Bridge using stainless steel rebar to last 120 years

Citation

(2002), "Bridge using stainless steel rebar to last 120 years", Anti-Corrosion Methods and Materials, Vol. 49 No. 6. https://doi.org/10.1108/acmm.2002.12849fab.002

Bridge using stainless steel rebar to last 120 years

Bridge using stainless steel rebar to last 120 years

Keywords: Stainless steel, Structures, Corrosion resistance

The Oregon Department of Transportation (ODOT), using highly alloyed stainless steel reinforcing bar in its concrete structures, is building a bridge that is expected to provide maintenance-free service for an amazing 120 years, nearly 2.5 times the service life of the bridge it is replacing.

Frank Nelson, bridge preservation managing engineer for ODOT, figures the taxpayers are getting a huge bargain. When finished by the end of 2003, the bridge will cost approximately $12.5 million. The stainless steel rebar, utilised in the most critical structural elements, accounts for only 13 per cent of the total bridge cost.

For that small increase, he observed, ODOT will save the cost of bridge replacement in 50 years. That is a sum likely to be $25 million dollars, or at least twice the cost of bridge construction today. As an alternative, the money saved could be used to build another bridge. Meanwhile, the new structure will require little more than routine examination.

The new bridge, carrying U.S. 101 over the Haynes Inlet Slough near the coastal town of Coos Bay, is using what is believed to be more stainless steel rebar than any bridge in North America - 362,878 kg or nearly 400 tons. Yet, this is not an ordinary stainless steel because it had to meet some very challenging requirements for corrosion resistance, strength and site seismicity.

Along the Oregon coast, the marine environment is very hostile to bridges. Salt-laden air and fog from the Pacific Ocean condense under the deck and T-beams of this bridge. Wind blows the chloride-containing moisture underneath the structures, initiating corrosion. Rain washes the chlorides off the road surface, but flushes away nothing below.

ODOT considered stainless-clad bar and epoxy coating of carbon steel rebar, but decided neither possessed sufficient durability nor long term resistance to chloride-induced corrosion. The concrete in which the rebar is embedded will eventually become contaminated with corrosive chlorides.

Extraordinary strength was required of the stainless to facilitate design of the new bridge and to deal with the potentially devastating seismic activity in this area. ODOT specified that the stainless alloy used had to have a minimum yield strength of 75 ksi (520 MPa). That strength level is new to bridge building and substantially higher than the 60 ksi (420 MPa) minimum yield strength required of the Type 316LN stainless used for rebar in ODOT’s Brush Creek and Smith River bridges replaced a few years ago. In addition, the alloy also had to provide high ductility (25 per cent elongation) so it could be effectively fabricated.

In view of the area’s geological history, Bridge Designer James Bollman had a study done to determine design seismicity and collapse criteria. Ground surface accelerations were intended to forecast a 1,000-year probability seismic event. Ground surface acceleration was calibrated at 1.05 g maximum, and peak bedrock acceleration at 0.54 g. The bridge, consequently, has been designed to remain serviceable with only a 10 per cent probability over its lifetime of the site seismicity exceeding the design seismicity.

It was clear that ODOT, its goal set on extending bridge life, wanted this to be its strongest bridge yet. With a higher strength stainless alloy than any it had used to date, Bollman also expected to enjoy an economic advantage of less stainless rebar weight than would have been required using 60 ksi alloy.

Stainless alloy choice

Farwest Steel Corp., Eugene, Ore., a steel distributor and rebar manufacturer, suggested trying Alloy 2205, a duplex stainless steel provided by Talley Metals Technology, Inc., Hartsville, S.C., a subsidiary of speciality materials expert Carpenter Technology Corp. ODOT was counting on Farwest Steel to fabricate the rebar it needed from rolled mill stock (Plates 2-8).

