High Strength Concrete

 Concrete is generally classified as Normal Strength Concrete (NSC), High Strength Concrete (HSC) and Ultra High Strength Concrete (UHSC). Indian Standard Recommendations about the Mix Designs is that the boundary of 35 MPa is between NSC and HSC. But in International High strength label was applied to concrete having strength above 40 MPa. More recently, the threshold rose to 50-60 MPa. Concrete of very high strength entered in the field of construction of high-rise buildings and long span bridges. 

The advent of Prestressed Concrete Technology has given impetus for making concrete of higher strength. In India, there are cases of using high strength concrete bridges. The first prestressed concrete bridge was built in Assam in 1949 for the Assam Rail Link at Siliguri in India. In fifty's a number of prestressed concrete structures were built using concrete of strength from 35 MPa to 45 MPa. 

There are other unconventional special methods for making high strength concrete. They are:   

• Seeding: These involves adding a small percentage of finely ground, fully hydrated Portland cement to the fresh concrete mix. The mechanism by which is supposed to aid strength development is difficult to explain. This method may not hold much promise. 
• Revibration: Concrete undergoes plastic shrinkage. Mixing water creates continuous capillary channels, bleeding, and water accumulates at some selected places. All these reduce the strength of concrete. Controlled revibration removes all most of these defects and increases the strength of concrete. 
• High Speed slurry mixing: This process involves the advance preparation of cement-water mixyure which is then blended with aggregate to produce concrete. Higher compressive strength obtained is attributed to more efficient hydration of cement particles and water achieved in the vigorous blending of cement paste. 
• Use of Admixtures: Use of water reducing agents are known to produce increased compressive strengths. 
• Inhibition of cracks: Concrete fails by the formation and propagation of cracks. If the propagation of cracks is inhibited, the strength will be higher. They appear to act as crack arresters without necessitating extra water for workability. Concrete cubes made in this way have yielded strength upto 105 MPa. 
• Sulphur Impregnation: Satisfactory high strength concrete have been produced by impregnating low strength porous concrete by sulphur. The process consists of moist curing the fresh concrete specimens for 24 hours, drying them at 120C for 24 hours. The sulphur-impregnated concrete has given strength upto 58 MPa. 
• Use of Cementitious aggregates: It has been found that use of cementitious aggregates has yielded high strength. When coarsely crushed, it makes a kind of aggregate known as ALAG. Using ALAG as aggregate, strength upto 125MPa has been obtained with water/cement ratio 0.32. 

Ultra High Strength Concrete 

As technology advances, it is but natural that concrete technologists are directing their attention beyond high strength concrete to ultra-high strength concrete. The following techniques are used for producing ultra-high strength concrete: 

Compaction by Pressure:- It has been pointed out earlier cement paste derives strength due to the combined effect of friction and bond. In ceramic material, grain size and porosity would be the most important parameters affecting friction and bond and hence the strength. 
Usually high strength has been generated in materials by employing "hot pressing" techniques and intermediate ranges of strengths have been achieved by applying high pressure at room temperature to Portland cement pastes. 

Helical Binding:-  This is an indirect method of achieving ultra-high strength in concrete. High tensile steel wire binding externally over the concrete cylinder results in good strength. 

Polymer Concrete:-  Impregnation of monomer into the pores of hardened concrete and then getting it polymerised by irradiation or thermal catalytic process, results in the development os very high strength. This method making ultra high strength concrete holds much promise. 

Reactive Powder Concrete:-  High Strength Concrete with strength of 100-120 MPa have been used for the construction of structural members. Concrete with 250-300 MPa are also used for non-structural applications such as flooring, safes and storage of nuclear wastes.