Crack prevention concrete additives

An intact polythene membrane within the concrete base of a building will prevent pressure driven flow of radon into the building from the soil, even if the concrete is cracked. Diffusive flow of radon into the building will also be reduced because of the comparatively low diffusion coefficient of radon in polythene ( v 10 7 cm2 s"1). No significant improvement was achieved by substituting a 50 ym sheet of mylar for polythene (mylar diffusion coefficient x 10"11 cm2 s"1). In this case additional difficulties were experienced in sealing the less flexible material to the walls. 

The addition of microsilica generally improves the frost resistance of concrete. However, at higher microsiUca additions the reduced permeability of the material may hamper moisture migration, and may cause a build-up of internal hydraulic pressure and cracking. To prevent this the amount of added microsiUca must not be excessively high. The use of an air-entraining admixture may also be necessary, to provide an adequate air-void system. 

Expansive cements based on calcium oxide are rarely used because of difficulties in controlling the extent and the kinetics of expansion. It has been suggested that expansive additives based on free CaO should be used to prevent early age thermal cracking in hardening concrete (Ebensperger and Springenschmidt, 1992). 

Unlike MgO-based binders, expansive cements (or expansive additives) based on ettringite formation or Ca(OH)2 formation from CaO are of little benefit for controlling cooling shrinkage in mass concrete, because they would undergo complete hydration before the maximum temperamre in the hardening concrete mix was reached. Consequently, the compressive stresses produced by these expansive reactions relax too soon to prevent crack formation in the concrete body in the course of cooling. 

Efflorescence of masonry, brick, concrete, and mortar can be eliminated by coating with a polyvinylalcohol solution in water. A solvent addition dming sihca glass production prevents cracking. The most effective solvents are methanol, ethanol, n-propanol, isopropanol, N,N-dimethylacetamide, N,N-dimethylformamide, methoxyethanol, and tetrafiu furyl alcohol. Oily stains on stonework can be cleaned with petroleum hydrocarbons. ... 

Fibres may also prevent the occurrence of large cracks, thus preventing percolation of water and contaminants into ceramic materials such as Portland cement mortars and concretes. So, corrosion of steel reinforcement or potential deterioration of concrete may be reduced with the addition of a variety of fibres to the Portland cement matrices. In addition, other enhanced... 

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