Sunday, 19 February 2012
Source: http://constructionduniya.blogspot.com/2012/02/new-materials-in-construction-concrete.html
The development and application of new materials in construction continually adds to the choices and decisions facing clients, designers and all responsible for building and construction. They continually seek greater and more reliable information about the serviceability in order that they meet more stringent design, safety and economic criteria.
Ever since Thomas Edison patented portland cement in 1907, it has been used for a variety of different uses. Sidewalks, buildings, sinks, and furniture are but a few of the products made from cement in the form of concrete. Cement is the dry powder that when mixed with other additives and water makes concrete. Over the past decade, new types of concrete and cement have been formulated that do everything from bend, to grow plants, and let light through.
In 2005, researchers at the University of Michigan created a bendable form of concrete that is "500 times more resistant to cracking and 40 percent lighter in weight." This new type of concrete has substituted the gross aggregate normally used in the making of concrete, for thin fibers. Projects in Japan, Korea, Switzerland, and Australia have already used this new bendable concrete. Unfortunately, the country in which it was created has been slow to adopt its use.
BETÃO ORGÂNICO was created in 2005 by "Lisbon-based architects and designers e-studio." This organic concrete blends organic and inorganic material together to create a living surface. Concrete retains water, as such the concrete is used as a "battery" to provide water during dry spells for the plant life growing on it. Rather than having grass growing between concrete slabs, it is now possible to have the grass grow on the concrete slabs. These slabs could be added to outside walls to create living siding and provide plants to soak up CO2.
LiTraCon is a Hungarian concrete product developed seven years ago by architect Ron Losonczi. By impregnating the concrete with optical glass fibers, light can be transmitted from the outside in or inside out. This concrete has the same strength as regular concrete and will continue to transmit light through walls up to twenty meters (twenty-two feet) thick.
Finally, Tececo has developed an eco-cement that absorbs CO2 from the environment. By adding reactive magnesia to the cement, water and CO2 are absorbed and harden. Other waste products, such as "fly and bottom ash, slags, plastics, paper glass etc" can also be added to the cement without affecting the CO2 absorption.
These new types of cement and concrete give architects and designers more choices for creating truly different looks. Normally, you think of ugly concrete walls or slabs. Now concrete can not only be bent, but used as a basis for plants and light effects.
Development of new types of concrete with improved performance is a very important issue for the whole building industry. This development is based on the optimisation of the concrete mix design, with an emphasis not only to the workability and mechanical properties but also to the durability and the reliability of the concrete structures in general. Appearance of the new types of concrete requires a revision and improvement of existing structural systems and actual building technologies. The economical aspect are of importance as well.
BASIC CONCEPT ABOUT CONCRETE
Concrete is a construction material composed of cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel limestone or granite, plus a fine aggregate such as sand), water, and chemical admixtures. The word concrete comes from the Latin word "concretus", which means "hardened" or "hard".
Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. Concrete is used to make pavements, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.
More concrete is used than any other man-made material in the world.[1] As of 2006, about 7 cubic kilometres of concrete are made each year—more than one cubic metre for every person on Earth.[2] Concrete powers a $US 35-billion industry which employs more than two million workers in the United States alone.[citation needed] More than 55,000 miles (89,000 km) of highways in America are paved with this material. The People's Republic of China currently consumes 40% of the world's cement/concrete production.
A superplasticizer is one of a class of admixtures called water-reducers that are used to lower the mix water requirement of concrete. Normal water-reducers based on lignosulphonic acids, hydroxycarboxylic acids or processed carbohydrates are capable of reducing water requirements by about 10 to 15 per cent. Incorporating larger amounts to produce higher water reductions results in undesirable effects on setting, air content, bleeding, segregation and hardening characteristics. Superplasticizers are chemically different from normal water-reducers, and are capable of reducing water contents by about 30 per cent. They are variously known as superplasticizers, superfluidizers, superfluidifiers, super water-reducers or high range water-reducers. Since they were first introduced in Japan about 15 years ago they have been used to produce several million cubic metres of concrete; in the construction of the Olympic stadium in Montreal alone, 5000 precast concrete units were produced utilizing superplasticizers.
The basic advantages of superplasticizers include, (1) high workability of concrete, resulting in easy placement without reduction in cement content and strength; (2) high strength concrete with normal workability but lower water content; and (3) a concrete mix with less cement but normal strength and workability.
Superplasticizers are broadly classified in four groups, viz, sulphonated melamine-formaldehyde condensates (SMF), sulphonated naphthalene-formaldehyde condensates (SNF), modified lignosulphonates (MLS), and others including sulphonic acid esters, carbohydrate esters, etc. variations exist in each of these classes and some formulations may contain a second ingredient. Most available data, however, pertain to SMF- and SNF-based admixtures. They are supplied either as solids or as aqueous solutions. In this Digest the dosage refers to the solid as a percentage of the weight of cement.
TYPES OF CONCRETE
Ever since Thomas Edison patented Portland cement in 1907, it has been used for a variety of different uses. Sidewalks, buildings, sinks, and furniture are but a few of the products made from cement in the form of concrete. Cement is the dry powder that when mixed with other additives and water makes concrete. Over the past decade, new types of concrete and cement have been formulated that do everything from bend, to grow plants, and let light through.
BENDABLE CONCRETE :
The new concrete is 500 times more resistant to cracking and 40 percent lighter in weight. The materials in the concrete itself are designed for maximum flexibility. The Engineered Cement Composites technology has been used already on projects in Japan, Korea, Switzerland and Australia, but has had slow adoption in the US. Traditional concrete presents many problems: lack of durability and sustainability, failure under severe loading, and the resulting expenses of repair. ECC should address most of those problems. The ductile, or bendable, concrete is made mainly of the same ingredients in regular concrete minus the coarse aggregate. It looks exactly like regular concrete, but under excessive strain, the ECC concrete gives because the network of fibers veining the cement is allowed to slide within the cement, thus avoiding the inflexibility that causes brittleness and breakage.
The Michigan Department of Transportation (MDOT) used the ECC to replace part of a bridge that crosses Interstate 94. The slab eliminated the need for expansion joints, which are moveable steel teeth that separate sections of regular concrete. With the ECC, a longer continuous slab is possible.
The Mihara Bridge, a new structure in Hokkaido, Japan, has a deck of ECC that is a mere 2 inches (5 centimeters) thick.
Studies suggest ECC should last twice as long as regular concrete, but the researchers said more tests are needed to confirm that claim. Professor Victor Li estimates that over the course of 60 years, with servicing and replacement costs considered, a bridge made of ECC could be cost 37 percent less than a traditional span.
