1 Prestressing Methods

1-1 Introduction

Prestressing can be defined as the application of a predetennined force or moment to a structural member in such a manner that the combined internal stresses in the member, resulting from this force or moment and from any antic­ipated condition of external loading, will be confined within specific limits. The prestressing of concrete, which is the subject of this book, is the result of applying this principle to concrete structural members with a view toward elimi­nating or materially reducing the tensile stresses in the concrete.

The prestressing principle is believed to have been well understood since about 1910, although patent applications related to types of construction involving the principle of prestress date back to 1888 (Abeles 1949). The early attempts at prestressing were abortive, however, owing to the poor quality of materials available in the early days as well as to a lack of understanding of the action of creep in concrete. Eugene Freyssinet, the eminent French engineer, generally is regarded as the first investigator to recognize the nature of creep in concrete and to realize the necessity of using high-quality concrete and high­tensile-strength steel to ensure that adequate prestress is retained. Freyssinet applied prestressing in structural application during the early 1930s. The history

J. R. Libby, Modern Prestressed Concrete© Van Nostrand Reinhold 1990

2 I MODERN PRESTRESSED CONCRETE

and the evolution of prestressing are controversial subjects and not well documented; so they are not discussed further in this book. The interested reader may find additional historical details in the references (Abeles 1949; Dobell 1950).

Many experiments have been conducted to demonstrate that prestressed concrete has properties that differ from those of nonprestressed reinforced concrete. Diving boards and fishing poles have been made of prestressed concrete to demonstrate the ability of this material to withstand large deflections without cracking. Of more significance, however, is the fact that prestressed concrete has proved to be economical in buildings, bridges, and other structures (under conditions of span and loading) that would not be practical or economical in reinforced concrete.

Prestressed concrete was first used in the United States (except in tanks) in the late 1940s. At that time, most U.S. engineers were completely unfamiliar with this mode of construction. Design principles of prestressed concrete were not taught in the universities, and the occasional structure that was constructed with this new material received wide pUblicity.

The amount of construction utilizing prestressed concrete has become tremendous and certainly will continue to increase. The contemporary structural engineer must be well informed on all facets of prestressed concrete. It is indeed unfortunate that the subject of prestressed concrete design and construction is not included in the undergraduate curriculum of many U.S. universities at this time.

1-2 General Design Principles

Prestressing, in its simplest form, can be illustrated by considering a simple, prismatic flexural member (rectangular in cross section) prestressed by a concentric force, as shown in Fig. 1-1. The distribution of the stresses at midspan is as indicated in Fig. 1-2. It is readily seen that if the flexural tensile stresses in the bottom fiber, due to the dead and live service loads, are to be eliminated, the uniform compressive stress due to prestressing must be equal in magnitude to the sum of these tensile stresses.

Live load

p _II I I I I I I I I I II I I I j j j I I. p

I.. L ~I Fig. 1 -1. Simple rectangular beam prestressed concentrically.

PRESTRESSING METHODS I 3

Compression Compression Compression Compression

\+0+\ = ~ Tension Tension

Fig. 1-2. Distribution of stresses at midspan of a simple beam concentrically prestressed.

There is a time-dependent reduction in the prestressing force, due creep and shrinkage of the concrete and relaxation of the prestressing steel (see Chapters 2, 3, and 7). If no tensile stresses are to be pennitted in the concrete, it is necessary to provide an initial prestressing force that is larger than would be required to compensate for the flexural stresses resulting from the selVice loads alone. The prestress loss, which is discussed in detail in Secs. 7-2 and 7-3, generally results in a reduction of the initial prestressing force by 10 to 30 percent. Therefore, if the stress distributions shown in Fig. 1-2 are desired after the loss of stress has taken place (under the effects of the final prestressing force), the distribution of stresses under the initial prestressing force would have to be as shown in Fig. 1-3.

Prestressing with the concentric force just illustrated has the disadvantage that the top fiber is required to withstand the compressive stress due to prestressing in addition to the compressive stresses resulting from the selVice loads. Furthennore, because prestressing must be provided to compress the top fibers, as well as the bottom fibers, if sufficient prestressing is to be supplied to eliminate all of the selVice load flexural tensile stresses, the average stress due to the prestressing force (the prestressing force divided by the area of the concrete section) must be equal to the maximum flexural tensile stress resulting from the selVice loads.

If this same rectangular member were prestressed by a force applied at a point one-third of the depth of the beam from the bottom of the beam, the distribution of the stresses due to prestressing would be as shown in Fig. 1-4. In this case, as in the previous example, the final stress in the bottom fiber due to prestressing

Compression Compression Compression Compression

\+0 +\ =

Tension Tension

Fig. 1-3. Distribution of stresses at midspan of a simple beam under initial concentric prestressing force.

