Reducing Bridge Damage Caused by Pavement Forces Part 1: Some Examples

Transcription

1 Reducing Bridge Damage Caused by Pavement Forces Part 1: Some Examples BY MARTIN P. BURKE, JR. Innumerable bridges both in the U.S. and abroad have been, and continue to be, damaged by the restrained growth 01 jointed rigid pavement. Yet the pavement growth/pressure phenomenon responsible for this damage is not described in bridge engineering textbooks nor in national bridge inspection training manuals, nor is it identified in the AASHTO Standard Specifications for Highway Bridges. As a consequence of this rack of recognition, it appears that many bridge engineers are unaware of this phenomenon; thus they continue to design and construct bridges that are vulnerable to extensive damage. Based on recent statements and recommendations in national pavement research reports and in published papers of some state pavement maintenance engineers, it is obvious that this phenomenon is also either neglected entirely by many pavement engineers, or its significance with respect to the long-term (longer than 15 years) function and durability of both pavements and bridges is not fully appreciated. As a result, some pavement maintenance practices have been advocated and adopted recently that will have a significantly adverse effect on the long-term integrity of both pavements and bridges. structures. This part of the article describes the damaging effect of the pavement growth/pressure phenomenon on three different bridges, and it provides a brief explanation of the phenomenon. The second part of this article, which will be published in the February 2004 issue of CI, contains documented background on long-term adverse pavement behavior and the effect of this behavior on end-jointed continuous bridges and joint less ( or integral) bridges. Part 2 also partially documents the general lack of awareness within the bridge engineering profession of this phenomenon. THREE BRIDGES Three radically different types of bridges-separated by both time and This situation led to the preparation of this two-part article. Because bridge and pavement engineering practices will continue to change with time, this article could give positive direction to that change by helping improve awareness of the growth/pressure phenomenon and its destructive potential; encouraging long-term research on this phenomenon; and motivating engineers to use more effective pavement and bridge practices to achieve safer and more durable

2 distance-shared a similar fate because they each are multiple span bridges with intermediate movement joints in their superstructures and they each were built in conjunction with jointed concrete pavement approaches. The behavior of these bridges can be considered somewhat characteristic of many similar bridges located throughout the U.S. The three bridges are the Old Third Street Viaduct of Cincinnati, OH; the John F. Kennedy Memorial Bridge of Louisville, KY, and Jeffersonville, IN; and the Pacos River Bridge of Carlsbad, NM. Old Third Street Viaduct A serious problem for the first of these bridges became evident alter its first 12 years of service. In the summer of 1970, bridge maintenance engineers noticed long, vertical cracks in the columns of Pier 1, the first pier at the north end of the structure. This bridge consisted of a series of deck slabs supported by continuous steel girders with short wall-type abutments and column-type piers. Pier 1, partially shown in Fig. 1, consists of a pair of short rectangular columns protruding up through a concrete walkway. The pier provides part of the vertical support for the first two superstructure spans, and its fixed bolster bearings provide complete longitudinal restraint for the first of several superstructure units. Because the design of this pier and its concrete quality were judged to be adequate for the load to be supported, and because maintenance engineers were confident that pier reinforcement was provided in accordance with plan and specification requirements, the integrity of the pier was restored by injecting epoxy into its cracks. Periodic inspection of these pier columns throughout the rest of the year revealed no new or extended cracks, suggesting that the epoxy injection repair had been successful. The following summer, however, new and similar cracks adjacent to the repaired cracks began to appear. So in lieu of further epoxy injection, maintenance personnel responded by applying the external hardware visible in Fig. 1. Subsequently, a broader examination of the structure was made to determine the cause(s) of these unusual cracks so that a more suitable maintenance response could be made. Clues to the cause of pier cracking were revealed when the condition of the adjacent wall-type abutment was considered. For example, the sliding plate-type deck joint was tightly closed, indicating a permanent 2-in.-horizontal (50 mm) abutment movement from the as-constructed position; the abutment breastwall was tilted toward the superstructure; there were large, essentially vertical cracks between the breastwall and turn-back wingwalls; the approach roadway consisted of jointed concrete pavement; and pier crack reappearance did not take place until warm summer temperatures were reached. Although there was no visible evidence of pavement or abutment backwall distress, and the approach pavement was only 12 years old, it nevertheless appeared highly likely that the pavement growth/pressure phenomenon was responsible for moving the abutment and cracking the first fixed pier of the bridge. More specifically, it appeared that the growing approach pavement jammed the north abutment toward the bridge superstructure, thereby moving and tilting the abutment and closing the movable joint between the abutment and first superstructure unit. Continual growth of the pavement subsequently moved both the abutment and first superstructure unit toward the second unit, thereby commencing closure of the movable deck joint between the first and second superstructure units. But the fixed bearings and the two short stiff columns of Pier 1 resisted longitudinal movement of the first superstructure unit. Because the forces that can be generated by the restrained growth of jointed pavement are so huge, and because these forces are somewhat

