Concrete deterioration

Concrete exposed to deicer salts, or to a marine environment is subjected to chloride and sodium loading. The ability of concrete to resist the penetration of chlorides and sodium is a primary design consideration in marine or cold environments. The ingress of chlorides into concrete is a major problem due to chloride-induced corrosion of the reinforcing steel and deicer salt scaling [a process by which a thin layer (< 1 mm) of concrete deteriorates from the surface of the concrete]. The penetration of sodium from sea water or deicer salts is generally... 

With portland cement concretes, deterioration takes the form of horizontal cracks, pop-outs, D-cracking, spalling and scaling. Salts used as deicing agents compound the problem. Early theories attributed the mechanism of failure to the 9 per cent volume increase when water converts to ice. "Critical saturation" -moisture filling more than 91 per cent of the voids was considered important. 

The inherent instability of sulfur-infiltrated concrete in aqueous media illustrated in this study may be the most important factor in utilization, because it will affect long-term durability of the concrete in many natural settings. The Ca(OH)2 produced by the hydration of portland cement is a principal reactant in the leaching process, and while it remains sulfur could be extracted, leaving the matrix vulnerable to other destructive processes. The removal rate of sulfur will vary greatly, depending mostly upon the pH of the immersion medium thus, the concrete deteriorates in alkaline sulfatic soils but is relatively stable in the corrosive neutral sulfatic solutions from the sodium sulfate plant. 

For concrete, deterioration due to freeze-thaw is caused bj freezing of pore water inside the concrete. If the pores are too small, then the expansion caused by freezing can exert stresses on the concrete that crack the concrete and thus cause deterioration. Air entrainment of 7-8 %, depending on the aggregate size, can essentially eliminate this freeze-thaw damage [3]. 

In addition to chloride ions, oxygen is also necessary for accelerated corrosion. Chemical, physical, and mechanical properties of concrete can have a significant effect on concrete deterioration by controlling the following ... 

The rate of cracking and spalling of the concrete when exposed to the expansion forces of the corrosion products. The effect of concrete properties on the corrosion and concrete deterioration processes of bridge structures has been... 

Durability of concrete is defined as an ability to ensure, with required factor of safety and for assumed period of time, the functions predicted in the design of concrete structure. There is no need to explain that the durability has a capital practical meaning. Therefore munerous works were focused on the mechanism of concrete deterioration henee this knowledge is a starting point toward the invention of methods aimed in prevention or retardation of the durability threatening processes. 

In reality the mechanism of concrete deterioration as a consequence of acid corrosion, if it is sulphuric acid, is the same as in case of sulphate corrosion. For this reason in both environments the matrix based on Portland cement with reduced Cj A content is more resistant. This example shows the imperfection of kind of corrosion classification, presented above. 

There are also the other reactive aggregates, namely gneiss and mica containing shales [41], In the interfacial transition zone, in the vicinity of aggregate surface— kaolinite and hydromicas, while from cement paste side—gel of sodium-calcium silicate hydrate, respectively are formed. However, in the case of serpentine concrete deterioration is due to the formation of brucite [75]. The clay minerals, such as chlorites, vermiculite, as well as micas and feldspars, are also included to reactive aggregate components. 

The deterioration of concrete can be the consequence of the presence of some aggregates components which, for example, as iron sulphide, decompose to give iron(lll) hydroxide and sulphuric acid [71]. This phenomenon will be presented later. Let us discuss now the studies of concrete deterioration mechanism caused by alkali silica reaction, the most important in practice. The two types of reactions can be distinguish ... 

It should be mentioned that in a warm, dry climate the chlorides improve the sulphate attack [280], Presumably, it is the reason, apart from the known effect of temperature on the rate of chemical reactions (Arrhenius mle), that in a warm climate the concrete corrosion occurs more rapidly than for example in the North Sea. The examples of disastrous quick concrete corrosion in the warm seas in the Middle East are known, as well as the excellent durability of drilling platforms situated on the North Sea [278]. The conditions governing in different geographic zones temperature, erosion, biological environment, are highly variable and therefore the mechanisms of concrete deterioration are greatly modified [61]. 

