Volume Changes of Concrete

Ai tcin [125] reminds that the increase of coarse aggregate content and decrease of fine aggregate results in reduction of final shrinkage. However, it should be remember that the coarse aggregate content increase has no effect on the absolute volume change of cement paste. The latter one depends only on the w/c ratio. 

As a result of concrete exposure to dry air or wind, the loss of some water content and decrease of its mass occurs. In the surface capillaries the menisci are produced and consequently the tensile stresses appear, reducing the apparent volume of concrete. This is known as drying shrinkage, accompanied by a loss of concrete mass. As it has been mentioned earlier, as higher w/c ratio as faster and more extensive shrinkage (Fig. 5.52), because water can easily evaporate from wide, joined capillaries system. The final drying shrinkage value is affected by porosity and pore stracture, as well as depends on the relative humidity of atmosphere. 

Concrete with high w/c ratio can reveal a chemical shrinkage ten times higher than the autogenous one [125]. However, in the case of concretes with low w/c the autoge- 

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Fig. 1.48 Volume changes of concretes containing a hydroxycarboxylic acid water-reducing agent under saturated conditions (Neville).

L Hermite, R.G. (1965) Volume change of concrete. Proceedings of International Conference on the Structure of Concrete and its Behaviour Under Load, 131-145. 

L Hermite RG (1960) Volume change of concrete. Proc. 4 Int. Symp. Chemistry of Cement, V-3, NBS Monograph 43 659-694... 

Carbonation leads to the reduction of volume changes induced by the moisture changes of concrete matured at ambient temperature or heat treated at normal pressure. However, the volume changes of autoclaved elements, as well as of the lime-sand brick are increased [39]. This stability of volume in the case of non-autoclaved concrete elements was applied in industrial practice [39]. 

Fig. 6.12 Comparison of 24 h early volume change using low and high dosages of expansive admixtures vs Type K cement. (Courtesy Mailvaganam, N., Nunes, S. and Bhagrath R. (1993). Effectiveness of expansive admixtures in structural grout compositions, Concrete International, 15(10), 38-43.

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. 

In estimating the economy that ensues by the use of an admixture, changes in mix composition of a unit volume of concrete and also the cost of... 

Infiltration of the permeable concrete with molten sulfur yields a matrix which is almost impermeable to water. The freeze-thaw durability tests have indicated this is so, and immersion tests showed that fully infiltrated specimens absorbed less than 0.3% water by volume over several months, although methanol immersion and vacuum poro-simeter measurements revealed that a pososity of over 5% was available for filling. A total shrinkage of about 13% (Table I) occurs when liquid sulfur crystallizes to the stable low-temperature S< form, but much of the volume change appears to be accommodated in closed pores and intercrystal inversion fractures which affect the permeability little. 

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