Permeability of concrete

The permeability of concrete is a guide to its durability (Section 1.5.2) but it can also be relevant to the design of structures which are intended to withstand a hydraulic head of water or other liquid. Extreme porosity is usually... 

The permeability of concrete to aqueous liquids is reduced by most air entraining agents [35] and this is illustrated in terms of the depth of penetration of water under a pressure of 8 N mm 2 m 48 h in Fig. 3.31. This is reflected in an improved resistance to attack by sulfate-bearing solutions which is indicated by the loss in ultrasonic wave velocity, as shown in Fig. 3.32 [37]. 

The permeability of concrete and the rates at which ions and gases diffuse in it are of major importance for durability. We shall consider only the behaviour of cement paste. 

Admixtures are sometimes used to reduce permeability of concrete (80—82). These include pore-filling materials such as chalk. Fuller s earth, or talc water repellents such as mineral oil, asphalt, or wax emulsions organic polymers (acryHc latexes, epoxies) and salts of fatty acids, especially stearates. 

While calcium chloride tends to decrease permeability of concrete, its corrosive influence and ability to increase shrinkage make it a potential hazard to long-term durability in reinforced concrete. 

F. R. Montgomery, The Autoclam for measuring the surface absorption and permeability of concrete on site , Int. Symp. on Advances in Concrete Technology, Athens, 1992, 107-132. 

Depending on which model is taken, various properties of the concrete have to be known beforehand. For example Parrotf s model [15] for prediction of corrosion initiation requires knowing the air permeability of concrete, the content of calcium oxide in the hydration products of cement and a coefficient that is a function of the relative humidity. For predicting the propagation time, the corrosion rate and the maximum acceptable corrosion penetration have to be known. In order to be able to use these models, test methods have to be clearly specified and the interdependencies between variables have to be verified. 

Superplasticizers are often essential in order to obtain durable reinforced-con-crete structures. They allow a high workability, which is essential to achieve proper compaction of fresh concrete especially in slender elements or in the presence of dense reinforcement, without increasing the amount of mixing water. The increase in workabihty is then achieved without changing the w/c ratio and thus without affecting the strength and permeability of concrete. 

Tawfiq K,Armaghani J,Vysyaraju JR (1996) Permeability of concrete subjected to cyclic loading. Transp Res Rec 1532 51... 

Whiting, D., Rapid Determination of the Chloride Permeability of Concrete, FHWA/RD-81/119,174p, Portland Cement Association, Skokie, Illinois (Aug. 1981)... 

There were attempts to relate the permeability of concrete to the properties of interfacial transition zone. However, the unambiguous results were not obtained. According to Roy [142], the constraction of interfacial transition zone surface does not play important role in concrete permeability, while Valenta [143] has quite opposite opinion. This problem will be discussed in Chap. 6 where the construction and properties of interfacial transition zone will be presented [144], In the light of the studies by Richet and Oliver [145] it is evident that the porosity of inteifacial transition zone in traditional concretes (w/c = 0.5 or more) has a significant influence on the permeability of concrete this permeability is a hundred times higher than in the case of cement paste and rises with the size of aggregate (Fig. 5.68). However, the effect of the transition zone on the diffusion of ions is not so evident, because the locally increased water content in this zone, decrease the w/c ratio in cement matrix outside it, which consequently limits the diffusion, thus a total effect can be negligible [145],... 

Porosity and then permeability of concrete contribute to the transport of gaseous or liquid media, as well as ions through this material. The latter one is of basic importance for the corrosion processes. 

Fig. 5.68 Effect of w/c ratio and aggregate size on permeability of concrete, (according to [145]) Bull R, OllivierJ. R, Chapter 3 in La Durabilite des Betons , page 66, fig. 3.4 Transport par diffusion dans un capillaire avec adsorption , Rublished by Rresses de I Ecole Nationale des Ronts et Chaussees, reproduced with the permission of the Rresses de I Ecole Nationale des Ronts et Chaussees...

Plasticizers and superplasticizers improve radically the pore stmcture (effect of w/c) and concrete becomes less permeable to air and water [159], Collepardi and Massida [159] found a capillary porosity and pore size lowering with decreasing permeability of concretes in which water reducers were used. The resistance to the sulphate attack was also improved [159]. 

