hence, concrete is often used for storm shelters.
Concrete does have some limitations despite its numerous advantages. Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking. Concrete remains the material of choice for many applications regardless of these limitations.
Hydration of Portland Cement
Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Concrete does not need to dry out in order to harden as commonly thought.
The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden). When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years. This very important reaction will be discussed in detail in this section.
Portland cement consists of five major compounds and a few minor compounds. The composition of a typical portland cement is listed by weight percentage in Table 2.

Cement Compound	Weight Percentage	Chemical Formula
Tricalcium silicate	50 %	Ca3SiO5 or 3CaO.SiO2
Dicalcium silicate	25 %	Ca2SiO4 or 2CaO.SiO2
Tricalcium aluminate	10 %	Ca3Al2O6 or 3CaO .Al2O3
Tetracalcium aluminoferrite	10 %	Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3
Gypsum	5 %	CaSO4.2H2O

Table 2: Composition of portland cement with chemical composition and weight percent. When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail.
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat
2 Ca₃SiO₅ + 7 H₂O ---> 3 CaO^.2SiO₂^.4H₂O + 3 Ca(OH)₂ + 173.6kJ
Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH^-) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.
The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier's principle). The evolution of heat is then dramatically increased.
The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower.

Figure 3: Schematic illustration of the pores in calcium silicate through different stages of hydration. The above diagrams represent the formation of pores as calcium silicate hydrate is formed. Note in diagram (a) that hydration has not yet occurred and the pores (empty spaces between grains) are filled with water. Diagram (b) represents the beginning of hydration. In diagram (c), the hydration continues. Although empty spaces still exist, they are filled with water and calcium hydroxide. Diagram (d) shows nearly hardened cement paste. Note that the majority of space is filled with calcium silicate hydrate. That which is not filled with the hardened hydrate is primarily calcium hydroxide solution. The hydration will continue as long as water is present and there are still unhydrated compounds in the cement paste.
Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate:
Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat
2 Ca₂SiO₄ + 5 H₂O---> 3 CaO^.2SiO₂^.4H₂O + Ca(OH)₂ + 58.6 kJ
The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Because these reactions do not contribute significantly to strength, they will be neglected in this discussion. Although we have treated the hydration of each cement compound independently, this is not completely accurate. The rate of hydration of a compound may be affected by varying the concentration of another. In general, the rates of hydration during the first few days ranked from fastest to slowest are:
tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate.
Refer to Demonstration 4
Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown below as a function of time.

Figure 4: Rate of heat evolution during the hydration of portland cement The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage. The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. This is particularly important for the construction trade who must transport concrete to the job site. It is at the end of this stage that initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.
Refer to Demonstration 5
Strength of Concrete
The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. Some pores will remain no matter how well the concrete has been compacted.


Figure 5: Schematic drawings to demonstrate the relationship between the water/cement ratio and porosity. The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph that follows.

Figure 6: A plot of concrete strength as a function of the water to cement ratio. Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability.
The physical characteristics of aggregates are shape, texture, and size. These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio.
Time is also an important factor in determining concrete strength. Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete's strength. It is common to use a 28-day test to determine the relative strength of concrete.
Concrete's strength may also be affected by the addition of admixtures. Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process. Some admixtures add fluidity to concrete while requiring less water to be used. An example of an admixture which affects strength is superplasticizer. This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.
SOME ADMIXTURES AND FUNCTIONS

TYPE	FUNCTION
AIR ENTRAINING	improves durability, workability, reduces bleeding, reduces freezing/thawing problems (e.g. special detergents)
SUPERPLASTICIZERS	increase strength by decreasing water needed for workable concrete (e.g. special polymers)
RETARDING	delays setting time, more long term strength, offsets adverse high temp. weather (e.g. sugar )
ACCELERATING	speeds setting time, more early strength, offsets adverse low temp. weather (e.g. calcium chloride)
MINERAL ADMIXTURES	improves workability, plasticity, strength (e.g. fly ash)
PIGMENT	adds color (e.g. metal oxides)

Table 3: A table of admixtures and their functions. Durability is a very important concern in using concrete for a given application. Concrete provides good performance through the service life of the structure when concrete is mixed properly and care is taken in curing it. Good concrete can have an infinite life span under the right conditions. Water, although important for concrete hydration and hardening, can also play a role in decreased durability once the structure is built. This is because water can transport harmful chemicals to the interior of the concrete leading to various forms of deterioration. Such deterioration ultimately adds costs due to maintenance and repair of the concrete structure. The contractor should be able to account for environmental factors and produce a durable concrete structure if these factors are considered when building concrete structures.

