Shrinkage 203 drying

The mechanism of shrinkage is complex and relates to the following phenomena  

The phenomena occurring during the adsorption and desorption of water vapour in the paste were discussed in the previous chapter. 

The radius of curvature in the capillaries is increasing as the content of condensate decreases simultaneously the compressive hydrostatic pressure inside the skeleton composed of gel particles increases. The capillary pressure becomes important to the relative humidity of about 40% at lower humidity water does not form menisci in the capillaries [63], 

The liquid in the capillaries of small radius exerts relatively high pressure, inversely proportional to the radius of meniscus  

The shrinkage resulting from the loss of capillary water results in the elastic deformation, depending on the elasticity modulus of the paste. The reduction of elasticity modulus with the increase of w/c ratio leads to the rise of this deformation. 

Attractive forces exist between the water in a pore and the solid surfaces when the liquid evaporates, the tension in the meniscus is transferred to the walls, and the pore tends to shrink. The shrinkage is not necessarily reversed when all the liquid has gone, because the pore may collapse. This effect could be important between about 90% and 45% RH. Above 90% it is unlikely to be important because the pores that arc being emptied arc wide, and the resulting stresses are small, and below about 45%, a stable meniscus cannot form. 

Due to unsatisfied bonding forces, the surface of a solid particle is under tension, as in a liquid. Adsorbed molecules decrease this tension, and if they are removed, the particle tends to contract. The greatest difference occurs when the last adsorbed layer is removed, and the effect should therefore be greatest below about 20% RH. It can only occur to the extent that closer packing is possible within the solid particle, e.g. by rearrangement of smaller units. Although discussed here in relation to a particle, it could presumably also occur in a convex portion of an irregularly shaped, continuous structure. 

If two solid surfaces are in contaet and the attractive forces between them are outweighed by those existing with moleeules of the liquid, the latter may be drawn in between them, so forcing them apart a disjoining pressure is said to exist. The effect occurs with many clay minerals. The main attractive forces in the latter case are probably ion-dipole forces between ions in the solid surfaces and water molecules, and the same could apply to C -S-H. 

This is essentially the same effeet as the last, but occurs between layers within a single particle. There could well be a continuum of effects, ranging from movement of water molecules between or away from the surfaces of adjacent separate particles to similar movement involving layers within a particle. An intermediate situation in such a continuum might be movement of water into or out of interlayer spaces at re-entrants at whieh layers of a eontinuous structure are splayed apart, such as those shown in Fig. 8.4. 

At low ij), the cmve is linear. At high values of , there is a critical value, where no fiirther shrinks e takes place, corresponding to liquid just filling the pores at the leatherhard point. This critical volume fraction, , occurs when the mechanical properties of the particle network is sufficiently rigid to resist the compressive capillary pressure. The liquid expansion of a ceramic green body, a, is defined by 

This equation assumes that the green body is isotropic (i.e., = 

When the green body is wet by the solvent, it has a compressive capillary force which holds it together. This capillary induced tension 

FIGURE 14JS Liquid expansiaii as a function of solids volume fraction with critical volume fraction where the particles are in cMitact 

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Supersulfiated cement (82) has a very low heat of hydration and low drying shrinkage. It has been used in Europe for mass concrete constmction and especially for stmctures exposed to sulfate and seawaters. 

A high soHds concentration is desirable to minimize the amount of time and energy required for forming and to minimize drying shrinkage. Using deflocculants, fluid slurries can be made using as Httle as 15—20 vol % Hquid. 

Current research related to biological additives is focused particularly on their influence on the properties of mortars, namely on porosity, tensile strength, compressive strength, drying shrinkage, etc. [23, 24, 26], The identification of proteinaceous additives used in historical buildings has been marginal for many years and no reliable methods are properly described in the literature. 

Fig. 3.36 The increase in compressive strength of plain and air-entrained concrete up to a period of 1 year. Table 3.25 Drying shrinkage of air-entrained concrete is no greater than that of plain concrete...

Air-entraining admixture Concrete proportions Air (%) Slump (cm) Drying shrinkage x lO -6 ... 

Table 4.9 Drying shrinkage of concrete containing varying proportions of a wax-emulsion-based dampproofer...

Fig. 5.37 CaCl2 increases drying shrinkage of concrete, although the moisture loss is decreased (Shideler).

Table 5.6 Drying shrinkage of concretes containing calcium chloride, triethanolamine and calcium formate...

Admixture Concentration (% by weight of Water- cement Cement content Drying shrinkage (%) at (day) ... 

The drying shrinkage of concrete containing calcium chloride is increased in comparison to plain concrete, even though the amount of moisture lost is less [22]. This is illustrated in Fig. 5.37 and it is thought that the reduced moisture loss will be due to the more advanced state of hydration in the specimens containing calcium chloride. The increased shrinkage must, therefore, be a characteristic of the type of cement hydration products formed. Under saturated conditions, such as total water immersion, the amount of expansion of the concrete is reduced when calcium chloride is present. 

Admixture types and addition levels which necessitate the use of increased water contents in the mix will increase drying shrinkage. 

The susceptibility of concrete to cracking due to drying shrinkage depends on whether the concrete is restrained or umestrained. If the concrete is umestrained, it can shrink freely and change volume without cracking. 

Fig. 6.22 Effect of pore meniscus and surface tension on the drying shrinkage of mortar (Baiogh [122]).

Fig. 6.23 Relationship between surface tension and drying shrinkage (Berke et al. [124]).

Increased tendency for drying shrinkage and differential thermal cracking. 

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