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Home > Cement & Concrete Basics > How Concrete is Made

How Concrete is Made

In its simplest form, concrete is a 

mixture of paste and aggregates, or 

rocks. The paste, composed of 

portland cement and water, coats 

the surface of the 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 newly mixed, strong and durable when hardened. These 

qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and 

superhighways, houses and dams. 

Proportioning

The key to achieving a strong, durable concrete rests in the careful proportioning and mixing 

of the ingredients. A mixture that does not have enough paste to fill all the voids between 

the aggregates will be difficult to place and will produce rough surfaces and porous 

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concrete. A mixture with an excess of cement paste will be easy to place and will produce a 

smooth surface; however, the resulting concrete is not cost-effective and can more easily 

crack. 

Portland cement's chemistry comes to life in the presence of water. Cement and water form 

a paste that coats each particle of stone and sand—the aggregates. Through a chemical 

reaction called hydration, the cement 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 ratio of water to cement. The water-cement ratio is the weight 

of the mixing water divided by the weight of the cement. High-quality concrete is produced 

by lowering the water-cement ratio as much as possible without sacrificing the workability of 

fresh concrete, allowing it to be properly placed, consolidated, and cured.

A properly designed mixture possesses the desired workability 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 that is drinkable and has no pronounced taste or odor may be 

used as mixing water for concrete. Excessive impurities in mixing water not only may affect 

setting time and concrete strength, but can also cause efflorescence, staining, corrosion of 

reinforcement, volume instability, and reduced durability. Concrete mixture specifications 

usually set limits on chlorides, sulfates, alkalis, and solids in mixing water unless tests can 

be performed to determine the effect the impurity has on the final concrete.

Although most drinking water is suitable for mixing concrete, aggregates are chosen 

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carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type 

and size of aggregate used depends on the thickness and purpose of the final concrete 

product

Relatively thin building sections call for small coarse aggregate, though aggregates up to six 

inches in diameter have been used in large dams. A continuous gradation of particle sizes is 

desirable for efficient use of the paste. In addition, aggregates should be clean and free 

from any matter that might affect the quality of the concrete.

Hydration Begins

Soon after the aggregates, water, and the cement are combined, the mixture starts to 

harden. All portland cements are hydraulic cements that set and harden through a chemical 

reaction with water call hydration. During this reaction, a node forms on the surface of each 

cement particle. The node grows and expands until it links up with nodes from other cement 

particles or adheres to adjacent aggregates. 

Once the concrete is thoroughly mixed 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, such as honeycombs and air pockets.

For slabs, concrete is left to stand until the surface moisture film disappears, then a wood 

or metal handfloat is used to smooth off the concrete. Floating produces a relatively even, 

but slightly rough, texture that has good slip resistance and is frequently used as a final 

finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by 

steel troweling.

Curing begins after the exposed surfaces of the concrete have hardened sufficiently to 

resist marring. Curing ensures the continued hydration of the cement so that the concrete 

continues to gain strength. Concrete surfaces are cured by sprinkling with water fog, or by 

using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods 

prevent evaporation of the water by sealing the surface with plastic or special sprays called 

curing compounds. 

Special techniques are used for curing concrete during extremely cold or hot weather to 

protect the concrete. The longer the concrete is kept moist, the stronger and more durable it 

will become. The rate of hardening depends upon the composition and fineness of the 

cement, the mix proportions, and the moisture and temperature conditions. Concrete 

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continues to get stronger 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.

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