LITRACON :
LiTraCon ("light transmitting concrete") is a translucent concrete building material made of fine concrete embedded with 5% by weight of optical glass fibers. It was developed in 2001 by Hungarian architect Aron Losonczi working with scientists at the Technical University of Budapest.
The days of dull, grey concrete could be about to end. The Hungarian architect has combined the world's most popular building material with optical fibre from Schott to create a new type of concrete that transmits light.
A wall made of "LitraCon" allegedly has the strength of traditional concrete and an embedded array of glass fibers that can display a view of the outside world, such as the silhouette of a tree, for example. Thousands of optical glass fibres form a matrix and run parallel to each other between the two main surfaces of every block. Shadows on the lighter side will appear with sharp outlines on the darker one. Even the colours remain the same. This special effect creates the general impression that the thickness and weight of a concrete wall will disappear. The hope is that the new material will transform the interior appearance of concrete buildings by making them feel light and airy rather than dark and heavy.
In theory, a wall structure built out of the light-transmitting concrete can be a couple of meters thick as the fibers work without any loss in light up to 20 m. Load-bearing structures can also be built from the blocks as glass fibers do not have a negative effect on the well-known high compressive strength of concrete. The blocks can be produced in various sizes with embedded heat isolation too. Thousands of optical glass fibers form a matrix and run parallel to each other between the two main surfaces of each block. The proportion of the fibers is very small (4%) compared to the total volume of the blocks. Moreover, these fibres mingle in the concrete because of their insignificant size, and they become a structural component as a kind of modest aggregate. Therefore, the surface of blocks remains homogeneous concrete. It can be produced as prefabricated building blocks and panels. Due to the small size of the fibers, they blend into concrete becoming a component of the material like small pieces of aggregates. In this manner, the result is not only two materials- glass in concrete- mixed, but a third new material which is homogeneous in its inner structure and on its main surfaces as well.
The glass fibers lead light by points between the two sides of the blocks. Because of their parallel position, the light-information on the brighter side of such a wall appears unchanged on the darker side. The most interesting form of this phenomenon is probably the sharp display of shadows on the opposing side of the wall.
If more and more buildings begin using this technology, more natural light can be used to light offices and stores. This could lead to huge drops in the amount of electricity used to light buildings, since they'd be naturally lit during the day. Also, people who get exposure to the sun are generally happier and more productive, so that is another reason for businesses to use this light-transmitting concrete.
TECECO (porecocrete porous concrete) :
One particular eco-friendly product that is generating much attention is - Porecocrete Porous Concrete from Asset Rehabilitation / TecEco. By adding reactive magnesia to the cement, water and CO2 are absorbed and harden. Other waste products, such as "fly and bottom ash, slag, plastics, paper glass etc" can also be added to the cement without affecting the CO2 absorption. TecEco porecocretes represent a large-scale market for eco-cement. Porecocrete porous pavements mimic nature. Eco-cement sets by absorbing carbon dioxide, as by design it allows the entry of abundant quantities of the gas through what is an open pore structure. Using recycled aggregates, concrete cannot get much more sustainable. The main potential use for porecocretes is to make porous pavement in cities so that people are less affected by drought. These are pavements with lots of holes in them, and with subsurface drainage and usually a capacity to store water underneath or in a reservoir. Surface runoff water either soaks into an aquifer in suitable terrain or is captured above an impervious layer and drained preferably to underground storage for further use. Before infiltrating into the subsoil or sub-surface drainage the process improves water quality by providing surface area and aerobic conditions for cleansing. Some of the main advantages of Porecocrete Porous Concrete are that water penetrates through quickly leaving drier and safer surfaces with no standing water, and a reduction in noise pollution as porous pavements also absorb noise. Then it leads to less maintenance on nearby buildings and superstructure, as aquifers would be more regularly replenished resulting in less variable ground moisture content, reduced ground movement with wet dry cycles. Porous pavements made with TecEco Eco-Cements would not be attacked by salts and would last considerably longer than conventional binders such as bitumen (or asphalt) and Portland cement.
Heat is absorbed by pavements during hot, sunny days and due to the fact that we have paved all the ground, large cities just get hotter and hotter. The solution is to let the ground breathe and porous pavements do just that.
In Australia, some parts of the US and several other places in the world, it has been noted that subdivisions made with porous pavements that also have street trees can be several degrees cooler than surrounding suburbs.
How do Eco-Cements Work?
Eco-Cements are made by blending reactive magnesia with conventional hydraulic cements like Portland cement. It is not recommended that large amounts of pozzolan are added to an Eco-Cement as the pozzolan will compete with the carbonation reaction of lime and tend to block the carbonation affect slowing it down. Eco-Cements are environmentally friendly because in permeable substrates the magnesium oxide will first hydrate using mix water and then carbonate forming significant amounts of strength giving minerals in a low alkali matrix. Many different wastes can be used as aggregates and fillers without reaction problems. The reactive magnesium oxide used in Eco-Cements is currently made from magnesite (a carbonate compound of magnesium) found in abundance. In future TecEco hope to make it from abundant magnesium in sea water using the Greensols process.
When added to concrete magnesia hydrates to magnesium hydroxide, but only in permeable materials like bricks, blocks, pavers and pervious pavements will it absorb CO2 and carbonate. The greater proportion of the elongated minerals that form is water and carbon dioxide. These minerals bond aggregates such as sand and gravel and wastes such as saw dust, slag, bottom ash, plastics, paper etc. Eco-Cement can include more waste than other hydraulic cements like Portland cement because it is much less alkaline, reducing the incidence of delayed reactions that would reduce the strength of the concrete. Portland cement concretes on the other hand can't include large amounts of waste because the alkaline lime that forms causes delayed and disruptive reactions
Eco-Cement Carbonation
The more magnesia added to Eco-Cement and the more permeable it is, the more CO2 that is absorbed. The rate of absorption of CO2 varies with the degree of permeability. Carbonation occurs quickly at first and more slowly towards completion. A typical Eco-Cement concrete block would be expected to fully carbonate within a year. Eco-Cement also has the ability to be almost fully recycled back into cement, should the concrete structure become obsolete. .
Steps involved in making Eco-Cement
1. Magnesite (a compound of magnesium) is heated in a kiln to around 600 to 750 degrees C.
The lower firing temperature of the Tec-Kiln makes it easier to use free energy such as wind or solar or even waste energy and TecEco plan to make a kiln that does not use fossil fuels and in which the CO2 gases produced from the magnesium carbonate as it decomposes is captured and contained for further use or safe disposal.
