MATERIALS RESEARCH
1. Reinforced concrete:
Definition:
Reinforced concrete is concrete in which reinforcing bars or other types of reinforcement have been integrated to improve one or more properties of the concrete. For many years, it has been utilized as an economical construction material in one form or another in buildings, bridges, and many other types of structures throughout the world. A large part of its worldwide appeal is that the basic constituent materials—cement, sand,aggregate, water, and reinforcing bars—are widely available and that it is possible to construct a structure using local sources of labor and materials.
In addition to being readily obtainable, reinforced concrete has been universally accepted because it can be molded essentially into any shape or form, is inherently rigid, and is inherently fire-resistant. With proper protection of the reinforcement, a reinforced concrete structure can be very durable and can have a long life even under harsh climatic or environmental conditions. Reinforced concrete structures have also demonstrated that they can provide a safe haven fromthe potentially devastating effects of earthquakes, hurricanes, floods, and tornadoes.
Typical Uses:
Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, ridges/overpasses, highways, runways, parking structures,dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats.
Properties:
Concrete is a brittle, composite material that is strong in compression and weak in tension. Cracking occurs when the concrete tensile stress in a member reaches the tensile strength due to externally applied loads, temperature changes, or shrinkage. Concrete members that do not have any type of reinforcement in them will typically fail very suddenly once the first tension cracks form because there is nothing to prevent
the cracks from propagating completely through the member.
Deterioration Mechanisms:
Most peoplecan identify the basic problems observed in concrete structures. A crack in a floor slab, a spall at the base of a column, or rust stains that discolor the underside of a beam are conditions present in many structures built with reinforced concrete.
Deterioration can be caused by an attack of the chemical makeup of concrete or from chemical reactions with embedded steel. Some of the different types of chemical attack include:
• Corrosion. Corrosion occurs from an electrical reaction within the concrete matrix. Exposure to oxygen and moisture are required for corrosion to occur. The electrical reaction causes the iron in the steel to oxidize. As the iron oxidizes, expansion inducing tensile stresses in the surrounding concrete. When the tensile strength of the concrete is exceeded, the concrete fails with a crack or spall. The alkalinity of the concrete normally protects the steel by significantly reducing the rate of corrosion; however, open cracks, reduction of concrete alkalinity, exposure to corrosive chemicals, and dissimilar metals can all increase the rate of corrosion.
• Chlorides. Chlorides are normally introduced to concrete structures through deicing salts or seawater. The chlorides penetrate the concrete, eventually making contact with the embedded steel. Once the chlorides combine with oxygen and moisture, corrosion of the steel occurs. With the presence of chlorides, the corrosion process is more aggressive, occurring even with high alkalinity and accelerating the corrosion process in concretes in which the alkalinity is reduced.
• Carbonation. Carbon dioxide in the air can react chemically with cement paste when moisture is present. The resulting chemical reaction reduces the pH of the concrete. The carbonation process penetrates the pores of the concrete, eventually penetrating to the embedded steel, increasing the potential for corrosion. Since the carbonation process penetrates the concrete, open cracks will accelerate the depth of carbonation penetration. High-quality concrete is less susceptible to the carbonation process.
• Alkali-Silica Reaction. Because of the intimate interaction between the cement paste and the coarse aggregate, using compatible materials is important. When certain types of aggregate are used, the silica in these stones can react with the hydroxides in the cement paste. When this chemical reaction takes place, a gel develops on the aggregate surface. When moisture is introduced to the gel, the gel reacts by expanding and inducing tensile stresses in the cement paste, causing cracking. The cracks allow additional moisture to enter the concrete, accelerating the reaction. Alkali-silica reaction is observed on the surface of the concrete by severe map cracking on the exposed surfaces.
• Freeze/Thaw. Freeze/thaw is the process that occurs when moisture within the pores of concrete freezes and expands. The expansion causes tensile forces to develop within the cement, causing cracking and scaling of the concrete. For freeze/thaw to occur, both moisture and freezing temperatures must be present. Freeze/thaw damage is exacerbated when the concrete is exposed to cyclic freezing and thawing. Freeze/thaw is resisted in concrete through air entrained into the concrete mix. Entrained air is the presence of large amounts of equally spaced microscopic air bubbles within the concrete mix. The freezing pore water expands into the spaces provided by the air bubbles.
2. Adobe:
Definition:
Adobe is a natural building material made from sand, clay, water, and some kind of fibrous or organic material (sticks, straw, and/or manure), which the builders shape into bricks (using frames) and dry in the sun. Adobe buildings are similar to cob and mudbrick buildings. Adobe structures are extremely durable, and account for some of the oldest existing buildings in the world. Compared to wooden buildings, adobe buildings offer significant advantages due to their greater thermal mass, in hot climates, but they are known to be particularly susceptible to earthquake damage.
Buildings made of sun-dried earth are common in West Asia, North Africa, West Africa, South America, southwestern North America, Spain, Eastern Europe and East Anglia, particularly Norfolk, known as clay lump. Adobe had been in use by indigenous peoples of the Americas in the Southwestern United States, Mesoamerica, and the Andean region of South America for several thousand years, although often substantial amounts of stone are used in the walls of Pueblo buildings.
Properties:
An adobe brick is a composite material made of clay mixed with water and an organic material such as straw or dung. The soil composition typically contains clay and sand. Straw is useful in binding the brick together and allowing the brick to dry evenly, thereby preventing cracking due to uneven shrinkage rates through the brick. Dung offers the same advantage and is also added to repel insects. The most desirable soil texture for producing the mud of adobe is 15% clay, 10-30% silt and 55-75% fine sand. Another source quotes 15-25% clay and the remainder sand and coarser particles up to cobbles 2-10 inches with no deleterious effect. Modern adobe is stabilized with either emulsified asphalt or Portland cement up to 10% by weight.
The clay content should be a mixture of no more that half expansive clays with the remainder non-expansive illite or kaolinite. Too much expansive clay results in uneven drying through the brick and cracking while too much kaolinite will make a weak brick.
Deterioration mechanisms:
- Basal Erosion (Rising Damp)
- Surface Erosion
- Displacement
- Cracks or Bulges
- Coating Failure
- Slump or Creep
- Collapse
3. Glass
Definition:
REFERENCES
- McGraw-Hill Professional: http://www.mhprofessional.com/downloads/products/0071638342/fanella_0071638350.pdf
- Matthew H. Banville: Assessment and Repair of Concrete Structures. July 2008
http://www.rci-online.org/interface/2008-07-banville.pdf
- James Garrison: Adobe; The materials, its deterioration, its coatings
http://missions.arizona.edu/sites/default/files/1%20Garrison-Adobe%20Characteristics.pdf
