Transcript
SPECIAL PURPOSES
CONCRETES
Ahmed Mohamed Sallam
Sara Almana Khider
Introduction
The properties of concrete depend on the quantities and qualities of its components. Because cement is the most active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining most economically the balance of properties desired for any particular concrete mixture.
History of the origin of cement
It is uncertain where it was first discovered that a combination of hydrated non- hydraulic lime and a pozzolan produces a hydraulic mixture , but concrete made from such mixtures was first used on a large scale by Roman engineers. They used both natural pozzolans (trass or pumice) and artificial pozzolans (ground brick or pottery) in these concretes.
Many excellent examples of structures made from these concretes are still standing, notably the huge monolithic dome of the Pantheon in Rome and the massive Baths of Caracalla.
Baths of Caracalla
The Pantheon - Rome
Cement production
Cement is made by heating limestone (calcium carbonate), with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix .
The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make Portland cement
Cement composition
The phase compositions in Portland cement are denoted by ASTM as tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). the early hydration of cement is principally controlled by the amount and activity of C3A . , the C3S and C2S will have the primary influence on long term development of structure .
Cements high in C3S (especially those that are finely ground) will hydrate more rapidly and lead to higher early strength. However, the hydration products formed will, in effect, make it more difficult for hydration to proceed at later ages, leading to an ultimate strength lower than desired in some cases. Cements high in C2S will hydrate much more slowly, leading to a denser ultimate structure and a higher long-term strength.
Physical Properties of Portland Cements
ASTM C 150 and AASHTO M 85 have specified certain physical requirements for each type of cement. These properties include 1) fineness, 2) soundness, 3) consistency, 4) setting time, 5) compressive strength, 6) heat of hydration, 7) specific gravity, and 8) loss of ignition. Each one of these properties has an influence on the performance of cement in concrete.
The fineness of the cement, for example, affects the rate of hydration. Greater fineness increases the surface available for hydration, causing greater early strength and more rapid generation of heat .
Types of Portland cement
Different types of Portland cement are manufactured to meet different physical and chemical requirements for specific purposes, such as durability and high-early strength. Eight types of cement are covered in ASTM C 150 and AASHTO M 85. These types and brief descriptions of their uses are listed in Table 1.
Cement type Use
I1 General purpose cement, when there are no extenuating conditions
II2 Aids in providing moderate resistance to sulfate attack
III When high-early strength is required
IV3 When a low heat of hydration is desired (in massive structures)
V4 When high sulfate resistance is required
IA4 A type I cement containing an integral air-entraining agent
IIA4 A type II cement containing an integral air-entraining agent
IIIA4 A type III cement containing an integral air-entraining agent
Ordinary Portland Cement
Ordinary Portland cement (OPC) is the most important type of cement.
The OPC was classified into three grades, namely 33 grade, 43 grade and53 grade depending upon the strength of the cement at 28 days when tested as per IS 4031-1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement,
if the strength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not less than 53 N/mm2, it is called 53 grade cement.
Uses :-
for general purposes cement , when there is no extenuating condition .
Rapid Hardening Cement
This cement is similar to ordinary Portland cement. As the name indicates it develops strength rapidly and as such it may be more appropriate to call it as high early strength cement.
Rapid hardening cement develops at the age of three days, the same strength as that is expected of ordinary Portland cement at seven days.
The rapid rate of development of strength is attributed to the higher fineness of grinding and higher C3S and lower C2S content.
Uses:In pre-fabricated concrete construction. Where formwork is required to be removed early for
reuse. Road repair works. In cold weather concrete where the rapid rate of
development of strength reduces the vulnerability of concrete to the frost damage.
Extra Rapid Hardening Cement
Extra rapid hardening cement is obtained by intergrinding calcium chloride with rapid hardening Portland cement.
The normal addition of calcium chloride should not exceed 2 percent by weight of the rapid hardening cement.
The strength of extra rapid hardening cement is about 25 per cent higher than that of rapid hardening cement at one or two days and 10–20 per cent higher at 7 days.