2. Grinding in the hot area of the Tec-Kiln will result in increased efficiency.
3. The heating process produces reactive magnesium oxide (magnesia).
4. The reactive magnesia powder is added to a pre-determined, but variable amount of hydraulic cement such as Portland cement, and if desired, supplementary cementitious materials like fly ash.
5. The resulting blended powder is Eco-Cement.
6. When mixed with water and aggregates such as sand, gravel and wastes, Eco-Cement concretes are ready for pouring into concrete, pressing into blocks or other uses.
BETAO ORGANICO :
This type of concrete was created in 2005 by "Lisbon-based architects and designers e-studio." This organic concrete blends organic and inorganic material together to create a living surface. Concrete retains water, as such the concrete is used as a "battery" to provide water during dry spells for the plant life growing on it. Rather than having grass growing between concrete slabs, it is now possible to have the grass grow on the concrete slabs. These slabs could be added to outside walls to create living siding and provide plants to soak up CO2.
SPRAYED CONCRETE :
The technique of sprayed concrete has been in use for over 50 years in construction, structural repairs and a variety of other applications. The use of properly applied sprayed concrete is now recognised as being a technically sound and economic method of applying concrete both for effective repairs and for new constructions. The sprayed concrete forms and excellent bond with itself, concrete and masonry. The material is compacted onto the substrate under its own momentum, resulting in a strong, dense product with good resistance to abrasion and weathering. Sprayed concrete is extremely versatile and as a free forming material lends itself to use in the construction industry. This imperviousness and low water cement ratio gives a durable concrete with a host of proven applications. In many cases sprayed concrete will out-perform traditional concrete both in strength and permeability. The elimination of form work, the speed of application, the small access required and the ability to have the spraying machine and materials over 200 metres from the point of application, result in a large cost saving over other techniques. With the ever-increasing structural loadings, the technique has proven particularly suitable for the strengthening of bridges, tunnels and culverts.
The Phaeno Science Center, designed by Zaha Hadid, is the largest building in Europe constructed from self-consolidating concrete, which requires no vibration to eliminate air pockets and even out distribution of aggregates. SCC can be placed at a faster rate with no mechanical vibration and less screeding, allows shorter construction periods, permits structural and architectural shapes and surface, not achievable with conventional concrete.
Emmanuel Combarel and Dominique Marrec, two French architects, used Ductal, a high-performance concrete created by Lafarge in 2001, to build the RATP Bus Center in Thiais,
HIGH PERFORMANCE CONCRETE:
It is a relatively new term used to describe concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:
Ease of placement
Compaction without segregation
Early age strength
Long-term mechanical properties
Permeability
Density
Heat of hydration
Toughness
Volume stability
Long life in severe environments
SHOTCRETE :
Shotcrete uses compressed air to shoot (cast) concrete onto (or into) a frame or structure. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork. It is sometimes used for rock support, especially in tunnelling. Today there are two application methods for shotcrete: the dry-mix and the wet-mix procedure. In dry-mix the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle. In wet-mix, the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying. For both methods additives such as accelerators and fiber reinforcement may be used.
The term Gunite is occasionally used for shotcrete, but properly refers only to dry-mix shotcrete, and once was a proprietary name.
PERVIOUS CONCRETE:
Pervious concrete is sometimes specified by engineers and architects when porosity is required to allow some air movement or to facillitate the drainage and flow of water through structures. Pervious concrete is referred to as "no fines" concrete because it is manufactured by leaving out the sand or "fine aggregate". A pervious concrete mixture contains little or no sand (fines), creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15% and 25% voids are achieved in the hardened concrete, and flow rates for water through pervious concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft²/ min or 200 L/m²/min), although they can be much higher.
Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures, but sufficient strength for many applications is readily achieved. Pervious concrete pavement is a unique and effective means to address important environmental issues and support sustainable growth. By capturing rainwater and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and meeting US Environmental Protection Agency (EPA) stormwater regulations. The use of pervious concrete is among the Best Management Practices (BMPs) recommended by the EPA, and by other agencies and geotechnical engineers across the country, for the management of stormwater runoff on a regional and local basis. This pavement technology creates more efficient land use by eliminating the need for retention ponds, swales, and other stormwater management devices. In doing so, pervious concrete has the ability to lower overall project costs on a first-cost basis.
ROLLER-COMPACTED CONCRETE :
It is sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block. Roller-compacted concrete is typically used for concrete pavement, but has also been used to build concrete dams, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours.
GLASS CONCRETE:
The use of recycled glass as aggregate in concrete has become popular in modern times, with large scale research being carried out at Columbia University in New York. This greatly enhances the aesthetic appeal of the concrete.
ASPHALT CONCRETE
Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing cement as the binder.
Base layer of asphalt concrete in a road under construction
RAPID STRENGTH CONCRETE
This type of concrete is able to develop high resistance within few hours after been manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process at record time, repair road surfaces that become fully operational in just few hours.
RUBBERIZED CONCRETE
While " rubberised concrete" is common, rubberized Portland cement concrete ("rubberized PCC") is still undergoing experimental tests, as of 2007 .
POLYMER CONCRETE
Polymer concrete is concrete which uses polymers to bind the aggregate. Polymer concrete can gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in only four hours. Polymer concrete is generally more expensive than conventional concretes.
Polymer concrete coating
High-Strength Concrete
A concrete of high strength can be made without admixtures provided it is mixed with low amounts of water and has desirable workability characteristics. At low water. cement ratios however, it is not easy to achieve good workability. As water reductions of about 25 to 30 per cent can be achieved by using superplasticizers without loss in workability characteristics, significantly higher initial and ultimate strengths are realized. Although high cement content may also be used to obtain high initial strengths in concrete, the higher heat developed by the chemical reactions produces undesirable cracks and shrinkage.
High early strength development, a characteristic of concrete made using superplasticizers at low water:cement ratios, is particularly advantageous in the production of precast units. For prestressed beams and units, which need overnight heat-curing, use of superplasticizers allows reduction in curing time and curing temperatures. High early strengths are particularly useful for placing concrete in traffic areas such as city roads and airport runways. Pumping at reduced water content is also facilitated by the use of superplasticizers.
In the early 1970s, experts predicted that the practical limit of ready-mixed concrete would be unlikely to exceed a compressive strength greater than 11,000 psi (76 MPa). Over the past two decades, the development of high-strength concrete has enabled builders to easily meet and surpass this estimate. Two buildings in Seattle, Washington, contain concrete with a compressive strength of 19,000 psi (131 MPa).