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Concrete Summary
Concrete is everywhere. Take a moment and think about all the concrete encounters you have had in the last 24 hours. All of these concrete structures are created from a mixture of cement and water with added aggregate. It is important to distinguish between cement and concrete as they are not the same. Cement is used to make concrete!
(cement + water) + aggregate = concrete
Cement is made by combining a mixture of limestone and clay in a kiln at 1450[[ring]] C. The product is an intimate mixture of compounds collectively called clinker. This clinker is finely ground into the powder form. The raw materials used to make cement are compounds containing some of the earth's most abundant elements, such as calcium, silicon, aluminum, oxygen, and iron.
Water is a key reactant in cement hydration. The incorporation of water into a substance is known as hydration. Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration. In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.
cement + water = hardened cement paste
The properties of this hardened cement paste, called binder, control the properties of the concrete. It is the inclusion of water (hydration) into the product that causes concrete to set, stiffen, and become hard. Once set, concrete continues to harden (cure) and become stronger for a long period of time, often up to several years.
The strength of the concrete is related to the water to cement mass ratio and the curing conditions. A high water to cement mass ratio yields a low strength concrete. This is due to the increase in porosity (space between particles) that is created with the hydration process. Most concrete is made with a water to cement mass ratio ranging from 0.35 to 0.6.
Aggregate is the solid particles that are bound together by the cement paste to create the synthetic rock known as concrete. Aggregates can be fine, such as sand, or coarse, such as gravel. The relative amounts of each type and the sizes of each type of aggregate determines the physical properties of the concrete.
sand + cement paste = mortar
mortar + gravel = concrete
Sometimes other materials are incorporated into the batch of concrete to create specific characteristics. These additives are called admixtures. Admixtures are used to: alter the fluidity (plasticity) of the cement paste; increase (accelerate) or decrease (retard) the setting time; increase strength (both bending and compression); or to extend the life of a structure. The making of concrete is a very complex process involving both chemical and physical changes. It is a material of great importance in our lives.

REFERENCE:

Abercrombie, S. Ferrocement: Building with Cement, Sand, and Wire Mesh. Schocken Books, NY, 1977.

Bye, G. C. Portland Cement: Composition, Production and Properties. Pergamon Press, NY, 1983.

Hewlett, P. C., and Young, J. F. "Physico-Chemical Interactions Between Chemical Admixtures and Portland Cement," Journal of Materials Education. Vol. 9, No. 4, 1987.

Introduction to Concrete Masonry. Instructor's Edition, Associated General Contractors of America, Washington D.C., Oklahoma State Department of Vocational and Technical Ed., Stillwater, 1988.

Kosmatka, Steven H., and Panarese, William C. Design and Control of Concrete Mixtures, Thirteenth edition, Portland Cement Association, 1988.

Materials Science of Concrete I, II, III. edited by Jan P. Skalny, American Ceramic Society, Inc, Westerville, OH, 1989.

Mindess, S., and Young, J.F. Concrete. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981.

Mitchell, L. Ceramics: Stone Age to Space Age. Scholastic Book Services, NY, 1963.

Rixom, M. R., and Mailuaganam, N. P. Chemical Admixtures for Concrete. R. & F.N. Spon, NY, 1986.

Roy, D. Instructional Modules in Cement Science. Pennsylvania State University, PA, 1985.

Sedgwick, J. "Strong But Sensitive" The Atlantic Monthly Vol. 267, No. 4, April 1991, pp 70-82.

