PSI - Issue 64

Antonio Bossio et al. / Procedia Structural Integrity 64 (2024) 56–64 Author name / Structural Integrity Procedia 00 (2019) 000–000

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5

1.50

Ω p,tr

Concrete Oxide Bar

1.00

S

2concrete

x

MC10 SFM EC2

Initial Bar Configuration

h rib

0.50

0

10

20

30

40

50

(a)

(b)

x [µm]

Fig. 3. (a) Different Ω p,tr ratios vs bar reduction, x , (b) Oxide-steel-concrete configuration at disengagement.

By solving the four-layer analytical model, it can be estimated how much the rebar needs to corrode before the concrete cover cracks completely. The corresponding internal pressure created by the oxide ( f l ) is evaluated, too. This pressure, in turn, tells how much the bond strength between the rebar and concrete will change. 4.1. Effect of concrete exposure class Different exposure conditions (represented by exposure classes, UNI EN 206-1) were used to reflect real-world situations. Minimum concrete strength and cover thickness allowed by MC10 were used for each exposure class (see Table 1). For example, Figure 4a shows the results for a 16mm diameter rebar in an environment with mild exposure (XC1). The graph shows the internal pressure ( f l ) exerted by the oxide pushing outwards on the concrete. As the rebar gets thinner due to oxidation (increasing bar reduction, x ), this pressure also increases. Bond strength ratio ( Ω p,tr ) compares the bond strength after corrosion to the original bond strength and it increases initially as the bar gets thinner (due to the pressure from the oxide). However, there's a limit to this. At higher levels of oxidation (thinner bar), the concrete cracks become wider, reducing the thickness of the uncracked concrete around the rebar. This makes the concrete less stiff and allows the pressure ( f l ) to drop again. Since the internal pressure is linked to how well the rebar bonds to the concrete, it's important to consider. But for most practical purposes, engineers are more interested in the direct relationship between how much the bar reduces ( x ) and how much the bond strength changes ( Ω p,tr ). This is shown in Figure 4b, also for harsher exposure conditions (XC4) as well.

Table 1. Concrete main parameters according to MC10. Concrete exposure class Concrete cover ( mm )

Concrete strength class ( MPa )

XC1 XC2 XC3

20 20 30

C20/25 C25/30 C30/37

4.2. Effect of bar diameter How the diameter of the rebar affects the bond behavior was also investigated at the same exposure class conditions. The results showed that the rebar diameter itself doesn't significantly impact how much the bar reduces due to oxidation (bar reduction, x ). However, Figure 5a reveals an interesting trend: for a similar amount of bar reduction, rebars with a smaller diameter experience a greater improvement in bond strength (higher Ω p,tr ratio). This suggests that, even though oxide affects all rebars similarly in terms of thickness loss, the bond strength improvement might be more pronounced for smaller diameter rebars.