Concrete Structures in Marine and Other Saline Environments

97 Technical Note CONCRETE STRUCTURES IN MARINE AND OTHER SALINE ENVIRONMENTS INTRODUCTION parameters. These include improved high performance Supplementary Cementitious Materials (SCMs) concretes The purpose of this Technical Note is to highlight various with lower water/cementitious material (W/C) ratio to technical requirements which are considered fundamental offer adequate chemical and dimensional stability. These to the construction of good quality concrete structures in SCMs concretes are used in conjunction with improved marine and other saline environments. Reinforced concrete construction practices and procedures for compaction, structures situated in marine environments (Fig. 1) such curing and cracking control. In addition, a range of as the ocean tidal and splash zones are subject to very other requirements included in specifications should aggressive deteriorating actions. The concrete itself is address various controls of source materials, chemical the primary defensive media and requires a high level of admixtures, and the impermeability of concrete. Such protective measures to deliver adequate durability for the performance requirements are normally tailored to suit life of the concrete structure. The need to design, specify local environments and materials. To this end the quality and construct durable concrete structures has been in the assurance and documentation requirements must provide forefront of concrete technology and worldwide attention for better control of the whole construction operation, at least over the past 15 years. Attention to durability including the vital interaction between the various of concrete has been the result of a growing number of technical and practical processes which have the potential durability - related problems in concrete structures, the main to compromise the overall intent of the durability strategy. one being the corrosion of steel reinforcement. The number This should allow the identifi cation and elimination of of reinforced concrete structures exposed to particular quality problems as early as possible, thus maintaining environmental conditions, undergoing serious deterioration the integrity of the specifi cation. In recognition that the has been steadily increasing in recent years, due mainly to durability of a structure may diminish over the years whilst material failures and inadequate design, construction and exposed to harsh conditions, this multi-stage protection maintenance practices. The strength of concrete alone is approach is intended to ensure that suffi cient redundancy no longer considered as the property which can guarantee exists to ensure the long term serviceability of the structure. long lasting structures. It is now recognised that a dense and It should be noted that the concrete durability section of impermeable concrete can best ensure long-term durability AS 5100 (Bridge Design)(4) is wholly based on Section 5 in harsh marine exposure conditions. of the AUSTROADS 1992 document whose technology pre-dates the advances in concrete durability made over The increased attention to the durability of concrete structures the past 20 years and is therefore inadequate for the design has also led to the progressive development of a durability and construction of new structures for marine and other based VicRoads structural concrete specifi cation Section aggressive environments. 610(1). Furthermore, in recognition of these developments a multi-stage durability protection strategy has been Protective Measures developed which incorporates a number of measures against Special protective measures which can form part of a multi- the possible ingress of chlorides in aggressive marine and stage approach to durability should include: other saline environments with specifi c emphasis on the long term durability of concrete structures in order to ensure • The use of concrete containing SCMs(5,6,7) (i.e. silica a minimum 100 years service life. A number of specifi c fume, fl y ash, slag) at the correct replacement levels; requirements representing the latest advances in concrete • Protection of all materials against chloride contamination technology and high performance concrete are included in prior to concrete placement; this strategy(2,3). • Use of a combination of water adding curing techniques; DURABILITY STRATEGY • Total isolation of all exposed in-situ concrete surfaces against the ingress of chlorides during the curing The overall durability strategy developed for the construction period; of bridges and other concrete structures in marine and other • Electrical continuity of the steel reinforcement; saline environments(2,3) must be underpinned by strict • Limitations to drying shrinkage and soluble salts, specification requirements with durability