In all situations, consideration must be given to the possibility of the deterioration of piled foundations due to aggressive substances in soils, in rocks, in ground-waters, in the sea and in river waters. Piles in a river or marine structures are also exposed to potentially aggressive conditions in the atmosphere, and they may be subjected to abrasion from shifting sand or shingle, or damage from floating ice or driftwood. In considering schemes for protecting piles against deterioration due to these influences, the main requirement is for detailed information at the site investigation stage on the environmental conditions. In particular, adequate information is required on the range of fluctuation of river or sea levels and of the groundwater table. In the latter case, the highest levels are required when considering the likely severity of sulphate attack on concrete piles or the corrosion of steel piles, and the lowest possible levels are of considerable importance in relation to the decay of timber piles. The possibility of major changes in groundwater levels due to, say, drainage schemes, irrigation, or the impoundment of water must be considered. In normal soil conditions, it is usually sufficient to limit chemical analyses of soil or groundwater samples to the determination of pH values, water-soluble sulphate content (mg per litre) and chloride content. Where the sulphate content exceeds 0.24% in soils it is advisable to determine the water-soluble sulphate content, expressing this in mg of SO4 per litre of water extracted. For brownfield sites, full chemical analyses are required to identify potentially aggressive substances. Methods of investigating and assessing brownfield sites are given by Harris et al., drawing attention to the health and safety precautions necessary, the need to employ specialist personnel, and care in selecting representative samples. Bacterial action can be an influence in the corrosion of steel piles. Samples of soil and groundwater should be obtained in sterilized containers, which are then sealed for transportation to the bacteriological laboratory for later analyses. Where steel piles are used for foundations in disturbed soils or fill material on land, an electrical resistivity survey is helpful in assessing the risk of corrosion and in the design of schemes for cathodic protection (see Section 10.4.2). Investigations for marine or river structures should include a survey of possible sources of pollution which might encourage bacteriological corrosion, such as contaminated tidal mudflats, discharges of untreated sewage or industrial effluents, dumping grounds for industrial or household refuse, and floating rubbish discharged from ships or harbour structures. The pattern of the sea or river currents should be studied and water samples were taken at various stages of spring and neap tides or at dry-weather and at flood and dry discharge stages in rivers. Particular attention should be paid to sampling water from currents originating at the areas of contamination previously identified. Chemical and bacteriological analyses should be made on the full range of samples to assess the daily or seasonal variation in potentially aggressive substances. Other items for study include the presence and activity of organisms such as weeds and barnacles, and molluscan or crustacean borers.
Durability and protection of concrete piles
Concrete piles inland structures
Properly mixed concrete compacted to a dense impermeable mass is one of the most permanent of all constructional materials and gives little cause for concern about its long-term durability in a non-aggressive environment. However, concrete can be attacked by sulphates and sulphuric acid occurring naturally in soils, by corrosive chemicals which may be present in industrial waste infill materials, and by organic acids and carbon dioxide present in groundwater as a result of decaying vegetable matter. Attack by sulphates is a disruptive process whereas the action of organic acids or dissolved carbon dioxide is one of leaching. Attack by sulphuric acid combines features of both processes. The naturally occurring sulphates in soils are those of calcium, magnesium, sodium and potassium. The basic mechanism of attack by sulphates in the ground is a reaction with hydrated calcium aluminate in the cement paste to form calcium sulpho-aluminate. The reaction is accompanied by an increase in the molecular volume of the minerals, resulting in the expansion and finally the disintegration of the hardened concrete. Other reactions can also occur, and in the case of magnesium sulphate, which is one of the most aggressive of the naturally occurring sulphates, the magnesium ions attack the silicate minerals in the cement in addition to the sulphate reaction. Ammonium sulphate, which attacks Portland cement very severely, does not occur naturally. However, it is used as a fertilizer and may enter the ground in quite significant concentrations, particularly in storage areas on farms or in the factories producing the fertilizer. Ammonium sulphate is also a by-product of coal-gas production and it can be found on sites of the abandoned gasworks. Because calcium sulphate is relatively insoluble in water, it cannot be present in sufficiently high concentrations to cause a severe attack. However, other soluble sulphates can exist in concentrations that are much higher than that possible with calcium sulphate. This is particularly the case where there is a fluctuating water table or flow of groundwater across a sloping site. The flow of groundwater brings fresh sulphates to continue and accelerate the chemical reaction. High concentrations of sulphates can occur in some peats and within the root mass of well-grown