![]() Chemical admixtures for concrete are described in ASTM C494. They include water-reducing agents, air-entraining agents, and setting agents. Chemical admixtures fall into three basic categories. Chemical and Mineral AdmixturesĪn admixture is a material, other than Portland cement, water and aggregate, that is used in concrete as it is mixed to modify the fresh or hardened concrete properties. Silica fume is used exclusively as an admixture. ![]() Similarly, ground granulated blast furnace slag reacts with water to form hydration products that provide the slag with cementitious properties.Ĭoal fly ash and ground granulated blast furnace slag can be blended with Portland cement prior to concrete production or added separately to a concrete mix (admixture). Silica fume is also a pozzolanic material consisting almost entirely (85 percent or more) of very fine particles (100 times smaller than Portland cement) that are highly reactive.Ĭoal fly ash produced during the combustion of subbituminous coal exhibits self-cementing properties (no additional activators, such as calcium hydroxide, are needed). Supplementary cementitious materials can affect the workability, heat released during hydration, the rate of strength gain, the pore structure, and the permeability of the hardened cement paste.Ĭoal fly ash that is produced during the combustion of bituminous coals exhibits pozzolanic properties. Self-cementing materials are materials that react with water to form hydration products without any activator. Descriptions of various kinds of pozzolans and their specifications are provided in ASTM C618. Pozzolanic materials are materials comprised of amorphous siliceous or siliceous and aluminous material in a finely divided (powdery) form, similar in size to Portland cement particles, that will, in the presence of water, react with an activator, typically calcium hydroxide and alkalis, to form compounds possessing cementitious properties. They typically include pozzolanic or self-cementing materials. Supplementary cementitious materials are sometimes used to modify or enhance cement or concrete properties. In terms of absolute volume, the cementing materials make up between about 7 and 15 percent of the mix, and water makes up 14 to 21 percent. Air is also a component of the cement paste, occupying from 1 to 3 percent of the total concrete volume, up to 8 percent (5 to 8 percent typical) in air entrained concrete. The paste normally constitutes about 25 to 40 percent of the total volume of the concrete. Portland cement combined with water forms the cement paste component of the concrete mixture. Most Portland cement particles are less than 0.045 mm (No. Portland cement is manufactured by crushing, milling, and blending selected raw materials containing appropriate proportions of lime, iron, silica, and alumina. Portland cement typically makes up about 15 percent of the PCC mixture by weight. Portland cements are hydraulic cements that set and harden by reacting with water, through hydration, to form a stonelike mass. Portland Cement and Supplementary Cementitious Materials ![]() Proper aggregate grading, strength, durability, toughness, shape, and chemical properties are needed for concrete mixture strength and performance. The coarse and fine aggregates used in PCC comprise about 80 to 85 percent of the mix by mass (60 to 75 percent of the mix by volume). In addition to these basic components, supplementary cementitious materials and chemical admixtures are often used to enhance or modify properties of the fresh or hardened concrete. ![]() The aggregate functions as a filler material, which is bound together by hardened Portland cement paste formed by chemical reactions (hydration) between the Portland cement and water. MATERIALSīasic components of PCC include coarse aggregate (crushed stone or gravel), fine aggregate (usually natural sand), Portland cement, and water. In either case, the PCC is then dumped, spread, leveled, and consolidated, generally using concrete slip-form paving equipment. Portland cement concrete is produced at a central plant and transported to the job site in transit mixers or batched into truck mixers directly and then mixed at the project site. In some cases the PCC slab may be overlaid with a layer of asphalt concrete. Portland cement concrete (PCC) pavements (or rigid pavements) consist of a PCC slab that is usually supported by a granular or stabilized base, and a subbase. Portland Cement Concrete Pavement INTRODUCTION
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