In the past, the method of protecting the main cables of suspension bridges against corrosion was by coating the steel with a red lead paste, wrapping the cables with galvanized, annealed wires, and applying a red lead paint. This method has met with a varying degree of success from excellent for the Brooklyn Bridge to poor for the General U. S. Grant Bridge at Portsmouth, Ohio. A potential defect in this system is that, as the cable stretches and shortens under live loads and temperature changes, some separation of adjacent turns of wire wrapping may occur. Depending on the degree of separation, the paint may crack and permit leakage of water and contaminants into the cable.
Alternative protection systems that have been used for some suspension bridges are as follows:
Bidwell Bar Bridge. This 1108 ft-span bridge has 11-in-dia., parallel, structural strand cables (Fig. 15.42). The protective system consists of the following components: plastic filler pieces, extruded from black polyethylene; a covering of nylon film; a ‘‘first pass’’ glass-reinforced acrylic-resin covering consisting of one layer of glass-fiber mat, two layers of glass cloth, and several coats of acrylic resin; a weather coat of acrylic resin;
and a finished coat of acrylic resin containing a sand additive to give the surface a rough texture. This type of covering was developed by Bethlehem Steel in conjunction with DuPont.
Newport, R.I., Bridge. The protective system for the cables of this bridge is the same as that described for the Bidwell Bar Bridge. However, since the cables consist of parallel wires, the black polyethylene filler pieces were not required.
General U. S. Grant Bridge, Ohio. The protective system comprises spiral-wrapped neoprene sheet and hypalon paint, a proprietary system developed by U. S. Steel.
Second Chesapeake Bay Bridge (William Preston Lane, Jr., Memorial). This has the same protective covering as applied to the General U. S. Grant Bridge.
Hennepin Avenue Bridge. The protective system consists of a wrapped neoprene sheet and hypalon paint system.
The Bidwell Bar Bridge was constructed in 1964 for the California Department of Water Resources. The protective cable covering has been performing satisfactorily. In the early 1970s, some corrosion was discovered at the cable bands, presumably resulting from shrinkage of the covering. Bethlehem Steel corrected the condition by rewrapping a short portion at the cable bands and recalking. A 1991 inspection indicated no distress in the cable covering.
The similar system applied to the Newport Bridge (installed in 1969) is still performing satisfactorily. A 1980 inspection indicated that some crazing of the top surface had occurred in some areas, but these were superficial and did not extend through the thickness. These areas were patched. There also were some signs of distress at the cable bands, in the calking groove. As a result of thermal contraction of the covering, the calking had worked loose (presumably the same condition as that in the Bidwell Bar Bridge). Repairs were made with a more resilient type calk that accommodates thermal movement.
The system developed by U. S. Steel and applied to the Second Chesapeake Bay Bridge in 1973, the General U. S. Grant Bridge in 1980, and the Hennepin Avenue Bridge in 1990, has been performing satisfactorily. This type of system was also used for rewrapping the Brooklyn Bridge cables in 1986.
Table 15.5 presents a partial listing of suspension bridges with appropriate statistics and the corrosion protection used for the main cables.
An area where the main cable is particularly vulnerable extends from the splay saddle to the eyebars in the anchorage blocks. The only corrosion protection available is the zinccoating of the wires. Depending on environmental conditions in the anchorage blocks, the galvanizing may have a life expectancy on the order of 20 years. Serious corrosion in this area occurred in the Brooklyn, Williamsburg, and Manhattan Bridges, requiring corrective measures. In the rehabilitation of one anchorage of the Manhattan Bridge, dehumidification equipment was included to control humidity in the anchorage block.
Suspender Corrosion. Generally, corrosion of suspenders is likely to occur at the anchorage sockets at the stiffening trusses and at retainer castings on top of those trusses. This may be attributed to two possible sources: salt spray from roadway deicing salts, or moisture that enters the interstices of the strand or rope at an upper level and trickles down to the socket or casting.
A 1974 report on the condition of the suspenders of the Golden Gate Bridge revealed that there was considerable reduction in suspender area due to corrosion that occurred as high as 150 ft above the roadway. This could be attributed to saltwater mist or fog.
For corrosion protection, U. S. Steel developed a procedure for extruding high-density black polyethylene over strands and rope. In many applications, this jacket also reduces vibration fatigue. For this purpose, particular attention is given to sealing and ends and minimizing the bending of wires at the nose of the socket.
Galvanizing. Wires can be protected against corrosion by galvanizing, a sacrificial coating of zinc that prevents corrosion of the steel so long as the coating is unbroken. Corrosion protection of the individual wires in a structural strand or rope is provided by various thicknesses of zinc coating, depending on the location of the wire in the strand or rope and the degree of corrosive environment expected. The effectiveness of the zinc coating is proportional to its thickness, measured in ounces per square foot of surface area of the uncoated wire. Class A zinc coating varies from 0.40 to 1.00 oz/ ft2 depending on the nominal diameter of the coated wire. A Class B or C coating is, respectively, 2 or 3 times as heavy as the Class A coating.
Generally, there are three basic combinations of coating: Class A coating throughout all wires; Class A coating for the inner wires and Class B for the outer wires; and Class A coating for the inner wires and Class C for the outer wires, depending on the degree of protection desired. Other coating thicknesses and arrangements are possible.
The heavier zinc coatings displace more of the steel area. This necessitates a reduction in rated breaking strength of strand or rope. ASTM A586 and A603 specify minimum breaking strengths required for various sizes of strand or rope in accordance with the three combinations of coating previously described. For other combinations of coating, the manufacturer should be consulted as to minimum breaking strength and modulus of elasticity.