4 I MODERN PRESTRESSED CONCRETE

Final stress

Initial stress

Fig. 1-4. Distribution of stresses due to prestressing force applied at lower third point of rectangular cross section.

should be equal in magnitude to the sum of the tensile stresses resulting from the service loads. By inspection of the two stress diagrams for prestressing (Figs. 1-2b and 1-4), it is evident that the average stress in the beam, prestressed with the force at the third point, is only one-half of that required to develop the amount required in the first example. In addition, the top fiber is not required to carry any compressive stress due to prestressing when the force is applied at the third point.

The economy that results from applying the prestressing force eccentrically is obvious. Further economy can be achieved when small tensile stresses are permitted in the top fibers. The tensile stresses may be due to prestressing alone or to the combined effects of prestressing and any service loads that may be acting at the time of prestressing. This is so because the required bottom-fiber prestress can be attained with a smaller prestressing force, which is applied at a greater eccentricity under such conditions. This principle is treated in greater detail in subsequent chapters.

In many contemporary applications of prestressing, the flexural tensile stresses due to the applied service loads are not completely nullified by the prestressing; nominal flexural tensile stresses are knowingly permitted under service load conditions. Economy of construction is the motivation for this practice as well. The use of flexural tensile stresses under service load conditions is considered in detail in this book.

1-3 Prestressing with Jacks

The prestressing force in the above examples could be created by placing jacks at the ends of the member, if there were abutments at each end sufficiently strong to resist the prestressing force developed by the jacks. Prestressing with jacks, which mayor may not remain in the structure, depending upon the circumstances, has been used abroad on dams, dry docks, pavements, and other special structures. This method has been used to a very limited degree in North America because extremely careful control of the design (including study of the behavior under overloads), construction planning, and execution of the

PRESTRESSING METHODS I 5

construction is required if the results obtained are to be satisfactory. Further­more, the loss of prestress resulting from this method is much larger than when other methods are used (see Sec. 3-15), unless frequent adjustments ofthe jacks are made, because the concrete is basically subjected to constant strain in this method of prestressing rather than to nearly constant stress as is the case in other methods. For these reasons, and because the types of structure to which this method of prestressing can be applied are very limited and beyond the scope of usual generalities, subsequent consideration of this method is not given in this book (Guyon 1953).

1-4 Prestressing with Pretensioned Tendons

Another method of creating the necessary prestressing force is referred to as pretensioning. Pretensioning is accomplished by stressing steel wires or strands, called tendons, to a predetermined stress, and then, while the stress is maintained in the tendons, placing concrete around the tendons. After the concrete has hardened, the tendons are released, and the concrete, which has become bonded to the tendons, is prestressed as the tendons. attempt to regain the length they had before they were stressed. In pretensioning, the tendons usually are stressed by the use of hydraulic jacks. The stress is maintained during the placing and curing of the concrete by anchoring the ends of the tendons to rigid, nonyielding abutments that may be as much as 500 ft or more apart. The abutments and appurtenances used in this procedure are referred to as a pretensioning bed or bench. In some instances, rather than using pretensioning benches, the steel molds or forms that are used to form the concrete members are designed in such a manner that the tendons can be safely anchored to the mold after they have been stressed. As the results obtained with each of these methods are identical, the factors involved in determining which method should be used are of concern to the fabricator of prestressed concrete, but do not usually affect the designer.

The tendons used in pretensioned construction must be relatively small in diameter because the bond stress between the concrete and a tendon is relied upon to transfer the force in the tendon to the concrete. If the bond stress exceeds the bond strength of the concrete, the tendon will slip, and the prestress will be lost. The ratio of the bond area (product of the circumference and length of the wire) to the cross-sectional area of a circular wire or bar is equal to 4L/d, where d is the diameter and L is the length over which the transfer is made; thus the bond area available per unit length of tendon decreases as the diameter increases. It follows that, for constant tendon stress, the bond stress increases as the tendon diameter increases. This explains why several tendons of small diameter normally area used in pretensioning concrete, rather than a few larger ones. Small-diameter strands composed of several small wires twisted around a straight center (core) wire are widely used in pretensioning concrete because of their excellent bond characteristics (see Sec. 6-6).

Pretensioning is widely used in the manufacture of prestressing concrete in

6 I MODERN PRESTRESSED CONCRETE

North America. The basic principles and some of the methods currently used here were imported from Europe, but much has been done to develop and adapt the procedures to the North American market. One of these developments was the introduction of pretensioned tendons that do not pass straight through the concrete member, but are deflected or draped into a path that approximates a curve. This procedure was first used on light roof slabs, but subsequently has been commonly used in the construction of large structural members. The use of deflected, pretensioned tendons is common in the production of large bridge girders.

