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    Corrugated Metal Culverts - Durability in New Zealand Conditions

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    louis dony

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    Corrugated Metal Culverts - Durability in New Zealand Conditions

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    louis dony
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    38 views3 pages

    Corrugated Metal Culverts - Durability in New Zealand Conditions

    Uploaded by

    louis dony

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
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    Corrugated Metal Culverts – Durability in NZ Conditions

    Corrugated Metal Culverts – Durability in New Zealand Conditions


    Corrugated steel pipes have been used for culverts in the United States since 1886 and
    aluminium pipes since the early 1960s. They have proven to be convenient, easy to install and
    generally more cost-efficient than concrete culverts. To improve the durability of metal culverts
    coatings like asphalt, concrete and - more recently, epoxy/polymers - have been applied. This
    summary introduces CSP® range of Multi-plate structures and outlines the methods for
    predicting the service life of corrugated metal culverts (both uncoated and coated) in various
    environmental conditions - with a particular focus on the New Zealand experience.

    Mechanisms of metal culvert deterioration


    Environmental deterioration of metal culverts is, broadly speaking, caused by three
    mechanisms:
    Soil-side corrosion which refers to the attack on the outside face of the buried culverts
    caused by soil contact. Both zinc and aluminium form protective films of oxides when exposed
    to corrosive conditions. These films will dissolve and re-form at a rate dependent on the nature
    and severity of the environmental attack. In a galvanised steel culvert, once the layer of zinc
    has been lost, the corrosion of the steel wall progresses more rapidly which is why aluminium
    culverts typically outlast galvanised steel in similar environmental conditions. The durability of
    the outside face of culverts can be extended by applying additional coatings. Cohesive or
    water-logged backfill material accelerates culvert corrosion.

    Ph 0800 655 200 or visit www.csp.co.nz May 2021 / Page 1


    Corrugated Metal Culverts – Durability in NZ Conditions

    Water-side corrosion affects the internal surface of the water-carrying metal culverts. Same as
    for the soil-side corrosion, aluminium tends to perform better than galvanised steel and
    additional coatings have been successfully used to extend the service life of culverts. The
    internal surface of culverts, above the water line, is typically less affected by corrosion than the
    section of walls in contact with water. The chemical composition of water flowing through the
    culvert affects the rate of water-side wall corrosion.
    Invert abrasion is caused by the solid particles (typically gravel and rock) transported by
    water. Aluminium tends to have better abrasion resistance than galvanised steel because,
    being a softer metal, it tends to absorb the impact of aggregate without chipping. Also, the
    corrosion resistance of aluminium is in-depth while steel deteriorates quickly once the
    protective layer of zinc has been stripped by abrasion. Durable coating layers like asphalt
    paving, epoxy/polymers or concrete enhance the abrasion resistance of metal culvert walls.
    The majority of culverts deteriorate starting from the invert, where water-side corrosion
    combines with abrasion to create a particularly aggressive combination of environmental
    conditions. For this reason invert coating is usually the most economical way of extending the
    service life of culverts.

    Assessment of environmental corrosivity


    The two main predictors of the severity of the environmental attack on metal culvert walls are
    the concentration of hydrogen ions pH and soil resistivity governed by the presence of
    chloride/sulfate ions. Other chemical elements affecting the durability of metal culverts are
    calcium (in the form of CaCO3), which slows the corrosion rate of steel, and heavy metals
    which are particularly detrimental for aluminium. Soil resistivity and pH index can be measured
    on site or in the laboratory to help designers specify the right product to achieve the required
    service life. Since abrasion greatly accelerates invert corrosion, it must also be taken into
    account.
    The pH readings and soil resistivity can be combined into one graph to indicate the expected
    service life of metal culverts in any particular environmental conditions. In this approach the
    end-of-life is defined as either the loss of say 25% of the culvert wall thickness to corrosion or
    as local perforation. Alternatively, tabled information or formulae describing the rate of wall
    thickness loss can be used by designers.

    ‘Sacrificial wall thickness’ durability design


    Strength checks in the AS/NZS 2041.1:2011 ‘Buried corrugated metal structures’ standard
    are based on the residual thickness of the culvert wall at the end of design life. This means that
    some wall thickness can be lost to corrosion (sacrificed) without affecting the safety of the
    structure. The same approach is used in the design of other structural elements in soil contact
    such as steel piles or pipelines, but culverts showing surface corrosion are often viewed as
    failing. This perception is based on everyday experience with steel components painted for
    aesthetics. Culverts are different in that their appearance is secondary to function which is why
    ‘sacrificial wall thickness’ has been commonly adopted as the most cost-efficient design
    approach.
    If aesthetics is a consideration, aluminium culverts are a better option than the galvanised steel
    product, as aluminium oxidation looks similar to metal so does not have a ‘rusty’ appearance.

    Ph 0800 655 200 or visit www.csp.co.nz May 2021 / Page 2


    Corrugated Metal Culverts – Durability in NZ Conditions

    CSP® culvert durability design


    To comply with the New Zealand legal framework and NZTA F3:2010 ‘Specification for pipe
    culvert construction’ document CSP® culverts are designed to AS/NZS2041.1:2011 ‘Buried
    corrugated metal structures’ standard. This includes strength checks for the residual (end-of-
    life) culvert wall thickness as well as compliance with the material suitability and invert
    protection provisions in Table 3.4. For added protection, the CSP® Multi-plate 100+ premium
    product range designed for corrosive environments features both the ‘sacrificial wall thickness’
    code allowance and full epoxy/polymer coating on both faces.

    CSP® NEW online culvert selection and pricing tool

    To assist culvert designers in choosing the right product to meet the durability requirements of
    a particular project CSP® has developed a convenient online design tool. Based on the
    environmental conditions on site (as assessed and input by the culvert designer) the tool
    selects the CSP® culvert material and trim to achieve the required design life. The price
    generated by the online tool is indicative only and does not include non-standard features like
    bevels, skews, elbows, risers etc. Cick here to access CSP® culvert selection and pricing tool.

    Bibliography:

    1 NCSPA ‘CSP Durability Guide’, May 2000


    2 NCSPA ‘Invert Abrasion Testing of CSP Coatings’, March 2002
    3 California DOT ‘Development of New Corrosion/Abrasion Guidelines for Selection
    of Culvert Pipe Materials’, November 2009
    4 AS/NZS2041.1:2011 ‘Buried Corrugate Metal Structures’ standard
    5 NCSPA ‘Pipe Selection Guide’, 2010

    Ph 0800 655 200 or visit www.csp.co.nz May 2021 / Page 3

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