In the opinion of experts, expressed during the workshop on the sulphate corrosion resistance, a low permeability of concrete is more important than the use of high sulphate resistant cement, in the case of sulphate corrosion hazard [65]. Therefore the w/c ratio maintained on a low level, together with higher cement content and proper curing of concrete at the early age of hardening, are of basic significance. 

The corrosion process consists in migration of chloride ions into the concrete with simultaneous OH ions diffusion in an opposite direction, from the leached cement hydrates. Therefore the porosity, and consequently the permeability of concrete is of fundamental importance. Cement matrix is a part of concrete which is subjected to the destructive action as a first one. The attack of chloride ions on cement paste will be then discussed first. However, some information concerning the effect of aggregate on the chloride ions diftusion is needed for comprehensive presentation of phenomena occurring in concrete. 

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

Analyzing these data it should be remember that the carbonation rate depends on the permeability of concrete, it means that is decreasing with lowering of w/c ratio... 

Table 6.9 Relative permeability of concrete with and without fly ash [310]...

Damage of concrete in aggressive media is accelerated by ice formation. There are several factors affecting the destraction of concrete as a result of capillary water freezing. Water does not freeze in the gel pores, because they are too small for stable ice nuclei formation. The monomolecular layer of adsorbed water, bound with the surface forces, does not freeze too. The temperature of water freezing in capillaries varies with capillary diameter, it is assumed as equal of about -15 °C. The permeability of concrete is an important factor, because water is penetrating into it as a result of capillary action. The process occurs significantly more rapidly when concrete is under the unilateral hydrostatic water pressure. 

There are traditional prejudices against the use of slag cement or Portland cement with fly ash addition in reiirforced concretes, however, they have not been proved experimentally [311, 352], On the contrary, a quite different behaviour of these materials was reported presumably this can be the consequence of reduced permeability of concrete and the resulting lower diffusion (see Table 6.9). However, apphcation of higher content and higher class cement has a positive influence on the dnrabihty of reinforcement. 

Low permeability of concretes produced from slag cements significantly decrease the diSusion of chlorides, irrespectively on the w/c ratio (Fig. 7.10) [1, 53] (see also Table 6.4). Only 2 cm thick concrete layer is practically impermeable for chlorides ditring two years exposition. 

The potassium or sodium aluminates dissolving in alkaline solution, are transformed in Al(OH)" ions [25], Due to the low permeability of concrete corrosion occurs slowly. Though the experiments show that that the rapid deterioration of calcium aluminate cement concrete occur at pH higher than 13, however, these concretes should not be used in environment of high pH. Aggregate containing soluble alkalis should also be avoided. 

Table 2. Workability, strength and resistance to chloride permeability of concretes. 

The permeability of concrete and along with it the migration of aggressive chemicals in... 

Permeability Due to the reduction in watencement ratio, the permeability of concrete is reduced by using WRA or HRWRA. Figure 5.11 illustrates that feature for a concrete with a given cement content and slump value when a commercial hydroxylated polymer is incorporated [28]. 

Figure 5.11 Permeability of concretes at a specified slump with and without hydroxylated polymer as WRA [28]...

The negative effect of carbonation on corrosion of reinforcing steel in concrete is well known. As to the sulfate attack on concrete, as a result of carbonation, the total pwrosity would be reduced and the permeability of concrete could be improved [ss-s ]. So, Gao [s l pointed out that the carbonation layer could mitigate diffusion of sulfate ions to some extent in the full immersion situation. 

Figure 7. Tlie chloride permeability of concrete contained the Pure-slag powder (Concrete 1 series) or the Compound-slag powder (Concrete 2 series) versus slag content...

For practical calculations, the permeability of concrete is considered as equal to that of hardened paste. In that assumption, the existence of microcracks and aggregate grains in concrete compensate each other for water flow. For higher concrete strength and low wic ratio the coefficient and permeability itself decrease considerably. A further decrease may be expected for high and very high performance concretes with low values of wIc ratio and with extensive use of microfillers. 

Measurement techniques for concrete cover are presented by Torrent (1999) and the gas permeability of concretes ranges are indicated in Table 11.5 where results of both methods are indicated the coefficient of... 

RILEM (1999) Technical recommendation tests for gas permeability of concrete, TC 116-PCD permeability of concrete as criterion of its durability. Materials and... 

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