The gain of strength will disappear with age and at 90 days the strength of extra rapid hardening cement or the ordinary Portland cement may be nearly the same.
Sulphate Resisting Cement
Ordinary Portland cement is susceptible to the attack of sulphates, in particular to the action of magnesium sulphate. Sulphates react both with the free calcium hydroxide in set cement to form calcium sulphate and with hydrate of calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% of the volume of the original aluminates.
Solid sulphate do not attack the cement compound. Sulphates in solution permeate into hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and even hydrated silicates.
The above is known as sulphate attack. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures in the zone of tidal variations.
To remedy the sulphate attack, the use of cement with low C3A content is found to be effective. Such cement with low C3A and comparatively low C4AF content is known as Sulphate Resisting Cement. In other words, this cement has a high silicate content. The specification generally limits the C3A content to 5 per cent.
Uses : Concrete to be used in marine condition; Concrete to be used in foundation and
basement, where soil is infested with sulphates;
Concrete used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;
Concrete to be used in the construction of sewage treatment works.
Low Heat Cement
It is well known that hydration of cement is an exothermic action which produces large quantity of heat during hydration.
Formation of cracks in large body of concrete due to heat of hydration has focused the attention of the concrete technologists to produce a kind of cement which produces less heat or the same amount of heat, at a low rate during the hydration process.
A low-heat evolution is achieved by reducing the contents of C3S and C3A which are the compounds evolving the maximum heat of hydration and increasing C2S.
A reduction of temperature will retard the chemical action of hardening and so further restrict the rate of evolution of heat. The rate of evolution of heat will, therefore, be less and evolution of heat will extend over a longer period.
Portland Blastfurnace Cement Portland Blastfurnace Cement contains up
to 70% ground granulated blast furnace slag, with the rest Portland clinker and a little gypsum. All compositions produce high ultimate strength, but as slag content is increased, early strength is reduced, while sulfate resistance increases and heat evolution diminishes. Used as an economic alternative to Portland sulfate-resisting and low-heat cements.
Portland Flyash Cement
contains up to 30% fly ash. The flyash is pozzolanic, so that ultimate strength is maintained. Because flyash addition allows a lower concrete water content, early strength can also be maintained. Where good quality cheap flyash is available, this can be an economic alternative to ordinary Portland cement.
Portland Pozzolan Cement Includes fly ash cement, since fly ash is
a pozzolan, but also includes cements made from other natural or artificial pozzolans. In countries where volcanic ashes are available (e.g. Italy, Chile, Mexico, the Philippines) these cements are often the most common form in use.
Portland Silica Fume cement Addition of silica fume can yield
exceptionally high strengths, and cements containing 5-20% silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer.
Calcium aluminate cements
are hydraulic cements made primarily from limestone and bauxite. The active ingredients are monocalcium aluminate CaAl2O4 (CA in Cement chemist notation) and Mayenite Ca12Al14O33 (C12A7 in CCN). Strength forms by hydration to calcium aluminate hydrates. They are well-adapted for use in refractory (high-temperature resistant) concretes, e.g. for furnace linings.
Influence of Portland Cement on Concrete PropertiesCement Property Cement Effects
Placeability Cement amount, fineness, setting characteristics
Strength Cement composition (C3S, C2S and C3A), loss on ignition, fineness
Drying Shrinkage SO3content, cement composition
Permeability Cement composition, fineness
Resistance to sulfate C3A content
Alkali Silica Reactivity Alkali content
Corrosion of embedded steel Cement Composition (esp. C3A content)
Other types of concretes
Polymer concrete
Polymer concrete is part of group of concretes that use polymers to supplement or replace cement as a binder. The types include polymer-impregnated concrete, polymer concrete, and polymer-Portland-cement concrete.
Composition In polymer concrete, thermosetting resins are
used as the principal polymer component due to their high thermal stability and resistance to a wide variety of chemicals. Polymer concrete is also composed of aggregates that include silica, quartz, granite, limestone, and other high quality material. The aggregate must be of good quality, free of dust and other debris, and dry. Failure of these criteria can reduce the bond strength between the polymer binder and the aggregate.
