Generally concrete is classified as Normal strength concrete (NSC), High strength concrete (HSC) and ultra-high strength concrete (UHSC). However there is no clear cut boundary for the classification. Indian Standard for mix design has suggested the boundary at 35 MPa (350 kg/cm2) between Normal and High strength concretes. However no boundary has been fixed between High strength concrete and ultra-high strength concrete.
In international forum about in 1970, High strength label was applied to concrete having compressive strength of about 40 MPa (400 kg/cm2). However, recently this limit has been raised to 50 to 60 MPa. Since about 1985 in the world concrete having compressive strength of 90 to 120 MPa has been used in the construction of long span bridges and high rise buildings.
The advent of pre-stressed concrete Technique has given the impetus for the production of high strength concrete. In India the first pre-stressed concrete bridge was built in 1949 for the Assam Rail Link Bridge at Siliguri. In fifty’s a number of pre-stressed concrete structures were constructed using concrete of compressive strength of 35 to 45 MPa. For the construction of pre-stressed concrete bridge over Ram Ganga near Moradabad, Central Building Research Institute had developed a Mix Proportion for the production of 50 MPa concrete.
However in general construction works, concrete having strength more than 35 MPa was not used. In the construction of Konkan Railway project and Mumbai Municipal Corporation Roads Construction probably concrete of strength more than 35 MPa was used. In Indian Construction scenario only during 90’s high strength concrete has stepped in.
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Later on concrete of strength varying from 45 to 60 MPa has been used in high rise buildings in Mumbai, Delhi and other Metropolitan cities. High strength concrete was also employed in the construction of bridges and fly-overs. Recently in the year 2000 in Mumbai concrete having strength 75 MPa was used in the construction of fly-over. In India high strength concrete has also been used at Kaiga power project. Here high performance concrete of 60 MPa with silica fume has been used.
For the obvious benefits of high strength concrete, the manufacture of high strength concrete will grow and find its due place in concrete construction. Thus the ready mixed concrete is developing fast in India. In the modern batching plants high strength concrete is produced in a mechanical manner. However one has to take care about mix proportioning, shape and kind of aggregates, use of supplementary cementitious materials as silica fumes and super plasticizers. Now with the use of modern equipments and understanding of the role of the constituent materials etc., the production of high strength concrete has become a routine matter.
Methods of Production of High Strength Concrete:
There are many special methods of making high strength concrete.
Some of them are as follows:
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1. Use of admixtures
2. Re-vibration
3. High speed slurry mixing
4. Prevention of cracks
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5. Sulphur impregnation or Sulphur filling
6. Use of cementitious aggregates, and
7. Seeding.
1. Use of Admixture:
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Substances used for reducing water contents are known as admixtures.
2. Re-Vibration:
The mixing water in concrete creates continuous capillary channels, bleeding and water accumulation at certain selected places. All these factors reduce the strength of concrete. Thus concrete undergoes plastic shrinkage. The controlled re-vibration controls all these defects and increases the strength of the concrete.
3. High Speed Slurry Mixing:
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In this process, first cement and water mixture is prepared and then aggregate is blended to produce concrete. The development of higher compressive strength is attributed to the more efficient hydration of cement particles and water saved in the vigorous blending of cement paste.
4. Inhibition or Prevention of Cracks:
It has been observed that development of cracks in concrete is an inherent phenomenon. Concrete fails due to the formation and propagation of cracks. If the development and propagation of cracks is checked, the strength of concrete will increase. If 2 to 3% of fine aggregate is replaced by polythene or polystyrene ‘Lenticules’ pieces 0.025 mm thick and 3 to 4 mm in diameter, will result in the increase of concrete strength. The polythene pieces act to check the propagation of cracks without necessitating extra water for workability. Concrete cubes prepared by this method have shown strength upto 105 MPa.
5. Sulphur Filling or Impregnation:
By impregnating low strength porous concrete by Sulphur, sufficiently high strength concrete has been produced.
