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Reflections on the Beginnings of Prestressed Concrete in America ineteen seventy-nine will be the 25-year Silver Jubilee of the founding of the Prestressed Concrete Institute. o Q Q To commemorate this important anniversary, the PCI o JOURNAL is presenting a series of papers on the early history (based on personal experiences and recollections) of Q prestressed and precast concrete in North America. These 0 Qo papers will be narrated by the persons who participated in the o early development of the industry. Q Part 1 (in the last issue) traced the events that led to the O O construction of the Walnut Lane Bridge and particularly the o significant role that Professor Gustave Magnel played in O Q introducing prestressed concrete to America. O o Q ci Part 2 (appearing in this issue) recounts the major o Q accomplishments American engineers made in launching the Q precast prestressed concrete industry. Q We believe that these series of articles will not only be o OQ fascinating to read but will serve as a historical record of one O L of the most exciting periods in the annals of construction. O oci p O p 0000000000000000000000000000000000 O Part 2 O O O 0 Dynamic American 0 0 engineers sustain 0 0 Magnets momentum 0 0 0 0 Charles C. Zoliman O Director of Engineering ci Urban Engineers of Maryland, Inc. ci ci Q Q O ci ci ci 0 Q In a free and competitive society such Q as ours, stagnation spells doom. ci (Evelyn H. Harris) he momentum generated by Pro- • What were the lessons we learned Tfessor Gustave Magnel (culminat- from the Walnut Lane Bridge? ing in the construction of the Walnut • How did Professor Magnels insis- Lane Bridge) carried across North tence on high quality materials and America. Reluctantly at first, then workmanship ultimately lead to the enthusiastically, American engineers acceptance of steel forms and exter- and contractors learned the neces- nal vibration, the horizontal mixer and high strength concrete? sary skills so as to be able to use prestressed concrete for their struc- • Who was responsible for the de- tures. Then in typical American fash- velopment and subsequent use of stress-relieved wire and seven-wire ion, to maximize the potential of strand, essentially an American inno- prestressed concrete, they con- vation which revolutionized the indus- ceived new construction methods, try? innovated new devices and im- • How did the first pretensioning plant proved existing prestressing tech- come into being in North America and niques. what were the early types of precast How did all this come about? concrete products? 30 • How did the prestressed block beam those early days, the influence of Pro- come about which is also a totally in- fessor Magnel continued to be felt. Re- dependent American development? garding one particular aspect of pre- • What were the events leading to the stressed concrete, the Professor was construction of the Tampa Bay uncompromising, namely, his insistence Bridge, an important structure which on workmanship and the quality of the sustained the momentum created by component materials of prestressed the Walnut Lane Bridge? concrete. • Why was the influence of William This particular point came to light Dean, an open-minded talented pub- dramatically in the spring of 1949 when lic servant with no real preferences, a group made up of those responsible so decisive to the growth of the for constructing the Walnut Lane Bridge young precast prestressed industry? visited Europe to inspect prestressed • How did standardization of bridge concrete construction (see the "After- beams get initiated? math" at the end of Part 1 of this series of papers). The above are some of the ques- Anthony Horst, general contractor for tions for which I will attempt to pro- the Walnut Lane Bridge, asked Profes- vide answers. But more importantly, sor Magnel during a dinner meeting, if it this narrative is an intensely human were possible to attain "zero slump." story reflecting courage, failure, in- With a smile, Magnel answered that ventiveness, coincidences, tragedy he would have liked to specify "minus and triumph. slump" for the concrete of the Walnut Lane Bridge. He continued, "Tomorrow, at my laboratory in Ghent, in your pre- Quality of Materials sence, I will batch no-slump concrete and Workmanship and you will see water coming to the sur- face." And thats exactly what hap- Despite the gradual "Americanization" pened! of prestressed concrete construction in Tony Horst, who was used to "pour- PCI JOURNAUJuly-August 1978 31 Fig. 1. Wood forms for Walnut Bridge. ing" 3000-psi (20.7 kPa) concrete with materials must meet relatively high a high slump of approximately 5 to 6 in. strength and durability standards.t] (127 to 152 mm) was duly impressed. [It On the other hand, Horst knew that a must be appreciated, of