Durability
Piling Handbook, 8th edition (revised 2008)
Durability
Chapter 3/1
Durability
3.1 Introduction Steel piling is widely used in permanent earth retaining and structural foundation works, and in the majority of circumstances it can be used in an unprotected condition. The degree of corrosion and whether protection is required depends upon the working environment - which can be variable, even within a single installation.
In general, marine environments are the most corrosive and variable. In the few metres of vertical zoning which most structures encompass, piles are exposed to underground, seawater immersion, inter-tidal, splash and marine atmospheric environments. For most environments characteristic corrosion rates have been established. However, in some cases, localised corrosion may occur, requiring detailed site examinations and data analysis.
This chapter outlines the corrosion performance and effective life of steel piling in various environments and reviews the protective measures that can be taken to increase piling life in aggressive environments.
3.2 Corrosion of piling in various environments
In determining the effective life of unprotected piles, the selection of piling section and the need for protection it is necessary to consider the corrosion performance of bare steels in different environments. The corrosion data given for the following environments indicate the loss of section where only one face is exposed to the environment. In practice, opposite sides of a piling structure may be exposed to different
environments. For example, one side of a harbour retaining wall may be subject to a marine environment whilst the opposite side is in contact with soil.
These situations are taken into account in 3.3, the tables of corrosion losses being based on those given in Eurocode 3:
Part 5.
3.2.1 Underground corrosion of steel piles
The underground corrosion of steel piles has been studied extensively. A review of published data, outlining mainly overseas experiences, concluded that the underground corrosion of steel piles driven into undisturbed soils is negligible, irrespective of the soil type and characteristics; the insignificant corrosion attack being attributed to the very low oxygen levels present in undisturbed soils. Pitting corrosion in the water table zone is frequently reported in the literature, but nowhere is this regarded as affecting the structural integrity of piling, except for excessive pitting found in some Norwegian marine sediments. Evaluations of
Piling Handbook, 8th edition (revised 2008)
Chapter 3/2
Durability
piles extracted from UK sites, ranging from canal and river embankments through harbours and beaches to a chemical slurry lagoon containing acid liquors (pH 2.8), also confirm negligible underground corrosion losses.
A further evaluation in Japan of test piles driven into natural soils at ten locations which were considered to be corrosive gave a maximum corrosion rate of 0.015 mm/side per year after ten years exposure.
An aspect of underground corrosion that can arise is that of microbial corrosion by sulphate-reducing bacteria, which is characterised by iron sulphide-rich corrosion products. Although this form of corrosion has been observed on buried steel structures, e.g. pipelines, there is no evidence from the literature or within ArcelorMittal experience that this is a problem with driven steel piles.
It is concluded that in natural, undisturbed soils steel pile corrosion is very slight and, for the purpose of calculations, a maximum corrosion rate of 0.012 mm/side per year can be used.
In the special case of recent-fill soils or industrial waste soils, where corrosion rates may be higher, protective systems should be considered.
3.2.2 Atmospheric corrosion
At inland sites, piles used for foundation work may also be used as support columns or retaining walls above ground level. In such cases bare steel will corrode in the atmosphere at a rate which depends upon the site environment. In order of increasing corrosivity, this can be broadly classified as rural, urban or industrial. Similarly, piling at coastal sites may be subject to a marine atmospheric environment.
Eurocode 3: Part 5 indicates that the atmospheric corrosion of steel averages approximately 0.01 mm/side per year and this value can be used for most atmospheric environments.
However, higher corrosion rates may be experienced in close proximity to the sea or when pollution produces very aggressive microclimates.
3.2.3 Corrosion in fresh waters
Fresh waters are very variable and can contain dissolved salts, gases or pollutants which may be either beneficial or harmful to steel. The term ‘fresh waters' is used to distinguish these from sea or estuarine brackish waters.
Chapter 3/3
Durability
The corrosion of steel in fresh waters depends upon the type of water, although acidity/alkalinity has little effect over the range pH 4 to pH 9, which covers the majority of natural waters. Corrosion losses from fresh water immersion generally are lower than for sea water and effective lives are normally proportionately longer.
However fresh waters are very variable and these variable conditions are reflected in the guidelines on corrosion rates given in Eurocode 3: Part 5.
3.2.4 Corrosion in marine environments
Marine environments normally encompass several exposure zones of differing aggressivity and the corrosion performance of marine structures in these zones requires separate consideration. Factors which can contribute to loss of pile thickness due to localised corrosion are also considered in 3.2.6. Information on loss of section thickness with time is given in tables 3.3.1 and 3.3.2.
Below the Bed-Level
Where piles are below the bed level very little corrosion occurs and the corrosion rate given for underground corrosion is applicable, i.e. 0.012 mm/side per year.