The process of impregnation is as follows:
(a) The fresh concrete specimens are cured moist for 24 hours.
(b) After moist curing they are dried at 120°C for 24 hours.
(c) After drying the specimens are immersed in molten sulphur under vacuum for two hours.
(d) After two hours of immersion, vacuum is released.
(e) The specimens are further immersed in molten sulphur for 1/2 hour for further infiltration of sulphur. Sulphur impregnated concrete has been found to give strength upto 58 MPa.
6. Use of Cementitious Aggregates:
The glassy slag clinker known as fondu is a cementitious material. When it is ground finely, it results in a kind of cement. When crushed coarsely it produces a kind of aggregate known as ALAG. Using ALAG as aggregate, concrete gives strength upto 125 MPa with water/cement ratio of 0.32.
7. Seeding:
This process involves adding a small percentage of finely ground, fully hydrated Portland cement to the fresh concrete mix. The Mechanism of developing high strength is difficult to explain. This method has not been found practical.
Production of Ultra High Strength Concrete:
The technological development has given impetus to the development of ultra-high strength concrete.
Techniques of Development of Ultra-High Strength Concrete:
Following techniques may be used for the production of ultra-high strength concrete:
1. Compaction by pressure
2. Helical binding
3. Polymerisation of concrete, and
4. Reactive powder concrete.
1. Compaction by Pressure:
The cement paste derives strength due to the combined effect of friction and bond. In ceramic (clay) materials, grain size and porosity are the most important parameters which affect friction and bond, hence the strength. Attempts have been made to reduce the grain size and porosity by the application of very high pressure at room temperature as well as at higher temperatures.
Unusually high strength has been generated in materials by using ‘hot pressing’ techniques. The intermediate ranges of strengths have been developed by applying high pressure at room temperature to port- land cement paste. At a temperature of 250°C, by subjecting cement paste to a pressure of 357 MPa (3570 kg/cm2).
Compressive strengths as high as 680 MPa and indirect tensile strength of 66 MPa has been obtained. The water/cement ratio used was 0.093. It has also been observed that hot pressed materials are volume stable. The micro structure of such materials is very compact. The porosity of such materials was found about 1.8%.
2. Helical Binding:
In this method high tensile steel wires are binded externally over the concrete cylinders. This results in good strength. This is an indirect method of achieving ultra-high strength in concrete.
3. Polymer Concrete:
The conventional hardened concrete after a period of moist curing contains a considerable amount of free water in voids. The water filled voids form a significant component of the total volume of concrete ranging from 5% in dense concrete to 15% in gap graded concrete. We know that 1% voids reduce the strength of concrete by 5%. Researchers have observed that by reducing the grain size and porosity in concrete by any means, the high strength concrete may be obtained.
Thus in the case of polymer concrete, a monomer is impregnated into the pores of the hardened concrete. After impregnation of monomer it is polymerized either by thermal catalytic process or by radiation process. On polymerization the monomer expands and fills the voids fully and holds there firmly forming a solid material, resulting in the development of very high strength.
4. Reactive Powder Concrete:
The concrete of 100 to 120 MPa strength has been used for construction of structural members and concrete with 250 to 300 MPa also has been used for nonstructural application such as flooring, safes and storage of nuclear wastes.
For the use of structural members, high ductility is needed along with high strength. Reactive powder concrete has been developed to meet both the requirements of strength and ductility. The strength of reactive powder concrete may vary from 200 MPa to 800 MPa with required ductility.
As we know the concrete is a heterogeneous material and the strength developed by cement paste is not fully retained when sand and aggregates are added. The Reactive powder concrete is made by using ground quartz less than 300 micron size, silica fume, synthesized precipitated silica, steel fibres about 10 mm in length and 180 micron in diameter in place of sand and aggregate i.e., the sand and coarse aggregate have totally been removed.