trees and hedgerows due to the movement and subsequent evaporation of sulphate-bearing groundwater drawn from the surrounding ground by root-action. The severity of the attack by soluble sulphates must be assessed by determining the soluble sulphate content and the proportions of the various cations present in an aqueous extract of the soil. These determinations must be made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%. The thaumasite form of sulphate attack which consumes the binding calcium silicate hydrates in Portland cement, thereby weakening the concrete, has been investigated extensively in recent years. The reaction requires the presence of sulphates, calcium silicate, carbonate, and water, and appears to be more vigorous at temperatures below 15oC. Carbonation of concrete due to atmospheric carbon dioxide acting on the calcium hydroxide in the concrete matrix causes a reduction in the pH rendering the concrete susceptible to sulphate reactions forming thaumasite. Free sulphuric acid may be formed in natural soil or groundwater as a result of the oxidation of pyrites in some peats, or in ironstone or alum shales. Sulphuric acid can also be present in industrial waste materials which have been contaminated by leakages from copper and zinc smelting works, and from dyeing processes. The acid has an effect on the cement in hardened concrete that is similar to that of sulphate attack, but the degradation may not result in significant expansion. Figure 10.3 shows the disintegration of the concrete in the shaft of a bored and cast-in-place pile caused by the seepage of sulphuric acid into porous fill material. In the UK, sulphates occurring naturally in soils are generally confined to the Keuper Marl (Mercia Mudstone), and to the Lias, London, Oxford, Kimmeridge and Weald Clays. They are also found in glacial drift associated with these formations. Sulphates may be present in the form of gypsum plaster in brick rubble fill. The sulphate content of the groundwater gives the best indication of the likely severity of sulphate attack, particularly that resulting from soluble sulphates. Where the water samples are taken from boreholes care should be taken to ensure that the sample is not diluted by water added to assist the drilling. If possible the groundwater should be sampled after a long period of dry weather. Groundwater flow across a sloping site through sulphate-bearing ground results in the highest concentration on the downhill side of the site and the flow may
Concrete piles in marine structures
Precautions against the aggressive action by seawater on concrete need only be considered in respect of precast concrete piles. Cast in-situ concrete is used only as a hearting to steel tubes or cylindrical precast concrete shell piles, where the tube or shell acts as the protective element. A rich concrete, well-compacted to form a dense impermeable mass, is highly resistant to aggressive action and, provided a cover of at least 50 mm is given to all reinforcing steel, precast concrete piles should have satisfactory durability over the normal service life of the structures they support. When the disintegration of reinforced concrete in seawater does occur it is usually most severe in the ‘splash zone’ and is the result of porous or cracked concrete caused by faulty design or poor construction. Evaporation of the seawater in the porous or cracked zone is followed by the crystallization of the salts and the resulting expansive action causes spalling of the concrete and the consequent exposure of the reinforcing steel to corrosion by air and water. The expansive reaction that occurs when corrosion products are formed on the steel accelerates the disintegration of the concrete. Freezing of seawater in porous or cracked concrete can cause similar spalling. However, where concrete piles are wholly immersed in seawater there is no degradation of properly made and well-compacted concrete. In an extensive review of the literature and the inspection of structures that had been in the sea for 70 years, Browne and Domone found no disintegration in permanently immersed reinforced concrete structures even though severe damage had occurred in the splash zone. They concluded that corrosion of the steel cannot occur with permanent immersion because the chloride present is restricted to a uniform low level and the availability of oxygen is low. Although seawater typically has a sulphate content of about 230 parts per 100 000, the presence of sodium chloride has an inhibiting or retarding effect on the expansion caused by its reaction with ordinary Portland cement. The latter material is, therefore, quite satisfactory for the manufacture of precast concrete piles for marine conditions but to avoid disintegration in the splash zone the concrete should have a minimum cement content of 360 kg/m and a maximum water/cement ratio of 0.45 by weight. Special Digest 1 does not provide recommendations for concrete exposed to seawater, but reference should be made to BS 6 349-1: 2000 Marine Structures. Air entrainment of concrete as a safeguard against frost attack on piles above the water line is unnecessary if the water/cement ratio is less than 0.45. The concrete in precast piles should be moist-cured for 7 days after the removal of the formwork (with a further 10 days exposure to air in order to be classified as ‘surface carbonated’). Great care should be taken in handling the piles to avoid the formation of transverse cracks which would expose the steel to corrosion in the splash zone. Coatings on precast concrete piles to protect them against deterioration in the splash zone are of little value since they are soon removed by the erosive action of waves, and by abrasion from floating debris or ice.