The primary difference between high-strength concrete and normal-strength concrete relates to the compressive strength that refers to the maximum resistance of a concrete sample to applied pressure. Although there is no precise point of separation between high-strength concrete and normal-strength concrete, the American Concrete Institute defines high-strength concrete as concrete with a compressive strength greater than 6000 psi (41 MPa).
Manufacture of high-strength concrete involves making optimal use of the basic ingredients that constitute normal-strength concrete. Producers of high-strength concrete know what factors affect compressive strength and know how to manipulate those factors to achieve the required strength. In addition to selecting a high-quality portland cement, producers optimize aggregates, then optimize the combination of materials by varying the proportions of cement, water, aggregates, and admixtures.
When selecting aggregates for high-strength concrete, producers consider the strength of the aggregate, the optimum size of the aggregate, the bond between the cement paste and the aggregate, and the surface characteristics of the aggregate. Any of these properties could limit the ultimate strength of high-strength concrete.
Admixtures
Pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. These materials impart additional strength to the concrete by reacting with portland cement hydration products to create additional C-S-H gel, the part of the paste responsible for concrete strength.
It would be difficult to produce high-strength concrete mixtures without using chemical admixtures. A common practice is to use a superplasticizer in combination with a water-reducing retarder. The superplasticizer gives the concrete adequate workability at low water-cement ratios, leading to concrete with greater strength. The water-reducing retarder slows the hydration of the cement and allows workers more time to place the concrete.
High-strength concrete is specified where reduced weight is important or where architectural considerations call for small support elements. By carrying loads more efficiently than normal-strength concrete, high-strength concrete also reduces the total amount of material placed and lowers the overall cost of the structure.
The most common use of high-strength concrete is for construction of high-rise buildings. At 969 ft (295 m), Chicago's 311 South Wacker Drive uses concrete with compressive strengths up to 12,000 psi (83 MPa) and is the tallest concrete building in the United States.
HIGH SRENGTH LIGHT WEIGHT CONCRETE
Lightweight concrete has been used successfully for many years for structural members in high-rise buildings. In addition to its lighter weight, which permits saving in dead load and this concrete provides better heat insulation than normal weight concrete. In recent years, the applications of high-strength concrete have increased, and high-strength concrete has now been used in many part of the world. However, not enough significant data of high-strength lightweight concrete with compressive strength in excess of 60 N/mm2 have been obtained. This report summarizes results of an experimental study of the properties of hardened high-strength lightweight concrete such as strength, drying shrinkage, durability and porosity, and provides important new information on the mix proportion and curing method of this concrete. These results are as follows; (a) In regard to porosity of lightweight aggregate, it was observed the tendency that expanded shale type has a lager radius than that of sintered fly ash type, it depends on aggregate characteristics, surface texture and void connection. (b) Different water content of lightweight aggregate gives influence to porosity of mortar matrix under drying condition. (c) Resistance of freezing and thawing or fire of light weight aggregate concrete is not necessary to advance under moisture condition, because of light weight aggregates, due to their cellular structure, capable of containing more water than normal weight aggregate. (d) As the consideration of the porosities and water content of hardened concrete, it was evaluated the properties of high-strength lightweight concrete. (author abst.)
Part of the results of an ongoing laboratory work carried out to design a structural lightweight high strength concrete ( SLWHSC ) made with and without mineral admixtures is presented. Basaltic-pumice ( scoria ) was used as lightweight aggregate.
A control lightweight concrete mixture made with lightweight basaltic-pumice (scoria) containing normal Portland cement as the binder was prepared. The control lightweight concrete mixture was modified by replacing 20% of the cement with fly ash and by replacing 10% of the cement with silica fume. A ternary lightweight concrete mixture was also prepared modifying the control lightweight concrete by replacing 20% of cement with fly ash and 10% of cement with silica fume. Two normal weight concretes (NWC) were also prepared for comparison purpose.
Fly ash and silica fume are used for economical and environmental concerns. Cylinder specimens were cast from the fresh mixtures to measure compressive and flexural tensile strength. The concrete samples were cured at 65% relative humidity with 20 °C temperature. The density and slump workability of fresh concrete mixtures were also measured.
Laboratory test results showed that structural lightweight concrete (SLWC) can be produced by the use of scoria. However, the use of mineral additives seems to be mandatory for production of SLWHSC. The use of ternary mixture was recommended due to its satisfactory strength development and environmental friendliness.
Future construction of concrete floating platforms for offshore oil exploration off the east coast of Canada will lead to a substantial increase in the use of high-strength lightweight (HSLW) concrete. HSLW concrete has been extensively used in Norway and other parts of Europe. HSLW concrete with its high durability and lightweight characteristics is a very much sought after material in the construction of concrete floating platforms.
SELF CONSOLIDATING CONCRETE
Self-consolidating concrete, also known as self-compacting concrete and SCC, is a highly flowable, non-segregating concrete that can spread into place, fill formwork and encapsulate even the most congested reinforcement, all without any mechanical vibration. As a high-performance concrete, SCC delivers these attractive benefits while maintaining all of concrete's customary mechanical properties and durability characteristics.
SCC is defined as a concrete mixture that can be placed purely by means of its own weight, with little or no vibration. Adjustments to traditional mix designs and the use of superplasticizers creates flowing concrete that meets tough performance requirements. If needed, low dosages of viscosity modifier can be used to eliminate unwanted bleeding and segregation.
Since its inception in the 1980s, the use of SCC has grown tremendously. The
development of high performance polycarboxylate polymers and viscosity modifiers have made it possible to create "flowing" concrete without compromising durability, cohesiveness, or compressive strength. The flowability of SCC is measured in terms of spread when using a modified version of the slump test (ASTM C 143). The spread (slump flow) of SCC typically ranges from 18 to 32 inches (455 to 810 mm) depending on the requirements for the project. The viscosity, as visually observed by the rate at which concrete spreads, is an important characteristic of plastic SCC and can be controlled when designing the mix to suit the type of application being constructed.
SCC's unique properties give it significant economic, constructability, aesthetic and engineering advantages. SCC is an increasingly attractive choice for optimizing site manpower (through reduction of labor and possibly skill level), lowering noise levels, and allowing for a safer working environment. SCC allows easier pumping (even from bottom up), flows into complex shapes, transitions through inaccessible spots, and minimizes voids around embedded items to produce a high degree of homogeneity and uniformity. That's why SCC allows for denser reinforcement, optimized concrete sections and shapes, and greater freedom of design while producing superior surface finishes and textures.
