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# How Concrete is Made

In its simplest form, concrete is a mixture of paste an d aggregates, or rocks. The paste, composed of portland cement and water, coats the surface of t he fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete. Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when new ly mixed, strong and durab le when hardened. These qualities explain why one material, concrete, can build skyscrapers, brid ges, sidewalks and superhighways, houses and dams. 

## Proportioning The key to achieving a strong, durable conc rete rests in the careful proportioning and mixing of the ingredients. A mixture that does not ha ve enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough surfaces and porous Page 1 of 5 How Concrete is Mad e

8 / 10 /201 5 http://www.cement.org/cement-concret e - b asics/how-concret e -is-mad econcrete. A mixture with an e xcess of cement paste will be easy to plac e and will produce a smooth surface; however, the resulting concrete is not cost-effective and can more easily crack. Portland cement's chemistry come s to life in the presence of water. Cement and water form a paste that coats each parti cle of stone and sand—the aggreg ates. Through a chemical reaction called hydration, the ce ment paste hardens and gains strength. The quality of the paste determines the character of the concrete. The strength of the paste, in turn, depends on the rati o of water to cement. The wate r-cement ratio is the weight of the mixing water divided by t he weight of the cem ent. High-quality conc rete is produced by lowering the water-cement ra tio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated, and cured. A properly designed mixture poss esses the desired work ability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.



## Other Ingredients Almost any natural water thatis drinkable and has no prono unced taste or odor may be used as mixing water for concrete . Excessive impurities in mixi ng water not only may affect setting time and concrete strengt h, but can also cause efflore scence, staining, corrosion of reinforcement, volume in stability, and reduced du rability. Concrete mixture specifications usually set limits on chlorides, sulfates, alkalis, a nd solids in mixing wa ter unless tests can be performed to determine the effect the impurity ha s on the final concrete. Although most drinking water is suitable for mixing concre te, aggregates are chosen Page 2 of 5 How Concrete is Mad e

8 / 10 /201 5 http://www.cement.org/cement-concret e - b asics/how-concret e -is-mad ecarefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of aggregat e used depends on the thickness and purpose of the final concrete product Relatively thin building secti ons call for small coarse aggre gate, though aggregates up to six inches in diameter have been used in large dams. A continuous gradati on of particle sizes is desirable for efficient use of the paste. In addition, aggregates should be clean and free from any matter that might affe ct the quality of the concrete. 

## Hydration Begins Soon after the aggregates, water, and the cement are combin ed, the mixture starts to harden. All portland cements are hydraulic cements that set an d harden through a chemical reaction with water call hydrat ion. During this reaction, a nod e forms on the su rface of each cement particle. The node grow s and expands until itlinks up with nodes from other cement particles or adheres toadjacent aggregates. Once the concrete is thoroughlymixed and workable it should be placed in forms before the mixture becomes too stiff. During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, su ch as honeycombs and air pockets. For slabs, concrete is left to stand until the surfac e moisture film dis appears, then a wood or metal handfloat is used to smooth off the concrete. Floatin g produces a relatively even, but slightly rough, texture t hat has good slip re sistance and is frequent ly used as a final finish for exterior slabs. If a smooth, hard, dense surface is re quired, floating is followed by steel troweling. Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures th e continued hydration of the cement so that the concrete continues to gain st rength. Concrete surfaces are cured bysprinkling with wa ter fog, or by using moisture-retaining fabric s such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with pl astic or special sprays called curing compounds. Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. Th e longer the concrete is kept mois t, the stronger and more durable it will become. The rate of hardening dep ends upon the compositi on and fineness of the cement, the mix proportions, an d the moisture and temperat ure conditions. Concrete Page 3 of 5 How Concrete is Mad e

8 / 10 /201 5 http://www.cement.org/cement-concret e - b asics/how-concret e -is-mad econtinues to get st ronger as it gets older. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Return to Cement & Concrete Basics

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