Construction and Value

Ladner Creek Trestle

Construction is an interesting aspect of social reality – it reflects (or merely “is”) the values of our time. The values of 1945 are very clearly “in” this Kettle Valley Railroad trestle – it was built both quickly and to last, without expense spared. Its importance likely concerned the strength of redundancy in Canadian rail transport during the war – the previous wooden trestle would be easier to destroy, or to be destroyed accidentally by fire. But despite the fact it was built quickly did not mean it was built without care or without heavy expense. The entire bridge was painted to protect it from corrosion. This paint is now peeling off, but the bridge remains in better shape than you might expect.

ladner trestle rustThe highway bridge over Ladner Creek, part of the Coquihalla Highway, is different from the trestle, and not merely in the way that it suits cars as the trestle suits trains. The bridge is made out of re-enforced concrete – a material that characterizes the highway age.

Ladner Creek Highway Bridge

Unlike steel, concrete can be made any shape, it can be strengthless fill, a polished floor, or it can be filled with metal rods and bear huge loads. We think it looks like rock, strong and permanent – but rock erodes (that’s why the Rocky mountains and the Laurentian mountains have different appearances), and concrete erodes many times more quickly than rock. Unlike rock, concrete is permeable to water and, categorically, anything soluble in water. Which means that if you strengthen your concrete with corrosion susceptible metal, the usable life of the resultant structure is limited not only by the erosion of the concrete itself but by the corrosion of the strengthening material inside. So, the use of concrete as a bridge building material means in advance a choice has been made about how long the structure can last. But this is not a value – every material you could pick would have some influence on how long the structure would last. Rather, the value exhibited in concrete structures such as this one is not knowing the state of repair of the structure. Since the essential corrosion is internal, no one using it sees the state it is in. Compare that with the picture of the rusting trestle above – if a bridge looks like this, every voting commuter knows the bridge is rusting. In comparison, sophisticated equipment and training is required to adequately assess what state a concrete structure is in. While we were able to discern many instances of salt weeping through the bridge deck, this is an indirect way of determining the corrosion and not an obvious sign of alarm to a lay eye (also, you can only see it if you happen to be standing under the bridge).

Of course, the highway bridge is built using some steel, in the form of I-beams sitting on pillars, holding up the bridge deck. These i-beams, however, were not painted. So, despite being 40 years newer than the trestle, they are significantly more corroded:

Highway Bridge I-beam corrosion

The metal is flaking off, like it might off a hulk rusting in a harbour. Crucially, however, this cannot be seen from the highway – because it is under the bridge. Only hikers would notice it, and only if they had parked on the wrong side of the road and had to bushwhack under the trestle to the trail-head. This evening I emailed Rodney Chapman, head of maintenance for the B.C. ministry of highways, and notified him of the corrosion and salt weeping at the Ladner Creek bridge, although I spared him my lecture on values. His response, if he chooses to get back to me, might help fill out this post by demonstrating not only our values in construction, but our reactions to those values when they are pointed out.

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3 thoughts on “Construction and Value

  1. Quite a good response concerning the highway bridge:

    Thank you Tristan Laing for your observations and photos as provided in your Tuesday, September 1, 2009 6:38 PM email to Rodney Chapman.

    The bridge was built in 1985. We inspect our bridges annually as part to the structures asset management program for the Ministry of Transportation and Infrastructure, South Coast Region. The last routine inspection was done by our Bridge Area Manager 06AUG09. Your observations match those contained in our inspection records. The bridge girders are weathering steel, the rust formed on the surface of the steel acts as a protective layer. Weathering steel is typically only painted in areas that are directly exposed to corrosive water leakage, typically near joints in the deck.

    The white staining you observed on the underside of the deck is efflorescence, the deposit of calcium carbonate leachate as water passes through cracks in the deck. This is part of the normal aging processes for concrete deck. When reinforcing steel corrodes the oxides that are created on the surface of the reinforcing have a 6 fold greater volume than the iron that is removed to create it. This leads to bursting stresses in the concrete that are exhibited by increased cracking. That allows more corrosive water into the cracks and the deterioration accelerates to the point where the cracking progresses horizontally, creating delaminations at the reinforcing layer. As the delaminations grow they pop out creating spalls in the concrete surface. This generally takes from 15-25 years to start. Then over the next 15-20 years it gets to the point where a major renewal investment is made to restore the structure.

    The conditions you observed are part of the normal aging processes for this type of bridge. There is no safety concern. We will continue to observe and record the conditions at this bridge and define the renewal investment needs. At the appropriate time, this structure will become a candidate for funding detailed condition assessment, structure evaluation and lifecycle renewal options analysis. The outcomes of that work are used to define the best renewal investment strategy, considering the thousands of structures we manage and the funding made available for capital renewal by the legislature.

    This bridge has a Bridge Assessment Index = 2.78 out of a possible 5.00, where 5 represents complete replacement required. It ranks 237 in the province. Typically the worst 200 are considered for renewal scoping funds in order to define the optimum renewal strategy and the timing. Once the scope and cost is defined the bridge is submitted as a candidate for design and construction funding. There are many contenders for the limited funds made available and so many structures reside in the renewal needs backlog waiting for funding support. During the waiting period structures may need to be posted to reduce the live loads allowed to cross it, or lane restrictions applied, or both. If necessary the Ministry will close a structure to public use. Protection of public safety is paramount for the Ministry staff responsible for managing structures.

    Thank you for bringing your observations to our attention.

    We trust this addresses your concerns, comments and questions are welcomed.

    Regards,

    Wim Jellema, P.Eng., C.Eng.

    Manager, Bridges & Structural Engineering

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