Sea Water Immersion Zone
Above the bed-level, and depending upon the tidal range and local topography, there may be a continuous seawater immersion zone in which, with time, piling exposed to unpolluted waters acquires a protective blanket of marine growth, consisting mainly of seaweeds, anemones and seasquirts. Corrosion of steel piling in immersion conditions therefore is normally low.
Tidal Zones
This zone lies between the low-water neap tides and high-water spring tides and tends to accumulate dense barnacle growths with fiIamentous green seaweeds. The marine growths again give some protection to the piling, by sheltering the steel from wave action between tides and by limiting the oxygen supply to the steel surface.
Corrosion investigations show that rust films formed in this zone contain lime, derived from barnacle secretions, which also helps to limit the long term corrosion rate of steels to a level similar to that of immersion zone corrosion.
Low Water Zone
At the low water level, where a lack of marine growth is observed, higher corrosion rates are often experienced. It has been established that, for piles in tidal waters, the low water level and the splash zone are regions of highest thickness losses.
Piling Handbook, 8th edition (revised 2008)
Chapter 3/4
Durability
Higher corrosion rates are sometimes encountered at the low water level because of specific local conditions and it is recommended that periodic inspection of these areas is undertaken.
Splash and atmospheric zones
Above the tidal zone are the splash and marine atmospheric zones, the former being subject to wave action and salt spray and the latter mainly to airborne chlorides. Unlike the tidal zone, these zones are not covered with marine growths. In the splash zone, which is a more aggressive environment than the atmospheric zone, corrosion rates are similar to the low water level, i.e. 0.075 mm/side per year. In this zone thick stratified rust layers can develop and at thicknesses above about 10 mm these tend to spall from the steel, especially on curved parts of the piles such as the shoulders and the clutches. However, it should be borne in mind that rust has a much greater volume than the steel from which it is derived and steel corrosion losses may amount to no more than 10% to 20% of the rust thickness.The boundary between the splash and atmospheric zones is not well defined;
however, corrosion rates diminish rapidly with distance above peak wave height and the mean atmospheric corrosion rate of 0.02 mm/side per year can be used for this zone.
3.2.5 Other environments
The corrosion performance of piling in natural environments has so far been considered. For environments such as industrial waste tips, land reclamation schemes or those affected by man-made pollution, guidelines on corrosion rates are given in Eurocode 3:Part 5.
3.2.6 Localised corrosion
Localised corrosion can occur particularly in marine environments and recently anomalous corrosion effects have been observed in parts of structures within a number of ports throughout Western Europe. In these cases, highly localised corrosion has occurred at the low water level which conforms to a specific pattern. This form of localised corrosion has become known as 'accelerated low water corrosion'.
Localised corrosion in the low water zone
This form of corrosion has been experienced on sheet piles, pipe piles, and H sections and on parts of fabricated structures, e.g.
angle and channel sections and plate material. These products have been produced by various manufacturers in Europe and Japan and therefore this phenomenon is not restricted to a particular section or steel manufacturer. Localised corrosion at the low water level has been investigated in Japan where it is termed
Chapter 3/5
Durability
'concentrated corrosion'. In a recent survey of port and harbour authorities throughout five Western European countries it was found that, on steel sheet piles, the localised corrosion followed a distinct pattern. In almost all cases the effect was confined to the outpans of sheet piled walls in a zone at, or just below, the mean low water level. The inpans were almost invariably unaffected. On 'U' shaped piles, this corrosion is most severe in the centre of the outpans, whilst for 'Z' shaped piles, the effect tends to be concentrated on the corners or webs of the outpans. Corrosion rates of 0.3 - 0.8 mm/year have been observed in these circumstances.
In extreme cases, pile thickness reductions in the outpan areas may lead to the premature formation of localised holes or slits in the steel. This can cause a reduction in structural integrity and in some cases, loss of fill material from behind the wall.
Factors affecting localised corrosion
In marine environments, localised higher rates of corrosion can be caused by several mechanisms, individually or in combination, as discussed below:-
a. Macro-cell effects have been found to occur on steel sheet piling in tidal waters where a range of corrosive environments is experienced. Research investigations have shown that potential differences exist between the various zones that occur in a marine environment such that the low water zone is anodic with respect to the tidal zone and that a corrosion peak occurs at the low water level due to the formation of a large differential aeration cell. The macro-cathode of this cell being in the tidal zone, where oxygen is available for the cathodic reduction reaction, and the macro-anode being in the adjacent low water zone. These macro-cell effects will vary depending upon local conditions.
b. Continual removal of the protective corrosion product layer through abrasion or erosion, by the action of fendering systems, propeller wash, bow-thrusters, waterborne sands and gravels or repeated stresses, can lead to intense localised corrosion. The area where the rust layer is continually removed becomes anodic to the unaffected areas, particularly in the low water zone where macro-cell effects are strongest.
c. In some cases, localised corrosion at the low water level has been associated with microbiological activity. A detailed evaluation of corrosion products from affected structures indicates the presence of compounds e.g. sulphides, which stimulate localised corrosion. It is considered that these compounds are associated with the presence of a consortia of bacteria including sulphate reducing bacteria and aerobic species.
