A rule-of-thumb as given below, may be used to find out the approximate yield of concrete from a given concrete mix.
If the proportion of concrete is a : b : c, i.e., if a parts of cement, b parts of sand and c parts of coarse aggregates are mixed by volume, the resulting concrete will have a volume of 2/3 (a + b + c).
Let w, a, b and c be absolute volumes of water, cement, fine aggregate and coarse aggregate respectively, then, w + a + b + c = 1.
Absolute volume = Weight of the materials / Apparent specific gravity x Unit wt of water Bulking of sand should be taken into account when volumetric proportioning of the aggregates is adopted. Otherwise, less quantity of concrete per bag of cement will be produced, which naturally will increase the cost of concrete. Also, there will be less quantity of fine aggregate in the concrete mix which may make the concrete difficult to place.
Let the bulking of sand be 25%. Then, for the concrete of proportion 1 : 2 : 4, the actual volume of sand to be used will be 1.25 × 2 instead of 2 per unit volume of cement. If this correction is not applied, the actual dry sand in the concrete will be 1 / 1.25 x 2 = 1.60, instead of 2 per unit volume of cement. The proportion of concrete will then be 1 : 1.60 : 4. This indicates that less quantity of concrete will be produced and in most cases, there will not be enough quantity of fine aggregate to give a workable mix.
Fiber Reinforced Concrete
Plain concrete possesses a very limited tensile strength, ductility and limited resistance to cracking. Internal microcracks are present in concrete and its poor tensile strength is due to propagation of such microcracks leading to the brittle failure of concrete. The tensile reinforcement used provides tensile strength to the concrete members but however do not increase the inherent tensile strength of concrete itself. It has been recognized that addition of small, closely spaced and uniform dispersed fibres to concrete acts as crack arrester and also improve its static and dynamic properties substantially. Such type of concrete is known as fibre reinforced concrete.
Some of the fibres used includes steel fibres, polypropylene, nylons, asbestos, glass, carbon etc. However steel fibres of diameter varying from 0.25 mm to 0.75 mm are mostly commonly employed.
Fibres with low modulus of elasticity such as nylons and polypropylene do not contribute much to the flexural strength, but they help in absorption of large energy and thus impart greater degree of toughness resistance to impact. High modules fibres such as steel, carbon and glass impart strength and stiffness to the composite.
Increase in volume of fibre increases the tensile strength and toughness of the composite (approximately linear variation). However higher percentage of fibre is likely to cause segregation and harshness in mix.
With increase in aspect ratio of fibre (ratio of length to diameter) upto 75, the ultimate, strength of
concrete increases linearly. However beyond 75, relative strength and toughness gets reduced.
Fibres aligned parallel to the loading direction offers more tensile strength and toughness as compared to perpendicular or randomly oriented particles.
Fibre-reinforced concrete is used for road pavements, industrial fittings, airfield overlays, explosive resistance structure etc. It also finds its application in pre-cast products like beams, boats, roof panels etc.