HIGH PERFORMANCE CONCRETE
In recent years, the terminology "High-Performance Concrete" has been introduced into the construction industry. This edition of Technical Talk explains high-performance concrete and how it differs from conventional concrete.
The American Concrete Institute (ACI) defines high-performance concrete as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely when using conventional constituents and normal mixing, placing and curing practices. A commentary to the definition states that a high-performance concrete is one in which certain characteristics are developed for a particular application and environment. Examples of characteristics that may be considered critical for an application are:
* Ease of placement
* Compaction without segregation
* Early age strength
* Long-term mechanical properties
* Permeability
* Density
* Heat of hydration
* Toughness
* Volume stability
* Long life in severe environments
Because many characteristics of high-performance concrete are interrelated, a change in one usually results in changes in one or more of the other characteristics. Consequently, if several characteristics have to be taken into account in producing a concrete for the intended application, each must be clearly specified in the contract documents.
A high-performance concrete is something more than is achieved on a routine basis and involves a specification that often requires the concrete to meet several criteria. For example, on the Lacey V. Murrow floating bridge in Washington State, the concrete was specified to meet compressive strength, shrinkage and permeability requirements. The latter two requirements controlled the mix proportions so that the actual strength was well in excess of the specified strength. This occurred because of the interrelation between the three characteristics. Other recent commercial examples where more than one characteristic was required are given in Table 1.
High-strength concrete A high-strength concrete is always a high-performance concrete, but a high-performance concrete is not always a high-strength concrete. ACI defines a high-strength concrete as concrete that has a specified compressive strength for design of 6,000 psi (41 MPa) or greater. According to a paper(1) by Paul Zia of North Carolina State University, other countries use a higher compressive strength in their definitions of high-strength concrete with 7,000 psi (48 MPa) minimum. Other countries also specify a maximum compressive strength, whereas the ACI definition is open-ended.
The specification of high-strength concrete generally results in a true performance specification in which the performance is specified for the intended application, and the performance can be measured using a well-accepted standard test procedure. The same is not always true for a concrete whose primary requirement is durability.
Durable concrete Specifying a high-strength concrete does not ensure that a durable concrete will be achieved. In addition to requiring a minimum strength, concrete that needs to be durable must have other characteristics specified to ensure durability. In the past, durable concrete was obtained by specifying air content, minimum cement content and maximum water-cement ratio. Today, performance characteristics may include permeability, deicer scaling resistance, freeze-thaw resistance, abrasion resistance or any combination of these characteristics. Given that the required durability characteristics are more difficult to define than strength characteristics, specifications often use a combination of performance and prescriptive requirements, such as permeability and a maximum water-cementitious material ratio to achieve a durable concrete. The end result may be a high-strength concrete, but this only comes as a by-product of requiring a durable concrete.
Concrete materials Most high-performance concretes produced today contain materials in addition to portland cement to help achieve the compressive strength or durability performance. These materials include fly ash, silica fume and ground-granulated blast furnace slag used separately or in combination. At the same time, chemical admixtures such as high-range water-reducers are needed to ensure that the concrete is easy to transport, place and finish. For high-strength concretes, a combination of mineral and chemical admixtures is nearly always essential to ensure achievement of the required strength. Examples of concrete mixes for durable and high-strength concrete are shown in Table 2.
Most high-performance concretes have a high cementitious content and a water-cementitious material ratio of 0.40 or less. However, the proportions of the individual constituents vary depending on local preferences and local materials. Mix proportions developed in one part of the country do not necessarily work in a different location. Many trial batches are usually necessary before a successful mix is developed.
High-performance concretes are also more sensitive to changes in constituent material properties than conventional concretes. Variations in the chemical and physical properties of the cementitious materials and chemical admixtures need to be carefully monitored. Substitutions of alternate materials can result in changes in the performance characteristics that may not be acceptable for high-performance concrete. This means that a greater degree of quality control is required for the successful production of high-performance concrete.
HIGH VOLUME FLY ASH CONCRETE
Fly ash is a by-product from coal-fired electricity generating power plants. The coal used in these power plants is mainly composed of combustible elements such as carbon, hydrogen and oxygen (nitrogen and sulfur being minor elements), and non-combustible impurities (10 to 40%) usually present in the form of clay, shale, quartz, feldspar and limestone. As the coal travels through the high-temperature zone in the furnace, the combustible elements of the coal are burnt off, whereas the mineral impurities of the coal fuse and chemically recombine to produce various crystalline phases of the molten ash. The molten ash is entrained in the flue gas and cools rapidly, when leaving the combustion zone (e.g. from 1500°C to 200°C in few seconds), into spherical, glassy particles. Most of these particles fly out with the flue gas stream and are therefore called fly ash. The fly ash is then collected in electrostatic precipitators or bag houses and the fineness of the fly ash can be controlled by how and where the particles are collected.
The use of fly ash in concrete
Fly ash can be used in concrete as a partial replacement for ordinary portland cement (opc). Fly ash can be introduced in concrete directly, as a separate ingredient at the concrete batch plant or, can be blended with the opc to produce blended cement, usually called portland-pozzolana cement (ppc) in India. Fly ash blended cements are produced by several cement companies in India.
Generally speaking, currently in the concrete industry, the percentage of fly ash as part of the total cementing materials in concrete normally ranges from 15 to 25%, although it can go up to 30-35% in some applications. The use of fly ash in concrete will improve some aspects of the performance of the concrete provided the concrete is properly designed. The main aspects of the concrete performance that will be improved by the use of fly ash are increased long-term strength and reduced permeability of the concrete resulting in potentially better durability. The use of fly ash in concrete can also address some specific durability issues such as sulphate attack and alkali silica reaction. However, a few additional precautions have to be taken to insure that the fly ash concrete will meet all the performance criteria.