Although many of the devices used in pretensioned construction are patented, the basic principle is in the public domain and has been so for many years. A detailed discussion of the construction procedures and equipment used in pretensioned construction is given in Chapter 15.

1-5 Prestressing with Post-tensioned Tendons

When a member is fabricated in such a manner that the tendons are stressed, and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to safely withstand the prestressing force, the member is said to be post-tensioned. Two types of tendons are used: bonded and unbonded.

Fully bonded post-tensioned tendons consist of bars, strands, or wires, in preformed holes, metallic ducts, or plastic tubes, that have been pressure-grouted after stressing. The tube is used to prevent the tendon from becoming bonded to the concrete at the time that the concrete is placed. After the concrete has been sufficiently cured, the tendon is stressed, and the tube is injected with grout. The cured grout effectively bonds the tendon to the tube and the concrete itself (the outside surface of the tube becomes bonded to the concrete when the concrete is placed). Rather than using metallic tubes, bonded tendons can be constructed by using holes formed in the concrete with removable rubber tubes or hoses; in this method the tendons are inserted into the preformed holes after the rubber tubes have been removed, and they are subsequently stressed and grouted. Another form of bonded tendons-which mayor may not be partially bonded to the concrete section, and are commonly known as external tendons­is used in special applications, as discussed in Sec. 6-8.

Unbonded tendons normally consist of strands or wires that are wrapped or encased in plastic after having been coated with a grease or a bituminous material. The grease sometimes contains a rust inhibitor to help protect the tendons from corrosion. Also, waterproof paper wrapping has been used rather than plastic. Unbonded tendons normally are assembled in a factory, shipped to the job site, and placed in the forms before the concrete is placed. They are not grouted after they have been stressed, so they do not become bonded to the concrete.

PRESTRESSING METHODS I 7

Bonded tendons are generally used in bridge construction, but unbonded tendons have been so used quite successfully. Unbonded tendons are most frequently used in building construction, but bonded tendons are sometimes used there also. The quantity of un bonded tendons used annually greatly exceeds the amount of bonded tendons used.

Post-tensioning offers a means of prestressing on the job site, which may be necessary in some instances. Very large building or bridge girders that cannot be transported from a precasting plant to the job site (because of their weight, size, or the distance between the plant and the job site) can be made by post­tensioning on the job site. Post-tensioning is used in precast as well as in cast­in-place construction. In addition, fabricators of pretensioned concrete will frequently post-tension members for small projects on which the number of units to be produced does not warrant the expenditures required to set up preten­sioning facilities. There are other advantages inherent in post-tensioned construction, which are discussed in subsequent chapters.

In post-tensioning, it is necessary to use some type of device to attach or anchor the ends of the tendons to the concrete section. Such devices usually are referred to as end anchorages or simply anchorages. The end anchorages, tendons, special jacking, and grouting equipment, if used, in post-tensioning concrete are collectively referred to as a post-tensioning system. There are several different systems in use. Chapter 16 contains a more detailed discussion of post-tensioning and post-tensioning systems.

1-6 Pretensioning vs. Post-tensioning

It is generally considered impractical to use post-tensioning on very short members because the elongation of a short tendon (during stressing) is small and requires very precise measurement. In addition, some post-tensioning systems do not function well with very short tendons. A number of short members can be made in series on a pre-tensioning bench without difficulty and with no need for precise measurement of the tendon elongation during stressing; relatively long tendon lengths result from making a number of short members in series.

It has been pointed out that very large members may be more economical when cast in place and post-tensioned, or when precast and post-tensioned near the job site, compared to transporting and handling large pretensioned structural members that are cast off-site.

Post-tensioning allows the tendons to be placed through structural elements on smooth curves of any desired path. Pretensioned tendons can be employed on other than straight paths, but not without expensive plant facilities and somewhat complicated construction procedures.

Because post-tensioning tendons can be installed in holes preformed in precast concrete elements or segments, they can be used to prestress a number of small

8 I MODERN PRESTRESSED CONCRETE

precast elements together to form a single large structural member. This technique, frequently referred to as segmental construction, is discussed in Sec. 8-3.

The cost of post-tensioned tendons, measured in either cost per pound of prestressing steel or cost per pound of effective prestressing force, generally is significantly greater than the cost of pretensioned tendons, because of the larger amount of labor required in placing, stressing, and grouting (where applicable) post-tensioned tendons, as well as the cost of special anchorage devices and stressing equipment. On the other hand, a post-tensioned member may require less total prestressing force than an equally strong pretensioned member. For this reason, one must be careful when comparing the relative costs of these modes of prestressing.

