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Chap1 - HOME PAGE CHAPTER 1 Fundamentals of Concrete...

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Concrete is basically a mixture of two components: aggre- gates and paste. The paste, comprised of portland cement and water, binds the aggregates (usually sand and gravel or crushed stone) into a rocklike mass as the paste hardens because of the chemical reaction of the cement and water (Fig. 1-1). Supplementary cementitious materials and chemical admixtures may also be included in the paste.* Aggregates are generally divided into two groups: fine and coarse. Fine aggregates consist of natural or manufac- tured sand with particle sizes ranging up to 9.5 mm ( 3 / 8 in.); coarse aggregates are particles retained on the 1.18 mm (No. 16) sieve and ranging up to 150 mm (6 in.) in size. The maximum size of coarse aggregate is typically 19 mm or 25 mm ( 3 / 4 in. or 1 in.). An intermediate-sized aggregate, around 9.5 mm ( 3 / 8 in.), is sometimes added to improve the overall aggregate gradation. The paste is composed of cementitious materials, water, and entrapped air or purposely entrained air. The paste constitutes about 25% to 40% of the total volume of concrete. Fig. 1-2 shows that the absolute volume of cement is usually between 7% and 15% and the water between 14% and 21%. Air content in air-entrained concrete ranges from about 4% to 8% of the volume. Since aggregates make up about 60% to 75% of the total volume of concrete, their selection is important. Aggregates should consist of particles with adequate strength and resistance to exposure conditions and should not contain materials that will cause deterioration of the concrete. A continuous gradation of aggregate particle sizes is desirable for efficient use of the paste. Throughout this text, it will be assumed that suitable aggregates are being used, except where otherwise noted. The quality of the concrete depends upon the quality of the paste and aggregate, and the bond between the two. In properly made concrete, each and every particle of aggregate is completely coated with paste and all of the spaces between aggregate particles are completely filled with paste, as illustrated in Fig. 1-3. * This text addresses the utilization of portland cement in the production of concrete. The term “portland cement” pertains to a calcium silicate hydraulic cement produced by heating materials containing calcium, silicon, aluminum, and iron. The term “cement” used throughout the text pertains to portland cement or blended hydraulic cement unless otherwise stated. The term “cementitious materials” means portland or blended cement, used with or without supplementary cementitious materials. HOME PAGE
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The freshly mixed (plastic) and hardened properties of concrete may be changed by adding chemical admixtures to the concrete, usually in liquid form, during batching. Chemical admixtures are commonly used to (1) adjust setting time or hardening, (2) reduce water demand, (3) in- crease workability, (4) intentionally entrain air, and (5) ad- just other fresh or hardened concrete properties.
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