The table given below is a paper presented by Dr Wilbert Langley and Dr Gordon Leaman at the sixth CANMET/ ACI / JCI International Conference, held May 31 - June 5, 1998. These are the actual mixes used in demonstration projects throughout Canada to prove the practicality of using high-volume fly ash concrete for a variety of projects. The Parklane Development in Halifax, Nova Scotia, Canada is a seven story structure and was built with 55% high-volume fly ash concrete (high strength mix given in the table below) . Cast-in-place columns and beams were poured with concrete specified to meet design strengths of between 4,350 psi at 28 days and 7,250 psi at 120 days. Actual strengths developed exceeded required strengths by 30%-40% on an average
HIGH VOLUME FLY ASH CONCRETE
All mixes contained air entraining admixtures and superplasticizers
|
| Conventional Mix | Low Strength 55% Replacement | Medium Strength 55% Replacement | High Strength 55% Replacement |
Total Cementitious Content (c+fa) | 483 | 374 | 566 | 660 |
Cement (lb) | 483 | 166 | 250 | 300 |
Class F Fly Ash (lb) | 0 | 208 | 316 | 360 |
Sand (lb) | 1334 | 1467 | 1250 | 1266 |
Stone (lb) | 1700 | 1834 | 1834 | 1850 |
Water (lb) | 220 | 185 | 198 | 185 |
Water to Cement Ratio | 0.46 | 0.49 | 0.35 | 0.28 |
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| ||
Compressive Strength (psi) |
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|
| |
| 3 day | 4,600 | 1,250 | 2,320 | 3,190 |
7 day | 5,000 | 1,750 | 3,040 | 4,900 |
28 day | 6,500 | 3,350 | 5,500 | 8,300 |
91 day | 7,100 | 4,050 | 7,700 | 10,900 |
365 day | 7,550 | 7,400 | 10,000 | - |
|
|
|
|
|
Set Time (hours:minutes) |
|
|
|
|
Initial | 6:25 | 8:25 | 5:35 | - |
Final | 7:50 | 11:15 | 7:40 | - |
In the US, the State of Wisconsin has been using a 60% Class F fly ash in concrete mix since 1989. HVFA concrete has now found a commercial niche in the Sydney construction market and is being trialed for the Sydney Olympic facilities. For the Crown Casino project, Connell Wagner required highly durable and low drying shrinkage concrete for the construction of the 55,000 square meter basement that was located below the water table
Another benefit of using fly ash in concrete is that fly ash makes beautiful, "architectural" concrete. Fly ash of today is light in color and its extreme workability ensures smoother finishes. That most famous of architecturally exposed concrete buildings, the Jonas Salk Institute, was built with fly ash concrete. I have seen the NCCBM building located at Ballabhgarh & it still looks beautiful even after having weathered so many years.
Addition of fly ash in plaster virtually eliminates defects like crazing, map cracking, drying shrinkage cracks, debonding, grinning, expansion & popping.
SHRINKAGE COMPENSATION CONCRETE
Use of Shrinkage Compensating Concrete (SCC) In Pre-Stressed Concrete
PRE-STRESSED CONCRETE:
There are characteristics of shrinkage compensating concrete (SCC) that are similar to the objectives and methodology behind pre-stressed concrete. Pre-stressed concrete is defined as a concrete member with a pre-determined compressive force, or moment, built into the member so that the internal stresses, designed as a result of the members intended use, will be equal to or less then the pre-stressing stresses built into the member. Post-tensioning and pre-tensioning are methods of achieving pre-stressed concrete.
The objectives and methodology of using SCC to enhance the properties of concrete is very similar to the objectives and methodology of using pre-stressed to accommodate structural loadings. Shrinkage cracking control, combined with the other inherent advantages of SCC, make SCC a better material for pre-stressed concrete members.
SHRINKAGE COMPENSATING COMPONENT
ACI recognizes two methods of achieving SCC, ettringite crystal development or calcium hydroxide platelet crystal development. The inherent characteristics of the calcium hydroxide platelet system, developed by the use of CONEX, is the better system of the two due to its inherent likeness of chemical action during hydration of the cement in use. The advantages of the platelet SCC method makes it well suited for use in pre-stressed concrete in general and in post-tensioned concrete in particular.
DESIGN CRITERIA:
The primary design objective of pre-stressed concrete is to place a compressive force in the concrete member that would prevent the concrete from going into tension and failing under design load conditions. Failure usually being defined as tension or stress cracking occurring in the concrete member. The primary objective for using pre-stressed concrete is economic. This is due to the fact that a pre-stressed member, of the same physical dimensions as a conventional reinforced concrete member, will have a greater load carrying capability. Conversely, for a given design loading, a pre-stressed member will be smaller in dimension and weight then a conventional concrete reinforced member.
SCC - HOW IT WORKS:
Shrinkage compensating concrete (SCC) has a case history of placements free of shrinkage cracks due to the "pre-stressing" action (restrained expansion or RE) created within the concrete during hydration and curing. During this period several phenomena are taking place simultaneously within the concrete. The most important being expansion of the concrete matrix due to the chemical reaction of CONEX creating development of calcium hydroxide crystals, and bonding of the concrete to the reinforcing. While this is occurring, the RE created causes the concrete to go into compression. The calcium hydroxide system of expansion, that is the formation of the platelets that create the expansion, is approximately at the same rate as the curing of the concrete. As long as compressive stresses within the concrete are greater then the tensile stresses the concrete will remain in compression, and tension cracks will not appear. This characteristic is taken advantage of by using SCC in the construction of cast in place slabs on grade (warehouse floors, pavements, secondary containment structures for hazardous materials) and structural members (bridge decks, primary containment structures, buildings, etc.). The benefit of SCC in slabs on grade is the ability to place larger sections (i.e. 20,000 ft2 / 2,000 m2) without contraction joints and with a reduction, and often elimination, of edge curling. The advantage of using SCC in bridge deck and containment structures is the increase in imperviousness of the concrete as well as the lack of shrinkage cracking.
SCC AS APPLIED TO PRE-STRESSED CONCRETE:
Currently the use of SCC is expanding into the pre-cast industry and also into pre-stressed concrete applications. While there are examples of cast in place pre-stressed applications of SCC, it is still a long way off from being in general use. This is perhaps due to the lack of published data detailing the use of SCC in post-tensioned applications. Tests should be done to establish the required expansion for desired results. These tests would establish the amount of compressive stress in the member resulting from different dosages of CONEX and related degrees of post-tensioning. This would allow other member characteristics to be adjusted accordingly. An obvious goal of using CONEX is to increase the quality of an existing concrete product and/or reduce the production cost of that concrete product. The addition of CONEX will increase the internal compressive stresses in the post-tensioned concrete member(s) if the proper restraint is provided. How this will impact the member design and/or production methods will need to be developed, but it presents a different look at the potential of shrinkage compensation in pre-stressed concrete.
In general, CONEX SCC is similar to and compatible with pre-stressed concrete, and acts interdependently with the cement in use, developing the following advantages:
1. A pre-stressed member with a higher degree of internal compression to assure greater crack control.
2. More impermeable concrete.
3. Better edge finishing.
4. Possibly a way to reduce production costs through less breakage and discard.
FIBER REINFORCEMENTS
NON-STEEL REINFORCEMENT :
Some construction cannot tolerate the use of steel. For example, MRI machines have huge magnets, and require nonmagnetic buildings. Another example are toll-booths that read radio tags, and need reinforced concrete that is transparent to radio.