The basic shape of an efficient pretensioned flexural member may be different from the most economical shape that can be found for a post-tensioned design. This is particularly true of moderate- and long-span members and somewhat complicates any generalization about which method is best under such condi­tions.

Post-tensioning generally is regarded as a method of making prestressed concrete at the job site, yet post-tensioned beams often are made in precasting plants and transported to the job site. Pretensioning often is thought of as a method of manufacturing that is limited to permanent precasting plants, yet on very large projects where pretensioned elements are to be utilized, it is not uncommon for the general contractor to set up a temporary pretensioning plant at or near the job site. Each method of making prestressed concrete has partic­ular theoretical and practical advantages and disadvantages, which will become more apparent after the principles are well understood. A final determination of the mode of prestressing that should be used on any particular project can be made only after careful consideration of the structural requirements and the economic factors that prevail for the particular project.

1-7 Linear vs. Circular Prestressing

The subject of prestressed concrete frequently is divided into linear prestressing, which includes the prestressing of elongated structures or elements such as beams, bridges, slabs, piles, and so, and circular prestressing, which includes pipe, tanks, silos, pressure vessels, and domes. This book has been confined to consideration of linearly prestressed structures. The reader interested in circular prestressed concrete structures will find considerable information in the technical literature of the American Concrete Institute, the American Society of Civil Engineers, the American Water Works Association, and the Prestressed Concrete Institute, as well as in civil engineering text and reference books.

PRESTRESSING METHODS I 9

1-8 Application of Prestressed Concrete

Prestressed concrete, when properly designed and fabricated, can be virtually crack-free under nonnal service loads as well as under moderate overload. This is believed to be an advantage in structures exposed to corrosive atmospheres in service. Prestressed concrete efficiently utilizes high-strength concretes and steels and is economical even with long spans. Reinforced concrete flexural members cannot be designed to be crack-free, cannot efficiently utilize high­strength concrete (except in compression members), and are not economical for long-span flexural members.

A number of other statements can be made in favor of prestressed concrete, but there are bona fide objections to the use of this material under specific conditions. An attempt is made to point out these criticisms in subsequent chapters. Among the more significant advantages of this material are that in many structural applications, prestressed concrete is lower in first cost than other types of construction, and, in many cases, if the reduced maintenance cost inherent in concrete construction is taken into account, prestressed concrete offers the most economical solution. Its benefits have been well confinned by the very rapid increase in the use of linear prestressed concrete that has taken place in the United States since its introduction in the late 1940s. It is well known that the advantages of low first cost and maintenance (real economy) outweigh intangible advantages that may be claimed except for very special conditions.

The precautions that engineers must observe in designing and constructing prestressed concrete structures differ from those required for reinforced concrete structures. Some of these precautions are discussed in this book, but others, such as those related to specific construction practices and the safety of workers, are not. The prudent engineer will keep infonned on such precautions and other considerations through the trad€" and technical literature.

lllustrations of prestressed structures and structural elements are given in Chapters 13 and 14, where the various types of building and bridge construction are described and compared.

1-9 Evolution of U.S. Design Criteria

A document entitled Criteria for Prestressed Concrete Bridges (Bureau of Public Roads 1955) presented the first criteria for the design of prestressed concrete published in the United States. This brief treatment of the design, materials, and construction of prestressed concrete, including a discussion of the provi­sions contained therein, was successfully used in the design of many of the early prestressed concrete bridges and buildings in the United States. A joint committee of members of the American Concrete Institute and the American

10 I MODERN PRESTRESSED CONCRETE

Society of Civil Engineers prepared a report containing tentative recommen­dations for prestressed concrete that was published in 1958 (ACI-ASCE 1958). This report served as the basis for the first provisions for prestressed concrete, contained in Building Code Requirements for Reinforced Concrete (ACI 318 1963); and all subsequent editions of this document (ACI 318) have contained provisions for prestressed concrete. Virtually all other U.S. specifications and codes for the design of bridges and buildings are based upon ACI 318 although many have individual differences in some of their provisions.

REFERENCES

Abeles, P. W. 1949. Principles and Practice of Prestressed Concrete. London. Crosby Lockwood & Son, Ltd.

ACI-ASCE Joint Committee 323. 1958. Tentative Recommendations for Prestressed Concrete. Journal of the American Concrete Institute 29(7):545-78.

ACI Committee 318. 1963. BUilding Code Requirements for Reinforced Concrete. Detroit. American Concrete Institute.

Bureau of Public Roads. 1955. Criteria for Prestressed Concrete Bridges. Washington, D.C. U.S. Government Printing Office.

Dobell, C. 1950. Patents and Code Relating to Prestressed Concrete. Journal of the American Concrete Institute 46(9):713-24.

Guyon, Y. 1953. Prestressed Concrete. New York. John Wiley & Sons, Inc.