Chapter OneSTRUCTURAL USE OF CONCRETE
This chapter presents some of the considerations for the use of concrete for structural purposes in building construction.
1.1 CONCRETE AS A STRUCTURAL MATERIAL
Concrete consists of a mixture that contains a mass of loose, inert particles of graded size (commonly sand and gravel) held together in solid form by a binding agent. That general description covers a wide range of end products. The loose particles may consist of wood chips, industrial wastes, mineral fibers, and various synthetic materials. The binding agent may be coal tar, gypsum, portland cement, or various synthetic compounds. The end products range from asphalt pavement, insulating fill, shingles, wall panels, and masonry units to the familiar sidewalks, roadways, foundations, and building frameworks.
This book deals primarily with concrete formed with the common binding agent of portland cement, and a loose mass consisting of sand and gravel. This is what most of us mean when we use the term concrete. With minor variations, this is the material used mostly for structural concrete-to produce building structures, pavements, and foundations.
Concrete made from natural materials was used by ancient builders thousands of years ago. Modern concrete, made with industrially produced cement, was first produced in the early part of the nineteenth century when the process for producing portland cement was developed. Because of its lack of tensile strength, however, concrete was used principally for crude, massive structures-foundations, bridge piers, and heavy walls.
In the late nineteenth century, several builders experimented with the technique of inserting iron or steel rods into relatively thin structures of concrete to enhance their ability to resist tensile forces. This was the beginning of what we now know as reinforced concrete. Many of the basic forms of construction developed by these early experimenters have endured to become part of our common technical inventory for building structures.
Over the years, from ancient times until now, there has been a steady accumulation of experience derived from experiments, research, and, most recently, intense development of commercial products. As a result, there is currently available to the building designer an immense variety of products under the general classification of concrete. This range is somewhat smaller if major structural usage is required, but the potential variety is still significant.
1.2 COMMON FORMS OF CONCRETE STRUCTURES
For building structures, concrete is mostly used with one of three basic construction methods. The first is called sitecast concrete, in which the wet concrete mix is deposited in forms at the location where it is to be used. This method is also described as cast-in-place or in situ construction.
A second method consists of casting portions of the structure at a location away from the desired location of the construction. These elements-described as precast concrete-are then moved into position, much as blocks of stone or parts of steel frames are.
Finally, concrete may be used for masonry construction-in one of two ways. Precast units of concrete called concrete masonry units (CMUs), may be used in a manner similar to bricks or stones. Alternately, concrete fill may be used to produce solid masonry by being poured into cavities in masonry produced with bricks, stone, or CMUs. The latter technique, combined with the insertion of steel reinforcement into the cavities, is widely used for masonry structures today. The use of concrete-filled masonry, however, is one of the oldest forms of concrete construction-used extensively by the Romans and the builders of early Christian churches.
Concrete is produced in great volume for various forms of construction. Building frames, walls, and other structural systems represent a minor usage of the total concrete produced. Pavements for sidewalks, parking lots, streets, and ground-level floor slabs in buildings use more concrete than all the building frameworks. Add the usage for the interstate highway system, water control, marine structures, and large bridges and tunnels, and building structural usage shrinks considerably in significance. One needs to understand this when considering the economics and operations of the concrete industry.
Other than pavements, the widest general use of concrete for building construction is foundations. Almost every building has a concrete foundation, whether the major above ground construction is concrete, masonry, wood, steel, aluminum, or fabric. For small buildings with shallow footings and no basement, the total foundation system may be modest, but for large buildings and those with many belowground levels, there may well be a gigantic underground concrete structure.
For above ground building construction, concrete is generally used in situations that fully realize the various advantages of the basic material and the common systems that derive from it. For structural applications, this means using the major compressive resistance of the material and in some situations its relatively high stiffness and inertial resistance (major dead weight). However, in many applications, the nonrotting, vermin- and insect-resistive, and fire-resistive properties may be of major significance. And for many uses, its relatively low bulk-volume cost is important.
Elements of Concrete Structures
Formation of a concrete structural system for a building usually consists of the assemblage of individual structural elements. Most commonly used structural systems are combinations of a few basic elements; these are:
Structural columns, piers, or other single supports
The actions of these individual elements and their various interactions for structural functions must be considered when designing building structures. Concrete is also widely used for foundations, and the common elements utilized for this purpose are:
Wall and single-column-bearing footings
Pile caps for clusters of piles
Piers, cast as columns in excavated holes
Consideration is given to each of these individual elements in this book. Some of the possibilities for their use in whole, assembled structures are illustrated in the building case study examples in Chapter 16.
Many special elements are also typically required for the completion of any building structure, such as pilasters, brackets, keys, pedestals, column caps, and so on. These are necessary, but essentially secondary, elements of the basic systems. Various situations for their use are illustrated in this book.
Many structures of more exotic forms can be realized with concrete beyond the simple systems treated in this book. Arches, domes, thin shells, folded plates, and other imaginative systems have been developed by designers who push the limits of the material's potentialities. We hope that readers may have the opportunity to work on such exciting structures at some time. Here, we start with the simplest, and most commonly used, structures.
1.3 PRIMARY SITUATIONS FOR INVESTIGATION AND DESIGN
A critical step in the visualization of structural behaviors is the consideration of the basic internal structural actions that occur in structural members. The five primary actions of internal structural resistance are tension, compression, shear, bending, and torsion. The structural functions of all the basic elements described previously can be developed with combinations of these basic internal actions.
There is another level down, of course, consisting of the basic stress actions that are a material's direct response to structural forces. Thus all the internal force actions can be produced from the basic stresses of tension, compression, and shear. For some materials the character of the stress is a critical concern since the material responds differently to the different stresses. Such is indeed the case with concrete, for which development of tension stress is a problem; this is the starting point for the design of reinforcement.
