7. Verification of Durability

Transcription

1 59 7. Verification of Durability The performance verification performe thus far has shown that the structural performance is higher than the performance requirements. That performance must not be lost because of eterioration of materials uring the esign service life of the structure (performance item 10 efine in Section 2.2). Verification is performe, therefore, to ascertain that the concrete an reinforcing bars use for the floor slabs an the beams meet the urability requirements. 7.1 Carbonation-Inuce Corrosion Verification concerning the corrosion of reinforcing bars ue to carbonation of the concrete is mae for the lower (bottom) reinforcing bars in the floor slab an in the beam in accorance with Section 2.2 of SSCS/Mate Calculation of esign value of carbonation epth The esign value of carbonation epth y is calculate using the following equation: y = γ α t (7.1.1) cb α = α k β e γ c where γ cb (=1.15) is a safety factor to account for the variation in the esign value of carbonation epth y, α is the esign carbonation rate (mm year -1/2 ), t is the esign service life, α k is the characteristic value of carbonation rate (mm year -1/2 ), β e (= 1.0) is a coefficient representing the extent of environmental action, an γ c (= 1.0) is the material factor for concrete. For the characteristic value of the carbonation rate coefficient α k, the following equation propose in Section of SSCS/Mate is use: α k = γ p α p α p = W/C (7.1.2) where γ p (= 1.1) is a safety factor to account for the accuracy in etermining α p an W/C is the water-to-cement ratio of concrete. Assuming that the water-to-cement ratio W/C of the concrete is 0.45, α k = 1.1 ( ) = 0.53 mm year -1/2 is obtaine. Therefore, the esign value of carbonation rate coefficient α is calculate as follows: α = = 0.53 mm year -1/2. Since the esign service life is 50 years, the esign value of carbonation epth y is calculate from Equation y = γ α t = = 4.3 mm cb Calculation of the critical epth for reinforcement corrosion The critical epth for reinforcement corrosion y lim is calculate from the following equation: y lim = c c k (7.1.3) where c is the concrete cover an c k is the remaining non-carbonate cover thickness.

2 60 Design value Limit value Verification Table Result of verification for carbonation in the slab W/C 0.45 α p (mm year -1/2 ) 0.48 γ p 1.1 α k (mm year -1/2 ) 0.53 γ c 1.0 β e 1.0 α (mm year -1/2 ) 0.53 t (year) 50 γ cb 1.15 y (mm) 4.3 c (mm) 70.0 c k (mm) 25.0 y lim (mm) 45.0 γ i 1.0 γ i y / y lim 0.10 Jugment OK in the beam Since the esign concrete cover over the bottom reinforcement in the floor slabs an beams is 70 mm an the remaining non-carbonate cover thickness c k is 25 mm (because of a salty environment), the critical epth for reinforcement corrosion y lim is obtaine as y lim = = 45 mm. In this calculation, as mentione in Section construction error in concrete cover is not taken into account Verification results Table summarizes the conitions for verification an the verification results. From these results, it can be conclue that the reinforcement in the floor slab an in the beam will not corroe ue to the carbonation of concrete. 7.2 Chlorie-Inuce Corrosion Verification concerning the corrosion of reinforcement ue to ingress of chlorie ions is mae with respect to the bottom reinforcement in the floor slabs an in the beams in accorance with Section 2.3 of SSC/Mate. This section eals only with the verification process for the bottom reinforcement in the floor slabs. Verification for the bottom reinforcement in the beams can be performe in a similar manner Calculation of chlorie ion concentration at reinforcement The esign value of chlorie ion concentration C at a reinforcement location is calculate using Equation It is assume here that the chlorie ion concentration of concrete uring mixing is zero. C 0.1c = γ clco 1 erf (7.2.1) 2 Dt where γ cl (= 1.3) is a safety factor to account for the scatter in C, C o is the chlorie ion concentration at the concrete surface, c is the concrete cover, D is the esign value of iffusion coefficient of chlorie ions in