3 proportional to the degree of restraint against pavement growth provided by the bridge, the cracking of the Pier 1 columns of this bridge was only the first indication of greater forces and more extensive future bridge fracturing unless pavement pressure relief joints were installed in the bridge approaches. As a first attempt at pressure relief, the approach pavement was cut transversely so that a 4-in.-wide (100 mm) plank of compressible polyethylene filler could be installed. This installation was made in March of Immediately following the cutting of pavement and the release of longitudinal pavement forces, the north wall type abutment tilted back 5/8 in. (16 mm) toward the bridge approach, and the slightly tilted Pier 1 rotated back to vertical. These responses provided clear visual evidence that the uncut and growing pavement was responsible for the closed deck joints and damage to the structure. In March of 1973, just 1 year after its installation, the original 4-in.-wide (100 mm) relief joint was measured and found to be only 2-1/2 in. wide (65 mm). This indicated pavement growth and compression of the polyethylene filler of 1-1/2 in. (40 mm) in 1 year's time. John F. Kennedy Memorial Bridge Similar trouble for the second of these bridges, the John F. Kennedy Memorial Bridge, was reported in local newspapers when the bridge was 27 years old. 1 This bridge consists of continuous through-truss main spans and continuous deck-type steel girders on each approach, with stub-type embankment-supported abutments and cap and round column piers. As described for the Old Third Street Viaduct, the deck joints at both ends of the northernmost two-span superstructure unit were closed; the supporting fixed pier was tilted 5 in. (130 mm) to the south, and the northernmost rocker bearings at the other piers were tilted in the same direction. The north abutment also had been moved south, probably about 7 in. (180 mm) (2 in. [50 mm] joint closure plus 5 in. [130 mm] pier movement). And like the Old Third Street Viaduct, the pavement growth/pressure phenomenon was probably responsible for most of this movement. In 1991, Indiana officials. reported that: "...The northern approach to the bridge has been tilting southward for 8 to 10 years, according to old inspection photographs. The leaning was never enough to cause alarm." 1 This bridge and its approach pavements were built in 1964, and it appears that the approach pavements were about 17 years of age when the growth of pavements had progressed far enough to have closed the movable deck joints, and moved all of the north approach bridge elements (abutment, superstructure, and pier) enough to have provoked public concern about the structure's stability. Apparently, the fixed pier columns (and presumably their pile-supported foundations) were tall and flexible enough to have tolerated 5 in. (130 mm) of movement and tilting without noticeable pier distress. In response to this magnitude of movement, the concrete approach pavements were cut and pressure relief joints were installed to eliminate