In some instances, the lifetime of the concrete structure is more important than its strength. Since corrosion is the main cause of failure of reinforced concrete, a corrosion-proof reinforcement can extend a structure's life substantially.
For these purposes some structures have been constructed using fiber-reinforced plastic rebar, grids or fibers. The "plastic" reinforcement can be as strong as steel. Because it resists corrosion, it does not need a protective concrete cover of 30 to 50 mm or more as steel reinforcement does. This means that FRP-reinforced structures can be lighter, have longer lifetime and for some applications be price-competitive to steel-reinforced concrete.
The main barrier to use of FRP reinforcement is the fact that it is neither ductile nor fire resistant. Structures employing FRP rebars may therefore exhibit a less ductile structural response, and decreased fire resistance.
However, the addition of short monofilament polypropylene fibers to the concrete during mixing may have the beneficial effect of reducing spalling during a fire. In a severe fire, such as the Channel Tunnel fire, conventionally reinforced concrete can suffer severe spalling leading to failure. This is in part due to the pore water remaining within the concrete boiling explosively; the steam pressure then causes the spalling
HISTORICAL PERSPECTIVE:
The concept of using fibers as reinforcement is not new. Fibers have been used as reinforcement since ancient times. Historically, horsehair was used in mortar and straw in mud bricks. In the early 1900s, asbestos fibers were used in concrete, and in the 1950s the concept of composite materials came into being and fiber reinforced concrete was one of the topics of interest. There was a need to find a replacement for the asbestos used in concrete and other building materials once the health risks associated with the substance were discovered. By the 1960s, steel, glass (GFRC), and synthetic fibers such as polypropylene fibers were used in concrete, and research into new fiber reinforced concretes continues today.
EFFECTS OF FIBER IN CONCRETE:
Fibers are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also lower the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete. Generally fibers do not increase the flexural strength of concrete, so it can not replace moment resisting or structural steel reinforcement. Some fibers reduce the strength of concrete.
The amount of fibers added to a concrete mix is measured as a percentage of the total volume of the composite (concrete and fibers) termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fiber length (l) by its diameter (d). Fibers with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fiber is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fiber usually segments the flexural strength and toughness of the matrix. However, fibers which are too long tend to "ball" in the mix and create workability problems.
Some recent research indicated that using fibers in concrete has limited effect on the impact resistance of concrete materials. This finding is very important since traditionally people think the ductility increases when concrete is reinforced with fibers. The results also pointed out that the micro fibers is better in impact resistance compared with the longer fibers.
The High Speed tunnel linings incorporated concrete containing 1 kg/m³ of polypropylene fibers, of diameter 18 & 32 μm, giving the benefits noted below.
Polypropylene fibers can:
Improve mix cohesion, improving pumpability over long distances
Improve freeze-thaw resistance
Improve resistance to explosive spalling in case of a severe fire
Improve impact resistance
Increase resistance to plastic
DEVELOPMENTS IN FIBER REINFORCED CONCRETE:
A new kind of natural fiber reinforced concrete (NFRC) made of cellulose fibers processed from genetically modified slash pine trees is giving good results. The cellulose fibers are longer and greater in diameter than other timber sources. Some studies were performed using waste carpet fibers in concrete as an environmentally friendly use of recycled carpet waste. A carpet typically consists of two layers of backing (usually fabric from polypropylene tape yarns), joined by CaCO3 filled styrene-butadiene latex rubber (SBR), and face fibers (majority being nylon 6 and nylon 66 textured yarns). Such nylon and polypropylene fibers can be used for concrete reinforcement
High performance fiber reinforced cementitious composites (HPFRCCs):
This particular class of concrete was developed with the goal of solving the structural problems inherent with today's typical concrete, such as its tendency to fail in a brittle manner under excessive loading and its lack of long term durability. The two important properties of HPFRCC's are
The remarkable ability to strain harden under excessive loading.
In layman's terms, this means they have the ability to flex or deform before fracturing, a behavior similar to that exhibited by most metals under tensile or bending stresses. Because of this capability, HPFRCCs are more resistant to cracking and last considerably longer than normal concrete.
Their low density.
A less dense, and hence lighter material means that HPFRCCs could eventually require much less energy to produce and handle, deeming them a more economic building material. Because of HPFRCCs' lightweight composition and ability to strain harden, it has been proposed that they could eventually become a more durable and efficient alternative to typical concrete.
HPFRCCs include the following ingredients: fine aggregates, a superplasticizer, polymeric or metallic fibers, cement, and water. Thus the principal difference between HPFRCC and typical concrete composition lies in HPFRCCs' lack of coarse aggregates. Typically, a fine aggregate such as silica sand is used in HPFRCCs.
One aspect of HPFRCC design involves preventing crack propagation, or the tendency of a crack to increase in length, ultimately leading to material fracture. This occurrence is hindered by the presence of fiber bridging, a property that most HPFRCCs are specifically designed to possess. Fiber bridging is the act of several fibers exerting a force across the width of a crack in an attempt to prevent the crack from developing further. This capability is what gives bendable concrete its ductile properties.
Proposed uses for HPFRCCs include bridge decks, concrete pipes, roads, structures subjected to seismic and non-seismic loads, and other applications where a lightweight, strong and durable building material is desired. Though HPFRCCs have been tested extensively in the lab and been employed in a few commercial building projects, further long term research and real world application is needed to prove the true benefits of this material.
The newly developed fiber reinforced concrete is named as Engineered Cementitious Composite (ECC).
It is 500 times more resistant to cracking
It is 40 percent lighter than traditional concrete
It can sustain strain-hardening up to several percent strain, resulting in a material ductility of at least two orders of magnitude higher when compared to normal concrete.
It has unique cracking behavior . When loaded to beyond the elastic range, ECC maintains crack width to below 100 µm, even when deformed to several percent tensile strains.
The basic mechanical properties of ECC are :
| ECC Material Properties |
|
|
| Ultimate Tensile Strength ( σCU ) |
| 4.6 MPa |
| Ultimate Strain ( εCU ) |
| 5.6 % |
| First Crack Stress ( σfc ) |
| 2.5 MPa |
| First Crack Strain ( εfc ) |
| .021 % |
| Modulus of Elasticity ( E ) |
| 22 GPa |
ECC's tensile strain hardening behavior has a capacity in the range of 3-7%,which means that unlike common concrete, which is brittle and breaks under that amount of strain, ECC will bend under the same stress, like a piece of sheet metal. The high ductility is achieved by optimizing the microstructure of the composite employing micromechanical models. ECC looks exactly like regular concrete, but under excessive strain, the ECC concrete bends because the distinctively coated matrix of fibers in the cement is allowed to slide within the cement. ECC is made using the same ingredients of regular concrete but without the use of coarse aggregate.