Plate 2 Completed Stage 1 of the new Haynes Inlet Slough Bridge from north bank of inlet, with retired timber trestle bridge at right and contractor’s partially dismantled workbridge in left background

Plate 3 Night pour of concrete on 2205 stainless steel and carbon steel rebar which has been used in the deck of the Haynes Inlet Slough Bridge

Plate 4 High strength 2205 stainless steel rebar with 2205 alloy mechanical couplers (two large diameter cylinders) has been used in a smaller quantity and in less space than possible with carbon steel rebar and lapped joints

Plate 5 Large section of rebar that has been made from both Carpenter 2205 alloy and carbon steel (brown in color)

Plate 6 Carpenter 2205 alloy rebar looking along the deck, down the centerline of a T-beam for the Haynes Inlet Slough Bridge

Plate 7 Taper threaded end of large diameter 2205 stainless steel rebar accommodates internally taper threaded ERICO LENTON^® mechanical coupler that has been made from the same alloy

Plate 8 In order to meet the increasing demands relating to ever smaller and more efficient terminal devices, with its 4-Chip-Stacked CSPs Sharp developed a system solution in package with no changes in form

Talley ″s 2205 stainless has a duplex microstructure, mixing austenite and ferrite phases, that reportedly gives the alloy the required 75 ksi yield strength. This is said to be 25 per cent more yield strength (per ASTM-A-955) than that of austenitic Types 316LN and 304 stainless steels (60 ksi) which are more common candidates for bridge rebar.

With improvements in hot rolling procedures, in fact, the company has been able to produce 2205 stainless steel bar in a yield strength range of 85-95 ksi. At the same time, while increasing strength, it has managed to exceed the 25 per cent elongation requirement that reflects ductility. That means its 2205 alloy rebar claims superior fatigue resistance. Thus the alloy can better withstand movement of the bridge and stress cycling under heavy truck traffic.

Talley delivers its spiral-ribbed rebar to Farwest Steel in the as-rolled condition, pickled and acid cleaned with surface free of oxides. The company, said to be the first alloy producer to hot roll this high strength product far from a routine procedure, has provided lengths of up to 30 feet in various sizes from 0.375 in. to 1.375 in. round (Nos. 3 through II).

With a careful balance of chromium, molybdenum and nitrogen content, 2205 stainless steel claims superior resistance to chloride pitting, crevice corrosion, stress corrosion cracking and general corrosion in many environments. Its resistance to corrosion is believed to be substantially better than that of the Type 316LN stainless ODOT used in previous bridge reconstruction.

Nominal analysis of 2205 alloy is: carbon 0.030 per cent max, manganese 2.0 per cent max, phosphorus 0.030 per cent max, sulphur 0.020 per cent max, silicon 1.0 per cent max, chromium 22.0 per cent, nickel 5.5 per cent, molybdenum 3.0 per cent, nitrogen 0.14 per cent, balance iron.

ODOT fully expects Talley’s 2205 stainless steel rebar to solve the corrosion problems it has experienced in vulnerable concrete coastal bridges. On a regular basis, inspectors used to drill into the concrete for core samples to measure chloride infiltration. They generally found that chloride ions had penetrated the hardened concrete of the most exposed structures, causing serious cracking, delamination and spalling.

Chloride intrusion, accelerated by the tensile cracking of the concrete caused by the relentless load of heavy moving traffic, then caused cracking between concrete and bars along the length of the original carbon steel rebar. With time and more traffic, the cracking caused rust, which occupies a volume greater than that of the original metal. This reaction led to spalling and, with pieces falling away, a further loss of bond between concrete and the steel. As structural members are weakened by corrosion, the increased stress on remaining members could lead to structural failure. With carbon steel rebar, structural failure has occurred in as short a time as 17 years. In contrast, the 2205 alloy is expected to last well over one hundred years.

Bridge design

Along with the duplex stainless alloy rebar, ODOT is using a much larger volume of 614,000 kg of grade 60 uncoated carbon steel rebar in the new bridge for substructure elements where corrosion is less of a problem. The two different steels are, for the most part, being used independently in different structural elements. Where they are used together, the stainless rebar is covered with a PE (polyethylene) sleeve at all points where the dissimilar metals intersect to prevent the possibility of galvanic corrosion.

The new bridge, 773 feet long and 85 feet wide, with rising, curving approaches, is a series of three spans of two-hinge cast-in-place concrete deck arches. An estimated 14,000 vehicles a day use the bridge, which carries the busy Oregon Coast Highway over an estaurine inlet. Hamilton Construction Co., Eugene, is the general contractor in charge of construction.