Weisburd, S. "Hard Science" Science News Vol. 134, No. 2, July 9, 1988, pp 24-26.

Structural integrity and failure

Structural integrity and failure is an aspect of engineering which deals with the ability of a structure to support a designed load (weight, force, etc...) without breaking, tearing apart, or collapsing, and includes the study of breakage that has previously occurred in order to prevent failures in future designs.

Structural integrity is the term used for the performance characteristic applied to a component, a single structure, or a structure consisting of different components. Structural integrity is the ability of an item to hold together under a load, including its own weight, resisting breakage or bending. It assures that the construction will perform its designed function, during reasonable use, for as long as the designed life of the structure. Items are constructed with structural integrity to ensure that catastrophic failure does not occur, which can result in injuries, severe damage, death, and/or monetary losses.

Structural failure refers to the loss of structural integrity, which is the loss of the load-carrying capacity of a component or member within a structure, or of the structure itself. Structural failure is initiated when the material is stressed beyond its strength limit, thus causing fracture or excessive deformations. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in structural engineering and structural design.

Introduction[edit]

Structural integrity is the ability of a structure or a component to withstand a designed service load, resisting structural failure due to fracture, deformation, or fatigue. Structural integrity is a concept often used in engineering, to produce items that will not only function adequately for their designed purposes, but also to function for a desired service life.

To construct an item with structural integrity, an engineer must first consider the mechanical properties of a material, such as toughness, strength, weight, hardness, and elasticity, and then determine a suitable size, thickness, or shape that will withstand the desired load for a long life. A material with high strength may resist bending, but, without adequate toughness, it may have to be very large to support a load and prevent breaking. However, a material with low strength will likely bend under a load even though its high toughness prevents fracture. A material with low elasticity may be able to support a load with minimum deflection (flexing), but can be prone to fracture from fatigue, while a material with high elasitcity may be more resistant to fatigue, but may produce too much deflection unless the object is drastically oversized.

Structural integrity must always be considered in engineering when designing buildings, gears or transmissions, support structures, mechanical components, or any other item that may bear a load. The engineer must carefully balance the properties of a material with its size and the load it is intended to support. Bridge supports, for instance, need good yield strength, whereas the bolts that hold them need good shear and tensile strength. Springs need good elasticity, but lathe tooling needs high rigidity and minimal deflection. When applied to a structure, the integrity of each component must be carefully matched to its individual application, so that the entire structure can support its load without failure due to weak links. When a weak link breaks, it can put more stress on other parts of the structure, leading to cascading failures.^[1][2]

History[edit]

The need to build structure with integrity goes back as far as recorded history. Houses needed to be able to support their own weight, plus the weight of the inhabitants. Castles needed to be fortified to withstand assaults from invaders. Tools needed to be strong and tough enough to do their jobs. However, it was not until the 1920s that the science of fracture mechanics, namely the brittleness of glass, was described by Alan Arnold Griffith. Even so, a real need for the science did not present itself until World War II, when over 200 welded-steel ships broke in half due to brittle fracture, caused by a combination of the stresses created from the welding process, temperature changes, and the stress points created at the square corners of the bulkheads. The squared windows in the De Havilland Comet aircraft of the 1950s caused stress points which allowed cracks to form, causing the pressurized cabins to explode in mid-flight. Failures in pressurized boiler tanks leading to boiler explosion were a common problem during this era, causing severe damage. The growing sizes of bridges and buildings began to lead to even greater catastrophes and loss of life. The need to build constructions with structural integrity led to great advances in the fields of material sciences and fracture mechanics.^[3][4]

Types of failure[edit]

Failure of a structure can occur from many types of problems. Most of these problems are unique to the type of structure or to the various industries. However, most can be traced to one of five main causes.