enhancing alkali-aggregate reactivity (AAR) and temperature May 2008 Page 1 of 5 Technical Note 97 differentials across a concrete element; Table 1: Exposure Classifi cations (1,4) & Concrete Mix • Application of protective coatings in the form of silane Design Parameters for Marine Environment impregnation, decorative/anti-carbonation, anti-graffi ti Concrete Exposure Concrete Grade/ W/C coatings and water tolerant epoxies in tidal zones; Members Classifi cation Ratio (Max) • Proper quality control including permeability testing Piles in terms of VPV during mandatory trial mixes as well Pile Caps VR450#/50-0.40 C as during the construction period; Columns (50 MPa) • Use of cathodic prevention(8) as part of the protection Crossheads Deck slab strategy (Fig.2); and B2 Approach slab VR400#/40-0.45 B2 • As part of a long term monitoring program, mild Abutments (40 MPa) B2 steel and carbon macrocell/galvanic current corrosion Fender walls B2 Wing walls # : Min. cementitious monitoring probes could be installed in a concrete B2 Above Deck content (Kg) member such as a pier column prior to placement of C the concrete (Fig.1). (Parapets etc) VR450#/50-0.40 Beams C (50 MPa) TYPES OF ENVIRONMENTAL AGGRESSIVENESS CONCRETE MIX DESIGN, CEMENTITIOUS TYPES Three distinct exposure zones consistent with corresponding potential corrosion zones have been identifi ed within a Concrete grades and mix proportions should comply with marine environment. For design purposes these exposure the requirements of Section 610. It is considered that zones can be categorised in terms of the exposure (5,6,7) (4) the use of SCMs such as silica fume, or combinations classifi cations given in AS 5100 (i.e. B2, C etc). The with fly ash or slag would be most appropriate. A exposure zones consistent with appropriate concrete mix 10% silica fume replacement as shown in Table 2 is designs, concrete grades, VPV values, and concrete cover proposed to provide adequate long-term impermeability to the steel reinforcement are defi ned as follows: with appropriate chemical admixtures for all structural components especially pile caps, piers and beams. Other a) Atmospheric zone – subject to salt-spray wind and possible combinations at medium replacement levels are weathering by the sun. Depending on the height of shown in Table 2 for piles, deck slabs, approach slabs, the structure, pier crossheads and superstructure fender walls and wing walls. Higher replacement levels usually fall into this category with an exposure as shown in Table 2 could be used although expert advice classifi cation of either B2 or C. Nevertheless, for low should be sought and consideration should be given to their level structures both beams and crossheads should be lower early strength development, longer formwork removal classifi ed as exposure classifi cation C in order to ensure times, lifting strengths and release of prestress. In particular that they are designed and constructed with a greater early stripping of formwork with no subsequent protection protective capability against the ingress of moisture (or isolation) of such concretes in marine environments, and waterborne chlorides. Above deck components would render them susceptible to chloride ingress at a very such as parapets must also be classifi ed as exposure early age as the initial VPV would be fairly high. Silica classifi cation C. fume concrete would give the best VPV results, followed b) Tidal/splash zone – subject to tidal water rich, in by fl y ash and then slag(5). The triple blends would provide chloride, and greatly infl uenced by the wetting and comparable results. The various combinations shown in drying processes which promotes ingress of water, Table 2 have been used very successfully for VicRoads waterborne chloride ions, and diffusion of oxygen (i.e. projects (Fig.1) including Patterson River Bridge, North exposure classifi cation C). Columns/piles, crossheads, Arm Bridge at Lakes Entrance, and Grey River Bridge on pile caps and bored piles exposed to tidal conditions Great Ocean Road. fall into this category. Piles should also be classifi ed as such for additional protective capability. Table 2: Typical Proportioning of Cementitious c) Submerged zone – where piles are underwater or Materials for Structures in Marine Environment are completely saturated, thus severely restricting the Recommended Proportioning of Cementitious Structural Concrete supply of oxygen, which is vital for corrosion initiation Material (% mass) in concrete mixes Members Preferred for all and subsequent propagation. • 90% GP/ 10% SF# concrete members • 90% GP/10% SF; or Piles, deck slabs, ap- The required concrete grades and mix design parameters • a triple blend combination of SF and FA for the various concrete components and typical exposure

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