Hamilton left the older, heavily timbered bridge in place to carry traffic while the east half of the new bridge was convicted in Phase I of the project. Now, in Phase II, the finished east half is carrying traffic, while the old bridge is being removed and the west half is built to join the east half While the old bridge had two lanes, the replacement bridge will have five.

To increase resistance to corrosive attack on the new bridge, Bollman had all bridge elements above the footings cast with microsilica concrete. This type of concrete is less permeable to corrosive chloride ions than conventional concrete. He specified also that Talley’s 2205 stainless alloy rebar be used to reinforce the deck, deck support T-beams and the rail curb on the edges of the bridge.

The deck and T-beams are typically the first to suffer the effects of corrosion because they are thinner than the main support members, and subjected to bending forces and dynamic loads from heavy trucks that cause significant stress cycling in both the rebar and concrete. Substructure elements, in compression under service load, do not experience stress cycling under service load into the tensile range. They are, therefore, less susceptible to damage from chloride intrusion.

The arch design harmonises with Oregon’s other coastal bridges and, in particular, the beautiful nearby Conde McCullough Memorial Bridge. The two-hinge arch design also allows for more slender arch ribs, which enhance the bridge appearance and also help meet the challenging seismic conditions. Without vertical bending, a more slender arch is possible at its support. This allows for smaller and less stiff supporting pedestals and footing, thus less cost for those foundation members.

All concrete members excepting the arch ends are monolithic, with reinforcement running completely through. This feature enhances the seismic energy absorptive capacity, which is a key element of bridge seismic resistance.

At the end of the ribs, hinges that have been made of 2205 stainless steel plate are submerged in brackish water of the estuary at high tide. These hinges preclude in-plane bending of the arch ribs at their supports for either live or dead load. Pins used with the hinges are made of a non-galling, corrosion resistant stainless steel.

Rebar path

After Farwest Steel receives hot rolled 2205 alloy rebar from Talley, it cuts pieces to specific lengths, bends some (because of the alloy’s good ductility) and adds LENTON taper threads to the ends of the No.10 bars. The fabricator requires Talley to comply with a straightness tolerance of no more than 1 in. deviation over a 5 foot length. Straightness is important because the company cuts a quantity of bar at a time in a tray, butting all the pieces against the same reference plate. The cut-off must produce bars of exactly the same length. Then the fabricated product is wrapped in plastic and shipped to Triad Steel Inc., subcontracting ironworker, at the bridge site.

Triad, based in Springfield, Ore., then positions the stainless rebar in forms in accordance with CAD placing drawings provided by Farwest Steel and ODOT. Responsible to Hamilton Construction, Triad then lap splices the 2205 alloy rebar with stainless tie wire, or joins lengths with LENTON mechanical couplers that have been made by ERICO, Inc., Solon, OH, from the same Talley alloy.

When 2205 duplex stainless rebar is joined by good lapping technique, the lapped joint has developed a yield strength of 75 ksi. However, the ERICO LENTON taper threaded 2205 alloy coupler, torqued onto the 2205 alloy rebar threaded by Farwest Steel, has developed an ultimate tensile strength of 100 ksi. In tests, strength as high as 130 UTS has been reached for LENTON. Under new construction codes, there is greater reliance on mechanically coupled rebar to transfer loads and stresses than there is on concrete, which can be adversely affected by salt, chlorides and other environmental conditions.

The LENTON mechanically coupled 2205 stainless rebar has been used extensively in the bottom of the T-beam which supports the bridge deck above the arch. It was chosen by ODOT for this application because its greater strength and corrosion resistance were needed here, and also because the coupled rebar, compared with lapped rebar, occupied less of the limited space available.

Slippage must be held to a minimum at the joint between the rebar and coupler because of the perpetual seismic threat and the tensile stress cycling from truck traffic. With the ERICO LENTON couplers, slipping motion has been held to 0.003 in. maximum at half the specified yield strength. This is said to be significantly better than the maximum 0.010 in. slip allowed by the ODOT specification. No other coupler system tested was found capable of meeting the tight slip specifications for this job. When the bridge is fully loaded, the general operating stresses are about half the yield strength.

Details available from: Talley Metals Technology, Inc., Tel: +1 800 334 8324; Fax: +1 610 736 8187; E-mail: cfranco@cartech.com; ERICO LENTON, Tel: +2 900 248 2677