April 28, 2016
The use of nickel-containing stainless steel rebar in bridges worldwide has been growing—although perhaps not as quickly as motorists wish when queuing impatiently while a lane or two of a bridge or elevated roadway is being dug up and repaired. The relatively quick corrosion of carbon or epoxy coated rebar results in cracking of the concrete and thus the need for major maintenance, leading to blocked off lanes of traffic while the concrete is removed and the rebar replaced. Today, transportation authorities are increasingly factoring in the indirect costs to society of traffic delays (wasted fuel, wasted time) as well as environmental costs (air pollution, landfill costs) when specifying construction materials. This holistic thinking makes the selection of stainless steel even more clearly justifiable.
Even without considering these less tangible factors, stainless steel is the best choice in many cases when looking at costs over the life of the bridge. When the Schaffhausen Bridge, which spans the Rhine in Switzerland, was planned in the mid-1990s, the design called for it to have an 80-year life with minimal maintenance and no loss of structural integrity. The use of conventional reinforcement materials, either coated or plain carbon steel, would mean major repairs roughly every 25 years, whereas the use of Type 304 (UNS S30400) stainless steel rebar would meet the 80 year criteria. By the selective use of stainless steel, the total cost of the bridge was increased by only 0.5%. Significant savings can be realized after only 25 years when stainless steel is used, with even greater savings accumulating over the life cycle of the structure. If this bridge had higher road salt loadings, or was near to seawater, a higher alloyed rebar material such as Alloy 2304 (S32304), Type 316L (S31603) or even Alloy 2205 (S32205) may have been needed, and that too would have only added marginally to the cost of the project. Many factors can affect the amount of chloride that will reach the rebar, including cement type, depth of cover, and water/cement ratio. This means that not all rebar in the structure needs to be in stainless steel. Rebar positioned deeper in the structure, or not otherwise exposed to higher levels of chlorides can be supplied in the lower cost unalloyed steel. Using a combination of stainless steel in the top layer and carbon steel in a lower layer of a bridge deck is a common practice. Tests have shown that galvanic corrosion is not an issue here.
Today, designers are specifying longer lifetimes for major bridges, as great as 125 years. The initial cost of some bridges may be a few percent higher with the selective stainless steel option, but in all cases, results in much lower life cycle cost.
The Allt Chonoglais bridge near Orchy, Scotland is a more recent example of stainless steel rebar use. The old bridge was found to be under strength for the future loads it would carry, and could not be economically repaired. De-icing salts over time had caused corrosion of the carbon steel rebar resulting in cracking of the concrete and leading to a reduction in structural integrity. A new and stronger structure was engineered with a requirement for a design life of 120 years. After considering the expected chloride levels over its lifetime, the Highways Agency and Transport Scotland chose Alloy 2304 (S32304) stainless steel for selected areas, such as the bridge deck, abutments, wing walls and bearing plinths. The total cost of the construction was £1.8 million (about US$2.5 million) with the new bridge open to traffic in August 2013. In 25 years or so, motorists driving on the A82 between Glasgow and Inverness will appreciate the forethought given by the Scottish engineers for using a nickel-containing stainless rebar that will last the 120 design life of the bridge.
Navy Pier Point Loma
All steel hulled sea-going vessels pick up a magnetic signature as they travel the globe. For naval vessels, this magnetism can be detrimental, as it can be used by enemy magnetic mines and other magnetic sensing equipment to identify and destroy them. Many navy vessels therefore undergo a deperming operation (also called degaussing) that removes that permanent magnetism and helps to camouflage them. The pier where this operation takes place should show no magnetic response either, hence the need for nickel-containing austenitic stainless steels. When the U.S. Navy selected concrete reinforcing (rebar) and stranded wire for their pier at Point Loma, San Diego, California, they used both Type XM-29 (UNS S24000) and Type 304 (S30400) stainless steel. The chosen alloys in the pier had to withstand the effects of the Pacific Ocean salt water diffusing into the concrete combined with warm temperatures. The magnetic permeability of Type XM-29 remains very low even after severe cold working, so the pre-stressing strand used in the square piling, pile caps and in the deck were also of this alloy. Where the stainless steel strand was only lightly stressed, Type 304 was used. In 2008, after 20 years of service, an inspection was made of the whole structure. The only minor corrosion found was on the Type 304 stainless steel where there had been insufficient concrete cover (less than 12mm). Where there was 25mm or greater cover, the Type 304 and all the Type XM-29 was found to be in excellent condition. Similar piers have been constructed at King's Bay, Georgia and Pearl Harbour, Hawaii.