Skip to main content

Posts

Prestressed concrete

The basic concept of prestresssing the concrete consists in introducing the artificially the compressive stresses in a structure before it is loaded. The tensile stresses in the prestressed concrete structure may be reduced to a great extent or even entirely eliniminated depending upon the magnitude of prestressing. In a prestressed concrete structure, the cost of supporting structure and foundation is reduced, dead load of structure is reduced and cracking of concrete is avoided. The high strength concrete and high tensile steel should be used in a prestressed concrete member. According to Indian standards, the cube strength of the concrete used should bot be less  than 35N/mm 2 . The ultimate strength of high tensile steel wires used in prestressing varies from 1500N/mm 2 for 8mm diameter bars to 2350N/mm 2 for 1.5 mm diameter bars. The various methods adopted in prestresssing are as follows: Pre-tensioning The method of providing desired amount of compressive stress in
Recent posts

Bond stress and Development length

When the steel bars are embedded in concrete, the concrete after setting, adheres to the surface of the bars and resist any force that tends to pull or push this rod. The intensity of the adhesive force is called bond stress . It is the longitudinal shear stress acting on the surface between the steel and the concrete. The term bond  is used to describe the means by which slip between the steel and concrete is prevented. The bond is provided by anchoring the bars properly and extending the bars beyond the point of maximum shear. OR Bond stress is defined as the shear force per unit of Nominal surface area of a reinforcement bar acting parallel to the bar on the interface between the bars and surrounding concrete. The bond between steel and concrete is mainly due to pure adhesive resistance, frictional resistance and mechanical resistance. The bond stress developed in concrete around the steel reinforcement is given by Design bond stress For deformed bars these values s

RC Columns

A Column may  be defined as element used primarily to support axial compression loads and with a height of at least three times its least lateral dimension. When the ratio of effective length of column to its least lateral dimension doesn't exceed 12, then it is termed as short column. When the ratio of effective length of column to its least lateral dimension exceeds 12, then the column is considered as long column or slender column. Pedestal: It is a short column of effective length not greater than 3 times of least lateral dimension. In case of pedestal in which the longitudinal reinforcement is not taken in account in strength calculations, nominal longitudinal reinforcement not less than 0.15 percent of the cross-sectional area shall be provided. Points to Remember The columns are reinforced with longitudinal, transverse or helical reinforcement. The longitudinal reinforcement is also termed as main steel. The transverse reinforcement is in the form of links

30 important objective questions of RCC

Ques 1) How is the deflection in RC beams controlled as per IS:456 ? (a) by using large aspect ratio (b) by using small modular ratio (c) by controlling span/depth ratio (d) by controlling water cement ratio Ques 2) In a cantilever beam carrying gravity load, main reinforcement is provided  (a) above the neutral axis (b) as vertical stirrups (c) as a helical reinforcement (d) below the neutral axis Ques 3) The minimum grade for hydraulic structure as per IS code recommendation is  (a) M15 (b) M20 (c)M30 (d)M40 Ques 4) Normally pre-stressing wires are arranged in the (a) upper part of the beam (b)lower part of the beam (c) centre (d) anywhere Ques 5) The ratio of permissible shear stress in LSM of design and WSM of design is  (a) 25:16 (b) 5:4 (c) 16:25 (d) 4:5 Ques 6)  Lever arm is the distance between  (a) centre of tensile and compressive zone (b) centre of tensile zone and bottom most layer of beam (c) The result

Lime

The Lime is calcium oxide (CaO) obtained by calcination of pure limestone (white chalk), kankar and other calcarious substances. It is not found in nature in the fress state. It is used as a materix for concrete, for plastering walls, ceiling etc,  For improving soil for agricultural purposes and Used in the production of glass.  It is also used as a flux in the manufacture of steel and Used in  the manufacture of paints Hydraulicity :- It is the property of lime by which sets in damp places, water or thick masonary walls where there is no circulation of air. Calcination :- It is the process in which lime is heated to redness in contact with air. Slaking :- Process of addition of water to quick lime is called slaking. Constituents of lime  Clay :-  Presence of clay in lime impacts hydraulicity to it and makes it insoluble in water. It should be in the range of 8 to 30% Soluble silicates: - Silicates in form of  calcium, magnesium and aluminium also impacts hydrauli

Ferro-cement OR Ferrocement

According to ACI (American concrete Institute), Ferrocement is a type of thin wall reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small size wire mesh. The mesh may be made of metallic or other suitable materials. Ferrocement differs from conventional concrete in that there is a higher ratio of steel to cement mortar. Ferrocement has many properties of steel and yet it will not rust. Although it looks and feels like concrete it can flex without cracking. By altering the steel/cement ratio we actually produce a material, which is superior to either steel or cement seperately. History of ferro-cement In 1940s, an Italian engineer Pier Luigi Nervi tested and presented in his paper, a new structural elements, an extremely thin plate of concrete made layers of small diameter wire mess and cement mortar with sand is used. In 1980, ACI gives standards of use and mixes  of ferro-cement. Prope

Orifice and hydraulic coefficients

The orifice is a small opening in the wall or base of a vessel through which the fluid flows. A mouthpiece is an attachment in the form of a small tube or pipe fixed to the orifice. Its length is usually two or three times the diameter of orifice. It is used to increase the amount of discharge. Hydraulic coefficients Coefficient of contraction :- It is defined as the ratio of area of jet at vena contracta (the points at which the streamlines first become parallel is called vena contracta) to the area of orifice. Coefficient of velocity :- It is defined as the ratio of the actual velocity of the jet at vena contracta to the theoretical velocity. Coefficient of discharge (C d ) :- It is defined as the ratio of the actual discharge through the orifice to the theoretical discharge. The coefficient of discharge is equal to the product of coefficient of contraction and coefficient of velocity. Coefficient of resistance :- It is defined as the ratio of loss of head in the orifice to