Tuesday, 24 May 2016

Stripping Down Corrosion

By Sam Parkin, New Business Engineer

One of the most common queries we get from structural engineers is how steel screw piles are designed to deal with corrosion (rusting).  While there are numerous ways of dealing with corrosion, Piletech typically recommend dealing with corrosion by the most powerful tool at our disposal: DESIGN.

There are three typical methods used for corrosion protection:
1.  Coating systems (including galvanising)
2. Cathodic protection
3. Sacrificial section area

In our experience galvanising and other coating systems are problematic as they get compromised and scratched off the pile during the installation process, or before they’re even being unloaded from the truck.  They also have a limited design life. Even worse, galvanising and coating systems can also promote localised corrosion intensity if a crack in the coatings forms. We’ve also seen systems such as Denso tape or HDPE sleeves – these options not only increase costs but increase programme and can cause H&S issues during installation. 

Cathodic protection involves setting up an electrochemical cell with the pile becoming the cathode and a sacrificial anode. This method of preventing corrosion is troublesome as the anode has a limited lifecycle and as such this method presents the need to continuously maintain the system over the design lifecycle of the piles. Initial setup costs for cathodic protection is also comparatively high.
Piletech now believe the best way to ensure quality assurance of the corrosion protection methodology is to simply allow sacrificial wall thickness to achieve the required design life for the piles.  It’s also cheaper, quicker, requires less QA, safer…. And it’s a lot easier – done with the flick of a pen before mobilising to site.

To calculate the required section thickness of the shaft and the helix you must first determine the design life of the piles, look at the surface area affected, and then estimate the rate at which the section will corrode. The design life is generally specified by the Structural Engineer; for permanent structures this is typically 50 or 100 years depending on the importance level of the structure.
With regard to the surface area considered, Piletech allow for corrosion to the outside face of the CHS shaft of a screw pile and to both faces of the pile helices. We don’t typically include for internal corrosion because the pile is concrete filled with a base cap to prevent soil entering into the pile thus eliminating any free oxygen required to cause corrosion.

The corrosion rate is either determined by soil testing on site, or it can be referenced in either AS2159:2009, HERA Design and Construct Bulletin No. 46 (reissued as Bulletin No. 62), NZS3404:2009 and/or The New Zealand Building Code.

The New Zealand Building Code verification method B1/VM4 states that the amount of section area deducted needs to take account of the aggressiveness of the soil and that further guidance can be found in AS2159:2009 Section 6.5 or the HERA Design and Construction Bulletin No. 46.

AS2159:2009 calls for a uniform corrosion allowance (mm/yr) which is dependent on the exposure classification. The exposure conditions depends on the PH, Chloride levels, Resistivity of the soil and the soil condition (high or low permeability soil). Using AS2159:2009 the steel sections of our piles typically fall into the non-aggressive (less than 0.01mm/year) or mild (0.01 - 0.02mm/year) category. 

HERA Design and Construct Bulletin No. 46 is specifically written relating to corrosion of steel sections in the ground and in water.  Prior to this report being issued, the go-to standard for corrosion rates was from a Steel Construction Institute (SCI) publication which outlined a rate of 0.015mm/year, regardless of soil conditions.  Thus the HERA report set out to test this rate in relation to the following variables:
  •   A natural soil pH ranging from 2.3-9.5
  • Soil Resistivity ranging between 300-50,000 Ohm’s
  • Soil N Values between 4-40
  • the majority of all the sites included intersecting water tables with both saline and fresh water with some exhibiting groundwater flow.

The key findings are outlined below:

1. The highest corrosion rates were found in permeable soils within 2.5 metres of the surface and which were at or above the water table. Even at the upper level the corrosion rate was below the 0.015mm/year outlined above.

2. Both mean and maximum corrosion rates decrease with time after installation of the pile.

3. There is no statistical significance found between the corrosion rate and any of the following parameters: soil type and permeability, N Value of the soil, pH of soil (there was a slight increase in corrosion rate below pH of 4), soil resistivity and nature of ground water.

4. The corrosion rate was similar on all faces of the steel pile.

5. The type of steel has no influence.

These conclusions indicate that the soil conditions have very little effect on the corrosion rate of the pile, and that using a 0.015mm/year corrosion rate is not only applicable, but conservative.

As a final reference, NZS3404:2009 - the structural steel code - has design corrosion rates depending on the location (exposed, water and soil), the type of fill encountered (controlled, uncontrolled) and the level of the water table. The two cases most relevant to Piletech screw piles are buried in fill below the permanent water table and the buried in controlled fill above the permanent water table both these cases have a design corrosion rate of 0.015 mm /year.  Which is in line with the other code requirements, as outlined above.

Thus, Piletech – unless directed otherwise – typically utilise a conservative corrosion rate of 0.03 mm / year for the outside of the pile shaft and 0.015 mm / year for both sides of the pile helix.  It’s also cheaper, quicker, with no QA hassles before our site teams mobilise to site.

Typical design values for corrosion over design life for Piletech screw piles
50 year design life (Shaft) = .03 x 50 = 1.5mm
100 year design life (Shaft) = .03 x 100 = 3mm
50 year design life (Helix) = .015 x 2 x 50 = 1.5mm
100 year design life (Helix) = .015 x 2 x 100 = 3mm