4 restraint against pavement growth and to minimize pressure transmission from the pavements to the bridge. Because embankment and subsoil consolidation and translation at the north approach mar also have contributed somewhat to substructure movement, motion detectors were implanted in the north abutment embankment. Those detectors were used to monitor possible embankment movements and to ensure that the installation of pavement pressure relief joints was a sufficient response to prevent further longitudinal movement of both the superstructure and the substructure elements. Subsequent monitoring of the detectors indicated that the embankments were stable and not contributing to the substructure movement. Pavement pressure relief joints are now being compressed by yearly pavement growth. Presumably, these relief joints will periodically be renewed to prevent future problems from this phenomenon. The rocker bearings have not as yet been righted but deck joints have been rebuilt to restore superstructure movements and minimize restraint stresses. Pacos River Bridge Similar trouble for the Pacos River Bridge came to a head when local maintenance personnel were no longer able to contend with this structure's unusual behavior and progressive deterioration. This bridge consists of eight deck-type rolled beams with simply supported spans and reinforced concrete wall-type piers and abutments. An inspection of the structure in 1991 revealed that the three easternmost movable deck joints were closed. The top of the east abutment had moved and tilted to the west about 5-1/2 in. (140 mm); the ends of the deck slabs were crushed; and rocker and bolster bearings were shifted and tilted so much that they were edge bearing on the bridge seats (Fig. 2). Bridge seats of the substructure units supporting the easternmost spans were badly cracked and fractured as well. Because of the magnitude of the movement at the east abutment, it would appear likely that the adjacent piers were tilted to the west as well. The complete history of the bridge and its concrete pavement approaches is uncertain. The bridge was constructed in 1941, but the date of construction of the present approach pavement has not been determined. A report about the bridge stated that, in 1983, the deck was repaired and overlaid, some repair work was done on the substructures, and bridge bearings and structural steel were repainted. Although the precise age of the approach pavement had not been determined, it was apparently old enough for its growth to have been responsible for the bridge's recent rapid deterioration. From an unpublished inspection report about this bridge, state bridge engineers concluded: "We believe that the [abutment tilting] deck crushing and bearing misalignments have been caused by the pavement shoving against the [abutment] backwall and bridge deck at the east end.we have noticed similar problems with pavement shoving at many other bridges. However, the amount of movement and subsequent damage to Bridge No is the worst we have seen." In addition to recommending various structure repairs, the immediate installation of pavement pressure relief joints in the bridge approaches was recommended. PAVEMENT GROWTH/PRESSUREPHENOMENON As described previously, serious trouble for the Old Third Street Viaduct, the John F. Kennedy Memorial Bridge, and the Pacos River Bridge became evident after these structures and approach pavements were 12, 17, and less than 30 years old, respectively. Because this trouble has been identified with the long-term behavior of jointed concrete

5 approach pavement, it is important that bridge design and maintenance engineers become familiar with the long-term behavior of such pavements. They also need to understand the pavement growth/pressure phenomenon, which is responsible for behavior that has such an adverse effect on bridge performance, integrity, and durability. lf a design only had to contend with the response of structures (pavement and bridges) to ambient temperature ranges, achievement of an efficient and functional transportation system would be relatively easy. Concrete shrinkage and the less-than-ideal effect of traffic maintenance practices, however, have compounded the problems faced by pavement and bridge maintenance engineers. Contraction joint contamination Transverse pavement contraction joints that are either partially sealed or unsealed open wider in response to lowering pavement temperatures and moisture levels. They become narrower in response to rising pavement temperature and moisture levels. However, after joints open and remain open at low temperature and moisture levels, compression-resistant fine roadway debris can infiltrate the crack below the sawcut joint and prevent subsequent complete closure to an asconstructed condition. This opening, infiltration of debris, and partial closing continues in sequence with daily and yearly temperature and moisture cycles. Of course, debris infiltration is facilitated where deicing chemicals are used to ensure dry pavements (and consequently, open contraction joints) during low winter temperatures. Infiltration of these joints by compression-resistant debris begins almost as soon as the crack forms below the saw cut. Unsealed joints are infiltrated from the top, sides, and bottom. For "sealed" joints (actually a misnomer because only the upper surfaces of such joints are initially sealed), initial infiltration begins at the open sides and bottoms. The movement of surface water that penetrates pavement and shoulder joints from above, and groundwater that seeps through shoulders and migrates along the subbase from below, facilitates this infiltration. As joint seals begin to fail because of a combination of age degradation, low temperature stiffening, traffic abrasion, neglect, and so forth, debris infiltration accelerates both from above and below. As a consequence of this contamination of contraction joints by compression-resistant debris, pavements will grow longitudinaliy in proportion to the amount of compressed debris that infiltrates and accumulates in the joints (if such growth is not resisted); or the pavement will both grow and be partially compressed where partial restraint against longitudinal growth is present. But where pavement growth is not possible, pressure generation commences and continues to accumulate until either the weakest pavement joints or the most vulnerable bridge elements are fractured, thereby relieving the built-up restraint stresses. Pressure generation The generation of longitudinal pavement pressures may be visualized as suggested in Fig. 3. Illustrated is an "idealized" chart of the yearly maximum longitudinally oriented compressive stress f' c in an extensive length of restrained pavement without movable joints (no pavement pressure-relief joints or movable bridge joints). Initially, the stress or pressure is insignificant as the joints are relatively clean and joint seals are intact and functioning. However, as years pass and joints begin to fill with debris, the yearly maximum pressure increases at a growing rate. As joints continue to fill, the accumulated compression-resistant debris minimizes infiltration of additional material, slowing the rate of joint infiltration and pressure generation. Somewhere along this hypothesized