From the biological factors the important role have bacteria, which are oxidizing sulphur and hydrated sulphides to the sulphates [98]. These mechanisms are not fully elucidated. The ammonium carbonate evolving mollusks can contribute in concrete deterioration too. 

The following properties are altered as a consequence of concrete carbonation shrinkage (see Sect. 5.3.2), strength, porosity, susceptibility to deformations, and resistance to the environmental impact. However, the pH of pore solution in concrete decreases and the passive film on steel is deteriorated therefore, the reinforcement is exposed to corrosion. This corrosion is probably the most frequent reason of concrete deterioration, because the mst formation causes the surrotmding concrete to crack and spall (see Sect. 6.4.11.). 

S. Brown deserves special recognition for his observational skills and interpretive acumen. Brown worked for Lone Star Research Laboratory in Hudson, New York, in the 1930s and in 1940 joined the research staff at the Portland Cement Association, where he spent approximately 25 years in cement and concrete investigations. Most of his scientific efforts were dedicated to the microscopical interpretation of clinker burning, cement hydration, and concrete deterioration. An unpublished report (Brown, 1936) contains the following interesting observations ... 

Reduced concrete deterioration due to alkali-silica reaction in mixes in which Portland cement has been partially replaced by fly ash has been widely reported (Hobbs, 1986, 1989 Meland, 1986 Shayan et ai, 1996). Fly ash seems to act mainly as an alkali diluter, lowering the amount of available alkalis in the system. The capability to reduce the alkali-aggregate expansion may vary in different ashes, and depends on their own alkali content and fineness. 

Yang, Q., Wu, X., and Httang, S. (1997) Concrete deterioration due to physical attack by salt crystallizatiorr, inProceedings 10th ICCC, Goteborg, paper 4iv032. 

Reinforced concrete deteriorates with time due to corrosion of the steel reinforcement and environmental effects on the concrete, excessive loading due to earthquakes and wind, coupled with increased loads on, say bridges, due to heavy traffic. These situations necessitate repairs. When steel had been used previously, there were a number of attendant drawbacks such as increased weight of steel—the steel plates had to be welded together—and there was considerable increase in overall thickness due to the protective jacket of concrete. 

Fig. 7. Visual observation of concrete deterioration. Condition 1 steady at 20°C and 54% relative humidity from 28 to 530 days (Phase I), and then 20 C and 32% relative humidity from 530 to 1132 days Condition 2 steady at 20°C and 82% relative humidity from 28 to 530 days (Phase I), and then from 530 to 1132 days...

Fig. 8. Visual observation of concrete deterioration exposed to Na2CQ3 and NaCl solutions PI...

K. J.Folliard, P. Sandberg, Mechanisms of Concrete Deterioration by Sodium Sulfate... 

Haynes, H. O Neill, R. and Mehta, P. K. Concrete Deterioration from Physical Attack by... 

Thaulow, Niels, Sahu, Sadananda. Mechanism of concrete deterioration due to salt... 

The visual symptoms of concrete deterioration are cracking, spalling, and disintegration. Each is visible and may occur individually or in combination.The American Concrete Institute has identified six primary factors that positively affect the durability of concrete structures these are ... 

Deterioration generally results from the nonapphcation of these listed items. Some of the more common forms of concrete deterioration are discussed in the following paragraphs. 

List the types of failures and their possible causes. Conditions such as spalling, cracking, and corrosion of the reinforcing steel will assist in determining the cause of the concrete deterioration ... 

Concrete itself can be destroyed by physical, mechanical, chemical or biological actions. Concrete deterioration can be the first step in corrosion of the reinforcement (e.g. when freeze-thaw processes induce cracking and spalling of the concrete cover). The chemical and physical attacks of concrete are described in detail in (Biczoc, 1986 Niirnberger, 1995). 

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