ECC has already been used by the Michigan Department of Transportation to patch a portion of the Grove Street Bridge deck over Interstate 94. The ECC patch was used as a replacement to the previously existent expansion joint that linked two deck slabs. Expansion joints, commonly used in bridges to allow for the seasonal expansion and contraction of the concrete decks, are an example of a ubiquitous construction practice that could eventually be eliminated through the use of bendable concrete. Other existent structures composed of ECC, include the Curtis Road Bridge in Ann Arbor, MI and the Mihara Bridge in Hokkaido, Japan. The deck of the Mihara Bridge, composed of bendable concrete, is only five centimeters thick and has an expected lifetime of one-hundred years.
Comparison to other composite materials:
| Properties | FRC | Common HPFRCC | ECC |
| Design Methodology | N.A. | Use high Vf | Micromechanics based, minimize Vf for cost and processibility |
Fiber | Any type, Vf usually less than 2%; df for steel ~ 500 micrometer | Mostly steel, Vf usually > 5%; df ~ 150 micrometer | Tailored, polymer fibers, Vf usually less than 2%; df < 50 micrometer |
| Matrix | Coarse aggregates | Fine aggregates | Controlled for matrix toughness, flaw size; fine sand |
| Interface | Not controlled | Not controlled | Chemical and frictional bonds controlled for bridging properties |
Mechanical Properties | Strain-softening: | Strain-hardening: | Strain-hardening: |
| Tensile strain | 0.1% | <1.5% | >3% (typical); 8% max |
| Crack width | Unlimited | Typically several hundred micrometers, unlimited beyond 1.5% strain | Typically < 100 micrometers during strain-hardening[1] |
Note: FRC=Fiber-Reinforced Cement. HPFRCC=High-Performance Fiber Reinforced Cementitious Composites
NANOMATERIALS
NANOTUBES AND NANOWIRES AS REINFORCEMENT:
In developing nano-composite materials, nanotubes and nanowires are expected to greatly improve the properties of the composites. Silicon carbide nanowires have been regarded as an excellent reinforcement for composites due to the high intrinsic strength of the materials. However, the silicon carbide nanowires have a smooth surface and are easily pulled out when the composites break because of the weak adhesion between the nanowires and the matrix. Therefore, we need to fabricate a contoured surface of the silicon carbide nanowires in order to improve the adhesion.
This led to the invention of a new type of silicon carbide nanowires – periodically twinned SiC nanowires, which have a contoured surface on the nanoscale. The nanowires – with a hexagonal cross section, a diameter of 50–300 nm and a length of tens to hundreds of micrometers – feature a zigzag arrangement of periodically twinned segments with a uniform thickness along the entire growth length. Computer simulation demonstrates that the zigzag columnar structure is formed by the stacking of hexagonal discs of {111} planes of SiC. A minimum surface energy and strain energy argument explain the formation of periodic twins in the SiC nanowires.
The twinning structure has made the nanowires exhibit different luminescence and chemical stability. A Chinese group showed that the silicon carbide nanowires with beaded morphology can greatly enhance the tensile strength of an epoxy composite. Therefore, the new type of twinned SiC nanowires is expected to find important applications in nano-composites.
MODIFICATION OF GLASSS FIBER USING PLAZMA POLYMERIZATION TECHNIQUE :
The plasma treated E-glass fiber improves the mechanical properties of acrylic resin denture base material, polymethylmethacrlyate (PMMA). Plasma surface treatment of fibers is used as reinforcement in composite materials to modify the chemical and physical properties of their surfaces with tailored fiber–matrix bonding strength.
Three different types of monomer 2-hydroxyethyl methacrylate (HEMA), triethyleneglycoldimethylether (TEGDME) and ethylenediamine (EDA) were used in the plasma polymerization modification of glass fibers. A radiofrequency generator was used to sustain plasma in a glass vacuum chamber. Glass fibers were modified at the same glow-discharge power of 25W and exposure time of 30min for each monomer. Fibers were incorporated into the acrylic with 1% (w/w) loading except control group. Specimens were prepared using a standard mould of 3cm×0.5cm×0.8cm in dimension with eight specimens in each group. Samples were subjected to a flexural strength test set up at a crosshead speed of 5mm/min. Scanning electron microscopy (SEM) was used to examine the microstructure and X-ray photoelectron
Concluding Remarks
It has long been a concrete technologist's dream to discover a method of making concrete at the lowest possible water: cement ratio while maintaining high workability. To a considerable extent this dream may be fulfilled with the advent of superplasticizers. They have added a new dimension to the application of admixtures, and have made it possible to produce concrete with compressive strength of the order of 90 MPa.
Superplasticizers have other possible applications. Energy conservation and diminishing supplies of high quality raw materials will increasingly necessitate the use of marginal quality cements and aggregates. In such instances the use of superplasticizers may permit production of concrete at low water:cement ratios that will be strong enough to meet normal performance requirements. There are literally countless possible applications of superplasticizers, for example, in the production of fly ash concrete, blast furnace slag cement concrete, composites with various types of fibres and lightweight concrete. In addition, the dispersing effect of superplasticizers is not limited to portland cement and may find application in other cementitious systems.
The fact that superplasticizers show remarkable advantages in producing concrete should not imply that there are no problems associated with their use. A satisfactory solution to the high rate of slump loss in superplasticized concrete is yet to be found and the relative effects of materials, production methods and external conditions that influence this phenomenon are not completely understood. Further study will be necessary of the compatibility of other admixtures such as retarders, accelerators and air-entraining agents in combination with superplasticizers. Though surface area, tricalcium aluminate, and sulphate contents seem to influence slump, no definite trend has been established.
Most available data on superplasticized concrete have been obtained using SMF- and SNF-based superplasticizers. Even within a single type, variations in behaviour have occurred, possibly because of the differences in molecular weight and in the type of cation associated with the superplasticizer. Consequently it is difficult to predict exactly the properties and behaviour of superplasticized concrete. As more data become available, especially on the long-term behaviour of these concretes, it will be possible to formulate standards and codes of practice. The future use of superplasticizers will, however, be dictated by cost factors (of admixture and operating costs) and by acceptance by industry
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