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Concrete is a mixture of sand and rock or similar inert material (aggregates) held together by a cementing material. Usually the cementing material is Portland cement, but sometimes binders such as asphalt or gypsum are used, in which case the concrete may be called asphaltic concrete or gypsum concrete. Properties of concrete are governed not only by the properties of its ingredients (cement, water, sand, and coarse aggregate) but also, to a great extent, by the relative proportions of these ingredients.
The proportions must be so selected as to produce a concrete mixture of desired workability, strength, durability, and economy. The most common aggregates are gravel and crushed stone, although cinders, blast-furnace slag, burned shale, crushed brick, or other materials may be used because of availability, or to alter such characteristics of the concrete such as workability, density, appearance, or conductivity of heat or sound. Usually aggregate which passes a sieve with 0. 187-inch openings (No. 4 sieve) is called fine aggregate, but that retained by a No.
sieve is coarse aggregate, although the division is purely arbitrary. If all the particles of aggregate are of the same size, or if too many fine particles are present, an excessive amount of cement paste will be required to produce a workable mixture; a range of sizes aids in the production of an economical mixture. The best concrete for a given use is usually the one which will provide the necessary strength and the desired workability at the lowest cost. Unless otherwise indicated, strength, as applied to concrete, refers to the ultimate compressive strength of the moist-cured concrete at the age of 28 days.
Most concretes are batched to provide an ultimate compressive strength of 2500 to 4000 psi after 28 days. The figure below shows a typical strength curve of concrete with the passage of time. The modulus of elasticity of concrete is about 1000 times the ultimate compressive strength. The strength of concrete depends chiefly on the water-cement ratio, with a low ratio producing a strong concrete. While only a small amount of water is required to complete the chemical reactions of setting concrete, more than this is used to make the concrete more workable.
The workability of concrete is usually measured by its slump. The standard method of measuring slump consists of placing the freshly-mixed concrete in a mold in the form of a truncated cone, 12 inches high, 8 inches in diameter at the bottom, and 4 inches in diameter at the top. The concrete is placed in the slump cone in three layers, each layer rodded thoroughly to compact it. When filled, the mold is immediately withdrawn by lifting it gently, and the slump of the concrete is measured at the vertical distance from the top of the mass to its original 12 inch height.
An increase in the amount of mixing water will increase the slump, but it will also decrease the strength and increase the tendency of the ingredients of the concrete to segregate unless more cement is added. Increasing the amount of cement paste increases the cost, so all three factors- strength, workability, and cost-are interrelated in a complex way. Procedure: 1. Concrete mixtures are commonly given as volume ratios as cement: sand: gravel. You will make two concrete mixtures at ratios given to you by the instructor.
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