Hydrophobic surface treatments, concrete

NTNU Internal test method KB 71127- Hydrophobic Surface Treatment. Measurements of Penetration Depth into Concrete, Trondheim (1998). 

Hydrophobic surface treatments, based mainly on silanes or siloxanes, are applied in order to reduce water (and chloride) ingress into concrete. The success of the treatment depends on the concrete moisture and on the quantity applied to ensure formation of a continuous water repellent layer in the pore system and on the surface tension in order to ensure a penetration of at least 2 mm (Polder, 1996). Hydrophobic treatments cannot protect concrete in permanently wet conditions. 

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. 

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. 

The application of inhibitors on the concrete surface requires the transport of the substance to the rebar where it has to reach a sufficiently high concentration to protect the steel against corrosion or reduce the rate of the ongoing corrosion. In this context only corrosion inhibitors that prolong the service life due to chemical or electrochemical interaction with the reinforcement are considered. Any other substances that may prevent the onset of corrosion or reduce ongoing corrosion by other means, such as surface treatment (e. g. hydrophobation) or additions that reduce the porosity of the concrete (e. g. fly ash, silica fume, waterproofing admixtures, etc.), are not considered to be corrosion inhibitors and are treated in other chapters. 

In order to test the protective efficiency of the surface treatments, a number of 0100 mm and 060 mm cores were later on drilled out from the concrete decks. For Structure 1, the cores were removed from the deck slab after approximately four years, while for Structure 2, the cores were removed from the deck beams after approximately two years of exposure. Shortly after all surface treatments, a number of 060 mm cores had also been removed for control of penetration depth of the hydrophobic agent. This control was carried out by use of visual observations based on splitting of the removed cores and spraying with pure water [9]. The control of chloride penetration later on was based on successively grinding off small powder samples, from which the chloride content was analyzed by use of a spectrophotometric method [10]. 

Neither for the concrete deck in Structure 2 it was possible to detect any penetration depth of the hydrophobic agent after surface treatment. The observation of a thin surface layer of the hydrophobic agent also here demonstrates the importance of the moisture conditions in the concrete at the time of surface treatment. If the moisture content in the concrete substrate is so high that only a thin surface layer of the hydrophobic agent forms, the long-term efficiency of such a protective measure may be reduced. Also other types of surface coatings may then be considered. 

Uses Hydrophobing agent tor internal or surface treatment of paints, mortar, concrete and paper Batter Bind S [Nat I. Starch Chem./Food Prods.]... 

When water comes into contact with a porous material such as concrete, it is absorbed rapidly by the underpressure in the pores caused by what is called capillary action. This action depends on the surface tension, viscosity, and density of the liquid, on the angle of contact between the liquid and the pore walls and on the radius of the pore. In concrete, the contact angle is small due to the presence of molecular attraction between the liquid and the substrate (that is, between water and cement paste). Under these conditions, a drop will spread on a flat surface, while the meniscus of a capillary pore will rise above the level of the surrounding liquid and be concave towards the dry side in Chapter 14 we will see how this aspect can be changed by hydrophobic treatment. 

Analogously, correlations exist between the coefficient of water permeabihty k and that of capillary absorption S, but these lose their validity if the surface of the concrete is subjected to a hydrophobic treatment, which will reduce considerably capillary absorption but not permeation. For concrete obtained with Portland cement, correlations between the coefficient of water permeability (k) and conductivity (o = 1/p) measured for a given value of relative humidity are available. On the other hand, conductivity varies greatly in concrete made with blended cements or carbonated concrete, while there is no significant change in water permeability. 

The cost of various techniques can only be given very roughly, and any estimate will be incomplete, since the actual cost will vary from one application to another. Furthermore, different types of prevention mechanisms are not directly comparable. Beyond this, it can be said that with respect to normal carbon-steel reinforcement, use of galvanized and epoxy-coated bars costs about twice as much, and the cost of stainless-steel reinforcement is about 5 to 10 times higher. The use of nitrite inhibitors in higher doses costs approximately 30 /m of concrete (volume). Coatings may vary from 7 to 50 /m of concrete surface, hydrophobic treatment costs about 10 /m. Cathodic prevention varies from 50 to 100 /m. ... 

Application of hydrophobic treatment to a concrete surface aims at reducing the capillary absorption of water and dissolved aggressive substances. Hydrophobising concrete leaves the pores open, so it does not affect the ingress of gaseous species. 

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