6 pressure generation curve, the pavement fractures adjacent to a joint, relieving some pressure, or the pavement blows up, relieving all of the pressure at the location of the blow-up. Illustrated in Fig. 3 is an idealized maximum yearly stress-generation curve for one particular pavement. Owing to the many factors that affect joint infiltration and compression, innumerable similar stress-time curves could be illustrated. This suggests that fracturing could occur at an earlier or later time, depending on the number of factors that combine to affect stress generation. 2,3 A number of different analytical approaches can be used to illustrate the huge compressive stresses associated with the partially restrained growth of jointed pavement, or for the restrained thermal elongation of such pavement. Generally, most reasonable assumptions about measured pavement behavior will yield stresses in the range of 1000 psi (7 MPa) or more. For a 24 ft x 9 in. (8.2 m x 230 mm) pavement, such pressures could result in a total longitudinal force of about 1300 ton (1200 tonne) or more than 25 times the forces usually assumed in the design of bridge abutments. throughout most of a pavement's cross section, maximum stresses (the upper flat portion of the pressure generation curve) can be achieved without progressive local fractures. Such pavements appear to be able to function indefinitely until localized pavement deterioration and/or spalling causes redistribution of stresses and local fracturing, or until an unusually hot and prolonged heat wave produces restraint stresses beyond the buckling capacity of the pavements. Then pavements erupt or blow-up instantaneously as illustrated in Fig. 4. The pavement blow-up shown in Fig. 4 is a clear indication of the huge compressive stresses that can be generated in a restrained jointed concrete pavement. Is it any wonder that forces considerably less than those that fractured this pavement were responsible for the damage, distortions, and dislocations inflicted on the three bridges noted previously? This article serves as a warning to those responsible for maintence of bridges where longitudinal pavement forces have not been moderated by the installation of pavement pressure relief joints. Occasionally, in some high-quality pavements, particularly those on straight, flat, or slightly depressed vertical alignments where pavement stresses are more uniformly distributed

7 References 1. "What's Wrong with the Kennedy Bridge...and How Might it be Fixed?" Louisville Courier-Journal, Oct. 24, Burke, M. P., Jr., "Pavement Pressure Generation: The Neglected Aspect of Jointed Pavement Behavior," Transportation Research Record 1627, TRB, National Research Council, Washington, DC, 1998, pp Burke, M. P., Jr., "Bridge Approach Pavements, Integral Bridges and Cycle Control Joints," Transportation Research Record 1113, TRB, National Research Council, Washington, DC, 1987, pp Selected far reader interest by the editors. FUENTE: Concrete International Enero, 2004