Hydrophobic admixtures

F. Tittarelh, G. Moriconi, The effect of silane-based hydrophobic admixture on corrosion ofgalvanized reinforcing steel in concrete, Corros. Sci. 52 (2010) 2958—2963. 

Hydrophobic Agents used as Concrete Admixture. In order to investigate the resistance of the concrete against chloride penetration, five concrete mixtures with increased additions of a silane-based hydrophobic admixture (Protectosil) [4], were produced (Table 1). The binder was based on a blended cement of type CEM II/A-V 42.5 with approximately 20 % fly ash in combination with silica fume (5.3 %). A total binder content of 412 kg/m gave a water/binder ratio of 0.40. The hydrophobic admixture was added to the fresh concrete mixture in the form of a powder in quantities of 0.5, 1.0, 2.0 and 4.0 % by weight of cement, respectively (P0.5 - P4.0). A reference mixture without any hydrophobic admixture (PO.O) was also produced. 

From all the results shown above, it appears that intermixed hydrophobic agents may be a valid alternative to the traditional surface hydrophobation of concrete. The results also clearly demonstrate, however, the importance of the concrete to properly dry out for the hydrophobic admixture to be effective. 

For curing periods of the concrete up to 90 days in water without any drying out before exposure, no beneficial effect of the silane-based hydrophobic admixture was observed. 

F. Tittarelli and G. Moriconi The Effect of Silane-Based Hydrophobic Admixture on Corrosion of Reinforcing Steel in Concrete, Cement and Concrete Research, Vol. 38 (2008) pp. 1354-1357. 

It has also been shown [254] that a commercial petroleum sulfonate surfactant which consists of a diverse admixture of monomers does not exhibit behavior typically associated with micelle formation (i.e., a sharp inflection of solvent properties as the concentration of surfactant reaches CMC). These surfactants exhibit gradual change in solvent behavior with added surfactant. This gradual solubility enhancement indicates that micelle formation is a gradual process instead of a single event (i. e., CMC does not exist as a unique point, rather it is a continuous function of molecular properties). This type of surfactant can represent humic material in water, and may indicate that DHS form molecular aggregates in solution, which comprise an important third phase in the aqueous environment. This phase can affect an increase in the apparent solubility of very hydrophobic chemicals. 

A study of the foaming capacities and stabilities [10] of a variety of air-entraining agents in a solution of cement extracts showed that commonly used anionic air-entraining agents, such as sodium dodecyl sulfate and sodium resinate (1) were visually precipitated from solution, (2) retained their ability to form stable foams after precipitation with only minor amounts of admixture left in solution, and (3) lost the major part of their ability to form stable foams after filtration. It was further shown from studies in cement pastes firstly that the admixture should be adsorbed on the solid particles of the paste with the non-polar ends of the molecule pointed towards the water phase, imparting a hydrophobic character to the cement... 

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. 

Fig. 4.5 The molecules or particles of the dampproofing admixture line the capillaries with a hydrophobic sheath.

Homopolymers are sometimes modified by a mechanical admixture of another homopolymer. As only about 5 % of pairs of all known polymers are mutually miscible, compatibility may be a problem in mixtures (blends). Copolymerization is technically applied to overcome, for example, the brittleness of polystyrene, polypropylene and PVC. It is also applied for improving the curing properties and modifying the viscoelastic properties of rubbers. By copolymerization, the relation between the hydrophobic and hydrophilic properties of macromolecules can also be modified. Their resistance to solvents may be enhanced. 

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. 

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

In order to investigate the protective effectiveness of a silane-based hydrophobic agent used as admixture to the fresh concrete, an experimental program was carried out, some results of which are briefly reported in the following. For comparison, some field investigations on the effectiveness of a traditional surface hydrophobation of two concrete harbor structures based on similar hydrophobic products are also briefly reported. 

The plastic viscosity of concrete mixture is increasing generally with air entrainment which is caused by adsorption of admixture molecules on cement grains the hydrophobic properties are thus obtained [358], Simultaneously, the bleeding is diminished [358],... 

---