Much has been made of the new Lesner Bridge with its emphasis on multimodal transit and its key role in the Hampton Roads transportation network — and while the new bridge is impressive, the old bridge is getting a new lease on life as it plays an important role in our ocean ecosystem. Up to 20, tons of concrete from the old Lesner Bridge structure will find new purpose as part of the Chesapeake Bay artificial reef program, where it will create appealing new habitats for reef dwelling fish and attract other ocean creatures.
As part of the reef known as the Cabbage Patch, the addition of these materials will further enhance an already popular fishing site and support the diversity of ocean life in the Chesapeake Bay. Primarily constructed to provide better fish habitats, artificial reefs have both environmental and economic impact as they rehabilitate stressed natural habitats and provide opportunities for recreational fishing and diving. In addition to serving as shelter and feeding ground for reef dwelling fish, artificial reefs in turn attract larger predator fish as well as other organisms that act as a food supply for various marine species.
The physical erosion is due to the wave action and floating objects.
The chemical attack is due to the sulphate, chloride and alkali ions present in the sea water. The sulfate ions reacts with calcium aluminum oxides in concrete to form ettringite, thus accelerating rupture development whereas the chloride ions leaches up to the reinforcement and accelerates the corrosion rate. The fly ash and silica fume in concrete reduces the sulphate and chloride attack and thus improves the durability and service life of the concrete structure 1. The durability of the concrete structure purely depends on the properties of concrete such as permeability, resistance to chemical attack and the resistance to the reinforcement corrosion.
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Adding admixtures in concrete reduces the action of external adverse environment on the concrete. A number of studies were already conducted for identifying the best combinations of adding fly ash and silica fume in concrete for the improvement of the performance against these deteriorations.
The previous researchers 1 - 3 argued that there is reduction in the compressive strength for the fly ash blended concrete. This drawback is rectified by the addition of silica fume in the mix which gives more dense concrete in the studies of microstructure behavior. In this study an attempt has been made to identify different experimental methods used to analyse the performance of multi blended concrete with fly ash and silica fume exposed to marine environment.
Diverse studies were also conducted to analyse the permeability and corrosion behavior of the blended concrete with different exposure conditions. The main objective of this paper to consolidate different methodologies adopted to study the behavior of multi blended concrete. Different materials that can be used as SCMs include fly ash, silica fume, ground granulated blast furnace slag GGBS , rice husk ash and other calcinated natural pozzolanic materials such as metakaolin. In that, the focus had been given to investigate the performance of fly ash and silica fume in the blended concrete.
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Fly ash is the combustion residue in coal-burning electric power plants, which flies out with the flue gas stream and is collected by mechanical separators, electrostatic precipitators or bag filters. Condensed silica fume, sometimes known simply as silica fume or micro silica, is produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys.
Chemical compositions of fly ash and silica fume from different studies are given in Table 1. It also has some slight variations depending upon the industry and the treatment process. The physical properties from different authors are furnished in Table 2 Specific gravity and specific surface area of cementing materials from various studies in which the physical properties of the supplementary Cementing materials disclose the fineness behaviour of silica fume are given. This property of silica fume is helpful to increase the density of concrete and lead to produce more impermeable concrete.
Table 2 Specific gravity and specific surface area of cementing materials from various studies Kayali and Elahi, et al 8 Rashad et al. Mechanical properties of blended concrete on marine atmosphere Nochaiya et al. XRD pattern of Fly ash detected amorphous phase and small crystalline phases as anhydrite, quartz, mullite, gehlenite and magnetite whereas the silica fume shows amorphous characteristics. The setting time of the mortar mix blended with fly ash was increased with increase in percentage of replacement of OPC.
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Whereas the setting times of Portland—fly ash cement pastes with the addition of silica fume were shorter than those of mixes without silica fume for the same fly ash content and all mixes meet the requirement set by the ASTM C The scanning electron micrograph studies disclosed that by introducing silica fume, the concrete became denser microstructure and thereby increased the compressive strength 1. Elahi et al. Based on the conclusions, it is suggested that the high performance concrete should be designed with different combinations of SCMs for different exposure conditions and exact choice of the materials should be based on the physical properties, durability and performance expected from the HPC.
It was concluded that replacement of OPC with HVFA with the existing curing practices may result in steady reduction in the mechanical properties. Intensive studies both laboratory based and Natural marine atmospheric exposure have already been conducted to study the mechanical properties of the blended concrete in marine environment. In this, the samples were exposed to different curing practices to vary the exposure conditions. The major curing practices found in the literatures are natural marine atmospheric exposure, permanent immersion in sea water and Alternate wet and dry cycles.
Both methods are having their own benefit and drawback. The effects on curing are studied by various investigators either in laboratory conditions or in actual marine atmosphere. The actual marine exposure studies could draw the long term effects on concrete even up to 10 years Whereas the laboratory studies lead to provide the results in short term monitoring by continuous data logging system.
The SEM analysis shows the formation of ettringite and the acceleration of rupture development As per Rusch, et al. Accelerated corrosion behavior study on blended concrete Ha, et al.
Different techniques adopted for the corrosion acceleration study are weight loss method, open circuit potential OCP measurements, Anodic polarization technique, impressed voltage technique, microcell corrosion studies, pH measurements and estimation of free chloride content. Mark G.
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Concrete in the Marine Environment
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