Underwater concretes

Typical uses include the production of non-dispersible underwater concrete and reduction of the accumulation of bleed water in mass concrete placed in deep forms. Consequently, AWAs are useful in mass concrete work because they prevent the formation of laitance on the surface of the concrete and thereby reduce the excessive cleaning between successive lifts. The admixtures also reduce the voids formed under horizontal reinforcing bars. Therefore, bond to steel increases and potential corrosion problems are reduced. The admixtures are also used in conjunction with WRAs in oil-well cementing grouts to reduce pipeline friction and rapid water loss and grouting of pre- and post-tensioned concrete ducts [47]. New valves and control devices under development in Europe and Japan used in conjunction with AWA will likely advance the field on underwater concrete. 

LMC is used in underwater concrete for both new construction and repair. The important requirements to obtain antiwashout capability, such as segregation resistance, flowability, self-leveling characteristics and lower bleeding are provided by the addition of viscosity-enhancing polymeric admixtures at polymer-cement ratios of 0.2-2.0%. These admixtures are water-soluble polymers, and classified under two groups, viz., cellulose types such as methyl cellulose and hydroxy ethyl cellulose and polyacrylamide types such as polyacrylamide and polyacrylamide-sodium acrylate [101]. 

Sakuta, M., Yoshioka, Y. andKaya, T. (1985). Use ofAcryyl- Type Polymer as Admixture for Underwater Concrete, ACI SP-89, ACI, Detroit, 261-78. 

Other applications include the construction of floor slabs where manipulation of the level of the admixture in the concrete will result in the production of bleed water should it be required to enhance finishing. The use of VEAs in grouting and underwater concrete applications is discussed below. 

Underwater concreting can be done by three methods the pumping of antiwashout concrete produced by the combined use of water reducing admixtures, superplasticizers and VEAs the use of tremies also using antiwashout concrete and the preplaced aggregate method discussed above. 

Often it is difficult to adjust the mixture proportions to achieve desired design parameters for all properties of concrete. Consequently the properties of colloidal underwater concrete are controlled by the addition of three chemical admixtures. Minimum water-cement ratios range from 0.36 to 0.40. Cement and fine-aggregate contents are usually higher than corresponding mixes placed on land, and silica fume may be used in conjunction with a superplasticizer or conventional water reducers to reduce segregation. The key to a non-dispersible concrete with self-leveling characteristics is the successful optimization of the VEA with the superplasticizer used to increase the slump. 

UNDERWATER CONCRETE - COMPATIBILITY PROBLEMS WITH VISCOSITY... 

Combinations of VEAs, WRAs and SPs are used in controlling sedimentation in highly flowable slurry like concrete and in underwater concrete used... 

Modem high-performance concretes (such as non-dispersible underwater concrete, self-compacting concrete, and concretes used at below freezing temperatures) have necessitated the use of special admixtures, often requiring on-site addition. Figure 7.57 shows a special dispensing unit with storage tank used for this purpose. 

UNDERWATER CONCRETE-COMPATIBILITY PROBLEMS WITH VISCOSITY ENHANCING ADMIXTURES (VEAs)... 

Protective coatings for coastal and underwater concrete structures and installations... 

Figure 8.7 Mechanism of antiwashoutability development by water-soluble polymer in antiwashout underwater concrete. ( 1991, JSCE, reprinted with permission.)... 

Development of low-temperature curable concrete and underwater concrete using acrylic binders [44-45]... 

Underwater concreting In underwater concreting the cement used must be coarsely ground to minimize the amount that becomes lost in the surrounding water. 

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