3 61 concrete, t is the esign service life, an erf is an error function. Because the amount of measurement ata obtaine in the environment in which this open-type wharf is locate is not sufficient, the chlorie ion concentration at the surface C o is efine accoring to Table of SSC/Mate. For the bottom reinforcement in the floor slabs, 9.0 kg/m 3 is use in view of the fact that the height from the mean monthly-highest water level (H.W.L) is about 2 m, an for the bottom reinforcement in the beams, 13.0 kg/m 3 is use because the beams is locate in the splash zone. The esign iffusion coefficient for chlorie ions in concrete D is calculate from the following equation: D 2 w w =γ cdk + Do l w (7.2.2) a w/l = 3 (σ se / E s + ε cs ) where γ c (= 1.0) is the material factor for concrete, D k is the characteristic value of iffusion coefficient of chlorie ions in concrete, D o (= 200 cm 2 /year) is a constant to represent the effect of cracks on transport of chlorie ions in concrete, w is the crack with, w a is the crack with limit specifie in Table 3.6.1, l is the crack spacing, σ se is the increment in stress of reinforcement from the state in which concrete stress at the portion of reinforcement is zero, E s is Young's moulus of reinforcement, an ε cs is compressive strain for evaluation of increment of crack with ue to shrinkage an creep of concrete. The characteristic value of the iffusion coefficient of chlorie ions in concrete D k is calculate by using the estimation formula for orinary Portlan cement-base concrete shown in Section of SSC/Mate. D k = γ p D p log D p = 3.9 (W/C) (W/C) 2.5 (7.2.3) where γ p (= 1.2) is the safety factor for the accuracy of D p. Since the water-to-cement ratio W/C of the concrete is 0.45, D k = = 1.07 cm 2 /year is obtaine. The values neee for the calculation of the crack with w an the ratio of crack with to crack spacing w/l are those calculate in Table for the bottom reinforcement parallel to the face line of the wharf at the mispan of the floor slabs, an those calculate in Table for the bottom reinforcement in the beams. From these values, the esign iffusion coefficient of chlorie ions D can be obtaine as D = (0.245 / 0.245) = 1.39 cm 2 /year. Therefore, since the esign concrete cover for the bottom reinforcement in the floor slabs is 70 mm an its esign service life is 50 years, the esign value of chlorie ion concentration C at the reinforcement location can be calculate as follows: C c = 1 cl Co erf = = 6.5 kg/m 2 erf Dt γ Threshol chlorie ion concentration for reinforcement corrosion For the threshol chlorie ion concentration C lim at the onset of reinforcement corrosion, a typical value of 1.2 kg/m 3, which is given in Section of SSC/Struc, is use. Accoring to the results of stuies on the relationship between the chlorie ion concentration in the superstructure of open-type wharves an reinforcement

4 62 corrosion an chlorie-inuce eterioration, it may be possible to use a value greater than 1.2 kg/m 3 as the threshol value, although the value of 1.2 kg/m 3 is use as a stanar one for this verification Verification results Table summarizes the conitions for verification an verification results for the bottom reinforcement in the floor slabs an the bottom reinforcement in the beams. The verification results inicate a high possibility of having corrosion an performance loss of the reinforcement in these members cause by chlorie ions uring the esign service life. It is, therefore, essential to consier a measure to ensure the require level of urability uring the esign service life. This is escribe in Chapter 9. Design value Table Result of verification for chlorie-inuce corrosion in the floor slab in the beam Type of cement OPC OPC W/C D p (cm 2 /year) γ p D k (cm 2 /year) γ c D o (cm 2 /year) w/l w a (mm) w (mm) D (cm 2 /year) c (mm) C o (kg/m 3 ) t (year) γ cl C (kg/m 3 ) Limit value C lim (kg/m 3 ) Verification γ i γ i C / C lim Jugment NG NG 7.3 Freezing an Thawing Verification concerning freezing an thawing may be omitte because the site of the open-type wharf is free from freezing an thawing (Section 2.4 of SSC/Mate). 7.4 Chemical Attack Verification concerning chemical attack may be omitte because the superstructure of the open-type wharf is free from chemical attack [Section 2.6 of SSC/Mate]. 7.5 Alkali-Aggregate Reaction Verification concerning alkali-aggregate reaction may be omitte by ensuring eterioration resistance to alkali-aggregate reaction through testing of concrete materials an so on [Section 2.6 of SSC/Mate].

5 Water Tightness Verification of water tightness may be omitte because water tightness is not require for the superstructure of the open-type wharf [Section 2.7 of SSC/Mate].