For our purposes here, it is useful to start with a basic element: the beam. This immediately presents all three basic stresses in the development of bending and shear for the basic beam action. And it makes a case for reinforcement, to develop significant internal tension for bending resistance, as well as an enhanced resistance to shear. The spanning slab represents essentially a variation on the basic beam function.
The second basic element to be considered is the column; that is the element whose basic task is resistance to compression. Variations here consist of the pier or pedestal (a very short column) and the bearing wall.
Finally, for the assembled system, a significant consideration is the interaction of elements in various framed configurations. This introduces the problem of joints or connections between elements, with the various force transfers necessary through the joints. It also involves consideration of the effects of one element on others to which it is connected; for example, the actions of adjacent beams on each other when continuity between spans occurs, and the interactions of beams and columns in a planar frame with continuous elements.
1.4 MATERIALS AND NATURE OF STRUCTURAL CONCRETE
This section presents discussions of the various ingredients of structural concrete and factors that influence the physical properties of the finished concrete. Other elements used to produce concrete structures are also discussed.
Common Forms of Structural Concrete
For serious structural usage, concrete must attain significant strength and stiffness, reasonable surface hardness, and other desired properties. While the mixture used to obtain concrete can be almost endlessly varied, the controlled mixes used for structural applications are developed within a quite limited set of variables. The most commonly used mix contains ordinary portland cement, clean water, medium-to-coarse sand, and a considerable volume of some fairly large pieces of rock. This common form of concrete will be used as a basis for comparison of mixes for special purposes.
Figure 1.1 shows the composition of ordinary concrete. The binder consists of the water and cement, whose chemical reaction results in the hardening of the mass. The binder is mixed with some aggregate (loose, inert particles) so that the binder coats the surfaces and fills the voids between the particles of the aggregate. For materials such as grout, plaster, and stucco, the aggregate consists of sand of reasonably fine grain size. For concrete the grain size is extended into the category of gravel, with the maximum particle size limited only by the size of the structure. The end product-the hardened concrete-is highly variable, due to the choices for the individual basic ingredients; modifications in the mixing, handling, and curing processes; and possible addition of special ingredients.
The cement used most extensively in building construction is portland cement. Of the five standard types of portland cement generally available in the United States and for which the American Society for Testing and Materials has established specifications, two types account for most of the cement used in buildings. These are a general-purpose cement for use in concrete designed to reach its required strength in about 28 days, and a high-early-strength cement for use in concrete that attains its design strength in a period of a week or less.
All portland cements set and harden by reacting with water, and this hydration process is accompanied by generation of heat. In massive concrete structures such as dams, the resulting temperature rise of the materials becomes a critical factor in both design and construction, but the problem is usually not significant in building construction. A low-heat cement is designed for use where the heat rise during hydration is a critical factor. It is, of course, essential that the cement actually used in construction correspond to that employed in designing the mix, to produce the specified compressive strength of the concrete.
Water must be reasonably clean, free of oil, organic matter, and any substances that may affect the actions of hardening, curing, or general finish quality of the concrete. In general, drinking-quality (potable) water is usually adequate. Salt-bearing seawater may be used for plain concrete (without reinforcing) but may cause the corrosion of steel bars in reinforced concrete.
A critical concern for the production of good concrete is the amount of water used. In this regard there are three principal concerns, as follows:
1. Having enough water to react chemically with the cement so that the hardening and strength gain of the concrete proceeds over time until the desired quality of material is attained.
2. Having enough water to facilitate good mixing of the ingredients and allow for handling in casting and finishing of the concrete.
3. Having the amount of water low enough so that the combination of water and cement (the paste) is not too low in cement to perform its bonding action. This is a major factor in producing high-grade concrete for structural applications.
The most common aggregates are sand, crushed stone, and pebbles. Particles smaller than 3/16 in. in diameter constitute the fine aggregate. There should be only a very small amount of very fine materials, to allow for the free flow of the water-cement mixture between the aggregate particles. Material larger than 3/16 in. is called the coarse aggregate. The maximum size of the aggregate particle is limited by specification, based on the thickness of the cast elements, spacing and cover of the reinforcing, and some consideration of finishing methods.
In general, the aggregate should be well graded, with some portion of large to small particles over a range to permit the smaller particles to fill the spaces between the larger ones. The volume of the concrete is, thus, mostly composed of the total aggregate, the water and cement going into the spaces remaining between the smallest aggregate particles. The weight of the concrete is determined largely by the weight of the coarse aggregate. Strength is also dependent, to some degree, on the structural integrity of the large aggregate particles.
While stone is the most common coarse aggregate, for various reasons other materials may be used. One reason for this may be the absence of available stone of adequate quality, but more often there is some desire to impart particular modified properties to the concrete. Some of these desired properties and the types of aggregates used to achieve them are discussed in this section.
Weight Reduction. For structural concrete, a common desire is for some reduction of the dead load of the structure. This is most often desired for concrete elements of spanning structures. Since the coarse aggregate typically constitutes at least two-thirds of the total mass of the concrete, any significant reduction in unit density of the coarse aggregate will result in a significant weight reduction of the finished concrete. If a relatively high strength is also desired, there is a limit to how much reduction can be achieved. Various natural and synthetic materials may be used as substitutes for the ordinary stone, but if reasonable strength and stiffness is critical, the maximum reduction is usually around 25 to 30%; that is, a reduction from a typical density of 145 to 150 pounds per cubic foot (pcf) to something just over 100 pcf. Lower finished densities may be achieved, but usually with significant loss of both strength and stiffness.