CHAPTER 6 CONCRETE

Concrete is one of the most important con- struction materials. It is comparatively economical, easy to make, offers continuity and solidity, and will bond with other materials. The keys to good-quality concrete are the raw materials required to make concrete and the mix design as specified in the project specifications. In this chapter, we'll discuss the characteristics of concrete, the ingredients of con- crete, concrete mix designs, and mixing concrete. We'll conclude the chapter with a discussion of precast and tilt-up concrete. At the end of the discussion, we provide helpful references. You are encouraged to study these references, as required, for additional information on the topics discussed.

CONCRETE CHARACTERISTICS

LEARNING OBJECTIVE: Upon completing this section, you should be able to define the characteristics of concrete.

Concrete is a synthetic construction material made by mixing cement, fine aggregate (usually sand), coarse aggregate (usually gravel or crushed stone), and water in the proper proportions. The product is not concrete unless all four of these ingredients are present.

CONSTITUENTS OF CONCRETE

The fine and coarse aggregates in a concrete mix are the inert, or inactive, ingredients. Cement and water are the active ingredients. The inert ingredients and the cement are first thoroughly mixed together. As soon as the water is added, a chemical reaction begins between the water and the cement. The reaction, called hydration, causes the concrete to harden. This is an important point. The hardening process occurs through hydration of the cement by the water, not by drying out of the mix. Instead of being dried out, concrete must be kept as moist as possible during the initial hydration process. Drying out causes a drop in water content below that required for satisfactory hydration of the cement. The fact that the hardening process does not result from drying out is clearly shown by the fact that concrete hardens just as well underwater as it does in air.

CONCRETE AS BUILDING MATERIAL

Concrete may be cast into bricks, blocks, and other relatively small building units, which are used in concrete construction. Concrete has a great variety of applications because it meets structural demands and lends itself to architectural treatment. All important building elements, foundations, columns, walls, slabs, and roofs are made from concrete. Other concrete applications are in roads, runways, bridges, and dams.

STRENGTH OF CONCRETE

The compressive strength of concrete (meaning its ability to resist compression) is very high, but its tensile strength (ability to resist stretching, bending, or twisting) is relatively low. Consequently, concrete which must resist a good deal of stretching, bending, or twisting - such as concrete in beams, girders, walls, columns, and the like - must be reinforced with steel. Concrete that must resist only compression may not require reinforcement. As you will learn later, the most important factor controlling the strength of concrete is the water-cement ratio, or the proportion of water to cement in the mix.

DURABILITY OF CONCRETE

The durability of concrete refers to the extent to which the material is capable of resisting deteriora- tion caused by exposure to service conditions. Concrete is also strong and fireproof. Ordinary structural concrete that is to be exposed to the elements must be watertight and weather-resistant. Concrete that is subject to wear, such as floor slabs and pavements, must be capable of resisting abrasion.

The major factor that controls the durability of concrete is its strength. The stronger the concrete, the more durable it is. As we just mentioned, the chief factor controlling the strength of concrete is the water-cement ratio. However, the character, size, and grading (distribution of particle sizes between the largest permissible coarse and the smallest permissible fine) of the aggregate also have important effects on both strength and durability. However,