Concrete Hydrophobation

Effect of Concrete Hydrophobation against Chloride Penetration... 

Keywords Concrete structures, durability, chloride penetration, protective measures, concrete hydrophobation, concrete admixtures, surface hydrophobation. 

The chemical materials used to produce dampproofers are able to form a thin hydrophobic layer within the pores and voids and on the surfaces of the concrete in one of three ways (1) reaction with cement hydration products ... 

Certain vegetable and animal fats have been used as dampproofers and again emulsions or pastes are preferred and typical formulations are given in Table 4.3 [10]. The fat can be white grease, tallow or soya bean oil and although they all produce hydrophobic concrete, different effects on compressive strength are obtained. 

Very finely divided wax emulsions are effective concrete dampproofing agents and are formulated so that the emulsion breaks down after contact with the alkaline concrete environment and forms a hydrophobic layer. 

It is believed that in the presence of dampproofing admixtures, the surfaces of the concrete, and the internal surfaces of the pores become coated with either a layer of molecules in the case of stearic acid and other fatty acids (Fig. 4.5b) or a layer of coalesced or separate particles of material in the case of waxes and bitumens, etc. (Fig. 4.5c). The end result in both cases is the production of hydrophobic surfaces exhibiting high contact angles to water, as shown in Fig. 4.6. 

Dampproofed concrete, therefore, should not show significant uptake of surface water in conditions of rain and wind up to about 100 km h T In fact, on prolonged exposure, some wetting does occur, because of defects in the hydrophobic coating, and the presence of larger voids in the concrete, up to perhaps 1 mm wide these are due to incomplete compaction, or to the nature of the concrete in the case of blocks. 

Ethyl silicate (tetraethoxysilane) is often used without modification as a water-repellent material for concrete and masonry in general. All, or nearly all. the ethoxyl groups are hydrolyzed by the moisture of the air to form cross-linked watcr-rcpcllcnt polymers. The material is applied in desirable thickness, dissolved in some volatile solvent which soon evaporates. Silicone resins which arc partially condensed before application, or even fully condensed, can also be used here. In the latter case, hardness is achieved on evaporation of the solvent. Certain silicone resins arc useful as hydrophobic agents for the impregnation of paper and fabrics. 

Very finely divided wax emulsions are effective concrete dampproofing agents and are formulated so that the emulsion breaks down after contact with the alkaline concrete environment and forms a hydrophobic layer. Waxes of melting point 57-60°C are used with an emulsifying agent based on sorbitan monostearate or ethoxylated sorbitan monostearate [2], The properties of a commercial product are given in Table 4.4 [5],... 

Polymeric fibers are popular for reinforcing concrete matrices because of their low density (more number of fibers for a prescribed volume fraction), high tensile strength, ease of dispersion, relative resistance to chemicals, and relatively low cost compared to other kinds of fibers. Polypropylene and polyolefin fibers are typically hydrophobic, resulting in a relatively poor bond with concrete matrices compared to some other types of fibers. Treatment of polypropylene with an aqueous dispersion of colloidal alumina or silica and chlorinated polypropylene enhances the affinity of these fibers toward cement particles. Treatment of polypropylene fibers with a surface-active agent provides better dispersion of the fibers and a stronger bond between cement and fiber. The earlier attempts at surface treatments of polypropylene fibers have had only limited success and have not been commercially attractive. 

The idea that a cleft is formed in the hydrophobic region of the protein stems from Mitchell s suggestion about a mobile membrane barrier, meant to explain the mechanism of operation of carriers that translocate hydrophilic metabolites [27,28]. Quite recently Mitchell [29] has presented a concrete version of the mobile membrane barrier principle. He assumes that a slight relative shift of a-helical transmembrane columns (e.g., rotation or tilting of the columns at a small angle) may result in the formation of a cleft allowing solutes to reach the membrane core. ... 

In order to seal cracks in wet conditions hydrophobic systems are used in conjunction with a hydrophilic polyurethane resin system. These are rapid foaming and setting resins which stop the flow of water when injected into cracks in concrete in the presence of moving water, providing an effective system of crack sealing in wet conditions. As soon as the hydrophilic resin stops the flow of water, the hydrophobic resin can be reinjected to produce a permanent seal. 

This type of adsorption is the basis for a number of important industrial processes, notably the separation of mineral ores by froth flotations (Somasun-daran, 1979), the de-inking of waste paper (Turai, 1982), the ultrapurification of fine powders for the chemical and ceramic industries (Mougdil, 1991), and the production of foamed concrete. In the last case, for the concrete to entrain air, it is not even necessary for the liquid phase to show any foaming. In most of the processes used above, surfactants, such as salts of long-chain carboxylic acids or long-chain amines, that adsorb with their polar or ionic heads oriented toward the solid and their hydrophobic groups oriented away from it, are used to make the surface of the solid hydrophobic. 

Basophob . [BASF AG] Aq. paraffin or polyethylene wax dispersions, sol ns. of fatty acid derivs. hydrophobic agents for internal or surface treatment of paints, mortars, concrete, and paper. 

One may ask, why a hydrophobic fiber, like polypropylene, was used with Portland cement instead of a more compatible fiber, such as flbrlllated polyvinyl alcohol Likewise, one may also ask why the latter has not been used to upgrade polyester concrete Presumably, the answer to the first question is, those with expertise in hydraulic cements are not well versed In organic polymer science and technology. 

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