DURABILITY of CONCRETE STRUCTURES

Transcription

1 DURABILITY of CONCRETE STRUCTURES Assist. Prof. Dr. Mert Yücel YARDIMCI This presentation covers the subjects in CEB Durable Concrete Structures Guideline and has been prepared by the graduate students under the supervision of Prof.Dr.Bülent BARADAN in Dokuz Eylul University. Part- VI Corrosion of reinforcement 1

2 Corrosion of metals involves an oxidation reaction, most simply expressed by the equation The proportions of oxygen and metal in the resulting compound will vary depending on the oxidation state of the metal. Corrosion in this form is only of minor concern for steel in civil engineering applications at ambient temperatures, since the reaction is typically slow. Corrosion becomes a problem for steel in conventional structures where water is present. In such circumstances, galvanic corrosion can occur, which is more damaging. 2

3 Chemistry of galvanic corrosion Galvanic, or wet, corrosion describes an electrochemical form of corrosion in which the close proximity of two different metals in contact with themselves and water containing an electrolyte leads to one of the metals corroding. Whether one or the other of the metals corrodes is dependent on the strength with which each metal s atoms are bound to each other. 3

4 Chemistry of galvanic corrosion An indication of this is indirectly obtained in terms of the metal s standard electrode potential, which is the potential difference between a metal electrode and a hydrogen electrode across an electrolyte solution junction under standard conditions. A more positive standard electrode potential denotes a material that is more prone to corrosion and is thus more active or anodic. Where the iron in steel is the more anodic of the two metals, it undergoes oxidation, which takes the form of ionisation at its surface: 4

5 Chemistry of galvanic corrosion 5

6 Chemistry of galvanic corrosion The presence of both water and oxygen is essential for galvanic corrosion to occur. Another important requirement is that the water in contact with the metal is capable of conducting electricity, which means that the presence of an electrolyte is necessary. The overall effect of galvanic corrosion is a loss of metal from the reinforcement, leading to a decline in the load-bearing capacity of a reinforced structural element. 6

7 Passivation The presence of concrete cover acts as a barrier to the movement of oxygen and substances capable of promoting corrosion towards the reinforcement, thus prolonging the life of the steel. However, the alkaline chemical environment in concrete also provides protection to the steel. This protection is known as passivation and occurs when, under conditions of high ph, a highly impermeable oxide layer of less than 1 μm in thickness forms at the steel surface. The layer acts to limit the accessibility of the steel surface to water, oxygen and corrosive species. 7

8 Passivation The stability of the passive layer is dependent on the ph of the pore solutions of the concrete, and a decrease in ph below around 11.5 will lead to the decomposition of the layer. Additionally, the passive layer can be destroyed in the presence of sufficient quantities of certain dissolved ions, with chloride ions being of greatest concern. 8

9 Steel corrosion in reinforced concrete The reactions involving in galvanic corrosion require both water and oxygen. Thus, the rate of reinforcement corrosion is largely dependent on the relative humidity within the concrete pores and the extent to which oxygen can access the steel surface. The importance of water is illustrated in the figure, where an increase in internal relative humidity leads to an increase in the rate of corrosion, expressed in terms of the corrosion current density (Icorr). 9

10 Steel corrosion in reinforced concrete 10

11 Steel corrosion in reinforced concrete Increasing temperature accelerates the rate of corrosion. 11

12 Steel corrosion in reinforced concrete We have seen that the electrochemical processes involved in galvanic corrosion require the transport of Fe 2/3+ and OH ions through solution. For this to progress at a rapid rate, the microstructure around the steel must permit the movement of these ions, and the levels of moisture present must be sufficiently high. Both of these factors determine the electrical resistivity of the concrete, and so this characteristic can be used as a measure of ion mobility. 12

13 Steel corrosion in reinforced concrete 13

14 Detrimental influences of steel corrosion in reinforced concrete The corrosion of steel reinforcement has two detrimental influences on the performance of structural concrete. The first is that the reinforcement itself undergoes a loss in cross-sectional area, which compromises its ability (and the ability of the reinforced concrete) to carry tensile stresses. The second is that the formation of rust at the steel surface eventually leads to the formation of cracks in the concrete cover. 14

15 Detrimental influences of steel corrosion in reinforced concrete The corrosion of reinforcement will only begin to produce a loss in the load-bearing capacity of a structural element beyond a certain level of mass loss from the steel. One of the reasons for this is that the initial formation of rust at the steel surface has the effect of enhancing the bond between the steel and the concrete. 15

16 Detrimental influences of steel corrosion in reinforced concrete 16

17 Detrimental influences of steel corrosion in reinforced concrete The corrosion products of steel are considerably less dense than the metal, which means that rust formation leads to an expansion in volume of up to four times. Generally, wider reinforcing bars located closer to the concrete surface will produce cracks earlier than narrower bars located at greater depths. Cracks may also result simply from the loss of load-bearing capacity, and the resulting increased structural deflection. 17

18 Detrimental influences of steel corrosion in reinforced concrete The overall effect of cracking is that the long-term deterioration in the load-bearing capacity of structural members typically follows the type of behaviour shown in the figure below. 18

19 Chloride Ingress Into Concrete One of the greatest threats to steel reinforcement in concrete is the chloride ion. Chlorides may enter concrete from the external environment via various mass transport processes. They can also be introduced as contaminants in constituent materials or as calcium chloride used as an accelerating admixture. The use of this compound is no longer permissible in reinforced and prestressed concrete, as a result of its corrosive nature. 19

20 Chloride Ingress Into Concrete One of the main reasons that chloride ingress into concrete is of such concern to engineers is the large number of opportunities for chlorides to come into contact with reinforced concrete. In the built environment, soluble chlorides are most commonly encountered from two sources: seawater de-icing salts on highways. 20

21 Chloride Ingress Into Concrete 21

22 Chloride Ingress Into Concrete 22

23 Role of chloride in corrosion When chloride ions reach the surface of steel reinforcement, they act to break down the passive layer at the surface and allow corrosion to progress. This depassivation process almost certainly involves the formation of chloride complexes with iron from the passive layer, such as in the following manner: 23

24 24

25 Protection from chloride induced corrosion Mix proportions and depth of cover Corrosion inhibitors and other admixtures Alternative reinforcement materials Reinforcement coatings Surface coatings 25

26 Protection from chloride induced corrosion Mix proportions and the depth of cover There are a number of approaches in formulating concrete mixes, which can be used to reduce the risk of chloride-induced corrosion in reinforced concrete. From the discussion of chloride ingress mechanisms, it can be deduced that the following strategies will limit the rate at which chlorides penetrate concrete: Reducing the volume of capillary porosity Reducing the pore diameter 26

27 Mix proportions and the depth of cover The most straightforward means of reducing the capillary porosity of concrete is simply to reduce the W/C ratio. Usually, this will also reduce pore diameters Reducing the pore diameter is best achieved through the combination of cement fraction particle sizes that produce a refined porosity. This is most commonly achieved through the use of combinations of Portland cement and cement components that are finer than PC and that undergo either pozzolanic or latent hydraulic reactions (FA, GGBS, SF, etc.). 27

28 The use of other cement components can potentially also increase the chloride binding capacity of the concrete. Figures 4.21 and 4.22 show how air permeability, chloride binding capacity, and chloride diffusion coefficient change with increasing levels of FA and GGBS. 28

29 29

30 EXPOSURE CLASSES FOR CHLORIDES 30

31 31