• The first, whether due to size, shape, or the choice of material, is that the structure is not strong and tough enough to support the load. If the structure or component is not strong enough, catastrophic failure can occur when the overstressed construction reaches a critical stress level.
• The second is instability, whether due to geometry, design or material choice, causing the structure to fail from fatigue or corrosion. These types of failure often occur at stress points, such as squared corners or from bolt holes being too close to the material's edge, causing cracks to slowly form and then progress through cyclic loading. Failure generally occurs when the cracks reach a critical length, causing breakage to happen suddenly under normal loading conditions.
• The third type of failure is caused by manufacturing errors. This may be due to improper selection of materials, incorrect sizing, improper heat treating, failing to adhere to the design, or shoddy workmanship. These types of failure can occur at any time, and are usually unpredictable.
• The fourth is also unpredictable, from the use of defective materials. The material may have been improperly manufactured, or may have been damaged from prior use.
• The fifth cause of failure is from lack of consideration of unexpected problems. Vandalism, sabotage, and natural disasters can all overstress a structure to the point of failure. Improper training of those who use and maintain the construction can also overstress it, leading to potential failures.^[3][4]

Notable integrity[edit]

• Akashi Kaikyō Bridge, a bridge
• Burj Khalifa, a building
• Ameralik Span, a span

Notable failures[edit]

Further information: List of structural failures and collapses

Bridges[edit]

See also: List of bridge disasters

Dee bridge[edit]

Main article: Dee bridge disaster

The Dee bridge after its collapse

On 24 May 1847 the new railway bridge over the river Dee collapsed as a train passed over it, with the loss of 5 lives. It was designed by Robert Stephenson, using cast iron girders reinforced with wrought iron struts. The bridge collapse was the subject of one of the first formal inquiries into a structural failure. The result of the inquiry was that the design of the structure was fundamentally flawed, as the wrought iron did not reinforce the cast iron at all, and that, owing to repeated flexing, the casting had suffered a brittle failure due to fatigue.^[5]

First Tay Rail Bridge[edit]

Main article: Tay Bridge disaster

The Dee bridge disaster was followed by a number of cast iron bridge collapses, including the collapse of the first Tay Rail Bridge on 28 December 1879. Like the Dee bridge, the Tay collapsed when a train passed over it causing 75 people to lose their lives. The bridge failed because of poorly made cast iron, and the failure of the designer Thomas Bouch to consider wind loading on the bridge. The collapse resulted in cast iron largely being replaced by steel construction, and a complete redesign in 1890 of the Forth Railway Bridge. As a result, the Forth Bridge was the first entirely steel bridge in the world.^[6]

First Tacoma Narrows Bridge[edit]

Main article: Tacoma Narrows Bridge (1940)

The 1940 collapse of the original Tacoma Narrows Bridge is sometimes characterized in physics textbooks as a classic example of resonance, although this description is misleading. The catastrophic vibrations that destroyed the bridge were not due to simple mechanical resonance, but to a more complicated oscillation between the bridge and winds passing through it, known as aeroelastic flutter. Robert H. Scanlan, father of the field of bridge aerodynamics, wrote an article about this misunderstanding.^[7] This collapse, and the research that followed, led to an increased understanding of wind/structure interactions. Several bridges were altered following the collapse to prevent a similar event occurring again. The only fatality was a dog named Tubby.^[6]

I-35W Bridge[edit]

Main article: I-35W Mississippi River bridge

Security camera images show the I-35W collapse in animation, looking north.

The I-35W Mississippi River bridge (officially known simply as Bridge 9340) was an eight-lane steel truss arch bridge that carried Interstate 35W across the Mississippi River in Minneapolis, Minnesota, United States. The bridge was completed in 1967, and its maintenance was performed by the Minnesota Department of Transportation. The bridge was Minnesota's fifth–busiest,^[8][9] carrying 140,000 vehicles daily.^[10]The bridge catastrophically failed during the evening rush hour on 1 August 2007, collapsing to the river and riverbanks beneath. Thirteen people were killed and 145 were injured. Following the collapse, the Federal Highway Administration advised states to inspect the 700 U.S. bridges of similar construction^[11] after a possible design flaw in the bridge was discovered, related to large steel sheets called gusset plates which were used to connect girders together in the truss structure.^[12][13] Officials expressed concern about many other bridges in the United States sharing the same design and raised questions as to why such a flaw would not have been discovered in over 40 years of inspections.^[13]

Buildings[edit]

See also: Category:Collapsed buildings.

