Special Properties of Concrete
Concrete is a porous material. Pores may be formed in concrete due to incomplete evacuation of entrained air in the course of compacting the concrete mix. It is impossible to produce absolutely dense concrete even by practicing dense placement of the concrete mix by vacuum treatment and repeated vibrating. Pores are formed in concrete also as a result of evaporation of water which fails to react with the cement constitution in the course of hardening.
The density of concrete can be increased not only by vacuum treatment, repeated vibrating or by reducing the content of evaporating water which fails to react with cement.
The placement of concrete of a high density can be ensured by the following means:
1. by selecting rationally graded aggregates ( with a minimum void age ) permitting a reduction in the amount of the introduced cement paste and, hence, a reduction in the water content of the mix;
2. by reducing the mobility of the concrete mix and, hence, by diminishing the cement paste constituent but this requires more intensive compacting;
3. by diminishing the water-cement ratio; this results in a smaller water content, increased density of the cement stone, provided more intensive compacting is practiced;
4. by applying cements binding a great amount of water in the course of hardening such as high-strength Portland cement, alumina cement, expanding cement, etc.; by introducing plasticizers such as soap-naphtha, acidol or alkali-treated wood tar, polymers and materials producing similar effects into the concrete mix.
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Admixtures for Concrete
Concrete can sometimes be improved by an admixture added to the cement, aggregates and water to modify one or more of the properties of the mix. Admixtures are not magic powders that can be added indiscriminately to poor concrete mixes to make good concrete. Neither can it be assumed that they will necessarily make good concrete better. The right admixture for the job must be used if the admixture is to do more good than harm. When a change is made to improve one property of concrete, some other properties will be affected, frequently adversely.
Principal admixtures are: air-entraining agents and water-reducing admixtures. Perhaps the most widely used admixtures are air-entraining agents. Air-entrainment is used to improve the resistance of concrete to damage from freezing and thawing. It also makes concrete slabs much more resistant to scaling where salts are used for deicing. It makes the mix more workable or at least more cohesive. It permits a substantial reduction in the water requirement and consequently the cement content in mass concrete and has helped with the temperature problem by reducing the amount of heat generated during setting of the cement. Air entrainment is generally considered to be the greatest advance in concrete technology in recent years.
Water-Reducing Admixtures
Use of water-reducing admixtures has expanded rapidly in the past few years. The name comes from the ability of these additives to reduce the mixing water required. Also they generally increase strength and they may take it possible to meet a strength requirement that could not otherwise be met with the cement and aggregate at hand.
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Corrosion of Concretes
Many times we know, that the concretes with a various course of corrosion are otherwise the same. They are produced from the same concrete, on the same producing equipment, by the same people, the concretes had the same treatment, static loadings and they have ( from the statistical point of view ) also the same physical/ mechanical parameters ( of strength ). Obviously it is valid, that we do not dimension the concretes correctly until now against corrosion.
The corrosion loading is not given in the concrete structures first of all by the influences of the environment and is not characterized by the present static solution. The environment is a continually changing resultant of the cumulated effect of various variables. Among them certainly belong the changing temperature and the temperature gradients, aggressive materials and further elements.
We are of the opinion, that the problems of the surface corrosion are rooted in deeper connections. We do not know, if it is possible to use various simplifications of the entrance parameters for further treatment, we do not know the laws of the parallel effect of various influences and decisively it is not possible to consider the concrete structures up to the age of 28 days.
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Hydration of Concretes
Concrete is a building material which originates by mixing, laying, compacting and treating the mix containing the cement (binders), aggregates, water, ingredients and admixtures. Under certain conditions this mix obtains in time mechanical properties. The set of all chemical, physical and mechanical reactions which are effective at the same time and whose resultant is permeated, is called hydration. There exist a long term efforts to penetrate deep into the knowledge of hydration processes. There are known many theories, many a partial knowledge. It is not the purpose of this paper to give their detailed summary. However, it is possible to generalize that the prevailing part of explanations supposes that the dominant factor is that of the reaction cementing materials and water. This reaction lies in progressive dissolution of the cementing materials (we do not mean cement only, but possible active aggregates or mixes of various cementing materials, etc.) and through the complicated process of heterogeneously forming gel and metastabile crystalloid structure.