course, that 30 batching plant at the site of his three- years ago the requirement for a 5000-psi span structure, which could produce (34.5 kPa) concrete on a job site was no-slump concrete, was impractical due extremely rare.] to lack of storage space for raw mate- Horst was obviously in a predicament. rials and working space. To install a How was he going to comply with Mag- batching plant would also be too costly nels requirement of no-slump concrete? since only a relatively small amount of On the one hand, he was aware that the high strength concrete was needed. professor was uncompromising where An additional consideration was that workmanship and quality were con- Horst realized that if prestressed con- cerned. [Professor Magnel knew from crete, particularly post-tensioning at the rigorous laboratory experiments and site, was ever to get off the ground in the long years of field experience that the United States, he needed the coopera- component materials of prestressed tion of the ready-mixed concrete indus- concrete, i.e., steel and concrete, "work try, which he would obviously receive by harder" (as compared to conventional using ready-mixed concrete for this first reinforced concrete) under high structure. stresses. Therefore, the quality of both Because of all the above consid- erations, Horst had no other choice but to use transit ready-mixed concrete for How appropriate a term for what is usually done at the job the Walnut Lane Bridge even though the site. However, one "pours" soup, not concrete. Quality haul from the concrete plant to the cast- concrete should be "placed." ing site was nearly an hours drive. Horst tThe concrete should have at least an ultimate strength of realized fully well that he would not get 5000 psi (34.5 kPa), consistently, and the cold drawn wires at least 220,000 psi (1517 kPa). , "no-slump" concrete out of the mixer. 32 rig. Z. Placing reinforcement in wood forms for Walnut Lane Bridge. To make matters worse, Sam Baxter nal vibrating in addition to internal vibrat- (at the afore-mentioned dinner) re- ing, was imperative. minded Horst that in accordance with This dual consolidation of the concrete the contract, Professor Magnel had the is essential if honeycombs and/or cold last word. No approval from Professor joints are to be avoided. Such vibrating, Magnel meant no approval from the City with high frequency and relatively low of Philadelphiatt and the Professor de- amplitude vibrators could only be effec- manded "no-slump" concrete! tively done through the use of properly Fortunately, Professor Magnel was braced and stiffened heavy gauge steel also an eminently practical man. He un- forms to which vibrators could be at- derstood the problem and eventually tached, permanently or temporarily. agreed to a maximum of 2-in. (51 mm) Preload Corp. had based their cost es- slump on condition "you are going to timates on the use of two sets of wood use steel forms so that, to insure com- forms (see Figs. 1 and 2) for the main paction, you can vibrate the concrete span girders and another two sets for energetically by means of external vibra- the approach span girders. They in- tion ... of course, you will do this in tended to place and internally vibrate the combination with internal vibration." concrete through side windows which This modified provision was not well were to be located slightly above the received by the Preload Corp. (fab- bottom flange of the girder in each panel ricators of the concrete girders). But Pro- between 14 ft 6 in. (about 5 m) on center fessor Magnel was drawing on his long diaphragms. experience in prestressed concrete work. He had learned that with low slump, but high strength concrete for ttQuoting from Baxters letter to A. W. Horst, June 17, 1949: "Since Professor Magnel was designated in the docu- -beams having relatively thin webs ift re- ments which accompanied the proposal, as the engineer lation to their depth (which makes plac- who would be responsible for this work, I must insist that the procedures and methods adopted will have his ap- ing of concrete difficult) energetic exter- proval." PCI JOURNAUJuly-August 1978 33 Preload Corp. objected to the use of steel forms because their high costs could not be depreciated with only seven uses for each of the four forms. Very reluctantly and against his better judg- ment, Professor Magnel resolved the impasse, by yielding to Preloads pres- sure. In doing so, however, he predicted there would be trouble and indeed there was! Preload attempted to provide what- ever external vibrating the wood forms could withstand without damage or dis- placement, hoping the less intense vi- brating "would do the job nevertheless." As Professor Magnel had anticipated, the abundance of concrete placing prob- lems throughout the entire concreting job was caused primarily by insufficient external vibrating.