Uses Polymer concrete may be used for new
construction or repairing of old concrete. The adhesion properties of polymer concrete allow patching for both polymer and cementitious concretes. The low permeability of polymer concrete allows it to be used in swimming pools, sewer pipes, drainage channels, electrolytic cells for base metal recovery, and other structures that contain liquids. It can also be used as a replacement for asphalt pavement, for higher durability and higher strength.
Ferrocement is a composite material which is used in
building or sculpture with cement, sand, water and wire or mesh material—often called a thin shell in North America.
Ferrocement has great strength and economy. It is fireproof, earthquake safe and does not rust, rot or blow down in storms. It has a broad range of applications which include home building, creating sculptures, repair of existing artifacts and building boats and ships.
Fibre reinforced concrete Fibre reinforced concrete (FRC) is concrete
containing fibrous material which increases its structural integrity. It contains short discrete fibres that are uniformly distributed and randomly oriented. Fibres include steel fibres, glass fibres, synthetic fibres and natural fibres. Within these different fibres that character of fibre reinforced concrete changes with varying concretes, fibre materials, geometries, distribution, orientation and densities.
Roller-Compacted Concrete
Roller-Compacted Concrete (RCC) or rolled concrete is a special blend of concrete that has the same ingredients as conventional concrete but in different ratios. It has cement, water, and aggregates, but RCC is much drier and essentially has no slump. RCC is placed in a manner similar to paving, often by dump trucks or conveyors, spread by bulldozers or special modified asphalt pavers. After placement it is compacted by vibratory rollers.
RCC is typically used for concrete pavement, but it is increasingly used to build concrete dams because the low cement content causes less heat to be generated while curing than do conventional massive concrete pours.
Self Compacting Concrete (SCC)
Unlike the conventional concrete, self compacting concrete doesn't require compacting using external force from mechanical equipments such as an immersion vibrator; instead SSC is designed in such as way that it gets compacted using its own weight and characteristics.
Once applied, the self compacting property enables the concrete to fully reinforce around the steel structures and completely fill the space within the framework.
The self compacting of concrete is achieved without losing any kind of strength, stability, or change in properties.
How is SCC made ?
Self compacting concrete is a type of concrete, which is not a product of mixing substances having different properties but a combination of several mixes having the same flow characteristics.
Manufacturing of a Self Compacting Concrete requires three main aspects to be fulfilled. They are as follows:
High amount of water reducing substance or super plasticizers is added for obtaining high flowing characteristics.
A type of aggregate mixture is added to gain the desired compactness. Note that the aggregate content is of round shape and proportional in size in order to increase the locking tendency of the concrete.
Alteration of fluid properties is done to ensure a cohesive mix which will keep the aggregate and paste together. These fluid properties can be achieved by adding a high quantity of fine content such as cement fly-ash or by adding viscosity modifying admixtures (VMA).
High performance concrete
High performance concrete (HPC) has been defined as concrete that possesses high workability, high strength and high durability.
The primary application for HPC have been structures requiring long service lives such as oil drilling platform, long span bridges and parking structures. HPC still requires good construction practice and good curing to deliver high performance.
High Performance Concrete (HPC) is a concrete made with appropriate materials combined according to a selected mix design; properly mixed, transported, placed, consolidated and cured so that the resulting concrete will give excellent performance in the structure in which it is placed, in the environment to which it is exposed and with the loads to which it will be subject for its design life.
Mix proportions for high-performance concrete (HPC) are influenced by many factors, including specified performance properties, locally available materials, local experience, personal preferences, and cost. With today’s technology, there are many products available for use in concrete to enhance its properties.
Heavyweight concrete
Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or manufactured aggregates such as iron or lead shot. The main land-based application is for radiation shielding (medical or nuclear). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures.
Low density concrete The use of low-density concrete provides
several advantages with the primary, and most obvious, benefit being a 15 to 25% savings in weight without sacrificing the overall strength of a structure. However, the use of low-density concrete can result in a considerable impact to the structural design and cost of the structure. Light weight aggregate are used to produce low density concrete , the reduced density is derived from voids within the aggregate particles
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