Thane building collapse[edit]

Main article: 2013 Thane building collapse

On 4 April 2013, a building collapsed on tribal land in Mumbra, a suburb of Thane in Maharashtra, India.^[14][15] It has been called the worst building collapse in the area.^{[16][nb 1]} 74 people died, including 18 children, 23 women, and 33 men, while more than 100 people survived. The search for additional survivors ended on 6 April 2013.^{[19][20][21][22]}

The building was under construction and did not have an occupancy certificate for its 100 to 150 low- to middle-income residents.^[23][24] Living in the building were the site construction workers and families. It was reported that the building was illegally constructed because standard practices were not followed for safe, lawful construction, land acquisition and resident occupancy.

By 11 April, a total of 15 suspects were arrested including builders, engineers, municipal officials and other responsible parties. Governmental records indicate that there were two orders to manage the number of illegal buildings in the area: a 2005 Maharashtra state order to use remote sensing and a 2010 Bombay High Court order. There were also complaints made to state and municipal officials.

On 9 April, a campaign began by the Thane Municipal Corporation to demolish area illegal buildings, focusing first on "dangerous" buildings. The forest department said that it will address encroachment of forest land in the Thane district. A call centre was established by the Thane Municipal Corporation to accept and track resolution of caller complaints about illegal buildings.

Savar building collapse[edit]

Main article: 2013 Savar building collapse

On 24 April 2013, Rana Plaza, an eight-storey commercial building, collapsed in Savar, a sub-district in the Greater Dhaka Area, the capital of Bangladesh. The search for the dead ended on 13 May with the death toll of 1,129.^[25] Approximately 2,515 injured people were rescued from the building alive.^[26][27]

It is considered to be the deadliest garment-factory accident in history, as well as the deadliest accidental structural failure in modern human history.^[24][28]

The building contained clothing factories, a bank, apartments, and several other shops. The shops and the bank on the lower floors immediately closed after cracks were discovered in the building.^[29][30][31] Warnings to avoid using the building after cracks appeared the day before had been ignored. Garment workers were ordered to return the following day and the building collapsed during the morning rush-hour.^[32]

Sampoong Department Store collapse[edit]

Main article: Sampoong Department Store collapse

On 29 June 1995, the 5-story Sampoong Department Store in the Seocho District of Seoul, South Korea collapsed resulting in the deaths of 502 people. In April 1995, cracks began to appear in the ceiling of the fifth floor of the store's south wing due to the presence of an air-conditioning unit on the weakened roof of the poorly built structure. On the morning of 29 June, as the number of cracks in the ceiling increased dramatically, the top floor was closed and managers shut the air conditioning off. The store management failed to shut the building down or issue formal evacuation orders; however, the executives themselves left the premises as a precaution. Five hours before the collapse, the first of several loud bangs was heard emanating from the top floors, as the vibration of the air conditioning caused the cracks in the slabs to widen further. Amid customer reports of vibration, the air conditioning was turned off but the cracks in the floors had already grown to 10 cm. At about 5:00 p.m. local time, the fifth floor ceiling began to sink; by 5:57 p.m., the roof gave way, and the air conditioning unit crashed through into the already-overloaded fifth floor, trapping more than 1,500 people and killing 502.

Ronan Point[edit]

Main article: Ronan Point

On 16 May 1968, the 22-story residential tower Ronan Point in the London Borough of Newham collapsed when a relatively small gas explosion on the 18th floor caused a structural wall panel to be blown away from the building. The tower was constructed of precast concrete, and the failure of the single panel caused one entire corner of the building to collapse. The panel was able to be blown out because there was insufficient reinforcement steel passing between the panels. This also meant that the loads carried by the panel could not be redistributed to other adjacent panels, because there was no route for the forces to follow. As a result of the collapse, building regulations were overhauled to prevent disproportionate collapse and the understanding of precast concrete detailing was greatly advanced. Many similar buildings were altered or demolished as a result of the collapse.^[33]