Piling Handbook, 8th edition (revised 2008)
Chapter 3/6
3.3 The effective life of steel sheet piles
The effective life of unpainted or otherwise unprotected steel piling, depends upon the combined effects of imposed stresses and corrosion.
Performance is clearly optimised where low corrosion rates exist at positions of high imposed stresses.
The opposite faces of a sheet pile may be exposed to different combinations of environments. The following tables indicate the mean loss of thickness due to corrosion for these environments in temperate climates over a given life span.
Table3.3.1 Loss of thickness [mm] due to corrosion for piles and sheet piles in soils, with or without groundwater
Notes:
1) The values given are only for guidance
2) Corrosion rates in compacted fills are lower than those in non- compacted ones. In compacted fills the figures in the table should be divided by two.
3) The values given for 5 and 25 years are based on measurements, whereas the other values are extrapolated.
Durability
d. Bi-metallic corrosion can occur where steel is electrically connected to other steels, metals or alloys, having nobler potentials or where weld metals are significantly less noble than the parent material. Corrosion is concentrated in the less noble steel, often at the junction between the dissimilar materials.
e. Discontinuous marine fouling by plants and animals can accelerate the corrosion rate in localised areas because of differential environmental conditions caused by their presence (resulting in the formation of differential aeration cells etc.) or possibly by their biological processes. However, dense continuous marine growth can stifle general corrosion by impeding the diffusion of oxygen to the steel surface.
f. Stray currents entering the structure from improperly grounded DC power sources can cause local severe localised damage at the point where the current leaves the structure.
Required design working life 5 years 25 years 50 years 75 years 100 years Undisturbed natural soils (sand, silt
clay, schist, ...) 0,00 0,30 0,60 0,90 1,20
Polluted natural soils and industrial
grounds 0,15 0,75 1,50 2,25 3,00
Aggressive natural soils (swamp,
marsh, peat, ...) 0,20 1,00 1,75 2,50 3,25
Non-compacted and non-aggressive
fills (clay, schist, sand, silt, ...) 0,18 0,70 1,20 1,70 2,20
Non-compacted and aggressive fills
(ashes, slag, ...) 0,50 2,00 3,25 4,50 5,75
Chapter 3/7
Durability
Table3.3.2 Loss of thickness [mm] due to corrosion for piles and sheet piles in fresh water or in sea water
Notes:
1) The values given are only for guidance.
2) The highest corrosion rate is usually found at the splash zone or at the low water level in tidal waters.
However, in most cases, the highest bending stresses occur in the permanent immersion zone,
3) The values given for 5 and 25 years are based on measurements, whereas the other values are extrapolated.
The corrosion losses quoted are extracted from Eurocode 3:part 5 and based upon investigations carried out over many years on steel exposed in temperate climates. While the values quoted are considered to be relevant to the design and performance of most sheet piling structures, in some
circumstances the designer may have local knowledge which leads to the adoption of higher values.
For combinations of environments where low water corrosion is involved, higher losses than those quoted have been observed at or just below the low water level mark and it is recommended that periodic inspection is undertaken.
Recent fill ground or waste tips will require special consideration.
Required design working life 5 years 25 years 50 years 75 years 100 years Common fresh water (river, ship canal, ...)
in the zone of high attack (water line) 0,15 0,55 0,90 1,15 1,40 Very polluted fresh water (sewage, industrial
effluent, ...) in the zone of high attack 0,30 1,30 2,30 3,30 4,30 (water line)
Sea water in temperate climate in the zone
of high attack (low water and splash zones) 0,55 1,90 3,75 5,60 7,50 Sea water in temperate climate in the zone
permanent immersion or in the intertidal zone 0,25 0,90 1,75 2,60 3,50
Piling Handbook, 8th edition (revised 2008)
Chapter 3/8
Durability
3.4 Example durability calculations
To establish the pile section needed for a given effective life in a specific environment it is necessary to follow the procedure below:
1 Establish the corrosion losses for each zone using tables 3.3.1, and 3.3.2.
2 Using the bending moment diagram Fig 3.4b establish the maximum bending moment in each corrosion zone.
3 Calculate the minimum required section modulus for each corrosion zone.
4 Using the graphs in 3.4.1, 3.4.2 and 3.4.3 determine the most appropriate section giving the required minimum section modulus after the loss of thickness calculated in step 1 above.
