Quality Concrete

The resistance of a structure against corrosion depends strongly on a wide range of properties that are generally taken together in the term quahty of concrete , which includes its composition and the care with which it is executed. Important concrete quality items are [7] ... 

Besides concrete quality, a minimum value of the concrete cover also has to be specified. Eurocode 2 [3] fixes minimum values ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-bearing environments, as shown in Table 11.5. It should be kept in mind that these values are minimum values that should be increased to obtain nominal values by 10 mm, to also take into consideration construction variability. Besides the protection of steel to corrosion, further requirements of minimum cover depth are fixed to ensure adequate transmission of mechanical forces and fire resistance. 

Research conducted in the UAE highlighted that the service life of reinforced concrete is significantly lower than in other parts of the world [15]. The service life of some typical structures is 35 years for offshore structures, 50 years for structures designed according to international and national codes, and 100 years for bridges and tunnels [16]. The harsh enviromnent (high temperature and humidity), the poor concrete quality, and the poor workmanship in the UAE are considered to be the main factors that are responsible for the rapid deterioration and aggressive corrosion of structures. This results in the short service lives of the structures constructed in the UAE. 

The diffusion constant D is determined by the concrete quality. At the carbonation front there is a sharp drop in alkalinity from pH 11-13 down to less than pH 8. At that level the passive layer, which we saw in Chapter 2 was created by the alkalinity, is no longer sustained so corrosion proceeds by the general corrosion mechanism as described in the Chapter 2. 

Empirically, a number of equations have been used to link carbonation rates, concrete quality and environment. Table 3.1 summarizes some of those equations and shows the factors that have been included. Generally there is a dependence. As discussed earlier the other factors are exposure, water/cement ratio, strength and CaO content (both functions of cement and, that is, alkali content). 

Therefore corrosion can be observed at 0.2% chloride if the concrete quality is poor and none seen above 1.0% or more if oxygen and water are excluded. If the concrete is very dry or totally saturated (as in (c) or (d)) then a change in conditions may lead to rapid corrosion. 

The electrical resistivity is an indication of the amount of moisture in the pores, and the size and tortuosity of the pore system. Resistivity is strongly affected by concrete quality, that is, cement content, water/cement ratio, curing and additives used. The chloride level does not strongly affect resistivity as there are plenty of ions dissolved in the pore water already and a few more chloride ions here or there does not make a big difference. However, chlorides in concrete can be hygroscopic, that is, they will encourage the concrete to retain water. This is why chlorides are often accused of reducing concrete resistivity. 

Equation (9.1) is a very simple equation to use if we can derive a value for the constant. However, there are deviations from the parabolic relationship. These include changing conditions such as humidity and temperature will change the coefficient, giving apparent deviation from the behaviour. Also the concrete quality changes with depth so the difficulty of diffusion will be variable. This leads to the generalized equation ... 

It is important to recognize that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the chlorides including capillary action and absorption as well as diffusion. Rapid initial absorption occurs when chloride laden water hits very dry concrete. In many circumstances these will only affect the first few millimetres of concrete. If so then the expedient of ignoring the first few millimetres of drillings and then calculating diffusion profiles will work. If the cover is low, the concrete cycles between very dry and wet or the concrete quality is low then the alternative transport mechanisms may overwhelm diffusion, at least to rebar depth. 

Impressed current, one concrete quality, chlorides in the initial mix... 

Water should be free of substances that may affect concrete quality. It can be water from any water supply company or other source provided it conforms to CEN EN 1008 (2002) for the EU. 

Several methods of speeding the inhibitor to the steel surface have been proposed. Drilling or grooving concrete to apply inhibitors is expensive and damaging. Simple spraying on to a dry surface vill probably be very dependent upon the concrete quality and the moisture level in the concrete,... 

Nischer, P. (1984) Influence of concrete quality and environment on carbonation, in Proc. of RILEM Sem. Durability of Concrete Structures under Normal Outdoor Exposure, 26—29 March, Hannover pp. 231—8. 

Figure 8-9. Influence of environment and concrete quality on the critical chloride content (according to CEB Bulletin 182).

In both structures reinforcement corrosion in terms of loss in cross section was negligibly small. The electrical continuity of the reinforcement, assessed by measuring the electrical resistance between two remote points of the area to be treated was found to be very good and no electrical short-circuits were detected. A few spalled areas, with cracks or visibly bad concrete quality were repaired conventionally with a suitable mortar prior to electrochemical treatment. 

Microscopic evaluation of hardened concrete quality ASTM C 856 petrographic examination... 

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