Transport Processes in Concrete

In this chapter the mechanisms of transport operating in concrete and the parameters that define them are discussed. Since the liquid present in the pores has an important influence both on the transport of the various aggressive species and in the degradation phenomena that can take place in concrete, it is worthwhile looking first at the composition of the pore solution and the physical forms of water in concrete as a function of environmental conditions. 

1) (The term permeation is preferred to indicate the mechanism of transport by the action of a pressure difference, in order to avoid confusion with the word permeability, often used (even in this text, though not always with complete accuracy) to indicate, in general, the 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

A certain amount of water is contained in the pores of the hydrated cement paste. The actual quantity of water in the pores of concrete, i. e. the moisture content, depends on the humidity of the surrounding environment Several ions produced by the hydration of cement are dissolved in the pore hquid, so that in reality it is a quite concentrated aqueous solution. 

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Figure 2.1 Principal factors involved in the transport processes in concrete, essential in the. phenomenon of corrosion...

The sensitivity of Magnetic Resonance (MR) to the local concentration, molecular dynamics and molecular environment of these nuclei make it well suited for the study of deterioration processes in concrete materials. Hydrogen (water), lithium, sodium, chlorine and potassium are all MR sensitive nuclei and play an important role in cement chemistry. The ability of MRI to spatially resolve and non-destructively examine test samples as a function of treatment or exposure has the potential to provide new insight to better understand deterioration mechanisms and mass transport properties of concrete materials. 

Crete surface to the bulk of the concrete. Permeability is high (Figure 1.6) and transport processes like, e. g., capillary suction of (chloride-containing) water can take place rapidly. With decreasing porosity the capillary pore system loses its connectivity, thus transport processes are controlled by the small gel pores. As a result, water and chlorides will penetrate only a short distance into concrete. This influence of structure (geometry) on transport properties can be described with the percolation theory [8] below a critical porosity, p, the percolation threshold, the capillary pore system is not interconnected (only finite clusters are present) above p the capillary pore system is continuous (infinite clusters). The percolation theory has been used to design numerical experiments and apphed to transport processes in cement paste and mortars [9]. 

Various forms of macro- and microelements differ in their ability to migrate and redistribute among the soil profile. The elements contained in clastic minerals are practically immobile. The elements, bound to finely dispersed clay minerals, are either co-transported with clay particles, or are involved in sorption-desorption processes. Part of the elements are found in concretions and also in very thin coating films of hydrated iron oxides some elements make a part of specially edaphic organic compounds. 

Each of the transport processes that leads to corrosion of the reinforcement and then governs its kinetics can be characterized by a parameter (D, S, K, p) that depends on the concrete properties and can be determined experimentally. Table 2.5 shows the parameters that are relevant to different situations. At least theoretically, these parameters can be used in the design of concrete structures to calculate the evolution in time of corrosion (initiation or propagation) or any other type of degradation as a function of concrete properties and environmental conditions. 

Correlations. Kinetics of the different transport processes that can take place in concrete, and thus the parameters that define them, are correlated among themselves because they depend on the porous structure of concrete. These correlations, however, are not of a general nature, but vary in relation to the composition or... 

Bazant developed a complete physical-mathematical model which describes the corrosion process in submerged concrete exposed to sea water (Bazant, 1979). A complete set of equations has been derived for the transport of oxygen and chloride ions through the concrete cover, the mass... 

On the other side, if there is no continuous network of capillaries in concrete, the transport of ions occurs by diffusion this process is several orders of magnitude slower than the capillary flow. Diffusion takes place under the difference of concentration and can be expressed by a Pick s first law ... 

The authors [62] formulated the fractional eqitation of transport processes, namely, the Eq. (23), where depending on concrete value a one distinguishes persistent (superdiffusive, l[Pg.331]

Specifically, the MACE experiments are intended to provide data on the nature and timescale of the melt/water quench process in conjunction with the time-dependent heat transport rates from the melt to the surroundings, i.e., to the overlaying water and the concrete basemat. 

The combination of kinetics, hydrodynamics and transport phenomena that provide the proper scale-up of bioreactors from laboratory to the industrial scale also has to be taken into account [4]. Different process solutions will be discussed within this chapter in detail, starting from concrete problem and illustrating possibilities to overcome this problem. 

Market and actors In Germany approximately 32 m. tons of cement are manufactured, of which about 16% are processed manually, 31% are used in factory manufacture of concrete parts and 53% are used in the manufacture of ready-mixed concrete. Around 5.5 m. tons are imported. The cement industry operates in a Fordist stmctured market for mass products (lengthy product cycles, dominated by price competition) with trends towards monopolies. Raw materials suppliers and downstream businesses (e.g. cement transportation enterprises) are also being controlled increasingly by the cement manufacturers. 

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