Galvanizing has some disadvantages. Depending on environmental conditions, for example, galvanizing may be expected to last only about 20 years. Also, the possibility that hot-dip galvanizing may cause hydrogen embrittlement is of concern. (There is some indication, however, that, with current technology, the hot-dip galvanizing method is not as likely to cause hydrogen embrittlement as previously.) In addition, it may be difficult to meet specifications for a Class C coating with the hot-dip method. Furthermore, wire with hotdip galvanizing may not have the fatigue resistance that wire coated by electrolytic galvanizing has.
Protection of Stays. In early cable-stayed bridges, stays, consisting of locked-coil or structural strands, were protected against corrosion by galvanizing and paint. Nevertheless, extensive corrosion occurred (S. C. Watson and D. G. Strafford, ‘‘Cables in Trouble,’’ Civil Engineering, vol 58, no. 4, April 1988, American Society of Civil Engineers). Contemporary stays, in contrast, are similar to external tendons generally used for posttensioned concrete.
They consist of prestressing steel, sheathing, corrosion-protection materials, and anchorages.
The Schillerstrasse footbridge in Stuttgart, Germany, completed in 1961, was the first cable-stayed bridge to employ a sheathed and cement-grout-injected stay system. The stays consist of a bundle of parallel prestressing wire encapsulated in a polyethylene (PE) pipe and injected with cement grout. The purpose of the PE sheathing is twofold: to provide a form for the cement grout and to serve as a corrosion barrier. The stays have been inspected on numerous occasions and have shown no signs of corrosion. The first use of this system in the United States was for the Pasco-Kennewick Bridge, completed in 1978. The stays of the bridge were inspected in 1990. After 12 years in service, the exposed wire was as bright and as good as the day it was installed, indicating that with proper care and procedures for installation, cementitious grout can be an effective corrosion inhibitor.
A sheathing of high-density polyethylene (HDPE) pipe is airtight. A 1⁄4-in thickness provides the same vapor barrier as a 35-ft-thick concrete wall. However, the HDPE pipe must be handled with care. If abused, as in the case of the Luling Bridge (related to excessive grout pressure), the pipe may, in time, develop longitudinal cracks. In addition, the cementgrout column may develop transverse cracks from cyclic tension in the stays, among other reasons. Thus, there is need to prevent direct access to the bare prestressing steel by corrosive agents.
Alternative materials to cement grout or means of providing additional corrosion barriers have been sought to increase corrosion protection of steel stays. Such materials as grease, wax, polymer-cement grout, and polybutadiene polyurethane have been tried with varying degrees of success.
Corrosion protection systems may be either two-phase or single-phase. In the two-phase method, the permanent corrosion protection is applied as the last operation of construction of the structure. This means that a temporary corrosion protection is required during a construction period that may be two to four years or longer in duration. The effectiveness of most temporary corrosion protection methods is short lived. If replenishment is overlooked or not accomplished, for whatever reason, there is a distinct risk of corrosion occurring before the permanent corrosion protection can be applied and the risk of having to replace the cable. There is currently a trend to a single-phase corrosion protection system that provides both the temporary and permanent system simultaneously, i.e., a system that provides protection from the manufacture of the cable throughout its service life.
The use of alternative materials for cementitious grout attempts to overcome the problem of grout cracking, and thus obviate the potential of a direct path for the corrosive agents to the steel, in the event the outside sheathing is compromised. However, the use of alternative materials for cementitious grout does not overcome the problem related to temporary corrosion protection of the steel strands.
To overcome the potential problems of a sheathed and grouted system, multiple barrier systems have been developed. The concept simply provides multiple corrosion barriers such that one or more materials take over the protective function for a material that has failed, or stated another way, provides increased redundancy in the corrosion protection system.
Generally, these additional barriers are provided by one of the following two methods:
• Individual greased and sheathed strands (the so-called monostrand method). It should be noted that the word grease as used in this context is generic, the material may be grease, wax, epoxy-tar or some other appropriate material.
• A coating applied directly to the strand such as galvanizing or epoxy.
Both of the above systems are installed or applied prior to shipment, thus, they are not only incorporated into the final total corrosion protection system, but they also provide the temporary corrosion protection during shipping, storage, after installation until the final grouting operation, and during service life.
In the search for corrosion protection methods and materials, consideration has been given to coatings applied directly to the prestressing steel. Galvanized prestressing wire has been used in some multibarrier systems. Galvanized prestressing steel should never be used where it is in direct contact with cementitious grout and the designer must be cognizant of the effects of galvanizing, if any, on the material properties of the steel.
A relatively recent development is an epoxy coated and filled strand whereby the interstices between the wires are filled with epoxy. This eliminates the concern for corrosive agents gaining access to the interior of the strand. Epoxy coating and filling of the individual strands provides both temporary and permanent corrosion protection to the strand, and eliminates the concern for aggressive corrosion agents reaching the prestressing steel as a result of cracked cement grout and potential cracks in the outside sheathing. So as not to compromise the effectiveness of the system, attention must be paid to the anchorage details. Special wedges are required that bite through the epoxy thickness and grip the prestressing strand.
The epoxy should not be stripped from the strand.
The so-called monostrand system as used for cable stays is an adaptation or transfer of technology of the post-tensioning monostrands that are used for parking garage or flat slab construction. The stay consists of a parallel bundle of 0.6 in diameter unbonded prestressing strands that are individually greased and sheathed, and enclosed in a HDPE external pipe and grouted. The corrosion protection of unbonded prestressing strand relies to a large extent on the prevention of moisture and corrosive materials from reaching the steel. Therefore, the sheathing on the individual strands must be completely watertight throughout its length, up to and including the anchorages.