Durability of concrete structures : investigation, repair, by Geoffrey Mays

By Geoffrey Mays

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It appears that the nature of the gels which cause deleterious expansion is dependent on the sodium-silica ratio. Therefore if the reactive particles in the aggregate are more than a certain level, or if they are so fine that sufficient reaction can take place, the gel formed will have a sodium-silica ratio which does not imbibe water as aggressively and therefore does not expand. A number of factors are necessary for the reaction to be deleterious. The soluble sodium content of the concrete must be high enough to give the pore liquid a critical concentration of sodium hydroxide.

The hydrates of this compound have a role in sulphate attack and also in the ingress of free chloride ions which affects the corrosion of embedded metal. Alternative cementing agents used with Portland cements react with its hydration products in a very complex way but generally to form C-S-H. HAC forms calcium aluminate hydrate but little or no calcium hydroxide. All the above reactions take time to complete and hydration, or at least molecular rearrangements, can still be occurring after months or even years.

Weathering Weathering is the deterioration of the outer skin of concrete caused by the effects of rain, frost, sunlight and atmospheric pollution. The result is a change Deterioration of concrete 25 Fig. 9 Section of concrete cement-rich (upper) surface. in appearance, usually considered to be for the worse with concrete and modern materials but sometimes acclaimed as the ‘noble patina of age’ with ‘traditional’ materials (Hawes, 1986). Fine particles of atmospheric dirt settle on to building surfaces and rain, also contaminated with dirt, falls on to and runs off the same surfaces.

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