Fig 3.4b Typical marine wall bending moments
Fig 3.4a Design cross section
Chapter 3/9
Durability
Using the design cross section and bending moment diagrams given in Figs 3.4a and 3.4b, assess the pile section needed to give a 50 year design life.
Depth Face Face Total thickness loss
1 2 over 50 year life (mm)
0 - 1m Soil Splash 4.35
1 - 5m Soil Tidal 2.35
5 - 6m Soil Low Water 4.35
6 - 12m Soil Immersion 2.35
12 - 18m Soil Soil 1.2
Depth Maximum ultimate bending moment
0 - 1m 10kNm/m
1 - 5m 440kNm/m
5 - 6m 520kNm/m
6 - 12m 590kNm/m
12 - 18m 370kNm/m
This example is based on the use of S390GP steel and hence a Yield Stress of 390N/mm2is used to determine the minimum section modulus needed for each corrosion zone.
Depth Minimum section modulus 0 - 1m 10x103x 1.2/390 = 31cm3/m 1 - 5m 440x103x 1.2/390 = 1354cm3/m 5 - 6m 520x103x 1.2/390 = 1600cm3/m 6 - 12m 590x103x 1.2/390 = 1815cm3/m 12 - 18m 370x103x 1.2/390 = 1139cm3/m
From the critical minimum section modulus requirements calculated in Step 3 and the appropriate thickness loss from step 1
(Zmin = 1815cm3/m, t = 2.35mm or Zmin = 1600cm3/m, t = 4.35mm) an appropriate pile section can be selected: in this case, AZ 25 in grade S390GP steel is to be adopted.
With application of a coating suitable for marine exposure conditions, an additional 20+ years can be anticipated.
Step 1
Step 2
Step 3
Step 4
Piling Handbook, 8th edition (revised 2008)
Chapter 3/10
Durability
3.4.1a Elastic section modulus against loss of thickness AZ piles
Chapter 3/11
Durability
3.4.1b Elastic section modulus against loss of thickness AZ piles
AZ 41-700 AZ 39-700 AZ 37-700
AZ 28-700 AZ 26-700 AZ 24-700
AZ 20-700 AZ 19-700 AZ 18-700 AZ 17-700 AZ 14-770-10/10 AZ 14-770 AZ 13-770 AZ 12-770
Piling Handbook, 8th edition (revised 2008)
Chapter3/12
Durability
3.4.2 Elastic section modulus against loss of thickness AU piles
Durability
Chapter3/13
3.4.3a Elastic section modulus against loss of thickness PU piles
PU 32
PU 28 PU 28-1
PU 22 PU 22-1
PU 18 PU 18-1 PU 15R PU 14R PU 13R PU 12 10/10 PU 12 PU 11R PU10R PU 9R PU 8R
Piling Handbook, 8th edition (revised 2008)
Durability
3.4.3b Elastic section modulus against loss of thickness GU piles
GU 18-400
GU 16-400 GU 15-500
GU 13-500 GU 12-500
GU 9-600 GU 8-600 GU 7-600
Chapter3/14
3.5 Protection for new and existing structures
In many circumstances steel corrosion rates are low and the use of protective systemsetc. is not necessary. However, there are circumstances where corrosion of steel piling can be more significant:
In these circumstances methods of increasing the effective life of a structure may need to be considered and the measures that can be taken include the following:
(a) Use of a heavier section
(b) Use of a high yield steel at mild steel stress levels
(c) Applying a protective organic coating or concrete encasement (d) Applying cathodic protection
If a sheet piling wall is to be constructed in an area which may be prone to localised corrosion, one or more of the specified measures to provide the desired effective life should be considered at the design stage to allow for the possibility of higher corrosion rates on unprotected steel piles particularly at or around the low water level. (Given the effects are highly localised, the additional expense involved in engineering a repair, when necessary, to account for the phenomenon is often modest in the context of the overall project cost).
Consideration should be given to the provision of an engineered solution to structures which are likely to be subject to abrasion or erosion. The effects of abrasion and erosion should also be taken into account when methods of corrosion protection are being considered, e.g. the use of a paint coating.
3.5.1 Measures for new structures 3.5.1.1 Use of a heavier section
Effective life can be increased by the use of additional steel thickness as a corrosion allowance. The extra steel thickness required depends upon the working life and environment of the piling structure. The thickness losses for steel piling in various service environments have been outlined in 3.3. In determining this corrosion allowance it is important to consider the stress distribution in the structure in order to locate the region where corrosion losses can be least tolerated. It is possible that the most corrosive zones will not coincide with the most highly stressed zone and therefore, in many circumstances, the use of a corrosion allowance can be a cost effective method of increasing effective life.
Alternatively, it may prove more economical to increase the pile thickness locally in the low water zone by the attachment of plates. Typically, these will need to be 2-3m in length.
Durability
Chapter 3/15