Concreting under water

For pipelines to be placed under water, it is necessary to provide negative buoyancy. This is commonly achieved by placing lightly reinforced concrete up to 150 mm thick over the 3-5 mm hot enamal coating on the steel. Joints at the welded tube ends have to be coated with a minimum of delay due to the high production rate required on the laying barge, and tapes have therefore found application at this point. Where submarine pipelines are pulled ... 

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

The lower water-cement ratio and higher cohesion produced at the desired compacting factor values in the mixes provide a more structured concrete under conditions of no applied force. This is of particular importance in slip-formed concrete where reduced edge slump is required. 

Tynes, W. (1967). Evaluation of Admixtures for Use in Concrete to be Placed Under Water,... 

To prevent evaporation, polymerization can be carried out in a closed system, such as an enclosed vessel, or the concrete can be wrapped in polyethylene sheet. Another possibility is carrying out the polymerization under water, which is especially attractive for decreasing monomer loss. The monomer in the specimen is usually not miscible with water so that it has little effect on the properties of the concrete-polymer. 

All the cements considered in this book fall into the category of hydraulic cements they set and harden as a result of chemical reactions with water, and if mixed with water in appropriate proportions continue to harden even if stored under water after they have set. Much the most important is Portland cement. Chapters I to 4 of the present work deal mainly with the chemistry of manufacture of Portland cement and with the nature of the resulting product. Chapters 5 to 8 deal mainly with the processes that occur when this product is mixed with water and with the nature of the hardened material. Chapters 9 to 11 deal with the chemistry of other types of cement, of admixtures for concrete and of special uses of cements. Chapter 12 deals with chemical and microstructural aspects of concrete, including ones relevant to processes that affect its durability or limit its service life. 

None (18) Stable out of contact with oxygen None stored under water in concrete tanks Smoke in sir phos-pborio acid (HaPOd dissolved in water Like matchee Solid pardcie burns flesh vapors very poisonous, cause none, decay smoke relatively harmless... 

Magnox cladding, which has been removed mechanically from spent fuel rods at the start of reprocessing operations, is contaminated with small pieces of fuel and will require treatment before disposal to the environment. At present, this waste is stored under water (to eliminate any fire risk) in large concrete silos and processes are now under development for the conditioning of this waste to make it suitable for disposal. The favored processing route comprises the following operations ... 

Some of the early uses of lime in construction and building have already been mentioned (section 1.3.2). A major development made by the Romans was a blend of slaked lime and volcanic ash, which would harden under water, called Roman Cement. The volcanic ash contained reactive silica and alumina which combined with the lime in the presence of controlled amounts of water to produce a solid mass bound by calcium silicates and aluminates. Such reactive materials are called pozzolans after Pozzuoli, a city near Naples. Roman Cement was mixed with aggregate to make a time concrete, which was used for a wide range of products and constructions. 

Precast concrete (Fig. 8-5a). Difficult to handle and transport shatters when driven into hard strata unless water jets are used strong as columns and end-bearing supports since they are steel-reinforced and/or prestressed useful where in-place casting is difficult under water corrosion resistant. 

Initial current densities in the range of 5-15 mA/m of reinforcing steel surface area are generally needed for protecting reinforced concrete structures exposed to the atmosphere. Much lower current densities are required under conditions that reduce the access of oxygen towards the surface of the steel, such as in water-saturated concrete. For elements operating under water, current densities typically in the range 0.2 to 2 mA/m are sufficient... 

In another study, Ogawa et al. (2005) studied the chloride ion penetration resistance of beams made with ordinary concrete and HPFRCC. In their study, 8-mm-thick HPFRCC plates were cast. A 100 X 100 X 400 mm mold was built from the cut pieces of the 8-mm-thick HPFRCC plates. Ordinary concrete was poured into this mold. An ordinary concrete specimen of the same size was also prepared. Both specimens were cured under water for 38 days and were soaked in artificial seawater containing 1.8% chloride ion concentration for 30 and 60 days. Test results showed that... 

The corrosion of concrete under sea water attack is a complex process and depends npon many physical and chemical factors, as well as upon the climate conditions. The placement of stmcture in relation to the sea is very important and the following situations can occur in practice (Fig. 6.68) [275] ... 

The addition of polymer to the concrete mixture causes the significant change of concrete properties [5], However, for these concretes quite different curing conditions are required, mainly initially in water or in very humid air and then in dry air. Therefore they cannot be used in under water concreting [5]. In practice these polymer-cement concretes are cured in the same way as cement concretes and for this reason they do not attain the assumed properties [5]. 

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