Oklahoma City bombing[edit]

Main article: Oklahoma City bombing

On 19 April 1995, the nine-story concrete framed Alfred P. Murrah Federal Building in Oklahoma was struck by a huge car bomb causing partial collapse, resulting in the deaths of 168 people. The bomb, though large, caused a significantly disproportionate collapse of the structure. The bomb blew all the glass off the front of the building and completely shattered a ground floor reinforced concrete column (see brisance). At second story level a wider column spacing existed, and loads from upper story columns were transferred into fewer columns below by girders at second floor level. The removal of one of the lower story columns caused neighbouring columns to fail due to the extra load, eventually leading to the complete collapse of the central portion of the building. The bombing was one of the first to highlight the extreme forces that blast loading from terrorism can exert on buildings, and led to increased consideration of terrorism in structural design of buildings.^[34]

Versailles wedding hall[edit]

Main article: Versailles wedding hall disaster

The Versailles wedding hall (Hebrew: אולמי ורסאי‎‎), located in Talpiot, Jerusalem, is the site of the worst civil disaster in Israel's history. At 22:43 on Thursday night, 24 May 2001 during the wedding of Keren and Asaf Dror, a large portion of the third floor of the four-story building collapsed.

World Trade Center Towers 1, 2, and 7[edit]

Main article: Collapse of the World Trade Center

In the September 11 attacks, two commercial airliners were deliberately crashed into the Twin Towers of the World Trade Center in New York City. The impact and resulting fires caused both towers to collapse in less than two hours. After the impacts had severed exterior columns and damaged core columns, the loads on these columns were redistributed. The hat trusses at the top of each building played a significant role in this redistribution of the loads in the structure.^[35] The impacts dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires. Temperatures became high enough to weaken the core columns to the point of creep and plastic deformation under the weight of higher floors. Perimeter columns and floors were also weakened by the heat of the fires, causing the floors to sag and exerting an inward force on exterior walls of the building. WTC Building 7 also collapsed later that day. According to the official report, the 47 story skyscraper collapsed within seconds due to a combination of a large fire inside the building and heavy structural damage from the collapse of the north tower.^[36][37]

Aircraft[edit]

Main article: Loss of structural integrity on an aircraft

See also: Category:Airliner accidents and incidents caused by in-flight structural failure

A 1964 B-52 Stratofortress test demonstrated the same failure that caused the 1963 Elephant Mountain & 1964 Savage Mountain crashes.

Repeated structural failures of aircraft types occurred in 1954, when two de Havilland Comet C1 jet airliners crashed due to decompression caused by metal fatigue, and in 1963-64, when the vertical stabilizer on four Boeing B-52 bombers broke off in mid-air.

Other[edit]

Warsaw Radio Mast[edit]

Main article: Warsaw radio mast

On 8 August 1991 at 16:00 UTC Warsaw radio mast, the tallest man-made object ever built before the erection of Burj Khalifa collapsed as consequence of an error in exchanging the guy-wires on the highest stock. The mast first bent and then snapped at roughly half its height. It destroyed at its collapse a small mobile crane of Mostostal Zabrze. As all workers left the mast before the exchange procedures, there were no fatalities, in contrast to the similar collapse of WLBT Tower in 1997.

Hyatt Regency walkway[edit]

Main article: Hyatt Regency walkway collapse

Design change on the Hyatt Regency walkways.

On 17 July 1981, two suspended walkways through the lobby of the Hyatt Regency in Kansas City, Missouri, collapsed, killing 114 and injuring more than 200 people^[38] at a tea dance. The collapse was due to a late change in design, altering the method in which the rods supporting the walkways were connected to them, and inadvertently doubling the forces on the connection. The failure highlighted the need for good communication between design engineers and contractors, and rigorous checks on designs and especially on contractor-proposed design changes. The failure is a standard case study on engineering courses around the world, and is used to teach the importance of ethics in engineering.^[39][40]